Lead-Free Perovskite Nanowire Array Photodetectors with Drastically Improved Stability in Nanoengineering Templates

Lead-Free Perovskite/Organic Semiconductor Vertical Heterojunction for Highly Sensitive Photodetectors

In recent years, photodetectors based on organic-inorganic lead halide perovskites have been studied extensively. However, the inclusion of lead in those materials can cause severe human health and environmental problems, which is undesirable for practical applications. Here, we report high-performance photodetectors with a tin-based perovskite/PEDOT:PSS vertical heterojunction. The device demonstrates a broadband photoresponse from NIR to UV. The maximum responsivity and gain are up to 2.6 × 10⁶ A/W and 4.7 × 10⁶, respectively. Moreover, a much shorter response time and higher detectivity can be achieved by reducing the thickness of PEDOT:PSS. The outstanding performance is due to the excellent optoelectronic properties of the perovskite and the photogating effect originating from the heterojunction. Furthermore, devices fabricated on flexible substrates can demonstrate not only high sensitivity but also excellent bending stability. This work opens up the opportunity of using lead-free perovskite in highly sensitive photodetectors with vertical heterojunctions.

Synthesis and optical applications of low dimensional metal-halide perovskites

Metal halide perovskites have received substantial attention in research communities due to their outstanding efficiency achievements in the field of photovoltaics, optoelectronics and electronics, exhibiting extraordinary optical, electrical and mechanical properties. The exceptional structural tunability enables perovskite material to possess low-dimensional form at the atomic level and extends their applications into optoelectronic and photonic fields. This review discusses the recent progress of synthetic routes and fundamental optoelectronic properties of low-dimensional metal halide perovskites. In addition, the focus is to highlight the potential applications of perovskites in various devices including solar cells, light-emitting diodes, lasers, waveguides and memory devices. Finally, outlooks and the challenges that face the development of the perovskite materials in the near future are also presented.

A Microchannel-Confined Crystallization Strategy Enables Blade Coating of Perovskite Single Crystal Arrays for Device Integration

Perovskite single crystals (PSCs) possess superior optoelectronic properties compared to their corresponding polycrystalline films, but their applications of PSCs in high-performance, integrated devices are hindered by their heavy thickness and difficulty in scalable deposition. Here, a microchannel-confined crystallization (MCC) strategy to grow uniform and large-area PSC arrays for integrated device applications is reported. Benefiting from the confinement effect of the microchannels, solution flow dynamics is well controlled, and thus uniform deposition of PSC arrays with suitable thickness is achieved, meaning they are applicable for scale-up device applications. The resulting PSCs possess excellent optoelectronic properties in terms of a long carrier lifetime (175 ns) and an ultralow defect density (2 × 10⁹ cm^-3 ), which are comparable to the corresponding bulk crystals. The unique embedded structure of PSCs within the microchannels allows the construction of a high-integration image sensor. This work paves the way toward high-throughput growth of PSCs for integrated optoelectronic devices.

Inverse Growth of Large-Grain-Size and Stable Inorganic Perovskite Micronanowire Photodetectors

Control of forward and inverse reactions between perovskites and precursor materials is key to attaining high-quality perovskite materials. Many techniques focus on synthesizing nanostructured CsPbX₃ materials (e.g., nanowires) via a forward reaction (CsX + PbX₂ → CsPbX₃). However, low solubility of inorganic perovskites and complex phase transition make it difficult to realize the precise control of composition and length of nanowires using the conventional forward approach. Herein, we report the self-assembly inverse growth of CsPbBr₃ micronanowires (MWs) (CsPb₂Br₅ → CsPbBr₃ + PbBr₂↑) by controlling phase transition from CsPb₂Br₅ to CsPbBr₃. The two-dimensional (2D) structure of CsPb₂Br₅ serves as nucleation sites to induce initial CsPbBr₃ MW growth. Also, phase transition allows crystal rearrangement and slows down crystal growth, which facilitates the MW growth of CsPbBr₃ crystals along the 2D planes of CsPb₂Br₅. A CsPbBr₃ MW photodetector constructed based on the inverse growth shows a high responsivity of 6.44 A W^-1 and detectivity of ∼10¹² Jones. Large grain size, high crystallinity, and large thickness can effectively alleviate decomposition/degradation of perovskites, which leads to storage stability for over 60 days in humid environment (relative humidity = 45%) and operational stability for over 3000 min under illumination (wavelength = 400 nm, light intensity = 20.06 mW cm^-2).

Three-Dimensional Perovskite Nanophotonic Wire Array-Based Light-Emitting Diodes with Significantly Improved Efficiency and Stability

Hybrid perovskites have emerged as promising candidates for highly efficient light-emitting diodes in the past few years due to their excellent crystallinity, high color purity, wide-range bandgap tunability, and solution processability. However, the reported device external quantum efficiency has not reached the level on par with that of conventional inorganic and organic light-emitting diodes. Moreover, device stability still needs substantial improvement. In this work, we demonstrate the fabrication of perovskite nanophotonic wire array-based light-emitting diodes with a capillary-effect-assisted template method. Compared with the planar control device, the nanostructured device demonstrates 45% improvement of external quantum efficiency from 11% to 16% owing to substantial enhancement on device light extraction efficiency verified by optical modeling. Intriguingly, it is also discovered that the nanostructured device possesses 3.89 times lifetime compared to the planar control device, due to effective template passivation. The results here have clearly shown that with a proper photonic device structure design, both the device performance and lifetime can be significantly improved.

