Charge Carrier Dynamics and Structural Defects in Layered Hybrid Perovskites
Naveen Venkatesan a, John Labram a, Rhys Kennard a, Ryan DeCrescent b, Hidenori Nakayama c, Clayton Dahlman a, Erin Perry a, Jon Schuller d, Michael Chabinyc a
a Materials, University of California Santa Barbara, University of California, Santa Barbara, 93106, United States
b Physics, University of California Santa Barbara, University of California, Santa Barbara, 93106, United States
c Electronics Materials and New Energy Laboratory, Mitsubishi Chemical Corporation
d Electrical and Computer Engineering, University of California Santa Barbara
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Poster, Naveen Venkatesan, 130
Publication date: 11th February 2019

Hybrid organic metal halide Ruddlesden-Popper (R-P) phases have recently been the subject of intense research efforts due to their good power conversion efficiencies in photovoltaics and controllable emission for light emitting diodes, while possessing better environmental stability compared to their three-dimensional counterparts. [1,2] Both the thin film structures and carrier dynamics of these layered perovskites are still poorly understood relative to the bulk methylammonium lead iodide. In this study, we use optical spectroscopy, X-ray scattering, and transmission electron microscopy to characterize the structures of these thin films of (C4H9NH3)2(CH3NH3)2Pb3I10 and (C4H9NH3)2(CH3NH3)3Pb4I13 on the meso- and nanoscales. [3] Additionally, the carrier dynamics of these phases were characterized by contactless time-resolved microwave conductivity (TRMC) measurements. [4] Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements suggest that these structures possess appreciable disorder in the layer stacking direction. Spatial photoluminescence measurements on the micron scale show no disorder or phase separation, so nanoscale imaging was required to image these defects. Transmission electron microscopy (TEM) measurements suggest the presence of significant phase impurities, contrary to previous reports suggesting single crystalline quality thin films. When using known crystal structures to index these SAED patterns, we see that the thin films comprise not only the targeted R-P phase, but also regions with lower and higher Pb-I sheet thickness (i.e. phase impurities). This phase intergrowth creates structural defects that interrupt layer stacking and is the cause broadening of in-plane diffraction peaks, causing them to be absent from previous GIWAXS measurements. Finally, because these films produce efficient photovoltaics despite this high degree of structure disorder, we measured the absorption coefficient using photothermal deflection spectroscopy (PDS) and find Urbach energies of 33 and 32 meV for the n = 3 and 4 R-P phases, respectively, compared to 19 meV for methylammonium lead iodide. Our results suggest that the absorbance of these thin films is dominated by the macroscopic n value, i.e. the targetted stoichiometry, but emission and photovoltaic performance is controlled by nano-inclusions of MAPbI3. This is because photogenerated charges and/or excitons tend to migrate toward lower bandgap phases prior to emission. This is further supported by TRMC measurements, which show short (~ns) lifetimes, indicative of intimately mixed, nanoscale phases, consistent with the structural picture from TEM. Despite the structural defects, the R-P films appear to maintain a low degree of electronic disorder suggesting that the Pb-I regions are electronically isolated from each other. 

Materials synthesis and structural characterization were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC-0012541. Work on devices was supported by the Defense Threat Reduction Agency under Award HDTRA1-15-1-0023. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Contract DE-AC02-76SF00515. The research reported here also made use of the shared facilities of the UCSB MRSEC (National Science Foundation DMR 1720256), a member of the Materials Research Facilities Network (www.mrfn.org ). R.M.K. gratefully acknowledges the National Defense Science and Engineering Graduate Fellowship for financial support. R.A.D. and J.A.S. acknowledge support from a National Science Foundation CAREER Award, under Award DMR-1454260. N.R.V. gratefully appreciates the help from D. Hanifi and Prof. A. Salleo with running and help with analysis of PDS.

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