Improved contact quality for silver-free silicon heterojunction solar cells by phosphoric acid treatment

Electroplating technology is considered as a promising alternative to low-temperature sintered silver pastes for the purpose of significantly reducing the cost of silicon heterojunction (SHJ) solar cells (SCs). However, achieving excellent contact quality between the plated grid and indium tin oxide (ITO) remains a big challenge to satisfy the requirements for fabrication and high performance. In this work, phosphoric acid (H3PO4) is used to modify the ITO surface to improve the contact quality of the plated grid/ITO and enhance the performance of SHJ SCs. It is found that the phosphate groups are adsorbed on the ITO surface to form a dipole layer after H3PO4 treatment. The wettability of H3PO4-treated ITO is substantially improved, resulting in a pinhole-free electroplated nickel (Ni) seed layer. Moreover, the work function of ITO rises by approximately 0.2 eV, which reduces the contact potential barrier between the ITO and the metal electrode. The contact resistivity (ρc) between the plated grid and H3PO4-treated ITO is reduced to approximately 0.4 mΩ·cm2, compared to 1.5 mΩ·cm2 of the control sample. In addition, the metal grid formed on the H3PO4-treated ITO exhibits a better uniformity. Finally, the SHJ cell with H3PO4 treatment process receives a remarkable power conversion efficiency (PCE) of 23.19%, which is approximately 1% (absolute) higher than that of the control device. Therefore, we demonstrate that modifying ITO by H3PO4 is a promising way to improve the contact quality for the electroplating metallization in SHJ SCs.