"Unleaded" Perovskites: Status Quo and Future Prospects of Tin-Based Perovskite Solar Cells

The tremendous interest focused on organic-inorganic halide perovskites since 2012 derives from their unique optical and electrical properties, which make them excellent photovoltaic materials. Pb-based halide perovskite solar cells, in particular, currently stand at a record efficiency of ≈23%, fulfilling their potential toward commercialization. However, because of the toxicity concerns of Pb-based perovskite solar cells, their market prospects are hindered. In principle, Pb can be replaced with other less-toxic, environmentally benign metals. Sn-based perovskites are thus the far most promising alternative due to their very similar and perhaps even superior semiconductor characteristics. After years of effort invested in Sn-based halide perovskites, sufficient breakthroughs have finally been achieved that make them the next runners up to the Pb halide perovskites. To help the reader better understand the nature of Sn-based halide perovskites, their optical and electrical properties are systematically discussed. Recent progress in Sn-based perovskite solar cells, focusing mainly on film fabrication methods and different device architectures, and highlighting roadblocks to progress and opportunities for future work are reviewed. Finally, a brief overview of mixed Sn/Pb-based systems with their anomalous yet beneficial optical trends are discussed. The current challenges and a future outlook for Sn-based perovskites are discussed.

Sn-Based Perovskite for Highly Sensitive Photodetectors

Organic-inorganic hybrid perovskites have emerged as promising functional materials for high-performance photodetectors. However, the toxicity of Pb and the lack of internal gain mechanism in typical perovskites significantly hinder their practical applications. Herein, a low-voltage and high-performance photodetector based on a single layer of lead-free Sn-based perovskite film is reported. The device shows broadband response from ultraviolet to near-infrared light with a responsivity up to 105 A W-1 and a high gain at a low operating voltage. The outstanding performance is attributed to the high hole mobility, p-doping nature, and excellent optoelectronic properties of the Sn-based perovskite. Moreover, the device is assembled on a flexible substrate and demonstrates both high sensitivity and good bending stability. This work demonstrates a route for realizing nontoxic, low-cost, and high-performance perovskite photodetectors with a simple device structure.

Significantly Enhancing Response Speed of Self-Powered Cu2ZnSn(S,Se)4 Thin Film Photodetectors by Atomic Layer Deposition of Simultaneous Electron Blocking and Electrode Protective Al2O3 Layers

Kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) thin film is a promising material for optoelectronic devices. In this work, we fabricate Mo/CZTSSe/CdS/ZnO/ITO (ITO, indium tin oxide) heterojunction photodetectors with favorable self-powered characteristics. The photodetector exhibits exceptional high-frequency photoresponse performance whose -3 dB bandwidth and rise/decay time have reached 1 MHz and 240/340 ns, respectively. For further improvement, ultrathin Al₂O₃ layer prepared via atomic layer deposition (ALD) process is introduced at the Mo/CZTSSe interface. The influence of ALD-Al₂O₃ layer thickness and its role on the photoresponse performance are investigated in detail. The interfacial layer proved to serve as a protective layer preventing selenization of Mo electrode, resulting in the reduction of MoSe₂ transition layer and the decrease of series resistance of the device. Accordingly, the -3 dB bandwidth is remarkably extended to 3.5 MHz while the rise/decay time is dramatically improved to 60/77 ns with 16 cycles of ALD-Al₂O₃ layer, which is 4-5 orders of magnitude faster than the other reported CZTSSe photodetectors. Simultaneously, it is revealed that the ALD-Al₂O₃ interfacial layer acts as an electron blocking layer which leads to the effective suppression of carrier recombination at the rear surface. Consequently, the responsivity and detectivity are enhanced in the entire range while the maximum values are up to 0.39 AW^-1 and 2.04 × 10¹¹ Jones with 8 cycles of ALD-Al₂O₃, respectively. Finally, the CZTSSe photodetector is successfully integrated into a visible light communication system and obtains a satisfying transfer rate of 2 Mbps. These results indicate the satisfying performance of CZTSSe-based thin film photodetectors with great potential applications for communication.

Completely Solvent-free Protocols to Access Phase-Pure, Metastable Metal Halide Perovskites and Functional Photodetectors from the Precursor Salts

Mechanochemistry is a green, solid-state, re-emerging synthetic technique that can rapidly form complex molecules and materials without exogenous heat or solvent(s). Herein, we report the application of solvent-free mechanochemical ball milling for the synthesis of metal halide perovskites, to overcome problems with solution-based syntheses. We prepared phase-pure, air-sensitive CsSnX3 (X = I, Br, Cl) and its mixed halide perovskites by mechanochemistry for the first time by reactions between cesium and tin(II) halides. Notably, we report the sole examples where metastable, high-temperature phases like cubic CsSnCl3, cubic CsPbI3, and trigonal FAPbI3 were accessible at ambient temperatures and pressures without post-synthetic processing. The perovskites can be prepared up to "kilogram scales." Lead-free, all-inorganic photodetector devices were fabricated using the mechanosynthesized CsSnBr1.5Cl1.5 under solvent-free conditions and showed 10-fold differences between on-off currents. We highlight an essentially solvent-free, general approach to synthesize metastable compounds and fabricate photodetectors from commercially available precursors.

Multiple-engineering controlled growth of tunable-bandgap perovskite nanowires for high performance photodetectors

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.

Direct Vapor-Liquid-Solid Synthesis of All-Inorganic Perovskite Nanowires for High-Performance Electronics and Optoelectronics

Controlled synthesis of lead halide perovskite (LHP) nanostructures not only benefits fundamental research but also offers promise for applications. Among many synthesis techniques, although catalytic vapor-liquid-solid (VLS) growth is recognized as an effective route to achieve high-quality nanostructures, until now, there is no detailed report on VLS grown LHP nanomaterials due to the emerging challenges in perovskite synthesis. Here, we develop a direct VLS growth for single-crystalline all-inorganic lead halide perovskite ( i.e., CsPbX₃; X = Cl, Br, or I) nanowires (NWs). These NWs exhibit high-performance photodetection with the responsivity exceeding 4489 A/W and detectivity over 7.9 × 10¹² Jones toward the visible light regime. Field-effect transistors (FET) based on individual CsPbX₃ NWs are also fabricated, where they show the superior hole mobility of up to 3.05 cm²/(V s), higher than other all-inorganic LHP devices. This work provides important guidelines for the further improvement of these perovskite nanostructures for utilizations.

