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

Highlights

• 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.

Abstract

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.

Introduction

Organic thin-film transistors (OTFTs) and circuits pose a promising way of manufacturing electronic systems with new properties, as they can be fabricated on flexible, large-area substrates [1], [2]. Applications range from organic active-matrix displays [3] and adaptive lighting panels to medical applications [4], wearable devices [5] or packaging [6]. Many of these applications require some sort of sensor, capable of extracting data from the environment, such as pressure, temperature, humidity or light. Imaging capabilities could potentially have the widest range of applications, as they can be used for example for photography, scanning, object or movement detection or range finding. Due to the high spatial resolution of typical image sensors, the required control and read-out logic tends to be sophisticated. Rigid image sensors have made a lot of progress in the past decades, and the majority of mobile consumer devices, such as smartphones and tablets, contain at least one of them. A flexible and printable version would be desirable and open up even more possible applications. Gesture or motion based interactions between humans and almost any surface or large-area scanning of objects are just some of the prospects. In 2005, Someya et al. demonstrated a flexible image sensor on a plastic substrate using organic photodiodes [7]. The fabrication process of organic imagers could, however, be simplified if efficient organic phototransistors (OPT) replaced the photodiodes. In the best case, these phototransistors are fabricated in exactly the same way as the logic transistors surrounding them without adding a substantial number of extra process steps. This would lead to an easy-to-integrate optical and electrical circuit on a common flexible substrate. This work aims to use existing, high-performance OTFTs [8] as photo-sensing elements, i.e., as OPTs, and poses the question if they are viable for a sensor application. In order to answer this question, the effect of illumination on the OTFTs has to be studied thoroughly. Furthermore, proper biasing sequences and read-out techniques have to be developed, which distinguish these opto-electric transducers from memory-type organic transistors [9], [10]. OPTs with various geometries and materials have already been investigated by other research groups within the past years [11], [12], [13], [14], [15], [16], [10]. However, the low-voltage operation of 2–3 V and the large effective mobility greater than 1.2 cm²/Vs at channel lengths as short as $1\mathrm{\mu }\text{m}$ demonstrated in [8] encourage further investigation.