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Advanced contact grid lines on silicon solar cells

IP.com Disclosure Number: IPCOM000184433D
Original Publication Date: 2009-Jun-24
Included in the Prior Art Database: 2009-Jun-24
Document File: 8 page(s) / 144K

Publishing Venue

IBM

Abstract

Contacting a silicon surface is still a technical challenge, especially in case of solar cell applications. The surface contact area has the high probability of recombination and other defects. Increasing the contact surface area by even etching into the contact layer reduces the know contact problems significant. Most likely the resistivity of the contact area is improved as well. The contact grid on the top side of the solar cells also causes quite some shadowing of the sun light. This again reduces the efficiency of the solar cell itself. Screen printed contact lines can’t be made ultimately thin due to process/technology restrictions. Plating technology enables much smaller contact lines, which can be even improved applying additional thinning. Also the contact etching out of a metal layer covering the silicon surface can be improved through structural thinning. An advanced line printing, with reduced line width and line embedded in the silicon surface would support better performance. There is no Ag based contact paste available which would support a line width printing in the range below 50µm. Therefore another method is discussed to achieve these line widths using other techniques. The advancement discussed below supports improved power output, due to reduced shadowing as well as improved contact resistance to the silicon surface.

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Advanced contact grid lines on silicon solar cells

New process flow description for recessed contact lines:

The publication describes the enlargement of the metal contact area i.e. on solar cell surface. The advantage of the described process is that the contact hole etching takes place before the diffusion process. The advantage is increased robustness of all follow up process steps. The contact holes can be realized through various technologies, like hard mask etching, pattern printing etc. The process flow starting from metal contact is shown below:

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Figure 1:

The mask enables the contact hole etching into the silicon bulk material. Next process step is the diffusion to generate the PN junction. This followed by the deposition of the antireflective coating (AR). The contacts are finally plated into the contact holes, or even printed etc., followed by the firing.

Figure 2: final device structure after contact and firing process

The larger contact area within the n-doped layer increases the performance significant, as outlined above. The process does have a stable process window based on the flow above.

Alternative process to generate contact holes:

In the alternative flow the contact holes are etched after the PN junction and AR layer creation. The disadvantage is, that the n-type layer is made very thin (µm

process flow showing how to make contact hole

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range), which causes problems to run a stable process window to etch the contact holes. Still the enhanced contact areas have lot's of benefits in terms of resistivity, recombination probability etc., as outlined above.

New process flow description for contact line width reduction:

The publication describes a so-called trimming process of the contact grid wires on the top side of solar cells. The conventional process flow (left) vs the improved one (right) is shown below:

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Figure 3: contact line trimming process flow

The polymer structure used for structuring determines after etching the metal electrode width. Additional trimming enables much smaller electrodes, which again reduce the shadowing effect and improve efficiency. The trimming can be realized using RIE technology or sputter etching with hard mask etc.. The trimming can be as far as long appropriate current density is secured for the wiring.

New process flow description for contact lines with reduced shadowing:

The shadowing through the contact grid lines is an issue in the solar cell performance. The contact lines are printed typically at a width of 120µm. This could be reduced to save effective silicon surface. The contacts not only build shadows due to the line width. The contact lines ar...