Efficient Flexible Solar Cell based on Composition‐Tailored Hybrid Perovskite

Organic–inorganic hybrid perovskites (OIHPs) are new photoactive layer candidates for lightweight and flexible solar cells due to their low‐temperature process capability; however, the reported efficiency of flexible OIHP devices is far behind those achieved on rigid glass substrates. Here, it is revealed that the limiting factor is the different perovskite film deposition conditions required to form the same film morphology on flexible substrates. An optimized perovskite film composition needs a different precursor ratio, which is found to be essential for the formation of high‐quality perovskite films with longer radiative carrier recombination lifetime, smaller density of trap states, reduced precursor residue, and uniform and pin‐hole free films. A record efficiency of 18.1% is achieved for the flexible perovskite solar‐cell devices made on an indium tin oxide/poly(ethylene terephthalate) substrate via a low temperature (≤100 °C) solution process.     

Methylphosphonium Tin Bromide: A 3D Perovskite Molecular Ferroelectric Semiconductor

3D ABX₃ organic–inorganic halide perovskite (OIHP) semiconductors like [CH₃NH₃]PbI₃ have received great attention because of their various properties for wide applications. However, although a number of low-dimensional lead-based OIHP ferroelectric semiconductors have been documented, obtaining 3D ABX₃ OIHP ferroelectric semiconductors is challenging. Herein, an A-site cation [CH₃PH₃]⁺ (methylphosphonium, MP) is employed to successfully obtain a lead-free 3D ABX₃ OIHP ferroelectric semiconductor MPSnBr₃, which shows clear above-room-temperature ferroelectricity and a direct bandgap of 2.62 eV. It is emphasized that MPSnBr₃ is a multiaxial molecular ferroelectric with the number of ferroelectric polar axes being as many as 12, which is far more than those of the other OIHP ferroelectric semiconductors and even the classical inorganic perovskite ferroelectric semiconductors BiFeO₃ (4 polar axes) and BaTiO₃ (3 polar axes). MPSnBr₃ is the first MP-based 3D ABX₃ OIHP ferroelectric semiconductor. This finding throws light on the exploration of other excellent 3D ABX₃ OIHP ferroelectric semiconductors with great application prospects.     

Highly Oriented Thin Films of 2D Ruddlesden‐Popper Hybrid Perovskite toward Superfast Response Photodetectors

2D hybrid perovskites have shown great promise in the photodetection field, due to their intriguing attributes stemming from unique structural architectures. However, the great majority of detectors based on this 2D system possess a relatively low response speed (≈ms), making it extremely urgent to develop new candidates for superfast photodetection. Here, a new organic–inorganic hybrid perovskite, (PA)₂(FA)Pb₂I₇ (EFA, where PA is n‐pentylaminium and FA is formamidine), which features the 2D Ruddlesden–Popper type perovskite framework that is composed of the corner‐sharing PbI₆ octahedra is reported. Significantly, photodetectors fabricated on highly oriented thin films, which exhibit a perfect orientation parallel to 2D inorganic perovskite layers, exhibit a superfast response time up to ≈2.54 ns. To the best of the knowledge, this figure‐of‐merit catches up with that of the top‐ranking commercial materials, and sets a new record for 2D hybrid perovskite photodetectors. Moreover, extremely high photodetectivity (≈1.73 × 10¹⁴ Jones, under an incident power intensity of ≈46 µW cm^?2), considerable switching ratios (>10³), and low dark current (≈10 pA) are also achieved in the detector, indicating its great potential for high‐efficiency photodetection. These results shed light on the possibilities to explore new 2D candidates for assembling future high‐performance optoelectronic devices.     

