Roberto Gómez

Recombination in Quantum Dot Sensitized Solar Cells

Quantum dot sensitized solar cells (QDSCs) have attracted significant attention as promising third-generation photovoltaic devices. In the form of quantum dots (QDs), the semiconductor sensitizers have very useful and often tunable properties; moreover, their theoretical thermodynamic efficiency might be as high as 44%, better than the original 31% calculated ceiling. Unfortunately, the practical performance of these devices still lags behind that of dye-sensitized solar cells. In this Account, we summarize the strategies for depositing CdSe quantum dots on nanostructured mesoporous TiO(2) electrodes and discuss the methods that facilitate improvement in the performance and stability of QDSCs. One particularly significant factor for solar cells that use polysulfide electrolyte as the redox couple, which provides the best performance among QDSCs, is the passivation of the photoanode surface with a ZnS coating, which leads to a dramatic increase of photocurrents and efficiencies. However, these solar cells usually show a poor current-potential characteristic, so a general investigation of the recombination mechanisms is required for improvements. A physical model based on recombination through a monoenergetic TiO(2) surface state that takes into account the effect of the surface coverage has been developed to better understand the recombination mechanisms of QDSCs. The three main methods of QD adsorption on TiO(2) are (i) in situ growth of QDs by chemical bath deposition (CBD), (ii) deposition of presynthesized colloidal QDs by direct adsorption (DA), and (iii) deposition of presynthesized colloidal QDs by linker-assisted adsorption (LA). A systematic investigation by impedance spectroscopy of QDSCs prepared by these methods showed a decrease in the charge-transfer resistance and increased electron lifetimes for CBD samples; the same result was found after ZnS coating because of the covering of the TiO(2) surface. The increase of the lifetime with the ZnS treatment has also been checked independently by open-circuit potential (V(oc)) decay measurements. Despite the lower recombination rates by electron transfer to electrolyte as well as the higher light absorption of CBD samples, only a moderate increase of photocurrent compared with colloidal QD samples is obtained, indicating the presence of an additional, internal recombination pathway in the closely packed QD layer.

Improving the performance of colloidal quantum-dot-sensitized solar cells

Solar cells based on a mesoporous structure of TiO2 and the polysulfide redox electrolyte were prepared by direct adsorption of colloidal CdSe quantum dot light absorbers onto the oxide without any particular linker. Several factors cooperate to improve the performance of quantum-dot-sensitized solar cells: an open structure of the wide bandgap electron collector, which facilitates a higher covering of the internal surface with the sensitizer, a surface passivation of TiO2 to reduce recombination and improved counter electrode materials. As a result, solar cells of 1.83% efficiency under full 1 sun illumination intensity have been obtained. Despite a relatively large short circuit current (J(sc) = 7.13 mA cm(-2)) and open circuit voltage (V(oc) = 0.53 V), the colloidal quantum dot solar cell performance is still limited by a low fill factor of 0.50, which is believed to arise from charge transfer of photogenerated electrons to the aqueous electrolyte.

Study of the charge displacement at constant potential during CO adsorption on Pt(110) and Pt(111) electrodes in contact with a perchloric acid solution

Abstract A new electrochemical approach has been made, employing the current—time transient responses when a CO adlayer is formed at a platinum electrode at various controlled potentials where CO oxidation does not take place. The case of Pt(110) is compared with those of Pt(111) and Pt(111) disordered after ten cycles of oxygen adsorption—desorption. In order to avoid interference with anion-specific adsorption, the study was carried out in a perchloric acid solution. There is good agreement between the charge measured by voltammetry in the absence of CO and the charges measured during the current—time transients. This is indicative that the latter charges are produced by the displacement of the species at the interface as a result of CO adlayer formations. The sign of the current transient has been found to depend on the potential at which CO adsorption is carried out. This dependence may be related to the nature of species which are present in the interfacial region, providing new complementary information that voltammetry cannot yield.

