Project description: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.
Project description:Methyl-ammonium lead iodide perovskite (MAPbI3) was synthesized in the form of micro-needles via a hydrothermal route at a low temperature of 100 °C in a two-step procedure for the first time. The results exhibit that the amount of the surfactant is crucial for the synthesis of the MAPbI3 nanostructures with well-controlled morphologies. In contrast to bulk MAPbI3, the one-dimensional (1-D) micro-needle perovskite with a diameter of 200 nm showed an improved hole injection from the perovskite to the hole transporting layer (HTL), providing a unique platform at the perovskite/HTL interface. The best performing device employing MAPbI3 perovskite micro-needles yielded stable and hysteresis-free devices with a best power conversion efficiency of (PCEbest) of 17.98%. The current findings highlight the potential of perovskite micro-needles as novel absorber systems and lay the basis for future commercialization.
Project description:So far, most techniques for modifying perovskite solar cells (PSCs) focus on either the perovskite or electron transport layer (ETL). For the sake of comprehensively improving device performance, a dual-functional method of simultaneously passivating trap defects in both the perovskite and ETL films is proposed that utilizes guidable transfer of Eu3+ in SnO2 to perovskite. Europium ions are distributed throughout the SnO2 film during the formation process of SnO2, and they can diffuse directionally through the SnO2/perovskite interface into the perovskite, while most of the europium ions remain at the interface. Under the synergistic effect of distributed Eu3+ in the SnO2 and aggregated Eu3+ at the interface, the electron mobilities of ETLs are evidently improved. Meanwhile, diffused Eu3+ ions passivate the perovskite to reduce trap densities at the grain boundaries, which can dramatically elevate the open-circuit voltage (V oc) of PSCs. Finally, the mainly PSCs coated on SnO2:Eu3+ ETL achieve a power conversion efficiency of 20.14%. Moreover, an unsealed device degrades by only 13% after exposure to ambient atmosphere for 84 days.
Project description:Paper offers a low-cost and widely available substrate for electronics. It possesses alternative characteristics to silicon, as it shows low density and high flexibility, together with biodegradability. Solution processable materials, such as hybrid perovskites, also present light and flexible features, together with a huge tunability of the material composition with varying optical properties. In this study, we combine paper substrates with halide-mixed perovskites for the creation of low-cost and easy-to-prepare perovskite-on-paper photodetectors with a broadband-tunable spectral response. From the bandgap tunability of halide-mixed perovskites we create photodetectors with a cut-off spectral onset that ranges from the NIR to the green region, by increasing the bromide content on MAPb(I1-xBrx)3 perovskite alloys. The devices show a fast and efficient response. The best performances are observed for pure I and Br perovskite compositions, with a maximum responsivity of ∼400 mA W-1 on the MAPbBr3 device. This study provides an example of the wide range of possibilities that the combination of solution processable materials with paper substrates offers for the development of low-cost, biodegradable and easy-to-prepare devices.
Project description:Understanding the formation chemistry of metal halide perovskites is key to optimizing processing conditions and realizing enhanced optoelectronic properties. Here, we reveal the structure of the crystalline precursor in the formation of methylammonium lead iodide (MAPbI3) from the single-step deposition of lead chloride and three equivalents of methylammonium iodide (PbCl2 + 3MAI) (MA = CH3NH3). The as-spun film consists of crystalline MA2PbI3Cl, which is composed of one-dimensional chains of lead halide octahedra, coexisting with disordered MACl. We show that the transformation of precursor into perovskite is not favored in the presence of MACl, and thus the gradual evaporation of MACl acts as a self-regulating mechanism to slow the conversion. We propose the stable precursor phase enables dense film coverage and the slow transformation may lead to improved crystal quality. This enhanced chemical understanding is paramount for the rational control of film deposition and the fabrication of superior optoelectronic devices.
Project description:Controllable growth of perovskite nanowires is very important for various applications in optical and electrical devices. Although significant progress has been achieved in the solution method, a deep understanding of the mechanics of growing perovskite nanowires is still lacking. Herein, we developed an electrochemical method for growing the perovskite nanowires and studied the growth processes systematically. The initial nucleation and crystal growth could be controlled by simply varying the additive solvents, thus leading to two stable size ratio distributions of the perovskite nanowires. Further, with compositional engineering, the bandgap of the perovskites could be tuned from 1.59 eV to 3.04 eV. All the as-grown perovskite nanowires displayed a unique structure with high crystallization quality, contributing to a very high responsivity of 2.1 A W-1 and a large on/off ratio of 5 × 103 for the photodetectors based on the CH3NH3PbBr3 nanowires. All of these findings demonstrate that the optimized solution method offers a new approach to synthesize perovskite nanowires for applications in photoelectric devices.
