Silicon solar cells are approaching their efficiency limit of 29% under the standard solar spectrum. In order to surpass this limit, a device is required that better manages the energy in each incoming energy packet (photon). One approach to this end is to split the energy of higher energy photons in two, such that two electron-hole pairs can be generated by one photon. This strategy has an upper limit of 45.9%. Organic Multiple Exciton Generation (OMEG) is executed by a photophysical process called singlet fission. A spin-0 (singlet) exciton is generated by a photon, and it decays into two spin-1 triplet excitons in a spin-conserving process. This talk will detail our progress towards developing OMEG augmented silicon solar cells (OMEGA-Si).
Layers of organic molecules are capable of generating multiple excitons per absorbed photon though a process known as singlet fission. As such, this process could be employed to fabricate a solar cell which circumvents the efficiency limits imposed by a single threshold design. Leveraging experience with silicon solar cells, the OMEGA Si project is developing a device that combines the maturity and high efficiency of crystalline silicon with the exciton multiplication afforded by singlet fission. This talk will communicate progress of the project including experiments to elucidate exciton and carrier transfer between the singlet fission layer and the underlying silicon solar cell.
The thermoradiative diode (TRD) is the symmetric counterpart to the photovoltaic solar cell that generates power via the net emission rather than absorption of light. While the TRD has enticing applications in night-sky power generation, there are also opportunities for power generation via waste heat recovery. However, while the theoretical limits for power generation are promising, the current technological limits have not been explored. Here we compare the electro-optical characteristics of HgCdTe photodiodes in operating in both thermoradiative and thermophotovoltaic (TPV) modes, supported by optical modelling. By contrasting thermoradiative and TPV operation using the same devices, we set realistic expectations for power generation using mid-infrared semiconductors.
We present the symmetric counterpart to the solar cell that generates power via the net emission rather than absorption of light. This thermoradiative diode (TRD) has enticing applications in night-sky power generation and waste heat recovery. However, while theoretical limits for night-sky power generation are promising, the current technological limits have not been explored. Here we present the electro-optical characteristics a HgCdTe photodiode in thermoradiative and photovoltaic operation, supported by theoretical calculations that include critical non-radiative processes.
The introduction of light scattering structures for efficient absorption of incident illumination is essential in ultra-thin solar cells given their reduced optical path length. The growing interest in these devices demands attaining high efficiencies through the identification of optimal designs to maximise the absorption of incident photons. A pathway towards such high efficiencies is the implementation of transparent scattering structures to minimise parasitic losses. We study the performance of these structures by focusing on dielectric/high-band-gap semiconductor scattering layers in an ultra-thin (80 nm) GaAs solar cell. Comparisons with absorptive scattering layers are enabled by presenting data for an equivalent device with metal/dielectric structures. Following a previously reported light management optimisation method which is guided by the dispersion of the avail- able waveguide modes, we find an improved performance for the transparent scattering layers. Our study also compares the light absorption enhancement offered by transparent photonic crystal diffractive arrays to that of transparent quasi-random geometries which target the diffracted power to the most favourable waveguide modes in the device. We find the former designs to have a superior performance in our device of interest, and the latter to suffer from greater reflection losses. Finally, our results also demonstrate the effectiveness of the optimisation method used and its applicability to multiple device architectures for the design of high-efficiency photovoltaics.
Ultra-thin (less than 100 nm thick) photovoltaics are proposed as an enabling technology for space power applications due to their intrinsic radiation tolerance. Outside of the Earth's atmosphere, spacecraft are bombarded with energetic electrons and protons which can cause dislocations in the lattice structure of their solar cell materials thus limiting mission lifetimes. Certain orbits that could be advantageous for imaging, security and network coverage of the Earth are currently inaccessible due to high levels of radiation making them inhospitable to space craft. Ultra-thin cells have superior radiation tolerance but lower optical absorption which necessitates the integration of a nanophotonic light-trapping structure. The first iteration of ultra-thin 80 nm absorber layer devices patterned by Displacement Talbot Lithography has shown promising electrical and optical performance.
Ultra-thin (< 100 nm absorber thickness) GaAs cells are a promising avenue for the design of solar cells with increased radiation tolerance for space applications. To address the high transmission loss through such thin absorber layers, rigorous coupled-wave analysis and a semi-analytical waveguide model are used to investigate the effectiveness of silver/dielectric hexagonal grating structures placed on the back of a thin (86 nm) GaAs cell. The grating is formed of silver disks in a dielectric (SiNχ), and simulations indicate an optimum period of 600-700 nm with a grating thickness around 100 nm. Using the results of external quantum efficiency and light current-voltage measurements of thin devices without light-trapping features, predicted efficiencies for cells with a grating structure are found to be up to double that of the cells without light-trapping designs, showing a significant potential for current enhancement through light-trapping.
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