A Phototransistor Leads to Artificial Synaptic Behaviors by Controlling Off-Current under Illumination

Organic phototransistors (OPTs) can amplify electrical output signals without additional devices and provide tunable optical spectral response. A lot of research is being conducted using the characteristic that the electrical characteristics of the OPT change depending on the light. Recent studies investigate the properties of these OPTs for various applications such as photodetectors, image sensors and neuromorphic devices. Here, we propose an organic semiconductor-based OPT device in which the off-current increases when light is applied to the OPT. The structure of OPT is a single transistor using p-type organic semiconductor dinaphtho[2,3-b:2′,3′- f]thieno[3,2-b]thiophene (DNTT) as a channel layer by thermally depositing it on a SiO₂/Si substrate. In addition, the photoresponse was improved by doping the entire surface of the material 2,2',2''-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) used as the hole transport layer of the organic light emitting diode (OLED). As a result, when the electrical characteristics of a single DNTT transistor were checked, an increase in off-current could be observed only under illumination of 300 nm to 400 nm, which is the DNTT photoresponse region. On the other hand, the TPBi-doped DNTT phototransistor shows an increase in off-current in all broadband wavelength ranges from 400nm to 1000nm. In addition, the change of current over time, we imitated artificial synaptic behavior by applying a relatively slow light response speed. As a result, a recognition rate of over 70% was achieved in all wavelength bands.