Efficient Flexible Organic Solar Cells with a Low-Temperature-Processed Al-Doped Zinc Oxide Electron Transport Layer

Zinc oxide (ZnO) has been widely used in inverted organic solar cells (I-OSCs) as the electron transport layer (ETL) due to its excellent electrical properties. However, the high-temperature process (>300 °C) applied to the ZnO ETL to enhance its electrical properties has been regarded as the bottleneck for application of this ETL to flexible I-OSCs due to the limited temperature tolerance of flexible substrates. In this work, we synthesized aluminum (Al)-doped ZnO with a sol–gel process (AZO) and successfully demonstrated highly efficient flexible I-OSCs with a low-temperature-processed (140 °C) AZO ETL (AZO 140 °C). Since Al doping of ZnO results in an enhanced electrical conductivity and reduced defects in ZnO even for low-temperature-treated materials, the rigid and flexible I-OSCs with AZO 140 °C ETLs exhibit power conversion efficiencies (PCEs) of 16.3 and 14.4%, respectively, which are higher than those of I-OSCs with low-temperature-processed (140 °C) ZnO ETLs. Furthermore, the unencapsulated I-OSC with the AZO ETL exhibits improved storage stability (T80 > 5000 h) compared to that with the ZnO ETL (T80 = 100 h), and an excellent thickness-insensitive PCE is also obtained for the I-OSCs with various thicknesses of the AZO ETL (22–190 nm). These results provide meaningful insight into reducing the process temperature of an ETL toward high-performance, flexible electronic systems.