Solar energy is the most abundant renewable source of energy, but it faces challenges to being widely implemented on the global energy scale. Applying solar broadly in the global energy industry calls for cost-effective energy storage in order to overcome the intermittency of sunlight. Solar fuel generation is an attractive path that captures much of the photon energy in the bonds of portable and energy-dense chemicals such as liquid hydrocarbons and hydrogen. This procedure can be achieved at global production scales through the carbon free-electrolysis of water to hydrogen or through a carbon-neutral combination of water-splitting with electrochemical carbon-dioxide reduction to liquid fuels.
For a solar cell to be capable of energetically driving an electrochemical reaction, it should provide adequate photovoltage in order to overcome the thermodynamic reaction potentials and the associated cell overpotentials. To provide this photovoltage near the cell maximum power point, a single-junction solar cell would require a band gap of greater than 2 eV. At this band gap, the device would be inherently inefficient due to an inability to absorb much of the solar radiation. Therefore, researchers have focused on achieving the required photovoltage through multi-band-gap tandem cells or by connecting a number of low band-gap solar cells in electrical series.
For a solar-driven electrosynthetic fuel generation reaction, versatility is needed in tuning the operating photovoltage to ideally match up with the required electrolysis load. Therefore, a non-monolithic tandem solar cell structure, designed with physically separable sub-cells, provides for the greatest versatility for designing the current-voltage output to match up with the required electrolysis load.
Researchers led by Joshua Spurgeon at the Conn Center for Renewable Energy Research at the University of Louisville simulated the current-voltage performance of a number of non-monolithic tandem photovoltaic arrangements encompassing serial and multi-terminal configurations, as a function of changing spectra with the time of the day and device-width-dependent resistance effects. The modelled cell energy-conversion behavior was investigated in conjunction with measured electrolysis load curves for carbon dioxide reduction products and hydrogen generation from water. Their research work is published in the Journal of Materials Chemistry A.
The authors simulated non-monolithic tandem solar cell behavior based on silicon as well as organometal halide perovskites in four-terminal and two-terminal configurations and coupled this with experimental data on water-splitting and carbon dioxide reduction. The results establish the performance of an integrated solar fuel system which accounts for the variable effects of changing illumination throughout the day. A suitably designed four-terminal system was modelled to match and often exceed the output of a two-terminal system.
The researchers observed that the four-terminal arrangement led to a 15.8% increase in daily hydrogen production with a 1.5eV/1.12eV system. There was also a 5.3% increase with a more ideal 1.74eV/1.12eV combination. They also simulated a four-terminal system to match the generation of formic acid and an increase in the production of ethylene by 20.4% in a copper-catalyzed carbon dioxide reduction process when compared to a two-terminal tandem arrangement. They also modelled the effects of series resistance in non-monolithic tandem gadgets. The results indicated more tolerance to the cell width in the four-terminal system. Overall, the simulations indicated that there are possible advantages to an integrated multi-terminal tandem solar fuels device and showed how the design parameters can be adjusted to maximize the system output.
Dr. Joshua Spurgeon, the corresponding author on this paper said: “There are a lot of exacting criteria that a system has to meet to efficiently use the energy of sunlight to drive electrolysis. The design for the photoactive components modeled in this work is a versatile strategy to maximize the solar-to-fuel efficiency for many different electrochemical reactions.”
R. Turner White, Bijandra Kumar, Sudesh Kumari and Joshua M. Spurgeon. Simulations of non-monolithic tandem solar cell configurations for electrolytic fuel generation. Journal of Materials Chemistry A, 2017, 5, 13112
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