Tuning dielectric-semiconductor interfacial properties in organic phototransistors for ultralow light detection

The quality of the dielectric-semiconductor interface plays a very crucial role in determining the photosensory performances of the organic field effect transistors. Herein, we have investigated the influence of polymer molecular weight on the photosensing behaviour of the organic transistors. In this study, we find high molecular weight PMMA to be more effective as gate dielectric in phototransistor applications compared to the low molecular weight PMMA due to the better morphological arrangements of the active semiconducting layer on top of its surface. The maximum values of photosensitivity and responsivity improved substantially from 3.5 x 10(4) and 8.3 x 10(3) A/W in the low molecular weight PMMA devices to 10(5) and 6.9 x 10(4) A/W in the high molecular weight PMMA devices, respectively, under same intensity light irradiation. Moreover, we have also proposed a simple, yet highly efficient interface modification technique to enhance the photosensitivity of the transistors under low light illumination through the plasma treatment of the dielectric layer. After the interface modification of the high molecular weight PMMA devices, the photosensitivity value enhanced significantly from 3.67 x 10(2) to 7.08 x 10(3) under an ultralow optical power of 5 mu W/cm(2). Post interface modification improvements in the device photosensitivity values were attributed to the plasma induced generation of polar surface functionalities which effectively enhance the number of free holes in the channel region by trapping the photo-generated electrons at the interface. This report, thus, represents a step forward towards improving the performance of the phototransistors for their applications in high performance organic optoelectronic devices.