Origin of Interface Limitation in Zn(O,S)/CuInS2‑Based Solar Cells

Copper indium disulfide CuInS2 grown under Cu rich conditions exhibits high optical quality but suffers predominantly from charge carrier interface recombination, resulting in poor solar cell performance. An unfavorable cliff like conduction band alignment at the buffer CuInS2 interface could be a possible cause of enhanced interface recombination in the device. In this work, we exploit direct and inverse photoelectron spectroscopy together with electrical characterization to investigate the cause of interface recombination in chemical bath deposited Zn O,S co evaporated CuInS2 based devices. Temperature dependent current voltage analyses indeed reveal an activation energy of the dominant charge carrier recombination path, considerably smaller than the absorber bulk band gap, confirming the dominant recombination channel to be present at the Zn O,S CuInS2 interface. However, photoelectron spectroscopy measurements indicate a small 0.1 eV spike like conduction band offset at the Zn O,S CuInS2 interface, excluding an unfavorable energy level alignment to be the prominent cause for strong interface recombination. The observed band bending upon interface formation also suggests Fermi level pinning not to be the main reason, leaving near interface defects as recently observed in Cu rich CuInSe2 as the likely reason for the performance limiting interface recombinatio

Surface and interface properties of chalcopyrite based solar cell structures as revealed by photoelectron spectroscopy

This thesis presents an investigation of the surface and interface structures in chalcopyrite based solar cell stacks with a particular focus on the impact of the novel and efficiency increasing RbF treatment on the chalcopyrite related interfaces and on the investigation of a wide bandgap chalcopyrite based stack to track performance limiting factors. Laboratory based x ray photoelectron spectroscopy XPS and synchrotron based hard x ray photoelectron spectroscopy HAXPES are used to examine the frontside and backside surface regions of different Cu In,Ga Se2 thin film solar cell absorbers, which were deposited at low temperatures i.e. substrate temperature during CIGSe deposition lt;500 C, compatible with the use of flexible substrates like polyamide and underwent NaF or combined NaF RbF post deposition treatment PDT , respectively. A Cu deficient surface region is found at both surfaces of all absorbers and is modelled as a [Cu] [In] [Ga] [Se] 1 5 8 surface compound covering stoichiometric 1 1 2 CIGSe. The application of NaF RbF leads an even more pronounced Cu depletion and to the incorporation of Rb in the depleted region at the frontside and backside of the absorbers. Additionally, strong indications are found for the NaF RbF PDT induced formation of a Rb In Se type RISe compound with a 1 1 2 stoichiometry, partially covering the absorber frontside surface, while not being present at the backside. The determined valence band maximum shows a shift away from the Fermi level towards the surface on both sides of the CIGSe absorber, which is reinforced by increased RbF PDT. Complementary, a RbInSe2 co evaporation PDT on high temperature deposited absorbers was investigated in comparison to an untreated and a RbF PDT absorber using HAXPES. The deposited layer was found to be significantly Rb deficient compared to a [Rb] [In] [Se] 1 1 2 stoichiometry, which is in agreement with pronounced Rb diffusion into the CIGSe absorber. While the application of two different fit models suggests that the formation of stoichiometric RbInSe2 and a separate In Se phase seems more likely, the formation of a strongly Rb poor Rb In Se phase cannot be excluded. To elaborate on the impact of the absorber buffer interface, a thickness series of CdS layer on those absorbers was investigated, revealing a pronounced S deficiency in the early stages of the CdS growth which is explained by the formation of Cd Se bonds. This S deficiency is increased for higher Rb contents on the absorber and might also be related to an initial phase of very limited CdS growth. Additionally, the wide bandgap CuInS2 Zn O,S interface was investigated using Cu poor and Cu rich CuInS2 absorbers to identify factors limiting device efficiency. Using XPS and HAXPES a rather inhomogeneous growth of the Zn O,S with respect to its thickness was found. The conduction band alignment, which was derived using inverse photoemission spectroscopy IPES , was found to show no significant offset, excluding it as a performance limiting factor

The Energy Level Alignment at the Buffer/Cu(In,Ga)Se2 Thin‐Film Solar Cell Interface for CdS and GaOx

Abstract Sputter‐deposited GaOx (i.e., oxygen‐deficient gallium oxide) films are evaluated as a potential replacement for the standard CdS buffer layers in Cu(In,Ga)Se2 (CIGSe) based thin‐film photovoltaics. The energy level alignment at the GaOx/CIGSe and CdS/CIGSe interfaces are compared by means of direct and inverse photoemission. For the GaOx/CIGSe a (0.04 ± 0.07) eV (i.e., a small spike‐like) conduction band offset (CBO) and a (−3.21 ± 0.19) eV (i.e., a large cliff‐like) valence band offset (VBO) are found, which suggests a nearly ideal charge‐selective contact. The derived GaOx band gap of (4.80 ± 0.25) eV confirms its utility as a highly transparent buffer layer. However, the GaOx (with x derived to be 1.1 ± 0.1) exhibits considerable (presumably) defect‐related occupied states above the valence band maximum. It is proposed that these states may increase charge carrier recombination and decrease open circuit voltage in respective devices; also explaining why solar cells with standard CdS buffer outperform devices with GaOx buffer, despite less ideal electronic interface properties (CBO: (−0.18 ± 0.07) eV, VBO: (−0.98 ± 0.15) eV) and the smaller CdS band gap of (2.35 ± 0.22) eV

Functionalized Nickel Oxide Hole Contact Layers: Work Function versus Conductivity

Nickel oxide (NiO) is a widely used material for efficient hole extraction in optoelectronic devices. However, its surface characteristics strongly depend on the processing history and exposure to adsorbates. To achieve controllability of the electronic and chemical properties of solution-processed nickel oxide (sNiO), we functionalize its surface with a self-assembled monolayer (SAM) of 4-cyanophenylphosphonic acid. A detailed analysis of infrared and photoelectron spectroscopy shows the chemisorption of the molecules with a nominal layer thickness of around one monolayer and gives an insight into the chemical composition of the SAM. Density functional theory calculations reveal the possible binding configurations. By the application of the SAM, we increase the sNiO work function by up to 0.8 eV. When incorporated in organic solar cells, the increase in work function and improved energy level alignment to the donor does not lead to a higher fill factor of these cells. Instead, we observe the formation of a transport barrier, which can be reduced by increasing the conductivity of the sNiO through doping with copper oxide. We conclude that the widespread assumption of maximizing the fill factor by only matching the work function of the oxide charge extraction layer with the energy levels in the active material is a too narrow approach. Successful implementation of interface modifiers is only possible with a sufficiently high charge carrier concentration in the oxide interlayer to support efficient charge transfer across the interface

Heavy Alkali Treatment of Cu(In,Ga)Se 2 Solar Cells: Surface versus Bulk Effects

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