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• 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]

[Display omitted] • A new paradigm in organic film engineering foresees exploiting axial coordination. • In-situ monitoring is required to assess the quality of the film growth. • Optical probes such as RAS reduce to minimum organic film damage. • We propose a strategy to apply RAS to these novel supramolecular structures. Ensuring the highest accuracy in determining the molecular assembling and the preservation of the chemical and physical properties of the molecules during the growth of organic layers requires a real-time monitoring of film formation. In this respect, optical techniques are preferred since they result in minimum organic film damage. Among these techniques, Reflectance Anisotropy Spectroscopy (RAS) proved to be the one with the highest sensitivity due to the development of intrinsic anisotropies in the optical response of molecular films, where molecular packing is driven by Van der Waals interactions. Recently, we proposed an original strategy to enable the growth of organic films via molecular self-assembly through highly directional coordination bonds. Herein, we consider a straightforward supramolecular structure employing axial bonds, based on the 1:1 assembly of a CoII porphyrin (CoTPP) and a linear ligand (DPNDI). This system is characterized by a rather isotropic structure that might, in principle, result in a weak RAS signal. Therefore, in the present work, we critically assess the range of applicability of such a spectroscopy. A number of other surface science techniques, including Low Energy Electron Diffraction (LEED), photoemission spectroscopies (PES and IPES) and Atomic Force Microscopy (AFM) is employed to fully characterize the axially coordinated molecular film. [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]