Elucidating the Impact of Thin Film Texture on Charge Transport and Collection in Perovskite Solar Cells

Organic–inorganic halide perovskites have emerged as one of the most promising materials for photovoltaic applications. Because of the polycrystalline nature of perovskite thin films, it is crucial to investigate the impact of microstructures on device performance. In this study, we employ ramp-annealing to tailor the texture of perovskite thin films via controlling the nucleation of perovskite grains. Electrochemical impedance spectroscopy studies further suggest that the thin film texture impacts not only the charge collection at the contact but also the carrier transport in the bulk perovskite layer. The combination of the two effects leads to enhanced performance in devices constructed of preferentially oriented perovskite thin films

Enhanced Lifetime of Cyanine Salts in Dilute Matrix Luminescent Solar Concentrators via Counterion Tuning

Organic luminophores offer great potential for energy harvesting and light emission due to tunable spectral properties, strong luminescence, high solubility, and excellent wavelength selectivity. To realize their full potential, the lifetimes of luminophores must extend to many years under illumination. Many organic luminophores, however, have a tendency to degrade and undergo rapid photobleaching, leading to the perception of intrinsic instability of organic molecules. In this work, we demonstrate that by exchanging the counterion of a heptamethine cyanine salt the photostability and corresponding lifetime of dilute cyanine salts can be enhanced by orders of magnitude from 10 h to an extrapolated lifetime of greater than 65,000 h under illumination. To help correlate and comprehend the underlying mechanism behind this phenomenon, the water contact angle and binding energy of each pairing were measured and calculated. We find that increased water contact angle, and therefore increasing hydrophobicity, generally correlates to improved lifetimes. Similarly, a lower absolute binding energy between cation and anion correlates to increased lifetimes. Utilizing the binding energy formalism, we predict the stability of a new anion and experimentally verify it with good consistency. Moving forward, these factors could be used to rapidly screen and identify highly photostable organic luminophore salt systems for a range of energy harvesting and light-emitting applications

Enhanced Electroluminescence Efficiency in Metal Halide Nanocluster Based Light Emitting Diodes through Apical Halide Exchange

Metal halide nanoclusters represent an attractive class of molecular building blocks for the design of functional materials with superior optical properties that can be utilized in a range of applications. Here, we demonstrate red and near-infrared light emitting diodes with a maximum external quantum efficiency >1%, utilizing phosphorescent octahedral molybdenum iodide nanoclusters. Efficiency improvement in these devices is realized by substituting heavier ligands in the apical nanocluster position that lead to the improvement in photoluminescence and exciton formation efficiencies in the nanoclusters. These results highlight how modulation of nanocluster salts with key terminal ligands has a profound effect on photoluminescence as well as electrical injection

Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange

The electronic properties of colloidal quantum dots (QDs) are critically dependent on both QD size and surface chemistry. Modification of quantum confinement provides control of the QD bandgap, while ligand-induced surface dipoles present a hitherto underutilized means of control over the absolute energy levels of QDs within electronic devices. Here, we show that the energy levels of lead sulfide QDs, measured by ultraviolet photoelectron spectroscopy, shift by up to 0.9 eV between different chemical ligand treatments. The directions of these energy shifts match the results of atomistic density functional theory simulations and scale with the ligand dipole moment. Trends in the performance of photovoltaic devices employing ligand-modified QD films are consistent with the measured energy level shifts. These results identify surface-chemistry-mediated energy level shifts as a means of predictably controlling the electronic properties of colloidal QD films and as a versatile adjustable parameter in the performance optimization of QD optoelectronic devices

Alkali Metal Halide Salts as Interface Additives to Fabricate Hysteresis-Free Hybrid Perovskite-Based Photovoltaic Devices

A new method was developed for doping and fabricating hysteresis-free hybrid perovskite-based photovoltaic devices by using alkali metal halide salts as interface layer additives. Such salt layers introduced at the perovskite interface can provide excessive halide ions to fill vacancies formed during the deposition and annealing process. A range of solution-processed halide salts were investigated. The highest performance of methylammonium lead mixed-halide perovskite device was achieved with a NaI interlayer and showed a power conversion efficiency of 12.6% and a hysteresis of less than 2%. This represents a 90% improvement compared to control devices without this salt layer. Through depth-resolved mass spectrometry, optical modeling, and photoluminescence spectroscopy, this enhancement is attributed to the reduction of iodide vacancies, passivation of grain boundaries, and improved hole extraction. Our approach ultimately provides an alternative and facile route to high-performance and hysteresis-free perovskite solar cells