Review on In-Free Recombination Junction Approaches for Two-Terminal Silicon/Perovskite Tandem Solar Cells

This contribution reviews the development of In-free recombination junction (RJ) approaches for two-terminal (2T) silicon/perovskite (PK) tandem solar cells, motivated by the increasing demand for scarce raw materials like indium. While the utilization of In-based transparent conductive oxide (TCO) thin films as RJ remains extensively used, in particular in devices reporting efficiencies greater than 30%, alternative In-free methods, including interlayer-free and nano-/poly-crystalline silicon tunnel junctions, have demonstrated their potential but are much less reported in literature. Four In-free RJ architectures are presented here, utilizing different techniques or materials and with a wide diversity in terms of integration in tandem devices: PIN and NIP configurations are reported, as well as two types of bottom cells (silicon heterojunction -SHJ-) and TOPCon-like) and different top cell processing (either solution- or vacuum-processed). A low-temperature tunnel RJ based on heavily doped nc-Si:H layers deposited by PECVD SHJ bottom cells was teste in PIN configuration tandem devices, either on a planar bottom cell and solution-processed top cell, achieving an efficiency of over 12%, or an a fully textured bottom cell with vacuum-processed top cell reaching efficiency of 16%. On planar TOPCon bottom cells, the deposition of a nc-Si:H(n+) layer by PECVD on the poly-Si:H(p+) allowed the formation of tunnel RJ in tandem NIP configuration resulting in an efficiency close to 17%. Second, a direct RJ structure allowing the recombination at the interface between the poly-Si:H(p+) and the SnO2 based electron transport layer of the top cell reaches an efficiency above 19%. Finally, a direct RJ architecture employing a NiOx layer directly sputtered on the poly-Si:H(n+) contact of a planar TOPCon bottom cell lead to a tandem efficiency above 20% in tandem PIN configuration.