Planar perovskite solar cells with long-term stability using ionic liquid additives.

Solar cells based on metal halide perovskites are one of the most promising photovoltaic technologies1-4. Over the past few years, the long-term operational stability of such devices has been greatly improved by tuning the composition of the perovskites5-9, optimizing the interfaces within the device structures10-13, and using new encapsulation techniques14,15. However, further improvements are required in order to deliver a longer-lasting technology. Ion migration in the perovskite active layer-especially under illumination and heat-is arguably the most difficult aspect to mitigate16-18. Here we incorporate ionic liquids into the perovskite film and thence into positive-intrinsic-negative photovoltaic devices, increasing the device efficiency and markedly improving the long-term device stability. Specifically, we observe a degradation in performance of only around five per cent for the most stable encapsulated device under continuous simulated full-spectrum sunlight for more than 1,800 hours at 70 to 75 degrees Celsius, and estimate that the time required for the device to drop to eighty per cent of its peak performance is about 5,200 hours. Our demonstration of long-term operational, stable solar cells under intense conditions is a key step towards a reliable perovskite photovoltaic technology.

Enhancing Perovskite Solar Cell Performance by Interface Engineering Using CH3NH3PbBr0.9I2.1 Quantum Dots.

To improve the interfacial charge transfer that is crucial to the performance of perovskite solar cells, the interface engineering in a device should be rationally designed. Here we have developed an interface engineering method to tune the photovoltaic performance of planar-heterojunction perovskite solar cells by incorporating MAPbBr3-xIx (MA = CH3NH3) quantum dots (QDs) between the MAPbI3 perovskite film and the hole-transporting material (HTM) layer. By adjustment of the Br:I ratio, the as-synthesized MAPbBr3-xIx QDs show tunable fluorescence and band edge positions. When the valence band (VB) edge of MAPbBr3-xIx QDs is located below that of the MAPbI3 perovskite, the hole transfer from the MAPbI3 perovskite film to the HTM layer is hindered, and hence, the power conversion efficiency decreases. In contrast, when the VB edge of MAPbBr3-xIx QDs is located between the VB edge of the MAPbI3 perovskite film and the highest occupied molecular orbital of the HTM layer, the hole transfer from the MAPbI3 perovskite film to the HTM layer is well-facilitated, resulting in significant improvements in the fill factor, short-circuit photocurrent, and power conversion efficiency.

High-performance perovskite photoanode enabled by Ni passivation and catalysis.

Lead halide perovskites have achieved phenomenal successes in photovoltaics due to their suitable bandgaps, long diffusion lengths, and balanced charge transport. However, the extreme susceptibility of perovskites to water or air has imposed a seemingly insurmountable barrier for leveraging these unique materials into solar-to-fuel applications such as photoelectrochemical conversion. Here we developed a CH3NH3PbI3-based photoanode with an ultrathin Ni surface layer, which functions as both a physical passivation barrier and a hole-transferring catalyst. Remarkably, a much enhanced photocurrent density, an unassisted photoelectrochemical conversion capability, and a substantially better stability against water have been achieved, which are exceeding most of the previously reported photoanodes as well as a similar CH3NH3PbI3-based device structure but without the Ni surface layer. Our study suggests many exciting opportunities of developing perovskite-based solar-to-fuel conversion.

Solar-Energy-Driven Photoelectrochemical Biosensing Using TiO2 Nanowires.

Photoelectrochemical sensing represents a unique means for chemical and biological detection, with foci of optimizing semiconductor composition and electronic structures, surface functionalization layers, and chemical detection methods. Here, we have briefly discussed our recent developments of TiO2 nanowire-based photoelectrochemical sensing, with particular emphasis on three main detection mechanisms and corresponding examples. We have also demonstrated the use of the photoelectrochemical sensing of real-time molecular reaction kinetic measurements, as well as direct interfacing of living cells and probing of cellular functions.

Solar-driven photoelectrochemical probing of nanodot/nanowire/cell interface.

