Fabrication of quantum dot-sensitized solar cells with multilayer TiO2/PbS(X)/CdS/CdSe/ZnS/SiO2 photoanode and optimization of the PbS nanocrystalline layer

In this work, two multilayer photoanode structures of TiO₂/PbS(X)/CdS/ZnS/SiO₂ and TiO₂/PbS(X)/CdS/CdSe/ZnS/SiO₂ were fabricated and applied in quantum dot-sensitized solar cells (QDSCs). Then, the effect of PbS QDs layer on the photovoltaic performance of corresponding cells was investigated. The sensitization was carried out by PbS and CdS QDs layers deposited on TiO₂ scaffold through successive ionic layer adsorption and reaction (SILAR) method. The CdSe QDs film was also formed by a fast, modified chemical bath deposition (CBD) approach. Two passivating ZnS and SiO₂ layers were finally deposited by SILAR and CBD methods, respectively. It was shown that the reference cell with TiO₂/CdS/ZnS/SiO₂ photoanode demonstrated a power conversion efficiency (PCE) of 3.0%. This efficiency was increased to 4.0% for the QDSC with TiO₂/PbS(2)/CdS/ZnS/SiO₂ photoelectrode. This was due to the co-absorption of incident light by low-bandgap PbS nanocrystalline film and also the CdS QDs layer and well transport of the charge carriers. For the CdSe included QDSCs, the PbS-free reference cell represented a PCE of 4.1%. This efficiency was improved to 5.1% for the optimized cell with TiO₂/PbS(2)/CdS/CdSe/ZnS/SiO₂ photoelectrode. The maximized efficiency was enhanced about 25% and 70% compared to the PbS-free reference cells with and without the CdSe QDs layer.

     

Optimization of CdSe layer on modified ZnO hierarchical spheres by spin-SILAR for efficient CdS/CdSe co-sensitized solar cells

In this study, after CdS quantum dots sensitized ZnO hierarchical spheres (ZnO HS), we used a simple process to deposit CdSe QDs on ZnO by spin-coating-based SILAR, and applied to photoanodes of quantum dots-sensitized solar cells. Before CdS and CdSe QDs deposition, the ZnO HS photoanodes were modified by Zn(CH₃COO)₂·2H₂O methanol solution to further enhance the open-circuit voltage and power conversion efficiency (PCE). The program of modifying photoanodes and the number of CdSe spin-SILAR cycles are evaluated on the optical and electrochemical properties of the cells. As a result, a high energy conversion efficiency of 2.49 % was obtained by using modified ZnO HS/CdS photoanode under AM 1.5 illumination of 100 mW cm^?2. And further decorated by the CdSe QDs, the ZnO HS/CdS/CdSe cell achieved a PCE of 5.36 % due to the modification of ZnO HS nanostructure, the enhanced absorption in the visible region, the lower recombination reaction and higher electron lifetime.     

Construction of TiO<Subscript>2</Subscript> NP@TiO<Subscript>2</Subscript> NT double-layered structural photoanode to enhance photovoltaic performance of CdSe/CdS quantum dots sensitized solar cells

TiO₂ nanoparticles (NP) at top of TiO₂ nanotube (TiO₂ NP@TiO₂NT) double-layered architecture is constructed to combine the advantages of TiO₂ NP and TiO₂ NT together. This composite TiO₂ NP@TiO₂NT architecture as photoanode possesses a larger surface area for more QDs loading, and highly tubular structure for electron swift transport. Based on this architecture, CdSe/CdS quantum dots (QDs) have been successfully synthesized by successive ionic layer adsorption reaction (SILAR) method for quantum dots-sensitized solar cell application. The photovoltaic performance of QDSSCs based on TiO₂ NP@TiO₂ NT have been investigated in contrast with bare TiO₂ NP and bare TiO₂ NT architectures with almost the same thickness. The results show that the power conversion efficiency (PCE) of QDSSCs could be enhanced using TiO₂ NP@TiO₂ NT and improved to 3.26%, which is 80% and 38% higher than QDSSCs based on bare TiO₂ NT and bare TiO₂ NP, respectively. The BET surface area, UV–vis absorption spectra, and incident photon to current conversion efficiency (IPCE) measurements results show the evidence that the TiO₂ NP@TiO₂ NT can combine advantages of TiO₂ NP and TiO₂ NT structures together and lead to a higher light harvesting efficiency, electron collecting efficiency, and efficient electron transport.     

