Multilayer Chlorophyll‐Based Unidirectional Bias‐Switchable Photodetector with Dual‐Mode Operation in Narrowband and Broadband

Abstract Photodetectors (PDs) generally operate in either broadband (BB) or narrowband (NB) mode. However, single‐mode PDs can no longer meet the diverse and complex demands of contemporary applications. In this study, an NB/BB dual‐mode PD is fabricated by sensitizing mesoporous TiO 2 (m‐TiO 2 ) with methyl trans ‐3 2 ‐carboxy‐pyropheophorbide‐ a (H 2 Chl‐1) and spin‐coating films of 13 1 ‐deoxo‐13 1 ‐dicyanomethylene‐pyropheophorbide‐ a (H 2 Chl‐2) and zinc methyl 3‐devinyl‐3‐hydroxymethyl‐pyropheophorbide‐ a (ZnChl). As the external electric field increases from −2 to −3 V, the operating mode of the PD shifts from NB to BB mode. This transition is attributed to the recombination of electrons and holes at the interface between m‐TiO 2 and H 2 Chl‐1, as well as the regulation by the external electric field. The PD demonstrates an NB photoresponse in the near‐ultraviolet region, with a full width at half maximum of 45 nm at −2 V. As the bias voltage increases, the PDs transition to a BB photoresponse mode, achieving a responsivity of 8.07 × 10 4 A W −1 and a specific detectivity of 5.82 × 10 12 Jones at −6 V. NB‐PD arrays are constructed for image sensing. BB‐PD arrays enable image random encryption and decryption functions by implementing “OR” logic gate. Furthermore, the integration of NB‐PD and BB‐PD modes enhances the security of information transmission.