Energetic alignment in nontoxic SnS quantum dot-sensitized solar cell employing spiro-OMeTAD as the solid-state electrolyte

An environmentally friendly solid-state quantum dot sensitized solar cell (ss-QDSSC) was prepared by combining colloidal SnS QDs as the sensitizer and organic hole scavenger spiro-OMeTAD (2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene) as the solid-state electrolyte, and the energy alignment of SnS and TiO₂ was investigated. The bandgap of colloidal SnS QDs increased with decreasing particle size from 14 to 4 nm due to an upshift of the conduction band and a downshift of the valence band. In TiO₂/SnS heterojunctions, the conduction band minimum (CBM) difference between TiO₂ and SnS was as large as ～0.8 eV; this difference decreased with decreasing particle size, but was sufficient for electron injection from SnS nanoparticles of any size into TiO₂. Meanwhile, the sensitizer regeneration driving force, that is, the difference between the valence band maximum (VBM) of SnS and the work function of the electrolyte, showed an opposite behaviour with the SnS size due to a downward shift of the SnS VB. Consequently, smaller SnS QDs should result in a more efficient charge transfer in heterojunctions, revealing the advantages of QDs vs larger particles as sensitizers. This prediction was confirmed by the improved photovoltaic performance of ss-QDSSCs modified with SnS nanoparticles, which peaked for 5–6 nm sized SnS nanoparticles due to the balance between electron injection and sunlight absorption.     

Hybrid photovoltaic cells with II–VI quantum dot sensitizers fabricated by layer-by-layer deposition of water-soluble components

We present the fabrication of all solid state heterojunction photovoltaic devices consisting of TiO₂ films sensitized by colloidal CdSe and CdTe quantum dots (QDs) and a hole transport layer of the conjugated polymer poly(9,9-dioctyl-fluorene-co-N-(4-butylenphenyl)diphenylamine). The sensitized films were prepared by alternating the layer-by-layer deposition of TiO₂ nanoparticles, water-soluble semiconductor QDs and polycations. Photovoltaic devices sandwiched between fluorinated tin oxide and gold electrodes showed a high rectification ratio and photovoltages of up to 1.15 V. Effective sensitization was observed for CdSe QDs, while incorporated CdTe QDs apparently had no such effect. These findings are explained by confinement effects in QDs.     

Growth and photovoltaic performance of SnS quantum dots

Tin sulphide (SnS) quantum dots of size ranging from 2.4 to 14.4 nm are prepared by chemical precipitation method in aqueous media. Growth of the SnS particles is monitored by controlling the deposition time. Both XRD and SAED patterns confirm that the particles possess orthorhombic structure. The uncapped SnS particles showed secondary phases like Sn₂S₃ and SnS₂ which is visible in the SAED pattern. From the electrochemical characterization, HOMO–LUMO levels of both TiO₂ and SnS are determined and the band alignment is found to be favorable for electron transfer from SnS to TiO₂. Moreover, the HOMO–LUMO levels varied for different particle sizes. Solar cell is fabricated by sensitizing porous TiO₂ thin film with SnS QDs. Cell structure is characterized with and without buffer layer between FTO and TiO₂. Without the buffer layer, cell showed an open circuit voltage (V_oc) of 504 mV and short circuit current density (J_sc) of 2.3 mA/cm² under AM1.5 condition. The low fill factor of this structure (15%) is seen to be increased drastically to 51%, on the incorporation of the buffer layer. The cell characteristics are analyzed using two different size quantum dots.     

Surface characterization of GSH-CdTe quantum dots

The surface characterization of CdTe QDs synthesized by a novel procedure using glutathione (GSH), low temperatures (60–90 °C) and K₂TeO₃ as the –Te precursor is reported. Fluorescence of the produced QDs is stable in the pH range 6–13 and QDs inside eukaryotic cells are highly fluorescent. The surface composition of GSH-CdTe QDs with different spectroscopic properties and particle size distributions was determined by XPS. The XPS analysis indicated that the QDs are essentially CdTe, although all nanoparticles contain 12–24% of CdO (and in one case also TeO₂). GSH decomposes with reaction time releasing small amounts of S⁻² ions that react with Cd(Te) to yield Cd(Te)S in a smaller amount than that of CdTe. Finally, the use of QDs in fluorescence mediated immunodetection of bacterial pathogens has been evaluated.     

