Effect of organic passivation on photoinduced electron transfer across the quantum dot/TiO2 interface

We report a study of the internal quantum efficiency (IQE) of CdSe quantum-dot (QD)-sensitized solar cells prepared by direct adsorption of pre-synthesized QDs, passivated with either tri-n-octylphosphine oxide (TOPO) or n-butylamine (BA), onto a nanocrystalline TiO(2) film.     

Raman and photoluminescence investigation of CdS/CdSe quantum dots on TiO2 nanoparticles with multi-walled carbon nanotubes and their application in solar cells

Raman spectroscopy and photoluminescence (PL) were used to investigate the improved short-circuit current density (J_SC) of CdS/CdSe quantum dot (QD)-sensitized solar cells with multi-walled carbon nanotubes (MWCNTs). Raman and PL experiments were carried out in order to explore the hot-electron and cold-electron injections, respectively. The experimental results showed that the concentration of MWCNTs influences the hot-electron and cold-electron injections from CdS/CdSe QDs to TiO₂ nanoparticles. Therefore, the improved J_SC in CdS/CdSe QD-sensitized solar cells can be explained as due to the better electron injections.     

Stamping transfer of a quantum dot interlayer for organic photovoltaic cells

An organophilic cadmium selenide (CdSe) quantum dot (QD) interlayer was prepared on the active layer in organic solar cells by a stamping transfer method. The mother substrate composed of a UV-cured film on a polycarbonate film with strong solvent resistance makes it possible to spin-coat QDs on it and dry transfer onto an active layer without damaging the active layer. The QD interlayers have been optimized by controlling the concentration of the QD solution. The coverage of QD particles on the active layer was verified by TEM analysis and fluorescence images. After insertion of the QD interlayer between the active layer and metal cathode, the photovoltaic performances of the organic solar cell were clearly enhanced. By ultraviolet photoelectron spectroscopy of CdSe QDs, it can be anticipated that the CdSe QD interlayer reduces charge recombination by blocking the holes moving to the cathode from the active layer and facilitating efficient collection of the electrons from the active layer to the cathode.     

A solid-state CdSe quantum dot sensitized solar cell based on a quaterthiophene as a hole transporting material

A hybrid quantum dot sensitized solar cell (QDSC) composed of CdSe quantum dots (QDs) as light harvesters and TiO(2) and 3,3'-didodecyl-quaterthiophene (QT12) as electron and hole conductors, respectively, has been fully processed in air. The sensitizer has been introduced into the TiO(2) nanoporous layer either by the successive ionic layer adsorption and reaction method or by attaching colloidal QDs either directly or through molecular cables (linkers). As previously observed for QDSCs based on liquid electrolytes, the efficiency depends on the way of QD attachment, the direct adsorption of QDs being the procedure yielding the best results. Thermal annealing was applied in order to enhance the device response under illumination. Remarkable open circuit potentials are attained (close to 1 V), leading to an efficiency of 0.34% (AM 1.5G) in initial tests. Although low, it ranks as one of the highest values reported for solid state QDSCs based on titanium dioxide and colloidal quantum dots.     

Controlling charge separation and recombination rates in CdSe/ZnS type I core-shell quantum dots by shell thicknesses

Type I core/shell quantum dots (QDs) have been shown to improve the stability and conversion efficiency of QD-sensitized solar cells compared to core only QDs. To understand how the shell thickness affects the solar cell performance, its effects on interfacial charge separation and recombination kinetics are investigated. These kinetics are measured in CdSe/ZnS type I core/shell QDs adsorbed with anthroquinone molecules (as electron acceptor) by time-resolved transient absorption spectroscopy. We show that the charge separation and recombination rates decrease exponentially with the shell thickness (d), k(d) = k(0)e(-βd), with exponential decay factors β of 0.35 ± 0.03 per ? and 0.91 ± 0.14 per ?, respectively. Model calculations show that these trends can be attributed to the exponential decrease of the 1S electron and hole densities at the QD surface with the shell thickness. The much steeper decrease in charge recombination rate results from a larger hole effective mass (than electron) in the ZnS shell. This finding suggests possible ways of optimizing the charge separation yield and lifetime by controlling the thickness and nature of the shell materials.     

