An international research team has built a triple-junction all-perovskite solar cell that uses a graphene oxide–self-assembled (SAM) bilayer hole-selective contact strategy to reduce optical losses, improve stability, and enhance the performance of tin-lead perovskite subcells.
Self-assembled monolayers (SAMs) are currently employed as state-of-the-art hole-transport layers in perovskite solar cells, owing to their low parasitic absorption, rapid charge extraction, and effective passivation of the perovskite buried interface. However, precisely controlling their thickness, molecular packing density, and orientation remains a significant challenge.
The scientists explained that, although SAMs enable high open-circuit voltage, they can also introduce significant charge extraction losses. Through systematic investigations of charge-carrier dynamics and buried interfaces, they identified that commonly used carbazole-based SAMs induce substantial ionic losses that hinder efficient charge extraction. To address these limitations, the team explored interface-engineering strategies based on graphene oxide (GO) integration.
They investigated, in particular, a SAM known as MeO-2PACz, GO, and GO/MeO-2PACz and found GO enables more conformal MeO-2PACz coverage compared with the standalone SAM on indium tin oxide (ITO) substrates. Furthermore, they ascertained that GO’s oxygen-containing functional groups enable strong interactions with the phosphonic acid anchoring groups of MeO-2PACz, promoting uniform SAM formation and improved interfacial passivation.
The research team built the top perovskite cell with an indium tin oxide (ITO) transparent conductive electrode, a nickel oxide (NiOx)/self-assembled monolayer (SAM) hole-transport layer and a wide-bandgap 2.00 eV perovskite absorber, a spacer based on phenyl-C61-butyric acid methyl ester (PCBM), a tin oxide (SnOx) electron-transport layer (ETL) and an another ITO layer.
The middle sub-cell incorporates a SAM-based hole-selective layer, a 1.55 eV or 1.60 eV intermediate-bandgap perovskite absorber, and an ETL made of fullerene (C60) and SnOx. The interconnection between the middle and top sub-cells is formed by a recombination layer based on either poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) or a bilayer configuration incorporating gold (Au) or ITO.
The top sub-cell employs a low-bandgap 1.25 eV perovskite absorber, followed by a C60/SnOx electron-transport bilayer and a conductive copper (Cu) rear electrode. This carefully engineered layer sequence enables efficient charge extraction and broad-spectrum photon harvesting in the triple-junction all-perovskite solar cell.
Photovoltaic performance was evaluated using current–voltage measurements, maximum power point tracking (MPPT), external quantum efficiency (EQE), and operational stability tests under simulated sunlight. Advanced spectroscopic and electrical techniques, including X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), bias-assisted charge extraction (BACE), and transient measurements, were used to analyze interfaces, charge dynamics, and ion migration.
The analysis revealed that the GO/SAM bilayer improves buried interface morphology, producing more uniform perovskite growth, reduced interfacial defects, and favorable crystal orientation. Compared with conventional PEDOT:PSS-based contacts, the bilayer was also found to reduce parasitic absorption and improve current generation in all-perovskite multi-junction devices developed for the experiments
By integrating the ITO/bilayer recombination junction into triple-junction devices, cell efficiency increased from 23.6% to 25.1%. Further optimization of the middle sub-cell bandgap enabled a champion efficiency of 27.3%. The bilayer architecture also improved operational stability, with the device retaining 90% of its initial power conversion efficiency after 770 hours of continuous illumination and outperforming PEDOT:PSS-based devices.
“Overall, our findings demonstrate the immense potential of SAM-based all-perovskite multi-junctions and bring this promising technology a step closer to higher industrial readiness levels,” the researchers said.
The research team explained that reducing charge-transport resistance and improving perovskite quality and energy-level alignment could enhance fill factor and open-circuit voltage, enabling effiencies beyond 30%.
The novel cell concept was presented in “Triple-junction all-perovskite solar cells with self-assembling hole contacts in all subcells,” published in Joule. The group comprised academics from Germany’s Universität Potsdam and the Swiss Federal Laboratories for Materials Science and Technology (Empa).
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