Significantly Enhanced Open Circuit Voltage and Fill Factor of Quantum Dot Sensitized Solar Cells by Linker Seeding Chemical Bath Deposition

We have significantly improved open circuit voltage and fill factor with a Pt counter electrode of quasi-solid state quantum dot sensitized solar cells (QDSSCs) by achieving compact coverage of QDs on a TiO₂ matrix through a linker seeding chemical bath deposition process, leading to 4.23% power conversion efficiency, nearly two times that with conventionally deposited control photoanode. The distinguishing characteristic of our linker seeding synthesis is that it does not rely on surface adsorption of precursor ions directly on TiO₂ (TiO₂∼Cd_<i>x</i>) but rather nucleates special ionic seeds on a compact linker layer (TiO₂-COORS-Cd_<i>x</i>), thereby resulting in a full and even coverage of QDs on the TiO₂ surface in large area. We have shown that the compact coverage not only helps to suppress recombination from electrolyte but also gives rise to better charge transport through the QD layer. This linker seeding chemical bath deposition method is general and expected to reinforce the hope of quasi-solid state QDSSCs as a strong competitor of dye-sensitized solar cells after further optimization and development

Secondary Branching and Nitrogen Doping of ZnO Nanotetrapods: Building a Highly Active Network for Photoelectrochemical Water Splitting

A photoanode based on ZnO nanotetrapods, which feature good vectorial electron transport and network forming ability, has been developed for efficient photoelectrochemical water splitting. Two strategies have been validated in significantly enhancing light harvesting. The first was demonstrated through a newly developed branch-growth method to achieve secondary and even higher generation branching of the nanotetrapods. Nitrogen-doping represents the second strategy. The pristine ZnO nanotetrapod anode yielded a photocurrent density higher than those of the corresponding nanowire devices reported so far. This photocurrent density was significantly increased for the new photoanode architecture based on the secondary branched ZnO nanotetrapods. After N-doping, the photocurrent density enjoyed an even more dramatic enhancement to 0.99 mA/cm² at +0.31 V vs Ag/AgCl. The photocurrent enhancement is attributed to the greatly increased roughness factor for boosting light harvesting associated with the ZnO nanotetrapod branching, and the increased visible light absorption due to the N-doping induced band gap narrowing of ZnO

Building High-Efficiency CdS/CdSe-Sensitized Solar Cells with a Hierarchically Branched Double-Layer Architecture

We report a double-layer architecture for a photoanode of quantum-dot-sensitized solar cells (QDSSCs), which consists of a ZnO nanorod array (NR) underlayer and a ZnO nanotetrapod (TP) top layer. Such double-layer and branching strategies have significantly increased the power conversion efficiency (PCE) to as high as 5.24%, nearly reaching the record PCE of QDSSCs based on TiO₂. Our systematic studies have shown that the double-layer strategy could significantly reduce charge recombination at the interface between the charge collection anode (FTO) and ZnO nanostructure because of the strong and compact adhesion of the NRs and enhance charge transport due to the partially interpenetrating contact between the NR and TP layers, leading to improved open-circuit voltage (<i>V</i>_oc) and short-circuit current density (<i>J</i>_sc). Also, when the double layer was subjected to further branching, a large increase in <i>J</i>_sc and, to a lesser extent, the fill factor (FF) has resulted from increases in quantum-dot loading, enhanced light scattering, and reduced series resistance

Unveiling Two Electron-Transport Modes in Oxygen-Deficient TiO₂ Nanowires and Their Influence on Photoelectrochemical Operation

Introducing oxygen vacancies (V_O) into TiO₂ materials is one of the most promising ways to significantly enhance light-harvesting and photocatalytic efficiencies of photoelectrochemical (PEC) cells for water splitting among others. However, the nature of electron transport in V_O-TiO₂ nanostructures is not well understood, especially in an operating device. In this work, we use the intensity-modulated photocurrent spectroscopy technique to study the electron-transport property of V_O-TiO₂ nanowires (NWs). It is found that the electron transport in pristine TiO₂ NWs displays a single trap-limited mode, whereas two electron-transport modes were detected in V_O-TiO₂ NWs, a trap-free transport mode at the core, and a trap-limited transport mode near the surface. The considerably higher diffusion coefficient (<i>D</i>_n) of the trap-free transport mode grants a more rapid electron flow in V_O-TiO₂ NWs than that in pristine TiO₂ NWs. This electron-transport feature is expected to be common in other oxygen-deficient metal oxides, lending a general strategy for promoting the PEC device performance

