Above 10% efficiency earth-abundant Cu2ZnSn(S, Se)4 solar cells by introducing alkali metal fluoride nanolayers as electron-selective contacts. (September 2018) - Chen, Cheng-Ying
Abstract: The present investigation mainly addresses the open circuit voltage (Voc) issue in kesterite based Cu2 ZnSn(S, Se)4 solar cells by simply introducing alkali metal fluoride nanolayers (~ several nm NaF, or LiF) to lower the work functions of the front ITO contacts without conventional hole-blocking ZnO layers. Kelvin probe measurements confirmed that the work function of the front ITO decreases from 4.82 to 3.39 and 3.65 eV for NaF and LiF, respectively, resulting in beneficial band alignment for electron collection and/or hole blocking on top electrodes. Moreover, a 10.4% power conversion efficiency (~ 11.5% in the cell effective area) CZTSSe cell with improved Voc of up to 90 mV has been attained. This demonstration may provide a new direction of further boosting the performance of copper chalcogenide based solar cells as well. Graphical abstract: The present investigation mainly addresses the open circuit voltage issue in kesterites based Cu2 ZnSn(S, Se)4 solar cells by introducing an alkali metal fluoride electron-selective nanolayer between ITO and CdS without a conventional hole-blocking ZnO layer. fx1 Highlights: The main novelty of this work is to use the alkali metal fluoride (AMF) nanolayers (NaF, or LiF) for the first time in the electron-selective contacts of chalcogenide based thin-film photovoltaics. The present investigation mainly addresses the open circuit voltage issue in CZTSSe solar cells by introducing AMF nanolayers to lower the work functions of the front ITO contacts without conventional hole-blocking ZnO layers. The 10.4 % PCE (~11.5% in the cell effective area) CZTSSe cell has been attained.