Hydrothermal Growth of TiO2 Nanorod Arrays and In Situ Conversion to Nanotube Arrays for Highly Efficient Quantum Dot‐Sensitized Solar Cells

TiO₂ nanorod (NR) and nanotube (NT) arrays grown on transparent conductive substrates are attractive electrode for solar cells. In this paper, TiO₂ NR arrays are hydrothermally grown on FTO substrate, and are in situ converted into NT arrays by hydrothermally etching. The TiO₂ NR arrays are reported as single crystalline, but the TiO₂ NR arrays are demonstrated to be polycrystalline with a bundle of 2–5 nm single crystalline nanocolumns grown along [001] throughout the whole NR from bottom to top. TiO₂ NRs can be converted to NTs by hydrothermal selective etching of the (001) core and remaining the inert sidewall of (110) face. A growth mechanism of the NR and NT arrays is proposed. Quantum dot‐sensitized solar cells (QDSCs) are fabricated by coating CdSe QDs on to the TiO₂ arrays. After conversion from NRs to NTs, more QDs can be filled in the NTs and the energy conversion efficiency of the QDSCs almost double.     

Chemical synthesis of CdS onto TiO2 nanorods for quantum dot sensitized solar cells

A quantum dot sensitized solar cell (QDSSC) is fabricated using hydrothermally grown TiO₂ nanorods and successive ionic layer adsorption and reaction (SILAR) deposited CdS. Surface morphology of the TiO₂ films coated with different SILAR cycles of CdS is examined by Scanning Electron Microscopy which revealed aggregated CdS QDs coverage grow on increasing onto the TiO₂ nanorods with respect to cycle number. Under AM 1.5G illumination, we found the TiO₂/CdS QDSSC photoelectrode shows a power conversion efficiency of 1.75%, in an aqueous polysulfide electrolyte with short-circuit photocurrent density of 4.04 mA/cm² which is higher than that of a bare TiO₂ nanorods array.     

Aqueous synthesis of CdSe and CdSe/CdS quantum dots with controllable introduction of Se and S sources

A new and convenient route is developed to synthesize CdSe and core–shell CdSe/CdS quantum dots (QDs) in aqueous solution. CdSe QDs are prepared by introducing H₂Se gas into the aqueous medium containing Cd²⁺ ions. The synthesized CdSe QDs are further capped with CdS to form core–shell CdSe/CdS QDs by reacting with H₂S gas. The gaseous precursors, H₂Se and H₂S, are generated on-line by reducing SeO₃ ^2? with NaBH₄ and the reaction between Na₂S and H₂SO₄, and introduced sequentially into the solution to form CdSe and CdSe/CdS QDs, respectively. The synthesized water-soluble CdSe and CdSe/CdS QDs possess high quantum yield (3 and 20 %) and narrow full-width-at-half-maximum (43 and 38 nm). The synthesis process is easily reproducible with simple apparatus and low-toxic chemicals. The relatively standard deviation of maxima fluorescence intensity is only 2.1 % (n = 7) for CdSe and 3.6 % (n = 7) for CdSe/CdS QDs. This developed route is simple, environmentally friendly and can be readily extended to the large-scale aqueous synthesis of QDs.     

TiO2 inverse opal based CdS/CdSe quantum dot co-sensitized solar cells

CdS and CdSe quantum dots were introduced as co-sensitizers into TiO₂ inverse opal quantum dot sensitized solar cells. Herein, the three-dimensionally ordered porous TiO₂ inverse opal film leads to a better infiltration of both sensitizers and hole transporting material, and the smaller surface area of TiO₂ inverse opal film is effectively offset by the incorporating of co-sensitization. It was found that the presence of CdS/CdSe co-sensitizers provides enhanced light absorption, and leads to a lower recombination rate of the electrons due to the stepwise structure of band edge in TiO₂/CdS/CdSe, which resulted in the observed enhanced photocurrent and energy conversion efficiency of the solar cells. A cell efficiency of 1.01 % has been attained.     

