An asymmetric zinc porphyrin (ZnPy) derivative bearing one benzoic acid and three 3-pyridines as meso-position substituents (zinc-5-(4-carboxyphenyl)-10,15,20-tri(3-pridyl)porphyrin, ZnMT3PyP) was used to sensitize graphitic carbon nitride (g-C3N4) for visible-light-driven photocatalytic H2 production. It was found that ZnMT3PyP exhibits more excellent photosensitization and stability on g-C3N4 than its counterpart bearing one benzoic acid and three phenyls (zinc-5-(4-carboxyphenyl)-10,15,20-triphenylporphrin, ZnMTPP) under visible light (λ > 420 nm) irradiation even though they have very similar physicochemical properties such as optical absorption capacities and energy band structures. Especially, ZnMT3PyP-Pt/g-C3N4 gives an apparent quantum yield (AQY) up to 25.1% at λ = 420 nm light illumination, greater than that (11.6%) of ZnMTPP-Pt/g-C3N4. The differences in photosensitization and stability between ZnMT3PyP and ZnMTPP are mainly due to the substitution of 3-pyridine for the phenyls in ZnMTPP, which leads to the electron transfers between ZnMT3PyP and g-C3N4 faster than that between ZnMTPP and g-C3N4. The present results provide a new insight applying porphyrin derivatives to the photocatalytic H2 production and open up a new path for further improving the conversion efficiency of solar energy to hydrogen energy through molecular designing.Keywords: Graphitic carbon nitride; Photocatalytic hydrogen production; Pyridine substituent; Zinc porphyrin derivative;

•CuPc(tBu)4 as additive is first introduced into the perovskite precursor solution.•The effects of CuPc(tBu)4 additive on the devices' photovoltaic performance are investigated.•CuPc(tBu)4 additive can improve the quality of perovskite layer.•Power conversion efficiency of the planar perovskite solar cell is enhanced from 15.3% to 17.3%.Solution processable planar heterojunction perovskite solar cell has drawn much attention as a promising low-cost photovoltaic device, and much effort has been made to improve its power conversion efficiency by choosing appropriate additives for the perovskite precursor solution. Different to those additives reported, a soluble and thermal stable tert-butyl substituted copper phthalocyanine (CuPc(tBu)4) as additive is first introduced into the perovskite precursor solution of a planar perovskite solar cell that is fabricated via the one-step solution process. It is found that the pristine device without CuPc(tBu)4 additive exhibits a power conversion efficiency of 15.3%, while an extremely low concentration (4.4 × 10−3 mM) of CuPc(tBu)4 in the precursor solution leads to the corresponding device achieving an enhanced power conversion efficiency of 17.3%. CuPc(tBu)4 as an additive can improve the quality of perovskite layer with higher crystallinity and surface coverage, then resulting in enhanced light absorption and reduced charge recombination, and thus the better power conversion efficiency. The finding presented here provides a new choice for improving the quality of perovskite layer and the photovoltaic performance of the planar heterojunction perovskite solar cells.Download high-res image (231KB)Download full-size image

A series of asymmetric zinc porphyrin (ZnPy) derivatives bearing different external substituents were synthesized and used to sensitize Pt-loaded graphitic carbon nitride (Pt/g-C3N4) for photocatalytic H2 production. Among them, ZnPy-1 has one benzoic acid and three phenyls as peripheral substituents, while ZnPy-2, ZnPy-3, and ZnPy-4 contain one benzoic acid and three pseudo-pyridines with different N-atom positions. The experimental results indicate that the pseudo-pyridine substitution for the phenyls in ZnPy-1 lead to enhanced photosensitization with an order of ZnPy-1 < ZnPy-2 < ZnPy-3 < ZnPy-4 under visible light (λ > 420 nm) irradiation. ZnPy-4-sensitized Pt/g-C3N4 (ZnPy-4-Pt/g-C3N4) exhibits the best average H2 production activity of 524 μmol h−1 with an extremely high turnover number (TON) of 11 089 h−1, which is much higher than that (328 μmol h−1) of ZnPy-2-Pt/g-C3N4 with a TON of 6942 h−1. Also, ZnPy-4-Pt/g-C3N4 shows a much higher apparent quantum yield (AQY) of 32.3% than that (11.5%) of ZnPy-2-Pt/g-C3N4 under 420 nm monochromatic light irradiation. The different N-atom positions in the pseudo-pyridines result in different interactions of the ZnPy dyes with a sacrificial reagent, which then strongly influences the photoactivity for H2 production. The present results demonstrate the molecular engineering aspect of ZnPy dyes in which fine-tuning of molecular structures is crucial for improving the photocatalytic H2 production activity of dye-sensitized semiconductors.

