Shift Of Conduction Band Edge Research Articles

Electronic Structure Characterization of La Incorporated Hf-Based High-<I>k</I> Gate Dielectrics by NEXAFS

The electronic structures of lanthunum (La) incorporated hafnium (Hf)-based oxides (HfLaO) and their silicate (HfLaSiO) films were investigated by the Near Edge X-ray Absorption Fine Structure (NEXAFS) technique. The oxygen (O) K-edge spectra, which reflected the hybridized Hf 5d state with the O 2p orbital, were found to reveal features of the unoccupied state of the metal oxides, as well as the conduction-band edge. We also found that, while La incorporation into the Hf-based oxides simply changed the features of the conduction-band structure, subsequent thermal annealing of the La-incorporated films led to a conduction-band edge shift due to an interface silicate reaction and/or local bond rearrangement depending on the La concentration and annealing temperature. The impact of La incorporation into the Hf-based high-k materials on the electronic structure is discussed by taking into account the intrinsic nature of these metal oxides.

Lithium-Modulated Conduction Band Edge Shifts and Charge-Transfer Dynamics in Dye-Sensitized Solar Cells Based on a Dicyanamide Ionic Liquid

Lithium ions are known for their potent function in modulating the energy alignment at the oxide semiconductor/dye/electrolyte interface in dye-sensitized solar cells (DSCs), offering the opportunity to control the associated multichannel charge-transfer dynamics. Herein, by optimizing the lithium iodide content in 1-ethyl-3-methylimidazolium dicyanamide-based ionic liquid electrolytes, we present a solvent-free DSC displaying an impressive 8.4% efficiency at 100 mW cm(-2) AM1.5G conditions. We further scrutinize the origins of evident impacts of lithium ions upon current density-voltage characteristics as well as photocurrent action spectra of DSCs based thereon. It is found that, along with a gradual increase of the lithium content in ionic liquid electrolytes, a consecutive diminishment of the open-circuit photovoltage arises, primarily owing to a noticeable downward movement of the titania conduction band edge. The conduction band edge displacement away from vacuum also assists the formation of a more favorable energy offset at the titania/dye interface, and thereby leads to a faster electron injection rate and a higher exciton dissociation yield as implied by transient emission measurements. We also notice that the adverse influence of the titania conduction band edge downward shift arising from lithium addition upon photovoltage is partly compensated by a concomitant suppression of the triiodide involving interfacial charge recombination.

Characterization of the Interface Properties and Processes in Solid State Dye-Sensitized Solar Cells Employing a Perylene Sensitizer

We recently reported on a perylene sensitizer, ID176, which performs much better in solid state dye-sensitized solar cells than in those using liquid electrolytes with iodide/tri-iodide as the redox couple (J. Phys. Chem. C2009, 113, 14595−14597). Here, we present a characterization of the sensitizer and of the TiO2/dye interface by UV−visible absorption and fluorescence spectroscopy, spectroelectrochemistry, photoelectron spectroscopy, electroabsorption spectroscopy, photoinduced absorption spectroscopy, and femtosecond transient absorption measurements. We report that the absorption spectrum of the sensitizer is red-shifted by addition of lithium ions to the surface due to a downward shift of the excited state level of the sensitizer, which is of the same order of magnitude as the downward shift of the titanium dioxide conduction band edge. Results from photoelectron spectroscopy and electrochemistry suggest that the excited state is largely located below the conduction band edge of TiO2 but that there are states in the band gap of TiO2 which might be available for photoinduced electron injection. The sensitizer was able to efficiently inject into TiO2, when a lithium salt was present on the surface, while injection was much less effective in the absence of lithium ions or in the presence of solvent. In the presence of the hole conductor 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-MeOTAD) and LiTFSI, charge separation was monitored by the emergence of a Stark shift of the dye in transient absorption spectra, and both injection and regeneration appear to be completed within 1 ps. Regeneration by spiro-MeOTAD is therefore several orders of magnitude faster than regeneration by iodide, and ID176 can even be photoreduced by spiro-MeOTAD.

