Bias-Modulated Multicolor Discrimination Enabled By Perovskite Photodetector with Back-to-Back Diode Configuration
Authors: Kim Hyeonghun , Kim Woochul , Pak Yusin , Jung Gun Young , Lee Sunghwan 
Published: July 2021

Photodetectors that convert incoming photons into electrical signals play a key role in wide emerging technologies, such as imaging, optical communication, bio-sensing, and environmental monitoring systems. Most conventional photo-detecting systems consist of broadband photodetectors and dissipative optical filters for measuring not only the brightness of illuminated light but also its spectral information. However, the integration of filters on top of the pre-fabricated detectors requires time-consuming and sophisticated alignment steps, consequently increasing the device cost. In addition, a considerable light loss caused by the filter (> 2/3 of the incident light) deteriorates the sensitivity of entire systems, particularly fatal in a relatively dark environment. Therefore, it is imperative to develop filter-free photodetectors (FFPs) with high sensitivity and specificity (narrow spectral response).
Very recently, organic/inorganic perovskites have been intensively studied and exploited as promising FFPs due to the outstanding optoelectronic properties of perovskites (e.g., high extinction coefficients, small exciton binding energies, long exciton diffusion lengths, and optical gap tuneability). Perovskite FFPs appear to be quite effective in achieving not only a narrow-band spectral response with full-width at half-maximum values < 100 nm but also sufficient external quantum efficiencies of 10-200%. However, the FFPs consisting of a single perovskite absorber are limited to a specific spectral response, which is not suitable for the discrimination of multi-colors. In particular, to mimic photoreceptors in the retina of the human eye, at least three individual FFPs with different light absorbers (i.e., for red (R), green (G), and blue (B) light) are necessary for the discrimination of the visible light spectrum. Therefore, the design of a single FFP with multi-spectral responses would be highly beneficial for creating an integrated and high-performance color imaging system.
Herein, we demonstrate, for the first time, a perovskite FFP adopting a back-to-back diode (p-i-n-i-p) configuration, in which a low-band n-i-p perovskite diode is vertically stacked on top of a high-band p-i-n perovskite diode. The unique device structure enables multi-spectral responses in the visible range through bias modulation. When a specific voltage is applied to the contact electrodes, one perovskite diode becomes active as a light absorber, while the other diode is inactive, and vice versa in the opposite voltage; thus, the FFP with the p-i-n-i-p configuration allows selective charge extraction from each perovskite by switching the bias polarity. This unique operation based on the two different perovskites can discriminate monochromatic RGB colors by a single device with a notably superior detectivity (> 1012 cm Hz1/2 W-1) to those of other conventional FFPs. In addition, we succeeded in discriminating mixed-color light using two p-i-n-i-p perovskite FFPs, which is more space- and cost-efficient than the conventional imaging system with three different dissipative color filters.

Figure 1