Tuning Polymer Semiconductor Morphology through Additive Engineering for a Stable Phototransistor

The application of PTB7-Th polymer as an efficient semiconductor material in a phototransistor (PT) has been demonstrated, with a strategy to improve the hole transport and enhance the device stability under dark as well as illumination conditions. The results have been established using two solvent additives, namely 1,8-diiodooctane (DIO) and 1,8-dibromooctane (DBrO) which have the ability to modulate the polymer morphology. These additives also improve the charge transport properties and help in achieving higher mobility in organic field effect transistors (OFETs). The DIO and DBrO-modified devices show an increase in mobility (0.09 and 0.22 cm2/V-s, respectively) with respect to the control device (without any additive). Further, the device stability in the dark as well as under illumination improves drastically for the DBrO-modified device, whereas photodegradation of the DIO-modified device is observed. The photophysical studies, AFM, FESEM, XRD, and electrical investigations were carried out to understand the effect of the solvent additive in lowering the contact resistance, suppressing traps, and improving the morphology. With the optimized concentration of DBrO as the solvent additive, a PTB7-Th based PT is implemented and a responsivity of 688 A/W is obtained.