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.     

Fast-grown CdS quantum dots: Single-source precursor approach vs microwave route

A cross-disciplinary protocol of characterization by joint techniques enables one to closely compare chemical and physical properties of CdS quantum dots (QDs) grown by single source precursor methodology (SSPM) or by microwave synthetic route (MWSR). The results are discussed in relation with the synthesis protocols. The QD average sizes, reproducible as a function of the temperatures involved in the growth processes, range complementarily in 2.8–4.5 nm and 4.5–5.2 nm for SSPM and MWSR, respectively. Hexagonal and cubic structures after X-ray diffraction on SSPM and MWSR grown CdS QDs, respectively, are tentatively correlated to a better crystalline quality of the latter with respect to the further ones, suggested by (i) a remarkable stability of the MWSR grown QDs after exposure to air during several days and (ii) no evidence of their fragmentation during mass spectrometry (MS) analyses, after a fair agreement between size dispersities obtained by transmission electron microscopy (TEM) and MS, in contrast with the discrepancy found for the SSPM grown QDs. Correlatively, a better optical quality is suggested for the MWSR grown QDs by the resolution of n > 1 excitonic transitions in their absorption spectra. The QD average sizes obtained by TEM and deduced from MS are in overall agreement. This agreement is improved for the MWSR grown QDs, taking into account a prolate shape of the QDs also observed in the TEM images. For both series of samples, the excitonic responses vs the average sizes are consistent with the commonly admitted empirical energy-size correspondence. A low energy PL band is observed in the case of the SSPM grown QDs. Its decrease in intensity with QD size increase suggests a surface origin tentatively attributed to S vacancies. In the case of the MWSR grown QDs, the absence of this PL is tentatively correlated to an absence of S vacancies and therefore to the stable behavior observed when the QDs are exposed to air.     

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.     

Bandgap Tuning with Thermal Residual Stresses Induced in a Quantum Dot

Lattice distortion induced by residual stresses can alter electronic and mechanical properties of materials significantly. Herein, a novel way of the bandgap tuning in a quantum dot (QD) by lattice distortion is presented using 4‐nm‐sized CdS QDs grown on a TiO₂ particle as an application example. The bandgap tuning (from 2.74 eV to 2.49 eV) of a CdS QD is achieved by suitably adjusting the degree of lattice distortion in a QD via the tensile residual stresses which arise from the difference in thermal expansion coefficients between CdS and TiO₂. The idea of bandgap tuning is then applied to QD‐sensitized solar cells, achieving ≈60% increase in the power conversion efficiency by controlling the degree of thermal residual stress. Since the present methodology is not limited to a specific QD system, it will potentially pave a way to unexplored quantum effects in various QD‐based applications.     

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 %).     

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.     

Cosensitized Quantum Dot Solar Cells with Conversion Efficiency over 12%

The improvement of sunlight utilization is a fundamental approach for the construction of high‐efficiency quantum‐dot‐based solar cells (QDSCs). To boost light harvesting, cosensitized photoanodes are fabricated in this work by a sequential deposition of presynthesized Zn–Cu–In–Se (ZCISe) and CdSe quantum dots (QDs) on mesoporous TiO₂ films via the control of the interactions between QDs and TiO₂ films using 3‐mercaptopropionic acid bifunctional linkers. By the synergistic effect of ZCISe‐alloyed QDs with a wide light absorption range and CdSe QDs with a high extinction coefficient, the incident photon‐to‐electron conversion efficiency is significantly improved over single QD‐based QDSCs. It is found that the performance of cosensitized photoanodes can be optimized by adjusting the size of CdSe QDs introduced. In combination with titanium mesh supported mesoporous carbon as a counterelectrode and a modified polysulfide solution as an electrolyte, a champion power conversion efficiency up to 12.75% (V_oc = 0.752 V, J_sc = 27.39 mA cm^?2, FF = 0.619) is achieved, which is, as far as it is known, the highest efficiency for liquid‐junction QD‐based solar cells reported.     