Increasing Photoluminescence Quantum Yield by Nanophotonic Design of Quantum-Confined Halide Perovskite Nanowire Arrays

High-photoluminescence quantum yield (PLQY) is required to reach optimal performance in solar cells, lasers, and light-emitting diodes (LEDs). Typically, PLQY can be increased by improving the material quality to reduce the nonradiative recombination rate. It is in principle equally effective to improve the optical design by nanostructuring a material to increase light out-coupling efficiency (OCE) and introduce quantum confinement, both of which can increase the radiative recombination rate. However, increased surface recombination typically minimizes nanostructure gains in PLQY. Here a template-guided vapor phase growth of CH₃NH₃PbI₃ (MAPbI₃) nanowire (NW) arrays with unprecedented control of NW diameter from the bulk (250 nm) to the quantum confined regime (5.7 nm) is demonstrated, while simultaneously providing a low surface recombination velocity of 18 cm s^-1. This enables a 56-fold increase in the internal PLQY, from 0.81% to 45.1%, and a 2.3-fold increase in OCEy to increase the external PLQY by a factor of 130, from 0.33% up to 42.6%, exclusively using nanophotonic design.

Strain-Mediated Phase Stabilization: A New Strategy for Ultrastable α-CsPbI3 Perovskite by Nanoconfined Growth

All-inorganic cesium lead triiodide (CsPbI₃ ) perovskite is considered a promising solution-processable semiconductor for highly stable optoelectronic and photovoltaic applications. However, despite its excellent optoelectronic properties, the phase instability of CsPbI₃ poses a critical hurdle for practical application. In this study, a novel stain-mediated phase stabilization strategy is demonstrated to significantly enhance the phase stability of cubic α-phase CsPbI₃ . Careful control of the degree of spatial confinement induced by anodized aluminum oxide (AAO) templates with varying pore sizes leads to effective manipulation of the phase stability of α-CsPbI₃ . The Williamson-Hall method in conjunction with density functional theory calculations clearly confirms that the strain imposed on the perovskite lattice when confined in vertically aligned nanopores can alter the formation energy of the system, stabilizing α-CsPbI₃ at room temperature. Finally, the CsPbI₃ grown inside nanoporous AAO templates exhibits exceptional phase stability over three months under ambient conditions, in which the resulting light-emitting diode reveals a natural red color emission with very narrow bandwidth (full width at half maximum of 33 nm) at 702 nm. The universally applicable template-based stabilization strategy can give in-depth insights on the strain-mediated phase transition mechanism in all-inorganic perovskites.

Structural and Functional Diversity in Lead-Free Halide Perovskite Materials

Lead halide perovskites have emerged as promising semiconducting materials for different applications owing to their superior optoelectronic properties. Although the community holds different views toward the toxic lead in these high-performance perovskites, it is certainly preferred to replace lead with nontoxic, or at least less-toxic, elements while maintaining the superior properties. Here, the design rules for lead-free perovskite materials with structural dimensions from 3D to 0D are presented. Recent progress in lead-free halide perovskites is reviewed, and the relationships between the structures and fundamental properties are summarized, including optical, electric, and magnetic-related properties. 3D perovskites, especially A₂ B⁺ B³⁺ X₆ -type double perovskites, demonstrate very promising optoelectronic prospects, while low-dimensional perovskites show rich structural diversity, resulting in abundant properties for optical, electric, magnetic, and multifunctional applications. Furthermore, based on these structure-property relationships, strategies for multifunctional perovskite design are proposed. The challenges and future directions of lead-free perovskite applications are also highlighted, with emphasis on materials development and device fabrication. The research on lead-free halide perovskites at Linköping University has benefited from inspirational discussions with Prof. Olle Inganäs.

Strong Cathodoluminescence and Fast Photoresponse from Embedded CH3NH3PbBr3 Nanoparticles Exhibiting High Ambient Stability

This study presents a comprehensive analysis of the strong cathodoluminescence (CL), photoluminescence (PL), and photoresponse characteristics of CH₃NH₃PbBr₃ nanoparticles (NPs) embedded in a mesoporous nanowire (NW) template. Our study revealed a direct correlation between the CL and PL emissions from the perovskite NPs (Per NPs), for the first time. Per NPs are fabricated by a simple spin-coating of a perovskite precursor on the surface of metal-assisted chemically etched mesoporous Si NW arrays. The Per NPs confined in the mesopores show blue-shifted and enhanced CL emission as compared to the bare perovskite film, while the PL intensity of Per NPs is dramatically high compared to that of their bulk counterpart. A systematic analysis of the CL/PL spectra reveals that the quantum confinement effect and ultralow defects in Per NPs are mainly responsible for the enhanced CL and PL emissions. Low-temperature PL and time-resolved PL analysis confirm the high exciton binding energy and radiative recombination in Per NPs. The room temperature PL quantum yield of the Per NP film on the NW template was found to be 40.5%, while that of Per film was 2.8%. The Per NPs show improved ambient air stability than the bare film due to the protection provided by the dense NW array, since a dense NW array can slow down the lateral diffusion of oxygen and water molecules in Per NPs. Interestingly, the Si NW/Per NP junction shows superior visible light photodetection and the prototype photodetector shows a high responsivity (0.223 A/W) with response speeds of 0.32 and 0.28 s of growth and decay in photocurrent, respectively, at 2 V applied bias, which is significantly better than the reported photodetectors based on CH₃NH₃PbBr₃ nanostructures. This work demonstrates a low-cost fabrication of CH₃NH₃PbBr₃ NPs on a novel porous NW template, which shows excellent photophysical and optoelectronic properties with superior ambient stability.

