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First author: Tack Ho Lee
Corresponding author: James R. Durrant
Communication unit: Imperial College London
Thesis DOI: https://doi.org/10.1002/aenm.202103698
Full text at a glance
Utilizing bulk heterojunctions (BHJs) of polymer donors and small molecule non-fullerene receptors, organic photovoltaic devices exhibit high performance, strong visible light and near-infrared absorption, and low energy loss, and are strong candidates for solar-powered water decomposition. However, the poor stability of small molecule receptors underwater limits their viability. In this paper, based on Y6:PM6 BHJ with another bifunctional PM6 layer transferred from water and the top layer of the Au/NiFe electrocatalyst, the authors demonstrate a stable and efficient organic photoanode for water oxidation. The additional PM6 layer serves to: 1) improve operational stability, and 2) inhibit the loss of compounding between the BHJ and the electrocatalyst layer. Compared with the anode without pm6 layer, these BHJ/PM6 primary anodes have an optocurrent density of 4.0 mA cm-2 at 1.23 VRHE and have good operational stability, maintaining a photocurrent ≥ 2 mA cm-2 for 1 h. Solar water oxidation experiments under near-infrared irradiation using these photoanodes have shown that incident photons are as efficient as 25% of the current under 770 nm illumination.
Background
Developing new technologies for sustainable synthetic fuels and chemicals is a key challenge to achieving net zero carbon emissions. Photochemical (PEC) water splitting has attracted widespread attention as a potentially low-cost, scalable pathway to solar-powered synthesis of green hydrogen as solar-to-electricity and power-to-fuel conversion are integrated in a semiconductor/electrolyte system. So far, although most PEC devices have used inorganic semiconductors to absorb sunlight, PEC devices based on organic semiconductors are receiving more and more attention. But these two types of semiconductors still have significant challenges in terms of performance, stability, energy level adjustability, scalability, and cost reduction.
Organic semiconductor-based bulk heterojunction (BHJs) photovoltaic devices are a promising alternative to inorganic photovoltaic devices due to their band gap tunableability (through molecular engineering) and solution processability. The power conversion efficiency of organic photovoltaic (OPV) devices has improved rapidly in recent years, and now exceeds 18%. The development of non-fullerene small molecule receptors (NF-SMAs) has particularly driven these efficiencies, due to their long exciton diffusion lengths and energy level tunability (narrow band gap, small energy offset, etc.). However, the poor stability of typical organic BHJ underwater limits its application in PEC water decomposition devices, especially since small molecules are easily dispersed or partially dissolved in aqueous solutions of optimized BHJ films, whereas polymers are not. Based on this, wired photovoltaic electrolyzer concepts or complex encapsulation methods have been used to separate or protect SMA: polymer blends from aqueous solutions. In PEC batteries, all-polymer BHJs improve the stability of organic photoelectrodes by excluding NF-SMAs from the blend. The all-polymer photocathode for water reduction has exhibited promising photocurrents of up to 8 mA cm-2, while the photoanodes for more thermodynamically and kinetically challenging water oxidation reactions are limited to a photocurrent of about 2 mA cm-2. Improving the performance of organic photoanodes remains a major challenge for achieving integral PEC water decomposition using tandem organic photocathodes and photoanodes. Another challenge is that the all-polymer photocathodes and photoandodes reported so far are largely limited to overlapping visible light absorption, which limits their use in organic PEC series devices. Therefore, the development of high-performance organic photoodes, especially near-infrared absorption organic photoanodes, is a key challenge that can make full use of the solar spectrum through complementary absorption with all-polymer photocathodes.
In this paper, the authors report a near-infrared light-collecting organic light anode based on polymer donors and NF-SMA BHJs for solar water oxidation. After BHJ deposition, a thin film transfer process is used to deposit a polymer overlay in water, which has the dual function of protecting BHJ from the aqueous electrolyte and preventing the return of photogenerated electrons to the anode/electrolyte interface. The resulting bifunctional polymer sandwich photoanode structure exhibits significant water oxidation properties and operational stability, which is a significant improvement over the previously reported organic photoanode.
Graphic and text analysis
Figure 1 a) Molecular structure of PM6 and Y6. b) Transfer of PM6 membrane from water to Y6: Schematic of PM6 membrane and corresponding photo. c) Absorption spectra of Y6:PM6 films with PM6 overlay (denoted Y6:PM6/PM6) and Y6:PM6 films. d) Schematic diagram of the structure of organic photoanode devices.
Figure 2 a) LSV scan of Y6:PM6/PM6 and Y6:PM6 photoanodes in 0.1 M KOH solution (pH 13) under intermittent 1 sunlight exposure. b) Normalized timing current curve at 1.23 VRHE in 0.1 M KOH solution (pH 13) under 1 sunlight. The digital photographs in the illustration show the oxygen release from the surface of the Y6:PM6/PM6 photoanode during the measurement. c) IPCE spectrum at 1.23 VRHE, Y6:PM6/PM6 photoanode in 0.1 M KOH solution (pH 13). d) Timing current curve (at 1.23 VRHE) in 1 M borate electrolyte (buffer pH 8.1), 0.1 M (pH 13) and 1 M KOH solution (pH 14) under continuous 1 m sunlight exposure.
Fig. 3 a) Negative current response of VRHE from 0.5 to 1.3 after lights off. b) The amount of negative charge is obtained as a function of applying bias voltage by integrating negative transient currents. c) Y6: PM6/PM6 photoanode and d) Y6: PM6 light anode energy level diagram. The PM6 layer prevents the electron reverse transfer of the Y6:PM6 BHJ.
Summary and outlook
In summary, by introducing a polymer interlayer between BHJ and the electrolyte, the efficient near-infrared absorption Y6:PM6 organic BHJ can be used as an organic photoanode for direct solar-powered water oxidation. Due to the poor underwater stability of small molecules or the need for extensive encapsulation work, organic light absorbers for water decomposition are limited to polymer semiconductors. The simple transfer of the hydrophobic polymer layer to water significantly improves the operational stability of the organic photoanode, which provides a wide range of options for organic semiconductors in underwater applications. In addition, ZnO nanoparticles as the bottom layer of the organic film improve stability, and the chemical deposition method of the OER catalyst can be selected to avoid the destruction of the organic film during photoanodic preparation. The optimized ITO/ZnO/Y6: PM6/PM6/Au/NiFeOOH photoanodes obtained significant Jph (4 mA cm−2) at 1.23 VRHE, and the stability of Jph ≥ 2 mA cm−2 was significantly improved at continuous sunlight at pH 13, reaching 4000 s. Under pH 8.1 and the same operating conditions, water oxidation has unprecedented operational stability after long-term storage in ambient air. In addition, the PM6 layer acts as an effective electron barrier layer, reducing the starting potential to around 400 mV by inhibiting accidental charge compounding at the anode/electrolyte interface. The stability of the solution-treated organic photoanode and the high performance of broadband absorption up to 900 nm indicate that the bifunctional polymer sandwich increases the possibility of solar-powered fuel production from PEC cells based on economical and scalable organic semiconductors.