Project description:Hot-carrier cooling (HCC) in metal halide perovskites above the Mott transition is significantly slower than in conventional semiconductors. This effect is commonly attributed to a hot-phonon bottleneck, but the influence of the lattice properties on the HCC behavior is poorly understood. Using pressure-dependent transient absorption spectroscopy, we find that at an excitation density below the Mott transition, pressure does not affect the HCC. On the contrary, above the Mott transition, HCC in methylammonium lead iodide is around 2-3 times faster at 0.3 GPa than at ambient pressure. Our electron-phonon coupling calculations reveal ∼2-fold stronger electron-phonon coupling for the inorganic cage mode at 0.3 GPa. However, our experiments reveal that pressure promotes faster HCC only above the Mott transition. Altogether, these findings suggest a change in the nature of excited carriers above the Mott transition threshold, providing insights into the electronic behavior of devices operating at such high charge-carrier densities.
Project description:Hybrid organic-inorganic perovskite materials are promising materials for photovoltaic and optoelectronic applications. Nevertheless, the construction of a computationally efficient potential model for atomistic simulations of perovskite with high fidelity to ab initio calculations is not a trivial task given the chemically complex nature of perovskite in terms of its chemical components and interatomic interactions. In the present study, we demonstrate that artificial neural network (ANN) models can be employed for efficient and accurate potential energy evaluation of MAPbI3 perovskite materials. The ANN models were trained using training sets composed of thousands of atomic images of tetragonal MAPbI3 crystals, with their respective energies and atomic forces obtained from ab initio calculations. The trained ANN models were validated by predicting the lattice parameters and energies/atomic forces of cubic MAPbI3 perovskite and had excellent agreement with ab initio calculations. The phonon modes could also be extracted using the trained ANN model with good agreement with ab initio calculations, provided that the atomic forces were incorporated into the training processes. Finally, we demonstrate that for a given system size, the trained ANN model offers 104 to 105 faster time consumption per energy evaluation relative to ab initio calculations using Vienna Ab initio Simulation Package, demonstrating the potential of the ANN model for exhaustively sampling the configuration spaces of chemically complex materials for predictions of thermodynamic properties and phase stabilities.
Project description:Methylammonium lead triiodide perovskite (CH₃NH₃PbI₃, MAPbI₃) has been emerging as an easy processing and benign defect material for optoelectronic devices. Fiber-like perovskite materials are especially in demand for flexible applications. Here we report on a kind of polyacrylonitrile (PAN)/MAPbI₃ composite fiber, which was electrospun from the mixing solution of PAN and MAPbI₃. The absorption edge and optical gap of the PAN/MAPbI₃ composite fibers can be easily tuned as the ratio of the perovskite changes. Both the moisture stability and the thermal stability of the perovskite are improved with the protection of PAN polymers. Flexible photodetectors based on this perovskite fiber were fabricated and analyzed. The photoresponse of the detector was highly sensitive to broadband visible light, and reached 6.5 μA W-1 at 700 nm with a voltage bias of 10 V. Compared with pure MAPbI₃ photodetectors, this composite fiber photodetector has much-improved stability and flexibility, which can even be used to detect motion-related angular changes.
Project description:Controlling the high-power laser transmittance is built on the diverse manipulation of multiple nonlinear absorption (NLA) processes in the nonlinear optical (NLO) materials. According to standard saturable absorption (SA) and reverse saturable absorption (RSA) model adapted for traditional semiconductor materials, the coexistence of SA and RSA will result in SA induced transparency at low laser intensity, yet switch to RSA with pump fluence increasing. Here, we observed, in contrast, an unusual RSA to SA conversion in quasi-two-dimensional (2D) perovskite film with a low threshold around 2.6 GW cm-2. With ultrafast transient absorption (TA) spectra measurement, such abnormal NLA is attributed to the competition between excitonic absorption enhancement and non-thermalized carrier induced bleaching. TA singularity from non-thermalized "Fermi Sea" is observed in quasi-2D perovskite film, indicating an ultrafast carrier thermalization within 100 fs. Moreover, the comparative study between the 2D and 3D perovskites uncovers the crucial role of hot-carrier effect to tune the NLA response. The ultrafast carrier cooling of quasi-2D perovskite is pointed out as an important factor to realize such abnormal NLA conversion process. These results provide fresh insights into the NLA mechanisms in low-dimensional perovskites, which may pave a promising way to diversify the NLO material applications.