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Published online by Cambridge University Press: 01 February 2011
We will describe the development and application of n-type microcrystalline silicon oxide (μc-SiOx:H) alloys as window layers in thin film silicon solar cells with microcrystalline silicon (μc-Si:H) absorber layers. Cells are prepared in n–i–p deposition sequence with illumination through the n-side. The layers were deposited by radio-frequency plasma enhanced chemical vapour deposition (RF-PECVD) at 185°C substrate temperature, using a mixture of phosphine (PH3), silane (SiH4), carbon dioxide (CO2) and hydrogen (H2) gases, at CO2 flows varied between 0.5 and 2 sccm and different thickness. Films were characterised by dark conductivity measurements, Photothermal Deflection Spectroscopy (PDS) and Raman spectroscopy to evaluate optical band gap E04, refractive index n and crystallinity ICRS, respectively. The results were compared with the data of alternative optimised window layers, such as n-type μc-Si:H and silicon carbide (μc-SiC:H) films. Also solar cells with conventional illumination through the p-side window were investigated for comparison. Solar cells were prepared with μc-SiOx:H n-layers of varied compositions and characterised by current-voltage (J-V) measurements under AM 1.5 illumination (and also under modified AM 1.5 illumination with red (OG590) and blue (OG7) filters) and reflectance measurements. The effects of the μc-SiOx n-layer composition and thickness on the performance of n-i-p cells were investigated and correlated with the optical, electrical and structural properties of the μc-SiOx:H n-layers. The results indicate that n-type μc-SiOx:H provides a sufficient combination of conductivity (up to 0.1 S/cm) and crystallinity (ICRS up to 30%) to function well as a doped layer for the internal electric field and the carrier transport and as a nucleation layer for the growth of the μc-Si:H i-layer. As a window layer, it also results in an enhanced spectral response, particularly in the long wavelength part of the spectrum of the solar cells, in comparison with the cells containing alternative window layers. An improved short circuit current density (Jsc) can be attributed to the wide optical gap E04 (around 2.3 eV) in the μc-SiOx:H window layers and reduced reflection in the long wavelength region of the spectrum. A minimum total reflectance of only 6% at 570nm wavelength was achieved with such μc-SiOx:H window layers. Using optimised n-type μc-SiOx:H as a window layer, an efficiency of 8.0% for 1cm2 cell area was achieved with 1 μm thick μc-Si:H absorber layer and Ag back reflector.
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