Selective Emitter for Solar Thermophotovoltaic Applications

. Selective Emitters (SEs) are the main components of solar thermophotovoltaic (STPV) systems; they act as intermediate thermal radiation emitters to shape the incident solar spectrum to match the wavelengths useful for the PV cell. In this work, we present the design, optimisation, fabrication, and characterisation of an SE based on a multilayer design made of SiN x , SiO 2 , and TiO 2 layers. The SE is optimised to work with PV cells based on III-V semiconductors, such as GaSb, InGaAs, and InGaAsS, the bests suitable for SPTV applications. The fabricated SE shows an emitter efficiency (η SE ) of 50% if matched with a PV cell with an energy bandgap of 0.63 eV.


Introduction
The current growth of renewable energy has renewed interest in thermo-photovoltaic (TPV) systems to recover high-temperature heat [1,2].A TPV system collects excess heat for directly conversion from thermal radiation to electricity through photovoltaics.One particularly promising source of heat is sunlight, which connects to solar thermophotovoltaic (STPV) systems [3,4], given their potential to reshape broadband solar radiation into more efficient, narrower band spectra for photovoltaic harvesting.The ideal thermal emitter is a selective emitter (SE), characterised by a sharp transition from high to low emissivity.Ideally, this emissivity change occurs at a photon energy approximately equal to the bandgap energy (Eg) of the associated PV cell.The precise choice of the optimal bandgap is determined by the thermal emission spectrum, controlled by the emitter temperature (TSE) and the spectral characteristics of the emitter surface [5].In current STPV systems, low bandgap PV cells are chosen, since the emitter temperature is below 1500 °C.The major choices are III-V semiconductors, typically GaSb (0.72 eV), InGaAs (0.6 -0.7 eV), and InGaAsSb (0.5 -0.6 eV).Here, a simple structure based on easy-to-fabricate multilayers (SiNx -SiO2 -TiO2) on a tungsten substrate is optimised to work at 1000 °C for the bandgap range of 0.55 -0.72 eV.Optical characterisations were performed to calculate the SE efficiency using the experimental materials' refractive indices.

Materials and methods
The emitter (fig. 1) was deposited on a tungsten (W) substrate using a magnetron sputtering system.The structure consists of three layers without metallic layers to reduce the oxidation issues that could occur during hightemperature applications [2].

Fig. 1. Architecture of the Selective Emitter
The thickness of each layer was obtained with a Genetic Algorithm developed on MATLAB®.This algorithm can automatically adjust each layer's thickness and calculate the stack reflectivity ρ(λ) at each iteration, using the transfer matrix method [6], assuming experimentally observed refractive indices.Moreover, according to Kirchhoff's law of thermal radiation and the principle of conservation of energy, the emissivity of an opaque object at thermal equilibrium satisfies the following relation: ε(λ) = α(λ) = 1-ρ(λ).Thus, the ηSE, defined as the ratio of the maximum electrical power generated by a solar cell exposed to the SE (Pmax) to that emitted by the SE itself (PSE), can be calculated following the equation: where λ represents the spectral wavelength, λg the PV cell bandgap wavelength, εSE(λ) the SE spectral emissivity, and IBB(λ,TSE) the spectral power density of a blackbody at TSE.The GA aims to maximise the value of ηSE by searching for the best combination of thicknesses of the various layers.Hence, the PV cell Eg (or λg) and TSE must be selected to optimise the SE structure.For this work, a fixed temperature of 1000 °C was chosen, while a variable bandgap energy range (Eg ∈ [0.55; 0.72] eV) was selected to include the entire III-V semiconductors region in the analysis.

Results
In Fig. 2, the emissivity of the as-deposited SE, (λ), is compared to that of the simulated and optimised SE ( (λ)).The deposited SE (solid black line in Fig. 2) shows high emissivity values mainly after 1000 nm, where the normalised blackbody spectral energy density at 1000 °C (grey area) exceeds 0.1.The two main measured peaks occur at 1130 nm and 1645 nm, and they appear similar to the expected values, i.e. those visible in the emissivity spectrum of the optimised SE (red dashed line) at 1120 nm and 1570 nm.At 2000 nm, both (λ) and (λ) drop to values below 0.1 and remain so until about 8000 nm, where the normalised blackbody spectral energy density at 1000 °C is reduced to values lower than 0.1.The rapid transition from high to low emissivity values ensures high emitter efficiency throughout the considered bandgap energy range.To better illustrate the operation of the SE and the differences between the structures, in Fig. 3 we show the emitter efficiency of the optimised ( ) and as-deposited ( ) SEs at 1000 °C as a function of PV cell bandgap energies.The optimized structure shows high emitter efficiency in the whole bandgap energy range chosen for the optimization.The values of at the edges of the bandgap range (i.e., 0.55 and 0.72 eV) are 0.50 and 0.51, respectively, while the maximum efficiency value of 0.54, is reached at 0.66 eV.For the deposited sample, a small reduction in emitter efficiency is observed: in fact, the maximum value reached at 1000 °C is 0.50 at Eg = 0.63 eV, while at Eg = 0.55 and 0.72 eV reaches 0.47 and 0.43, respectively.Despite the slight reduction, the structure retains significant values of emitter efficiency throughout the bandgap energy range chosen for the optimization.

Conclusion
In this work, we propose a selective emitter (SE) to be easily fabricated via sputtering deposition of SiNx, SiO2, and TiO2 on a W substrate.A genetic algorithm optimises the SE efficiency for a chosen operating temperature of 1000 °C and an extended range of bandgap energies to ensure proper functioning with the most commonly used PV cells for SPTV systems.The deposited sample behaves very similarly to what was expected from simulations, reaching a maximum efficiency of 50% at 0.63 eV (at 1000 °C).In conclusion, the proposed structure demonstrated high efficiency and versatility, as it can efficiently operate in the energy bandgap range of 0.55 to 0.72 eV.

Fig. 2 .
Fig. 2. Left axis: Spectral emissivity of the as-deposited (solid black line) and optimised (dashed red line) selective emitter.Right axis: normalised spectral energy density of a blackbody at 1000 °C (grey area).

Fig. 3 .
Fig. 3. Emitter efficiency of the as-deposited (solid black line) and optimized (dashed red line) selective emitter at 1000 °C.