Efficient metal halide perovskite light-emitting diodes with significantly improved light extraction on nanophotonic substrates

Metal halide perovskite has emerged as a promising material for light-emitting diodes. In the past, the performance of devices has been improved mainly by optimizing the active and charge injection layers. However, the large refractive index difference among different materials limits the overall light extraction. Herein, we fabricate efficient methylammonium lead bromide light-emitting diodes on nanophotonic substrates with an optimal device external quantum efficiency of 17.5% which is around twice of the record for the planar device based on this material system. Furthermore, optical modelling shows that a high light extraction efficiency of 73.6% can be achieved as a result of a two-step light extraction process involving nanodome light couplers and nanowire optical antennas on the nanophotonic substrate. These results suggest that utilization of nanophotonic structures can be an effective approach to achieve high performance perovskite light-emitting diodes.

The rapid development of halide perovskite light-emitting diodes mainly relies on the optimization of the active layer and charge injection layers. Here Zhang et al. incorporate three-dimensional nanophotonic substrates to enhance light out-coupling and achieve high external quantum efficiency of 17.5%.

Geometrical optimisation of core-shell nanowire arrays for enhanced absorption in thin crystalline silicon heterojunction solar cells

Background: Elongated nanostructures, such as nanowires, have attracted significant attention for application in silicon-based solar cells. The high aspect ratio and characteristic radial junction configuration can lead to higher device performance, by increasing light absorption and, at the same time, improving the collection efficiency of photo-generated charge carriers. This work investigates the performance of ultra-thin solar cells characterised by nanowire arrays on a crystalline silicon bulk. Results: Proof-of-concept devices on a p-type mono-crystalline silicon wafer were manufactured and compared to flat references, showing improved absorption of light, while the final 11.8% (best-device) efficiency was hindered by sub-optimal passivation of the nanowire array. A modelling analysis of the optical performance of the proposed solar cell architecture was also carried out. Results showed that nanowires act as resonators, amplifying interference resonances and exciting additional wave-guided modes. The optimisation of the array geometrical dimensions highlighted a strong dependence of absorption on the nanowire cross section, a weaker effect of the nanowire height and good resilience for angles of incidence of light up to 60°. Conclusion: The presence of a nanowire array increases the optical performance of ultra-thin crystalline silicon solar cells in a wide range of illumination conditions, by exciting resonances inside the absorber layer. However, passivation of nanowires is critical to further improve the efficiency of such devices.

Air-stable lead-free hybrid perovskite employing self-powered photodetection with an electron/hole-conductor-free device geometry

In this study, we have demonstrated the fabrication of one of the few lead-free perovskite photodetectors reported to date. In particular, we fabricated the photodetector based on a methylammonium bismuth iodide, MA3Bi2I9 (MABI), structure that works under a self-powered mode of operation. The optoelectronic properties of the photodetector were investigated systematically. The photoresponse of the photodetector was carefully studied and compared with the literature, which demonstrated the capability of the MABI structure for detecting light at a very low incident irradiance of 10 μW cm-2. Also, it exhibited a fast and reproducible response speed along with high photosensitivity (∼105) and detectivity (∼1012 Jones) at low operating voltages (0 V and 0.8 V) through the large effective device area. It was also revealed that the MABI photodetector had good stability after storage for two months at ambient conditions. It is therefore concluded that our photodetector fabricated with a MABI perovskite structure using a simple geometry has great potential to further improve the optoelectronic properties when proper electron/hole-transporting layers are fused for proper charge extraction. This will have significant benefits for next-generation cost-competitive optoelectronic technology to address the scalability issue.

Wet-Chemical Synthesis of Surface-Passivated Halide Perovskite Microwires for Improved Optoelectronic Performance and Stability

One-dimensional (1D) halide perovskite materials with intrinsic high carrier mobility and long diffusion length hold great promises for high-performance optoelectronic devices, in which the passivation of the surface defects is of significance for further boosting its optoelectronic performance as well as its moisture stability. Herein, we demonstrate a simple room-temperature wet-chemical synthetic protocol for perovskite microwires with controlled morphologies and passivated surface states. This strategy allows for facile assembly of hydrophobic 1 H,1 H-perfluorooctylamine (PFA) molecules on the surface of the perovskite microwires owing to the coordination binding between the amino groups of PFA and Pb²⁺. Both steady and time-resolved photoluminescence measurements revealed that the passivation of PFA greatly benefit for the inhibition of the photogenerated carriers recombination. The constructed perovskite microwire-based photodetectors have shown increased detectivity of 4.99 × 10¹¹ jones and responsivity of 1.27 A/W (light power density of 1 mW/cm²). Moreover, the hydrophobic fluorocarbon alkyl chains endow the perovskite microwires with higher resistance toward moisture. Such coating of a water-resisting layer resulted in greatly enhanced stability of perovskite microwires under the humidity of 55 ± 5% over 30 days. We thus believe that our work is of importance for the development of 1D halide perovskite photodetectors with highly improved performance and stability.

Recent advances in one-dimensional halide perovskites for optoelectronic applications

Metal-halide perovskites have emerged as efficient, low-cost energy materials owing to their remarkable optoelectronic properties. In particular, the dimensionality and morphology of crystallites may have a striking influence on their chemical and physical properties and therefore affect their optoelectronic applications. One-dimensional halide perovskites have superior carrier transportation in one dimension, high crystalline quality, and consequently, high quantum efficiencies and long carrier diffusion lengths, which are important for the performance of perovskite-based nanoscale optoelectronic and photonic devices. In this review, we highlight recent advances in the synthesis of one-dimensional halide perovskites and their unique properties as well as their novel optoelectronic applications. This review aims to provide an overview of the achievements in synthesis techniques and nanoscale optoelectronic applications based on one-dimensional perovskite nanocrystals.