Space-Confined Growth of MoS₂ Nanosheets within Graphite: The Layered Hybrid of MoS₂ and Graphene as an Active Catalyst for Hydrogen Evolution Reaction

Since the electrocatalytic activity of layered molybdenum disulfide (MoS₂) for hydrogen evolution reaction (HER) closely depends on its exposed edges, the morphology and size of the material are critically important. Herein, we introduce a novel solvent-evaporation-assisted intercalation method to fabricate the hybrid of alternating MoS₂ sheets and reduced graphene oxide layers, in which the nanosize of the MoS₂ nanosheets can be effectively controlled by leveraging the confinement effect within the two-dimensional graphene layers. Significantly, the resulting MoS₂/reduced graphene oxide (RGO) composite shows excellent catalytic activity for HER characterized by higher current densities and lower onset potentials than the conventional pre-exfoliated RGO supported MoS₂ nanosheets. Further experiments on the effect of oxidation degree of graphene, the crystallinity of MoS₂, and the exposed active site density on the HER performance of the MoS₂/RGO composites show that there is an optimum condition for the catalytic activity of HER due to a balance between the numbers of exposed active sites of MoS₂ and the internal conductive channels provided by graphene

Mesoporous TiO₂ Single Crystals: Facile Shape‑, Size‑, and Phase-Controlled Growth and Efficient Photocatalytic Performance

In this work, we have succeeded in preparing rutile and anatase TiO₂ mesoporous single crystals with diverse morphologies in a controllable fashion by a simple silica-templated hydrothermal method. A simple in-template crystal growth process was put forward, which involved heterogeneous crystal nucleation and oriented growth within the template, a sheer spectator, and an excluded volume, i.e., crystal growth by faithful negative replication of the silica template. A series of mesoporous single-crystal structures, including rutile mesoporous TiO₂ nanorods with tunable sizes and anatase mesoporous TiO₂ nanosheets with dominant {001} facets, have been synthesized to demonstrate the versatility of the strategy. The morphology, size, and phase of the TiO₂ mesoporous single crystals can be tuned easily by varying the external conditions such as the hydrohalic acid condition, seed density, and temperature rather than by the silica template, which merely serves for faithful negative replication but without interfering in the crystallization process. To demonstrate the application value of such TiO₂ mesoporous single crystals, photocatalytic activity was tested. The resultant TiO₂ mesoporous single crystals exhibited remarkable photocatalytic performance on hydrogen evolution and degradation of methyl orange due to their increased surface area, single-crystal nature, and the exposure of reactive crystal facets coupled with the three-dimensionally connected mesoporous architecture. It was found that {110} facets of rutile mesoporous single crystals can be considered essentially as reductive sites with a key role in the photoreduction, while {001} facets of anatase mesoporous single crystals provided oxidation sites in the oxidative process. Such shape- and size-controlled rutile and anatase mesoporous TiO₂ single crystals hold great promise for building energy conversion devices, and the simple solution-based hydrothermal method is extendable to the synthesis of other mesoporous single crystals beyond TiO₂

Solution-Processed, Barrier-Confined, and 1D Nanostructure Supported Quasi-quantum Well with Large Photoluminescence Enhancement

Planar substrate supported semiconductor quantum well (QW) structures are not amenable to manipulation in miniature devices, while free-standing QW nanostructures, <i>e</i>.<i>g</i>., ultrathin nanosheets and nanoribbons, suffer from mechanical and environmental instability. Therefore, it is tempting to fashion high-quality QW structures on anisotropic and mechanically robust supporting nanostructures such as nanowires and nanoplates. Herein, we report a solution quasi-heteroepitaxial route for growing a barrier-confined quasi-QW structure (ZnSe/CdSe/ZnSe) on the supporting arms of ZnO nanotetrapods, which have a 1D nanowire structure, through the combination of ion exchange and successive deposition assembly. This resulted in highly crystalline and highly oriented quasi-QWs along the whole axial direction of the arms of the nanotetrapod because a transition buffer layer (Zn_<i>x</i>Cd_1–<i>x</i>Se) was formed and in turn reduced the lattice mismatch and surface defects. Significantly, such a barrier-confined QW emits excitonic light ∼17 times stronger than the heterojunction (HJ)-type structure (ZnSe/CdSe, HJ) at the single-particle level. Time-resolved photoluminescence from ensemble QWs exhibits a lifetime of 10 ns, contrasting sharply with ∼300 ps for the control HJ sample. Single-particle PL and Raman spectra suggest that the barrier layer of QW has completely removed the surface trap states on the HJ and restored or upgraded the photoelectric properties of the semiconductor layer. Therefore, this deliberate heteroepitaxial growth protocol on the supporting nanotetrapod has realized a several micrometer long QW structure with high mechanical robustness and high photoelectric quality. We envision that such QWs integrated on 1D nanostructures will largely improve the performance of solar cells and bioprobes, among others

