Sub-bandgap Radiation Research Articles

Spectral regulation in thermophotovoltaic devices

Thermophotovoltaic (TPV) device converts thermal radiation into electricity output through photovoltaic effect. High-efficiency TPV devices have extensive applications in grid-scale thermal storage, full-spectrum solar utilization, distributed thermal-electricity cogeneration, and waste heat recovery. The key to high-efficiency TPV devices lies in spectral regulation to achieve band-matching between thermal radiation of the emitters and electron transition of the photovoltaic cells. The latest advances in nanophotonics, materials science, and artificial intelligence have made milestone progress in spectral regulation and recording power conversion efficiency of up to 40% of TPV devices. Here we systematically review spectral regulation in TPV devices at the emitter end as well as the photovoltaic cell end. At the emitter end, spectral regulation is realized through thermal metamaterials and rare-earth intrinsic emitters to selectively enhance the in-band radiation and suppress the sub-bandgap radiation. At the photovoltaic cell end, spectral regulation mainly focuses on recycling the sub-bandgap thermal radiation through optical filters and back surface reflectors located at the front and back of the photovoltaic cells, respectively. We emphasize the light-matter interaction mechanisms and material systems of different spectral regulation strategies. We also discuss the spectral regulation strategies in near-field TPV devices. Finally, we look forward to potential development paths and prospects of spectral regulation to achieve scalable deployment of future TPV devices.

Numerical modeling and performance enhancement of micro combustor powered thermophotovoltaic systems using high contrast gratings

• Steady-state 3D modeling is done for parallel plate micro combustor TPV system. • Incorporated SiC porous foam enhanced power density by an order of magnitude. • HCGs are used to tailor emitted spectral radiation increased power density by 50%. • Reflector confine power to one side of micro combustor which doubled power density. Thermophotovoltaic (TPV) systems offer an efficient way to directly convert thermal energy into electricity using the radiation emitted from a high temperature source. Compared to the conventional photovoltaic (PV) systems, TPVs have additional elements such as filters to tailor and shape the radiated energy impinging on the PV cell to improve the conversion efficiency. High contrast grating (HCG) structures are integrated within the combustor walls which act as a selective filter. The filter plays a major role by suppressing the sub-band gap radiation and reflecting it back to the combustion source. Typically, amorphous silicon (a-Si) periodic gratings with quartz as a substrate material are optimized for gallium antimonide (GaSb) PV cells. In this work, we have carried out detailed Multiphysics simulations to study the performance of GaSb, Si and InGaAsSb PV cells when operating in a micro combustor emitting non uniform radiation source operating in a temperature range of 500–2000 K. Results show that the proposed TPV system can have a power density of 250 mW/cm 2 using GaSb PV cell. Further, significantly more photons can be converted to useful power with a higher cut-off wavelength PV cell such as InGaAsSb. Therefore, the power density can be as high as 410 mW/cm 2 . This study shows that the TPV systems employing low bandgap PV cells can have > 20 times higher power density compared to conventional silicon PV cells.

Thermophotovoltaic efficiency of 40%

Thermophotovoltaics (TPVs) convert predominantly infrared wavelength light to electricity via the photovoltaic effect, and can enable approaches to energy storage1,2 and conversion3–9 that use higher temperature heat sources than the turbines that are ubiquitous in electricity production today. Since the first demonstration of 29% efficient TPVs (Fig. 1a) using an integrated back surface reflector and a tungsten emitter at 2,000 °C (ref. 10), TPV fabrication and performance have improved11,12. However, despite predictions that TPV efficiencies can exceed 50% (refs. 11,13,14), the demonstrated efficiencies are still only as high as 32%, albeit at much lower temperatures below 1,300 °C (refs. 13–15). Here we report the fabrication and measurement of TPV cells with efficiencies of more than 40% and experimentally demonstrate the efficiency of high-bandgap tandem TPV cells. The TPV cells are two-junction devices comprising III–V materials with bandgaps between 1.0 and 1.4 eV that are optimized for emitter temperatures of 1,900–2,400 °C. The cells exploit the concept of band-edge spectral filtering to obtain high efficiency, using highly reflective back surface reflectors to reject unusable sub-bandgap radiation back to the emitter. A 1.4/1.2 eV device reached a maximum efficiency of (41.1 ± 1)% operating at a power density of 2.39 W cm–2 and an emitter temperature of 2,400 °C. A 1.2/1.0 eV device reached a maximum efficiency of (39.3 ± 1)% operating at a power density of 1.8 W cm–2 and an emitter temperature of 2,127 °C. These cells can be integrated into a TPV system for thermal energy grid storage to enable dispatchable renewable energy. This creates a pathway for thermal energy grid storage to reach sufficiently high efficiency and sufficiently low cost to enable decarbonization of the electricity grid.

