Mark T. Winkler

Light-induced water oxidation at silicon electrodes functionalized with a cobalt oxygen-evolving catalyst

Integrating a silicon solar cell with a recently developed cobalt-based water-splitting catalyst (Co-Pi) yields a robust, monolithic, photo-assisted anode for the solar fuels process of water splitting to O2 at neutral pH. Deposition of the Co-Pi catalyst on the Indium Tin Oxide (ITO)-passivated p-side of a np-Si junction enables the majority of the voltage generated by the solar cell to be utilized for driving the water-splitting reaction. Operation under neutral pH conditions fosters enhanced stability of the anode as compared to operation under alkaline conditions (pH 14) for which long-term stability is much more problematic. This demonstration of a simple, robust construct for photo-assisted water splitting is an important step towards the development of inexpensive direct solar-to-fuel energy conversion technologies.

Crystalline silicon photovoltaics: a cost analysis framework for determining technology pathways to reach baseload electricity costs

Crystalline silicon (c-Si) photovoltaics are robust, manufacturable, and Earth-abundant. However, barriers exist for c-Si modules to reach US

Optical designs that improve the efficiency of Cu2ZnSn(S,Se)4 solar cells

0.50–0.75/Wp fabrication costs necessary for subsidy-free utility-scale adoption. We evaluate the potential of c-Si photovoltaics to reach this goal by developing a bottom-up cost model for c-Si wafer, cell, and module manufacturing; performing a sensitivity analysis to determine research domains that provide the greatest impact on cost; and evaluating the cost-reduction potential of line-of-sight manufacturing innovation and scale, as well as advanced technology innovation. We identify research domains with large cost reduction potential, including improving efficiencies, improving silicon utilization, and streamlining manufacturing processes and equipment, and briefly review ongoing research and development activities that impact these research domains. We conclude that multiple technology pathways exist to enable US

Insulator-to-Metal Transition in Sulfur-Doped Silicon

0.50/Wp module manufacturing in the United States with silicon absorbers. More broadly, this work presents a user-targeted research and development framework that prioritizes research needs based on market impact.

Hall mobility of cuprous oxide thin films deposited by reactive direct-current magnetron sputtering

We present an optical-design approach that improves the short-circuit current and efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells by tuning the thickness of the upper-device layers in order to maximize transmission into the CZTSSe absorber. The design approach combines optical modeling of idealized planar devices with a semi-empirical approach for treating the impact of surface roughness. Experimentally, we demonstrate that the new device architecture — which emphasizes thinner CdS and transparent-conducting layers — increases short-circuit current by about 10% in champion-caliber devices. These improvements are directly realized in the power-conversion efficiencies of CZTSSe devices, resulting in a certified improvement in the overall record power-conversion efficiency for CZTSSe from 11.1% to 12.0%. We also report comparable improvements for devices with band gaps in the range of 1.1–1.3 eV.

Insulator-to-Metal Transition in Selenium-Hyperdoped Silicon: Observation and Origin

We observe an insulator-to-metal transition in crystalline silicon doped with sulfur to nonequilibrium concentrations using ion implantation followed by pulsed-laser melting and rapid resolidification. This insulator-to-metal transition is due to a dopant known to produce only deep levels at equilibrium concentrations. Temperature-dependent conductivity and Hall effect measurements for temperatures T>1.7  K both indicate that a transition from insulating to metallic conduction occurs at a sulfur concentration between 1.8 and 4.3×10(20)  cm(-3). Conduction in insulating samples is consistent with variable-range hopping with a Coulomb gap. The capacity for deep states to effect metallic conduction by delocalization is the only known route to bulk intermediate band photovoltaics in silicon.

