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Multi-APprOach for high efficiency integrated and inteLLigent cONcentrating PV modules (Systems)

Final Report Summary - APOLLON (Multi-APprOach for high efficiency integrated and inteLLigent cONcentrating PV modules (Systems))

Executive Summary:

The project APOLLON (Multi-APprOach for high efficiency integrated and inteLLigent CONcentrating PV), carried out in the European Seventh Framework Program, has concerned the optimisation and development of Concentrating Photovoltaic (CPV) systems based on refractive and reflective optics. The different technology paths have been followed with an integrated approach, by addressing the research on the whole development chain of the Concentrating Photovoltaic Technology: from the improvement of the Metal-Organic-Chemical-Vapor Deposition (MOCVD) technique, which is used for the growth of semiconductor materials composing the photovoltaic devices, till to the final construction of a prototype concentrating photovoltaic system. In order to achieve the higher potential for a future commercial exploitation of the CPV technology, the APOLLON Consortium has been formed by aggregating 17 Partners (coming from 8 European countries), joining the two "wings" of technological innovation: the research centres and the industry, fixing the final objective to develop an innovative, high concentration CPV system, with a target cost of 2 Euro/W. Different optics have been developed and compared, from Fresnel and hybrid prismatic/Fresnel to spectrum splitting solutions. The ultimate product developed in APOLLON has been a concentrating mirror-based photovoltaic system which adopts new tracking strategies based on new Intelligent Concentrator (IC) PV modules, which, at the end of the project development, reached 30 % efficiency (850 W/m2 and 25°C ) with high efficiency InGaP/InGaAs/Ge multi-junction (MJ) solar cells. The new IC PV modules have integrated patent pending sun pointing sensors, which allow tracking the sun with high precision (<0.1° experimental validated), and DC/DC devices with Maximum Power Point algorithm which are able to maximize the production of the electric power generated by each CPV module. Patent pending new optics working at high concentration factor (around 800 X) have also been developed. The APOLLON Consortium has also investigated new technological route enabling to expand the combination of semiconductor materials to be used in the multi-junction solar device, in particular demonstrating the possibility to deposit in the same MOCVD growth chamber both elements of group IV and elements of groups III and V of the periodic table. Combining III-V and Ge alloys is expected to show further progress in solar cell efficiency surpassing the 45 % value. Research on SiGe Virtual substrate has been also carried out in order to decrease the device cost. In order to check the reliability status of the CPV technology , IEC 62108 tests as well as die shear test and pull test have been applied on the solar cell receivers. A Round Robin methodology has been chosen for the module CPV characterization. Module Testing has been subject of intensive research, since many of the necessary testing methodologies related to Concentrator modules still require development and verification. APOLLON partners worked for contributing to the actual standard on CPV by proposing new theoretical approach to model the electrical behaviour of a CPV module equipped with multi-junction solar cell and by developing new methodology for the determination of outdoor solar cell temperature. New finding on the influence of the electrical operating condition of the module prior to measurement of IV characteristic has been showed as well. It is finally important to highlight the contribution of this project on the measurement campaign of Solar Direct Radiation (DNI) in joint collaboration with the European FP7 project on Research Infrastructures SOPHIA. The initiative initiated in APOLLON has actrated the interest of many European Institutions that will allow enriching the European data-base of the solar radiation, as well as increasing the accuracy of these measurements, thus providing to end-users more accurate capability in energy forecast. Thanks to the holist approach followed in APOLLON, the project has reached its main objective to increase the competitiveness of the CPV technology, more than halving the cost assumed at the beginning of the research and development activity. The environmental assessment has shown that the APOLLON technology is also competitive on the sustainability front: energy payback time is about a year, while the carbon footprint is about 20 g CO2 equivalents per kWh of electricity generated
See attachment PDF Publishable Summary Final Report

Project Context and Objectives:
Concentrating photovoltaics (CPV) can play a key role towards the transition to a sustainable global energy system, owing to the possibility to reach high system efficiency value and produce low environment impact. CPV has also high potential to achieve cost reduction for solar-generated electricity, since it is possible to decrease, in function of the concentration factor, the area of expensive solar cells, by using cheaper optical elements concentrating the light. At the same time, reducing the area needed for the semiconductor material, CPV offers a good answer to the problem of the scarcity of the material. When the European Commission launched in 2007 on the THEME ENERGY the call “Concentrating photovoltaics: cells, optics, modules” a large penetration of the CPV technology in the photovoltaic market was still missing: the production of CPV system, in fact, lied far back in the 1MWp range. There were several reasons that had hindered a large-scale commercialisation of CPV systems, and they were related, mainly, to the following items:

1. The existence of a large variety of the CPV system typology
2. High Cost/Watt
3. Reliability and lack of international norm and standardisation on CPV
4. Lack of initiatives for supporting the industrialisation of CPV

1. CPV offered such a large variety of technical possibilities, ranging from low concentration (C< 100 sun), trough medium (100 < C < 300 sun) up to high concentration (C> 300 suns) systems, with single or two axis tracker, with reflective elements (mirror) or refractive elements (lenses) , , , , , , , ; this large variety of solutions had scattered the efforts in different technological path avoiding a fast development towards industrial application. There was, however, a clear tendency to focalise the attention on high concentration applications, and in particular, on two groups of systems:
- Point focus (PF) systems based on refractive optics
- Point focus or Dense Array (DA) systems based on reflective optics

Both CPV systems presented advantages and disadvantages, thus any of them demonstrated superiority with respect to the other. For this reason, it was important to deepen in Europe the knowledge of both technological paths. With respect to the previous European Projects, one innovative character of APOLLON has been the possibility to address the research and development activities with a Multi-approach methodology, that is, considering both refractive and reflective CPV systems and therefore allowing a benchmarking between the different PV technologies, getting a better understanding of the main obstacles that needed to be overcome for their successful market penetration.

2. A higher competitiveness with the “flat plate” module photovoltaic technology was required to the CPV technology in order to get a stronger penetration in the energy market. The research activities of the APOLLON Project have taken their impulse starting from two main reference documents: the Photovoltaic Technology Research Advisory Council report, edited in 2005, with the title: “A Vision for Photovoltaic Technology”, and the Strategic research agenda for Photovoltaic Solar Energy Technology edited in 2007 and later on in 2011. The first one allowed defining the general overall economic objective to be reached by the CPV technology, while the second ones, helped to fix the objective on each singular component of the CPV systems. In the Photovoltaic Technology Research Advisory Council report it was stated that the generation cost of PV electricity from grid-connected systems would have been halved by 2010-2015. This meant that the typical “flat plate” turn-key system price of 5 €/W could reach the value of 2.5 €/W within the aforementioned period. This prevision had to be kept in mind to address the economic horizon for CPV: it was clear that CPV systems could become a winning photovoltaic technology if a cost of 2 €/W could be achieved in the same period. The main concern in the APOLLON project has then been to set up research activities whose scope was to reach better CPV module performances and, at the same time, to achieve a substantial cost reduction. The Strategic research agendas for Photovoltaic Solar Energy Technology have been used as main reference documents to fix the project objectives on every system component: the solar cells, the optic concentrator, the module and the tracker. Therefore, in the APOLLON project an integrated-approach has been adopted, joining all the actors’ chain, from Universities, SME, industries and final End-User to assure the necessary expertise mixture capable to face all the technological and economic critical issues related to each component of the CPV system.

3. CPV technology needed to overcome its perceived lack of reliability. Long-term stability of CPV systems was an important goal to be demonstrated. It demanded for accelerated testing procedures and long-term-outdoor testing. The existence and acceptance of well-designed and robust standards covering all aspects of Concentrator PV systems was still missing at the starting date of the project, therefore, having in mind that for a successful commercialisation of the CPV technology it was essential to implement the work on norms and standard, in the APOLLON project the testing activities have occupied a very important role. In particular accelerated testing of receiver have been foreseen, following the tests standard described in the IEC 62108, involving also partners with experience in space application, where severe tests on solar cells were routinely carried out. Furthermore, taking into account that the methodology for the outdoor module testing was still under the PV community evaluation, in the APOLLON Project a Round-Robin CPV module testing methodology has been adopted and new testing methodologies applied. In this way, the co-operation of the different Partners involved in outdoor CPV characterisation has provided new characterization methods useful for the development of standardised tests and allowed to supply reliable data for the evaluation of the module performances and energy payback time.

4. While in USA, since early 1991, Arizona Public Service (APS) was installing and routinely operating Amonix high CPV systems to support the industrialisation of this technology , in Europe, an important initiative for sustaining an industrial development of CPV sector only started at the end of 2006 with the Institute for Concentrated Photovoltaic System (ISFOC). To further favour the industrialisation of the CPV technology, in the APOLLON project an important end-user has been involved. CPV technology found is better success as utility scale application, therefore the presence in the Consortium of an end-user could promote a stronger confidence on the CPV technology in this application sector and therefore its deeper future penetration in the energy production market. In order to favour the steps toward an industrialization of the CPV technology, the development activities have also been addressed to assure an higher integration between optic and receivers setting up technologies with low environmental impact.

A synthesis of the project objectives starting from the described contest is reported in Figure 1, while a detailed description of the project targets on each CPV component in reported in Figure 2.
See attachment PDF Publishable Summary Final Report

Project Results:
The APOLLON Project has been carried out in two developing phases: in the first one, an optimization of the existing refractive and reflective CPV technologies, from the cell component up to the system level, has been foreseen, while, in the second one, the research towards more advanced solutions and a more substantial progress beyond the state-of-the-art has been pursued. In the first phase of the project, in particular , two main CPV technologies have been developed and compared: the Mirror Based Spectrum Splitting one (which make use of mirror and diachromic optics concentrating the solar radiation onto two separate arrays of photovoltaic cells, respectively made of “low gap” and “high gap” ) and the Fresnel based one (which uses lenses to concentrate the light on monolithic multi-junction solar cells). At the end of the first phase, in order to assess the progress made by the project, with the scope to maintain the targets aligned with the technological improvement in this field and address the research towards realistic perspectives for the exploitation of results, a Mid Term Review Meeting was planned and an EC Review Report with the help of experts was issued. In consideration of the before mentioned assessments, the APOLLON Consortium re-addressed the research towards the development of a New Mirror Based CPV system solution (abandoning the complex and expensive splitting technology), while maintaining the MJ concept as the heart of the power generation. (see Figure 3).

Figure 3 APOLLON project implementation

Thanks to the holist approach followed in APOLLON, a considerable know-how has been developed in the project regarding the full manufacturing chain of the CPV technology: starting from the MOCVD growth technology till the system environmental impact and cost analysis, including Si and III-V multi-junction (MJ) solar cells, optics, receivers, module testing methodology, tracking issues and measurement campaign for DNI. Priority has been given to find out innovative solutions which could increase the competitiveness level of the CPV technology, both in terms of increasing the system performances and decreasing the system cost per Watt, in agreement with the main project objective. The main results achieved since the beginning of the project are summarized in Table 1.