Preparation, optical properties and solar cell applications of CdS quantum dots synthesized by chemical bath deposition

We reported on temperature-dependent photoluminescence (PL) studies of CdS quantum dots (QDs, ~5 nm in diameter) grown onto TiO₂ nanorod arrays by chemical bath deposition. By constructing a liquid-junction solar cell, a power conversion efficiency of 0.88 % was demonstrated. In addition, we observed anomalous emission behavior, specifically a ‘Λ’-shaped (blueshift–redshift) temperature dependence of the peak energy for CdS related PL with increasing temperature. From a study of the integrated PL intensity as a function of temperature, it was revealed that thermionic dissociation of excitons (carriers) out of local potential minima into higher energy states was the dominant mechanism leading to the quenching behavior of the QDs related PL. At 110 K, the localized excitons were totally dissociated and converted to the free excitons. Our results shed light on the exciton-dissociation process in CdS QDs and can be used for the proposed solar cell application.     

Enhanced photoelectrochemical properties of TiO2 nanorod arrays decorated with CdS nanoparticles

TiO₂ nanorod arrays (TiO₂ NRAs) sensitized with CdS nanoparticles were fabricated via successive ion layer adsorption and reaction (SILAR), and TiO₂ NRAs were obtained by oxidizing Ti NRAs obtained through oblique angle deposition. The TiO₂ NRAs decorated with CdS nanoparticles exhibited excellent photoelectrochemical and photocatalytic properties under visible light, and the one decorated with 20 SILAR cycles CdS nanoparticles shows the best performance. This can be attributed to the enhanced separation of electrons and holes by forming heterojunctions of CdS nanoparticles and TiO₂ NRAs. This provides a promising way to fabricate the material for solar energy conversion and wastewater degradation.     

Front-side illuminated CdS/CdSe quantum dots co-sensitized solar cells based on TiO₂ nanotube arrays

We fabricated a front-side illuminated CdS/CdSe quantum dots co-sensitized solar cell based on TiO(2) nanotube arrays. The freestanding TiO(2) nanotube arrays were first detached from anodic oxidized Ti foils and then transferred to the fluorine-doped tin oxide to form photoanodes. An opaque Cu(2)S with high electrochemical activity was used as the counter electrode. A photovoltaic conversion efficiency as high as 3.01% under one sun illumination has been achieved after optimizing the deposition time of CdSe quantum dots and the length of the TiO(2) nanotube arrays. It is observed that the power conversion efficiency of quantum dots sensitized solar cells from the front-side illumination mode (3.01%) is much higher than that of the back-side illumination mode (1.32%) owing to the poor catalytic activity of Pt to polysulfide electrolytes and light absorption by the electrolytes for the latter.     

Surface modification of ZnO nanorods with CdS quantum dots for application in inverted organic solar cells: effect of deposition duration

Incorporating cadmium sulfide quantum dots (CdS QDs) onto ZnO nanorod (ZNRs) has been investigated to be an efficient approach to enhance the photovoltaic performance of the inverted organic solar cell (IOSC) devices based on ZNRs/poly (3-hexylthiophene) (P3HT). To synthesize CdS/ZNRs, different durations of deposition per cycle from 1 to 9 min were used to deposit CdS via SILAR technique onto ZNRs surface grown via hydrothermal method at low temperature on FTO substrate. In typical procedures, P3HT as donor polymer were spun-coating onto CdS/ZNRs to fabricate IOSC devices, followed by Ag deposition as anode by magnetron sputtering technique. Incorporation of CdS QDs has modified the morphological, structural, and optical properties of ZNRs. Incorporation of CdS QDs onto ZNRs also led to higher open circuit voltage (V_oc) and short circuit current density (J_sc) of optimum ZNRs/CdS QDs devices due to the increased interfacial area between ZNRs and P3HT for more efficient exciton dissociation, reduced interfacial charge carrier recombination as a result of lower number of oxygen defects which act as electron traps in ZnO and prolonged carrier recombination lifetime. Therefore, the ZNRs/CdS QDs/P3HT device exhibited threefold higher PCE (0.55%) at 5 min in comparison to pristine ZNR constructed device (0.16%). Overall, our study highlights the potential of ZNRs/CdS QDs to be excellent electron acceptors for high efficiency hybrid optoelectronic devices.     

