Highly Transparent Dual-Sensitized Titanium Dioxide Nanotube Arrays for Spontaneous Solar Water Splitting Tandem Configuration

Vertically aligned one-dimensional (1D) titanium dioxide (TiO₂) arrays on transparent conducting oxide (TCO) substrates, which can act as host electron transport materials for low bandgap materials, were synthesized via a hydrothermal reaction combined with a controlled chemical etching process. By controlling the chemical etching conditions, we can maximize the light transmission properties of the 1D TiO₂ arrays, which is beneficial for the front electrode in photoelectrochemical (PEC) tandem configurations. As a result, dual sensitization to form 1D TiO₂@CdS@CdSe (CdS and CdSe coated 1D TiO₂) results in excellent photocurrent density, as well as transparency, and the resulting material is able to pass unabsorbed photons through the front electrode into the rear bias solar cell. Owing to the improved light transmission in combination with the increased specific surface area of the obtained 1D TiO₂ arrays from the controlled etching process, a high-efficiency PEC tandem device with ∼2.1% was successfully fabricated for unassisted hydrogen evolution. Efficient PEC tandem device was fabricated for unassisted solar hydrogen generation using highly transparent composite electrode composed of dual sensitization to form 1D TiO₂@CdS@CdSe

Controlled Dissolution of Polystyrene Nanobeads: Transition from Liquid Electrolyte to Gel Electrolyte

The widespread commercialization of dye-sensitized solar cells (DSSCs) remains limited because of the use of highly volatile liquid electrolytes. Recently, gel-type quasi-solid electrolytes containing a polymer additive or inorganic nanomaterial have shown promising results in terms of the cell efficiency. However, most gel electrolytes have serious obstacles for pore-filling because of their high viscosity. Herein, we report the first observation of the transition from a liquid to a gel electrolyte after filling the cell with the liquid electrolyte using the controlled dissolution of polystyrene nanobeads on the counter electrode, suggesting that the pore-filling problem can be diminished in quasi-solid state DSSCs. The time-resolved solidification allows for the preparation of the gel electrolyte without interfering with the cell performance. The optimal DSSC composed of the gel electrolyte exhibits almost the same power conversion efficiency as the liquid electrolyte based DSSC when measured using an AM1.5G solar simulator at 100 mW/cm² light illumination. Moreover, the long-term stability of the DSSC was greatly improved

Highly Conductive Freestanding Graphene Films as Anode Current Collectors for Flexible Lithium-Ion Batteries

The electrodes in lithium-ion batteries (LIBs) are typically films that are arranged on metal foil current collectors with a thickness of several tens of μm. Here, we report on the preparation of a thick free-standing graphene film synthesized by CVD as an alternative to Cu foil as an anode current collector. As a model system, MoS₂ anodes with a flower-like morphology were anchored onto the surface of the thick graphene film. A hybrid and binder free anode without a conventional metal current collector exhibited an excellent capacity value of around 580 mAh/g (@50 mA/g) and reasonable charge/discharge cyclability. The work presented here may stimulate the use of graphene films as replacements for conventional current collectors and additive free electrode in LIBs

Stamping Transfer of a Quantum Dot Interlayer for Organic Photovoltaic Cells

An organophilic cadmium selenide (CdSe) quantum dot (QD) interlayer was prepared on the active layer in organic solar cells by a stamping transfer method. The mother substrate composed of a UV-cured film on a polycarbonate film with strong solvent resistance makes it possible to spin-coat QDs on it and dry transfer onto an active layer without damaging the active layer. The QD interlayers have been optimized by controlling the concentration of the QD solution. The coverage of QD particles on the active layer was verified by TEM analysis and fluorescence images. After insertion of the QD interlayer between the active layer and metal cathode, the photovoltaic performances of the organic solar cell were clearly enhanced. By ultraviolet photoelectron spectroscopy of CdSe QDs, it can be anticipated that the CdSe QD interlayer reduces charge recombination by blocking the holes moving to the cathode from the active layer and facilitating efficient collection of the electrons from the active layer to the cathode

Highly Efficient Monolithic Dye-Sensitized Solar Cells

Monolithic dye-sensitized solar cells (M-DSSCs) provide an effective way to reduce the fabrication cost of general DSSCs since they do not require transparent conducting oxide substrates for the counter electrode. However, conventional monolithic devices have low efficiency because of the impediments resulting from counter electrode materials and spacer layers. Here, we demonstrate highly efficient M-DSSCs featuring a highly conductive polymer combined with macroporous polymer spacer layers. With M-DSSCs based on a PEDOT/polymer spacer layer, a power conversion efficiency of 7.73% was achieved, which is, to the best of our knowledge, the highest efficiency for M-DSSCs to date. Further, PEDOT/polymer spacer layers were applied to flexible DSSCs and their cell performance was investigated

Efficient Hole Extraction from Sb₂S₃ Heterojunction Solar Cells by the Solid Transfer of Preformed PEDOT:PSS Film

Here, we report significant improvements of <i>V</i>_oc and FF in Sb₂S₃ quantum dot (QD)-based, solid-state heterojunction solar cells prepared from the solid transfer of preformed PEDOT:PSS hole extraction layers. Despite the moderate optical properties of Sb₂S₃ QDs, the solid state QD solar cells suffer from poor power conversion efficiency (PCE) resulting from the disappointing <i>V</i>_oc and the high series resistance since there is inefficient charge extraction from QDs to the metal top electrode. In order to improve the hole extraction performance, a significantly uniform PEDOT:PSS (poly­(3,4-ethylenedioxythiophene)/poly­(styrenesulfonate)) layer was transferred on the hole transport layer (P3HT, poly­(3-hexylthiophene-2,5-diyl)) by using a simple solid-transfer method. In contrast with conventional spin-cast methods, the hydrophilic PEDOT:PSS layer was uniformly coated on the hydrophobic P3HT layer without any significant detriment to P3HT film properties. Due to improved contact surface for the Au top electrode and hole conductance resulting in significantly improved charge extraction, the power conversion efficiency was dramatically enhanced. Furthermore, the thickness of the PEDOT:PSS film was precisely optimized by layer-by-layer solid transfer, and thereby the PCE of the PEDOT:PSS solid-transfer device (30 nm) was improved by 25.7% in comparison to the PEDOT:PSS spin-cast device and by 76% in comparison to the PEDOT:PSS free device