Bifacial illuminated PbS quantum dot-sensitized solar cells with translucent CuS counter electrodes

This paper reports the optimization of the TiO₂ photoanode and the fabrication of bifacial illuminated PbS quantum dot-sensitized solar cells (QDSSCs) with translucent CuS counter electrodes. TiO₂ photoanode is prepared by introducing a compact TiO₂ layer between FTO substrate and TiO₂ film with titanium diisopropoxide bis(acetylacetonate) in 1-butanol and post-treatment with TiCl₄ aqueous solution; then, PbS quantum dots (QDs) are deposited on the surface of TiO₂ film by successive ionic layer adsorption and reaction method; also, by means of control of the TiO₂ surface charge, QD density of TiO₂ film is improved by adding triethanolamine into the cationic precursor solution. Using this optimized TiO₂ photoanode and a translucent CuS counter electrode, a bifacial illuminated PbS QDSSC is fabricated. The preparation conditions are optimized and the surface morphology and electrochemical properties of TiO₂ photoanodes are characterized. The bifacial illuminated PbS QDSSC achieves a power conversion efficiency of 2.16 %, which is increased by 48.97 % compared with the single illuminated QDSSC.     

Synthesis of highly luminescent CdTe/ZnO core/shell quantum dots in aqueous solution

The synthesis and photoluminescence (PL) properties of aqueous CdTe/ZnO core/shell quantum dots (QDs) have been investigated by using thiolglycolic acid as a capping reagent. The highlighted contribution of the present study was CdTe QDs coated with a ZnO shell by controlling the hydrolysis process of Zn(OAc)₂. The QDs benefitted from overcoming the high lattice mismatch between CdTe and ZnO. The PL peak wavelength of the CdTe/ZnO QDs with high PL quantum yields up to 88% was located in a range between 547 and 596 nm by adjusting the size of CdTe cores and the thickness of ZnO shells. The results of X-ray diffraction analysis and transmission electron microscopy observation indicate that the dot-shaped CdTe/ZnO QDs (566 nm) with an average size of 2.2 nm in diameter belong to the cubic CdTe crystal structure. Due to the passivation of surface defects, it is found that the luminescence decay curves accord with a biexponential decay model of exciton and trap radiation behavior. The average PL lifetimes of CdTe (571 nm) and CdTe/ZnO (596 nm) QDs at room temperature are 27.3 and 35.1 ns, respectively.     

Fluorescent and nonlinear optical features of CdTe quantum dots

The study of photoluminescence and nonlinear optical properties of red (emitted at 650 nm), yellow (emitted at 570 nm) and green (emitted at 530 nm) CdTe quantum dots (QD) spin coated on quartz substrate that had been prepared by changing the ratio between octadecylphosphonic acid and octadecence within 0.1:1–1:1 was carried out. Spectral width of the emission spectra indicates an enhancement with the increasing of QDs sizes, namely ca. 25, 28 and 50 nm for the QD size of 2.5, 3.5 and 5 nm, correspondingly. The entire QDs samples feature a spherical morphology with a relatively narrow size distribution. The optical second harmonic generation (SHG) stimulated by coherent bicolor treatment at 1,540 nm and its second harmonic generation was studied versus the laser light power density and incident angle.     

Photoluminescence studies of InAs/GaAs quantum dots covered by InGaAs layers

Photoluminescence (PL), PL excitation (PLE), and time-resolved PL were used to study effects of InGaAs layers on the optical properties of InAs/GaAs quantum dots (QDs). A rich fine structure in the excited states of confined excitons (up to n = 4 quantum states) was observed, providing useful information to study the quantum states in the InAs/GaAs QDs. A significant redshift of the PL peak energy for the QDs covered by InGaAs layers was observed, attributing to the decrease of the QD strain and the lowing of the quantum confinement.     

