High-Performance Flexible Hybrid Photosensor Circuits Made by Low-Temperature Solution Processed Metal-Oxide and Organic TFTs
Authors: Kim Jaehyun , Kim Jaekyun , Kang Jingu , Kim Myung-Gil , Kim Yong-Hoon , Park Sung Kyu 
Published: February 2020

A wearable, skin-like sensory device offers collection of meaningful information from our body and from the surrounding atmosphere, in an imperceptible manner. By a synergic combination of highly sensitive organic materials with high-performance inorganic devices at a low temperature, skin-like sensory devices with performances beyond current expectations are possible. Here we demonstrate large-area and skin-like ultra-thin organic/metal-oxide heterogeneous photosensor arrays with high photoresponsivity and photodetectivity, monolithically integrated at a low-temperature (<150°C) using a solution process. An integration of spatially isolated organic sensing devices and metal oxide devices via the alternating photo-conversion process enabled in-pixel signal amplification capability and photodetectivity up to 1.9x1014 Jones. The 10x10 multiplexed photosensor arrays are successfully implemented featuring ultra-thin (1~3-µm-thick), extremely light weight (~2.52 g m-2) and mechanically stable properties under a bending radius of less than 1 mm, which demonstrates a facile platform for large-area and skin-compatible high-precision sensor applications.

Recently, a low-temperature process (< 150°C) which uses photochemical activation to fabricate solution-based metal-oxide materials was introduced, comparable to the processing temperature of organic materials. As a result, high-performance solution-processed metal-oxide devices could be implemented on organic platforms, even on a low-thermal budget polymeric substrate. More importantly, solution processing offers a possibility to deposit diverse functional materials with a cost-competitive scalability.

For photosensor applications, organic semiconductors with a low-to-mid bandgap are preferred since they can absorb photons in a wide range of wavelengths, from infrared to visible light. However, single-organic-phototransistor pixels have a fundamental limitation in terms of the photo-current, since their operation regime generally lies in off-state gate bias condition to maximize the photodetectivity. Furthermore, the relatively low light absorption coefficient and carrier mobility (0.1~1 cm2/Vs) also have limited the output photo-current of the organic devices. Therefore, circuit-level hybridization with high-performance, stable inorganic devices is necessary, such as by inserting a current boosting function in the sensor pixel, to fundamentally increase the photo-current and simultaneously improve the photodetectivity.

In particular, a solution-processable organic semiconductor, 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT) exhibits a rather high photo-response to visible light, however, the relatively low carrier mobility as well as the lack of proper doping methods are still problematic, compared to inorganic counterparts. To solve this issue, organic-inorganic hybrid systems have received lots of attention due to its potentials to overcome the fundamental limits of organic sensor systems and also in aim to expand the application areas using the organic sensors.

For the solution-processed discrete organic and metal oxide TFTs on flexible polyimide substrates, we annealed InGaZnO (IGZO) semiconductor by deep-ultraviolet (DUV) process at 150℃, which is the highest process temperature for TFT fabrication. Our experiments shows that the transfer curves of discrete IGZO TFT exhibited almost minimal Vth or the threshold voltage shift by an exposure of halogen light while diF-TESADT showing the obvious Vth shift. Based on these individual TFTs, respectively, we constructed the hybrid circuits that have the discrete sensory and amplifying functions by virtue of organic and oxide TFTs performance.

In other words, we propose a simple voltage divider composed of two TFTs: one is organic TFT and the other is oxide TFT (LT1). So, VDD is divided by two TFTs’ resistance such that VO = VDD ∙ RLT1 / (RLT1+Rorg), and output voltage goes into another oxide TFT’s (HT2) gate input voltage. Therefore we can amplify the organic TFT’s small drain current by increasing a ratio of LT1 and HT2. As a result, once the illumination intensity-dependent current from diF-TESADT OTFT is translated into the gate potential of HT2, the output current (IOUT) of HT2 is modulated according to its transfer characteristics, resulting in about multiple orders of magnitude current amplification. It should be noted that IOUT of hybrid circuitry was increased only at illuminated condition while remained low in the dark condition. Various amplified high output currents can also be obtained from the hybrid photosensor compared to a single diF-TESADT phototransistor according to the circuit design.

In conclusion, large-area and skin-compatible, high-detective diF-TESADT/IGZO heterogeneous photosensor arrays were realized by using a simple low-temperature solution and alternating photo-conversion process. The monolithically integrated organic sensor arrays including metal-oxide boosting circuitry demonstrated unprecedented photosensitivity and photodetectivity, showing the possibility of organic sensor devices in versatile industrial applications. More importantly, beyond the photo-sensing performances, the organic/metal-oxide heterogeneous photosensor arrays are likely to be processed over a large-area and highly uniform multiplexed structures, showing most suitable and facile route for ultra-thin and skin-like high-precision sensor systems.

Figure 1