A Robust Nonvolatile Resistive Memory Device Based on a Freestanding Ultrathin 2D Imine Polymer Film

Here, the synthesis of a wafer‐scale ultrathin 2D imine polymer (2DP) film with controllable thickness from simple benzene‐1,3,5‐tricarbaldehyde (BTA) and p‐phenylenediamine (PDA) building blocks is reported using a Schiff base polycondensation reaction at the air–water interface. The synthesized freestanding 2DP films are porous, insulating, and more importantly, covalently linked, which is ideally suited for nonvolatile memristors that use a conductive filament mechanism. These devices exhibit excellent switching performance with high reliability and reproducibility, with on/off ratios in the range of 10² to 10⁵ depending on the thickness of the film. In addition, the endurance and data retention capability of 2DP‐based nonvolatile resistive memristors are up to 200 cycles and 8 × 10⁴ s under constant voltage stress at 0.1 V. The intrinsic flexibility of the covalent organic polymer enables the fabrication of a flexible memory device on a polyimide film, which exhibits as reliable memory performance as that on the rigid substrate. Moreover, the 2DP‐based memory device shows outstanding thermal stability and organic solvent resistance, which are desirable properties for applications in wearable devices.     

Realizing Reduced Imperfections via Quantum Dots Interdiffusion in High Efficiency Perovskite Solar Cells

Realization of reduced ionic (cationic and anionic) defects at the surface and grain boundaries (GBs) of perovskite films is vital to boost the power conversion efficiency of organic–inorganic halide perovskite (OIHP) solar cells. Although numerous strategies have been developed, effective passivation still remains a great challenge due to the complexity and diversity of these defects. Herein, a solid-state interdiffusion process using multi-cation hybrid halide perovskite quantum dots (QDs) is introduced as a strategy to heal the ionic defects at the surface and GBs. It is found that the solid-state interdiffusion process leads to a reduction in OIHP shallow defects. In addition, Cs⁺ distribution in QDs greatly influences the effectiveness of ionic defect passivation with significant enhancement to all photovoltaic performance characteristics observed on treating the solar cells with Cs_0.05(MA_0.17FA_0.83)_0.95PbBr₃ (abbreviated as QDs-Cs5). This enables power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.     

2D Ruddlesden–Popper Polycrystalline PerovskitePyro-Phototronic Photodetectors

Two-dimensional (2D) Ruddlesden–Popper (RP) layered halide perovskite has attracted wide attentions due to its unique structure and excellent optoelectronic properties. With inserting organic cations, inorganic octahedrons are forced to extend in a certain direction, resulting in an asymmetric 2D perovskite crystal structure and causing spontaneous polarization. The pyroelectric effect resulted from spontaneous polarization exhibits a broad prospect in the application of optoelectronic devices. Herein, 2D RP polycrystalline perovskite (BA)₂(MA)₃Pb₄I₁₃ film with excellent crystal orientation is fabricated by hot-casting deposition, and a class of 2D hybrid perovskite photodetectors (PDs) with pyro-phototronic effect is proposed, achieving temperature and light detection with greatly improved performance by coupling multiple energies. Because of the pyro-phototronic effect, the current is ≈35 times to that of the photovoltaic effect current under 0 V bias. The responsivity and detectivity are 12.7 mA W⁻¹ and 1.73 × 10¹¹ Jones, and the on/off ratio can reach 3.97 × 10³. Furthermore, the influences of bias voltage, light power density, and frequency on the pyro-phototronic effect of 2D RP polycrystalline perovskite PDs are explored. The coupling of spontaneous polarization and light facilitates photo-induced carrier dissociation and tunes the carrier transport process, making 2D RP perovskites a competitive candidate for next-generation photonic devices.     

A Strategy to Design High‐Density Nanoscale Devices utilizing Vapor Deposition of Metal Halide Perovskite Materials

The demand for high memory density has increased due to increasing needs of information storage, such as big data processing and the Internet of Things. Organic–inorganic perovskite materials that show nonvolatile resistive switching memory properties have potential applications as the resistive switching layer for next‐generation memory devices, but, for practical applications, these materials should be utilized in high‐density data‐storage devices. Here, nanoscale memory devices are fabricated by sequential vapor deposition of organolead halide perovskite (OHP) CH₃NH₃PbI₃ layers on wafers perforated with 250 nm via‐holes. These devices have bipolar resistive switching properties, and show low‐voltage operation, fast switching speed (200 ns), good endurance, and data‐retention time >10⁵ s. Moreover, the use of sequential vapor deposition is extended to deposit CH₃NH₃PbI₃ as the memory element in a cross‐point array structure. This method to fabricate high‐density memory devices could be used for memory cells that occupy large areas, and to overcome the scaling limit of existing methods; it also presents a way to use OHPs to increase memory storage capacity.     

