Photoelectric devices using an organic semiconductor attract much attention because of their unique features such as thinness, lightness, and flexibility. In general, semiconductor devices consist of inorganic materials typified by silicon. On the one hand, fabricating “flexible” devices with inorganic materials is essentially difficult owing to their rigidity. On the other hand, organic semiconductors can work in a thin film of 100 nm and be formed on flexible substrates such as plastic or ultra-thin glass films by using ordinary processing techniques such as printing or evaporation. We have investigated an organic photodiode (OPD) as a photosensor and an organic light-emitting diode (OLED) as a light source for an organic photoelectric device.
Fig.1 Organic photoelectric device
Fig. 2 Working mechanism of OPD and OLED
The basic structure and operating mechanism of the organic photoelectric device is as follows. Both OPDs and OLEDs have a simple structure where the organic semiconductor is sandwiched between an anode and a cathode. In OPDs, the organic layer includes an electron donor molecule and acceptor molecule. When these molecules absorb photons, electrostatically coupled electron-hole pairs called excitons are generated. The excitons diffuse into the donor-acceptor interface and dissociate into the free carrier. Finally, the hole and electron are collected at each electrode and are detected as an electrical signal. Since organic semiconductors have intrinsically similar properties to insulators, the organic layer needs to be extremely thin. This structure suggests that OPDs have conductivity along the vertical (thickness) direction while having high resistance along the horizontal axis. This in turn suggests that electrical crosstalk is barely observed compared to conventional inorganic devices. Moreover, organic semiconductors show a high photoelectric conversion efficiency in thin films because of a high optical absorption coefficient. This characteristic also supports the suitability of organic materials for use as thin devices.
In OLEDs, on the other hand, the hole and electron injected from each electrode recombine in a luminescent organic molecule, and photoemission is observed. OLEDs are suitable for surface emitting light sources in large areas by aid of a sandwiched structure using a transparent electrode and a simple fabrication process. This can achieve high illumination uniformity and a preferred pattern design.
We have developed a flexible OPD with photosensitivity in the near-infrared (NIR) region. The wavelength region around 800 nm is called the “biological optical window” because biological components such as water or hemoglobin (Hb) have less of an absorbance in the NIR region. Thus, light scattering is more dominating rather than absorption, and we can obtain a variety of information about living bodies. We aim to apply the flexible OPD to bio-sensing devices to reduce stress in measurements.
Fig. 3 (a) Flexible OPD
Fig. 3 (b) Spectral response
Fig. 4 Biological optical window
We have also demonstrated a large-area NIR-OLED with an arbitrary emitting pattern. Our NIR-OLEDs have an extremely high operational stability even under operation at high current density,* in addition to a radiant power of 1 mW. By taking advantage of the surface emitting device, we can provide a promising light source that irradiates the target uniformly for sensing or for darkfield observation.
■NIR image of our OLED
Fig. 5 Emission spectrum
Fig. 6 (a) Conventional LED
Fig. 6 (b) Surface emitting OLED
*T. Yamanaka, H. Nakanotani, S. Hara, T. Hirohata, C. Adachi, “Near-infrared organic light-emitting diodes for biosensing with high operating stability,” Appl. Phys. Express 10, 074101 (2017).
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