Final Report Summary - AMON-RA (Architectures, materials, and one-dimensional nanowires for photovoltaics - research and applications)
Executive summary:
We report on the activities carried out and the results obtained within the Seventh Framework Programme (FP7) project 'Architectures, materials and one-dimensional nanowires for PVs - research and applications' (AMON-RA) contract no. 214814. The main goal of the project was to demonstrate a dual-junction photovoltaic (PV) cell on silicon substrate based on nanowire architecture in a cooperative effort of 6 partners. In AMON-RA, we combined the excellence in materials science at Lund University (ULUND) and the world-leading experience in PVs at the Fraunhofer Institute for Solar Energy Systems (ISE) with the capabilities in processing of nanowire materials and the ambitions in fabricating nanowire-based PV components at the company Sol Voltaics AB (SOL). All this was supported by state of the art materials and device characterisation and structure modelling. Novel ground-breaking PV materials was developed by ULUND by the use of nanowires that were supplied to the Technical University of Denmark (DTU) and the Johannes Kepler University Linz (JKU) for detailed structural characterisation, SOL processed the PV structures, which were measured by ISE, supported by modelling from ISE and University of Kassel (UKAS). The ultimate ambition of the partners was to evaluate the potential of a proposed and dramatically different solar cell architecture which promises to exceed the current price / performance limitations of commercially available cells and to obtain a thorough understanding of fundamental and device-relevant physical properties of nanowires applied to PV applications.
Project context and objectives:
We report on the activities carried out and the results obtained within the FP7 AMON-RA project. The main goal of the project was to demonstrate a dual-junction PV cell on silicon substrate based on nanowire architecture in a cooperative effort of six partners. In AMON-RA, we combined the excellence in materials science at ULUND and the world-leading experience in PVs at ISE with the capabilities in processing of nanowire materials and the ambitions in fabricating nanowire-based PV components at the company SOL. All this was supported by state of the art materials and device characterisation and structure modelling. Novel ground-breaking PV materials was developed by ULUND by the use of nanowires that were supplied to DTU and the JKU for detailed structural characterisation, SOL processed the PV structures, which were measured by ISE, supported by modelling from ISE and UKAS. The ultimate ambition of the partners was to evaluate the potential of a proposed and dramatically different solar cell architecture which promises to exceed the current price / performance limitations of commercially available cells and to obtain a thorough understanding of fundamental and device-relevant physical properties of nanowires applied to PV applications.
The project was based on a collaboration between theoreticians that simulated and experimentalists that fabricated and characterised the structures. This project was carried out in a true European spirit with an increasing interaction between the different partners over the course of the project.
We can report here that:
- the project was running at high speed;
- most project goals have been achieved;
- some results have been exceeding the envisioned goals;
- however, not all anticipated milestones could be reached.
The project has led to the first steps of the development of a new type of solar cell structure based on semiconductor nanowires that have the potential to exceed all reported solar cell efficiencies. However, with the end of the project no financial support is at hand to continue this development which certainly would increase the competitiveness of Europe in this very important industrial and energy sector.
Project results:
Progress in the project
The project was organised in 7 work packages (WPs), with the seventh being management. The WPs had been carefully chosen in terms of congruent competence areas. Each WP involved activities of more than one partner and the leading partner was chosen from the most experienced one in that area. It was planned and executed that all WPs run over the entire period of the project, except WP5, which dealt with characterisation of fabricated solar cell structures and thus started upon availability of the first device that had been fabricated (month 12). From that point on all WPs were running in parallel until the end of the project.
The main idea behind AMON-RA was to make arrays of light-harvesting nanowires on silicon substrates. The structure intended was composed of nanowires including tandem PV cells, based on heteroepitaxy of III-V semiconductors. We intended to investigate two different architectures: The first, called 'Light Guiding', consisting of core-shell nanowires with a diameter equal to or greater than ? / n, in which the sunlight shall be guided through materials with decreasing band gap. The second architecture, called 'Effective Medium', consisting of denser arrays of thinner wires, where the light waves will experience an effectively averaged medium as they propagate downwards. The connected Milestone MS7 that was intended to make a decision on the final path of the project was already reported with MS4 when we agreed to proceed to research with the most promising architecture, effective medium, to determine its potential and limitations.