Dithiafulvenyl Unit as a New Donor for High-Efficiency Dye-Sensitized Solar Cells: Synthesis and Demonstration of a Family of Metal-Free Organic Sensitizers

This work identifies the dithiafulvenyl unit as an excellent electron donor for constructing D−π–A-type metal-free organic sensitizers of dye-sensitized solar cells (DSCs). Synthesized and tested are three sensitizers all with this donor and a cyanoacrylic acid acceptor but differing in the phenyl (<b>DTF-C1</b>), biphenyl (<b>DTF-C2</b>), and phenyl–thiopheneyl–phenyl π-bridges (<b>DTF-C3</b>). Devices based on these dyes exhibit a dramatically improved performance with the increasing π-bridge length, culminating with DTF-C3 in η = 8.29% under standard global AM 1.5 illumination

A Quasi-Quantum Well Sensitized Solar Cell with Accelerated Charge Separation and Collection

Semiconductor-sensitized solar cell (SSSC) represents a new generation of device aiming to achieve easy fabrication and cost-effective performance. However, the power of the semiconductor sensitizers has not been fully demonstrated in SSSC, making it actually overshadowed by dye-sensitized solar cell (DSSC). At least part of the problem is related to the inefficient charge separation and severe recombination with the current technologies, which calls on rethinking about how to better engineer the semiconductor sensitizer structure in order to enhance the power conversion efficiency (PCE). Herein we report on using for the first time a quasi-quantum well (QW) structure (ZnSe/CdSe/ZnSe) as the sensitizer, which is quasi-epitaxially deposited on ZnO tetrapods. Such a novel photoanode architecture has attained 6.20% PCE, among the highest reported to date for this type of SSSCs. Impedance spectra have revealed that the ZnSe/CdSe/ZnSe QW structure has a transport resistance only a quarter that of, but a recombination resistance twice that of the ZnSe/CdSe heterojunction (HJ) structure, yielding much longer electron diffusion length, consistent with the resulting higher photovoltage, photocurrent, and fill factor. Time-resolved photoluminescence spectroscopy indicates dramatically reduced electron transfer from ZnO to the QW sensitizer, a feature which is conducive to charge separation and collection. This study together with the impedance spectra and intensity modulated photocurrent spectroscopies supports a core/shell two-channel transport mechanism in this type of solar cells and further suggests that the electron transport along sensitizer can be considerably accelerated by the QW structure employed

Enhanced Performance of Polymeric Bulk Heterojunction Solar Cells via Molecular Doping with TFSA

Organic solar cells based on bis­(trifluoromethanesulfonyl)­amide (TFSA, [CF₃SO₂]₂NH) bulk doped poly­[<i>N</i>-9′′-hepta-decanyl-2,7-carbazole-<i>alt</i>-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT):C₇₁-butyric acid methyl ester (PC₇₁BM) were fabricated to study the effect of molecular doping. By adding TFSA (0.2–0.8 wt %, TFSA to PCDTBT) in the conventional PCDTBT:PC₇₁BM blends, we found that the hole mobility was increased with the reduced series resistance in photovoltaic devices. The p-doping effect of TFSA was confirmed by photoemission spectroscopy that the Fermi level of doped PCDTBT shifts downward to the HOMO level and it results in a larger internal electrical field at the donor/acceptor interface for more efficient charge transfer. Moreover, the doping effect was also confirmed by charge modulated electroabsorption spectroscopy (CMEAS), showing that there are additional polaron signals in the sub-bandgap region in the doped thin films. With decreased series resistance, the open-circuit voltage (<i>V</i>_oc) was increased from 0.85 to 0.91 V and the fill factor (FF) was improved from 60.7% to 67.3%, resulting in a largely enhanced power conversion efficiency (PCE) from 5.39% to 6.46%. Our finding suggests the molecular doping by TFSA can be a facile approach to improve the electrical properties of organic materials for future development of organic photovoltaic devices (OPVs)