Surface Effect on 2D Hybrid Perovskite Crystals: Perovskites Using an Ethanolamine Organic Layer as an Example

Despite the remarkable progress of optoelectronic devices based on hybrid perovskites, there are significant drawbacks, which have largely hindered their development as an alternative of silicon. For instance, hybrid perovskites are well-known to suffer from moisture instability which leads to surface degradation. Nonetheless, the dependence of the surface effect on the moisture stability and optoelectronic properties of hybrid perovskites has not been fully investigated. In this work, the influence of the surface effect of 2D layered perovskites before and after mechanical exfoliation, representing rough and smooth surfaces of perovskite crystals, are studied. It is found that the smooth 2D perovskite is less sensitive to ambient moisture and exhibits a considerably low dark current, which outperforms the rough perovskites by 23.6 times in terms of photodetectivity. The superior moisture stability of the smooth perovskites over the rough perovskites is demonstrated. Additionally, ethanolamine is employed as an organic linker of the 2D layered perovskite, which further improves the moisture stability. This work reveals the strong dependence of the surface conditions of 2D hybrid perovskite crystals on their moisture stability and optoelectronic properties, which are of utmost importance to the design of practical optoelectronic devices based on hybrid perovskite crystals.

Nanobeam X-ray Fluorescence Dopant Mapping Reveals Dynamics of in Situ Zn-Doping in Nanowires

The properties of semiconductors can be controlled using doping, making it essential for electronic and optoelectronic devices. However, with shrinking device sizes it becomes increasingly difficult to quantify doping with sufficient sensitivity and spatial resolution. Here, we demonstrate how X-ray fluorescence mapping with a nanofocused beam, nano-XRF, can quantify Zn doping within in situ doped III-V nanowires, by using large area detectors and high-efficiency focusing optics. The spatial resolution is defined by the focus size to 50 nm. The detection limit of 7 ppm (2.8 × 10¹⁷ cm^-3), corresponding to about 150 Zn atoms in the probed volume, is bound by a background signal. In solar cell InP nanowires with a p-i-n doping profile, we use nano-XRF to observe an unintentional Zn doping of 5 × 10¹⁷ cm^-3 in the middle segment. We investigated the dynamics of in situ Zn doping in a dedicated multisegment nanowire, revealing significantly sharper gradients after turning the Zn source off than after turning the source on. Nano-XRF could be used for quantitative mapping of a wide range of dopants in many types of nanostructures.

Metal Halide Perovskites: Synthesis, Ion Migration, and Application in Field-Effect Transistors

The past several years have witnessed tremendous developments of metal halide perovskite (MHP)-based optoelectronics. Particularly, the intensive research of MHP-based light-emitting diodes, photodetectors, and solar cells could probably reform the optoelectronic semiconductor industry. In comparison, in spite of the large intrinsic charge carrier mobility of MHPs, the development of MHP-based field-effect transistors (MHP-FETs) is relatively slow, which is essentially due to the gate-field screening effect induced by the ion migration and accumulation in MHP-FETs. This work mainly aims to summarize the recent important work on MHP-FETs and propose solutions in terms of the development bottleneck of perovskite-based transistors, in an attempt to boost the research of MHP transistors further. First, the advantages and potential applications of MHP-FETs are briefly introduced, which is followed by a detailed description of the MHP crystalline structure and various material fabrication techniques. Afterward, MHP-FETs are discussed, including transistors based on hybrid organic-inorganic perovskites, all-inorganic perovskites, and lead-free perovskites.

Lead-free hybrid perovskites for photovoltaics

This review covers the state-of-the-art in organo-inorganic lead-free hybrid perovskites (HPs) and applications of these exciting materials as light harvesters in photovoltaic systems. Special emphasis is placed on the influence of the spatial organization of HP materials both on the micro- and nanometer scale on the performance and stability of perovskite-based solar light converters. This review also discusses HP materials produced by isovalent lead(II) substitution with Sn2+ and other metal(II) ions, perovskite materials formed on the basis of M3+ cations (Sb3+, Bi3+) as well as on combinations of M+/M3+ ions aliovalent to 2Pb2+ (Ag+/Bi3+, Ag+/Sb3+, etc.). The survey is concluded with an outlook highlighting the most promising strategies for future progress of photovoltaic systems based on lead-free perovskite compounds.

Significantly improved black phase stability of FAPbI3 nanowires via spatially confined vapor phase growth in nanoporous templates

The formamidinium lead iodide (FAPbI3) perovskite has attracted immense research interest as it has much improved stability than methylammonium lead iodide (MAPbI3) while still maintaining excellent optoelectronic properties. Compared to MAPbI3, FAPbI3 has shown an elevated decomposition temperature and a slower decomposition process and therefore it is considered as a more promising candidate for future high-efficiency and reliable optoelectronic devices. However, these excellent optoelectronic properties only exist in the alpha phase and this phase will spontaneously transform into an undesired delta phase with much poorer optoelectronic properties regardless of the environment. This is the main challenge for the application of the FAPbI3 perovskite. Herein, we report a novel strategy to stabilize the cubic black phase of FAPbI3 by using nanoengineering templates. Without further treatment, the black phase can be held over 7 months under ambient conditions and 8 days in an extreme environment with a Relative Humidity (RH) of 97%. A systematic study further reveals that this great improvement can be attributed to the spatial confinement in anodized alumina membrane (AAM) nanochannels, which prohibits the unwanted α-to-δ phase transition by restricting the expansion of NWs in the ab plane, and the excellent passivation against water molecule invasion. Meanwhile, we also demonstrate the potency of these NWs in practical applications by configuring them into photodetectors, which have shown reasonable response and excellent device stability.

