# Near-perfect photon utilization in an air-bridge thermophotovoltaic cell

## Abstract

Thermophotovoltaic cells are similar to solar cells, but instead of converting solar radiation to electricity, they are designed to utilize locally radiated heat. Development of high-efficiency thermophotovoltaic cells has the potential to enable widespread applications in grid-scale thermal energy storage1,2, direct solar energy conversion3,4,5,6,7,8, distributed co-generation9,10,11 and waste heat scavenging12. To reach high efficiencies, thermophotovoltaic cells must utilize the broad spectrum of a radiative thermal source. However, most thermal radiation is in a low-energy wavelength range that cannot be used to excite electronic transitions and generate electricity. One promising way to overcome this challenge is to have low-energy photons reflected and re-absorbed by the thermal emitter, where their energy can have another chance at contributing towards photogeneration in the cell. However, current methods for photon recuperation are limited by insufficient bandwidth or parasitic absorption, resulting in large efficiency losses relative to theoretical limits. Here we demonstrate near-perfect reflection of low-energy photons by embedding a layer of air (an air bridge) within a thin-film In0.53Ga0.47As cell. This result represents a fourfold reduction in parasitic absorption relative to existing thermophotovoltaic cells. The resulting gain in absolute efficiency exceeds 6 per cent, leading to a very high power conversion efficiency of more than 30 per cent, as measured with an approximately 1,455-kelvin silicon carbide emitter. As the out-of-band reflectance approaches unity, the thermophotovoltaic efficiency becomes nearly insensitive to increasing cell bandgap or decreasing emitter temperature. Accessing this regime may unlock a range of possible materials and heat sources that were previously inaccessible to thermophotovoltaic energy conversion.

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## Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

## References

1. 1.

Amy, C., Seyf, H. R., Steiner, M. A., Friedman, D. J. & Henry, A. Thermal energy grid storage using multi-junction photovoltaics. Energy Environ. Sci. 12, 334–343 (2019).

2. 2.

Datas, A., Ramos, A., Martí, A., del Cañizo, C. & Luque, A. Ultra high temperature latent heat energy storage and thermophotovoltaic energy conversion. Energy 107, 542–549 (2016).

3. 3.

Lenert, A. et al. A nanophotonic solar thermophotovoltaic device. Nat. Nanotechnol. 9, 126–130 (2014).

4. 4.

Bierman, D. M. et al. Enhanced photovoltaic energy conversion using thermally based spectral shaping. Nat. Energy 1, 16068 (2016).

5. 5.

Ungaro, C., Gray, S. K. & Gupta, M. C. Solar thermophotovoltaic system using nanostructures. Opt. Express 23, A1149–A1156 (2015).

6. 6.

Rephaeli, E. & Fan, S. Absorber and emitter for solar thermo-photovoltaic systems to achieve efficiency exceeding the Shockley-Queisser limit. Opt. Express 17, 15145–15159 (2009).

7. 7.

Harder, N.-P. & Würfel, P. Theoretical limits of thermophotovoltaic solar energy conversion. Semicond. Sci. Technol. 18, S151–S157 (2003).

8. 8.

Seyf, H. R. & Henry, A. Thermophotovoltaics: a potential pathway to high efficiency concentrated solar power. Energy Environ. Sci. 9, 2654–2665 (2016).

9. 9.

Fraas, L. M., Avery, J. E. & Huang, H. X. Thermophotovoltaic furnace–generator for the home using low bandgap GaSb cells. Semicond. Sci. Technol. 18, S247–S253 (2003).

10. 10.

Bianchi, M., Ferrari, C., Melino, F. & Peretto, A. Feasibility study of a thermo-photo-voltaic system for CHP application in residential buildings. Appl. Energy 97, 704–713 (2012).

11. 11.

Durisch, W. & Bitnar, B. Novel thin film thermophotovoltaic system. Sol. Energy Mater. Sol. Cells 94, 960–965 (2010).

12. 12.

Licht, A., Pfiester, N., DeMeo, D., Chivers, J. & Vandervelde, T. E. A review of advances in thermophotovoltaics for power generation and waste heat harvesting. MRS Adv. 4, 2271–2282 (2019).

13. 13.

Zhou, Z., Sakr, E., Sun, Y. & Bermel, P. Solar thermophotovoltaics: reshaping the solar spectrum. Nanophotonics 5, 1–21 (2016).

