Esterification of Indoline-Based Small-Molecule Donors for Efficient Co-evaporated Organic Photovoltaics

In this study, we attempted to develop a protocol for fabricating efficient solid-state organic photovoltaics based on materials used currently for dye-sensitized solar cells. Three typical indoline dyes, namely, D131, D102, and D149, were employed as electron donors in conjunction with C70 fullerene in solution-processed planar-heterojunction (PHJ) organic solar cells (OSCs). The PHJ cells based on these dyes exhibited similar external quantum efficiencies over the entire spectral range, resulting in identical short-circuit photocurrents. The open-circuit voltages of the PHJ cells were consistent with the highest occupied molecular orbital level of the corresponding indoline dye. The D102-based PHJ cell exhibited the highest power conversion efficiency, of up to 3.1%. The efficiency was limited by the light-harvesting capability of the solar cell, given that the short diffusion length (∼5 nm) of D102 limited the thickness of the active layer; the diffusion length was determined through an optical simulat...