Importance of Electron Transport Ability in Naphthalene Diimide-Based Polymer Acceptors for High-Performance, Additive-Free, All-Polymer Solar Cells

We report a systematic investigation of the correlations between the electron mobility of polymer acceptors and the photovoltaic performances of all-polymer solar cells (all-PSCs) by using a series of naphthalene diimide (NDI)-based polymer acceptors. Polymer acceptors typically have much lower electron mobility than fullerenes, which is one of the main factors in limiting the performance of all-PSCs. In addition, the anisotropic charge transport properties of the polymers require careful control of their packing structure and orientation suitable for their use in all-PSCs. To control the planarity of the polymer backbone and enhance electron mobility, we introduce three different electron-rich units (i.e., thiophene (T), bithiophene (T2), and thienylene-vinylene-thienylene (TVT)) into the NDI-based polymers. Particularly, P(NDI2OD-TVT) polymers exhibit the highest electron mobility (2.31 cm(2) V-1 s(-1)) in organic field-effect transistors owing to various factors including enhanced degree of coplanarity, strong intermolecular interactions, and facilitated three-dimensional (3-D) charge transport. In addition, the superb electron transport capability of P(NDI2OD-TVT) leads to a well-balanced hole/electron mobility ratio in all-PSC blends. Thus, all-PSCs based on the P(NDI2OD-TVT) acceptor exhibit a high power conversion efficiency of 4.25% without any solvent additives or thermal treatments. We suggest that the high electron transport ability of the polymer acceptor is important requirement for producing high-performance, additive-free all-PSCs.