Spectral filtering of sub-bandgap radiation using all-dielectric gratings for thermophotovoltaic applications

Filtering of the sub-band spectral radiation is an attractive technique to overcome the lower efficiencies of direct conversion thermophotovoltaic technology. The poor performance of these systems is due to the relatively small portion of the incident energy being above the bandgap of photovoltaic cell. To effectively filter the majority of the sub-bandgap radiation and re-employ it as regenerative heat, a viable solution is to design an efficient spectrally selective filter ideally matched to the photovoltaic cell’s bandgap. Here, we have explored a high contrast amorphous silicon grating on a quartz substrate. Quartz due to its inherent nature inhibits transmission of sub-band gap radiation in the infrared (IR) region (>4.75 μm), whereas gratings further filter radiation above 1.8 μm suitable for GaSb photovoltaic cell. The optimized filter is fabricated using direct write laser lithography, and optical characterization result shows that 72% of incident radiation in unconvertible region (>1.80 μm) is filtered and recycled. Further, the thermal characterization results of IR filter carried out using a ceramic heater has shown the drop in effective temperature from 1074.9 to 813.2 K in above bandgap region. This suppressed radiation has contributed to an absolute increase in source body temperature by 16.0 K resulting in increase in the above bandgap radiation available for thermophotovoltaic conversion. The proposed spectral filtering design can be tailored to solar cells of any bandgap and is scalable for employment in various thermophotovoltaic applications.

Thermodynamic performance of near-field electroluminescence and negative electroluminescent refrigeration systems

Electroluminescent (EL) and negative electroluminescent (NEL) devices are radiative thermoelectric energy converters that use electric power for refrigeration. For the EL system, we apply a forward bias to the emitter that we want to cool, whereas a reverse bias voltage is applied to the hot absorber for the NEL system. In this work, we derive the thermodynamic limits of the cooling power density and coefficient of performance (COP) of near-field EL and NEL refrigeration systems based on entropy analysis that considers near-field effects. We show numerically that operating the EL and NEL systems in the near-field regime could increase the cooling power density and the COP bounds to a certain extent. As the vacuum gap decrease from 1000 to 10 nm, the near-field effects improve the performance of the NEL system all the time, but the performance of the EL system increases to the optimal value and then decreases. In addition, the increase in temperature difference weakens the performance of both refrigeration systems greatly. Moreover, we also investigate the effects of the absence of sub-bandgap thermal radiation on the performance of the EL and NEL systems. Our work indicates significant opportunities for evaluating the performance of near-field radiative thermoelectric energy converters from the perspective of thermodynamic limits. Meanwhile, these results establish the targets for cooling power density and COP of the near-field EL and NEL systems.

