Design, synthesis, and structural characterization of the first dithienocyclopentacarbazole-based n-type organic semiconductor and its application in non-fullerene polymer solar cells

Abstract

Ladder-type dithienocyclopentacarbazole (DTCC) cores, which possess highly extended π-conjugated backbones and versatile modular structures for derivatization, were widely used to develop high-performance p-type polymeric semiconductors. However, an n-type DTCC-based organic semiconductor has not been reported to date. In this study, the first DTCC-based n-type organic semiconductor (DTCC–IC) with a well-defined A–D–A backbone was designed, synthesized, and characterized, in which a DTCC derivative substituted by four p-octyloxyphenyl groups was used as the electron-donating core and two strongly electron-withdrawing 3-(dicyanomethylene)indan-1-one moieties were used as the terminal acceptors. It was found that DTCC–IC has strong light-capturing ability in the range of 500–720 nm and exhibits an impressively high molar absorption coefficient of 2.24 × 10⁵ M⁻¹ cm⁻¹ at 669 nm owing to effective intramolecular charge transfer and a strong D–A effect. Cyclic voltammetry measurements indicated that the HOMO and LUMO energy levels of DTCC–IC are −5.50 and −3.87 eV, respectively. More importantly, a high electron mobility of 2.17 × 10⁻³ cm² V⁻¹ s⁻¹ was determined by the space-charge-limited current method; this electron mobility can be comparable to that of fullerene derivative acceptors (μ_e ∼ 10⁻³ cm² V⁻¹ s⁻¹). To investigate its application potential in non-fullerene solar cells, we fabricated organic solar cells (OSCs) by blending a DTCC–IC acceptor with a PTB7-Th donor under various conditions. The results suggest that the optimized device exhibits a maximum power conversion efficiency (PCE) of up to 6% and a rational high V_OC of 0.95 V. These findings demonstrate that the ladder-type DTCC core is a promising building block for the development of high-mobility n-type organic semiconductors for OSCs.