• Energy Research

For decades, progress in the field of optical (including solar) energy conversion was dominated by advances in the conventional concentrating optics and materials design. In recent years, however, conceptual and technological breakthroughs in the fields of nanophotonics and plasmonics combined with a better understanding of the thermodynamics of the photon energy-conversion processes reshaped the landscape of energy-conversion schemes and devices. Nanostructured devices and materials that make use of size quantization effects to manipulate photon density of states offer a way to overcome the conventional light absorption limits. Novel optical spectrum splitting and photon-recycling schemes reduce the entropy production in the optical energy-conversion platforms and boost their efficiencies. Optical design concepts are rapidly expanding into the infrared energy band, offering new approaches to harvest waste heat, to reduce the thermal emission losses, and to achieve noncontact radiative cooling of solar cells as well as of optical and electronic circuitries. Light–matter interaction enabled by nanophotonics and plasmonics underlie the performance of the third- and fourth-generation energy-conversion devices, including up- and down-conversion of photon energy, near-field radiative energy transfer, and hot electron generation and harvesting. Finally, the increased market penetration of alternative solar energy-conversion technologies amplifies the role of cost-driven and environmental considerations. This roadmap on optical energy conversion provides a snapshot of the state of the art in optical energy conversion, remaining challenges, and most promising approaches to address these challenges. Leading experts authored 19 focused short sections of the roadmap where they share their vision on a specific aspect of this burgeoning research field. The roadmap opens up with a tutorial section, which introduces major concepts and terminology. It is our hope that the roadmap will serve as an important resource for the scientific community, new generations of researchers, funding agencies, industry experts, and investors.

AbstractElectrochemical water splitting has attracted considerable attention for the production of hydrogen fuel by using renewable energy resources. However, the sluggish reaction kinetics make it essential to explore precious‐metal‐free electrocatalysts with superior activity and long‐term stability. Tremendous efforts have been made in exploring electrocatalysts to reduce the energy barriers and improve catalytic efficiency. This review summarizes different categories of precious‐metal‐free electrocatalysts developed in the past 5 years for alkaline water splitting. The design strategies for optimizing the electronic and geometric structures of electrocatalysts with enhanced catalytic performance are discussed, including composition modulation, defect engineering, and structural engineering. Particularly, the advancement of operando/in situ characterization techniques toward the understanding of structural evolution, reaction intermediates, and active sites during the water splitting process are summarized. Finally, current challenges and future perspectives toward achieving efficient catalyst systems for industrial applications are proposed. This review will provide insights and strategies to the design of precious‐metal‐free electrocatalysts and inspire future research in alkaline water splitting.

AbstractHybrid organic–inorganic perovskite solar cells (PSCs) are one of the most promising candidates for next generation photovoltaics. Further improvement in their performance, particularly efficiency, durability and reproducibility, requires a deep understanding of recombination losses during fabrication and within a device itself. In this work, we report a contactless, imaging‐based procedure to spatially resolve electronic properties of PSCs including implied open‐circuit voltage (iVoc) and its temperature coefficient, ideality factor (nid) and activation energy of recombination (EA) by employing illumination intensity and temperature‐dependent photoluminescence. The illumination intensity dependence of iVoc allows the extraction of nid whereas its temperature dependence allows the extraction of the temperature coefficient and EA. This imaging approach is then applied to investigate changes of these electronic parameters on fully and partially fabricated devices.

The rapid advancement of thin-film photovoltaic (PV) technology increases the real possibility of large-area Si-based tandems reaching 30% efficiency, although light in these devices must be managed carefully. We identify the optical requirements to reach high efficiencies. Strict conditions are placed on material parasitic absorption and transmission of contacts: Absorption of 20% of sub-bandgap light leads to the required top-cell efficiencies of 18% at a bandgap of 1.5 eV to break even and 23% to reach tandem efficiencies of 30%. Perovskite-silicon tandem cells present the first low-cost devices capable of improving standalone 25% efficiencies and we quantify the efficiency gains and reduced thickness afforded by wavelength-selective light trapping. An analytical formalism for Lambertian tandem light trapping is introduced, yielding stringent requirements for wavelength selectivity. Applying these principles to a perovskite-based top cell characterized by strong absorption and high luminescence efficiency we show that tandem efficiencies greater than 30% are possible with a bandgap of E g = 1.55 eV and carrier diffusion lengths less than 100 nm. At an optimal top-cell bandgap of 1.7 eV, with diffusion lengths of current vapor-deposited CH 3 NH 3 PbI x Cl 1-x perovskites, we show that tandem efficiencies beyond 35% are achievable with careful light management.

Multi-junction tandem design has been proven to be an effective means to further improve the efficiency of solar cells. However, its share in the photovoltaics market at present is tiny, since the most efficient tandem device comprises III-V semiconductors, which entail the use of expensive fabrication processes. The advent of perovskite solar cells, which have revitalized the PV field with their unprecedented pace of development, promises to address this bottleneck. Perovskite materials could not only serve as the top subcell absorber for commercial solar cells including Si and copper indium gallium selenide, but could work efficiently as bottom subcells owing to highly tuneable bandgaps which extend down to the range of ~1.2 to 1.5 eV. The highest-efficiency perovskite tandem to date was achieved by pairing a perovskite top cell with a Si bottom cell in a four-terminal configuration, yielding 26.4%. This review gives an overview of recent progress on the main tandem structures, and describes the detailed design improvements that have resulted in new record efficiencies. Ultimately, commercialization of these tandem solar cells relies on the scalability of perovskite technology. We, therefore, highlight the development of large-scale tandems and approaches to produce perovskite modules. We also point out the critical aspects that will require further effort and provide guidelines for future developments. The potential obstacles that will hamper the commercialization of perovskite tandems, if not adequately addressed, namely device stability and toxicity, are then critically examined. Finally, the substantial opportunities that perovskite materials open up for other solar devices with a tandem configuration are mentioned, which are attracting increasing attention.

This work provides a robust technoeconomic analysis of a range of direct solar hydrogen generation (DSHG) systems to understand the opportunities and challenges of deploying DSHG to reduce the cost of renewable hydrogen production.

Approximately half the cost of a finished crystalline silicon solar module is due to the silicon itself. Combining this fact with a high-efficiency potential makes thin-film crystalline silicon solar cells a growing research area. This paper, written in two parts, aims to outline world-wide research on this topic. The subject has been divided into techniques which use native substrates and techniques which use foreign substrates. Light trapping, vapour- and liquid-phase deposition techniques, cell fabrication and some general considerations are also discussed with reference to thin-film cells.

ABSTRACTDielectric scattering structures are a promising way of trapping light in solar cells. Titanium dioxide is a particularly attractive candidate material because of its high refractive index and ability to be deposited on a finished solar cell. Here, we present an experimental demonstration of photocurrent enhancement in thin film recrystallised silicon solar cells using TiO2 pillar arrays fabricated on the rear of the cells using nanoimprint lithography. A short circuit current enhancement of 19% is measured experimentally, and excellent agreement with numerical simulations is obtained. We show numerically that by replacing the Ag capping present on the cells with a detached rear Ag back reflector, the enhancement could reach 37%. Copyright © 2012 John Wiley & Sons, Ltd.