Position-Induced Efficient Doping for Highly Doped Organic Thermoelectric Materials

Electrical doping is essentially required for high-performance organic thermoelectric (TE) materials; however, the doping efficiency eta(d) has not been extensively investigated in highly doped organic semiconductors (OSCs). Here, it is demonstrated that the distribution of dopant molecules in a specific position in highly doped OSCs affects the eta(d), which is critically related to the Seebeck coefficient S and the electrical conductivity sigma. Poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) films are p-doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) by either solution-sequential (SSq) doping or vapor doping. SSq doping deposited F4TCNQ only in the amorphous domains of PBTTT films, whereas vapor doping deposited it in both the amorphous and crystalline domains. F4TCNQ molecules in the crystalline domains exhibited a high eta(d) and led to a rapid increase of the power factor with increasing sigma: S-2 sigma proportional to sigma(0.76). These results provide guidance for the efficient doping of highly doped OSCs and emphasize the importance of doping efficiency in obtaining high-performance organic TE materials.