William R. Erwin

Light trapping in mesoporous solar cells with plasmonic nanostructures

Plasmon resonances in metal nanostructures have been extensively harnessed for light trapping in mesoporous solar cells (MSCs), including dye-sensitized solar cells (DSSCs) and recently in perovskite solar cells (PSCs). By altering the geometry, dimension, and composition of metal nanostructures, their optical characteristics can be tuned to either overlap with the sensitizer absorption and enhance light harvesting, or absorb light at a wavelength complementary to the sensitizer enabling broadband solar light capture in MSCs. In this comprehensive review, we discuss the mechanisms of plasmonic enhancement in MSCs including far-field coupling of scattered light, near-field coupling of localized electromagnetic fields, hot electron transfer, and plasmon resonant energy transfer. We then summarize the progress in plasmon enhanced DSSCs in the past decade and decouple the impact of metal nanostructure shape, size, composition, and surface coatings on the overall efficiency. Further, we also discuss the recent advances in plasmon-enhanced perovskite solar cells. Distinct from other published reviews, we discuss the significance of femtosecond spectroscopies to probe the fundamental underpinnings of plasmon enhanced phenomena and understand the mechanisms that give rise to energy transfer between metal nanoparticles and solar materials. The review concludes with a discussion on the challenges in plasmonic device fabrication, and the promise of low-loss semiconductor nanocrystals for plasmonic enhancement in MSCs that facilitate light capture in the infrared.

Surface engineered porous silicon for stable, high performance electrochemical supercapacitors

Silicon materials remain unused for supercapacitors due to extreme reactivity of silicon with electrolytes. However, doped silicon materials boast a low mass density, excellent conductivity, a controllably etched nanoporous structure, and combined earth abundance and technological presence appealing to diverse energy storage frameworks. Here, we demonstrate a universal route to transform porous silicon (P-Si) into stable electrodes for electrochemical devices through growth of an ultra-thin, conformal graphene coating on the P-Si surface. This graphene coating simultaneously passivates surface charge traps and provides an ideal electrode-electrolyte electrochemical interface. This leads to 10–40X improvement in energy density, and a 2X wider electrochemical window compared to identically-structured unpassivated P-Si. This work demonstrates a technique generalizable to mesoporous and nanoporous materials that decouples the engineering of electrode structure and electrochemical surface stability to engineer performance in electrochemical environments. Specifically, we demonstrate P-Si as a promising new platform for grid-scale and integrated electrochemical energy storage.

All Silicon Electrode Photocapacitor for Integrated Energy Storage and Conversion

We demonstrate a simple wafer-scale process by which an individual silicon wafer can be processed into a multifunctional platform where one side is adapted to replace platinum and enable triiodide reduction in a dye-sensitized solar cell and the other side provides on-board charge storage as an electrochemical supercapacitor. This builds upon electrochemical fabrication of dual-sided porous silicon and subsequent carbon surface passivation for silicon electrochemical stability. The utilization of this silicon multifunctional platform as a combined energy storage and conversion system yields a total device efficiency of 2.1%, where the high frequency discharge capability of the integrated supercapacitor gives promise for dynamic load-leveling operations to overcome current and voltage fluctuations during solar energy harvesting.

Morphological modulation of bimetallic nanostructures for accelerated catalysis

Bimetallic nanostructures are of significant technological interest due to their ability to uniformly combine properties of two distinct metals, giving rise to multimodal characteristics. In this work, we have synthesized Au/Ag core/shell bimetallic nanostructures and investigated the role of temperature in controlling the morphological evolution and the corresponding impact on catalytic transformation. By increasing the reaction temperature from 35 °C to 80 °C, the edge morphologies of Au/Ag nanostructures evolved from rounded to sharp corners, which directly impact the catalytic properties. The size of the bimetallic nanostructures also increased when the temperature was raised due to faster Ag+ reduction along specific crystallographic planes, giving rise to red shifts in the plasmon resonance. The catalytic activity of Au/Ag nanostructures was compared to commercially purchased Ag nanospheres for the reduction of 4-nitrophenol to 4-aminophenol with NaBH4. The reaction rate for 4-nitrophenol reduction was significantly higher on Au/Ag-NSs relative to the Ag nanospheres, while the induction time was lowest on the Ag nanospheres. These observations were attributed to the simultaneous effects of (i) surface area available for catalytic reaction, (ii) crystallographic facets supporting the nanostructures, (iii) surface ligands, and (iv) composition of the metal nanostructures.

Engineered porous silicon counter electrodes for high efficiency dye-sensitized solar cells.

