2. Experimental
2.1. Sample information
Twelve industrially grown mc-Si bricks, which were partially produced by different manufacturers, were investigated. The twelve bricks were classified into two classes with the help of the initial grain structure images of these bricks (Fig. 1). Class fine-grained bricks (A–F) have a similar uniform small grain size comparable to the hp mc-Si images which were already presented in literature [7]. Class coarse-grained bricks (G–L) have an inhomogeneous grain size distribution with some large and some small grains. Wafers from the twelve bricks were processed in one batch into solar cells by Hanwha QCells using the PERC technology. The maximum solar cell efficiencies of the investigated wafers are presented in Table 1. All class fine-grained bricks have higher maximum solar cell efficiencies (18.5%–18.8%) compared to the class coarse-grained bricks (17.6%–18.4%). In the following, it will be analysed whether the higher solar cell efficiency of the class fine-grained bricks can be correlated to parameters of the grain structure like grain size, grain orientation and grain boundary type.
2.2. Characterization methods
For the determination of the grain size-, grain orientation-, and grain boundary type distribution on full wafer scale 156 × 156 mm2 two measurement systems were used. Firstly, the grains were automatically detected by using the grain detector (GEMINI tool from INTEGO) [11]. The measurement principle is based on reflectivity. 20 LED modules, which are placed under an angle of 90°, 70°, and 60° to the wafer surface, irradiate the wafers surface subsequently followed by an automatic imaging processing step which determines the position and the size of each grain. The minimum detectable grain size for the present investigations was around 0.075 mm2. A description of the grain size distribution was done by using the so-called coefficient of variation CVgrain size (GS). The CVGS is defined as the standard deviation of the grain sizes divided by the mean grain size.
The crystallographic orientation for grains larger than 3 mm2 was determined in a second step by using the Laue scanner [12]. This system irradiates each grain with white x-rays and a Laue pattern is detected. A post processing of the Laue pattern is done automatically by the system. As a result the grain orientation in growth direction is obtained. Furthermore, the grain boundary type is calculated if the grain orientation of neighbouring grains was measured. In the following, the grain orientation results were represented by inverse pole figures IPF. The measured grain orientations were simplified to hkl values up to 20 and circles were drawn into the IPF. The centre of a circle represents the grain orientation itself and the circle diameter is proportional to the area fraction of the grain orientation with regard to the totally measured wafer area. The grain orientation distribution is also described by using the coefficient of variation CVgrain orientation (GO). In this case the CVGO is defined as the standard deviation of the area fractions of the grain orientations divided by the mean value.
After measuring the grain structure, band to band photoluminescence imaging PLI on as-cut non passivated wafers was performed to investigate the area fraction of recombination active defects. The PLI measurements were done with the OPTECTION imaging tool using a 175W laser (wavelength = 790 nm) and a 40s exposure time. An image post processing quantifies the area fraction of the recombination active defects.
2. Experimental2.1. Sample informationTwelve industrially grown mc-Si bricks, which were partially produced by different manufacturers, were investigated. The twelve bricks were classified into two classes with the help of the initial grain structure images of these bricks (Fig. 1). Class fine-grained bricks (A–F) have a similar uniform small grain size comparable to the hp mc-Si images which were already presented in literature [7]. Class coarse-grained bricks (G–L) have an inhomogeneous grain size distribution with some large and some small grains. Wafers from the twelve bricks were processed in one batch into solar cells by Hanwha QCells using the PERC technology. The maximum solar cell efficiencies of the investigated wafers are presented in Table 1. All class fine-grained bricks have higher maximum solar cell efficiencies (18.5%–18.8%) compared to the class coarse-grained bricks (17.6%–18.4%). In the following, it will be analysed whether the higher solar cell efficiency of the class fine-grained bricks can be correlated to parameters of the grain structure like grain size, grain orientation and grain boundary type.2.2. Characterization methodsFor the determination of the grain size-, grain orientation-, and grain boundary type distribution on full wafer scale 156 × 156 mm2 two measurement systems were used. Firstly, the grains were automatically detected by using the grain detector (GEMINI tool from INTEGO) [11]. The measurement principle is based on reflectivity. 20 LED modules, which are placed under an angle of 90°, 70°, and 60° to the wafer surface, irradiate the wafers surface subsequently followed by an automatic imaging processing step which determines the position and the size of each grain. The minimum detectable grain size for the present investigations was around 0.075 mm2. A description of the grain size distribution was done by using the so-called coefficient of variation CVgrain size (GS). The CVGS is defined as the standard deviation of the grain sizes divided by the mean grain size.The crystallographic orientation for grains larger than 3 mm2 was determined in a second step by using the Laue scanner [12]. This system irradiates each grain with white x-rays and a Laue pattern is detected. A post processing of the Laue pattern is done automatically by the system. As a result the grain orientation in growth direction is obtained. Furthermore, the grain boundary type is calculated if the grain orientation of neighbouring grains was measured. In the following, the grain orientation results were represented by inverse pole figures IPF. The measured grain orientations were simplified to hkl values up to 20 and circles were drawn into the IPF. The centre of a circle represents the grain orientation itself and the circle diameter is proportional to the area fraction of the grain orientation with regard to the totally measured wafer area. The grain orientation distribution is also described by using the coefficient of variation CVgrain orientation (GO). In this case the CVGO is defined as the standard deviation of the area fractions of the grain orientations divided by the mean value.After measuring the grain structure, band to band photoluminescence imaging PLI on as-cut non passivated wafers was performed to investigate the area fraction of recombination active defects. The PLI measurements were done with the OPTECTION imaging tool using a 175W laser (wavelength = 790 nm) and a 40s exposure time. An image post processing quantifies the area fraction of the recombination active defects.
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