Effects fo electron transport ability of naphthalene diimide-based polymer acceptors on the performances in polymer/polymer blended solar cells

Abstract

High-performance all-polymer solar cells (all-PSCs) have been fabricated by using novel naphthalene diimide (NDI)-based alternating copolymers as electron acceptors. 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(NDI-TVT) polymers exhibit the highest electron mobility ($2.31 cm^{2} V^{-1} s^{-1}$) in organic field-effect transistors (OFETs) owing to the various factors including enhanced degree of coplanarity, strong intermolecular interactions, facilitated three-dimensional (3-D) charge transport. In addition, the superb electron transport capability of P(NDI-TVT) leads to the well-balanced hole/electron mobility ratio in all-PSC blends. Thus, all-PSCs based on the P(NDI-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 that can allow 3-D charge transport pathways and well-balanced hole/electron mobility ratio is highly effective for producing high-performance, additive-free all-PSCs.