Titanium Silicide—A Promising Candidate as the Recombination Layer for Perovskite/TOPCon Tandem Solar Cells

To overcome the theoretical efficiency limits of single-junction solar cells, concept of multi-junction or tandem solar cells has been proposed. In tandem solar cells, two subcells with different energy bandgaps are stacked and connected in series. During the operation, two different types of carriers are transported from each subcell and recombined at the interlayer. The interlayer, which bridges two different solar cell technologies, determines the performance of tandem devices. Since the ideal open circuit voltage (V_OC) is equal to the sum of the V_OC of each subcell, recombination of carriers at the interlayer without any potential loss is required for a high-efficiency tandem device. Barrier-free interlayers can be realized using recombination layers. Especially, transparent conductive oxide (TCO) materials have been widely employed as recombination layers, attributed to their high transmittance and conductivity. However, application of TCO has following limitations: (1) sputtering damage on bottom cell and (2) scarcity of indium. Therefore, the development of TCO-free recombination layers is necessary.
In this work, titanium silicide (TiSi₂) was proposed as recombination layer for perovskite/tunnel oxide passivated contact (TOPCon) 2-T tandem solar cells as an alternative to conventional TCO-based recombination layers for the first time. TiSi₂ was fabricated in two steps: (1) Ti deposition and (2) oxidation. Thin Ti layer was directly deposited on the p⁺–Si and subsequently oxidized at 600 °C for 30 min in an O₂ atmosphere. Ti film was oxidized to TiO₂ (denoted as ox-TiO₂), and TiSi₂ was simultaneously formed at the Ti-Si interface. Diffusion was a key mechanism in the formation of TiSi₂ in this work. Therefore, the reaction formation mechanism was interpreted based on the diffusion theory and experimental results. The optical and electrical properties of the ox-TiO₂ and TiSi₂ layer were optimized, respectively, by controlling the initial Ti thickness (5~100 nm). With the initial Ti 50 nm, the lowest optical reflectance and highly ohmic contact between the TiO₂ and p⁺–Si layers with a contact resistivity of 161.48 mΩ*cm² were obtained. In contrast, the TCO interlayer shows Schottky behavior with much higher contact resistivities. It is demonstrating the remarkable potential of TiSi₂ as recombination layer. Furthermore, as the recombination layer of TiSi₂ and the electron transport layer (ETL) of TiO₂ are formed simultaneously, the process steps for tandem fabrication became simpler. Finally, the MAPbI₃/TOPCon tandem device yielded an efficiency of 16.23%, marking the first reported efficiency for a device including silicide-based interlayer.
As a proof-of-concept stage, this work focused on analyzing the properties of TiSi₂ and demonstrating the TiSi₂-applied tandem device. Formation mechanism of TiSi₂ and its properties as a recombination layer were explored. Therefore, the basic structures of subcells (MAPbI₃ top cell without any treatment & TOPCon bottom cell without passivation layer) were combined into tandem architecture. In other words, additional optimizations of subcells are essential for improving the device efficiency further. Despite that, the efficiency exhibited in this work certainly implies that silicide-based materials are promising candidates for recombination layers in perovskite/TOPCon tandem solar cells.