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 %).     

Simple indoline based donor–acceptor dye for high efficiency dye-sensitized solar cells

A simple metal-free donor–acceptor type sensitizer U01, bearing strong electron donor indoline-triphenylamine was synthesized for panchromatic sensitization of TiO₂ nanocrystalline film. Photovoltaic properties of U01 showed remarkably enhanced light harvesting due to the presence of strong electron donor and robust structure. The new U01 sensitized solar cell exhibited a photovoltaic performance: a short-circuit photocurrent density (J_sc) of 10.70 mA cm⁻², an open-circuit photovoltage (V_oc) of 0.758 V and a fill factor (FF) of 0.74, corresponding to an overall conversion efficiency of 6.01% under standard global AM 1.5 solar light condition. Our results suggest that indoline-triphenylamine based robust D–A molecular architecture is a highly promising class of panchromatic sensitizers for improvement of the performance of dye-sensitized solar cells (DSCs).     

TiO2 nanowires sensitized with CdS quantum dots and the surface photovoltage properties

TiO₂ nanowires prepared by thermal annealing of anodized Ti foil were sensitized with CdS quantum dots (QDs) via chemical bath deposition (CBD). Microstructural characterizations by SEM, TEM and XRD show that the CdS nanocrystals with the cubic structure have intimate contact to the TiO₂ nanowires. The amount of CdS QDs can be controlled by varying the CBD cycles. The experiment results demonstrate that the surface photovoltage (SPV) response intensity was significantly enhanced and the surface photovoltage response region was also expanded obviously for the TiO₂ NWs sensitized by CdS QDs.     

Hydrothermal growth of double-layer TiO2 nanostructure film for quantum dot sensitized solar cells

A double-layer (DL) film with a TiO₂ nanosheet-layer on a layer of TiO₂ nanorod-array, was synthesized on a transparent conductive fluorine-doped tin oxide substrate by a two-step hydrothermal method. Starting from the precursors of NaSeSO₃, CdSO₄ and the complex of N(CH₂COOK)₃, CdSe quantum dots (QDs) were grown on the DL-TiO₂ substrate by chemical bath deposition method. The samples were characterized by X-ray diffraction, Scanning electron microscopy, Energy dispersion spectroscopy, and their optical scattering property was measured by light reflection spectrometry. Some CdSe QDs sensitized DL-TiO₂ films serve as the photoanodes, were assembled into solar cell devices and their photovoltaic performance were also characterized. The short circuit current and open-circuit voltage of the solar cells range from 0.75 to 4.05 mA/cm² and 0.20 − 0.42 V under the illumination of one sun (AM1.5, 100 mW/cm²), respectively. The photocurrent density of the DL-TiO₂ film is five times higher than that of a bare TiO₂ nanorod array photoelectrode cell.     

The electromagnetic properties of the Mn doped Ge/Si quantum dots diodes

We present the electromagnetic properties of Mn doped Ge quantum dots (QDs)/Si electromagnetic diode. The Ge:Mn QDs were grown with a GeH₄/Ar mixed gas under a constant flow at 500 °C by means of a plasma enhanced chemical vapor deposition (PECVD) process. They were then doped with different concentrations of Mn using a magnetron sputtering technique and annealed at 600 °C. The Ge:Mn QD samples show wildly open smooth hysteresis loops. The remnant magnetization M_r and saturation magnetic intensity M_s are functions of the doping concentration of Mn. The electromagnetic diodes fabricated in this way exhibit perfect electromagnetic, current–voltage (I–V) and capacitance–voltage (C–V) properties. The largest voltage and magnetic resistance differences with and without magnetic field are up to 4 V and 169 kΩ, respectively. These electromagnetic properties of the Ge_1?xMn_x QDs/Si diodes can be used to make various electromagnetic devices, including switches and storages devices.