Theoretical limiting efficiencies have a critical role in determining technological viability and expectations for device prototypes, as evidenced by the photovoltaics community's focus on detailed balance. However, due to their multicomponent nature, photoelectrochemical devices do not have an equivalent analogue to detailed balance, and reported theoretical efficiency limits vary depending on the assumptions made. Here we introduce a unified framework for photoelectrochemical device performance through which all previous limiting efficiencies can be understood and contextualized. Ideal and experimentally realistic limiting efficiencies are presented, and then generalized using five representative parameters—semiconductor absorption fraction, external radiative efficiency, series resistance, shunt resistance and catalytic exchange current density—to account for imperfect light absorption, charge transport and catalysis. Finally, we discuss the origin of deviations between the limits discussed herein and reported water-splitting efficiencies. This analysis provides insight into the primary factors that determine device performance and a powerful handle to improve device efficiency.
Metasurfaces offer significant potential to control far-field light propagation through the engineering of the amplitude, polarization, and phase at an interface. We report here the phase modulation of an electronically reconfigurable metasurface and demonstrate its utility for mid-infrared beam steering. Using a gate-tunable graphene-gold resonator geometry, we demonstrate highly tunable reflected phase at multiple wavelengths and show up to 237° phase modulation range at an operating wavelength of 8.50 μm. We observe a smooth monotonic modulation of phase with applied voltage from 0° to 206° at a wavelength of 8.70 μm. Based on these experimental data, we demonstrate with antenna array calculations an average beam steering efficiency of 23% for reflected light for angles up to 30° for this range of phases, confirming the suitability of this geometry for reconfigurable mid-infrared beam steering devices. By incorporating all nonidealities of the device into the antenna array calculations including absorption losses which could be mitigated, 1% absolute efficiency is achievable up to 30°.
Tandem junction (n-p + -Si/ITO/WO 3 /liquid) core-shell microwire devices for solar-driven water splitting have been designed, fabricated and investigated photoelectrochemically. 0.0068% and 0.0019% when the cathode compartment was saturated with Ar or H 2 , respectively, due to the non-optimal photovoltage and band-gap of the WO 3 that was used in the demonstration system to obtain stability of all of the system components under common operating conditions while also insuring product separation for safety purposes. Broader contextDirect photoelectrochemical conversion of sunlight into a storable, energy-dense fuel has the opportunity to provide a predictable, carbon-neutral energy source to displace current carbon-based technologies. Solar hydrogen generation via water splitting is an important goal because the voltage requirements for this process are well matched to the maximum power point of high-efficiency tandem photovoltaics. In addition to including light-absorbing materials that provide sufficient voltage for water splitting, an integrated solar fuels device requires catalysts connected to the light absorbers and an ionic transport pathway between the anode and cathode to complete the circuit while maintaining product separation below the lower explosive limits, for safety purposes. Integration of these different active materials is important to further development of this technology. Single-crystalline Si microwire arrays represent an architecture that can allow the system operation and integration requirements to be met, because photoactive Si microwires have been previously embedded into ionically conductive, gasblocking membranes. However, Si microwires do not produce enough photovoltage for unassisted water splitting even in a tandem Si-based structure. We describe a Si microwire based tandem junction device that produces sufficient photovoltage for unassisted water splitting, by use of WO 3 in a core-shell tandem structure. This system provides a proof-of-principle for this design, which can be improved signicantly through the incorporation of higher efficiency wide band-gap semiconductors as they become available and are stable under the same conditions as the rest of the components of the device.
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