In This Issue
Summer Issue of The Bridge on Energy, the Environment, and Climate Change
July 3, 2015 Volume 45 Issue 2

The Solar Opportunity

Monday, July 6, 2015

Author: Nathan S. Lewis and Daniel G. Nocera

Solar energy utilization poses a vexing conundrum: at present, we cannot afford to use it, but eventually we probably cannot afford not to use it. The promise rests in the unmatched size of the solar resource: more energy from the sun strikes the Earth in one hour than all of the energy consumed on the planet in an entire year (DOE 2005; Lewis and Nocera 2006).

As with all energy sources, challenges for solar energy use reside in the “cost of extraction.” The diffuseness of solar energy, typically providing a yearly averaged power density of about 200 W/m2 at representative midlatitudes, requires the coverage of relatively large areas with a sunlight capture system, in turn requiring very inexpensive but high-performance materials and balance of systems to be viable. Additionally, to contribute to a large fraction of a global energy system, use of solar energy requires concomitant development of an accompanying technological approach to provide tera-watt (TW)-days of reliable, robust, persistent, scalable, and cost-effective energy storage.

Three main technologies for solar energy use involve

  • solar electricity production by photovoltaics,
  • solar thermal systems that produce electricity or process heat as their primary outputs, and
  • nonbiological systems that mimic the process of photosynthesis through direct production of fuel from sunlight.

The status of these technologies, and the physical principles that underlie their operation, were described in a widely distributed report commissioned by the US Department of Energy, Basic Research Needs for Solar Energy Utilization (DOE 2005). This article provides an update on many significant subsequent developments, and identifies the challenges and associated opportunities for further research and engineering, in each of the three approaches.

Solar Electricity

The cost of solar panels, measured in dollars per peak watt of power produced, has declined along a remarkably impressive curve, decreasing by about 20 percent for each doubling of production volume of solar panels globally (Fraunhofer ISE 2014; Jean et al. 2015). Nevertheless, solar electricity is still an extremely expensive electricity generation technology at utility scale, especially when the full costs of installation as well as capital and operational costs are considered (CCST 2011; Jean et al. 2015; NAS 2010). The potential for further major cost reductions, through both evolutionary and disruptive technology development, continues to stimulate fervent research and development (R&D) efforts in solar electricity.

Typical Materials

Devices based on crystalline silicon (Si) dominate the solar panel market (Fraunhofer ISE 2014; Jean et al. 2015). Whereas the fundamental physical design of Si-ased photovoltaics has not markedly changed, significant effort has been devoted to device engineering, including novel surface passivation layers, improved contact geometries to reduce parasitic recombination processes, and the use of heterojunction overlayers. These efforts have produced steady, incremental improvements in the efficiency of Si-based photovoltaic (PV) modules. In parallel, significant cost reductions have been obtained in manufacturing costs, along with economies of scale associated with the automation of multibillion-dollar factories that yield greater gigawatt/year panel production volumes.

Gallium arsenide (GaAs) and III-V-based PV single-junction devices have been engineered through the use of sophisticated light management techniques and junction and surface passivation methods, along with epitaxial growth techniques, to produce very high-performance devices, with efficiencies greater than 29 percent (Manners 2012; NREL 2015) approaching the theoretical limit of 32 percent for a conventional single-junction device under unconcentrated sunlight. But the increased complexity of these devices results in much more expensive manufacturing costs, so such systems have not achieved significant market penetration.

Other than space-based power applications where weight as opposed to cost is at a premium, highly engineered III-V multijunction devices are currently considered viable only for terrestrial applications in conjunction with high-concentration-factor optics using solar tracking and optical concentrating systems.

Opportunities with Other Materials

Opportunities exist for obtaining improved efficiencies through spectral splitting approaches and novel designs for both 1-D and 2-D optical concentration and tracking systems and structures, and through new approaches to the growth of high-quality, high-performing III-V monolithic devices and structures.

Inorganic Thin-Film Materials

Inorganic thin-film materials use polycrystalline materials grown by a less costly, scalable manufacturing process. Cadmium telluride (CdTe)–based devices offer lab-scale efficiencies of more than 20 percent (NREL 2015) and module efficiencies of more than 12 percent (Fraunhofer ISE 2014), and are manufacturable because CdTe sublimes congruently. Devices have been obtained using engineered cadmium sulfide (CdS)/CdTe heterostructures that provide grain boundary passivation as well as control over junction recombination at the CdS/CdTe interface. Thanks to their low manufacturing cost, CdTe-based thin-film modules have captured a significant market share (~5 percent) of shipped PV modules (Fraunhofer ISE 2014).

