Synthesis and Characterizations of Organometallic Lead Halide Perovskite Single Crystals for Photovoltaic Applications

Abstract

Solar energy is a clean source of energy that can help to fulfill the increasing global energy demand. Among light-harvesting devices, perovskite solar cells (PSCs) have been a focus of interest among next-generation photovoltaic (PV) technologies due to their incredible conversion efficiency (certified PCE of ~25.2%) along with their lower cost and ease of fabrication. However, many transport barriers and defect trap states at the interfaces and grain boundaries have negative effects on PSCs. It decreases their efficiency and stability and increases the hysteresis effect. From the literature survey, the above drawbacks can be minimized by controlling the morphology, grain boundary, grain size, charge recombination, and density of defect states in the perovskite layer by different surface passivation. However, the growth of high-quality single-crystal (SC) perovskite films is a great strategy for the fabrication of defect-free PSCs with photovoltaic parameters close to the theoretical limit, which resulted in high efficiency and superior stability of the device. Thus, considering the potential improvement in terms of stability and performance of PSCs using single-crystalline perovskite absorbing layer and different passivation layers, this work focuses on synthesizing perovskite single crystals and passivation layer for photovoltaic applications. Methylammonium lead iodide (MAPbI3) is the most commonly used photo absorber in PSCs. We have chosen MAPbI3 perovskite single crystal in the present work and modified them with guanidinium (GUA) cation and bromide halides. Following single crystals (SCs) were synthesized via inverse temperature crystallization (ITC) synthesis root. 1. MAPbI3 and Methylammonium lead bromide (MAPbBr3) SCs 2. GUAxMA1-xPbI3 SCs 3. MAPb(I1-x Brx)3 SCs (x = 0.04, 0.08, 0.12 and 0.16) However, a water-resistive lead sulfate layer is used as a surface passivator for improving the stability against moisture. XRD patterns of grounded crystals showed a single perovskite phase. There is no grain boundary (from SEM microstructure study). Temperature stability of the MAPbI3 SCs ware examined by temperature depended XRD patterns. From the detailed bias- and temperature-dependent studies, we found that the low-frequency capacitance values are influenced by ion density and mobility. Consequently, single-crystalline MAPbI3 depicts an activation energy of ~0.53–0.54 eV with an exceptionally low electronic trap density of ~0.96 × 1010 cm–3. The small substitution of Methylammonium (MA) with guanidinium (GUA) decreases the activation energy for iodide ion migration in comparison to pristine MAPbI3 SCs. The presence of large GUA cations in the 3D perovskite structure induces lattice enlargement, which perturbs the atomic interactions within the perovskite lattice. Consequently, the GUAxMA1−xPbI3 crystal exhibits a higher degree of hysteresis during current-voltage (J–V) measurements than the single crystalline MAPbI3 counterpart. It is found that I/Br alloying structure effectively slows down the degradation of MAPbI3 SCs over time. Single-phase cubic structure was obtained from XRD study when a certain amount of Br (x=0.12) incorporated in MAPbI3 SCs, leading to a tetragonal to the cubic phase transition. We find that the smaller Br atoms decrease the lattice spacing of the MAPbI3 SCs, which restricts the ion migrations and increases the activation energy, leading to improve stability of MAPbI3 SCs. The calculated activation energy of ~0.34±0.007 and ~0.51±0.008 eV is associated with the tetragonal and cubic phases of MAPbI3 SCs. A clear tetragonal-cubic phase transition of the MAPbI3 SCs was noticed in the 50-55 oC temperature range. On the other hand, tetragonal-cubic phase transition temperatures are decreasing with Br inclusion, and the most crucial benefits in terms of phase stability are found for MAPb(I0.88Br0.12)3 SCs. We fabricated MAPb(I1-x Brx)3 SCs based photodetectors (PDs) and studied their response and stability. The best values reached for MAPb(I0.88Br0.12)3 SC PDs under white light with an intensity of ~0.5 mW cm-2, and the values of responsivity and specific detectivity are ~2.049 mA/W and ~15.19 × 1010 jones, respectively. More importantly, the photocurrent of MAPb(I0.88Br0.12)3 SCs based photodetectors (PDs) decrease by only ~54% without encapsulation, while the photoresponse of MAPbI3 PDs is reduced by ~81% after 365 days. Additionally, the responsivity of PDs with 12% Br doping is decreased by only ~15.4% compared to a ~37.5 % reduction for pure MAPbI3 SC-based planar PDs after 10 days of continuous operation. We studied the role of lead sulfate (PbSO4) as an effective passivator in MAPbI3 and MAPbBr3 SCs. Using impedance spectroscopy, we evaluated the ion migration and electrical properties of lead sulfate-passivated MAPbI3 and MAPbBr3 SCs. We found that the low-frequency impedance response that is assigned to the ionic motion in the MAPbBr3 and SCs is strongly affected by the inorganic PbSO4 surface treatment. The activation energy corresponding to the ion migration of MAPbBr3 and MAPbI3 SC increased from ~0.28 to ~0.36 eV and ~0.54 and ~0.59 eV after PbSO4 surface treatment, respectively. The temperature-dependent I–V hysteresis of the MAPbBr3 SCs upon PbSO4 passivation was also measured. We found that such PbSO4 surface treatment stabilizes the crystal surface and improves the hysteresis properties of the crystals at elevated temperatures.