Enhanced Grain Size and Crystallinity in CH3NH3PbI3 Perovskite Films by Metal Additives to the Single-Step Solution Fabrication Process

Bandgap Pressure Coefficient of a CH3NH3PbI3 Thin Film Perovskite

Recent scientific interest in examining the bandgap evolution of a MAPbI₃ hybrid perovskite by applying hydrostatic pressure has mostly focused on a room-temperature tetragonal phase. In contrast, the pressure response of a low-temperature orthorhombic phase (OP) of MAPbI₃ has not been explored and understood. In this research, we investigate for the first time how hydrostatic pressure alters the electronic landscape of the OP of MAPbI₃. Pressure studies using photoluminescence combined with calculations within density functional theory at zero temperature allowed us to identify the main physical factors affecting the bandgap evolution of the OP of MAPbI₃. The negative bandgap pressure coefficient was found to be strongly dependent on the temperature (α_120K = -13.3 ± 0.1 meV/GPa, α_80K = -29.8 ± 0.1 meV/GPa, and α_40K = -36.3 ± 0.1 meV/GPa). Such dependence is related to the changes in the Pb-I bond length and geometry in the unit cell as the atomic configuration approaches the phase transition as well as the increasing phonon contribution to octahedral tilting as the temperature increases.

Preparation of Micron-sized Methylamine-PbCl3 perovskite grains by controlling phase transition engineering for selective Ultraviolet-harvesting transparent photovoltaics

Selective ultraviolet-harvesting transparent perovskite solar cells (T-PSCs) have attracted great interest because of their high transmittance and unique photovoltaic properties, especially in the fields of smart windows for power generation and building glass. However, owing to the unsatisfactory solubility of PbCl₂ in most conventional solvents, preparing transparent methylammonium lead chloride (MAPbCl₃) films with high quality and sufficient thickness by conventional methods poses a substantial challenge for their application deployment in T-PSCs. In this work, two novel strategies based on an ion-exchange procedure for controlling phase transition engineering (CPTE) are proposed. For CPTE-2, an optimized cubic phase MAPbCl₃ film with a large grain size and high full coverage is prepared by transforming the tetragonal phase MAPbI₃ precursor into the cubic phase MAPbCl₃. Establishing relevant models based on crystal parameters investigates the formation mechanism of this high-quality MAPbCl₃ film. Accordingly, the resultant T-PSCs exhibit remarkable film quality and micron-sized grains and reach an optimum efficiency of 0.33% (J_SC = 0.66 mA cm^-2, V_OC = 1.14 V, and FF = 43.72%).

The Progress of Additive Engineering for CH3NH3PbI3 Photo-Active Layer in the Context of Perovskite Solar Cells

Methylammonium lead triiodide (CH3NH3PbI3/MAPbI3) is the most intensively explored perovskite light-absorbing material for hybrid organic–inorganic perovskite photovoltaics due to its unique optoelectronic properties and advantages. This includes tunable bandgap, a higher absorption coefficient than conventional materials used in photovoltaics, ease of manufacturing due to solution processability, and low fabrication costs. In addition, the MAPbI3 absorber layer provides one of the highest open-circuit voltages (Voc), low Voc loss/deficit, and low exciton binding energy, resulting in better charge transport with decent charge carrier mobilities and long diffusion lengths of charge carriers, making it a suitable candidate for photovoltaic applications. Unfortunately, MAPbI3 suffers from poor photochemical stability, which is the main problem to commercialize MAPbI3-based perovskite solar cells (PSCs). However, researchers frequently adopt additive engineering to overcome the issue of poor stability. Therefore, in this review, we have classified additives as organic and inorganic additives. Organic additives are subclassified based on functional groups associated with N/O/S donor atoms; whereas, inorganic additives are subcategorized as metals and non-metal halide salts. Further, we discussed their role and mechanism in terms of improving the performance and stability of MAPbI3-based PSCs. In addition, we scrutinized the additive influence on the morphology and optoelectronic properties to gain a deeper understanding of the crosslinking mechanism into the MAPbI3 framework. Our review aims to help the research community, by providing a glance of the advancement in additive engineering for the MAPbI3 light-absorbing layer, so that new additives can be designed and experimented with to overcome stability challenges. This, in turn, might pave the way for wide scale commercial use.

Impact of Vertical Inhomogeneity on the Charge Extraction in Perovskite Solar Cells: A Study by Depth-Dependent Photoluminescence

In perovskite solar cells (PSCs), the vertical inhomogeneities which include uneven grains, voids, and grain boundaries are closely linked to the underlying charge transport layer which controls the nucleation and grain growth in the perovskite film. Herein, the vertical inhomogeneity of perovskite films in the device structure is analyzed by depth-dependent photoluminescence (PL) achieved with different excitation wavelengths. An analytical representation between vertical inhomogeneity and depth-dependent PL, parametrized with a factor, b, is introduced to understand the relation between inhomogeneity and charge recombination. Lower values of b correlate to lower vertical inhomogeneity and hence reduced recombination. The analytical representation is validated in two sets of devices that show remarkable differences in perovskite film morphology, device based on mesoporous TiO₂ and planar SnO₂. By exploring the morphological properties and the PL emission from different depths across the device structures, we show that the lower vertical inhomogeneity leads to more efficient charge carrier extraction in planar SnO₂-based devices. Moreover, the SnO₂-based devices exhibit lower Urbach energy, which concurs with the slow transient photovoltage decay, suggesting less defects and recombination losses. This work provides a broader understanding of the impact of vertical inhomogeneity on the charge extraction efficiency and presents a methodology to study quantitatively the inhomogeneity of perovskite films in device structures.