Augmented photoelectrochemical response of CdS/ZnS quantum dots sensitized hematite photoelectrode

A visible light active and stable photoelectrode has been developed by depositing a passivating layer of ZnS QDs on CdS QDs sensitized hematite photoelectrode (Hematite‐CdS/ZnS) for PEC generation of hydrogen. Photoelectrochemical properties, in terms of stability and efficiency, have been investigated on the various hematite photoelectrodes sensitized with CdS QDs and CdS/ZnS QDs by varying number of SILAR cycles. I–V characteristics show that two layers of ZnS QDs deposited over three layers of CdS could enhance PEC response of hematite and efficiency by a factor of 3 and 11 respectively. Chronoamperometry measurement ensures that after adding a layer of ZnS QDs, CdS sensitized hematite film turns out to be a stable photoelectrode in the electrolyte. Prepared photoelectrodes have been characterized by XRD, SEM, HRTEM and UV–Vis spectrophotometer for various structural, morphological and optical properties to analyze PEC results. Mott–Schottky analysis and incident photon to current conversion efficiency (IPCE) measurements of sensitized hematite photoelectrode supported the improved PEC response of CdS/ZnS QDs sensitized hematite thin films. Copyright © 2016 John Wiley & Sons, Ltd.     

A reduced graphene oxide interface layer for improved power conversion efficiency of aqueous quantum dots sensitized solar cells

In quantum dots sensitized solar cells (QDSSCs), the back-transport reaction, resulting from the recombination between the photo-generated electrons in transparent conductive oxide (TOC) substrates and the polysulfide species electrolytes, significantly impairs the power conversion efficiency. To solve this, a reduced graphene oxide (rGO) interface layer has been for the first time introduced between the TOC substrate and the porous TiO₂ film in a modified CdS QDSSC by a spray-coating method. This rGO interface layer can effectively suppress the back-transport reaction by inhibiting the contact between TOC substrates and electrolytes. Consequently, an improvement in power conversion efficiency of ca. 20% has been achieved on the solar cell. Besides, an interesting threshold effect has also been found on this interface layer, which leads to much variated behavior of this rGO layer on the suppression of the back-transport reaction. Consequently, the optimum rGO interface layer has been obtained with a spray time of 2 min, which leads to the best photovoltaic properties.     

Phosphorus-doped silicon quantum dots for all-silicon quantum dot tandem solar cells

Doping of Si quantum dots is important in the field of Si quantum dots-based solar cells. Structural, optical and electrical properties of Si QDs formed as multilayers in a SiO₂ matrix with various phosphorus (P) concentrations introduced during the sputtering process were investigated for its potential application in all-silicon quantum dot tandem solar cells. The formation of Si quantum dots was confirmed by transmission electron microscopy. The addition of phosphorus was observed to modify Si crystallization, though the phosphorus concentration was found to have little effect on quantum dot size. Secondary ion mass spectroscopy results indicate minimal phosphorus diffusion from Si QDs layers to adjacent SiO₂ layers during high-temperature annealing. Resistivity is significantly decreased by phosphorus doping. Resistivity of slightly phosphorus-doped (0.1 at% P) films is seven orders of magnitude lower than that of intrinsic films. Dark resistivity and activation energy measurements indicate the existence of an optimal phosphorus concentration. The photoluminescence intensity increases with the phosphorus concentration, indicating a tendency towards radiative recombination in the doped films. These results can provide optimal condition for future Si quantum dots-based solar cells.     

Plasmon-enhanced photocurrent in dye-sensitized solar cells

Plasmonic structures of FTO/TiO₂/NPs-Ag and FTO/NPs-Ag/TiO₂ electrodes were fabricated by sputter technology and the sol–gel & spin coating procedure. These electrodes with similar optical absorptions in the visible region enhanced by the surface plasmon resonance of silver nanoparticles have different photovoltaic properties, revealing that the significant design can be used to identify the favorably enhanced direction of plasmonic structure resulting from plasmonic scattering to trap light which confines light within the active TiO₂ layer to promote dye absorption in dye-sensitized solar cells (DSSCs). In the FTO/TiO₂/NPs-Ag, a 60% enhancement in photocurrent and an improvement in photovoltage were observed and the increased incident photon-to-photocurrent efficiency (IPCE) was consistent with the enhanced absorption spectrum. However, the photovoltaic properties of the FTO/NPs-Ag/TiO₂ were similar to those of the standard electrode. This concept is potentially applicable to new kinds of solar cells.     

