Micah S. Glaz

High-Performance and Environmentally Stable Planar Heterojunction Perovskite Solar Cells Based on a Solution-Processed Copper-Doped Nickel Oxide Hole-Transporting Layer

An effective approach to significantly increase the electrical conductivity of a NiOx hole-transporting layer (HTL) to achieve high-efficiency planar heterojunction perovskite solar cells is demonstrated. Perovskite solar cells based on using Cu-doped NiOx HTL show a remarkably improved power conversion efficiency up to 15.40% due to the improved electrical conductivity and enhanced perovskite film quality. General applicability of Cu-doped NiOx to larger bandgap perovskites is also demonstrated in this study.

Room Temperature Electrodeposition of Molybdenum Sulfide for Catalytic and Photoluminescence Applications

An elegant method for the electrodeposition of MoS2 thin films using room temperature ionic liquids (RTIL) as an electrolyte was developed. Simple molecular precursors of Mo and S were added in different concentrations to tune the composition and deposition process. The electrodeposition of MoS2 was confirmed with both Raman spectroscopy and XPS. Analysis showed that the electrodeposited MoS2 films form a flower shape morphology with edge active sites that promote the hydrogen evolution reaction (HER). Furthermore, this technique enables selective tuning of the film thickness and demonstrates high photoluminescence activity with a decrease in the number of layers.

Zr Incorporation into TiO2 Electrodes Reduces Hysteresis and Improves Performance in Hybrid Perovskite Solar Cells while Increasing Carrier Lifetimes

We investigate zirconium (Zr) incorporation into the titanium dioxide (TiO2) electron-transporting layer used in organometal halide perovskite photovoltaics. Compared to Zr-free controls, solar cells employing electrodes containing Zr exhibit increased power conversion efficiency (PCE) and decreased hysteresis. We use transient photovoltage and photocurrent extraction to measure carrier lifetimes and densities and observe longer carrier lifetimes and higher charge densities in devices on Zr-containing electrodes at microsecond times as well as longer persistent photovoltages extending from ∼milliseconds to tens of seconds. We characterize the surface stoichiometry and change in work function and reduction potential of the TiO2 upon incorporation of Zr and discuss the charge recombination at the TiO2 interface in the context of these variables. Finally, we show that the combination of Zr-TiO2 electrode modification with device pyridine treatment leads to a cumulative improvement in performance.

Copper indium gallium selenide (CIGS) photovoltaic devices made using multistep selenization of nanocrystal films.

The power conversion efficiency of photovoltaic devices made with ink-deposited Cu(InxGa1-x)Se2 (CIGS) nanocrystal layers can be enhanced by sintering the nanocrystals with a high temperature selenization process. This process, however, can be challenging to control. Here, we report that ink deposition followed by annealing under inert gas and then selenization can provide better control over CIGS nanocrystal sintering and yield generally improved device efficiency. Annealing under argon at 525 °C removes organic ligands and diffuses sodium from the underlying soda lime glass into the Mo back contact to improve the rate and quality of nanocrystal sintering during selenization at 500 °C. Shorter selenization time alleviates excessive MoSe2 formation at the Mo back contact that leads to film delamination, which in turn enables multiple cycles of nanocrystal deposition and selenization to create thicker, more uniform absorber films. Devices with power conversion efficiency greater than 7% are fabricated using the multiple step nanocrystal deposition and sintering process.

Influence of Composition on the Performance of Sintered Cu(In,Ga)Se2 Nanocrystal Thin‐Film Photovoltaic Devices

Thin-film photovoltaic devices (PVs) were prepared by selenization using oleylamine-capped Cu(In,Ga)Se2 (CIGS) nanocrystals sintered at a high temperature (>500 °C) under Se vapor. The device performance varied significantly with [Ga]/[In+Ga] content in the nanocrystals. The highest power conversion efficiency (PCE) observed in the devices studied was 5.1 % under air mass 1.5 global (AM 1.5 G) illumination, obtained with [Ga]/[In+Ga]=0.32. The variation in PCE with composition is partly a result of bandgap tuning and optimization, but the main influence of nanocrystal composition appeared to be on the quality of the sintered films. The [Cu]/[In+Ga] content was found to be strongly influenced by the [Ga]/[In+Ga] concentration, which appears to be correlated with the morphology of the sintered film. For this reason, only small changes in the [Ga]/[In+Ga] content resulted in significant variations in device efficiency.

