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Decoupling interrelated parameters for designing high performance thermoelectric materials.

The world's supply of fossil fuels is quickly being exhausted, and the impact of their overuse is contributing to both climate change and global political unrest. In order to help solve these escalating problems, scientists must find a way to either replace combustion engines or reduce their use. Thermoelectric materials have attracted widespread research interest because of their potential applications as clean and renewable energy sources. They are reliable, lightweight, robust, and environmentally friendly and can reversibly convert between heat and electricity. However, after decades of development, the energy conversion efficiency of thermoelectric devices has been hovering around 10%. This is far below the theoretical predictions, mainly due to the interdependence and coupling between electrical and thermal parameters, which are strongly interrelated through the electronic structure of the materials. Therefore, any strategy that balances or decouples these parameters, in addition to optimizing the materials' intrinsic electronic structure, should be critical to the development of thermoelectric technology. In this Account, we discuss our recently developed strategies to decouple thermoelectric parameters for the synergistic optimization of electrical and thermal transport. We first highlight the phase transition, which is accompanied by an abrupt change of electrical transport, such as with a metal-insulator and semiconductor-superionic conductor transition. This should be a universal and effective strategy to optimize the thermoelectric performance, which takes advantage of modulated electronic structure and critical scattering across phase transitions to decouple the power factor and thermal conductivity. We propose that solid-solution homojunction nanoplates with disordered lattices are promising thermoelectric materials to meet the "phonon glass electron crystal" approach. The formation of a solid solution, coupled with homojunctions, allows for synergistically enhanced thermoelectric properties. This occurs through a significant reduction of thermal conductivity, without the deterioration of thermopower and electrical conductivity. In addition, we introduce the concept of spin entropy in wide band gap semiconductor nanocrystals, which acts to fully disentangle the otherwise interconnected quantities for synergistically optimized thermoelectric performance. Finally, we discuss a new concept we developed that is based on an ultrathin-nanosheet composite that we fabricated from ultrathin nanosheets of atomic thickness. These retain the original strong two-dimensional electron gas (2DEG) and allow for decoupled optimization of the three thermoelectric parameters, which improves thermoelectric performance.