WP1 had the objective to control the growth of the nanowires. The effects of growth parameters on composition, doping and structure were addressed. 4 partners were involved with ULUND and SOL having the main task to grow wires while DTU and JKU were mainly involved in characterisation of the grown wires, which served as necessary feedback to the growers. ULUND was the lead partner.
The main goals of WP1 were to:
- develop the incorporation of dopants during nanowire epitaxial growth;
- optimise catalyst-assisted and non-catalyst-assisted growth on III-V substrates;
- achieve epitaxial growth of III-V nanowires on Si;
- control and optimise III-V nanowire heterostructure segment growth and composition in nanowires, both axially and radially;
- optimise core shell growth for nanowire surface passivation and light guiding;
- evaluate the growth feasibility of III-V nanowire materials in materials combinations predicted for optimal solar cells;
- development of Esaki tunnel diode for incorporation into dual junction nanowire solar cells.
Much work had been done in this WP and results were reported in a number of deliverables (D1.1 1.2 1.3 1.4 and 1.5) and had been important for all milestones except MS4 and MS9. The growth activities have been continuously developed with a tremendous improvement. While we started with aerosol nanoparticles we have developed all parts necessary to reach the final goal of a dual-junction nanowire solar cell structure. Work to grow from imprint-defined seed particles started in the last period of the project. Those results will enable us to grow the structures that, according to the simulations made within the project, will lead to higher efficiencies in a future continuation of the project, if we can find funding for this. The new samples really boosted the efficiency of our structures. Still, the structures used during the final period of the project were not perfect.
It should be mentioned here that the transition from the aerosol particles to the imprinted seeds had been rather complicated. We have been working extensively on imprinted substrates and had to overcome a number of obstacles that prohibited good, reliable, uniformly and repeatedly structures. Not only the preparation of the substrate was a problem but also the larger diameter and the higher density of the seeds changed the growth and we needed to adjust growth parameters. We thus worked on two lanes:
a) aerosol-seeded growth for the further development of post-growth processing and measurement techniques; and
b) development of the imprint-seeded growth.
We have studied catalyst-assisted and non-catalyst-assisted growth of nanowires. While the catalyst-assisted growth has reached a high degree of understanding, the non-catalyst-assisted growth was not as successful and thus terminated.
We have realised controlled doping of the nanowires, both p-and n-type, especially in indium phosphide (InP) nanowires.
We have reached a high degree of epitaxial growth of III-V nanowires on silicon substrates. Here, we succeeded for the growth of Ga-based nanowires, i.e. GaAsxPy, a nucleation yield very close to 100 %. For In-based wires, i.e. InAsxP 1-x, a yield close to 50 % was achieved. However, we did not achieve to grow dual-junction nanowire structures on silicon. We developed an in situ etching process that allows the complete control over axial and radial growth. This process was developed for In-based wires but is rather generic and was applicable for other materials as well.
Several types of heterostructures have been produced. Nanowires with a GaAs nucleation, followed by gallium arsenide phosphide (GaAsP) sections with varying As and P contents can be produced by adjusting the ratio between AsH3 and PH3 in the reactor atmosphere. The same approach was used to make InAsP wires with an InP nucleation. Switching of the group-III material has been demonstrated by growing a gallium phosphide (GaP) section on InP nanowires nucleated from a Si substrate, as well as an InP section on InGaP wires grown on a Si substrate with an epitaxial GaP coating. Shell growth for Ga-based nanowires has been demonstrated by an increase of the temperature after growth of the nanowires to deposit a layer of GaP. This resulted in an epitaxial layer of GaP shell surrounding the wire.
We have demonstrated single nanowire Esaki tunnel diodes with high performance. Our non-optimised devices display peak current densities of up to 329 A / cm2, and we intend to exploit this design for nanowire-based multi-junction solar cells in the InAsP-GaAsP materials system.
We can also report the growth of radial gallium indium phosphide (GaInP) with varied materials composition on InP as needed for one way of passivating the active wire. Here, we also found a clear obstacle for the further development of the architecture with a light guiding shell due to requirements on lattice matching found. Together with the modelling this enabled us to decide rather early on pursuing only the development of one architecture within the project.
Also, the growth of dual junction PV structures in the GaInP materials system has reached a high degree of maturity.
The developments in the growth have, together with the developments in post-growth pro-cessing and measurement techniques, led to a number of real-world characterisation of finalised PV structures, with astonishing device efficiency.
