Article

# Energy Level Alignment in CdS Quantum Dot Sensitized Solar Cells Using Molecular Dipoles

Authors:
• Applied Materials, Rehovot, Israel
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## Abstract

The energy levels of CdS quantum dots (QDs) can be shifted in a systematic fashion with respect to the TiO(2) bands using molecular dipoles. Dipole moments pointing toward the QD surface shift the energy levels toward the vacuum level (a), thus enabling electron injection from excited QD states into the TiO(2) conduction band at lower photon energies compared to QDs with adsorbed molecular dipoles which are pointing away from the QD surface (b). In CdS QD sensitized solar cells this leads to a dipole dependent shift of the photovoltage onset and the photocurrent.

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... Similar qualitative trends were observed for CdS and PbS QDs coordinated by benzenethiol derivatives. [73,74] However, dipoles, despite representing a simple, effective description of the ligands inducing a shift of the QD energy levels, [19,[71][72][73] cannot explain the concomitant ligand-induced broadband enhancement of the QD absorption. As already discussed for the changes of both the QD optical band gap and absorption coefficients, purely electrostatic arguments do not provide a comprehensive explanation of the effects exerted by the ligands on the overall QD electronic structure. ...
... Similar qualitative trends were observed for CdS and PbS QDs coordinated by benzenethiol derivatives. [73,74] However, dipoles, despite representing a simple, effective description of the ligands inducing a shift of the QD energy levels, [19,[71][72][73] cannot explain the concomitant ligand-induced broadband enhancement of the QD absorption. As already discussed for the changes of both the QD optical band gap and absorption coefficients, purely electrostatic arguments do not provide a comprehensive explanation of the effects exerted by the ligands on the overall QD electronic structure. ...
Article
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At the size scale at which quantum confinement effects arise in inorganic semiconductors, the materials’ surface‐to‐volume ratio is intrinsically high. This consideration sets surface chemistry as a powerful tool to exert further control on the electronic structure of the inorganic semiconductors. Among the materials that experience the quantum confinement regime, those prepared via colloidal synthetic procedures (the colloidal quantum dots –and wires and wells, too–) are prone to undergo surface reactions in the solution phase and thus represent an ideal framework to study the ensemble impact of surface chemistry on the materials’ electronic structure. It is here discussed such an impact at the ground state by using the absorption spectrum of the colloidal quantum dots as a descriptor. The experiments show that the chemical species (the ligands) at the colloidal quantum dot surface induce changes to the optical band gap, the absorption coefficient at all wavelengths, and the ionization potential. These evidences point to a description of the colloidal quantum dot (the ligand/core adduct) as an indecomposable species, in which the orbitals localized on the ligands and the core mix in each other’s electric field. This description goes beyond conventional models that conceive the ligands on the basis of pure electrostatic arguments (i.e., either as a dielectric shell or as electric dipoles) or as a mere potential energy barrier at the core boundaries.
... The bulk CdS band gap is 2.42eV. Photo sensitization is due to absorption of photon energy which leads to generation of photoexcited electron and hole in the layer of Quantum dots [139]. ...
Thesis
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In this thesis, photovoltaic redox reactions have been realized through polyurethane ionomer in Quantum dot sensitized solar cells. Thermoplastic polyurethanes are composed of hard segments and soft segments contents. Electrolyte active pendant groups have been introduced on different chemical environment of hard segment contents. Free structured polyurethane ionomer has been realized as electrolyte matrix for photovoltaic reaction in Quantum dot sensitized solar cell. In other words, semiconducting behaviour was realized through variation of chain structure on polyurethane ionomer backbone. Electrolyte active redox groups played a crucial role in photovoltaic reactions. Functionalized redox active urethane linkages function as redox mediating center for conversion of solar energy through Quantum dot sensitized solar cells. Quantum confinement regulates energy gap and energy levels in polyurethane ionomer. Polyurethane ionomer exhibits ionic conductivity because of variation of composition and density of pendant groups between hard segments and soft segments. In this thesis, we have focused on functionalization of hard segment content within polyurethane chain. Photovoltaic redox mediation has been explored via sulfonated polyurethane with different chemical environments in Quantum dot sensitized solar cells. Functionalized polyurethanes have been attracted great attention towards development of ionomer gel electrolyte (matrix) because of corrosion resistant, efficient adhesive nature and better electrochemical stability. Semiconducting behaviour has been achieved due to presence of ionomeric unit (pendant group) on hard segment content. Thus, various polyurethane ionomers have been studied via structural variation across hard segment contents to investigate the solar characteristic performance. The major concluding remarks of the thesis are explained below chapter wise. In chapter 3 Polyurethane has been synthesized through reaction of (HMDI+PTMG+EDA) at hard segment contents of 30%. Polyurethane hard segments contents (urethane and urea linkages) have been functionalized with sulfonating agents (NaH+ϒ-propane sultone) to generate ionic segments contain mononegative charge. In practice, ionic segment contains Na+ as counter ion. The degree of hard segment functionalization increases with increase in weight ratio of sulfonating agents. Various spectroscopic technologies have been used to investigate structural and functional features. Polyurethane ionomer (sulfonated polyurethane) showed almost spherical size and texture in TEM micrograph. Electrolyte active group embedded polyurethane showed spherical atmosphere due to possible minimization of surface energy. Thermal resistance property was enhanced because of presence of covalently linked ionic segments. The introduction of ionic pendant group causing crystalline regions in the polyurethane ionomers. The variation of functionalization has been proved through UV-visible absorption spectra. The functionalized polyurethane showed red shifted absorption band and absorption peak shifted due to variation of HOMO-LUMO energy gap. HOMO-LUMO energy gaps and energy levels were controlled through optimization of functionalizing agents. Pristine polyurethane showed electrochemical inactive features. However, ionic segment offers charged redox active center to polyurethane chain and redox active behaviour was characterized with clear signature of oxidation and reduction peaks. Hydrophilic pendant group offered electrical regions on the surface of polyurethane chain. Solution phase ionic conductivity has been studied via tuning the composition of electrolyte active pendant anion across polyurethane hard segments. Ionomer gel electrolyte was prepared in a mixture of highly polar organic solvents. Disodium salt of ethylene diamine tetraaceticacid stabilized the surface structure of CdS QDs. Quantum confinements effect were investigated through blue shifted absorption spectra of synthesized CdS particle. Particle size was analysed through DLS and TEM measurements. The average size of 4 nm and band gap (Eg = 2.69eV) were established in CdS particle. The photovoltaic device was fabricated through layer structure design using spin coating and doctor blade technique. The fabricated device (QDSSC) showed photovoltaic characteristic curve and photovoltaic reaction was realized through measurement of photocurrent density and open circuit potential. The electrolyte features was realized in the open circuit potential range (0.45-0.65V) in QDSS cell. The optimized photovoltaic conversion efficiency was analysed 1.25% with well structural stabilization efficiency in ionomer electrolyte. In chapter 4 polyurethanes have been synthesized through structural variation of chain extenders (diamine and diol based) at constant HSC of 30%. Electrolyte activity was explored via creating structural, functional and size difference in polyurethane ionomers. Oxygenic rich polyurethane was also synthesized using PCL-diol in place of PTMG during polymerization reaction. The functionalization reaction was optimized with constant weight ratio of sulfonating agents. The ionomeric segments were generated on different hard segment contents of polyurethane chain. The degree of ionization level was characterized using NMR, FTIR and UV-visible absorption spectra. The constant weight ratio of sulfonating agents offered different energy gap (HMO-LUMO) which was probably due to different degree of reactivity of hard segments in polyurethane chains. Thermal resistance and glass transition temperature were characterized with TGA and DSC measurements. Hydrophilic ionic segments (pendant group) offered different degree of ionic conductivity because of different degree of segmental motion and structural movements of polyurethane ionomer chains. The lifetime of free electrons were measured on the different surface of functionalized hard segments of polyurethane ionomers. Ionomer gel electrolytes were prepared using 20% and 30% (w/v) polyurethane ionomers in mixture of highly polar solvents. 3-mercaptopropeonic acid functionalized the surface of CdS particles. Size confinement effects were analyzed through blue shifted absorption spectra. The optimized band gap (Eg = 2.71eV) and average size 12-15 nm were stabilized with 0.65M MPA. Solar device has been fabricated using spin coating and doctor blade techniques. The ionomer gel electrolyte was sand whiched between surface treated (PANi or SGO coated photoanode) and counter electrodes. Photovoltaic characteristic curves were obtained through J-V measurements under illumination of 100 mW/cm2 intense White LED light. Photovoltaic effect was realized through redox reaction of ionomer gel electrolyte in QDSS cells. The QDSS cells open circuit potentials were analyzed in the range of 0.2 - 0.65V for different ionomer electrolytes. The optimized device (QDSSC) showed maximum power conversion efficiency of 1.16% with carbon black decorated polyurethane ionomer gel. In chapter 5 GO implanted polyurethane ionomers have been developed as gel electrolytes with content 0.2%, 0.5% and 1% GO. The optimized PUI-GO (0.5%) has been studied extensively in QDSSCs. The gel polyelectrolyte activity was realized due to presence of pendant anions (sulfonate and carboxylate ion) linked on composite polyurethane backbone. The GO implanted ionomers were characterized with 1H NMR, FT-IR, TGA, DSC and UV-visible spectroscopy. Electrochemical characteristics have been studied with the help of CV and EIS measurements. The surface morphologies (size, surface texture, chemical interaction and interfacial wettability) have been investigated with SEM, AFM and TEM measurements. Structure-function characteristic features have been correlated with surface morphology. RGO has been coated on conductive surface of FTO to improve electron injection and transport properties. Quantum dots sensitized solar cell has been fabricated with MPA caped CdS Quantum dots as photosensitizer. The polyelectrolyte activity of developed structure was studied comparatively with pristine polyurethane ionomer (PUI). The optimum GO content improved the electrical regions in polyurethane ionomer. The fabricated Quantum dot sensitized solar cell consist a configuration FTO-RGO/TiO2/MPA-CdS/PUI-GO/FTO-Pt for photovoltaic characterization. The optimized QDSS cell showed conversion efficiency of 1.63% with open circuit potential of 0.594V with efficient passivation (retards charge recombination) and redox activity between photoanode and counter electrode.
