Novel dopant-free carbazole- and phenothiazine-appended indolo[3,2-a]carbazole-based small molecules as efficient hole-transport materials for perovskite solar cells

Abstract

Developing efficient, stable, cost-effective hole-transport materials (HTMs) is critical for advancing high-performance perovskite solar cells (PSCs). In this study, we introduced novel, dopant-free fused heterocyclic indolo[3,2-a]carbazole (IC)-based HTMs, namely, C₁₂Cbz-IC and C₁₂Phz-IC, which are functionalized with electron-donating carbazole (Cbz) and phenothiazine (Phz) moieties, respectively. These IC-based HTMs are synthesized via a cost-effective Buchwald–Hartwig amination and exhibit superior hole mobility, enhanced hydrophobicity, and low-lying HOMO energy levels (−5.4 eV for C₁₂Cbz-IC and −5.5 eV for C₁₂Phz-IC). These characteristics facilitate efficient hole extraction and charge transport in PSCs. When incorporated into PSCs without dopants, C₁₂Cbz-IC and C₁₂Phz-IC achieved impressive power conversion efficiencies of 16.41% and 15.43%, respectively. A key advantage of these materials is their significantly low production cost, ∼50% that of the benchmark Spiro-OMeTAD-based HTM. Furthermore, stability tests demonstrated that after 1000 hours of dark storage, Spiro-OMeTAD-based devices retained only 34% of their initial efficiency, whereas C₁₂Cbz-IC and C₁₂Phz-IC maintained 51% and 54% of their initial efficiency, respectively. These findings highlight the potential of IC-based HTMs as cost-effective and stable alternatives to Spiro-OMeTAD, offering a promising pathway toward the development of next-generation, high-efficiency PSCs.