Nanoscale Observation of the Influence of Solvent Additives on All-Polymer Blend Solar Cells by Photoconductive Atomic Force Microscopy

We investigated the role of trace amounts of solvent additives in the performance improvement of all-polymer blend solar cells, composed of poly(3-hexylthiophene) (P3HT) as the donor and a naphthalene diimide-based copolymer (P(NDI2OD-T2)) as the acceptor, by photoconductive atomic force microscopy (PC-AFM). Nanoscale observation of the evolution of the photocurrent generation and distribution demonstrates that the addition of 0.5 vol % 1,8-diiodooctane (nonaromatic additive) and 1-chloronaphthalene (aromatic additive) has a negligible effect on the scale of phase segregation but leads to an increase in photocurrent owing to an increase in the charge transport efficiency, which is achieved by increasing the ordering of P3HT with extended interchain conjugation. In particular, the formation of a pathway that transports photogenerated charge carriers is found to be critical for increasing the photocurrent in the device. The PC-AFM characterization elucidates the interplay between the additive-mediated microstructures of the polymer:polymer blends and their local and macroscopic device performances.