High-Performance Silicon-Compatible Large-Area UV-to-Visible Broadband Photodetector Based on Integrated Lattice-Matched Type II Se/n-Si Heterojunctions

A gold-induced NH₄Cl-assisted vapor-based route is proposed and developed to achieve vertically aligned submicron Se crystals on lattice-matched (111)-oriented silicon substrates, based on which a high-performance large-area silicon-compatible photodetector is constructed. Thanks to the energy band structure and the strongly asymmetrical depletion region, the fabricated Se/Si device maintains a similar wavelength cutoff to that of selenium devices before the IR region, along with a high-performance broadband photoresponse in the UV-to-visible region. The large-area photodetector maintains a very low leakage current under a -2 V bias, and a high on/off ratio of 10³-10⁴ is obtained with a high photocurrent of 62 nA at 500 nm. A photoresponse is clearly observed when the bias voltage is removed. The pulse response precisely provides a high response speed (τ_rise + τ_fall ≈ 1.975 ms), exceeding the fastest Se-based photodetectors in current reports. The enhanced photoelectric properties and the self-power photoresponse mainly derive from the integrated high-quality Se/n-Si p-n heterojunctions with both lattice match and type II energy band match.

A Novel Conductive Mesoporous Layer with a Dynamic Two-Step Deposition Strategy Boosts Efficiency of Perovskite Solar Cells to 20

Lead halide perovskite solar cells (PSCs) with the high power conversion efficiency (PCE) typically use mesoporous metal oxide nanoparticles as the scaffold and electron-transport layers. However, the traditional mesoporous layer suffers from low electron conductivity and severe carrier recombination. Here, antimony-doped tin oxide nanorod arrays are proposed as novel transparent conductive mesoporous layers in PSCs. Such a mesoporous layer improves the electron transport as well as light utilization. To resolve the common problem of uneven growth of perovskite on rough surface, the dynamic two-step spin coating strategy is proposed to prepare highly smooth, dense, and crystallized perovskite films with micrometer-scale grains, largely reducing the carrier recombination ratio. The conductive mesoporous layer and high-quality perovskite film eventually render the PSC with a remarkable PCE of 20.1% with excellent reproducibility. These findings provide a new avenue to further design high-efficiency PSCs from the aspect of carrier transport and recombination.

Size-tunable CsPbBr3 perovskite ring arrays for lasing

Morphology engineering is a decisive factor for the optoelectronic properties of nanocrystals. Differing from morphologies with a solid interior, ring-like structures have a unique internal space which is not only available for loading chemicals, but also useful for controlling the field distribution. Herein, the perovskite array family welcomes a new member - CsPbBr3 ring arrays. This work solves several fundamental problems for fabricating CsPbBr3 ring arrays: (i) developing a simple method using 2D colloidal crystal templates to achieve ring arrays of CsPbBr3, (ii) finding two ways, say changing the template size or annealing of the template, to accurately tune the ring size of the array in a wide range from 2.6 μm to 16.9 μm, and (iii) investigating the dynamics of perovskite rings, which indicates a shrinking process towards the template spheres before the crystallization of the perovskites. Finally, the application of CsPbBr3 perovskite ring arrays to the field of lasers is shown.

Remarkable long-term stability of nanoconfined metal-halide perovskite crystals against degradation and polymorph transitions

Metal-halide perovskites are promising candidates to advance optoelectronic devices but are known to suffer from rapid material degradation. Here we demonstrate that nanoconfinement is an effective strategy for the long-term stabilization of metal-halide perovskite MAPbI3 crystals against humidity-induced degradation and temperature-induced polymorph transitions. Two-dimensional X-ray diffraction patterns of MAPbI3 films reveal an unprecedented air-stability of up to 594 days in non-chemically modified, non-passivated MAPbI3 films deposited on substrates imposing complete 2D confinement on the tens of nanometers length scale. Temperature-dependent X-ray diffraction analysis and optical spectroscopy further reveal the suppression of temperature-dependent phase transitions in nanoconfined MAPbI3 crystals. Most notably, the high-temperature cubic phase of MAPbI3, typically stable at temperatures above 327 K, remains present until a temperature of 170 K when the perovskite crystals are nanoconfined within the 100 nm diameter pores of anodized aluminum oxide templates. Photoluminescence mapping confirms that nanoconfined MAPbI3 crystals exhibit spatial uniformity on the tens of microns length scale, suggesting that nanoconfinement is an effective strategy for the formation of high-quality, stable MAPbI3 crystals across large areas.

Organic/Inorganic Metal Halide Perovskite Optoelectronic Devices beyond Solar Cells

Investigations of organic-inorganic metal halide perovskite materials have attracted extensive attention due to their excellent properties including bandgap tunability, long charge diffusion length, and outstanding optoelectronic merits. Organic-inorganic metal halide perovskites are demonstrated to be promising materials in a variety of optoelectronic applications including photodetection, energy harvesting, and light-emitting devices. As perovskite solar cells are well studied in literature, here, the recent developments of organic-inorganic metal halide perovskite materials in optoelectronic devices beyond solar cells are summarized. The preparation of organic-inorganic metal halide perovskite films is introduced. Applications of organic-inorganic metal halide perovskite materials in light-emitting diodes, photodetectors, and lasers are then highlighted. Finally, the recent advances in these optoelectronic applications based on organic-inorganic metal halide materials are summarized and the future perspectives are discussed.

A lead-free two-dimensional perovskite for a high-performance flexible photoconductor and a light-stimulated synaptic device

Organo-lead halide perovskites have emerged as promising materials for high-performance photodetectors. However, the toxicity of lead cations in these materials limits their further applications. Here, a flexible photoconductor is developed based on lead-free two-dimensional (2D) perovskite (PEA)2SnI4via a one-step solution processing method. The flexible transparent electrodes are patterned from rGO/(PEDOT:PSS) hybrid films. The stability and reproducibility of the devices are significantly improved on adding 30 mol% SnF2 to the perovskite. The flexible photoconductors show a photoresponsivity of 16 A W-1 and a detectivity of 1.92 × 1011 Jones under 470 nm illumination, which are higher than those of most of the similar devices. Besides, the devices possess much better mechanical flexibility and durability than the flexible devices with an Au electrode. Finally, this flexible photoconductor is applied as a light-stimulated synaptic device and can mimic the short-term plasticity of biological synapses. This is the first study to report that lead-free 2D perovskite can be used in flexible photoconductors and synaptic devices.