Table 1. Overview of the results achieved in the APOLLON project

CPV Component or technology Results achieved
MOCVD technology
 New temperature tuning capability to reduce the wafer curvature in strained structures and better control the fast temperature changes at the interfaces between phosphide and arsenide materials.
 New gas injection system and susceptor to grow III-V and IV elements in the same growth chamber.
Solar cells

Solar cell with high CPV-Cell Economical Performance Index
( Efficiency ([ŋ], Concentration factor [Cf] Yield [Y], Cost [C])
 Si-c (suitable for ultrasonic welding): ŋ= 20%, Cf= 20 suns, Y =96%.
 SJ InGaP (for spectra splitting system): ŋ= 16%, Cf=400 suns, Y =96%.
 DJ InGap/GaAs (for spectra splitting system): ŋ= 29.92 C=750 suns, Y =78% (working tunnel diodes till 5000 suns).
 TJ InGaP/InGaAs/Ge: ŋ= 35.3%, Cf=2000 suns, Y=78%, (working tunnel diodes till 4000 suns), C < 0.6 €/W.
 New robust grid design to minimize the current dis-uniformity generated by grid interruptions.
 New concepts: Bifacial InGaP/GaAs/SiGe MJ cell (demonstrated the possibility to growth SiGe in the same MOCVD reactor for III-V growth).
 SiGe virtual substrates with EPD = 3.2 • 105 cm-2.
Optics

Fresnel lens: ŋopt = 82.5 @ 836 suns, α = ± 0.5°. ŋopt = 84.7% @ 256 suns, α = ± 0.5°, reaching with SOE, Cf= 700 suns.
 Prismatic/Fresnel hybrid optics (patented): ŋopt ~81% @ 500 suns, with SOE improved α: from ±0,5° to ±0,9°.
 Two spectral zone concentrator: ŋopt =77%, α = ±0,9° on InGaP target Cf =625 suns and ŋopt =78.5%, α = ±1,5° on Si target Cf =42 suns. Patent pending.
 Mirror based concentartor: ŋopt = 82.5 Cf= 836 suns, α = ± 0.5°. Patent pending.
Note ŋopt = optical efficiency; α = acceptance angle

Receivers
 Chip on board (COB) and Chip of flex (COF) receivers suitable for high-throughput module assembly production line.
 Wire pull test and die shear test successfully accomplished
 Prequalification testing (thermal cycling) according to EN 62108 performed.

Modules
 PF module with ŋ =24% (STC) .
 Mirror based spectra spitting module with ŋ =17% (STC).
 New intelligent mirror based concentrating modules (ICM) with internally integrated position sensitive detectors (PSD) and Maximum power point tracking devices (MPPT), ŋ =30% (STC). Patent pending.

Tracking
 Development of intelligent sensor for accurate tracking.
 Development of the maximum power point tracking electronic for CPV module.
 Development of “alignment tester” for the correct installation of the CPV module on the tracker structure.
 Development of tracking system based on close loop (driven by the ICM) and open loop logic.

CPV system environmental impact and cost analysis

A Life Cycle Assessment (LCA) has been carried out
 Energy payback time (EPBT) = 1 year (for Catania, Sicily, 1794 kWh/m2/yr), 30 year system lifetime, and a power rating at 850 W/m2.
 The Apollon final design is about 20 g CO2 equivalents per kWh of electricity generated.
 Mega-modules, each with six 32-cell modules mounted within a single housing, are envisioned for future commercialization. With these modules the installed system cost would turn out around € 3.07/Wp; the Apollon project target of € 2.00/Wp can be reached by a full automation of the manufacturing processes, by increasing the module efficiency to 34% and applying cheaper material for the primary mirror and module housing.
 Further cost reduction can be obtained maintaining the process yield >80% and by increasing the concentration factor till 1500 suns.

Module testing methodology
 Round Robin performed to supply reliable date to the module supplier. Efficiency value were found in agreement within 1.8% (STD %). Discrepancies has been attributed to the measurement apparatus more than to spectral conditions.
 New theoretical approach to model the electrical behaviour of a CPV multi-junction (MJ) module and to evaluate the cells junction temperature of the module in operating outdoor conditions.
 New finding on the influence of the electrical operating conditions of modules prior to measurement of IV characteristics to minimize possible measurement artefacts that this parameter can introduce.
 Set up of Real Time Monitoring and Diagnostic System (RTMDS) test facilities

Measurement Campaign on DNI
 Definition of an outdoor methodology for measuring with the maximum level of accuracy the spectral irradiance distribution of solar spectrum.
 The result of this campaign have been made available also to the European project SOPHIA, for the DNI European data base. In 2013 Institutions: 15 (It , Es, Nl, De); Participants: 22; Instruments: 100.

1.3.1. Solar cells development
Several approaches are under development to rise the CPV efficiency system value, mainly adopting discrete or/and monolithic multi-junction solar cell technologies. The formers are used in CPV systems adopting the splitting of the solar light. In this case, the spectrum is separated in two or more wavelength ranges, each one concentrated on different kinds of materials (solar cells), grown on different substrates, and having different energy gap values to increase the light harvesting. This solution let the CPV system developer the freedom to select the energy gag of the materials without any constrain of compatibility between them. The monolithic multi-junction solar cell technologies, on the other hand, use a common substrate over which the different materials with different energy gap are grown. In this last case, the materials utilized in the multi-junction structures should present similar lattice constant, in order to avoid introducing in the device detrimental defects. Since in APOLLON, both the presented technological approaches has been considered and compared, different solar cell technologies have been developed, in particular the silicon based and III-V based.
Since the beginning of the project, the approach of APOLLON aimed to find research paths able to increase the solar cell efficiency value, by improving, as well, the values of other solar cell parameters (concentrator factor, yield and the cost) in order to get economical affordable solutions. The main objective of the solar cell development can be summarized in the aim to increase the cell economic performance index value (CEPI) .

As far as the silicon cells are concerned, in Apollon, concentrator “laser grooved” cSi solar cells have been developed by NAREC and ENEA. Scope of the development has been: a) to modify the bus bar and grid design of the solar cell in order to produce a simplified contact schemes suitable for highly throughput automated assembling line; b) to optimize the device structure to increase the efficiency value. Several parts of the solar cell structure were optimized: ARC coating thickness, emitter, back contact (in order to increase the reflectance and then the Infrared spectral response ). An example of the simulation performed to optimize the Sic cell is reported in Figure 4.

Figure 4. On the left: 1-D, 2-D, 3-D cSi solar cell modeling and their fitting to experimental data; on the right: Optimization of emitter junction depth and finger spacing to different concentration regimes (b) applied to filtered AM1.5 solar spectrum ʎ> 600 nm.

Several solar cell design contact scheme have been produced to meet the requests of the CPV system developer (see Figure 5). Thanks to the integrated approach, a close and fruitful interaction has been produced between the solar cell manufacture and the system developer in order to reach the optimized solution.

Figure 5. Different cell layout scheme and solar cell design produced during the development phase

According to the target of the project. Sic have been realized with efficiency value=20.5% @30sun with yield=95%.

In the first phase of the project, for the “high energy gap” target of the Mirror Based Spectrum Splitting system, ENE has developed Single junction (SJ) InGaP and dual junction (DJ) InGaP/GaAs solar cells, while triple junction (TJ) InGaP/InGaAs/Ge solar cells have been developed for the Fresnel based CPV system. The performance obtained are reported in Table 2 .

Table 2. Performances of the III-V solar cells produced for the Mirror Based Spectrum Splitting Fresnel based CPV systems

Cell type DIA EFF (%) Concentration factor Yield (%)
InGaP SJ 16 400 96
InGaP/InGaAs DJ 29.2 750 78
InGaP/InGaAs/Ge TJ 35.5 750 78.8

The TJ solar cells have been characterized at high concentration, showing excellent efficiency of 35,3% @ 2000 suns (see Figure 6) . The tunnel diode connecting the different part of the device, have been checked till 4000 suns.

Figure 6. TJ InGaP/InGaAs/Ge ENE solar cells performances versus concentration

In the second phase of the project, several research activities have been implemented with the scope to increase the CEPI ( see. Figure 7).

Figure 7. Block diagram of the different research activities followed in APOLLON to increase the cell economic performance index value.

The WP2.2 aimed mainly to increase the solar cell efficiency values at high concentration, the WP2.3 to decrease the cell cost, while the WP 2.4 aimed to increase the wafer yield and solar cell efficiency.


Development of a new high flexibility MOCVD growth chamber, WP 2.4

Nowadays most of the III-V based MJ solar cells utilized in CPV systems are obtained by growing different layers of semiconductor material by using the Metal Organic Vapour Phase Epitaxy (MOVPE) technique. It is worthwhile remembering that when the concentrator technology based on III-V solar cells was initially proposed in the 70’s, the solar cell manufacturing was based on the Liquid Phase Epitaxial technique (LPE). At that time, III-V concentrating solar cells were only GaAs single junction (SJ) devices. At the end of the 80’s, the activities on CPV entered in a “dormant” period, as new technologies, like amorphous silicon, were publicized to be the final solution for reducing the cost of the PV energy. A renewed attention on III-V cells rose up from 1990 on, when the MOVPE technique replaced the LPE growth one, owing to the better composition and thickness layer control allowed by the former. Higher cell efficiency values could be obtained and the development of multi-junction concept taken off. This brief historical review helps to introduce the importance of a constant evolution in the growing technique to boost the performances of solar cell technology.


The Problem of Wafer Bowing during Multi-junction Solar Cell Growth

The uniform growth of III-V semiconductor layers and structures depends on high thermal homogeneity of the wafer surface. Growth rate, crystal quality and composition, especially in ternary and quaternary materials highly depend on the local temperature of the surface at which the species are incorporated into the crystal lattice. So far, supposing semiconductor wafers under strain-free conditions, thus lying flat in the wafer’s carries pocket (wafer satellite), MOCVD reactors optimization was directed towards providing a uniformly heated graphite wafer carrier (wafer susceptor) in order to get a flat thermal gradient across the said pocket, and then along the wafer’s diameter. However in the course of the deposition of multi-junction solar cells strain-free conditions are not generally present, since several layers with different thermal expansion coefficient are usually joined together, furthermore, in order to improve the efficiency of the solar cell structures, strain engineering has been more and more applied. Therefore, during the MJ solar cell structure growth, the bow of the wafers is always present and it is expected to change during the growth of such structures. This, in turn, causes a loss of contact between wafer and wafer satellite, causing undesired thermal gradients between wafer center and edge (see Figure 8) and, as a final result, non-uniform electrical and optical properties in solar cell layers.

Figure 8. Bowing of a wafer in the wafer carrier's pocket can lead to loss of thermal contact of the wafer to the heated graphite (figure top for convex and concave bow). This leads to undesired thermal non-uniformity on the wafer surface causing inhomogeneous growth of the semiconductor materials (figure bottom).

In order to apply strain engineering without compromising the wafer yield a new design of the MOCVD reactor growth chamber was needed.

MOCVD with New Temperature Tuning Capability

The requirement of removing the supposition of semiconductor wafers under strain-free conditions resulted in a paradigm change in MOCVD equipment design, since the changing thermal gradient induced by the loss of mechanical contact between wafer and graphite had to be compensated in-situ, i.e. during growth, by adjusting the center-to-edge temperature of the wafer carrier to keep the wafer surface temperature constant and uniform. Basis for development were the RSE specifications that AIXTRON applied on AIX 2800G4 Planetary Reactor platform which features a main RF-heated susceptor carrying 12 four-inch wafer carriers. Those wafer carriers, called “satellites”, are rotated on a gas-foil. Several approaches to influence the thermal gradient across these satellites were investigated. They ranged from mechanically adjusting the RF-coil heights via RF-field displacement till using the gas foil (GFR=Gas Foil Rotation) itself for temperature control. Numerical CFD (Computational Fluid Dynamics) simulations yielded the best results and maximal satellite temperature profile tuning capabilities for the GFR approach. Different inert gases, such as H2 or N2, have quite different thermal transport coefficients, implying that by administering H2, N2 or a mixture thereof to the center and edge of the satellite gas foil separately, the thermal transport from the homogenously heated susceptor to the satellite can be modified. Since the gas flow and composition can be adjusted at any time during the growth process, the MOCVD reactor can adapt to any strain-induced wafer bow and can, thus, keep the wafer surface temperature constant by boosting thermal transport either at the center or the edge of the satellite. Demonstration of the concept has been proved in APOLLON considering the temperature tuning capability on Ge wafers, which are mostly used for the fabrication of MJ cells. CFD simulation has also been performed to predict the temperature distribution over Ge wafer in function of the gas foil composition underneath the satellite introduced separately in the inner and outer zone.

