Electrode formation in a crystalline silicon solar cell is one of the important processes for collecting electrons generated in the solar cell. In particular, the metal-silicon contact interface property of the solar cell has a great influence on the electrical properties and light conversion efficiency of the device. The electrodes enable collection of the generated electron hole pairs, and serve to connect the solar cell by attaching ribbons on them when manufacturing the module. In order for the electrodes to play these roles, the interface property between the electrodes and the silicon substrate is very important. In general, the metal-silicon interface consists of glass frit, Ag crystallite and Ag colloid. They affect the rate of recombination depending on the formation conditions and generate leakage current. Therefore, it is possible to increase the efficiency of the solar cell by understanding the basic principles of the metal-silicon contact formation mechanism and reducing the underlying cause of leakage current generation.
The mechanism of metal-silicon contact formation has been analyzed in detail and the principle of Ag crystallite formation has been presented. In addition, the effect of Ag crystallite on the recombination rate has been analyzed by observing the metal-silicon interface through TEM analysis. In general, the mechanism of metal-silicon contact formation is known as the redox reaction of metal and silicon. Although it is known that the oxidation-reduction reactions of PbO, Ag, and Si grow Ag crystallite when the firing temperature increases, an accurate analysis of the metal-silicon contact formation mechanism has not been made yet. In addition, most metal-silicon contact studies focus on properties after contact formation. Therefore, this paper has tried to help understand the mechanism of metal-silicon contact formation and has analyzed the underlying causes of increasing the recombination rate.
The first step is to analyze the contact characteristics between glass frit-Si and Ag-Si. Ag film of about 1 μm thickness at 10-6 Torr was deposited on the sample using the evaporation method. Glass frit used glass frit paste with low glass transition temperature. Glass frit has been printed on the sample surface using a screen printing method. The sample has been firing processed at Tpeak = 850 °C, and surface morphology and electrical properties have been analyzed through SEM and QSSPC measurements. Ag film and Si reactions have occurred only in very limited area. The low concentration of Ag and O ions has caused this result. In the Ellingham diagram, Ag and O reactions can be confirmed through Gibbs free energy change. The increased recombination rate of the reaction of Ag film and Si has been analyzed through QSSPC measurement.
The second step is to analyze the Ag crystallite formation mechanism. The formed Ag crystallite affects the recombination rate and contact resistance of the solar cell. However, despite the fact that Ag crystallite affects solar cell characteristics, an accurate analysis of the Ag crystallite contact formation mechanism has not been made yet. The reason for that is that Ag crystallite is difficult to be structurally analyzed and various changes occur simultaneously depending on the properties of metal paste and process conditions.
In order to analyze the cause of the formation of Ag crystallite, the firing temperature has been subdivided to observe the growth of Ag crystallite, and crystal structure and interfacial properties have been observed using TEM. As a result, it has been confirmed that when Si is etched in inverted pyramid by PbO and then cooled, Ag crystallite grows epitaxially along the Ag-Pb phase diagram. It has been observed through TEM that the Ag crystallite formed along the Ag-Pb phase diagram contains Pb participating in the reaction at the upper area. At the Ag and Si interface observed using TEM, Ag and Si atoms have 3 to 4 matching. These results indicate that Ag is grown epitaxially on the Si surface when Ag and Si are in contact, and confirm that the lattice mismatch is very low. The lattice mismatch between Ag and Si interface is very low at 1.7%.
Final step is to calculate the change of the recombination current density according to the growth of Ag crystallite. The recombination current density has been calculated based on the recombination lifetime formula. The calculated recombination current density parameters are J0e, J02 and J0.metal. J0e increases approximately from 100 fA/cm2 to 210 fA/cm2 when the firing temperature increases from 750 to 950 °C. J02 increases approximately from 0.5 nA/cm2 to 2 nA/cm2. J0.metal decreases approximately from 120 fA/cm2 to 40 fA/cm2 when the temperature increases from 750 to 800 ℃. Then, as the firing temperature increases to 950 °C, J0.metal increases approximately from 40 fA/cm2 to 60 fA/cm2. When the firing temperature increases from 750 to 950 ℃, the size and distribution of Ag crystallite increase radically. However, J0.metal does not show the tendency to increase the size and distribution of Ag crystallite. This is because Ag crystallite achieved epitaxial growth on the Si surface. 1.7 % lattice mismatch of Ag-Si interface indicates that the dangling bond of Ag-Si interface is very low. Therefore, Ag crystallite has very little effect on increasing the recombination current density. In addition, based on these results, it can be confirmed that the PbO content in the metal paste is related to an increase in the recombination current density.
In this study, the mechanism of metal-silicon contact formation has been analyzed to investigate the cause of the increase in metal recombination current density. Based on the results of Ag-Si reaction and glass frit-Si reaction, It has been confirmed that glass frit etched Si and formed an inverted pyramid pit. In addition, it has been confirmed that the Ag crystallite is formed along the Ag-Pb phase diagram. Ag crystallite have grown epitaxially with a lattice mismatch of about 1.7% on the Si surface. Low lattice mismatch does not significantly affect the recombination current density increase. These results are expected to have a great impact on the analysis of the recombination current density at the metal-silicon contact and the fabrication of the metal paste. However, metal-silicon contact formation is not only affected by those discussed in this study but also by doping concentration, defects in the silicon surface. Based on the contents mentioned in this paper, further studies will be conducted to improve the understanding of the metal-silicon contact formation mechanism.