Overcoming Space‐Charge Effect for Efficient Thick‐Film Non‐Fullerene Organic Solar Cells

TLDR

Non‑fullerene organic solar cells have reached up to 14 % power‑conversion efficiency, yet their high performance is usually limited to ~100 nm active layers because low electron mobility hinders roll‑to‑roll processing of thicker films. This study introduces a non‑fullerene OSC that maintains a 9.1 % PCE even with a 300 nm active layer by optimizing device architecture to mitigate space‑charge effects. By employing an inverted architecture that generates excitons near the cathode, the electron transport distance is shortened, reducing electron accumulation in thick films. Optical modeling shows most incident light is absorbed near the transparent electrode, and the resulting thick‑film device achieves efficient performance, offering guidelines for further development of high‑efficiency thick‑film non‑fullerene OSCs.

Abstract

Abstract Organic solar cells (OSCs) containing non‐fullerene acceptors have realized high power conversion efficiency (PCE) up to 14%. However, most of these high‐performance non‐fullerene OSCs have been reported with optimal active layer thickness of about 100 nm, mainly due to the low electron mobility (≈10 −4 –10 −5 cm 2 V −1 s −1 ) of non‐fullerene acceptors, which are not suitable for roll‐to‐roll large‐scale processing. In this work, an efficient non‐fullerene OSC based on poly[(5,6‐difluoro‐2,1,3‐benzothiadiazol‐4,7‐diyl)‐alt‐(3,3′″‐di(2‐octyldodecyl)‐2,2′;5′,2″;5″,2′″‐quaterthiophen‐5,5′′′‐diyl)] (PffBT4T‐2OD):EH‐IDTBR (consists of electron‐rich indaceno[1,2‐b:5,6‐b′]dithiophene as the central unit and an electron‐deficient 5,6‐benzo[c][1,2,5]thiadiazole unit flanked with rhodanine as the peripheral group) with thickness‐independent PCE (maintaining a PCE of 9.1% with an active layer thickness of 300 nm) is presented by optimizing device architectures to overcome the space‐charge effects. Optical modeling reveals that most of the incident light is absorbed near the transparent electrode side in thick‐film devices. The transport distance of electrons with lower mobility will therefore be shortened when using inverted device architecture, in which most of the excitons are generated close to the cathode side and therefore substantially reduces the accumulation of electrons in the device. As a result, an efficient thick‐film non‐fullerene OSC is realized. These results provide important guidelines for the development of more efficient thick‐film non‐fullerene OSCs.