Improving the Photocurrent in Quantum-Dot-Sensitized Solar Cells by Employing Alloy PbxCd1-xS Quantum Dots as Photosensitizers

Ternary alloy Pb_xCd_1−xS quantum dots (QDs) were explored as photosensitizers for quantum-dot-sensitized solar cells (QDSCs). Alloy Pb_xCd_1−xS QDs (Pb_0.54Cd_0.46S, Pb_0.31Cd_0.69S, and Pb_0.24Cd_0.76S) were found to substantially improve the photocurrent of the solar cells compared to the single CdS or PbS QDs. Moreover, it was found that the photocurrent increases and the photovoltage decreases when the ratio of Pb in Pb_xCd_1−xS is increased. Without surface protecting layer deposition, the highest short-circuit current density reaches 20 mA/cm² under simulated AM 1.5 illumination (100 mW/cm²). After an additional CdS coating layer was deposited onto the Pb_xCd_1−xS electrode, the photovoltaic performance further improved, with a photocurrent of 22.6 mA/cm² and an efficiency of 3.2%.

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.

A study of optical absorption of cysteine-capped CdSe nanoclusters using first-principles calculations

Optical absorption of cysteine-capped CdSe nanoclusters varies with cluster size, ligands and solvents.

Toward highly efficient CdS/CdSe quantum dot-sensitized solar cells incorporating a fullerene hybrid-nanostructure counter electrode on transparent conductive substrates

The structural and catalytic properties of fullerene counter electrode for CdS/CdSe-QDSCs are systematically investigated. An efficiency of 4.18% is achieved.

CdS sensitized TiO2 photoanodes for quantum dot-sensitized solar cells by hydrothermal assisted chemical bath deposition and post-annealing treatment

A hydrothermal assisted chemical bath deposition method was adopted to directly deposit CdS quantum dots on TiO₂ photoanodes in QDSCs for the first time. An efficiency of 1.78% is achieved with appropriate post-annealing treatment.

Synthesis of honeycomb-like mesoporous pyrite FeS2 microspheres as efficient counter electrode in quantum dots sensitized solar cells

Honeycomb-like mesoporous pyrite FeS2 microspheres, with diameters of 500-800 nm and pore sizes of 25-30 nm, are synthesized by a simple solvothermal approach. The mesoporous FeS2 microspheres are demonstrated to be an outstanding counter electrode (CE) material in quantum dot sensitized solar cells (QDSSCs) for electrocatalyzing polysulfide electrolyte regeneration. The cell using mesoporous FeS2 microspheres as CE shows 86.6% enhancement in power conversion efficiency (PCE) than the cell using traditional noble Pt CE. Furthermore, it also shows 11.4% enhancement in PCE than the cell using solid FeS2 microspheres as CE, due to the mesoporous structure facilitating better contact with polysulfide electrolyte and fast diffusion of redox couple species in electrolyte.

Carbon fiber/Co9S8 nanotube arrays hybrid structures for flexible quantum dot-sensitized solar cells

Recently, hybrid carbon materials and inorganic nanocrystals have received an intensive amount of attention and have opened up an exciting new field in the design and fabrication of high-performance catalysts. Here we present a novel kind of hybrid counter electrode (CE) consisting of a carbon fiber (CF) and Co9S8 nanotube arrays (NTs) for fiber-shaped flexible quantum dot-sensitized solar cells (QDSSCs). The growth mechanisms of Co(CO3)0.35Cl0.20(OH)1.10 nanowire arrays (NWs) on the CFs were discussed, and the catalytic activity of the CF, Pt and Co9S8/CF hybrid structure (Co9S8@CF) were elucidated systematically as well. An absolute energy conversion efficiency of 3.79% has been demonstrated under 100 mW cm(-2) AM 1.5 illumination by using Co9S8@CF as a CE. This work not only demonstrates an innovative approach for growing cobalt sulfide NTs on flexible substrates that can be applied in flexible devices for energy harvesting and storage, but also provides a kind of hybrid structure and high-efficiency CE for QDSSCs.

Controlled (110) and (101) crystallographic plane growth of single crystalline rutile TiO2 nanorods by facile low cost chemical methods

A new approach has been employed to grow large rutile TiO₂ nanorods (NRs) with a high number density by low cost chemical methods.

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.

Spray deposition of water-soluble multiwall carbon nanotube and Cu2ZnSnSe4 nanoparticle composites as highly efficient counter electrodes in a quantum dot-sensitized solar cell system

In this paper, low-cost counter electrodes (CEs) based on water-soluble multiwall carbon nanotube (MWCNT) and Cu2ZnSnSe4 nanoparticle (CZTSe NP) composites have been successfully introduced into a quantum dot-sensitized solar cell (QDSC) system. Suitable surface modification allows the MWCNTs and CZTSe NPs to be homogeneously dispersed in water, facilitating the subsequent low-temperature spray deposition of high quality composite films with different composite ratios. The electrochemical catalytic activity of the composite CEs has been critically compared by electrochemical impedance spectroscopy and Tafel-polarization analysis. It is found that the composite CE at the MWCNT : CZTSe ratio of 0.1 offers the best performance, leading to an optimal solar cell efficiency of 4.60%, which is 50.8% higher than that of the Pt reference CE. The as-demonstrated higher catalytic activity of the composite CEs compared to their single components could be ascribed to the combination of the fast electron transport of the MWCNTs and the high catalytic activity of CZTSe NPs.

