Molecular Doping Inhibits Charge Trapping in Low-Temperature-Processed ZnO toward Flexible Organic Solar Cells

There has been a growing interest in the development of efficient flexible organic solar cells (OSCs) due to their unique capacity to provide energy sources for flexible electronics. To this end, it is required to design a compatible interlayer with low processing temperature and high electronic quality. In this work, we present that the electronic quality of the ZnO interlayer fabricated from a low-temperature (130 degrees C) sol-gel method can be significantly improved by doping an organic small molecule, TPT-S. The doped TPT-S, on the one hand, passivates uncoordinated Zn-related defects by forming N-Zn bonds. On the other hand, photoinduced charge transfer from TPT-S to ZnO is confirmed, which further fills up electron-deficient trap states. This renders ZnO improved electron transport capability and reduced charge recombination. By illuminating devices with square light pulses of varying intensities, we also reveal that an unfavorable charge trapping/detrapping process observed in low-temperature-processed devices is significantly inhibited after TPT-S doping. OSCs based on PBDB-T-2F:IT-4F with ZnO:TPT-S being the cathode interlayer yield efficiencies of 12.62 and 11.33% on rigid and flexible substrates, respectively. These observations convey the practicality of such hybrid ZnO in high-performance flexible devices.

Automated summary

Improvements in the electronic quality of the ZnO interlayer were achieved by doping TPT-S, leading to enhanced electron transport capability and reduced charge recombination. The doping of TPT-S significantly inhibited unfavorable charge trapping/detrapping process in low-temperature-processed devices. OSCs using ZnO:TPT-S as the cathode interlayer demonstrated high efficiencies on both flexible and rigid substrates.