Investigation of the dependence of a screen-printed solar cell open circuit voltage on the al paste composition

The back surface field (BSF) induced by a highly Al doped p⁺ region is beneficial in reducing the back surface recombination velocity (BSRV) in a conventional industrial type silicon solar cell. The effectiveness of the BSF in controlling the BSRV depends on its thickness and uniformity. The Al-BSF thickness is said to depend on the screen-printed Al thickness while the annealing method, rapid or slow firing, controls the uniformity. However, even though screen-printed Al thickness of 25-30 μm is generally used in the industry and fired rapidly in the belt furnace, there is still a difference in open circuit voltage (V_oc) and efficiency depending on the source of Al paste. In order to understand the source of variation in V_oc, we fabricated large area (239 cm²) Cz silicon solar cells on commercial 65 ohm/sq emitter using three Al pastes from three vendors, A, B and C. We used a commercial front Ag paste with a known firing profile to circumvent the influence of the front contacts on the cell performance. The best V_oc for cells with Al pastes from vendors A, B and C were 636 mV, 635mV, and 637 mV, respectively. The SEM analysis revealed the average BSF thickness for vendors A, B, and C pastes as 7.3 μm, 5.9 μm and 6.9 μm, respectively. The long wavelength IQE and the LBIC analysis were similar for all the three pastes. This shows that, irrespective of the Al vendor, if the front side of the cell is similarly processed, and provided the BSF is uniform and greater than 4 μm, similar V_oc can be achieved. Therefore, the Al pastes from different vendors are similar in performance provided the front side of the cell is processed with adequate control.