Formation of PV absorber layer materials using electrochemical versions of atomic layer deposition
Czerniawski, Justin Michael
MetadataShow full item record
This dissertation discusses the layer-by-layer deposition of Cu2Se and In2Se3 using potential pulse atomic layer deposition (PP-ALD) and optimization studies of CdTe using electrochemical atomic layer deposition (E-ALD). PP-ALD is an electrodeposition methodology combining concepts from techniques such as sequential monolayer deposition (SMD) and E-ALD in order to create E-ALD quality films at increased deposition rates. During PP-ALD cycles, potentials are alternated between cathodic and anodic potentials, foregoing cyclic voltammetry (CV) or solution alternation required in SMD and E-ALD. The cathodic potential is positioned at a co-electrodeposition potential for a time sufficient to form fractions of a monolayer (ML). The Anodic potential oxidatively strips elemental excess creating stoichiometric deposit surfaces. During pure co-dep, potentiostatic or galvanostatic, localized excess of an element results in stoichiometry variations. Since the Cathodic pulse time limits the amount deposited, PP-ALD avoids burying elemental excess as it remains accessible to stripping during the Anodic pulse. This is an ALD method since it is based on repeated application of surface limited reactions to grow deposits an atomic layer at a time. Electrochemical surface limited reactions are frequently referred to as underpotential deposition (UPD), which occurs at a potential that takes advantage of compound formation energetics to form a deposit of one element on another. In PP-ALD, the anodic potential produces UPD since only thermodynamically stable compound remains. The resulting Cu2Se and In2Se3 film properties were characterized using coulometry, electron probe microanalysis (EPMA), CV, x-ray diffraction (XRD), and spectroscopic ellipsometry. In2Se3 deposits made from pH 1 solution were preferable to those made from pH 3 solution due to stoichiometry and homogeneity improvements. A PP-ALD solution made with Se+6 precursor was vetted as a replacement for the more reactive Se+4 ion. Selenic acid solution resulted in 0.1-0.3 ML of Se deposition when compared to selenous acid, which deposited 0.6 ML at the same potential. The lack of deposition limits the practicality of selenic acid as a replacement PP-ALD precursor. Finally, CdTe deposits were improved by excluding O2 and H2 gas formed during the deposition process through the use of CO2 purging and less permeable gasket material.