Fabrication of CdS quantum dot sensitized solar cells via a pressing route

CdS quantum dots have been self-assembled on the surface of dispersed nanocrystalline TiO₂ particles, and a light-harvesting electrode has been fabricated from the resulting sensitized P25 particles using the pressing route. The spectroscopic and photochemical properties of photosensitized nanocrystalline TiO₂ electrodes were studied. The results indicate that electrode preparation by the pressing route may lead to partial loss or damage of the CdS coating and creation of regions that are inaccessible to the redox electrolyte. Nevertheless, the pressing method using pre-coated powders shows promise as a low cost method for the preparation of photoelectrodes in sensitized-solar cells.     

n-Type silicon quantum dots and p-type crystalline silicon heteroface solar cells

Heteroface devices have been realized by depositing phosphorus-doped silicon (Si) quantum dots (QDs) (n-type) on a p-type crystalline silicon substrate. To compare the quantum confinement effect, different sizes (3, 4, 5, and 8±1 nm) of Si QD were fabricated, whose optical energy bandgaps are in the ranges of 1.3–1.65 eV. The electrical and photovoltaic properties of heterojunction devices were characterized by illuminated and dark I–V measurements, C–V measurements, and spectral response measurements. The diodes showed a good rectification ratio of 5×10⁶ for 4 nm Si QDs at the bias voltage of ±1.0 V at 298 K. The ideality factor and junction built-in potential deduced from current–voltage (I–V) and capacitance–voltage (C–V) plots are 1.86 and 0.847 V for 3 nm QD device, respectively. From the illuminated IV characteristics, the open circuit voltages were 556, 540, 512, and 470 mV for mean QD diameters 3, 4, 5, and 8±1 nm, respectively. Temperature-dependant dark I–V measurements suggest that the carrier transport in the devices is controlled by recombination in the space-charge region. This study indicates the silicon QDs can be good candidates for all-silicon tandem solar cells.     

Synthesis and characterization of boron-doped Si quantum dots for all-Si quantum dot tandem solar cells

Multiple layers of Si quantum dots (QDs) in SiO₂ with a narrow size distribution were synthesized by a co-sputtering technique. Structural, electrical and optical properties of Si QD/SiO₂ multilayer films with various boron (B) concentrations introduced during the sputtering process were studied. X-ray photoelectron spectroscopy (XPS) revealed B-B/B-Si bonding, which suggests possible boron inclusion in the nanocrystals. The addition of boron was observed to suppress Si crystallization, though the boron concentration was found to have little effect on the QD size. Reductions in film resistivity were observed with the increase in boron concentration, which is believed to be a consequence of an increase in carrier concentration. This is supported by a large decrease in the activation energy accompanying the drop in resistivity, consistent with the Fermi energy moving towards the valence bands. The photoluminescence (PL) intensity was found to decrease with increase in boron concentration.     

Temperature dependence of photocurrent components on enhanced performance GaAs/AlGaAs multiple quantum well solar cells

The performance of Al_0.36Ga_0.64As p/i/n solar cells with multiple quantum wells (MQW) of GaAs/Al_0.36Ga_0.64As in the i-region has been investigated at various temperatures, ranging from −10°C to 100°C, and compared with that of conventional solar cells composed of either the quantum well material (GaAs) or the barrier material (Al_0.36Ga_0.64As) alone. The dark currents of the MQW cells were found to lie between those of the conventional cells. The increase of dark current with temperature was accompanied by a slight decrease of the diode ideality factor. A linear dependence of open-circuit voltage (V_oc) on temperature was observed for all cells when illuminated with a 100W halogen lamp. V_oc for the MQW cells was found to be independent of the number of wells, lying between the V_oc's for the two conventional cells. The MQW cells exhibited performance improvement with temperature when compared to the conventional cells and there was a significant enhancement in the short-circuit current with temperature of those MQW cells that exhibited poorer performance at lower temperatures. Theoretical calculations have quantified the contribution of the tunneling current component to the total observed photocurrent at the various temperatures examined. It was found that tunneling currents are present at all temperatures and can be the dominant component in MQW cells of thinner wells at low temperatures. These results suggest that GaAs/Al_0.36Ga_0.64As MQW structures, of good-quality material, when processed as conventional solar cells with antireflective coatings should deliver more output power under intense illumination than conventional solar cells composed of the quantum well material alone.     

