Jan Ebser

Emissivity-corrected power loss calibration for lock-in thermography measurements on silicon solar cells

This paper describes power loss calibration procedures with implemented emissivity correction. The determination of our emissivity correction matrix does neither rely on blackbody reference measurements nor on the knowledge of any sample temperatures. To describe the emissivity-corrected power calibration procedures in detail, we review the theory behind lock-in thermography and show experimentally that the lock-in signal is proportional to the power dissipation in the solar cell. Experiments show the successful application of our emissivity correction procedure, which significantly improves the informative value of lock-in thermography images and the reliability of the conclusions drawn from these images.

μXRF investigations on the influence of solar cell processing steps on iron and copper precipitates in multicrystalline silicon

The material quality of multicrystalline silicon is influenced by crystal defects and contaminations like transition metal precipitates. During solar processing these defects can be restructured and change their electrical activity. The purpose of this work is to study the impact of different solar cell processing steps on the distribution and electric activity of transition metal precipitates like iron and copper. Therefore, neighbouring wafers of a multicrystalline silicon ingot, intentionally contaminated with iron and copper were investigated by μXRF (X-Ray Fluorescence Microscopy) at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, to determine the distribution of transition metal precipitates. Afterwards, several solar cell processing steps were applied to these samples. The same sample areas were measured by μXRF again to determine the influence of the applied processing steps on the observed transition metal precipitates. Therefore, a different behaviour of iron and copper precipitates could be observed as expected, due to their different dissolution and diffusion coefficients in silicon. Additionally, the same processing steps were applied to a second set of samples to evaluate the effect of processing steps on the minority charge carrier lifetime and the recombination activity of grain boundaries.

p+-doping analysis of laser fired contacts for silicon solar cells by Kelvin probe force microscopy

Local rear contacts for silicon passivated emitter and rear contact solar cells can be established by point-wise treating an Al layer with laser radiation and thereby establishing an electrical contact between Al and Si bulk through the dielectric passivation layer. In this laser fired contacts (LFC) process, Al can establish a few μm thick p+-doped Si region below the metal/Si interface and forms in this way a local back surface field which reduces carrier recombination at the contacts. In this work, the applicability of Kelvin probe force microscopy (KPFM) to the investigation of LFCs considering the p+-doping distribution is demonstrated. The method is based on atomic force microscopy and enables the evaluation of the lateral 2D Fermi-level characteristics at sub-micrometer resolution. The distribution of the electrical potential and therefore the local hole concentration in and around the laser fired region can be measured. KPFM is performed on mechanically polished cross-sections of p+-doped Si regio...

>20% efficient 80 µm thin industrial-type large-area solar cells from 100 µm Sawn c-Si Wafers

Reducing the thickness of crystalline Si wafers to be processed into solar cells yields several significant benefits: PV module manufacturing cost can be reduced and the required diffusion length of minority carriers is smaller. The latter in turn enables a higher efficiency potential and a larger spread of Si materials to be employed for rear junction solar cell concepts which are advantageous for n-type devices. Industrial-type 80 μm thin large-area rear junction solar cells manufactured from 100 μm wire-sawn wafers exhibit an independently certified efficiency of 20.1% with VOC of 672 mV.

Surface Texturing for Silicon Solar Cell Application Using ICP-PECVD Plasma Technique

This work is focused on plasma-induced texturing using a single chamber inductively coupled plasma – plasma-enhanced chemical vapour deposition (ICP-PECVD) lab-tool. Plasma treatment using pure NF3 under low pressure conditions leads to high etch rates (1.2–1.4 μm/min) enabling fast surface texturing. 6 min plasma treatment on wafers with saw damage decreases the effective reflectivity Reff from 31% to 13.7% without anti-reflection coating (ARC), which is lower than the value for a common alkaline surface texture. For planar (KOH pre-treated) samples Reff can be reduced from 38% to 17.8% after 4 min plasma treatment. Lifetime measurements reveal a postprocess lifetime of 250 μs without any further damage removal etching (DRE). This refers to a low density of plasma-induced defects. Furthermore, it is demonstrated that plasma textured surfaces enable excellent contact formation for screen-printing of Ag/Al pastes on boron emitters leading to low specific contact resistances below 5 mΩcm even for low set peak firing temperatures (TSet < 800°C). These values are much lower than values realized with a common isotropic wet acidic etch and slightly lower than values realized with a common alkaline surface texture, leading to reduced power losses due to lowered series resistance. In addition, the specific contact resistance on POCl3 emitters by screen-printing of commercial available Ag pastes is determined to be below 1 mΩcm, nearly independent of the set peak firing temperature.

P + Doping Analysis of Laser Fired Contacts by Raman Spectroscopy

Laser firing of contacts is a simple method to establish local rear contacts of silicon PERC (passivated emitter and rear contact) solar cells. The silicon bulk is contacted point-wise by Al driven by the laser through the rear dielectric layer. The Al is deposited on the full area by physical vapor deposition or screen-printing. Al and B, if the Al paste contains B additives, can thereby establish a p-doped Si region below the Laser Fired Contact (LFC), which helps to lower carrier recombination because of the local back surface field and also to reduce contact resistance. In this work Raman spectroscopy is used to detect and investigate the p-layer established by laser firing through screen-printed Al. Scanning Raman measurements allow spatially resolved determination of the free hole concentration in the contact area. In a line scan through a LFC, the step in doping concentration between the lowly doped bulk Si and the highly doped LFC area is clearly seen by a high local hole concentrations in the range of 10 cm in the LFC region. This shows that scanning Raman spectroscopy is a useful method for the microscopic understanding of LFCs and optimization of the process parameters.

Spatially Resolved Analysis of Selectively Doped Regions via Confocal Raman Microscopy

More or less all proposed highly efficient solar cell concepts use quite successfully laterally selective boron doping. However, quality control regarding achieved doping level, lateral extension, etc. on real devices or precursors gets more and more complicated with these small structures as the typical characterisation techniques simply do not work well on small scale. High resolution mapping confocal Raman spectroscopy is considered a possible technique to tackle this challenge. In this contribution it is demonstrated on IBC precursor structures that the contrast in local doping level can be finely resolved via mapping Raman spectroscopy, even though the determined absolute value is found to be too small. It is discussed on the basis of a few fundamental calculations that the depth sensitivity is accountable for this drawback.