Orbital-Engineering-Based Screening of π-Conjugated d⁸ Transition-Metal Coordination Polymers for High-Performance n-Type Thermoelectric Applications

Extraordinary progress has been achieved in polymer-based thermoelectric materials in recent years. New emerging π-conjugated transition-metal coordination polymers are one of the best n-type polymer-based thermoelectric materials. However, the microscopic descriptions on geometric structures, orbital characteristics, and most importantly, thermoelectric properties remain elusive, which has seriously hampered the experimentalists to draw a straightforward design strategy for new n-type polymer-based thermoelectric materials. Herein, we assess the n-type thermoelectric properties of 20 π-conjugated d8 metal center coordination polymers and rationalize their thermoelectric properties in terms of molecular geometry, orbital nature, and electron–phonon coupling based on first-principles calculations. An explicit screening rule for high-performance n-type π-conjugated transition-metal coordination polymeric thermoelectric materials was found, i.e., smaller metal center d orbital component ratio in the conduction band minimum, weaker electron–phonon coupling, higher intrinsic mobility, and thereby higher thermoelectric power factor can be achieved. Guided by this rule, poly(Pd-C2S4) and poly(Ni-C2Se4) show very high power factors. We built a map of high-performance π-conjugated transition-metal coordination polymers for n-type thermoelectric applications, which will help to accelerate the screening and design of innovative n-type thermoelectric polymers.