Zinc phthalocyanine (ZnPc) derivatives containing bulky 2,6-diphenylphenoxy peripheral substituents with different structures and symmetries are used as sensitizers of graphitic carbon nitride (g-C3N4) for photocatalytic H2 production. It is found that an A2BC type asymmetrical ZnPc derivative (Zn-di-PcNcTh-1) containing four 2,6-diphenylphenoxy substituents, a thiophene unit and a fused benzene ring bearing one carboxylic group shows an obvious red-shift in the Q-band absorption with a broader absorption spectrum as compared to its symmetrical analogue (Zn-tetrad-Pc-1) containing eight 2,6-diphenylphenoxy substituents. The asymmetrical Zn-di-PcNcTh-1 with an additional carboxyl group exhibits a higher dye-loaded amount and stable grafting on g-C3N4 than the symmetrical Zn-tetrad-Pc-1, and thus causing more efficient interfacial electron transfer in Zn-di-PcNcTh-1/g-C3N4. Especially, an impressively high apparent quantum yield (3.05%) can be obtained under 730 nm monochromatic light irradiation, higher than that (1.14%) of Zn-tetrad-Pc-1 without a carboxyl group. The present results not only provide a significant advance in the molecular engineering aspect of ZnPc derivatives for effectively utilizing the red/near-IR light of sunlight, but also exhibits a promising strategy for improving the solar-to-hydrogen conversion efficiency.

•Novel asymmetric ZnPc derivative was prepared as dye of DSSCs.•Zn-tri-PcNc-9 with six diphenylthiophenol groups has broad Q-band absorption.•CDCA as coadsorbent can retard ZnPc molecule aggregates and charge recombination.•Zn-tri-PcNc-9-sensitized cell containing CDCA yields 3.61% efficiency.•S atoms in ZnPc’s substituents expand the solar cell’s red/near-IR responsive range.Asymmetric zinc phthalocyanine derivative (Zn-tri-PcNc-9) bearing one carboxylic and six bulky diphenylthiophenol groups was synthesized as a sensitizer for dye-sensitized solar cells. The obtained Zn-tri-PcNc-9 exhibits strong and expanded Q-band absorption peak (at ∼741 nm) in the red/near-IR light (600–800 nm) range, and its photovoltaic performance in sensitizing TiO2-based solar cell can be significantly improved by using chenodeoxycholic acid (CDCA) as co-adsorbent due to the retarded charge recombination. Under an optimal sensitization condition, the corresponding solar cell exhibits an efficiency of 3.61%, which is improved by 144% as compared to the solar cell without CDCA. Moreover, Zn-tri-PcNc-9-sensitized solar cell shows a maximum incident photo-to-current conversion efficiency of 31.6% at ∼730 nm, red-shifted by ∼20 nm as compared to that (∼710 nm) of its O-substituted analog (Zn-tri-PcNc-8) bearing six diphenylphenoxy groups, suggesting an effective solution to expand the red/near-IR responsive range of the ZnPc-sensitized solar cell, and also demonstrating a possibility for future panchromatic sensitizing agents in dye-sensitized solar cells.

