The rapid development of Internet of Things and big data has made the conventional storage devices face the need of reforming. Rather than using electrical pulses to store data in one of two states, photomemory exploiting optical stimulation to store light information emerges as a revolutionary candidate for the optoelectronic community. However, fully optically driven photomemory with fast data transmission speed and outstanding energy saving capability suffers from less exploration. Herein, a transistortype photomemory using a 2D Cs 2 Pb(SCN) 2 Br 2 /polymer hybrid floating gate is explored and three host polymers, polystyrene, poly(4-vinylphenol), and poly(vinylpyrrolidone) (PVP), are investigated to understand the relationship between polymer matrix selection and memory performance. All devices show a photoinduced recovery memory behavior but with two distinctly different photomemory behaviors. In addition to the demonstration of a regular nonvolatile photomemory showing a high on/off ratio of >10 6 over 10 4 s, an unusual fully optically driven memory behavior is intriguingly accomplished in the Cs 2 Pb(SCN) 2 Br 2 /PVP photomemory. Using white light as the driver of programming and a blue laser as the main performer of erasing, this device can be switched between two distinguishable states and possesses acceptable data discriminability, as evidenced by its fully optically driven writing (programing)-reading-erasing-reading switching function that shows an on/off current ratio of 10 3 . This study not only presents the first 2D perovskite-based photomemory but also shows a novel fully optically driven memory that has been rarely reported in the literature.

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In this review, the latest developments in low dimensional perovskites and different approaches to constructing 2D perovskites are discussed, involving the traditional way of altering A-site cation and the latest way of varying X-site anion.

Two‐dimensional (2D) tin (Sn)‐based perovskites have recently received increasing research attention for perovskite transistor application. Although some progress is made, Sn‐based perovskites have long suffered from easy oxidation from Sn2+ to Sn4+, leading to undesirable p‐doping and instability. In this study, it is demonstrated that surface passivation by phenethylammonium iodide (PEAI) and 4‐fluorophenethylammonium iodide (FPEAI) effectively passivates surface defects in 2D phenethylammonium tin iodide (PEA2SnI4) films, increases the grain size by surface recrystallization, and p‐dopes the PEA2SnI4 film to form a better energy‐level alignment with the electrodes and promote charge transport properties. As a result, the passivated devices exhibit better ambient and gate bias stability, improved photo‐response, and higher mobility, for example, 2.96 cm2 V−1 s−1 for the FPEAI‐passivated films—four times higher than the control film (0.76 cm2 V−1 s−1). In addition, these perovskite transistors display non‐volatile photomemory characteristics and are used as perovskite‐transistor‐based memories. Although the reduction of surface defects in perovskite films results in reduced charge retention time due to lower trap density, these passivated devices with better photoresponse and air stability show promise for future photomemory applications.

Pb 2+ ; X = Cl − , Br − , I − ) have demonstrated impressive performance. Capitalizing on their tunable dimensionality, layered perovskites such as 2D and 2D/3D (or quasi-2D) structures have also emerged significantly expanding semiconducting material candidates due to the wide diversity of potential A-site cation and/or X-site anion spacer selections. [9,10] The unique advantages of layered lead halide perovskites are high stability, quantum confinement effect, and tunable photophysical properties. They are increasingly sought after for optoelectronic research in recent years. [11][12][13] The most popular layered lead halide perovskites are based on the Ruddlesden-Popper (RP) structure with the general formula of A n+1 B n X 3n+1 (n = 1, 2, …), where A-site-substituted layered RP-type perovskites are generally hybrid organic-inorganic compounds containing long organic chains. These long organic chains however are insulating. Added to this, the spatial confinements of charge carrier and dielectric contrast between perovskite stack and spacer [11,12] result in a large exciton binding energy, typically ten times larger than the those of 3D perovskites. [14] Most of the reported 2D Ruddlesden-Popper (RP) lead halide perovskites with the general formula of A n+1 B n X 3n+1 (n = 1, 2, …) comprise layered perovskites separated by A-site-substituted organic spacers. To date, only a small number of X-site-substituted RP perovskites have been reported. Herein, the first inorganic-cation pseudohalide 2D phase perovskite single crystal, Cs 2 Pb(SCN) 2 Br 2 , is reported. It is synthesized by the antisolvent vapor-assisted crystallization (AVC) method at room temperature. It exhibits a standard single-layer (n = 1) Ruddlesden-Popper structure described in space group of Pmmn (#59) and has a small separation (d = 1.69 Å) between the perovskite layers. The SCN − anions are found to bend the 2D Pb(SCN) 2 Br 2 framework slightly into a kite-shaped octahedron, limiting the formation of a quasi-2D perovskite structure (n > 1). This 2D single crystal exhibits a reversible first-order phase transformation to 3D CsPbBr 3 (Pm3m #221) at 450 K. It has a low exciton binding energy of 160 meV-one of the lowest for 2D perovskites (n = 1). A Cs 2 Pb(SCN) 2 Br 2 -single-crystal photodetector is demonstrated with respectable responsivity of 8.46 mA W −1 and detectivity of ≈1.2 × 10 10 Jones at a low bias voltage of 0.5 V.