An Energetic and Kinetic View on Cyclopentadithiophene Dye-Sensitized Solar Cells: The Influence of Fluorine vs Ethyl Substituent

A comprehensive understanding on the structure−property relationship of metal-free organic chromophores in dye-sensitized solar cells will form the basis of further design of advanced materials and realization of considerable enhancement of device performance, boosting the widespread utilization of solar energy at an affordable cost. In this paper we report profound influences of the fluorine versus ethyl substituent of the cyclopentadithiophene conjugated spacer of push−pull photosensitizers, upon the energetic and kinetic characteristics of dye-sensitized solar cells. Joint electrochemical and spectral measurements reveal that with respect to the fluorine-containing dye, the ethyl counterpart exhibits a 0.18 V more negative excited-state redox potential upon anchoring on titania, which overwhelms the 0.01 eV upward conduction band edge shift of the dye-coated titania film. The ethyl-correlated more favorable dye/titania interface energetics benefits a remarkably higher electron injection yield, which is verified with the transient emission and photocurrent action spectrum measurements. Furthermore, analysis on the electrical impedance data affords a notably smaller recombination reaction order for the cell made from the ethyl-substituting chromophore than that of the fluorine counterpart, accounting for a remarkably suppressed interfacial charge recombination with triiodide and thus explaining the observed higher open-circuit photovoltage.

Preparation of highly efficient gel-state dye-sensitized solar cells using polymer gel electrolytes based on poly(acrylonitrile-co-vinyl acetate)

A highly efficient gel-state electrolyte was fabricated by using poly(acrylonitrile -co-vinyl acetate) (PAN–VA) as the gelator of an 3-methoxypropionitrile (MPN)-based liquid electrolyte and was applied in dye-sensitized solar cells (DSSCs). The VA segaments act to dissolve the copolymer into the electrolyte, forming a gel-state structure. The electric conductivity of the gel-state electrolyte is comparable to that of the liquid electrolyte, attributed to the enhancement effect of the AN segments to the dissociation of LiI and 1-propyl-2,3-dimethylimidazolium iodide (DMPII). This effect also leads to a slightly downward shift of the TiO2 conduction band edge toward positive potentials. The energy conversion efficiency of the DSSC achieved by using this gel-electrolyte is 8.34%, which is 97% the value of the liquid-state cell (8.63%).

Anion-correlated conduction band edge shifts and charge transfer kinetics in dye-sensitized solar cells with ionic liquid electrolytes

In solvent-free ionic liquid electrolytes for efficient dye-sensitized solar cells, iodide and non-iodide melts are ordinarily blended in order to attain a high dynamic fluidity and thereby meet a fast mass transport of electroactive species. This common tactic could bring forth a prominent impact of various anions on cell efficiency by altering photocurrent and/or photovoltage. Herein we report evident effects of the dicyanamide versus tetracyanoborate anion on the energetics of titania conduction band edge and the kinetics of multi-channel charge-transfer reactions in cells employing a high absorption coefficient ruthenium sensitizer C106. A slightly shorter photoluminescence lifetime of C106 grafted on alumina is probed for the tetracyanoborate-based reference cell with respect to the dicyanamide counterpart. However, owing to a more favourable thermodynamic driving force of ∼90 meV, the tetracyanoborate anion prompts an almost 3-fold faster electron injection from the excited-state dye to titania than dicyanamide, leading to a higher charge separation yield, which is in good agreement with an almost indistinguishable ratio of external quantum efficiency enhancement in the whole spectral response region. Compared to tetracyanoborate, the presence of dicyanamide at the titania/electrolyte interface evokes a 27-fold smaller interfacial electron exchange rate (K) with triiodide, accounting for the open-circuit photovoltage variation observed in current-voltage measurements.