We report a nitrogen-doped carbon nanodot (N-Cdot)/TiO2 nanowire photoanode for solar-driven, real-time, and sensitive photoelectrochemical probing of the cellular generation of H2S, an important endogenous gasotransmitter based on a tunable interfacial charge carrier transfer mechanism. Synthesized by a microwave-assisted solvothermal method and subsequent surface chemical conjugation, the obtained N-Cdot/TiO2 nanowire photoanode shows much enhanced photoelectrochemical photocurrent compared with pristine TiO2 nanowires. This photocurrent increase is attributed to the injection of photogenerated electrons from N-Cdots to TiO2 nanowires, confirmed by density functional theory simulation. In addition, the charge transfer efficiency is quenched by Cu(2+), whereas the introduction of H2S or S(2-) ions resets the charge transfer and subsequently the photocurrent, thus leading to sensitive photoelectrochemical recording of the H2S level in buffer and cellular environments. Moreover, this N-Cdot-TiO2 nanowire photoanode has been demonstrated for direct growth and interfacing of H9c2 cardiac myoblasts, with the capability of interrogating H2S cellular generation pathways by vascular endothelial growth factor stimulation as well as inhibition.

Surface plasmon resonance enhanced real-time photoelectrochemical protein sensing by gold nanoparticle-decorated TiO2 nanowires.

Recently developed photoelectrochemical (PEC) sensing systems represent a unique potential detection method for real-time analysis of chemical/biological molecules, while the low absorption of TiO2 nanomaterials in the visible wavelength region and the slow surface charge transfer efficiency limit the ultimate sensitivity. Here we develop a gold nanoparticle-decorated TiO2 nanowire sensor for PEC detection of protein binding. The direct attachment of Au nanoparticles to TiO2 nanowires offers strong surface plasmon resonance for electrochemical field effect amplification, yielding a ~100% increase of photocurrent density. In addition, the surface functionalization of gold nanoparticles allows for direct capturing of target proteins near the Au/TiO2 interface and thus substantially enhances the capability of attenuation of energy coupling between Au and TiO2, leading to much-improved sensor performance. As a proof of concept, cholera toxin subunit B has been robustly detected by the TiO2-Au nanowire sensor functionalized with ganglioside GM1, with a high sensitivity of 0.167 nM and excellent selectivity. Furthermore, the real-time feature of photoelectrochemical sensing enables direct measurement of binding kinetics between cholera toxin subunit B and GM1, yielding association and disassociation rate constants and an equilibrium constant K(d) of 4.17 nM. This surface plasmon resonance-enhanced real-time, photoelectrochemical sensing design may lead to exciting biodetection capabilities with high sensitivity and real-time kinetic studies.

Reversible chemical tuning of charge carriers for enhanced photoelectrochemical conversion and probing of living cells.

A facile, solution method for reversible tuning of oxygen vacancies inside TiO2 nanowires, in which the reducing treatment of TiO2 by NaBH4 leads to 2.4-fold increase of photocurrent density, compared to pristine TiO2 nanowires, is reported. Subsequent oxidizing treatment using KMnO4 or annealing in air can reset the photocurrent density to the original values. The incident photo-to-current conversion efficiency measurement exhibits that the reduced TiO2 nanowires present both enhanced photoactivity in both UV and visible regions. Density functional theory calculations reveal that the oxygen vacancies in the reduced TiO2 cause defect states in the band structure and result in enhanced carrier density and conductivity. In addition, the enhanced solar energy-driven photoelectrochemical conversion allows real-time, sensitive chemical probing of living cells that are directly grown on the TiO2 nanowire photoanodes. As proofs-of-concept, after functionalized with horseradish peroxidase (HRP) on the surface, the reduced TiO2 NWs demonstrate sensitive, real-time monitoring of the H2O2 levels in several distinctive living cell lines, with the lowest detectable H2O2 concentration of 7.7 nM. This reversible tuning of oxygen vacancies suggests a facile means for transition metal oxides, with enhanced photoconversion activity and electrochemical sensitivity.

WO3-reduced graphene oxide composites with enhanced charge transfer for photoelectrochemical conversion.

