Thin Film-PV: Micro Analytics and Laser Processing

© Fraunhofer CSP
Detailed view of two troughs of a µ-TLM structure in LSM height profile imaging.

Failure diagnostics and nanoscopic examinations of thin-film solar modules require extremely high-resolution analytics. Electrical, optical and microstructural methods are continuously further developed in order to evaluate processes and materials in thin-film photovoltaics (CdTe, CIGS, Si, OPV). We determine the reliability of thin-film solar modules based on spatially resolved yield/loss analyses and analyze the cause of failures in open-field and laboratory installations. We design and manufacture special test structures by applying laser structuring and coating technology.

Partners from the PV industry, plant operators, and equipment manufacturers are the ones who define the R&D objectives. By applying electro-optical localization and preparation methods and material-scientific analytics, the causes for defects can be elucidated down to their atomic dimensions in a very short time. Based on a fundamental understanding of the processing conditions and the resulting electrical properties of the material the team Thin Film Characterization develops proposals for »inline« diagnostics.

Services

  • Defect diagnostics in thin-film PV: localization, target preparation and microstructural analysis
  • Element analytics in film stacks: quantitative depth-profile analyses with TOFSIMS, XPS, TEM
  • Laser structuring and contract coating (TCOs, rear contacts)
  • Electrical micro characterization
  • Optical simulation/characterization

Examples

© 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.

Defect Diagnostics in Crystalline Si Thin Films

Defects and contaminations in in thin-film solar cells are localized by applying electro-optical methods and REM/EBIC (a). Target preparation methods permits the identification of defects buried in the layer structure (b) and to draw conclusions on faults in processing. More advanced element-analytical methods like ToF-SIMS, XPS, TEM/EDX help to classify the various defect types.

© Fraunhofer CSP
REM image of a P1 scribe lasered in femtoseconds in a glass pane coated with approx. 600nm TCO.

Laser Processing in Thin-film PV

In the processing of all thin-film PV concepts (CIGS, CdTe, a-Si/µc-Si, etc.) the quality and dimension of the laser structures for the monolithic interconnection of the film stack is of great importance for the module's later performance. Laser ablation can be optimized by adjusting the laser parameters to the layer system and structuring step. As far as efficiency losses and reliability are concerned, heat-loss zones as well as melting and material residues resulting from the laser ablation process deserve special attention. For these purposes, short-pulse (ns) and ultra-short pulse (fs) lasers with variable wavelengths are used. Subsequent microstructural analyses (Raman spectrometry, laser scanning, microscopy, etc.) as well as accompanying optical-thermal simulations serve the purpose of process control and understanding the ablation behavior.

© Fraunhofer CSP
Three-dimensional view of a trough structure for µ-TLM, imaged by means of laser scanning microscopy (LSM).

Resistance Measurement in TCO Layer Systems

In the process and quality control of thin-film photovoltaics, we need a simple measurement procedure that is suited to determine the specific resistance of thin films lying on top of layers with a higher conductivity. In the scope of a research project, scientists at the Fraunhofer CSP developed a modified transfer-length measurement method (TLM). The microscopic test structure it requires was introduced into the layer stack by means of ion milling or fs-laser ablation. With the subsequent TLM measurement on a micrometer scale, the specific resistance of a TCO buffer layer can be determined directly inside a stacked layer.

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

Anti-Soiling Evaluation of Glass Coating

In regions with high airborne dust loads (e.g. deserts, cities) dust depositions on the glass surfaces of PV modules are the cause significant yield losses. Anti-soiling layers, for example, can be applied to minimize the otherwise necessary cleaning effort. A characterization of the microstructure is useful for understanding and testing the function of these dust-repellent layers. For example, under real desert conditions, cementation processes occur in which dust particles are virtually "caked" onto the glass surface. We realize the simulation of such soiling processes in the laboratory with the aid of a test facility which has been developed exclusively for this purpose.

© Fraunhofer CSP
Atomically resolved structure of the CIGS film.

Stacking Faults in the Atomic Structure of CIGS Films

We produce artifact-free cross-sections of susceptible thin-film systems such as CIGS solar cells and apply preparation methods especially developed for this purpose. They allow for the high-resolution imaging of the atomic structure by transmission electron microscopy (TEM). The images reveal local material defects as irregularities in the ordered crystal structure of the atoms.

© Fraunhofer CSP
Visualization of position of pn transition (red) in the cross-section of a thin-film solar cell by means of REM/EBIC.

Target Preparation and Microstructure Analysis

Methods used in defect diagnostics are applied to analyze thin-film solar cells made of CIGS or CdTe, for example. Defective areas can be isolated and examined selectively by means of focused ion beam (FIB) target preparation. The combination of structural analysis and electrical methods (FIB-EBIC) enables a direct correlation of electrical and structural properties and hence the identification of defect causes on a microstructural scale.