I. Konovalov

Electrical properties of Cu}In}S absorber prepared on Cu tape (CISCuT)

Abstract An In precursor on a Cu tape, sulfurized under special transient conditions, shows an internal structure with at least four different semiconducting layers. This structure has a rectifying I–V characteristic also without any buffer layer. The space-charge region is located near the top surface in the upper part of the In-rich layer having a columnar structure, and in the top layer. Deeper-lying layers provide a good contact to the In-rich layer, but show no carrier collection themselves. An EBIC investigation and an original thermopower measurement revealed a pnp-structure with the n-type layer being the In-rich one. The lower part of the film contains a short-circuited p–n junction, switched in series in the opposite direction. One proposal for the band structure of the absorber is presented.

Valence band offset at interfaces between CuI and indium sulfides

The energy differences between In 3d5/2, In 4d5/2, and S 2p3/2 core levels are almost the same in β-In2S3, CuIn5S8, AgIn5S8, CuInS2, In6S7, and InS semiconductors. The system of these core levels provides a reference energy that can be used to put the valence band edges of these materials on a common energy scale. The experimental study shows that this common energy scale remains valid within ±0.2 eV for prediction of valence band discontinuities at heterointerfaces between these (and potentially other) indium sulfides and CuI. CuI/CuIn5S8 heterojunction showed no valence band offset and it may be interesting for applications as a diode.

Electronic Properties of Cu-In-S Solar Cells on Cu-Tape Substrate

Thin film solar cells obtained by the “CIS on copper tape” technique are investigated. This technique promises a high throughput capability, but the efficiency of the cells is still about 5 % only. The model of the band structure of the absorber has been introduced into the model of the whole cell. Parameters of the model were determined experimentally by use of quantitative EBIC profiling, C-V doping profiling, Hall measurements, and AFM. The structure has been simulated using SCAPS-1D software. Results of the simulation show a good correlation to the measured I-V and DSR data of the cell. The benefits and drawbacks of the cell structure as well as factors limiting its efficiency are discussed.

Design, Actual Performance, and Electrical Stability of CISCuT-Based Quasi-Endless Solar Cell Tapes

Solar cells based on Cu-In-S-phase absorber layers on a quasi-endless copper tape were made in a series of consecutive roll-to-roll processes, for which a base line has recently been set up. The so called “as grown cell absorber”, which consists of a stack of CuIn 5 S 8 and CuInS 2 layers, is completed to a solar cell by a CuI buffer layer of about 70 nm thickness and a 1 μm thick ZnO-TCO window layer. The design of the whole cell including the phase constitution of the absorber layer and the actual standard electrical characteristics will be presented. Considering the first stability tests and the experimental possibilities of the base line, an outlook will be given concerning the ability and perspectives of using these cell tapes for module production.

Activation Energy of Local Currents in Solar Cells Measured by Thermal Methods

A new technique based on dynamic precision contact thermal measurements of the local current density of solar cells in the dark was developed to determine and map the activation energy of local currents. Because the activation energy or, more generally, the temperature dependence of the current density at constant bias in the dark is determined by the conduction mechanism, measurements of the activation energy may help to distinguish between various possible conduction mechanisms dominating in diAerent regions of the cell. An application of this technique to multicrystalline silicon solar cells is presented. #1998 John Wiley & Sons, Ltd.

Small valence band offset in (010) InS∕CuI heterojunction diodes

Heterojunctions between the (010) facet of an orthorhombic InS single crystal and evaporated CuI show a remarkably small valence band offset of 0.15eV (cliff). This minor band offset allows rather good injection conditions for holes. In accordance to this result, the current-voltage characteristics of the device are rectifying and show a large forward voltage.

CuInS 2 /PEDOT Photovoltaic Structure

Structures based on combination of electrically conductive polymers with inorganic semiconductors are currently intensively investigated with the aim to prepare low-cost, largearea and flexible photovoltaic devices. In this study, multilayer structures consisting of CuInS 2 (CIS) and poly(3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonate (PSS) thin films were prepared and investigated for photovoltaic applications. Polycrystalline CIS absorber layers were synthesized on top of a layered structure on Cu tape substrate using socalled non-vacuum CISCuT technique. Thin PEDOT buffer layers doped with PSS were deposited onto KCN etched and vacuum annealed CIS films. The deposition was performed using the spin-casting technique from an aqueous dispersion of PEDOT/PSS mixed with Nmethylpyrrolidone, isopropanol, glycerin and epoxysilane additives. Optimal deposition parameters for stable PEDOT films with a good adherence to the surface of CIS were selected experimentally. The morphology and thickness of prepared films and structures was determined using SEM technique. Average film thickness was about of 1.5 μm for CIS and 50 nm for PEDOT films. Current-voltage and impedance characteristics were measured. Significant photovoltage and photocurrent of the photovoltaic structures were observed under standard illumination intensity. The best structure showed an open-circuit voltage of 510 mV and a shortcircuit current density of 20.2 mA/cm 2 .

Local partial densities of states in CuInS2 upper valence band determined by soft-x-ray emission spectroscopy: Evidence for In 5p contribution

The CuL2,3, InM4,5, and SL1 soft-x-ray emission spectra of single-crystalline CuInS2 were measured using synchrotron radiation as excitation source. These spectra essentially reflect the local partial densities of states (LPDOS) of Cu 3d, In 5p, and S 3p valence states, respectively. They correspond to features in the total density of states of the upper valence band as revealed by valence-band photoelectron spectrum. On common binding-energy scale the SL1 and InM5 spectra display broad peaks positioned slightly below the Cu 3d-related peak center and a shoulder above the Cu peak center, extending towards the valence-band maximum. A density-functional calculation of the LPDOS confirms two components occurring in both S 3p and In 5p partial densities of states. From the similarity of the positions and the intensity ratios of these two components, an admixture of In 5p states to the S 3p states in the upper valence band is suggested, providing an explanation of the abnormally small band gap of ternary coppe...

Band alignment of type I at (100)ZnTe/PbSe interface

A junction of lattice-matched cubic semiconductors ZnTe and PbSe results in a band alignment of type I so that the narrow band gap of PbSe is completely within the wider band gap of ZnTe. The valence band offset of 0.27 eV was found, representing a minor barrier during injection of holes from PbSe into ZnTe. Simple linear extrapolation of the valence band edge results in a smaller calculated band offset, but a more elaborate square root approximation was used instead, which accounts for parabolic bands. PbSe was electrodeposited at room temperature with and without Cd2+ ions in the electrolyte. Although Cd adsorbs at the surface, the presence of Cd in the electrolyte does not influence the band offset.

X-ray photoconductivity due to trap-sensitive relaxation of hot carriers

We observed the temperature-dependent modulation of the electrical conductivity in ZnO thin films under periodic illumination by soft x rays. At specific temperatures, small variations of the excitation energy near the x-ray absorption edges resulted in large element-specific variations of the conductivity modulation. The emission rate of electrons at traps E1/E2 and E3/E4 in ZnO at these specific temperatures roughly equals the excitation frequency. We conclude that relaxation of electrons, excited from localized core levels into the conduction band, predominantly happens into trap states with the same localization. The experimental results were explained using symmetry selection rules and local transition probabilities.