Graphene multilayers with interlayer covalent bonds for structural applications and CVD graphene in perovskite solar cells with improved stability

Please contact Lisa Spicer at lms8@psu.edu for Zoom information.

Abstract:

Interlayer bonded graphene multilayers

Building upon our experimental capabilities for growth and transfer of high-quality, single-crystalline CVD graphene (mm scale crystals) transferred from Cu foils, and wafer-scale transfer of single-crystalline epitaxial graphene from SiC wafers [1,1], we have investigated fabrication processes for graphene multilayers with precise control over the twist angle of the stacked graphene layers and, thus, the interlayer covalent bond density, as predicted by computational studies [1]. We have generated understanding of the fundamental mechanisms enabling the formation of C-C interlayer bonds in graphene bilayers and multilayers, and used it to study and optimize relevant fabrication processes paying attention to their future friendliness to manufacturing. In addition, polycrystalline CVD graphene grown over relatively large areas (>10” scale) was also used to form such multilayer metamaterials. Random twist angles and, thus, variable densities of interlayer sp3 bonds characterize these multilayers. Nevertheless, such bond density variation should average out after several layers (notably, ca. 20,000 layers are required for personnel ballistic armor applications).

The twist angle(s) of bilayer or few-layer graphene have been directly characterized by TEM and/or LEED. The structures of the pristine layers have been compared to those of the metamaterials formed after application of the various processes that induce interlayer covalent bonds. Detailed characterization using spectroscopic techniques (Raman, NIR and UV/Vis spectroscopy, XPS/UPS), microscopy techniques (TEM, SEM, AFM) and diffraction techniques (LEED, SAD-TEM, XRD) has been performed. The mechanical properties of multilayer graphene-diamond nanocomposites are currently being characterized and compared to pristine graphene multilayers. AFM-based nanoindentation and friction techniques have been employed in our initial approach.

CVD Graphene in Perovskite Solar Cells with Improved Stability

Despite the impressive efficiency and facile fabrication of hybrid organic-inorganic metal halide perovskite solar cells, their environmental stability remains an important obstacle to their commercialization. We have used high quality, large-area CVD graphene as barrier layer in standard methylammonium lead iodide perovskite solar cells to improve their stability by (i) preventing moisture ingress into the perovskite, and (ii) blocking the diffusion of silver ions from the electrode to the perovskite. [1] Graphene is transferred to the perovskite using a novel, orthogonal solvent-assisted process. Upon exposure to humidity for one week, cells with graphene barrier retained 93% of their initial PCE, whereas cells without graphene only 46%. Similarly, after heat treatment in an inert atmosphere, cells with graphene barrier showed no decrease in PCE, whereas those without graphene decreased to ~75% of initial PCE. Large area CVD Graphene is shown to be a prime candidate for improving the environmental stability of perovskite solar cells, in general, and is compatible with flexible, large area perovskite photovoltaic systems and roll-to-roll fabrication schemes.

Bio

Prof. Dimitrakopoulos received his Ph.D. in Materials Science from Columbia University in 1993. He is a Professor of Chemical Engineering at UMass Amherst, working on: Graphene and phosphorene synthesis; perovskite photovoltaics; phosphorene memory; graphene metamaterials; graphene microfluidics; and massively exfoliated graphene. From 1995 to 2013, he was an RSM at IBM T. J. Watson Research Center, where he worked on organic and hybrid semiconductors, epi-graphene growth and optoelectronics, and porous ULK dielectrics for chip interconnects. From 1993 to 1995 he was a postdoc at Philips Research, NL, working on pentacene transistors. He is a Fellow of the National Academy of Inventors (2020).