OPD / OLED

"Flexible" photoelectric device

Technical overview

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.

Application

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

Further Potential of Organic Semiconductor

We also focus on the unique optical properties of the organic semiconductor. For example, in some organic emitters studied in the field of OLEDs, charge carriers are generated in a solid-state film by photo-irradiation. In other words, an organic molecule designed as an emitter for OLEDs contains the characteristic of a light-receiving material (References 2-4). The charge carriers generated in the film are stored stably for a long time due to the internal electrostatic potential induced by polarization of the organic molecule and can be extracted again as electroluminescence (charge recombination) by applying an external field. Furthermore, the location and the amount of the charge carrier are simultaneously retained, thus enabling a unique charge storage device with three advantageous features—light-receiving, charge storage, and emission—in a single organic molecule. Figure 7 shows an example of the charge storage ability by irradiating patterned light into the device. The charge carriers are generated only at the exposed area, so the input image can be reproduced as electroluminescence simply by applying the external voltage even after the arbitrary hold time has passed. Notably, the spatial information of the photo-generated carriers is retained even though the device does not have a pixelized structure.

Figure 7. Demonstration of charge storage device

References

1. 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).

2. T. Yamanaka, H. Nakanotani and C. Adachi, Slow recombination of spontaneously dissociated organic fluorophore excitons. Nat. Commun. 10, 5748 (2019).

3. T. Yamanaka, H. Nakanotani and C. Adachi, Significant role of spin-triplet state for exciton dissociation in organic solids. Sci. Adv. 8, eabj9188 (2022).

4. T. Yamanaka, H. Nakanotani, K. Nakamoto and C. Adachi, Electron lifetime of over one month in disordered organic solid-state films. Adv. Mater. 35, 2210335 (2023).

OPD / OLED

"Flexible" photoelectric device

Technical overview

Application

Further Potential of Organic Semiconductor

References

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