There are long-term concerns about the availability of ample Te resources to reach TW scale and the toxicity of cadmium if it is released into the environment, but these factors are being acceptably addressed at the present installation level and with current sealing processes of the insoluble CdTe material in modules.

Copper indium gallium selenium (CuInGaSe2; CIGS) and related materials systems have been demonstrated to yield efficiencies greater than 21 percent in small test cells. But the materials have been very challenging to manufacture while preserving efficiencies greater than 15 percent at module scale because of the need for precise control over the stoichiometry of this multielement material over large areas.

Thin-film materials advantageously allow the production of lightweight, potentially flexible modules that could help reduce installation costs if the highest reported conversion efficiencies can be maintained at module production scale. Alternative, earth-abundant light absorbers that could provide alternative materials options for a scalably manufacturable PV technology relative to the CdTe- and CIGS-based systems are also being explored, including detailed examinations of zinc phosphide (Zn3P2), group II-IV-VI materials such as zinc tin nitride (ZnSnN2), and copper(I) oxide (Cu2O).

Organic Solar Cells

Organic solar cells offer the promise of a flexible, processible, lightweight materials system that could entail very low manufacturing and installation costs. The canonical system is derived from an interpenetrating network of organic hole conductors such as polyphenylene-vinylene with functionalized buckyball-based materials acting as the light absorber and electron donor. The overall energy conversion performance has proven to be a complex function not only of the molecular composition and structure of the components, but also of their morphology, interfaces, impurities at the ppm-ppb levels and of the inorganic/organic contacts between the active elements and the metallized electrical contact layers in a fully operational device structure.

Intense research efforts have led to a steady improvement in device efficiency, with the best systems exhibiting efficiencies of more than 11 percent on small-area devices (NREL 2015). Obtaining demonstrated long-term stability at these efficiency levels is still challenging.

Dye-Sensitized Solar Cells

Dye-sensitized solar cells use an inorganic transition-metal molecular species, generally based on derivatives of ruthenium [Ru(II)] bipyridyl complexes, to inject electrons into a nanocrystalline titanium dioxide (TiO2)–based particle network, while effecting the oxidation of an electron donor, typically iodide, in either a liquid or gel-type electrolyte (DOE 2005).

Efficiencies for small-area devices can approach 12 percent (NREL 2015), although long-term stability at these efficiency levels, especially over large areas, remains to be established. Ongoing studies seek to understand the fundamental processes that underpin efficient injection of photogenerated electrons into the mesoscopic material, the transport mechanisms of carriers through the nanocrystalline films, kinetic processes for charge-carrier recombination relative to ionic motion, and timescales for electron injection and hole capture by the electrolyte, as well as variation in these properties with the nature of the donor, electrolyte, dye, and composition and morphology of the support.

The newest family of light absorber materials is based on hybrid inorganic/organic materials that consist of lead salts with ammonium cations arranged in a perovskite structure. The efficiencies of these systems on small test samples have skyrocketed from 3 percent to over 20 percent in a matter of 2–3 years, in a remarkable set of advances (Gunther 2015; NREL 2015). These materials offer a new paradigm for solar light absorbers because they exhibit unusually high photovoltages despite the lack of structural periodicity or long-range crystallinity and order in the absorbing phase.

These systems will likely provide new insights into fundamental semiconducting chemistry and physics, though the existing materials systems pose technological challenges: their long-term stability is questionable, and the highest reported efficiencies are fleeting in nature. In addition, the perovskite salts are soluble in water, potentially yielding toxic, soluble lead ions upon dissolution. Both the efficiency limits and the generalizability of the approach to other materials systems are ripe topics for further research.

Quantum Dots

An alternative focus for R&D involves quantum dots and related materials that could ultimately provide efficiencies in excess of the conventional theoretical Shockley-Queisser device limit. Such materials involve lead sulfide (PbS), lead selenide (PbSe), and related systems (Ning et al. 2014), as well as materials that offer the possibility of generation of multiple charge carriers from a single photon, in an inverse Auger process. Notably, both inorganic and organic systems under suitable conditions have been shown to produce multiple excitons from a single photon (Congreve et al. 2013; Schaller and Klimov 2004).

Such approaches, along with intermediate-band systems, are in the early development stage, but offer the potential of very high efficiencies with materials that have a thin-film structure and thus low eventual manufacturing costs.

Solar Thermal Systems

Solar thermal systems involve the concentration of sunlight by optical focusing and/or optically reflective surfaces. The localized regions of heat are used to either perform chemical reactions or heat up a thermal fluid such as oil or a molten salt, with the hot fluid then used on demand to drive a turbine and produce electricity. The optical focusing and concentration systems for such installations are considered mature.

