Synthesis of a Conjugated Polymer with Broad Absorption and Its Application in High-Performance Phototransistors

An amorphous copolymer (PBDT-BBT) of 4,8-bis(2-thienyl)benzo[1,2-b:4,5-b']dithiophene and 5,5'-bibenzo[c][1,2,5]thiadiazole was synthesized by Stile coupling polymerization. PBDT-BBT exhibited good solution processability, excellent thermal stability with decomposition temperature of 437 degrees C, broad absorption (300-800 nm), deep HOMO level (-5.7 eV), and LUMO level (-3.7 eV). The microstructure order of PBDT-BBT thin films is not susceptible to thermal annealing temperature (80-200 degrees C). Field-effect transistors based on this polymer exhibited a charge-carrier mobility of 6 x 10(-3) cm(2) V-1 s(-1), threshold voltage of -1 V, and on/off current ratio of 106 without any post-treatments. Thin film phototransistors of PBDT-BBT exhibited a photoresponsivity of 3200 mA W-1 and photocurrent/dark current ratio of 4 x 10(5).