... 72,73 On the other hand, several reports made on QDSSC architectures, where a similar dipolar capping of the QDs was employed, revealed a null effect on modulating V oc . 206,207 Based on the combined analysis of interfacial dynamics at QD−MO interfaces by TRTS and interfacial energetics by UPS, Wang et al. explained this apparent contradiction by revealing a lack of work function modulation induced by "QD dipolar capping" due to Fermi level pinning at the strongly coupled QD−MO interface. This conclusion does not mean that the idea of employing dipolar capping to control the Fermi level and the relative band alignment is unfeasible at QD−MO interfaces, but it would require the prevention of the Fermi level pinning at the interface. ...
Article
Full-text available
Electron transfer at a donor-acceptor quantum dot-metal oxide interface is a process fundamentally relevant to solar energy conversion architectures as, e.g., sensitized solar cells and solar fuels schemes. As kinetic competition at these technologically relevant interfaces largely determines device performance, this Review surveys several aspects linking electron transfer dynamics and device efficiency; this correlation is done for systems aiming for efficiencies up to and above the ∼33% efficiency limit set by Shockley and Queisser for single gap devices. Furthermore, we critically comment on common pitfalls associated with the interpretation of kinetic data obtained from current methodologies and experimental approaches, and finally, we highlight works that, to our judgment, have contributed to a better understanding of the fundamentals governing electron transfer at quantum dot-metal oxide interfaces.
... In solar cell applications, semiconductor materials are used for light absorption [8][9][10]. One of the most essential ways of using a quantum dot as light absorber material in photovoltaic devices is due to their ability to inject charge carriers between titanium dioxide (i.e., introducing electrons) and electrolyte (i.e., introducing holes) to understand the significance of energy level arrangements [11]. II-VI semiconductor quantum dots have received great interest due to their remarkable optical qualities such as low half peak width, high brightness, tunable emission, and broad absorption properties [12]. ...
Article
Full-text available
Cadmium Sulfide quantum dots were prepared in three solvents such as distilled water, ethanol, and isopropyl alcohol respectively. Solvent dependence of cadmium sulfide quantum dots was investigated under XRD, TEM, UV, PL, and J–V characteristics. The results showed CdS quantum dots with sizes ranging from 7.5, 8.2, to 9 nm which enabled the control of the optical properties and consequently the solar cell performance. The optical absorption spectrum was observed near the ultra-violet region. Absorbance spectra reveal that there is a blue shift in the absorbance as the quantum dot size decreases due to the quantum confinement effect. For photoluminescence spectra, the emission peaks were observed at 430 nm, 468 nm, and 470 nm respectively. Among the solvents, cadmium sulfide quantum dots prepared with ethanol has the highest power conversion efficiency of 0.68%.
... In solar cell applications, semiconductor materials are used for light absorption [7][8][9]. One of the most essential ways of using a quantum dot as light absorber material in photovoltaic devices is due to their ability to inject charge carriers between titanium dioxide (i.e., introducing electrons) and electrolyte (i.e., introducing holes) to understand the signi cance of energy level arrangements [10]. Comparatively less attention was provided for inorganic semiconductor materials like quantum dots (eg: CdS) [11,12] because the deposition of quantum dots on the surface of mesoporous titanium dioxide (m-TiO 2 ) is very less so it is necessary to increase the deposition of quantum dots and it can be done by self-assembly binding method [13,14]. ...
Preprint
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Cadmium Sulfide quantum dots were prepared in three solvents such as distilled water, ethanol, and isopropyl alcohol respectively. Solvent dependence of cadmium sulfide quantum dots was investigated under XRD, TEM, UV, PL, and I-V characteristics. The results showed CdS quantum dots with sizes ranging from 7.5 nm, 8.2 nm, and 9 nm which enabled the control of the optical properties and consequently the solar cell performance. The optical absorption spectrum was observed near the ultra-violet region. Absorbance spectra reveal that there is a blue shift in the absorbance as the quantum dot size decreases due to the quantum confinement effect. For photoluminescence spectra, the emission peaks were observed at 430 nm, 468 nm, and 470 nm respectively. Among the solvents, cadmium sulfide quantum dots prepared with ethanol has the highest power conversion efficiency of 0.68%.
... The light absorption in QDSCs is mainly performed by one or several successive, different QDs layers (Zhengji et al., 2012;Liua et al., 2020;Emin et al., 2011;Esparza et al., 2015;Gonzalez-Pedro et al., 2010), CdS (Zhang et al., 2012;Toyoda et al., 2013), CdSe Liua et al., 2020;Toyoda et al., 2013;Shalom et al., 2009), PbS (Liu and Kamat, 1993;Sh Pan et al., 2017;Ahangarani Farahani and Marandi, 2017), ZnSe (Xinga et al., 2020;Asgari Fard and Dehghani, 2019), CdTe Huang et al., 2016;Mobedi et al., 2014) and other QDs have been explored for sensitization/co-sensitization of the photoelectrodes. The power conversion efficiencies were achieved in the range of 0.53%-12% (Nideep et al., 2019;Liu and Yu, 2009;Gopi et al., 2016Gopi et al., , 2015Yang et al., 2015) and 2.8%-10% (Sh Liua et al., 2020;Chen et al., 2014aChen et al., , 2014bZhou et al., 2016;Yuan et al., 2016;Mu et al., 2013) for the single QDs sensitization or co-application of several QDs layers, respectively. ...
Article
In this research CdSe0.4Te0.6 NCs were synthesized in aqueous solution through a cheap, simple, and modified hot injection chemical precipitation method. The reflux time was altered in a wide range for the synthesis of these NCs with different size and bandgap energies. Then they were applied as a co-sensitizing layer together with CdS NCs film in the photoanod of quantum dot sensitized solar cells (QDSCs). It was shown that the QDSC with TiO2 NCs/CdSeTe(7 h)/CdS/ZnS photoelectrode demonstrated the maximum power conversion efficiency (PCE) of 4.2% in AM1.5 solar irradiation. The selected CdSeTe NCs prepared in 7 h of the reflux time were utilized in a second growth process to form core–shell CdSeTe-CdS NCs. The shell formation time was widely altered and NCs were applied in the photoanod of corresponding QDSCs. It was demonstrated that the cell with TiO2 NCs/CdSeTe(7 h)-CdS(100 min)/CdS/ZnS photoelectrode revealed the best efficiency of 4.6% in the experiments. This was due to the surface passivation of the sensitizing NCs and better charge collection efficiency.
