Molecular Linkers to QDs Sensitised Solar Cells

Molecular Linkers to QDs Sensitised Solar Cells

A key issue governing efficient electron transfer between two semiconductors is interfacial electronic energy alignment. The ability to tune electronic energy alignment of QDs allows one to optimize the band positions for efficient photon-to-current conversion [1-3]. Colloidal CdSe quantum dots (QDs) of different sizes, prepared by a solvothermal route, have been employed as sensitizers of nanostructured TiO₂ electrode based solar cells. Three different bi-functional linker molecules have been used to attach colloidal QDs to the TiO₂ surface: mercaptopropionic acid (MPA), thioglycolic acid (TGA), and cysteine. The linker molecule plays a determinant role in the solar cell performance, as illustrated by the fact that the incident photon to charge carrier generation efficiency (IPCE) could be improved by a factor of 5–6 by using cysteine with respect to MPA. The photovoltaic properties of QD sensitized electrodes have been characterized for both three-electrode and closed two-electrode solar cell configurations. For three-electrode measurement a maximum power conversion efficiency near 1% can be deduced, but this efficiency is halved in the closed cell configuration mainly due to the decrease of the fill factor (FF) [4].

We have investigated the role of linker molecules in quantum-dot-sensitized solar cells (QDSSCs) using density-functional theory (DFT) and experiments. Linkers not only govern the number of attached QDs but also influence charge separation, recombination, and transport. Understanding their behaviour is therefore not straightforward. DFT calculations show that mercaptopropionic acid (MPA) and cysteine (Cys) exhibit characteristic binding configurations on TiO₂ surfaces. This information is used to optimize the cell assembly process, yielding Cys-based cells that significantly outperform MPA cells, and reach power conversion efficiencies (PCE) as high as 2.7% under AM 1.5 illumination. Importantly, the structural information from theory also helps understand the cause for this improved performance [5].

CdSe nanoparticles (NPs) capped with cysteinate (Cys), 3-mer-captopropionate (MP), and mercaptosuccinate (MS) were adsorbed to TiO₂ from basic aqueous dispersions. Native capping groups served as molecular linkers to TiO₂. Thus, the materials-assembly chemistry was simplified and made more reproducible and environmentally benign. The electronic properties of CdSe and the electron-transfer reactivity at CdSe-linker-TiO₂ interfaces varied with the structure and functionality of the capping groups. Cys-capped CdSe NPs exhibited a narrow and intense first excitonic absorption band cantered at 422 nm, suggesting that they were magic-sized Nano crystals (MSCs) with diameters less than 2 nm. MP- and MS-capped CdSe NPs had broader and lower-energy absorption bands, which are typical of regular quantum dots. Photocurrent action spectra of Nano crystalline TiO₂ films functionalized with Cyst-CdSe, MP-CdSe, and MS-CdSe overlaid closely with absorption spectra, indicating that excitation of CdSe gave rise to the injection of electrons into TiO₂. Under white-light illumination, the global energy-conversion efficiency for Cyst-capped CdSe ((0.45 (0.11) %) was 1.2-to-6-fold greater than for MP-and MS-capped CdSe. Similarly, the absorbed photon-to-current efficiency was 1.3-to-3.3-fold greater. These differences arose from linker-dependent variations of electron-injection and charge-recombination reactivity. Transient absorption measurements indicated that electron injection from Cys-capped CdSe was more efficient than from MS-capped CdSe. In addition, charge recombination at CdSe-MS-TiO₂ interfaces was complete within hundreds of nanoseconds, whereas the charge-separated-state lifetime at CdSe-Cys-TiO₂ interfaces was on the order of several microseconds. Thus, Cys-capped CdSe MSCs are readily attached to TiO₂ and exhibit unusual electronic properties and desirable electron-transfer reactivity [6].

The efficiency of the QD sensitized solar cells fabricated with such assemblies also strongly depends on the linkers used and follows the trends observed for the charge transfer [7].

