Graphite-Based Localized Heating Technique for Growing Large Area Methylammonium Lead Bromide Single Crystalline Perovskite Wafers and Their Charge Transfer Characteristics

Development of a reproducible technique to grow large area single crystalline perovskite wafers is an open research gap in the field of single crystalline perovskite solar cells. A graphite-based localized heating technique for growing large area methylammonium lead bromide (CH₃NH₃PbBr₃; MAPBr) single crystalline thin film (SCTF) on different buffer layers, such as glass/indium doped tin oxide (ITO), glass/ITO/poly(triaryl amine) (PTAA), and glancing angle deposition (GLAD) coated glass/ITO/TiO₂ substrates is reported, and their charge transport properties are discussed. It is observed that the localized heating technique can confine the supersaturation of the precursor mainly to the center of the substrate, leading to a restricted number of nucleations within a specific area on the substrate. Here, such 2-3 seed crystals obtained initially are allowed to grow to a larger size of up to 65 mm². The X-ray diffraction (XRD) analysis indicated that the large area SCTF is an actual single crystal and not a heterogeneous group of small crystals merged together with a crystallinity index (CI) of 92.60 ± 0.11% which was comparable to that of the bulk single crystal (97.74 ± 0.47%). The atomic force microscopy (AFM) image depicted a smooth SCTF surface (R_a = 4.37 ± 0.01 nm), and the wave-like pattern is attributed to the substrate morphology, implying that the topography of the substrate plays a crucial role in obtaining a planar SCTF. The XRD, UV-visible, photoluminescence (PL), Raman, and FTIR spectra analyses revealed that the large area SCTF is phase pure and free of residual impurities. The charge injection characteristics of the SCTFs grown on different buffer layers were investigated using PL emission (PLE) and PL decay analyses. The decrease in the PLE intensity for the SCTFs grown on PTAA and TiO₂ substrates implied exciton quenching behavior, indicating the injection of the photogenerated charge carriers into the charge transfer layers (CTLs). The decrease of the fast decay component from τ₁ = 4.77 ± 0.18 ns for glass to τ₁ = 3.32 ± 0.07 ns for TiO₂ and τ₁ = 3.15 ± 0.33 ns for PTAA is ascribed to the interfacial recombination of the charges accumulated at the CTL/perovskite interface. These results propose that the localized heating technique can be employed for growing large area single crystalline perovskite wafers for optoelectronic and photovoltaic device applications.