A strained InGaP/Ge growth was designed in order to introduce a curvature in the wafer and analyze the temperature difference evolution between the center and the edge of the wafer during the temperature tuning experiment. For this purpose, advanced diagnostic “in situ” tools were used allowing measuring the wafer curvature during the growth and the true (emissivity corrected) wafer surface temperature in the wafer center and edge (see Figure 9)

Figure 9. Temperature in the center and edge of the wafer along with curvature behavior during the experiment described in the text. The curvature is measured in the center of the wafer. It is possible to distinguish 4 zones: 1) Growth of the strained InGaP layer 2), Growth interruption and stabilization, 3) First variation of the H2/N2 mixture underneath the satellite, 4) Second variation of the H2/N2 mixture underneath the satellite .
Once the InGaP was grown with a slightly different lattice parameter with respect to Ge, the wafer bent. After observing the initial wafer bending, the growth process was interrupted and a proper time was left for allowing the system to reach the equilibrium. After the deposition interruption, an equilibrium status was reached, in which the stress induced by lattice mismatched was compensated by the wafer curvature. At this point the value of the curvature reached the constant value of 500 km-1 and the difference between the surface temperature of the centre and the edge of the wafers was around 20°C. Owing to the wafers deformation, the surface edge temperature of the wafer decreased with respect to the center (concave bending). Subsequently two different H2/N2 mixtures were introduced underneath the satellite (zone 3 and 4 of Figure 9), while continuing measuring the evolution of wafers temperatures and curvature. After the first change, the curvature increased, till reaching the value of 800 km-1, while the difference between the surface temperature of the centre and the edge of the wafers reached 30°C; with the second change, the curvature and the temperature differences were zeroed. The results indicates the presence of thermal gradient on the back side of the wafer allows controlling the wafer deformation and therefore applying strain engineering without compromising the wafer yield.

Another important application of the new temperature tuning capability is the possibility to control the rapid change of temperature which take place at the As-based and P-based interface, found in the InGaP/InGaAs/Ge multi-junction solar structures . It is worthwhile to point out, in fact, that a very fast temperature control can be obtained with the new temperature e tuning capability, as this capability is applied, by using different gas foil mixtures, at the satellite level, where the thermal inertia is drastically reduced, while such a temperature control would be impossible if applied at the susceptor level, as usually happens in conventional MOCVD reactors, since, in this case, the greater material‘s mass would imply too high thermal inertia for a fast temperature control.

The Problem of Group IV and Group III-V Semiconductor Growth Incompatibility

So far, the possibility to grow Germanium and its alloy in the same MOCVD growth chamber to be used for the subsequent III-V MJ growth has been hindered by the “Carry over effect ”: since Ge is a dopant for III-V compounds, after epitaxial Ge deposition, Ge can continuously evaporate from the MOCVD reactor walls and susceptor limiting the formation of the N/P junction in the subsequent growth of III-V layers. The same detrimental effect can be expected in the subsequent growth of the Ge-junction in the next growth run as III-V-atoms then covering the reactor surfaces also act as dopants in group-IV semiconductors. In addition, the growth conditions for the growth of Ge and GaAs can be quite different in terms of growth pressure and temperature. In order to face the challenge of Ge desorption from the reactor walls and remove the growth incompatibility of Group IV and group III-V semiconductors, a new design of the MOCVD reactor growth chamber was needed.

The APPOLLON SOLUTION: Reactor and deposition Optimization for Group-IV and III-V Semiconductor Growth in the Same MOCVD Chamber.

The design of the MOCVD reactor has been optimized for both Ge and III-V growth conditions and in order to reduce desorption of atoms from the reactor walls and susceptor. To allow maximal flexibility and introduce the possibility of an inert gas flow at the large ceiling of the reactor, a triple injector was chosen. This inlet features a group-III inlet sandwiched at top and bottom between two group-V injectors, which can also be used to individually add inert carrier gas.

Figure 10. Schematic (left) and implementation (right) of the 3-fold flexible gas inlet.

One objective of the optimization was to find the ideal diameter for this injector in conjunction with the optimal temperature profile the gas experiences when flowing from the injector to the exhaust. The different decomposition kinetics of the precursors IBuGe, TMGa, TMIn, TMAl, TBP, TBAs, PH3 and AsH3 have been taken into account and the temperature profile tuned accordingly. Using numerical CFD computation a relatively large injector was found as the optimum solution (see Figure 10). To check Group IV and Group III-V Semiconductor Growth compatibility, SiGe growths have been carried out with Silane and IBuGe in the same APOLLON growth chamber utilized for the growth of III-V compounds. Before and after SiGe deposition, a GaAs layer, 10 µm thick, slightly doped with silicon has been deposited on a GaAs undoped substrate, as a marker run for the assessment of the Ge carry over effect . No attempt has been made to optimize the mobility in the marker run executed before the SiGe deposition and after several triple junction growth runs. As a precaution taken to decrease the Ge carry over effect, one III-V-based coating run has been carried out after SiGe deposition. The results of the Hall characterization are reported in Table.3.

Table 3. Hall characterization for the assessment of the Ge carry over effect.

RUN/growth parameters and Hall results T(°C) V/III ratio Growth rate (µm/h) Hall mobility (cm2/V s) at 300 K/77 K Background carrier concentration (cm-3) at 300 K/77 K
RUN 263, GaAs before SiGe deposition 650 21 7 4187/5492 8.9*1016/ 7.6 1016
RUN 279, GaAs after SiGe deposition 650 21 7 2631/2565 4.6*1017/5 1017

The results show that by using a proper designed MOCVD growth chamber and adequate growth procedure is possible to reduce the carry-over of Ge to a level suitable for subsequent III-V N-P junction growth. The selection of an adequate solar cell polarity helps further to minimize the effect of Ge incorporation in III-V based layers. Another indirect proof of the reduced carry over effect, have been obtained growing several triple junction cells InGaP/InGaAs/Ge after SiGe deposition. The fill factor of such cells presented values over 87% demonstrating the high quality of the top cell limiting junction. Therefore it is possible to deposit group IV and III-V elements of the periodic table in the same high throughput MOCVD systems, expanding the band gap engineering possibilities to increase light harvesting and consequently the solar cell efficiency, without introducing substantial cost increase in the growth process, which would be required if separate growth chambers for group IV and groups III-V elements were utilized.

Solar cell structure developments and new concepts, WP .2.2

First of all Solar cell modelling has been implemented following an hybrid approach: by using ATLAS (by Silvaco) for band diagram calculation and simpler simulation of single junction solar cell, while applying the Hovel analytical model with a Matlab software platform for the performance evaluation of multi-junction solar cells. Different InGaP/InGaAs/Ge solar cell structures have been designed with improved top energy gap and realized for getting high peak tunnel diode and high efficiency. In order to increase the solar cell reliability, a new front contact grid more resistant to grid interruption has been modelled (see Figure 11) and then realized by electron beam lithography and by rapid thermal annealing process

Figure 11. Equivalent circuit utilized to simulate the solar cell grid performances and results of the simulation carried out by RSE on the effect of the solar cell grid interruption on the efficiency. The “carpenter” geometry has been selected
Anti-reflection coating (ARC) has been modelled, and realized by Ion Beam Assisted Deposition in order to avoid further thermal annealing. The oxide layers have been analysed with reflectivity measurement, X-ray photoelectron spectroscopy and Atomic Force Microscopy in order to optimize the deposition process. Nano-structure morphological modifications of ARC surface have also been investigated in order to further improve coating performances. Most of the growth activity has been dedicated to the qualification of the new MOCVD reactor till arriving to develop triple junction cell in line with the devices developed in the first phase of the project. The more remarkable result surely concerns the possibility to grow in the same MOCVD growth chamber III-V and IV elements, that more than compensate the discrete results on the efficiency reached during the development work. By all means, the successful development of TJ cell with efficiency values >35% in the same MOCVD reactor also used to growth IV elements has surely constituted a fundamental and starting proof which has opened the route toward more promising multi-junction structures. A new kind of bifacial solar cell based on III-V and IV elements has been proposed in the road map to further increase the solar cell efficiency (see Figure 12.)

Figure 12. On the left, sequence for the realization of the bifacial (BFC) SiGe-GaAs-InGaP solar cell; on the right, solar cell road map .

Low temperature Ge and SiGe epitaxy, WP 2.3

The driving force to combine III-V on Si is the cost reduction reachable by using the cheaper silicon substrates instead of the more expensive Ge or GaAs ones. The main difficulties to realize the III-V epitaxy deposition of Si are due to the different lattice constants and thermal expansion coefficients of the two semiconductors. One of the solutions under development “to match” the two semiconductors, it is to use the so called “SiGe virtual substrates” technology, in which with a proper growth technique, Ge is deposited on Si with a reduced number of defect, over which the remained solar cell structure can be subsequently grown.

Heteroepitaxy of highly mismatched structures involve many physical aspects which influence the growth mode of the epilayer. The different surface energies between the film and the substrate mainly determine the behaviour of the growing film mode. The presence also of the elastic strain induced by the lattice mismatch and the kinetic processes involved during non-equilibrium epitaxial growth, cause the growth to deviate from ideal equilibrium conditions. In case of Ge/Si heteroepitaxy, the difference in thermal expansion coefficient between Silicon (2.6x10-6 °C-1 ) and Germanium (5.8 x10-6 °C-1) cause the wafer to bend and eventually crack within the epilayer which can compromise the subsequent overgrowth of electronic devices onto the layer surface. This effect, in particular, is enhanced in thermally driven CVD epitaxial processes, in which high growth temperature are required in order to overcome the activation energy for precursor dissociation. Thus the implementation of low thermal-budget epitaxial processes is mandatory and low-energy plasma-enhanced chemical vapour deposition is the elective technology to achieve this task. In APOLLON, The University of Ferrara has investigated the realization of SiGe buffer layers by of Low Energy PECVD (LEPECVD) trying to produce Ge layers with surface threading dislocation density compatible with the heteropitaxial III-V triple junction solar cells. Three tasks revealed to be the drivers for this research: the achievement of fully relaxed Ge capping layers having a thickness of at least 5 µm, the reduction of the surface threading dislocations density (TDD) down to a value lower than 1x106 cm-2, in order to avoid the detrimental effects of crystal defects leading to minority carrier recombination, and finally the achievement of Virtual substrates (VS) having a surface roughness compatible with the realization mirror like morphologies. In the framework of the project it was possible to realize thick SiGe virtual substrates (6.7 µm-thick) having a dislocation density in the range of 5x105 cm-2, in agreement with the target of the project, as revealed from the etch pit density count ( see
Figure 13), however the typical residual surface roughness was in the range of 50-90 nm RMS, and it was judged too high to get epilayers with mirror like morphologies.

Figure 13. Scanning electron microscope etch pit count.

Pre-polishing of the SiGe wafers is been evidence as a key step in the preparation of the virtual SiGe wafers prior of epitaxial growth. It is claimed that enough thick (5µm) Ge layers are required over Si to accomplish properly the polishing step and get a roughness as much as possible comparable with that of standard Ge wafers.

