Instead of designing electronic devices to wastefully dissipate their excess heat, a new quantum-dot based thermo-electric effect aims to harvest that heat to generate enough electricity to double battery lifetimes.

Every electronic device wastes energy dissipating heat that it could be using to extend battery life, according to the Massachusetts Institute of Technology (MIT). By combining micro-gap thermal effects with nanoscale quantum dots, MIT researchers recently demonstrated thermophotovoltaic materials that recover lost heat by generating electricity with it.

Thermophotovoltaics is traditionally a low-efficiency process, but MIT researchers have found two ways to boost the efficiency and current carrying capability simultaneously. Currently a commercial version of MIT's thermophotovoltaic material that boosts its performance by 10-times is due out next year from MTPV Corp. (Micron-gap Thermal Photo-Voltaics Corp., Boston). MTPV was founded by Bob DiMatteo to commercialize the micro-gap innovation invented at MIT and subsequently developed by DiMatteo at Draper Laboratories (Cambridge). Now a second round of improvements wrought by quantum dots has been reported by MIT professor Peter Hagelstein, promising a second 10-fold boost in performance, Together, the two improvements boost thermophotovoltaic materials to 100-times better performance than today. 

Thermophotovoltaic polymers generate electricity by reabsorbing emitted heat photons in photovoltaic layers that MTPV has boosted in performance with microgap dimensions. Quantum dots were recently demonstrated by Hagelstein, and his graduate student Dennis Wu, to further increase that performance by harnessing the dipole quantum effects of near-surface electric fields.

Quantum effects only kick in when materials--here electrons--are confined into spaces sized so small that overall bulk properties of the material are swamped out. In Hagelsteins' current formulation, two quantum dots with a thin barrier between them, allows conversion of thermal energy into electricity at an efficiency levels near 90 percent of the theoretical maximum.

The layered material works by separating its hot side from its cold side with a nanoscale air gap that couples dipoles on either side through Coulomb interactions. On the cold side, electrons in a quantum dot are boosted in energy enough to tunnel through an oxide barrier to the a nearby quantum dot, where they generate an electrical current.

The technique works because of near-field coupling by evanescent waves that only propagate short distances, thereby making their contributions negligible in conventional thermophotovoltaic materials. By making the thermophotovoltaic microgap ultra-thin, DiMatteo and colleagues at MTPV have shown that throughput power per unit area can be greatly increased. In addition, Hagelstein's recent demonstration shows that performance can be further improved by harnessing thermal fluctuations in the near-surface electric fields, thereby coupling  dipoles on the hot side to the cold side by promoting carriers. The smaller the quantum dots can be made, the higher the material's efficiency becomes, hinging commercial success on perfecting practical fabrication methods for ultra-small quantum dots.

Funding was provided by Draper Laboratory and MTPV Corp.