WP2 had the objective to develop the process technology necessary to make functional PV cells from nanowires. This required innovative steps, development of suitable processes, as well as adaptation and application of established processes. WP2 was also responsible for the processing involved in production of PV cells for further testing and evaluation. Three partners were involved in this WP, with SOL being the main and the lead partner, assisted by ISE and DTU.
The main goals of WP2 were to:
- create low-resistive transparent contacts to nanowire arrays;
- develop a metallic contact grid for maximum efficiency;
- passivate nanowire surfaces to reduce recombination and resistance.
Results from the work in this WP have been reported in four deliverables, D2.1 - 2.4 and have been important for four milestones, MS2, 4, 7 and 10.
As mentioned above, the pre-growth patterning has been an essential part of the development work. We started out with aerosol-defined patterns that served for many of the developments in WP1 but could not be used to optimise the structures. We have developed an imprint method to generate patterns that more resemble the predicted optimised structures and have been more successful with the imprinted patterns during the last period of the project.
Post-growth patterning has reached such a degree of maturity that we are able to routinely fabricate samples from the grown structures that can go to the device characterisation. Still, the processing is not optimal and we foresee a much higher efficiency of the final PV structure in case we would have the possibility to improve processing further.
WP3 had the objective to obtain a thorough understanding of fundamental and device relevant physical properties of nanowires intended for PV applications. A precise characterisation of the grown nanowires is crucial to understand and optimise the growth schemes. It is crucial to develop efficient techniques to measure the energy spectrum, and energy absorption spectrum thereof, and to understand the consequences for the electronic and structural properties. Important device parameters like absorption efficiency and carrier concentration needed to be evaluated for these nanowire structures. 5 partners were involved in this WP with JKU being the main and the lead partner, assisted by ULUND, UKAS, ISE and DTU. The main issues were nanowire shape and size, chemical composition, crystal structure and defect characterisation, especially for heterostructures and wires with different doping levels.
Results from the work in this WP have been reported in 5 deliverables, D3.1 - 3.5 and have been important for almost all milestones except MS8 and 10.
Many different methods have been applied to characterise the structures, especially different microscopy, X-ray, and luminescence techniques. The measurements gave plenty of insights into the morphological and structural properties and the physics of the wires. The intense communication between the growers from ULUND and SOL with the physicists from JKU and DTU ensured that the observations had a direct input in the growth experiments. From the morphological and structural observations, the growers could modify the process parameters in order to improve the structures. However, one of our internal dreams we could not achieve: to characterise one single fully processed structure by x-ray, electron microscopy as well as electrically.
WP4 had the objective to develop novel models, analyse the physics, and to develop the ultimate performance design for nanowire solar cells. The methods were physics-based and aimed to be predictive, i.e. by knowing the geometry, materials, and microscopic material parameters of the solar cell, the electrical and optical characteristics are calculated as in an experiment. Model verification was ensured by detailed comparison to experimental data determined in WP3 and WP5. The results were used to analyse and interpret experimental results, study the underlying physical mechanisms and develop novel designs at the computer.
Originally, it was planned that two partners (UKAS and ISE) would work in this WP, however, even ULUND joint this task. The reason being on one hand that UKAS had some delays since the group moved from Zürich to Kassel in the first months of the project and on the other hand that ULUND had some extra capacity for such simulations. UKAS was the main and lead partner in this WP.
The main goals of WP4 were:
- the definition of suitable bandgap energies for single-and dual-junction nanowire solar cells;
- the definition of suitable device structure including doping levels and barrier layers;
- the definition of optimum configuration for wire length, spacing and diameter;
- the understanding of optical and electrical loss mechanism in single-and dual-junction nanowire solar cells with / without light guide structure;
- the improvement of electro-optical modeling tools for nanowire materials.
Results from the work in this WP were reported in four deliverables, D4.1 - 4.4 and had been important for four Milestones (MS2, 4, 7 and 10).
Extensive simulations have been performed by all three partners with the emphasis to actually split the work between the 3 partners involved and use the strength of each partner to accomplish a more complete picture. The optical generation function for nanowire solar cells have been analysed and the electromagnetic mechanisms for light capture in a nanowire array under solar illumination have been clarified. Surely, even in this WP more results could be achieved by a continued support of the project.
