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Plasmons Generating Nanocomposite Materials (PGNM) for 3rd Generation Thin Film Solar Cells

Final Report Summary - SOLAMON (Plasmons Generating Nanocomposite Materials (PGNM) for 3rd Generation Thin Film Solar Cells)

Executive summary:

Within the Seventh Framework Programme (FP7), the SOLAMON project, launched in February 2009, targets the conversion efficiency enhancement of thin-film solar cells through optical plasmonic effects. The research consortium, coordinated by CEA-Liten, gathers five partners among the major European thin film solar cells players together with the Australian pioneer in the field of improvement of solar cell performances through nanomaterials. SOLAMON activities address various thin film solar cells like a-Si-H, organic and dye sensitised cells. The quantitative objective of SOLAMON is an increase of 20 % in the short circuit current density of the solar cells. For that purpose, the concept of Plasmon generating nanocomposite materials (PGNM) has been introduced, consisting in metallic nanoparticles (NP) of well controlled characteristics embedded in a matrix. This nanocomposite associated with the active layer is expected to boost the absorption of sun light and lead to a more efficient electrical carrier generation.

PGNM synthesis

The deposition of well controlled small Ag NPs inside various matrices was demonstrated. The deposition of larger NPs promoting far field Plasmon resonance scattering was also made possible on the basis of a new source design (patent pending). Despite a broader size distribution, good process control and reproducibility were achieved. Furthermore, going beyond the scope of SOLAMON, preliminary experiments demonstrated the feasibility of Ag / Al203 core shell NPs that could be advantageously used in PGNM-based solar cells for electrical management purpose.

PGNMs based solar cell design

The building blocks required for a complete modelling of PGNM-based solar, namely the MMP simulation code for the determination of PGNM optical properties, the optical Sunshine simulator and electrical ASPIN code have been integrated successfully. The EQE and I / V curves of the different cells integrating or not PGNMs were simulated and some of them experimentally validated. At the present state of progress and taking into account only the far field Plasmon resonance effect, we already get first evidence of the potential interest of PGNMs into dye sensitised cells. In return, for organic and a-Si:H solar cells, no noticeable improvement was foreseen with sub-30 nm Ag NPs very near or directly inside the active layer. However, preliminary modelling refinements taking better into consideration the near field Plasmon effect should improve these predictions.

PGNMs integration in solar cells

The PGNMs layers were successfully integrated in the different cell technological chains (a-Si;H, OSC and DSSC) since most PGNMs based cells were found to correctly operate. Some of them like organic cells seem to exhibit some improvement (+5 %) of their photocurrent and efficiency. However, the experimental results confirmed the modelling predictions that embedding Ag NPs can strongly impact the electrical chain of the solar cell; this was found obvious for the a-SiH cells, much less in the organic cells. For Dye-sensitised solar cell (DSSC)s, an additional chemical compatibility issue between electrolyte and Ag NPs was evidenced. However, the new core-shell technological solutions could solve both, electrical and chemical issues.

A first assessment of the costs of the vacuum NP technology shows that, at the present level of technological maturity, the additional cost of PGNM based a-Si:H solar cell would be negligible.

Conclusion:

After two years of research, the basic tools necessary to the development of Plasmon-based solar cells have been successfully implemented, in particular a patented low temperature large NP deposition technology, an optical and electrical model dedicated to the simulation of PGNM-based solar cell structures and a complete technological chain now operational for NPs integration in thin film solar cells. The results obtained so far showed that the main objective of SOLAMON could not been reached (+20 % increase in photocurrent), mainly because of enhanced electrical recombination issues. That is why, on the basis of the SOLAMON foreground, new technological solutions are proposed to produce large passivated core-shell metallic NPs. Two technological patents have been filled for the production of this new kind of NPs.

Project context and objectives:

As the world energy needs continue to rise and concerns about the environmental impact of traditional energy technologies become more pressing, photovoltaic energy increasingly appears as a key energy technology for mankind. Currently, the wafer based crystalline Si technology dominates the high efficiency grid connected market, with a share of more than 90 % and an industrial conversion efficiency of over 15 % for both monocrystalline and multicrystalline Si. However the cost of this generation I solar cells (module cost USD 3 / Wp source: DrKW Equity research 79209-(N1): Solar Industry) still remains too high to compete with the other classical sources of electricity generation.

Thin film solar cells, the so-called generation II solar cells, have the potential to significantly decrease the cost of photovoltaics for other market applications, down to USD 1 / Wp in 2013. For thin film amorphous silicon (a-Si:H market: large surfaces, Building integrated photovoltaics BIPV), organic and DSSCs (markets: consumer goods, nomad applications), competitiveness on the markets is strongly linked to the cost-reduction which can be achieved through the development of low cost materials and thin film concepts. However, their conversion efficiency remains lower compared to the first generation ones since current R / D efficiencies range from 5 to 7 %, typically. The next step, leading to generation III solar cells needs a gap in conversion efficiency to achieve a cost target of USD 0.5 / Wp, crucial for the market penetration.

