Surface engineering of Cu(In,Ga)Se2 by KF and CdS – A study by high-energy photoemission spectroscopy
Başlık çevirisi mevcut değil.
- Tez No: 508153
- Danışmanlar: Prof. Dr. M . CH. LUX-STEINER
- Tez Türü: Doktora
- Konular: Fizik ve Fizik Mühendisliği, Physics and Physics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2017
- Dil: İngilizce
- Üniversite: Freıe Unıversıtat Berlın
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
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Özet (Çeviri)
Thin film solar cells based on polycrystalline Cu(In,Ga)Se2 (CIGSe) absorbers have reached a maximum conversion efficiency of 22.8% on the laboratory scale, already exceeding efficiencies of solar cells based on polycrystalline silicon. However, many questions about the exact surface and near-surface composition remain and a complete picture of the formation of the interface to the CdS buffer layer is still under debate. In this work, first, the interface formation between CIGSe and CdS was investigated by means of hard x-ray photoelectron spectroscopy (HAXPES) depending on the Cu concentration of the CIGSe. For this purpose, a Cu-poor and a Cu-rich absorber with bulk Cu to In+Ga ratios of 0.80 and 0.95, respectively, were compared. It was shown that the Cd diffusion into the Cu-poor absorber is a two-step process, where Cu atoms are actively exchanged by Cd impurities during the diffusion process; whereas in the Cu-rich sample, the Cu atoms are not directly involved in the diffusion process. In addition, it was observed that by having a lower activation energy for Cd diffusion in the Cu-poor samples, Cd atoms completely disappeared from the near-surface region after an annealing process up to 400 °C, whereas a small amount of Cd could be still measured within the same depth of Cu-rich sample after the same annealing process. Another difference in the interface formation of Cu-poor and Cu-rich CIGSe/CdS samples upon the annealing process was found to be a Ga diffusion towards the CIGSe surface. In contrast to the Cu-rich sample, a significant Ga diffusion towards the CIGSe surface was observed in case of the Cu-poor samples at elevated temperatures. Second, the impact of KF post-deposition treatment of CIGSe on the CIGSe/CdS interface was investigated by means of depth-resolved HAXPES. Besides a strong intermixing at the interface, an increased Cu depletion due to the KF-PDT was observed in combination with an increased accumulation of Cd and S. A general shift of about 0.15 eV to higher binding energies of the CIGSe valence band at the absorber surface as well as the CIGSe and CdS related core levels was measured on the KF treated sample. This phenomenon was attributed to the impact of additional cadmium, which acts as donor and releases further electrons into the conduction band of the absorber. Then, the electrons accumulate at the CdS surface after having passed the interface region. This additional surface charge leads to a pronounced shift in the photoemission spectra as observed on the KF treated CIGSe absorber compared to the non-treated absorber.
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