Abstract
We report an optoelectronic device consisting of a solution-processed indium gallium zinc oxide (IGZO) thin-film transistor and vacuum-deposited small organic molecules. Depending on the configurations of the organic materials, either bulk heterojunction or planar heterojunction (PHJ), the device assumes the functionality of either a photosensor or a photoinduced memory, respectively. Under λ = 625 nm light illumination, the photosensor shows response and recovery time of ∼50 ms, responsivity of ∼5 mA/W, sensitivity above 104, and a linear response. The mechanism of the photoinduced memory is studied experimentally and verified using a device simulation. We find that the memory is due to long charge retention time at the organic PHJ interface which is stable for over 9 days. It is correlated with the low leakage current found in ordered organic junctions having low subgap tail states. The presented integration of the PHJ with the transistor constitutes a new design of write-once-read-many-times memory device that is likely to be attractive for low-cost applications.
Highlights
In recent decades, there has been a rapid development of organic and metal oxide semiconductor materials
We show that depending on the configuration of the organic layers in the phototransistor, one can achieve phototransistors acting either as a photosensor or as a photoinduced memory device
We study different structures of organic capping layers deposited on top of indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) and their different functionalities as well as the physical mechanisms that govern the device characteristics
Summary
There has been a rapid development of organic and metal oxide semiconductor materials. We show that depending on the configuration of the organic layers in the phototransistor, one can achieve phototransistors acting either as a photosensor or as a photoinduced memory device. We first demonstrate a photosensor by using an organic bulk heterojunction (BHJ) structure on top of IGZO.[17] The fabricated device showed typical photosensing characteristics, with ∼50 ms response and recovery time, decent responsivity, and a linear response at a gate voltage (Vg) of −0.8 V. Changing the BHJ structure to a planar heterojunction (PHJ) structure, a photoinduced write-once-read-many (WORM) memory was successfully demonstrated. We consider this change of functionality to be the main result of this paper. We have found that the charges were retained in the top organic layer and correlated the charge retention/leak mechanism with subgap states in organic PHJ
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