Photoinduced charge carrier dynamics and spectral band filling in organometal halide perovskites

In recent years, the solution-processed organic-inorganic perovskite solar cells have attracted considerable attention because of their advantages of high energy conversion efficiency, low cost, and easily processing. Organometallic halide perovskite solar cells have gradually demonstrated particular superior properties in energy field due to their excellent photoelectric properties. This has been triggered by the unprecedented increase in its overall power conversion efficiency reaching 23% in just a few years, and it is becoming a direct competitor against the existing leading technology silicon. In this paper, 5-AVA-doped organometal halide perovskite films, (5-AVA)_0.05(MA)_0.95PbI₃ and (5-AVA)_0.05(MA)_0.95PbI₃/Spiro-OMeTAD, are prepared by the two-step method. The generation and recombination mechanism of charge carriers in two kinds of film samples are discussed in detail. The bivalent band structure of perovskite film material CH₃NH₃PbI₃ is determined by ultraviolet-visible absorption spectra of perovskite film (5-AVA)_0.05(MA)_0.95PbI₃ and (5-AVA)_0.05(MA)_0.95PbI₃/Spiro-OMeTAD. We investigate the photocarrier dynamics and band filling effects in these two organometal halide perovskite films by using femtosecond transient absorption spectroscopy. For (5-AVA)_0.05(MA)_0.95PbI₃, the photoinduced bleach recovery at 760 nm reveals that band-edge recombination follows second-order kinetics, indicating that the dominant relaxation pathway is via the recombination of free electrons and holes. With regard to the perovskite film (5-AVA)_0.05(MA)_0.95PbI₃ and (5-AVA)_0.05(MA)_0.95PbI₃/Spiro-OMeTAD, the signal is photoinduced absorption from 550 nm to 700 nm. As the delay time increases, the electrons and holes are recombined, which results in a red shift of absorption spectrum in (5-AVA)_0.05(MA)_0.95PbI₃. This can be referred to as Moss-Burstein band filling model. In contrast, the electrons and holes of (5-AVA)_0.05(MA)_0.95PbI₃/Spiro-OMeTAD perovskite film sample are separated after photoexcitation. The holes rapidly transfer to the hole transport layer of Spiro-OMeTAD. It will lead to an increase in sample absorbance and a rapid recovery of bleaching signals. Consequently, electron-hole recombination is no longer a dominant pathway to the relaxation of photocarriers and the band filling effect is not significant in the composite film. Our findings provide a valuable insight into the understanding of the charge carrier dynamics and spectral band filling in mixed perovskites. These results conduce to the understanding of the intrinsic photo-physics of semiconducting organometal halide perovskites with direct implications for photovoltaic and optoelectronic applications, and provide a reference for the future research of perovskite solar cells.