Dithienylbenzothiadiazole-Based Donor-Acceptor Organic Semiconductors and Effect of End Capping Groups on Organic Field Effect Transistor Performance

Donor-Acceptor-Donor (D-A-D) based conjugated molecules 4,7-bis(5-(4-butoxyphenyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (BOP-TBT) and 4,7-bis(5-(4-trifluoromethyl)phenyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (TFP-TBT) using thiophene-benzothiadiazole-thiophene central core with trifluoromethyl phenyl and butoxyphenyl end capping groups were designed and synthesised via Suzuki coupling. Optical, electrochemical, thermal, and organic field effect transistor (OFET) device properties of BOP-TBT and TFP-TBT were investigated. Both small molecules possess two absorption bands. Optical band gaps were calculated from the absorption cut off to be in the range of 2.06–2.25 eV. Cyclic voltammetry indicated reversible oxidation and reduction processes and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated to be in the range of 5.15–5.40 eV and 3.25–3.62 eV, respectively. Upon testing both materials for OFET, trifluoromethylphenyl end capped material (TFP-TBT) shows n-channel behaviour whereas butoxyphenyl end capped material (BOP-TBT) shows p-channel behaviour. Density functional theory calculations correlated with shifting of HOMO-LUMO energy levels with respect to end capping groups. Vacuum processed OFET of these materials have shown highest hole carrier mobility of 0.02 cm2/Vs and electron carrier mobility of 0.004 cm2/Vs, respectively using Si/SiO2 substrate. By keeping the central D-A-D segment and just by tuning end capping groups gives both p- and n-channel organic semiconductors which can be prepared in a single step using straightforward synthesis