Lead halide perovskites have gained extensive attention in the photodetectors (PDs) field, but the problems of intrinsic toxicity and poor stability still hamper their practical applications. Furthermore, ultraviolet (UV) PDs have rarely been explored due to challenges, including the lack of suitable materials and the costly and complicated fabrication processes. In order to solve the above problems, herein it is reported a novel all-inorganic lead-free halide CsAg 2 I 3 single crystal (SC) with pure phase and high quality. This CsAg 2 I 3 SC shows ultra-stable crystal structure, steady chemical properties, and good optical properties, and it can be stable up to 893 K. Its 1D crystal structure and soft crystal lattice support the formation of self-trapped excitons, leading to a broadband emission at 610 nm with a large Stokes shift of 315 nm (2.23 eV). The strong absorption for UV light combined with direct bandgap, large Stokes shift, and ultra-high stability promises an environment-friendly, low-cost, and high-performance UV PD based on the 1D all-inorganic silver (I) halides.

Singlet fission (SF) is a spin-allowed exciton multiplication process, in which a photogenerated singlet separates efficiently into two free triplets. Herein, we report an experimental study on the solution-phase intermolecular SF (xSF) in a prototype radical dianion system of PTCDA2−, which is produced from its neutral precursor PTCDA (i.e., perylenetetracarboxylic dianhydride) via a two-step consecutive photoinduced electron transfer mechanism. Our ultrafast spectroscopic results enable a comprehensive mapping of the elementary steps involved in the solution-phase xSF process of photoexcited PTCDA2−. Along the cascading xSF pathways, the three intermediates including excimer 1(S1S0), spin-correlated triplet pair 1(T1T1), and spatially separated triplet pair 1(T1·S0·T1) have been identified, with their formation/relaxation time constants being determined. This work demonstrates that the solution-phase xSF materials can be extended to charged radical systems and that the three-step model usually adopted to describe the crystalline-phase xSF can also be valid in describing solution-phase xSF.

We present a mechanistic study of a PTCDA 2− /TiO 2 dyesensitized photocatalytic system, in which the stable radical dianion PTCDA 2− is formed via a two-step consecutive photoinduced electron transfer from its neutral precursor PTCDA (i.e., perylenetetracarboxylic dianhydride). Photoexcitation of PTCDA 2− brings forth an interesting behavior known as vibrationally excited-state-selective, visible-light photocatalytic hydrogen evolution reaction (HER). In conjunction with the information gleaned from optical spectroscopy and ultrafast dynamics, we reveal that an intermediate complex (IC) state with a lifetime of ∼12 ps exists in the vicinity of a certain vibrationally excited state of PTCDA 2− . Such a unique IC state mediates the interfacial electron transfer (IET) channel from the specific excited state of PTCDA 2− to the conduction band continuum of TiO 2 . As an outcome, the effective IC-mediated IET process in this photocatalytic system leads to a remarkable HER rate that reaches ∼4660 μmol g −1 h −1 .