DTBDT‐Based Polymer Hole Transport Materials for Low Voltage Loss CsPbI₂Br Perovskite Solar Cells

Abstract The power conversion efficiency (PCE) of CsPbI₂Br perovskite solar cells (PSCs) is still far from the theoretical efficiency due to the pronounced losses in open‐circuit voltage (V_OC). The V_OC loss can be mitigated by employing an appropriate hole transport layer (HTL), which facilitates energy level alignment and minimizes interface recombination losses. In this work, two D‐π‐A type polymers are chosen, PE64 and PE65, as HTLs, where pentacyclic dithieno[2,3‐d; 2′,3′‐d “]benzo[1,2‐b; 4,5‐b”]dithiophene (DTBDT) as the D‐unit and quinoxaline (Qx) as the A‐unit. It is demonstrated that the polymer PE65 with chlorinated thiophene side chain on the DTBDT unit has an optimized molecular arrangement, improved energy level matching, and enhanced passivation with CsPbI₂Br, effectively reducing the losses caused by radiative and non‐radiative recombination in CsPbI₂Br PSCs. Finally, the CsPbI₂Br PSCs utilizing PE65 as HTL achieve a power conversion efficiency (PCE) of 17.60% with a high V_OC of 1.44 V. Furthermore, the PE64 and PE65 are also employed to construct inter‐connecting layers (ICLs) for tandem solar cells (TSCs). The CsPbI₂Br/D18:Y6 TSCs based on PE65‐ICL yield a PCE of 22.32% with a high V_OC of 2.25 V. This work demonstrates that pentacyclic DTBDT‐based polymers are also promising HTLs for high‐performance PSCs and TSCs.