ZnO nanorod/CdS nanocrystal core/shell-type heterostructures for solar cell applications

ZnO/CdS core/shell nanorod arrays were fabricated by a two-step method. Single-crystalline ZnO nanorod arrays were first electrochemically grown on SnO(2):F (FTO) glass substrates. Then, CdS nanocrystals were deposited onto the ZnO nanorods, using the successive ion layer adsorption and reaction (SILAR) technique, to form core/shell nanocable architectures. Structural, morphological and optical properties of the nanorod heterojunctions were investigated. The results indicate that CdS single-crystalline domains with a mean diameter of about 7 nm are uniformly and conformally covered on the surface of the single-crystalline ZnO nanorods. ZnO absorption with a bandgap energy value of 3.30 ± 0.02 eV is present in all optical transmittance spectra. Another absorption edge close to 500 nm corresponding to CdS with bandgap energy values between 2.43 and 2.59 eV is observed. The dispersion in this value may originate in quantum confinement inside the nanocrystalline material. The appearance of both edges corresponds with the separation of ZnO and CdS phases and reveals the absorption increase due to CdS sensitizer. The photovoltaic performance of the resulting ZnO/CdS core/shell nanorod arrays has been investigated as solar cell photoanodes in a photoelectrochemical cell under white illumination. In comparison with bare ZnO nanorod arrays, a 13-fold enhancement in photoactivity was observed using the ZnO/CdS coaxial heterostructures.     

Effects of junction formation conditions on the photovoltaic properties of sintered CdS/CdTe solar cells

Microstructures and properties of sintered CdS films on glass substrates and sintered CdTe films on polycrystal CdS substrates have been investigated. The CdS films, which contained 9 wt % CdCl₂ as a sintering aid and were sintered at 650° C for 1 h in nitrogen, are transparent and have an average grain size of 15m and an electrical resistivity of 0.5cm. The CdTe films, which were coated on the sintered CdS substrate and were sintered above 610° C for 1 h in nitrogen, have a dense structure with an average grain size larger than 5m. All polycrystal CdS/CdTe solar cells were fabricated by this successive coating and sintering method. The sintering temperature of CdTe films on the sintered CdS films was varied from 585 to 700° C. Compositional interfaces and p-n juctions are formed during sintering. The highest solar efficiency (7.18%) was found in a solar cell made by sintering the composite layer of glass-CdS-CdTe at 625° C for 1 h. A fabrication temperature below 610° C resulted in poor solar cell efficiencies due to the porous structure of the CdTe films and above 650° C also resulted in poor efficiencies due to the formation of a CdS_1-x Te_x layer at the interface and a large p-n junction depth.     

The effects of the growth parameters of the CdS window layer on the photovoltaic properties of MOCVD-grown CdS/CdTe solar cells

This paper forms part of an ongoing study aimed at producing high-efficiency polycrystalline photovoltaic cells by a single integrated process using metal organic chemical vapor deposition (MOCVD). Relationships between CdS growth variables, final microstructure and device performance parameters are established. CdTe grain sizes of 3–5 μm diameter can be achieved using CdS growth temperatures of 275 °C or below, even in the absence of nucleation delay. Short-circuit photocurrent depends on CdS growth temperature and dopant concentration.     

Cellulose acetate fibers covered by CdS nanoparticles for hybrid solar cell applications

