Conjugated Polyelectrolyte Doping Strategy for Defect Passivation in ZnO Electron Transport Layers for Efficient Inverted Organic Solar Cells

Conjugated polymer electrolytes (CPEs) based on naphthalene diimide derivatives, including PFN-NDI-Br, PFN-NDI-Br-OH, and PFN-NDI-I, have been extensively explored for use in non-fullerene inverted organic solar cells (iOSCs). Initially, these CPEs were employed as cathode interlayers to improve device efficiency by enhancing electron extraction and transport. However, experimental results showed no significant positive effect on the power conversion efficiency (PCE) of non-fullerene iOSCs when CPEs were used as the cathode interlayer. An alternative strategy was developed to address this limitation by doping these CPEs into the ZnO electron transport layer (ETL). This doping strategy is proved to be highly effective, particularly with PFN-NDI-I doping, which increased the PCE from 15.1% to 17.1%. The improvement is due to enhanced charge transport and collection by reducing trap states within the ZnO lattice. The interaction between CPE anions and Zn atoms reduced defect sites, leading to higher electrical conductivity in the ZnO layer. Additionally, optical simulations revealed that CPE doping shifted the optical electric field toward the active layer, enhancing light absorption and further boosting device performance. In addition to these findings, Doping ZnO with CPEs has led to significant improvements in device performance. For instance, PFN-NDI-Br-doped ZnO increased the PCE to 16.3%, with simultaneous enhancements in open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF). Moreover, devices incorporating PFN-NDI-Br-OH and PFN-NDI-I as dopants demonstrated further increases in PCE to 16.4% and 17.1%, respectively. These improvements can be attributed to several factors, including the release of free electrons through interactions between the anions of the CPEs and Zn within the ZnO lattice, enhancing the conductivity of the ETL. In particular, PFN-NDI-I-doped ZnO reduction in work function (WF) of ZnO, resulting in improved Jsc and Voc. Additionally, the reduction Urbach energy and trap states facilitated more efficient charge transport and minimized recombination losses, while the CPE-doped ZnO provided effective defect passivation, leading to higher FF compared to devices using pristine ZnO. Overall, these results highlight a promising method for overcoming the limitations associated with CPEs as interlayers and addressing unwanted interactions with non-fullerene acceptors. Doping ZnO with PFN-NDI derivatives significantly improved the electrical and optical properties of the ETL, leading to higher overall PCE. These findings indicate the potential of PFN derivatives in optimizing the ETL and addressing limitations in non-fullerene iOSCs.