Microscopic Theory of Intersubband Thermophotovoltaics Mauro F. Pereira

1. Microscopic Theory of Intersubband Thermophotovoltaics Mauro F. Pereira Theory of Semiconductor Materials and Optics Materials and Engineering Research Institute Sheffield Hallam University S1 1WB Sheffield, United Kingdom Department of Physics Jazan University, Jazan, Saudi Arabia M.Pereira@shu.ac.uk

2. Outline • The Solar Paradox • Challenges for next generation solar cells • Nonequilibrium Green's Functions approach to absorption and gain • ISB Thermophotovoltaics • Summary

3. Solar Potential Average power > 100 W/m2 in populated areas

4. The Solar Paradox • Infinitely abundant energy • Fusion reactor • Solar constant: 1360 W/m2 (CN@6000K) • Surface incidence: ~ 1000 times the need of primary energy • Sub products at the origin of > 90% of commercial energy • A resource negligibly exploited for energy production

5. Conventional PVs - Problems to be Solved • Light with Energy below Eg will not be absorbed • Excess photon energy above Eg is lost in form of heat • Possible solutions : • multi-junction • Intermediate bands • hot carrier solar cells • TPVs

6. Third Generation PV Challenges

7. III-V Multi-Junction Solar Cell

8. Challenges - Multi-juntcion and IB Slide courtesy of S. Tomić Further microscopic analysis is required with full quantum transport and optics - NGF method is ideal!

9. Challenges - Hot Carrier PVs Energy Loss Mechanisms Solutions sought Nanostructures to reduce cooling rate due to phonon emission Energy selective contacts allowing carrier transmission at a single energy level - however difficult to achieve good selectivity and high current densities • Heat transfer to lattice (LO Phonon emission) • Heat leakage to contacts as they are extracted from the absorber • The NGF method used for complex QCL structures is ideal to address those difficulties

10. Thermophotovoltaics Convert IR radiation (heat) into electricity • technology very closely related to MJPV Many potential applications • Portable, low emission generators for military and civilian use • Generation from ‘waste’ industrial heat • Domestic boilers • Automotive industry Market size (2000 estimate) $85 – 265 million possible for non-auto

11. TPV Structures

12. Calculated Photocurrent • photon flux at 1 sun and 1.5 am • photocarrier generation at depth z • photocurrent Ref: V. Aroutiounian et , J. Appl. Phys. 89, 2268–2271 (2001).

13. Theoretical Approach to obtain the microscopic optical response - Nonequlibrium Keldysh Greens Functions (NGF) • Dyson equation solvers for realistic structures • Many Body + Nonequilibrium + Bandstructure engineering = +

14. Both coherent transport and scattering described on the same microscopic footing with Green's functions. • Relation to the (single particle) density matrix • The GF's contain more information than conventional semiconductor Bloch equations derived directly from

15. Other GF's complete the picture • Spectral function • Lehman representation for the retarded GF

16. electron-electron selfenergy GWapproximation • impurity scattering selfenergy second born approximation • interface roughness second born approximation

17. converged? no yes initial guess:Gret(ω,k,α,β)G<(ω,k,α,β) evaluate:Σret(ω,k,α,β)Σ<(ω,k,α,β) evaluate anew:Gret(ω,k,α,β)G<(ω,k,α,β) new guess:Gret(ω,k,α,β)G<(ω,k,α,β) evaluatecurrent densitiespopulations

18. Projected Greens Functions Equation - Intersubband

19. Correlation Contribution • Dynamically screened, nondiagonal and frequency dependence dephasing mechanisms are described.

20. Gain/Absorption Calculated through the Optical Susceptibility (Imaginary Part)

21. Conduction x Valence Subband Structures

22. Summary of the Numerical Method • Solve the 8 × 8 K∙P Hamiltonian for QWs • Solve the selfconsistent loop for the selfenergy and G< (occupation functions). • Solve the integral equation for the polarization by numerical matrix inversion • Calculate the absorption • Calculate the semiclassical photocurrent

23. ISB Thermophotovoltaics TE Mode Tsource = 1000 K (a) 5 nm QW (b) 10 nm QW solid: many body effects dashed: free carriers bottow and top curves in each panel: 1 and 3 × 1012 carriers/cm2 extra features on absorption due to a combination of nonparabolicity and many body effects M.F. Pereira, JOSAB 28, 2014 (2011)

24. ISB Thermophotovoltaics TM Mode - carriers at 300K Tsource = 1000 K (a) 5 nm QW (b) 10 nm QW solid: many body effects dashed: free carriers bottow and top curves in each panel: 1 and 3 × 1012 carriers/cm2 Strong redistribution of oscillator strength due to many body effects

25. Interplay of Irradiance and QW ISB Absorption

26. ISB Thermophotovoltaics TE Mode - carriers at 300 K Doping: 3 × 1012 carriers/cm2 (a) 5 nm QW (b) 10 nm QW solid: many body effects dashed: free carriers bottow and top curves in each panel: Tsource= 500 and 1000 K If the peak flux overlaps with certain spectral regions, the many body effects are highlighted

27. ISB Thermophotovoltaics TM Mode - carriers at 300 K Doping: 3 × 1012 carriers/cm2 (a) 5 nm QW (b) 10 nm QW solid: many body effects dashed: free carriers bottow and top curves in each panel: Tsource= 500 and 1000 K If the peak flux overlaps with certain spectral regions, the many body effects are highlighted

28. ISB Thermophotovoltaics TE vs TM (max) Mode in the far infrared - carriers at 300 K Doping: 3 × 1012 carriers/cm2 (a,c) 5 nm QW (b,d) 10 nm QW solid: many body effects dashed: free carriers (a,b) Tsource= 500 (c,d) Tsource= 1000 K There is a region in the far infrared where TE > TM even without considering projection losses on TM, which are unavoidable.

29. ISB Thermophotovoltaics • There is a region in far infrared where the TE mode that does not require prisms and couplers dominates even though the MIr dipole is much larger for TM. • Many-body corrections are important if high densities are reached - hot carrier devices??? • Full nonequilibrium required for hot carriers - forthcoming.