Anisotropic Charge Transport in Cu(In,Ga)Se₂ by Heavy Alkali Postdeposition Treatment for Reducing Cell-to-Module Efficiency Loss in Monolithically Integrated Photovoltaic Modules

The recent efficiency boosting of Cu(In,Ga)Se₂ (CIGS) solar cells is undoubtedly triggered by heavy alkali postdeposition treatments (PDTs). However, the effects are not obvious under monolithically integrated CIGS modules where various current-shunting sources can deteriorate the device performance. Herein, It is reported that KF PDT can effectively suppress the major shunting sources caused by P1 and P3 laser scribing for monolithic interconnection, reducing the cell-to-module (CTM) efficiency gap in CIGS photovoltaics. CIGS with NaF PDT exhibits nearly isotropic and high hole mobilities, causing a large CTM efficiency loss. CIGS with additional KF PDT, on the other hand, reveals much lower in-plane hole mobility than the out-of-plane component, significantly increasing the P1 shunt resistance without exacerbating the photocarrier extraction in the active area. It is suggested that such anisotropic charge transport is due to carrier scattering by low-conductivity phases at the CIGS grain boundaries. Furthermore, passivation of the front junction by KF PDT raises the tolerance to P3 scribing-induced damage, increasing the P3 shunt resistance while preserving the junction property unlike the NaF PDT case. The work implies that the recent trend of employing heavy alkali PDTs for a high-efficiency cell is also crucial for designing a high-efficiency CIGS module.