Electron microscope image of a dirty glass surface
© Fraunhofer CSP
Electron microscope image of a dirty glass surface showing chlorine-containing salt compounds in green color.

Surface and Film Characterization

The electronic properties of solar cells are influenced by thin passivation and contact layers and their interfaces with the light-absorbing areas. We offer high-resolution and highly sensitive layer analysis of solar cells and other components of PV modules. Thus, the relationships between process variations and layer compositions on passivation layer stacks on solar cells can be traced and layer thicknesses can be measured on the nanometer scale. In addition, we have the ability to trace the causes of specific module defects down to the atomic level after localization with electro-optical methods and then by means of electron microscopy (SEM, FIB, TEM with EBIC, EDX), which is often the basis for understanding and eliminating the causes of defects.

Services

  • High-resolution structure and element analysis in the layer stack: Quantitative surface and depth profile analysis with ToF-SIMS, XPS, TEM
  • Defect diagnostics in thin-film PV: localization, target preparation and microstructure analysis
  • Fabrication (magnetron sputtering) and characterization of functional thin films
  • Voltage degradation tests and high voltage leakage current tests

Examples

TEM image PERC solar cell
© Fraunhofer CSP
High-resolution TEM image of the backside passivation layer stack of a PERC solar cell. from: S. Lange, A. Hähnel, V. Naumann, C. Hagendorf, Elemental analyses and excess oxygen investigations by STEM-EELS at AlOx/Si interfaces in PERC solar cells, Abstract SiliconPV2020

High-resolution Layer Characterization

In current cell concepts, thin layers are used on the front and back of the solar cells, which serve to passivate the absorber layers, but also for contacting and charge carrier transport. Basic properties of the layer systems such as element composition and layer thickness have a large impact on the electronic properties of the solar cells. Since the layers often have thicknesses in the range of a few to several tens of nanometers and the surfaces are often textured, high-resolution electron microscopy techniques are required to investigate layer properties with nanometer accuracy. We use transmission electron microscopy (TEM) in combination with energy dispersive X-ray spectroscopy (EDX), electron energy loss spectroscopy (EELS) and electron diffraction (SAED) to analyze the cross-sectional lamellae, which are only 50 to 100 nanometers thick and are prepared from the samples beforehand. Complementary to these methods, information about any impurities on the surfaces or in the layers is obtained by means of depth profiles using X-ray photoelectron spectroscopy (XPS) and, with maximum detection sensitivity, by time-of-flight secondary ion mass spectrometry (ToF-SIMS).

Testing device PIDcon
© Fraunhofer CSP
With the PIDcon test device, potential-induced degradation (PID) can be detected at the solar cell level.
Hugo-Junkers-Preis-Finalist
© Saxony-Anhalt
Hugo-Junkers-Preis-Finalist 2017

PID test equipment for unencapsulated solar cells

For the evaluation of unencapsulated solar cells and module encapsulation materials with regard to their susceptibility to potential-induced degradation (PID), an integrated test device was developed at the Fraunhofer CSP. The patented test procedure is commercially available as PID cell tester "PIDcon" from Freiberg Instruments. With PIDcon it is possible to carry out PID tests on solar cells within a few hours and without the complex use of expensive climate chambers.

Based on this know-how, a PID module test device is currently being developed in cooperation with Freiberg Instruments to market maturity for use in the open field. PIDcheck allows modules to be tested for quality even after they have been installed in the field and to make statements about the susceptibility of modules to the PID effect. With a duration of four to eight hours, the device takes considerably less time to use than previous methods.

 
Further information on www.pidcon.com

characterization of dust-soiled glass in a test field in Qatar
© Fraunhofer CSP
REM, FIB, TEM & EDX characterization of dust-soiled glass in a test field in Qatar.

Anti-soiling evaluation of glass coatings

In regions with high levels of airborne dust (e. g. deserts, cities) the dust deposits on the glass surfaces of the PV modules result in significant yield losses. This process is also known as "soiling". Anti-soiling layers can be used to minimize the cleaning effort. Microstructural characterization is helpful for understanding and checking the function of these dust-repellent layers. Under real desert conditions, for example, cementation processes take place in which dust particles are literally "baked" onto the glass. The simulation of such contamination processes in the laboratory is carried out at the Fraunhofer CSP using a specially developed test stand.

Electro-optical methods and REM/EBIC
© Fraunhofer CSP
Electro-optical methods and REM/EBIC (a) help to localize defects and contaminations in thin-film solar cells. Target preparation methods permits the identification of defects buried in the layer structure (b) and to draw conclusions on process faults.

Layer Diagnostics in Thin-film PV

Defects and impurities in thin-film solar cells are located using electro-optical methods and REM/EBIC (a). Target preparation methods can be used to find buried defects in the layer structure (b), which allow conclusions to be drawn about defects in the process. Advanced elemental analytical methods such as ToF-SIMS, XPS, TEM/EDX help to classify different types of defects.