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

This study presents the development of low-voltage (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\le 2.0$ </tex-math></inline-formula> 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<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{{2}}$ </tex-math></inline-formula>) 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 (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu _{\max }$ </tex-math></inline-formula>) of 0.21 cm2/V<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula>s with an on/off ratio of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 10^{{4}}$ </tex-math></inline-formula>, showcasing performance competitive with solution-based phototransistors. Additionally, the threshold voltage (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula>) shifts from 1.80 to 1.10 V under NIR illumination of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$730~\mu $ </tex-math></inline-formula>W/cm2, revealing a strong dependency of the device’s threshold voltage on NIR light intensity.