State of the art and perspective of inorganic photovoltaic: the research activity at MIBSOLAR

Inorganic materials dominated the PV market from the beginning. PV devices based on silicon are the most common solar cells currently being produced and it is mainly due to silicon technology that the PV has been growing by 40% per year over the last decade. Cost, sustainability and environmental issues are at the core of the PV silicon based industry activities. An additional step in the silicon solar cell development is on-going and it is related to a further efficiency improvement through defect control, device optimization, surface modification, nanotechnology approaches. Inorganic thin film PV technologies have been under development in the last decades as a low cost alternative to bulk c-Si. Incidentally, this technology is the most suitable for Building Integrated Photovoltaics (BIPV) and for large-scale production since the module is the final stage of an in-line process that does not require the assembly of discrete smaller cells. In the frame of inorganic thin film Cu(In,Ga)Se2 (CIGS) solar cells is one of the most promising in terms of the efficiency record (20.3 %), nearly reaches the one of Si solar cell [1] and of stability. Despite the promise of CIGS thin-film and also of CdTe photovoltaic technologies with respect to reducing cost per watt of solar energy conversion, these approaches rely on elements that are either costly and/or rare in the earth's crust (e.g. In, Ga, Te) or that present toxicity issues (e.g., Cd), thereby potentially limiting these technologies in terms of future cost reduction and production growth. The new material Cu2ZnSnS4 (CZTS), in which indium and gallium are replaced by the readily available elements zinc and tin, is very promising in terms of reduced cost and sustainability but an intense research activities to optimize the growth process and the material quality are still necessary before transferring the device on the market. In the MIB-SOLAR center of UNIMIB, a new method for chalcogenide thin film (CIGS) deposition on glass and flexible substrates has recently been developed, in collaboration with a small enterprise [2, 3]. Up to now the process has been optimized for CIGS films with a thickness of around 2 micron, obtaining a cell efficiency of 14% on glass and 8.3% on stainless steel substrate, respectively, in less than 3 years of activity. In the last year we started a new activity regarding CZTS thin film [4]. Two main growth deposition methods are under investigation and testing: 1) Sputtering process and 2) Sol gel method This article points out the most important advances and current technologies related to inorganic PV devices. In the meantime the most challenging and promising strategies acting to increase the ratio between the efficiency and the cost of inorganic solar cells in our laboratory will be described. Some novel concepts under development in MIBSOLAR centers, which could have an important role in the future of PV will be also described. NOTE BIBLIOGRAFICHE [1] P.Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W.Wischmann, M.l Powalla Prog. Photovolt: Res. Appl. 19:894–897 (2011) [2] Acciarri, M. Binetti S., Le Donne A., Lorenzi B., Caccamo L., Miglio L., Moneta R. Marchionna S., Meschia M. Cryst. Res. Technol. 46, No. 8, 871–876 (2011) [3] European Patent 2013 n° EP 13425019.0