The project objective was to contribute to the field of thin film silicon technology based on amorphous and microcrystalline silicon photoactive layers by findings that can lead to increased thin film silicon module efficiencies while ensuring lower costs and long-term stability. This project had four axes which focused on layers with new or better properties (e.g. new materials to be incorporated into devices), on improved processes (e.g. new plasma regimes leading to higher quality layers), on improved devices (integration of all layers into state-of-the-art devices), and on enhanced cell and module reliability. The project focused on the use of advanced materials and optical structures to increase device efficiency. Mixed-phase p and n doped silicon oxide layers are now implemented in most devices and we were able to reveal their nanoscale filamentary structure responsible for their unique properties. In particular, it could be shown that these layers reduce the parasitic absorption (increase in current) and improve the performance of devices deposited on substrate with high roughness. These layers are now routinely used for most amorphous and microcrystalline cells. A new understanding of the impact of plasmas on the growth on rough substrates was acquired, allowing disentangling effects linked to bulk material issue from effects linked to porous zones appearing on rough substrates. Strong accent was also on improving cell stability by minimizing light induced degradation and gaining better control over the morphology evolution which is important for the growth of high quality silicon layers. Much effort went into the development of new concepts for the transparent front electrode to improve transparency and conductivity, including the introduction of non-intentionally doped zinc oxide, zinc oxide bilayers and hydrogenated indium oxide. But also new concepts to overcome the trade-off between light trapping and electrical cell performance were introduced including nanoimprinting, nanomoulding and multi-scale electrode architectures. Combining many individual novel concepts and improvements, a micromorph cell with an excellent initial efficiency of 14.1% was demonstrated within the project, a 1.5% absolute improvement with respect to the final results of the preceding 2005-2007 OFEN project. Stabilized efficiencies increased moderately from 11.1% to 11.5% for solar cells on glass. Noteworthy the trend toward thinner bottom cells (from 3 um to 1.2 um), allows identical stabilized efficiencies but opens the perspective for higher throughput in industry. Microcrystalline silicon solar cell efficiency improved from 9.9% to 10.9% compared to the preceding project period. Based on the project findings, we expect now to be able to bring the stabilized efficiencies to significantly higher level. Globally, the findings of the project should contribute to lower the cost of thin film silicon modules, while continuing increasing efficiencies. Production costs of 0.35 to 0.44 €/Wp should soon be possible for modules containing only 1-1.5 micron of silicon and two zinc oxide layers, making it further one of the most attractive technology for a world with a significant solar electricity share.
