Naturally Derived Chlorophyll Absorption Layers for RGB Color Recognition in IGZO Phototransistors

With the continuous growth of various Internet of Things (IoT)-based industries – including mobile smart devices, autonomous vehicles, and medical equipment – there has been a sustained interest in the technologies that recognize and communicate information in real-time. Particularly, as a significant portion of information processing is visual, the study of optoelectronic devices, such as photosensors that detect visual information, has become increasingly important. Metal-oxide semiconductors have been a focal point as potential optoelectronic devices due to their advantages, such as high uniformity over large areas, high optical transparency, high mobility, and low off current. Past research has primarily aimed to expand the inherently limited light absorption range(ultraviolet and blue light) of these semiconductors, which results from their wide bandgap (&gt;3.0 eV). This was usually achieved by introducing light-absorbing layers or by forming subgap states. However, most strategies focused merely on broadening the detection range into the visible or infrared spectrum without the ability to distinguish specific wavelengths of the absorbed light. It has been shown that even when absorbing different wavelengths of light, a consistent photo-response is generated based on its intensity, making it impossible to identify the specific wavelength of absorbed light. Therefore, conventional photosensor devices need additional color filters to identify different wavelengths, increasing costs and complicating high-density integration.<br/>In this study, we propose an indium-gallium-zinc oxide (IGZO) phototransistor integrated with naturally derived chlorophyll as the light-absorbing layer to differentiate between different wavelengths without needing for color filters. Chlorophyll's characteristic green hue arises from its predominant absorption of blue and red light, while reflecting much of the green light. Utilizing the unique absorption properties of chlorophyll, we enhanced the light response range of the IGZO phototransistor. Specifically, absorption in the chlorophyll layer produces electron-hole pairs. These generated electrons then migrate to the IGZO channel layer, altering the transistor's electrical characteristics. For a chlorophyll concentration of 0.06 M, we measured the phototransistor's characteristics under red (635 nm), green (532 nm), and blue (405 nm) light irradiation. At an intensity of 1 mW/mm², results indicated photoresponsivity of 1607/641/3881 AW^-1, photosensitivity of 5.80 × 10⁷/3.51 × 10⁵/2.35 × 10⁸, and detectivity of 3.86 × 10¹²/1.63 × 10¹¹/3.58 × 10¹³ Jones for red, green, and blue wavelengths, respectively. This is in contrast to conventional photodetectors, which typically show a light responsivity order of blue&gt;green&gt;red; our findings presented an order of blue&gt;red&gt;green. Exploiting this distinctive characteristic, combining a conventional phototransistor with our chlorophyll-induced version allows the design of an NMOS (N-type metal oxide semiconductor) inverter logic circuit capable of differentiating between 3 colors of blue, green, and red without color filters. Using SPICE (Simulation Program with Integrated Circuit Emphasis) simulations and the phototransistor's experimental results, we demonstrated that within a light intensity range of 0.1~5 mW/mm², the output voltage responded with low, high, and mid-values for red, green, and blue light inputs, respectively.<br/>In conclusion, the IGZO phototransistor utilizing chlorophyll as an absorption layer has demonstrated the potential to differentiate between red, green, and blue light. This suggests the possibility of simplifying the actual fabrication process by eliminating color filters in the sensor. Moreover, it implies the potential for various applications, such as light-controlled logic circuits.