One-step preparation and assembly of aqueous colloidal CdS(x)Se(1-x) nanocrystals within mesoporous TiO2 films for quantum dot-sensitized solar cells

Structural evolution from the CdSSe alloy to the CdS/CdSe core/shell in Cd(S and Se) composite quantum dots and its impact on the performance of sensitized solar cells

CdSSe alloy and CdS/CdSe core/shell quantum dots (QDs) are widely studied in quantum dot solar cells (QDSSCs). However, to date, there have been no detailed comparative investigations into the cell performance between CdSSe alloy and CdS/CdSe core/shell structures prepared with the same preparation process. In this work, the performances of CdSSe alloy and CdS/CdSe core/shell QDSSCs, which are prepared with the same SILAR (successive ionic layer adsorption and reactions) process, are investigated in detail. By simply tuning the layer numbers and arrangement sequence of the CdS and CdSe layers, a series of QDs, including CdSSe alloy structures, CdS/CdSe multilayer structures, and CdS/CdSe core/shell structures, are successfully prepared with a layer-by-layer technique, while maintaining a similar morphology. Based on these QD sensitized TiO₂ photoanodes, QDSSCs are assembled. The CdS/CdSe core/shell QDSSCs yield a maximum power conversion efficiency of 5.08% under AM 1.5 illumination of 100 mW cm^-2, which is increased by 77% in comparison with that of CdSSe alloy QDSSCs (2.87%). The significantly enhanced photovoltaic performance of QDSSCs with core/shell architectures is mainly attributed to their high short-circuit current density, which arises from the enhanced absorption intensity. In addition, the CdS/CdSe core-shell contributes to the attenuation of the interfacial charge recombination rate and prolongs the electron lifetime, resulting in more efficient charge collection in QDSSCs.

Preferentially oriented Ag-TiO2 nanotube array film: An efficient visible-light-driven photocatalyst

Ag-TiO₂ nanotube array films with the preferential orientation of crystals were fabricated on ITO glass by magnetron sputtering and anodization. Comprehensive characterization was performed to ascertain the composition and microstructure characteristics of thin films. The photocatalytic activities were evaluated through the reduction of hexavalent chromium (Cr₂O₇^2- (Cr (VI)) as a model compound under visible light irradiation. XRD and XPS studies reveal the development of preferred orientation along [001] in anatase TiO₂ nanotubes by adjusting the Ag content during magnetron sputtering. Such unusual behavior is attributed to the minimization of anatase (001) surface energy assisted by Ag. The Ag-TiO₂ nanotube arrays having preferred crystal orientation exhibit superior separation/transfer of photo-induced charges. Furthermore, the Ag-TiO₂ nanotube arrays show improved absorption of visible light due to the SPR effect induced by Ag and the formation of heterojunction between the TNAs and Ag₂O. TNA-3Ag exhibits the highest photocatalytic activities by removing 99.1 % Cr (VI) in 90 min under visible light illumination.

The Size Effect of TiO2 Hollow Microspheres on Photovoltaic Performance of ZnS/CdS Quantum Dots Sensitized Solar Cell

Size controllable TiO₂ hollow microspheres (HMS) were synthesized by a carbonaceous spheres (CS) template method. Based on TiO₂ HMS, the ZnS/CdS quantum dots (QDs) were loaded to form a ZnS/CdS@TiO₂ HMS photoanode for quantum dots sensitized solar cell (QDSSC). The size effects of TiO₂ HMS on photovoltaic performance were investigated, and showed that TiO₂ HMS with sizes ~560 nm produced the best short-circuit current density (J_sc) of 8.02 mA cm⁻² and highest power conversion efficiency (PCE) of 1.83%, showing a better photovoltaic performance than any other QDSSCs based on TiO₂ HMS with size ~330 nm, ~400 nm, and ~700 nm. The improvement of photovoltaic performance based on ~560 nm TiO₂ HMS which can be ascribed to the enhanced light harvesting efficiency caused by multiple light reflection and strong light scattering of TiO₂ HMS. The ultraviolet-visible (UV-vis) spectra and incident photo to the current conversion efficiency (IPCE) test results confirmed that the size of TiO₂ HMS has an obvious effect on light harvesting efficiency. A further application of ~560 nm TiO₂ HMS in ZnS/PbS/CdS QDSSC can improve the PCE to 2.73%, showing that TiO₂ HMS has wide applicability in the design of QDSSCs.