WP5 had the objective to characterise nanowire solar cells under standard test conditions in order to determine the efficiency of these devices and to access the behaviour and quality of the different layers in the structure. The device characterisation was also an important input to WP4 to validate the theoretical device simulation by experimental results. New measurement setups have to be developed to measure multi-junction nanowire solar cells accurately. 2 partners worked in this WP with ISE being the main and lead partner, assisted by UKAS. This was especially important since no other nanowire group in the world that works on PV structures has been able to perform such measurements.
The main goals of WP5 were:
- the precise IV and quantum efficiency measurements of single-junction and dual-junction nanowire solar cells with/without light guide structure;
- the characterisation of nanowire solar cells under concentrated illumination;
- to evaluate influence of process technology and characterisation on nanowire solar cell performance.
Three deliverables were within this WP, D5.1 - 5.3 and the results were important to 6 milestones (MS3 and MS6 - 10). Characterisation of fully processed nanowire PV structures was accomplished with the new measurement set-up developed by ISE during the first reporting period. For the best devices a top efficiency of 13.8 % under 1 sun illumination was determined. This result is better than any so far reported results for so-called emerging PV segment according to the NIST efficiency data.
WP6 had the objective to communicate the findings of the project to the scientific and technical community, to handle IPR and confidentiality issues, and to produce a roadmap for future development and deployment of nanowire solar cells. Three partners had tasks in this WP with SOL being the main and lead partner, assisted by ULUND and ISE.
Before communicating findings to the public and in order to handle IPR and confidential issues in a correct way, all partners posted their manuscripts on the internal web sites to ensure that all partners could comment and react. The dissemination was mainly done via journal articles and conference contributions.
WP7 was the management WP where ULUND had the main and lead role, assisted by UKAS, SOL and JKU.
As illustrated by the deliverables and milestones, the project was running accordingly to the anticipated time line. All Deliverables and Milestones have been reported to the European Commission (EC). We can conclude that the project AMON-RA was running on a high speed generating good and promising results as outcome of a true cooperation between the partners and which was even enhanced during the final course of the project.
Potential impact:
The project has led to the first steps of the development of a new type of solar cell structure based on semiconductor nanowires that has the potential to exceed all reported solar cell efficiencies.
The main result got a very high visibility in the scientific literature due to an article published in Science.
List of websites: http://www.amonra.eu(si apre in una nuova finestra)
We report on the activities carried out and the results obtained within the Seventh Framework Programme (FP7) project 'Architectures, materials and one-dimensional nanowires for PVs - research and applications' (AMON-RA) contract no. 214814. The main goal of the project was to demonstrate a dual-junction photovoltaic (PV) cell on silicon substrate based on nanowire architecture in a cooperative effort of 6 partners. In AMON-RA, we combined the excellence in materials science at Lund University (ULUND) and the world-leading experience in PVs at the Fraunhofer Institute for Solar Energy Systems (ISE) with the capabilities in processing of nanowire materials and the ambitions in fabricating nanowire-based PV components at the company Sol Voltaics AB (SOL). All this was supported by state of the art materials and device characterisation and structure modelling. Novel ground-breaking PV materials was developed by ULUND by the use of nanowires that were supplied to the Technical University of Denmark (DTU) and the Johannes Kepler University Linz (JKU) for detailed structural characterisation, SOL processed the PV structures, which were measured by ISE, supported by modelling from ISE and University of Kassel (UKAS). The ultimate ambition of the partners was to evaluate the potential of a proposed and dramatically different solar cell architecture which promises to exceed the current price / performance limitations of commercially available cells and to obtain a thorough understanding of fundamental and device-relevant physical properties of nanowires applied to PV applications.
Project context and objectives:
We report on the activities carried out and the results obtained within the FP7 AMON-RA project. The main goal of the project was to demonstrate a dual-junction PV cell on silicon substrate based on nanowire architecture in a cooperative effort of six partners. In AMON-RA, we combined the excellence in materials science at ULUND and the world-leading experience in PVs at ISE with the capabilities in processing of nanowire materials and the ambitions in fabricating nanowire-based PV components at the company SOL. All this was supported by state of the art materials and device characterisation and structure modelling. Novel ground-breaking PV materials was developed by ULUND by the use of nanowires that were supplied to DTU and the JKU for detailed structural characterisation, SOL processed the PV structures, which were measured by ISE, supported by modelling from ISE and UKAS. The ultimate ambition of the partners was to evaluate the potential of a proposed and dramatically different solar cell architecture which promises to exceed the current price / performance limitations of commercially available cells and to obtain a thorough understanding of fundamental and device-relevant physical properties of nanowires applied to PV applications.
