M. Ch. Lux-Steiner

High-sensitivity quantitative Kelvin probe microscopy by noncontact ultra-high-vacuum atomic force microscopy

We present quantitative measurements of the work function of semiconductor and metal surfaces prepared in ultrahigh vacuum (UHV) using a combination of UHV noncontact atomic force microscopy and Kelvin probe force microscopy. High energetic and lateral resolution is achieved by using the second resonance frequency of the cantilever to measure the electrostatic forces, while the first resonance frequency is used to simultaneously obtain topographic images by the frequency modulation technique. Spatially resolved work-function measurements reveal a reduced work function in the vicinity of steps on highly oriented pyrolytic graphite. On the GaAs(110) surface it could be demonstrated that defect states in the forbidden band gap cause a local pinning of the Fermi level along monolayer steps. On p-WSe2(0001) work-function variations due to the Coulomb potential of single dopant sites were resolved.

On the function of a bathocuproine buffer layer in organic photovoltaic cells

The role of bathocuproine (BCP) buffer layer inserted between active layer and Al contact in photovoltaic cells based on phthalocyanine (Pc) and C60 was investigated. Photoluminescence (PL) experiments show exciton quenching at the C60–Al interface to be strongly reduced by inserting BCP. Current-voltage characteristics of photovoltaic cells with planar geometry front electrode/Pc∕C60∕BCP∕Al show that BCP dramatically improves electron transport out of the C60 film into the Al electrode. The tenfold increase in power conversion efficiency (η) with BCP can mostly be attributed to the latter effect. BCP does not improve η in photovoltaic cells with blend film geometry front electrode/Pc:C60∕Al.

Radiative recombination via intrinsic defects in CuxGaySe2

A detailed analysis of the radiative recombination processes in CuxGaySe2 epitaxial layers is presented aiming at an investigation of the intrinsic defect levels as a function of chemical composition. CuxGaySe2 is grown by metalorganic vapor phase epitaxy to allow a precise control of composition. Temperature and excitation intensity dependent photoluminescence is used to identify different recombination mechanisms and to determine the ionization energies of the defect levels involved. Defect-correlated optical transitions in Cu-rich epilayers are described in a recombination model consisting of two acceptor and one donor levels showing ionization energies of (60±10) meV, (100±10) meV, and (12±5) meV, respectively. The identification of a shallow compensating donor in CuxGaySe2 and the assignment of the 100 meV state to an acceptor are the most important new aspects in this model. Photoluminescence properties of layers showing Ga-rich compositions are discussed in a model of highly doped and highly compen...

The eta-solar cell with CuInS2: A photovoltaic cell concept using an extremely thin absorber (eta)

Diffusion length of charge carriers within the absorbing material is one of the important restricting properties for the efficiency of solar cell devices. A new cell design using an extremely thin absorber (eta-solar cell) is prepared to obtain an effective separation of charge carriers within the depletion layer. It could be figured out that the properties of CuInS2 (CIS) strongly depend on the porosity of the base layer. Multiple scattering within the porous structure is evident. Moreover, it can be demonstrated that there is a maximum in short-circuit current density for a medium thickness of the absorbing layer.

High-resolution work function imaging of single grains of semiconductor surfaces

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.

Kelvin probe force microscopy for the nano scale characterization of chalcopyrite solar cell materials and devices

Abstract Kelvin probe force microscopy allows to determine not only the surface topography as does atomic force microscopy, but in addition also delivers images of the surface work function on a nanometer scale. Operation in ultrahigh vacuum improves the lateral and energy resolution and allows to obtain absolute work function values. In this paper we will introduce the method and give examples for the application to solar cell materials and devices. We review examples where the surface of an oriented CuGaSe2 film showed distinct work function values for differently oriented facets of single grains, with differences as high as 250 meV, possibly affecting the power conversion efficiency of a solar cell. A cross-sectional study of a complete solar cell device based on the CuGaSe2 absorber material revealed the formation of an additional MoSex layer between the Mo back contact and the absorber. We will present results of measurements at individual grain boundaries of the absorber material. Furthermore, band bending effects at these grain boundaries are discussed and compared to results from transport studies.

Kelvin probe force microscopy in ultra high vacuum using amplitude modulation detection of the electrostatic forces

We present a detailed study of contact potential difference measurements in ultra high vacuum using Kelvin probe force microscopy. A dependence of the contact potential difference on the tip-sample distance was measured and is explained by an inhomogeneous work function of the highly doped silicon cantilever. On semiconducting samples the measured contact potential difference additionally depends on the AC sample voltage. By investigating this effect for different AC voltages and tip-sample distances, we can conclude that bias-induced bandbending has to be considered in a quantitative analysis of the contact potential difference on semiconductors. Using amplitude modulation detection of the electrostatic forces at the second resonance frequency, a high sensitivity can be achieved at low AC voltages. Spatially resolved contact potential difference measurements on submonolayer fullerene islands grown on graphite were used to determine the lateral resolution of approximately 20 nm.

Improved performance of thin film solar cells based on Cu(In, Ga)S2

Abstract Different approaches to the multi-source evaporation of Cu(In,Ga)S2 for thin film solar cells have been studied. Indirect synthesis via a copper-free precursor film leads to superior transport properties but also to an inherent grading of the [Ga]/([In]+[Ga]) ratio. Solar cells prepared from these absorbers show confirmed total area conversion efficiencies up to 12.3%, exceeding the previous record for selenium-free chalcopyrite based cells. A more homogeneous gallium depth distribution and a higher absorber band gap have been achieved by modifying the preparation sequence. An efficiency of more than 10% is demonstrated for the first time with high gap chalcopyrite absorbers as required for tandem configurations.

CuGaSe2 solar cell cross section studied by Kelvin probe force microscopy in ultrahigh vacuum

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.

Contacts to a solar cell with extremely thin CdTe absorber

Abstract The concept for a solar cell with extremely thin absorber (eta-cell) comprises a porous TiO2 substrate covered by a CdTe film with a local thickness of 150–250 nm. At the present stage of development this type of cell exhibits a photovoltage between 600 and 700 mV and a photocurrent of 5–9 mA/cm2 under AM1.5 illumination, indicating the feasibility of the concept. The fill factor is, however, only in the range of 20%, due to a strong voltage-dependence of the photocurrent. This paper discusses the influence of the front and back contacts on the limited cell performance and shows that the insertion of a buffer layer at the front contact gives an improved fill-factor.