Novel Mg‐ and Ga‐doped ZnO/Li‐Doped Graphene Oxide Transparent Electrode/Electron‐Transporting Layer Combinations for High‐Performance Thin‐Film Solar Cells

Herein, a novel combination of Mg‐ and Ga‐co‐doped ZnO (MGZO)/Li‐doped graphene oxide (LGO) transparent electrode (TE)/electron‐transporting layer (ETL) has been applied for the first time in Cu2ZnSn(S,Se)4 (CZTSSe) thin‐film solar cells (TFSCs). MGZO has a wide optical spectrum with high transmittance compared to that with conventional Al‐doped ZnO (AZO), enabling additional photon harvesting, and has a low electrical resistance that increases electron collection rate. These excellent optoelectronic properties significantly improved the short‐circuit current density and fill factor of the TFSCs. Additionally, the solution‐processable alternative LGO ETL prevented plasma‐induced damage to chemical bath deposited cadmium sulfide (CdS) buffer, thereby enabling the maintenance of high‐quality junctions using a thin CdS buffer layer (≈30 nm). Interfacial engineering with LGO improved the Voc of the CZTSSe TFSCs from 466 to 502 mV. Furthermore, the tunable work function obtained through Li doping generated a more favorable band offset in CdS/LGO/MGZO interfaces, thereby, improving the electron collection. The MGZO/LGO TE/ETL combination achieved a power conversion efficiency of 10.67%, which is considerably higher than that of conventional AZO/intrinsic ZnO (8.33%). MGZO/LGO TE/ETL are combined in CZTSSe thin‐film solar cells to solve the chronicle problems of conventional Al‐doped ZnO/ZnO TE/ETL combination. The MGZO/LGO combination enables a considerably improved conversion efficiency by enhancing the electron collection rate and suppressing the open‐circuit voltage deficit.