14. 14.

Ferrari, C., Melino, F., Pinelli, M., Spina, P. R. & Venturini, M. Overview and Status of Thermophotovoltaic Systems. Energy Proc. 45, 160–169 (2014).

15. 15.

Daneshvar, H., Prinja, R. & Kherani, N. P. Thermophotovoltaics: fundamentals, challenges and prospects. Appl. Energy 159, 560–575 (2015).

16. 16.

Sakakibara, R. et al. Practical emitters for thermophotovoltaics: a review. J. Photon. Energy 9, 032713 (2019).

17. 17.

Bauer, T. Thermophotovoltaics. Green Energy and Technology Vol. 7 (Springer, 2011).

18. 18.

Werth, J. J. Thermo-photovoltaic converter with radiant energy reflective means. US patent 3,331,707 (1963).

19. 19.

Swanson, R. M. Recent developments in thermophotovoltaic conversion. In 1980 International Electron Devices Meeting 186–189 (IRE, 1980).

20. 20.

Omair, Z. et al. Ultraefficient thermophotovoltaic power conversion by band-edge spectral filtering. Proc. Natl Acad. Sci. USA 116, 15356–15361 (2019).

21. 21.

Wernsman, B. et al. Greater than 20% radiant heat conversion efficiency of a thermophotovoltaic radiator/module system using reflective spectral control. IEEE Trans. Electron Dev. 51, 512–515 (2004).

22. 22.

Fourspring, P. M., Depoy, D. M., Rahmlow, T. D., Lazo-wasem, J. E. & Gratrix, E. J. Optical coatings for thermophotovoltaic spectral control. Appl. Opt. 45, 1356–1358 (2006).

23. 23.

Woolf, D. N. et al. High-efficiency thermophotovoltaic energy conversion enabled by a metamaterial selective emitter. Optica 5, 213–218 (2018).

24. 24.

Shemelya, C., Demeo, D. F. & Vandervelde, T. E. Two dimensional metallic photonic crystals for light trapping and anti-reflective coatings in thermophotovoltaic applications. Appl. Phys. Lett. 104, 021115 (2014).

25. 25.

Rinnerbauer, V. et al. High-temperature stability and selective thermal emission of polycrystalline tantalum photonic crystals. Opt. Express 21, 11482–11491 (2013).

26. 26.

Arpin, K. A. et al. Three-dimensional self-assembled photonic crystals with high temperature stability for thermal emission modification. Nat. Commun. 4, 2630 (2013).

27. 27.

Wang, Y. et al. Hybrid solar absorber-emitter by coherence-enhanced absorption for improved solar thermophotovoltaic conversion. Adv. Opt. Mater. 6, 1800813 (2018).

28. 28.

Nakagawa, N., Ohtsubo, H., Waku, Y. & Yugami, H. Thermal emission properties of Al2O3/Er3 Al5O12 eutectic ceramics. J. Eur. Ceram. Soc. 25, 1285–1291 (2005).

29. 29.

Ferguson, L. G. & Dogan, F. A highly efficient NiO-doped MgO matched emitter for thermophotovoltaic energy conversion. Mater. Sci. Eng. B 83, 35–41 (2001).

30. 30.

Burger, T., Sempere, C., Roy-Layinde, B. & Lenert, A. Present efficiencies and future opportunities in thermophotovoltaics. Joule https://doi.org/10.1016/j.joule.2020.06.021 (2020).

31. 31.

Burger, T., Fan, D., Lee, K., Forrest, S. R. & Lenert, A. Thin-film architectures with high spectral selectivity for thermophotovoltaic cells. ACS Photon. 5, 2748–2754 (2018).

32. 32.

Mahorter, R. G., Wernsman, B., Thomas, R. M. & Siergiej, R. R. Thermophotovoltaic system testing. Semicond. Sci. Technol. 18, S232 (2003).

33. 33.

Green, M. A. et al. Solar cell efficiency tables (Version 55). Prog. Photovolt. Res. Appl. 28, 3–15 (2020).

34. 34.

Crowley, C. J. Thermophotovoltaic converter performance for radioisotope power systems. AIP Conf. Proc. 746, 601–614 (AIP, 2005).

35. 35.

