Publication date: 5th November 2025
Single-junction perovskite solar cells are approaching the Shockley–Queisser efficiency limit, making tandem architectures essential for further performance gains. In two-terminal perovskite/silicon tandem devices, the series-connected sub-cells constrain the overall photocurrent, highlighting the need to maximize the photocurrent of the wide-bandgap perovskite top cell.
Fullerene C60 remains the dominant electron transport material (ETM) owing to its high electron mobility and excellent thermal and photostability; however, its parasitic absorption in the visible region (350–550 nm), corresponding to a photocurrent loss of approximately 1.2 mA cm⁻², fundamentally limits the achievable short-circuit current density (Jsc) in wide-bandgap perovskite devices.
In this study, phenazine-based ETMs are designed and synthesized as low–optical-loss alternatives to C60. Owing to their extended π-conjugated phenazine framework, these materials exhibit wide optical bandgaps and suppressed visible-light absorption while maintaining suitable energy-level alignment for electron extraction, as confirmed by UV–vis absorption spectroscopy and ultraviolet photoelectron spectroscopy (UPS). Their thermal robustness and uniform interfacial coverage on perovskite films are verified by thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, and Kelvin probe force microscopy.
Devices fabricated in an ITO/HTL/FA₀.₈Cs₀.₂Pb(I₀.₈Br₀.₂)₃/ETL/BCP/Ag configuration reveal that pristine phenazine-based ETMs yield reduced open-circuit voltage (Voc), fill factor (FF), and device stability compared to C60-based devices. These limitations are effectively overcome by introducing a bilayer ETL architecture incorporating a thin C60 interlayer beneath the phenazine derivative. Notably, the fluorine-substituted phenazine derivative in a C60 (10 nm)/F6 (10 nm) bilayer improves device efficiency from 12.76% to 18.57% with enhanced stability, achieving performance comparable to that of C60-based devices. These results demonstrate that bilayer interfacial engineering enables the practical integration of low–optical-loss ETMs into wide-bandgap perovskite top cells for tandem photovoltaics.
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