Pure Organic Semiconductor‐Based Photoelectrodes for Water Splitting

Abstract

Currently, photoelectrochemical water‐splitting research is dominated by inorganic and organic–inorganic hybrid photoelectrodes. Although organic semiconductors have several advantages over their inorganic counterparts, the development of photoelectrodes based on pure organic materials has been lagging for the last decade. Recent improvements in molecular design, synthesis, and processing of organic semiconductors, such as polythiophenes, graphitic carbon nitrides, conjugated acetylenic polymers, alternating donor–acceptor‐conjugated polymers, and N‐containing fused‐ring small molecules, significantly enhance the performance of these photolectrodes without added cocatalysts. Although this research has been conducted over the past few years, this overlooked topic still stays unexplored, with more efficient materials yet to be discovered. Herein, the early development and recent advances of exclusively organic photoelectrodes for water splitting are summarized and brief perspectives for future improvements are provided.

Conjugated Acetylenic Polymers Grafted Cuprous Oxide as an Efficient Z‐Scheme Heterojunction for Photoelectrochemical Water Reduction

“…[ 1,2 ] In the last few years, numerous environmentally friendly, inexpensive, and efficient semiconductor photoelectrodes have been developed. [ 3–6 ] Up to now, the field of PEC water splitting is dominated by inorganic semiconductors consisting of earth abundant elements (e.g., silicon, [ 7–12 ] metal oxides, [ 13–16 ] metal sulfides, [ 17,18 ] dichalcogenide, [ 19 ] etc.). However, the high cost and poor stability of noble metal (e.g., Pt) as cocatalysts seriously restrict the practical applications of inorganic semiconductors for the PEC hydrogen evolution reaction (HER).…”

“…Compared with traditional inorganic semiconductors, organic semiconductors (e.g., graphitic carbon nitride [g‐C 3 N 4 or polyheptazine], [ 20–23 ] polythiophene, [ 24,25 ] conjugated covalent organic frameworks [COFs], [ 26 ] conjugated acetylenic polymers [CAPs] [ 27–29 ] ) have attracted increasing attentions benefitting from their tunable bandgaps, engineered band edge positions, and molecular‐level desirable active centers. [ 6 ] However, the PEC HER performance of current organic photocathodes falls far behind inorganic counterparts mainly due to their severe recombination of photoinduced holes and electrons.…”

“…Recently, diverse strategies have been explored for promoting the charge separation of organic semiconductors, mainly focusing on the heterojunction or homojunction construction. [ 6,23,24,28,30–32 ] Among them, the formation of a type‐II heterojunction, attributable to the effective separation of photoinduced electron–hole pairs through the band bending between two semiconductors, is one of the most facile ways for enhancing the PEC performance ( Figure a). [ 23,30,32 ] Unfortunately, for the type‐II heterojunction, the reduction and oxidation reactions occur under lower potentials, respectively, resulting in a significantly reduced redox activity.…”

High-performance and stable photoelectrochemical water splitting cell with organic-photoactive-layer-based photoanode

“…Moreover, an organic PEC system can work without applying bias by band position engineering, indicating that a solar cell is not required, which has been considered a requirement for PEC commercialization. In addition, organic semiconductors can be used for both photoanodes and photocathodes by simply converting their order of deposition [19][20][21][22] . Despite these merits, organic semiconductors have not attracted attention as photoactive materials for PEC water splitting due to their low stability in aqueous solutions 20 .…”

“…1). Furthermore, they lost their performances in a few minutes and even for the case of recent stable organic photoelectrodes stabilized by TiO 2 layers, less than 50% initial photocurrent is maintained after 1 h 21 .…”