between CIGS and the ITO substrate, which is gradually relaxed with increasing CIGS thickness.
Similar problems were also observed on Al-doped ZnO back contacts.
By reducing the thickness of the CIGS absorbers, from 1.6 μm to 1.2 μm, higher total current densities can be achieved by bifacial illumination, even if the efficiency drops because of lower FF and Voc. However this is an important step towards another important application of thin-film technology for BIPV: semi-transparent solar cells for PV windows. This target could be reached by fabricating ultra-thin CIGS devices (~300 nm).
Finally, one of the main objectives of this thesis was to investigate the fabrication of Cd-free solar cells by using only vacuum-based techniques. For this purpose, Zn(O,S) deposited by atomic layer deposition was studied as an alternative buffer layer for LTPED-grown CIGS, in collaboration with the Ångström Laboratory at Uppsala University (Uppsala, Sweden). The performance of CIGS-based solar cells is strongly influenced by the interface layer of Zn(O,S), since both the conduction band offset and the defect density at the interface control FF and Voc. Atomic layer deposited Zn(O,S) films are known to grow as a mixture of ZnO grains and a S-rich amorphous phase, with a larger sulphur content in the interface layer. In this thesis, from the characterization of devices with different Zn(O,S) composition and thickness, the ALD growth of Zn(O,S) appeared to be influenced by the CIGS composition. By analysing solar cells with LTPED-grown CIGS having GGI=30% and GGI=37.5% as a function of the Zn(O,S) composition, the FF trends suggested that the GGI strongly affects the device behaviour. First, the FF maxima were obtained at different sulphur content of the buffer layer, then, while for GGI=30%
the FF was similar to the CdS reference, for GGI=37.5% it dropped by more than 10 points. The first result can be explained by a larger S content of the interface layer, while a generally lower interface quality could be responsible for the dramatic FF drop. Thereby, the Ga content of the CIGS surface is suspected to affect the formation of Zn(O,S) interface layer, promoting more S content and/or even a different nucleation. In fact, hints of island nucleation were found in ZnS buffer layer on co-evaporated CIGS. These hints are suspected to affect Zn(O,S) as well, in a way that depends on the surface composition of both CIGS and Zn(O,S). The phenomenon may result in poor morphology for very-thin films: only when the islands grow enough and merge together covering the whole CIGS surface, the interface recombination can be reduced, thus maximizing both FF and Voc maximized. Every absorber batch is expected to have a unique surface (in terms of GGI, grains dimension and orientations) which hence brings to a special nucleation of the interface layer of Zn(O,S) and to unique properties of the pn-junction.
With LTPED-CIGS having GGI=30%, Zn(O,S) buffers resulted in devices comparable or even more efficient than the CdS reference, whose efficiency can be extended by the application of a AR coating.
The improvements must be confirmed by means of higher-quality absorber to test whether the ALD-Zn(O,S) can be alternative to CdS even for the most efficient (η>16%) solar cells.
In order to improve the robustness of LTPED process and the electrical performance of CIGS cells, some features of the absorber layer must be investigated in the near future. One of the most interesting aspects is the unintentional Ga-grading of the record device. It will be crucial to understand which is the driving force of this grading, in order to maximize its effect on the cell efficiency. Moreover, the model for the GGI calculation from the Raman depth profile is being further refined to improve the accuracy and the reliability of this non-destructive technique. Samples with intentionally-graded CIGS are being deposited by means of LTPED using targets with different compositions; they are intended to correlate the Raman-peak shift with the GGI and to lay the groundwork for LTPED-grown absorbers with optimized double Ga-grading, aiming to efficiency improvements.
According to literature, further improvements are then expected by introducing a post-deposition treatment of KF. In fact, it is widely accepted that KF treatments can improve the interface between CIGS and CdS as well as the buffer uniformity, thus resulting in higher Voc and FF due to lower surface recombination. Moreover, since a thinner CdS is needed to obtain a conformal coverage of K-doped CIGS, an indirect increase of the short-circuit current can also be obtained. Although the role of KF with alternative buffer materials is not yet clear, the advantages for LTPED-grown CIGS solar cells are potentially the same that has allowed several laboratories to overcome the 20% efficiency threshold over the last 2-3 years.
During the PhD, I was involved in other research projects, not presented in this thesis, aimed to the development of CIGS by LTPED.
An unconventional substrate heating by Joule effect (136) was developed to preserve the substrate from over-heating and to limit the heat losses during the CIGS growth. Joule-heating is based on the application of a DC electrical power directly through the Mo back contact of the cell: electrical energy is converted into heat by Joule effect and produces a localized heating of the sample surface, i.e. of the Mo layer only. Devices based on CIGS grown by LTPED with Joule-heating demonstrated the same device features than the one realized by conventional IR heating. This novel process is aimed to lower the energy cost for the manufacturing of thin-film solar cells and to enable the deposition onto polyimide or other thermolabile plastic substrates.
In another project, LTPED epitaxy of CIGS was explored and 1.6μm-thick single-crystal CIGS films were successfully grown by means of LTPED on Ge at very low temperature (270 °C), in order to compare the optical and electrical characteristics of poly- and mono-crystalline CIGS. The only defects found were twin boundaries along the (112) direction while photoluminescence spectra exhibited no InCu peak and CV analysis revealed a free carrier concentration much larger than the Mo reference. Since the density of compensating defects is reduced in CIGS epilayer than in polycrystalline CIGS films, single-crystal CIGS promise better performances as absorber layer in single and multi-junction thin-film solar cells.
Eventually, roll-to-roll deposition of CIGS is currently being exploited to grow CIGS on flexible tapes, such as metal (steel, Mo, etc.), ultra-thin glass or even plastics like polyimide. A vacuum chamber has been specifically designed for the deposition of highly-efficient CIGS on flexible substrates by LTPED:
an in-situ control of the film thickness by IR interference gives the feedback to activate two external motors which move the tape to realize the roll-to-roll deposition.
Summarizing, the present work confirmed LTPED technique is a desirable process for industrial manufacture of low-cost and high-quality CIGS. In fact, LTPED is the only known process able to fabricate CIGS for highly-efficient (η=17%) thin-film solar cells at low temperature (250 °C) and in a single-stage deposition.