Hybrid direct carbon fuel cell-thermoradiative systems for high-efficiency waste-heat recovery

Medium-grade waste heat released from direct carbon fuel cell contains enormous useful energy, which can be recycled for secondary power generation. However, conventional solid-state devices for harvesting moderate-grade waste heat, such as thermophotovoltaics and thermoelectric converters, are limited by low power generation and conversion efficiency. Here a hybrid system consisting of a direct carbon fuel cell and a Gallium antimonide thermoradiative device is proposed, where the effects of thermal coupling between the subsystems, over-potential losses in the fuel cell, non-radiative recombination and sub-bandgap radiation in the thermoradiative device, and various heat losses within the system are included. The thermoradiative device recycles the waste heat produced in the direct carbon fuel cell to generate additional electricity, thus enabling a significant improvement in power generation. The performance characteristics and the parametric design strategies of the hybrid system are investigated. The simulation results predict a maximum power density of 516 W·m−2 at the fuel cell temperature of 972 K, which exhibits a 1.48 times improvement compared with the standalone direct carbon fuel cell. This work reveals the potential of the direct carbon fuel cell-thermoradiative device hybrid system for waste heat recycling and opens up a new avenue to boost the energy conversion efficiency of carbon fuel cells.

Indium tin oxide and gold nanoparticle solar filters for concentrating photovoltaic thermal systems

The combination of concentrating solar power and photovoltaic technologies in hybrid photovoltaics thermal systems have the potential of increasing the overall efficiency of solar energy by utilizing non-Photovoltaic wavelengths as dispatchable thermal energy for use as industrial process heat and/or electricity generation using a thermal engine. One way of achieving this requires a thermally-stable spectral filter capable of effectively transmitting photovoltaic radiation while absorbing non-Photovoltaic radiation. Here, we utilize indium tin oxide and gold nanoparticles in a silane-based heat transfer fluid capable of withstanding temperatures up to 340 °C while maintaining spectral transparency at photovoltaic wavelengths. To promote compatibility with the heat transfer fluid and achieve greater thermal stability of the nanoparticles, surface modifications were performed for both the indium tin oxide and the gold nanoparticles. It was found that the greatest solution stability was achieved with the use of a (6–7% aminopropylmethylsiloxane)-dimethylsiloxane copolymer surface ligand. Interestingly, when heating the nanoparticles, a spectral blue-shift was observed in the localized surface plasmon resonance peaks of both nanoparticles along with an increase in absorptivity for indium tin oxide. Analysis of the filter performance revealed that a filter optimized for c-Si photovoltaic receiver transmits 73% of bandgap radiation and absorbs 78% of sub-bandgap radiation. The increasing solar absorption properties of the nanoparticle at higher temperatures helped the filter maintained an ideal performance up to 250 °C, even with lower concentrations of nanoparticles. At 300 °C, however, the filter performance reduced due to the disappearance of the gold peak, which is attributed to the inferring free tin ions.

One-Chip Near-Field Thermophotovoltaic Device Integrating a Thin-Film Thermal Emitter and Photovoltaic Cell.

Thermal radiation transfer between two objects separated by a subwavelength gap (near-field thermal radiation transfer) can be orders of magnitude larger than that in free space, which is attracting increasing attention with respect to both fundamental nanoscience and its potential for high-power-density and high-efficiency conversion of heat to electricity in thermophotovoltaic (TPV) systems. However, the realization of near-field thermal radiation transfer in TPV systems involves significant challenges because it requires a subwavelength gap and large temperature difference between the emitter and the PV cell while minimizing the heat transfer that does not contribute to the photocurrent generation. To overcome these challenges, here we demonstrate a one-chip near-field TPV device consisting of a thin-film Si emitter and InGaAs PV cell with an intermediate Si substrate, which enables the suppression of the heat transfer due to sub-bandgap radiation by free carriers and surface modes. Through the one-chip integration and thermal isolation using Si process technologies, we realize a deep subwavelength gap (<150 nm) between the emitter and the intermediate substrate without using any external positioners while maintaining a large temperature difference (>700 K). Compared to the equivalent device operating in the far-field regime, we achieve 10-fold enhancement of the photocurrent in the PV cell without degrading the open-circuit voltage and fill factor, demonstrating the potential of our one-chip device for the future applications of near-field thermal radiation transfer.

