Domains In CdS Research Articles

High-field domains in CdS adjacent to a junction of p-type solar cells

A thin cover layer (150 Å preferred) of copper-doped CdS, when applied on top of any p-type solar cell, can connect this cell directly to an electron-blocking electrode without a pn-junction and increases the open circuit voltage close to its theoretical value; in the example of a CdS/CdTe cell, it increases Voc to its extrapolated value at T = 0 K of the band gap of 1.45 eV. This is caused by a high-field domain that is attached to the junction and limits the field to below tunneling to prevent junction leakage and connects to the CdS that has turned p-type. The large Debye length exceeding the thickness of the CdS forces a direct connection to the electron-blocking cathode with holes tunneling into the metal. The difference of junction-attached high-field domains to the electrode-attached domains, which were described earlier, are given and the consequences are delineated by increasing the conversion efficiency from 8% to 16% in CdTe, while also causing some series resistance limitation. The effect of the added CdS layer is discussed by drawing a to-scale model of the CdS/CdTe solar cell from all experimentally available data and the assumption of the continuity of the hole current. A small jump of the valence band downward is caused by interface recombination. The assistance of high-field domains in CdS is also exemplified by the results of an extremely simple production procedure of the CdS/Cu2S solar cells.

Polymer/Nanocrystal Hybrid Solar Cells: Influence of Molecular Precursor Design on Film Nanomorphology, Charge Generation and Device Performance.

In this work, molecular tuning of metal xanthate precursors is shown to have a marked effect on the heterojunction morphology of hybrid poly(3-hexylthiophene-2,5-diyl) (P3HT)/CdS blends and, as a result, the photochemical processes and overall performance of in situ fabricated hybrid solar cells. A series of cadmium xanthate complexes is synthesized for use as in situ precursors to cadmium sulfide nanoparticles in hybrid P3HT/CdS solar cells. The formation of CdS domains is studied by simultaneous GIWAXS (grazing incidence wide-angle X-ray scattering) and GISAXS (grazing incidence small-angle X-ray scattering), revealing knowledge about crystal growth and the formation of different morphologies observed using TEM (transmission electron microscopy). These measurements show that there is a strong relationship between precursor structure and heterojunction nanomorphology. A combination of TAS (transient absorption spectroscopy) and photovoltaic device performance measurements is used to show the intricate balance required between charge photogeneration and percolated domains in order to effectively extract charges to maximize device power conversion efficiencies. This study presents a strong case for xanthate complexes as a useful route to designing optimal heterojunction morphologies for use in the emerging field of hybrid organic/inorganic solar cells, due to the fact that the nanomorphology can be tuned via careful design of these precursor materials.

Microstructural and Chemical Investigation of PVD-CdS/PVD-<inline-formula><tex-math> $\hbox{CuIn}_{\hbox{1-x}} {\hbox{Ga}}_{\hbox{x}} {\hbox{Se}}_{\hbox{2}}$</tex-math></inline-formula> Heterojunctions: A Transmission Electron Microscopy Study

We report on a detailed transmission electron microscopy (TEM) study of physical-vapor-deposited (PVD) CdS /CuIn 1-x Ga x Se 2 (CIGS) heterojunctions prepared at the Mia-Soleproduction line. High-resolution TEM images of the heterointerface reveal the coexistence of CdS domains of cubic and hexagonal phases. Both are shown to grow epitaxially on the CIGS surface. Twin boundaries in the CIGS were observed to propagate into the epitaxial CdS and continue through the whole CdS layer. Scanning TEM in combination with energy dispersive X-ray spectroscopy shows the presence of Cu in the CdS up to -20 nm from the heterojunction. These results provide insights into the PVD-CdS/CIGS heterointerface and suggest that buffer layer crystallinity sufficient to produce photocurrent generation may be obtained with further process optimization.

Tuning the Morphology of Au/CdS Nanocomposites through Temperature-Controlled Reduction of Gold-Oleate Complexes

A general synthetic strategy for controlling the shape of gold domains grown onto CdS semiconductor nanocrystals is presented. The colloidal growth of Au nanoparticles is based on the temperature-controlled reduction of Au-oleate complexes on the surface of CdS and allows for precise tuning of nanoparticle diameters from 2.5 to 16 nm simply by adjusting the temperature of the growth solution, whereas the shape of Au/CdS nanocomposites can be controllably switched between matchsticks and barbells via the reaction rate. Depending on the exact morphology of Au and CdS domains, fabricated nanocomposites can undergo evaporation-induced self-assembly on a substrate either through end-to-end coupling of Au domains, resulting in the formation of one-dimensional chains or via side-by-side packing of CdS nanorods, leading to the onset of two-dimensional superlattices.

