Donor-Acceptor Conjugated Polymers Based on Bisisoindigo: Energy Level Modulation toward Unipolar n-Type Semiconductors

Four bisisoindigo (bis-IID) derivatives containing different numbers of electron withdrawing fluorine (F) or nitrogen (N) atoms and eight alternating copolymers of them with 2,2'-bithophene (BT) or 3,3',4,4'-tetrafluoro-2,2'-bithiophene (4FBT) were synthesized. Depending on the numbers of F and N atoms incorporated, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the polymers were feasibly tuned in the ranges of -5.51 to -6.06 eV and -3.64 to -4.01 eV, respectively. Top gate and bottom contact (TGBC) organic thin-film transistors (OTFTs) were fabricated to study the semiconducting properties of the polymers. Five polymers, i.e., P0F-BT, P2F-BT, P4F-BT, P2N2F-BT, and P0F-4FBT that contain 0-4 F and N atoms in the repeating unit, with HOMO and LUMO energy levels above -5.90 and -3.80 eV, respectively, all showed ambipolar OTFT characteristics. Three other polymers P2F-4FBT, P4F-4FBT, and P2N2F-4FBT that contain 6 or 8 F and N atoms with HOMO levels below -5.90 eV and LUMO levels below -3.80 eV were all unipolar n-type semiconductors, with electron mobilities up to 0.93, 0.71, and 1.24 cm(2)/V.s, respectively.