... TiO 2 NCs (Tian et al., 2013), TiO 2 nanorods (NRs) Zhang et al., 2016; and large size TiO 2 spheres or hollow spheres (HSs) . There have been various semiconductor quantum dots were applied and reported on the photoelectrodes as sensitizer, including CdS Ahangarani Farahani and Marandi, 2017;Shalom et al., 2009), Ag 2 S Liu et al., 2013;, CdSe (Lee and Lo, 2009;Okur et al., 2010), CdTe , CdSeTe , Sb 2 S 3 (Choi et al., 2014;Zhang et al., 2013), PbS (Ahangarani Bakueva et al., 2004), PbSe (Kitada et al., 2009;Park and Yoon, 2010), ZnSe (Karanikolos et al., 2006;Seol et al., 2010) and other nanocrystals (NCs). Besides, some articles are reported based on using two or more kinds of semiconductor NCs as sensitizer, for example, CdS/CdSe (Lee and Lo, 2009), CdS/CdTe ; CdSe/CdTe , CdS/PbS (Ahangarani , CdS/CdSe/ZnS (Chang and Lee, 2007), using different methods of deposition of quantum dots on the photoelectrodes Tubtimtae et al., 2011). ...
Article
Incident photon-to-current conversion efficiency (IPCE) of CdS based quantum dot sensitized solar cells (QDSCs) is a major drawback in their low efficiency. Here we report a new trend in fabrication of QDSC with enhanced IPCE with application of as prepared Ag2S and Ag2[email protected] QDs as a co sensitizer. Ag2S and Ag2[email protected] QDs were synthesized first using a microwave method and were examined by means of XRD, TEM and UV–Vis spectroscopy. Then the prepared QDs were applied for the fabrication of QDSCs using a novel drop casting method. The results revealed a significant enhancement of IPCE as well as short current density (12.5 mA·cm⁻²) and cell efficiency (3%). The enhancement is directly related to well absorption of Ag2S with low band gap in the visible range and effective injection of photo-excited electrons to CdS compact layer according to suggested band diagram. Ag2[email protected] co-sensitizer showed an enhancement in photocurrent density due to successful passivation of Ag2S QDs and suppressing recombination on their surface.
... 14 Cyclic voltammetry analysis of polymer electrolyte provides energy levels alignment with photosensitizer, that is, energy difference between valence band (VB) of QD and highest occupied molecular orbital (HOMO) of polyelectrolyte is very less in comparison to the energy gap between lowest unoccupied molecular orbital (LUMO) of polymer and conduction band (CV) of QDs. 16,17 The energy barrier between LUMO of polymer and CV of photosensitizer is responsible for electron blocking layer, that is, excited photoelectrons from VB of QD cannot reach to LUMO of polymer that results in good performance of QDSSC. ...
Article
Full-text available
A hole conducting layer for quantum dot sensitized solar cell (QDSS) as a function of redox behavior has been reported. Polyurethanes, comprising hard and soft segments, have been functionalized for its use in solar cell application. Functionalization has been confirmed through NMR and FTIR studies. The functionalization of hard segment results in incorporation of ionic moieties which enhances its electrical conductivity, electrochemical and optical properties and displays a crucial role as a hole transport materials for QDSS cells due to proper work function and reduces energy barrier at the interface of active layer and counter electrode leading to reversible charge transport without decomposition. Cadmium sluphide (CdS) quantum dot has been synthesized using capping agent and the size (4 nm) and shape (spherical) has been confirmed through various techniques including TEM, AFM, SEM and DLS. Energy diagram of whole system has been revealed by measuring HOMO-LUMO and VB-CB energy gap through cyclic voltammetry and UV-vis spectrophotometry. The proper energy level alignment with electron transport layer and electron collecting layer provides suitability to transport hole for continuous harvesting of light. Solar cell device has been fabricated using successful layered design of functionalized polyurethane. The incorporation of a thin polyanilene (PANi) layer helps reducing the electron transport toward reverse direction (cathode) by adjusting the LUMO energy gap of polymer gel electrolyte and confirms re-excitation of dropped electron towards quantum dots (photo anode) through quenching under continuous illumination. The device with structure FTO/TiO2/CdS/PANi/PGE/Pt exhibits a photo current density of (Jsc ~ 2.20 mA/cm2), open circuit voltage Voc of 0.60 V, fill factor of 0.78 and photovoltaic conversion efficiency (PCE) of 1.25% using functionalized polyurethane.
... In the case of catalysis, enhanced catalytic activity can be achieved through shapecontrolled synthesis of metal crystals with more exposed active facets [7]. For quantum dot (QD) solar cells, a larger photocurrent is attainable through moderate energy level alignment of QDs using molecular dipoles modification [8]. ...
Article
The ability to arbitrarily regulate semiconductor interfaces provides the most effective way to modulate the performance of optoelectronic devices. However, less work has been reported on piezo-modulated interface engineering in all-oxide systems. In this paper, an enhanced photoresponse of an all-oxide Cu 2 O/ZnO heterojunction was obtained by taking advantage of the piezotronic effect. The illumination density-dependent piezoelectric modulation ability was also comprehensively investigated. An 18.6% enhancement of photoresponse was achieved when applying a –0.88% compressive strain. Comparative experiments confirmed that this enhancement could be interpreted in terms of the band modification induced by interfacial piezoelectric polarization. The positive piezopotential generated at the ZnO side produces an increase in space charge region in Cu 2 O, thus providing an extra driving force to separate the excitons more efficiently under illumination. Our research provides a promising method to boost the performance of optoelectronics without altering the interface structure and could be extended to other metal oxide devices.
... There are two main methods to link QDs electrically to ZnO NWs: a) Chemical covalent bonding of the QDs with or without bifunctional linkers [30,36,37]. b) QD growth directly from the precursor solution, i. e. direct growth of the semiconductor QDs on the electrode surface by successive ionic layer adsorption and reaction (SILAR). ...
... During the last years, materials scientists have been showed enough interest and made great efforts to develop and apply the transition-metal chalcogenides. Transition-metal chalcogenides can be widely used in sensors, catalyst, lithium battery; optical devices, solar cell, and so on [1][2][3] . Copper sulfides, an important member of the transition-metal chalcogenides, is one of the p-type compound semiconductors with narrow band gap. ...
Conference Paper
A composite of CuS-graphene was prepared via a mild experimental condition, using copper nitrate, EDTA, glucose, carbon disulfide, ethylenediamine and graphene oxide as the raw materials. The composite was characterized by X-ray power diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and scanning electron microscope. In this process, graphene oxide was reduced to graphene with the formation of CuS.
... The greater the electrondonating character of the ligand's substituent, the more destabilized the VB and CB energy levels became due to the greater negative dipole of the ligand, and therefore, electron transfer to TiO 2 became more energetically favorable ( Figure 9). 119 Within photovoltaic devices based on InAs QD-poly-(phenylenevinylene) active material, the devices using methylthiophenolate capping ligands for the QDs resulted in an order of magnitude higher photocurrents than similar devices with trioctylphosphine (TOP)-capped InAs. The authors claimed that this result was due to favorable energetic alignment at the heterojunction for the QDs with dipolar methylthiophenolate ligands. ...
Article
The subject of this review is the colloidal quantum dot (QD) and specifically the interaction of the QD with proximate molecules. It covers various functions of these molecules, including (i) ligands for the QDs, coupled electronically or vibrationally to localized surface states or to the delocalized states of the QD core, (ii) energy or electron donors or acceptors for the QDs, and (iii) structural components of QD assemblies that dictate QD-QD or QD-molecule interactions. Research on interactions of ligands with colloidal QDs has revealed that ligands determine not only the excited state dynamics of the QD but also, in some cases, its ground state electronic structure. Specifically, the article discusses (i) measurement of the electronic structure of colloidal QDs and the influence of their surface chemistry, in particular, dipolar ligands and exciton-delocalizing ligands, on their electronic energies; (ii) the role of molecules in interfacial electron and energy transfer processes involving QDs, including electron-to-vibrational energy transfer and the use of the ligand shell of a QD as a semipermeable membrane that gates its redox activity; and (iii) a particular application of colloidal QDs, photoredox catalysis, which exploits the combination of the electronic structure of the QD core and the chemistry at its surface to use the energy of the QD excited state to drive chemical reactions.
... There are two kinds of method to construct CDS. One method is based on the reduction [6,7], and the other one is based on the increase [8,9]. The first method construct a CDS from one node with maximum degree, and color it, then all of the neighbors of this node is colored with another color to formation initial CDS, then reduce some nodes by some rules to reduce the size of CDS .And another method first computes a dominate set and then selection some nodes to connect the nodes of dominate set. ...
... CdS nanoparticles due to their interesting size dependent optical and electronic properties and potential applications in Photonics and Electronics have been the subject for several studies for a long time [1,2]. The optical and electrical properties of CdS Quantum Dots and thin films have been extensively studied [3,4,5]. Many techniques are used for synthesis of CdS nanoparticles but most of these processes are costly, and require reaction condition and long reaction time. ...