Hidden energy levels? Carrier transport ability of CdS/CdS1-xSex quantum dot solar cells impacted by Cd-Cd level formation

In quantum dot sensitized solar cells (QDSSC), a cascade energy level structure controlled by assembly of cadmium-chalcogenide quantum dots can remarkably improve the sunlight harvesting and charge carrier lifetime. Despite the advantages of using co-sensitizers, energy conversion efficiencies are still low. An increased understanding of the causes of the low photoconversion efficiency (PCE) will contribute to the development of a straightforward approach to improve solar cell performance by exploiting co-sensitization. Herein we discuss how an excess of cadmium causes structural disorder and defect levels impacting the PCE of QDSSC devices. Thus, outer CdS1-xSex/inner CdS QD-co-sensitized B,N,F-co-doped-TiO2 nanotubes (BNF-TNT) were prepared. Chalcogenides were deposited by the SILAR method on BNF-TNT, varying the load of CdS as the inner sensitizer, while for CdS1-xSex, five SILAR cycles were used (5-CdS1-xSex), controlling the nominal S/Se molar ratio of the ternary alloy. Cd defects named as Cd-Cd energy levels were observed during CdS sensitization. Although incorporation of outer CdS1-xSex provides a tunable band gap to achieve good band alignment for carrier separation, Cd-Cd energy levels in the sensitizers act as recombination centers, limiting the overall electron flow at the BNF-TNT/CdS/CdS1-xSex interface. A maximum PCE of 2.58% was reached under standard AM 1.5G solar illumination at 100 mW cm-2. Additional limitations of SILAR as a deposition strategy of QDs are also found to influence the PCE of QDSSC.

The role of boron in the carrier transport improvement of CdSe-sensitized B,N,F-TiO2 nanotube solar cells: a synergistic strategy

The presence of boron intensifies the synergistic effect of doping and sensitization to improve charge carrier transport into solar devices.

Chemical, structural and photovoltaic properties of graded CdSxSe1−x thin films grown by chemical bath deposition on GaAs(100)

Chemically graded Cd(S,Se) thin film and photovoltaic cell illustration.

Theoretical investigation of energy gap bowing in CdSxSe1−x alloy quantum dots

To investigate energy gap bowing in homogeneously alloyed CdS_xSe_1−x quantum dots (QDs) and to understand whether it is different from bulk, we perform density functional theory based electronic structure calculations for spherical QDs of different compositions x (0 ≤ x ≤ 1) and of varying sizes (2.2 to 4.6 nm).

Effects of Carbon Allotrope Interface on the Photoactivity of Rutile One-Dimensional (1D) TiO2 Coated with Anatase TiO2 and Sensitized with CdS Nanocrystals

The assembly of a large-bandgap one-dimensional (1D) oxide-conductive carbon-chalcogenide nanocomposite and its surface, optical, and photoelectrochemical properties are presented. Microscopy, surface analysis, and optical spectroscopy results are reported to provide insights into the assembly of the nanostructure. We have investigated (i) how the various carbon allotropes (C60), reduced graphene oxide (RGO), carbon nanotubes (CNTs), and graphene quantum dots (GQDs) can be integrated at the interface of the 1D TiO2 and zero-dimensional (0D) CdS nanocrystals; (ii) the carbon allotrope and CdS loading effects; (iii) the impact of the carbon allotrope presence on 0D CdS nanocrystals; and (iv) how they promote light absorbance. Subsequently, the functioning of the integrated nanostructured assembly in a photoelectrochemical cell has been systematically investigated. These studies include (i) chronoamperometry, (ii) impedance measurements or EIS, and (iii) linear sweep voltammetry. The results indicate that the presence of a GQD interface shows the most enhancement in the photoelectrochemical properties. The optimized photocurrent values were respectively noted to be 2.8, 2.2, 1.9, and 1.6 mA/cm(2), indicating JGQD > JRGO > JCNT > Jfullerene. Furthermore, the annealing conditions have indicated that ammonia treatment leads to an increase in the photoelectrochemical responses when using any form of the carbon allotropes.

