Performance of fuc-Si:H Solar Cells with Amorphous P-Layer
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electrodes. At photon energies above 1.8 eV the optical absorption coefficient a was determined from transmittance (7) and reflectance (R) spectra where optical interference effects are suppressed [6]. The constant photocurrent method (CPM) [7] was used to obtain information on the absorption spectra between 0.9 and 2.0 eV. In addition, electron microscopy has been used for the characterization of selected samples. The solar cells (1 cm 2) consisted of a TCO / p (a-Si:H or a-SiC:H) / i (ýtc-Si:H) / n (a-Si:H) / metal structure. The i-layers were deposited under identical conditions as the jic-Si:H films. The typical i-layer thickness was - 2000 A. Current / voltage (I/V) curves under illumination and in the dark as well as the quantum efficiency were measured. For investigation of the light induced degradation the cells were exposed at 25°C to a 500 mW/cm 2 AM 1.5 illumination for 160 min. RESULTS The crystallinity of the films was investigated by Raman scattering. Samples deposited at silane concentrations equal or lower than 5% show crystalline fractions in the Raman spectra. A decrease of the crystallinity is observed for higher rf-power and higher silane concentrations. Following Luysberg et al. [8] the Raman spectra are fitted by Gaussian peaks centred at about 520 cm-l, 510 cm-1 and 480 cm-'. The peak located at 480 cm-1 has to be attributed to the amorphous phase while the other peaks at about 510 cm-1 and 520 cm-1 are attributed to the crystalline phase. In the following the ratio of the integrated crystalline intensity IC= 1520 + 1510 to the amorphous plus crystalline integrated intensity 'A + IC will be taken as a semiquantitive measure of the crystalline volume fraction Xc = Ic / (IA + Ic). The fitting procedure yields the value of Xc with an accuracy of ± 1% but it does not allow to determine an absolute value of the crystalline volume fraction. XC reflects only changes of the crystallinity. Luysberg deduced from a comparison of Raman data and high resolution transmission electron microscopy (TEM) images that values of Xc =_60 % correspond to true crystalline volume fractions of about 90 % [8]. X-ray diffraction measurements indicate an average crystalline grain size below 100 A. The room temperature dark conductivity GD of all microcrystalline samples is in the , range between 10-8 Q-1 cm-I and 1 10-7 J2-1 cm- . In the initial state the dark D increasing 15 and photo
crystallinity
E
10
a) , \"higher
0
of all
micro-
crystalline volume fraction is stable.
5-f
increasing Crystallinity 0.2 0.4
conductivity
crystalline samples are similar regardless of differences in their crystallinity but upon light soaking the photoconductivity of the samples with the crystalline volume fractions below - 60 % decreases as in the case of amorphous silicon whereas the photoconductivity of the film with the Thus, for the deposition of the i-layers of p-i-n structures the condition which in films resulted in the highest crystalline volume fraction Xc = 65%, i.e. a silane
xconcentration 0.6
of 2.5 % and an rf-pow
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