Article
Full-text available
In this paper we report successful synthesis CdS quantum dots & CdS/ZnS core-shell nanoparticles by Microwave assisted method. A domestic microwave oven (LG 8080) is used for synthesis of the nanocomposites. The as prepared samples were characterized by X-Ray Diffraction (XRD), High resolution Transmission Electron Microscopy (HRTEM), Scanning Electron Microscopy (SEM), Energy-Dispersive X Ray Spectra (EDX), UV-Visible and Pl Spectroscopy. The HRTEM images confirm nanoformation of the prepared samples. The average particle size from the HRTEM study is found to be 3-5 nm for CdS quantum dots and 5-15 nm for CdS/ZnS core-shell nanoparticles. From HRTEM image core/shell formation of the as-synthesized samples are confirmed.
... 9 Surface ligands have been shown to shift the absolute energy positions of the valence band maximum (VBM) and conduction minimum (CBM) of QDs, and a growing body of evidence suggests that the magnitude of the energy shift can be characterized as a function of the dipole moment between the surface and linking group and the intrinsic dipole moment of the ligand itself. [10][11][12] By changing the capping ligand one can shift band energies of CdSe, 10,[13][14][15] CdS, 16 PbS, 4,11,17,18 as well as other nanocrystals. 19 Most recently, researchers have been using ligands to shift the absolute energy levels of QDs and to form an energy level gradient for efficient charge separation. ...
Article
Through a systematic approach we show that the insertion of a thin alumina layer in between a PbS QD layer and an Au substrate can eliminate Fermi level pinning. In this study band edge energies of different sized PbS QD monolayers with different cross-linkers were measured by using ultraviolet photoelectron spectroscopy and electrochemistry. When PbS QDs were immobilized directly on the Au, the measured valence band maximum was found to be insensitive to changes in the QD size or cross-linker indicating Fermi level pinning of the QD valence band to the Au Fermi level. After insertion of a thin film of alumina in between the PbS quantum dot monolayer film and the Au substrate, the measured valence band position revealed a shift that depended on ligand and QD size. These results identify a general method for eliminating Fermi level pinning in QDs and an approach for predictably controlling the energetics at QD–metal interfaces which is beneficial for improving the performance of QD based solar cells.
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Light-driven proton directional transport is important in living beings as it could subtly realize the light energy conversion for living uses. In the past years, 2D materials-based nanochannels have shown great potential in active ion transport due to controllable properties, including surface charge distribution, wettability, functionalization, electric structure, and external stimuli responsibility, etc. However, to fuse the inorganic materials into bio-membranes still faces several challenges. Here, we proposed peptide-modified WS2 nanosheets via cysteine linkers to realize tunable band structure and, hence, enable light-driven proton transmembrane transport. The modification was achieved through the thiol chemistry of the –SH groups in the cysteine linker and the S vacancy on the WS2 nanosheets. By tuning the amino residues sequences (lysine-rich peptides, denoted as KFC; and aspartate-rich peptides, denoted as DFC), the ζ-potential, surface charge, and band energy of WS2 nanosheets could be rationally regulated. Janus membranes formed by assembling the peptide-modified WS2 nanosheets could realize the proton transmembrane transport under visible light irradiation, driven by a built-in potential due to a type II band alignment between the KFC-WS2 and DFC-WS2. As a result, the proton would be driven across the formed nanochannels. These results demonstrate a general strategy to build bio-semiconductor materials and provide a new way for embedding inorganic materials into biological systems toward the development of bioelectronic devices.
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In this work CdSe0.4Te0.6 NCs were synthesized in aqueous solution by a novel combinative chemical precipitation/microwave-activated method. The microwave time was interestingly altered in a short-time range for the synthesis of these NCs with different sizes and bandgap energies. Then they were applied as the co-sensitizing quantum dots layer in the photoanode of the CdS QDs sensitized solar cells (QDSCs). It was displayed that the QDSC with TiO2 NCs/CdSeTe(0.5hR)/CdS/ZnS photoelectrode demonstrated a power conversion efficiency (PCE) of 1.65% in AM 1.5 solar irradiation. The selected CdSeTe(0.5hR) NCs were prepared in 0.5 h of the reflux time in chemical precipitation method. Then they were utilized in a second microwave-activated growth process to achieve larger sizes of the CdSeTe NCs. The microwave time was changed in the range of 0–2.5 h in the experiments and synthesized CdSeTe 0.5hR + 0–2.5hM NCs were applied in the photoanode of the corresponding QDSCs. It was shown that the QDSC with TiO2 NCs/CdSeTe(0.5hR + 0.5hM)/CdS/ZnS photoelectrode revealed a maximum power conversion efficiency of 5%. The considerable point was that the CdSeTe(0.5hR + 0.5hM) NCs demonstrated 76% higher photoluminescence (PL) quantum yield (QY) than the best situation of particles which were normally prepared in 7 h of the reflux time. Besides, the growth was properly carried out in just 1 h of this proposed combinative method. The better crystalline quality and appropriate absorption edge were introduced as the main reasons for highest power conversion efficiency. The short synthesis time was also clarified as the advantage of this proposed approach compared to the normal colloidal synthesis in longer reflux times.
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Many colloidal quantum dot (QD)-based devices involve charging of the QD, either via intentional electronic doping or via electrical charge injection or photoexcitation. Previous research has shown that this charging can give rise to undesirable electrochemical surface reactions, leading to the formation of localized in-gap states. However, little is known about the factors that influence the stability of charged QDs against surface oxidation or reduction. Here, we use density functional theory to investigate the effect of various ligands and solvents on the reduction of surface Cd in negatively charged CdSe QDs. We find that X-type ligands can lead to significant shifts in the energy of the band edges but that the in-gap state related to reduced surface Cd is shifted in the same direction. As a result, shifting the band edges to higher energies does not necessarily lead to less stable electron charging. However, subtle changes in the local electrostatic environment lead to a clear correlation between the position of the in-gap state in the bandgap and the energy gained upon surface reduction. Binding ligands directly to the Cd sites most prone to reduction was found to greatly enhance the stability of the electron charged QDs. We find that ligands bind much more weakly after reduction of the Cd site, leading to a loss in binding energy that makes charge localization no longer energetically favorable. Lastly, we show that increasing the polarity of the solvent also increases the stability of QDs charged with electrons. These results highlight the complexity of surface reduction reactions in QDs and provide valuable strategies for improving the stability of charged QDs.
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This study explored the size dependence of colloidal CdSe nanocrystals (NCs) on the photovoltaic properties of CdSe NC/poly(3-hexylthiophene) (P3HT) hybrid bulk-heterojunction (BHJ) solar cell devices. The size-dependent photovoltaic performance was achieved by utilizing CdSe supraquantum dots (SQDs), which are three-dimensionally interconnected colloidal superstructures composed of hundreds of CdSe quantum dots (QDs). The average size of the SQDs can span tens of nanometers, which allow the formation of percolation networks in BHJ films. The open-circuit voltage of the devices was observed to be proportional to the size of the SQDs because of their ideal percolation networks. The photocurrents were determined by the competition between the charge separation and charge transport abilities controlled by the SQD sizes. Overall, the 46 nm-sized CdSe SQD-device demonstrated the highest power conversion efficiency (PCE) of 0.95%, which was 3.2 times higher than that of the control 4.3 nm-sized CdSe QD device. However, further increasing the SQD size resulted in a decrease in the PCE because of the inherent carrier recombination loss within the SQDs. To overcome this "Goldilocks problem," we tuned the energy level at the surface region of the CdSe SQDs via electron-donating 4-methylthiophenol (MTP) ligand exchange. The MTP-treated CdSe SQDs further improved the device performance by enhancing the charge separation and increasing the energy level offset at the CdSe SQD/P3HT interface.
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Due to exceptional electron-accepting ability, light-absorption and delocalized conjugated structure, buckminsterfullerene (C60) has attracted fascinating interests in the field of organic solar cells. However, poor delocalization and accumulation of electrons for pristine C60 in physiological aqueous solution and difficulties in conjugation with biomolecules limit its extended photovoltaic ap-plications in bioassay. Herein, we reported the non-covalent coupling of C60 to an electronically complementary porphyrin-derived metal-organic framework (PCN-224) with carboxyl-group terminals. Such assembly not only offered a friendly interface for biocon-jugation but also resulted in a long-range ordering C60@PCN-224 donor-acceptor system that demonstrated an unprecedented pho-tocurrent enhancement up to 10 times with respect to each component. As an example, by further cooperating with Nanobodies, the as-prepared C60@PCN-224 was applied to a photoelectrochemical (PEC) immunosensor for S100 calcium-binding protein B with by far the most promising detection activities. This work may open a new venue to unlock the great potential of C60 in PEC bio-sensing with excellent performances.
