TPV Cell Air-Bridge Structure for Out-of-Band Photon Reflection
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Thermophotovoltaic (TPV) cells face significant efficiency losses due to out-of-band (OOB) photon reflectance below 95%, leading to approximately 10% absolute efficiency reduction, as conventional reflectors and filters struggle to achieve unity reflectance, resulting in spectral inefficiencies and parasitic absorption.
Innovation Solution
The integration of air-bridges between thin-film active layers and reflectors in TPV cells, which includes a spacer layer with cavities filled with air or semiconductor materials, enhances OOB reflectance to nearly 99%, minimizing absorption losses and maximizing refractive index mismatch, thereby improving power conversion efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional reflectors and filters are used to achieve high OOB reflectance, then spectral control is improved, but reflectance remains below 95% leading to efficiency losses
Solution Approach 1:
An air-bridge layer is introduced as an intermediary between the TPV cell and the back-surface reflector. This air-bridge consists of a spacer layer with low-refractive-index material (such as air, porous material, or low-index dielectric) that acts as an optical intermediary to enhance OOB photon reflection. The air-bridge creates a refractive index mismatch that prevents OOB photons from entering the cell, thereby reducing absorption losses and achieving reflectance exceeding 95%.
2Loss of energy
If selective reflectors with high OOB reflectance are used, then spectral efficiency is improved, but manufacturing complexity increases
Solution Approach 1:
The air-bridge structure is implemented locally at the interface between the TPV cell and the back-surface reflector, rather than requiring complex modifications throughout the entire optical system. The spacer layer is deposited only in specific regions to create the air-bridge, allowing selective enhancement of OOB reflectance where needed while maintaining simplicity in other parts of the device.
Solution Approach 2:
The invention changes the refractive index parameter at the cell-reflector interface by introducing a low-refractive-index air-bridge layer. This parameter change (from high-index semiconductor to low-index air/porous material) creates the optical effect needed for enhanced OOB reflection, achieving high spectral efficiency through a simple parameter modification rather than complex structural design.
3Loss of energy
If metallic back-surface reflectors are used, then OOB reflectance is improved, but parasitic absorption occurs
Solution Approach 1:
The air-bridge layer serves as an optical intermediary that prevents direct contact between the TPV cell and the metallic reflector. This intermediary layer with low refractive index (air, porous material, or low-index dielectric) eliminates the parasitic absorption that would occur in metallic reflectors while maintaining high OOB reflectance through refractive index mismatch.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach increases TPV power conversion efficiency beyond 31% by reducing OOB absorption losses by over four times, making low-cost semiconductors like silicon viable for thermophotovoltaic applications while maintaining high spectral efficiency and reducing sensitivity to bandgap and emitter temperature.
Implementation Method 1
maximizing refractive index mismatch, thereby improving power conversion efficiency
Implementation Method 2
sufficiently high energy photons (so-called in-band photons) radiated from a thermal emitter excite electronic transitions in the cell. Photogenerated charge carriers are subsequently separated and extracted as electricity
Implementation Method 3
OOB thermal radiation in a TPV can be recycled back to the closely positioned emitter using spectral control
Data Source
AI summary
To reach high efficiencies, thermophotovoltaic cells must utilize the broad spectrum of a radiative thermal source. One promising approach to overcome this challenge is to have low-energy photons reflected and reabsorbed by the thermal emitter, where their energy can have another chance at contributing toward 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. This work demonstrates nearly perfect reflection of low-energy photons (˜99%) by embedding an air layer within the TPV cell. This result represents a four-fold reduction in parasitic absorption relative to existing TPV cells. As out-of-band reflectance approaches unity, TPV efficiency becomes nearly insensitive to cell bandgap and emitter temperature. Accessing this regime unlocks a range of possible materials and heat sources that were previously inaccessible to TPV energy conversion.