Kevin D. Dobson

Stability of CdTe/CdS thin-film solar cells

Abstract The recent literature regarding the stability of CdTe/CdS photovoltaic cells (as distinguished from modules ) is reviewed. Particular emphasis is given to the role of Cu as a major factor that can limit the stability of these devices. Cu is often added to improve the ohmic contact to p-CdTe and the overall cell photovoltaic performance. This may be due to the formation of a Cu 2 Te/CdTe back contact. Excess Cu also enhances the instability of devices when under stress. The Cu, as Cu + , from either Cu 2 Te or other sources, diffuses via grain boundaries to the CdTe/CdS active junction. Recent experimental data indicate that Cu, Cl and other diffusing species reach (and accumulate at) the CdS layer, which may not be expected on the basis of bulk diffusion. These observations may be factors in cell behavior and degradation, for which new mechanisms are suggested and areas for future study are highlighted. Other possible Cu-related degradation mechanisms, as well as some non-Cu-related issues for cell stability are discussed.

Controlling Growth Chemistry and Morphology of Single-Bath Electrodeposited Cu ( In , Ga ) Se2 Thin Films for Photovoltaic Application

Single-bath electrodeposition of polycrystalline Cu(In,Ga)Se 2 thin films for photovoltaic applications is described. Cu(In,Ga)Se 2 was deposited onto Mo electrodes from low concentration aqueous baths containing CuCl 2 , InCl 3 , GaCl 3 , and H 2 SeO 3 . Buffering the solutions to pH ∼ 2.5 stabilized bath chemistry and improved Cu(In,Ga)Se 2 film composition. Bath concentrations were shown to affect composition of deposited films, with a bath [Se 4+ ]/[Cu 2+ ] ratio of 1.75 required to maintain suitable deposited Se and Cu levels, while [In 3+ ] could be adjusted to control deposited In and Ga. Deposited films initially exhibited significant cracking, which was prevented by lowering the [Se 4+ ] in the bath, and contained Cu 2-x Se as secondary phases, resembling cauliflower florets, embedded in the film surfaces. The formation of these secondary phases was overcome by pretreating the Mo electrodes with a short 1 min deposition from the Cu(In,Ga)Se 2 bath. This, coupled with a multipotential deposition regime, led to growth of smooth, compact, crack-free films of near stoichiometric values. Mechanisms of film growth and morphology control are discussed. All as-deposited films exhibit low crystallinity, and for device processing require recrystallization by annealing in an H 2 Se atmosphere. Promising preliminary results of electrodeposited Cu(In,Ga)Se 2 devices are presented.

Electroless Ni and NiTe2 ohmic contacts for CdTe/CdS PV cells

Abstract The preparation and characterization of electroless Ni and NiTe 2 back-contacts to CdTe/CdS solar cells are described. NiTe 2 , formed by a modification of the electroless Ni process, produces cells with PV properties superior (efficiencies >10% have been obtained) to those contacted with electroless Ni. NiTe 2 -contacts offer promising cell-stability in dry air, but exhibit reversible degradation on exposure to ambient H 2 O vapor.

Comparative Study of Tungsten Monocarbide and Platinum as Counter Electrodes in Polysulfide-Based Photoelectrochemical Solar Cells

The stability and the electrocatalytic activity of polycrystalline tungsten monocarbide (WC) and platinum (Pt) electrodes in a concentrated polysulfide electrolyte were examined in an electrochemical half-cell and an integrated photoelectrochemical cell (PEC) device. Cyclic voltammetry and linear sweep voltammetry measurements found WC electrodes to be approximately twice as active as Pt electrodes toward polysulfide reduction. Electrochemical and X-ray photoelectron spectroscopy (XPS) measurements indicated that WC was stable in a polysulfide electrolyte, with no appreciable W sulfide formation. In contrast, XPS showed significant levels of sulfide species on the Pt surface, which might explain the differences in the electrochemical behavior of the two electrode materials. The performance of integrated PEC devices comprised of thin-film CdSe photoanodes and WC or Pt counter electrodes in a polysulfide electrolyte was consistent with that predicted from electrochemical half-cell measurements.

Growth Mechanisms of Electrodeposited CuInSe2 and Cu(In,Ga)Se2 Determined by Cyclic Voltammetry

Electrodeposition (ED) of CuInSe 2 -based thin films from a buffered single-bath on dcsputtered Mo layers has been investigated. In order to understand the film growth, cyclic voltammetry (CV) was used to identify mechanisms leading to the formation of CuInSe 2 and Cu(In,Ga)Se 2 . Similar CV data were observed for deposition from Cu-Se, Cu-In-Se, Cu-Ga-Se, and Cu-In-Ga-Se baths. A preliminary mechanism for CuInSe 2 and Cu(In,Ga)Se 2 film growth has been proposed from CV, composition and glancing incidence x-ray diffraction (GIXRD) data. Incorporation of In into the growing films occurs via reaction with H2Se, formed by reduction of the initially deposited Cu3Se 2 , to form In2Se3, which is in turn rapidly assimilated into the film by reaction with copper selenides to form CuInSe 2 . The incorporation of Ga may occur via a similar mechanism, however, the precipitation of Ga(OH)3 can not be ruled out as a possible route for Ga uptake. All as-deposited ED CuInSe 2 –based films have poor crystallinity and require annealing in H 2 Se prior to device processing. Preliminary device results are presented, reporting a conversion efficiency of 6.5% and 6.2% for electrodeposited CuInSe 2 and Cu(In,Ga)Se 2 , respectively.