Effects of Mg salt-modified TiO 2 on the photovoltaic performance of dye-sensitized solar cells

This work reports on the preparation of Mg salts [MS; MgCO 3 and Mg(CH 3COO) 2]-modified TiO 2 electrodes by very simple dip coating and effects on the photovoltaic properties of dye-sensitized solar cells (DSSCs) with them. The DSSC with MS-modified TiO 2 layer showed an increase in short-circuit current ( J sc ) and open-circuit voltage ( V oc) , resulting in a power conversion efficiency (PCE) of 8.48% compared to that (7.27%) of reference device with bare TiO 2. The improved J sc value was attributed to the increased dye adsorption. Electrochemical impedance spectroscopy, dark current–voltage and incident photon-to-current conversion efficiency studies revealed that the V oc enhancement was caused by the conduction band edge shift of TiO 2 to a more negative potential due to the formation of a surface dipole.

A hybrid functional scheme for defect levels and band alignments at semiconductor–oxide interfaces

AbstractWe introduce a theoretical scheme to study defect energy levels and band alignments at semiconductor–oxide interfaces. The scheme relies on hybrid functionals to overcome the band gap underestimation typically achieved with semilocal density functionals. For atomically localized defects, the more accurate description achieved with hybrid functionals does not lead to significant shifts of the charge transition levels, provided these levels are referred to a common reference potential. This result effectively decouples the shifts of the band edges with respect to the defect levels. We also show that relative shifts of conduction and valence band edges as determined by exact nonlocal exchange lead to band offsets in excellent agreement with experimental values for several semiconductor–oxide interfaces. The proposed scheme is illustrated through a series of applications, including the dangling bond defects in silicon and germanium, the charge state of the O2 molecule during silicon oxidation, and the oxygen vacancy in Si–SiO2–HfO2 stacks.

Influence of Lithium Ion Concentration on Electron Injection, Transport, and Recombination in Dye-Sensitized Solar Cells

Varying Li+ concentration in the electrolyte of dye-sensitized solar cells equipped with compact TiO2 blocking layers is found to alter the mean slopes of semilogarithmic open-circuit photovoltage−intensity and dark current−voltage plots. Almost identical values of ideality factor or transfer coefficient are required to fit data in the dark and under illumination for each Li+ concentration. It is found that cell characteristics become progressively more “ideal” as Li+ concentration is increased, with a transfer coefficient of ca.1 for 1 M Li+ in the electrolyte. We find that trends in photovoltage−intensity data are well fitted using a model which assumes that electron transfer to acceptor species in the electrolyte occurs from both the conduction band of the TiO2 and an exponential distribution of band gap surface states. Changes in the mean ideality factor and linearity of semilogarithmic photovoltage−intensity plots can be rationalized by considering the variation in overlap between occupied donor states (conduction band and surface states) with electron acceptor states in the electrolyte, as the conduction band edge is shifted positive by increasing Li+ concentration. In accordance with previous studies, this positive shift in conduction band edge is also found to cause a dramatic increase in the photocurrent generation efficiency of the cells, especially in the long-wavelength region of the photocurrent action spectrum. It is argued that this improvement in photocurrent is predominantly due to an increase in wavelength-dependent electron injection efficiency, as opposed to an increase in electron collection efficiency.

Temperature-dependent Hall and photoluminescence evidence for conduction-band edge shift induced by alloying ZnO with magnesium

This work discusses the effect of conduction-band edge shift induced by alloying ZnO with magnesium. Temperature-dependent Hall and temperature-dependent photoluminescence measurements are used to characterize the epitaxial Zn1−xMgxO thin films grown on (111) Si using intervening epitaxial Lu2O3 buffer layers, which prove that the addition of Mg in ZnO shifts the conduction-band edge to higher energy, thus increasing the activation energy of the defect donor states and reducing the n-type background carrier concentration.