Substrate- and time-dependent photoluminescence of quantum dots inside the ultrathin polymer LbL film

The photoluminescence of CdSe/ZnS quantum dots (QDs) in different configurations at solid surfaces (glass, silicon, PDMS, and metals) is considered for three types of organization: QDs directly adsorbed on solid surfaces, separated from the solid surface by a nanoscale polymer film with different thickness, and encapsulated into a polymer film. The complete suppression of photoluminescence for QDs on conductive metal surfaces (copper, gold) indicated a strong quenching effect. The temporal variation of the photoluminescent intensity on other substrates (glass, silicon, and PDMS) can be tuned by placing the nanoscale (3-50 nm) LbL polymer film between QDs and the substrate. The photooxidation and photobleaching processes of QD nanoparticles in the vicinity of the solid surface can be tuned by proper selection of the substrate and the dielectric nanoscale polymer film placed between the substrate and QDs. Moreover, the encapsulation of QD nanoparticles into the polymer film resulted in a dramatic initial increase in the photoemission intensity due to the accelerated photooxidation process. The phenomenon of enhanced photoemission of QDs encapsulated into the ultrathin polymer film provides not only the opportunity for making flexible, ultrathin, QD-containing polymer films, transferable to any microfabricated substrate, but also improved light emitting properties.     

Quantum-dot-sensitized solar cells fabricated by the combined process of the direct attachment of colloidal CdSe quantum dots having a ZnS glue layer and spray pyrolysis deposition

We were able to attach CdSe quantum dots (QDs) having a ZnS inorganic glue layer directly to a mesoporous TiO(2) (mp-TiO(2)) surface by spray coating and thermal annealing. Quantum-dot-sensitized solar cells based on CdSe QDs having ZnS as the inorganic glue layer could easily transport generated charge carriers because of the intimate bonding between CdSe and mp-TiO(2). The application of spray pyrolysis deposition (SPD) to obtain additional CdSe layers improved the performance characteristics to V(oc) = 0.45 V, J(sc) = 10.7 mA/cm(2), fill factor = 35.8%, and power conversion efficiency = 1.7%. Furthermore, ZnS post-treatment improved the device performance to V(oc) = 0.57 V, J(sc) = 11.2 mA/cm(2), fill factor = 35.4%, and power conversion efficiency = 2.2%.     

CdSe Quantum Dots and N719‐Dye Decorated Hierarchical TiO2 Nanorods for the Construction of Efficient Co‐Sensitized Solar Cells

Three‐dimensional hierarchical TiO₂ nanorods (HTNs) decorated with the N719 dye and 3‐mercaptopropionic or oleic acid capped CdSe quantum dots (QDs) in photoanodes for the construction of TiO₂ nanorod‐based efficient co‐sensitized solar cells are reported. These HTN co‐sensitized solar cells showed a maximum power‐conversion efficiency of 3.93 %, and a higher open‐circuit voltage and fill factor for the photoanode with 3‐mercaptopropionic acid capped CdSe QDs due to the strong electronic interactions between CdSe QDs, N719 dye and HTNs, and the superior light‐harvesting features of the HTNs. An electrochemical impedance analysis indicated that the superior charge‐collection efficiency and electron diffusion length of the CdSe QD‐coated HTNs improved the photovoltaic performance of these HTN co‐sensitized solar cells.     

Toward highly efficient CdS/CdSe quantum dots-sensitized solar cells incorporating ordered photoanodes on transparent conductive substrates

A series of ordered photoanodic architectures (including ordered TiO(2) nanotube arrays (TNT), ZnO nanorods, ZnO/TiO(2) core/shell nanostructures) for CdS/CdSe sensitized solar cells (QDSCs), were fabricated directly on transparent conductive oxide glasses by a facile sol-gel assisted template process. The morphologies, optical and electrical properties of TNTs and CdS/CdSe co-sensitized TNTs have been demonstrated. The effect of CdSe deposition time on the cell performance was clarified, and the growth mechanism of the CdSe quantum dots on the surface of the TNTs has been proposed as well. Furthermore, the evolution of open-circuit photovoltage (V(oc)) towards CdSe deposition time has been investigated by electrochemical impedance spectroscopy (EIS). A promising light-to-electricity conversion efficiency of up to 4.61% has been achieved with 3 μm long TNT arrays, which is the best record for sandwich-type ordered TNT-based QDSCs.     

