The transparent four-terminal perovskite solar cell employs an ion-modulated spiro-MeOTAD hole transport layer, which passivates interfacial defects, enhances carrier dynamics, and allows for a tunable work function in wide-bandgap perovskites. When integrated into mechanically stacked 4T tandems with an n-TOPCon cell, the device achieved an overall efficiency of 28.4–30.2%, along with improved open-circuit voltage and fill factor.
Image: Indian Institute of Technology Bombay
Researchers at the Indian Institute of Technology Bombay have fabricated a transparent four-terminal (4T) perovskite solar cell based on a hole transport layer (HTL) that suppresses interfacial recombination while simultaneously enhancing photoluminescence quantum yield and quasi-Fermi level splitting.
“The cell also exhibits a dynamically tunable work function, enabling effective coupling with band gap varying perovskite absorbers,” the research’s corresponding author, Dinesh Kabra, told pv magazine. “Across three perovskite compositions, the device shows substantial improvement in open-circuit voltage and fill factor, independent of bandgap. Implemented in transparent n-i-p configured perovskite solar cells, the universal HTL enhances efficiency and operational stability, enabling a 30.2% combined efficiency when optically coupled in four-terminal tandems with commercial n-TOPCon silicon cells.”
“Our strategy obviates the need for bandgap-specific HTL engineering, thereby mitigating halide segregation, a key limitation of wide-bandgap perovskites in tandem photovoltaics,” he went on to say. “Decoupling transport-layer compatibility from absorber composition allows perovskite formulations to be selected based on intrinsic stability and optoelectronic quality rather than interfacial constraints. Hence, it enables absorber optimization independent of charge-selective layer matching; this universal HTL redefines tandem device design principles and provides a scalable route toward commercially viable, highly stable and efficient perovskite-silicon photovoltaics.”
In the study “Bandgap-tunable transparent perovskite solar cells for 4T Si/perovskite tandem photovoltaics with PCE > 30% via rational interface management,” published in the Royal Society of Chemistry, the scientists explained that the cell was fabricated with a HTL made of ion-modulated spiro-OMeTAd, noting that spiro-OMeTAD for perovskite cell applications is usually doped with a compound known as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to enhance hole extraction and conductivity. This kind of doping, however, requires time-intensive air-oxidization for 24 hours, which reportedly represents an obstacle to the commercial production of perovskite PV devices.
The ion-modulated, radical-doped spiro-MeOTAD HTL can reportedly achieve optimal work function tuning via 4-tert-butyl-1-methylpyridinium bis(trifluoromethanesulfonyl)imide (TBMPTFSI) salt modulation, offering enhanced stability. Unlike molecular structure engineering for energy level adjustment, this approach provides a simpler and more controllable way to align energy levels and reduce interfacial defects.
Image: Indian Institute of Technology Bombay
The tandem device was built with a top perovskite cell made with a substrate made of glass, an electron transport layer (ETL) made on tin oxide (SnO₂), a perovskite absorber, the spiro-MeOTAD hole transport layer (HTL), an indium zinc oxide (IZO) layer serving as the top transparent electrode (TE), and silver (Ag) metal grids.
Image: Indian Institute of Technology Bombay
According to the researchers, theptimization of the TBMPTFSI concentration, ranging from 15 to 20 %, along with careful adjustment of HTL spin-coating speeds, led to significant improvements in efficiency, open-circuit voltage and fill factor for each perovskite composition. Incorporating the Ion-Spiro HTL notably increased carrier lifetimes and reduced Shockley–Read–Hall recombination constants compared to the conventional HTL, indicating fewer interfacial defects. Optical characterization confirmed minimal changes in the perovskite band edges, while photoluminescence quantum yield (PLQY) measurements further corroborated the reduced defect densities in Ion-Spiro devices.
The researchers integrated the perovskite into mechanically stacked four-terminal (4T) tandem configurations with n-TOPCon silicon solar cells, and the tandem device achieved an overall efficiency of 28.4–30.2 %. Measurements of external quantum efficiency (EQE), transmission, and integrated JSC closely matched the J–V and optical analyses, validating the performance improvements afforded by the ion-modulated HTL. Finally, stability tests under heat, continuous illumination, and maximum power point tracking showed that devices incorporating the Ion-Spiro HTL exhibited slightly enhanced robustness, consistent with the reduced density of interfacial defects.
“Notably, the introduction of ion-modulated spiro-MeOTAD with an optimized work function significantly enhanced surface defect tolerance, thereby influencing carrier dynamics, resulting in improved 2–5% open-circuit voltage and 6–7% fill factor,” said Kabra. “These findings highlight the essential impact of interfacial defect passivation using ion-modulated spiro-MeOTAD in achieving high-efficiency, stable perovskite solar cells for tandem applications, offering a promising route toward next-generation photovoltaic technologies.”
 
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