Inverted perovskite solar cells using dimethylacridine-based dopants.

Doping of perovskite semiconductors1 and passivation of their grain boundaries2 remain challenging but essential for advancing high-efficiency perovskite solar cells. Particularly, it is crucial to build perovskite/indium tin oxide (ITO) Schottky contact based inverted devices without predepositing a layer of hole-transport material3-5. Here we report a dimethylacridine-based molecular doping process used to construct a well-matched p-perovskite/ITO contact, along with all-round passivation of grain boundaries, achieving a certified power conversion efficiency (PCE) of 25.39%. The molecules are shown to be extruded from the precursor solution to the grain boundaries and the bottom of the film surface in the chlorobenzene-quenched crystallization process, which we call a molecule-extrusion process. The core coordination complex between the deprotonated phosphonic acid group of the molecule and lead polyiodide of perovskite is responsible for both mechanical absorption and electronic charge transfer, and leads to p-type doping of the perovskite film. We created an efficient device with a PCE of 25.86% (reverse scan) and that maintained 96.6% of initial PCE after 1,000 h of light soaking.

Vertical Heterogeneous Integration of Metal Halide Perovskite Quantum-Wires/Nanowires for Flexible Narrowband Photodetectors.

Charge collection narrowing (CCN) has been reported to be an efficient strategy to achieve optical filter-free narrowband photodetection (NPD) with metal halide perovskite (MHP) single crystals. However, the necessity of utilizing thick crystals in CCN limits their applications in large scale, flexible, self-driven, and high-performance optoelectronics. Here, for the first time, we fabricate vertically integrated MHP quantum wire/nanowire (QW/NW) array based photodetectors in nanoengineered porous alumina membranes (PAMs) showing self-driven broadband photodetection (BPD) and NPD capability simultaneously. Two cutoff detection edges of the NPDs are located at around 770 and 730 nm, with a full-width at half-maxima (fwhm) of around 40 nm. The optical bandgap difference between the NWs and the QWs, in conjunction with the high carrier recombination rate in QWs, contributes to the intriguing NPD performance. Thanks to the excellent mechanical flexibility of the PAMs, a flexible NPD is demonstrated with respectable performance. Our work here opens a new pathway to design and engineer a nanostructured MHP for novel color selective and full color sensing devices.

Teaching an Old Anchoring Group New Tricks: Enabling Low-Cost, Eco-Friendly Hole-Transporting Materials for Efficient and Stable Perovskite Solar Cells.

As a key component in perovskite solar cells (PVSCs), hole-transporting materials (HTMs) have been extensively explored and studied. Aiming to meet the requirements for future commercialization of PVSCs, HTMs which can enable excellent device performance with low cost and eco-friendly processability are urgently needed but rarely reported. In this work, a traditional anchoring group (2-cyanoacrylic acid) widely used in molecules for dye-sensitized solar cells is incorporated into donor-acceptor-type HTMs to afford MPA-BT-CA, which enables effective regulation of the frontier molecular orbital energy levels, interfacial modification of an ITO electrode, efficient defect passivation toward the perovskite layer, and more importantly alcohol solubility. Consequently, inverted PVSCs with this low-cost HTM exhibit excellent device performance with a remarkable power conversion efficiency (PCE) of 21.24% and good long-term stability in ambient conditions. More encouragingly, when processing MPA-BT-CA films with the green solvent ethanol, the corresponding PVSCs also deliver a substantial PCE as high as 20.52% with negligible hysteresis. Such molecular design of anchoring group-based materials represents great progress for developing efficient HTMs which combine the advantages of low cost, eco-friendly processability, and high performance. We believe that such design strategy will pave a new path for the exploration of highly efficient HTMs applicable to commercialization of PVSCs.

Tungsten-based highly selective solar absorber using simple nanodisk array.