Nanotextured Spikes of α-Fe2O3/NiFe2O4 Composite for Efficient Photoelectrochemical Oxidation of Water

We demonstrate for the first time the application of p-NiFe₂O₄/n-Fe₂O₃ composite thin films as anode materials for light-assisted electrolysis of water. The p-NiFe₂O₄/n-Fe₂O₃ composite thin films were deposited on planar fluorinated tin oxide (FTO)-coated glass as well as on 3D array of nanospike (NSP) substrates. The effect of substrate (planar FTO and 3D-NSP) and percentage change of each component (i.e., NiFe₂O₄ and Fe₂O₃) of composite was studied on photoelectrochemical (PEC) water oxidation reaction. This work also includes the performance comparison of p-NiFe₂O₄/n-Fe₂O₃ composite (planar and NSP) devices with pure hematite for PEC water oxidation. Overall, the nanostructured p-NiFe₂O₄/n-Fe₂O₃ device with equal molar 1:1 ratio of NiFe₂O₄ and Fe₂O₃ was found to be highly efficient for PEC water oxidation as compared with pure hematite, 1:2 and 1:3 molar ratios of composite. The photocurrent density of 1:1 composite thin film on planar substrate was equal to 1.07 mA/cm² at 1.23 V_RHE, which was 1.7 times higher current density as compared with pure hematite device (0.63 mA/cm² at 1.23 V_RHE). The performance of p-NiFe₂O₄/n-Fe₂O₃ composites in PEC water oxidation was further enhanced by their deposition over 3D-NSP substrate. The highest photocurrent density of 2.1 mA/cm² at 1.23 V_RHE was obtained for the 1:1 molar ratio p-NiFe₂O₄/n-Fe₂O₃ composite on NSP (NF1-NSP), which was 3.3 times more photocurrent density than pure hematite. The measured applied bias photon-to-current efficiency (ABPE) value of NF1-NSP (0.206%) was found to be 1.87 times higher than that of NF1-P (0.11%) and 4.7 times higher than that of pure hematite deposited on FTO-coated glass (0.044%). The higher PEC water oxidation activity of p-NiFe₂O₄/n-Fe₂O₃ composite thin film as compared with pure hematite is attributed to the Z-path scheme and better separation of electrons and holes. The increased surface area and greater light absorption capabilities of 3D-NSP devices result in further improvement in catalytic activities.

Large-Grain Tin-Rich Perovskite Films for Efficient Solar Cells via Metal Alloying Technique

Fast research progress on lead halide perovskite solar cells has been achieved in the past a few years. However, the presence of lead (Pb) in perovskite composition as a toxic element still remains a major issue for large-scale deployment. In this work, a novel and facile technique is presented to fabricate tin (Sn)-rich perovskite film using metal precursors and an alloying technique. Herein, the perovskite films are formed as a result of the reaction between Sn/Pb binary alloy metal precursors and methylammonium iodide (MAI) vapor in a chemical vapor deposition process carried out at 185 °C. It is found that in this approach the Pb/Sn precursors are first converted to (Pb/Sn)I₂ and further reaction with MAI vapor leads to the formation of perovskite films. By using Pb-Sn eutectic alloy, perovskite films with large grain sizes up to 5 µm can be grown directly from liquid phase metal. Consequently, using an alloying technique and this unique growth mechanism, a less-toxic and efficient perovskite solar cell with a power conversion efficiency (PCE) of 14.04% is demonstrated, while pure Sn and Pb perovskite solar cells prepared in this manner yield PCEs of 4.62% and 14.21%, respectively. It is found that this alloying technique can open up a new direction to further explore different alloy systems (binary or ternary alloys) with even lower melting point.

Structural effects on optoelectronic properties of halide perovskites

This review gives a perspective on different synthetic methodologies for the preparation of halide perovskites and highlights the structural effects on their optoelectronic properties.

Perovskite Nanowire Extrusion

The defect tolerance of halide perovskite materials has led to efficient optoelectronic devices based on thin-film geometries with unprecedented speed. Moreover, it has motivated research on perovskite nanowires because surface recombination continues to be a major obstacle in realizing efficient nanowire devices. Recently, ordered vertical arrays of perovskite nanowires have been realized, which can benefit from nanophotonic design strategies allowing precise control over light propagation, absorption, and emission. An anodized aluminum oxide template is used to confine the crystallization process, either in the solution or in the vapor phase. This approach, however, results in an unavoidable drawback: only nanowires embedded inside the AAO are obtainable, since the AAO cannot be etched selectively. The requirement for a support matrix originates from the intrinsic difficulty of controlling precise placement, sizes, and shapes of free-standing nanostructures during crystallization, especially in solution. Here we introduce a method to fabricate free-standing solution-based vertical nanowires with arbitrary dimensions. Our scheme also utilizes AAO; however, in contrast to embedding the perovskite inside the matrix, we apply a pressure gradient to extrude the solution from the free-standing templates. The exit profile of the template is subsequently translated into the final semiconductor geometry. The free-standing nanowires are single crystalline and show a PLQY up to ∼29%. In principle, this rapid method is not limited to nanowires but can be extended to uniform and ordered high PLQY single crystalline perovskite nanostructures of different shapes and sizes by fabricating additional masking layers or using specifically shaped nanopore endings.