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As one of the most promising third-generation photovoltaics devices, quantum dots sensitized solar cells (QDSCs) have attracted much more attention due to their easy fabrication, low cost and potential high efficiency, etc. Enormous substantial efforts have been taken to boost the photoelectrical conversion efficiencies (PCEs) and device stability consistently from optimizing precisely materials structure and device architecture. Throughout the development process of QDSCs, it is noteworthy that metal chalcogenide based semiconductors are key materials in the aspect of capturing the sunlight as sensitizers, catalytic electrolyte reduction as counter electrodes (CEs), and interface charge transport as interface modification layers, and so on. Herein, we systematically reviewed the recent progress of metal chalcogenides based QDSCs in practical applications from three main functional points, that was QD sensitizers, counter electrodes (CEs), and interface modification layers. Besides, we outlined the fundamental structure, operation principle, and brief history of this sensitized solar cells. Finally, the state of existing challenges and future prospects for QDSCs employing various metal chalcogenides were also discussed.
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In this work CdTe NCs with different sizes were synthesized through a hot-injection chemical precipitation method. Then they were applied as the co-sensitizers in the photoelectrode of the CdTe and CdS sensitized solar cells. The photoelectrodes were composed of a nanocrystalline TiO2 layer formed of hydrothermally grown TiO2 NCs with dominant size of 25 nm. This layer was sensitized with CdS and CdTe NCs via the successive ionic layer adsorption and reaction (SILAR) and drop casting methods. The average size of utilized CdTe NCs was altered in the range of 2.7–3.4 nm in the experiments. Polysulfide electrolyte and CuS counter electrodes were also applied in conventional structure of these QDSCs. Different photoanodes were fabricated with various sizes of the as prepared CdTe NCs. Then the effect of corresponding bandgap energies and band edge positions on the photovoltaic performance of the CdS/CdTe sensitized solar cells was investigated. According to the results, a maximum efficiency of 3.8% was achieved for the QDSC with CdTe NCs prepared at 5h of refluxing process (3.2 nm). This efficiency was increased about 72% compared to the reference cell which was sensitized with just CdS NCs. The optimization was also addressed based on the energy band diagram of the composing components of the fabricated QDSCs.
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Organic-inorganic hybrid perovskite has been subject of intense investigation due to their attractive optical and electronic properties, e.g., direct bandgap, high absorption coefficient and ambipolar charge transport. Such properties allowed the application of this material in solar cells and light emitting diodes efficiently. Thus, the development of new synthesis routes that allow the production of materials with the appropriate characteristics for each application is extremely important for the development of this area of research. Therefore, in this Ph.D. work, we’ll present results on the synthesis and characterization of perovskite films and nanocrystals obtained from new methodologies, which are based on thin films of lead sulfide (PbS) and lead iodide (PbI2) deposited by rf-sputtering and on quantum dots of PbS. In the first synthesis route, amorphous PbS thin films deposited by sputtering were converted to PbI2 thin films by the iodination process at room temperature. This procedure resulted in a complete structural change, as attested by XRD measurements. The PbI2 films were converted into CH3NH3PbI3 by immersing them in a solution of methylammonium iodide (CH3NH3I). The second route consisted of depositing directly films of PbI2 by sputtering. The conversion into CH3NH3PbI3 also was performed by immersing the films in a CH3NH3I solution. These two methods allowed us to synthesize CH3NH3PbI3 thin films with good optical and structural properties and with complete substrate coverage, without evidence of cracks or holes, as verified by scanning electron microscopy images. Such methodologies have the potential to pave the way for the large-scale production of reproducible and high-efficiency CH3NH3PbI3 solar cells. The third route was devoted to producing perovskite nanocrystals using PbS quantum dots as precursors. This approach was performed through iodination of PbS quantum dots. This produced PbI2 nanowires of about 50 μm in length and 200 nm in diameter. The conversion in perovskite nanocrystals was accomplished through immersion of the PbI2 nanowires into a solution of CH3NH3I. This procedure generated perovskite nanocrystals of about 5 μm in length and 400 nm in width.
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Ultraviolet photoelectron spectroscopy (UPS) is the most widely used technique to determine the ionization energy (IE) of electronic materials, as this parameter is critically important for the energy level alignment in electronic and optoelectronic devices. For organic semiconductor IE assessment, molecules are typically evaporated and polymers spin‐coated onto a conductive substrate, and then measured by UPS. For substrates that possess a constant work function over large area, the determination of IE from the measured UPS data is straight forward. However, if the substrate is heterogeneous (intentionally or unintentionally) in local work function, the conventional method to determine IE yields erroneous results and a more careful data evaluation is necessary. While the secondary electron cutoff (SECO) can exhibit area‐averaged values, the valence levels are split in energy according to the local work function. Here, we demonstrate the possible pitfalls of heterogeneous substrates by employing well‐controlled model systems, and show how appropriate data analysis can still yield correct IE values. Ultraviolet photoelectron spectroscopy is an invaluable technique to determine the ionization energy of electronic materials. However, for samples featuring inhomogeneous local work function, determining the ionization energy using conventional method might yield erroneous results. By employing model samples, the authors demonstrate the impact of heterogeneity on the spectral features and show how appropriate data analysis can still yield correct ionization energy values.
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It has been well established that polymer additives in electrolyte can impede the charge recombination processes at the photoanode/electrolyte interface, and improve performance, especially Voc, of the resulting sensitized solar cells. However, there are few reports about the effect of electrolyte additives on counter electrode (CE) performance. Herein, we systematically investigated the effect of polyethylene glycol (PEG) additives with various molecular weights (Mw from 300 to 20 000) in polysulfide electrolyte on the performance of two representative CdSe and Zn–Cu–In–Se (ZCISe) quantum dot sensitized solar cells (QDSCs), and explored the mechanism of the observed effects. Electrochemical impedance spectroscopy measurements indicate that all PEG additives can improve the charge recombination resistance at the photoanode/electrolyte interface, therefore suppressing the unwanted charge recombination process, and enhancing the Voc of the resulting cell devices accordingly. On the CE side, with the increase of Mw of PEG additives, the initial effect of reducing the charge transfer resistance at the CE/electrolyte interface evolves into an increasing resistance; accordingly the initial positive effect on FF turns into negative one. Accordingly, low Mw PEG can improve efficiency for both CdSe (increasing from 6.81% to 7.60%) and ZCISe QDSCs (increasing from 9.26% to 10.20%). High Mw PEG is still effective for CdSe QDSCs with an efficiency of 7.38%, but falls flat on ZCISe QDSCs (with an efficiency of 9.11%).
Chapter
Energy shortage and environmental pollution are two major problems of mankind in the 21st century. Photovoltaic (PV) devices are kinds of the possible and promising choices among the group of the energy conversion systems. Among them, quantum dots‐sensitized solar cells (QDSSCs) are considered to be one kind of the low‐cost solar cells, which encompass devices whose maximum conversion efficiency is above the 32% limit for single junction converters in AM 1.5 sunlight. Conventional solar cells with inflexible substrates are limited in many unique commercial applications, while flexible solar cells on plastic films and thin metal foils are already being made due to the advantages of lightweight, translucent, flexibility and low cost, which possess more and wider application areas. Energy conversion in solar cell consists of the generation of photo‐induced electron and hole pairs in semiconductor by the absorption of light and the separation of charges. Therefore, it is very important to understand the influence factors for the photovoltaic performance in flexible QDSSCs. In the chapter, we firstly give a brief introduction of the basic concepts and working mechanism of quantum dots and the QDSSCs, and then discuss in detail the performance parameters of flexible QDSSCs, including the choosing of the QDs types, the fabrication of the flexible photo‐anode films, sensitization method, the interfacial engineering technique and counter electrodes. Finally, forecasts of the future development of flexible QDSSCs are reviewed.
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A synergetic approach of employing smooth mesoporous TiO2 microsphere (SμS-TiO2)-nanoparticulate TiO2 (np-TiO2) composite photoanode, and size and defect controlled CdSe quantum dots (QD) to achieve high efficiency (η) in a modified Grätzel solar cell, quantum dot sensitized whisperonic solar cells (QDSWSC), is reported. SμS-TiO2 exhibits whispering gallery modes (WGM) and assists in enhancing the light scattering. SμS-TiO2 and np-TiO2 provide conductive path for efficient photocurrent charge transport and sensitizer loading. The sensitizer strongly couples with the WGM and significantly enhances the photon absorption to electron conversion. The efficiency of QDSWSC is shown to strongly depend on the size and defect characteristics of CdSe QD. Detailed structural, optical, microstructural and Raman spectral studies on CdSe QD suggest that surface defects are prominent for size ~2.5 nm, while the QD with size > 4.5 nm are well crystalline with lower surface defects. QDSWSC devices exhibit an increase in η from ≈0.46% to η ≈ 2.74% with increasing CdSe QD size. The reported efficiency (2.74%) is the highest compared to other CdSe based QDSSC made using TiO2 photoanode and I-/I3- liquid electrolyte. The concept of using whispering gallery for enhanced scattering is very promising for sensitized whisperonic solar cells.