1.3.2. Optics
The optics in CPV technology have the fundamental function to concentrate the solar flux on the solar cell, allowing a reduction of the semiconductor material needed to produce a given electrical power. With a constant aperture cross-section of the optical system, the higher is the geometrical concentration factor , the lower is the surface of the semiconductor materials needed to produce the same power. For this reason is of great interest to develop optics having high concentration factors. In the Apollon project different solutions for the optical concentrators have been investigated with the aim of reaching high concentration factors, wider acceptance angles , high optical efficiency, long-term stability and a low manufacturing costs. For this purpose, both refractive and reflective optics have been designed, realized and tested by the different partners of the project. Among the mirror based solutions, a comparison has been carried out between the spectrum splitting based solutions and the simple mirror based systems. In particular SolarTec, SE SRTIIE, CRP and ENEA worked on concentrator systems based on a refractive primary lens, while C-Power, UNIFE, RSE and ASSE developed mirror based concentrators. All these systems foresee a secondary optical element to homogenise the power density distribution on the cell plane and to increase the angular acceptance of the system.

Optimization of refractive optics

Silicon Fresnel lens coupled with a secondary optical element have been optimized. The Fresnel geometry has been studied by SolarTec taking into account several aspects like the cell size, the influence of the working temperature, the misalignment of the impinging light beam and the spectral behaviour related to the different spectral response of each sub-cell in a triple junction device. At the same time SE SRTIIE made an accurate study, focused on the optimization of a cone shaped reflective secondary optical elements. The efficiency of the whole concentrator system has been calculated as a function of different focal lengths of the primary optical elements and of the SOE half-angles aperture also in presence of misalignments. SolarTec made a final lens parquet (see Figure 14) composed by 144 silicon Fresnel lens (array 12x12) glued on glass. Indoor test have shown an optical efficiency of the lens of about 84%. SE SRTIIE realized SOE prototypes using a reflective sheets produced from the German Company Alanod GMBH and named «Miro high reflective 95». The SOEs have been integrated on the solar cell receiver (see next paragraph).

Figure 14. On the left: SolarTEc lens parquet. On the right: the hybrid lens designed by ENEA and realized by CRP.

An alternative option to the Fresnel lens, was developed by ENEA. It is a novel kind of optic, called hybrid lens (see Figure 14), having the same size and focal length of the optimized Fresnel lens. It is realized by an array of square refractive prisms in the central area and the typical Fresnel grooves at the four corners. In the peripheral areas, the prisms are replaced by the Fresnel grooves in order to reduce the optical losses due to the total length of the prisms edges. Its design was optimized to provide the best performances when operating with the SOE produced by SE SRTIIE. The used material is PMMA and for this reason the lens do not need to be covered by any protecting glass sheet, thus allowing the manufacturing of lighter modules. The lens was produced by CRP by using an injection-compression moulding process by using a metallic insert mounted on an electrical 70ton injection moulding machine, equipped for the injection-compression mode. The moulding process parameters like mould temperature, plastic injection temperature and pressure, as well the holding pressure, have been optimized in order to improve the optical quality of the plastic lens. The optical characterization have been carried out on the whole concentrator system composed by the hybrid lens and the conical secondary collector. The indoor tests have been performed using an optical bench in the ENEA laboratories generating a light beam having the same angular divergence of the sun. The maximum lens efficiency that was measured was about 82% with an acceptance angle of ±0,9°; also a good power density uniformity on the target was achieved.

Development of mirror based concentrators with spectrum splitting

The C-Power Srl company developed a concentrator system realized with reflective optics and spectrum splitting filters which separates and redirects different light wavelengths towards specific semiconductors solar cells operating at different band-gaps (Figure 15). In parallel to the system developed by of CPower, the University of Ferrara (UniFe) developed another type of mirror based concentrator operating with spectrum splitting. The adopted concentrator architecture was based on a Cassegrain geometry (Figure15). The CPower optic is composed by an asymmetric parabolic mirror deflecting the incident solar radiation toward a secondary optical element that concentrates the light both on the first receiver, consisting on a silicon cell, and on a second receiver composed by a high gap InGaP III-V solar cell. A dichroic mirror is placed between the solar cell devices so to allow near-infrared light reaching the Silicon cell, while at the same time to reflect the remaining part of the solar spectrum toward an optical guide redirecting this radiation to the InGaP solar cell.
.
Figure 15. On the left: schematic cross section of the C-Power concentrator; On the right: Cassegrain Concentrator developed by UNIFE

In this way two different levels of concentration are obtained, allowing the operation of silicon solar cells with a medium concentration factor and on a limited portion of the optical spectrum, while the InGaP solar cell was used at higher concentration so under high light flux. The geometrical concentration reached for the InGaP cell is about 620x while the concentration at the first receiver is about 40x. The acceptance angles of the two receivers are different and, because of the asymmetrical geometry, they are not the same along the different axis; it is obtained an angular acceptance of ±3.0° on the Si target and ±1.5 on the InGaP target. CPower Mirror based spectrum splitting prototypes showed an optical efficiency approximately of 70%

New Mirror based Optics

The optical design has been performed by both CRP and RSE, working in parallel on two similar optical schemes to optimize the concentrator performances. After the design phase a comparison of the performances obtained for the two concentrators was performed and the RSE optical scheme was chosen for the manufacturing phase. The main optical performances for the selected configuration and the comparison with the starting specifications are reported in Table 4.

Table 4. Design and actual performances of the Apollon mirror based concentrator.

Optical efficiency Optical angular acceptance (α) Average concentration Maximum concentration CAP
Starting specifications
(Amendment) >82%
Milestone M39: > 80% ±1.5°
Milestone M39: >±1° 500 Not specified 0.59 0.39
Design performance 84.1% ±0.86° 847 1500 0.44
Measured performance 82.5% ±0.5° 836 2076 0.25

The concentrator design targets were modified with respect to the starting ones with the scope to maintain them aligned with the technological improvement in this field overall and address the research towards realistic perspectives for the exploitation of results. In particular, the market trend suggested to design the concentrator with a concentration factor greater than 500 suns (such as 1000 suns) with an illumination uniformity that should guarantee a maximum concentration factor of 2000 suns. The design phase foreseen the simulation of the primary mirror (POE) in function of the focal length, the radius of curvature, the off-axis distance and the concentrator length. In particular by increasing the POE length in the lower part of the paraboloid, it was possible to improve the collection efficiency for certain misalignment angles.

Figure 16. On the left the Improved design of the POE. By increasing the length of the POE a higher angular acceptance is obtained. On the right 3D view homogenizer with innovative aperture

Different SOE have been considered: standard pyramidal, RSE pyramidal , standard conical, RSE conical , BK7 pyramidal TIR prism, RSE mixed conical-pyramidal (several types). The final selected design was a proper modification of a classical homogenizer having an innovative patent pending input aperture to take advantage from the asymmetrical aberrations produced by the primary mirror, thus improving the angular acceptance of the system, without reducing its maximum optical efficiency (see Figure 16). The advantage in the innovative SOE design is reported in Figure 17. The simulations considered in the optical design only refer to the concentrator optical elements. In order to validate the concentrator design, it was necessary to verify that the incidence angles of the rays impinging on the cell surface well fit the related specifications for the used PV cell. For the selected optical configuration it has been calculated the distribution of the power impinging on the cell as a function of the incidence angles on the cell surface. The results of the simulation are reported in Table 5.

Figure 17. Left: SOE efficiency as a function of the two misalignment angles, Right: angular acceptance of the whole concentrator, comparison between the angular acceptance obtained when using a standard pyramidal SOE vs. a RSE SOE.

Table 5. Distribution of the optical power impinging on the cell surface as a function of the incident angle of the rays on the PV cell.

Incidence angles on the cell surface < 60° 60° ÷ 70° >70°
Percentage of the incident power [%] 97.7 1.8 0.5
Relative solar cell response 1 0.87 0

A proper module window treatment has been also considered in the optical design of the concentrator in order to improve the light transmission. Nanostructured window surfaces, with anti-soiling properties, surely deserve attention in the next research projects in order to improve the energy generated by the CPV module.

The benefits of the designed concentrator can be summarized as the following:
 no central obstruction is present on the input aperture of the concentrator so better optical and collection efficiency can be achieved;
 the light impinges on the mirrors with high incident angles thus improving the reflection efficiency;
 the optical system can be easily downscaled (the minimum size is mainly limited by the precision of the robotized industrial mounts);
 the PV cell is located on the bottom side of the concentrator which is a cooler position, not illuminated by the sun; such position also allows an easy replacement of the photovoltaic cell.

Manufacturing of the concentrator
Primary Optical Element (POE)
To reduce the costs of the optical components the mirrors are made by deformation of commercial reflective sheets to the desired paraboloid shape. The optically best reflective sheets (with silver-based deposition) are more difficult to deform, the challenge is to shape these sheets without creating buckling waves and without damaging the reflective layers. Alternative production process, such as the deposition of silver and oxide layers on a suitable substrate (glass, polycarbonate, aluminium) that already has the desired paraboloid shape has been however considered. Such production process was rejected because have been evidenced drawbacks respectively related to fragility, excessive costs and not correct shape/roughness of the substrate. The final choice is an Alanod MIRO 27 reflective sheet, deformed to paraboloid shape, and with subsequent application of a silver+oxide final reflective layer.


Figure 18. Picture of one type of Primary Mirror (yellowish protective film still present).

Secondary Optical Element (SOE)
The Secondary Optics for the Apollon project, designed and patented by RSE, is a square pyramid shaped reflector, properly modified at the entrance side. The design foresees a silver based reflective surface. The small sizes make the manufacturing process quite challenging. Several ways to produce this object have been reviewed. After performing several tests the method chosen is to cut the developed shape from a commercial reflective sheet and then fold it into the desired shape.

Figure 19. Secondary Optics, a reflective obliquely truncated square cone. Source: ASSE s.r.l.

1.3.3. Receivers and Extended Unit Receivers

In a PV concentration system, The sun power not transformed into electric power appears as heat. Under high luminous flux, this heat must be removed as effectively as possible to keep the cell temperature not too high. Typically an heat sink must therefore be coupled to the receiver, in order to allow cells to operate at temperatures of no more than 80°C. For this reason the selection of the receiver substrate plays a fundamental role in the reliability of the whole systems.
The solar receiver is mainly composed by a substrate on which are assembled one or more solar cells, bypass diode and connectors. The substrate is a crucial choice that affects the performances of the device, thus different materials are available and both electrical and thermal properties were taken into consideration during the APOLLON project.
Solar receivers require high thermal conductivity materials, so regular electronic substrates such as glass-reinforced epoxy laminate (FR4) are not suitable. Four main classes of high thermal conductivity materials have been compared in term of cost and performances ( low thermal resistance):
 IMS (Insulated Metal Substrate)
 Thick film on ceramic substrates
 Direct bonded copper (DBC)
 Thick film on aluminium
Alumina substrates (of the class of ceramic substrates) have been selected for their high reliability and the most cost effective solutions. After substrate selection, the receiver assembly has been carried out in three steps:
 Die attach
 Wire Bonding
 SMT assembly
Where the first to steps are related to Chip On Board (COB) technology, the last one refers to the Surface Mounting Technology device with Sn alloy.

Die attach
Chip attachment is the first process in die assembly. The chip is adhered to the die pad on the board, after which wire bonding is done using a wedge or ball bonder. One of the challenges of COB package technology is the bare chip placement. The need is to assemble complex devices at high speeds, with high consistency, and at low costs. Accurate placement of the bare die on the substrate with ever finer pitch and denser routing layouts is necessary. For wire bonded COB, inaccurate chip placement may cause insufficient adhesive coverage around the die and therefore imperfect die attach which cause yield problems. Bonding materials requirements include high adhesion, high thermal conductivity, high electrical conductivity, and acceptable process temperature. The die attach has been performed by CRP using epoxy silver filled adhesive applying a dot-dispense method, using equipment and process similar to the SMT bottom-side attachments. A pressure/pulse-time controller dispenses a specific amount of adhesive. The chip is picked and placed into the wet adhesive and seated to desired bond line thickness. The applied automatic die bonder provides precise control over the dispense quantity and placements, chip orientation, placement accuracy (x,y), down pressure to seat chip in adhesive.