Effect of counter electrode, thickness and sintering temperature of TiO 2 electrode and TBP addition in electrolyte on photovoltaic performance of dye sensitized solar cell using pyronine G (PYR) dye

Dye sensitized solar cells (DSSCs) assembled with nano-crystalline TiO 2 adsorbed with pyronine G (PYR) dye as photoanode and polar solvent-treated poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) coating on a conductive glass (fluorine-doped thin oxide, FTO), as a counter electrode were studied. It was found that the DSSC using carbon black (0.2 wt%) modified DMSO-PEDOT:PSS conductive coating as a counter electrode resulted highest power conversion efficiency. This is attributed to the role of DMSO in enhancing the conductivity of a PEDOT:PSS film and carbon black plays a vital role in increasing the catalytic activity of the film due to its larger surface area. The solar cell performance was studied as a function of sintering temperature and thickness of TiO 2 layer, dye sensitization time of TiO 2 electrode and electrolyte composition. The DSSCs consist of TiO 2 electrode sintered at a temperature of 450 °C shows higher photocurrent attributed to the formation of higher crystalline film. This enhancement in the interconnection between particles results a swift diffusion of electrolyte and good transportation of electrons in the film. All types of DSSC possess the same dependence of performance on the photoanode thickness, i.e. the efficiency increases with the anode thickness to a maximum value, and then decreases slightly when the thickness of photoanode increases further. The effect of pyridine derivative addition in electrolyte on the performance of photovoltaic parameters of DSSCs has also been investigated. A reduced interface defect density and a resulting reduction of charge carrier recombination were found to be the main mechanism for an increased open circuit voltage of DSSCs with TBP addition in the electrolyte. The negative shift of conduction band edge of TiO 2 after the addition of TBP in electrolyte was also to be a factor in improving the open circuit voltage. The over all power conversion efficiency of the DSSCs with TBP in the electrolyte is about 5.24%.

Thermopiezoelectric effects on optoelectronic properties of CdTe/ZnTe quantum wires

AbstractWe investigate thermopiezoelectric field distributions and their effects on electronic properties of the highly strained CdTe/ZnTe quantum wires. The full 3D coupled thermoelectromechanical formulation, consisting of balance equations for heat transfer, electrostatics and the mechanical field, is developed. Effects of thermal loadings on piezoelectric fields are determined for rectangular CdTe/ZnTe quantum wire systems. The conduction band edge shifts due to thermoelectromechancial loadings are calculated. A single band Schrödinger equation is used to determine eigenstates for electrons and the analysis of classical effects are presented. Piezoelectric quantities are observed to be relatively less sensitive to thermal loadings. However, these effects are observed to be important in the determination of electronic/optoelectronic properties of quantum wire systems. The results presented here help improve our understanding of the classical effects on electronic structures and optoelectronic properties of CdTe/ZnTe quantum wires. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Efficient Dye-Sensitized Solar Cell Based onoxo-Bacteriochlorin Sensitizers with Broadband Absorption Capability

Two dye sensitizers, methyl trans-32-carboxy-8-deethyl-7-ethyl-8-oxo-pyropheophorbide-a (BChlorin-1) and methyl trans-32-carboxy-7-demethyl-8-methyl-7-oxo-pyropheophorbide-a (BChlorin-2), with stable bacteriochlorin skeletons were synthesized and applied to dye-sensitized solar cells. Both sensitizers absorb the light all over the visible region owing to partial saturation of the two pyrrole rings on the Qx transition dipole. When they were deposited on a TiO2 film, the J-aggregates of the sensitizers are partially formed to give broad and red-shifted Qy bands. The surface coverage of TiO2 film by BChlorin-2 is much larger than by BChlorin-1, suggesting the former sensitizer forms more serious aggregation on the surface of TiO2, and this could cause more exciton annihilation to reduce the photocurrent of solar cell. The frontier molecular orbitals of both sensitizers obtained from the DFT calculations show no distinguishable difference. Extended calculations on the dye−TiO2Na model system suggest that additional LUMO + 2 orbital in BChlorin-1 may also contribute to the difference in photocurrent. The larger photovoltage in BChlorin-1 sensitized solar cell was attributed to a less efficient charge recombination in the dye−TiO2 interface to give a longer electron lifetime (τ). Additional 4-tert-butylpyridine in the electrolyte significantly reduced the photocurrent and the solar energy-to-electricity conversion efficiency (η) of the solar cells, especially when BChlorin-2 was employed as a sensitizer. This dramatic decrease was attributed to the shift of conduction band edge (CBE) of TiO2 to a negative potential above the molecular Fermi level (MFL) of the sensitizers and suppressed the electron injection from the MFL of sensitizer to CBE of TiO2. Coadsorption of BChlorin-1 with chenodeoxycholic acid (CDCA) could break the dye aggregate and improve the incident photon-to-current conversion efficiency at the absorption bands maxima. BChlorin-1 sensitized solar cells coadsorbed with CDCA gave a longer electron lifetime and a larger diffusion coefficient than the cell without CDCA. By coadsorbing with 5 mM CDCA in solution, the BChlorin-1 sensitized solar cell gave a highest performance with short-circuit photocurrent = 18.4 mA cm−2, open-circuit photovoltage = 0.54 V, fill factor = 0.66, and η = 6.6% under the air mass AM 1.5 (100 mW cm−2) illumination.

