Researchers in France have developed very-low-bandgap thermophotovoltaic (TPV) cells featuring a barrier layer designed to suppress dark current. TPV, not to be confused with photovoltaic-thermal (PVT) technology, converts infrared (IR) radiation from heat sources directly into electricity using PV cells. Potential applications include thermal energy storage and the conversion of industrial process heat into electricity.
The technology has attracted scientific interest for decades because it can potentially capture a broader portion of the solar spectrum and has the technical potential to exceed the Shockley-Queisser limit of conventional photovoltaics. However, efficiencies reported to date have been too low for commercial viability, as TPV devices continue to suffer from optical and thermal losses.
“Our work reports on the first fabrication and characterization of a very-low-bandgap TPV cell (0.23 eV at 200 K), made of an indium arsenide/indium arsenide antimonide (InAs/InAsSb) type-II superlattice absorber, which is the material now widely used for photodetectors,” corresponding author Rodolphe Vaillon told pv magazine. “In other words, we have transformed an IR photodetector into a TPV cell, capable of operating at higher temperatures than those usual for IR photodetectors.”
Vaillon added that materials and structures used in IR photodetectors, such as InAs/InAsSb type-II superlattice barrier structures, appear to be good candidates for TPV energy conversion. “We show that the barrier structure is performing better than the standard p-intrinsic-n (PIN) structure at operating temperatures larger than 150 K. A photovoltaic effect is even observed at room temperature,” he said.
Very-low-bandgap TPV cells are suited to converting lower-energy infrared radiation, making them potentially useful for lower-temperature heat sources while also helping to maximize power generation. Their main drawback, however, is high dark current – unwanted electrical current that flows through a cell even in the absence of incoming radiation – which reduces voltage and power output.
High dark current typically requires very-low-bandgap cells to operate at cryogenic temperatures, such as around 77 K (−196 C). To reduce this cooling requirement, the researchers introduced an aluminum arsenide antimonide (AlAsSb) barrier layer. The barrier creates a band offset that restricts unwanted carrier flow through the device and suppresses surface-current leakage, thereby reducing dark current and enabling the cell to maintain better performance at higher operating temperatures.
The researchers fabricated two InAs/InAsSb type-II superlattice TPV cell designs: a conventional PIN structure used as a reference and a barrier structure incorporating a 200 nm AlAsSb layer. They processed the wafers into both small “micro” mesa cells and 1 × 1 cm² “macro” cells using metallization and wet etching. The micro cells also underwent sidewall passivation and received dielectric coatings.
The researchers characterized the grown layers using high-resolution X-ray diffraction, atomic force microscopy and photoluminescence measurements, while the finished devices underwent temperature-dependent current-voltage and dark-current testing. They then measured photovoltaic performance under infrared radiation from an 800 C thermal emitter positioned 14 cm from the cells.
“Once processed, the cells exposed to an 800 C emitter displayed improved performance by more than tenfold when compared to standard PIN ones (for temperatures above 150 K),” the researchers said. “The barrier layer, specific to this cell design, was also found to contribute greatly to the suppression of surface current leaking when associated with a passivation step (sulfuration).”
At a cell temperature of 123 K (−150 C), the barrier cells recorded open-circuit voltages of around 23 mV and fill factors of about 0.2, although one measurement produced an open-circuit voltage of 0.15 V. The researchers also demonstrated that a passivated barrier micro cell continued to exhibit a photovoltaic effect at 273 K (around 0 C), with an open-circuit voltage of about 75 mV.
“The fabrication process is robust, but improvements could still be expected by working on the characterization setup (especially the view factor) or by increasing the absorber bandgap to see how it affects the dark current,” the researchers concluded.
The results were presented in “Fabrication and characterization of very-low bandgap InAs/InAsSb thermophotovoltaic cells,” published in Solar Energy Materials and Solar Cells. Researchers from the French National Centre for Scientific Research (CNRS), the University of Montpellier, the University of Orléans, and the University of Toulouse contributed to the study.
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