Thermoelectrics versus thermophotovoltaics: two approaches to convert heat fluxes into electricity

Direct conversion of heat fluxes into electricity is usually done by thermoelectric generators (TEGs). For hot-side temperatures above 1000 K, thermal radiation carries a high energy density, comparable with the energy density extracted from TEGs and therefore a direct conversion of thermal radiation into electricity, named thermophotovoltaics (TPV), would also be an option. This paper compares both methods with respect to efficiency and extractable power density. The physical limits are estimated under simplified but realistic boundary conditions. For TPV the radiative detailed balance limit under black body radiation, which was calculated for different hot-side temperatures from 310 K to 3000 K for an optimized bandgap of the applied material was used. But, since very narrow bandgaps leading to strong non-radiative recombination mechanisms, the bandgap was limited to . The effect of suppressing sub-bandgap radiation as well as an enhanced radiation density in the nearfield (near-field TPV) were also included. The TEG efficiency and power density was calculated under thermal matching conditions with a heat transfer coefficient of and an average device . The results are compared with experimental data for TPV and TEGs from literature. It is shown, that up to 600 K hot-side temperature TEGs are superior to TPV, due to a significant higher power density. Above 1000 K TPV profits from higher efficiency by a similar power density. But above 2000 K TPV suffers from cell heating. The range 600 K to 1000 K is currently captured by high temperature thermoelectrics, but near-field TPV (NF-TPV) has good chances to compete with TEGs in this temperature range in the future.

Thin-Film Architectures with High Spectral Selectivity for Thermophotovoltaic Cells

Thermophotovoltaic (TPV) systems are a promising technology for distributed conversion of high-temperature heat to electricity. To achieve high conversion efficiency, the transport of sub-bandgap radiation between the thermal emitter and PV cell should be suppressed. This can be achieved by recycling sub-bandgap radiation back to the emitter using a spectrally selective cell. However, conventional TPV cells exhibit limited sub-bandgap reflectance. Here we demonstrate thin-film In0.53Ga0.47As-based structures with high spectral selectivity, including record-high average sub-bandgap reflectance (96%). Selectivity is enabled by short optical paths through a high-quality material fabricated using epitaxial lift-off, high-reflectance back surfaces, and optimized interference. In addition, we use a parallel-plate TPV model to evaluate the impact of specific structural features on performance and to optimize the cell architecture. We show that a dielectric spacer between InGaAs and the Au back surface is an impo...

Spectrally Selective Mirrors with Combined Optical and Thermal Benefit for Photovoltaic Module Thermal Management

Waste heat generated during daytime operation of a solar module will raise its temperature and reduce cell efficiency. In addition to thermalization and carrier recombination, one major source of excess heat in modules is the parasitic absorption of light with sub-bandgap energy. Parasitic absorption can be prevented if sub-bandgap radiation is reflected away from the module. We report on the design considerations and projected changes to module energy yield for photonic reflectors capable of reflecting a portion of sub-bandgap radiation while maintaining or improving transmission of light with energy greater than the semiconductor bandgap. Using a previously developed, self-consistent opto-electro-thermal finite-element simulation, we calculate the total additional energy generated by a module, including various photonic reflectors, and decompose these benefits into thermal and optical effects. We show that the greatest total energy yield improvement comes from photonic mirrors designed for the outside o...

Role of spectral non-idealities in the design of solar thermophotovoltaics.

To bridge the gap between theoretically predicted and experimentally demonstrated efficiencies of solar thermophotovoltaics (STPVs), we consider the impact of spectral non-idealities on the efficiency and the optimal design of STPVs over a range of PV bandgaps (0.45-0.80 eV) and optical concentrations (1-3,000x). On the emitter side, we show that suppressing or recycling sub-bandgap radiation is critical. On the absorber side, the relative importance of high solar absorptance versus low thermal emittance depends on the energy balance. Both results are well-described using dimensionless parameters weighting the relative power density above and below the cutoff wavelength. This framework can be used as a guide for materials selection and targeted spectral engineering in STPVs.

