Researchers at HZB, University of Potsdam, Technical University Berlin and Empa have developed a graphene oxide (GO)/self-assembled monolayer (SAM) bilayer hole-transport architecture that addresses key efficiency and stability bottlenecks in tin-lead (Sn–Pb) perovskite solar cells and enables high-performance all-perovskite triple-junction devices.
All-perovskite multi-junction solar cells (APMSCs) are attractive due to their high theoretical efficiencies - approximately 44% for double-junction and 52% for triple-junction configurations. Recent progress has relied heavily on narrow-band-gap (~1.25 eV) Sn-Pb perovskite bottom cells. However, their performance has remained constrained by the widely used hole transport layer PEDOT:PSS, which introduces both optical and chemical losses. Its parasitic absorption (typically from 30-50 nm thick layers) reduces current density by 0.5-1.4 mA cm−2, while its acidic and hygroscopic nature accelerates degradation through interfacial reactions that promote iodine vacancy formation and oxidation of I− and Sn2+.
Self-assembled monolayers have emerged as promising alternatives due to their negligible absorption and strong interface passivation in lead-based systems. Yet, their implementation in Sn-Pb perovskites has lagged. The researchers show that while SAMs can deliver high open-circuit voltage, they also introduce significant charge extraction losses. Mechanistically, commonly used carbazole-based SAMs lead to ionic losses and non-uniform buried interfaces, which suppress efficient hole transport and increase non-radiative recombination.
To overcome these limitations, the team introduced a GO/SAM bilayer, where the GO acts as an electronically and morphologically favorable foundation for the SAM. This bilayer improves surface coverage, promotes more homogeneous perovskite film formation, and enhances crystallite orientation at the buried interface. As a result, both electronic and ionic losses are mitigated, leading to improved charge extraction and reduced recombination.
In single-junction Sn-Pb devices, this approach enabled a power conversion efficiency of 22.1% with an open-circuit voltage of 0.88 V. When integrated into monolithic triple-junction architectures - described by the team as a complex stacked structure analogous to a “Big Mac” - the bilayer replaces PEDOT:PSS at the critical interface between the middle and bottom subcells. This reduces near-infrared parasitic absorption losses by approximately 0.5 mA cm−2 and enhances photogeneration in the Sn-Pb subcell to 10.3 mA cm−2.
The resulting triple-junction devices achieve a power conversion efficiency of 27.3% (stabilized at 27.0%), placing them among the highest-performing APMSCs reported to date. Importantly, the chemically benign GO/SAM interface also delivers substantial stability gains: devices retain 90% of their initial efficiency after 770 hours of continuous operation at 25°C, significantly outperforming PEDOT:PSS-based counterparts (T90 of 380 hours).
The results highlight that the buried interface between the Sn-Pb absorber and the hole transport layer is a critical lever for both efficiency and durability. By resolving the charge extraction and interfacial uniformity limitations of SAMs, the GO/SAM bilayer provides a viable pathway toward more stable and efficient multi-junction perovskite solar cells. Further improvements in series resistance, perovskite film quality, and energy band alignment could push efficiencies beyond 30%.