In this paper, we report the design, fabrication, and characterization of a tungsten-based metamaterial selective solar absorber (SSA) combining a metal-insulator-metal (MIM) structure and simple nanodisk array. The as-fabricated absorber absorbs strongly and selectively in the wavelength range of 0.5-1.75 µm with a characterized absorptance of more than 90%, which drops abruptly to less than 12.6% beyond 2.5 µm. In addition, this broadband and highly selective absorber is polarization-insensitive under incidence of normal plane waves. Moreover, the solar selectivity remains invariable up to 40°, which is beneficial for solar thermal applications. These properties are verified theoretically and experimentally in the present work. Further analysis on energy dissipation reveals the underlying physics and optical performances.

Ruthenium acetylacetonate in interface engineering for high performance planar hybrid perovskite solar cells.

As it already made huge effect in the commercialization of silicon and other photovoltaics, interface engineering is dispensable in the current and future evolution of hybrid perovskite solar cells (PSCs) techniques. In order to solve carriers' recombination and detention at the cathode side of planar PSCs, in this work, Ruthenium acetylacetonate (RuAcac) was successfully adopted as a reliable and stable cathode interfacial layer (CIL) to improve the inverted planar PSCs. The power conversion efficiency of the optimal devices was enhanced from 12.74% for the control device without RuAcac, to 17.15% for the RuAcac based devices, with an open circuit voltage of 1.077 V, a short circuit current density of 21.28 mA/cm2, and fill factor of 74.7% correspondingly. A series of photon-physics and microscopy protocols, including EQE, UPS, XPS, PL and SKPM, were used to discover the function of RuAcac CIL. Those results confirms an identical conclusion that RuAcac enables the formation of quasi-ohmic contact at the cathode side by eliminating the energy level barrier between the LUMO of PCBM and Fermi level of silver electrode. The low temperature and facile processed Ruthenium acetylacetonate in this work definitely offer us a robust interface-engineering way for the perovskite solar cells and also their commercialization.

Wide-Range Tunable Fluorescence Lifetime and Ultrabright Luminescence of Eu-Grafted Plasmonic Core-Shell Nanoparticles for Multiplexing.

Wide-range, well-separated, and tunable lifetime nanocomposites with ultrabright fluorescence are highly desirable for applications in optical multiplexing such as multiplexed biological detection, data storage, and security printing. Here, a synthesis of tunable fluorescence lifetime nanocomposites is reported featuring europium chelate grafted onto the surface of plasmonic core-shell nanoparticles, and systematically investigated their optical performance. In a single red color emission channel, more than 12 distinct fluorescence lifetime populations with high fluorescence efficiency (up to 73%) are reported. The fluorescence lifetime of Eu-grafted core-shell nanoparticles exhibits a wider tunable range, possesses larger lifetime interval and is more sensitive to separation distance than that of ordinary Eu-doping core-shell type. These superior performances are attributed to the unique nanostructure of Eu-grafed type. In addition, these as-prepared nanocomposites are used for security printing to demonstrate optical multiplexing applications. The optical multiplexing experiments show an interesting pseudo-information "a rabbit in a well" and conceal the real message "NKU."

Eliminating performance loss from perovskite films to solar cells.

Preoptimizing perovskite films may generally improve the performance of the final perovskite solar cells (PSCs). However, the research on whether the film optimization fully contributes to the enhancement of the final PSCs has been long neglected. We demonstrated that the preparation of metal electrodes by high-vacuum thermal evaporation, an unavoidable step in almost all device fabrication processes, will damage the surface of perovskite films, resulting in component escape, defect density rebound, carrier extraction barrier, and film stability deterioration. Therefore, the prepared perovskite film and the final film actually working in devices are not exactly the same, and the contribution of film optimization to the device improvement was weakened. We designed a bilayer structure composed of graphene oxide and graphite flakes to eliminate the unwanted film inconsistencies and thus save the film optimization loss. Therefore, the efficient PSCs with power conversion efficiency of 25.55% were obtained, which demonstrated negligible photovoltaic performance loss after operating for 2000 hours.

Simple and robust phenoxazine phosphonic acid molecules as self-assembled hole selective contacts for high-performance inverted perovskite solar cells.