A zinc phthalocyanine (ZnPc) derivative (Zn-tri-PcNc-8) containing tri-benzonaphtho-condensed porphyrazine with one carboxylic and six diphenylphenoxy peripheral substitutions was designed and synthesized as a sensitizer for dye-sensitized solar cells (DSSCs). For the purpose of extending the absorption spectra while minimizing the formation of ZnPc molecular aggregates, bulky 2,6-diphenylphenoxy groups were used as electron donor moieties, and the carboxylic group as an anchoring group to graft the sensitizer onto the semiconductor. It was found that a TiO2-based solar cell sensitized by Zn-tri-PcNc-8 shows a maximum incident photon-to-current conversion efficiency in the red/near-IR light range (650–750 nm), and a solar cell sensitized at near room temperature (30 °C) for 48 h exhibits the best efficiency (3.01%). The efficiency was much higher than that (1.96%) for a solar cell sensitized by its analogue (Zn-tri-PcNc-2) having one carboxyl and three tert-butyl groups without chenodeoxycholic acid (CDCA), indicating that the introduction of six bulky diphenylphenoxy substitutions with large steric hindrance in the ZnPc macrocycle can effectively suppress the molecular aggregates, thus resulting in an improved conversion efficiency. The present results shed light on an effective solution to adjust the ZnPc property via chemical modification such as changing the “push–pull” effect and adding large steric hindrance substituents to further improve the efficiency of the phthalocyanine-sensitized solar cell.

•Novel asymmetric ZnPc derivatives were prepared as dye of DSSCs.•Zn-tri-PcNc-5 with six diphenylthiophenol groups shows more Q-band redshift than Zn-tri-PcNc-4.•Zn-tri-PcNc-4 with six diphenylphenoxy groups yielded 3.22% efficiency.•Zn-tri-PcNc-4-sensitized cell has higher efficiency than Zn-tri-PcNc-5-sensitized one.Asymmetric ZnPc derivatives with two carboxyl and six diphenylphenoxy or diphenylthiophenol groups were synthesized as dye of DSSCs. Those ZnPcs exhibit strong red/near-IR light absorption, and Zn-tri-PcNc-5 with six diphenylthiophenol groups shows obvious redshift in the Q-band and enhanced absorbance compared to Zn-tri-PcNc-4 with six diphenylphenoxy groups, while Zn-tri-PcNc-4 yielded a 3.22% efficiency in sensitizing TiO2-based solar cell, much higher than that (1.30%) of the S-substituted analog (Zn-tri-PcNc-5). The decreased efficiency of Zn-tri-PcNc-5 is due to the molecular orbital shift to negative direction, stemmed from S atoms instead of O atoms in the six substituents of Zn-tri-PcNc-4, which leads to insufficient driving force for the electron injection. The present results demonstrate that the optimization of molecular orbital levels of ZnPcs via changing the substituents’ “push–pull” effect is an effective approach to improve the ZnPc-sensitized cell performance.

•A theoretical investigation of ZnPca, ZnPcb and ZnPcc has been conducted.•The photovoltaic performances of the self-assembled complexes were investigated.•The n-butyoxyl groups bring significant influence on the properties of the complexes.•The theoretical research and the experiment prove the directional charge transfer.A theoretical investigation of self-assembled donor–acceptor dyads (ZnPca, ZnPcb and ZnPcc) formed by axial coordination of zinc phthalocyanines appended with 4-carboxyl pyridine has been conducted with the density functional theory (DFT) method and time-dependent DFT (TD-DFT) calculations. A comparison between the molecular structures, atomic charges, molecular orbitals, UV–vis spectra and infrared (IR) spectra has been studied. Further, as sensitizers for the TiO2-based dye-sensitized solar cells, the photovoltaic performances have been investigated. The ZnPcc-sensitized solar cell exhibits a higher conversion efficiency than the ZnPcb and ZnPca-sensitized ones under AM 1.5 G solar irradiation, while the ZnPca-sensitized cell performs the poorest due to the lack of peripheral substituents (n-butyoxyl groups) which can be confirmed by the result of the theoretical research. It shows that the directionality of charge transfer in the self-assembled donor–acceptor dyads is important and benefit for the efficiency of the DSSC.