The project was based on a collaboration between theoreticians that simulated and experimentalists that fabricated and characterised the structures. This project was carried out in a true European spirit with an increasing interaction between the different partners over the course of the project.
We can report here that:
- the project was running at high speed;
- most project goals have been achieved;
- some results have been exceeding the envisioned goals;
- however, not all anticipated milestones could be reached.
The project has led to the first steps of the development of a new type of solar cell structure based on semiconductor nanowires that have the potential to exceed all reported solar cell efficiencies. However, with the end of the project no financial support is at hand to continue this development which certainly would increase the competitiveness of Europe in this very important industrial and energy sector.
Project results:
Progress in the project
The project was organised in 7 work packages (WPs), with the seventh being management. The WPs had been carefully chosen in terms of congruent competence areas. Each WP involved activities of more than one partner and the leading partner was chosen from the most experienced one in that area. It was planned and executed that all WPs run over the entire period of the project, except WP5, which dealt with characterisation of fabricated solar cell structures and thus started upon availability of the first device that had been fabricated (month 12). From that point on all WPs were running in parallel until the end of the project.
The main idea behind AMON-RA was to make arrays of light-harvesting nanowires on silicon substrates. The structure intended was composed of nanowires including tandem PV cells, based on heteroepitaxy of III-V semiconductors. We intended to investigate two different architectures: The first, called 'Light Guiding', consisting of core-shell nanowires with a diameter equal to or greater than ? / n, in which the sunlight shall be guided through materials with decreasing band gap. The second architecture, called 'Effective Medium', consisting of denser arrays of thinner wires, where the light waves will experience an effectively averaged medium as they propagate downwards. The connected Milestone MS7 that was intended to make a decision on the final path of the project was already reported with MS4 when we agreed to proceed to research with the most promising architecture, effective medium, to determine its potential and limitations.
WP1 had the objective to control the growth of the nanowires. The effects of growth parameters on composition, doping and structure were addressed. 4 partners were involved with ULUND and SOL having the main task to grow wires while DTU and JKU were mainly involved in characterisation of the grown wires, which served as necessary feedback to the growers. ULUND was the lead partner.
The main goals of WP1 were to:
- develop the incorporation of dopants during nanowire epitaxial growth;
- optimise catalyst-assisted and non-catalyst-assisted growth on III-V substrates;
- achieve epitaxial growth of III-V nanowires on Si;
- control and optimise III-V nanowire heterostructure segment growth and composition in nanowires, both axially and radially;
- optimise core shell growth for nanowire surface passivation and light guiding;
- evaluate the growth feasibility of III-V nanowire materials in materials combinations predicted for optimal solar cells;
- development of Esaki tunnel diode for incorporation into dual junction nanowire solar cells.
Much work had been done in this WP and results were reported in a number of deliverables (D1.1 1.2 1.3 1.4 and 1.5) and had been important for all milestones except MS4 and MS9. The growth activities have been continuously developed with a tremendous improvement. While we started with aerosol nanoparticles we have developed all parts necessary to reach the final goal of a dual-junction nanowire solar cell structure. Work to grow from imprint-defined seed particles started in the last period of the project. Those results will enable us to grow the structures that, according to the simulations made within the project, will lead to higher efficiencies in a future continuation of the project, if we can find funding for this. The new samples really boosted the efficiency of our structures. Still, the structures used during the final period of the project were not perfect.
It should be mentioned here that the transition from the aerosol particles to the imprinted seeds had been rather complicated. We have been working extensively on imprinted substrates and had to overcome a number of obstacles that prohibited good, reliable, uniformly and repeatedly structures. Not only the preparation of the substrate was a problem but also the larger diameter and the higher density of the seeds changed the growth and we needed to adjust growth parameters. We thus worked on two lanes:
a) aerosol-seeded growth for the further development of post-growth processing and measurement techniques; and
b) development of the imprint-seeded growth.
We have studied catalyst-assisted and non-catalyst-assisted growth of nanowires. While the catalyst-assisted growth has reached a high degree of understanding, the non-catalyst-assisted growth was not as successful and thus terminated.
We have realised controlled doping of the nanowires, both p-and n-type, especially in indium phosphide (InP) nanowires.
