Role of Bandgap Grading for the Performance of Microcrystalline Silicon Germanium Solar Cells
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tempered for half an hour at 150'C and 175°C. The open circuit voltage (V0 c), short circuit current (1,c), fill factor (FF) and efficiency (rI) were measured under AMI .5 light illumination. The quantum efficiency (QE) was determined by DSR-measurements under short circuit conditions. MATERIAL PROPERTIES
-5
C-
20%o_ 400/
In Fig. 1 Raman spectra of ktc-Si] 5 Gex alloys deposited with different germane content are plotted. Three strong peaks are observed which are attributed to Si-Si (-500 cm-), Si-Ge (-400 cm-), Ge-Ge (-300 cm-') optical modes according to [5]. The variation of the width of the peaks can either be caused by differences in disorder, differences in strain distribution or by
size effects of small crystallites. The Raman spectra show no signal of an amorphous phase. Moreover, since no peak at 520 cm is detected 300 350 400 450 500 no indication of silicon clusters is found in the ranan shift [cmt l] investigated samples. Because a linear relation between the shift of the position of the Si-Si Fig. 1: Raman spectra of ýtc-SiGe:H with mode and the germanium content in the solid indicated germane content. phase is reported for (poly-)crystalline SiGe [6,7] and gtc-SiGe:H [3], we used the Si-Si peak positions for the evaluation of the composition (Tab. 1). The content in the solid phase is smaller than the content of GeH 4 and decreases with higher germanium content. No influence of the H2-dilution on the Raman peak position in the region between 1:120 and 1:320 has been found. ;ý 60%/
Tab. 1: Germanium content in the solid phase determined from Raman spectroscopy for different germane content in the gas phase. Germane content in the gas phase Germanium content x in the solid phase
20% 0.21
Optical absorption spectra derived by PDS spectra are shown in Fig. 2. An increase of the absorption is observed over the whole region of energy with rising germanium content. Since a plot of 4(ac) as a function of E does not always yield straight lines over a significant energy range the gap is defined as rx(E.) - 10 cml which is close to the indirect bandgap [3]. The evaluation of the optical gap is further complicated by an enhanced absorption below the gap. Similar subgap absorption is found in ktc-Si:H, but its origin is not yet clear, Defect states, strain and potential fluctuations are likely causes for this absorption. Neglecting these deviations we can evaluate a bandgap of 0.95 eV for the sample prepared with 40% germane in
592
140% 0.37
160% 0.51
170% 0.56
101 103
I1
10•
2-_102 10 1 0
100
1.0
1.5
2.0
2.5
energy [eV Fig. 2: Optical absorption spectra for different composition.
the gas phase and a bandgap of 0.89 eV for the 60% germane films. When the H 2 dilution is enhanced for samples with a constant germanium content the deviation 1.0 of the absorption spectra from a square 0 root behavior near the bandgap increases a) 0.8 which we attribute to an increase of the internal stress. With decreasing H 2 dilution N an increase of the absorption coefficient at 0.6 higher energies is observed
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