Fabrication of NIR Sensitive-Low Operating Voltage Phototransistor With Unidirectional Organic Polymer

This study presents the development of low-voltage ( $\le 2.0$ V) near-infrared (NIR) sensitive organic phototransistors (OPTs) using an economical unidirectional floating film transfer method (UFTM) technique to process the poly[2,5-(2-octyldodecyl)-3,6-diketopyrrolopyrrole-alt-5,5-(2,5-di(thien-2-yl) thieno[3,2-b]-thiophene)] (DPP-TTT) active layer. This layer exhibits strong red/NIR absorption within the 600–900 nm spectrum. The devices utilize an ion-conducting lithium alumina (Li-Al2O3) gate dielectric, applied via solution processing, and a semiconducting layer prepared through the FTM method, enabling the deposition of unidirectional organic polymers. The high areal capacitance (410 nf/cm $^{{2}}$ ) of the Li-Al2O3 thin film limits the OPTs’ operating voltage to 2 V. These devices demonstrate high NIR (830 nm) photosensitivity (215%) and photoresponsivity (28.16 A/W) in the accumulation mode. In darkness, the OPTs achieve a saturation mobility ( $\mu _{\max }$ ) of 0.21 cm2/V $\cdot $ s with an on/off ratio of $\sim 10^{{4}}$ , showcasing performance competitive with solution-based phototransistors. Additionally, the threshold voltage ( ${V}_{\text {th}}$ ) shifts from 1.80 to 1.10 V under NIR illumination of $730~\mu $ W/cm2, revealing a strong dependency of the device’s threshold voltage on NIR light intensity.