CuGa(x)Se(y) chalcopyrite-related thin films grown by chemical close-spaced vapor transport (CCSVT) for photovoltaic application: Surface- and bulk material properties, oxidation and surface Ge-doping

[摘要]：Device-grade ternary Cu-Ga-Se chalcopyrite thin films used for photovoltaic energy conversion have been prepared by a novel chemical close-spaced vapor transport (CCSVT) technique developed for a deposition on areas of up to 10 x 10 cm(2). A two-step process has been developed which allows the fine tuning of the film composition and the electronic properties. The extension of deposition times in the two-step process led to final film compositions with [Ga]/[Cu] ratios ranging from 0.9 to 5.7, allowing the study of the structural phase transitions. In this paper the main focus of interest is related to the material properties of the device-grade thin films prepared by CCSVT technique. We present our recent studies on (i) the growth, compositional, structural and electronic structural properties, (ii) the degradation under ambient conditions and (iii) the feasibility of n-type doping this p-type semiconducting material by germanium. Thin films were grown with chalcopyrite (1:1:2) and CuGaSe(2)-related defect compound structures (DC) with stoichiometries of CuGa(3)Se(5) and CuGa(5)Se(8). In order to derive the DC structure, X-ray and neutron powder diffraction investigations have been carried out on powders of these CuGaSe2-related compounds grown by elemental synthesis (powder) and CCSVT (thin films), respectively. We found no hints for an ordering of defects, as proposed in the past and giving name to the so-called Ordered Defect Compounds (ODC) in this and related structures. From our results a growth model is presented for CuGa(3)Se(5) formation in gallium-rich CCSVT-grown CuGa(x)Se(y) films. The chemical and electronic surface and interface structure of CuGaSe(2) thin films with bulk [Ga]/[Cu] ratios between 0.94 and 1.39 is investigated by X-ray and UV-excited photoelectron spectroscopy (XPS and UPS, respectively). A transition of the Cu:Ga:Se surface composition from 1:1:2 for the Cu-rich bulk sample to 1:3:5 for the sample with the highest bulk [Ga]/[Cu] ratio is observed. Simultaneously, a downward shift of the valence band maximum position with respect to the Fermi energy is found. The comparison of the estimated conduction band minimum with that of CdS reveals the formation of a pronounced "cliff-like" conduction band offset at the respective interface. Furthermore, the CuGaSe(2) thin film degradation under ambient as well as under thermal conditions of CuGaSe(2) thin films has been studied by XPS. During thermal oxidation, the formation of predominantly Ga(2)O(3) and some amount of SeO(2) were observed, but no copper oxides could be detected in the near-surface region of the thin films. The same oxides are found after native oxidation in air under ambient conditions. An additional sodium oxide compound formed at the thin film surface, Na(x)O and Na(2)CO(3) after thermal and native oxidation, respectively. Germanium ion implantation technique of the near-surface region of CuGaSe(2) thin films has been used in order to prove the feasibility of n-type doping. In photoluminescence (PL) studies, the occurrence of a new emission line is identified as Ge related and explained as a donor-acceptor-pair (DAP) recombination. The precise role the Ge is playing in this doping of CuGaSe(2) is revealed by X-ray absorption spectroscopy (XANES and EXAFS) and ab initio calculations based on the density functional theory. The studies indicate that the incorporated Ge atoms preferentially occupy Ga sites when relaxation around the dopant is taken into account. Additionally, our corresponding theoretical band structure model predicts the existence of additional localized electronic acceptor and donor defect bands within the band gap of CuGaSe(2) originating from a strong covalent interaction between Ge 4s and Se 4p states for Ge atoms tetrahedrally surrounded by the Se nearest-neighbor atoms. A theoretically predicted anti-bonding Ge-Se4sp(3) defect band appearing well above the Fermi level for the Ge(Ga)(1+) point defect system can be di

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