Determination of the Exciton Binding Energy in CdSe Quantum Dots

The exciton binding energy (EBE) in CdSe quantum dots (QDs) has been determined using X-ray spectroscopy. Using X-ray absorption and photoemission spectroscopy, the conduction band (CB) and valence band (VB) edge shifts as a function of particle size have been determined and combined to obtain the true band gap of the QDs (i.e., without an exciton). These values can be compared to the excitonic gap obtained using optical spectroscopy to determine the EBE. The experimental EBE results are compared with theoretical calculations on the EBE and show excellent agreement.

Photoinduced electron transfer from a terrylene dye to TiO 2: Quantification of band edge shift effects

A terrylene chromophore exhibiting a high extinction coefficient has been developed as a sensitizer for photovoltaic applications. The photophysical and photochemical properties of the dye were analyzed both experimentally and theoretically. Terrylene-sensitized nanocrystalline TiO 2 solar cells yielded good photocurrents providing more than 60% in external quantum efficiency. The photoinduced electron transfer from the dye to TiO 2 was found to be very sensitive to conduction band edge shifts in TiO 2 induced, either by changes in the composition of the redox electrolyte or by UV-illumination. This sensitivity was observed in quantum efficiencies for photocurrent generation of terrylene-sensitized solar cells and in photoinduced absorption experiments. The conduction band shifts were quantified using charge extraction methods. The observed sensitivity of the injection efficiency suggests that photoinduced electron transfer occurs from the relaxed excited state, possibly due to poor electronic coupling between TMIMA excited states and TiO 2 conduction band states.

Temperature tunability of size in CdS nanoparticles and size dependent photocatalytic degradation of nitroaromatics

Size tunability of thiophenol capped CdS nanoparticles (NPs) has been achieved by controlling the temperatures in situ. Synthesis at 5 °C produced stable particles of smallest size having narrow size distribution and high photoluminescence quantum yield. The photoluminescence of thiophenol capped CdS NPs was quenched by the nitroaromatic compounds. The Stern–Volmer constant of dinitrobenzene was about 15-fold higher than nitrobenzene, which indicates that introduction of nitro groups in the benzene ring increases the quenching efficiency. Further, the as-prepared CdS NPs were found to display size dependent photocatalytic activity towards degradation of nitroaromatics. The catalytic efficiency of CdS quantum particles was quintupled with decrease in particle size from 5.8 to 3.8 nm. An empirical equation has been derived to correlate the catalytic efficiency of the nanoparticles with the twin factors operating in the quantum confinement regime: (i) change in surface to volume ratio and (ii) shift in conduction band edge.

Finite-Size Effect on Band Structure and Photoluminescence of Semiconductor Nanocrystals

Based on a model for size-dependent atomic cohesive energy of nanocrystals, a simple and unified model, without any adjustable parameter, has been established for finite size effect on shifts of valence and conduction band edges [DeltaEV(D) and DeltaEC(D)], and bandgap expansion [DeltaEG(D)| of semiconductor nanocrystals, where Delta denotes the expansion and D is the size of nanocrystals. In the light of this model, DeltaEV(D), DeltaEC(D), and DeltaEG(D) functions increase with dropping D in comparison with the corresponding bulk counterparts. The available experimental results of Si, CdS, ZnSe, and ZnTe nanocrystals confirm the validity of model.