In this context, significant progress in material performances and innovative thin films concept can contribute significantly to the cost target achievement.

To overcome this paradigm between efficiency and low thickness of the active material, a substantial increase of knowledge and development of new materials enhancing light trapping is needed. A very promising way to improve the light trapping and absorption of photovoltaic solar cells is to exploit the strong scattering and absorption of light by metal nanoparticles due to the generation of localised surface plasmons. As a matter of fact, thin film device performance is further improved due to either the enhanced electric field around nanoparticles or the scattering of light into waveguide-propagating modes along the film, the wavelength of maximum light absorption or scattering being tuneable with the nanoparticle size, shape, and local dielectric environment. Enhanced absorption schemes have a great potential for several thin film technologies, e.g. amorphous silicon, organic and dye-sensitised thin film solar cells where charge transport particularly puts severe constraints on the active layer thickness.

This concept is clearly described as a priority for novel PV technologies (A Strategic Research Agenda for photovoltaic solar energy technology, European photovoltaic technology platform, European Communities, 2007, ISBN 978-92-79-05523-2, p 46-47.

The objective of the SOLAMON project, is to develop high potential PGNM which will pave the way to the generation III solar cells (high efficiency and low cost).

The objective is an augmentation in the external quantum efficiency resulting in an increase of 20 % in the short circuit current density of the thin film solar cells.

To achieve such an ambitious goal, the project will focus on the development of fully tailored building block nanoparticles able to generate a plasmon effect for enhanced solar absorption in thin film solar cells. Such nanoparticles designed for an optimum absorption will be integrated in solar cells matrix using a recently developed room temperature deposition process designed by Mantis Deposition. This step will result in the specific design of PGNM for solar cells using a knowledge based approach coupling modelling (University of Ljubljana) at both scales: nanoscopic (plasmonic structure) and macroscopic (solar cells). SOLAMON will address three different classes of solar cells: a-Si:H thin films, organics and dye sensitised. Developing the PGNM on these three classes aims at maximising the project impact because scientific background acquired on these technologies could be easily transferred to other ones. However, the a-Si:H based technology dedicated to the BIPV market will be firstly considered when a strategic choice occurs, keeping in mind that, even of large economic importance, the two other technologies do not have the same key BIPV environmental and social impact.

The optimal design of nanoparticles (nature, shape, size distribution, surface density, refractive index of the host matrix) will be approached by multipolar modeling for effective implementation in the three state of the art thin film technologies, a-Si:H, organic and DSSCs. The selection will use the calculation of both the scattering efficiency and enhanced electric field in the wavelength range relevant for each technology.

Finely controlled nanoparticles (pure metal and alloy) will be deposited at room temperature by an exclusive process design by Mantis Deposition. This work will be supported by the plasmons modeling performed at the nanoparticle scale.

Objective 1: Nanoparticles size tunable between 10 and 120 nm with 90 % of nanoparticles in +/- 10 % around the mean size value.

Optical and electrical modeling of the light absorbing structure integrating nanoparticles embedded in a given matrix will be performed to optimise the nanoparticles localisation and to quantify the expected improvement on both external quantum efficiency and short circuit current density. This approach will on one hand serve as a prediction tool for improvement potential and on the other hand will result in the specific design of PGNM for thin film solar cells.

Si, TiO2 and organic based PGNM will be synthesised by coupling the Mantis technology to processes classically used to deposit the active layer. Absorption of these nanocomposites, constituting the 'heart' of the solar cell, will be characterised.

Objective 2: 25 fold peak increase in absorption in the active layer in the wavelength range of interest: 700-800 nm for a-Si-H and dye-sensitised, and 650-800 nm for organic (best performance at UNSW: 16 folds in the 1000 nm wavelength range).

Proof of concept will be demonstrated through the implementation of PGNM in all three types of solar cells and measurement of both the external quantum efficiency and photocurrent enhancement.

Project objective: 20 % augmentation in the short circuit current density resulting in the increase of overall efficiency (a-Si-H, DSSC, and organic cells).

SOLAMON gathers the key actors to address material knowledge based development, material modelling and material synthesis process. The complementary partners will therefore answer the needs of all 3 types of systems: organic (CEA), a-Si:H thin film (TUD) and DSSCs (SOLAR). Clearly focused on a concept to feasibility approach, SOLAMON combines the most promising technique of nanoparticles deposition with an integrated modelling study. SOLAMON will use the exclusive expertise of the world leader in the field of plasmonic theory: the University of New South Wales (Australia).

List of websites: http://www.solamon.eu/

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