CdSe/Cd1−x Zn x S core/shell quantum dots with tunable emission: growth and morphology evolution

We demonstrate an organic synthesis to fabricate hydrophobic core/shell CdSe/Cd_1?x Zn _x S quantum dots (QDs) with tunable photoluminescence (PL) between green and red at relatively low temperature using trioctylphosphine S reacted directly with cadmium and zinc acetate. A seeded growth strategy was used for preparing large CdSe cores. Large CdSe cores revealed a rod-like morphology while small one exhibited a spherical shape. Being coated with a Cd_1?x Zn _x S shell on spherical CdSe cores with an average size of 3.9 nm in diameter, core/shell QDs exhibited a cubic morphology (a length of 5 nm). In contrast, the core/shell QDs created using a small core (3.3 nm in diameter) show a spherical morphology. Namely, the anisotropic aggregation behavior of CdS monomers on CdSe cores occurs when the rod-like core is coated with a Cd_1?x Zn _x S shell. CdS interlayer plays an important role for such morphology evolution because all CdSe cores with a pure ZnS shell exhibited a spherical morphology. The PL properties of CdSe/Cd_1?x Zn _x S core/shell QDs depended strongly on the size and morphology of the cores. The QDs revealed a narrow and tunable PL spectrum. It is believed that this facile strategy can be extended to synthesize other core–shell QDs at low temperature.     

A novel method for fabrication of CdS quantum dot-sensitized solar cells

In this report, a novel and facile in situ gas–solid reaction method has been developed for the deposition of CdS quantum-dots (QDs) on a mesoscopic TiO₂ film. In the approach, cadmium nitrate solution was first coated on mesoporous TiO₂ films, and subsequently transformed into CdS QDs (with size about 2–3 nm) by reaction with hydrogen sulfide (H₂S) gas generated in a closed container at room temperature. Different from the conventional solution techniques, this method offers new opportunities for rapid and facile deposition of CdS QD-coated TiO₂ films without the introduction of the by-products. With the CdS QDs-decorated TiO₂ active electrodes, the liquid and solid solar cells were fabricated with power conversion efficiencies (PCEs) of 1.90 and 0.80%, respectively.     

Double-sided transparent electrodes of TiO2 nanotube arrays for highly efficient CdS quantum dot-sensitized photoelectrodes

A novel double-sided CdS quantum dots-sensitized TiO₂ nanotube (TNT)/ITO photoelectrode is fabricated to improve the energy conversion efficiencies of quantum dots-sensitized solar cells (QDSCs). Our experimental results show that the double-sided CdS quantum dots-sensitized TNT/ITO photoelectrodes show enhanced light absorption. As a consequence, the photoelectrochemical response of the CdS/TNT/ITO photoelectrode is much improved compared with single-sided CdS sensitized TNT arrays on Ti substrate (i.e., CdS/TNT/Ti photoelectrode). An optimum conversion efficiency of 7.5 % is achieved by the double-sided CdS(15)/TNT/ITO photoelectrode, which is an enhancement of about 120 % when compared with the single-sided CdS/TNT/Ti photoelectrode. Our results demonstrate that the energy conversion efficiencies of QDSCs can be improved by designing a new photoelectrode structure.     

CdS quantum dot sensitized nanocrystalline Gd-doped TiO2 thin films for photoelectrochemical solar cells

CdS quantum dot sensitized Gd-doped TiO₂ nanocrystalline thin films have been prepared by chemical method. X-ray diffraction analysis reveals that TiO₂ and Gd-doped TiO₂ nanocrystalline thin films are of anatase phase. The absorption spectra revealed that the absorption edge of CdS quantum dot sensitized Gd-doped TiO₂ thin films shifted towards longer wavelength side (red shift) when compared to that of CdS quantum dot sensitized TiO₂ films. CdS quantum dots with a size of 5 nm have been deposited onto Gd-doped TiO₂ film surface by successive ionic layer adsorption and reaction method and the assembly of CdS quantum dot with Gd-doped TiO₂ has been used as photo-electrode in quantum dot sensitized solar cells. CdS quantum dot sensitized Gd-doped TiO₂ based solar cell exhibited a power conversion efficiency of 1.18 %, which is higher than that of CdS quantum dot sensitized TiO₂ (0.91 %).     

Dye sensitized solar cells using freestanding TiO2 nanotube arrays on FTO substrate as photoanode

The front-side illuminated dye-sensitized solar cells with TiO₂ nanotube arrays/FTO based photoanode were fabricated by a simple detachment and transfer method. The morphology and crystalline phase of the TiO₂ nanotubes were studied by scanning electron microscopy and X-ray diffraction. A photoconversion efficiency of 1.78% was obtained using TiO₂-nanotube/FTO glass as a photoanode. It was found that the average tubes diameter increased with anodizing voltage, and higher photoconversion efficiency was obtained for nanotubes synthesized at the higher voltage.     

