Narrowband photodetectors can sense only a small spectral range of light and thus realize the color discrimination. The ability of spectral discrimination for narrowband photodetectors makes them attractive in many applications including biomedical sensing, machine vision and imaging. Up to date, a number of strategies have been developed to realize narrowband photodetections, including: (1) combining an optical filter with a broadband photodetector; (2) using an absorber with a narrowband absorption; (3) intentionally enhancing the absorption in a selected wavelength range by the plasmonic effect; and (4) manipulating the external quantum efficiency via charge collection narrowing (CCN). Despite narrowband photodetectors based on these approaches have been demonstrated with an appropriate set of performance metrics, they still have some disadvantages. To this end, a low-cost narrowband photodetector with a great architectural simplicity and a wider tunable spectral response range is much desired.
Two-dimensional (2D) perovskites exhibit strong electron-phonon coupling leads to the formation of self-trapped states within bandgap. Those self-trapped states in 2D perovskites are excitonic in nature and possess weak optical transition strength with a longer lifetime. With these unique features, self-trapped states in 2D perovskites with distinguished weak absorption peak below free exciton absorption peak naturally form another absorption onset to realize the CCN via recombination losses for narrowband photodetection, thus bypass the necessity to specially design the active materials. Furthermore, the bandgap energy of 2D perovskites can be readily tuned within entire visible wavelength range by tuning the halide anion and the layer number of 2D perovskites. Along with the extremely large anisotropy between the in-plane and out-of-plane electrical conductivity in 2D perovskites, we anticipate that the narrowband photodetectors of 2D perovskite single crystal vertical structures can exhibit excellent performance with high external quantum efficiency, low dark current and high on-off ratio in the entire visible wavelength range.
Recently, we demonstrated narrowband photodetectors based on 2D perovskites single-crystal plates with high external quantum efficiency of 200%, ultralow dark current of 10-12 A and high on-off ratio of 103 across the entire visible spectrum. The full-width at half-maximum are less than 60 nm across the entire visible spectrum, and especially less than 20 nm in the blue wavelength range. Compared with 3D counterparts, our 2D perovskite based narrowband photodetectors show a greatly enhanced EQE and can sustain two orders of magnitude higher electrical field and be made two orders of magnitude thinner. The excellent performance of our narrowband photodetectors can be ascribed to the strong electron-phonon interaction induced self-trapped states within bandgap and extremely low electrical conductivity in the out-of-plane direction, both of which efficiently facilitate CCN of the external quantum efficiency. By simply using the naturally formed two absorption onsets in 2D perovskites, we can avoid to specially design materials to obtain two different absorption regimes. Our findings provide an alternative paradigm to utilize the self-trapped states to achieve narrowband photodetections with a high external quantum efficiency for many image applications and open the exciting potential of 2D perovskites for next-generation optoelectronics.
The related paper has been published in Nature Communications. See details: Junze Li, Jun Wang, Jiaqi Ma, Hongzhi Shen, Lu Li, Xiangfeng Duan & Dehui Li*. Self-trapped state enabled filterless narrowband photodetections in 2D layered perovskite single crystals. Nat. Commun. 2019, 10, 806.