Wire Bonding
Wire bonding is the most widely used technique for electrical chip interconnections in the microelectronics industry. Wire bonding is currently a fully automated process. Both automatic thermo-sonic and ultrasonic bonding machines are in worldwide use. Automatic wire bonding uses pattern recognition to locate reference marks (fiducial marks) on both chip and the package . The position of the reference marks relative to the bonding pads is stored in the machine memory and, once aligned, the machine automatically bonds all connections according to the pre-programmed sequence at a rate up to 10 bonds per second. Wire bonding was done at CRP laboratories, equipped with an automatic wire bonder

Figure 20. Particular of the ceramic substrate after wire bonding

Surface Mounting Technology
After cell assembly, connectors and bypass diode has to be integrated in the receiver. Components are suitable for soldering with Sn alloy solder paste. Since the whole conductor is left uncovered, components may move during soldering phase, so on small components as the diode, a glue dot is dispensed, allowing the component to remain in position during soldering phase. The soldering process was done by CRP in a vapor phase oven, the advantage of such solution is because of its perfect inert gas atmosphere and the wetting with lead free solder paste, is almost as good as with leaded solder alloys. The final assembled receiver is shown in Figure 21.

Figure 21. Assembled receiver

COF receivers (SE SIIRTE)

Within frameworks of the APOLLON project, Chip on Flex (COF) receiver were developed and experimental samples of the receivers were manufactured by SE SIIRTE. Integration of the receiver with SOEs is shown in Figure 22. In COF-receivers developed by SE SRTIIE, all cells’ connections are carried out with the help of aluminium-polyimide flexible film boards and done by ultrasonic welding. The aluminium-polyimide flexible boards combine in themselves functions of a mechanical support, flexible electric connections, isolating element and an effective heat-conducting path. The aluminium-polyimide system is an adhesiveless foil dielectric. For reduction of deformations caused by cyclic changes of temperature, the aluminium foil is welded through holes to the back of the cells and assembled with thermally compensated loops.

Figure 22. COF-receivers with integrated truncated pyramid and truncated cone SOEs. These solution were applied on the Fresnel base module in the first phase of the project.

Extended unit receiver

In the APOLLON module, an extended unit receiver, made of the solar receiver plus the integrated SOE and heat sink was realized as well. CRP in co-operation with ASSE, after a first analysis of the state of the art, designed and realized the solar receiver for the new mirror based system. ASSE was in charge to manufacture the extended unit receiver, therefore taking care of assembling the receiver, the SOE and the heat sink. For the prototype modules of the Apollon project, the extended unit receivers have been fixed to the base sheet of the module by means of bolts so that they can be removed and replaced, thus allowing the experimenting required during the prototype phase (see Figure 23). In future commercial modules replacement of the single receivers will probably not be possible.

Figure 23. Extended Unit Receiver. On the left the assembled unit, on the right a sketch of the different parts. A Picture of the extended receiver is also depicted.

Prior arriving to the final extended unit design showed in Errore. L'origine riferimento non è stata trovata., several solutions have been analysed, in particular for the heat sink, the receiver support and for an adequate integration of the SOE. In the Extended Unit Receiver, adhesive bonding is used (see Errore. L'origine riferimento non è stata trovata.) between:
 the SOE and the SOE Support
 the SOE Support and the Receiver
 the Receiver and the Receiver Support r
 the Receiver Support and the Heat Sink.
The last two bonding are critical from the thermal point of view (cell cooling) and require a thin layer of adhesive with a good thermal conductivity. Mainly the choice of the adhesive has been influenced by the thermal conductivity, by the particle size, by the costs, by the ease of handling (storage, mixing, application, curing).

Receiver and Extended unit receiver Lifetime testing

Lifetime testing of the solar receivers has been carried out by CRP following the reference standard IEC 62108, which determines the test to be applied in order to stress the device both thermally and electrically. The purpose of the test is to determine the ability of the receivers to withstand thermal mismatch of the different materials involved in the realization and fatigue caused by repeated changes of temperature. For this purpose, samples of the solar receiver are placed in the climatic chamber and stressed with a thermal profile. Electric power is applied to the junction in order to recreate the effect of ageing caused by current flow in normal operation.
Different receivers substrates have been benchmarked. Thermal grease was used to match thermally the aluminium heat sink to the substrates, as in the final module. After 500 cycles, the solar receivers were optically inspected and the dark I-V current was measured comparing it to the beginning measure. During the test no cell experienced a current flow interruption. From the comparison of the dark I-V curves, it was possible to identify a type of substrates that did not survive the test (see Figure 24).


Figure 24. Dark I-V measures pre and post thermal stress. Voltage values on the abscissa, in [V], and current values in the ordinate, in [mA]. On the left, typical I-V curve of the receivers that survived the test; on the right, typical I-V curve of receivers which did not survive the test.

The passage from cold to hot state, and vice-versa, caused condensation of the humidity on the cell surface, and when an electric field is applied, silver migration occurs. The silver migration causes a decrease in shunt resistance, which increases the leakage current with the cell in operating condition. This situation should also occur in real life condition when the sealing of the PV module is not adequate. Substrates with silver thickness double than others have of course more chance to see a failure .
Life time testing has also been carried out by SE SRTIIE on the extended unit receivers, showing in some cases, the detachment of the receivers from the receiver support. After visual inspection (see Figure 25) it is possible to observe a non-homogeneous distribution of the adhesive bonding on the receiver support.

Figure 25. Receiver support with non-homogeneously distributed adhesive bonding

The high solar cell operating temperature estimated during the outdoor characterization can be possible attributed to the no idea thermal interface between the receiver and the receiver support.

Receiver Die shear and Wire pull tests

Die shear test results shows that for all the analysed samples the test is passed, due to the fact that the detachment force is over 5 kg, which is the minimum allowed value from the regulation (Mil-Std-883 Method 2019.7). Wire pull test results shows that for all the analysed samples the test is passed, due to the fact that the breaking force is over 7 g, which is the minimum allowed value from the regulation test (MIL STD-883).

Bond joints pull-strength investigations were performed also on COF receivers by wire pull test.During investigations an optimal values of the main bond parameters were defined. At an optimal bond parameters, pull-strength of the bond joints was not less than 25-30 g.

1.3.4. Module technologies and new tracking logics


Different module technologies have been developed in APOLLON, based on the above described refractive and reflective optics. In particular, in the first phase of the project SOLARTEC Fresnel based modules have been compared with the CPOWER and University of Ferrara Mirror based spectra splitting modules (see Figure 26).


Figure 26. Different CPV module technologies: a) Fresnel based technology, b) Mirror based spectra splitting with Cassegrain geometry (UNIFE) and CPower Mirror based spectra splitting technology before c) and after optimization d).

At the end of the optimization phase, considerable improvement with respect to the starting technologies were obtained (see Table 6 ).

Table 6. Comparison among Module outdoor performances before and after the optimization of the CPV technologies. Efficiency calculated in operating conditions.

Module type Eff(%) DNI(W/m2) Deltaopt-st
PFst 16 830
PFopt 22 940 +37%
MBS2st 10.3 883
MBS2opt 14.6 774 +41%
Legend: st= starting, opt= optimized

MBS technology showed the biggest improvement even the performance of the optimized prototype MBS modules were lower than expected: the overall efficiency was of about 15%; the losses in the performances were mainly due to optical and mechanical mismatches. In spite of the possibility to improve the module performances, the cost analysis carried out by the CPV developer, CPower, showed that the higher complexity associated to this technology discouraged its commercial exploitation in short term period. Therefore the APOLLON Consortium decided to invest the research efforts towards the development of a new mirror based solution, eliminating the complexity and related costs of the spectrum splitting one. The new APOLLON module, developed in the second phase of the project, incorporated also innovative solutions adopted to face another important issue related to the CPV technology: the tracking reliability. Very often it is not realised that improvement on module efficiency can be useless if sun tracking is inaccurate and not reliable. This topic was not addressed in the Work programme on concentrating photovoltaics, but it has been evaluated by the APOLLON Consortium of importance to get economically-attractive concentrating photovoltaics, therefore innovative and economical affordable solutions have been researched and incorporated in the new APOLLON module.

The Problem of the cost of the Tracking Accuracy Sensors (TAS)

One of the most advanced tracking strategy makes use of closed-loop sun tracking control based on TAS that can measure the misalignment between the normal to the module plane and the direction of the sun light rays that hit the module surface. In this case, the accuracy of the solar tracker depends on the accuracy of the TAS. Among TAS, the Position Sensitive Sensors (PSD) and CCD sensors exhibit the best characteristics in term of resolution and precision. However, high precision PSD requires high precision analogue electronics that is capable of acquiring the signals and minimize any effects of environmental noise. This introduce a problem of cost. Also CCD sensor requires a digital acquisition system that is not cheap (such as a PC or a FPGA).

The Problem of the installation of the tracking accuracy sensors (TAS)

One of the main problems common to all tracking accuracy sensors or PSD is that they must necessarily be installed on a plane that is coplanar to the photovoltaic module: the maximum misalignment error between the sensor and module planes must be much smaller than the module acceptance angle. If the two planes are not coplanar, in fact, the solar tracker would control the perfect sensor alignment to the Sun but not the actual module alignment to the Sun (see Figure 27). During the first installation of the tracker, the technicians, by using proper adjustment screws, make the sensor plane coplanar to the module plane. However, any thermal effects may create an angular mismatch between the two planes which alters the proper functioning of the sun pointing sensor.

Figure 27. Angular mismatch between the PSD and the CPV module planes which causes the not accurate pointing of the module to the Sun.

The problem of the residual mismatch losses

In spite of the best efforts to track the sun, variable misalignments and consequently, residual mismatch losses, can be always present along the time. These are due, for example, to the possible different misalignments of the CPV modules located in the different position on the tracker frame. In fact, module installed on the external part of the tracker frame can suffer of bigger misalignment owing to the tracker’s frame flexure. To reduce the problem of residual mismatch losses, in APOLLON maximum power point (MPPT) devices integrated in the CPV module have been developed.

APOLLON progress with respect to the State of the art

In APOLLON a wider significance of the tracking issue has been introduced, not just meaning the tracking of the sun but even the capacity to track the maxim power point (MPPT) of the module. A new technological approach is proposed that transfers the tracking issue at the module level bring to the development of Intelligent Concentrating Module technology. (ICMs)
The PSD sensor unit developed by RSE solves the problem described above, because it is integrated into the CPV modules ( in particular it is installed in module backplane within the waterproof box) and it is a low cost device. It is composed by four main components ( see Figure 28):
• CCD sensor with output projection data, which is a CCD that can provide the projection of light on reference axis of the sensor;
• Optical collimator which transfers a bright spot on the sensor surface;
• Electronic board which acquires and process CCD data and outputs the misalignment errors ERR_AZs (°) and ERR_ELs (°).
• Waterproof box

Figure 28. PSD RSE sensor unit consisting of a waterproof box, optical collimator, CCD image sensor and electronic processing board (patent pending).

The CCD sensor is a particular commercial low cost CCD, consisting of an electrical charge matrix (pixels). It is equipped with an electronic board that is able to provide directly the projection of the light on orthogonal axes x-y (see Figure 29). This circuit provides an output voltage proportional to the electric charge that is storied along the rows and columns of the sensor. An analog to digital converter quantizes the light in 256 levels (8 bit if set) or 1024 (when set to 10bit) and converts the value in digital format.