The Electrochemistry of Nanostructured Titanium Dioxide Electrodes

Several of the multiple applications of titanium dioxide nanomaterials are directly related to the introduction or generation of charge carriers in the oxide. Thus, electrochemistry plays a central role in the understanding of the factors that must be controlled for the optimization of the material for each application. Herein, the main conceptual tools needed to address the study of the electrochemical properties of TiO(2) nanostructured electrodes are reviewed, as well as the electrochemical methods to prepare and modify them. Particular attention is paid to the dark electrochemical response of these nanomaterials and its direct connection with the TiO(2) electronic structure, interfacial area and grain boundary density. The physical bases for the generation of currents under illumination are also presented. Emphasis is placed on the fact that the kinetics of charge-carrier transfer to solution determines the sign and value of the photocurrent. Furthermore, methods for extracting kinetic information from open-circuit potential and photocurrent measurements are briefly presented. Some aspects of the combination of electrochemical and spectroscopic measurements are also dealt with. Finally, some of the applications of TiO(2) nanostructured samples derived from their electrochemical properties are concisely reviewed. Particular attention is paid to photocatalytic processes and, to a lesser extent, to photosynthetic reactions as well as to applications related to energy from the aspects of both saving (electrochromic layers) and accumulation (batteries). The use of TiO(2) nanomaterials in solar cells is not covered, as a number of reviews have been published addressing this issue.

Validity of double-layer charge-corrected voltammetry for assaying carbon monoxide coverages on ordered transition metals : comparisons with adlayer structures in electrochemical and ultrahigh vacuum environments

Abstract A coulometric procedure enabling the reliable and accurate evaluation of saturated CO coverages, θ sat CO , on Pt-group transition-metal electrodes is outlined, and applied to CO adlayers on ordered low-index platinum, rhodium, and iridium surfaces in acidic aqueous media. Along with voltammetric data, the method utilizes previously described measurements of the charge displaced upon CO adsorption. The reverse of this charge, Q dis , together with the “background” charge Q b flowing between a suitable pair of electrode potentials in the absence of CO, constitutes the overall “double-layer” correction Q dl to the total voltammetric charge Q tot measured for the electrooxidation of adsorbed CO between the same potentials. Significantly, the Q dis as well as the Q b component of Q dl typically constitutes moderate or even large corrections to Q tot , so that the deduced θ sat CO values are noticeably (20–30%) smaller than some voltammetric-based estimates reported earlier. However, the revised coulometric θ sat CO values are in consistently good agreement with the corresponding coverages obtained by means of an infrared spectrophotometric procedure. These θ sat CO values are compared with adlayer structural information obtained recently from in situ scanning tunneling microscopy along with infrared spectroscopy, and also with structural data for corresponding adlayers in ultrahigh vacuum (UHV). In most cases, the electrochemical and UHV-based θ sat CO values are not greatly different (within 5–10%), even though the CO binding site arrangements are often dissimilar in these two environments. The role of the electrode potential in affecting θ sat CO under some conditions via alterations in binding-site energetics, however, is noted for the Pt(111)/CO system.

Potentiostatic charge displacement by exchanging adsorbed species on Pt(111) electrodes—acidic electrolytes with specific anion adsorption

Abstract The voltammetric contribution of some specifically adsorbed anions (acetate, oxalate, chloride and bromide) is revised for the case of Pt(111) electrodes by means of experiments of displacement by CO in acidic medium. It is found that the usual states correspond to the reversible adsorption/desorption of hydrogen whereas the so-called unusual states would correspond to the adsorption/desorption of anions. This means that the maximum coverage of hydrogen adatoms prior to hydrogen evolution is around two-thirds of a complete monolayer of Pt(111). From the potential dependence of the voltammetric profiles the electrosorption valency of the different anionic species is calculated.