Co-sensitization by using two or more dyes with complementary absorption spectra as sensitizers of a semiconductor to expand the spectral response range of dye-sensitized solar cells (DSSCs) is an effective approach to enhance device performance. To improve the light-harvesting capability in the near-infrared (IR) region, zinc phthalocyanine (Zn-tri-PcNc-1) was applied to combine with a D-π-A triarylamine–bithiophene–cyanoacrylate-based organic dye (DH-44) for fabrication of a co-sensitized solar cell. The resulting co-sensitized device shows an efficient panchromatic spectral response feature in the range of 320–750 nm and gives an overall conversion efficiency of 6.61%, which is superior to that of the individual dye-sensitized solar cells under standard AM 1.5G one sun irradiation. The above results represent a clear advance toward efficient and low-cost co-sensitized solar cells with a panchromatic spectral response.Keywords: Co-sensitization; dye; Dye-sensitized solar cell; Spectral responseD;

Zinc phthalocyanine (ZnPc) derivatives with asymmetric (Zn-tri-PcNc-2) or symmetric (Zn-tetrad-Nc) structure, which possess wide spectral response in the visible/near-IR light region, are synthesized and utilized as a sensitizer of graphitic carbon nitride (g-C3N4) with 0.5 wt% Pt-loading for photocatalytic H2 production. The experimental results indicate that Zn-tri-PcNc-2 exhibits much better photosensitization on g-C3N4 than Zn-tetrad-Nc under visible/near-IR light although Zn-tetrad-Nc possesses wider and stronger optical absorption property than Zn-tri-PcNc-2. Zn-tri-PcNc-2-Pt/g-C3N4 exhibits an average H2 production rate of 132 μmol h−1, which is much better than that (26.1 μmol h−1) of Zn-tetrad-Nc-Pt/g-C3N4 under visible-light (λ ≥ 500 nm) irradiation. Moreover, Zn-tri-PcNc-2-Pt/g-C3N4 also shows much higher apparent quantum yield (AQY) than Zn-tetrad-Nc-Pt/g-C3N4 under red/near-IR light irradiation. Especially, Zn-tri-PcNc-2-Pt/g-C3N4 exhibits impressively higher AQY (1.07%) than that (0.22%) of the Zn-tetrad-Nc-Pt/g-C3N4 under 700 nm monochromatic light irradiation. The much better photoactivity of Zn-tri-PcNc-2-Pt/g-C3N4 than Zn-tetrad-Nc-Pt/g-C3N4 is caused by the asymmetric structure of Zn-tri-PcNc-2, which can result in the electronic orbital directionality of its excited state, much faster photogenerated electron transfer to g-C3N4, and higher red/near-IR light utilization efficiency as compared to Zn-tetrad-Nc-Pt/g-C3N4. The present results provide an important insight into the effects of molecular structure and optical absorption property of phthalocyanine derivatives on the photoactivity of the dye-sensitized semiconductor, and also guide us to further improve the solar energy conversion efficiency by optimizing the molecular structure and effectively utilizing the visible/near-IR light of sunlight.

Novel highly asymmetric zinc tetraazaporphyrin (TAP) derivatives (Zn-tri-TAPNc and Zn-tri-PcNc) with one carboxyl and three tert-butyl peripheral substituent groups were synthesized. A highly asymmetric zinc phthalocyanine (ZnPc) derivative (Zn-tri-PcNc) has a benzo-annelated ring which contains tribenzonaphtho-condensed tetraazaporphyrin with the same peripheral substituents as Zn-tri-TAPNc. As a sensitizer for the TiO2-based dye-sensitized solar cell, Zn-tri-PcNc derived from the benzo-annelation of the TAP macrocycle showed improved light harvesting and electron injection efficiency, which can retard the charge recombination, resulting in a great improvement in the incident photon-to-current conversion efficiency (IPCE). The Zn-tri-PcNc-sensitized solar cell exhibited a higher conversion efficiency (2.89%) than the Zn-tri-TAPNc-sensitized one (1.20%) under AM 1.5G solar irradiation. The present results on the TAP macrocycle's benzo-annelation demonstrate that optimization of molecular structure via changing the peripheral substituent group's “push–pull” effect and enlarging the conjugated π-system is an effective approach to improve the performance of the tetraazaporphyrin-based dye-sensitized solar cell.

