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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 />

Seite 100 von 288<br />

ATHLET, D. Brémaud, ETH Zurich<br />

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