Graphene-Based Hole-Selective Layers for High-Efficiency, Solution-Processed, Large-Area, Flexible, Hydrogen-Evolving Organic Photocathodes

Abstract

Regio-regular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT), the work-horse of organic photovoltaics, has been recently exploited in bulk heterojunction (BHJ) configuration with phenyl-C61-butyric acid methyl ester (PCBM) for solution-processed hydrogen-evolving photocathodes, reaching cathodic photocurrents at 0 V vs. RHE (J 0V vs RHE ) of up to 8 mA cm -2 . The photoelectrochemical performance of these photocathodes strongly depends on the presence of the electron (ESL) and hole (HSL) selective layer. While TiO 2 and its sub-stoichiometric phases are consolidated ESL materials, the currently used HSLs (e.g., MoO 3 , CuI, PEDOTT:PSS, WO 3 ) suffer electrochemical degradation under hydrogen evolution reaction (HER)-working conditions. In this work, we use solution-processed graphene derivatives as HSL to boost efficiency and durability of rr-P3HT:PCBM-based photocathodes, demonstrating record-high performance. In fact, our devices show cathodic J 0V vs RHE of 6.01 mA cm -2 , onset potential (V o ) of 0.6 V vs. RHE, ratiometric power-saved efficiency (φ saved ) of 1.11% and operational activity of 20 hours in 0.5 M H 2 SO 4 solution. Moreover, the designed photocathodes are effectively working in different pH environments ranging from acid to basic. This is pivotal for their exploitation in tandem configurations, where photoanodes operate only in restricted electrochemical conditions. Furthermore, we demonstrate the scalability of our all-solution processed approach by fabricating a large-area (~9 cm 2 ) photocathode on flexible substrate, achieving remarkable cathodic J 0V vs RHE of 2.8 mA cm -2 , V o of 0.45 V vs. RHE and φ saved of 0.31%. This is the first demonstration of highefficient rr-P3HT:PCBM flexible photocathodes based on low-cost and solution-processed manufacturing techniques.

Scalable Production of Graphene Inks via Wet‐Jet Milling Exfoliation for Screen‐Printed Micro‐Supercapacitors

“…Few‐layer graphene (<5 layers), instead, has a 2D 1 peak more intense than the 2D 2 . Taking into account the intensity ratios of the 2D 1 and 2D 2, it is possible to roughly estimate the flake thickness . Figure e shows the comparison between the Raman spectra (normalized to the G peak intensity) of the graphite and WJM‐graphene.…”

MoS2 Quantum Dot/Graphene Hybrids for Advanced Interface Engineering of a CH3NH3PbI3 Perovskite Solar Cell with an Efficiency of over 20%

“…201,202 Noteworthy, the %c of C sp 2 in f-RGO is almost the same of that of native RGO (75.6% 183 ), indicating that the functionalization process do not alter the basal properties of the RGO flakes. 183 The component peaking at ~286.9 eV is assigned to C-O bonds (%c = 6.9%). A residue of C sp 3 is still present (peak centered at 284.7 eV, %c ~7.6%) as well as carboxylate carbon O-C=O bonds (peak centered at 287.7 eV, %c ~1%).…”

“…The ratio between the sum of the %c of SH free and S-S bnds related to the MPTS and that of C bonds on f-RGO is 3%, and estimate the percentage extent of the functionalization of RGO with MPTS. 183 The interaction between the MPTS and the RGO flakes was confirmed by complementary Fourier-transform infrared (FTIR) measurements. After RGO functionalization, in the f-RGO FTIR spectrum (see SI, Figure S4), the Si-O-Si stretching band appears at 1078 cm -1 shifted and broaden compared to pure MPTS (1089 cm -1 ), indicating the coupling between the alkoxy silane groups and the oxygen groups of RGO, as proposed in Scheme 1.…”

“…In fact, MPTS are polar molecules able to decrease the surface energy of native RGO in alcohol-based solvents (∼46.1 mN m -1 in ethanol), 208,209 enhancing dispersion stability and hindering formation of aggregates during films deposition. 183 In addition, XPS analysis also evidences that S 2p doublet related to S-S bonds (centered at ~164.5 eV) create an interconnection between the MPTS molecules and MoS2 QDs, since the corresponding %c increases from 10% of the total S content in f-RGO 167,205 to ~21% of the total S content in MoS2 QDs-f-RGO.…”

A Three‐Dimensional Architecture for Hydrogen‐Evolving, Host/Guest, Hybrid Organic/Inorganic Photocathodes Based on Nanolamellar MoO3 Scaffolds

“…Semiconducting polymers offer several advantages owing to their p ‐ or n‐ type character, controllable band gap energy, ease of fabrication by means of printing or roll‐to‐roll deposition techniques and the verified stability of their charge generation efficiency in aqueous electrolytes . Recent literature showed seminal explorations on the use of organic semiconductors in photocatalytic water splitting together with their application as absorbers in hydrogen evolution photocathodes also demonstrating high efficiency using non‐precious catalysts . Recently, our group developed hybrid organic/inorganic photocathodes that obtained promising results in terms of efficiency, stability and scalability of the synthetic approach .…”