In this work, we demonstrate for the first time, the use of porous silicon (P-Si) as counter electrodes in dye-sensitized solar cells (DSSCs) with efficiencies (5.38%) comparable to that achieved with platinum counter electrodes (5.80%). To activate the P-Si for triiodide reduction, few layer carbon passivation is utilized to enable electrochemical stability of the silicon surface. Our results suggest porous silicon as a promising sustainable and manufacturable alternative to rare metals for electrochemical solar cells, following appropriate surface modification.

Interplay of structural and compositional effects on carrier recombination in mixed-halide perovskites

In this work, we investigate the effects of grain structure and bromide content on charge transport in methylammonium lead iodide/bromide (MAPb(I1−xBrx)3) perovskites by examining the steady-state (ss-PL) and time-resolved (tr-PL) photoluminescence of planar films with varying grain size and bromide content. We controlled the perovskite grain structure via solvent engineering with N,N-dimethylformamide (DMF) or dimethyl sulfoxide : γ-butyrolactone (DMSO : GBL), and the resultant grain morphology was independent of Br doping between 0–33%. Carrier lifetimes ranged from 29–52 ns with increasing Br content for DMF-produced perovskite films, and from 2–9 ns for films obtained with DMSO : GBL. Analysis of XRD and photoluminescence data suggests this order-of-magnitude difference in lifetimes is attributable to the nature of iodide-rich recombination nuclei within the bulk perovskite. By modulating the precursor solvent and Br composition, the influence of low-bandgap recombination nuclei can be minimized to enhance charge transport and lengthen the carrier lifetime in mixed-halide perovskite films.

Mixed Halide Hybrid Perovskites: A Paradigm Shift in Photovoltaics

Perovskite solar cells (PSCs) have improved at an unprecedented rate, reaching over 22% power conversion efficiency in less than a decade, and have generated a wealth of high impact work in the literature studying the properties of these unique materials. While the highest performance and best progress have been observed with methylammonium lead triiodide perovskites, their low tolerance to moisture and rapid degradation when exposed to UV light and high temperatures have posed a major setback to their commercialization. Recently, mixed halide hybrid perovskites (MHHPs) where I− is substituted with Br− and Cl− have driven a paradigm shift in PSCs, synergistically combining high performance with high ambient stability. In this comprehensive review, we summarize the progress made in the past 5 years in methylammonium-based organic–inorganic MHHPs and cesium-based all inorganic MHHPs. We discuss the fabrication approaches for halide substitution and the resulting crystal structure, long-term stability and durability, and device performance. Unique from other reviews, we focus this review on the optical properties of MHHPs, including light-induced phase segregation and alterations in the optical band gap and photoluminescence with halide substitution. We also summarize trends in carrier dynamics, including carrier thermalization, recombination, and charge transfer probed with time-resolved optical spectroscopies. Furthermore, we provide a short overview on mixed cation hybrid perovskites, where methylammonium is substituted with formamidinium/cesium or Pb is substituted with less toxic and benign metals to improve both the stability and the environmental impact of PSCs. We discuss the optical characteristics of mixed cation perovskites and approaches to lower nonradiative recombination with cation substitution. We conclude this review with a discussion on innovative future directions that will propel research on MHHPs as a game changer in perovskite-based optoelectronics.

Embedding solar cell materials with on-board integrated energy storage for load-leveling and dark power delivery (Presentation Recording)

This work will discuss our recent advances focused on integrating high power energy storage directly into the native materials of both conventional photovoltaics (PV) and dye-sensitized solar cells (DSSCs). In the first case (PV), we demonstrate the ability to etch high surface-area porous silicon charge storage interfaces directly into the backside of a conventional polycrystalline silicon photovoltaic device exhibiting over 14% efficiency. These high surface area materials are then coupled with solid-state ionic liquid-polymer electrolytes to produce solid-state fully integrated devices where the PV device can directly inject charge into an on-board supercapacitor that can be separately discharged under dark conditions with a Coulombic efficiency of 84%. In a similar manner, we further demonstrate that surface engineered silicon materials can be utilized to replace Pt counterelectrodes in conventional DSSC energy conversion devices. As the silicon counterelectrodes rely strictly on surface Faradaic chemical reactions with the electrolyte on one side of the wafer electrode, we demonstrate double-sided processing of electrodes that enables dual-function of the material for simultaneous energy storage and conversion, each on opposing sides. In both of these devices, we demonstrate the ability to produce an all-silicon coupled energy conversion and storage system through the common ability to convert unused silicon in solar cells into high power silicon-based supercapacitors. Beyond the proof-of-concept design and performance of this integrated solar-storage system, this talk will conclude with a brief discussion of the hurdles and challenges that we envision for this emerging area both from a fundamental and technological viewpoint.