A high performance inverted organic solar cell with a low band gap small molecule (p-DTS(FBTTh2)2) using a fullerene derivative-doped zinc oxide nano-film modified with a fullerene-based self-assembled monolayer as the cathode

Abstract

We have successfully integrated two effective strategies to improve the power conversion efficiency (PCE) of bulk heterojunction inverted small molecule solar cells (i-SMSCs) by doping a ZnO cathode with a fullerene derivative (ZnO–C60) followed by fullerene derivative self-assembled monolayer (SAM) modification on its surface in contact with the active layer. Such ZnO–C60 gives a fullerene-derivative-rich interface in contact with the active layer and enhanced surface conductivity relative to pristine ZnO (from 0.015 to 1.09 S cm⁻¹) and bulk electron mobility (from 1.23 ± 0.39 × 10⁻⁴ to 6.43 ± 0.35 × 10⁻³ cm² V⁻¹ s⁻¹). Using this ZnO–C60 as the cathode, the device with the active layer p-DTS(FBTTh₂)₂:PC₇₁BM gives a higher PCE of 8.3% than that using ZnO without doping, 6.08%. Further incorporation of fullerene derivatives (NPC60-OH and NPC70-OH) as a SAM on ZnO–C60 effectively passivates the electron traps on the ZnO–C60 surface, resulting in increased electron mobility. The device using the ZnO–C60 nanofilm with phenol substituted C70 (NPC70-OH) as the SAM gives a further promoted PCE of up to 9.14%, which is the best value among the reported values in i-SMSCs.