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Published in Advanced Materials : https://doi.org/10.1002/adma.201802080

Just Accepted - Advanced Materials

Si photocathodes integrated with Ag-supported dendritic Cu catalysts are used to perform light-driven reduction of CO2 to C2 and C3 products in aqueous solution. A back illumination geometry with an n-type Si absorber was used to permit the use of absorbing metallic catalysts. Selective carrier collection was accomplished by a p+ implantation on the illumination side and an n+ implantation followed by atomic layer deposition of TiO2 on the electrolyte site. The Ag-supported dendritic Cu CO2 reduction catalyst was formed by evaporation of Ag followed by high-rate electrodeposition of Cu to form a high surface area structure. Under simulated 1 sun illumination in 0.1 M CsHCO3 saturated with CO2, the photovoltage generated by the Si (∼600 mV) enables C2 and C3 products to be produced at -0.4 vs. RHE. Texturing of both sides of the Si increases the light-limited current density, due to reduced reflection on the illumination side, and also deceases the onset potential. Under simulated diurnal illumination conditions photocathodes maintain over 60% faradaic efficiency to hydrocarbon and oxygenate products (mainly ethylene, ethanol, propanol) for several days. After 10 days of testing, contamination from the counter electrode is observed, which causes an increase in hydrogen production. This effect is mitigated by a regeneration procedure which restores the original catalyst selectivity. A tandem, self-powered CO2 reduction device was formed by coupling a Si photocathode with two series-connected semitransparent CH3NH3PbI3 perovskite solar cells, achieving an efficiency for the conversion of sunlight to hydrocarbons and oxygenates of 1.5% (3.5% for all products).

Single crystalline ternary ZnS(x)Se(1-x) nanowires with uniform chemical stoichiometry and accurately controllable compositions (0≤x≤ 1) were synthesized through a simple and yet effective one-step approach with a specially designed modification. Energy-gap-tuning via compositional change was achieved for a direct band gap from 2.6 to 3.6 eV. Raman spectroscopy studies revealed typical two-mode behavior indicative of high miscibility in the alloyed compound. Moreover, the enhanced electrical-conductivity and gating effect behavior after the formation of ternary alloy enable their application in nano/micro-field effect transistor devices. In addition, the slow recombination rate in the photo-response process indicates their potential for photoelectric applications.

The family of solution-processed tin-based perovskites is demonstrated as a new and superior near-infrared gain medium. Due to the large electron-hole bimolecular recombination associated with tin and the reduced trap density with SnF2 treatment, these lead-free "green" perovskites yield stable coherent light emission extending to approximate to 1 mu m at strikingly low thresholds.

Photoelectrochemieal water splitting half reactions on semiconducting photoelectrodes have received much attention but efficient overall water splitting driven by a single photoelectrode has remained elusive due to stringent electronic and thermodynamic property requirements. Utilizing a tandem configuration wherein the total photovoltage is generated by complementary optical absorption across different semiconducting electrodes is a possible pathway to unassisted overall light-induced water splitting. Because of the low photovoltages generated by conventional photovoltaic materials (e.g., Si, CIGS), such systems typically consist of triple junction design that increases the complexity due to riptoelectrical trade-offs and are also not cost-effective. Here, we show that a single solution processed organic inorganic halide perovskite (CH3NH3PI3) solar cell in tandem with a Fe2O3 photoanode can achieve overall, unassisted water splitting with a solar-to-hydrogen conversion efficiency of 2.4%. Systematic dectro-optical studies were performed to investigate the performance of tandem device. It was found that the overall efficiency was limited by the hematite's photocurrent and onset potential. To understand these limitations, we have estimated the intrinsic solar to chemical conversion efficiency of the doped and undoped Fe2O3 photoanodes. The total photopotentid generated by our tandem system (1.87 V) exceeds both the thermodynamic and kinetic requirements (1.6 V), resulting in overall water splitting without the assistance of an electrical bias.

Despite their extremely good performance in solar cells with efficiencies approaching 20% and the emerging application for light-emitting devices, organic-inorganic lead halide perovskites suffer from high content of toxic, polluting, and bioaccumulative Pb, which may eventually hamper their commercialization. Here, we present the synthesis of two-dimensional (2D) Cu-based hybrid perovskites and study their optoelectronic properties to investigate their potential application in solar cells and light-emitting devices, providing a new environmental-friendly alternative to Pb. The series (CH3NH3)(2)CuClxBr4-x, was studied in detail, with the role of Cl found to be essential for stabilization. By exploiting the additional Cu d-d transitions and appropriately tuning the Br/C1 ratio, which affects ligand-to-metal charge transfer transitions, the optical absorption in this series of compounds can be extended to the near-infrared for optimal spectral overlap with the solar irradiance. In situ formation of Cu+ ions was found to be responsible for the green photoluminescence of this material set. Processing conditions for integrating Cu-based perovskites into photovoltaic device architectures, as well as the factors currently limiting photovoltaic performance, are discussed: among them, we identified the combination of low absorption coefficient and heavy mass of the holes as main limitations for the solar cell efficiency. To the best of our knowledge, this is the first demonstration of the potential of 2D copper perovskite as light harvesters and lays the foundation for further development of perovskite based on transition metals as alternative lead-free materials. Appropriate molecular design will be necessary to improve the material's properties and solar cell performance filling the gap with the state-of-the-art Pb-based perovskite devices.

We report a regenerative iron oxide nanorod solar cell sensitized with squarine (SQ02) dye, which improves the overall light harvesting due to the dye's complementary absorption. The use of illumination sources which enabled selective excitation of Fe2O3 and the dye independently allowed the decoupling of transport and light absorption in Fe2O3. From absorbed photon-to-current conversion efficiency (APCE) calculations, it was found out that about 80% (at lambda = 680 nm) of the absorbed photons in SQ02 dye are converted to electrons and transported without any further loss into the FTO electrode. Impedance spectroscopy revealed that bulk recombination is still a prominent problem while utilizing hematite as a photoanode in a dye sensitized configuration.

