Device Stability and Light-Soaking Characteristics of High-Efficiency Benzodithiophene–Thienothiophene Copolymer-Based Inverted Organic Solar Cells with F-TiO_x Electron-Transport Layer

Organic solar cells (OSC) based on low-band-gap thienothiophene-benzodithiophene copolymer have achieved relatively high efficiency (7-9%) in recent times. Among this class of material, poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB-7) is one of the high-efficiency materials reported for OSC. However, this material seems to be intrinsically unstable compared to the commonly used workhorse polymer, poly(3-hexylthiophene) (P3HT), especially when illuminated in air. Inverted device architecture is usually adopted to improve device stability, but the device stability using PTB-7 is not yet well-understood. In this work, a systematic degradation study on a PTB-7:PC71BM-based inverted OSC employing F-TiO(x) as electron-transport layer (ETL) was conducted for the first time. Air stability, photostability in inert atmosphere, and photostability under ambient conditions of the device were separately carried out to understand better the polymer behavior in inverted OSC. The device's air stability with different polymer absorber layers was studied by exposing the devices in air for up to 1500 h. Because of the long and easily cleavable alkoxy side chains in the polymer backbone, a PTB-7:PC71BM-based inverted OSC device is highly susceptible to oxygen and moisture when compared to a P3HT:PC61BM-based device. In addition, with the presence of F-TiO(x) ETL, a significant reduction in light-soaking time was also observed in PTB-7:PC71BM inverted OSC for the first time. The TiO(x)/organic interface was found to be responsible for the reduction in the light-soaking time.