Heteroaromatic ring-modified naphthalimide interlayer materials for efficient polymer solar cells†

We designed and synthesized four naphthalimide (NI)-based organic cathode interlayer materials (CIMs) end-capped with electron-withdrawing heteroaromatic rings, namely, NTDC, NTR, NTRIC and NTTC. Compared with NTN (bis(N-dimethylaminopropyl)-4-thiophene-1,8-naphthalimide), which is a previously reported CIM, these four CIMs exhibited red-shifted absorption spectra, slightly elevated energy levels, enhanced π–π stacking, and increased surface wettability and roughness. In particular, NTR terminated with a rhodanine unit exhibited increased electron mobility and conductivity, uniformly distributed rod-like surface morphology, superior thermal stability, and stabilized NTR/electron acceptor contact. Moreover, binary PSCs based on the NTR interlayer achieved a power conversion efficiency of up to 18.9% and exhibited enhanced device thermal stabilities. This study provides an effective approach to design efficient and thermally stable NI-based CIMs through heteroaromatic terminal-group strategy for application in high-efficiency non-fullerene PSCs.