8 research outputs found
Assessment of Chemical and Electronic Surface Properties of the Cu2ZnSn(SSe)4 after Different Etching Procedures by Synchrotron-based Spectroscopies
Kesterite Cu2ZnSn(S,Se)4 absorber layers with different [S]/([S]+[Se]) ratios were studied using XPS, UPS, Hard X-ray (HIKE) photoemission and the Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS). The samples were prepared by IREC using sequentially sputtered metallic precursor stacks with metal ratios of [Cu]/([Zn]+[Sn])=0.80, [Zn]/[Sn]=1.20 followed by annealing under S+Se+Sn atmosphere. Different etching procedures were used depending on the sample's composition. It is shown that the surface composition varies from that of the bulk, especially for the Se-rich samples. Contamination with sulfur is detected after using a Na2S etching solution for the pure Se kesterite. A Cu-depleted surface was found for all samples before and after etching. HIKE measurements show a higher [Zn]/[Sn] ratio in the near surface region than on the very surface. This is explained by the fact, the etching procedure removes secondary phases from the very few surface layers, while some of ZnS(e) is still buried underneath. In order to investigate the band gap transition from the pure sulfide (1.5 eV) to the pure selenide (1eV), the valence and conduction band of the respective absorbers were probed. According to UPS and HIKE measurements, the relative distance between Fermi level (Ef) and valance band maximum (VBM) for sulfide sample was 130 meV larger than for selenide. Using NEXAFS on the copper, zinc and tin edges, the development of the conduction band with increasing [S]/([S]+[Se]) ratios was studied. Stoichiometric powder samples were used as reference materials. © 2015 Published by Elsevier Ltd.Peer ReviewedPostprint (published version
Surface Engineering of Cu In,Ga Se2 by KF and CdS a Study by High Energy Photoemission Spectroscopy
Oberflächenmodifikation von Cu(In,Ga)Se2 durch KF und CdS und deren Charakterisierung mit Hochenergie-Photoelektronenspektroskopie
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.Chalcopyrit-Dünnschichtsolarzellen haben im Labormaßstab einen maximalen
Wirkungsgrad von 22.8 % erreicht. Allerdings bleiben viele offene Fragen über
die genaue Oberfläche und oberflächennahe Zusammensetzung, und ein
vollständiges Bild von der Ausbildung der Grenzfläche zum CdS-Pufferschicht
wird noch diskutiert. In dieser Arbeit wurde zunächst die Grenzflächenbildung
zwischen CIGSe und CdS mittels HAXPES (engl. hard x-ray photoelectron
spectroscopy) in Abhängigkeit von der Cu-Konzentration des CIGSe untersucht.
Es wurde gezeigt, dass die Cd-Diffusion in die Cu-armen Absorber ein
zweistufiger Prozess ist, bei dem Cu-Atome während des Diffusionsprozesses
aktiv durch Cd-Störatome ausgetauscht werden; wohingegen in der Cu-reichen
Probe die Cu-Atome nicht direkt am Diffusionsprozess beteiligt werden. Darüber
hinaus wurde beobachtet, dass Cd-Atome nach einem Temperprozess bis zu 400 °C
durch eine niedrigere Aktivierungsenergie aus dem oberflächennahen Bereich von
Cu-armen Probe vollständig verschwunden sind; wohingegen eine kleine Menge von
Cd innerhalb der gleichen Tiefe der Cu-reichen Probe nach dem gleichen
Temperprozess gemessen werden konnte. Ein weiterer Unterschied in der
Grenzflächenbildung von Cu-armen und Cu-reichen CIGSe/CdS-Proben beim Tempern
wurde gefunden: Eine Ga-Diffusion in Richtung der CIGSe Oberfläche. Im
Gegensatz zur Cu-reichen Probe wurde eine signifikante Ga-Diffusion bei
erhöhten Temperaturen im Falle der Cu-armen Proben beobachtet. Zweitens wurden
die Auswirkungen der KF-PDT (engl. post-deposition treatment) von CIGSe auf
die CIGSe/CdS-Grenzfläche mittels tiefenaufgelöster HAXPES untersucht. Neben
einer starken Vermischung an der Grenzfläche wurde eine erhöhte Cu-Verarmung
aufgrund der KF-PDT in Kombination mit einer erhöhten Akkumulation von Cd und
S beobachtet. Eine Verschiebung des CIGSe-Valenzbandes an der Oberfläche von
etwa 0,15 eV zu höheren Bindungsenergien sowie der CIGSe- und CdS-bezogenen
Kernniveaus wurde an der KF-behandelten Probe gemessen. Dieses Phänomen wurde
den Auswirkungen von zusätzlichem Cadmium zugeschrieben, das als Donator wirkt
und weitere Elektronen in das Leitungsband des Absorbers freisetzt. Dann
akkumulieren die Elektronen an der CdS-Oberfläche, nachdem sie den
Grenzflächenbereich passiert haben. Diese zusätzliche Oberflächenladung führt
zu einer deutlichen Verschiebung der Photoemissionsspektren, wie sie auf dem
KF-behandelten CIGSe-Absorber im Vergleich zum unbehandelten Absorber
beobachtet wird
Assessment of Chemical and Electronic Surface Properties of the Cu2ZnSn(SSe)4 After Different Etching Procedures by Synchrotron-based Spectroscopies
Assessment of Chemical and Electronic Surface Properties of the Cu2ZnSn(SSe)4 after Different Etching Procedures by Synchrotron-based Spectroscopies
Kesterite Cu2ZnSn(S,Se)4 absorber layers with different [S]/([S]+[Se]) ratios were studied using XPS, UPS, Hard X-ray (HIKE) photoemission and the Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS). The samples were prepared by IREC using sequentially sputtered metallic precursor stacks with metal ratios of [Cu]/([Zn]+[Sn])=0.80, [Zn]/[Sn]=1.20 followed by annealing under S+Se+Sn atmosphere. Different etching procedures were used depending on the sample's composition. It is shown that the surface composition varies from that of the bulk, especially for the Se-rich samples. Contamination with sulfur is detected after using a Na2S etching solution for the pure Se kesterite. A Cu-depleted surface was found for all samples before and after etching. HIKE measurements show a higher [Zn]/[Sn] ratio in the near surface region than on the very surface. This is explained by the fact, the etching procedure removes secondary phases from the very few surface layers, while some of ZnS(e) is still buried underneath. In order to investigate the band gap transition from the pure sulfide (1.5 eV) to the pure selenide (1eV), the valence and conduction band of the respective absorbers were probed. According to UPS and HIKE measurements, the relative distance between Fermi level (Ef) and valance band maximum (VBM) for sulfide sample was 130 meV larger than for selenide. Using NEXAFS on the copper, zinc and tin edges, the development of the conduction band with increasing [S]/([S]+[Se]) ratios was studied. Stoichiometric powder samples were used as reference materials. © 2015 Published by Elsevier Ltd.Peer Reviewe