Jian Liu, Yongqiang Shi, Jingjin Dong, Mohamad I. Nugraha, Xinkai Qiu, Mengyao Su, Ryan C. Chiechi, Derya Baran, Giuseppe Portale, Xugang Guo, L. Jan Anton Koster
ACS Energy Lett., 4, 7, pp. 1556-1564, (2019)
Molecular doping of organic semiconductors creates Coulombically bound charge and counterion pairs through a charge-transfer process. However, their Coulomb interactions and strategies to mitigate their effects have been rarely addressed. Here, we report that the number of free charges and thermoelectric properties are greatly enhanced by overcoming the Coulomb interaction in an n-doped conjugated polymer. Poly(2,2′-bithiazolothienyl-4,4′,10,10′-tetracarboxydiimide) (PDTzTI) and the benchmark N2200 are n-doped by tetrakis (dimethylamino) ethylene (TDAE) for thermoelectrics. Doped PDTzTI exhibits ∼10 times higher free-charge density and 500 times higher conductivity than doped N2200, leading to a power factor of 7.6 μW m–1 K–2 and ZT of 0.01 at room temperature. Compared to N2200, PDTzTI features a better molecular ordering and two-dimensional charge delocalization, which help overcome the Coulomb interaction in the doped state. Consequently, free charges are more easily generated from charge–counterion pairs. This work provides a strategy for improving n-type thermoelectrics by tackling electrostatic interactions.