Researchers from the National University of Singapore and AGH University of Krakow have developed a novel fabrication strategy for double-sided tunnel oxide passivated contact (DS-TOPCon) silicon bottom cells, designed for use in perovskite/silicon tandem (PST) solar cells. The team demonstrated that carefully controlled heated indium tin oxide (ITO) deposition can significantly reduce sputtering-induced damage while enhancing conductivity and transparency - two critical requirements for efficient tandem integration.
In DS-TOPCon architectures, both the front and rear surfaces of the silicon wafer feature passivating silicon oxide (SiOx)/polycrystalline silicon (poly-Si) stacks, replacing the conventional single-sided (SS-TOPCon) structure. The result is a fully passivated, symmetric cell that minimizes recombination losses and raises the open-circuit voltage. This approach also provides improved carrier selectivity and mechanical stability - qualities that make DS-TOPCon particularly well suited to high-efficiency, monolithic tandem devices.
However, because thin poly-Si layers have limited lateral conductivity, a transparent conductive oxide (TCO) layer must be applied to maintain good electrical contact and optical coupling to the perovskite top cell. The researchers found that conventional ITO sputtering often degrades the excellent surface passivation of the DS-TOPCon stack. To address this, they developed a novel process for heated ITO deposition at around 250 °C, allowing in-situ annealing within the sputtering chamber. At this optimized temperature, the ITO films displayed higher carrier mobility and improved optical transparency without introducing significant interface damage to either n- or p-type TOPCon contacts. Deposition at higher temperatures led to hydrogen effusion and dopant deactivation, which reduced performance.
Using an industrial M2-sized Czochralski (Cz) silicon wafer (244 cm²), the team fabricated DS-TOPCon bottom cells that achieved 16.7% efficiency under 1-Sun conditions. These devices incorporated a 1.5 nm thermally grown SiOx tunneling layer, in-situ doped hydrogenated amorphous silicon precursors annealed at 850 °C to form poly-Si, and hydrogenation through SiNx firing at 700 °C. Approximately 75 nm of ITO was sputtered on both sides as the contact and interconnection layer, followed by low-temperature screen printing of silver contacts (200 °C curing).
When integrated with a perovskite top cell, the resulting 16 cm² monolithic tandem achieved a power conversion efficiency (PCE) of 22.1%, with an open-circuit voltage (Voc) of 1.727 V, current density (Jsc) of 17.9 mA/cm², and a fill factor (FF) of 71.7%. Importantly, the team emphasized that this result demonstrates one of the largest area DS-TOPCon-based tandems reported to date using industrial-grade Cz wafers and fully screen-printed contacts - a step toward scalable manufacturing.
Looking ahead, the researchers plan to incorporate fire-through rear contacts and adopt thicker, more conductive poly-Si layers to further reduce series resistance. They also noted the potential of these devices for urban and rooftop applications, where higher energy yield per unit area is crucial. With their combination of industrial compatibility, improved passivation stability, and tandem readiness, heated-ITO DS-TOPCon cells represent an advancement toward commercially viable, high-efficiency perovskite/silicon tandem photovoltaics.
