Comparing between steady-state excitonic transitions and ultrafast polaronic photoexcitations in layered perovskites: the role of electron–phonon interaction

We have studied four 2D layered perovskites, including OA2PbI4 (RP phase), ODAPbI4 and BDAPbI4 (DJ phase), (GA)MAPbI4 (ACI phase), where OA is [(CmH2m+1)NH3](m = 8), ODA is [NH3(CH2)mNH3](m = 8), BDA is [NH3(CH2)mNH3](m = 4), and GA is [C(NH2)3]; RP, DJ, and ACI means Ruddlesden–Popper, Dion–Jacobson and alternating cations in the interlayer, respectively. The temperature dependence of absorption and photoluminescence (PL) spectra have been measured. From which the average phonon energy (electron-phonon interaction strength) is analyzed as around 34 (80), 47 (184), 50 (402), and 63 (758) with the unit of meV for OA2PbI4, ODAPbI4, BDAPbI4, and (GA)MAPbI4, respectively. Larger phonon energy indicates the involvement of more phonons in organic spacer layer, with the corresponding stronger electron-phonon interaction. Furthermore, ultrafast transient absorption spectroscopy proves that, when the excitation photon energy is serval hundred meV higher than bandgap, the excitons still are the major photoexcitations in OA2PbI4, but polarons are major one in ODAPbI4, BDAPbI4, and (GA)MAPbI4 films, no matter the excitonic transitions dominate the absorption at their band edges. This work proves the organic spacers can regulate electron–phonon interaction then optoelectronic properties in 2D perovskites profoundly, which have implications toward future rational design for relevant devices.