Development Of Organic Polymer Semiconductor-Based Photoelectrode Devices And Their Photoelectrocatalytic Performance Study

The development of novel photoelectrocatalytic materials for high-efficiency photoelectrochemical（PEC）hydrogen evolution has achieved great attention in recent years.In the past decades,extensive research efforts have been devoted to the fabrication of photoelectrodes using inorganic semiconductors.However,their band structures and photoelectronic properties are typically predetermined by the compositions and crystal structures.Moreover,it is commonly required to use high temperature or other harsh synthesis/deposition conditions to achieve the high crystallinity nanostructures to ensure unobstructed internal charge transport.In contrast,the energy level and optoelectronic properties of organic semiconductors（OSs）can be predesigned from the molecular level with much larger flexibility than most inorganic semiconductors.In addition,processing OSs is much easier as it usually requires much lower temperatures and can use purely solution-based techniques,thus enabling low-cost,high-throughput,and large-scale production,which has potential commercialization value.Despite these advantages,their use as photoelectrode catalysts for PEC water splitting has been rarely reported,and their PEC performance is far from dissatisfactory,which is limited by their short exciton diffusion length,sluggish reaction kinetics,and durability issues.More recently,organic semiconductors（OSs）have already been well-established in organic photovoltaic devices,light-emitting diodes,organic field-effect transistors,more and more novel OSs with excellent optoelectronic properties have been synthesized,which provide great opportunities for them to succeed in achieving high PEC performance.The purpose of this thesis is to investigate the relationship between the structure and properties of conjugated OSs and to explore new methods to achieve OSs functionalization and their application.On the other hand,we mainly focus on the development of novel strategies to achieve high-efficiency photoelectrodes in a photoelectrochemical cell.Therefore,the research work of this thesis is mainly divided into four parts as follows:（1）PFDPABT was firstly used as a photocathode for PEC process.After modification by mixing with carbon nanotubes,Pt deposition,and addition of Ti O₂interfacial layer,the photocurrent of PFDPABT was increased apparently.The onset potential of the photoelectrode was increased by about 100 m V after Pt deposition.These results suggest that these surface/interface modification can effectively improve the PEC performance of PFDPABT by enhancing its electrical conductivity,increasing the catalytic reaction active sites,and promoting carrier separation.（2）A novel and universal method was developed to uniformly and compactly deposit the hydrophilic Ru O2 layer onto the hydrophobic BHJ（J72:N2200）by improving the droplet adhesiveness and roughness of the BHJ surface with the assistance of Pt decoration.The as-obtained FTO/BHJ（Pt）/Ru O₂photocathode delivered a photocurrent of-9.0 m A/cm² at 0 V vs.RHE with an onset potential as positive as 1.0 V vs.RHE,and the ABPE reaches 2.8%at 0.5 V vs.RHE,which is superior to other all-polymer-based photocathodes.The efficient electron extraction ability and catalytical property of Ru O₂,which makes a hole transfer layer and electron transfer layer insignificant in constructing the photoelectrode structure.A series of controlled experiments were conducted to determine the optimal processing condition of the simplified photocathodes,and the importance of uniform and proper loading of Ru O₂ layer on the performance of PEC was demonstrated.（3）Another three couples of BHJ（PM6:Y6,PM6:BTP-BO-4Cl,J71:ITIC）were used as photoactive layers to fabricate photocathodes（FTO/PM6:Y6（Pt）/Ru O₂,FTO/PM6:BTP-BO-4Cl（Pt）/Ru O₂ and FTO/J71:ITIC（Pt）/Ru O₂）by the same method,and their PEC performance were studied.Therein,the as-obtained FTO/PM6:Y6（Pt）/Ru O₂photocathode achieves the highest photocurrent density of-15m A/cm² at 0 V vs.RHE for HER compared with previous reported BHJ based photocathodes.The ABPE is up to 3.7%at 0.4 V vs.RHE.Moreover,the electrochemical impedance spectroscopy（EIS）and open circuit potential（OCP）characterization further proved that the uniform Ru O₂ layer plays important role in charge extraction,transfer,and catalysis.（4）A novel strategy to achieve electron-withdrawing group modification of OSs by self-photocatalysis was proposed.A cheap and mild sodium sulfite,PFDPABT,and lamps can be employed for the successful generation of sulfite radicals to perform the sulfonation in an aqueous solution.The rate of sulfonation can be controlled by adjusting the concentration of sulfite and light intensity.The as-obtained S-PFDPABT has a lower LUMO level,improved conductivity,and hydrophilicity,and switched photocurrent direction in a PEC cell.In addition,a reasonable mechanism of self-photocatalysed sulfonation was proposed according to the density functional theory（DFT）calculation and experimental characterization.Most importantly,the change of hydrophilicity before and after sulfonation can be applied in photoresist-free photolithography.In summary,the work in this thesis provides some basis for the development of new OSs materials by self-photocatalysis method,the search for photovoltaic electrode materials with excellent properties,and the rational design and optimization of photovoltaic electrodes.