Applying l-cystine as an electron transport layer toward efficient organic solar cells

The electron transport layer (ETL) is essential to improve device performance of organic solar cells (OSCs). Here, l-cystine, which is inexpensive and environmentally friendly, was successfully employed in OSCs of PTB7-Th:PC71BM as an ETL. The introduction of l-cystine can achieve an enormous enhancement in performance: Voc (open circuit voltage) from 0.74 to 0.79 V, Jsc (short-circuit current density) from 14.76 to 16.89 mA cm2, fill factor from 51.45% to 70.73%, and power conversion efficiencies from 5.62% to 9.44%, which is better than PFN (8.52%) as an ETL. The excellent PCE is attributed to the WF of Al was shifted from −4.23 to −3.87 eV by l-cysteine modified, thereby more effectively extracting and collecting carriers. This study demonstrates that the natural and non-toxic l-cystine is a strong competitor for ETL in high-efficiency OSCs.