Marked effects of indolyl vs. indolinyl substituent on solid-state structure, carrier mobility and photovoltaic efficiency of asymmetrical squaraine dyes

Abstract

Two solution-processed asymmetrical squaraines (ASQs) with cyclopenta[b]indolinyl (1a) and cyclopenta[b]indolyl (1b) as end cappers have been designed and synthesized. Although the internal molecular structure variations are minimal, the presence of the cyclopenta[b]indolinyl group endows 1a more planar molecular structure, which results in a much more compact solid-state structure (density is 1.317 g cm⁻³ for 1a but is 1.187 g cm⁻³ for 1b), dramatically affecting charge transport in the thin films. The hole mobility of 1a:PC₇₁BM blended film is about 7 times higher than that of 1b:PC₇₁BM. Consequently, the maximum power conversion efficiency (PCE) value of the organic photovoltaic cells (OPVs) based on 1a of up to 4.1%, approximately 80% higher than that of 1b, is one of the highest PCEs achieved for ASQ-based bulk-heterojunction (BHJ) OPVs.