In this work cellulose acetate (CA) fibers with a diameter of approximately 1 μm were immersed in a cadmium sulfide (CdS) precursor solution. After 3 h the original white color CA fibers became yellow and maintained the same form, suggesting the deposition of CdS on fiber surface. SEM images showed that CA fibers were covered by uniformly sized CdS nanoparticles of approximately 100 nm. XRD and optical absorption spectra indicated that they contained mostly cubic crystalline phase with the optical band gap of 2.43 eV. CdS coated CA fibers, called CdS(CA) fibers, were dispersed in a polar dispersant (dimethyl sulfoxide, DMSO) and then mixed with a poly(3-hexylthiophene) (P3HT) solution in a non-polar solvent (dichlorobenzene, DCB). The mixture was cast onto a transparent conductive glass substrate (Indium–Tin–Oxide, ITO), and after solvent evaporation a thin layer of CdS(CA)–P3HT composite was formed. It is observed that the volume relation between the polar dispersant and non-polar solvent influences the solubility of the P3HT product in the composite coating and the photovoltaic performance of the corresponding cell as well. The mass ratio between CdS(CA) fibers and P3HT in the composite layer affects the optical absorption of the composite. The best photovoltaic performance was obtained in CdS(CA)–P3HT based cells with a volume relation between DCB and DMSO of 3.5–1, a mass ratio between CdS(CA) and P3HT of 1:1, and a rapid drying process for composite coatings.     

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.     

Synthesis of monodisperse CdS nanowires and their photovoltaic applications

Cadmium sulphide (CdS) nanowires with a monodisperse diameter of 3.6 nm and an aspect ratio of 10–170 were successfully synthesized using a simple and reproducible hot coordinating solvents method. The morphology and optical properties of the CdS nanocrystals were investigated using transmission electron microscopy (TEM), high-resolution TEM (HRTEM) ultraviolet–visible (UV–Vis) absorption spectroscopy and photoluminescence (PL) spectroscopy. It was found that using a long alkyl chain phosphonic acid-octadecylphosphonic acid (ODPA) causes a low diffusion rate and low reactivity which help to control the morphology of the nanocrystals. The timing of the injection process was also found to have critical effect on the morphology of the nanocrystals. Sharp peaks in both the UV–Vis absorption and PL spectra indicate that the size distribution of the diameter is nearly monodisperse. The photovoltaic properties of photovoltaic devices made with a blend of our nanowires and poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) were also investigated. Devices made with the nanowires were found to have double the I_sc observed in devices made with lower aspect ratio CdS nanorods. The possible reason of low photocurrent and high V_oc is maybe due to the presence of ligand in the nanocrystals.     

Enhanced photovoltaic performance of solar cell based on front-side illuminated CdSe/CdS double-sensitized TiO2 nanotube arrays electrode

Free-standing TiO₂ nanotube (NT) arrays have been prepared by a two-step anodization method. These translucent TiO₂ NT arrays can be transferred to the fluorine-doped tin oxide glass substrates to form front-side illuminated TiO₂ NT electrodes. The TiO₂ NT electrodes were double-sensitized by CdSe/CdS quantum dots (QDs) through successive ionic layer adsorption and reaction (SILAR) process. The absorption range of the TiO₂ NT electrode was extended from ~380 to 700 nm after sensitization with CdSe/CdS QDs. The SILAR cycles were investigated to find out the best combination of CdS and CdSe QDs for photovoltaic performance. The power conversion efficiency of 2.42 % was achieved by the CdSe(10)/CdS(8)/TiO₂ NT solar cell. A further improved efficiency of 2.57 % was obtained with two cycles of ZnS overlayer on the CdSe(10)/CdS(8)/TiO₂ NT electrode, which is 45.19 % higher than that of back-side illuminated solar cell. Furthermore, the ZnS(2)/CdSe(10)/CdS(8)/TiO₂ NT solar cell possesses a higher stability than CdSe(10)/CdS(8)/TiO₂ NT solar cell during the same period. The better photovoltaic performance of the ZnS(2)/CdSe(10)/CdS(8)/TiO₂ NT solar cell has demonstrated the promising value to design quantum dots-sensitized solar cells with double-sensitized front-side illuminated TiO₂ NT arrays strategy.     

Dye modification with photovoltaic enhancement for CdS quantum dots sensitized solar cell based on three-dimensional ZnO spheres

A large amount of ZnO with a three-dimensional sphere-like morphology has been synthesized by a facile hydrothermal route and applied as the photoanode material in CdS quantum dots sensitized solar cells (QDSSCs). After the modification of the dye co-sensitized process, an overall power conversion efficiency of 2.32 % with a short-circuit current density of 9.25 mA/cm² was obtained in ZnO/CdS/dye-QDSSC, which shows 66.9 and 49.4 % respective improvement over that of pure ZnO/CdS–QDSSC. This result is attributed to its superiority in light absorption and charge–hole separation for the ZnO/CdS/dye-QDSSC.     