Tuning photocurrent response through size control of CdSe quantum dots sensitized solar cells

The photovoltaic characterization of CdSe quantum dots sensitized solar cells (QDSSCs) by tuning band gap of CdSe quantum dots (QDs) through size control has been investigated. Fluorine doped tin oxide (FTO) substrates were coated with 20 nm in diameter TiO₂ nanoparticles (NPs). Pre-synthesized colloidal CdSe quantum dots of different sizes (from 4.0 to 5.4 nm) were deposited on the TiO₂-coated substrates using direct adsorption (DA) method. The FTO counter electrodes were coated with platinum, while the electrolyte containing I^?/I ₃ ^? redox species was sandwiched between the two electrodes. The current density-voltage (J-V) characteristic curves of the assembled QDSSCs were measured for different dipping times, and AM 1.5 simulated sunlight. The maximum values of short circuit current density (J_sc) and conversion efficiency (η) are 1.62 mA/cm² and 0.29 % respectively, corresponding to CdSe QDs of size 4.52 nm (542 nm absorption edge) and of 6 h dipping time. The variation of the CdSe QDs size mainly tunes the alignment of the conduction band minimum of CdSe with respect to that of TiO₂ surface. Furthermore, the J_sc increases linearly with increasing intensity of the sun light, which indicates the sensitivity of the assembled cells.     

Reduced graphene oxide and CdTe nanoparticles co-decorated TiO2 nanotube array as a visible light photocatalyst

TiO₂ nanotube array (TiO₂ NT) was co-decorated by reduced graphene oxide (RGO) and CdTe nanoparticles (NPs) through a simple one-step electrodeposition process. RGO film was formed on the top surface of TiO₂ NT and CdTe NPs homogeneously dispersed within the RGO sheets and on the inner/outer walls of TiO₂ NT. Resulting from the synergetic effect of RGO and CdTe, the photocatalytic activity of the ternary RGO/CdTe–TiO₂ NT photocatalyst far exceeded those of bare TiO₂ NT, RGO-TiO₂ NT, and CdTe–TiO₂ NT photocatalysts in the degradation of herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) under simulated solar light or visible light irradiation. After 180-min UV–Vis (or visible light) irradiation, almost 100 % (or 96 %) 2,4-D removal efficiency was achieved on RGO/CdTe–TiO₂ NT, much higher than 42 % (or 2 %) on bare TiO₂ NT, 58 % (or 10 %) on RGO–TiO₂ NT, and 52 % (or 41 %) on CdTe–TiO₂ NT. This study will inspire better design of advanced photocatalysts with high visible-light photocatalytic activity.     

Enhanced photovoltaic performance of quantum dot sensitized solar cells with Ag-doped TiO2 nanocrystalline thin films

Ag-doped titanium dioxide (TiO₂) nanocrystalline thin films have been prepared by the sol–gel dip coating method and used as photoanode to fabricate quantum dot sensitized solar cells. The X-ray diffraction studies reveal the formation of anatase phase without any impurity phase. The surface morphology studied using scanning electron microscope shows uniform distribution of particles. The optical band gap was found to be 3.5 and 3.4 eV for CdS quantum dot sensitized TiO₂ and CdS quantum dot sensitized Ag-doped TiO₂ thin film respectively. The Ag-doped TiO₂ based solar cell exhibited a power conversion efficiency of 1.48 % which is higher than that of TiO₂ (0.9 %).     

Formation of aligned ZnO nanotube arrays by chemical etching and coupling with CdSe for photovoltaic application

High density aligned ZnO nanotube (NT) arrays were synthesized using a facile chemical etching of electrochemically deposited ZnO nanorods (NRs). The influence of etching time and solution concentration on the ZnO NT formation was investigated. Moreover, cadmium selenide (CdSe) nanoparticles as sensitizers were assembled onto the ZnO NT and NR arrays for solar cell application. A conversion efficiency (η) of 0.44% was achieved for CdSe/ZnO NT-based solar cell under the white light illumination intensity of 85 mW/cm². An 8% enhancement in η was observed between the CdSe/ZnO NT-based and NR-based solar cell due to the enhancement of the photocurrent density. ZnO NT arrays have been proved to have a superior ability as compared with ZnO NR arrays when employed as a semiconductor film.