Semi-transparent Perovskite Solar Cells for Four-Terminal Perovskite/CIGS Tandem Solar Cells

Tandem solar cells that combine perovskite (PVK) top cells and Si, Cu(In,Ga)(Se,S)2 (CIGS), and other bottom cells have attracted much attention for increasing the efficiency of solar cells. To use the PVK solar cells as the top cells, their metal electrode needs to be replaced with a transparent conductive layer such as indium tin oxide (ITO) deposited by sputtering, where thermally evaporated MoOx needs to be introduced to protect the underlying hole transport layer (HTL) from sputtering damage. In this study, it was revealed that the effect of ion bombardment during ITO sputtering on widely used spiro-OMeTAD was not detrimental to the device performance, whereas it improved the power conversion efficiency (PCE) owing to the better band alignment caused by oxidation of the HTL. By eliminating the use of the MoOx buffer layer, we were able to develop semi-transparent PVK cells with higher durability and transmittance. PCEs of 19.5% (certified 19.3%) and 26.2% were achieved for a 1 cm2 buffer-free semi-transparent PVK cell and four-terminal PVK/CIGS tandem solar cells, respectively.