Spray sensor - solution-processable organic semiconductor
Researchers at Technische Universität München (TUM), in Germany, have created a solution-processable organic semiconductor for devices such as high-speed cameras and night vision systems.
The problem with the active pixel sensors in standard imaging technology is the low-fill factor. Lead researcher Dr Daniela Baierl explains, ‘Usually, read-out electronics fill about 70% of each pixel. Only about 30% remains for light detection, since the light cannot be transmitted through the opaque electronics. By employing a technique to vertically deposit photoactive material covering all the electronics, it is possible to reach a fill factor of almost 100%.’
She adds that the deposition of organic materials in their liquid phase makes them compatible with a wide range of substrates and CMOS chips. Since deposition does not involve a vacuum or high temperature, the process is cheaper.
To make the sensors, the team starts with a CMOS chip that can read out and process photogenerated signals. ‘We spray the different organic layers on these chips. First, the photoactive layer (1-(3-methoxycarbonyl)propyl-1-phenyl[6,6] C61 [PCBM]: poly(3-hexylthiophene)[P3HT]) is applied. This is followed by another polymer that is needed for the suppression of dark (leakage) current. The third layer consists of a highly conductive (sheet resistance ~80Ohm/sq) and transparent layer of the organic material PEDOT:PSS. It is applied (like the other layers) as a large-area layer, covering all pixels in one processing step. The process is then concluded with encapsulation to hinder moisture and oxygen diffusion into the sensor.’
The spraying process takes about 30–60 seconds, while the annealing of the organic layers takes about 5–10 minutes per layer. Baierl claims the deposition of solution-processable polymers is applicable to many surfaces, including flexible substrates, and that whole wafers could eventually be produced in a single-step process.
Thus far, the team has used the polymer-based photoactive materials P3HT and PCBM for the bulk-heterojunction structure. They have also used a squaraine dye, which is sensitive to light in the near-infrared region.
According to Baierl, the spray sensors ‘can replace the conventional sensors in almost every application, but the most promising one would be in night vision cameras. They will allow the development of cheap near-infrared spectroscopy organic imagers for surveillance purposes and automotive night vision systems.’
Further tweaks must be made before the system can be brought to industry. The encapsulation of the organic sensors will be improved, as they are prone to early degradation. The team is also looking to incorporate multi-colour images, as the prototype does not contain a colour filter. When these issues have been resolved, the researchers hope to process the organic materials on passive pixel CMOS chips with a small pixel array, and to demonstrate that the concept works on commercial CMOS chips.