Pb–Sn–Cu Ternary Organometallic Halide Perovskite Solar Cells

Exploiting organic/inorganic hybrid perovskite solar cells (PSCs) with reduced Pb content is very important for developing environment‐friendly photovoltaics. Utilizing of Pb–Sn alloying perovskite is considered as an efficient route to reduce the risk of ecosystem pollution. However, the trade‐off between device performance and Sn substitution ratio due to the instability of Sn²⁺ is a current dilemma. Here, for the first time, the highly efficient Pb–Sn–Cu ternary PSCs are reported by partial replacing of PbI₂ with SnI₂ and CuBr₂. Sn²⁺ substitution results in a redshift of the absorption onset, whereas worsens the film quality. Interestingly, Cu²⁺ introduction can passivate the trap sites at the crystal boundaries of Pb–Sn perovskites effectively. Consequently, a power conversion efficiency as high as 21.08% in inverted planar Pb–Sn–Cu ternary PSCs is approached. The finding opens a new route toward the fabrication of high efficiency Pb–Sn alloying perovskite solar cells by Cu²⁺ passivation.     

In Situ Grazing-Incidence Wide-Angle Scattering Reveals Mechanisms for Phase Distribution and Disorientation in 2D Halide Perovskite Films

2D hybrid halide perovskites with the formula (A′)₂(A)_n-1Pb_nI_3n+1 have remarkable stability and promising efficiency in photovoltaic and optoelectronic devices, yet fundamental understanding of film formation, key to optimizing these devices, is lacking. Here, in situ grazing-incidence wide-angle X-ray scattering (GIWAXS) is used to monitor film formation during spin-coating. This elucidates the general film formation mechanism of 2D halide perovskites during one-step spin-coating. There are three stages of film formation: sol–gel, oriented 3D, and 2D. Three precursor phases form during the sol–gel stage and transform to perovskite, first giving a highly oriented 3D-like phase at the air/liquid interface followed by subsequent nucleations forming slightly less oriented 2D perovskite. Furthermore, heating before crystallization leads to fewer nucleations and faster removal of the precursors, improving orientation. This outlines the primary causes of phase distribution and perpendicular orientation in 2D perovskite films and paves the way for rationally designed film fabrication techniques.     

Highly Sensitive,Fast Response Perovskite Photodetectors Demonstrated in Weak Light Detection Circuit and Visible Light Communication System

Organic–inorganic hybrid perovskite (OIHP) photodetectors have presented unprecedented device performance mainly owing to outstanding material properties. However, the solution‐processed OIHP polycrystalline thin films with defective surface and grain boundaries always impair the key parameter of photodetectors. Herein, a nonfullerene passivation layer exhibits more efficient passivation for OIHP materials to dramatically reduce the trap density of state, yielding a dark current as low as 2.6 × 10^?8 A cm^?2 under ?0.1 V. In addition, the strong absorption in near‐infrared (NIR) region of nonfullerene/C₆₀ heterojunction broadens the detectable range to over 900 nm by effective charge transport, ultimately leading to a specific detectivity of 1.45 × 10¹² and 7.37 × 10¹¹ cm Hz^1/2 W^?1 at 650 and 820 nm, respectively. Encouragingly, the response speed of 27 ns is obtained at 0.6 mm² of device area by removing constrain from the resistance–capacitance constant. Moreover, the prominent practical application of the photodetector is demonstrated in a weak light detection circuit and a visible light communication system. It is believed that the OIHP photodetectors with high sensitivity, NIR photoresponse, and ultrafast speed would pave the way to commercial applications.     