Raman, V. K., Burger, T. & Lenert, A. Design of thermophotovoltaics for tolerance of parasitic absorption. Opt. Express 27, 31757–31772 (2019).

36. 36.

Peumans, P., Yakimov, A. & Forrest, S. R. Small molecular weight organic thin-film photodetectors and solar cells. J. Appl. Phys. 93, 3693 (2003).

37. 37.

Lee, K., Shiu, K.-T., Zimmerman, J. D., Renshaw, C. K. & Forrest, S. R. Multiple growths of epitaxial lift-off solar cells from a single InP substrate. Appl. Phys. Lett. 97, 101107 (2010).

38. 38.

Chancerel, F. et al. Epitaxial lift-off of InGaAs solar cells from InP substrate using a strained AlAs/InAlAs superlattice as a novel sacrificial layer. Sol. Energy Mater. Sol. Cells 195, 204–212 (2019).

39. 39.

Chen, K., Zhao, B. & Fan, S. MESH: A free electromagnetic solver for far-field and near-field radiative heat transfer for layered periodic structures. Comput. Phys. Commun. 231, 163–172 (2018).

40. 40.

Cuevas, J. C. & García-Vidal, F. J. Radiative heat transfer. ACS Photon. 5, 3896–3915 (2018).

41. 41.

Forrest, S. R., Leheny, R. F., Nahory, R. E. & Pollack, M. A. In0.53Ga0.47As photodiodes with dark current limited by generation-recombination and tunneling. Appl. Phys. Lett. 37, 322–325 (1980).

42. 42.

Riben, A. R. & Feucht, D. L. Electrical transport in nGe-pGaAs heterojunctions. Int. J. Electron. 20, 583–599 (1966).

43. 43.

Dupré, O., Vaillon, R. & Green, M. A. Thermal Behavior of Photovoltaic Devices (Springer, 2017).

## Acknowledgements

We thank P. Herrera-Fierro for assistance with device characterization, and P. Reddy for discussions. This material is based upon work supported by the Army Research Office (ARO) under award number W911NF-19-1-0279, and the National Science Foundation (NSF) Graduate Research Fellowship under grant number NSF DGE 1256260.

## Author information

Authors

### Contributions

D.F. and T.B. planned the experiments, built the experimental setup, and conducted the experiments and simulations. S.M. and B.L. conducted the simulations. S.R.F. and A.L. planned and supervised the project and analysed the data. All authors contributed to the preparation of the manuscript and analysed the data.

### Corresponding authors

Correspondence to Andrej Lenert or Stephen R. Forrest.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 Au BSR and air-bridge cell structures and efficiency estimation.

a, Schematic of a conventional thin-film TPV cell with Au reflector. The top and bottom InGaAs contact layers are left unetched in the Au BSR cell to reduce the contact resistance. b, Schematic of a thin-film TPV cell with air-bridge reflector. The air cavity thickness in the air-bridge TPV cell is 600 nm. OOB reflectance is affected by the air cavity thickness and the thickness optimization is provided in Extended Data Table 1. c, Hypothetical Au BSR cell structure with no top and bottom InGaAs contact layers. d, Simulated PCE of the hypothetical Au BSR cell in c and the air-bridge cell in b.

### Extended Data Fig. 2 Air-bridge cell profilometry.

a, b, Schematic of an air-bridge TPV cell after the substrate removal and before the top contact grid lines patterning (a) and the top surface profilometry measurement of the cell in a (b). The designed air cavity thickness is 700 nm. TPV epitaxial layers bow slightly downward by about 100 nm (~0.1% of the lateral span of each air cavity), resulting in an actual air cavity thickness of ~600 nm in the air-bridge cell.

### Extended Data Fig. 3 Absorption and EQE spectra.

a, Simulated (orange) and measured (blue) absorption spectra of the Au BSR cell using FTIR spectroscopy. The optical cavity formed by the Au reflector and the TPV thin films leads to increased absorption by the Au reflector by creating several interference peaks. b, Simulated (orange) and measured (blue) absorption spectra of the air-bridge TPV (structure in Extended Data Fig. 1) using FTIR. The air-bridge TPV features a lossless reflective semiconductor–air interface, effectively eliminating most parasitic OOB absorption. c, Simulated absorption spectrum using transfer matrix methods (integrating incidence angle from 0° to 30°) and the measured EQE of the air-bridge cell at a 15° angle of incidence. Photons with E > 1.35 eV are partially absorbed in the 200-nm-thick front InP window layer (bandgap energy Eg = 1.35 eV) and do not contribute to current in the TPV active material (InGaAs), leading to the discrepancy at high photon energies.

