Jörg Isenberg

Impact of metal silicide precipitate dissolution during rapid thermal processing of multicrystalline silicon solar cells

Synchrotron-based analytical x-ray microprobe techniques were employed to study the dissolution of iron, copper, and nickel silicide precipitates at structural defects in cast multicrystalline silicon in response to rapid thermal processing (RTP). A direct correlation was observed between iron silicide precipitate dissolution, increased minority carrier recombination, and decreased device performance after high-temperature (1000°C) RTP. In contrast, iron precipitates comparable in size to as-grown material remained after lower-temperature RTP (860°C); in this case the material exhibited higher minority carrier diffusion length and better solar cell performance. RTP at both temperatures effectively dissolved nickel and copper silicide precipitates. It is concluded that iron dissolved from structural defect reservoirs detrimentally affects the cell performance, likely by forming distributed point defects and smaller precipitates. For cast multicrystalline silicon, higher performance can be expected by inhib...

Observation of transition metals at shunt locations in multicrystalline silicon solar cells

By employing a combination of analytical tools including lock-in thermography and synchrotron-based x-ray fluorescencemicroscopy,transition metals have been identified at shunting locations in two types of low-cost multicrystalline silicon (mc-Si) solar cellmaterials: cast multicrystalline and ribbon growth on substrate (RGS). At a shunting location in the cast mc-Si cell, silver and titanium, both contact strip materials, have been identified at the shunting location, suggesting a process-induced error related to contact metallization. At a shunting location in the RGS cell, a material-specific shunting mechanism is described, involving channels of inverse conductivity type, where copper and iron are found. The possible roles of these metals in this shunting mechanism are discussed. These results illustrate the wide range of physical mechanisms involved with shunting in solar cells.

Quantifying the effect of metal-rich precipitates on minority carrier diffusion length in multicrystalline silicon using synchrotron-based spectrally resolved x-ray beam-induced current

Synchrotron-based, spectrally resolved x-ray beam-induced current (SR-XBIC) is introduced as a technique to locally measure the minority carrier diffusion length in semiconductor devices. Equivalence with well-established diffusion length measurement techniques is demonstrated. The strength of SR-XBIC is that it can be combined in situ with other synchrotron-based analytical techniques, such as x-ray fluorescence microscopy (μ-XRF) and x-ray absorption microspectroscopy (μ-XAS), yielding information about the distribution, elemental composition, chemical nature, and effect on minority carrier diffusion length of individual transition metal species in multicrystalline silicon. SR-XBIC, μ-XRF, and μ-XAS measurements were performed on intentionally contaminated multicrystalline silicon, revealing a strong correlation between local concentrations of copper and nickel silicide precipitates and a decrease of minority carrier diffusion length. In addition, the reduction of minority carrier diffusion length due t...

Carrier density imaging (CDI): a spatially resolved lifetime measurement suitable for in-line process-control

Carrier density imaging (CDI) is introduced as a new, spatially resolved carrier lifetime measurement technique in solar cell production. CDI provides the actual local lifetimes as compared to standard lifetime mapping techniques (e.g. MW-PCD) which yield the differential lifetime only. Most important, CDI is an extremely fast measurement technique: A measurement on a 100/spl times/100 mm/sup 2/ wafer under low-level injection conditions can be performed on a timescale of seconds whereas a standard MW-PCD map needs about 2 hours for a measurement with identical resolution even if high injection conditions are chosen. The combination of a spatially resolved and fast measurement predestines CDI for in-line process control. It is possible to achieve actual lifetime maps with overall measurement times on the order of seconds on as cut samples as well as on emitter-diffused wafers without any surface passivation. Improvements for further reduction of measurement time are discussed.

Shunt-analysis of epitaxial silicon thin-film solar cells by lock-in thermography

Lock-in thermography has been applied for shunt-analysis on epitaxial silicon thin-film solar cells. The solar cell material was made by epitaxial deposition of the base layer on highly doped monocrystalline (Czochralski) and multicrystalline silicon substrates in an APCVD-system. Solar cells were prepared in a laboratory-type and an industrial-type process. Characterization of the solar cells by infrared lock-in thermography and microscopy revealed a clear correlation between shunts and epitaxial defects in case of the lab-type solar cells. Furthermore an increased concentration of shunts located under the emitter grid lines of the industrial-type solar cells compared to the lab-type solar cells was observed. The analysis by thermography thus gave insight into the quality of the epitaxial layers and into problems concerning a transfer of the solar cell process from laboratory to industrial scale manufacturing.

Correlation of spatially resolved lifetime measurements with overall solar cell parameters

Lifetime mappings are common tools for assessing the quality of mc-silicon for solar cell production. Nevertheless it is a difficult problem to directly relate lifetime mappings to overall solar cell parameters. This paper intends to show that this correlation is possible quantitatively. We have correlated actual low-level injection lifetimes obtained by carrier density imaging (CDI) measurements with overall cell parameters of solar cells processed on adjacent wafers. The 2D lifetime-structure is taken account for by appropriate weighing functions that include the whole information given in the frequency distribution of bulk lifetimes. Thus a one dimensional cell model (PC1D) can be applied. Good general agreement between predicted and measured cell parameters has been achieved, deviations are discussed. Further insight into the gettering behavior of block-east and Bridgman-grown mc-silicon was attained.

Shunt imaging in solar cells using low cost commercial liquid crystal sheets

A simple low cost set-up to determine the location of shunts in solar cells has been developed. Our manual system can reproducibly detect all major shunts in large area reverse biased solar cells at a rate of 1 cell every 12 seconds and allows therefore the imaging of large numbers of cells in a short time. The system is based on the use of commercial liquid crystal sheets which are brought into thermal contact with the solar cell by vacuum. The colour change of the sheet is an indication of the local heating of the cell at shunt locations under reverse bias at typical voltages between -2 and -10 V. The detected shunts are the same as the ones found in lock-in infrared thermography at reverse bias. We discuss some of the system specifications and limitations and give several application examples, with a special emphasis on edge isolation.

Spatially resolved assessment of power losses due to bulk material quality and metallization problems

The main advantage of illuminated lock-in thermography (ILIT) over standard (dark) lock-in thermography is the ability to measure at actual operation conditions of solar cells. Thus ILIT allows a quantitative and spatially resolved investigation of the sum of all power losses in a solar cell at actual operation conditions with one measurement. The quantitative influence of low bulk material quality on solar cell performance is investigated. Also the locations of high contact resistance of the frontside metallization and of high series resistance within the metallization were determined with ILIT. The results were compared with Corescan results for the same cells and a good correlation between the locations determined with both methods was found.

Thermographic imaging of free carrier density in silicon for solar cells

The measurement of free carrier density in silicon is a crucial parameter for the characterisation of silicon solar cell material. Carrier Density Imaging (CDI) is a valuable tool to obtain spatially resolved images of the free carrier density distribution. This article describes the experimental setup of CDI for absorption mode and recently developed emission mode measurements. The theoretical dependence of the absorption and emission of infrared radiation on the free carrier density is discussed. Results of absorption and emission mode measurements are presented and the advantages of the new emission mode are elaborated.