Controllable in situ photo-assisted chemical deposition of CdSe quantum dots on ZnO/CdS nanorod arrays and its photovoltaic application

Compound semiconductors have been widely applied in the energy field as light-harvesting materials, conducting substrates and other functional parts. Nevertheless, to effectively grow them in various forms toward objective applications, limitations have often been met to achieving high growth rate, simplicity of method and controllability of growing processes simultaneously. In this work, we have grown a uniform CdSe layer on ZnO/CdS nanorod arrays by a novel in situ photo-assisted chemical deposition method. The morphology and quality of the as-formed material could be significantly influenced by tuning the optical parameters of the injected light. Due to the effect of injected light on the key reactions during the growth, a modified natural light with removal of the UV and IR components seems to be more suitable than monochromic light. An efficiency of 3.59% was achieved without any additional treatment, significantly higher than the efficiency of 2.88% of the sample by conventional CBD method under similar conditions with growth rate one order of magnitude higher. In general, the result has suggested its potential importance for other compound materials and opto-electronic devices.

Well-dispersed NiS nanoparticles grown on a functionalized CoS nanosphere surface as a high performance counter electrode for quantum dot-sensitized solar cells

The integration of design and encapsulation strategies using a combination of methods has been recognized as one of the most effective strategies to realize practical applications of various quantum-dot sensitized solar cell materials.

Semi-Transparent ZnO-CuI/CuSCN Photodiode Detector with Narrow-Band UV Photoresponse

The ZnO homogeneous pn junction photodiode is quite difficult to fabricate due to the absence of stable p-type ZnO. So exploring reliable p-type materials is necessary to build a heterogeneous pn junction with n-type ZnO. Herein, we develop a simple and low-cost solution-processed method to obtain inorganic p-type CuI/CuSCN composite film with compact morphology, high conductivity, and low surface state. The improved performance of CuI/CuSCN composite film can be confirmed based on high-rectification ratio, responsivity, and open voltage of ZnO-CuI/CuSCN photodiode UV detectors. Moreover, photodiodes with novel top electrodes are investigated. Compared with commonly used Au and graphene/Ag nanowire (NWs) electrode, poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (

PEDOT

PSS) electrode prepared by Meyer rod-coating technique opens one route to obtain a semitransparent photodiode. The photodiode with

PEDOT

PSS as the top electrode under reverse illumination has the highest photocurrent density due to higher UV transmittance of

PEDOT

PSS transparent electrode compared with ITO glass. The low-energy consumption, and high responsivity, UV to visible rejection ratio and air stability make this ZnO-CuI/CuSCN photodiode quite promising in the UV-A detection field.

Surface engineering of ZnO nanostructures for semiconductor-sensitized solar cells

Semiconductor-sensitized solar cells (SSCs) are emerging as promising devices for achieving efficient and low-cost solar-energy conversion. The recent progress in the development of ZnO-nanostructure-based SSCs is reviewed here, and the key issues for their efficiency improvement, such as enhancing light harvesting and increasing carrier generation, separation, and collection, are highlighted from aspects of surface-engineering techniques. The impact of other factors such as electrolyte and counter electrodes on the photovoltaic performance is also addressed. The current challenges and perspectives for the further advance of ZnO-based SSCs are discussed.

TiO2 nanoparticle/ZnO nanowire hybrid photoanode for enhanced quantum dot-sensitized solar cell performance

A hybrid structured TiO₂/ZnO photoanode was composed of highly ordered ZnO nanowires (NWs) and small TiO₂ nanoparticles (NPs) filling the gaps among ZnO NWs.