Scanning photocurrent microscopy of lateral organic bulk heterojunctions

Scanning confocal photocurrent microscopy has been used to characterize carrier collection efficiency in lateral bulk heterojunction devices. By analyzing the photocurrent mappings within these devices, the lateral extents of the space charge regions has been measured and reported. Modulation via white light bias or increased voltage bias is also shown to increase the size of the space charge regions.

Fast time-resolved electrostatic force microscopy: Achieving sub-cycle time resolution

The ability to measure microsecond- and nanosecond-scale local dynamics below the diffraction limit with widely available atomic force microscopy hardware would enable new scientific studies in fields ranging from biology to semiconductor physics. However, commercially available scanning-probe instruments typically offer the ability to measure dynamics only on time scales of milliseconds to seconds. Here, we describe in detail the implementation of fast time-resolved electrostatic force microscopy using an oscillating cantilever as a means to measure fast local dynamics following a perturbation to a sample. We show how the phase of the oscillating cantilever relative to the perturbation event is critical to achieving reliable sub-cycle time resolution. We explore how noise affects the achievable time resolution and present empirical guidelines for reducing noise and optimizing experimental parameters. Specifically, we show that reducing the noise on the cantilever by using photothermal excitation instead of piezoacoustic excitation further improves time resolution. We demonstrate the discrimination of signal rise times with time constants as fast as 10 ns, and simultaneous data acquisition and analysis for dramatically improved image acquisition times.

Nanoscale fluorescence imaging with quantum dot near-field electroluminescence

We demonstrate near-field fluorescence excitation and imaging with a quantum dot (QD) light emitting diode (QDLED) integrated at the tip of a scanning probe. The tip-embedded QDLED is employed in a near-field scanning optical microscopy setup to directly excite a secondary colloidal QD sample. Electrically pumped QDs enable multi-color, self-illuminating probes with no conventional optics needed for light coupling. Monolayer QDs stamped at the very tip of a micromachined silicon probe facilitates precise position control of the ultra-thin (10–15 nm) light source. Sensitivity of fluorescence intensity to the QDLED–QD sample distance was measured down to 50 nm order, demonstrating spatially resolved imaging.

Device physics and operation of lateral bulk heterojunction devices.

Measurements of lateral bulk heterojunction (BHJ) devices have recently been reported as a means to characterize charge transport and recombination properties within organic photovoltaic (OPV) materials. These structures allow for the direct measurement of the lateral extents of the space charge regions, potential and electric field profiles, current versus voltage characteristics, and other physical and chemical properties. This article describes numerical simulations that show three different transport regimes present within lateral BHJ devices and two different experimental methods, which verify those findings. These measurement techniques utilize typical confocal microscopy tools as well as steady-state current versus voltage measurements on high aspect ratio nanofabricated structures in order to probe the material properties between the electrodes. Experimental results show that the lateral extents of space charge regions within these devices are approximately 1-5 μm, which are related to the drift lengths of the charge carriers, and that the mechanism of bimolecular recombination is shown to be a bulk material property. The results within this article describe a series of methods to evaluate charge transport and recombination along the in-plane direction in BHJ films and provide complementary insights to those obtained from vertical-device-based measurements.

Perylene diimide functionalized polynorbornene: a macromolecular scaffold for supramolecular self-assembly

We have synthesized and characterized a polymer comprising a polynorbornene backbone with perylene diimide (PDI) pendant groups. The polymer exhibits intramolecular self-assembly into molecular aggregates that can be controlled via solvent quality. In a medium that solvates the polymer backbone well the PDI chromophores self-assemble into characteristic H-type aggregates aligned along the polymer backbone. In contrast, in a solvent medium that does not solvate the polymer well the polynorbornene backbone collapses into a globular conformation preventing the aggregation of the PDI chromophores, which display monomer-like photophysics. The polymer photophysics are recorded under a variety of conditions, including lifetimes and relative quantum yields. Furthermore, we utilize the weakly coupled H-aggregate model to quantify the exciton bandwidth of the polymer structure and demonstrate that polymer thin films can retain the same chromophore aggregated morphology as the fully solvated polymer.