We have reached a high degree of epitaxial growth of III-V nanowires on silicon substrates. Here, we succeeded for the growth of Ga-based nanowires, i.e. GaAsxPy, a nucleation yield very close to 100 %. For In-based wires, i.e. InAsxP 1-x, a yield close to 50 % was achieved. However, we did not achieve to grow dual-junction nanowire structures on silicon. We developed an in situ etching process that allows the complete control over axial and radial growth. This process was developed for In-based wires but is rather generic and was applicable for other materials as well.
Several types of heterostructures have been produced. Nanowires with a GaAs nucleation, followed by gallium arsenide phosphide (GaAsP) sections with varying As and P contents can be produced by adjusting the ratio between AsH3 and PH3 in the reactor atmosphere. The same approach was used to make InAsP wires with an InP nucleation. Switching of the group-III material has been demonstrated by growing a gallium phosphide (GaP) section on InP nanowires nucleated from a Si substrate, as well as an InP section on InGaP wires grown on a Si substrate with an epitaxial GaP coating. Shell growth for Ga-based nanowires has been demonstrated by an increase of the temperature after growth of the nanowires to deposit a layer of GaP. This resulted in an epitaxial layer of GaP shell surrounding the wire.
We have demonstrated single nanowire Esaki tunnel diodes with high performance. Our non-optimised devices display peak current densities of up to 329 A / cm2, and we intend to exploit this design for nanowire-based multi-junction solar cells in the InAsP-GaAsP materials system.
We can also report the growth of radial gallium indium phosphide (GaInP) with varied materials composition on InP as needed for one way of passivating the active wire. Here, we also found a clear obstacle for the further development of the architecture with a light guiding shell due to requirements on lattice matching found. Together with the modelling this enabled us to decide rather early on pursuing only the development of one architecture within the project.
Also, the growth of dual junction PV structures in the GaInP materials system has reached a high degree of maturity.
The developments in the growth have, together with the developments in post-growth pro-cessing and measurement techniques, led to a number of real-world characterisation of finalised PV structures, with astonishing device efficiency.
WP2 had the objective to develop the process technology necessary to make functional PV cells from nanowires. This required innovative steps, development of suitable processes, as well as adaptation and application of established processes. WP2 was also responsible for the processing involved in production of PV cells for further testing and evaluation. Three partners were involved in this WP, with SOL being the main and the lead partner, assisted by ISE and DTU.
The main goals of WP2 were to:
- create low-resistive transparent contacts to nanowire arrays;
- develop a metallic contact grid for maximum efficiency;
- passivate nanowire surfaces to reduce recombination and resistance.
Results from the work in this WP have been reported in four deliverables, D2.1 - 2.4 and have been important for four milestones, MS2, 4, 7 and 10.
As mentioned above, the pre-growth patterning has been an essential part of the development work. We started out with aerosol-defined patterns that served for many of the developments in WP1 but could not be used to optimise the structures. We have developed an imprint method to generate patterns that more resemble the predicted optimised structures and have been more successful with the imprinted patterns during the last period of the project.
Post-growth patterning has reached such a degree of maturity that we are able to routinely fabricate samples from the grown structures that can go to the device characterisation. Still, the processing is not optimal and we foresee a much higher efficiency of the final PV structure in case we would have the possibility to improve processing further.
WP3 had the objective to obtain a thorough understanding of fundamental and device relevant physical properties of nanowires intended for PV applications. A precise characterisation of the grown nanowires is crucial to understand and optimise the growth schemes. It is crucial to develop efficient techniques to measure the energy spectrum, and energy absorption spectrum thereof, and to understand the consequences for the electronic and structural properties. Important device parameters like absorption efficiency and carrier concentration needed to be evaluated for these nanowire structures. 5 partners were involved in this WP with JKU being the main and the lead partner, assisted by ULUND, UKAS, ISE and DTU. The main issues were nanowire shape and size, chemical composition, crystal structure and defect characterisation, especially for heterostructures and wires with different doping levels.
Results from the work in this WP have been reported in 5 deliverables, D3.1 - 3.5 and have been important for almost all milestones except MS8 and 10.
