Trevor D. Hull

Infrared Spectroscopic Study of Vibrational Modes in Methylammonium Lead Halide Perovskites.

The organic cation and its interplay with the inorganic lattice underlie the exceptional optoelectronic properties of organo-metallic halide perovskites. Herein we report high-quality infrared spectroscopic measurements of methylammonium lead halide perovskite (CH3NH3Pb(I/Br/Cl)3) films and single crystals at room temperature, from which the dielectric function in the investigated spectral range is derived. Comparison with electronic structure calculations in vacuum of the free methylammonium cation allows for a detailed peak assignment. We analyze the shifts of the vibrational peak positions between the different halides and infer the extent of interaction between organic moiety and the surrounding inorganic cage. The positions of the NH3(+) stretching vibrations point to significant hydrogen bonding between the methylammonium and the halides for all three perovskites.

Local Polar Fluctuations in Lead Halide Perovskite Crystals

Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low-frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in the cubic phase of both hybrid (CH_{3}NH_{3}PbBr_{3}) and all-inorganic (CsPbBr_{3}) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations indicate that head-to-head Cs motion coupled to Br face expansion, occurring on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr_{3}.

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

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 (LDe–) and holes (LDh+) to be directly imaged with SPCM (LDe– = 10–28 μm, LDh+ = 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 (krad = 5 ± 3 × 10–10 cm3/s) that is consistent with the predicted radiative rate. First-order recombination at low photoexcited carrier density (knrp-type = 1.0 ± 0.3 × 105 s–1, knrn-type = 1.5 ± 0.3 × 105 ...

Unbalanced Hole and Electron Diffusion in Lead Bromide Perovskites

We use scanning photocurrent microscopy and time-resolved microwave conductivity to measure the diffusion of holes and electrons in a series of lead bromide perovskite single crystals, APbBr3, with A = methylammonium (MA), formamidinium (FA), and Cs. We find that the diffusion length of holes (LDh+ ∼ 10-50 μm) is on average an order of magnitude longer than that of electrons (LDe- ∼ 1-5 μm), regardless of the A-type cation or applied bias. Furthermore, we observe a weak dependence of LD across the A-cation series MA > FA > Cs. When considering the role of the halide, we find that the diffusion of holes in MAPbBr3 is comparable to that in MAPbI3, but the electron diffusion length is up to five times shorter. This study shows that the disparity between hole and electron diffusion is a ubiquitous feature of lead halide perovskites. As with organic photovoltaics, this imbalance will likely become an important consideration in the optimization of lead halide perovskite solar cells.

Interplay between organic cations and inorganic framework and incommensurability in hybrid lead-halide perovskite CH3NH3PbBr3

Organic-inorganic coupling in the hybrid lead-halide perovskite is a central issue in rationalizing the outstanding photovoltaic performance of these emerging materials. Here we compare and contrast the evolution of structure and dynamics of the hybrid CH3NH3PbBr3 and the inorganic CsPbBr3 lead-halide perovskites with temperature, using Raman spectroscopy and single-crystal X-ray diffraction. Results reveal a stark contrast between their order-disorder transitions, abrupt for the hybrid whereas smooth for the inorganic perovskite. X-ray diffraction observes an intermediate incommensurate phase between the ordered and the disordered phases in CH3NH3PbBr3. Low-frequency Raman scattering captures the appearance of a sharp soft mode in the incommensurate phase, ascribed to the theoretically predicted amplitudon mode. Our work highlights the interaction between the structural dynamics of organic cation CH3NH3+ and the lead-halide framework, and unravels the competition between tendencies of the organic and inorganic moieties to minimize energy in the incommensurate phase of the hybrid perovskite structure.

Dynamic disorder in ABX 3 (A=3D CH 3 NH 3 , Cs; B=3D Pb; X=3D Br 3 , CI 3 ) perovskites

Lead-halide hybrid perovskite crystals have emerged as potentially materials for solar cells due to their power conversion efficiency over 20% [1]. In this work we conduct a comparative study between two hybrids (CH<inf>3</inf>NH<inf>3</inf>PbBr<inf>3</inf> and CH<inf>3</inf>NH<inf>3</inf>Pba<inf>3</inf>) and an all-inorganic lead-halide perovskite (CsPbBr<inf>3</inf>). Both have the general ABX<inf>3</inf> perovskite formula, with similar band gap (2.2 eV) and similar structural phase sequence, orthorhombic at low temperature, changing to tetragonal and then to cubic as temperature is increased [2]. Theoretical studies suggested that dynamic disorder, imposed by the rotation of anisotropic organic molecules in the inorganic octahedral network, plays an important role in the high photovoltaic activity presented by this perovskites [3,4].

Correction to “Limits of Carrier Diffusion in n-Type and p-Type CH3NH3PbI3 Perovskite Single Crystals”

J. Phys. Chem. Lett. 2016, 7 (17), pp 3510−3518. DOI: 10.1021/acs.jpclett.6b01308 D the preparation of our subsequent manuscript, we discovered that the model that we used in this work to determine the sensitivity constant for time-resolved microwave conductivity (TRMC) measurements does not yield the correct mobility−yield product because it did not take into account crystal thickness. To more accurately determine the mobility− quantum yield product, we calculate the sensitivity constant by numerically simulating the experimental crystal distribution on the substrate in the microwave cavity. We simulate a small change (10−3−10−2 S/m) in conductivity in the top 5−50 μm of the crystal to represent the generation and diffusion of photogenerated carriers in TRMC. Using our corrected model, we find that the mobility (electron + hole)−quantum yield product for the p-type sample discussed in the main text decreases to 42 ± 5 cm V−1 s−1, with a resulting diffusion length (electron +

The nature of dynamic disorder in lead halide perovskite crystals(Conference Presentation)

We combine low frequency Raman scattering measurements with first-principles molecular dynamics (MD) to study the nature of dynamic disorder in hybrid lead-halide perovskite crystals. We conduct a comparative study between a hybrid (CH3NH3PbBr3) and an all-inorganic lead-halide perovskite (CsPbBr3). Both are of the general ABX3 perovskite formula, and have a similar band gap and structural phase sequence, orthorhombic at low temperature, changing first to tetragonal and then to cubic symmetry as temperature increases. In the high temperature phases, we find that both compounds show a pronounced Raman quasi-elastic central peak, indicating that both are dynamically disordered.