Molecular Chemistry-Controlled Hybrid Ink-Derived Efficient Cu₂ZnSnS₄ Photocathodes for Photoelectrochemical Water Splitting

To realize economically competitive hydrogen production through photoelectrochemical (PEC) water splitting, it is essential to develop an efficient photoelectrode consisting of earth-abundant constituents in conjunction with low-cost solution processing. Cu₂ZnSnS₄ (CZTS) has received significant attention as a promising photocathode owing to its abundance and good absorption properties. However, the efficiency of the solution-processed CZTS photocathode is not yet comparable to its counterparts. Here, a hybrid ink, obtained by careful control of precursor mixing order, was used to produce a highly efficient CZTS photocathode. The molecular chemistry-controlled hybrid ink formulation, particularly the roles of thiourea–Sn²⁺ complexation, was elucidated by liquid Raman spectroscopy. The hybrid ink-derived CZTS thin films modified with conformal coating of an n-type TiO₂/CdS double layer and a Pt electrocatalyst achieved an exceptionally high photocurrent of 13 mA cm^–2 at −0.2 V versus a reversible hydrogen electrode under 1 sun illumination. The modified photocathodes showed relatively stable H₂ production with faradaic efficiency close to unity

Stacked Porous Iron-Doped Nickel Cobalt Phosphide Nanoparticle: An Efficient and Stable Water Splitting Electrocatalyst

Exploration of proficient electrocatalyst from earth-abundant nonprecious metals in lieu of noble metal-based catalysts to obtain clean hydrogen energy through large-scale electrochemical water splitting is still an ongoing challenge. Herein, iron-doped nickel cobalt phosphide nanoplate arrays grown on a carbon cloth (NiCoFe_<i>x</i>P/CC) are fabricated using a simple hydrothermal route, followed by phosphorization. The electrochemical analysis demonstrates that the NiCoFe_<i>x</i>P/CC electrode possesses high electrocatalytic activity for water splitting in alkaline medium. Benefits from the synergistic effect between the metal centers, two-dimensional porous nanoplates, and unique three-dimensional electrode configuration of NiCoFe_<i>x</i>P/CC provide small overpotentials of 39 at 10 mA cm^–2 and 275 mV at 50 mA cm^–2 to drive the hydrogen evolution reaction and oxygen evolution reaction, respectively. Furthermore, the assembled two-electrode (NiCoFe_<i>x</i>P/CC∥NiCoFe_<i>x</i>P/CC) alkaline water electrolyzer can achieve 10 mA cm^–2 current density at 1.51 V. Remarkably, it can maintain stable electrolysis over 150 h. The excellent activity and stability of this catalyst is proved to be a economical substitute of commercial noble metal-based catalysts in technologies relevant to renewable energy

Conflicted Effects of a Solvent Additive on PTB7:PC₇₁BM Bulk Heterojunction Solar Cells

Recently, polymer–fullerene based bulk heterojunction (BHJ) solar cells, which contain blends of poly­({4,8-bis­[(2-ethylhexyl)­oxy]­benzo­[1,2-b:4,5-<i>b</i>′]­dithiophene-2,6-diyl}­{3-fluoro-2-[(2-ethylhexyl)­carbonyl]­thieno­[3,4-<i>b</i>]­thiophenediyl}) (PTB7) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), have been widely studied due to exhibiting high power conversion efficiency (PCE) and well-defined nanomorphology. Because of the short exciton diffusion pathway (less than 10 nm) in organic thin films, the optimization of PTB7:PC₇₁BM BHJ with optimized morphology is very important between the donor and acceptor. In order to increase nanoscale phase separation, the chemical additives of 1,8-diiodooctane (DIO) have been used in PTB7:PC₇₁BM blend systems. However, the mechanism studies of DIO in BHJ solar cells and its effectiveness on device stability are unclear. In this study, we fabricated polymer solar cells (PSCs) based on PTB7:PC₇₁BM BHJ with various DIO concentrations to investigate not only correlation between device performances and different morphologies, but also the influence of additives on device stabilities. Positive effects of DIO, which were induced by efficient charge separation in BHJ at optimized blending ratio, are proved by the results of time-resolved photoluminescence (TRPL), and negative effects of DIO on a device stability have been investigated according to the ISOS-D-1 protocol

Unveiling the Crystal Formation of Cesium Lead Mixed-Halide Perovskites for Efficient and Stable Solar Cells

Thermal instability of organic–inorganic hybrid perovskites will be an inevitable hurdle for commercialization. Recently, all-inorganic cesium lead halide perovskites, in particular, CsPbI₂Br, have emerged as thermally stable and efficient photovoltaic light absorbers. However, the fundamental properties of this material have not been studied in detail. The crystal formation behavior of CsPbI₂Br is investigated by examining the surface morphology, crystal structure, and chemical state of the perovskite films. We discover a previously uncharacterized feature that the formation of black polymorph through optimal annealing temperature proves to be critical to both solar cell efficiency and phase stability. Our optimized planar heterojunction solar cell exhibits a <i>J–V</i> scan efficiency of 10.7% and open-circuit voltage of 1.23 V, which far outperforms the preceding literature