CdS Heterojunction Research Articles

How do Buffer Layers Affect Solar Cell Performance and Solar Cell Stability?

ABSTRACTBuffer layers are commonly used in the optimization of thin-film solar cells. For CuInSe2-and CdTe-based solar cells, multilayer transparent conductors (TCOs, e.g., ZnO or SnO2) are generally used in conjunction with a CdS heterojunction layer. Optimum cell performance is usually found when the TCO layer in contact with the CdS is very resistive or almost insulating. In addition to affecting the open-circuit voltage of a cell, it is commonly reported that buffer layers affect stress-induced degradation and transient phenomena in CdTe- and CuInSe2-based solar cells. In amorphous silicon solar cells, light-induced degradation has a recoverable and a nonrecoverable component too, and the details of the mechanism may depend on the p-type contact layer. Because of the similarity of transients and degradation in dissimilar material systems, it is suggested that degradation and recovery are driven by carriers rather than by diffusing atomic species. The question that must be addressed is why, not how, species diffuse and atomic configurations relax differently in the presence of excess carriers. In this paper, I suggest that the operating conditions of a cell can change the carrier transport properties. Often, excess carriers may enhance the conductance in localized regions (“filaments”) and buffer layers; limiting current flow into such filaments may therefore control the rate and amount of degradation (or recovery).

Depth profiling sulphur in bulk CdTe and [formula omitted] thin film heterojunctions

Polycrystalline CdTeCdS heterojunction solar cells are a possible candidate for the low cost, high efficiency conversion of solar energy. The formation of an intermediate CdS x Te 1− x layer during a high temperature annealing stage is believed to increase optical absorption and decrease cell efficiency. S diffusion in single crystal CdTe has been investigated by NRA using the 32S (d,p o) 33S nuclear reaction, at a deuteron energy of 2 MeV. Details of the NRA depth profiling procedure are given, which was found to be relatively straightforward and suitable for use on a small Van de Graaff accelerator. The resulting diffusion parameters are compared to those obtained by SIMS using a Cs + primary ion beam, examining negative secondary ions. The diffusion coefficients were found to be 1.1 × 10 −15cm 2 s −1 at 450°C and ∼8 × 10 −15cm −1 s at 550°C. S diffusion in thin films was also investigated by 2 MeV 4He + RBS on annealed polycrystalline CdSCdTe multilayers.

Effects of Cu on CdTe / CdS Heterojunction Solar Cells with Au/Cu Contacts

solar cells were fabricated by depositing an Au/Cu contact layer with various thicknesses and deposition conditions on polycrystalline structures. Cu has a dual effect on the cell performance; it helps in the formation of better ohmic contacts to and increases the acceptor doping concentration, but excess Cu could diffuse all the way to the interface to form recombination centers and shunt paths and degrade cell performance. Both secondary ion mass spectroscopy and capacitance‐voltage measurements confirm the incorporation of Cu into the bulk of the films. Cd outdiffusion toward the surface of was observed during the Au/Cu deposition. The thickness of Cu plays a critical role in the solar cell performance because both the series and shunt resistances decrease with the increase in Cu thickness. Carrier transport analysis showed that depletion region recombination dominates the current transport in the solar cells with an Au/Cu contact. The transport mechanism remains the same in spite of the amount of Cu incorporation into the bulk and interface. Higher Au/Cu deposition rates result in a greater excess of Cd at the surface, leaving more Cd vacant sites below the surface. This causes an increase in dopant concentration but also results in a higher defect density and reduced cell performance.

Test of band offset commutativity by photoemission from an i n s i t u grown ZnTe/CdS/ZnTe quantum well

We have characterized the ZnTe–CdS heterojunction by valence and core level photoemission with synchrotron radiation, using quantum wells grown in situ on ZnTe(110) substrates. The valence band offset of∼ΔEv=0.9 eV shows that this heterojunction is of the staggered type (type II), rarely encountered in experimental studies. We find evidence for chemical reaction involving Te at the interface. The band offsets at both sides of the CdS quantum well exhibit small differences on the order of 0.1 eV, which are also reflected in the dipole contribution at the interfaces. This study, while reporting only small deviations from commutativity, suggests that dipole contributions of varying magnitude, caused by interface reactions, influence the magnitude of the band offset.

Radiation degradation of solar cells based on InPCdS heterojunctions

The influence of 20.6 MeV proton and 1 MeV electron radiation with integral flux densities 10 12 cm −2 and 5×10 15 cm −2, respectively, on InPCdS solar cells was investigated. Stability of parameters of the InPCdS solar cells to radiation is demonstrated to be due to a low magnitude of the damage coefficient of pInP and to the unilateral collection of carriers from the narrow gap basic material (InP).

