Microwave assisted CdSe quantum dot deposition on TiO2 films for dye-sensitized solar cells

CdSe quantum dot (QD ) sensitized TiO(2) films have been fabricated using a one-step microwave assisted chemical bath deposition (MACBD) technique and used as photoanodes for quantum dot sensitized solar cells. This technique allows direct and rapid deposition and a good contact between the CdSe and TiO(2) films. The photovoltaic performances of the cells with CdSe deposited at different times are investigated. The results show that cells based on MACBD deposited TiO(2)/CdSe electrodes achieve a maximum short circuit current density of 12.1 mA cm(-2) and a power conversion efficiency of 1.75% at one Sun (AM 1.5 G, 100 mW cm(-2)), which is comparable with those fabricated using conventional techniques.     

染料敏化太阳电池TiO2光阳极研究进展

纳米TiO2是目前性能最为优良的染料敏化太阳电池(DSSC)光阳极材料。文章系统综述了优化纳米TiO2光阳极的染料吸附、电子传输、再生染料等性能的技术和方法,主要包括纳米TiO2光阳极薄膜微结构的调控、TiO2光阳极的离子/元素掺杂、TiO2光阳极的表面包覆、TiO2光阳极的表面处理等方面的国内外研究进展,并分析了目前TiO2光阳极存在的主要问题及未来的发展方向。     

染料敏化太阳能电池丝网印刷浆料中有机造孔剂对电池性能的影响

本文采用丝网印刷工艺制备染料敏化太阳能电池的氧化钛多孔薄膜,并将其组装成电池器件.主要研究了丝网印刷浆料中造孔剂-乙基纤维素和松油醇的含量对氧化钛薄膜光电性能的影响.光电性能测试和扫描电镜结果显示,当乙基纤维素N50和N10的比例为1:0、松油醇的含量为75%时,薄膜的结构和光电转换性能结果最优,单层光电转换效率从0.84%提升到1.23%.     

Effective harvesting,detection, and conversion of IR radiation due to quantum dots with built-in charge

We analyze the effect of doping on photoelectron kinetics in quantum dot [QD] structures and find two strong effects of the built-in-dot charge. First, the built-in-dot charge enhances the infrared [IR] transitions in QD structures. This effect significantly increases electron coupling to IR radiation and improves harvesting of the IR power in QD solar cells. Second, the built-in charge creates potential barriers around dots, and these barriers strongly suppress capture processes for photocarriers of the same sign as the built-in-dot charge. The second effect exponentially increases the photoelectron lifetime in unipolar devices, such as IR photodetectors. In bipolar devices, such as solar cells, the solar radiation creates the built-in-dot charge that equates the electron and hole capture rates. By providing additional charge to QDs, the appropriate doping can significantly suppress the capture and recombination processes via QDs. These improvements of IR absorption and photocarrier kinetics radically increase the responsivity of IR photodetectors and photovoltaic efficiency of QD solar cells.     

用于敏化太阳能电池的二氧化钛光阳极材料

敏化太阳能电池是当前清洁能源领域的研究热点之一,有望成为第三代太阳能电池。二氧化钛作为敏化太阳能电池的光阳极材料之一被广泛研究。主要综述了近20 a来二氧化钛光阳极材料的结构进展,并从电子注入效率、电子传输和基底电荷收集效率方面评述了各种结构的应用特点。另外,描述了当代三明治状二氧化钛工作电极的超薄保形覆盖层、工作层和阻挡层。然后,重点介绍了用于电子传输工作层的二氧化钛一维纳米阵列的制备方法及特点。最后,展望了光阳极结构与合成方法的未来发展趋势。     

Improving Si solar cell performance using Mn:ZnSe quantum dot-doped PLMA thin film