Adsorption of CdSe QDs on TiO₂

CdSe quantum dots (QDs) were introduced as an interlayer in the poly (3-hexylthiophene) (P3HT)/TiO₂ Nano rods arrays (NRAs) hybrid solar cells. The presence of CdSe QDs was found to provide enhanced light absorption, assisting charge separation at the P3HT/TiO₂ interface, and leading to a lower recombination rate of the electrons due to the stepwise structure of band edge in P3HT/CdSe/TiO₂, which accounts for the observed enhanced photocurrent and photo voltage of the hybrid solar cells. As a result, the efficiency of P3HT/CdSe QDs/TiO₂ NRAs hybrid cell is greatly higher than that of P3HT/TiO₂ NRAs based cell [8].

Three different types of solar cells that capitalize salient properties of semiconductor Nano crystals have emerged: (I) metal-semiconductor or Schottky junction photovoltaic cell, (ii) semiconductor nanostructure-polymer solar cell, and (iii) semiconductor sensitized quantum dot solar cell (Figure 1.1).

Figure 1.1 Schematic diagram showing the strategies to develop quantum dot (semiconductor Nano crystal) based solar cells: (a) metal-semiconductor junction, (b) polymer-semiconductor, and (c) semiconductor-semiconductor systems. Adapted from[8].

The electronic characteristics Size-employees of the QDs semiconductor are watched like one of the most attractive characteristics to obtain a gradient of the opening of the bandage in solar cells of the point of the quantum. The dependency of the tariff of the injection of the constant load as large as particle demonstrates the possibility of modulating the tariff of transference of the electron between CdSe and the TiO2 particles doing quantization of the size effects [9-10].

Picture 1.2 demonstrates the dependency of the opening of energy of the registry. Whereas the size of particle diminishes, we see a tariff of transference heightened of the electron. eV of the impelling force of0.8 is in or near the energy of the reorganization and therefore we counted on a normal region of Marcus of who the index of the transference of the electron increases with the impelling force. This small difference of the obtained particle energy diminishing so large is sufficient to increase the tariff of transference of the electron in almost 3 orders of the magnitude.

Figure 1.2 Left: The dependence of electron transfer rate constant on the energy difference between the conduction bands. Right: Scheme illustrating the principle of electron transfer from two different size CdSe quantum dots into TiO₂ (From ref 11.)

In the paper, we have prepared TiO2/MPA/CdSe/ZnS by the successive ionic adsorption and the reaction (SILAR) of the layer. The results demonstrate that a significant improvement of the light to the effectiveness of the conversion of the electrical energy of the point of the quantum sensitized the solar cells with an electrolyte of polysulfide which can be reached as to use the functional superficial modificantes of I SAW (SH-R-COOH, Mercaptopropionic acid (MPA)). The injected carriers of the load in a CdSe-modified film of TiO2/MPA can be gathered in an electrode that leads to generate photocurrent. The compound of TiO2/MPA/CdSe/ZnS, employee like anode of the photo in an electrochemical cell of the photo, exhibits a photon to load ç of the effectiveness of the carrier generation, present short circuit JSC, voltage VOICE of the opened circuit, and factor FF of the embankment corresponds under the parameters the 0.909%, 6.67 mA/cm², 0.43 V [12].

We have characterized the injection of the electron of the excited point’s photo of the quantum of the CDes (QDs) to nanoparticles TiO2 based on the separation of antiparticle within the bound assemblies molecular. The injection of the electron and the recombination Deidra of the load were characterized by the emission of stationary state that extinguished, the time-solve emission nanosecond, and the absorption of the passer-by of the nanosecond. The production of the injection of the electron diminished with the increase of the length of MAA chain and of the Inter separation of the particle. The injection of the electron happened in multiple calendars. A fast component (<10-8 s) explained to majority of the injection, whereas the rest happened in time scale of the microsecond. We attributed kinetic multi exponential of the injection to the transference of the electron of a range of the conduction-bandage and catcher states. The recombination Deidra of the load happened in time scale of the microsecond, and the kinetic one was independent of the length of MAA chain. Our results reveal that the ways of the decontamination of excite-bean and the dihedron electron-transfer the reactivity of assemblies tied of nanoparticles can systematically be tempering varying the Inter separation of the particle.