Figure 29: CCD image sensor, equipped with a specific electronic board, provides directly the projection of the bright light spot on the axes x-y.
Since the CCD selected by RSE allows measuring directly the light profile on the x-y axes (256 + 256 = 512 measurements), while more common CCDs require the measurement of the positions of all the pixels of the matrix (in this case they are 256 x 256 = 65536), the RSE sensor unit requires an acquisition system characterized by a sample frequency that is 128 times less than that required by a conventional CCD and it is capable of storing a number of variables 128 times smaller than a traditional CCD. These two advantages are fundamentals, because they contribute significantly to reduce the cost of the electronics that is used for the acquisition and processing of data coming from the CCD. The optical collimator and the electronic board have been entirely designed and developed by RSE. The former allows getting high angular tracking resolution. In particular, the theoretical minimum measurable misalignment angle is equal to 0.012°. The electronic board allows performing the following different functions:
 Interfacing with the CCD through a digital communication protocol defined in the datasheet by the manufacturer of the CCD.
 Capturing light profiles along the axes x-y and storing their values in 512 temporary registers.
 Calculating the coordinates of the center of gravity of the light spot.
 Calculating the values of the azimuth angle errors of ERR_AZs (°) and elevation ERR_ELs(°).
 Automatically controlling the integration time for optimal scanning of the pixels of the CCD in relation to the intensity of the incident light.
 Recognizing the status of non-alignment of the module or lack of sunlight.
 Communicating by a protocol defined by RSE on the RS485 bus. On this bus various sensors can be connected to at the same time with their own address.

The new PSD unit developed is then compact, cheap and allow getting high tracking accuracy ( patent pending).
PSD sensors have integrated in two ASSE prototypes modules and several experimental test sessions have been performed to check the tracking accuracy (see Figure 30). Hereafter the results of a typical test performed on a full day is reported. The I-V curve of one module and the misalignment errors measured by both PSD sensors have been logged with a sample time of 30s. Only one PSD sensor has been used for the closed loop tracker control. During the test-day the misalignments between the planes of the two PSDs has been under 0.04°.

Figure 30. Apollon (ASSE) modules installed on the solar tracker with integrated PSD sensors.


In Figure 31 the azimuth and elevation angular misalignment values measured by PSDs during the test-day are showed.

Figure 31. Azimuth and elevation errors measured by PSD1 and PSD2 during a test-day.

The experimental results carried out by the outdoor testing of the Apollon modules and the monitoring of two PSD sensors indicate that the PSD sensor is able to control the tracker maintaining the module aligned to the Sun with error within the range of +/- 0.1°.

APOLLON progress on DC/DC control

The New DC-DC converter has been developed by TECNALIA and it is located on the rear side of every single module in order to extract its maximum power and thus eliminate the mismatching losses among the modules. Additionally, the device includes monitoring and communication functions to inform about its instantaneous performance. All this CPV generator monitoring information can be gathered and analysed by a centralized control. A Power Line Communications (PLC) system has been designed to allow this communication between the CPV modules and a central control unit, through the positive and negative terminals of the power output of DC-DC converters. This way, most important magnitudes of CPV modules can be known individually: voltage and current of CPV module and voltage and current of the DC-DC converter.

Figure 32. CPV MPPT Device. Final prototype

A Solution for an accurate module installation: External calibration devices

When the modules are mounted in the tracker they have to be properly installed and adjusted in order to get all the modules in the same tracker frame plane. As a consequence, a new hardware device has been developed by TECNALIA in order to visualize the error simultaneously provided by PSD sensors located in every installed module on the tracker. This “tester” alignment reading device is composed of a black and white screen and several buttons to navigate inside the menu of the system. The device is connected to a RS-485 bus and reads the data provided by each PSD sensor connected to the mentioned bus, in a master–slave scheme. This way, the error provided by each sensor is shown in the screen of the device and the alignment of all the modules can be easily achieved during the installation process.

Figure 33. External calibration devices. On the right the a Display of the azimuth and elevation errors in XY axis

1.3.5. Module characterization and new standard

Module characterization

A round-robin outdoor methodology has been applied to establish the performance of the developed CPV modules in order to provide an evaluation of the uncertainties of measurements between different test locations. To provide a fair comparison between the different results, the performance of each set of tests was normalised to a set of standard test conditions (STC). These were:
 Direct normal irradiance: 900 Wm-2
 Cell temperature: 25 C
Due to the uncertainties involved in measuring the DNI spectrum, no correction was applied for change in the spectral conditions. All tests were performed at solar noon +/- 1.5 hours to avoid large variations in spectrum.
The results of the I-V inter-comparison are reported in

Figure 34. Comparison of the corrected IV characteristics from the different partner test sites. Measurement performed on the Fresnel based module produced by SOLARTEC.

Table 7. STC Performance Summary Table

Institute Open Circuit Voltage [V] Short Circuit Current [A] Max. power [W] Efficiency [%]
JRC 48.15 1.277 50.05 24.14
RSE 48.05 1.284 48.64 23.46
UCY 48.53 1.265 50.31 24.27
ENEA 48.39 1.277 50.01 24.12

Average 48.28 1.275 49.75 24.00
St. Dev. 0.219 (0.5%) 0.01 (0.6%) 0.753 (1.5%) 0.364 (1.5%)

From the characterisations performed, there is a reassuring level of agreement between the different partner’s results. In particular the results obtained at JRC and UCY show a very good agreement in terms of efficiency and power output of the module. Some additional series resistance present in the RSE results has lowered the power output slightly, but the voltage and current outputs also seem in good agreement. The round-robin has therefore improved confidence in the module performance measurements, and also has suggested that the effect of different spectral conditions need not be corrected for, as long as reasonable care is taken to perform the measurements under similar atmospheric conditions and time of day.
A round Robin has been applied also on the new mirror based modules developed in the second phase of the project. Due to the large size of the complete module, four mini-modules were prepared which sufficiently replicated the configuration of the full size module to be suitable to perform characterisation and pre-qualification tests in their place. These were distributed to different test locations. The partners involved in this round-robin were: JRC, RSE, UCY and ENEA. Two of these prototype modules were sent to the JRC for pre-qualification testing, whilst the other two were sent to ENEA and UCY after initial testing by RSE at ASSE’s facilities. The two modules used for round-robin testing were both tested outdoors for short periods. It was found that the modules displayed a peak efficiency of up to 30% at standard test conditions, in line with the targets of the APOLLON project, including the losses due to the shunted cell. A summary of the results is reported in Errore. L'origine riferimento non è stata trovata.. This represents a significant improvement over the previous performance measurements of the original technology optics and receivers. It is worthwhile to point out that unlike the previous round-robin, within which each institution tested the same module, this time a series of prototype mini-modules were tested, which were taken to be representative of the actual module performance. In this case, a direct comparison of the module results is not instructive, but the average performance of the set of modules as a whole can be used as an indication of the overall performance of the design.

Table 8. STC Performance Summary Table.

Institute Module Open Circuit Voltage [V] Short Circuit Current [A] Max. power [W] Efficiency [%]
JRC MC4-1 12.40 2.74 26.96 28.16
RSE MC4-2 12.89 2.71 28.73 30.00
UCY MC4-2 11.74 2.48 21.74 22.70
ENEA MC4-3 11.9 2.58 25.30 27.7

Average 12.23 2.63 25.68 27.14
St. Dev. 0.52(4.3%) 0.12 (4.6%) 2.98 (11.6%) 3.12 (11.5%)

Standard: determination of the solar cell temperature

The performance of concentrator photovoltaic modules are strongly affected by the cell junction temperature, that mainly depends on the heat transfer between solar cell and thermal spread heater, as well as on the ambient temperature and on the average wind speed.
In Apollon Project a new method similar to the “Voc method”, based on the comparison between the Voc value in experimental conditions and the Voc value in standard reference condition (Vocr) (IEC62670 third edition) has been proposed for the determination of the solar cell temperature. It presents the following improvements:
 It takes into account the variation of module voltage temperature coefficient,  with the current;
 It doesn’t need the knowledge of Vocr

The measurement procedure to be applied, foreseen by the APOLLON Project Method (APM) are reported in Figure 35.

Figure 35. Scheme of the APM method for the estimation of junction temperature.

The indoor tests aim to the determine the voltage temperature coefficient in function of the dark current. Once such relationship is known, the operating junction temperature in outdoor conditions can be determined simply by measuring the electrical variables Voc and Isc, that are associated to any outdoor I-V curve. The APM is based upon a single junction equivalent modelling of the Multi-Junction CPV module whose parameters (equivalent ideality factor neq, series resistance Rs, inverse saturation current Ioeq) have to be determined by an appropriate identification procedure. The APM requires during the indoor tests, the positioning thatof a temperature sensor as closer as possible to the cell in order to allow measuring of the cell temperature when the thermal heating can be controlled or by a dark current that is injected into the module or by a climatic room.
The APM method is characterized by the following steps:
 Measurement of the dark I-V curve at a given temperature Tj, by using a dedicated thermocouple.
 Identification of the electrical parameters of the single junction equivalent model.
 Calculation of the indoor I-V curve depurated from series resistance voltage dropm by using the series resistance identified in the previous step.
 Determination of the voltage-temperature coefficient β in function of the dark current values.
 Estimation of junction temperature in experimental conditions

The validation of the APM method is reported in Figure 36 which shows the graphs of a commercial CPV backside temperature, Tback, (directly measured by a thermocouple) and the maximum junction temperature estimated by the APM method. The measured module backside temperature is lower than the junction temperature, as expected, and the daily trend of the estimated junction temperature follows quite well the daily trend of the measured module backside temperature.

Figure 36. Graph of module backside temperature, Tback, maximum junction temperature, determined by the APM method, air temperature, and wind velocity.


1.3.6. Environmental & Economic Assessment

Concentrating photovoltaics (CPV) may be viewed as the pursuit of a very sustainable idea to minimize the semiconductor material by managing and concentrating the light with other more sustainable materials. By conserving the amount of energy-converting materials, there is a cost window to use more expensive, more highly efficient PV semiconductor devices. From a sustainability and economic viewpoint, the addition of extra materials for the tracker, the housing, and the light management needs to compensate, economically and in terms of sustainability, for the PV semiconductor device material that they displace. A life-cycle environmental assessment was performed on the final Apollon concentrating photovoltaic module design, with the aim to determine energy payback time (EPBT), the carbon footprint of the system. The costs for making the prototypes and for manufacturing commercial systems is reported. The results are benchmarked against other concentrating photovoltaic systems. Ways to reduce the environmental impact in the material and production design choices are highlighted.

Environmental assessment results

In the final module prototype, aluminium in the module structure and optics dominates with the most embedded energy and stronger greenhouse gas emissions (see Figure 37)

Figure 37. Lefts: Breakdown of the cumulative energy demand of the 32-cell module components. Right: Breakdown of the greenhouse gas emissions of the 32-cell module components.

The electronics contribute with 11% of the greenhouse gas emissions. The steel fasteners and brackets take up a 7% contribution. The 1% of specialty or precious metals contribute 6% of the emissions. The energy payback time (EPBT) is calculated for a location in Catania, Sicily (1794 kWh/m2/yr), a 30 year system lifetime, and is graphically shown in Figure 38. The Apollon final CPV design’s energy payback time is about a year – a respectable value for any renewable electricity generating system. The carbon footprint is an estimate of the grams of CO2 emitted per kWh of electricity generation. The Apollon final design is about 20 g CO2 equivalents per kWh of electricity generated. This value, like the EPBT, are within the range of single junction non-concentrating silicon PV modules, and is ‘competitive’ on the sustainability front.

Figure 38. Comparison of the energy payback times of two commercial CPV systems with starting, optimized and final (ASSE) Apollon CPV prototypes.

Figure 39. Comparison of the carbon footprint (g CO2 eq/kWh) with two commercial CPV systems, and starting, optimized and final (ASSE) Apollon CPV prototypes.