CdSe quantum dot-dispersed DOBAMBC: an electro-optical study

Cadmium selenide quantum dot (CdSe QD) has been used as a dopant in ferroelectric liquid crystal (FLC) 2-methylbutyl 4-(4-decyloxybenzylideneamino) cinnamate (DOBAMBC). Effect of CdSe QD in DOBAMBC on its different electro-optical (E-O) properties has been studied in the SmC* phase. The optical micrographs recorded for the pure and composite material are showing good dispersion of QDs in the FLC matrix. Micrographs of unaligned sample cell revealed that CdSe QDs induce homeotropic alignment of FLC molecules. An appreciable change in the value of E-O parameters like tilt angle, spontaneous polarisation and response time with shifting of SmA–SmC* phase transition temperature has been observed for CdSe QD–DOBAMBC composite. The observed properties of composite system have been discussed on the basis of surface properties of QDs in FLC system.     

Fortification of CdSe quantum dots with graphene oxide. Excited state interactions and light energy conversion

Graphene based 2-D carbon nanostructures provide new opportunities to fortify semiconductor based light harvesting assemblies. Electron and energy transfer rates from photoexcited CdSe colloidal quantum dots (QDs) to graphene oxide (GO) and reduced graphene oxide (RGO) were isolated by analysis of excited state deactivation lifetimes as a function of degree of oxidation and charging in (R)GO. Apparent rate constants for energy and electron transfer determined for CdSe-GO composites were 5.5 × 10(8) and 6.7 × 10(8) s(-1), respectively. Additionally, incorporation of GO in colloidal CdSe QD films deposited on conducting glass electrodes was found to enhance the charge separation and electron conduction through the QD film, thus allowing three-dimensional sensitization. Photoanodes assembled from CdSe-graphene composites in quantum dot sensitized solar cells display improved photocurrent response (~150%) over those prepared without GO.     

Quantum dot solar cells. harvesting light energy with CdSe nanocrystals molecularly linked to mesoscopic TiO2 films

By using bifunctional surface modifiers (SH-R-COOH), CdSe quantum dots (QDs) have been assembled onto mesoscopic TiO(2) films. Upon visible light excitation, CdSe QDs inject electrons into TiO(2) nanocrystallites. Femtosecond transient absorption as well as emission quenching experiments confirm the injection from the excited state of CdSe QDs into TiO(2) nanoparticles. Electron transfer from the thermally relaxed s-state occurs over a wide range of rate constant values between 7.3 x 10(9) and 1.95 x 10(11) s(-1). The injected charge carriers in a CdSe-modified TiO(2) film can be collected at a conducting electrode to generate a photocurrent. The TiO(2)-CdSe composite, when employed as a photoanode in a photoelectrochemical cell, exhibits a photon-to-charge carrier generation efficiency of 12%. Significant loss of electrons occurs due to scattering as well as charge recombination at TiO(2)/CdSe interfaces and internal TiO(2) grain boundaries.     

Iridium-complex modified CdSe/ZnS quantum dots; a conceptual design for bi-functionality toward imaging and photosensitization

We report the design and synthesis of Ir-complex functionalized CdSe/ZnS quantum dots (QDs), in which the QD plays a key role in imaging, while the Ir-complex acts as a sensitizer to produce singlet oxygen; this conceptual design presents a novel scheme in both bio-imaging and photodynamic therapy.     

Graphene quantum dots assisted photovoltage and efficiency enhancement in CdSe quantum dot sensitized solar cells

CdSe quantum dot sensitized solar cells(QDSCs) modified with graphene quantum dots(GQDs) have been successfully achieved in this work for the first time. Satisfactorily, the optimized photovoltage(Voc) of the modified QDSCs was approximately 0.04 V higher than that of plain CdSe QDSCs, consequently improving the photovoltaic performance of the resulting QDSCs. Served as a novel coating on the CdSe QD sensitized photoanode, GQDs played a vital role in improving Vocdue to the suppressed charge recombination which has been confirmed by electron impedance spectroscopy as well as transient photovoltage decay measurements. Moreover, different adsorption sequences, concentration and deposition time of GQDs have also been systematically investigated to boost the power conversion efficiency(PCE) of CdSe QDSCs. After the coating of CdSe with GQDs, the resulting champion CdSe QDSCs exhibited an improved PCE of 6.59% under AM 1.5G full one sun illumination.     

