Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

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After optimizing the In2S3 buffer deposition parameters, the CIGS solar cell made on glass substrate
were developed and a maximum efficiency of 14.1% has been achieved. With an aim to develop
flexible solar cell with thermally evaporated In2S3 buffer layer, the knowledge obtained from solar cells
made on glass substrate has been transferred to flexible polyimide substrates.
Scanning electron microscopic (SEM) investigations
The microstructure and surface coverage efficiency of In2S3 buffer layer deposited on CIGS surface
was studied by SEM. Figure 1.1 a and 1.1 b show the plan and cross-section of the polyimide/
Mo/CIGS/(~30nm) In2S3 stack. It was observed that there are some spots where the In2S3 film does
not uniformly cover the rough CIGS surface, this could result in limiting the high cell efficiencies.
Furthermore the presence of intermediate residual Na(F) layer could be a problem between the CIGS
and buffer layer, which originates from the NaF post deposition treatment after CIGS growth.
In order to improve the CIGS surface coverage and remove NaF residuum from the absorber surface,
an additional processing step was included, where the CIGS surface was etched in ammonium
hydroxide solution.
Figure 1.1: Surface morphology of the In2S3 film deposited on CIGS surface (a), and cross-section
micrograph showing the presence of ~30 nm thin In2S3 buffer layer on top of the CIGS film (b).
Solar cell characteristics
The cell parameters of both, the cell with In2S3 and the cell with CdS buffer layer, respectively are
listed in table 1.1. Further the I-V curves of both cells are shown in figure 1.2. It can be seen that due
to the higher band gap of the indium sulfide buffer layer, a higher short circuit current Jsc is achieved
compared to the flexible reference cell with chemical bath deposited CdS buffer layer. However, the fill
factor and open circuit voltage are still below the values of the reference cell. A best cell efficiency of
10.1% was achieved for the flexible CIGS cell with indium sulfide buffer layer. This value is 76% of the
13.2 % efficiency of the reference cell. The result was a project milestone and perhaps the first report
of CdS free flexible CIGS solar cell on polymer with efficiency approaching 10%.
Table 1.1: Photovoltaic parameters of flexible CIGS solar cells with In2S3 and CdS buffer layers.
Voc (mV) Jsc (mAcm -2 ) FF (%) � (%)
In2S3 Buffer 530 31.8 59.6 10.1
CdS reference 632 29.9 69.9 13.2
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ATHLET - Advanced Thin Film Technologies for Cost Effective Photovoltaics, A. N. Tiwari, ETH Zürich