Abstract
Hybrid metal-halide perovskites are promising new materials for use in solar cells; however, their chemical stability in the presence of moisture remains a significant drawback. Quasi two-dimensional (2D) perovskites that incorporate hydrophobic organic interlayers offer improved resistance to degradation by moisture, currently still at the cost of overall cell efficiency. To elucidate the factors affecting the optoelectronic properties of these materials, we have investigated the charge transport properties and crystallographic orientation of mixed methylammonium (MA)-phenylethylammonium (PEA) lead iodide thin films as a function of the MA-to-PEA ratio and, thus, the thickness of the "encapsulated" MA lead-halide layers. We find that monomolecular charge-carrier recombination rates first decrease with increasing PEA fraction, most likely as a result of trap passivation, but then increase significantly as excitonic effects begin to dominate for thin confined layers. Bimolecular and Auger recombination rate constants are found to be sensitive to changes in electronic confinement, which alters the density of states for electronic transitions. We demonstrate that effective charge-carrier mobilities remain remarkably high (near 10 cm2V-1s-1) for intermediate PEA content and are enhanced for preferential orientation of the conducting lead iodide layers along the probing electric field. The trade-off between trap reduction, electronic confinement, and layer orientation leads to calculated charge-carrier diffusion lengths reaching a maximum of 2.5 μm for intermediate PEA content (50%).
Highlights
The popularity of hybrid metal−halide perovskites for photovoltaic and other optoelectronic applications has risen steadily since their relatively recent debut in the field.[1]
One challenge that hampers the widespread commercialization of perovskite-based solar cells is the chemical stability of the perovskite thin film
The stability was dramatically improved, the maximum recorded power conversion efficiencies (PCEs) of these cells did not exceed 8%. It is unclear from these two initial solar cell studies whether the low efficiencies are the result of unoptimized processing or fundamental problems with charge transport in 2D perovskites, a few other studies have suggested that the 2D nature of these perovskites should not limit device performance,[23,24] and others have reported improved device efficiencies of both photovoltaic and lightemitting devices with synthetic manipulation of the 2D materials.[25−27] one of the original studies compared the performance of solar cells based on 2D
Summary
For the film with intermediate amount (50%) of PEA, a maximum in charge-carrier diffusion length is reached, at a remarkably good value of 2.5 μm For this intermediate regime, excitonic effects are still relatively subtle, monomolecular recombination is lowest because of effective trap passivation, and the chargecarrier mobility is relatively high as a result of enhanced ordering of layers in the direction of the electric-field probe. Our study demonstrates that 2D perovskites with optimized alignment of lead iodide transport layers can exhibit an ideal combination of reduced trap-related recombination and decent charge-carrier mobilities, yielding water-resistant materials for highly efficient photovoltaic cells. Figures showing a comparison of steadystate photoluminescence spectra, time-dependent photoluminescence, GIWAXS patterns in qxy−qz for (PEA)2PbI4, XRD patterns, (PEA)2PbI4 in the eclipsed and staggered structures, attenuation lengths for each composition, powder XRD patterns (Cu-Kα1) for each material composition, and azimuthal integration of GIWAXS patterns in qxy−qz. (PDF)
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