Stability Factors in Graphene-Based Perovskite Solar Cells Studied By in Situ Spectroscopy
Authors: Acik Muge , Darling Seth B. 
Published: February 2020

Power conversion efficiency in perovskite-based solar cells has recently improved to ≥20%, however, there is insufficient understanding of the underlying optoelectronic device function. Among all perovskite materials as candidates for the light harvesters in such solar devices, organolead halide perovskites, MAPbX3 (X=I, Br, Cl), have stood out with their outstanding optoelectronic properties such as tunable bandgaps, long electron-hole diffusion length and high electron/hole mobility. Indeed, replacement of ETL/HTL with graphene-derived materials (graphene oxide, reduced graphene oxide, n/p-doped graphene, etc.) has emerged as a pathway to improve device performance, and replaced traditional use of TiO2 and Al2O3 in these devices. Nevertheless, unclear film growth, nucleation and degradation mechanisms at the graphene/perovskite hybrid interfaces require understanding of interfacial working mechanisms and perovskite film formation, particularly over oxide-based electron/hole conductors. Moreover, graphene/perovskite structure-property relationships are not well understood due to unclear chemistry/poor characterization at the interfaces of ETL/perovskite/HTL hybrids. To explore interfacial working mechanisms and perovskite film formation in graphene-derived perovskite solar cells, we performed variable temperature (≤600°C) in situ spectroscopy (infrared absorption, micro-Raman, UV-vis-NIR, x-ray photoelectron and luminescence). Our studies targeted perovskite/graphene interfaces and perovskite growth mechanisms to overcome detrimental effects of stability factors such as incomplete lead precursor conversion, inconsistent crystallite formation/film uniformity, and weak cation-anion-solvent coordination. Effect of film thickness, lead content, stoichiometry control, underlayer/overlayer composition, and perovskite growth temperature were optimized for better film efficiency and charge transport. To address film scalability and stability, we studied opto-thermal changes in reduced graphene/graphite oxide (RGO) upon halide-based (CH3NH3PbI3, CH3NH3PbBr3, CH3NH3PbCl3) perovskite deposition, and performed spectroscopic analysis derived from the intensity and peak areas of perovskite vibrational normal modes of C-H (~2800-3200 cm-1) and N-H (~2000-2800 cm-1) and their interfacial reactions with oxygen functional groups in RGO. Controlled perovskite formation was achieved at room temperature for bromide/chloride-based perovskites resulting improved chemical stability with heat (vs. iodide derivative) that were decomposed at ≥150°C. Poor perovskite formation was monitored on RGO resulting in film degradation in air (O2, H2O) by in situ characterization; additional insights were derived from defect analysis from ID/IG ratio variation at perovskite/RGO interfaces. Film morphology and composition was examined by ex situ XRD, SEM, TEM, and AFM studies.
Keywords: In situ spectroscopy, graphene/perovskite solar cells, perovskite formation
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“Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Office of Science User Facility under Contract No. DE-AC02-06CH11357. M.A. also acknowledges support from the Joseph Katz Named Fellowship at Argonne National Laboratory.”