Researchers from Italy's University of Salerno, Warsaw University in Poland and Lithuania's Center for Physical Sciences and Technology have developed graphene - ITO hybrid transparent electrodes aimed at improving charge transport in next-generation multijunction space solar cells.
Multijunction GaInP/GaAs/Ge solar cells are the dominant photovoltaic technology for space applications, delivering initial efficiencies of around 30% under the AM0 spectrum. These devices rely on stacked p-n junctions with different bandgaps to capture a broader portion of the solar spectrum, but their performance remains constrained by front electrode losses. Transparent conducting oxides such as indium tin oxide (ITO) are widely used, yet they suffer from an inherent trade-off between electrical conductivity and optical transparency, along with mechanical brittleness. To address these limitations, the researchers introduced a hybrid architecture that integrates monolayer graphene with conventional ITO. Graphene, known for its high carrier mobility and optical transparency, was synthesized via cold-wall chemical vapor deposition and transferred onto pre-patterned, commercially available ITO-coated glass substrates (approximately 100 nm thick) using a thermal release tape method. The goal was to enhance lateral conductivity and charge carrier mobility while preserving the transparency required for efficient light absorption in multijunction devices.
Raman spectroscopy confirmed successful graphene integration and high material quality. The hybrid films exhibited characteristic D, G, and 2D peaks at approximately 1344 cm⁻¹, 1583 cm⁻¹, and 2693 cm⁻¹. The low D-band intensity indicated minimal defects, while subtle spectral shifts - such as a slight blueshift of the G band and a downward shift of the 2D band - suggested charge-transfer interactions and carrier doping at the graphene - ITO interface. The narrowing of the 2D peak further pointed to strong interfacial coupling, with only a minor increase in the D/G ratio after transfer, confirming that structural integrity was largely preserved.
At the nanoscale, electrical characterization using Tunneling Atomic Force Microscopy (TUNA-AFM) revealed substantial improvements in charge transport. Measurements performed with a platinum-coated probe under a 1-2 V DC bias showed that bare ITO surfaces exhibited localized conduction primarily at grain boundaries, with tunneling currents ranging from about −950 fA to 940 fA. In contrast, graphene-coated ITO surfaces displayed smoother morphology and continuous conductive pathways, with tunneling currents increasing to between −1.6 pA and 1.5 pA.
This corresponds to an approximately 60% increase in nanoscale tunneling current, directly indicating enhanced local charge transport. The improvement is attributed to graphene’s high in-plane conductivity combined with strong interfacial coupling, which facilitates both lateral carrier transport and vertical tunneling across the electrode. Importantly, repeated measurements confirmed the reproducibility of these results and demonstrated uniform electrical behavior across the hybrid surface.
The findings highlight the potential of graphene - ITO hybrid electrodes to overcome key limitations of conventional transparent electrodes in space photovoltaics. By improving electrical continuity without compromising optical performance or surface uniformity, these structures offer a promising route toward lightweight, durable, and high-efficiency solar cells for aerospace applications. While the present work focuses on nanoscale characterization, further device-level studies will be required to fully assess performance gains in operational solar cells.