Economic Assessment: Case study

The continuous market expansion of the traditional silicon technology has driven down the cost much more than expected, bringing cSi fixed power plants to be sold in 2013 at 1.40-1.70 € /kWp and Tracked cSi power plants to 1.7-2 €/kWp. In order to assess the level of competitiveness reached by the CPV technology (HCPV) a case study has been analysed. In particular the levelized cost of energy (LCOE) has been evaluated considering three different locations, having a different amount of diffused light: Ragusa (Italy), Giza (Egypt) and Tucson (USA). The benchmarking among the different technologies (C-Si either fixed or tracked and HCPV ) has been accomplished assuming to set up PV power plants on a fixed terrain area of 100 x 200 m2, with two possible layouts: dense (91 trackers); sparse (55 trackers). Three qualities of HCPV modules have been considered: 30%, 32% and 34% STD efficiency.

Figure 40. Terrain layouts used for the calculation of LCOE and energy production simulation F: fixed c-Si with 5x29 support structures and a total of 5220 modules organized in 290 strings; T1: dense layout with 91 trackers carrying 3276 c-Si modules or 7280 HCPV-modules; T2: sparse layout with 55 trackers carrying 1980 c-Si modules or 4400 HCPV-modules.

The main conclusion of the analysis are hereafter reported:
 Tracked c-Si and HCPV in the dense layout always have a slightly higher LCOE than in the sparse layout because shading losses are higher in the first case. For a fixed-size terrain however the dense layout leads to a higher energy production. The choice of the density of the trackers on a given terrain is a compromise between lower LCOE and higher energy production
 For c-Si (either fixed or tracked) the calculated LCOE’s for Ragusa vary from 0.091 to 0.104 €/kWh. For HCPV the LCOE’s for the same site vary from 0.120 to 0.128 €/kWh. Consequently HCPV has difficulties in being competitive in Ragusa, which is a zone with DNI = 5.4 kWh/m2/day and near 29% of diffuse light.
 For Tucson, which is a zone with DNI = 6.9 kWh/m2/day and only 21% of diffuse light, the calculated LCOE’s change in favor of HCPV with respect to c-Si. In particular for HCPV-modules with 34% efficiency and system install costs of €1.90 / kWp the LCOE are € 0.099 /kWh which is close to being competitive with fixed and tracked c-Si systems, depending on the install costs of the latter.
In order to understand how to address the future efforts to increase the CPV technology competitiveness, un analytical simplified cost analysis has been developed.

Economic Assessment: analytical simplified cost analysis

The simplified cost analysis has been developed in two main steps:
1) By considering
a. the solar cell un-yielded cost constant, regardless the efficiency value,
b. the module, the tracker and the fixed costs constant, regardless the concentration factor.
These assumptions allowed evaluating the system cost in function of three terms:

The first term of equation (1) takes into account the solar cell cost, the second term takes into account the tracker and module costs, while the third one takes into account the costs which do not depend on the solar cell efficiency value, like the inverter and BOS costs. , and are constants depending on the cost of the different CPV components, while, , represents the solar cell efficiency. The CEPI is the cell economic performance index value given by:

CEPI = h*Cf*Y/C
Where:
 = cell efficiency (%)
Cf = concentration factor
Y = process yield
C = un-yielded cell cost [Euro/cm2]
2) By considering the necessary decrease of the solar cell, module, tracker and fix costs to reach a more competitive technology
The analysis concerning the first step has been based on the following assumptions. Like in the car sector, where year by year it is possible to find cars at the same cost with improved performances (for example, in terms of engines which consume less fuel), we can assume that we can reach higher solar cell efficiency value at the same solar cell cost. The same hypothesis can hold for the CPV module: we can assume that the CPV module technology can improve in term of getting modules operating at higher concentration factor with comparable acceptance angle, maintaining fixed the cost. In particular, in the cost analysis, the module area has been maintained constant, regardless the concentration factor, by assuming to install optics with comparable aperture area but adopting solar cells with different area, as smaller as the concentration factor increases. By maintaining the area of the module constant, the tracker area is also constant, regardless the concentration factor, therefore, by considering that we need similar tracking accuracy, regardless the concentration factor, (since we assume to get similar module acceptance angle) we can also keep the tracker cost constant, regardless the concentration factor. The main results of a such cost analysis are reported in Figure 41 and Figure 42.

Figure 41. CPV cost analysis: influence of solar cell efficiency, concentration factor and process yield. The range of the nowadays best MJ cell efficiency values is depicted. Irradiation 850W/m2.

Figure 42. CPV cost analysis: possible scenario considering the decrease of the solar cell, module, tracker and fix costs. The range of the nowadays best MJ cell efficiency values is depicted. Irradiation= 850 W/m2.

In conclusion, when the concentration factor and process yield values are low (low value of CEPI), the first term of equation (1) plays a relevant role in the total system cost, therefore, it can happen that high efficient systems can cost more than systems having lower efficiency value. Therefore the risk of introducing project targets only regarding the solar cells efficiency value is that of inducing a possible false idea on the competitiveness of the CPV technology. By keeping the process yield high enough (>80%) and by decreasing the solar cell, module, tracker and fix costs, it is possible to reach a completive technology at concentration factor ≥ 1000 suns. A further system cost decrease can be obtained till 1500 suns, over this values, further concentration values increment do not produce any substantial system cost decrease.

2.1.7. Spectral radiometer inter-comparison campaign

Since the CPV system efficiency calculation relays on the correct measurement of the direct solar irradiation and of its spectral distribution, in the APOLLON project a spectral radiometer inter-comparison campaign was implemented. Thanks to the dissemination action adopted, this initiative attracted the interest of other European Institutions not directly involved in the project. During the life time of the project, three inter-comparison campaigns took place:
 2011; ENEA, Portici (NA), I
Institutions: 8 (It & Cy); Participants: 12, 20 Instruments
 2012: ENEL, Catania (CT), I
Institutions: 8 (It , Es, Jp); Participants: 15, 30 Instruments
 2013: ISFOC, Puertollano (CR), Spain
Institutions : 15(,It , Es, Nl, De); N.22 Participants N.100 solar Instruments.

Figure 43. Set-up overview of the solar instrument tested in Puertollano.

Figure 44. Successful expansion of the spectral radiometer inter-comparison campaign, which starting from Italy has nowadays involved several European Institutions.

The spectral radiometer inter-comparison campaign carried out in APOLLON and the fruitful link with other European projects, like SOPHIA, has been a very successful initiative, it has allowed increasing the accuracy in DNI measurements as well as to enrich the European Data base on DNI providing to end-users more accurate capability in energy forecast.

1.3.7. Conclusion
The European collaboration among the partners of the project has achieved more than would have otherwise been possible, joining the different competence necessary to have an integrated approach, therefore addressing the research on the whole development chain of the Concentrating Photovoltaic Technology: from the improvement of the Metal-Organic-Chemical-Vapor Deposition (MOCVD) technique, which is used for the growth of semiconductor materials composing the photovoltaic devices, till to the final construction of a prototype concentrating photovoltaic system.

The re-orientation of the research activity on the second phase of the project has brought as a final result the development of new Mirror based CPV system, adopting new tracking strategies based on a new type of Intelligent Concentrating Module (ICM), 30% efficient, with integrated position sensitive detector and Maximum Power Point Tracking (MPPT) devices.

Thanks to the holist approach followed in APOLLON, the project has reached its main objective to increase the competitiveness of the CPV technology, more than halving the cost assumed at the beginning of the research and development activity.

See attachment PDF Publishable Summary Final Report

Potential Impact:

Europe has a large but distributed potential in CPV technology. Only by joining forces and adopting high level of ambition we can build a strong sector. In particular the technology transfer can be accelerated by a close cooperation between science and industry. In APOLLON the consortium has been set up with the aim of joining the two “ wings” of the technological innovation: the research centres and the industries or SMEs. This equilibrate mixture has been the first necessary steps towards the future commercial exploitation of the CPV technology.

The research in APOLLON has embraced all the development chain of the CPV system, starting from the improvement of the MOCVD system, which is used for the deposition of semiconductor materials composing the photovoltaic device, till to arrive to the final construction of a prototype concentrating photovoltaic system. The outcome of the project is then represented by several products. A number of companies are involved in the exploitation phase, as regards component production and final system commercialisation. In facilitate the managing of the knowledge, the intellectual property, and all the other innovation-related activities arising from the project, an Exploitation Management Committee has been formed by the industrial partners of the Consortium and the Coordinator.
The assessment of the all industrial perspectives of the project and the exploitation plan is reported also in the “Technology Implementation Plan” (TIP) report, which has been prepared by the Exploitation Management Committee (Delivery D8.11)..

The different products output of this project have been the following:
 Improved MOCVD growth equipment
 Solar cells with high economic performance index
 Improved optics ( lenses and mirror based concentrator); High throughput injection-compression moulding technology for optics production
 Improved solar cell COB and COF receivers
 New tracking strategies based on new Intelligent module integrated PSD and MPPT devices, Improved trackers and control logic
 New Mirror based CPV system
 Improved measurement methodology and techniques

AIXTRON sees good opportunities for the exploitation of the results achieved in the APOLLON project. The lessons learned when optimizing the MOCVD reactor for growth of group-IV and III-V semiconductors in one structure can, apart from the obvious growth of novel multi-junction solar cells, be applied to a variety of multicomponent semiconductor growth problems. Those range from interface sharpness in conventional III-V semiconductor structures to carry-over effects in doping, either within the same growth run or from run to run. The reactor design rules and technologies researched in APOLLON, especially in regard to the radial temperature profile of the reactor, help tailoring the decomposition and deposition of precursors and materials to take place in the exact spot where they are needed for growth, while keeping undesired – and therefore carry-over prone – deposition in other parts of the chamber minimal.
Additionally, the in-situ wafer temperature profile adjustment researched in APOLLON can be adapted to other material systems as well. For instance, the InGaN material system used for Solid-State-Lighting shows a huge emission wavelength dependency of -1.4 nm/K with local growth temperature, mostly due to its immiscibility gap. As wafer sizes become larger, those wafers are inherently subject to deformation during growth, causing the same kind of loss of thermal contact to the carrier as observed for the multi-junction solar cells in APOLLON. If not mitigated in some way, this loss of contact can lead to large emission wavelength variations across the LED wafer. The in-situ temperature profiling puts another, elegant method into the toolbox of reactor design. Thus, the technologies researched in APOLLON can benefit AIXTRON’s competitiveness in a variety of markets from solar via transistor and high power applications to LED. However, customers in those markets expect hassle-free and reliable functionality of their MOCVD system on a 24/7 basis. Hence, additional development of those technologies into products is required. This timeline is expected to be on the order of 4-5 years and is, thus, in line with the timelines and requirements of these markets.

The demonstrated possibility to grow in the same MOCVD reactor chamber both III-V and IV elements of the period table by RSE will allow expanding the band gap engineering possibilities to increase the solar cell efficiency to values > 45% (for example by combining the SiGeSn-1eV alloys with GaAs and InGaP), without introducing a substantial cost increase in the growth process, which would have been required if separate growth chambers for group IV and groups III-V elements were utilized, as happened so far.


NAREC 20% efficient Silicon Solar cells with infrared response enhancement suitable for ultrasonic welding are expected to be exploited both in the new emerging spectra splitting and low concentration Photovoltaic systems. ENE III-V solar cells have been developed for working at high concentration factor (till 4000 suns ) in order to decrease the impact of cell cost on the overall CPV system cost. These cells can be exploited in CPV system operating at high concentration factor

Primary optic (Fresnel and Hybrid lenses) and secondary optic (reflective cones) have been improved increasing the optical acceptance angle near to ± 1°, at a concentration ratio >700, maintaining the optical efficiency in the range of 81%. A mirror based solution has been patented by RSE and it is foreseen its exploitation and the new Mirror based systems developed in the project. The improvement in the lens performance, and in particular on the acceptance angle, encourages to address the future research and development towards the design and fabrication of lens with higher concentration capability to further reduce the CPV system cost. In order to improve the molding of optical components, CRP exploited injection-compression molding, that allows getting better cavity replication and optical finishing of molded lens. This technology is proposed to future lens producer for its reduced manufacturing costs and enhanced system efficiency.

