Engineering the Hole Transport Layer with a Conductive Donor-Acceptor Covalent Organic Framework for Stable and Efficient Perovskite Solar Cells.

Spiro-OMeTAD doped with lithium-bis(trifluoromethylsulfonyl)-imide (Li-TFSI) and butyl-pyridine (-BP) is widely used as a hole transport layer (HTL) in n-i-p perovskite solar cells (PSCs). Spiro-OMeTAD based PSCs typically show poor stability owing to the agglomeration of Li-TFSI, the migration of lithium ions (Li), and the existence of potential mobile defects originating from the perovskite layer. Thus, it is necessary to search for a strategy that suppresses the degradation of PSCs and overcomes the Shockley Queisser efficiency limit via harvesting excess energy from hot charge carrier. Herein, two covalent organic frameworks (COFs) including BPTA-TAPD-COF and a well-defined donor-acceptor COF (BPTA-TAPD-COF@TCNQ) were developed and incorporated into Spiro-OMeTAD HTL. BPTA-TAPD-COF and BPTA-TAPD-COF@TCNQ could act as multifunctional additives of Spiro-OMeTAD HTL, which improve the photovoltaic performance and stability of the PSC device by accelerating charge-carrier extraction, suppressing the Li migration and Li-TFSI agglomeration, and capturing mobile defects. Benefiting from the increased conductivity, the addition of BPTA-TAPD-COF@TCNQ in the device led to the highest power conversion efficiency of 24.68% with long-term stability in harsh conditions. This work provides an example of using COFs as additives of HTL to enable improvements of both efficiency and stability for PSCs.