Indium tin oxide (ITO) has been extensively used as a transparent conductive oxide (TCO) material due to its excellent optical and electrical properties. The ITO layer is applied on the front and rear sides of the front-back contact (FBC) SHJ cells. As the front TCO layer, ITO provides anti-reflective features and facilitates the lateral transport of electrons. Conversely, a well-engineered rear TCO layer facilitates favorable band alignment for charge carrier collection and enhances the reflection of near-infrared light. However, the consumption of indium for solar cell applications competes with the growing demand across other industries. Furthermore, indium is scarce and expensive, contributing to the increase in capital expenditure. As a result, research into the feasibility of eliminating the TCO layer has gained significant interest in the past few years. While the concept of TCO-free solar cells has been demonstrated in limited studies with efficiencies exceeding 20%, TCO-free FBC-SHJ solar cells employing copper- based metallization remain unexplored. This work extends the TCO-free contacts by integrating the copper-plated metallization on the front side of SHJ cells.

Direct contact between silver and silicon, following the elimination of the TCO layer, fails to withstand wet-chemical processing due to weak adhesion of silver to silicon. A thin titanium layer was introduced between the silicon and silver layers to improve adhesion. In addition, direct metal deposition onto the silicon surface induced sputtering-related defects, which degrade the passivation quality of the solar cells. Thus, the effect of annealing treatment on the surface passivation is studied. Furthermore, annealing plays a crucial role in decreasing the contact resistivity on the front side, which is essential for efficient electron extraction.

In this study, an efficiency of 22.62% was achieved for front TCO-free FBC-SHJ solar cells with copper-plating metallization, attributed to an open-circuit voltage (Voc) of 720 mV and a fill factor (FF) of 80%. It was observed that Voc and FF were influenced by the annealing duration and the thickness of the hydrogenated nanocrystalline silicon oxide layer, respectively. On the other hand, efficiencies of 19.24% are attained for rear TCO-free FBC-SHJ solar cells featuring a localized-area front ITO layer with Ti/Ag back contact, respectively. Electrical characterizations revealed negligible passivation quality degradation upon front TCO localization. However, the introduction of rear Ti interlayer decreased the short-circuit current density (Jsc) due to the lower near-infrared wavelengths reflectivity of titanium compared to silver. Combining the optimized front and rear TCO-free developments yielded a completely TCO-free contact featuring front copper-plated metallization with an efficiency of 21.19%. The device demonstrated good passivation quality, evidenced by Voc exceeding 710 mV and FF above 80%. Future development may consider the feasibility of depositing anti-reflective coating on the rear side to increase the Jsc further. In addition, the stability of TCO-free devices under prolonged light exposure remains a concern, as indicated by a 7% FF relative decrease after 100 hours of continuous illumination.