A device-oriented study of CdS films deposited by a hot-wall technique

A device-oriented study was carried out on thick CdS films (25–30 μm) suitable for solar-cell applications deposited on glass substrates, using a hot-wall technique at a pressure of 1 × 10−6 Torr (1 Torr = 133 Pa). Films were characterized using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The degree of preferred orientation was observed using a simplified version of the Schulz method. Films grown under nominally identical conditions showed two types of growth habit. Type-I films were composed of crystallites having their c axis in a cone with a half angle of 6° about the substrate normal. In Type-11 films, the growth direction was within 11° of the normal to the substrate. Surface grain dimensions varied from 2 to 6 μm, and the room-temperature resistivity varied from 1 to 5 Ω∙cm for the following deposition parameters: substrate temperatures of 230–250 °C, hot-wall temperatures of 370–400 °C, and a deposition rate of 1.2 μm∙min−1. Front-wall CuxS–CdS heterojunctions were made using a hot CuCl dip. Current–voltage characteristics were measured at room temperature. Without optimizing the cell design, open-circuit voltages of 0.45 V and short-circuit currents of 20–30 mA∙cm−2 under AM2 illuminations were observed.

Deposition and properties of zinc telluride and cadmium zinc telluride films

Thin films of ZnTe and Cd1−xZnxTe (x<0.5) have been deposited by the direct combination of the elements on the surface of W/graphite, ceramic, and glass substrates at 550–600 °C in a hydrogen or helium atmosphere. Their microstructure, crystallographic, optical, and electrical properties were studied. Because of the greater stability of ZnTe than CdTe, the Zn/Cd molar ratio in Cd1−xZnxTe films is always less than that in the gas phase. The optical band gap of ZnTe has been determined to be 2.25 eV and that of Cd1−xZnxTe is a linear function of the composition. Preliminary work on Cd1−xZnxTe /CdS heterojunctions has also been carried out.

Analysis of the electrical characteristics of sprayed Cu 2SCdS heterojunctions

In this paper we first present an analysis of current-voltage characteristics obtained in the dark and under illumination; this allows us to determine the cell parameters as well as their dependence on illumination intensity for different wavelengths. Capacitance measurements were carried out to give us more information.

Electrochemical study of the thermal treatment effects on Cu xS thin films

The effects of thermal treatments on the stoichiometry and photovoltaic propierties of Cu xS thin films have been studied. It has been observed that the evaporation of a thin copper films on Cu xS, followed by thermal treatment in vacuum, improves the Cu xS CdS heterojunctions due to a rire in the stoichiometry and fill factor.

Electron and hole traps in the Cu xS-CdS heterojunction

DLTS and ODLTS measurements have been performed on both undoped and copper doped CdS and on Cu x S−CdS heterojunctions. Three electron traps with ionisation energies of 0.18, 0.36 and 0.44 eV were found in undoped material. In copper doped CdS two additional hole traps attributed to the copper levels, with ionisation energies of 0.34 and 0.93 eV were found. The 0.93 eV hole trap was also observed in the heterojunction, indicating that copper had diffused across the junction.

Photoconductive properties of the evaporated CdTe-sintered CdS heterojunction

CdTeCdS heterostructures were prepared by evaporating CdTe onto photoconductive sintered CdS films doped with Cu. In the photoconductive spectrum an additional sensitivity peak appeared at 840 nm. The photoconductive mechanism was identified as photoconduction in the CdS film due to injected carriers through the CdTeCdS heterojunction. The dependence of the 840 nm sensitivity on Cu doping rate was described and the sensitivity spectrum due to the injected carriers was separated from the total spectrum.

Photodetection properties of Cu 2SCdS heterojunction solar cells

Cu 2SCdS heterojunctions were made by dipping evaporated or sprayed CdS layers in a CuCl solution. The spectral responses of these cells were dependent on the illumination level and the electrical bias. When the cells were reverse biased and illuminated with CdS band gap radiation, the reverse current drastically increased. This is a consequence of the shrinking of the junction barrier width by the trapping of photoholes in the CdS space charge region by negatively charged copper centres. The reverse current, which is a Zener tunnelling current, is very sensitive to this shrinking. Similar behaviour was observed when the cells were forward biased; however, the enhancement of the direct current was weaker. The result is that the cells operate as a switch activated by CdS band gap radiation.

Effects of the Basal Plane of CdS Single Crystal on the Growth of Cu2S and the Electric Properties of the Cu2S–CdS Solar Cells

Different growth behaviour of the CuxS layer was obtained experimentally when the CuxS layer was grown chemically on the basal plane of the CdS. The growth process of the layer is discussed by assuming the different diffusion velocities of Cd element in the CuxS and Cu2S. The electric properties of the non-heat treated single crystal Cu2S–CdS heterojunction diode are also discussec with respect to the variation of the constituent elements in the transition region of the heterojunction diode.