In order to sensitize the great semiconductors of the opening of the bandage such as TiO2 or ZnO with CdSe QDs, two main methods have been used:

1. Growth of the QDs directly in the great surface of the semiconductor of the opening of the bandage.
2. Bind colloidal QDs previously synthesized to a functional molecule.

The highest effectiveness’s of the cell have been obtained with the previous method. Niitsoo and other [13] disclosed an effectiveness of 2.8% under 1 illumination of the sun in a configuration of the three-electrode, because the chemical bath deposited the CDes/the porous solar cells sensitized CdSe TiO2. Diguada and other [14] obtained an effectiveness of the cell of 2,7% but now in a closed configuration of the cell (system of the two-electrode), even though in this case the obtained FFs is the poor men, under 0,45 stops more most of the samples. In both works, an electrolyte of polysulfide has been used as the pair’s redox to regenerate the photo excited the holes in the QDs [15-20].

The quantum of the semiconductor for example CdSe, PbSe, and InAs with its new opportunities of the bandage of the harmonious supply of the openings to harvest slight energy in the visible and infrared regions of the solar light. The early efforts demonstrated the capacity of chemically and electrochemical they deposited nanostructures of the CDes and CdSe in TiO2, Son2, and ZnO to generate visible slight the irradiation inferior photocurrent. Making use effects of quantization of the size, one can easily temper the answer of the photo of the electrode. We report here modulation of the Inter particle electron to transfer rate by varying the QD particle size. Femtosecond transient absorption measurements, elucidating the size-dependent electron injection rate from excited CdSe into TiO2 nanoparticles, plods described.

Figure 1A shows the absorption spectra of different sized CdSe particles in toluene. These particles prepared using a previously reported procedure exhibit absorption in the visible with a characteristic exciton absorption peak. By comparing the absorption behaviour to the size-dependent absorption curve, we estimated the particle diameter for five different size particles. As the particle size decreases from 7.5 to 2.4 nm, the first (1S3/21Se) exciton peak shifts (Figure 1.3 A) from 645 nm (1.92 eV) to 509 nm (2.44 eV). Controlling particle size therefore provides a convenient way to modulate the band energies and the energy of the charge carriers.

Figure 1.3 (A) Absorbance spectra of CdSe quantum dots in toluene. (Y-axis offset is introduced for clarity.) (B) Transient absorption spectra recorded 2 ps following the 387 nm laser pulse excitation of different size CdSe quantum dots in 1:1 ethanol/tetrahydrofuran (THF). Adapted from[21].

 Abundance of the electrons are at the moment four approaches available for the modulation of the transference. The first approach is studied in detail by Watson and the fellow workers and provides evidence that the nature of molecules of linker between the Nano-particles QDs and TiO2 of the CDes influences efficiency of the kinetic one and of and the transference diedra of the electron of excite-been. It was found that the tariff of the injection of the electron of CdSe QDs to TiO2 depends CdSe QDs as large as. The size of CdSe that controls QDs therefore provides an advisable way to modulate the energies of the bandage-opening. In the third approach, an alternating way to modulate the tariff of the injection of the electron is to temper the edge of the bandage of TiO2 (figure. 1.4). One has settled down that protonation induced pH of the superficial groups is useful to change of position the edge of the bandage of TiO2. With respect to molecules of the dye, the QDs is ideal to study pH lead kinetic of the transference of the electron due to the robustness of Wendish them of QDs towards increase of pH. The fourth strategy that is used to heighten the efficiency of the conversion and the photo bases on the molecular modification of the surfaces of the semiconductor. It was demonstrated that the molecular graft is long-range in tempering the levels of energy of QDs. Barea and others., demonstrated that the united molecular dipoles in QDs can control the injection and the recombination in QDSSCs [21-29].

Fig. 1.4: (a) Scheme describing the shift (DG) in the conduction-band of TiO₂ at different pH conditions. (b) Electron transfer rate versus DG. Data are taken with permission from [30-32].

