Band Engineering and Majority Carrier Switching in Isostructural Donor-Acceptor Complexes DPTTA-F TCNQ Crystals ( = 1, 2, 4).

Three isostructural donor-acceptor complexes DPTTA-F TCNQ ( = 1, 2, 4) are investigated experimentally and theoretically. By tuning the number of F atoms in the acceptor molecules, the resulting complexes display a continuous down shift of the valence band maximum, conducting band minimum, and optical bandgap. The majority carriers convert from hole (DPTTA-FTCNQ), balanced hole, and electron (DPTTA-FTCNQ) to electron (DPTTA-FTCNQ). This result shows that band engineering can be realized easily in the donor-acceptor complex systems by tuning the electron affinity of the acceptor. The bandgaps of these three complexes vary from 0.31 to 0.41 eV; this narrow bandgap feature is crucial for achieving high thermoelectric performance and the unintentional doping in DPTTA-FTCNQ leads to the effective suppression of the bipolar cancelling effect on the Seebeck coefficient and the highest power factor.