Self-Powered Wide-Narrow Bandgap-Laminated Perovskite Photodetector with Bipolar Photoresponse for Secure Optical Communication

Perovskite photodetectors with bipolar photoresponse characteristics are expected to be applied in the field of secure optical communication (SOC). However, how to realize the perovskite photodetector with bipolar response remains challenging. Herein, by introducing bismuth iodide (BiI3 ) into Sn-Pb mixed perovskite precursor solution, 2D perovskite FA3 Bi2 I9 is spontaneously formed at the bottom to realize a wide-narrow bandgap-laminated perovskite film. Wavelength-dependent bipolar response is realized based on the absorption difference of the photoactive region with different bandgap combined with the carrier competition of the homotypic transport layer adopted in the as-fabricated photodetector. Under the visible/near-infrared (NIR) light irradiation, the bottom/top of the film generates a higher carrier concentration, where electrons are easier to be separated and transported by the SnO2 /PC61 BM to the bottom/top electrodes, respectively, resulting in a negative and positive bipolar response. Finally, based on positive NIR signal as the effective signal and negative visible signal as the interference signal, the SOC system is realized, where the positive NIR signal is well hidden by the negative visible signal. This work provides a simple and feasible strategy for fabrication of laminated perovskite films to achieve bipolar response.

Identification and Mitigation of Transient Phenomena That Complicate the Characterization of Halide Perovskite Photodetectors

Halide perovskites have shown promise to advance the field of light detection in next-generation photodetectors, offering performance and functionality beyond what is currently possible with traditional inorganic semiconductors. Despite a relatively high density of defects in perovskite thin films, long carrier diffusion lengths and lifetimes suggest that many defects are benign. However, perovskite photodetectors show detection behavior that varies with time, creating inconsistent device performance and difficulties in accurate characterization. Here, we link the changing behavior to mobile defects that migrate through perovskites, leading to detector currents that drift on the time scale of seconds. These effects not only complicate reproducible device performance but also introduce characterization challenges. We demonstrate that such transient phenomena generate measurement artifacts that mean the value of specific detectivity measured can vary by up to 2 orders of magnitude even in the same device. The presence of defects can lead to photoconductive gain in photodetectors, and we show batch-to-batch processing variations in perovskite devices gives varying degrees of charge carrier injection and photocurrent amplification under low light intensities. We utilize the passivating effect of aging to reduce the impact of defects, minimizing current drifts and eliminating the gain. This work highlights the potential issues arising from mobile defects, which lead to inconsistent photodetector operation, and identifies the potential for defects to tune photodetection behavior in perovskite photodetectors.

Defect recombination suppression and carrier extraction improvement for efficient CsPbBr3/SnO2heterojunction photodetectors

Perovskite materials with excellent optical and electronic properties have huge potential in the research field of photodetectors. Constructing heterojunctions and promoting carrier transportation are significant for the development of perovskite-based optoelectronics devices with high performances. Herein, we demonstrated a CsPbBr₃/SnO₂heterojunction photodetector and improved the device performances through post-annealing treatment of SnO₂film. The results indicated that the electrical properties of SnO₂films will make an important impact on carrier extraction, especially for type-II heterojunction. As the electrons transfer layer in CsPbBr₃/SnO₂type-II heterojunction, defects related to oxygen vacancy should be the key factor to affect carrier concentration, induce carriers' limitation and recombination rate. Under proper annealing temperature for SnO₂layer, the recombination rate can decrease to 1.37 × 10²¹cm³s and the spectral responsivity will be highly increased. This work can enhance the understanding on the photoresponse of perovskite photodetectors, and will be helpful for the further optimization and design of optoelectronic devices based on the perovskite heterojunction.

Tunable Multiband Halide Perovskite Tandem Photodetectors with Switchable Response

Photodetectors with multiple spectral response bands have shown promise to improve imaging and communications through the switchable detection of different photon energies. However, demonstrations to date have been limited to only two bands and lack capability for fast switching in situ. Here, we exploit the band gap tunability and capability of all-perovskite tandem solar cells to demonstrate a new device concept realizing four spectral bands of response from a single multijunction device, with fast, optically controlled switching between the bands. The response to monochromatic light is highly selective and narrowband without the need for additional filters and switches to broader response bands on applying bias light. Sensitive photodetection above 6 × 10¹¹ Jones is demonstrated in all modes, with rapid switching response times of <250 ns. We demonstrate proof of principle on how the manipulation of the modular multiband detector response through light conditions enables diverse applications in optical communications with secure encryption.