The deposition of CdSe in the CDes CD-rich films that it was deposited first in the TiO2 film, or salinization of the CDes CD-rich leans with the solution of selenosulphate improves the parameters of the cell. The measures of the spectral response of Photocurrent indicate the losses photocurrent due to the poor effectivenesses of the collection, as it is hard by the spectral dependency of the intensity of the illumination. The effectiveness’s of the cell up to 2, 8% under solar conditions have obtained [33].

This paper aims to review the updated development of the aforesaid technologies applied to the catalytic production of hydrogen of the TiO2 photo. In agreement with the studies it disclosed in Literature, ion-implantation of the metal and the sensibilización of the dye is very effective methods to extend the phantom that it activates to visible range. Therefore, they play an important role in the development of catalytic production of photo-efficient hydrogen [34].

That it understands the adsorption of the point of the quantum of CdSe (QD) phenomena in the films microscopic TiO2 it is important to improve the operation of the solar cells sensitized point of the quantum (QDSSCs). A kinetic model of the adsorption has been developed to clarify both Langmuir-like secondary processes of the adsorption of monolayer and the aggregation of QD. The retirement of surface-limits oxide of trioctylphosphine as well as the use of acid mercaptopropionic 3 (MPA) as linker molecular it improved the adsorption of QDs toluene-suspended on the TiO2 films. The prolonged exhibition of a TiO2 film to a suspension of CdSe QD gave rise to the particle assembly added without concerning the adsorption method. A greater cover of TiO2 was reached with a smaller QDs due to the reduced pressures of the size. The spectroscopy of high speed transitory absorption demonstrated one more a faster injection of the electron in TiO2 de QDs directly fixed by adsorption (kET = 7, 2 109 s -1) compared with MPA-bound QDs (kET = 2, 3 109 s -1). Presented/displayed the kinetic details of the adsorption in this study are useful to control the adsorption of CdSe QD in TiO2 and to design the anodes of the photo efficiency of and for QDSSCs [35].

Molecular Attachment with TiO₂

Using the functional superficial modificantes of BI (SH-R-COOH), the points of the quantum of CdSe (QDs) have been mounted on the films microscopic TiO2. On visible the slight excitation, CdSe QDs injects electrons in the crystallites of TiO2 Nano. The transitory absorption of Femtosecond as well as the emission that extinguishes experiments confirms the injection of the excited state of CdSe QDs in nanoparticles TiO2. The transference of the thermal electron of s-been relaxed happens on an ample range of the constant values of the tariff between 7, 3 109 and 1, 95 1011 s-1. The injected carriers of the load in a CdSe-modified film TiO2 can be gathered in an electrode that leads to generate photocurrent. The compound of TiO2-CdSe, when it is used like anode of the photo in an electro-chemical cell of the photo, exhibits photon-to-load the effectiveness of the generation of carrier of 12%. The significant loss of electrons happens due to the dispersion as well as the recombination of the load in the interfaces and the internal limits of grain TiO2/CdSe [36].

Sensitized films dye. The crystalline films of Nano done of wide bandage open the semiconductors, such as TiO2, only respond in region UV. To use the dyes that absorb strongly in visible the slight phantom to sensitize the films is unidirectional to extend its answer of the photo. For example, in a crystalline film porous TiO2 de Nano, the effective superficial area can more be doubles 1000 than in great colloidal particles. This makes slight the absorption extreme-efficient, even with monolayer mere of the dye fixed by adsorption to each particle [37]. We mentioned on that front reaction that it promoted and the absorbencia the reagent providing suitable quality and the amount of active sites is the third main directorate that the investigation has taken to heighten operation 239 of the catalyst [of Nano]. It reviewed Literature with respect to this aspect. In 2011, they summarized the basic strategies for the engineering of the crystalline facet of the catalysts of the photo, particularly TiO2 crystals [38]. We referred to reader to its article for a discussion detailed on history, the basic strategies of the synthesis, the crystalline prediction of the form, and the catalytic activity of the photo of the several parameters of the synthesis. Of interest, of which they discuss much is intersected with the other directions of the investigation of the enhancement, demonstrated here in figure. 1.5:

Fig. 1.5: This diagram illustrates the interrelationships between crystal structure, surface chemistry, and size of Nano catalysts [40]. (Reproduced with permission from Royal Society of Chemistry 2011.)

