Margaret Young

The nature of photoinduced phase transition and metastable states in vanadium dioxide

Photoinduced threshold switching processes that lead to bistability and the formation of metastable phases in photoinduced phase transition of VO2 are elucidated through ultrafast electron diffraction and diffusive scattering techniques with varying excitation wavelengths. We uncover two distinct regimes of the dynamical phase change: a nearly instantaneous crossover into an intermediate state and its decay led by lattice instabilities over 10 ps timescales. The structure of this intermediate state is identified to be monoclinic, but more akin to M2 rather than M1 based on structure refinements. The extinction of all major monoclinic features within just a few picoseconds at the above-threshold-level (~20%) photoexcitations and the distinct dynamics in diffusive scattering that represents medium-range atomic fluctuations at two photon wavelengths strongly suggest a density-driven and nonthermal pathway for the initial process of the photoinduced phase transition. These results highlight the critical roles of electron correlations and lattice instabilities in driving and controlling phase transformations far from equilibrium.

Angle dependence of transparent photovoltaics in conventional and optically inverted configurations

Integration of transparent photovoltaics into the building envelope creates unique opportunities to reduce the levelized electricity cost of solar power. However, this integration warrants consideration of the angular dependence of these devices as illumination around the building envelope is rarely at normal incidence. Here we correctly update transfer-matrix and equations to accurately model the quantum efficiency and optical properties under oblique illumination. We use this model to demonstrate the various angular performance characteristics possible for proof-of-concept optimizations of transparent planar-heterojunction solar cells and discuss considerations needed to fully account for optical, electrical, and positional configurations in this optimization.

Impact of Ultrathin C60 on Perovskite Photovoltaic Devices

Halide perovskite solar cells have seen dramatic progress in performance over the past several years. Certified efficiencies of inverted structure (p-i-n) devices have now exceeded 20%. In these p-i-n devices, fullerene compounds are the most popular electron-transfer materials. However, the full function of fullerenes in perovskite solar cells is still under investigation, and the mechanism of photocurrent hysteresis suppression by fullerene remains unclear. In previous reports, thick fullerene layers (>20 nm) were necessary to fully cover the perovskite film surface to make good contact with perovskite film and avoid large leakage currents. In addition, the solution-processed fullerene layer has been broadly thought to infiltrate into the perovskite film to passivate traps on grain boundary surfaces, causing suppressed photocurrent hysteresis. In this work, we demonstrate an efficient perovskite photovoltaic device with only 1 nm C60 deposited by vapor deposition as the electron-selective material. Utilizing a combination of fluorescence microscopy and impedance spectroscopy, we show that the ultrathin C60 predominately acts to extract electrons from the perovskite film while concomitantly suppressing the photocurrent hysteresis by reducing space charge accumulation at the interface. This work ultimately helps to clarify the dominant role of fullerenes in perovskite solar cells while simplifying perovskite solar cell design to reduce manufacturing costs.

Efficient zinc sulfide cathode layers for organic photovoltaic applications via n-type doping

We demonstrate efficient zinc sulfide cathode window layers in thin-film organic photovoltaics enabled by n-type doping zinc sulfide (ZnS) with aluminum sulfide (Al2S3) directly through co-deposition. By optimizing the Al2S3 concentration, the power conversion efficiency is improved from 0.6% ± 0.2% in undoped ZnS window layer devices to 1.8% ± 0.1%, identical to control devices. The mechanism for this performance enhancement is shown to stem from the enhanced conductivity and interface energetics of ZnS upon n-type doping. This work expands the catalog of efficient, inorganic, non-toxic, cathode side window layers that could be effective in a range of thin-film photovoltaic technologies.

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.

Aqueous‐Containing Precursor Solutions for Efficient Perovskite Solar Cells

Abstract Perovskite semiconductors have emerged as competitive candidates for photovoltaic applications due to their exceptional optoelectronic properties. However, the impact of moisture instability on perovskite films is still a key challenge for perovskite devices. While substantial effort is focused on preventing moisture interaction during the fabrication process, it is demonstrated that low moisture sensitivity, enhanced crystallization, and high performance can actually be achieved by exposure to high water content (up to 25 vol%) during fabrication with an aqueous‐containing perovskite precursor. The perovskite solar cells fabricated by this aqueous method show good reproducibility of high efficiency with average power conversion efficiency (PCE) of 18.7% and champion PCE of 20.1% under solar simulation. This study shows that water–perovskite interactions do not necessarily negatively impact the perovskite film preparation process even at the highest efficiencies and that exposure to high contents of water can actually enable humidity tolerance during fabrication in air.

Anions for Near-Infrared Selective Organic Salt Photovoltaics

Organic molecular salts are an emerging and highly tunable class of materials for organic and transparent photovoltaics. In this work, we demonstrate novel phenyl borate and carborane-based anions paired with a near-infrared (NIR)-selective heptamethine cation. We further explore the effects of anion structures and functional groups on both device performance and physical properties. Changing the functional groups on the anion significantly alters the open circuit voltage and yields a clear dependence on electron withdrawing groups. Anion exchange is also shown to selectively alter the solubility and film surface energy of the resulting molecular salt, enabling the potential fabrication of solution-deposited cascade or multi-junction devices from orthogonal solvents. This study further expands the catalog and properties of organic salts for inexpensive, and stable NIR-selective molecular salt photovoltaics.

Evaluation of C1A1Pc synthesis methods for transparent organic photovoltaic

Two processes for chloroaluminum phthalocyanine (ClAlPc) synthesis using phthalonitrile and phthalic anhydride are compared based on ClAlPc purity, cost analysis of reactants, and performance for low-cost organic photovoltaic applications. Purity of ClAlPc is characterized through UV-vis and HPLC-mass spectrometry. The UV-vis spectra of ClAlPc from phthalic anhydride and phthalonitrile show strong Q-band absorption at 670 nm and 677 nm respectively, and 670 nm for reference ClAlPc. Also, liquid chromatogram of ClAlPc from two precursors and reference ClAlPc have the same retention time at 7 min with 580.16 m/z and the same isotope distribution. The power conversion efficiency of ClAlPc photovoltaic devices varies from 0.22% to 1.9% depending on the precursor. The best devices made with material synthesized using the phthalonitrile process have comparable efficiency to reference devices.