Involvement of carriers in the size-dependent magnetic exchange for Mn:CdSe quantum dots

The magnetic behavior for Mn:CdSe (0.6%) quantum dots (QDs) exhibits size-dependent magnetic exchange mediated by the concentration of intrinsic carriers, which arise from surface states. High temperature paramagnetic behavior that can be fit to a Brillouin function with weak low temperature antiferromagnetic (AFM) coupling is observed for the large Mn:CdSe (5.0 and 5.8 nm) QDs. The 2.8 and 4.0 nm Mn:CdSe QDs display a size-independent blocking temperature (T(B)) at 12 K, decreasing coercivity with increasing size, and a lowering of the activation barrier for spin relaxation as the QD is increased in size. The magnetic behavior is inconsistent with classical domain theory behavior for a superparamagnet (SPM) but can be accounted for in a carrier-mediated RKKY model. Fitting the susceptibility data reveals a Pauli-paramagnetic (PPM) component that is believed to arise from the presence of carriers. The carrier density is observed to scale with the surface to volume ratio in the QDs, indicating the carriers arise from surface states that are weakly localized resulting in the onset of long-distance carrier-mediated RKKY exchange inducing overall ferrimagnetism in the Mn:CdSe QDs when the carrier concentration is above a critical threshold.     

Uncovering the role of the ZnS treatment in the performance of quantum dot sensitized solar cells

Among the third-generation photovoltaic devices, much attention is being paid to the so-called Quantum Dot sensitized Solar Cells (QDSCs). The currently poor performance of QDSCs seems to be efficiently patched by the ZnS treatment, increasing the output parameters of the devices, albeit its function remains rather unclear. Here new insights into the role of the ZnS layer on the QDSC performance are provided, revealing simultaneously the most active recombination pathways. Optical and AFM characterization confirms that the ZnS deposit covers, at least partially, both the TiO(2) nanoparticles and the QDs (CdSe). Photoanodes submitted to the ZnS treatment before and/or after the introduction of colloidal CdSe QDs were studied by electrochemical impedance spectroscopy, cyclic voltammetry and photocurrent experiments. The corresponding results prove that the passivation of the CdSe QDs rather than the blockage of the TiO(2) surface is the main factor leading to the efficiency improvement. In addition, a study of the ultrafast carrier dynamics by means of the Lens-Free Heterodyne Detection Transient Grating technique indicates that the ZnS shell also increases the rate of electron transfer. The dual role of the ZnS layer should be kept in mind in the quest for new modifiers for enhancing the performance of QDSCs.     

Photoluminescence of a freely suspended monolayer of quantum dots encapsulated into layer-by-layer films

A single monolayer of CdSe/ZnS quantum dots (QDs) has been encapsulated into a 60 nm free-suspended layer-by-layer (LbL) film. The QD monolayer showed a low light-emission within this film in contact with supporting solid substrates, but the manifold increase of photoluminescencence intensity was observed when the film was lifted and freely suspended over the microfabricated cylindrical cavities. This phenomenon was discussed in relationship with the effect of the elimination of the surface quenching enhanced by optical reflection from highly reflective silicon cavities. We suggest that a significant increase of the photoluminescence intensity of QD monolayers suspended over the microfabricated array can be interesting for future diagnostic and sensing applications.     

Electrochemical impedance spectra of CdSe quantume dots sensitized nanocrystalline TiO_2 solar cells

Quantum dots sensitized nanocrystalline TiO2 solar cells (QDSSCs) are promising third-generation photovoltaic devices.In comparison with conventional dye-sensitized solar cells (DSSCs),the efficiency of QDSSCs is still very low (about 3%).In this paper,the electrochemical impedance spectroscopy technology has been adopted to investigate the quasi-Fermi level and the carrier dynamics of the colloidal CdSe QDs sensitized TiO2 eletrode with S2-/Sxredox electrolytes and the series resistance of the QDSSCs.In co...