Poly(lauryl methacrylate) (PLMA) thin film doped with Mn:ZnSe quantum dots (QDs) was spin-deposited on the front surface of Si solar cell for enhancing the solar cell efficiency via photoluminescence (PL) conversion. Significant solar cell efficiency enhancements (approximately 5% to 10%) under all-solar-spectrum (AM0) condition were observed after QD-doped PLMA coatings. Furthermore, the real contribution of the PL conversion was precisely assessed by investigating the photovoltaic responses of the QD-doped PLMA to monochromatic and AM0 light sources as functions of QD concentration, combined with reflectance and external quantum efficiency measurements. At a QD concentration of 1.6 mg/ml for example, among the efficiency enhancement of 5.96%, about 1.04% was due to the PL conversion, and the rest came from antireflection. Our work indicates that for the practical use of PL conversion in solar cell performance improvement, cautions are to be taken, as the achieved efficiency enhancement might not be wholly due to the PL conversion.     

Impacts of Post-metallisation Processes on the Electrical and Photovoltaic Properties of Si Quantum Dot Solar Cells

As an important step towards the realisation of silicon-based tandem solar cells using silicon quantum dots embedded in a silicon dioxide (SiO₂) matrix, single-junction silicon quantum dot (Si QD) solar cells on quartz substrates have been fabricated. The total thickness of the solar cell material is 420 nm. The cells contain 4 nm diameter Si quantum dots. The impacts of post-metallisation treatments such as phosphoric acid (H₃PO₄) etching, nitrogen (N₂) gas anneal and forming gas (Ar: H₂) anneal on the cells’ electrical and photovoltaic properties are investigated. The Si QD solar cells studied in this work have achieved an open circuit voltage of 410 mV after various processes. Parameters extracted from dark I–V, light I–V and circular transfer length measurement (CTLM) suggest limiting mechanism in the Si QD solar cell operation and possible approaches for further improvement.     

A novel preparation of small TiO₂ nanoparticle and its application to dye-sensitized solar cells with binder-free paste at low temperature

TiO(2) nanoparticles with diameter <10 nm were synthesized by a facile, non-hydrothermal method at low temperature. A porous TiO(2) film electrode consisting of the obtained small TiO(2) nanoparticles and commercial TiO(2) nanoparticles without any organic binder was prepared at low temperature. The photovoltaic performance of the solar cell based on the TiO(2) electrode was investigated by the current-voltage and electrochemical impedance spectra. All the experimental results indicate that the addition amount of the small TiO(2) nanoparticles in the binder-free paste affects the photovoltaic performance of the photoelectrode greatly. The overall energy conversion efficiency of the optimized binder-free photoelectrode achieves 3.53% without high-temperature sintering. Additionally, the performance of the small particles derived from this facile method can be comparable with that of small ones obtained from traditionally hydrothermal method, indicating the small particles in our study can be applied to flexible dye-sensitized solar cells. And the present low-temperature preparation of photoelectrode containing small TiO(2) nanoparticles shows an encouraging performance on both conductive glass and plastic substrates and could be suited in the industrial and large-scale application due to its low energy cost and relatively high conversion efficiency.     

Au Nanoparticles as Interfacial Layer for CdS Quantum Dot-sensitized Solar Cells

Quantum dot-sensitized solar cells based on fluorine-doped tin oxide (FTO)/Au/TiO₂/CdS photoanode and polysulfide electrolyte are fabricated. Au nanoparticles (NPs) as interfacial layer between FTO and TiO₂ layer are dip-coated on FTO surface. The structure, morphology and impedance of the photoanodes and the photovoltaic performance of the cells are investigated. A power conversion efficiency of 1.62% has been obtained for FTO/Au/TiO₂/CdS cell, which is about 88% higher than that for FTO/TiO₂/CdS cell (0.86%). The easier transport of excited electron and the suppression of charge recombination in the photoanode due to the introduction of Au NP layer should be responsible for the performance enhancement of the cell.     

