Bifacial Kesterite Solar Cells on FTO Substrates

The thin film kesterite-type Cu2ZnSnSe4 (CZTSe) and Cu2ZnSn(SxSe1-x)(4) (CZTSSe) chalcogenides are promising materials due to their composition based on earth abundant elements and proven photovoltaic performance in solar cells. The use of transparent back electrodes for photovoltaic applications, adds to this family of thin film devices a plethora of possible technological applications including generating electrical power also from the rear side, thus becoming bifacial devices. This work reports a strategy to improve the transparent back electrode/absorber interface that consists of the insertion of a molybdenum (Mo) or a Mo-doped with sodium (Mo:Na) nanolayer between the transparent electrode and the kesterite absorber. The CZTSe and CZTSSe absorbers have been fabricated onto fluorine doped tin oxide (FTO) substrates from sputtered metals followed by reactive annealing on Se+Sn and Se+S+Sn atmosphere respectively, at 525 and 550 degrees C annealing temperatures. By Raman scattering of the back interface through the transparent substrate (lambda = 532 and 785 nm), the formation of Mo(SxSe1-x)(2) was found on the devices with the corresponding Mo nanolayers correlating with their improved photovoltaic parameters without detrimental electrode/absorber interfacial reaction. A gain in bifacial (front) efficiency from 3.1% (2.6%) in the FTO/CZTSSe device to 7.7% (6.3%) in the FTO/Mo/CZTSSe solar cell with an overall improvement of the optoelectronic parameters demonstrates the effectiveness of this strategy. The use of transparent back contacts and interface engineering are of high interest, not only for bifacial devices but also for several advanced photovoltaic applications such as building integration and high efficiency tandem devices. The strategy herein presented can potentially boost the performance of such technologies.