### Extended Data Fig. 4 Dispersion calculation.

a, b, Simulated energy transmission coefficient for s-polarized (τs) and p-polarized (τp) light for the conventional Au BSR cell (a) and the air-bridge cell (b). The summed transmission probability (τs + τp) is plotted for in-plane wavevectors (k) normalized by free-space wavevectors (k0) at various photon energies ($$\hbar$$ω). Here, the solid, white line represents the free-space light line, and the dashed, white line represents the light-line in the medium. The red, dashed line (a) represents the analytical solution for the surface plasmon polariton (SPP) at the Au back surface.

### Extended Data Fig. 5 Characterization setups.

a, b, Schematic of the TPV efficiency measurement setup using a SiC globar emitter (a) and a true blackbody emitter (b). c, Schematic of the emitter emissivity calibration setup. d, Calibrated SiC globar emissivity at 1,455 K and 1,297 K, indicating that the emissivity is stable within the measured temperature range.

### Extended Data Fig. 6 Device current density–voltage characteristics.

a, Current density–voltage characteristics of the air-bridge TPV measured under 1,455-K SiC globar illumination for various Jsc values. Detailed parameters obtained from the curves are presented in Extended Data Table 2. b, Current density–voltage characteristics of the air-bridge TPV in the dark. The reverse bias current is dominated by tunnelling from −3 V to −1 V, and by shunt resistance, and by generation and recombination of electron–hole pairs from −1 V to 0 V. The forward bias current is dominated by generation and recombination from 0 V to 0.2 V, by diffusion current from 0.2 V to 0.5 V, and by series resistance above 0.5 V. See Methods section ‘Current density–voltage characterization and diode equation fitting’.

### Extended Data Fig. 7 PCE loss analysis.

PCE versus source temperature T. Red stars represent the measured PCE of the air-bridge TPV. a, Simulated PCE (black) with VF = 0.134, Ain = 0.61, Rout = 0.99 and Rs = 26 mΩ cm2. Other curves show the change in PCE when varying one of the above parameters. When T < 1,200 K, PCE increases with T, while improving Rout provides the highest improvements to PCE. At T > 1,500 K, PCE is limited by Rs. Without improving Rs, changes that increase the current will decrease PCE. b, Simulated PCE with Rs = 1 mΩ cm2. Rout benefits PCE the most at T < 1,200 K, while VF dominates at T > 1,200 K. Thus, positioning the cell closer to the emitter is important at high T. c, Simulated PCE curves with Rs = 1 mΩ cm2 and VF = 1. Still, Rout benefits PCE the most at low T, while Rout and Ain are equally important when T is high. Further enhancement of PCE requires improving the TPV material quality by decreasing the diffusion saturation current, generation and recombination lifetime, shunt resistance and so on.

### Extended Data Fig. 8 PCE at VF = 1.

a, PCE and Jsc versus emitter temperature given VF = 1 and globar emission spectrum. b, PCE and Jsc versus emitter emissivity given VF = 1 and emitter temperature of 1,455 K.

### Extended Data Fig. 9 Concentric cylindrical TPV configuration.

a, Simulated PCE and Jsc versus din/dout in a concentric emitter-cell configuration consisting of the air-bridge cell and a 1,455-K SiC globar emitter. din/dout refers to the ratio of the inner diameter to the outer diameter in an infinitely long cylindrical TPV configuration. b, Simulated PCE versus radiation sink fraction for three emitter-cell TPV pairs. The dotted line represents a selective emitter (εe,in = 0.9, εe,out = 0.1) with a non-selective cell (εc = 1, Eg = 0.74 eV, with electronic properties matching those of the air-bridge cell). The dashed line represents a non-selective emitter (εe = 1) with the air-bridge cell, and solid line: a selective emitter (0.9/0.1) with the air-bridge cell. The temperature of the emitter is 1,455 K in all cases.

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Fan, D., Burger, T., McSherry, S. et al. Near-perfect photon utilization in an air-bridge thermophotovoltaic cell. Nature 586, 237–241 (2020). https://doi.org/10.1038/s41586-020-2717-7

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