Flexible low-voltage organic phototransistors based on air-stable dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT)

• Use of low-voltage, high-mobility organic TFTs as phototransistors.
• Large but slow threshold-voltage shift based on charge trapping at the dielectric.
• Absorption-limited and trapping-limited regimes depending on illumination.
• Maximum operation frequency is larger in phototransistors with shorter channels.
• Demonstration of a flexible gesture recognition system.

Photosensitive elements based on organic thin-film transistors readily integrated into flexible, large-area organic circuits open up new scopes in light-sensing applications. In this work, high-performance, low-voltage organic thin-film transistors based on dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) are thoroughly characterized with respect to their optical functionality. The fundamental light-dependent effect, i.e., a large but slow threshold-voltage shift based on charge trapping in the aluminum oxide of the gate dielectric or at the semiconductor–dielectric interface, is analyzed depending on various parameters, such as biasing conditions, integration time, wavelength and power of the incoming light as well as the channel length. An optimized 3-phase operation consisting of reset, integration and read-out is proposed in order to maximize reproducibility, sensitivity and responsivity of the phototransistors. Two distinct regimes, an absorption-limited and a trapping-limited regime, are identified depending on the density of available trappable electrons, which is determined by the optical input power and the integration time. The maximum operation frequency is found to be larger in phototransistors with shorter channel lengths. Based on these results, an organic gesture recognition system with a refresh rate of 1 Hz was designed, implemented and successfully tested.

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