Geometric Effect of the Photo Responsivity of Organic Phototransistors

Organic phototransistors exhibit considerably higher photoresponsivity than diode-like photodetectors owing to gate-field-effect amplification. However, the conventional definition of photoresponsivity (R) fails to accurately capture the photoresponsivity trends of transistor-based photodetectors. This study systematically investigates the impact of device geometry-specifically the width-to-length (W/L) ratio and photosensitive area-on the responsivity and photocurrent of organic phototransistors. The experimental results reveal that increasing the W/L ratio or decreasing the device area substantially enhances responsivity. A detailed analysis based on the definition of responsivity is presented herein. Finally, we introduce a channel-width-normalized responsivity to compensate for geometric effects, enabling a more accurate evaluation of device performance across different device structures. Overall, our results indicate the potential for optimizing organic phototransistors by tuning their geometric parameters.