Compositional Engineering of Rare Earth Metal Doped ZnO as Electron Transport Layers for Next-Generation Solar Cells /

Abstract

Dye-sensitized and perovskite solar cells are considered emerging innovations due to their significant performance improvements over the last decade. The performance efficiency of the perovskite solar cells depends on the performance of the perovskite absorber layer, electron transport layer (ETL), hole transport layer (HTL), and contacts. ZnO is commonly used as an ETL for both PSCs and DSSCs due to the direct band gap of 3.32 eV and favorable band alignment. Efficient PSCs and DSSCs require ZnO films with fewer defects and minimized recombination losses. In this research work, to reduce defects and recombination loses, ZnO was doped with 2%, 4%, and 6% La and Ce using the sol-gel technique, and ETLs were fabricated through spin coating. The band gap of ZnO decreased with 2% Ce and 4% La incorporation into the ZnO lattice, leading to improved crystallinity, reduced defects, and the formation of more uniform films. Microstrain and dislocation density reached their lowest values in 2% Ce-ZnO and 4% La-ZnO, enhancing crystal growth. When perovskite absorber layer Cs0.10MA0.90Pb(I0.9Br0.10)3 was deposited over 2% Ce-ZnO and 4% La-ZnO, pinholes were minimized, and the contact angle increased, improving charge carrier extraction, reducing recombination losses, and enhancing moisture stability. Finally, HTL-free PSCs were fabricated using La- and Ce- doped ZnO ETLs, Cs0.10MA0.90Pb(I0.9Br0.10)3 and MAPbI₃ absorber layers, and a carbon electrode. The power conversion efficiency (PCE) peaked at 13.38% and 13.74% for 4% La ZnO/MAPbI₃ and 2% Ce-ZnO/MAPbI₃, respectively, compared to 11.29% for pristine ZnO/MAPbI₃, due to simultaneous improvements in Jsc and FF. The highest PCE of 14.05% was observed in the 4% La-ZnO/Cs0.10MA0.90Pb(I0.9Br0.10)3 configuration, due to synergistic effects from 10% CsBr doping in MAPbI₃ and 4% La doping in ZnO, which enhanced crystallinity, mobility, conductivity, charge extraction, and overall performance. Similarly, 2% Ce-ZnO/ Cs0.10MA0.90Pb(I0.9Br0.10)3 achieved a maximum efficiency of 14.12% due to uniform film morphology, reduced defects, and minimized recombination losses. DSSCs using 2%, 4%, and 6% La and Ce-doped ZnO ETLs as photoanodes with N719 dye as a sensitizer were also fabricated. The 4% La-ZnO cell exhibited Voc of 0.73 V, Jsc of 11.92 mA/cm², and an efficiency of 4.38%, outperforming pristine and other La-doped ZnO cells. Similarly, the 2% Ce-ZnO cell achieved the highest efficiency of 5.04%, with an open circuit voltage (Voc) of 0.72 V, short circuit current density (JSC) of 13.1 mA/cm², and a fill-factor (FF) of 0.53, indicating a substantial increase in current generation compared to other Ce-ZnO based cells. Therefore, the optimal doping of Ce and La in ZnO effectively improves its properties as an ETL, enhancing the performance of both PSCs and DSSCs.