Barrier Model of Cu2S–CdS Heterojunction Diodes Estimated from Open-Circuit Photovoltage Spectrum and Cathodoluminescence

Barrier Model of Cu2S–CdS Heterojunction Diodes Estimated from Open-Circuit Photovoltage Spectrum and Cathodoluminescence

Luminescent properties of CuGaS2 doped with Cd or Zn

The photoluminescence spectra of CuGaS2 crystals doped with Cd or Zn vary continuously from a green emission (0.51 μ) to an orange emission (∼ 0.59 μ) depending upon the concentrations of Cd and Zn. The room-temperature internal quantum efficiency is reproducibly 0.2%. The electroluminescent efficiencies of CuGaS2: CdS heterojunction LEDs fabricated by using these substrates are not appreciably higher than the best efficiencies reported earlier.

Light-emitting mechanism of ZnTe–CdS heterojunction diodes

ZnTe–CdS heterojunction LED, which was prepared on the ZnTe substrate by CdS vapor growth, showed the red light emission at 300°K. The EL spectra had a peak at 1.78–1.80 eV at 300°K, and two peaks at about 2.10 and 1.55 eV at 77°K. The existence of the ZnxCd1−xTe mixed crystal with a graded band structure at the heterojunction interface was shown from the results of the photovoltage spectra and transmittance analysis of the heterojunction. The formation of the mixed crystal was found to be due to Cd diffusion into the ZnTe substrate during the epitaxial growth. From photoluminescence measurements, it is concluded that the red emission of ZnTe–CdS LED may originate from the ZnxCd1−xTe mixed crystal layer. Thus, efficient red emission could be obtained in our ZnTe–CdS heterojunctions on account of the mixed crystal layer, which reduced high-energy barriers for electrons and holes.

Heat treatment effects in Cu2S–CdS heterojunction photovoltaic cells

The photovoltaic properties of single-crystal Cu2S–CdS heterojunctions have been investigated as a function of heat treatment by detailed measurements of the dependence of short-circuit current on photon energy, temperature, and the state of optical degradation or enhancement. A coherent picture is formulated for the relationship between enhancement and optical degradation and for their effect on the transport of short-circuit photoexcited current and dark forward-bias current in the cell. Optical degradation in the Cu2S–CdS cell is shown to be closely identical to optical degradation of lifetime in a homogeneous CdS: Cd: Cu crystal, which indicates that the CdS: Cu layer near the junction interface controls carrier transport in the cell. It is proposed that both the photoexcited short-circuit current and the dark forward-bias current are controlled by a tunneling-recombination process through interface states. Changes in the junction profile resulting from enhancement and/or optical degradation modify the electric field at the barrier to modulate both currents.

Preparation and Properties of Nonheat‐Treated Single Crystal Cu2 S ‐ CdS Heterojunctions

The photoelectronic properties of single‐crystal heterojunctions have been investigated. The heterojunctions were formed on the A or Bfaces of conducting single crystals by dipping in a cuprous chloride solution at 75°C. No separate heat‐treatment was given. The cells typically had monochromatic quantum efficiencies greater than 10% in the region 0.5–0.9micro;. The photoresponse is due to carrier pair generation in the , with transfer of electrons across the interface into the . The forward current at and below room temperature is dominated by tunneling of electrons through interface states. A general band diagram of the heterojunction is discussed. Consistent differences in the behavior of A and Bface samples were noted with respect to the growth kinetics of the , the maximum photovoltage at 300°K, the I–V characteristics, and the junction capacitance. The effect of uniaxial stress on the capacitance of A and Bface samples suggests a piezoelectric mechanism.

Electrical and photoelectric characteristics of pSi - nCdS heterojunctions: A G Rokakh and N M Tsukerman, Izv VUZ Fiz, No 10, 1971, 145–146 ( in Russian)

Electrical and photoelectric characteristics of pSi - nCdS heterojunctions: A G Rokakh and N M Tsukerman, Izv VUZ Fiz, No 10, 1971, 145–146 ( in Russian)

Memory characteristics of p type-germanium/n type-cadmium-sulphide heterojunctions

The V/I characteristics of p Ge-n CdS heterojunctions exhibit a hysteresis or memory characteristic which is the result of two diode conduction states of resistances 106 and 10Ω. The device switches from the high to the low state in less than 30 ns under reverse bias, and resetting to the high-resistance state occurs in less than 100 ns when a threshold forward bias current is exceeded. In either state, the device retains its state for periods greater than two weeks and appears to have potential application as a fast, nonvolatile memory element.