Aluminum-silicon Interdiffusion in Screen Printed Metal Contacts for Silicon based Solar Cells Applications☆

Abstract In this work we propose a detailed investigation of the Al – Si interdiffusion that occurs during the firing process of the Al-Si back contact of silicon based solar cells. The investigation is based on high resolution scanning electron microscopy (SEM) and compositional microanalysis with energy dispersive X-Ray microanalysis (EDX). We have found a dependence of Si precipitation in the Al matrix depending on the microstructure of the Al screen printable paste. We suggest a gettering effect promoted by the larger Al particles lying within the Al paste being able to affect the Al paste resistivity, the Al distribution within the BSF region of the solar cell, thus affecting the solar cell performances and finally the Al paste thermal expansion coefficient. Finally we demonstrate that the presence of the glass frit reduces the surface tension and, homogenizes the diffusion process. Reduction of surface tension decreases the internal pressure and increases the Si interdiffusion in Al

IInnovative Low Temperature Sintering, High Adhesion, Low Contact Resistance Screen Printable Silver Paste for a-Si:H/c-Si Heterojunctions

Amorphous/crystalline silicon heterojunction is the most attractive technique to obtain high efficiency solar cells. The emitter of these cells is usually covered by Transparent Conductive Oxides (TCO) that ensures lateral conductivity and Anti Reflection Coating (ARC). Then the cell front side is completed by a metal grid, made by screen printed silver, sintered at temperature below 200°C. Both the TCO and the grid strongly influence the cell series resistance as well as the cell efficiency. The choice of TCO needs to be tuned in order to ensurethe lowest sheet resistance and an appropriate workfunction to fit the emitter requirements. Consequentlythe front screen printed grid needs to be optimized to produce the lowest specific contactresistance with the underneath TCO. In this work we show the results obtained developing an innovative low temperature curing, polymeric based, screen printable silver paste. High linear conductivity, low specific contact resistivity and strong adhesion to TCO have been reached. Specific contact resistivity as low as 0.15mΩcm2 and printed Ag bulk resistivity of 20µΩcm have been reached on TCO film having sheet resistance in the range between 40 and 200 Ω/sq

Reduced screen printed aluminum laydown for low cost silicon solar cells

The majority of PV market is covered by p-type c-Si based solar cells, having metal contacts produced by screen printing. Common screen printable Aluminum pastes, due to their chemical properties are able to form deep and effective Back Surface Filed (BSF), providing also low surface recombination velocity. According to the actual industrial state of the art the wet laydown of screen printable Al paste is in the range of 1.4-1.6g per cell, nearly 6-7 mg per printed cm2. This reflects in a cost per cell of 0.02-0.035€. In the roadmap to reduce the solar cell manufacturing costs, a lower laydown is a goal, which also could reflect in lower cell bowing after firing process. However it is not immediate to obtain the same electric performances reducing the Al available to form the BSF. In this paper we investigate the possibility to produce a lower laydown of Al paste which ensures the same electrical performances of a standard Al paste thickness, through the understanding of BSF formation mechanism and electrical properties of the paste in correlation with its formulation. To optimize the effect of the available metal, we have produced several Al pastes by screening different Al powder granulometry mix and frits to obtain good values of Voc, FF and BSF homogeneity comparable to those produced by standard screen printable Al pastes