Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE
Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE
Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE
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The XRD measurements on InxSy layers deposited on SLG revealed that all the films were amorphous<br />
like as no distinct diffraction peaks of any of the In2S3 phases were detected in any of the films (asgrown<br />
and annealed) evaporated from powder-A and powder-B which may also be attributed to small<br />
layer thicknesses (30-60 nm).<br />
As shown in Fig. 6a and b, a ~30 nm thick evaporated InxSy layer on CIGS was clearly distinguishable<br />
in the SEM cross-section image (Fig. 6a). Also evident is that the InxSy layer uniformly covered the<br />
CIGS surface, but occasional discontinuity occurred at the edges of the CIGS grains. The morphology<br />
showed no differentiable structure in evaporated buffer layer as the thickness was rather small<br />
(Fig. 6b).<br />
Solar cell characteristics<br />
The solar cells were developed with InxSy buffer layers. For studying the solar cell characteristics,<br />
fresh In2S3 powder was filled in the crucible for each buffer deposition run. From the first evaporation<br />
run, powder B (�= 11.6%) yielded better cell properties than powder A (�= 8.6%) as given in Table I.<br />
High S-loss from powder-B can cause modifications in CIGS by adding sulfur in CIGS. However, at<br />
this stage it remains speculative as further measurements are necessary to prove it conclusively.<br />
The solar cells with In2S3 buffer layer show, as expected, higher spectral response in short wavelength<br />
region compared to cell with CdS buffer layer (Fig. 7).<br />
It has been earlier reported that the annealing treatment can modify the CIGS/InxSy interface and<br />
hence the solar cell properties. Therefore, we investigated the cells properties affected by in-situ<br />
annealing after buffer deposition step. As given in table II, the PV properties started degrading at and<br />
above PBDVA temperature of 200°C, primarily due to lowering in FF and Voc. At 250°C, all PV<br />
parameters were very low.<br />
Powder<br />
�<br />
[%]<br />
FF<br />
[%]<br />
Voc<br />
[mV]<br />
Jsc<br />
[mA/cm 2 ]<br />
A 8.6 55 570 27.4<br />
CdS-Ref. 11.9 67.2 591 30<br />
B 11.6 62 654 28.6<br />
CdS-Ref. 12.4 67.4 627 29.4<br />
Table I: The PV parameters of the cells with ~30 nm thin InxSy buffer layer evaporated from powder-A<br />
and -B and their respective CBD-CdS reference cells.<br />
Figure 7: External quantum efficiency of the cell with InxSy and CdS buffer layers.<br />
It is well known that the tetragonal In2S3 has spinal-type crystal structure with large defect density. As<br />
a result of this, Cu can not only occupy the vacant sites in the In2S3 defect structure, but also it can<br />
substitute In atoms [12]. This causes a Cu depletion and In enrichment at the CIGS side under the<br />
influence of temperature. There is an optimum temperature of annealing of the cell, below which<br />
efficiency improves and above that efficiency drops due to excessive interdiffusion and formation of an<br />
undesirable phase [11]. In addition, the interdiffusion can also occur due to difference in chemical<br />
potential across heterojunction layers. However, we observed inconsistent results with air annealing of<br />
the completed cell.<br />
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ATHLET, D. Brémaud, ETH Zurich<br />
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