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

7/9
3) Semitransparent CIGS solar cells
Wide-gap solar cells grown on a transparent conducting oxide (TCO) back contact layer could be used
as top cells in tandem devices, or as bi-facial devices illuminated from both sides using a mirror, or as
semi-transparent solar cells (solar windows).
Previous experiments have shown:
� In order to create a quasi-ohmic contact between the CIGS absorber and the TCO (ZnO:Al) back
contact the application of MoSe2 layer as an interface layer is necessary.
� Since the ZnO:Al layer acts as a sodium barrier, an additional sodium supply to the CIGS
absorber is necessary in order to enhance the cell performance.
� The CIGS absorber on top of the TCO layer can only be grown at comparable low substrate
temperatures (450°C). Processing of Ga-rich or even CGS layers, therefore may become a
challenging task, since these layers need high energies of formation and show slow elemental
inter-diffusion.
In a first step CIGS absorbers were prepared on soda lime glasses with 1µm Mo back contact in order
to understand the different growth behaviour of the absorber layer during the three-stage process.
Structural investigations of the absorber surface revealed, that the selenium partial pressure required
for the growth of Ga-rich and CGS absorbers must be different from standard growth process. Further
investigations are necessary to determine the optimum conditions. Further it was observed, that the
elemental inter-diffusion and Cu incorporation and therefore also the layer formation during the 2nd
growth stage is hindered at higher Ga concentrations.
CIGS layers of different compositions were deposited under identical conditions on Mo coated glass
substrates and on SLG with ZnO:Al back contact. The substrate temperature (Tsub) was held at 400 °C
during the first stage of the CIGS process for both types of substrates. In the beginning of the second
stage, Tsub was increased to enhance diffusion of Cu into the (In,Ga)2Se3 precursor. In case of Mo
coated SLG, Tsub,max was 580 °C, but in case of ZnO:Al coated substrates, the substrate temperature
was increased to 450 °C only to ensure maintenance of good optical and electrical properties of the
transparent back contact.
Figure 3.3: SEM images of cross-sections
of CIGS samples on Mo coated SLG with
different Ga contents x: a) x = 0.00, b) x =
0.22, c) x = 0.65, d) x = 0.79. CIGS
deposition at Tsub,max = 580 °C.
Figure 3.4: SEM images of cross-sections
CIGS samples on ZnO:Al coated SLG wi
different Ga contents x: a) x = 0.00, b) x = 0.2
c) x = 0.56, d) x = 0.76. CIGS deposition
Tsub,max = 450 °C.
SEM images of the cross-section of various samples clearly show that with increasing Ga content the
grain size in CIGS decreases for both substrate types and growth conditions.
111/290
ATHLET - Advanced Thin Film Technologies for Cost Effective Photovoltaics, A. N. Tiwari, ETH Zürich