Quantum rod-sensitized solar cell: nanocrystal shape effect on the photovoltaic properties

The effect of the shape of nanocrystal sensitizers in photoelectrochemical cells is reported. CdSe quantum rods of different dimensions were effectively deposited rapidly by electrophoresis onto mesoporous TiO(2) electrodes and compared with quantum dots. Photovoltaic efficiency values of up to 2.7% were measured for the QRSSC, notably high values for TiO(2) solar cells with ex situ synthesized nanoparticle sensitizers. The quantum rod-based solar cells exhibit a red shift of the electron injection onset and charge recombination is significantly suppressed compared to dot sensitizers. The improved photoelectrochemical characteristics of the quantum rods over the dots as sensitizers is assigned to the elongated shape, allowing the build-up of a dipole moment along the rod that leads to a downward shift of the TiO(2) energy bands relative to the quantum rods, leading to improved charge injection.     

Deposition and properties of CBD and CSS CdS thin films for solar cell application

We deposited cadmium sulfide (CdS) thin films using the chemical-bath deposition (CBD) and close-spaced sublimation (CSS) techniques. The films were then treated in CdCl₂ vapor at 400 °C for 5 min. The CSS CdS films had hexagonal structure, and good crystallinity. The CdCl₂ treatment did not produce major changes, but there was a decrease in the density of planar defects. The untreated CBD CdS films had cubic structure and poorer crystallinity than the CSS films. After the CdCl₂ treatment, these films recrystallized to the hexagonal phase, resulting in better crystallinity and a lower density of planar defects. The conformal coverage and the presence of bulk oxygen are the key issues in making the CBD films more suitable for photovoltaic applications.     

Annealing effects on the chemical deposited CdS films and the electrical properties of CdS/CdTe solar cells

CdS layers grown by chemical bath deposition (CBD) are annealed in the oxygen and argon-hydrogen atmosphere respectively. It has been found that the open circuit voltage of the CdS/CdTe solar cell increases when the CBD CdS is annealed with oxygen before the deposition of CdTe by close spaced sublimation (CSS), while the performance of the solar cell decreases when the CBD CdS is annealed with argon-hydrogen. Electronic properties of the CdS films are investigated using X-ray photo-electron spectroscopy (XPS), which indicates that the Fermi level is shifting closer to the conduction band after annealing in the oxygen and consequently a higher open circuit voltage of the solar cell can be obtained.     

Electrical properties of vacuum annealed CdS films

Cadmium sulphide (CdS) films were evaporated in vacuum on glass substrates maintained at room temperature. These films were later annealed in vacuum at temperatures in the range 30 to 300° C. The variation of d.c. electrical conductivity was studied in the temperature range 100 to 300 K. While the conductivity data in the range 100 to 150 K were observed to follow Mott's variable range hopping process, the conductivity in the high-temperature region (150 to 300 K) could be explained by Seto's model.     

Optical properties of ZnO/ZnS and ZnO/ZnTe heterostructures for photovoltaic applications

Although ZnO and ZnS are abundant, stable, and environmentally benign, their band gap energies (3.44, 3.72 eV, respectively) are too large for optimal photovoltaic efficiency. By using band-corrected pseudopotential density functional theory calculations, we study how the band gap, optical absorption, and carrier localization can be controlled by forming quantum-well-like and nanowire-based heterostructures of ZnO/ZnS and ZnO/ZnTe. In the case of ZnO/ZnS core/shell nanowires, which can be synthesized using existing methods, we obtain a band gap of 2.07 eV, which corresponds to a Shockley-Quiesser efficiency limit of 23%. On the basis of these nanowire results, we propose that ZnO/ZnS core/shell nanowires can be used as photovoltaic devices with organic polymer semiconductors as p-channel contacts.     

