Search

• Hard X-ray photoelectron spectroscopy, density functional theory studies combined. • In-O, In-F and Ga-F formation on Cu(In,Ga)Se 2 surfaces with KF and RbF treatments. • Greater S-Se intermixing at the interface of KF-treated Cu(In,Ga)Se 2 with CdS. • Alkali metal defects at absorber/buffer interface influence fill factor of devices. • Valence band shifts due to alkali treatment increase interfacial type-inversion. In this work, the alkali-induced chemical and electronic modifications observed at the KF- and RbF-treated Cu(In,Ga)Se 2 (CIGSe)/CdS interfaces are correlated to a Density Functional Theoretical (DFT) model of the alkali metal induced point defects at a CuInSe 2 /CdS interface. Analysed with hard X-ray photoelectron spectroscopy (HAXPES), the near-interface regions showed a Cu-poor, In-rich and stoichiometric CdS composition for the KF-CIGSe/CdS interface and a Cu-poor, In, S-rich composition for the RbF-CIGSe/CdS interface. The DFT-calculated defect formation energies and valence band offsets (VBO) at the defect-induced interfaces indicate towards possible formation of specific defects at the KF- and RbF-treated CIGSe/CdS interfaces. Cu vacancies indicated by the Cu-poor stoichiometry of the alkali-treated interfaces contribute to an increase in the acceptor densities (N A). Possible formation of K Cu and Rb Cu defects could result in lower N A at the interfaces because of the Cu vacancies being filled up by K and Rb atoms. Na Cd and excess Cd Cu defects at the KF-CIGSe/CdS interface and only Cd Cu defects at the RbF-CIGSe/CdS interface might have formed that would result in higher donor densities (N D) at the interfaces. These factors, which showed enhanced type-inversion when applied in device simulations, resulted in fill factor (FF) and open-circuit voltage (V oc) gains in devices. [ABSTRACT FROM AUTHOR]

• Auger parameter analysis utilized to detect Na speciation at Cu(In,Ga)Se 2 surfaces. • Na from post deposition treatment formed (Na x Cu 1-x)(In y O z) complex compound. • No interaction between Na from soda-lime glass and Cu(In,Ga)Se 2 elements was found. • Na from post deposition treatment might not always be beneficial for solar cells. • Na from soda-lime glass resulted in type-inversion at the absorber/buffer interface. Sodium is known to play a crucial role in thin-film solar cells based on Cu(In,Ga)Se 2 (CIGSe) absorbers, however, the chemical state of Na in CIGSe is still under discussion. In this work, an Auger parameter analysis has been carried out to identify the differences in the chemical states of Na in CIGSe, originating from two different sources: the soda-lime glass substrate (Na-SLG) and from a post-deposition treatment (Na-PDT). In Na-PDT CIGSe, a Na state with extra positive charges, an In y O z phase, a reduced [Cu]/[In] or [Cu]/[Se] ratio at the surface and most favorable formation of Na Cu defects indicates the possible formation of a (Na x Cu 1-x)(In y O z) complex compound. In Na-SLG CIGSe, only Na states possibly located at the grain boundaries at the absorber surface could be identified. Both CIGSe with sputtered Zn(O,S) buffer have been studied to gain an insight on the influence of the absorber surface states on the energy band line-up at the respective absorber/buffer interfaces. [ABSTRACT FROM AUTHOR]

Either metallic Na or NaF were deposited onto Cu(In,Ga)Se 2 surfaces and studied by photoelectron spectroscopy and surface photovoltage spectroscopy without breaking the ultra-high vacuum. The deposition of elemental Na at room temperature led to the formation of an intermediate Cu and Ga rich layer at the CIGSe surface, whereas for NaF the composition of the CIGSe surface remained unchanged. A metal like surface induced by an inverted near surface region with a reduced number of defect states was formed after the deposition of Na. Under the chosen experimental conditions, the near surface layer was independent on the amount of Na and stable in time. In contrast, the usage of NaF weakened the inversion and led to an increased band bending compared to the untreated CIGSe sample. The SPV signals decreased with proceeding time after the deposition of NaF. [ABSTRACT FROM AUTHOR]