Hole Transport Layer‐Free Low‐Bandgap Perovskite Solar Cells for Efficient All‐Perovskite Tandems

Abstract Low‐bandgap (LBG, E_g ≈1.25 eV) tin‐lead (Sn‐Pb) perovskite solar cells (PSCs) play critical roles in constructing efficient all‐perovskite tandem solar cells (TSCs) that can surpass the efficiency limit of single‐junction solar cells. However, the traditional poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole transport layer (HTL) in LBG PSCs usually restricts device efficiency and stability. Here, a strategy of employing 2‐aminoethanesulfonic acid (i.e., taurine) as the interface bridge to fabricate efficient HTL‐free LBG PSCs with improved optoelectronic properties of the perovskite absorbers at the buried contacts is reported. Taurine‐modified ITO substrate has lower optical losses, better energy level alignment, and higher charge transfer capability than PEDOT:PSS HTL, leading to significantly improved open‐circuit voltage (V_OC) and short‐circuit current density of corresponding devices. The best‐performing LBG PSC with a power conversion efficiency (PCE) of 22.50% and an impressive V_OC of 0.911 V is realized, enabling all‐perovskite TSCs with an efficiency of 26.03%. The taurine‐based HTL‐free TSCs have highly increased stability, retaining more than 90% and 80% of their initial PCEs after constant operation under 1‐sun illumination for 600 h and under 55 °C thermal stress for 950 h, respectively. This work provides a facile strategy for fabricating efficient and stable perovskite devices with a simplified HTL‐free architecture.