Performance of Near-Field Thermophotovoltaic Cells Enhanced With a Backside Reflector

Thermophotovoltaic (TPV) systems are very promising for waste heat recovery. This work analyzes the performance of a near-field TPV device with a gold reflecting layer on the backside of the cell. The radiative transfer from a tungsten radiator, at a temperature ranging from 1250 K to 2000 K, to an In0.18Ga0.82Sb TPV cell at 300 K is calculated using fluctuational electrodynamics. The current generation by the absorbed photon energy is modeled by the minority carrier diffusion equations considering recombination. The energy conversion efficiency of the cell is determined from the generated electrical power and the net absorbed radiant power per unit area. A parametric study of the cell efficiency considering the gap spacing and other parameters is conducted. For an emitter at temperature 1250 K, the efficiency enhancement by adding a mirror, which reduces the sub-bandgap radiation, is shown to be as much as 35% relative to a semi-infinite TPV cell. In addition, the potential for further improvement by reducing surface recombination velocity from that of a perfect ohmic contact is examined. The cell performance is shown to increase with decreasing gap spacing below a critical surface recombination velocity.

Time-resolved surface photovoltage measurements atn-type photovoltaic surfaces: Si(111) and ZnO(101¯0)

The time-resolved change in the surface potential upon photoexcitation has been measured in two $n$-type photovoltaics, Si (111) 7\ifmmode\times\else\texttimes\fi{}7 and ZnO ($10\overline{1}0$), using two different laser pump-synchrotron x-ray probe methodologies. Taken together, these experiments allow the dynamics of the surface photovoltage (SPV) to be monitored over timescales of subnanoseconds to milliseconds. The timescales for the photoinduced change in the SPV are dramatically different in the two samples, with measured SPV decay time constants of 6.6 \ensuremath{\mu}s for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7\ifmmode\times\else\texttimes\fi{}7 surface are well modeled by a self-decelerating relaxation model involving the recombination of carriers by thermionic emission across the surface depletion layer on nanosecond timescales. In the case of ZnO ($10\overline{1}0$), a persistent photoconductivity (PPC) is observed, which is influenced by oxygen annealing conditions during sample preparation. Persistent photoconductivity is also observed when the surface is illuminated with subband-gap (405 nm) radiation, revealing that defect states approximately 340 meV above the valence band edge are directly associated with the PPC. We demonstrate that, under the conditions of our experiment, PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO.

Identifying different mechanisms of circular photogalvanic effect in GaAs/Al0.3Ga0.7As two dimensional electron gas by photo-modulation technique

We investigate the circular photogalvanic effect (CPGE) excited by sub-bandgap radiation in a GaAs/Al0.3Ga0.7As two dimensional electron gas and tune its amplitude by synchronously imposing an above-bandgap unpolarized light at normal incidence. With this photo-modulation technique, we identify two microscopic mechanisms of CPGE according to the dramatic change of apparent Rashba and Dresselhaus effects. We suggest the optical transitions to be Franz-Keldysh and intraband regime, respectively. Both regimes coexist in conventional CPGE and the intraband regime dominates at sufficient modulation power.

Bias Dependence of Sub-Bandgap Light Detection for Core–Shell Silicon Nanowires

We experimentally demonstrate a vertically arrayed silicon nanowire-based device that exhibits voltage dependence of photoresponse to infrared sub-bandgap optical radiation. The device is fabricated using a proximity solid-state phosphorus diffusion method to convert the surface areas of highly boron-doped silicon nanowires into n-type, thus forming a radial core-shell p-n junction structure. Prominent photoresponse from such core-shell Si nanowires is observed under sub-bandgap illumination at 1310 nm. The strong bias dependence of the photoresponse and other device characteristics indicates that the sub-bandgap absorption is attributed to the intrinsic properties of core-shell Si nanowires rather than the surface states. The attractive characteristics are based on three physical mechanisms: the Franz-Keldysh effect, quasi-quantum confinement effect, and the impurity-state assisted photon absorption. The first two effects enhance carrier tunneling probability, rendering a stronger wave function overlap to facilitate sub-bandgap absorption. The last effect relaxes the k-selection rule by involving the localized impurity states, thus removing the limit imposed by the indirect bandgap nature of Si. The presented device uses single-crystal silicon and holds promise of fabricating nanophotonic systems in a fully complementary metal-oxide-semiconductor (CMOS) compatible process. The concept and approach can be applied to silicon and other materials to significantly extend the operable wavelength regime beyond the constraint of energy bandgap.

