ZnO电子传输层对于反型结构聚合物太阳电池光浴效应的影响

采用金属氧化物电子传输层(ETL)的聚合物光伏器件在制备完成之初通常性能表现低下, J-V曲线呈异常“S”形. 当器件受白光持续照射后, 该不良状况会逐渐好转, 此过程称为光浴(light-soaking). 光浴现象普遍被认为是ETL界面问题所致. 从器件结构着手, 研究了ZnO 纳米颗粒ETL相邻的两个界面在光浴问题上的作用. 制备了功能层相同的(电极除外)正型、反型器件及复合ETL结构器件, 发现光浴现象仅出现于包含ZnO/ITO界面的反型器件中, 证明该界面是导致光浴现象的主要原因. 分析认为: ZnO颗粒表面O₂吸附形成的电子陷阱增加了ITO/ZnO势垒厚度, 使得光生电子无法逾越而成为空间电荷积累, 从而导致器件初始性能不佳. 器件经光照后, ETL内部受激而生的空穴电子对填补了ZnO缺陷, 提升了ETL的电荷选择性并减小了界面势垒厚度, 被束缚的光生电子得以隧穿至ITO电极, 反型器件性能最终得以改善.

Effect of ZnO electron-transport layer on light-soaking issue in inverted polymer solar cells

A common phenomenon of polymer solar cells with metal oxide electron-transport layers (ETLs), known as “light-soaking” issue, is that the as-prepared device exhibits an anomalous S-shaped J-V characteristic, resulting in an extremely low fill factor (FF) and thus a poor power conversion efficiency. However, the S-shape disappears upon white light illumination with UV spectral components, meanwhile the performance parameters of the device recover the normal values eventually. This behavior appears to be of general validity for various metal oxide layers regardless of the synthesis and fabricating processes. Its origin is still under debate, while the ETL interface problems have generally been claimed to be the underlying reason so far. In this paper, both conventional and inverted cells with using ZnO nanoparticles (NPs) as ETL are fabricated to clarify the interface effect of the ETL on the light soaking procedure. The inverted device shows a typical light-soaking issue with an initial FF less than 20% as expected, whereas the J-V curves of the conventional cell remain regular shapes throughout the test. This result indicates that the ITO/ZnO interface is a key reason of S-shaped J-V characteristics, which is further verified via the use of Cs₂CO₃/ZnO ETL. The insert of Cs₂CO₃ layer isolates the ITO electrode from contacting with ZnO layer, and the kink disappears in the as-prepared device with this bi-layered ETL inverted structure. Our explanation for the result above is that the oxygen impurities absorbed onto the surface of ZnO NPs during fabrication process, behave as strong electron traps, and thus increasing the width of the energy barrier (EB) at the interface of ITO/ZnO. Subsequently, photogenerated electrons accumulate in the ZnO layer adjacent to the interface, resulting in extremely poor performance. Upon white light illumination, however, the trap sites are filled by photogenerated carriers within the ZnO layer, and therefore narrowing the EB. As the barrier width becomes thin enough to be freely tunneled through, a good selectivity behavior of ZnO ETL is reached, leading to a fully remarkable recovery in device performances.