Si solid-state quantum dot-based materials for tandem solar cells

The concept of third-generation photovoltaics is to significantly increase device efficiencies whilst still using thin-film processes and abundant non-toxic materials. A strong potential approach is to fabricate tandem cells using thin-film deposition that can optimise collection of energy in a series of cells with decreasing band gap stacked on top of each other. Quantum dot materials, in which Si quantum dots (QDs) are embedded in a dielectric matrix, offer the potential to tune the effective band gap, through quantum confinement, and allow fabrication of optimised tandem solar cell devices in one growth run in a thin-film process. Such cells can be fabricated by sputtering of thin layers of silicon rich oxide sandwiched between a stoichiometric oxide that on annealing crystallise to form Si QDs of uniform and controllable size. For approximately 2-nm diameter QDs, these result in an effective band gap of 1.8 eV. Introduction of phosphorous or boron during the growth of the multilayers results in doping and a rectifying junction, which demonstrates photovoltaic behaviour with an open circuit voltage (V_OC) of almost 500 mV. However, the doping behaviour of P and B in these QD materials is not well understood. A modified modulation doping model for the doping mechanisms in these materials is discussed which relies on doping of a sub-oxide region around the Si QDs.     

Enhanced light scattering and photovoltaic performance for dye-sensitized solar cells by embedding submicron SiO2/TiO2 core/shell particles in photoanode

The introduction of light scattering in the photoanodes of dye-sensitized solar cells is one of the most effective approaches to enhance their photovoltaic performance. In this work, we prepared submicron SiO₂/TiO₂ core/shell particles and embedded these particles in the nanostructured TiO₂ photoanodes for light to scatter in the dye-sensitized solar cells. Due to the large difference in the refractive index between the SiO₂ core and the TiO₂ shell, the embedded submicron SiO₂/TiO₂ core/shell particles showed strong light scattering effect. Light absorbance of the dyed photoanode with the embedded SiO₂/TiO₂ particles for light scattering was found to be three times stronger than the one without light scattering particles over a wide wavelength range. The power conversion efficiency of dye-sensitized solar cells was increased by about 50% after the introduction of light scattering SiO₂/TiO₂ core/shell particles in the photoanode. This work will provide a base for further enhancement in the photovoltaic performance of dye-sensitized solar cells by optimizing the submicron SiO₂/TiO₂ core/shell particles and the photoanodes.     

Porous CdS-sensitized electrochemical solar cells

In inorganic semiconductor (such as CdS)-sensitized solar cells, isolated nanoparticles (including quantum dots) or Porous semiconducting layers are particularly efficient and effective in extracting charge carriers generated by solar energy, without a serious recombination among sensitizers. In this study, porously structured CdS was formed by spray pyrolysis deposition (SPD) using an excess cadmium chloride and thiourea aqueous mixture solution onto an mp-TiO₂ substrate pre-heated to 450 °C in an air atmosphere and subsequent washing of the excess cadmium chloride using deionized water. As expected, the power conversion efficiency of a photoelectrochemical solar cell fabricated with the porous CdS was greatly improved, to 1.71%, the highest efficiency ever reported for CdS-sensitized solar cells employing polysulfide as an electrolyte. This improvement in performance is attributed to the efficient transport of the charge carriers generated in CdS.     

Micron-sized silica mesh for enhancing the performance of quantum dot sensitized solar cells

Three dimensional silica mesh structures are prepared through a new and simple method for enhancing the quantum dot sensitized solar cells performance and stability.Silica patterns are made on the top of the TiO₂ photoanodes and a marked improvement in light scattering properties of meshed structures is confirmed by diffuse reflectance spectroscopy measurements. This improvement enhances the current density and consequently the cells ‘efficiency. Parameters of electron transport in cells are explored by electrochemical impedance spectroscopy (EIS). According to the EIS results, silica mesh declines the recombination rate in cells in a clear way. Here more than 50% efficiency improvement is obtained in meshed structures in comparison to cells with normal TiO₂ photoanode structures. The insulated silica mesh, reduces the electrolyte's deleterious effect on the semiconductor layers and the cells’ stability is improved.     

