Alkoxy-Induced Near-Infrared Sensitive Electron Acceptor for High-Performance Organic Solar Cells

We develop a fused-ring electron acceptor (IOIC3) based on naphtho[1,2-b:5,6-b′]dithiophene core with alkoxy side-chains and compare it with its counterpart (IOIC2) with alkyl side-chains. Change in the side-chains affects electronic, optical, charge transport, and morphological properties of the analogues. Because of π-conjugative effect and σ-inductive effect of the oxygen atoms, IOIC3 exhibits a slightly upshifted HOMO level (-5.38 eV) and a downshifted LUMO level (-3.84 eV) relative to IOIC2 (HOMO = -5.41 eV, LUMO = -3.78 eV), leading to red-shifted absorption and smaller optical bandgap of 1.45 eV than that of IOIC2 (1.54 eV). IOIC3 exhibits a higher electron mobility of 1.5 × 10^-3 cm² V^-1 s^-1 than IOIC2 (1.0 × 10^-3 cm² V^-1 s^-1). Organic solar cells (OSCs) based on PTB7-Th:IOIC3 exhibit power conversion efficiency (PCE) as high as 13.1%, significantly higher than that of PTB7-Th:IOIC2 (9.33%). The semitransparent OSCs based on PTB7-Th:IOIC3 afford PCEs of up to 10.8% with an average visible transmittance (AVT) of 16.4%, higher than those of PTB7-Th:IOIC2 (PCE = 7.32%, AVT = 13.1%).