The Novel Additive 1‐Naphthalenethiol Opens a New Processing Route to Efficiency‐Enhanced Polymer Solar Cells

Abstract

Polymer solar cells (PSCs) based on fullerene derivatives often require additives to optimize active layer morphology. Here, the novel additive 1‐naphthalenethiol (SH‐na) is proposed for processing the PSC active layer of PTB7:PC 71 BM. Spin‐casting with SH‐na as additive achieves a power conversion efficiency (PCE) of 7.3%, compared to 6.7% for preparations containing the conventional 1,8‐diiodooctane additive. Dipping of the active layer into a methanol solution of critical SH‐na concentration increases the PCE further to 8.75%. This is mainly due to an improved open‐circuit voltage (from 0.72 to 0.79 V) together with a high achieved fill factor of 0.70. The improved PCE is correlated to the morphology optimization according to measurements of grazing incidence small/wide‐angle X‐ray scattering, neutron reflectivity, atomic force microscopy, Fourier transform infrared spectroscopy, and X‐ray photoelectron spectroscopy. The integrated results suggest that the halogen‐free additive SH‐na can form hydrogen bonds with both PTB7 and PC 71 BM, resulting in substantially improved PTB7 crystallization and multi‐length‐scale PC 71 BM dispersion for appropriate aggregation and networks. The subsequent dipping treatment with SH‐na further modifies the active layer morphology for a more PC 71 BM‐enriched surface and better PC 71 BM networks in the bulk film for an optimized electron‐to‐hole mobility ratio of 2.04, hence resulting in improved device performance.