Finely Tuned Cores in Star-Shaped Zwitterionic Molecules for Interface Engineering of High-Performance Polymer Solar Cells

Developing stable and cheap organic cathode interlayers (OCIs; to replace metal cathode and expensive OCIs for enhancing the device stability and reducing the manufacturing cost) is an important topic for commercial applications of polymer solar cells (PSCs). Herein, four one-step synthesized organic electron transport layers (G-Series electron-transport layers [ETLs]) with a novel star-shaped molecular structure consisting of a series of different heteroatom atoms as cores and sulfobetaine ions as a terminal substituent are explored. The energy levels can be finely tuned by applying different heteroatom atoms as cores. With the conventional device structure with poly[[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl) carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-Th) as a donor and [6,6]-phenyl-C₇₁-butyric-acid-methyl-ester (PC₇₀BM) as an acceptor, the G-C2-based devices exhibit a power conversion efficiency (PCE) of 8.90% with Al as the top electrode, much higher than that of the corresponding Ca/Al-based device (7.43%). Furthermore, G-Series-based solar cells are also more stable than the reference device based on Ca. In addition, these easy-to-get ETLs can be widely suitable for other PSCs based on different active layer systems. This work not only shows a new strategy for fine-tuning energy levels of nonconjugated zwitterionic molecules but also provides simple and stable ETLs for low-cost and high-performance PSCs.