Formation and Characterization of Electroless-Deposited NiTe2 Back Contacts to CdTe /CdS Thin-Film Solar Cells

The preparation and characterization of NiTe 2 back contacts to CdTe/CdS cells are described. NiTe 2 is formed on p-CdTe by electroless deposition at low reductant concentrations, producing devices that are superior (conversion efficiencies often exceeding 10%) to those contacted with electroless Ni. X-ray photoelectron spectroscopy and X-ray diffraction showed the product to be nickel(II) ditelluride (Ni 2+ Te 2 2 ), produced, in the catalytic presence of Al 3+ ions, by the reaction of Ni 2+ ions with a Te-rich CdTe surface, which is formed by an in situ etch in the chemical hath. Preliminary results indicate that Ni, from NiTe 2 , is not mobile within the cell structure, which is expected to he favorable for stability. Cells exhibit degradation when exposed to H 2 O vapor. However, this effect was found to he reversible, with performance improving to near predegradation values following cell transfer to dry air conditions.

A new photoelectrochemical test cell and its use for a combined two-electrode and three-electrode approach to cell testing.

This paper describes the design, assembly, and operation of a photoelectrochemical (PEC) test cell that is relatively easy to construct and well suited for testing photoelectrode/counterelectrode combinations in a reproducible manner. The design of the cell permits measurements to be made in both two-electrode and three-electrode arrangements. The benefits of conducting both two-electrode and three-electrode measurements are illustrated using data obtained from the new test cell for a PEC system based on a polysulfide electrolyte, CdSe(0.8)Te(0.2) photoanode, and tungsten monocarbide counterelectrode. It is shown that linear sweep voltammograms measured in three-electrode mode can be used to describe current transients recorded in a two-electrode cell modified by the addition of a reference electrode.

How CDTE Solar Cells Operate: Determining Collection using Bifacial Device Characterization

The fundamental mechanisms governing carrier transport and in CdTe solar cells are not well established. Effects of diffusion length (L), depletion width (W), primary heterojunction vs. back junction are not well correlated with the CdTe thickness (t), or back contact. Bifacial analysis provides quantitative insight into CdTe device operation by separating the effects of front and back junction. Back spectral response (SRB) was analyzed to evaluate L and W. Front spectral response (SRF) is nearly unaffected by L or W. SR B and back Jsc are higher for thinner cells as SR B is limited by diffusion across the field free region that is smaller for thinner cells. Bifacial characterization results indicate a photosensitive back barrier. There is no evidence of a back junction and we conclude that a single junction determines recombination current. These results establish that performance for AM1.5 light through front contact is determined primarily by voltage dependent collection, not diffusion length

When, Why and Where are CdTe/CdS Solar Cells Stable?

The role of Cu in CdTe/CdS solar cell instability remains the subject of much debate. The investigation of a range of ‘Cu’-contacted CdTe/CdS cells, which had received various thermal stress treatments, is described. Cells that were stressed in air exhibit strong current-voltage (I-V) rollover and junction degradation. No such degradation was observed for ‘Cu’-contacted cells that had been stressed in dry-N 2 atmosphere. Cu is found to diffuse rapidly through the cell structure during back contact annealing and to accumulate in the CdS layer. With stress, significant levels of Cu dope the grain bulk, producing (with Cl) high resistance, photoconducting CdS. This behavior is independent of stress atmosphere and is, therefore, unlikely to (initially) be a dominating mechanism for cell degradation. Our results suggest simple air oxidation of the back contact interface to be a likely origin of I-V rollover in CdTe/CdS cells.

Thermochemical and kinetic aspects of Cu2ZnSn(S,Se)4 thin film growth by reacting Cu-Zn-Sn precursors in H2S and H2Se

Electrodeposited thin-film Cu-Zn-Sn samples were reacted in varying ratios of mixed Ar/H2S/H2Se to form Cu2ZnSn(S,Se)4 (CZTSSe). Reactions were carried out at atmospheric pressure, reaching a temperature of 550 °C. Sulfur/selenium ratios in reacted films were determined as a function of [H2S]/[H2Se] in the reaction atmosphere. Relative S/Se incorporation into the CZTSSe films was used to estimate the Gibbs free energy of quaternary Cu2ZnSnSe4 to be ΔGCZTSe823K≈−675 kJ·mol−1. When chalcogen species are supplied from hydride gas sources, formation of the selenide quaternary is favored over the sulfide.