Research efforts are focused on the development of new types of thermal storage materials that can provide higher amounts of stored heat per unit volume of the thermal fluid while also being affordable at scale. Because only the direct component of sunlight can be focused, these installations require a region with a high direct normal insolation value (e.g., a desert). Generation costs have proven to be greater than $0.15/kWh when all-in installation and operational expenses are included (CCST 2011; NAS 2010), and these costs have not declined substantially over time.

An alternative approach is to use the focused solar heat to drive chemical reactions, at theoretical second-law system conversion efficiencies that can exceed 60–70 percent. In one approach, the solar heat is used to promote the Fischer-Tropsch catalyzed production of synfuel from syngas (CO and H2).

The cost of solar-derived heat must compete with the very inexpensive cost of heat produced from combustion of natural gas. Alternatively, solar heat can be used to drive an endothermic fuel-forming process such as water splitting; for example, molten zinc can be reacted with steam to produce H2 and zinc oxide (ZnO). In a separate process, the ZnO is heated to release O2 and regenerate the molten zinc (Weimer et al. 2009). Both of the process steps have been demonstrated to occur individually in the laboratory.

The engineering challenges involve obtaining a system geometry that lets focused light enter the reaction cavity and does not let much of the high-temperature heat escape, while simultaneously allowing for the input flow of the reactant and egress of the product gases in continuous or suitably large batch process modes. Additionally, the metal and metal oxide reactants must be confined in a safe and inexpensive reactor design that uses materials that are robust, inexpensive, and compatible with the reactor process operation at extremely high temperatures.

Solar Fuels: Nonbiological Production Systems

Nonbiological systems that directly produce fuel from sunlight are much less technologically developed than solar electricity or solar thermal systems, and no demonstration systems or deployed solar fuel installations presently exist. The production of fuels from sunlight offers the potential to provide a technological solution to scalable grid storage, by subsequent on-demand combustion or use of the solar fuel as a feedstock for a fuel cell, and also offers a scalable technology for the carbon-neutral production of fuels for the 40 percent of global transportation that requires a high energy density liquid fuel to function.

Chlorophyll-Based and Related Light Absorbers

One approach to solar fuel production involves the use of molecular components related to those used in natural photosynthesis. Various chlorophyll-based and related light absorbers have been assembled in conjunction with precisely connected electron donors and acceptors to achieve separation of the light-induced electron-hole pairs produced by absorption of sunlight by the chromophore of interest. These systems have advanced understanding of both electron transfer processes in molecular systems and the fundamental charge separation and transport processes in natural photosynthetic systems.

For sustained fuel production, these molecular assemblies will require coupling the photogenerated charges to multielectron/proton catalysts that can effect the bond-breaking and bond-forming processes needed to produce fuels, with the concomitant evolution of O2 to allow for completion of a sustainable fuel production cycle. Fully functional molecular assemblies have not yet been achieved and are a target of chemical research efforts.

Challenges involve avoiding back reactions at all stages of the process, obtaining a persistent separation of the products, coupling one-electron charge separation processes to the requisite multielectron catalysts that can operate in a mutually compatible environment with each other and with the light-absorbing molecules, and achieving robustness of the entire assembly under sustained solar illumination.

Semiconductor Nanoparticles

An alternative approach uses particles of inorganic semiconductors, coupled to heterogeneous cocatalysts, to perform the light absorption, charge separation, and fuel-forming processes (DTI 2009). Upon ultraviolet illumination, materials such as strontium titanate (SrTiO3), doped TiO2, and other large band-gap metal oxides, in the presence of cocatalysts such as platinum (Pt) and ruthenium(IV) oxide (RuO2) or iridium(IV) oxide (IrO2), can effect the solar-driven splitting of water into H2 and O2.

Materials with improved response in the visible region, such as CdS or indium phosphide (InP), are generally either unstable to passivation or corrosion and/or do not provide sufficient driving force to effect one or both of the half-reactions involved with water splitting or other photochemically based fuel production. Tantalum (oxy)nitride (TaON) with RuOx and chromium(III) oxide (Cr2O3) cocatalysts provide a photoresponse that extends into the visible region of the solar spectrum (Maeda et al. 2013), thus improving the solar conversion efficiency.

Challenges involve identifying a suitable photocatalytic material that combines efficiency and stability, and eventually developing an approach to avoid coevolution of stoichiometric, potentially explosive, mixtures of H2(g) and O2(g).