Electrical characterization and carrier transport mechanisms of GaAs p/i/n devices for photovoltaic applications

GaAs p/i/n diodes made by Metal-Organic Vapour Phase Epitaxy were examined by electrical measurements for evaluating the optimum i-region for use as solar cells. Four series of samples were prepared and studied each one with a different i-region width. The performance of the devices was examined by means of Admittance spectroscopy as well as classical current–voltage and capacitance–voltage characterization, allowing the calculation of the minority carriers lifetime (τ_eff) and the diodes ideality factors. The values of the τ_eff were found to lie between 8.7 ps and 0.14 ns for i-region widths between 0 and 0.8 μm. These results were used to model the multilayer structure with the two-diode representation and explain the conductance mechanisms inside the diodes. This modeling showed that the recombination/generation currents were dominating in forward biased diodes and the ohmic loss current in reverse bias.     

ZnO-CdS core-shell quantum dots sensitized solar cell: influence of crystalline and amorphous CdS structures in photovoltaic performance

ZnO nanoparticles (NPs) coated with amorphous and crystalline CdS quantum dots (QDs) were successfully synthesized through chemical bath deposition (CBD) process. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) have been utilized to characterize the samples morphology and structural properties. The conduction band of CdS QDs is much higher than the ZnO conduction band facilitates electron transfer process through cascade system. The thickness and crystallinity of the CdS QDs coated on ZnO NPs critically controls the electron diffusion length and photovoltaic performance of the solar cell. The red shift from 506 to 524 nm, increased optical absorption in the UV-visible range and electron diffusion length limited by the thickness of the amorphous/crystalline CdS QDs coated on ZnO NPs film, influences the performance of the QDs sensitized solar cell (QDSSCs) under one sun illumination intensity (AM 1.5, 100 mW/cm2). The results discuss the CBD process controlled growth of CdS QDs on ZnO NPs and its influence on the photovoltaic performance of QDSSCs.     

Optical and electronic properties of fluorene/thiophene/benzothiadiazole pseudorandom copolymers for photovoltaic applications

A new family of 9,9-bisalkylfluorene (F)/thiophene (T)/benzothiadiazole (B) π-conjugated copolymers for organic solar cells is reported. The structure of the reported copolymers is pseudorandom: in turn each F, T, B monomer unit is alternated to the other randomly distributed two units. Voltammetric, UV–visible, and photoluminescence measurements have been carried out to assess the optical and electronic properties of the synthesized materials. The occurring of photoinduced charge transfer towards a fullerene electron acceptor was investigated by photoluminescence quenching and light-induced electron spin resonance experiments. The copolymer having alternating thiophene monomer units and randomly distributed fluorene and benzothiadiazole units exhibits the most promising characteristics; the photophysic study shows that such polymer/fullerene blend could represent a novel and cheaper material to be used as convenient donor–acceptor system for polymer solar cells.     

Shape effect on electronic and photovoltaic properties of CdS nanocrystals

Changes in electronic and photovoltaic properties of semiconductor nanocrystals predominantly due to changes in shape are discussed here. Cadmium sulfide (CdS) semiconductor nanocrystals of various shapes (tetrapod, tetrahedron, sphere and rod) obtained using an optimized solvothermal process exhibited a mixed cubic (zinc blende) and hexagonal (wurtzite) crystal structure. The simultaneous presence of the two crystal phases in varying amounts is observed to play a pivotal role in determining both the electronic and photovoltaic properties of the CdS nanocrystals. Light to electrical energy conversion efficiencies (measured in two-electrode configuration laboratory solar cells) remarkably decreased by one order in magnitude from tetrapod --> tetrahedron --> sphere --> rod. The tetrapod-CdS nanocrystals, which displayed the highest light to electrical energy conversion efficiency, showed a favorable shift in position of the conduction band edge leading to highest rate of electron injection (from CdS nanocrystal to the wide band gap semiconductor viz. titanium dioxide, TiO2) and lowest rate of electron-hole recombination (higher free electron lifetimes).