A solution-processed organic semiconductor photocatalyst sheet has achieved unassisted solar-driven water splitting by utilizing tailored molecular underlayers to direct both reduction and oxidation reactions from a single light-absorbing film. Under simulated one-sun illumination without an applied external electrical bias, the integrated photocatalyst sheet demonstrated continuous overall water splitting for more than two hours, generating hydrogen at a stabilized production rate of 1.3 micromoles per hour per square centimeter. This performance corresponds to an unassisted solar-to-hydrogen conversion efficiency of 0.1 percent while maintaining a stoichiometric hydrogen-to-oxygen evolution ratio of two to one. Direct solar water splitting offers a sustainable pathway to produce clean hydrogen fuel, yet converting laboratory photoelectrodes into scalable systems has faced persistent material bottlenecks.
The central operating mechanism of the photocatalyst sheet relies on an underlayer engineering strategy that establishes functional microdomain architectures beneath an organic bulk heterojunction blend. When deposited over different molecular contacts on a fluorine-doped tin oxide substrate, this identical bulk heterojunction layer selectively drives either the hydrogen evolution reaction or the oxygen evolution reaction. Modifying the underlying substrate with a self-assembled molecular layer of 2PACz configures the organic film as an efficient photocathode for proton reduction. Conversely, functionalizing the underlying substrate with a phosphonic acid derivative, NI-PA, reorients internal charge extraction directionality, causing the same polymer blend to operate as an efficient photoanode for water oxidation. This approach yields photocathode and photoanode half-cells that display remarkably well-matched photocurrent characteristics.
Quantitative photoelectrochemical measurements confirmed the complementary performance of the individual half-cell configurations and their integrated microdomain sheets. In three-electrode half-cell assessments, the 2PACz-modified photocathode delivered a photocurrent density of 4.1 milliamperes per square centimeter at 0 volts versus the reversible hydrogen electrode for the hydrogen evolution reaction. Under identical illumination conditions, the NI-PA-modified photoanode generated a photocurrent density of 2.7 milliamperes per square centimeter at 1.23 volts versus the reversible hydrogen electrode for the oxygen evolution reaction. When integrated into a complete photocatalyst sheet incorporating 1,000-micrometer 2PACz domains alongside complementary NI-PA domains, the combined device demonstrated simultaneous reduction and oxidation activity during photoelectrochemical evaluation. The full architecture recorded a photocurrent density exceeding 2 milliamperes per square centimeter at 0.1 volts versus the reversible hydrogen electrode and 1.4 milliamperes per square centimeter at 1.23 volts, establishing an operational photocurrent crossover potential of 0.56 volts versus the reversible hydrogen electrode.
Fabricating the co-planar photoelectrode domains required systematic optimization of underlayer geometry and interface dimensions. The researchers established controlled charge-selective contact microdomains on fluorine-doped tin oxide substrates using polydimethylsiloxane stamp-based contact printing. When evaluating the influence of domain sizing on overall water-splitting performance, the team observed that miniaturizing the patterned features below 1,000 micrometers led to a noticeable decline in solar conversion efficiency. Detailed analysis revealed that smaller domain geometries substantially increase the specific perimeter length between adjacent, oppositely functionalized substrate regions.
To establish that this device architecture does not rely strictly on cleanroom lithography, the authors developed a scalable spray-deposition technique for microdomain underlayer formation. A custom mist deposition system was constructed around an aerosol generator utilizing a vibrating mesh operating at full power amplitude to create a fine microdroplet mist of 2PACz dissolved in ethanol at a concentration of 2 millimolar. Liquid condensate and oversized droplets were trapped in a bottom drainage reservoir while an argon carrier gas directed the entrained aerosol mist onto pre-cleaned, activated fluorine-doped tin oxide substrates resting on a hot plate at 110 degrees Celsius. Deposition proceeded through repeated 45-second spray exposures separated by three-minute thermal annealing intervals, followed by a final ten-minute bake at 140 degrees Celsius and ultrasonic washing in ethanol. Subsequent overnight immersion in a 1 millimolar solution of NI-PA functionalized the remaining exposed substrate areas, demonstrating a high-throughput, solution-based manufacturing pathway for patterned molecular underlayers.
Synthesizing the high-performance organic semiconductor materials required precise organometallic polymerization procedures. The donor polymer PBDTTTPD was synthesized by combining 100 milligrams of the distannylated monomer BDT-Sn with one equivalent of the dibrominated monomer TPD-Br in a round-bottom flask, utilizing 2 mole percent tris(dibenzylideneacetone)dipalladium(0) and 8 mole percent tri(o-tolyl)phosphine as the catalyst system. The degassed reaction mixture was dissolved in 20 milliliters of chlorobenzene and 2 milliliters of dimethylformamide under an argon atmosphere and refluxed at 150 degrees Celsius for 48 hours. After precipitation into methanol, crude impurities, residual catalytic residues, and low-molecular-weight oligomers were removed through successive Soxhlet extractions using methanol, acetone, and hexane. The purified polymer donor was recovered through chloroform extraction and reprecipitated in methanol, yielding 93 percent of the dry product. A parallel synthetic pathway was utilized to synthesize the alternative polymer acceptor PNDITCVT from dibrominated NDI and distannylated TCVT monomers, incorporating an additional dichloromethane Soxhlet fraction to achieve an isolated yield of 81 percent.