Hybrid Organic-Inorganic Perovskite Photodetectors

Hybrid organic-inorganic perovskite materials garner enormous attention for a wide range of optoelectronic devices. Due to their attractive optical and electrical properties including high optical absorption coefficient, high carrier mobility, and long carrier diffusion length, perovskites have opened up a great opportunity for high performance photodetectors. This review aims to give a comprehensive summary of the significant results on perovskite-based photodetectors, focusing on the relationship among the perovskite structures, device configurations, and photodetecting performances. An introduction of recent progress in various perovskite structure-based photodetectors is provided. The emphasis is placed on the correlation between the perovskite structure and the device performance. Next, recent developments of bandgap-tunable perovskite and hybrid photodetectors built from perovskite heterostructures are highlighted. Then, effective approaches to enhance the stability of perovskite photodetector are presented, followed by the introduction of flexible and self-powered perovskite photodetectors. Finally, a summary of the previous results is given, and the major challenges that need to be addressed in the future are outlined. A comprehensive summary of the research status on perovskite photodetectors is hoped to push forward the development of this field.

From Nonluminescent Cs4PbX6 (X = Cl, Br, I) Nanocrystals to Highly Luminescent CsPbX3 Nanocrystals: Water-Triggered Transformation through a CsX-Stripping Mechanism

We report a novel CsX-stripping mechanism that enables the efficient chemical transformation of nonluminescent Cs₄PbX₆ (X = Cl, Br, I) nanocrystals (NCs) to highly luminescent CsPbX₃ NCs. During the transformation, Cs₄PbX₆ NCs dispersed in a nonpolar solvent are converted into CsPbX₃ NCs by stripping CsX through an interfacial reaction with water in a different phase. This process takes advantage of the high solubility of CsX in water as well as the ionic nature and high ion diffusion property of Cs₄PbX₆ NCs, and produces monodisperse and air-stable CsPbX₃ NCs with controllable halide composition, tunable emission wavelength covering the full visible range, narrow emission width, and high photoluminescent quantum yield (up to 75%). An additional advantage is that this is a clean synthesis as Cs₄PbX₆ NCs are converted into CsPbX₃ NCs in the nonpolar phase while the byproduct of CsX is formed in water that could be easily separated from the organic phase. The as-prepared CsPbX₃ NCs show enhanced stability against moisture because of the passivated surface. Our finding not only provides a new pathway for the preparation of highly luminescent CsPbX₃ NCs but also adds insights into the chemical transformation behavior and stabilization mechanism of these emerging perovskite nanocrystals.

All Inorganic Cesium Lead Iodide Perovskite Nanowires with Stabilized Cubic Phase at Room Temperature and Nanowire Array-Based Photodetectors

Alluring optical and electronic properties have made organometallic halide perovskites attractive candidates for optoelectronics. Among all perovskite materials, inorganic CsPbX₃ (X is halide) in black cubic phase has triggered enormous attention recently owing to its comparable photovoltaic performance and high stability as compared to organic and hybrid perovskites. However, cubic phase stabilization at room temperature for CsPbI₃ still survives as a challenge. Herein we report all inorganic three-dimensional vertical CsPbI₃ perovskite nanowires (NWs) synthesized inside anodic alumina membrane (AAM) by chemical vapor deposition (CVD) method. It was discovered that the as-grown NWs have stable cubic phase at room temperature. This significant improvement on phase stability can be attributed to the effective encapsulation of NWs by AAM and large specific area of these NWs. To demonstrate device application of these NWs, photodetectors based on these high density CsPbI₃ NWs were fabricated demonstrating decent performance. Our discovery suggests a novel and practical approach to stabilize the cubic phase of CsPbI₃ material, which will have broad applications for optoelectronics in the visible wavelength range.

High-Performance Red-Light Photodetector Based on Lead-Free Bismuth Halide Perovskite Film

In this study, we developed a sensitive red-light photodetector (RLPD) based on CsBi₃I₁₀ perovskite thin film. This inorganic, lead-free perovskite was fabricated by a simple spin-coating method. Device analysis reveals that the as-assembled RLPD was very sensitive to 650 nm light, with an on/off ratio as high as 10⁵. The responsivity and specific detectivity of the device were estimated to be 21.8 A/W and 1.93 × 10¹³ Jones, respectively, which are much better than those of other lead halide perovskite devices. In addition, the device shows a fast response (rise time: 0.33 ms; fall time: 0.38 ms) and a high external quantum efficiency (4.13 × 10³%). It is also revealed that the RLPD has a very good device stability even after storage for 3 months under ambient conditions. In summary, we suggest that the CsBi₃I₁₀ perovskite photodetector developed in this study may have potential applications in future optoelectronic systems.

Ultrahigh-Responsivity Photodetectors from Perovskite Nanowire Arrays for Sequentially Tunable Spectral Measurement

Compared with polycrystalline films, single-crystalline methylammonium lead halide (MAPbX₃, X = halogen) perovskite nanowires (NWs) with well-defined structure possess superior optoelectronic properties for optoelectronic applications. However, most of the prepared perovskite NWs exhibit properties below expectations due to poor crystalline quality and rough surfaces. It also remains a challenge to achieve aligned growth of single-crystalline perovskite NWs for integrated device applications. Here, we report a facile fluid-guided antisolvent vapor-assisted crystallization (FGAVC) method for large-scale fabrication of high-quality single-crystalline MAPb(I_1-xBr_x)₃ (x = 0, 0.1, 0.2, 0.3, 0.4) NW arrays. The resultant perovskite NWs showed smooth surfaces due to slow crystallization process and moisture-isolated growth environment. Significantly, photodetectors made from the NW arrays exhibited outstanding performance in respect of ultrahigh responsivity of 12 500 A W^-1, broad linear dynamic rang (LDR) of 150 dB, and robust stability. The responsivity represents the best value ever reported for perovskite-based photodetectors. Moreover, the spectral response of the MAPb(I_1-xBr_x)₃ NW arrays could be sequentially tuned by varying the content of x = 0-0.4. On the basis of this feature, the NW arrays were monolithically integrated to form a unique system for directly measuring light wavelength. Our work would open a new avenue for the fabrication of high-performance, integrated optoelectronic devices from the perovskite NW arrays.