In the quest for a more sustainable alternative for Cu(In,Ga)(S,Se)² (CIGS) solar cells, Solliance partner imec has fabricated and characterized Cu2ZnSnSe4 (CZTSe)-based solar cells. Under AM1.5G illumination, the best 1x1cm² cell shows an efficiency of 6.3 percent, a result that is comparable to state-of-the-art lab reports. These first promising results pave the way towards an improved solar cell structure that can eventually equal CIGS-based cells in efficiency.
Since the start of its research activities on alternative materials for thin-film photovoltaics (PV) in 2011, imec has achieved first promising results on CZTSe solar cells. Efficiencies have been demonstrated that are comparable to the best lab results reported elsewhere. The kesterite CZTSe, just like CZTS, is an emerging alternative chalcogenide solar cell absorber which, in contrast to CIGS, does not suffer from abundancy issues. The bandgaps of these two kesterites, 1.5-1.6eV for CZTS and 0.9eV for CZTSe, makes a combined material system almost ideal for a multi-junction thin-film solar cell that can compete with CIGS cells in terms of efficiency. To reach a comparable cell efficiency of 20 percent, further improvements on the layer and cell structures of the CZTSe and CZTS absorbers are mandatory.
Imec fabricated its CZTSe layers by sputtering Cu, Zn and Sn metal layers on Mo/glass substrates, and subsequent annealing in an H2Se containing atmosphere. Absorbers of different stoechiometry were fabricated. Cross-section scanning electron microscopy (XSEM) shows that the polycrystalline absorber layers are approximately 1µm thick and that the typical grain size is of the order of 1µm. The selenized samples were then processed at Helmholtz Zentrum Berlin into solar cells using a standard process flow for chalcogenide solar cells.
The highest efficiency obtained on a 1x1cm² cell is 6.3 percent, with a maximum short circuit current of 31.3mA/cm², an open circuit voltage of 0.39V and a fill factor of 52 percent. First results indicate that the doping density, as measured from drive level capacitance profiling (DLCP), increases exponentially with the Zn/Sn ratio. As these results look very promising, further research is ongoing to improve the structure and efficiency of these sustainable alternatives for CIGS solar cells.
These results were presented at the European Materials Research Society (e-MRS) Spring Meeting 2012. The Flemish ‘Strategisch Initiatief Materialen’ (SIM) SoPPoM program is acknowledged for its collaboration.
Imec’s thin-film solar cell activities are integrated in the Solliance collaboration platform. Solliance’s goal is to strengthen the position of the Eindhoven-Leuven-Aachen triangle (ELAt region) as a world player in thin-film PV. Solliance aims to realize this ambition by joint use of state-of-the-art infrastructure, alignment of research programs, and close cooperation with the solar business community. It focuses on three main thin-film technologies: thin-film Si, alternatives for CIGS, and organic PV.
Imec’s CZTSe solar cell with 6.3-percent efficiency: (first) XSEM image and (second) I-V characteristics under AM1.5G illumination.