Achieve Better Performance of Inverted Perovskite Solar Cells by Using the Fluorinated Polymer as the Electron Transporting Layer

Conjugated polymers are considered as promising candidates for the use of electron transport layers (ETLs) to enhance both the device performance and stability of the inverted planar perovskite solar cells (PSCs). Therefor; constant efforts are dedicated to the design and evaluation of more efficient polymeric ETLs. Herein, two dithienochrysene diimide (DTCDI)-based polymers P(DTCDI-DTBT) and P(DTCDI-DTDFBT) have been synthesized by copolymerizing the DTCDI unit with 4,7-di(thiophen-2-yl)-benzo[c][1,2,5]thiadiazole (DTBT) and 4,7-di(thiophen-2-yl)-difluorobenzo[c][1,2,5]thiadiazole (DTDFBT) moieties, respectively, and explored as ETLs for inverted PSCs. It was found that the fluorinated polymer P(DTCDI-DTDFBT) exhibited lowest unoccupied molecular orbital energy levels similar to P(DTCDI-DTBT) but superior electron transport ability. Due to strong interaction between F atoms and the perovskite film, the better passivation effect and smaller charge transfer resistance can be revealed from the characterization of P(DTCDI-DTDFBT)-based devices when they were used as an ETL, and the inverted PSCs using P(DTCDI-DTDFBT) as the ETL showed a power conversion efficiency of 16.3%, much higher than that of P(DTCDI-DTBT)-based devices (11.8%). Moreover, the devices relying on P(DTCDI-DTDFBT) also demonstrated a considerable improvement in the stability of the PSC devices, retaining 87% of its initial performance after being exposed at an ambient environment for 168 h. This work indicates that fluorination could be an effective way for developing efficient polymeric ETLs to enhance the performance and stability of the inverted PSCs.