For inverted perovskite solar cells (PSCs), the interfacial defects and mismatched energy levels between the perovskite absorber and charge-selective layer restrain the further improvement of photovoltaic performance. Interfacial modification is a powerful tool for defect passivation and energy level turning by developing new charge-selective materials. Herein, we report three new molecules, 2BrCzPA, 2BrPTZPA, and 2BrPXZPA as self-assembled hole selective contacts (SA-HSCs) by an economical and efficient synthetic procedure. Benefiting from the stronger electron-donating ability of phenothiazine and phenoxazine compared to that of carbazole, 2BrPTZPA and 2BrPXZPA showed more matched energy levels and decreased energy loss. In addition, the ITO substrate coated with 2BrPTZPA and 2BrPXZPA could induce higher-quality perovskite crystal growth without obvious grain boundaries in the vertical direction. Consequently, the corresponding inverted PSCs with decreased trap state density achieved high power convention efficiencies (PCEs) of 22.06% and 22.93% (certified 22.38%) for 2BrPTZPA and 2BrPXZPA, respectively. Furthermore, the 2BrPXZPA-based device with encapsulation retained 97% of the initial efficiency after 600 h of maximum power point tracking under one sun continuous illumination. Finally, 2BrPXZPA was also used for the surface modification of NiOx, and the inverted PSC based on the NiOx/2BrPXZPA bilayer achieved a higher PCE of 23.66% with an open circuit voltage of 1.21 V. This work extends the design strategy of SA-HSCs for efficient and stable inverted PSCs and promotes the commercialization process.

High-efficiency, flexible and large-area red/green/blue all-inorganic metal halide perovskite quantum wires-based light-emitting diodes.

Metal halide perovskites have shown great promise as a potential candidate for next-generation solid state lighting and display technologies. However, a generic organic ligand-free and antisolvent-free solution method to fabricate highly efficient full-color perovskite light-emitting diodes has not been realized. Herein, by utilizing porous alumina membranes with ultra-small pore size as templates, we have successfully fabricated crystalline all-inorganic perovskite quantum wire arrays with ultrahigh density and excellent uniformity, using a generic organic ligand-free and anti-solvent-free solution method. The quantum confinement effect, in conjunction with the high light out-coupling efficiency, results in high photoluminescence quantum yield for blue, sky-blue, green and pure-red perovskite quantum wires arrays. Consequently, blue, sky-blue, green and pure-red LED devices with spectrally stable electroluminescence have been successfully fabricated, demonstrating external quantum efficiencies of 12.41%, 16.49%, 26.09% and 9.97%, respectively, after introducing a dual-functional small molecule, which serves as surface passivation and hole transporting layer, and a halide vacancy healing agent.

Heterogeneous 2D/3D Tin-Halides Perovskite Solar Cells with Certified Conversion Efficiency Breaking 14.

As the most promising lead-free one, tin-halides based perovskite solar cells still suffer from the severe bulk-defect due to the easy oxidation of tin from divalent to tetravalent. Here, a general and effective strategy is delivered to modulate the microstructure of 2D/3D heterogeneous tin-perovskite absorber films by substituting FAI with FPEABr in FASnI3 . The introduction of 2D phase can induce highly oriented growth of 3D FASnI3 and it is revealed in the optimal 2D/3D film that 2D phase embraces 3D grains and locates at the surfaces and grain boundaries. The FPEA+ based 2D tin-perovskite capping layer can offer a reducing atmosphere for vulnerable 3D FASnI3 grains. The unique microstructure effectively suppresses the well-known oxidation from Sn2+ to Sn4+ , as well as decreasing defect density, which leads to a remarkable enhanced device performance from 9.38% to 14.81% in conversion efficiency. The certified conversion efficiency of 14.03% announces a new record and moves a remarkable step from the last one (12.4%). Besides of this breakthrough, this work definitely paves a new way to fabricate high-quality tin-perovskite absorber film by constructing effective 2D/3D microstructures.

Dialkylamines Driven Two-Step Recovery of NiOx/ITO Substrates for High-Reproducibility Recycling of Perovskite Solar Cells.

