Charge-Carrier Dynamics in 2D Hybrid Metal-Halide Perovskites.
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%).
Department of Physics, University of Oxford, Clarendon Laboratory , Parks Road, Oxford, OX1 3PU, United Kingdom.
Rebecca L
RL
Milot
Department of Physics, University of Oxford, Clarendon Laboratory , Parks Road, Oxford, OX1 3PU, United Kingdom.
Rebecca J
RJ
Sutton
Department of Physics, University of Oxford, Clarendon Laboratory , Parks Road, Oxford, OX1 3PU, United Kingdom.
Giles E
GE
Eperon
Department of Physics, University of Oxford, Clarendon Laboratory , Parks Road, Oxford, OX1 3PU, United Kingdom.
Amir Abbas
AA
Haghighirad
Department of Physics, University of Oxford, Clarendon Laboratory , Parks Road, Oxford, OX1 3PU, United Kingdom.
Josue
J
Martinez Hardigree
Oxford Photovoltaics Ltd. , Unit 6, Begbroke Science Park, Woodstock Road, Oxford, OX5 1PF, United Kingdom.
Laura
L
Miranda
Department of Physics, University of Oxford, Clarendon Laboratory , Parks Road, Oxford, OX1 3PU, United Kingdom.
Henry J
HJ
Snaith
Department of Physics, University of Oxford, Clarendon Laboratory , Parks Road, Oxford, OX1 3PU, United Kingdom.
Michael B
MB
Johnston
Department of Physics, University of Oxford, Clarendon Laboratory , Parks Road, Oxford, OX1 3PU, United Kingdom.
Laura M
LM
Herz
Charge-carrier mobility; GIWAXS; THz spectroscopy; photovoltaics; recombination