Investigation on new CuInS2/carbon composite counter electrodes for CdS/CdSe cosensitized solar cells

The search for semiconductor-sensitized solar cell (SSC) counter electrode alternatives has been a continuous effort and long ongoing work, while the studies in counter electrode kinetic performance and stability are important to improve the overall efficiency. Here, a ternary chalcopyrite compound CuInS2 is first employed as counter electrode (CE) material for CdS/CdSe cosensitized solar cells. Besides, in order to increase the electron transfer activity at the counter electrode/electrolyte interface and stability, an appropriate amount of active carbon/carbon black mixture is introduced to afford CuInS2/carbon composite electrodes. Electron transfer processes in CuInS2-based electrodes are investigated in detail with the aid of electrochemical impedance spectroscopy and I-E measurement. Up to 4.32% of the light-to-electricity conversion efficiency has been achieved for the CdS/CdSe SSCs with the CuInS2/carbon composite electrode. Besides, a preliminary long-term stability test reveals that the new CuInS2/carbon composite counter electrode exhibits good stability after being kept in the dark at room temperature and without current flow for 1000 h.

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

In the field of quantum dots (QDs)-sensitized solar cells, semiconductor QDs sensitizer with a moderate band gap is required in order to sufficiently match the solar spectrum and achieve efficient charge separation. At present, changing the size of QDs is the main method used for adjusting their band gap through quantum size effect, however, the pore sizes of mesoporous TiO2 film set a limit on the allowed size of QDs. Therefore, the tuning of electronic and optical properties by changing the particle size could be limited under some circumstances. In this paper, high-quality aqueous CdS(x)Se(1-x) QDs sensitizer is successfully synthesized and effectively deposited on a mesoporous TiO2 film by a one-step hydrothermal method. In addition to size, alloy QDs provide composition as an additional dimension for tailoring their electronic properties. The alloy composition and band gap can be precisely controlled by tuning the precursor (Se/Na2S·9H2O) ratio while maintaining the similar particle size. By using such CdS(x)Se(1-x) sensitized TiO2 films as photoanodes for solar cell, a maximum power conversion efficiency of 2.23% is achieved under one sun illumination (AM 1.5 G, 100 mW cm(-2)).

Panchromatic quantum-dot-sensitized solar cells based on a parallel tandem structure

A tandem-structure sensitized solar cell, comprising different inorganic semiconductor quantum dots (QDs) as sensitizers in two different compartments, has been designed for the first time with the aim of extending the light-absorption range of current technologies. In this system, the CdS/CdSe co-sensitized quantum-dot solar cell (QDSC) is in the upper part, whereas the PbS/CdS co-sensitized QDSC is in the lower part; these are connected in parallel with each other. In the middle of the tandem solar cell, a Cu2 S mesh counter electrode is employed. By optimizing the electrode thickness and QD-deposition time, short-circuit photocurrent density values of as high as 25.12 mA cm(-2) have been achieved; this value is nearly equal to the sum of the two constituent QD-sensitized devices and gives rise to a solar power-conversion efficiency of 5.06 %.

Composite counter electrode based on nanoparticulate PbS and carbon black: towards quantum dot-sensitized solar cells with both high efficiency and stability

PbS/carbon black (CB) composite counter electrode (CE) has been fabricated by a low cost and low temperature processable method using the wet chemistry synthesized PbS nanoparticles. The nanosized PbS in the composite CE provides a large area of catalytic sites, and the chain-type CB framework acts as an excellent electrical tunnel for fast electron transport from an external circuit to highly catalytic PbS nanoparticles. The optimized PbS/CB composite CE shows a charge transfer resistance (R(CT)) as low as 10.28 Ω cm², which is an order of magnitude lower than the value obtained in the previous study on pure PbS CE. The CdS/CdSe quantum dot-sensitized solar cells with the PbS/CB composite CE achieve a photovoltaic conversion efficiency of 3.91% and no degradation of the efficiency over 1000 h under room conditions.

Large-scale synthesis of Cu2SnS3 and Cu(1.8)S hierarchical microspheres as efficient counter electrode materials for quantum dot sensitized solar cells

Exploration of new catalytic semiconductors with novel structures as counter electrode materials is a promising approach to improve performances of quantum dot sensitized solar cells (QDSSCs). In this work, nearly mono-disperse tetragonal Cu(2)SnS(3) (CTS) and rhombohedral Cu(1.8)S hierarchical microspheres with nanometer-to-micrometer dimensions have been synthesized respectively via a simple solvothermal approach. These microspheres are also demonstrated as efficient counter electrode materials in solar cells using ZnO/ZnSe/CdSe nanocables as photoanode and polysulfide (S(n)(2-)/S(2-)) solution as electrolyte. While copper sulfide is regarded as one of the most effective counter electrode materials in QDSSCs, we demonstrate the CTS microspheres to show higher electrocatalytic activity for the reduction of polysulfide electrolyte than the Cu(1.8)S microspheres. This contributes to obvious enhancement of photocurrent density (J(SC)) and fill factor (FF). Power conversion efficiency (PCE) is significantly enhanced from 0.25% for the cell using a pure FTO (SnO(2):F) glass as counter electrode, to 3.65 and 4.06% for the cells using counter electrodes of FTO glasses coated respectively with Cu(1.8)S and CTS microspheres.