Investigations on conducting polymer coating strategy towards development of high sensitivity organic phototransistors

Abstract

Photodetectors are an indispensable part of modern technologies and are widely used in various opto-sensing and imaging applications, artificial vision, healthcare systems and environment monitoring and surveillance systems. Currently, the inorganic Si-based photodetectors and photomultipliers have dominated the electronics market due to their extremely mature processing techniques and low cost. However, due to the inherent rigidity of the semiconductor, their applications are severely limited in the areas that demand flexible and bendable photodetectors. One such domain is the healthcare domain wherein the optical sensors need to be placed in close proximity to the human skin to record the feeble optical signals. Owing to their lucrative advantages in terms of flexibility, easy solution-processibility, low cost and large area fabrication, organic photodetectors have emerged as a suitable alternative to the inorganic photodetectors for such applications. Among the organic photodetectors, the organic phototransistors (OPTs), in particular, have been heavily researched upon due to their low dark current levels, enhanced signal to noise ratios (SNRs) and high gain mechanism which lead to highly sensitive photodetectors. The performance of OPTs is significantly determined by the charge carrier injection/collection efficiency at the electrode/polymer interfaces and their transport in the semiconducting polymer bulk. Effects like metal diffusion are predominant in devices employing metal electrodes deposited on top of an underlying organic layer which are detrimental to the device performance. Increased metal diffusion leads to high contact resistances at the metal/polymer interfaces which hamper device performance. In the first part of work, the effect of metal diffusion on the organic semiconductor layer has been investigated through the optical, structural characterizations and fabrication of sandwich type devices. Furthermore, factors like polymer regio-regularity and film coating technique have been judiciously selected and investigated in order to limit the interfacial metal diffusion. In the next part, polymer thin film based bottom gate top contact (BGTC) type OPTs have been fabricated utilizing optimized film coating conditions. The fabricated OPTs have demonstrated high photosensitivity of the order of 104 and photoresponsivity of 17.6A/W towards green (λ = 522 nm) and red illuminations (λ = 630 nm) which were found to be much higher than many of the previously reported polymer thin film based OPTs fabricated from other organic materials. The results mark high suitability of the fabricated OPTs towards opto-sensing and imaging applications.