Thin-film transistors play a key role in organic image sensors by controlling the flow of information to readout electronics. However, they limit pixel size, making it difficult to scale photosensors to high resolutions. New research aims to eliminate the need for transistors in organic sensors by incorporating a switch mechanism directly inside the photodetector, integrating sense and control into a single device.

The device works in the same way as traditional photodetectors. A two-dimensional array of pixels records the brightness of an image, but instead of using an embedded transistor to disrupt photodetector output, the detectors themselves are switched on one at a time.

A blocking layer is incorporated at the bottom of each photodetector stack, which frustrates charge transport to ensure that a pixel in an off state is unresponsive to irradiance. Devices without a blocking layer behave as regular photodiodes.

The researchers fabricated 45 individual switchable pixels by thermal evaporation, which they used to simulate a 900-pixel array. Each pixel was independently tested and calibrated to analyze uniformity and predict scalability.

The difference between the experimentally obtained and simulated images was nearly undetectable. The authors said this implies the method can be generalized to fabricate larger non-planar, flexible and stretchable image sensors using organic materials.

“The ability to shape image sensors in the future will bring tremendous benefits to optical systems,” said author C. Kyle Renshaw.

Currently, device response and detectivity are lower than in traditional image sensors, and the pixel pitch is larger. However, this work gives proof of concept for the scalability of transistor-free, flexible and lightweight organic sensors.

“This introduces a new paradigm for image sensors enabled by the tailored design of multifunctional devices,” Renshaw said.

Source: “Organic photodetectors with frustrated charge transport for small-pitch image sensors,” by Z. Ma and C. K. Renshaw, Journal of Applied Physics (2019). The article can be accessed at