Versatile Pendant Polymer for Selective Charge Carrier Transport via Controlling the Supramolecular Self-Assembly

Polyvinyl carbazole (P0)-based pendant polymers were synthesized by modifying carbazole motifs with pyrene derivatives (P1 and P4) to manipulate the bandgap and frontier orbital energy levels. To establish the electronic properties of pendant polymers according to structural differences, the polymers were utilized as additional hole transport layers in planar-type perovskite solar cells and organic photovoltaic cells. When P4 with thiophene-pyrene pendant was used as hole transport layer, all device parameters, except open-circuit voltage, were significantly improved in comparison with P0 and P1 (conjugated with t-butyl pyrene derivatives). Since P4 had more electrically conductive thiophene units than benzene units with fewer alkyl groups, the supramolecular assembly of P4 was found to be more favorable in electronic devices. Furthermore, devices with P4 demonstrated lower dark current than others, which could potentially be useful for charge carrier transport and sensitive photo detecting devices.

Ultrafast, Self-Powered, and Charge-Transport-Layer-Free Ultraviolet Photodetectors Based on Sequentially Vacuum-Evaporated Lead-Free Cs2AgBiBr6 Thin Films

Researchers have focused on perovskite-based ultraviolet photodetectors due to their significance in fundamental scientific and practical applications. However, toxicity and instability hold back their mass production and commercialization. The lead-free Cs₂AgBiBr₆ double perovskite, promised to be an alternative, is fabricated mostly by spin coating, which restricts the practical application in high-resolution image sensors. Herein, we demonstrate a sequential vacuum evaporation method for the fabrication of the Cs₂AgBiBr₆ film. A self-powered ultraviolet photodetector based on the evaporated Cs₂AgBiBr₆ thin film is further constructed without any carrier-transport layers, for the first time. The best-performing device has a high on/off ratio of 6.6 × 10³, and its response time is fast, less than 6.13 μs. Moreover, the as-prepared devices exhibit salient stability under harsh operational conditions (continuous illumination, high temperature, and humidity). In addition, the pixelated image sensor containing a 25 × 25 Cs₂AgBiBr₆ photodetector array achieves a proof-of-concept special pattern recognition. Our work paves the way for new-generation ultraviolet image sensors composed of environmentally friendly and high-performance perovskite photodetector arrays.

The Effects of Temperature on the Growth of a Lead-Free Perovskite-Like (CH3NH3)3Sb2Br9 Single Crystal for An MSM Photodetector Application

We have fabricated a photodetector based on (CH₃NH₃)₃Sb₂Br₉ (MA₃Sb₂Br₉) lead-free perovskite-like single crystal, which plays an important role in the optoelectronic characteristics of the photodetector as a perovskite-like photosensitive layer. Here, MA₃Sb₂Br₉ single crystals were synthesized by an inverse temperature crystallization process with a precursor solution at three different growth temperatures, 60 °C, 80 °C, and 100 °C. As a result, a MA₃Sb₂Br₉ single crystal with an optimum growth temperature of 60 °C presented a low trap density of 2.63 × 10¹¹ cm^-3, a high charge carrier mobility of 0.75 cm² V^-1 s^-1, and excellent crystal structure and optical absorption properties. This MA₃Sb₂Br₉ perovskite-like photodetector displayed a low dark current of 8.09 × 10^-9 A, high responsivity of 0.113 A W^-1, and high detectivity of 4.32 × 10¹¹ Jones.

Enhanced Weak-Light Detection of Perovskite Photodetectors through Perovskite/Hole-Transport Material Interface Treatment

Enhancement in weak-light detection and other photodetection properties was observed for organic-inorganic halide perovskite photodetectors as a result of benzylammonium iodide (BzAI) treatment at the methylammonium lead triiodide (MAPbI₃) and hole-transport layer (HTL) interface. After treatment, growth of the two-dimensional Ruddlesden-Popper perovskite phase was observed at the MAPbI₃ surface, which shifted the overall surface work function upwards and thus effectively facilitated charge transfer across the MAPbI₃/HTL interface. As a result, the fully fabricated device with 10 mg/mL (BzAI/isopropanol) treatment exhibited shorter rise time (t_rise) and decay time (t_decay) of 53 and 38 μs, respectively, compared to t_rise and t_decay of 214 and 120 μs, respectively, for the pristine MAPbI₃ sample. In addition, the BzAI-treated device exhibited larger linearity compared to the pristine MAPbI₃ sample, demonstrating a high and stable specific detectivity of 1.49 × 10¹³ to 2.14 × 10¹³ Jones under incident light intensity of 10^-3 to 10⁰ mW/cm², respectively.