The molecules also were found to be rough perpendiculars to (110) TiO2 surfaces. The proton lost of formic acid on the adsorption thinks to tie to tend a bridge on oxygen atoms to form oxhidrilo that tended a bridge on. Others suggested after the excessive layers of TMA the proton it is limited only weak the oxygen that tends a bridge on and can oscillate between two oxygen atoms [41-42].

Fig. 1.6: The three adsorption geometries deduced for format on the rutile TiO₂ (110) surface. A is the geometry thought to be adopted by the majority of adsorbed format, deduced from PhD and NEXAFS. B is a minority species inferred from NEXAFS and STM measurements and C from STM. Adapted from[43].

The adsorption of acid phosphonic in the (110) surfaces of anatase TiO2 (101) and of rutilo (has been investigated by means of calculations tight-that tied density-functional-based efficient. We studied the geometries and the energies of the adsorption of several models of the adsorption to reach the clarification of the discrepancy in finding experimental of a preferred obligatory state. In this paper we demonstrated that there are several structures of the adsorption probably to being present in the specific surfaces TiO2. Those structures have exclusively a Saw configuration. They have similar energies of the adsorption but diverse geometries. For the complexes of monodentate, we found a tendency strong of the geometry of the adsorption that relaxes towards the Saw coordination. Also, they perceivably have smaller energies of the adsorption. In addition, we extensively demonstrated the trustworthiness of the method of SCC-DFTB for this chemical system that opens the way for the studies of the adsorption in more complex materials of Titania [43].

The fast component of the injection with 4MBA was approximately as efficient as through acids mercaptoalkanoic short-chain. Our results reveal that the ways of the decontamination of excite-been and the dihedrons electron-transfer the reactivity of assemblies tied of NPs can be temperings varying the molecular characteristics of linkers [44].

Figure 1.7 shows the Ti 2p, O 1s, N 1s, and C 1s photoelectron spectra recorded from clean anatase TiO2(101) and anatase TiO2(101) following the adsorption of a monolayer (1 ML) of pABA at a photon energy (hν) of 1000 eV. These spectra were recorded on bending magnet beam line D1011. We define 1 ML in this case as saturation coverage, where all available 5-fold-coordinated Ti sites on the surface are occupied. The coverage is monitored from the relative intensity of the molecule-to-substrate O 1s signal.31 Figure 2a shows the Ti 2p spectra, which consist of two peaks due to spin−orbit splitting: the Ti 2p3/2 peak at a binding energy of 458.9 eV and the Ti 2p1/2 peak at 464.5 eV. The small peak at a binding energy of 457.4 eV is evidence of residual Ti3+, which for anatase TiO2 (101) is thought to arise from subsurface oxygen vacancies32 or possibly from step edges on the (101) surface.33 Following exposure to pABA, the Ti 2p spectrum shows no significant change apart from a rigid shift of 0.2 eV to lower binding energy, which is also observed in the O 1s spectra in Figure 2b, and a small reduction in the intensity of the Ti3+-derived peak. The shift is due to adsorb ate-induced upward band bending and has been observed following the adsorption of other molecules on the TiO2 anatase surface.18,34 The reduction in the intensity of the Ti3+-derived peak in the Ti 2p spectrum may suggest some degree of re-oxidation of surface Ti3+ upon adsorption of the amino acid, as has been observed following the adsorption of similar molecules.34 However, it is also possible that the reduction in the intensity of the Ti3+ peak in the Ti 2p spectrum relative to the Ti4+ peak is due to preferential adsorption of the molecule at O-vacancy sites, where Ti3+ is located [45-48].

Figure 1.7: Core-level synchrotron radiation photoemission spectra of a clean anatase TiO₂ (101) single crystal and the anatase TiO₂ (101) single crystal following the adsorption of ∼1 ML of PABA. (a) Ti 2p. Adapted from[48].