Because of the toxicity of water-soluble lead, the recycling of organic-inorganic lead-halides perovskite solar cells (PSCs) has attracted increasing attention. Here, we report a highly reliable two-step process to recycle cost-dominated indium-tin-oxide (ITO) substrates coated with NiOx and regenerate their based PSCs by function of dialkylamines. The champion recycled PSC can achieve 20% in conversion-efficiency, higher than 17.92% of the fresh one. Strikingly, the regenerated devices can remain superior to the fresh ones in the first 7 of 10 recycles. The comprehensive X-ray photoelectronic spectroscopy analysis reveals that dipropylamine has a suitable interaction with NiOx surfaces by Ni-N coordination, enabling its effective interfacial passivation and template effect of high-quality growth of perovskites. That leads to the suppressed nonradiative recombination of both interfacial and bulk, and finally improves the device performances. The dialkylamines driven two-step recycling process offers a promising and highly reproducible strategy to recycle PSCs, especially the cost-dominated NiOx/ITO substrates.

Interfacial stabilization for inverted perovskite solar cells with long-term stability.

Perovskite solar cells (PSCs) commonly exhibit significant performance degradation due to ion migration through the top charge transport layer and ultimately metal electrode corrosion. Here, we demonstrate an interfacial management strategy using a boron chloride subphthalocyanine (Cl6SubPc)/fullerene electron-transport layer, which not only passivates the interfacial defects in the perovskite, but also suppresses halide diffusion as evidenced by multiple techniques, including visual element mapping by electron energy loss spectroscopy. As a result, we obtain inverted PSCs with an efficiency of 22.0% (21.3% certified), shelf life of 7000 h, T80 of 816 h under damp heat stress (compared to less than 20 h without Cl6SubPc), and initial performance retention of 98% after 2000 h at 80 °C in inert environment, 90% after 2034 h of illumination and maximum power point tracking in ambient for encapsulated devices and 95% after 1272 h outdoor testing ISOS-O-1. Our strategy and results pave a new way to move PSCs forward to their potential commercialization solidly.

Self-Powered and Broadband Lead-Free Inorganic Perovskite Photodetector with High Stability.

Metal halide perovskite materials have opened up a great opportunity for high-performance optoelectronic devices owing to their extraordinary optoelectronic properties. More than lead halide ones, stable and nontoxic bismuth halide perovskites exhibit more promise in their future commercialization. In this work, we developed for the first time photodetectors based on full-inorganic Cs3Bi2I9-xBrx perovskites and modulate their performance by varying x in the composition systematically. Among those self-powered photodetectors, those based on Cs3Bi2I6Br3 shows the best performance with excellent photosensitivity of 4.1 × 104 at zero bias as well as the responsivity and detectivity reaching 15 mA/W and 4.6 × 1011 Jones, respectively. More strikingly, the full-inorganic perovskite photodetectors exhibit excellent stability in the ambient environment and can maintain over 96% of the initial value after 100 days owing to the high stability of the core perovskite film. The paper definitely paves an alternative and promising strategy for the design of future commercial photodetectors that are self-powered, stable, nontoxic, etc.

Piezoelectric Energy Harvester Based on LiNbO3 Thin Films.

This paper reports the results of the influence of the energy of laser pulses during laser ablation on the morphology and electro-physical properties of LiNbO3 nanocrystalline films. It is found that increasing laser pulse energy from 180 to 220 mJ results in the concentration of charge carriers in LiNbO3 films decreasing from 8.6 × 1015 to 1.0 × 1013 cm-3, with the mobility of charge carriers increasing from 0.43 to 17.4 cm2/(V·s). In addition, experimental studies of sublayer material effects on the geometric parameters of carbon nanotubes (CNTs) are performed. It is found that the material of the lower electrode has a significant effect on the formation of CNTs. CNTs obtained at the same growth time on a sample with a Cr sublayer have a smaller diameter and a longer length compared to samples with a V sublayer. Based on the obtained results, the architecture of the energy nanogenerator is proposed. The current generated by the nanogenerator is 18 nA under mechanical stress of 600 nN. The obtained piezoelectric nanogenerator parameters are used to estimate the parameters of the hybrid-carbon-nanostructures-based piezoelectric energy converter. Obtained results are promising for the development of efficient energy converters for alternative energy devices based on lead-free ferroelectric films.