Sensitive mid-infrared detection in wide-bandgap semiconductors using extreme non-degenerate two-photon absorption

Identifying strong and fast nonlinearities for today’s photonic applications is an ongoing effort 1 . Materials 2–5 and devices 6–9 are typically sought to achieve increasing nonlinear interactions. We report large enhancement of two-photon absorption through intrinsic resonances using extremely non-degenerate photon pairs. We experimentally demonstrate two-photon absorption enhancements by factors of 100–1,000 over degenerate two-photon absorption in direct-bandgap semiconductors. This enables gated detection of sub-bandgap and sub-100 pJ mid-infrared radiation using large-bandgap detectors at room temperature. Detection characteristics are comparable in performance to liquid-nitrogen-cooled HgCdTe (MCT) detectors. The temporal resolution of this gated detection by two-photon absorption is determined by the gating pulse duration.

Enhanced near-infrared response of a-Si:H solar cells with β-NaYF 4:Yb 3+ (18%), Er 3+ (2%) upconversion phosphors

A near-infrared to visible upconversion phosphor (β-NaYF 4:Yb 3+ (18%), Er 3+ (2%)) has been applied at the back of a thin film hydrogenated amorphous silicon (a-Si:H) solar cell in combination with a white back reflector to investigate its response to sub-bandgap infrared irradiation. Current–voltage measurements were performed on the solar cells. A maximum current enhancement of 6.2 μA was measured on illumination with a 980 nm diode laser at 28 mW. This corresponds to an external quantum efficiency (EQE) of 0.03% of the solar cell. A small part, 0.01%, was due to the direct absorption of sub-bandgap radiation but the larger part originates from upconversion. These experiments constitute a proof-of-principle for the utilization of photon upconversion in thin film solar cells. A close match between the non-linear behavior of the upconversion material and the EQE was found by varying the intensity of the laser light.

Emission of Negative Potassium Ions from Single Crystal Potassium Bromide during Exposure to 248-nm Excimer Laser Radiation

Many wide bandgap materials yield charged and neutral emissions when exposed to sub-bandgap laser radiation at power densities below the threshold for optical breakdown and plume formation. In previous work, we reported the observation of negative alkali ions from several alkali halides under comparable conditions. Within our detection sensitivity, we observed no negative halogen ion emission, in spite of the high electron affinities of the halogens. Significantly, the positive and negative alkali ions showed a high degree of spatial and temporal overlap. Our conclusion, based on a detailed study of KCl emissions, was that K− is formed by the sequential attachment of two electrons to K+. Here we provide further evidence for this mechanism based on a study of KBr.

CMOS-Compatible All-Si High-Speed Waveguide Photodiodes With High Responsivity in Near-Infrared Communication Band

Submicrometer silicon photodiode waveguides, fabricated on silicon-on-insulator substrates, have photoresponse from <1270 to 1740 nm (0.8 AW <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at 1550 nm) and a 3-dB bandwidth of 10 to 20 GHz. The p-i-n photodiode waveguide consists of an intrinsic waveguide 500times250 nm where the optical mode is confined and two thin, 50-nm-thick, doped Si wings that extend 5 mum out from either side of the waveguide. The Si wings, which are doped one p-type and the other n-type, make electric contact to the waveguide with minimal effect on the optical mode. The edges of the wings are metalized to increase electrical conductivity. Ion implantation of Si <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> 1times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> at 190 keV into the waveguide increases the optical absorption from 2-3 dBmiddotcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> to 200-100 dBmiddotcm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and causes the generation of a photocurrent when the waveguide is illuminated with subbandgap radiation. The diodes are not damaged by annealing to 450 degC for 15 s or 300 degC for 15 min. The photoresponse and thermal stability is believed due to an oxygen stabilized divacancy complex formed during ion implantation