Dopamine Adsorbed on TiO₂

The results of the photoemission suggest it molecule of the dopamine fixed by adsorption to the surface to geometry of dentate, giving by result the retirement of the states of the opening of the bandage in the bandage of the TiO2 valence. Using the effect of the reflector, the phantoms of K-edge NEXAFS of the coal indicate that the fenilos ring in molecules of the dopamine normal is oriented to the surface. A combination of experimental results and calculation indicates the aspect of the new vacant states that appear after the adsorption. The possible paper of these states in load-transfers the mechanism of the system dopamine-TiO2 is discussed [49].

TiO2 is of technological interest in uses including catalysis of the photo, solar biomateriales, and cells photovoltaic of the novel. Many of these uses use TiO2 in a form of particulate of Nano. TiO2 is used like a semiconducting n-type in the dye-sensitized solar cells (DSSCs). Because TiO2 absorbs the light in region UV, in DSSCs the surface is covered with an organic molecule that absorbs in the visible region of the phantom. TiO2 is also highly biocompatible, that makes ideal for the use in medical uses. You implant Titanium have been very to be guessed right due to the TiO2 layer that the forms in its surfaces. Nanoparticles of TiO2-functionalized have studied for a number of biological and environmental uses including bioelectronics, the materials anti-incrust antes, and the bacteria of the slaughter. Dopamine, demonstrated in picture 1.30: it is of frequent use in these systems to modify the surfaces of nanoparticles TiO2 where it is used like molecule of the anchor to which other molecules, such as chains of polymer can be grafted. The molecules of the dopamine can also facilitate transference of the load between nanoparticles TiO2 and the biological system.

Chen and others. it studied a system able to inhibit the growth of the cell of particular the pathogenic bacteria (bacteria that cause diseases) when he was illuminated with light UV. This system is made up of nanoparticles with a base of oxide of the iron and a rind titanium of dioxide. The molecules of the dopamine one same-were mounted on the TiO2 rind and used to anchor succinic anhydride on the surface. This alternadamente was used to unite the inmunoglobulina G (IgG) to nanoparticles. Nanoparticles TiO2 with the DNA anchored on the surfaces of nanoparticle via the dopamine has studied for the uses of the recognition of the DNA. These systems could potentially provide the base of the sensors for the hibridación of the DNA (a technique of the recognition of the DNA) and for the detection of the proteins that tied to the DNA. In these potential biomedical uses, the direction and the stability of the adsorbent ones are clearly important.

Figure 1.9: (Left) Dopamine molecule with carbon atoms labeled from 1 to 8 for reference throughout the article. (Right) Geometry-optimized cluster model of dopamine on an anatase TiO₂ (101) surface. Red spheres represent O atoms, gravy spheres represent carbon atoms, small white spheres represent H atoms, blue spheres represent N atoms, and light-gravy spheres represent Ti atoms. Adapted from[49].

Catechol adsorption on TiO₂ is of interest as a light-harvesting molecule for solar cells. Catechol does not absorb light below 4.2 eV (300 nm), which is much larger than the 3.2 eV (370 nm) band gap of TiO₂. However, catechol-sensitized TiO₂ nanoparticles have an absorption onset of around 3 eV (420 nm). This shift has been found to differ depending on the specific catechol molecule used. The molecule-to-surface charge transfer in this system is thought to differ from that in DSSCs. In the DSSC systems mentioned above, the dye absorbs the incident photon, resulting in electron hole separation in the dye. The electron then undergoes rapid transfer to the TiO₂ conduction band. In the case of the catechol-TiO₂ system, however, it has been proposed by Persson et al. that instead of the electron being excited in the adsorb ate and then being transferred into the TiO₂ conduction band there is a direct catechol-to-TiO₂ charge transfer (i.e., the electron is directly photo injected from catechol into the conduction band of TiO₂ without the participation of excited states in catechol). This direct charge transfer is believed to be an excitation from the highest occupied π orbital in catechol to the Ti^4þ (3d) levels at the bottom of the conduction band of TiO₂ [49-54].

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