An international research team led by Germany’s Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) has fabricated a perovskite-silicon tandem solar cell based on a transparent conductive oxide (TCO) made of zinc-doped tin oxide (ZTO) in an effort to replace counterparts made of indium tin oxide (ITO).
“The novelty of this work lies in demonstrating ZTO, an indium-free TCO, as a viable recombination layer for fully textured perovskite-silicon tandem solar cells on industrially relevant TOPCon bottom cells,” corresponding author Sadaf Ghasemi told pv magazine. “Unlike
conventional ITO, which relies on non-sustainable and scarce indium that is unsuitable for mass production, ZTO matches ITO performance using the same scalable DC sputtering process from rotary targets in an inline tool.”
“We systematically studied structural, chemical, and optoelectrical properties of ZTO, aluminum-doped zinc oxide (AZO), and ITO,
and investigated their influence on the hole transport layer (HTL) formation,” she went on to say. “We found ZTO’s superior compatibility with hybrid-processed perovskite top cells on TOPCon-based perovskite-silicon tandem solar cells, without efficiency penalty versus ITO.”
In the study “Indium-Free Recombination Junctions on Tunnel Oxide Passivating Contacts for Fully Textured Perovskite/Silicon Tandem Solar Cells,” published in RRL Solar, Ghasemi and her colleagues explained that their analysis was performed on ohmic n-TOPCon substrates consisting of textured n-doped silicon wafers with TOPCon layers on both sides, which are hydrogenated after crystallization.
AZO, ITO, and ZTO transparent conductive oxides were deposited on both sides by DC sputtering under previously optimized conditions, followed by a curing anneal to mitigate sputter-induced damage. ll layers were sputtered at 30 nm and optimized for minimal damage while ensuring good electrical contact, with properties evaluated before and after a 300 C annealing step.
Image: Fraunhofer ISE
The impact of processing was evaluated using spatially averaged implied open-circuit voltage (iVOC) from photoluminescence (PL) measurements before sputtering, after deposition, and after curing. AZO induced the strongest initial passivation loss, while ITO and ZTO caused only moderate degradation; however, all materials recovered to high open-circuit values after annealing. Contact resistivity measurements revealed that AZO exhibits the lowest values and remains stable after curing, whereas ITO and ZTO show higher resistivity that increases moderately upon annealing, while still remaining suitable for tandem device integration.
Hall-effect measurements further showed that ITO is the most conductive TCO due to its high carrier mobility. ZTO improves after annealing as a result of increased carrier concentration, whereas AZO suffers from pronounced mobility loss and a corresponding rise in sheet resistance. Overall, these results demonstrated that sputter parameters and annealing strongly tune the structural and electrical properties of the TCOs, while subsequent HTL processing largely homogenizes interfacial surface behavior across all materials.
Substrate crystallinity and surface-related parameters such as contact angle and work function were found to be poor predictors of full device performance, as AZO deviates significantly despite apparently favorable interfacial characteristics.
The three tandem device configurations based on ITO, AZO, and ZTO exhibited distinct photovoltaic performance. AZO-based devices achieve high open-circuit voltage but poor efficiency due to low short-circuit current density and fill factor, likely caused by contact transport limitations or interfacial instability. In contrast, ITO- and ZTO-based devices show consistently high performance with comparable efficiencies, indicating minimal selectivity losses and efficient hole extraction.
Overall, both ITO and ZTO are viable recombination layers, with ZTO identified as particularly promising for tandem integration due to its comparable electronic and optical performance. “Under current-matching conditions, ITO- and ZTO-based devices achieve comparable efficiencies of 27–28%,” Ghasemi concluded.
The research group comprised academics from the University of Freiburg in Germany and the University of Twente in the Netherlands.
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