Ultrafast Carrier Dynamics in Type II ZnSe/CdS/ZnSe Nanobarbells

We employ femtosecond transient absorption spectroscopy to get an insight into ultrafast processes occurring at the interface of type II ZnSe/CdS heterostructured nanocrystals fabricated via colloidal routes and comprising a barbell-like arrangement of ZnSe tips and CdS nanorods. Our study shows that resonant excitation of ZnSe tips results in an unprecedently fast transfer of excited electrons into CdS domains of nanobarbells (<0.35 ps), whereas selective pumping of CdS components leads to a relatively slow injection of photoinduced holes into ZnSe tips (tau(h)= 95 ps). A qualitative thermodynamic description of observed electron processes within the classical limit of Marcus theory was used to identify a specific charge transfer regime associated with the ultrafast electron injection into CdS. Potential photocatalytic applications of the observed fast separation of carriers along the main axis of ZnSe/CdS barbells are discussed.

Synthesis of ZnSe/CdS/ZnSe Nanobarbells Showing Photoinduced Charge Separation

We report on a colloidal synthesis of barbell-shaped nanocrystals comprising a type II heterojunction of ZnSe and CdS domains and showing compelling evidence of photoinduced charge separation at the interface of ZnSe and CdS materials. The nanobarbells were fabricated in a two-step procedure by growing ZnSe caps onto polar facets of CdS nanorods. Under present synthetic conditions, minimal growth of ZnSe shell in the lateral direction and focusing of the barbell length distribution were observed. Formation of epitaxial interfaces between nearly lattice-matched ZnSe and CdS crystal phases was primarily evidenced by the observation of “spatially indirect” fluorescence and long radiative lifetimes corresponding to the decay of charge transfer states.

Efficient Photoinduced Charge Injection from Chemical Bath Deposited CdS into Mesoporous TiO2 Probed with Time-Resolved Microwave Conductivity

We use the contactless time-resolved microwave conductivity (TRMC) technique to investigate the photoconductance of CdS-coated mesoporous TiO2 and ZrO2. The CdS domains were grown directly on the surface of the oxide by a chemical bath deposition method. Mobile charge carriers are generated with a low yield following photoexcitation of CdS grown on ZrO2; there is no injection into the oxide and the short-lived photoconductance signal is attributed to the hole in CdS. In contrast, illumination of CdS domains grown on TiO2 results in very efficient electron injection into the TiO2 nanoparticles, producing long-lived charge carriers. We show that the quantum yield for electron injection into TiO2 per photon absorbed by the CdS sensitizer is close to unity. The photoconductance action spectrum follows the absorption of the CdS domains, which indicates that the charge injection efficiency does not depend on wavelength in the range where the CdS absorbs (425–550 nm).

Suppression of shock excitation and nonlinear process of acoustic domains in CdS caused by a two-step pulse application

The acoustic domains in CdS caused by the two-step pulse are studied with Ef an ultrasonic injection method and Brillouin scattering measurement. When the first-step field Ef is smaller than the synchronous field Ea i.e., Ef?Ea , the factor m of fm ( f; frequency) of the lattice attenuation (αl) increases with increasing Ef and we finally obtained m = 2 at Ef = Ea. This means that the deviation from f2.0 is caused not only by the nonlinear process but also by the shock excitation. On the other hand, when Ef≳Ea, the amplitude of the current waveform decreased drastically and the maximum intensity of the frequency spectra becomes weak and it’s frequency becomes higher. This means that the nonlinear process is suppressed.

Optical Transmission Modulation Mechanisms Caused by Acoustic Domains in CdS

An acoustic domain in CdS crystals modulates a radiated optical beam near the intrinsic absorption edge. This paper reports that the equivalent band-gap alteration caused by the scattering effect and intrinsic absorption are superimposed and cause optical modulation. The equivalent band-gap alteration is dependent on sample thickness but the intrinsic absorption is caused by the Franz-Keldysh effect and is not affected by sample thickness.

Frequency Spectrum of Acoustic Domain Caused by Variable Rise-Time Pulse

This paper is concerned with the behavior of acoustic domain in CdS with use of a variable rise-time of applied pulse. For a rise-time pulse slower than 6 µs, amplified fluxes are from thermal phonons. The peak and width of the flux distributions can be well explained by the small-signal theory. Signal shapes by Brillouin scattering become a trapezoid when the shock is relaxed. The lattice attenuation measured by the ultrasonic injection technique is proportional to the square of acoustic frequency (0.8 GHz to 2 GHz)

Resonant Forbidden Brillouin Scattering in CdS

Resonant forbidden Brillouin scattering by the TA phonon domains in CdS has been observed near the fundamental absorption edge. The results are interpreted with a dielectric theory based on the second derivative of the dielectric constant.

