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Grain Boundary Properties Conclusion


Collection of high-quality EBSD grain size and grain orientation data from as-received CIGS devices was limited due to the surface roughness, and many grains could not be properly indexed. T is issue was circumvented by ion-polishing the CIGS samples at a glancing angle to reduce surface roughness. T e quality of EBSD patterns aſt er ion-polishing was adequate for indexing, although the patterns were then collected at a depth of ~200–250 nm from the original surface. T e CIGS grain size did not have


a signifi cant impact on device perfor- mance (at this level of effi ciency), and other factors are expected to play a dominant role. T e EBSD results indicated that the majority of grain boundaries in these devices were high-angle boundaries. T e EBSD results also showed that the CIGS fi lms did not have a strong texture, which agrees with previous X-ray diff raction measurements for devices grown using a two-step selenization process. Typically, CIGS thin fi lms grown by co-evaporation will oſt en display a <220/204> and <112> grain texture. T e SEM and EBSD data indicated that there was a signif- icant variation in the porosity among the diff erent devices, but this did not lead to diff erences in device performance. T e extent to which the high-angle grain boundaries and porosity will limit device performance at higher effi ciency levels is under investigation.


Acknowledgements


The authors wish to thank S. I. Wright and M.M. Nowell of EDAX for their guidance in the EDSB analysis.


References [1] T Kato . “Recent Research Progress of High-effi ciency CIGS Solar Cell in Solar Frontier,” 7th International Workshop on CIGS Solar Cell Technology (IW-CIGSTech 7) 32nd EU PVSEC, June 20–24, 2016, ICM, Munich, Germany.


[2] S Philipps and W Warmuth . Fraunhofer Institute for Solar Energy, Photovoltaics Report , Feb. 26, 2016.


[3] S Siebentritt , Sol Energ Mat Sol C 95 ( 2011 ) 1471 – 76 . [4] R Caballero et al ., Prog Photovolt: Res Appl 21 ( 2013 ) 30 – 46 .


[5] M Nichterwitz et al ., T in Solid Films 517 ( 2009 ) 2554 – 57 .


[6] D Abou-Ras et al ., J Appl Cryst 40 ( 2007 ) 841 – 48 . [7] MA Contreras , T in Solid Films 511– 12 ( 2006 ) 51 – 54 .


38


Figure 9 : Misorientation angles for grains in Devices 1 and 3 shown in (a) and (b). The “Red” lines show the low-angle grain boundaries, whereas high-angle grain boundaries are denoted by the blue lines. Special high-symmetry grain boundaries of 60 o are shown in green. Bar graphs in (c) and (d) show the distribution of the misorientation angles between all grains in each device. Voids are excluded in the grain boundary analysis.


[8] D Abou-Ras et al ., Phys Stat Sol (RRL) 2 ( 3 ) ( 2008 ) 135 – 75 .


[9] U Rau et al ., Appl Phys A 96 ( 2009 ) 221 – 34 . [10] SI Wright and MM Nowell , Microsc Microanal 12 ( 2006 ) 72 – 84 .


[11] ASTM International, SI Wright et al., presentation “Grain Size by EBSD,” http://www.astm.org/COMMIT/ e04_presentations/EBSD_GrainSize (accessed March 7, 2018).


[12] LN Brewer and JR Michael , Microscopy Today 18 ( 02 ) ( 2010 ) 10 – 15 .


[13] T Maitland and S Sitzman . Electron Backscatter Diff raction (EBSD) Technique and Materials Characterization Examples . in Scanning Microscopy for Nanotechnology, Techniques and Applications eds W Zhou, ZL Wang, Z.L., Springer-Verlag , New York , ( 2007 ), pp 41 – 75 .


[14] SI Wright and MM Nowell . personal communication. [15] R Baier . “Electronic grain boundary properties in polycrystalline Cu(In,Ga)Se 2 semiconductors for thin fi lm solar cells” thesis, Freien University, Berlin, 2012. [16] G Hanna et al ., Appl Phys A 82 ( 2006 ) 1 – 7 .


www.microscopy-today.com • 2018 May


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