Chip on flex (COF) receivers with integrated secondary optics elements have been developed with an estimated production cost of 0.56 €/W in order to reach a module cost around 1 €/W. This cost reduction, in line with project target, has been an important step which allowed increasing the competitiveness of the CPV technology. COF-technology is a high throughput assembling techniques due to use of the automated process such as positioning of the chips, ultrasonic bonding, gluing. This technology is commercially proposed by SE SR TIIE to CPV module producers

Automated assembly of solar cells based on Chip on Board process has been developed by CRP. The process mainly consists of two steps: solar cell attach and wire bonding. The technology is proposed for its reduced manufacturing costs / high precision placement of the components / enhanced system efficiency/ enhanced thermal management to CPV module producers or solar cell producers that has not in house COB technology


New “tracking” strategies has been implemented in the new Intelligent Concentrating Modules (ICM), with integrated position sensitive detector and Maximum Power Point Tracking (MPPT) devices. These activities have seen the participation of different partners, in particular of ASSE, RSE TECNALIA. The ICM along with the new tracking system developed in APOLLON are expected to increase the CPV system energy production, both because more accurate sun tracking is foreseen (with lower energy consumption) and also for the extra power recovering allowed by the MPTT devices, in case of current mismatch among modules. The ICM is expected to be commercialized by ASSE, in agreement with RSE and TECNALIA, after the next foreseen industrialization phase.
As far as the tracking control is concerned, as well as the DC-DC converter with specialized MPPT algorithm, TECNALIA sees at first, as a potential user ASSE. TECNALIA will not be the final manufacturer of the tracking control, but the technology provider. In this sense, TECNALIA is looking for an Spanish company whose business turns around the electronic manufacturing, and which would be responsible for the manufacturing and commercialization, depending on ASSE strategy. However, there is another option which consists in licensing the technology to a third party, who will commercialize the control worldwide. ASSE would be one of these clients, but not the only one. This way the market size would be much larger and success opportunities greater.

A new HCPV-system with off-axis mirror-based modules with >30% STC-efficiency ha sbene developed by ASSE. Potential customers are Utility scale renewable energy producers and managers. Electricity plants > 1 MWp. Predominantly. The European Research project Apollon allowed ASSE s.r.l. to gain important additional experience on the construction of HCPV-modules, in particular on mirror-based modules, and on the construction of trackers for HCPV-systems. Furthermore the research clarified the possibilities, and the difficulties, for a future development and commercialization of such modules. This development has to take place in the context outlined above and summarized in the points below:
 The commercial modules to be developed shall have a CSTC-efficiency of 34% or better. The results of the Apollon project indicate that an optical efficiency of 82% can be reached on single receiver level. A good quality control should enable to limit the mismatch losses between the receivers of a module to 5%. This implies, that concentrator cells with efficiencies >34% /0.82 /0.95= 43.6% shall be commercially available. This can reasonably be expected since a record cell efficiency of 44.1% has already been certified
 The installation costs for the new HCPV-system shall be well below € 1.90 / Wp, maybe as low as € 1.50 / Wp because during the development period installation costs are expected to go further down for fixed- and tracked c-Si systems and for CPV-systems of the competitors. Market developments shall be continuously monitored. To reach the cost target all system aspects shall be optimized together: module, tracker and BoS shall be dimensioned perfectly together in order to minimize costs. In addition the system shall be designed for low-maintenance and for low self-consumption of energy.
 Although equipped with a huge experience in the sector, ASSE s.r.l. would essentially be a newcomer in the HCPV-market. Market researchers state that new players will need from 5 to 6 years to establish themselves and to become self-sustaining [10]. This period is more or less compatible with the view of ASSE s.r.l.: a couple of years for the industrialization phase and another 2-3 years for qualification of the product according to IEC 62108 and for the installation and monitoring of at least 3 pilot plants world-wide of up to 500 kWp.
 Financing shall be found to bridge these 5 to 6 years prior to market entrance. The economic partner(s) shall be sufficiently solid to invest from M€ 50 to M€ 100 during this period. These investments shall cover research and development, product qualification, automated production lines, installation of pilot plants and marketing. Considering the current situation in the CPV-market, in the first years it will not be an easy task to obtain a full financing from private investors: a significant part of the financial support should come from public funding, for example through public tenders for medium-large scale demonstration projects..
 Considering the above, an entrance in the market around the year 2018 can be foreseen. In that moment the global CPV market is currently predicted to be between 500 and 1000 MWp/year. Assuming a 1% market share in the first years of introduction of the new product, this implies that an initial production from 5 to 10 MWp / year has to be taken into account. System costs shall be calculated aiming at a much larger share.
 In the period 2013-2018 the current major CPV-players will install large systems, demonstrating that costs can be competitive, demonstrating that the HCPV-technology works and is reliable, and as a consequence improve bankability- and insurance-conditions for CPV.
 The new CPV-system shall mainly be designed as a ground-mounted system for areas with DNI > 6 kWh/m2/day (> 2200 kWh/m2/year) and a low percentage of diffuse light.

The research activity carried out on modelling and CPV module outdoor testing methodology is expected to provide a valuable feedback for improving the actual standard on CPV. The new measurement procedures on the determination of the operating solar cell temperature will be transferred to IEC Technical Committee 82 WG7. The spectral radiometer inter-comparison campaign carried out in APOLLON and the fruitful link with other European project like SOPHIA, has attracted the interest of several European Institutions and has allowed increasing the accuracy in DNI measurements as well as to enrich the European Data base on DNI providing to end-users more accurate capability in energy forecast.

Since in the APOLLON Consortium the “supply and demand” have been joined, because the CPV system supplier (ASSE) has been involved along with an End-User (Enel E&R) interested to the utilisation of the CPV technology, an important cross fertilization action took place: while the End-User gained more confidence on the CPV technology and on the new test procedures for CPV modules, the CPV system supplier could address the research and development towards the technological aspects judged still weak by the End User (mainly, increasing module efficiency and tracking reliability).

The involvement of Enel E&R refers mainly to the WP 5 focused on testing and performance evaluation of the CPV system and components developed during the project. Enel as End-User had the responsibility to install and test the system prototypes developed in order to assess the performance, the durability and reliability of whole systems and components. Enel as electric utility wants to investigate the possible industrial exploitation of the CPV technologies by performing cost-benefit analysis. Moreover in the frame of WP5 ENEL E&R had been strongly involved in research activities related to the definition of an outdoor experimental procedure for measuring with the maximum level of accuracy the spectral distribution of solar spectrum. The precise determination of solar spectrum distribution permits the calculation of the solar power density contained in the different wavelength bands where Multi Junctions PV cells are reactive. During the APOLLON project Enel E&R developed a Real Time Monitoring and Diagnostic System (RTMDS) to perform long term tests on the Dense array prototype which will be exploited in the next projects. Enel E&R organized the preparatory work on a specific area of the laboratory to install the new Mirror Based Prototype developed by ASSE. This are will be exploited for the next testing

The innovative character of the APOLLON project has been evidenced by several patenting, in particular on the MOCVD temperature tuning capability, on the new optimized CPV mirror spectrum based concentrator, on the intelligent module, on the primary and secondary mirror of the new mirror based module. The possible transfer of the patent of the innovative sun pointing sensor to companies that operate in the field of microelectronics is expected to bring the cost of the component to some euro per piece, allowing an high diffusion of the product.


The environmental assessment and in particular the breakdown of the embedded energy of the module components has evidenced that the aluminium dominates with the most embedded energy. The use of aluminium is a quite sustainable material choice, because it is highly recyclable, however, advanced manufacturing techniques, such as 3D printing, may eventually be able to provide a solution for making an aluminium, structurally sound, very light-weight.


The cost analysis has shown that further efforts have to be addressed to decrease the CPV system cost to be competitive with the conventional silicon technology. The installation costs for the new HCPV-system shall be well below € 1.90/Wp may be as low as € 1.50/Wp because during the next development period installation costs are expected to go further down for fixed-and tracked c-Si systems. It has been demonstrated that when the concentration factor and process yield values are low, the solar cells cost plays a relevant role in the total system cost, therefore, it can happen that high efficiency systems can cost more than systems having lower efficiency value. In other words, the risk of introducing project targets only regarding the solar cells efficiency value would be that of inducing a possible false idea on the competitiveness of the CPV technology. Future research programs addressed to increase the competitiveness of the CPV technology should fix targets on solar cell efficiency values taking also in due account the working concentration level and process yield. By keeping the process yield high enough (>80%) and by decreasing the solar cell, module, tracker and fix costs, it is possible to reach a competitive CPV technology, with respect to the other PV technologies, at concentration factor ≥ 1000 suns. According to the starting assumption of the cost analysis, a further system cost decrease can be obtained till 1500 suns, over this value, further concentration values increment do not produce any substantial system cost decrease.


A further remarkable impact of the project comes from the solar cell development activity for which has been accomplished in one of the research Institution of the Consortium a successful setup of new infrastructures, equipment and advanced instrumentation. This initiative has been possible thanks to the considerable investments deployed to better answer to the commitments in APOLLON and to better support the industrial development. Around the EC contribution, a considerable amount of economic resources has been concentrated for a better implementation of the project and for a better exploitation of the results. The knowhow developed along with the investments made by to build up a research facility for advanced solar cell development forefront in this field, constitute nowadays a winning blend which provides an important contribution addressed to enhance industrial competitiveness in Europe. Being the Consortium geographically distributed all around Europe, the exploitation of the results is expected to be wide not only in term of commercial products but even in term of territorial extension.

1.4.2. Dissemination
A wide dissemination action took place to allow promoting an effective exploitation of the project results. The APOLLON web site (http://www.apollon-eu.org) is up dated to disseminate on the web the results about the project. Articles, press releases, presentation and announcements have been made regarding the project APOLLON. A list of the main dissemination initiatives is reported in Table 9.
Of particular relevance:
• The final APOLLON workshop planned to take place in Paris during the 28th European Photovoltaic Conference
• The preparation of a “white book” on CPV . This report has been compiled by the members of the APOLLON Project and it represents the culmination of 5 years work from 2008 to 2013, bringing together the results concerning all the development chain of the Concentrating Photovoltaic technology, from the growth technology to the system characterisation and environmental impact. This, in particular, includes:
o The work on the development of a new MOCVD growth chamber with a new heating system, gas injection system and wafer susceptor design to get a better control on the wafers deformation during the deposition of the different semiconductor layers of a multi-junction solar cell and to allow depositing new materials with reduced carry over effects
o The improvement on silicon and III-V based solar cells structures to increase the cell economic performance index
o The development and comparison between different optic solutions: Fresnel, Hybrid lenses, Mirrors with or without dichroic filters with different secondary optics
o The realization of new intelligent concentrating modules (ICM) adopting internally integrated position sensitive detectors and maximum power point tracking devices
o The testing of receiver and modules, including new methodology for contributing to the actual standard on CPV trough mismatch analysis and the determination of outdoor solar cell temperature
o The results of the Spectral radiometer inter-comparison campaign to increase the accuracy in DNI measurements
o The Environmental and cost impact analysis
o The possible development scenarios of the Concentrating Photovoltaic technology and where the research has to be further addressed
With this initiative The APOLLON Consortium hope to have given a better understanding of the progress of the Concentrating Photovoltaic technology and its future impact.

See attachment PDF Final Report

List of Websites:

http://www.apollon-eu.org