Lanthanide-Induced Photoluminescence in Lead-Free Cs2AgBiBr6 Bulk Perovskite: Insights from Optical and Theoretical Investigations.

Emphasis was recently placed on the Cs2AgBiBr6 double perovskite as a possible candidate to substitute toxic lead in metal halide perovskites. However, its poor light-emissive features currently make it unsuitable for solid-state lighting. Lanthanide doping is an established strategy to implement luminescence in poorly emissive materials, with the additional advantage of fine-tuning the emission wavelength. We discuss here the impact of Eu and Yb doping on the optical properties of Cs2AgBiBr6 thin films, obtained from the solution processing of hydrothermally synthesized bulk crystalline powders, by combining experiments and density functional theory calculations. Eu(III) incorporation does not lead to the characteristic 5D0 → 7F2 emission feature at 2 eV, while only a weak trap-assisted sub-band gap radiative emission is reported. Oppositely, we demonstrate that incorporated Yb(III) leads to an intense and exclusive photoluminescence emission in the near-infrared as a result of the efficient sensitization of the lanthanide 2F5/2 → 2F7/2 transition.

Oxygen Pressure Influence on Properties of Nanocrystalline LiNbO3 Films Grown by Laser Ablation.

Energy conversion devices draw much attention due to their effective usage of energy and resulting decrease in CO2 emissions, which slows down the global warming processes. Fabrication of energy conversion devices based on ferroelectric and piezoelectric lead-free films is complicated due to the difficulties associated with insufficient elaboration of growth methods. Most ferroelectric and piezoelectric materials (LiNbO3, BaTiO3, etc.) are multi-component oxides, which significantly complicates their integration with micro- and nanoelectronic technology. This paper reports the effect of the oxygen pressure on the properties of nanocrystalline lithium niobate (LiNbO3) films grown by pulsed laser deposition on SiO2/Si structures. We theoretically investigated the mechanisms of LiNbO3 dissociation at various oxygen pressures. The results of x-ray photoelectron spectroscopy study have shown that conditions for the formation of LiNbO3 films are created only at an oxygen pressure of 1 × 10-2 Torr. At low residual pressure (1 × 10-5 Torr), a lack of oxygen in the formed films leads to the formation of niobium oxide (Nb2O5) clusters. The presented theoretical and experimental results provide an enhanced understanding of the nanocrystalline LiNbO3 films growth with target parameters using pulsed laser deposition for the implementation of piezoelectric and photoelectric energy converters.

Influence of mixed organic cations on the structural and optical properties of lead tri-iodide perovskites.

Perovskites with mixed organic cations possess better properties in some aspects as compared to their pure counterparts. However, the structural and optical properties of these mixed-type perovskites have been rarely investigated. In this study, probable structures of mixed organic cation perovskites, MAxFA1-xPbI3 (x = 1, 0.8, 0.6, 0.4 and 0.2), were comparatively discussed. Saturation of excitonic emission at 1.66 eV indicates limited orthorhombic phase at 30 K in MAPbI3, which confirms the co-existence of orthorhombic and tetragonal phases at low temperatures. Based on the comprehensive temperature- and power-dependent measurements, it is found that defects are activated in mixed organic cation perovskites under low excitation power at room temperature, whereas this process is not observed in pure MAPbI3. At high excitation power, free exciton recombination is suppressed due to exciton-exciton interaction for all samples. Analysis of stability against temperature based on these photoemission processes shows that the structure with comparable organic cation proportion is more stable; this can be explained by uniformly distributed strains existing at the boundaries between MAPbI3 and FAPbI3 molecules. These analyses of structural and optical properties of mixed organic cation perovskites are meaningful in dictating the future optoelectronic devices.