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Metal halide perovskite solar cells (PSC) exhibit outstanding power conversions efficiencies when fabricated as mm-sized devices, but creation of high-performing large-area PSCs that are stable under operating conditions on a sufficiently long timescale still presents a significant challenge. We demonstrate herein that modification of the interface between the perovskite and spiro-OMeTAD hole-transporting material with commercially available para-substituted benzenthiol molecules facilitates fabrication of cm-sized PSCs with both improved efficiency and stability. Comprehensive analysis using specialised and conventional physical characterisation techniques has been undertaken to demonstrate that band alignment at the perovskite surface can be tuned to improve the solar cell efficiency via adsorption of benzenethiols with a significant dipole moment. Moreover, modification of the perovskite with cyano-substituted benzenethiol enhances charge extraction and reduces charge recombination in the devices. These effects enable improvements in the power conversion efficiency of PSCs from 19.0 to 20.2 % and from 18.5 to 19.6 % under 1 sun AM 1.5G irradiation with 0.16 and 1.00 cm2 aperture, respectively. Most importantly, benzenethiol-modified perovskite solar cells retain more than 80 % of the initial performance after 185 h of continuous operation at 50 % relative humidity and 50 °C device temperature under 1 sun irradiation, while devices with no interfacial modification undergo continuous deterioration down to 35 % of the initial efficiency. These significant improvements are provided by a very simple and perfectly reproducibile modification procedure that can be readily adopted in other types of PSCs.
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Organolead iodide perovskites, CH 3 NH 3 PbI 3 , have attracted the attention of researchers around the world due to their optical and electrical properties. Their main characteristics include, direct band-gap (1.4 to 3.0 eV), large absorption coefficient in the visible spectrum, long carrier diffusion length and ambipolar charge transport. Aside that, perovskite thin films can be produced with low cost and are compatible with large-scale manufacture. Perovskite thin films have been synthesized mainly by spin-coating technique and thermal evaporation, which can be executed in one or two steps. Aiming to increase the light absorption, nanostructured perovskite thin films are also under intense study, since the nanostructures can absorb more light than a flat film. Thus, in this work, we reported the synthesis of perovskite (CH 3 NH 3 PbI 3 ) nanorods by means of conversion of lead sulphide quantum dots (PbSQD). The perovskite nanorods were grown by exposing the PbSQD to a highly concentrated iodine atmosphere and then dipping the resulting film in methylammonium iodide (CH 3 NH 3 I) solution. The first step converts completely the PbSQD into lead iodide (PbI 2 ) nanowires, ≈50 µm long and ≈200 nm diameter, through substitution of sulphur by iodine atoms and subsequent aggregation of the particles. The later step converts the PbI 2 nanowires in perovskite nonorods (≈5 µm long and ≈400 nm diameter). The perovskite nanorods present a regular geometry along all its length. A preferential alignment of nanorods to the substrate plane was observed. The preliminary results show that we can control the size of nanorods through exposition time of PbSQD to iodine, which change the size of PbI 2 nanowire as well. The conversion process was studied by x-ray diffraction, optical absorption, photoluminescence and scanning electron microscopy.
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A versatile methodology for the production of organic surfactant-free metal chalcogenide microparticles consisting of nano crystallites at room temperature in a short time is described. The reaction of various metal sources with $$\hbox {LiBH}_{4}$$ in the presence of either S or Se yielded their corresponding CuS, $$\hbox {Cu}_{2}\hbox {S}$$, CdS and $$\hbox {Cu}_{2\hbox {-}\mathrm{z}}\hbox {Se}$$ microparticles. These micron size particles are aggregates of nano crystallites. The reactivity of $$\hbox {LiBH}_{4}$$ and supersaturated condition helped in the formation nanocrystals. The first observation of metal source dependent morphology of particles produced under identical reaction condition is also discussed. The morphology of CuS particles obtained in these reactions was varying with the change of metal source used in the reaction. Interestingly, the reactions producing metal chalcogenide microparticles also yielded borane $$(\hbox {BH}_{3})$$ as a side product. Graphical Abstract SYNOPSIS Surfactant-free CuS, $$\hbox {Cu}_{2}\hbox {S}$$, CdS and $$\hbox {Cu}_{2\hbox {-}\mathrm{x}}\hbox {Se}$$ microparticles were produced at room temperature within one hour of reaction time under supersaturated condition. Morphology of CuS microparticles obtained in these reactions was varying with a change of metal ion source used in the reaction. Open image in new window
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Herein, we have investigated the effect both of the bifunctional linker (L1, L2, L3, and L4) and ZnO morphology (porous nanoparticles (NPs), nanowires (NWs), and nanotubes (NTs-A and NTs-Z)) on the electron injection in CdSe QDs sensitized solar cell by first-principle simulation. Via calculating the partitioned interfaces formed by different components (linker/QDs and ZnO/linker), we found that the electronic states of QDs and every ZnO substrate are insensitive upon any linker capped, while the frontier orbitals of L1-L4 (with increased delocalization) manifest a systematical negative-shift. Because of the lowest unoccupied molecular orbital (LUMO) of L1 compared to its counterparts aligned in the region of the virtual states of QDs or substrate with a high density of states, it always yields a stronger electronic coupling with QDs and varied substrates. After characterization of the complete ZnO/linker/QDs system, we found that the electron injection time (τ) depends on both of the linker and substrate vastly. On the one hand, L1 bridged QDs and every substrate always achieve the shortest τ than its counterparts associated cases apparently. On the other hand, NWs supported systems always yield the shortest τ no matter what the linker is. Overall, the NWs/L1/QDs system achieves the fastest injection by ~160 fs. These essentially stem from the shortest molecular length of L1 decreasing the distance between QDs and substrate, subsequently improving the interfacial coupling. Meanwhile, the NWs supported cases generate the less sensitive virtual states for both of the QDs and NWs, ensuring a less variable interfacial coupling. These combined can provide a comprehension on the effect contributed from linker and oxide semiconductor morphology on charge transfer for aim of choosing appropriate component with fast directional electron injection.
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The synthesis of luminous nanocrystals via easily handled methods has received tremendous attention in recent years, especially toward scaling-up fabrication. In this work, we present a gram-scale, cost-effective and eco-friendly method for synthesis of water-soluble Fe:CdS nanocrystals with highly tunable properties. The structural and optical properties of the as-synthesized nanocrystals are systematically explored. The obtained Fe:CdS nanocrystals possess cubic zinc blende structure, and the Fe:CdS nanocrystals are nearly spherical and show narrow size distribution. Utilizing thioglycollic acid as the capping agent finally facilitates the water solubility of Fe:CdS nanocrystals, which makes them useful for biomedical applications. The influence of reaction temperature on the optical properties of the as-prepared nanocrystals is investigated in detail. The particle size and optical properties of the resulting Fe:CdS nanocrystals are found to be easily manipulated by simply adjusting the reaction temperature, in which the emission peaks of the Fe:CdS nanocrystals red-shift with an increase in the particle sizes of the nanocrystals. Compared to bulk CdS, the dramatic blue shift of absorption peaks induced by reaction temperature indicates strong quantum confinement effect. More amusingly, the band gap of the as-formed nanocrystals can be tailored in a broad range of 3.40 to 3.10 eV by altering the reaction temperature. The approach displayed here is preponderant in that the precursors are nontoxic and easily available, the particle size and optical properties of the resulting nanocrystals are readily tailored. This work offers a reliable and green synthetic route toward water-soluble and highly luminescent nanocrystals with tunable optical properties.
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Reducing the donor-acceptor excess energy (ΔGET) associated with electron transfer (ET) across quantum dot (QD)/oxide interfaces can boost photoconversion efficiencies in sensitized solar cell and fuel architectures. One proposed path for engineering ΔGET losses at interfaces refers to the tuning of sensitizer workfunction by exploiting QD dipolar molecular capping treatments. However, the change in workfunction per Debye in QD solids has been reported to be ~20-fold larger when compared to the effect achieved in QD sensitized architectures. The origin behind the modest workfunction tunability in QD sensitized oxides remains unclear. Here, we investigate the interplay between QD dipolar molecular capping, interfacial QD-oxide ET rates and QD workfunction in PbS QD/SnO2 sensitized interfaces. We find that interfacial QD-to-oxide ET is invariant to the nature and strength of the specific QD dipolar capping treatment. Photoelectron spectroscopy reveals that the resolved invariance in ET rates is the result of a lack of QD workfunction (and hence ΔGET) tuning, despite effective molecular dipolar capping. We conclude that Fermi level pinning precludes tuning donor-acceptor energetics by dipolar molecular capping in strongly coupled quantum dot sensitized oxides.
