RESUMO
The use of a bulk heterojunction of organic semiconductors to drive photoelectrochemical water splitting is an emerging trend; however, the optimum energy levels of the donor and acceptor have not been established for photoanode operation with respect to electrolyte pH. Herein, we prepare a set of donor polymers and non-fullerene acceptors with varying energy levels to probe the effect of photogenerated electron injection into a SnO2-based substrate under sacrificial photo-oxidation conditions. Photocurrent density (for sacrificial oxidation) up to 4.1 mA cm-2 was observed at 1.23 V vs reversible hydrogen electrode in optimized photoanodes. Moreover, we establish that a lower-lying donor polymer leads to improved performance due to both improved exciton separation and better charge collection. Similarly, lower-lying acceptors also give photoanodes with higher photocurrent density but with a later photocurrent onset potential and a narrower range of pH for good operation due to the Nernstian behavior of the SnO2, which leads to a smaller driving force for electron injection at high pH.
RESUMO
The direct conversion of solar energy into chemical fuels, such as hydrogen, via photoelectrochemical (PEC) water splitting requires the efficient oxidation of water at a photoanode. While transition metal oxides have shown a significant success as photoanodes, their intrinsic limitations make them the bottleneck of PEC water splitting. Recently, initial research reports suggest that organic semiconductors (OSCs) could be possible alternative photoanode materials in both dye-sensitized and thin film photoelectrode configurations. Herein we review the progress to date, with a focus on the major issues faced by OSCs: stability and low photocurrent density in aqueous photoelectrochemical conditions. An outlook to the future of OSCs in photoelectrochemistry is also given.
RESUMO
As organic semiconductors attract increasing attention to application in the fields of bioelectronics and artificial photosynthesis, understanding the factors that determine their robust operation in direct contact with aqueous electrolytes becomes a critical task. Herein we uncover critical factors that influence the operational stability of donor:acceptor bulk heterojunction photocathodes for solar hydrogen production and significantly advance their performance under operational conditions. First, using the direct photoelectrochemical reduction of aqueous Eu3+ and impedance spectroscopy, we determine that replacing the commonly used fullerene-based electron acceptor with a perylene diimide-based polymer drastically increases operational stability and identify that limiting the photogenerated electron accumulation at the organic/water interface to values of ca. 100 nC cm-2 is required for stable operation (>12 h). These insights are extended to solar-driven hydrogen production using MoS3, MoP, or RuO2 water reduction catalyst overlayers where it is found that the catalyst morphology strongly affects performance due to differences in charge extraction. Optimized performance of bulk heterojunction photocathodes coated with a MoS3:MoP composite gave 1 Sun photocurrent density up to 8.7 mA cm-2 at 0 V vs RHE (pH 1). However, increased stability was gained with RuO2 where initial photocurrent density (>8 mA cm-2) deceased only 15% or 33% during continuous operation for 8 or 20 h, respectively, thus demonstrating unprecedented robustness without a protection layer. This performance represents a new benchmark for organic semiconductor photocathodes for solar fuel production and advances the understanding of stability criteria for organic semiconductor/water-junction-based devices.
RESUMO
The development of facile routes to prepare fully conjugated block copolymers (BCPs) from diverse monomers is an important goal for advancing robust bulk-heterojunction (BHJ) organic photovoltaics (OPVs). Herein we introduce a synthetic strategy for step-growth BCPs employing 1,2-bis(trialkylstannyl)ethene as one monomer, which, in addition to offering improved backbone planarity, directly yields a vinylene-terminated macromonomer suitable for Heck-Mizoroki coupling. The benefits of our strategy, which facilitates the preparation of functionalized macromonomers suitable for BCP synthesis, are demonstrated with a representative BCP based on a diketopyrrolopyrrole (DPP) copolymer coded pBDTTDPP as the donor block and a perylenediimide (PDI) copolymer coded as pPDIV as the acceptor block. Feed ratio optimization affords control over the macromonomer chain-end functionalities and allows for the selective formation of a tri-BCP consisting of pPDIV-b-pBDTTDPP-b-pPDIV, which is employed in a single-component BHJ OPV. Devices achieved a power conversion efficiency of 1.51% after thermal stress at 150 °C compared to 0.02% for a control device consisting of a comparable blend of pBDTTDPP and pPDIV. The difference in performance is ascribed to the morphological stability of the BHJ when using the BCP.
RESUMO
Melt-processing of organic semiconductors (OSCs) is a promising environmentally-friendly technique that can alleviate dependence on toxic chlorinated solvents. While melt-processed single-component OSC devices (e.g. field-effect-transistors) have been demonstrated, multi-component bulk heterojunctions (BHJs) for organic photovoltaics (OPVs) remain a challenge. Herein, we demonstrate a strategy that affords tunable BHJ phase segregation and domain sizes from a single-phase homogeneous melt by employing strongly-crystalline small-molecule OSCs together with a customized molecular compatibilizing (MCP) additive. An optimized photoactive BHJ with 50 wt% MCP achieved a device power conversion efficiency of ca. 1% after melting the active layer at 240 °C (15 min, followed by slow cooling) before deposition of the top electrode. BHJ morphology characterization using atomic force and Kelvin probe microscopy, X-ray diffraction, and photo-luminescence measurements further demonstrate the trade-off between free charge generation and transport with respect to MCP loading in the BHJ. In addition, a functional OPV was also obtained from the melt-processing of dispersed micron-sized solid BHJ particles into a smooth and homogeneous thin-film by using the MCP approach. These results demonstrate that molecular compatibilization is a key prerequisite for further developments towards true solvent-free melt-processed BHJ OPV systems.
