Interface recombination in a complex multilayered thin‐film solar structure causes a disparity between the internal open‐circuit voltage (VOC,in), measured by photoluminescence, and the external open‐circuit voltage (VOC,ex), that is, a VOC deficit. Aspirations to reach higher VOC,ex values require a comprehensive knowledge of the connection between VOC deficit and interface recombination. Here, a near‐surface defect model is developed for copper indium di‐selenide solar cells grown under Cu‐excess conditions. These cell show the typical signatures of interface recombination: a strong disparity between VOC,in and VOC,ex, and extrapolation of the temperature dependent q·VOC,ex to a value below the bandgap energy. Yet, these cells do not suffer from reduced interface bandgap or from Fermi‐level pinning. The model presented is based on experimental analysis of admittance and deep‐level transient spectroscopy, which show the signature of an acceptor defect. Numerical simulations using the near‐surface defects model show the signatures of interface recombination without the need for a reduced interface bandgap or Fermi‐level pinning. These findings demonstrate that the VOC,in measurements alone can be inconclusive and might conceal the information on interface recombination pathways, establishing the need for complementary techniques like temperature dependent current–voltage measurements to identify the cause of interface recombination in the devices.

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Effective defect passivation of semiconductor surfaces and interfaces is indispensable for the development of high efficiency solar cells. In this study we systematically investigated the surface and grain boundary properties of CuInSe 2 (CISe) with scanning tunneling microscopy (STM) and spectroscopy (STS) after different surface treatments such as potassium cyanide (KCN) etching, pre-electrolyte treatment with cadmium ions, and annealing in ultrahigh vacuum (UHV). We show that air exposed samples with a subsequent KCN etching step exhibits a highly defective surface. However, a Cd pre-electrolyte treatment passivates most of these defects, which manifests itself by a reduction of the high conductance in the STS measurements at positive sample biases. The origin of the improvement can be traced back to an increase in surface band bending, which leads to a type inversion, induced by a change in the concentration of Cu vacancies. We observe a defect passivation at the CISe surface and at the grain boundaries. Our results give a direct explanation of why the CdS buffer layer in CISe thin film solar cells is of utmost importance for high efficiency devices.

In-depth understanding and subsequent optimization of the contact layers in thin film solar cells are of high importance in order to reduce the amount of nonradiative recombination and thereby improve device performance. In this work, the buried MoSe 2 /CuInSe 2 interface of stoichiometric absorbers is investigated with scanning tunneling spectroscopy and Kelvin probe force microscopy combined with compositional measurements acquired via photo-electron spectroscopy after a mechanical lift-off process. We find that the local density of states, as measured with scanning tunneling spectroscopy, is similar to the front-side of ultra-high vacuum annealed CISe absorbers. The grain boundaries exhibit a weak upward band bending, opposite to Cu-poor CuGaSe 2 , and we measure an increased Cu accumulation at the rear CISe surface compared to the bulk composition and a non-zero concentration of Cu on the Mo-side. Grazing incidence X-ray diffraction measurements corroborate that a small amount of a Cu x Se secondary phase is present at the MoSe 2 /CuInSe 2 interface in contrast to reports on Cu-poor material. Our findings shed new light into the complex interface formation of CuInSe 2 -based thin film solar cells grown under Cu-rich conditions. KEYWORDSCuInSe 2 , Kelvin probe force microscopy, scanning tunneling microscopy INTRODUCTIONCu(In,Ga)Se 2 (CIGSe) thin film solar cells offer high power conversion efficiencies (PCE), 1 low energy payback time, and long-term stability. 2Several major breakthroughs in the last decades allowed this material system to surpass the 20% efficiency benchmark on rigid and flexible substrates 1,3 and PCEs as high as 23.35% were reported. 1 One of the important steps that allowed for high PCE was the introduction of a Gallium back-gradient, which effectively reduced the recombination at the Mo/MoSe 2 back-contact. 4,5 The optimization of this interface [Correction added on 11 January 2021, after first online publication: surname of 'Harry Mönig' has been corrected in this version.]is still an area of intensive research due to the following reasons: The Molybdenum back-contact interface has a poor optical reflectivity and the recombination velocity is high, 6 which means that effective passivation strategies are indispensable. This is especially true for ultra-thin devices 7 where bandgap grading is not feasible.Pure CuInSe 2 (CISe) solar cells have several advantages compared to CIGSe such as an easier manufacturing process and a lower bandgap, which makes this material more attractive for tandem applications.However, the PCE was stuck for a long time at 15% 8 without post deposition treatment. Recently, KF treatment improved this number to 16%, 9 which is still far away from the record CIGSe absorbers. It was shown that the likely reason for the lower CISe solar cell performanceThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

In this study, cross-sectional surface potential imaging of n+/semi-insulating GaAs junctions is investigated by using amplitude mode kelvin probe force microscopy. The measurements have shown two different potential profiles, related to the difference in surface potential between the semi-insulating (SI) substrate and the epilayers. It is shown that the contact potential difference (CPD) between the tip and the sample is higher on the semi-insulating substrate side than on the n-type epilayer side. This change in CPD across the interface has been explained by means of energy band diagrams indicating the relative Fermi level positions. In addition, it has also been found that the CPD values across the interface are much smaller than the calculated values (on average about 25% of the theoretical values) and increase with the electron density. Therefore, the results presented in study are only in qualitative agreement with the theory.