Grain Boundary Properties
Figure 7 : Inverse pole fi gure EBSD data (a) to (c) show grain size and orientation for Devices 1, 2, and 3, respectively. A single dilation clean-up step was used as described in the text. Samples were oriented so that the stainless steel rolling direction is aligned with the long direction of the page. Regions with a Confi dence Index < 0.1 are shown in black.
Low-angle grain boundaries are defined as adjacent grains with only 2 o –15 o misorientation. The misorientation angle between grains is shown in Figures 9 a and 9 b for Devices 1 and 3. Most grain boundaries were high-angle (shown in blue), and special high-symmetry grain boundaries of 60 o are denoted by green color. An alternate way of presenting the data is to show the distribution of misorientation angles between adjacent grains as shown in Figures 9 c and 9 d, which indicated that few low-angle grain boundaries were present (red lines).
The misorientation angles shown in Figures 9 c and 9 d exhibit a broad distribution centered on 40 o –45 o , which is consistent with a random orientation [ 11 ]. This result agrees with X-ray diffraction measurements conducted on similar devices grown using a two-step selenization process. Conversely, thin films of CIGS grown by co-evaporation often display <220/204> and <112> grain textures [ 7 ].
Discussion
There are several proposed models used to explain the properties of grain boundaries in CIGSe [ 3 , 7 , 15 ]. In the electronic barrier model, trapped charge at the grain boundary causes local bending of the conduction and valence bands adjacent to the grain boundary. In this
model, a downward band bending toward GBs would favor recombination of carriers, while an upward band bending would repel minority carriers and reduce recombination [ 15 , 16 ].
Baier et al. investigated the symmetry of GBs that influences GBs electronic properties [ 15 ]. Their results showed that special (Σ = 3) grain boundaries typically were neutral (zero potential), while the non- Σ = 3 grain boundaries had a wider range in potential from positive to negative. Hannah et al. also investigated CIGS samples, and their results indicated that samples with a random texture had a more negative contact potential near the grain boundaries compared to the samples with a strong (220 / 204) texture [ 16 ]. These studies suggest the grain boundary orientation and grain boundary defects may limit device performance as predicted by Rau et al. [ 9 ].
Kelvin probe force microscopy (KPFM) is one method that has been used to obtain spatially resolved information of the work function at the grain boundary and at adjacent grains [ 15 , 16 ]. Similar experiments are currently in progress to characterize the surface potential of CIGS devices near the heterojunction interfaces and grain boundaries using KPFM and EBSD in order to better understand the correlation between grain boundary misori- entation and device efficiency.
Figure 8 : Grain sizes in µm and log normal fi ts for Devices 1–3 in (a) to (c), respectively. Device 1 had an average grain size of 0.6 µm, whereas the average grain size for the other two devices was 0.4 µm.
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