Further Advances in UHD

Three-layer Organic-film Color Image Sensor

For compact, high-resolution single-chip cameras

NHK is conducting R&D on imaging devices utilizing organic films that convert light to electrical signals, toward implementation of compact, high-resolution, single-chip cameras. We have developed a color image sensor with layered organic films.

Typical image sensors for single-chip color cameras use a pixel mosaic pattern with color filters for red, green and blue (Fig. 1). This method generates an image by collecting color information from multiple pixels, so the resolution of the image and utilization of light is lower than that of the actual pixel devices.

In this research, we have developed a three-layer color image sensor using organic films that detect only blue and only green light, layered vertically over a CMOS*1 image sensor that detects red light (Fig. 2).

Fig. 1: Conventional typical single-chip color image sensor
Fig. 2: Three-layer color image sensor and operating principles

Incident light passes the first organic layer, which absorbs only the blue light component and converts to an electrical signal, and is transparent to the green and red components. The second organic layer absorbs only the green component, and the red component is detected by the CMOS image sensor. The organic layers are combined with transparent thin-film transistors, and the signals output from each of the layers can be combined to reproduce a color image. The developed sensor’s photo and specifications are shown in Fig. 3 and Table 1. The reproduced image example and sensor’s spectral characteristics are shown in Fig. 4.

Table 1 : Three-layer color image sensor specifications
Device structure Organic film for blue (Layered over glass substrate)
+ organic film for green (layered over red CMOS image sensor)*
Pixels 320 (H) × 240 (V)
Pixel pitch 20 μm
Frame rate 60 frames/s

* The blue organic film is placed facing green organic film.

A two-layer sensor was fabricated first.
(An organic film for green over CMOS chip for red)

Then, an organic film for blue made separately on a glass substrate was placed facing the organic film for green to complete the three-layer structure.

Fig. 3: Fabricated image sensor



Fig. 4: Image reproduced by three-layer color image sensor (a) and spectral characteristics (b)

This structure enables all color information of red, green and blue to be obtained within a single pixel, achieving a high-resolution image sensor that uses light more efficiently. We will continue to work reducing the pixel size and increasing the number of pixels, and accelerate R&D toward realizing a compact, high-resolution, single-chip camera.