TiO2/CdS/CdSe quantum dots co-sensitized solar cell with the staggered-gap (type-II) heterojunctions for the enhanced photovoltaic performance

The effect of co-sensitization and ZnS passivation on the photovoltaic performance of CdS quantum dot sensitized solar cells (QDSSCs) were investigated. The deposition of CdS, CdSe quantum dots (QD) and ZnS passivation on TiO₂ photoanode was carried out by successive ionic layer adsorption and reaction (SILAR) method. CdS/CdSe co-sensitization developed two staggered type-II heterojunctions at TiO₂/CdS and CdS/CdSe interfaces and resulted a cascade energy band structure. This suitable band alignment facilitated the double charge transfer mechanism at each heterojunction and transported the electrons easily into the photoanode. The narrow bandgap sensitizers CdS and CdSe significantly improved the potential utilization of solar spectrum with more charge carrier generation. ZnS passivation on QD surface suppressed electrode/electrolyte interfacial charge recombination and facilitated more electron injection from QDs into TiO₂ photoanode. The EDAX elemental mapping results inferred that CdS, CdSe and ZnS have efficiently covered the TiO₂ surface. TiO₂/CdS and CdS/CdSe interfaces and the amorphous nature of ZnS could be verified with HRTEM images. Hence, the co-sensitization and surface passivation played a significant role to enhance the PCE of CdS QDSSC from 1.9% to 4.05%.     

Improved conversion efficiency of Ag2S quantum dot-sensitized solar cells based on TiO2 nanotubes with a ZnO recombination barrier layer

We improve the conversion efficiency of Ag2S quantum dot (QD)-sensitized TiO2 nanotube-array electrodes by chemically depositing ZnO recombination barrier layer on plain TiO2 nanotube-array electrodes. The optical properties, structural properties, compositional analysis, and photoelectrochemistry properties of prepared electrodes have been investigated. It is found that for the prepared electrodes, with increasing the cycles of Ag2S deposition, the photocurrent density and the conversion efficiency increase. In addition, as compared to the Ag2S QD-sensitized TiO2 nanotube-array electrode without the ZnO layers, the conversion efficiency of the electrode with the ZnO layers increases significantly due to the formation of efficient recombination layer between the TiO2 nanotube array and electrolyte.     

ZnO photoanodes with different morphologies grown by electrochemical deposition and their dye-sensitized solar cell properties

In this article, we grew zinc oxide (ZnO) samples with different morphologies, e.g. film, nanowire and nanosheet, with electrochemical deposition (ECD) by controlling the precursor concentration and the growth mechanism was also discussed. The morphology influence on the photovoltaic conversion efficiency of the dye-sensitized solar cells (DSSC) assembled with different ZnO photoanodes was investigated by measuring current density–voltage (J–V) curve, quantum efficiency (QE) spectrum and electrochemical impedance spectrum (EIS). It was found that the DSSC constructed with ZnO nanowire array as photoanode can absorb more dye, improve the photon utilization rate and provide rapid collection channels for the photoexcited carriers. Therefore, the photovoltaic conversion efficiency of ZnO nanowire DSSC was improved.     

大小颗粒TiO_2复合制备纳晶薄膜电极及其性能研究

选用大小粒径分别为200nm和21nm的TiO2颗粒,采用刮涂法制备了几种不同条件的TiO2薄膜电极,研究了大小颗粒TiO2的复合方式和质量比对其所组装染料敏化太阳能电池光电性能的影响。应用红外吸收光谱仪和扫描电子显微镜对TiO2薄膜电极进行了表征,在100mW/cm2(AM 1.5G)光照下,测试了电池的光电性能。结果表明:将大颗粒TiO2作为光散射层,且大颗粒TiO2和小颗粒TiO2质量比为1∶3时,所制薄膜不但可以保持纳米粉体高比表面积的优点,同时可以提高对太阳光的散射率,用其组装的电池光电性能最好,转换效率达到2.46%。     

