Search

Cross-sectional Kelvin probe force microscopy (KPFM) in ultrahigh vacuum is used to characterize the electronic structure of InGaAs∕InP quantum wells. The KPFM signal shows clear peaks at the position of the quantum wells and exhibits a systematic trend for different wells. It is demonstrated that KPFM is capable of detecting quantum wells as narrow as 5nm. Evidence for carrier accumulation in the quantum wells is observed. A complete quantitative analysis of the quantum well properties is shown to be impeded by tip averaging effects and due to surface/interface states.

Kelvin probe force microscopy in ultrahigh vacuum was used to image the electronic structure of thin-film solar cell cross sections based on as-grown Cu-rich CuGaSe2 absorbers. We observe different secondary phases in the absorber film. A p-type degenerate Cu2−xSe phase is identified by a higher work function (Φ∼5.35eV) than CuGaSe2 (Φ∼5.1eV), allowing good contrast mappings of both phases within the absorber film. Besides entire Cu2−xSe crystallites we also observed this secondary phase segregated as an interfacial layer along CuGaSe2 grain boundaries. An additional high-work function phase at the CuGaSe2∕window junction is attributed to the formation of an improper CuS buffer layer during chemical bath processing. The detrimental effect of these secondary phases on the solar cell performance is discussed.

Kelvin probe force microscopy under ultrahigh vacuum conditions has been used to image the electronic structure of a Mo/CuGaSe2/CdS/ZnO thin film solar cell. Due to the high energy sensitivity together with a lateral resolution in the nanometer range we obtained detailed information about the various interfaces within the heterostructure. The absolute work function of the different materials was measured on a polished cross section. To obtain a flat and clean surface we optimized the sputtering process with Ar ions. The presence of an additional MoSe2 layer between the Mo backcontact and the CuGaSe2 absorber layer was observed.

The size reduction of modern electronic devices creates a growing demand for characterization tools to determine material properties on a nanometer scale. The Kelvin probe force microscope is designed to obtain laterally resolved images of the sample’s work function. Using a setup in ultrahigh vacuum, we were able to distinguish work function variations for differently oriented crystal facets of single grains on a semiconductor surface. For the tetragonal solar cell material CuGaSe2 the experiments demonstrate differences as low as 30 meV between (102) and (111) oriented surfaces and up to 255 meV between (112) and (110) surfaces. This influences the band bending of solar cell heterostructures and consequently also the solar power conversion efficiency.

The diffusion behavior of Zn in solar-grade Cu(In,Ga)Se2 (CIGSe) is found to be similar to that in epitaxial CuInSe2 (CISe), which indicates that grain boundaries only play a minor role as segregation sites and fast-transport pathways. The diffusivity obeys the Arrhenius equation DZn = 3.8 × 10−3exp(−1.24 eV∕kBT) cm2 s−1. Surprisingly, the Zn65 diffusion profiles obtained by the radiotracer technique exhibit anomalous shapes with a second maximum near the CI(G)Se-substrate interface. The observations may be indicative of an interstitial-substitutional diffusion mechanism.

The symmetry-dependence of electronic grain boundary (GB) properties in polycrystalline CuInSe2 thin films was investigated in a combined study applying scanning electron microscopy, electron backscatter diffraction, and Kelvin probe force microscopy. We find that highly symmetric Σ3 GBs have a higher probability to be charge neutral than lower symmetric non-Σ3 GBs. This symmetry-dependence can help to explain the large variations of electronic properties found for GBs in Cu(In,Ga)Se2.

Chalcopyrite structures are candidates for efficient intermediate-band solar cells in thin-film technology. Here, we examine a material combination of CuInSe2 dots embedded in CuGaS2 matrix and show that epitaxial growth leads to distinctive tetrahedral nanostructures. Our model calculations provide us with the optimal nanodot size to reach the maximum efficiency—in principle up to 61%. The optimal quantum dot satisfies the known physical constraints, and it is in excellent qualitative agreement with our grown samples.

Thin films of Cu3BiS3 were prepared by coevaporation. Hall-effect, Seebeck-effect, and surface photovoltage measurements show that Cu3BiS3 is a p-type semiconductor with Hall-mobility, free carrier concentration, and thermo-electric power of 4 cm2/V s, 2×1016 cm−3, and 0.73 mV/K, respectively. The work function was determined by Kelvin probe force microscopy to be (4.37±0.04) eV before and (4.57±0.01) eV after deposition of a thin In2S3 layer. Transient surface photovoltage measurements at variable excitation wavelength showed the importance of defect states below the band gap for charge separation and the opportunity for surface defect passivation by a very thin In2S3 layer. The band bending at the Cu3BiS3/In2S3 interface was obtained. The role of grain boundaries for charge transport and charge separation is discussed.

The chemical structure of the CdS/CuGaSe2 chalcopyrite solar cell buffer/absorber interface is investigated by combining element depth profiling using elastic recoil detection analysis and surface-near bulk sensitive x-ray emission spectroscopy. Significant Cd and S concentrations (≥0.1 at. %) are found deep in the absorber bulk. The determined high Cd and S diffusion coefficient values at 333 K of 3.6 and 3.4×10−12 cm2/s, respectively, are attributed to diffusion along CuGaSe2 grain boundaries. As a result, a three-dimensional buffer/absorber interface geometry is proposed.

In view of the outstanding performance of polycrystalline thin film solar cells on the basis of Cu In,Ga Se2, the electrical activity at grain boundaries currently receives considerable attention. Recently, Kelvin probe force microscopy KPFM has been applied to characterize of the properties of individual grain boundaries, observing a drop in the surface potential in many cases. We present finite element simulations of the electrostatic forces to assess the experimental resolution of KPFM. Depending on the tip sample distance, the observed drop in the work function amounts to only a fraction of the real surface potential drop. The simulations are considered for different grain boundary models and consequences for the quantitative evaluation of experimental results are discussed

Employing higher oscillation modes of microcantilevers promises higher sensitivity when applied as sensors, for example, for mass detection or in atomic force microscopy. Introducing a special cantilever geometry, we show that the relation between the resonance frequencies of the first and second resonance modes can be modified to separate them further or to bring them closer together. In atomic force microscopy the latter is of special interest as the photodiode of the beam deflection detection limits the accessible frequency range. Using finite element simulations, we optimized the design of the modified cantilever geometry for a maximum reduction of the frequency of the second oscillation mode with respect to the first mode. Cantilevers were fabricated by silicon micromachining and subsequently utilized in an ultrahigh vacuum Kelvin probe force microscope imaging the surface potential of C-60 on graphite. (c) 2006 American Institute of Physics.

We report on the buffer/absorber interface formation in highly efficient (14.5%, air mass 1.5) ZnO∕CdS∕Cu(In,Ga)Se2 solar cells with a physical vapor deposited CdS buffer. For Se-decapped Cu(In,Ga)Se2 (CIGSe) absorbers we observe sulfur passivation of the CIGSe grain boundaries during CdS growth and at the interface a thermally stimulated formation of a region with a higher band gap than that of the absorber bulk, determining the height of the potential barrier at the CdS∕CIGSe interface. For air-exposed CIGSe samples the grain boundary passivation is impeded by a native oxide/adsorbate layer at the CIGSe surface determining the thermal stability of the potential barrier height.