Materials and concepts for inorganic photovoltaics

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• Materials engineering for electronics

• Bulk Si solar cells

The photovoltaic industry will continue using for a very long time crystalline silicon wafers but their growth method (metallurgic Si, ribbon Si) and their impurity content (O, C, Al, Fe, Ti, B, P…) necessitate a very strong decrease of their thickness (<100 µm) in order to minimise the impact of the minority carrier diffusion length. The gettering, surface passivation, texturing, and metallisation steps become very important. Besides, the development of N type Si for photovoltaics is becoming a good alternative. Our research efforts will be based on developing innovative processes for surface passivation (Al2O3 dielectrics, AlN ...), for texturing (reactive plasma), doping and local metallisation (implantation, laser, lamps). The electric properties of metallurgic Si wafers and Si ribbon will be correlated with the impurity contents in these wafers before and after treatment.
Ongoing projects: ANR-BIFASOL ; AMI-DEMOS, EUROGIA-LAPSIS
Academic collaborations: INES-Chambery, INL-Lyon, ILV-Versailles, IUMN-Lille …
Collaborations with industry: PHOTOWATT, SOLARFORCE, EXCICO, IREPA-Laser …

• Thin film silicon cells

Thin film materials belong to the second generation of photovoltaic technologies. In particular, crystalline silicon provides several advantages: abundance (even in the gas phase), non-toxicity, easy recycling, chemical and thermal stability. However the disadvantages need to be tackled: indirect gap requiring high thicknesses, recombination defects, production costs. Our activities in this field are based on

o Investigation of new processes of elaboration of crystalline Si films on flexible substrates (metallic alloys ...), for instance the direct deposition of Si from a controlled plasma gas or the use of an ink containing silicon nanoparticles followed by sintering. The structural and electronic properties need to be correlated with the photovoltaic parameters of these structures.

o Development of methods for optical management in ultrathin Si in order to increase the path of photons and charge generation. Internal dielectric reflective films (ex. a-SiON:P, a-SiON:B), photonic crystals, metallic nanoparticle structures.

Projects: ANR-SILASOL ; FP7-POLYSIMODE
Academic collaborations: IMEC-B, HZB-DE, FhgISE-DE, INES-Chambery…
Collaborations with industry : PHOTOWATT, SUNTECH, IREPA-Laser, EXCICO …

• New concepts for photovoltaics

Photon conversion by luminescence

Increasing the conversion efficiency requires the complete use of the solar spectrum by the conversion layer. One of the possible solutions is impurity cells or intermediate band cells, in which a modification of the active part of the cell is necessary. Another original idea consists in the modification of the incident spectrum by energy conversion of its photons, either by down-conversion (DC) or up-conversion (UC). In the first case thermalised photons are recovered and in the second case the non absorbed photons are recovered. Our investigations are:

o Development of conversion films based on silicon oxynitride containing silicon nanocristals doped with rare earth elements.

o Development of transparent conductive oxide films (TCOs) based on ZnO doped with one or several rare earth elements (Tb, Yb, Nd…) in order to allow the required conversion properties. Studying the charge transfers between ZnO films and silicon nanoparticles is also planned.

Academic collaborations: IPCMS-Strasbourg, IJL-Nancy, METU-Turquie…

Plasmonics structures
The use of plasmonics in photovoltaics is very recent and relies on the possibility that metallic nanoparticles dispersed on a surface or at the rear can enhance the electromagnetic field and thus increase the absorption in thin films, in particular for silicon. We use either a chemical method (deposition of Ag and annealing) or a physical method (ionic implantation of Ag or Al in a dielectric matrix). The bottlenecks are the control of size and density, the demonstration of efficient conversion and the integration of the process in the final cell (bulk Si or thin film).
Academic collaborations: IJL-Nancy, IPCMS-Strasbourg, UTT- Troyes

Silicon tandem structures
The bandgap of crystalline silicon can be tuned by nanostructuring. The objective is to obtain silicon tandem cells by putting side by side Si materials with different nanoparticle sizes or nanowires.
Our research is two-fold:

o Structure containing silicon nanoparticles dispersed in order in a dielectric matrix, which effective bandgap is controlled by the size of nano-objects. The scientific challenges are the control of doping in these nanostructures (either in-situ during the magnetron sputtering deposition or ex-situ by ionic implantation and thermal or laser annealing) and the measurement of the consequences on the optical and electronic properties. The major technological challenge is the fabrication of a cell on these structures.

o Silicon nanowire structures by CVD deposition of multilayers containing nanoparticles but in conditions enabling the percolation of these nanoparticles. Our future work will be base on the elaboration of Si nanowires by etching of dielectric matrixes, the doping of these nanowires, the determination of the optical properties of these vertical structures and eventually the realisation of cells.

Academic collaborations: IJL-Nancy, LMPO-Metz

III-V tandem structures on silicon
The use of tandem cells, by putting side by side several semiconductors absorbing a part of the solar spectrum, seems a promising way given the conversion efficiencies already obtained in this way. We investigate the fabrication of novel multijunction cells combining the advantages of silicon and III-V materials. We aim to develop InGaN alloy cells on silicon substrates in order to convert a large part of the solar spectrum and convert it into electric charges. The scientific objectives are the understanding of InGaN alloy growth phenomena on Si substrates using buffer layers. The technological objectives are the realisation of tandem cells in which the conversion efficiencies will exceed 30%, which is the theoretical limit for homojunction cells. The environmental objectives are the use of less raw materials (Si, In, Ga, ...) for better performances.
Projects: ANR-NOVAGAINS
Academic collaborations: LGEP-Gif, GergiaTech-Metz,INL-Lyon…
Collaborations with industry: NOVATIONS