Flexible,Semitransparent, and Inorganic Resistive Memory based on BaTi0.95Co0.05O3 Film

Perovskite ceramics and single crystals are commonly hard and brittle due to their small maximum elastic strain. Here, large‐scale BaTi_0.95Co_0.05O₃ (BTCO) film with a SrRuO₃ (SRO) buffered layer on a 10 µm thick mica substrate is flexible with a small bending radius of 1.4 mm and semitransparent for visible light at wavelengths of 500–800 nm. Mica/SRO/BTCO/Au cells show bipolar resistive switching and the high/low resistance ratio is up to 50. The resistive‐switching properties show no obvious changes after the 2.2 mm radius memory being written/erased for 360 000 cycles nor after the memory being bent to 3 mm radius for 10 000 times. Most importantly, the memory works properly at 25–180 °C or after being annealed at 500 °C. The flexible and transparent oxide resistive memory has good prospects for application in smart wearable devices and flexible display screens.     

Supramolecular Framework Constructed by Dendritic Nanopolymer for Stable Flexible Perovskite Resistive Random-Access Memory

The 3D supramolecular framework (3D-SF) is constructed in this work through the hydrogen bond assisted self-assembly of spherical dendritic nanopolymer to regulate the flexibility, stability, and resistive switching (RS) performance of perovskite resistive random-access memory (RRAM). Herein, the 3D-SF network acts as the perovskite crystallization template to regulate the perovskite crystallization process due to its coordination interaction of functional groups with the perovskite grains, presenting the uniform, pinhole-free, and compact perovskite morphology for stable flexible RRAM. The 3D-SF network in situ stays at the perovskite intergranular boundaries to crosslink the perovskite grains. The RS performance of 3D-SF-modified perovskite RRAM device is evidently improved to the ON/OFF ratio of 10⁵, the cycle number of 500 times, and the data retention time of 10⁴ s. The 50-days exposure of unencapsulated RRAM device at ambient environment still makes the ON/OFF ratio to be kept at ≈10⁴, indicating the potential of long-term stable multilevel storage in the high-density data storage. The bending action under different radius also does not change the RS performance due to the excellent bending-resistant ability of 3D-SF-modified perovskite film. This work explores a novel polymer additive strategy to construct the 3D supramolecular framework for stable flexible perovskite optoelectronic devices.     

Ultrathin Ruddlesden–Popper Perovskite Heterojunction for Sensitive Photodetection

Thanks to their unique optical and electric properties, 2D materials have attracted a lot of interest for optoelectronic applications. Here, the emerging 2D materials, organic–inorganic hybrid perovskites with van der Waals interlayer interaction (Ruddlesden–Popper perovskites), are synthesized and characterized. Photodetectors based on the few‐layer Ruddlesden–Popper perovskite show good photoresponsivity as well as good detectivity. In order to further improve the photoresponse performance, 2D MoS₂ is chosen to construct the perovskite–MoS₂ heterojunction. The performance of the hybrid photodetector is largely improved with 6 and 2 orders of magnitude enhancement for photoresponsivity (10⁴ A W^?1) and detectivity (4 × 10¹⁰ Jones), respectively, which demonstrates the facile charge separation at the interface between perovskite and MoS₂. Furthermore, the contribution of back gate tuning is proved with a greatly reduced dark current. The results demonstrated here will open up a new field for the investigation of 2D perovskites for optoelectronic applications.     