One dimensional nanostructure/nanoparticle composites as photoanodes for dye-sensitized solar cells

Dye-sensitized solar cells (DSCs) show potential as a low cost alternative to silicon solar cells. Power conversion efficiencies exceeding 12% have been achieved for DSCs. Typical DSCs are based on TiO(2) nanoparticle photoanodes, which have numerous grain boundaries, surface defects and trap states as electrons transport from one particle to the other. Such defects and trap states increase back charge transfer (charge recombination) from the photoanode to electrolyte. One dimensional (1D) nanostructures such as nanofibers, nanorods, nanowires, and nanotubes can offer direct and fast electron transport to the electron collecting electrode. However, these 1D nanostructures have a major disadvantage of having insufficient surface area and inefficient dye attachment. To solve this challenge, mixtures of TiO(2) nanoparticles and 1D nanostructures (e.g. nanofibers, nanorods, nanowires, and nanotubes) are used to take advantage of the large surface area of nanoparticles and efficient charge transport of 1D nanostructures. In this article, we review the recent developments in using mixtures of 1D nanostructures and nanoparticles as photoanodes for efficient DSCs. Various randomly oriented and vertically aligned 1D nanostructures and their composites with nanoparticles are discussed. Future increase of efficiency in DSCs using 1D nanostructure/nanoparticle composites will rely on the optimization of diameters of 1D nanostructures, control of ratios of 1D nanostructures and nanoparticles, increase of crystallinity, and reduction of surface defects on the 1D nanostructures. This work will provide guidance for designing and growing appropriate 1D nanostructures, and combining them with nanoparticles at an optimal ratio for efficient DSCs.     

Reduced charge recombination in a co-sensitized quantum dot solar cell with two different sizes of CdSe quantum dot

An efficient photoelectrode is fabricated by sequentially assembling 2.5 nm and 3.5 nm CdSe quantum dots (QDs) onto a TiO2 film. As revealed by UV-vis absorption spectroscopy, two sizes of CdSe QD can be effectively adsorbed on the TiO2 film. With a broader light absorption range and better coverage of CdSe QDs on the TiO2 film, a power conversion efficiency of 1.26% has been achieved for the TiO2/CdSe QD (2.5 nm)/CdSe QD (3.5 nm) cell under the illumination of one Sun (AM 1.5G, 100 mW cm(-2)). Electrochemical impedance spectroscopy shows that the electron lifetime for the device based on TiO2/CdSe QD (2.5 nm)/CdSe QD (3.5 nm) is longer than that for devices based on TiO2/CdSe QD (2.5 nm) and TiO2/CdSe QD (3.5 nm), indicating that the charge recombination at the interface is reduced by sensitizing with two kinds of CdSe QDs.     

CdSe nanocrystal sensitized ZnO core-shell nanorod array films: Preparation and photovoltaic properties

ZnO/CdSe core-shell nanorod array films were synthesized via a two-step method. ZnO nanorod array films were first grown on a TCO substrate, and then CdSe nanocrystals were deposited on the nanorods to form core-shell structured films. The resulting films were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV-vis absorption spectroscopy. Especially, dark-field images and transmission electron diffraction of the TEM were used to study the morphology and the chemical nanostructure of the ZnO/CdSe core-shell nanorods in detail. We investigated the photovoltaic performance of the resulting ZnO/CdSe core-shell nanorod array films as solar cell photoanodes. Parameters, such as the length of the ZnO nanorods, the shell phase structure and the deposition time of the CdSe nanocrystals were found to affect the photovoltaic performance of the solar cell. This study provides a facile method to prepare nanocomposite photoanodes of solar cells, and gives some insight about the fundamental mechanisms that improve the performance.