Perovskite solar cells (PSCs) have experienced rapid development in the past period of time, and a record efficiency of up to 25.7% has been yielded. At present, the PSCs with the planar structure are the most prevailing, which not only can significantly simplify the device fabrication process but also reduce the processing temperature. Particularly, the electron transport layer (ETL) plays a critical role in boosting the device performance of planar PSCs. ZnO is a promising candidate as the ETL owing to its high transparency, suitable energy band structure, and high electron mobility. Moreover, ZnO is easy to be processed at a low cost and low energy. This review mainly summarized the recent advances in the application and strategic optimization of ZnO ETL for planar PSCs. The basic properties of ZnO, including energy levels, mobility, processability, trap defects, as well as chemical stability, are clearly clarified. The most available deposition means for preparing ZnO ETLs were also described briefly. Finally, we presented the challenges and guidelines for utilizing ZnO as ETL on efficient planar PSCs.

Plastic electrodes are desirable for the rapid development of flexible organic electronics. In this article, a plastic electrode has been prepared by employing traditional conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and plastic substrate polyethersulfone (PES). The completed electrode (Denote as HC-PEDOT:PSS) treated by 80% concentrated sulfuric acid (H₂SO₄) possesses a high electrical conductivity of over 2673 S/cm and a high transmittance of over 90% at 550 nm. The high conductivity is attributed to the regular arrangement of PEDOT molecules, which has been proved by the X-ray diffraction characterization. Temperature-dependent conductivity measurement reveals that the HC-PEDOT:PSS possesses both semiconducting and metallic properties. The binding force and effects between the PEDOT and PEI are investigated in detail. All plastic solar cells with a classical device structure of PES/HC-PEDOT:PSS/PEI/P3HT:ICBA/EG-PEDOT:PSS show a PCE of 4.05%. The ITO-free device with a structure of Glass/HC-PEDOT:PSS/Al4083/PM6:Y6/PDINO/Ag delivers an open-circuit voltage (V_OC) of 0.81 V, short-circuit current (J_SC ) of 23.5 mA/cm², fill factor (FF) of 0.67 and a moderate power conversion efficiency (PCE) of 12.8%. The above results demonstrate the HC-PEDOT:PSS electrode is a promising candidate for all-plastic solar cells and ITO-free organic solar cells.