DTBDT-based polymer hole transport materials for low voltage loss CsPbI${}_2$Br perovskite solar cells

The power conversion efficiency (PCE) of CsPbI${}_2$Br 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 CsPbI${}_2$Br, effectively reducing the losses caused by radiative and non-radiative recombination in CsPbI${}_2$Br PSCs. Finally, the CsPbI${}_2$Br 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 CsPbI${}_2$Br/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. (© 2024 Wiley‐VCH GmbH)