Understanding the Impact of Cu-In-Ga-S Nanoparticles Compactness on Holes Transfer of Perovskite Solar Cells.

Although a compact holes-transport-layer (HTL) film has always been deemed mandatory for perovskite solar cells (PSCs), the impact their compactness on the device performance has rarely been studied in detail. In this work, based on a device structure of FTO/CIGS/perovskite/PCBM/ZrAcac/Ag, that effect was systematically investigated with respect to device performance along with photo-physics characterization tools. Depending on spin-coating speed, the grain size and coverage ratio of those CIGS films on FTO substrates can be tuned, and this can result in different hole transfer efficiencies at the anode interface. At a speed of 4000 r.p.m., the band level offset between the perovskite and CIGS modified FTO was reduced to a minimum of 0.02 eV, leading to the best device performance, with conversion efficiency of 15.16% and open-circuit voltage of 1.04 V, along with the suppression of hysteresis. We believe that the balance of grain size and coverage ratio of CIGS interlayers can be tuned to an optimal point in the competition between carrier transport and recombination at the interface based on the proposed mechanism. This paper definitely deepens our understanding of the hole transfer mechanism at the interface of PSC devices, and facilitates future design of high-performance devices.

A low-temperature-annealed and UV-ozone-enhanced combustion derived nickel oxide hole injection layer for flexible quantum dot light-emitting diodes.

Sol-gel derived nickel oxide (NiOx) has been extensively investigated as the hole injection layer (HIL) for many optoelectronic devices because of its advantages of high environmental stability and low cost fabrication. Conventional sol-gel synthesis of NiOx requires high annealing temperature to convert precursors into crystal lattices, which limits its application in flexible devices. To address this issue, a low-temperature (150 °C) combustion method is used to synthesize NiOx. Besides, UV-ozone treatment is further performed to improve the electrical properties of low-temperature-grown NiOx, which leads to the formation of nickel oxyhydroxide (NiO(OH)) surface dipoles and Ni vacancies and thus modifies the energy structure and increases the conductivity of NiOx. Moreover, the formation of surface NiO(OH) induces a vacuum level shift and thus reduces the hole injection barrier. Owing to the enhanced hole injection, solution-processed green QLEDs with optimized UV-ozone treated NiOx HILs exhibit maximum current efficiencies of 45.8 cd A-1 and external quantum efficiencies of 10.9%, which outperform those of the devices with poly(3,4-ethylene dioxythiophene) : poly(4-styrenesulfonate) (PEDOT:PSS) HILs. Meanwhile, these devices also show better long-term stability with a 3.2-fold longer half-life time than that of the PEDOT:PSS-based devices. The demonstrated low-temperature-annealed and UV-ozone enhanced NiOx HILs would enable the realization of flexible QLEDs with high brightness, efficiency and stability.

Printable Fabrication of a Fully Integrated and Self-Powered Sensor System on Plastic Substrates.

Wearable and portable devices with desirable flexibility, operational safety, and long cruising time, are in urgent demand for applications in wireless communications, multifunctional entertainments, personal healthcare monitoring, etc. Herein, a monolithically integrated self-powered smart sensor system with printed interconnects, printed gas sensor for ethanol and acetone detection, and printable supercapacitors and embedded solar cells as energy sources, is successfully demonstrated in a wearable wristband fashion by utilizing inkjet printing as a proof-of-concept. In such a "wearable wristband", the harvested solar energy can either directly drive the sensor and power up a light-emitting diode as a warning signal, or can be stored in the supercapacitors in a standby mode, and the energy released from supercapacitors can compensate the intermittency of light illumination. To the best of our knowledge, the demonstration of such a self-powered sensor system integrated onto a single piece of flexible substrate in a printable and additive manner has not previously been reported. Particularly, the printable supercapacitors deliver an areal capacitance of 12.9 mF cm-2 and the printed SnO2 gas sensor shows remarkable detection sensitivity under room temperature. The printable strategies for device fabrication and system integration developed here show great potency for scalable and facile fabrication of a variety of wearable devices.