Synergistic HT4N/Spiro-OMeTAD dual-hole-transport strategy for efficient and stable wide-bandgap perovskite solar cells

Abstract Wide-bandgap perovskite solar cells (WBG-PSCs) are indispensable top subcells in perovskite-based tandem solar cells (TSCs) targeting power conversion efficiencies (PCEs) beyond 35 %. Their performance, however, is limited by the strong thickness dependence of the absorber and the poor thermal robustness of conventional hole-transport layers (HTLs). Here, we theoretically explore the potential of 2,3-thienoimide-ended hexyl-substituted quaterthiophene (HT4N), an oligothiophene derivative, as an efficient HTL in WBG-PSCs. Compared to conventional HTLs, HT4N offers significant advantages due to its superior hole mobility and deep HOMO level, which enhance charge extraction and interface performance by improving energy-level alignment at the perovskite/HTL interface. Integrating a 40 nm HT4N interlayer with 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (Spiro-OMeTAD) in a dual-HTL configuration, combined with a 600 nm-thick perovskite absorber, achieves a champion PCE of 24.16% and an open-circuit voltage (VOC) of 1.37 V, surpassing the single-HTL device (21.69%). The HT4N layer effectively suppresses interfacial recombination, reduces parasitic absorption in Spiro-OMeTAD, and improves thermal stability, with the dual-HTL device retaining >97% of its initial PCE at 400 K, while single-HTL devices degrade to ~80%. Extending this approach to semitransparent WBG-PSCs using indium zirconium oxide (IZRO) as the transparent electrode achieves a PCE of 22.30%. These findings establish HT4N as a promising hole-transport material and demonstrate that dual-HTL engineering offers a powerful route to highly efficient, thermally robust, and tandem-compatible WBG-PSCs.