High Seebeck Coefficient and Power Factor in n‐Type Organic Thermoelectrics

The n‐type thermoelectric properties of [6,6]‐phenyl‐C 61 ‐butyric acid methyl ester (PCBM) are investigated for different solution‐based doping methods. A novel inverse‐sequential doping method where the semiconductor (PCBM) is deposited on a previously cast dopant 4‐(1,3‐dimethyl‐2,3‐dihydro‐1H‐benzoimidazol‐2‐yl)‐ N , N ‐diphenylaniline film to achieve a very high power factor PF ≈ 35 µW m −1 K −2 with a conductivity σ ≈ 40 S m −1 is introduced. It is also shown that n‐type organic semiconductors obey the −1/4 power law relation between Seebeck coefficient S and σ that are previously found for p‐type materials. An analytical model on basis of variable range hopping unifies these results. The power law for n‐type materials is shifted toward higher conductivities by two orders of magnitude with respect to that of p‐type, suggesting strongly that n‐type organic semiconductors can eventually become superior to their p‐type counterparts. Adding a small fraction lower lowest unoccupied molecular orbital material (core‐cyanated naphthalene diimide) into PCBM leads to a higher S for inverse‐sequential doping but not for bulk doping due to different morphologies.