D. Gal

Electrochemical deposition of zinc oxide films from non-aqueous solution : a new buffer/window process for thin film solar cells

Abstract Zinc oxide is a wide band gap semiconductor with wide application in thin film devices such as n-type window layers for thin film solar cells, piezoelectric and luminescent devices, and for catalytic applications. We have cathodically electrodeposited films of ZnO by reduction of dissolved oxygen in a non-aqueous solution (dimethylsulfoxide) containing a Zn salt. This method allows a large deposition potential window and gives films with high transparency, good crystallinity and adherence. The ZnO was electrodeposited on Cu(In,Ga)Se2 as a buffer layer. The resulting solar cells gave higher light-to-electricity conversion efficiencies (>11%) than those made with conventional r.f. sputtered insulating ZnO.

Band diagram of the polycrystalline CdS/Cu(In,Ga)Se2 heterojunction

Contact potential difference measurements in the dark and under illumination are used to derive the conduction band offset (ΔEc) in a solar cell quality junction formed by chemical bath deposition of CdS on a polycrystalline thin film of Cu(In,Ga)Se2. Our experimental measurements and the estimates made for dipole contributions show that the junction is of type II, i.e., without a spike in the conduction band ( ΔEc=80 meV±100 meV). This is consistent with the high performance of the actual solar cell. However, it differs from most previous results on junctions based on single crystals and/or vacuum deposited CdS, which indicated the existence of a conduction band spike.

Electrochemical Deposition of ZnSe and (Zn,Cd)Se Films from Nonaqueous Solutions

Films of (Zn,Cd)Se and ZnSe were electrodeposited from dimethylsulfoxide solutions of elemental Se and the perchlorates of Zn and Cd. The composition of the mixed selenides could be controlled by variation of the deposition current density, utilizing diffusion control of the low concentrations of Cd(ClO{sub 4}){sub 2}. The film composition was measured by X-ray photoelectron spectroscopy and compared with the measured bandgap values extracted from optical transmission spectra. Photoelectrochemical photocurrent spectroscopy and contact potential difference (Kelvin probe) measurements both showed changes in apparent conductivity type with Zn concentration in the electrolyte from n-type (low Zn) concentration to p-type (high Zn). This phenomenon is discussed in terms of preferential surface trapping of either electrons or holes at the nanocrystallite surface.

Size-quantized CdS films in thin film CuInS2 solar cells

Semiconductor quantum dots and wires are the subject of great interest, mainly due to their size-dependent electronic structures, in particular increased band gap and therefore optoelectronic properties. We have electrodeposited films of size-quantized CdS (∼4 to 5 nm cross section by 15 nm height) as a buffer layer on CuInS2. The resulting CuInS2/CdS thin-film solar cells gave increased photocurrents and higher light-to-electricity conversion efficiencies (>11%) than those made with conventional nonquantized CdS films. This was due mainly to the increased band gap of the quantized CdS, allowing more light to reach the active CuInS2 layer.

BAND GAP DETERMINATION OF SEMICONDUCTOR POWDERS VIA SURFACE PHOTOVOLTAGE SPECTROSCOPY

Surface photovoltage spectroscopy (SPS) is introduced as a powerful tool for band gap determination of semiconductor powders. The main advantage of SPS is that scattering and reflection do not interfere with the spectra. Therefore, it does not suffer from the inherent limitations of transmission/reflection based spectroscopies, most notably diffuse reflectance spectroscopy (DRS). The principles of the approach are presented and its usefulness is demonstrated by comparing it with DRS for band gap determination of GaAs, InP, CdTe, CdSe, and CdS semiconductor powders.

Nanocrystal‐Size Control of Electrodeposited Nanocrystalline Semiconductor Films by Surface Capping

The crystal size of nanocrystalline CdS and CdSe films, electrodeposited from dimethyl sulfoxide solutions containing a Cd salt and elemental S or Se, is shown to depend on the nature of the anion of the Cd salt. Relatively strongly adsorbing anions, such as chloride, result in a smaller nanocrystal size than relatively nonadsorbing anions such as perchlorate. This difference in nanocrystal size is explained by blocking (or capping) of the growing crystals by the adsorbed ions. Very strongly adsorbing species, such as alkyl phosphines, result in even smaller crystal size (3.5 nm average diameter).

Effect of air annealing on the electronic properties of CdSCu(In,Ga)Se2 solar cells

Abstract The effect of air annealing on state-of-the-art, solar-cell-quality CdS Cu(In,Ga)Se 2 heterojunctions has been studied using contact potential difference and surface photovoltage measurements. The annealing treatment is shown to have no significant effect on the band lineup of the heterojunction. However, the surface photovoltage spectral response increases markedly upon air annealing. These results can be reconciled if air annealing of the junctions leads mainly to elimination of recombination centers, rather than to changes in the built-in voltage or in the band lineup. We also show that ZnO deposition has an effect on the surface photovoltage that is similar to that of air annealing.

ENGINEERING THE INTERFACE ENERGETICS OF SOLAR CELLS BY GRAFTING MOLECULAR PROPERTIES ONTO SEMICONDUCTORS

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.

Surface photovoltage measurements in liquids

We present a simple, compact, and robust arrangement for surface photovoltage measurements of free semiconductor surfaces immersed in liquids. It is based on the classical Kelvin probe arrangement, where the semiconductor sample is put in a liquid-containing, electrically insulating vessel, with an optically transparent window, situated between the sample and the Kelvin probe. At the price of permitting relative, rather than absolute, contact potential difference values, this modification enables easy, routine surface photovoltage measurements of semiconductors in any kind of liquid ambient. The validity and efficiency of this approach are demonstrated by surface photovoltage spectra obtained from the p-InP(100) surface in various liquid etchants.

Band diagram and band line‐up of the polycrystalline CdS/Cu(In,Ga)Se2 heterojunction and its response to air annealing

Contact potential difference measurements in the dark and under illumination are used to derive the conduction band offset (ΔEC) in the CdS/Cu(In,Ga)Se2 junction before and after air annealing. This junction was formed by chemical bath deposition of CdS on state‐of‐the‐art, solar cell quality, polycrystalline thin films of Cu(In,Ga)Se2. Based on our experimental measurements and estimates made for dipole contributions, we find that the junction in the cell is of type II, i.e. without a spike in the conduction band (ΔEC=80 meV±100 meV). Air annealing improves cell performance and also markedly increases the surface photovoltage spectral response. However, the annealing treatment is shown to have no significant effect on the band line‐up of the heterojunction, which remains type II. This indicates that air annealing of these junctions does not change the built‐in voltage or the band line‐up.