We reported herein a facile and scalable preparation process for MoS2-decorated ZnxCd1-xS hybrid photocatalysts for hydrogen generation. ZnxCd1-xS nanopowder was first prepared from commercially available precursors employing a solution based process. MoS2 hydrogen evolution reaction catalyst was then loaded onto the ZnxCd1-xS nanopowder via a photo-assisted deposition process which employed mild conditions (room temperature, atmospheric pressure and visible light illumination). Thus, this process represents an important advantage in the large scale production of semiconductor/MoS2 hybrid photocatalysts in comparison to the conventional method relying on thermal decomposition of (NH4)(2)[MoS4] precursor at high temperature and under H2S pressure. The best Zn0.2Cd0.8S/MoS2 3% showed two hundred-and-ten times (210 times) faster hydrogen generation rate on visible light illumination compared with that obtained for un-treated Zn0.2Cd0.8S. That was the most impressive catalytic enhancement ever recorded for a semiconductor photocatalyst decorated with a noble metal free electrocatalyst.

The effect of TBP on Co(II/III) redox electrolyte was examined. Photocurrent of the device can be limited in electrolyte with high TBP concentration due to its viscous nature. The higher amount of TBP in electrolyte increases the viscosity of electrolyte and consequently slows down the diffusion of Co-III species. This is a unique observation for Co(II/III) electrolyte which is not observed in I-/I-3(-) electrolyte. An increase in TBP concentration from 0.1 to 0.5 M in a cobalt electrolyte containing 0.1 M Co(bpy)(3)(TFSI)(2) and 0.033 M Co(bpy)(3)(TFSI)(3) produced a 90 mV improvement in open-circuit potential (V-oc). Using electrochemical impedance spectroscopy (EIS), this enhancement could be attributed to the reduced interfacial recombination (33%) as well as a negative shift in the conduction 0 band level of TiO2 (67%). Although the influence of TBP in iodide/triiodide and cobalt-complex electrolytes is similar, the increase of viscosity at relatively high concentration of TBP in Co electrolyte should be taken into consideration in order to accomplish high efficiency DSCs based on Co-complex electrolytes.

Photoelectrochemical (PEC) cells are attractive for storing solar energy in chemical bonds through cleaving of water into oxygen and hydrogen. Although hematite (alpha-Fe2O3) is a promising photoanode material owing to its chemical stability, suitable band gap, low cost, and environmental friendliness, its performance is limited by short carrier lifetimes, poor conductivity, and sluggish kinetics leading to low (solar-to-hydrogen) STH efficiency. Herein, we combine solution-based hydrothermal growth and a post-growth surface exposure through atomic layer deposition (ALD) to show a dramatic enhancement of the efficiency for water photolysis. These modified photoanodes show a high photocurrent of 3.12 mAcm(-2) at 1.23V versus RHE, (> 5 times higher than Fe2O3) and a plateau photocurrent of 4.5 mAcm(-2) at 1.5 V versus RHE. We demonstrate that these photoanodes in tandem with a CH3NH3PbI3 perovskite solar cell achieves overall unassisted water splitting with an STH conversion efficiency of 3.4%, constituting a new benchmark for hematite-based tandem systems.

The good electrical and morphological characteristics of TiO2 nanofibers and the high extinction coefficient of CH3NH3PbI3 perovskite are combined to obtain a solar cell with a power conversion efficiency of 9.8%. The increase of the film thickness dramatically diminishes the performance due to the reduction in porosity of the TiO2 nanofiber framework. The optimum device (similar to 413 nm film thickness) is compared to a planar device, where the latter produces higher V-oc but lower J(sc), and consequently lower efficiency at all measured light intensities.

Many candidate materials for photoelectrochemical water splitting will be better employed by decoupling optical absorption from carrier transport. A promising strategy is to use multiple thin absorber layers supported on transparent, conducting materials; however there are limited such materials that are both pH stable and depositable on arbitrary high surface area substrates. Here we present the first 3D porous niobium doped tin oxide (NTO) electrodes fabricated by atomic layer deposition. After high temperature crystallization the NTO is transparent, conductive, and stable over a wide range of pH. The optimized films have high electrical conductivity up to 37 S/cm concomitant with a low optical attenuation coefficient of 0.99 mu m(-1) at 550 nm. NTO was deposited onto high surface area templates that were subsequently coated with hematite Fe2O3 for the photoelectrochemical water splitting. This approach enabled near-record water splitting photocurrents for hematite electrodes employing a host-guest strategy.

The utilisation of Cu2O photocathodes for photoelectrochemical water splitting requires their stabilisation due to photocorrosion in aqueous electrolytes. Ultrathin films of wide band gap semiconducting oxides deposited by atomic layer deposition (ALD) on top of cuprous oxide can perform the dual function of both facilitating charge extraction (through the creation of a p–n junction) and protecting the absorber material from the aqueous electrolyte, thereby suppressing corrosion in favor of hydrogen generation. The factors that determine the photocurrent performance as well as the stability of these photoelectrodes are examined. Specifically, the influence of ALD deposition temperature, electrolyte pH, electrolyte composition as well as post-deposition annealing treatments was studied. The successful development of protective overlayers must fulfil the dual requirements of favourable band alignments as well as chemical stability. At long time scales, the deactivation of the photocathodes proceeds through etching of the amorphous overlayer, accompanied by the loss of the platinum catalyst particles. Through the deposition of a semi-crystalline TiO2 overlayer, 62% stability over 10 hours of testing has been demonstrated without re-platinization.