Highly Controlled Codeposition Rate of Organolead Halide Perovskite by Laser Evaporation Method

Organolead-halide perovskites can be promising materials for next-generation solar cells because of its high power conversion efficiency. The method of precise fabrication is required because both solution-process and vacuum-process fabrication of the perovskite have problems of controllability and reproducibility. Vacuum deposition process was expected to achieve precise control; however, vaporization of amine compound significantly degrades the controllability of deposition rate. Here we achieved the reduction of the vaporization by implementing the laser evaporation system for the codeposition of perovskite. Locally irradiated continuous-wave lasers on the source materials realized the reduced vaporization of CH₃NH₃I. The deposition rate was stabilized for several hours by adjusting the duty ratio of modulated laser based on proportional-integral control. Organic-photovoltaic-type perovskite solar cells were fabricated by codeposition of PbI₂ and CH₃NH₃I. A power-conversion efficiency of 16.0% with reduced hysteresis was achieved

Glancing Angle Deposition of Copper Iodide Nanocrystals for Efficient Organic Photovoltaics

We report a simple method to achieve efficient nanostructured organic photovoltaics via patterning copper iodide (CuI) nanocrystals on indium tin oxide by glancing angle deposition. The strong interfacial interaction between zinc phthalocyanine (ZnPc) and CuI leads to the formation of nanopillar arrays with lying-down molecular order, which greatly improve light absorption and surface roughness for exciton dissociation. Optimized ZnPc/C₆₀ bilayer cell has a power conversion efficiency of 4.0 ± 0.1%, which is about 3-fold larger than that of conventional planar cell

Understanding Device-Structure-Induced Variations in Open-Circuit Voltage for Organic Photovoltaics

We investigate the structural influences on the device performance, especially on open-circuit voltage (<i>V</i>_OC) in squaraine (SQ)/fullerene (C₆₀) bilayer cells. Simply changing the SQ thickness could lead to 40% variation in <i>V</i>_OC from 0.62 to 0.86 V. The ionization potential (IP) of SQ films and recombination at the anode surface as well as donor/acceptor (D/A) interface sensitively vary with film thicknesses, which account for the shifts in <i>V</i>_OC. The anode recombination can be effectively suppressed by preventing direct contact between C₆₀ and the anode with a buffer layer, delivering an elevated <i>V</i>_OC. Through polarized infrared–multiple-angle incidence resolution spectroscopy measurement, the molecular structure of SQ films is found to gradually evolve from lying-down on indium–tin oxide substrates with noncentrosymmetric orientation at low thicknesses to random structure at high thicknesses. The different molecular orientation may yield different strengths of electronic coupling, which affects the charge-carrier recombination and thus <i>V</i>_OC. Moreover, the oriented SQ films would spontaneously compose aligned dipole moments at the D/A interface because of the strong dipolar effects in SQ molecules identified by density functional theory calculations, whereas no aligned interfacial dipole moment exists in the random structure. The resulting interfacial dipole moments would form an electric field at the D/A interface, leading to variations in the IP and thus impacting <i>V</i>_OC. Our findings demonstrate that <i>V</i>_OC in organic photovoltaic cells is critically associated with the molecular orientation that affects the charge-carrier recombination and interfacial dipole alignment, which should be seriously taken into consideration for the design of organic molecules and optimization of the cell efficiency

Crystallization Dynamics of Organolead Halide Perovskite by Real-Time X‑ray Diffraction

We analyzed the crystallization process of the CH₃NH₃PbI₃ perovskite by observing real-time X-ray diffraction immediately after combining a PbI₂ thin film with a CH₃NH₃I solution. A detailed analysis of the transformation kinetics demonstrated the fractal diffusion of the CH₃NH₃I solution into the PbI₂ film. Moreover, the perovskite crystal was found to be initially oriented based on the PbI₂ crystal orientation but to gradually transition to a random orientation. The fluctuating characteristics of the crystallization process of perovskites, such as fractal penetration and orientational transformation, should be controlled to allow the fabrication of high-quality perovskite crystals. The characteristic reaction dynamics observed in this study should assist in establishing reproducible fabrication processes for perovskite solar cells

Adjustment of Conduction Band Edge of Compact TiO₂ Layer in Perovskite Solar Cells Through TiCl₄ Treatment

Perovskite solar cells (PSCs) without a mesoporous TiO₂ layer, that is, planar-type PSCs exhibit poorer cell performance as compared to PSCs with a porous TiO₂ layer, owing to inefficient electron transfer from the perovskite layer to the compact TiO₂ layer in the former case. The matching of the conduction band levels of perovskite and the compact TiO₂ layer is thus essential for enhancing PSC performance. In this study, we demonstrate the shifting of the conduction band edge (CBE) of the compact TiO₂ layer through a TiCl₄ treatment, with the aim of improving PSC performance. The CBE of the compact TiO₂ layer was shifted to a higher level through the TiCl₄ treatment and then shifted in the opposite direction, that is, to a lower level, through a subsequent heat treatment. These shifts in the CBE were reflected in the PSC performance. The TiCl₄-treated PSC showed an increase in the open-circuit voltage of more than 150 mV, as well as a decrease of 100 mV after being heated at 450 °C. On the other hand, the short-circuit current decreased after the treatment but increased after heating at temperatures higher than 300 °C. The treated PSC subjected to subsequent heating at 300 °C exhibited the best performance, with the power conversion efficiency of the PSC being 17% under optimized conditions

Epitaxial Growth of C₆₀ on Rubrene Single Crystals for a Highly Ordered Organic Donor/Acceptor Interface

The highly ordered epitaxial growth of C₆₀ films on rubrene single crystals was demonstrated. The C₆₀ crystals growth commensurate with rubrene (001) surface lattice was confirmed by reflection high energy electron diffraction and X-ray diffraction and grazing incident wide-angle X-ray scattering. Depending on growth conditions, several surface morphologies (rounded, hexagonal, and triangular grains) of C₆₀ grains were observed at the initial growth process. Large grains with layer-by-layer step and terrace structure of C₆₀ were observed at the terrace region of rubrene (001) surface. The growth of high-crystallinity C₆₀ films was achieved at high substrate temperature and slow deposition rate. Long migration length on the substrate which was given by enough substrate temperature enabled the formation of large single crystalline domains