Inverted organic solar cells integrated with room temperature solution-processed bismuth sulfide electron selective layer

Conventional electron selective layer (ESL) such as zinc oxide (ZnO) and titanium dioxide (TiO2) usually requires high annealing temperature (>300 °C) to produce high quality thin films. Without high annealing temperature, the solar cell shows poor performance. In addition, a device fabrication process that requires high annealing temperature is also not suitable to apply in flexible substrate. To address this issue, we propose bismuth sulfide (Bi2S3) to replace the conventional ESL. The advantage of using Bi2S3 ESL is it can be synthesized at room temperature using a simple solution process. The power conversion efficiency (PCE) of the optimized device with poly (3-hexylthiophene-2,5-diyl) (P3HT) electron donor and phenyl-C61-butyric acid methyl ester (PCBM) electron acceptor can achieve up to 2.32%, which is comparable to the device based on ZnO and TiO2 using the same absorbing layer. Interestingly, the result shows that Bi2S3 enhanced charge extraction to the FTO cathode but did not contribute to the photocurrent generation. Furthermore, it also acted as an effective electron selective layer by suppressing leakage current and charge carriers recombination of the device. The device with optimum immersion duration (30 min) exhibited almost seven times increase in PCE with respect to that without Bi2S3 ESL. This solution-processed Bi2S3 ESL is considered to be suitable for low-cost flexible optoelectronics in future.