20.1 % Certified Efficiency of Planar Hole Transport Layer‐Free Carbon‐Based Perovskite Solar Cells by Spacer Cation Chain Length Engineering of 2D Perovskites

The planar triple-layer hole transport layer (HTL)-free carbon-based perovskite solar cells (C-PSCs) have outstanding advantages of low cost and high stability, but are limited by low efficiency. The formation of a 3D/2D heterojunction has been widely proven to enhance device performance. However, the 2D perovskite possesses multiple critical properties associated with 3D perovskite, including defect passivation, energy level, and charge transport properties, all of which can impact device performance. It is challenging to find a powerful means to achieve comprehensive regulation and trade-off of these key properties. Herein, we propose a chain-length engineering of alkylammonium spacer cations to achieve this goal. The results show that the 2D perovskite formed by short-chain alkylammonium cations primarily acts to passivate defects. With the increase in cation chain length, the 2D perovskite achieves a more matched energy level with 3D perovskite, enhancing the built-in electric field and promoting charge separation. However, the further increase in chain length impedes the charge transport due to the insulativity of organic cations. Comprehensively, the 2D perovskite formed by tetradecylammonium cations achieves the optimal balance of defect passivation, interface charge separation, and charge transport. The planar HTL-free C-PSCs exhibit a new record efficiency of 20.40 % (certified 20.1 %).