Abstract
Amorphous silicon (a-Si:H) / micro-crystalline silicon (μc-Si:H), "micromorph" tandem solar cells have been investigated using a detailed electrical - optical model. Although such a tandem has good light absorption over the entire visible spectrum, the a-Si:H top cell suffers from strong light-induced degradation (LID). To improve matters, we have replaced a-Si:H by hydrogenated polymorphous silicon (pm-Si:H), a nano-structured silicon thin film with lower LID than a-Si:H. But the latter’s low current carrying capacity necessitates a thicker top cell for current-matching, again leading to LID problems. The solution is to introduce a suitable intermediate reflector (IR) at the junction between the sub-cells, to concentrate light of the shorter visible wavelengths into the top cell. Here we assess the suitability of N-type micro-crystalline silicon oxide (μc-SiOx:H) as an IR. The sensitivity of the solar cell performance to the complex refractive index, thickness and texture of such a reflector is studied. We conclude that N-μc-SiOx:H does concentrate light into the top sub-cell, thus reducing its required thickness for current-matching. However the IR also reflects light right out of the device; so that the initial efficiency suffers. The advantage of such an IR is ultimately seen in the stabilized state since the LID of a thin top cell is low. We also find that for high stabilized efficiencies, the IR should be flat (having no texture of its own). Our study indicates that we may expect to reach 15% stable tandem micromorph efficiency.
Highlights
A "micromorph" tandem combines a hydrogenated amorphous silicon (a-Si:H) top sub-cell with a hydrogenated micro-crystalline silicon bottom one
The structure studied is nano porous SiOx ARC/ glass/ ZnO/ top a-Si:H or polymorphous silicon (pm-Si):H PIN cell/ intermediate reflector (IR) at the junction between sub cells/ bottom μc-Si:H PIN cell/ ZnO/ Ag, when growth is from the P-layer side, or with a MgF2 ARC on the top ZnO, when layer growth is in the opposite direction, beginning with Ag on glass (Fig. 1)
The performances of "micromorph" tandems having two types of N-type μc-SiOx:H intermediate reflectors of different refractive indices have been compared to a case having no IR at the junction between sub-cells
Summary
In order to attain superior stabilized state properties, we may use hydrogenated polymorphous silicon (pm-Si:H) as the intrinsic layer in the top sub-cell in place of a-Si:H. We may reduce the thickness of the a-Si:H top cell absorber to attain improved stabilized state properties This would lead to further current mismatch between the top and bottom sub-cells. It may be pointed out that the N- μc-SiOx:H deposited at ENEA was not meant to be used as an IR, but rather as the N-layer in PIN solar cells In this theoretical study we have used it as an IR, to see what effect a different set of complex refractive index (CRIND - n and κ) values of N- μc-SiOx:H may have on the suitability of such a material for use as an IR
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