Many different methods have been applied to characterise the structures, especially different microscopy, X-ray, and luminescence techniques. The measurements gave plenty of insights into the morphological and structural properties and the physics of the wires. The intense communication between the growers from ULUND and SOL with the physicists from JKU and DTU ensured that the observations had a direct input in the growth experiments. From the morphological and structural observations, the growers could modify the process parameters in order to improve the structures. However, one of our internal dreams we could not achieve: to characterise one single fully processed structure by x-ray, electron microscopy as well as electrically.
WP4 had the objective to develop novel models, analyse the physics, and to develop the ultimate performance design for nanowire solar cells. The methods were physics-based and aimed to be predictive, i.e. by knowing the geometry, materials, and microscopic material parameters of the solar cell, the electrical and optical characteristics are calculated as in an experiment. Model verification was ensured by detailed comparison to experimental data determined in WP3 and WP5. The results were used to analyse and interpret experimental results, study the underlying physical mechanisms and develop novel designs at the computer.
Originally, it was planned that two partners (UKAS and ISE) would work in this WP, however, even ULUND joint this task. The reason being on one hand that UKAS had some delays since the group moved from Zürich to Kassel in the first months of the project and on the other hand that ULUND had some extra capacity for such simulations. UKAS was the main and lead partner in this WP.
The main goals of WP4 were:
- the definition of suitable bandgap energies for single-and dual-junction nanowire solar cells;
- the definition of suitable device structure including doping levels and barrier layers;
- the definition of optimum configuration for wire length, spacing and diameter;
- the understanding of optical and electrical loss mechanism in single-and dual-junction nanowire solar cells with / without light guide structure;
- the improvement of electro-optical modeling tools for nanowire materials.
Results from the work in this WP were reported in four deliverables, D4.1 - 4.4 and had been important for four Milestones (MS2, 4, 7 and 10).
Extensive simulations have been performed by all three partners with the emphasis to actually split the work between the 3 partners involved and use the strength of each partner to accomplish a more complete picture. The optical generation function for nanowire solar cells have been analysed and the electromagnetic mechanisms for light capture in a nanowire array under solar illumination have been clarified. Surely, even in this WP more results could be achieved by a continued support of the project.
WP5 had the objective to characterise nanowire solar cells under standard test conditions in order to determine the efficiency of these devices and to access the behaviour and quality of the different layers in the structure. The device characterisation was also an important input to WP4 to validate the theoretical device simulation by experimental results. New measurement setups have to be developed to measure multi-junction nanowire solar cells accurately. 2 partners worked in this WP with ISE being the main and lead partner, assisted by UKAS. This was especially important since no other nanowire group in the world that works on PV structures has been able to perform such measurements.
The main goals of WP5 were:
- the precise IV and quantum efficiency measurements of single-junction and dual-junction nanowire solar cells with/without light guide structure;
- the characterisation of nanowire solar cells under concentrated illumination;
- to evaluate influence of process technology and characterisation on nanowire solar cell performance.
Three deliverables were within this WP, D5.1 - 5.3 and the results were important to 6 milestones (MS3 and MS6 - 10). Characterisation of fully processed nanowire PV structures was accomplished with the new measurement set-up developed by ISE during the first reporting period. For the best devices a top efficiency of 13.8 % under 1 sun illumination was determined. This result is better than any so far reported results for so-called emerging PV segment according to the NIST efficiency data.
WP6 had the objective to communicate the findings of the project to the scientific and technical community, to handle IPR and confidentiality issues, and to produce a roadmap for future development and deployment of nanowire solar cells. Three partners had tasks in this WP with SOL being the main and lead partner, assisted by ULUND and ISE.
Before communicating findings to the public and in order to handle IPR and confidential issues in a correct way, all partners posted their manuscripts on the internal web sites to ensure that all partners could comment and react. The dissemination was mainly done via journal articles and conference contributions.
WP7 was the management WP where ULUND had the main and lead role, assisted by UKAS, SOL and JKU.
As illustrated by the deliverables and milestones, the project was running accordingly to the anticipated time line. All Deliverables and Milestones have been reported to the European Commission (EC). We can conclude that the project AMON-RA was running on a high speed generating good and promising results as outcome of a true cooperation between the partners and which was even enhanced during the final course of the project.
Potential impact:
The project has led to the first steps of the development of a new type of solar cell structure based on semiconductor nanowires that has the potential to exceed all reported solar cell efficiencies.
The main result got a very high visibility in the scientific literature due to an article published in Science.
List of websites: http://www.amonra.eu(si apre in una nuova finestra)
