Nanostructured thin films for multibandgap silicon triple junction solar cells

Abstract

By implementing nanostructure in multiband-gap proto-Si/proto-SiGe/nc-Si:H triple junction n-i-p solar cells, a considerable improvement in performance has been achieved. The unalloyed active layers in the top and bottom cell of these triple junction cells are deposited by Hot-Wire CVD. A significant current enhancement is obtained by using textured Ag/ZnO back contacts instead of plain stainless steel. We studied the correlation between the integrated current density in the long-wavelength range (650-1000 nm) with the back reflector surface roughness and clarified that the rms roughness from 2D AFM images correlates well with the long-wavelength response of the cell when weighted with a Power Spectral Density function. For single junction 2-μm thick nc-Si:H n-i-p cells we improved the J sc from 15.2 mA/cm 2 for plain stainless steel to 23.4 mA/cm 2 using rough back reflector. We introduced profiling of the H 2 dilution during growth of the nc-Si:H layer to prevent a transition to amorphous growth. The efficiency for a single junction n-i-p cell reached 8.5%, the highest reported value for HWCVD cells of this kind. Moreover, these cells show to be totally stable under light-soaking tests. Combining the above techniques, an efficiency of 10.9% has been obtained for triple junction cells (J sc = 8.35 mA/cm 2 , V oc = 1.98 V, FF = 0.661). By using effective light trapping techniques and three different band-gap materials, the required thickness could be kept small (∼ 2.5 μm total).

Surface-properties relationship in sputtered Ag thin films: Influence of the thickness and the annealing temperature in nitrogen

“…When the objective is to achieve metallic electrodes with high roughness and electrical conductivity, as required for back-contacts in thin-film solar cells, Ag layer thicknesses above 200 nm and temperatures above 400 • C are typically used [7,17,18] energetic costs for the electrode preparation. Besides, an effective monitoring of the resultant surface morphology is desirable, as well as the accurate prediction of light distribution in the rough coating, especially to check the homogeneity of the electrode properties on large areas.…”

Hydrogenated microcrystalline silicon germanium as bottom sub-cell absorber for triple junction solar cell

“…In attempts to further improve the efficiency and stability, multijunction thin film silicon solar cells have been tested a lot and are being developed rapidly [3][4][5].…”

“…Generally, in the standard structure of the multi-junction thin film silicon solar cell, mc-Si:H has been selected as the bottom subcell material [3][4][5]. However, a relatively thick bottom sub-cell (4 2000 nm) is required to ensure current matching between the sub-cells due to its weak infrared absorption [5].…”

Very thin and stable thin-film silicon alloy triple junction solar cells by hot wire chemical vapor deposition

“…Having a 10-cell average conversion efficiency (g) > 10%, the performance of these devices exceeds that of earlier efforts to fabricate triple junction a-Si:H/lc-Si:H/lc-Si:H solar cell with HWCVD as the method for all absorbers. 19 The performance is lower than the optimized PECVD-made triple junction solar cells developed by United Solar, mainly due to the difference in the open circuit voltage (V oc ). This is expected, because we use lower band gap materials.…”