Room-temperature sub-band gap optoelectronic response of hyperdoped silicon

Cuprous oxide (Cu2O) is a promising earth-abundant semiconductor for photovoltaic applications. We report Hall mobilities of polycrystalline Cu2O thin films deposited by reactive dc magnetron sputtering. High substrate growth temperature enhances film grain structure and Hall mobility. Temperature-dependent Hall mobilities measured on these films are comparable to monocrystalline Cu2O at temperatures above 250 K, reaching 62 cm2/V s at room temperature. At lower temperatures, the Hall mobility appears limited by carrier scattering from ionized centers. These observations indicate that sputtered Cu2O films at high substrate growth temperature may be suitable for thin-film photovoltaic applications.

Modeling integrated photovoltaic–electrochemical devices using steady-state equivalent circuits

Hyperdoping has emerged as a promising method for designing semiconductors with unique optical and electronic properties, although such properties currently lack a clear microscopic explanation. Combining computational and experimental evidence, we probe the origin of sub-band-gap optical absorption and metallicity in Se-hyperdoped Si. We show that sub-band-gap absorption arises from direct defect-to-conduction-band transitions rather than free carrier absorption. Density functional theory predicts the Se-induced insulator-to-metal transition arises from merging of defect and conduction bands, at a concentration in excellent agreement with experiment. Quantum Monte Carlo calculations confirm the critical concentration, demonstrate that correlation is important to describing the transition accurately, and suggest that it is a classic impurity-driven Mott transition.

Modeling the Cost and Minimum Sustainable Price of Crystalline Silicon Photovoltaic Manufacturing in the United States

Room-temperature infrared sub-band gap photoresponse in silicon is of interest for telecommunications, imaging and solid-state energy conversion. Attempts to induce infrared response in silicon largely centred on combining the modification of its electronic structure via controlled defect formation (for example, vacancies and dislocations) with waveguide coupling, or integration with foreign materials. Impurity-mediated sub-band gap photoresponse in silicon is an alternative to these methods but it has only been studied at low temperature. Here we demonstrate impurity-mediated room-temperature sub-band gap photoresponse in single-crystal silicon-based planar photodiodes. A rapid and repeatable laser-based hyperdoping method incorporates supersaturated gold dopant concentrations on the order of 10(20) cm(-3) into a single-crystal surface layer ~150 nm thin. We demonstrate room-temperature silicon spectral response extending to wavelengths as long as 2,200 nm, with response increasing monotonically with supersaturated gold dopant concentration. This hyperdoping approach offers a possible path to tunable, broadband infrared imaging using silicon at room temperature.

Interfaces between water splitting catalysts and buried silicon junctions

Significance This article extends the construction of direct solar-to-fuels devices, such as the artificial leaf based on crystalline silicon. Because a single Si junction has insufficient potential to drive water splitting, it cannot be used for direct solar-to-fuels conversion. This paper performs an equivalent circuit analysis for multiple series-connected devices. The predictive utility of the model is demonstrated in the case of water oxidation at the surface of a Si solar cell, using a cobalt–borate oxygen evolving catalyst. Considering recent cost reductions of Si solar cells, this paper offers a path to the construction of low cost solar-to-fuels devices. We describe a framework for efficiently coupling the power output of a series-connected string of single-band-gap solar cells to an electrochemical process that produces storable fuels. We identify the fundamental efficiency limitations that arise from using solar cells with a single band gap, an arrangement that describes the use of currently economic solar cell technologies such as Si or CdTe. Steady-state equivalent circuit analysis permits modeling of practical systems. For the water-splitting reaction, modeling defines parameters that enable a solar-to-fuels efficiency exceeding 18% using laboratory GaAs cells and 16% using all earth-abundant components, including commercial Si solar cells and Co- or Ni-based oxygen evolving catalysts. Circuit analysis also provides a predictive tool: given the performance of the separate photovoltaic and electrochemical systems, the behavior of the coupled photovoltaic–electrochemical system can be anticipated. This predictive utility is demonstrated in the case of water oxidation at the surface of a Si solar cell, using a Co–borate catalyst.