Multistep Photoluminescence Decay Reveals Dissociation of Geminate Charge Pairs in Organolead Trihalide Perovskites

Abstract

cation process, [9][10][11][12] fundamental understanding of the perovskite properties ensuring high photovoltaic performance still lags behind.Basic research has primarily focused on the elucidation of charge transfer, transport, and collection properties. [13][14][15][16] This in turn allowed determination of important parameters such as electron and hole diffusion lengths, lifetimes, surface recombination rates, etc. In addition, significant efforts have been made aiming to determine whether the photoexcitation in organometal trihalide perovskites creates free charges or bound electron-hole pairs (excitons). [17][18][19][20][21][22][23] Previous investigations have shown that excitons split into charge carriers within a few picoseconds. [24,25] Signatures of the direct free charge generation in MAPbI 3 perovskite materials were reported in refs. [18,22] and [23]. Meanwhile, existence of exciton-carrier duality was demonstrated by Sheng et al. [19] Despite significant controversy concerning this issue, [17,23,[26][27][28] the current prevailing view favors branching between free carriers and excitons [19,20,29] with binding energy within a 2-150 meV range [2,27,[30][31][32] depending on temperature, dielectric constant, and crystal size. [20,29,33,34] However under operating conditions free charges represent the dominant species in perovskite solar cells. [20,23,35] Despite many studies on the subject, the evidence on the exact mechanism of the charge carrier generation remains controversial.In this work, we aim to elucidate the nature and initial dynamics of photogenerated charge carriers in methylammonium lead iodide perovskites (MAPbI 3 ) by means of picosecond time-resolved photoluminescence (PL) spectroscopy at various temperatures (20-300 K). While photoluminescence originates from recombination of the carriers, its time and temperature dependencies allow indirect evaluation of photogenerated carrier dynamics. Our studies allowed us to separate geminate and nongeminate recombination contributions, and we argue that geminate recombination dynamics is determined by the loss of spatial correlation between initially generated charge pairs. ResultsWe investigated perovskite photoluminescence dynamics in their films deposited on glass, fluorine-doped tin oxide (FTO), Charge carrier dynamics in organolead iodide perovskites is analyzed by employing time-resolved photoluminescence spectroscopy with several ps time resolution. The measurements performed by varying photoexcitation intensity over five orders of magnitude enable separation of photoluminescence components related to geminate and nongeminate charge carrier recombination and to address the dynamics of an isolated geminate electronhole pair. Geminate recombination dominates at low excitation fluence and determines the initial photoluminescence decay. This decay component is remarkably independent of the material structure and experimental conditions. It is demonstrated that dependences of the geminate and nongeminate radiative recombination components on...

Properties of Excitons and Photogenerated Charge Carriers in Metal Halide Perovskites

“…A special case that the dissociation process associating with the photogenerated weakly bound geminate charge pairs instead of excitons, has also been reported by Augulis et al for MAPbI 3 perovskite . Besides, in perovskite nanostructures, Yang et al demonstrated that, in ultrathin CsPbBr 3 nanosheets with thickness of 1.8–20 nm, the exciton dissociation was strongly influenced by the correlation between the exciton binding energy and the inter‐nanosheets electronic coupling .…”

Device Physics of Hybrid Perovskite Solar cells: Theory and Experiment

“…For PSCs, these measurements report values from tens to, unusually, hundreds of nanoseconds, extracted from the fitting to a mono-exponential law that characterizes the dominant radiative recombination path. Nevertheless, in some studies, a twotime constant PL decay has been found [78][79][80][81] with the consequent consideration of competing recombination mechanisms. In this context, the work by Kanemitsu and co-workers [30] particularly showed that PL decay curves are sensitive to the excitation fluence, as in Figure 8c, and that the PL decay dynamics are well explained by the free-carrier band-to-band recombination rather than the exciton recombination.…”

Energy Barriers Restrict Charge Carrier Motion in MAPI Perovskite Films

“…[ 15 ] Weak temperature dependence was observed for the MAPI photoluminescence decay. [ 28 ] Figure 4a, however, shows that the photocurrent drops much faster at low temperatures. We also note a higher photocurrent at a longer time scale (several µs) at low temperatures.…”