Patent Application
20230178673
The properties of both materials, a-Si and poly-Si are intensely studied experimentally through years [1-10]. With offering simplicity for fabricating poly-Si wafers and ribbons on one hand, these materials suffer from low absorption coefficient. On the other hand, thin film poly-Si quickly heats up under sun irrigation by high energy photons. As a result, these solar cells are losing 1% efficiency with every 10° C. increase of the temperature. Very simple technology of making a-Si layers produces materials with high density of dangling bonds, i.e., very high recombination rate of photocarriers generated by sunlight. Hydrogenation of a-Si films significantly improves performance of these solar cells. However, thin film a-Si solar cells are effectively blind to part of the sun spectrum with photon energy less than 1.8 eV. In the design of heterostructure solar cells combining a-Si with poly-Si we were motivated to avoid mentioned above deficiencies of these materials.
With offering simplicity for fabricating poly-Si wafers and ribbons on one hand, these materials suffer from low absorption coefficient. On the other hand, thin film poly-Si quickly heats up under sun irrigation by high energy photons. As a result, these solar cells are losing 1% efficiency with every 10° C. increase of the temperature. Very simple technology of making a-Si layers produces materials with high density of dangling bonds, i.e., very high recombination rate of photocarriers generated by sunlight. Hydrogenation of a-Si films significantly improves performance of these solar cells. However, thin film a-Si solar cells are effectively blind to part of the sun spectrum with photon energy less than 1.8 eV. In the design of heterostructure solar cells combining a-Si with poly-Si we were motivated to avoid mentioned above deficiencies of these materials.
In design of our solar cell, a thin highly doped layer of a-Si is used. However, high density of dangling bonds, about 1020cm-3, in a-Si [5-6] causes high recombination rate of photocarriers. To reduce this high recombination rate commercial layers of a-Si are subjected to hydrogenation during processing. It is known [7] that plasma hydrogenation carries high processing costs. In the underlying a-Si p-type 0.1 µm thick layer, the doping gets to a high level ranging from 1017 to 109cm-3 to suppress activities of dangling bonds and reduce recombination rate of photocarriers.
The heterojunction is completed by a 100 µm thick base of poly-Si under the a-Si layers. As per the illustrations provided in
The complete structure of the solar cell was studied using finite element analysis to find its response to AM1.5 solar spectrum. The intensity of this solar spectrum was assumed to be 1000 W/m2. The output current vs voltage characteristics is presented in
The other parameters of the solar that resulted from the study arc- a) Maximum Power (Pm)= 20.94 mW/cm2, b) Voltage between the electrodes during maximum power production (Vm)= 0.6 V, c) Output current from the solar cell during maximum power production (Im)=34.9 mA/cm2 and d) Overall efficiency of the solar cell (Eff) =20.5%.
Most important is the fact, that our design does not carry crystalline materials and therefore is free of complicated consideration about lattice match between adjacent layers. The main specificity of the proposed design is suppression of recombination rates in both, amorphous and polycrystalline layers the structure. Instead of using hydrogenation of dangling bonds, which is costly technology, we propose to use diffusion of impurities in thin amorphous layers and ion implantation of polysilicon.
The production of epi-all are layers of our solar cell could be done by Liquid Phase Epitaxy (LPE). LPE is known to be the simplest and least costly epitaxy. There is no preferential etching direction of amorphous silicon surface. Typical donors and acceptors used in production of commercial diodes can be used in a-Si/poly-Si solar cell.
Configuration of a-Si/poly-Si solar cell final structure can be produced with donor doping at the amorphous, top layer, and acceptor doping of the poly- silicon, or vice versa with acceptor doping of the amorphous, top layer, and donor doping of the poly- silicon layer.
The manufacturing of a-Si/poly-Si solar cell could use simple liquid epitaxy or any of commercial production steps such as SSP (Standard Screen Printed), PERC (Passivated Emitter Rear Contact) or IBC (Interdigitated back contact solar cells)
The accompanied drawings have been vividly described in the description of invention. This section describes the aspects of the drawings that are relevant to the design of the proposed solar cell.