Research on the Effect of Different Sizes of ZnO Nanorods on the Efficiency of TiO2-Based Dye-Sensitized Solar Cells

TiO2-based dye-sensitized solar cells doped with different sizes of ZnO nanorods were fabricated and studied by photoelectrochemical measurements. The results show that the solar conversion efficiency of the dye-sensitized solar cells after the addition of ZnO nanorods (1 wt %) was increased by about 15% compared to that without ZnO nanorods. The effect of different sizes of ZnO nanorods on the charge carrier transport properties has been studied in the composite semiconductor film by means of a transient photovoltage technique. The result indicates that the carrier diffuse rate in the cis-di(isothiocyanato)-bis(4,4‘-dicarboxy-2,2‘-bipyridine)ruthenium(II)-sensitized TiO2/ZnO film electrode was about 1 to 3 orders of magnitude faster than that in the TiO2 electrode. The cyclic voltammograms suggest that the conduction band edge shifts toward the vacuum level after the addition of ZnO nanorods in working electrodes, which may be the main cause of the enhancement of the open-circuit photovoltage (Voc). The experimental results indicate that the addition of ZnO nanorods can improve charge carrier transport, decrease recombination, enhance Voc, and increase the efficiency of energy conversion.

Influence of the TiCl4Treatment on Nanocrystalline TiO2Films in Dye-Sensitized Solar Cells. 2. Charge Density, Band Edge Shifts, and Quantification of Recombination Losses at Short Circuit

Chemical bath deposition of TiO2 from TiCl4 is an often used treatment that improves the photocurrent from dye-sensitized TiO2 solar cells. In this paper, charge density and kinetic data are used to show that the main effects of this treatment are an 80 mV downward shift in the TiO2 conduction band edge potential and a 20-fold decrease in the electron/electrolyte recombination rate constant. Together, these changes increase the quantum efficiency of charge separation at the interface, thus providing the observed increase in the photocurrent. The reduction in the recombination rate constant allows a greater concentration of electrons to accumulate at Voc, thus offsetting the Voc loss otherwise expected from the conduction band edge shift. Photocurrent transients and charge extraction data are used to show that the TiCl4 treatment has little effect on the transport of electrons at short circuit. The electron/electrolyte recombination rate constant at short circuit has been measured with the CCTPV (Constant Current Transient PhotoVoltage) technique. The results further confirm that any improvements in transport could not cause the beneficial effect of the TiCl4 treatment. Verification of the CCTPV technique is undertaken by comparison to transient absorption and by a model of the technique. Charge separation in dye-sensitized cells concerns two steps, charge injection and dye regeneration. Transient optical experiments to determine which process is improved by the TiCl4 treatment are discussed.

Investigations of the electron energy structure and phase composition of porous silicon with different porosity

Energy structure of the valence and conduction bands together with phase composition of porous silicon (por-Si) obtained at different etching times were investigated using ultrasoft X-ray emission spectroscopy (USXES) and X-ray absorption near-edge structure (XANES) techniques. USXES and XANES spectra represent local partial density of states in valence and conduction band, correspondingly. XANES data were obtained with the use of synchrotron radiation at synchrotron BESSY II. Porous silicon layers were obtained on different types of single-crystalline silicon (1 0 0) and (1 1 1) substrates by standard electrochemical etching in HF solution with etching times from 1 to 10 min. USXES and XANES investigations show formation of Si nanocrystals covered by amorphous phase together with nonstoichiometric silicon sub-oxide SiO x ( x ∼ 1.3) in the surface layers. XANES investigations show that the increase of sample porosity leads to the shift of conduction band edge to the higher energy range that corresponds to increase of the band gap width in porous silicon up to ∼1.4 eV.