Hybrid structures between semiconducting metal oxides and carbon with rational synthesis represent unique device building blocks to optimize the light absorption and charge transfer process for the photoelectrochemical conversion. Here we demonstrate the realization of a WO3-reduced graphene oxide (RGO) nanocomposite via hydrothermal growth of ultrathin WO3 nanoplates directly on fluorine-doped tin oxide (FTO) substrates, followed by in situ photo-reduction to deposit RGO layers on WO3 nanoplate surface. Photoanodes made of the WO3-RGO nanocomposites have achieved a photocurrent density of 2.0 mA cm(-2) at 1.23 V vs. reversible hydrogen electrode (RHE), which is among the highest reported values for photoanodes based on hydrothermally grown WO3. Electrochemical impedance spectroscopy reveals that the increase of photoactivity is attributed to the enhanced charge transfer by the incorporation of RGO, thus suggesting a general approach for designing other metal oxide-RGO hybrid architectures.

Controlled Sn-doping in TiO2 nanowire photoanodes with enhanced photoelectrochemical conversion.

We demonstrate for the first time the controlled Sn-doping in TiO(2) nanowire (NW) arrays for photoelectrochemical (PEC) water splitting. Because of the low lattice mismatch between SnO(2) and TiO(2), Sn dopants are incorporated into TiO(2) NWs by a one-pot hydrothermal synthesis with different ratios of SnCl(4) and tetrabutyl titanate, and a high acidity of the reactant solution is critical to control the SnCl(4) hydrolysis rate. The obtained Sn-doped TiO(2) (Sn/TiO(2)) NWs are single crystalline with a rutile structure, and the incorporation of Sn in TiO(2) NWs is well controlled at a low level, that is, 1-2% of Sn/Ti ratio, to avoid phase separation or interface scattering. PEC measurement on Sn/TiO(2) NW photoanodes with different Sn doping ratios shows that the photocurrent increases first with increased Sn doping level to >2.0 mA/cm(2) at 0 V vs Ag/AgCl under 100 mW/cm(2) simulated sunlight illumination up to ~100% enhancement compared to our best pristine TiO(2) NW photoanodes and then decreases at higher Sn doping levels. Subsequent annealing of Sn/TiO(2) NWs in H(2) further improves their photoactivity with an optimized photoconversion efficiency of ~1.2%. The incident-photon-to-current conversion efficiency shows that the photocurrent increase is mainly ascribed to the enhancement of photoactivity in the UV region, and the electrochemical impedance measurement reveals that the density of n-type charge carriers can be significantly increased by the Sn doping. These Sn/TiO(2) NW photoanodes are highly stable in PEC conversion and thus can serve as a potential candidate for pure TiO(2) materials in a variety of solar energy driven applications.

A piperidinium salt stabilizes efficient metal-halide perovskite solar cells.

Longevity has been a long-standing concern for hybrid perovskite photovoltaics. We demonstrate high-resilience positive-intrinsic-negative perovskite solar cells by incorporating a piperidinium-based ionic compound into the formamidinium-cesium lead-trihalide perovskite absorber. With the bandgap tuned to be well suited for perovskite-on-silicon tandem cells, this piperidinium additive enhances the open-circuit voltage and cell efficiency. This additive also retards compositional segregation into impurity phases and pinhole formation in the perovskite absorber layer during aggressive aging. Under full-spectrum simulated sunlight in ambient atmosphere, our unencapsulated and encapsulated cells retain 80 and 95% of their peak and post-burn-in efficiencies for 1010 and 1200 hours at 60° and 85°C, respectively. Our analysis reveals detailed degradation routes that contribute to the failure of aged cells.

Lasing from lead halide perovskite semiconductor microcavity system.

Organic-inorganic halide perovskite semiconductors are ideal gain media for fabricating laser and photonic devices due to high absorption, photoluminescence (PL) efficiency and low nonradiative recombination losses. Herein, organic-inorganic halide perovskite CH3NH3PbI3 is embedded in the Fabry-Perot (FP) microcavity, and a wavelength-tunable excitonic lasing with a threshold of 12.9 µJ cm-2 and the spectral coherence of 0.76 nm are realized. The lasing threshold decreases and the spectral coherence enhances as the temperature decreases; these results are ascribed to the suppression of exciton irradiative recombination caused by thermal fluctuation. Moreover, both lasing and light emission below threshold from the perovskite microcavity (PM) system demonstrate a redshift with the decreasing temperature. These results provide a feasible platform based on the PM system for the study of light-matter interaction for quantum optics and the development of optoelectronic devices such as polariton lasers.