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Self-assembled monolayers (SAMs) of benzoic acid based molecules are used to modify the metal oxide–polymer interface in a hybrid poly-3-hexylthiophene (P3HT)/TiO2 photovoltaic device structure. The effect of SAMs on current density is in accordance with expectation from the driving force for charge separation of metal oxide–polymer interface in a hybrid poly-3-hexylthiophene (P3HT)/TiO2 photovoltaic device. However, the effect of monolayers on open circuit voltage is quite unexpected from the interfacial energetics as all the monolayers improve the open circuit voltage in spite of different sign of the interfacial dipole for different SAMs. This suggests that the monolayers have additional functions. Overall device performance is enhanced by more than a factor of two using a SAM with permanent dipole pointing towards the TiO2 surface or pointing towards polymer when compared to a control device with no interface modifiers. This study concludes that the SAM layer has two functions that are to shift the position of the conduction band of the porous TiO2 relative to the polymer HOMO level so as to influence interfacial charge separation and to act as a barrier layer, insulating back electron transfer from the TiO2 to the polymer. Both effects can benefit the performance of hybrid polymer metal oxide solar cells.
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CdS and CdSe quantum dots are commonly used together as co-sensitizers for quantum dot-sensitized solar cells, and CdS QDs are usually deposited on TiO2 films as an intermediate layer to facilitate the subsequent deposition of CdSe QDs. A modified Successive Ionic Layer Adsorption and Reaction (SILAR) technique with the addition of a triethanolamine (TEA) additive into a cationic precursor solution was utilized to optimize the CdS intermediate layer to enhance the performance of the obtained CdS/CdSe co-sensitized solar cells. Due to the increased light harvesting ability as well as the improved charge separation efficiency, our solar cell with a TEA-CdS interlayer demonstrated an enhanced power conversion efficiency with an improved short-circuit current and an increased open-circuit voltage. Utilizing a newly reported counter electrode based on CuS formed on a Ti sheet, the eventual power conversion efficiency of TEA-CdS/CdSe QDSSC was further improved to as high as 5.70%.
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In the typical solution-based synthesis of colloidal quantum dots (QDs), it always resorts to some surface treatment, ligand exchange processing or post-synthesis processing, which might involve some toxic chemical regents injurious to the performance of QD sensitized solar cells. In this work, the CuInS2 QDs are deposited on the surface of one-dimensional TiO2 nanorod arrays by the pulsed laser deposition (PLD) technique. The CuInS2 QDs are coated on TiO2 nanorods without any ligand engineering, and the performance of the obtained CuInS2 QD sensitized solar cells is optimized by adjusting the laser energy. An energy conversion efficiency of 3.95% is achieved under one sun illumination (AM 1.5, 100 mW cm-2). The improved performance is attributed to enhanced absorption in the longer wavelength region, quick interfacial charge transfer and few chance of carrier recombination with holes for CuInS2 QD-sensitized solar cells. Moreover, the photovoltaic device exhibits high stability in air without any specific encapsulation. Thus, the PLD technique could be further applied for the fabrication of QDs or other absorption materials.
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Layered metal sulphides (LMSs) such as MoS2, WS2 and SnS2 have attracted much attention in the field of photocatalysis due to their excellent properties. Herein, a facile and effective liquid exfoliation solvothermal method for fabricating TiO2/LMS (LMS = MoS2, WS2 or SnS2) photocatalysts has been developed. The optimum molar ratio of Ti–Mo, Ti–W and Ti–Sn was determined to be 50:0.8, 50:0.1 and 50:0.1, respectively. The optical properties of TiO2/LMS with a matching solar spectrum contribute to converting the solar energy to chemical energy by photon-driven photocatalytic reactions. The combined effect of liquid exfoliation and solvothermal reforming has been demonstrated as an effective method to obtain high efficiency photocatalysts using bulk metal sulphides as sensitizers. The binding site of TiO2 and the LMS at the interface of a composite photocatalyst was investigated by the density functional theory (DFT) method at a molecular cluster level, and the calculation results showed that firm structures were formed at the interfaces of TiO2 nanoparticles and the LMS. The photocatalytic activity evaluation of TiO2/LMS showed that the LMS played the crucial role in separation of photogenerated e−/h+ pairs and utilization of photons for enhancing the photocatalytic activity of TiO2. The study of the electron transfer mechanism indicated that the synergetic effect of superoxide radicals (·O2−) and hydroxyl radicals (·OH) plays the leading role in the dye degradation process.
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Quantum dot-sensitized solar cells (QDSCs) are promising solar energy conversion devices, which could be a low-cost alternative to the prevailing photovoltaic technologies. The nanocrystalline quantum dot (QD) light absorber exhibits high molar extinction coefficient with tunable photo-responses, compared with molecular dyes. However, the power conversion efficiencies (PCEs) of QDSCs are generally below 9.5%, which lags far behind its molecular sensitizer counterpart (up to 13%), let alone the perovskite solar cell whose efficiency is already above 20%. The low PCE has been attributed to large free energy loss of sensitizer regeneration and inefficient charge separation occurring at the QD/electrolyte interfaces, for which various interfacial engineering strategies have been reported to enhance the PCE and cell stability. Herein we review the recent progress on the interfacial engineering of QDSCs and provide future prospects for the development of highly efficient and stable QDSCs.
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Back reactions were impressed effectively by double blocking barrier, organic molecules and ZnS on the photoanode of quantum dot (QDs) sensitized solar cells (QDSSCs), thereby achieving higher conversion efficiency. In this work, four different organic molecules were applied in QDSSCs, the efficiency increased by two fold from 2.21% to 4.25% when co-modification with 4-tert-butyl pyridine (TBP) and ZnS were sequentially applied. The incident photon-to-current efficiency (IPCE) and parameters obtained from impedance spectroscopy (IS) such as recombination resistance (Rrec), chemical capacitance (Cμ), electron lifetimes were consistent with the measured photovoltaic performance. We speculated that organic molecules mainly inhibit the charge recombination of injected electrons in the TiO2 with electrolyte, because of its electron-donating property of tert-butyl group. The selective site of modification has been tested for assessing the dominant mechanism underlying the improvement of solar cell characteristics.
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Dye-sensitized solar cells (DSSCs) have attracted significant attention throughout the world from both academic and industrial fields as a promising alternative to conventional solid-state photovoltaic devices since a report by O’Regan and Grätzel in 1991 [1]. To improve the overall efficiency and long-term stability of DSSCs, several groups have investigated various sensitizers, photoanode materials, counterelectrodes, and redox systems. Light absorbers, such as transition metal complexes and organic molecules, have been designed and tested in DSSCs. In these cells the photoexcited dye injects electrons into the conduction band of TiO2, then the oxidized dye cations are regenerated by electron donation from the electrolyte or, alternatively, by hole injection into an organic hole transporting material for the solid-state counterpart [2, 3]. Although the results obtained so far are very impressive, further improvements in both efficiency and stability by introducing new materials and engineering their interfaces are anticipated. Inorganic semiconducting materials “quantum dots (QDs)” are attracting increasing attention because of their technological importance in solar energy conversion, light emitting diodes, and sensor applications [4]. The attractive properties of QDs, i.e., their size dependent optical, electronic, and mechanical properties, coupled with the available synthetic protocols allow these materials to be integrated into various types of solar cell. In this chapter we focus mainly on incorporating the QDs into mesoscopic TiO2 based solar cells.
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Gold nanoclusters (Au NCs) with molecule-like behavior have emerged as a new light harvester in various energy conversion systems. Despite several important strides made recently, efforts toward the utilization of NCs as a light harvester have been primarily restricted to proving their potency and feasibility. In solar cell applications, ground-breaking research with a power conversion efficiency (PCE) of more than 2% has recently been reported. Because of the lack of complete characterization of metal cluster-sensitized solar cells (MCSSCs), however, comprehensive understanding of the interfacial events and limiting factors which dictate their performance remains elusive. In this regard, we provide deep insight into MCSSCs for the first time by performing in-depth electrochemical impedance spectroscopy (EIS) analysis combined with physical characterization and density functional theory (DFT) calculations of Au NCs. In particular, we focused on the effect of the size of the Au NCs and electrolytes on the performance of MCSSCs and reveal that they are significantly influential on important solar cell characteristics such as the light absorption capability, charge injection kinetics, interfacial charge recombination, and charge transport. Besides offering comprehensive insights, this work represents an important stepping stone toward the development of MCSSCs by accomplishing a new PCE record of 3.8%.