Highly Efficient 2D/3D Hybrid Perovskite Solar Cells via Low‐Pressure Vapor‐Assisted Solution Process

The fabrication of multidimensional organometallic halide perovskite via a low‐pressure vapor‐assisted solution process is demonstrated for the first time. Phenyl ethyl‐ammonium iodide (PEAI)‐doped lead iodide (PbI₂) is first spin‐coated onto the substrate and subsequently reacts with methyl‐ammonium iodide (MAI) vapor in a low‐pressure heating oven. The doping ratio of PEAI in MAI‐vapor‐treated perovskite has significant impact on the crystalline structure, surface morphology, grain size, UV–vis absorption and photoluminescence spectra, and the resultant device performance. Multiple photoluminescence spectra are observed in the perovskite film starting with high PEAI/PbI₂ ratio, which suggests the coexistence of low‐dimensional perovskite (PEA₂MA_n?1Pb_nI_3n+1) with various values of n after vapor reaction. The dimensionality of the as‐fabricated perovskite film reveals an evolution from 2D, hybrid 2D/3D to 3D structure when the doping level of PEAI/PbI₂ ratio varies from 2 to 0. Scanning electron microscopy images and Kelvin probe force microscopy mapping show that the PEAI‐containing perovskite grain is presumably formed around the MAPbI₃ perovskite grain to benefit MAPbI₃ grain growth. The device employing perovskite with PEAI/PbI₂ = 0.05 achieves a champion power conversion efficiency of 19.10% with an open‐circuit voltage of 1.08 V, a current density of 21.91 mA cm^?2, and a remarkable fill factor of 80.36%.     

Highly Reproducible Large‐Area Perovskite Solar Cell Fabrication via Continuous Megasonic Spray Coating of CH3NH3PbI3

A simple, low‐cost, large area, and continuous scalable coating method is proposed for the fabrication of hybrid organic–inorganic perovskite solar cells. A megasonic spray‐coating method utilizing a 1.7 MHz megasonic nebulizer that could fabricate reproducible large‐area planar efficient perovskite films is developed. The coating method fabricates uniform large‐area perovskite film with large‐sized grain since smaller and narrower sized mist droplets than those generated by existing ultrasonic spray methods could be generated by megasonic spraying. The volume flow rate of the CH₃NH₃PbI₃ precursor solution and the reaction temperature are controlled, to obtain a high quality perovskite active layer. The devices reach a maximum efficiency of 16.9%, with an average efficiency of 16.4% from 21 samples. The applicability of megasonic spray coating to the fabrication of large‐area solar cells (1 cm²), with a power conversion efficiency of 14.2%, is also demonstrated. This is a record high efficiency for large‐area perovskite solar cells fabricated by continuous spray coating.     

Highly Efficient and Stable Solar Cells Based on Crystalline Oriented 2D/3D Hybrid Perovskite

Highly efficient and stable 2D/3D hybrid perovskite solar cells using 2‐thiophenemethylammonium (ThMA) as the spacer cation are successfully demonstrated. It is found that the incorporation of ThMA spacer cation into 3D perovskite, which forms a 2D/3D hybrid structure, can effectively induce the crystalline growth and orientation, passivate the trap states, and hinder the ion motion, resulting in improved carrier lifetime and reduced recombination losses. The optimized device exhibits a power conversion efficiency (PCE) of 21.49%, combined with a high V_OC of 1.16 V and a notable fill factor (FF) of 81%. More importantly, an encapsulated 2D/3D hybrid perovskite device sustains ≈99% of its initial PCE after 1680 h in the ambient atmosphere, whereas the control 3D perovskite device drops to ≈80% of the original performance. Importantly, the device stability under continuous light soaking (100 mW cm^?2) is enhanced significantly for 2D/3D perovskite device in comparison with that of the control device. These results reveal excellent photovoltaic properties and intrinsic stabilities of the 2D/3D hybrid perovskites using ThMA as the spacer cation.     

Valence band and catalytic activity of Au nanoparticles in Fe2O3/SiO2/Si(100) environment

A 10-nm-thick gold film was evaporated onto a SiO₂/Si(100) substrate and was implanted by Ar⁺ ions at 40 keV and 10¹⁵ at/cm² dose creating island like Au nanoparticles. An 80-nm-thick gold film was also deposited the onto same substrate and considered as reference.Fe₂O₃ film was deposited onto the gold nanoparticles and the gold/oxide interface was modeled. The valence band and the structure were measured by means of photoelectron spectroscopy (XPS) and by transmission electron microscopy (TEM), respectively. The catalytic activity was detected by CO oxidation, which was higher after the deposition of Fe₂O₃ layer onto Au nanoparticles than that on a continuous Au film. This observation was correlated to the nanosize and the redistributed valence band density of states of gold in the Au/Fe₂O₃ interface.     

