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Unformatted text preview: What is reported here? What is new? Relation to prior work Why does it matter? INTRODUCTION Copper chalcopyrite compounds, notably the Cu(In,Ga)Se2 (CIGS) alloys are promising materials for high performance thin film solar cells. Recent results have demonstrated that small area devices based on these materials can exhibit at least 20% efficiency.1,2 The cells show improved performance when produced from polycrystalline materials rather than from single crystals,3, 4 and the device results exhibit little dependence on the grain size.5 … Prior works have examined an alternative method for producing CdS junctions by evaporation, which generally yields lower performance devices. Although no direct measurement of CdS diffusion in grain boundaries was made, it was inferred that CdS had diffused into grain boundaries based on scanning Kelvin probe and other measurements. This was suggested to be due to grain boundary diffusion, in agreement with our prior study showing explicitly that CdS was present in intergranular voids deep within CuInSe2. … In this paper, the structural and compositional differences in grain boundaries of single and bilayer films grown at 400ºC or 580 ºC as well as the CIGS and CdS junction are reported. The results provide new information on how single layer growth processes may differ from multistage processes and the role of deposition temperature. More importantly the results show how CdS penetration of the grain boundaries in the CBD process can effectively wrap the heterojunction around the grains producing a three-dimensional nanostructured device. The results support a model proposed by Noufi et al. but which has not hitherto been verified though detailed experimental data. The mechanism by which surface doping of the grain boundaries is obtained is directly shown by nanoprobe EDS measurements, which demonstrate that the cation/anion ratio in the grain boundary changes as CdS appears, suggesting that the CdS-filled boundaries are cation rich and consequently should be ntype. Experimental Who made the samples? How were samples made? Can reference other Sources if appropriate How were analyses done? Why is this a good method? How long do samples last? The samples analyzed and described here were complete solar cell devices provided by the Institute of Energy Conversion (IEC) at the University of Delaware. The CIGS layers were produced by multisource evaporation on Mocoated soda-lime glass using bilayer and single layer recipes at two different substrate temperatures Ts=580°C and 400°C. The resulting layers were coated with undoped CdS by CBD, an undoped ZnO layer, and an n+-doped indium tin oxide transparent conductor. A detailed description of the processing conditions may be found in Shafarman et al. The deposition parameters as well as the resulting performance of the devices studied are summarized in Table 1. Structural and compositional information was obtained from examination of both plan-view and cross-sectional samples in a transmission electron microscope. Preparation of electron transparent samples followed conventional processes. For the cross-sectional samples, slices were cut from the devices and a sandwich structure was formed by gluing two pieces face-to-face, grinding to ~30µm thickness, and Ar ion-milling at liquid nitrogen temperature to electron transparency. The milling ion energy was 5 keV and the angle of the beam with respect to the surface was 15°. Under these conditions the ion range is ~1 nm into the surface. Typically ion damage and ion-induced movement of atoms is restricted to this range. These instruments provide small diameter electron probes (1.2 and 1.6 nm, respectively) and are optimized for nanochemical analysis. The electron transparent samples were found to be acceptable for at least seven days but to avoid slow oxidation, new specimens were prepared if a sample was older than this. What specific result are you describing? What does the data show? Does it agree with prior literature? How did you check your result? Summarize what the Data means What new science results From this experiment? What do the results tell You about a problem of Interest? How can you generalize The results to other Questions?...
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This note was uploaded on 09/15/2013 for the course MSE 182 taught by Professor Braun during the Fall '08 term at University of Illinois, Urbana Champaign.

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