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FeS2-sensitized ZnO/ZnS nanorod arrays were fabricated and used as the photoanodes for quantum-dot-sensitized solar cells (QDSSCs). The cell performance of the ZnO/ZnS nanorod arrays after sensitization was better than that of ZnO-based nanorod arrays without sensitizing treatment. Pyrite FeS2 was found to be an effective photosensitizer for QDSSCs. Various QDSSCs were assembled using different counter electrodes, such as Pt, FeS2 nanorods and FeS2 nanoparticles, and comparisons of cell performance as well as catalytic activity were made among them.
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Colloidal CdSe quantum dots (QDs) of different sizes, prepared by a solvothermal route, have been employed as sensitizers of nanostructured TiO(2) electrode based solar cells. Three different bifunctional linker molecules have been used to attach colloidal QDs to the TiO(2) 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).
Article
Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined and new entries since July 2008 are reviewed. Efficiencies are updated to the new reference solar spectrum tabulated in IEC 60904-3 Ed. 2 revised in April 2008 and an updated list of recognised test centres is also included.
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Here we present a CdS quantum dot sensitized solar cell based on a mesoporous TiO2 film with remarkable stability using I−/I3− electrolyte. Chemical Bath Deposition (CBD) was used to deposit the CdS quantum dots within the porous network. We show that a thin coating of the QD sensitized film with an amorphous TiO2 layer strongly improves the performance and photostability of the solar cell. We propose that the coating passivates QD surface states which act as hole traps and are responsible for photodegradation of the device. In addition, this coating decreases the recombination of electrons from the CdS quantum dots and the mesoporous TiO2 into the electrolyte solution. We obtain a significant improvement of all cell parameters resulting in a total light to electric power conversion efficiency of 1.24%.
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The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future research efforts toward utilization of tailored nanostructures will be an important challenge for the development of next generation solar cells.
Article
The liquid junction dye-sensitized solar cell (DSSC) has reached laboratory solar efficiencies of 11%. In contrast, the semiconductor-sensitized analogue (SSSC) has, up to now, exhibited a maximum efficiency of 2.8%. This begs the questions: is this difference fundamental? Will SSSCs always be inferior to DSSCs? We discuss the differences between the two types of cells, considering typical charge transfer times for the various current generating and recombination processes. Three main factors that could contribute to differences between the two types of cells are discussed: multiple layers of absorbing semiconductor on the oxide, the different electrolytes normally used for the two types of cell, and charge traps in the absorbing semiconductor. Entropic effects and the irreversible electron injecting nature of the normally used Ru dye to TiO2 are also briefly considered. We conclude that although the DSSC does possess some fundamental advantages, we can expect large improvements in efficiency of the SSSC, possibly reaching values comparable to the DSSC.
Article
Chemical solution deposited (CD) CdSe films possess a nanocrystalline structure and exhibit quantum size effects due to the small crystal size. This results in a blue shift of the optical spectra. It has been observed that there is a certain critical ratio between the complexing agent (nitrilotriacetate) and Cd concentrations used in preparing the films, denoted as R(c), above which there is a pronounced red shift of the optical spectra of the films. Using X-ray diffraction and electron microscopy, this red shift was correlated with an increase in crystal size. This sharp change suggested a changeover in the CD mechanism. Optical absorption spectra and laser scattering measurements of the deposition solutions in the absence of selenosulfate showed that Cd(OH)(2) was present in solutions below R(c) (often only after an induction period during which the solution pH increased), but not above R(c), although a visible Cd(OH)2 suspension was not apparent under normal deposition conditions, even below R(c). X-ray photoelectron spectroscopy indicated the presence of adsorbed colloidal Cd(OH)(2) on the glass substrates only under conditions where Cd(OH)(2) was also present in solution. It is proposed that, below R(c), the CD mechanism is initiated on the Cd(OH)(2) colloidal particles adsorbed on the substrate while, above R(c), deposition occurs directly on the substrate by initial CdSe formation, without any mediation by Cd(OH)(2). The change in crystal size at R(c) is explained by the change in mechanism. Similar behavior was obtained for CdS and PbSe, showing the generality of the conclusions.
Article
We present a concise, although admittedly non-exhaustive, but hopefully didactic review and discussion of some of the central and basic concepts related to the energetics of surfaces and interfaces of solids. This is of particular importance for surfaces and interfaces that involve organic molecules and molecular films. It attempts to pull together different views and terminologies used in the solid state, electrochemistry, and electronic device communities, regarding key concepts of local and absolute vacuum level, surface dipole, work function, electron affinity, and ionization energy. Finally, it describes how standard techniques like photoemission spectroscopy can be used to measure such quantities.
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The electronic properties of semiconductor surfaces can be controlled by binding tailor-made ligands to them. Here we demonstrate that deposition of a conducting phase on the treated surface enables control of the performance of the resulting device. We describe the characteristics of the free surface of single crystals and of polycrystalline thin films of semiconductors that serve as absorbers in thin film polycrystalline, heterojunction solar cells, and report first data for actual cell structures obtained by chemical bath deposition of CdS as the window semiconductor. The trend of the characteristics observed by systematically varying the ligands suggests changes in work function rather than in band bending at the free surface, and implies that changes in band line-up, which appear to cause changes in band bending, rather than direct, ligand-induced band bending changes, dominate.
Article
CdSe is homogeneously deposited into nanoporous TiO2 films and used in liquid junction photoelectrochemical solar cells. The effect of the deposition parameters on the cell are studied, in particular differences between ion-by-ion and cluster deposition mechanisms. CdSe deposition on a Cd-rich CdS film that was deposited first into the TiO2 film, or selenization of the Cd-rich CdS layer with selenosulphate solution improves the cell parameters. Photocurrent spectral response measurements indicate photocurrent losses due to poor collection efficiencies, as shown by the strong spectral dependence on illumination intensity. Cell efficiencies up to 2.8% under solar conditions have been obtained.
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Molecular modification of dye-sensitized, mesoporous TiO2 electrodes changes their electronic properties. We show that the open-circuit voltage (V(oc)) of dye-sensitized solar cells varies linearly with the dipole moment of coadsorbed phosphonic, benzoic, and dicarboxylic acid derivatives. A similar dependence is observed for the short-circuit current density (I(sc)). Photovoltage spectroscopy measurements show a shift of the signal onset as a function of dipole moment. We explain the dipole dependence of the V(oc) in terms of a TiO2 conduction band shift with respect to the redox potential of the electrolyte, which is partially followed by the energy level of the dye. The I(sc) shift is explained by a dipole-dependent driving force for the electron current and a dipole-dependent recombination current.
Article
We demonstrate tuning of the electronic level positions with respect to the vacuum level in colloidal InAs nanocrystals using surface ligand exchange. Electrochemical as well as scanning tunneling spectroscopy measurements reveal that the tuning is largely dependent on the nanocrystal size and the surface linking group, while the polarity of the ligand molecules has a lesser effect. The implications of affecting the electronic system of nanocrystal through its capping are illustrated through prototype devices.
Article
Different-sized CdSe quantum dots have been assembled on TiO2 films composed of particle and nanotube morphologies using a bifunctional linker molecule. Upon band-gap excitation, CdSe quantum dots inject electrons into TiO2 nanoparticles and nanotubes, thus enabling the generation of photocurrent in a photoelectrochemical solar cell. The results presented in this study highlight two major findings: (i) ability to tune the photoelectrochemical response and photoconversion efficiency via size control of CdSe quantum dots and (ii) improvement in the photoconversion efficiency by facilitating the charge transport through TiO2 nanotube architecture. The maximum IPCE (photon-to-charge carrier generation efficiency) obtained with 3 nm diameter CdSe nanoparticles was 35% for particulate TiO2 and 45% for tubular TiO2 morphology. The maximum IPCE observed at the excitonic band increases with decreasing particle size, whereas the shift in the conduction band to more negative potentials increases the driving force and favors fast electron injection. The maximum power-conversion efficiency </=1% obtained with CdSe-TiO2 nanotube film highlights the usefulness of tubular morphology in facilitating charge transport in nanostructure-based solar cells. Ways to further improve power-conversion efficiency and maximize light-harvesting capability through the construction of a rainbow solar cell are discussed.
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