The material quality of the silicon used for solar cells is highly dependent on the feedstock used and the crystallization process. Fraunhofer ISE is preparing detailed analyses of efficiency-limiting electrical characteristics with regard to maximum achievable efficiency, recombination lifetime, impurity contents and crystallographic structure.
Material quality may be determined as a function of block height, process temperatures and gettering steps. We develop and simulate adapted processes for material improvement and analyze all wafer-based silicon materials such as mono- and multicrystalline silicon, silicon from alternative feedstock (in particular UMG silicon, compensated silicon) and ribbon-drawn material.
There are two main routes available for analyses at Fraunhofer ISE:
- Wafer and cell analysis on an industrial scale with relatively large piece quantities on the PV-TEC research line
- Clean room processing, combined with basic and advanced material characterization.
The first route combines rapid inline and offline characterization of wafers and solar cells with industrially compatible processes on large piece quantities, in order to determine material quality and efficiency potential on an adequate statistical basis. The second route permits deeper insight into efficiency-limiting material defects thanks to advanced material characterization and highly efficient cell processing to estimate maximum efficiency potential.
Further research priorities include investigations into Cz degradation, material properties in compensated material, chemical materials analysis and the precipitation behavior of metallic impurities with multidimensional simulations and experiments on synchrotron sources.
Methods and equipment
In addition to standard basic characterization methods, the following options are available in particular:
- Front-end inspection (geometry, chipping, thickness topography)
- Analysis of microcracks, precipitates
- Base resistance
- Lifetime measurement
- Photoluminescence imaging: (dislocation densities on as-cut wafer, lifetime, iron distribution)
- Electroluminescence imaging
- Luminescence spectroscopy (PL/EL)
- Dislocation density analysis (EPD)
- SEM analysis: crystal orientation and grain boundary classification (EBSD), recombination activity (EBIC), chemical composition (EDX), luminescence (CL)
- Thermal imaging: shunt analysis, lifetime measurement, trap analysis
- Fourier spectroscopy: oxygen/carbon analysis
- Hall measurements
- Chemical analysis for metals and dopants (ICP-OES, AAS)
- Simulation of temperature processes with Sentaurus