[Paper Review] Near-Field Thermophotovoltaic Energy Conversion with Thin-film Tandem Cells

Payam Sabbaghi, Qing Ni+1 more|arXiv (Cornell University)|Jul 30, 2020

Thermal Radiation and Cooling Technologies1 references4 citations

TL;DR

This study proposes a near-field thermophotovoltaic (TPV) system using thin-film tandem cells of p-doped GaSb and n-doped InAs on a gold backreflector to enhance energy conversion efficiency. By leveraging fluctuational electrodynamics and multilayer dyadic Green's functions, the system achieves 41% efficiency and 468.8 kW/m² power output at a 50 nm vacuum gap, outperforming single-junction cells by up to 87% in power and 10% in efficiency.

ABSTRACT

The performance of a near-field thermophotovoltaic system with a tandem-cell structure composed of thin-film p-doped GaSb and n-doped InAs sub-cells on a gold backside reflector is theoretically investigated. The temperatures of the Ga-doped ZnO emitter and the tandem cells are set as 1800 K and 300 K, while the thicknesses of GaSb and InAs sub-cells are considered as 1.5um and 0.5um, respectively. Fluctuational electrodynamics along with the multilayer dyadic Green's function is used to study near-field radiative heat transfer with the consideration of photon chemical potential, whereas radiative recombination and nonradiative Auger recombination are taken into account for evaluating the electrical performance of the tandem cells. At a vacuum gap of 50 nm, it is found that the tandem cells with independent charge collections achieve electrical power output 468.8 kW/m2 at a conversion efficiency of 41%, generating relatively 87% (or 21%) more power with about absolute 5% (or 10%) higher efficiency than the single GaSb (or InAs) cell of the same 2-um thickness. The physical mechanism of near-field spectral heat transfer is elucidated with energy transmission coefficient, while the current-voltage characteristics of sub-cells are discussed in detail. This work will pave the way to enhance near-field thermophotovoltaic energy conversion performance with tandem or multi-junction cells.

Motivation & Objective

• To enhance near-field thermophotovoltaic (TPV) energy conversion efficiency beyond single-junction cells.
• To investigate the performance of thin-film tandem cells composed of GaSb and InAs sub-cells under near-field radiative transfer.
• To analyze the impact of photon chemical potential and recombination mechanisms on electrical output in tandem TPV systems.
• To optimize tandem cell design for maximum power and efficiency at high emitter temperatures (1800 K) and small vacuum gaps (50 nm).

Proposed method

• Theoretical modeling using fluctuational electrodynamics to calculate near-field radiative heat transfer across a 50 nm vacuum gap.
• Employment of multilayer dyadic Green's functions to model electromagnetic fields and energy transmission in the tandem cell structure.
• Incorporation of photon chemical potential effects in the radiative transfer analysis to account for non-equilibrium conditions.
• Modeling of electrical performance using radiative and Auger nonradiative recombination rates in GaSb and InAs sub-cells.
• Independent charge collection in sub-cells to maximize current and voltage output.
• Comparison of tandem cell performance with single GaSb and InAs cells of equivalent total thickness (2 µm).

Experimental results

Research questions

• RQ1 What is the maximum power output and efficiency achievable in a near-field TPV system using GaSb/InAs tandem cells at a 50 nm gap?
• RQ2 How does the tandem cell architecture improve performance compared to single-junction GaSb or InAs cells under the same conditions?
• RQ3 What role does photon chemical potential play in near-field radiative heat transfer in the TPV system?
• RQ4 How do radiative and Auger recombination mechanisms affect the current-voltage characteristics of the tandem sub-cells?
• RQ5 What is the physical mechanism behind the enhanced spectral energy transfer in the near-field regime?

Key findings

• At a 50 nm vacuum gap, the tandem cell achieves a power output of 468.8 kW/m² with a conversion efficiency of 41%.
• The tandem cell generates 87% more electrical power than a single GaSb cell of the same total thickness.
• The tandem cell achieves 10% higher efficiency than a single InAs cell, despite the latter having a smaller bandgap.
• The energy transmission coefficient analysis reveals strong spectral selectivity in near-field heat transfer, enabling efficient coupling to the sub-cell bandgaps.
• The current-voltage characteristics show that independent charge collection in sub-cells significantly enhances output by minimizing current mismatch.
• The study confirms that near-field effects dominate at sub-100 nm gaps, enabling substantial performance gains over far-field TPV systems.