The molecular contact material NI-PA was synthesized in an isolated laboratory reaction to provide the selective electron-collecting underlayer for the photoanode regions. The preparation involved reacting 158.46 milligrams of 1,8-naphthalic anhydride with 100 milligrams of 2-aminoethylphosphonic acid in the presence of 0.5 grams of imidazole inside a 25-milliliter reaction vessel heated to 140 degrees Celsius for 30 minutes. Upon cooling to room temperature, the crude reaction mixture was treated with 5 milliliters of an equal-volume solution of ethanol and 2 molar hydrochloric acid. The resulting solid precipitate was isolated by filtration, washed successively with 10 milliliters of the acidified ethanol solution and 10 milliliters of pure ethanol, and dried under reduced pressure. This procedure yielded 87 milligrams of NI-PA as a purified white solid, representing an isolated chemical yield of 36 percent. Proton nuclear magnetic resonance spectroscopy in deuterated dimethyl sulfoxide confirmed product purity, displaying characteristic multiplet and triplet resonances matching expected molecular shifts.
Substrate functionalization protocols were executed under controlled thermal and chemical conditions to achieve robust molecular chemisorption onto conductive glass. Transparent fluorine-doped tin oxide glass substrates were sequentially cleaned through 20-minute ultrasonic baths in acetone, deionized water, and isopropanol before undergoing 15 minutes of ultraviolet-ozone surface activation. Microdomain contact printing utilized flexible polydimethylsiloxane stamps molded from photoresist patterns dry-etched into silicon wafers and treated with a fluorinated silane monolayer to prevent adhesion. Stamps loaded with 2 millimolar 2PACz solution were dried at 90 degrees Celsius, pressed onto activated substrates, and baked at 140 degrees Celsius for ten minutes before sonication in ethanol to remove unbound molecules. Complementary regions were functionalized by immersing the patterned substrates overnight in a 1 millimolar solution of NI-PA dissolved in dimethylformamide, followed by secondary thermal annealing at 140 degrees Celsius and ultrasonic washing in fresh dimethylformamide to guarantee stable monolayer anchoring.
Completing the integrated photocatalyst sheet involved the sequential deposition of the photoactive polymer blend and dual catalytic surface top-layers. Bulk heterojunction films were deposited onto the patterned underlayers by dynamic spin-coating at 3,000 revolutions per minute from chloroform solutions containing PBDTTTPD and PNDIHDT at a 1.5 to 1 weight ratio, followed by hot-plate annealing at 100 degrees Celsius for ten minutes. A platinum co-catalyst for the hydrogen evolution reaction was selectively electrodeposited onto the p-type photocathode domains from an argon-degassed electrolyte containing 1 milligram per milliliter potassium hexachloroplatinate and 0.5 molar sodium sulfate under simulated one-sun illumination at 0 volts versus silver/silver chloride. Finally, lithium iridium oxide co-catalyst nanoparticles, prepared by calcining iridium acetylacetonate and lithium hydroxide at 400 degrees Celsius, were spray-coated from an isopropanol-water suspension and annealed at 120 degrees Celsius.
Electrochemical and gas evolution measurements were conducted in specialized photoelectrochemical reactors to rigorously verify overall water splitting. Unassisted photocatalytic gas production was evaluated inside a hermetically sealed 20-milliliter glass purge reactor containing 20 milliliters of the pH 9 buffer electrolyte under continuous nitrogen purging at 6 milliliters per minute. The reaction vessel was irradiated with simulated one-sun illumination from a calibrated 500-watt xenon lamp. Evolved gas streams were routed directly into the automated sample loop of a gas chromatograph equipped with a barrier discharge ionization detector, sampling reaction products at five-minute intervals. Calibration protocols using platinum electrodes confirmed quantitative detection, demonstrating sustained, stoichiometric production of hydrogen and oxygen gas over continuous two-hour operational testing periods.
Extensive physical, electronic, and structural characterizations corroborated the morphology and interface behavior across all device layers. Surface topologies and localized contact potential differences across microdomain boundaries were mapped via atomic force microscopy and Kelvin probe force microscopy using platinum-coated cantilevers.
Subject of Research: Solar-driven water splitting using organic semiconductor photocatalyst sheets
Article Title: An organic semiconductor-based photocatalyst sheet for overall solar-driven water splitting
Article References: Diercks, N. J., Park, J. S., Yum, J.-H., Meng, Y., Bouchez, A. E., Sorin, F., Cho, H.-H., & Sivula, K. (2026). An organic semiconductor-based photocatalyst sheet for overall solar-driven water splitting. Nature Energy. https://doi.org/10.1038/s41560-026-02160-0
Image Credits: AI Generated
DOI: 10.1038/s41560-026-02160-0
Keywords: organic semiconductors, photocatalyst sheet, solar-driven water splitting, bulk heterojunction, hydrogen evolution reaction, oxygen evolution reaction, charge extraction layer, microdomain patterning, 2PACz, NI-PA, PBDTTTPD, PNDIHDT, photoelectrochemistry, solar-to-hydrogen conversion, PDMS contact printing, PTAA nanoparticles, lithium iridium oxide
News Source: Denise Maddox. (November 12, 2026). Patterned Organic Semiconductor Sheet Achieves Solar Water Splitting Without External Bias. Scienmag.
