Publikationen / Ergebnisse
(Französisch)
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Ces cinq dernières années ont permis l’industrialisation à grande échelle des technologies à base de silicium couches mince (SCM), grâce à des efforts industriels importants, impliquant plusieurs entreprises suisses. Les perspectives de coût de production pour le SCM sont excellentes (nettement moins de 0.5€/W sur support verre), mais pour assurer l’implémentation à très grande échelle (>> 100 GW) dans un environnement compétitif et ainsi bénéficier des avantages uniques des SCM (matériaux abondants et non toxique, bon rendement énergétique, temps de retour en énergie minimum) , il est impératif d’améliorer significativement le rendement des dispositifs. Grâce aux récents résultats de recherche, les facteurs limitant fortement l’efficacité ont été identifiés. Ce projet propose une manière concrète de procéder pour que ces facteurs ne soient plus limitant. L’objectif global du projet est la réalisation de dispositifs à plus haut rendement (jusqu’à 20% de gain relatif en état stabilisé), stables à l’environnement et utilisant peu de matière première (jusqu’à -30% pour l’absorbeur). La méthodologie choisie implique un travail important sur le couplage des cellules dans les jonctions multiples : ce couplage se traduit par une perte des propriétés électriques comparées au potentielle des cellules individuelles. An moyen de substrat optiquement diffusant, mais favorables à la croissance, de nouvelles couches dopées et couches tampons plus transparentes et plus adéquates pour les substrats rugueux, ainsi que par des procédés plasma permettant le dépôt de matériaux plus denses, des rendements stables de près de 14% sont visés, avec un potentiel pour atteindre 15%. En parallèles les meilleures procédés et matériaux développés seront appliqués sur les technologies de cellules à hétérojonction amorphe-crystalline qui offrent la perspective de procédés simples pour des cellules dépassant les 22% de rendement.
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Schlussbericht
(Englisch)
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The global goal of this project, which started in March 2012 and ended in February 2015, was to further increase the conversion efficiency of thin film silicon photovoltaic devices, with parallel application of the new processes to high efficiency crystalline Si cells. For this purpose, six axes were defined that ranged from a better understanding of the key fabrication parameters to an improved reliability and characterization of the developed devices. All device efficiencies could be significantly improved during the project, both at laboratory and industry levels. In particular, much progress has been made towards a better understanding of amorphous and microcrystalline silicon material properties, partly thanks to dedicated characterization and development of new deposition regimes. In particular, the impact of excitation frequency on microcrystalline silicon material quality was demonstrated through a careful comparison between cells deposited with VHF (40.68 MHz) and RF (13.56 MHz). While both frequencies allow for the growth of very good bulk material quality, the efficiency of the cells prepared at VHF is, in the range considered here, typically lower. Electron microscopy measurements evidenced the fact that, for similar microcrystalline silicon grains quality, the absorber layer deposited at RF is denser than when deposited at VHF. However, in turn, VHF sustains better material quality at larger growth rate on rough morphology such as on rough front electrode or, in the case of a tandem, after the amorphous silicon top cell. Additionally, developments were undertaken with fluorinated precursors leading to a new type of absorber layer microstructure, with larger grains and enhanced absorption in the near infrared, resulting in very high short-circuit current densities. For amorphous silicon, a detailed screening of various deposition processes and corresponding material properties was carried out. This resulted in cells with particularly low gap and consequently high current density, but also in a high bandgap solar cell dedicated to the use as top cell in multi-junction devices leading to triple-junction voltages up to 1.93 V. A lot of work was also pursued to further improve light management and opto-electrical coupling, as required for such thin (of the order of 3 µm) devices. In particular, a novel approach to model the basic mechanism governing light trapping was developed, underlining the important role of parasitic absorption in the non-active layers. More transparent, silicon oxide based, doped and buffer layers are hence now routinely implemented in thin-film silicon solar cells, which further present the advantage of improving the electrical performance on rough morphologies, thanks to a “shunt-quenching” effect. In addition, work has been pursued in further optimizing the front and back electrodes with e.g. the development of metallic back reflectors. The reliability of solar cells and modules, and more particularly its link to various external stresses such as oxygen or water was also under scrutiny. In particular, simulations of the water penetration with time in PV modules could successfully be simulated, as a function of encapsulant material, module configuration and climatic location. Since water ingress is a common source of possible failure for most photovoltaic (PV) technologies (thin-film silicon, crystalline silicon, CIGS,..), these simulations provide crucial inputs to better predict long-term outdoor performance of PV modules. Thanks to all these developments, significant progress was made in terms of conversion efficiency, with a certified world record of 10.7% for a microcrystalline single-junction solar cell (achieved in 2013), followed by a certified micromorph tandem efficiency of 12.6% in 2014, right below the 12.7% official world record achieved right after by the National Institute of Advanced Industrial Science and Technology (AIST). Ultimately, an initial efficiency over 14% was reached in tandem (collaboration with Delft University) and triple-junction configuration (without germanium), while first tests of quadruple junctions provided insights on the roadmap to follow to reach stabilized efficiencies above 14% with already available building blocks. On the industrial level, an outstanding stabilized module efficiency of 12.3% on 1.43 m2 was demonstrated by TEL Solar. Finally, several of the layers developed in this project, including transparent conductive (TC) layers to be used as electrodes, were applied and tested in crystalline silicon heterojunction solar cells. Efficiencies as high as 22.4% were achieved with the standard design, while an efficiency of 22% could be reached with a back-contacted solar cell design. Development of new TC layers based on Indium Zinc Oxide (IZO) led to very promising results with very high opto-electronic properties together with a high thermal stability. The results of this project hence open further perspectives for the use of the developed processes and layers in higher efficiency c-Si solar cells.
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