Independent Memcapacitive Switching Triggered by Bromide Ion Migration for Quaternary Information Storage

Memcapacitors are emerging as an attractive candidate for high‐density information storage due to their multilevel and adjustable capacitances and long‐term retention without a power supply. However, knowledge of their memcapacitive mechanism remains unclear and accounts for the limited implementation of memcapacitors for multilevel memory technologies. Here, repeatable and reproducible quaternary memories fabricated from hybrid perovskite (CH₃NH₃SnBr₃) memcapacitors are reported. The device can be modulated to at least four capacitive states ranging from 0 to 169 pF with retention for 10⁴ s. Impressively, an effective device yield approaching 100% for quaternary memory switching is achieved by a batch of devices; each state has a sufficiently narrow distribution that can be distinguished from the others and is superior to most multilevel memories that have a low device yield as well as an overlapping distribution of states. The memcapacitive switching stems from the modulated p–i–n junction capacitance triggered by Br^? migration, as demonstrated by in situ element mapping, X‐ray photoelectron spectra, and frequency‐dependent capacitance measurements; this mechanism is different from the widely reported memristive switching involving filamentary conduction. The results provide a new way to produce high‐density information storage through memcapacitors.     

2D Perovskite Sr2Nb3O10 for High-Performance UV Photodetectors

2D perovskites, due to their unique properties and reduced dimension, are promising candidates for future optoelectronic devices. However, the development of stable and nontoxic 2D wide-bandgap perovskites remains a challenge. 2D all-inorganic perovskite Sr₂Nb₃O₁₀ (SNO) nanosheets with thicknesses down to 1.8 nm are synthesized by liquid exfoliation, and for the first time, UV photodetectors (PDs) based on individual few-layer SNO sheets are investigated. The SNO sheet-based PDs exhibit excellent UV detecting performance (narrowband responsivity = 1214 A W⁻¹, external quantum efficiency = 5.6 × 10⁵%, detectivity = 1.4 × 10¹⁴ Jones @270 nm, 1 V bias), and fast response speed (t_rise ≈ 0.4 ms, t_decay ≈ 40 ms), outperforming most reported individual 2D sheet-based UV PDs. Furthermore, the carrier transport properties of SNO and the performance of SNO-based phototransistors are successfully controlled by gate voltage. More intriguingly, the photodetecting performance and carrier transport properties of SNO sheets are dependent on their thickness. In addition, flexible and transparent PDs with high mechanical stability are easily fabricated based on SNO nanosheet film. This work sheds light on the development of high-performance optoelectronics based on low-dimensional wide-bandgap perovskites in the future.     

(111)-Oriented Pb(Zr0.52Ti0.48)O3 thin film on Pt(111)/Si substrate using CoFe2O4 nano-seed layer by pulsed laser deposition

Perovskite Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin film with perfect (111)-orientation was achieved on CoFe₂O₄ seeded-Pt(111)/Ti/SiO₂/Si substrate by pulsed laser deposition technique using target with limited excess Pb. Pyrochlore phase formation was suppressed on Pt by CoFe₂O₄ nano-seed layer (~7 nm), and perovskite PZT was achieved at temperature as low as 550 °C. CoFe₂O₄ seed layer that has perfect (111)-orientation acts as a promoter for perfectly (111)-orientated growth of PZT. PZT film grown at 600 °C has higher degree of crystalline orientation, lower surface roughness, and compacted microstructure in comparison to the film grown at 550 °C. The PZT film has a nano-size grain-feature structure with grain size of about 40–60 nm. Perovskite formation was also confirmed by ferroelectric measurement. The ferroelectric properties of PZT film grown at 600 °C is higher than that grown at 550 °C which could be attributed to the enhancement of the crystalline orientation, crystallinity, and microstructure of the film.