Effect of increasing amount of steps on the potential of zero total charge of Pt(111) electrodes

This paper reports results on the interaction of CO with Pt(s)[n(111)×(111)] electrodes in perchloric acid medium. Charge displacement experiments allow the estimation of the potential of zero total charge (pztc) values for this series of stepped surfaces. The pztc values decrease linearly with the step density for n≥5, while for shorter terraces a plateau is observed. Knowledge of the pztc is a key point in the evaluation of the CO coverage by means of coulometric measurements. Using this method, we have calculated the CO surface concentration at saturation for a series of stepped surfaces. The resulting values are similar regardless of the step density, but they are significantly lower than those for the Pt(111) basal plane.

Electrochemical behaviour of platinum surfaces containing (110) sites and the problem of the third oxidation peak

Abstract The voltammetric behaviour of flame-treated Pt(110) electrodes has been studied for different cooling atmospheres. It can be stated that avoiding thermally adsorbed oxygen during the cooling step gives rise to samples with the lowest density of surface defects. Electrochemical criteria for checking the cleanliness of this orientation have been proposed. They involve the voltammetric characteristics of both hydrogen and oxygen adsorption—desorption processes. The monitoring of surface cleanliness by the two adspecies is due to the ambiguous interpretation of hydrogen adsorption—desorption only, whose voltammetric profile is not affected significantly by surface contaminants, while the change of the hydrogen electrical charge could be ascribed to two independent origins, either contamination or reconstruction lifting. Then, the problem of the so-called peak III is treated by considering Pt(110) prepared with different surface conditions and the stepped surfaces containing (110) sites on terraces or steps, respectively. It may be concluded that formation of peak III occurs if surface domains of (110) sites with a minimum size are present. Conversely, (110) sites far apart on the surface cannot give rise to peak III. A mechanism for the formation of a well defined type of site at high hydrogen coverage is discussed which accounts for the irreversibility of the adsorption desorption process ascribed to peak III where hydrogen is the desorbing species.

On the voltammetric and spectroscopic characterization of nitric oxide adlayers formed from nitrous acid on Pt(h,k,l) and Rh(h,k,l) electrodes

Cyclic voltammetry and in situ FTIR spectroscopy have been employed to characterize NO adlayers at platinum and rhodium single-crystal electrodes. These adlayers, which are generated upon surface decomposition of nitrous acid, give infrared spectra similar to those observed for the same surface at high NO coverages under UHV conditions. The NO stretching frequency turned out to be potential dependent with a significant upward shift as the electrode potential increased. The analysis of the absolute spectra obtained for NO-covered Pt(111) and Rh(111) electrodes shows a linear variation of the band center frequency of linearly bonded NO with slopes of 65 and 20cm−1 V−1, respectively. In addition to this potential dependence of the NO stretching frequency, the spectra obtained with the Rh(111) electrode suggests the existence of a change from bridge to linearly bonded NO as the electrode potential increases. Parallel voltammetric experiments allowed the determination of the surface coverage.

Toward Antimony Selenide Sensitized Solar Cells: Efficient Charge Photogeneration at spiro-OMeTAD/Sb2Se3/Metal Oxide Heterojunctions

Photovoltaic devices comprising metal chalcogenide nanocrystals as light-harvesting components are emerging as a promising power-generation technology. Here, we report a strategy to evenly deposit Sb2Se3 nanoparticles on mesoporous TiO2 as confirmed by Raman spectroscopy, energy-dispersive X-ray spectrometry, and transmission electron microscopy. Detailed study of the interfacial charge transfer dynamics by means of transient absorption spectroscopy provides evidence of electron injection across the Sb2Se3/TiO2 interface upon illumination, which can be improved 3-fold by annealing at low temperatures. Following addition of the spiro-OMeTAD hole transporting material, regeneration yields exceeding 80% are achieved, and the lifetime of the charge separated species is found to be on the millisecond time scale (τ50% ∼ 50 ms). These findings are discussed with respect to the design of solid-state Sb2Se3 sensitized solar cells.