Semiconducting Photoelectrodes

The most advanced solar fuel generation systems to date use semiconducting photoelectrodes, which combine light absorption, charge separation, and catalysis in a single material (DOE 2005).

Water-Splitting

To construct an integrated semiconducting photoelectrode, electrocatalysts must be integrated with an inherently unstable semiconductor. Metallic electrocatalysts generally block light and/or are porous, allowing the corrosive electrolyte to permeate to the underlying semiconductor (Haussener et al. 2012; Hu et al. 2014; Jin et al. 2014; Sun et al. 2015a,b). Self-healing catalysts of manganese, cobalt, and nickel (Nocera 2012) enable complete, assembled photovoltaic structures, protected by transparent conductive oxide layers, to be immersed in water and produce atmospheric-pressure stoichiometric mixtures of H2(g) and O2(g) using earth-abundant materials (Esswein et al. 2012; Pijpers et al. 2011; Reece et al. 2011).

In alkaline media, where low-resistance, intrinsically safe systems can be constructed, integrated semiconducting photoanodes have been recently developed and shown to provide operational stability and efficiency for water oxidation while operating continuously for thousands of hours under simulated sunlight (Sun et al. 2015a,b).

Research on semiconducting photoelectrodes will continue to focus on new materials for the light absorbers, separators or membranes, and electrocatalysts; new device morphologies that enable synergistic integration of the functions of the components in ways that provide cost and/or performance advantages relative to the properties of the discrete materials themselves; and new systems architectures that enable intrinsically safe operation with minimal use of scarce materials and robust product separation, to simultaneously provide robustness, efficiency, safety, and cost-effectiveness for the system as a whole. Cost-effectiveness is an especially acute challenge as the levelized cost of hydrogen from steam reforming of natural gas is $2/kg, and photovoltaics combined with grid-connected electrolyzers yield a functionally equivalent competitive H2 production technology to direct solar-driven water splitting.

CO2 Reduction

Solar fuel systems are also being investigated for the direct reduction of CO2 to liquid transportation fuels, a very challenging multielectron and multiproton process.

A low overpotential catalyst for CO2 reduction involves the use of substituted pyridiniums on certain electrode surfaces to yield, at low current densities, methanol and other alcohols, whereas metal electrodes require high overpotentials, are generally unstable at the required reducing potentials, and produce a wide array of organic products requiring expensive separation and concentration processes (Benson et al. 2008). Oxide-derived nanoparticles have been shown to reduce CO to ethanol as a major product, possibly allowing for a two-step conversion involving reduction of CO2 to CO followed by the production of liquid fuels (Li et al. 2014).

H2 derived from electrolysis has been used as a feedstock for an engineered bacterial system to effect the production of dilute aqueous solutions of isopropanol from sunlight (Torella et al. 2015), complementing studies that have coupled the enzyme formate dehydrogenase to a semiconductor electrode to directly produce fuel from sunlight.

A significant challenge is the instability of enzymes in vitro. Other challenges for CO2 reduction involve the use of an expensive and/or dilute reactant (400 ppm of CO2 in the atmosphere vs. 55 M of water for production of H2) (APS 2011), separation of the products from the liquid electrolyte, limitations due to small mass fluxes for uptake of atmospheric CO2 by aqueous and nonaqueous fluids, and operation under aerobic conditions without significant reduction of O2 at the cathode or reaction of the products or intermediates of CO2 reduction with either the evolved or ambient O2.

Promise and Potential

Much progress has been made toward developing a scalable technology for the use of solar energy, but significant challenges remain for all current technological approaches. Most of the approaches offer the potential to provide much higher efficiencies, much lower costs, improved scalability, and new functionality relative to the embodiments of solar energy conversion systems that have been developed to date. Research, engineering, and manufacturing will need to be pursued in harmony to allow realization of the full potential of solar energy.

Acknowledgments

NSL acknowledges support from the National Science Foundation (CHE-1214152), the Department of Energy Office of Science through the Joint Center for Artificial Photosynthesis (grant DE-SC0004993), and the DOE Office of Science (grant DE-FG02-03ER15483). DGN acknowledges support from the US DOE Office of Science (grant DE-SC0009565), Air Force Office of Scientific Research (grant FA9550-09-1-0689), and the TomKat Trust.

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About the Author:Nathan S. Lewis is George L. Argyros Professor of Chemistry in the Division of Chemistry and Chemical Engineering, Beckman Institute and Kavli Nanoscience Institute, California Institute of Technology. Daniel G. Nocera (NAS) is Patterson Rockwood Professor of Energy in the Department of Chemistry and Chemical Biology at Harvard University.