High Seebeck Coefficient in Mixtures of Conjugated Polymers

A universal method to obtain record‐high electronic Seebeck coefficients is demonstrated while preserving reasonable conductivities in doped blends of organic semiconductors through rational design of the density of states (DOSs). A polymer semiconductor with a shallow highest occupied molecular orbital (HOMO) level‐poly(3‐hexylthiophene) (P3HT) is mixed with materials with a deeper HOMO (PTB7, TQ1) to form binary blends of the type P3HTx:B1‐x (0 ≤ x ≤ 1) that is p‐type doped by F4TCNQ. For B = PTB7, a Seebeck coefficient S = 1100 µV K−1 with conductivity σ = 0.3 S m−1 at x = 0.10 is achieved, while for B = TQ1, S = 2000 µV K−1 and σ = 0.03 S m−1 at x = 0.05 is found. Kinetic Monte Carlo simulations with parameters based on experiments show good agreement with the experimental results, confirming the intended mechanism. The simulations are used to derive a design rule for parameter tuning. These results can become relevant for low‐power, low‐cost applications like (providing power to) autonomous sensors, in which a high Seebeck coefficient translates directly to a proportionally reduced number of legs in the thermogenerator, and hence in reduced fabrication cost and complexity. Record‐high Seebeck coefficients are achieved for p‐type‐doped blends of common conjugated polymers. The method used is based on a rational design of the density of states, such that the characteristic hop occurs from the compound with the shallower HOMO to the compound with the deeper HOMO.