Graphene and Semiconductors

Graphene incorporated perovskite-based solar cells have been demonstrated for low-cost and scalable production of renewable energy as an alternative to metal oxide or polymer -based perovskite photovoltaics. The varying PCE at ~0.62-18% motivates us to study graphene/perovskite interfaces since graphene oxide (GO) or its reduced form (RGO) recently emerge as an electron (ETL) or hole (HTL) transport layers in these devices. Organic-inorganic methylammonium lead halides, MAPbTx (T=I, Br, Cl and MA=CH3NH3 are deposited on graphene films as light harvesting layers because of their exciting optoelectronic properties: tunable bandgap, long electron-hole diffusion lengths and high electron/hole mobility. Nevertheless, halide-based perovskites require in situ characterization to understand perovskite growth mechanisms at GO/MAPbTx film interfaces. Understanding the origin of perovskite degradation mechanisms is also lacking at the ETL/perovskite/HTL interfaces due to limited materials characterization. In particular, incomplete lead precursor conversion, inconsistent crystallite formation, weak cation-anion-solvent coordination, uncontrolled film thickness, lead content, and stoichiometry are detrimental effects for device reliability and stability.To address stability issues, we examined degradation, nucleation and growth mechanisms during annealing in RGO after perovskite deposition from a DMF solution (Fig. 1). First, GO thin films (3-5 layers) were deposited by vacuum filtration, next followed by spin coating of perovskites, and then annealed on GO. Chemical interactions were interpreted at perovskite/RGO interfaces for the grain size, orientation, boundaries, and surface effects using variable-temperature (≤400°C) in situ infrared absorption spectroscopy, Raman, XPS, SEM, XRD, and AFM. MAPbIx growth on GO indicated Pb-I formation upon MAPbIx deposition. Pb(NO3)2 formed at GO/MAPbIx interfaces after annealing, and modified by unsaturated Pb states. Oxygen-induced chemical reactions occurred at ≤150°C that removed epoxides and hydroxyls of GO films yielding defective RGO because of MAPbIx degradation. In contrast, MAPbBrx growth on GO resulted in improved chemical stability with heat. Metallic Pb states were cleared after annealing