The equivalent gap alteration by the scattering caused by acoustic domains in CdS

An acoustic domain in CdS crystals scatters a radiated optical beam near the intrinsic absorption edge. This paper presents work showing that the equivalent band-gap alteration caused by the scattering effect is observed in 600–1000-μm-thick samples. This alteration is caused by the following: (i) In the CdS crystal the optical transmission of incident light with e∥c is larger than that for incident light with e⊥c (e: polarization of the incident light; c: c axis of the crystal). (ii) When the incident light is scattered by the shear acoustic wave in the sample, the plane of polarization is rotated by 90°. Therefore, the incident light with e⊥c behaves as if it were e∥c and the optical transmission increases.

Mechanism of optical modulation caused by acoustic domains in CdS

Mechanism of optical modulation caused by acoustic domains in CdS

Relation between Phase Memory Effect and Velocity of Trap-Controlled High-Field Domain in CdS

The phase memory effect and the dependence of the domain velocity upon both the light intensity and the bias voltage are investigated in bulk and evaporated CdS. “Incomplete” phase memory effect is found in these CdS. It is found that the dependence of the domain velocity vd upon both the light intensity B and the bias voltage V is represented by vd=cBxVz (x=0.6∼1.0, z=0.7∼1.0, V>Vth), where c is a constant, and Vth is the threshold voltage of the domain formation. The dependence of the domain velocity is explained by assuming that the domain velocity is just the average effective velocity of all the photo-excited electrons. The phase memory effect is explained by this dependence. It is concluded that the mechanism of the phase memory effect must contain at least both the deep hole traps and the deep electron traps.

Change in the energy gap caused by acoustic domains in CdS

This paper shows that tha altered energy gap caused by the acoustic domains in CdS lies between the fundamental optical band gaps e⊥c and e∥c (e: polarization of the incident light; c: c axis of the crystal). The experimental results take into account the errors for both the finite beam diameter and the rotation of polarization of incident light due to scattering.

Properties of Excited Surface-Wave Domains in CdS

The round-trip surface-wave domains are generated on the basal plane of CdS by exciting a surface wave externally. The effect of a transparent adhesive tape and observation by an optical probe method prove these domains to be surface-wave domains. The domain properties are controlled by the excited wave. That is, the excited domain has a narrower spectrum width than the spontaneous domain, and the fmax of the excited domain shifts to the first subharmonic frequency of the excited wave regardless of the carrier density of the sample. The excited wave, the strain of which is greater than 10-5, amplifies surface waves having frequencies near its first subharmonic by the nonlinear interaction through space charges. Then these waves are amplified further by the interaction with drifting electrons, and build up the full-grown excited domain.

Observation of three mechanisms of optical modulation caused by acoustic domains in CdS

This paper presents experimental results concerning optical attenuation caused by acoustic domains in CdS. Three mechanisms are assumed: These are (i) shift in the energy gap, (ii) light scattering by acoustic flux, and (iii) rotation of the plane of polarization of the incident light. These mechanisms are experimentally distinguished. And it is found that in the case of e∥c (where e is the polarization of the incident light and c the c axis of the crystal) three kinds of optical modulation exist, but in the case of e⊥c only the effect of scattering is substantial.

Optical transmission modulation and light scattering by acoustoelectric domains in CdS

The optical transmission modulation observed in the presence of acoustoelectric domains has been investigated in CdS and compared with the light scattering from the acoustic flux. Calculations of the decrease in the optical transmission due to light scattering show that light scattering may account for all the transmission modulation observed in the presence of acoustoelectric domains. The existing arguments for the presence of high ac-electric fields connected with the domains are discussed, and it is concluded that these arguments probably are not correct. [Russian Text Ignored].

Brillouin scattering, piezobirefringence, and dispersion of photoelastic coefficients of CdS and ZnO

We have measured the dispersion of the Brillouin scattering from acoustoelectrical domains in CdS and ZnO. These spectra are compared with the birefringence spectra obtained by applying uniaxial stress. The resonant cancellation of the Brillouin scattering occurs at the spectral position of the isotropic point of the stress-induced birefringence. From these spectra it is concluded that the Brillouin scattering in CdS and ZnO is determined by elasto-optic effects alone. The spectra of some of the photoelastic coefficients have been determined. A model dielectric constant is derived where both ground-state excitons and unbound continuum exciton states contribute. From this dielectric constant and the quasicubic model we calculate all the six independent photoelastic coefficents. By comparison with the experimental results the shear deformation potentials of the valence bands are obtained. It is found that the exchange interaction between the excitons may change the values of the photoelastic coefficients in ZnO about 10%.

Boundary Effect of the Sample on the Brillouin Scattering Intensity Induced by Acoustoelectric Domains in CdS

The Brillouin scattering intensity has been investigated using slow transverse acoustic (T2 mode) waves amplified through acoustoelectric effect in CdS. The experiments have been performed under the two scattering configurations; the incident light polarizations are parallel and perpendicular to the c -axis. The Brillouin scattering cross sections are also derived including the indirect photoelastic effect due to piezoelectrically induced electro-optic effect. The measured scattering intensities are found to agree with the theoretical analysis when the boundary effect of the sample is taken into account.