WO3 nanoflakes for enhanced photoelectrochemical conversion.

We developed a postgrowth modification method of two-dimensional WO3 nanoflakes by a simultaneous solution etching and reducing process in a weakly acidic condition. The obtained dual etched and reduced WO3 nanoflakes have a much rougher surface, in which oxygen vacancies are created during the simultaneous etching/reducing process for optimized photoelectrochemical performance. The obtained photoanodes show an enhanced photocurrent density of ∼1.10 mA/cm(2) at 1.0 V vs Ag/AgCl (∼1.23 V vs reversible hydrogen electrode), compared to 0.62 mA/cm(2) of pristine WO3 nanoflakes. The electrochemical impedance spectroscopy measurement and the density functional theory calculation demonstrate that this improved performance of dual etched and reduced WO3 nanoflakes is attributed to the increase of charge carrier density as a result of the synergetic effect of etching and reducing.

Indirect growth of mesoporous Bi@C core-shell nanowires for enhanced lithium-ion storage.

In this paper, we propose a facile synthetic strategy for uniform bismuth@carbon (Bi@C) core-shell nanowires, which are prepared via controlled pyrolysis of Bi2S3@glucose-derived carbon-rich polysaccharide (GCP) nanowires under an inert atmosphere. Carbonization of GCP and pyrolysis of Bi2S3 into Bi occur at 500 °C and 600 °C, respectively, which increase the specific surface area and the pore volume of the nanowires, thus allowing accommodation of more lithium ions. Meanwhile, the carbon shell serves as a buffer layer to relieve large volume expansion-contraction during the electrochemical alloy formation, and can also efficiently reduce the aggregation of the nanowires. As a proof-of-concept, the Bi@C core-shell nanowire anodes manifest enhanced cycling stability (408 mA h g(-1) after 100 cycles at a current density of 100 mA g(-1)) and rate capacity (240 mA h g(-1) at a current density of 1 A g(-1)), much higher than pure bismuth microparticles and corresponding Bi2S3@C nanowires.

Silicon nanowires for biosensing, energy storage, and conversion.

Semiconducting silicon nanowires (SiNWs) represent one of the most interesting research directions in nanoscience and nanotechnology, with capabilities of realizing structural and functional complexity through rational design and synthesis. The exquisite control of chemical composition, structure, morphology, doping, and assembly of SiNWs, in both individual and array format, as well as incorporation with other materials, offers a nanoscale building block with unique electronic, optoelectronic, and catalytic properties, thus allowing for a variety of exciting opportunities in the fields of life sciences and renewable energy. This review provides a brief summary of SiNW research in the past decade, from the SiNW synthesis by both the top-down approaches and the bottom-up approaches, to several important biological and energy applications including biomolecule sensing, interfacing with cells and tissues, lithium-ion batteries, solar cells, and photoelectrochemical conversion.

Unconventional 0-, 1-, and 2-dimensional single-crystalline copper sulfide nanostructures.

We report the synthesis of several unconventional 0-, 1- and 2-dimensional copper sulfide nanostrucutures by the chemical vapor deposition method. The key factor for morphology and structure control of a variety of copper sulfide products is the tuning of deposition and growth temperature to fit for the surface energy barriers and promote different growth directions. At a high growth temperature (480 °C) that provides enough thermal energy, a 0-D octahedral copper sulfide single crystal structure was synthesized. At a slightly lower growth temperature (460 °C), a new 1-D copper sulfide nanorod structure with a nanocrystal head was discovered for the first time. At a much lower growth temperature (150 °C), 2-D copper sulfide nanoflakes with a single crystal hexagonal structure were obtained. These novel structural varieties of copper sulfide can lead to discovering more unconventional material structures and growth mechanisms of other transitional metal chalcogenides, and may allow for new copper sulfide based devices and applications.