Limits of Carrier Diffusion in n-Type and p-Type CH₃NH₃PbI₃ Perovskite Single Crystals

Abstract

Using a combination of scanning photocurrent microscopy (SPCM) and time-resolved microwave conductivity (TRMC) measurements, we monitor the diffusion and recombination of photoexcited charges in CH3NH3PbI3 perovskite single crystals. The majority carrier type was controlled by growing crystals in the presence or absence of air, allowing the diffusion lengths of electrons (L D e–) and holes (L D h+) to be directly imaged with SPCM (L D e– = 10–28 μm, L D h+ = 27–65 μm). TRMC measurements reveal a photogenerated carrier mobility (μh + μe) of 115 ± 15 cm2 V–1 s–1 and recombination that depends on the excitation intensity. From the intensity dependence of the recombination kinetics and by accounting for carrier diffusion away from the point of photogeneration, we extract a second-order recombination rate constant (k rad = 5 ± 3 × 10–10 cm3/s) that is consistent with the predicted radiative rate. First-order recombination at low photoexcited carrier density (k nr p‑type = 1.0 ± 0.3 × 105 s–1, k nr n‑type = 1.5 ± 0.3 × 105 s–1) is slower than that observed in CH3NH3PbI3 thin films or in GaAs single crystals with AlGaAs passivation layers. By accounting for the dilution of photogenerated carriers upon diffusion, and by combining SPCM and TRMC measurements, we resolve disagreement between previous reports of carrier diffusion length.

Substrate-dependent electronic structure and film formation of MAPbI3 perovskites

“…Inside the perovskite layer we can distinguish three regions: (i) the interface region in which the passivation layer is formed and changes in vacuum level position are observed due to the modification of the substrate by surface reactions as well as charge transfer between the substrate and the overlayer (interface dipole); (ii) the band-bending region, where a gradual shift of the perovskite valence band is observed due to charge redistribution, and finally (iii) the steady state region with final band alignment. The downward band bending, which is present in all four cases, indicates that the prepared MAPbI 3 is inherently n-type as often reported in literature15253. However, the intrinsic charge-carrier density is rather low resulting in wide depletion regions, otherwise the same Fermi level position close to the CB would be reached on all substrates after the 200 nm of deposition.…”

Carrier diffusion in thin-film CH3NH3PbI3 perovskite measured using four-wave mixing

“…The first observation of micron-scale carrier diffusion lengths in CH 3 NH 3 PbI 3−x Cl x 17-19 stimulated a comprehensive research effort aimed at understanding the nature of carrier transport. [20][21][22][23][24][25][26][27][28][29][30][31][32][33] Carrier mobilities and/or diffusion lengths have been studied using electrical techniques (e.g. AC Photo Hall, 25 spacecharge-limited current, 31 impedance spectroscopy, 21 , and spatially-resolved electron-beam-induced current 27 ) as well as optical techniques that rely on electrical contacts such as photoluminescence quenching 17,18 and scanning photocurrent microscopy.…”

“…AC Photo Hall, 25 spacecharge-limited current, 31 impedance spectroscopy, 21 , and spatially-resolved electron-beam-induced current 27 ) as well as optical techniques that rely on electrical contacts such as photoluminescence quenching 17,18 and scanning photocurrent microscopy. 26,33 Some of these techniques offer the ability to probe transport in a working solar cell device, however imperfectly characterized interface energetics and ambiguities tied to the model-dependent extraction of transport characteristics impede the determination of the physical processes limiting device performance.…”

Metal Halide Perovskites: Synthesis, Ion Migration, and Application in Field‐Effect Transistors

“…Semonin et al synthesized MAPbI 3 single crystals via vapor‐diffusion, in which MAI and PbI were dissolved in stabilized hydriodic acid with added hypo‐phosphorous acid. The MAPbI 3 single crystals with size of 1–8 mm were obtained after several days under an argon environment . Figure c presents an optical image of a MAPbI 3 single crystal with graphite electrical contacts for scanning photocurrent microscopy (SPCM) measurement.…”

“…Since metal halide perovskites (MHPs) have been utilized by Kojima et al as efficient light absorber in solar cells, MHP‐based optoelectronics have in recent years enjoyed skyrocketing developments in light‐emitting diodes (LEDs), solar cells, and photodetectors . However, in comparison, MHP‐based field‐effect transistors (MHP‐FETs) have drawn relatively less research efforts, in spite of the intrinsic large charge carrier mobilities and ambipolar properties of MHPs . This could be mainly blamed for the gate‐field screening effect induced by the ion migration and accumulation at the perovskite/dielectric interface, which makes it tedious to obtain a reliable room‐temperature operated MHP‐FET.…”