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Liu et al. Microstructures 2023;3:2023001 https://dx.doi.org/10.20517/microstructures.2022.23 Page 11 of 21

understand the origin and decay of activity, more control experiments are required to investigate the effect
of CD structure on PEC performance.

Organic hole transfer materials
Currently, because of the tunable electrical properties of organic semiconductors or organic molecular
materials, the construction of inorganic/organic composite photoanode has attracted increasing attention.
Organic long-chain polymer hole transport materials are widely used in solar cells, such as spiro-OMeTAD
(2,2′,7,7′-tetrakis(N,N-p-dimethoxyphenylamino)-9,9′-spirobifluorene) and PTAA (poly-[bis(4-
[74]
phenyl)(2,4,6-trimethylphenyl)amine]) . In addition, some research has shown that polyimide polymers
have high thermal and chemical stability. Polyimide film electrodes with the semiconductor characteristics
of a narrow band gap and suitable band structure have good catalytic ability and stability for the OER in
strongly alkaline electrolytes. Thus, the integration of semiconductors with functional polymer layers can be
used to achieve efficient and stable photoelectrodes under alkaline conditions. Gao et al. coated a metal-free
[75]
poly(p-phenylene pyromellitimide) (PI) film on a BiVO photoanode by in-situ polymerization . The PI
4
film not only acts as a good OER catalytic layer but also promotes the transfer of photogenerated holes on
the surface of the photoanode. The photocurrent density of the obtained PI/BiVO was increased by ~2.5
4
times compared to a pristine BiVO photoanode . Gu et al. reported a metal-free bifunctional polyaniline
[75]
4
(PANI)/CD electrocatalyst capable of producing hydrogen under light, in which PANI as a p-type
semiconductor was used to solve the problem of insufficient protons .
[76]
Small molecular materials that are soluble in water or polar solvents are more suitable for photoelectric
catalytic systems. Currently, many research works have reported that organic hole-transporting molecular
materials can be used to modify photoanodes as cocatalysts for hole modulation to improve charge
separation efficiencies, such as ferrocene ligands, phenothiazine (PTZ) and trifluoroacetic acid [77-80] . A
suitable hole-accepting ligand should satisfy the energy requirement for continuous hole transfer and
interact closely with the photoelectrode to enable ultrafast hole transfer. Niu et al. systematically studied the
role of carbazole-derived hole transport molecules anchoring on the surface of a CdS QD/TiO film . As
[80]
2
shown in Figure 7, dithiol-functionalized carbazole and a ruthenium coordination compound (RubdaS)
were used as a hole transfer molecule and OER catalyst, respectively. These results demonstrated the
importance of hole transport molecules for rapid hole transfer. A series of hole transport molecules,
including carbazole, triphenylamine (TPA) and PTZ, were further anchored on the surface of a BiVO /CdS
4
photoanode. The photogenerated holes from the photoanode can be extracted rapidly by these molecules.
Among these different hole transport molecules, the obtained BiVO /CdS-TPA/CoBi exhibited an ultralow
4
onset potential of 0.15 V vs. a RHE and the highest photocurrent density of 5.2 mA/cm . Wu et al.
2[81]
reported the transfer kinetics of photogenerated holes in CdS nanorods (NRs) by PTZ molecules adsorbed
on the surface . In the presence of PTZ, the trapped holes are transferred to the PTZ to form PTZ radical
+
[82]
cations and this hole transfer rate is found to be much faster than the slow electron-trapping hole
recombination in CdS NRs. This proved that the adsorbed PTZ can effectively extract trapped holes in CdS
NRs . In addition, Li et al. fabricated CdSe QDs modified with PTZ hole-accepting ligands and found that
[82]
the PTZ modification of the CdSe QDs could significantly enhance the PEC H evolution efficiency .
[83]
2
RESEARCH METHODS FOR KINETICS OF HOLE TRANSFER
In the semiconductor photocatalytic process, the lifetimes of the kinetic processes, such as migration,
transport and recombination, generally range from a few nanoseconds to a few microseconds, whereas the
timescales of photocatalytic reaction process are microseconds to seconds. Therefore, to effectively study the
transport and transfer of charges in photoelectrodes, time-resolved analytical testing techniques are
necessary.