Currently, crystalline-Si (c-Si) based devices rule the photovoltaic (PV) market, accounting for about 96% of the total annual production versus 4% for thin films based technologies (namely, CdTe, Cu(In,Ga)Se2 (CIGS) and a-Si). In spite of the strong market gap between Si and thin films technologies, the development of PV absorbers proper for thin films based devices is nowadays even more crucial than in the past for future applicationsboth in Building/Product Integrated Photovoltaicsand in tandem devices. Furthermore, the availability of many raw materials used in thin film solar devices is seriously decreasing, while both energy and technology needs for the daily life are strongly increasing, which makes material saving crucial. The most studied alternatives to CdTe and CIGS in the last years were Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe), where more abundant and less expensive elements like Zn and Sn are used in place of In and Ga. More recently, further alternatives based on earth abundant elements emerged, among them Cu2MnSnS4 (CMTS) and Cu2FeSnS4 (CFTS).
Our research activities deal with the above-mentioned PV absorbers and related solar devices. In detail:
SILICON Under the realistic assumption that c-Si based PV modules will dominate the PV market in the coming decade, our research activity has been focused on the further increase of Si solar cells efficiency (studying the effect of defects mainly by spectroscopic techniques), on the characterization of low price and high quality solar grade silicon feedstock and finally on new initiatives to build high efficiency tandem solar cells.
CIGS and CuGaS2 (CGS) thin films on glass and flexible substrates (like plastic foils) are grown by an innovative hybrid sputtering-evaporation approach (combining the advantages of both techniques) and tested both in single junction and tandem devices.
CZTS, CFTS and CMTS are prepared mainly by a soft-chemical route involving the coordination of the metals into the solution thanks to the use of DMSO as solvent and thiourea as sulphur source, making it very appealing due to the absence of further organic additives and external sulphur sources. The precursors solution is directly deposited by drop-casting onto the substrate without the use of further expensive and/or industrially non-compatible instruments, making the whole procedure appealing for industrial green application.
For all these PV absorbers, acomprehensive structural and spectroscopic characterization (including scanning electron microscopy, Raman spectroscopy, X-ray diffractionand photoluminescence) is performed. All the new absorber layers are tested in prototype solar devices.
Thermoelectricity is a way to convert heat into electricity without the use of any movable part. As such,thermoelectric generators are suitable, especially when miniaturized, to harvest low-temperature heatand to make it available as electric power to distributed sensor networks or to other portable devices.
Bottom-up and top-down nanotechnology has played a major role in the enhancement of the efficiency of thermoelectric materials. Over the last decade, we have developed methods to obtain silicon nanowires and nanolayers, and to enhance bulk thermoelectric properties by controlled precipitation ofsecond phases in nanocrystalline silicon thin films. Research on thermoelectrics is currently oriented along two main lines, namely (a) silicon-based thermoelectric integrated devices working in the medium temperature range to supply electric power to wireless devices and (b) the development of novel mixed organic-inorganic nanocomposites to harvest body heat in portable (wearable) sensors.