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

Figure 1.2: Comparison of I-V characteristics of solar cells with In2S3 (dashed-dotted line) and CdS
(solid line) buffer layers.
2) Ultrasonic spray pyrolysis (USP)
Highest efficiency CIGS based thin film solar cells are reported using a CdS buffer layer grown by a
chemical bath deposition (CBD) method. However, there are several physical, economical and
ecological reasons to substitute the buffer layer material as well as the deposition process. A
promising alternative buffer layer material for Cu(In,Ga)(S,Se)2 solar cells is In2S3. In the following the
growth characteristics of ultrasonic spray pyrolysis (USP) deposited indium sulfide layers and the
development of a 12.4 % efficiency cell are presented.
Ultrasonic spray pyrolysis is a low-cost, non-vacuum thin film deposition method where a chemical
precursor solution is ultrasonically excited using oscillating piezo-crystals to generate very small
droplets in micron or sub-micron size range. These droplets are subsequently transported by a carrier
gas through a tube and funnel system onto a heated substrate where the precursors thermally
decompose and form a compound. For depositing indium sulfide layers indium(III)chloride (InCl3) and
thiourea ((NH2)2CS), dissolved in methanol are used as precursors. The most important deposition
parameters affecting the layer properties are the molar ratio R[In]:[S] of indium and sulfur in the
precursor solution and the substrate surface temperature Tsurf. Figure 2.1 shows a simplified
schematic of the newly developed USP system. An additional spray chamber preventing external
turbulences and an x-y-table for the funnel allowing more homogeneous films and up-scaling were
added to the set up. Together with a runback for big droplets in the feed pipe these modifications
resulted in a reproducible and reliable deposition process.
Figure 2.1) Simplified schematic of the USP setup. The
solution is nebulized in the ultrasonic actuator and the
droplets are carried with N2 carrier gas through the feed
pipe and the funnel onto the heated substrate
Thin-film solar cells were prepared by spraying indium sulfide on a SLG/Mo/Cu(In,Ga)(S,Se)2 stack.
Subsequently a bi-layer of i-ZnO/ZnO:Al was sputtered onto the buffer layer as front electrode and a
Ni/Al grid was evaporated as secondary contact for better carrier collection. The solar cells were
analyzed by measuring External Quantum Efficiency (EQE) and current density-voltage (J-V).
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ATHLET - Advanced Thin Film Technologies for Cost Effective Photovoltaics, A. N. Tiwari, ETH Zürich
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