Testing of dye sensitized TiO2 solar cells I: Experimental photocurrent output and conversion efficiencies

Recently, a paper was published by the Lausanne Group headed by Dr. M. Graetzel which reported a simple low cost 7% efficient photo electrochemical solar cell made from a trinuclear Ru dye complex adsorbed on the very rough surface of a colloidal TiO₂ film. In the current paper, a verification of this result is presented using procedures described in the literature. Measurements are reported in stimulated and natural sunlight which confirm that the efficiency is indeed in the range previously reported. Predicted Air Mass 1.5 photo currents are compared to those obtained from fabricated dye sensitized cells. Although current densities of 12 mA/cm² and voltages of over 0.6 V are measured,it is found that corresponding fill factors, less than 0.6, limit the performance of the cell under solar illumination. The basic economics of such a device are outlined and it is proposed that cell costs of $ 0.6 per peak watt could be possible if the longevity of the cell is at least 15 years.     

CdSe quantum dots sensitized nanoporous hematite for photoelectrochemical generation of hydrogen

A novel system of CdSe quantum dots (QDs) sensitized porous hematite (α-Fe₂O₃) films has been investigated as a potential photoelectrode for hydrogen generation via photoelectrochemical (PEC) splitting of water. Before sensitization, nanoporous hematite thin films were prepared by spray pyrolysis. Characterizations for crystalline phase formation, crystallite size, absorption spectra, and flatband potential were carried out to analyze PEC data. Loading time of sensitizer to hematite thin films was found to be crucial in affecting its PEC properties. Film having sensitizer loading time as 42 h exhibited best photocurrent density of 550 μA cm⁻² at 1.0 V versus SCE. Current study, for the first time, explores the possibility of using low band gap QDs sensitization on a low band gap film, hematite in PEC splitting of water.     

High-efficiency polycrystalline CdTe thin-film solar cells

Cadmium telluride is a promising photovoltaic material for thin-film solar cells. However, further improvements on performance and reproducibility of devices have been limited by the conventional SnO₂/poly-CdS/poly-CdTe device structure used for more than 30 years. In this paper, we review partial R&D approaches at NREL to understand the issues related to the conventional device structure and to develop several novel materials and a modified device structure for minimizing these issues. We have achieved a CdTe polycrystalline thin-film solar cell demonstrating an NREL-confirmed, total-area efficiency of 16.5% by using new materials and the modified device structure. To apply the high-efficiency CdTe cell fabrication technique, we developed two manufacturing processes for producing high-efficiency CdTe modules with the potential of high throughput and low cost.     

Photocurrent in CdTe NIP solar cells

A simple analytical model for the voltage dependence of the photocurrent in CdTe n–i–p solar cells is presented. The physical model is corroborated with a numerical solution of Poisson and the two continuity equations under illumination and shows excellent agreement with the numerical data. The new model is compared with previously reported models of Bube and Crandall. The new model illuminates the loss mechanism of carriers near the front interface of CdTe solar cells. It is shown that the photocurrent is high when the carrier velocity due to the electric field is higher than the product of the absorption coefficient and the diffusion coefficient, almost regardless of the interface recombination velocity value. At lower electric field values, the interface recombination velocity has a stronger effect on the photocurrent. Experimental conditions leading to a reduction of the electric field and a corresponding decrease in the collection efficiency in CdTe n–i–p solar cells are discussed.     

