Disentangling the effect of the hole-transporting layer, the bottom, and the top device on the fill factor in monolithic CIGSe-perovskite tandem solar cells by using spectroscopic and imaging tools

Abstract We present monolithic copper–indium–gallium–diselenide (Cu(In,Ga)Se₂, CIGSe)-perovskite tandem solar cells with air- or N₂-transferred NiO_x:Cu with or without self-assembled monolayer (SAM) as a hole-transporting layer (HTL). A champion efficiency of 23.2%, open-circuit voltage (V{}_oc) of 1.69 V, and a fill factor (FF) of 78.3% are achieved for the tandem with N₂-transferred NiO_x:Cu + SAM. The samples with air-transferred NiO_x:Cu + SAM have V{}_oc and FF losses, while those without SAM are heavily shunted. We find via x-ray and UV photoelectron spectroscopy that the air exposure leads to non-negligible loss in the Ni²⁺ species and changes in the NiO_x:Cu’s work function and valence band maxima, both of which can negatively impact the V{}_oc and the FF of the tandems. Furthermore, by performing dark lock-in thermography, photoluminescence (PL), and scanning electron microscopy studies, we are able to detect various morphological defects in the tandems with poor performance, such as ohmic shunts originating from defects in the bottom CIGSe cell, or from cracking/delaminating of the perovskite top cell. Finally, by correlating the detected shunts in the tandems with PL-probed bottom device, we can conclude that not all defects in the bottom device induce ohmic shunts in the tandems since the NiO_x:Cu + SAM HTL bi-layer can decouple the growth of the top device from the rough, defect-rich and defect-tolerant bottom device and enable high-performing devices.