Fig 4. The efficiency and fill factor measurements according to different peak temperature of contact co firing for Ag and Ni contacts
To overcome the high contact resistivity of Ni and reduce the high cost of Ag contacts, Ag is used as a seed layer with Ni and Cu forming the bulk of the metal in a bi-layer contact type. The average electrical output parameters and the Suns-VOC measurement results for the single layer along with bi-layer contact types are presented in detail with respect to their optimized firing temperature in Table II. For reference, a screen-printed Ag contact solar cell is presented in Table III. The in-house prepared Ag paste performs the best among the other types with an average efficiency of 21.7%. The second best performing structure is Ag/Cu bi-layer contact stack which achieves 21.32% efficiency and FF of 81.4%. This result implies that even though the samples are fired at a peak temperature of 770°C, there is no Cu contamination in the cell, as supported by high shunt resistance (RSH) values of 150000. The J0, metalvalue of 48fA/cm2 and the RC of 5.88mΩ∙cm2 for the Ag/Cu contact, compared to reference Ag contact of 41fA/cm2 and 4.63Ω∙cm2, also strongly indicate that Ag is successfully blocking the Cu diffusion into the junction. The total series resistance (RS) of the same contact, obtained by the light IV measurement, on the other hand, shows an additional resistive effect that may be due to the line resistance of the stack contact as a whole. This conclusion is further substantiated by the microstructure analysis depicted in Fig. 5. The elemental examination reveals that the Cu remains on top of the Ag layer, indicating that Cu, despite its aggressive diffusion properties, has not permeated underneath the Ag. The Ag/Ni contacts also result in promising electrical performance. The FF of 81.2% indicates that the contact formation is complete and there is no shunt in the cell considering the contact stack of Ag/Ni yielded close to maximum shunt resistance. However, the contact resistance and total series resistance is slightly higher than the others, possibly due to some delamination between Ag and Ni. For the Ni as a single contact, in addition to above-mentioned analysis, the cell is clearly shunted and degraded due to use of same glass frit in each paste and the peak firing temperature of 800°C, which is clearly pushing the contact proximity closer to the junction. This is supported by the low shunt resistance of 12050 Ω∙cm2.
Table II. The average electrical output parameters of best performed cells with respect to peak temperature