Bifacial configurations for CdTe solar cells

We present a different back contact for CdTe solar cell by the application of only a transparent conducting oxide (TCO), typically ITO, as a back electrical contact on all-PVD CdTe/CdS photovoltaic devices that acts as a free-Cu stable back contact and at the same time allows to realize bifacial CdTe solar cells, which can be illuminated from either or both sides. Also devices with thin CdTe layers (from 2 μm down to 1 μm) have been prepared to improve the conversion efficiency on the back side illumination, which is limited by the collection of carriers far away from the junction and to reduce the amount of material in the CdTe device. Reproducible solar cells exceeding 10% efficiency on the front side illumination and exceeding 3% on the back side illumination are reported.     

砷化镓量子点太阳电池及材料的研究现状

介绍了目前砷化镓量子点太阳电池的研究现状,主要包括:量子点太阳电池理论分析、量子点结构材料生长与性能表征、量子点太阳电池器件结构设计与制备技术三方面。此外,在对目前研究中所存在的难点问题进行分析后,认为制备高质量的量子点结构材料以及优化量子点太阳电池结构设计是目前获得高性能量子点电池的关键。     

High efficiency ultra-thin sputtered CdTe solar cells

Large scale manufacturing of CdTe PV modules at the GW/yr level may be constrained due to the limited availability of the relatively rare (Te) element and the volume of potentially hazardous (Cd) material being used in the typically 3–8 μm thick CdTe absorber layer. However, we find that it is possible to reduce the CdTe layer thickness without much compromise in efficiency. The CdS/CdTe solar cells were fabricated using magnetron sputtering with ultra-thin CdTe layers in the range of 0.5–1.28 μm. The ultra-thin films and cells were characterized using X-ray diffraction (XRD), optical transmission, scanning electron microscopy (SEM), current–voltage and quantum efficiency measurements. These results were compared with those of standard 2.3 μm thick CdTe sputtered cells. Different post-deposition processing parameters were required for cells with ultra-thin and standard CdTe thicknesses to achieve high efficiency. Ultra-thin CdTe cells showed crystallographic texture and CdTe_1−xS_x alloy formation after CdCl₂ treatment very similar to standard CdTe cells. Optimization of the post-deposition CdCl₂ treatment and back-contact processing yielded cells of 11.2% efficiency with 0.7 μm CdTe compared to 13.0% obtained with standard 2.3 μm CdTe cells.     

Recent developments in evaporated CdTe solar cells

Recent developments in the technology of high vacuum evaporated CdTe solar cells are reviewed. High-efficiency solar cells of efficiencies up to 12.5% have been developed on soda-lime glass substrates with a low-temperature (<450 °C) process. This simple process is suitable for in-line production of large-area solar modules on glass as well as on flexible polymer films with a roll-to-roll deposition process. Flexible and lightweight CdTe solar cells with a record efficiency of 11.4% have been developed in a superstrate configuration, and 3.5% efficiency mini-modules have been realised in a preliminary development. Deposition of high-temperature stable ITO front contact layer on polyimide is important for high-efficiency cells, as the layer should withstand processing steps maintaining its high electrical conductivity and optical transparency. Another development is an application of a transparent conducting oxide (TCO) ITO as a back electrical contact on CdTe leading to first bifacial CdTe solar cells, which can be illuminated from either or both sides. Accelerated long-term stability tests show that light soaking improves the efficiency of CdTe solar cells with ITO back contacts and performance does not degrade.Stability of CdTe solar cells has been measured after irradiation with high-energy protons and electrons of different fluences. These solar cells exhibit superior radiation tolerance compared to conventional Si and GaAs solar cells for space applications. Because of extreme stability, and high specific power (kW/kg) of flexible solar cells, CdTe has a promising potential for space applications.     

