Unveiling the Impact of Interfaces and Impurities on Photogenerated Charge Trapping in Phototransistors with Diverse Organic Semiconductor Active-Layer Architectures

The fabrication of efficient phototransistors relies on understanding the trapping of photogenerated charge carriers in localized electronic states (known as trap sites) which creates an additional electric field in the active layer. These sites are mostly located at interfaces and impurities within the active layer and play a crucial role in controlling the device performance. Hence, they are crucial considerations in the design of high-responsivity phototransistors. This paper reports on the impact of active-layer interfaces and impurities on the photoresponse behavior of phototransistors based on PTCDI-C₅ (n-type) and C₈-BTBT (p-type) organic semiconductor layers. Trap sites are introduced into various active layers via vacuum evaporation, solution processing, and hybrid processes. The mechanism of charge trapping is elucidated using ultraviolet photoelectron spectroscopy, providing insights into the electron band energy structure at the interfaces. The findings reveal that both interfaces and impurities can significantly affect the photoresponse behavior of the devices. Impurities are found to consistently enhance the photoresponse, whereas interfaces can induce either positive or negative photoresponses, depending on their spatial orientation and bias polarity. This study establishes an important link between the active-layer structure and the photoresponse of devices and provides valuable insights for the design and optimization of high-performance phototransistors.