Achieving enhanced linear and nonlinear optical absorption in a (PEA) ₂ PbI ₄ /WS ₂ heterojunction by efficient energy transfer

The interactions of charge transfer (CT) and nonradiative energy transfer (ET) in heterojunctions of two‐dimensional (2D) transition metal dichalcogenides and quasi‐2D single crystal perovskite thin films have the potential applications in sensor, energy harvesting and solar cells. However, the CT and ET between them are not clear. Herein, we examine the ET in a (PEA) 2 PbI 4 /WS 2 (PEA stands for phenethylamine and (PEA) 2 PbI 4 is abbreviated as PEPI) heterojunction using combined ultrafast spectroscopy and nonlinear optical absorption measurements. The ET from PEPI to WS 2 predicted by band alignment is first observed with photoluminescence spectroscopy and then revealed by femtosecond transient absorption spectroscopy to exhibit a high ET efficiency approximating 68%. The efficient ET from PEPI to WS 2 is also manifested in the large enhancement of nonlinear optical absorption of PEPI by a two‐photon process due to extra relaxation of pathway of excitons in the heterojunction. Our observation could have important implications for investigating and optimizing the energy delivery of 2D heterojunctions and related devices.