Solar cells convert light energy into electricity, and the principle of imaging sensors is to convert light into electronic signals. If you can combine the two into one, you can get an imaging sensor with its own solar charging capabilities. University of Michigan researchers recently claimed that They have already made this sensor. The sensor can shoot 15 frames per second with only daytime sunlight.
The head of the research team, Euisik Yoon, professor of electrical engineering and computer science at the University of Michigan, said that when combined with a microprocessor and wireless transceiver system, this sensor can create a compact camera that can transmit images.
Previous light-powered imaging sensors can be roughly divided into two categories. The first category is to replace some of the sensor's pixels with photovoltaic cells. This method is no problem in principle, but increasing the number of photovoltaic cells must reduce the number of pixels. , and vice versa. The second category is to let the pixel switch between the imaging state and the power generation state. In principle this is also feasible, but the system will be more complex and will inevitably limit the maximum number of frames per second of the sensor.
Professor Yoon and postdoctoral Sung-Yun Park created the third method. They noticed that many photons arrived at the camera, but they were not converted into electricity by photodiodes, but crossed the gap between the photodiodes and passed the energy to The substrate. Then, two people placed a second layer of diode behind the photodiode as a photovoltaic layer to convert these electrons into electrical energy. Thus, the photon energy reaching the sensor was used more fully.
Since photovoltaic diodes have been in use for a long time, they have never been exploited to generate electricity by leaking photon energy. Therefore, they do not occupy valuable imaging pixel space and do not require complex switching operations.
The sensor is made using a standard CMOS process, but its pixel structure and electrical characteristics are quite different from conventional sensors. First, the new system's pixels contain a PN junction, and a power-generating diode under the photodiode. Second, Traditional pixels use negatively charged electrons as carriers for charge. The new system uses positively charged holes as charge carriers for both imaging and power generation. Holes move faster than electrons, but they do not affect imaging.
On the left is an image taken at 7.5 frames per second, and on the right is an image taken at 15 frames per second.
The new sensor has a pixel size of 5 microns and a power generation capacity of 998 picowatts per square millimeter per lux, surpassing all the self-generating sensors so far. The clear-sky brightness is 60,000 lux, enough for the system to operate at 15 frames per second. Shooting. Ordinary lighting conditions are 20,000-30,000 lux. The system's maximum shooting speed is 7.5 frames per second. The standard video frame rate is 30 frames per second, but this is not required.
In addition, the research team stated that there is room for reduction in the energy consumption of the system. The team has experimented with a variety of low-power technologies, including automatic adjustment of the frame rate based on light, in order to allow the system to provide higher frames in the same light. rate.
The next goal of the project will be a practical self-charging wireless transmission camera.