Enhanced H2 evolution and the interfacial electron transfer mechanism of titanate nanotube sensitized with CdS quantum dots and graphene quantum dots

Synergistic the modulation of photon absorption capability and interfacial charge transfer of the photocatalyst are highly required for developing high-performance heterojunction photocatalysts. The ternary CdS-graphene quantum dots-titanate nanotubes (CdS-GQDs-TNTs) nanocomposite have been prepared by an in situ growth method. The physicochemical characterization reveals that the GQDs are firmly decorated on both inner and outer surface of TNT through the formation of Ti–O–C chemical bonding, and CdS QDs are loaded on the outer surface of TNTs through strong interfacial interaction. The intimate integrated CdS-GQDs-TNTs nanocomposite exhibits much superior photocatalytic performance toward H₂ production compared with binary GQDs-TNTs and pure TNTs photocatalyst, which can be attributed to the combined interaction of the stronger visible light harvesting, the longer lifetime of photogenerated electron−hole pairs, faster interfacial charge transfer rate, fast and long-distance electron transport pass. The interfacial charge transfer mechanism of CdS-GQDs-TNTs ternary composite are proposed based on photoelectrochemical measurements.     

Stability of CdTe/CdS thin-film solar cells

The recent literature regarding the stability of CdTe/CdS photovoltaic cells (as distinguished from modules) is reviewed. Particular emphasis is given to the role of Cu as a major factor that can limit the stability of these devices. Cu is often added to improve the ohmic contact to p-CdTe and the overall cell photovoltaic performance. This may be due to the formation of a Cu₂Te/CdTe back contact. Excess Cu also enhances the instability of devices when under stress. The Cu, as Cu⁺, from either Cu₂Te or other sources, diffuses via grain boundaries to the CdTe/CdS active junction. Recent experimental data indicate that Cu, Cl and other diffusing species reach (and accumulate at) the CdS layer, which may not be expected on the basis of bulk diffusion. These observations may be factors in cell behavior and degradation, for which new mechanisms are suggested and areas for future study are highlighted. Other possible Cu-related degradation mechanisms, as well as some non-Cu-related issues for cell stability are discussed.     

Dye sensitized solar cells on paper substrates

This article reports for the first time in the literature, a dye sensitized solar cells with 1.21% efficiency (V_oc=0.56 V, J_sc=6.70 mA/cm² and F.F.=0.33) on paper substrates. The current dye sensitized solar cell technology is based on fluorine doped SnO₂ (FTO) coated glass substrates. The problem with the glass substrate is its rigidity and heavy weight. Making DSSCs on paper opens the door for both photovoltaic and paper industries. The potential of using mature paper making and coating technologies will greatly reduce the current PV cost. Paper substrate based DSSCs not only offer the advantages of flexibility, portability and lightweight but also provide the opportunities for easy implantation to textile. In this study, a low temperature process is developed to coat uniform nickel on paper substrate as the metal contact to replace the traditional expensive FTO. The Ni paper showed excellent conductivity of 8-10 Ω/□. It is found that the control of metal oxide electrode morphology is critical to solar cell performance. The TiO₂ film has the tendency to crack on Ni coated paper, which resulted in the shunt of the device and no solar cell efficiency was obtained. ZnO film on the other hand had good morphology tolerance on Ni coated paper and yielded solar cell efficiency of 1.21% (V_oc=0.56 V, J_sc=6.70 mA/cm² and F.F.=0.33) under AM 1.5 (activation area is 0.16 cm²). The control sample of ZnO solar cell on FTO glasses has the efficiency of 2.66% (V_oc=0.64 V, J_sc=9.97 mA/cm² and F.F.=0.42).     

Photovoltaic properties of close-space sublimated CdTe solar cells

CdS/CdTe solar cells were fabricated by close-space sublimation with a screen-printed Te-rich CdTe source and their photovoltaic properties were investigated by varying the substrate temperature, cell area, and thicknesses of CdTe and ITO layers. The resistivity of CdTe layers employed in this study was 3×10⁴ Ω cm. The optimum substrate temperature and thickness for CdTe deposition were 600°C and 5 μm, respectively. The CdTe bulk resistance degraded the cell performance above 6 μm. As the cell area increased the V_oc remained almost constant, while the J_sc and FF were strongly degraded because of the increase of the lateral resistance of the ITO layer. The optimum thickness of the ITO layer in this study was 300–450 nm. In this experiment we obtained an efficiency of 9.4% in the 0.5 cm² cells. The series resistance of the cell should be further reduced to increase the fill factor and improve the efficiency.