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Grain Boundary Properties


8 mm working distance, and a magnification of 10 k×. Experimental EBSD conditions were as follows: map size area = 12 μ m × 12 μ m; pixel size (medium) = 0.04 μ m, the bin size = 5 × 5 ; gain = 300, exposure time = 3.66 msec, frame averaging = 5, frames per second = 270 fps ( ~55 patterns per sec with frame averaging of 5×). T e EBSD signal originates from approximately the top


~20 nm of the sample, and therefore the quality of EBSD data is sensitive to crystal perfection near the surface [ 10 – 13 ]. T e CIGS structure is tetragonal, with a c/a of ~2.0, but the structure was indexed assuming a pseudo cubic symmetry to improve the indexing [ 13 ]. T e orientation of each pixel is calculated during pattern collection and assigned a confi dence index based on the quality of the pattern. Aſt er the entire data set of EBSD patterns is collected, each pixel is assigned to a specifi c grain. For pixels that had a low confi dence index, a standard clean-up procedure was applied to assign pixels to grains. T e clean-up procedure consisted of single “grain dilation” for each data set using the following criteria: (1) a grain must contain multiple rows of pixels, (2) single dilation iteration, (3) pixel size of 0.04 µm (40 nm), (4) minimum grain size of fi ve pixels, and (5) an inter-grain tolerance angle of 5 degrees. Grains smaller than 0.1 µm were not included in grain-size analysis. Similar EBSD work by Abou-Ras et al. used an Ar ion milling procedure to prepare cross-sectional CIGS samples to minimize the beam damage. T e pixel size used for their investigation was 10–50 nm, and the authors excluded grains with diameters  0.2 µm [ 8 ]. Atomic force microscopy . Samples of etched and ion-polished CIGS were imaged using peak force (PF) tapping AFM techniques. T e AMF images were obtained on a Bruker Icon using a Nanoscope V controller (soſt ware v.8.15). Cantilevers employed were Bruker Scanasyst (air) with the following settings: PF set point = 1.2 V; noise threshold = 0.5 nm; PF amplitude = 300 nm; Z range = 12 μm; PF engagement set point = 0.15 V. Scan sizes were 50 µm × 50 µm. All images were captured at 1024 lines of resolution. All images were produced using SPIP v.6.3.5 soſt ware (Scanning Probe Image Processor, Image Metrology, Denmark). A 2nd order average


Table 1 : NuvoSun cell performance. Device effi ciency (in %) and open circuit voltage (V oc in volts) for each device. The solar cell effi ciency refers to the effi ciency of converting light into electricity. Open-circuit voltage, the maximum voltage available from a solar cell, is also a measure of performance. Commercial devices with polycrystalline silicon have open-circuit voltages on the order of ~ 600 mV, whereas organic solar devices can have open-circuit voltages in excess of 600 mV.


Device ID Device 1 Device 2 Device 3


Efficiency 10.9 11.4 11.0


Voc


0.613 0.613 0.590


plane fi t with a zero-order LMS and mean set to zero plane fi t was used to fl atten the images. All postprocessing measure- ments were done using SPIP soſt ware as well.


Results


CIGS cells : process and device properties . T e grain properties (size, texture, and relative orientation) were investigated using EBSD of cells that were grown under diff erent reactor conditions but which had a similar level of performance ( Table 1 ). Device effi ciencies ranged from 10–11% effi ciency, and the open circuit voltage (V oc ) ranged from 590 mV to 629 mV.


Surface roughness of as-received NuvoSun CIGS . AFM was used to investigate the morphology and roughness of the as-received CIGS surfaces for the various devices. Figures 2 a- 2 c show AFM images of the surfaces that indicated the grain structure for Device 1 was more faceted compared to Devices 2 and 3. The average surface roughness of the CIGS films, as measured by AFM techniques, varied from 60 nm to 94 nm. Figure 3 shows SEM images of CIGS film surfaces. The SEM images also revealed variations in surface structure and roughness among the samples, and all of the as-received samples contained sub-micron porosity, typically located at grain boundary junctions. EBSD of NuvoSun cells . Grain orientation maps, derived from the EBSD patterns, are usually displayed in inverse pole


Figure 2 : AFM images for Devices 1 to 3 in images (a) to (c). Fields of view are 10 µm × 10 µm. The average surface roughness (Ra) values for the cells were determined from 50 µm × 50 µm regions. The Ra values by AFM for Devices 1–3 were 60.1 nm, 73.8 nm, and 94.7 nm, respectively.


34 www.microscopy-today.com • 2018 May


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