•Highly asymmetric zinc phthalocyanine derivative is used as dye of the solar cell.•Chenodeoxycholic acid (CDCA) as coadsorbent hinders the phthalocyanine aggregation.•Dye-adsorption temperature significantly impacts the solar cell's performance.•An optimal dye adsorption condition leads to efficient electron injection.•Efficiency of solar cell is improved by 47% as compared to the cell without CDCA.Highly asymmetric zinc phthalocyanine derivative (Zn-tri-PcNc) containing tribenzonaphtho-condensed porphyrazine with one carboxyl and three tert-butyl (t-Bu) substituent groups is used as a sensitizer to fabricate dye-sensitized solar cells (DSSCs), and the effects of chenodeoxycholic acid (CDCA) as a coadsorbent and the dye adsorption temperature on the solar cell's performance are investigated. It is found that CDCA coadsorption can hinder the dye aggregation, which is beneficial for improving the electron injection efficiency and retarding the charge recombination, and thus resulting in the enhancement of the short-circuit current density and open-circuit photovoltage. Moreover, the dye adsorption temperature on electrode also shows a significant impact on the photovoltaic performance of the solar cell, and an optimal dye adsorption condition of the TiO2 electrode is found to be 5 × 10−5 M Zn-tri-PcNc ethanol solution containing 7.5 mM CDCA at 5 °C, which can contribute to the maximum conversion efficiency of 2.89% with short-circuit current density of 9.42 mA cm−2 and open-circuit photovoltage of 0.48 V, improved by 47% as compared to the solar cell fabricated with TiO2 electrode sensitized by the zinc phthalcoyanine in absence of CDCA.

Although dye-sensitized semiconductors for photocatalytic H2 production have been reported for more than 20 years, the spectral response is still focused on the region of 400–600 nm and it is almost never extended to >600 nm. The present work successfully uses zinc phthalocyanine (Zn-tri-PcNc) and naphthalocyanine (Zn-tetra-Nc) to sensitize TiO2 for photocatalytic H2 production, which expands the spectral response of dye-sensitized semiconductors by harvesting light from 600 to 800 nm. Zn-tri-PcNc/TiO2 has a H2 production amount of 567.4 μmol with a turnover number (TON) of ∼7565, which is much higher than that of Zn-tetra-Nc/TiO2 (236.5 μmol H2 with a TON of ∼3153). In particular, Zn-tri-PcNc-sensitized TiO2 shows an apparent quantum yield (AQY) of up to 0.2% under 700 nm monochromatic light irradiation and ∼0.1% under monochromatic light irradiation between 500 and 800 nm, indicating its efficient visible/near-IR-light-driven photocatalytic H2 production. Future work on the panchromatic responsive dye-sensitized semiconductor system can be carried out using phthalocyanines co-sensitized with other dyes to utilize the whole visible/near-IR light of sunlight.

Dye-sensitized solar cells (DSSCs) were fabricated by using porous ZnO electrodes derived from home-made ZnO nanoparticles. Electrochemical impedance spectra and open-circuit photovoltage decay curves measurements were performed to investigate the photoelectrochemical characteristics of ZnO films annealed at different temperatures. The experimental results indicate that the effects of the bulk traps and the surface states within the ZnO films on the recombination processes of the photoinjected electrons in DSSCs depend on the annealing temperature. The DSSC based on the ZnO electrode annealed at 400 °C exhibits an optimal energy conversion efficiency of 3.92% under the illumination of one sun simulated sunlight because the farthest decrease in the effects of both bulk traps and surface states at this film can maintain a lower charge recombination probability. This result indicates that the ZnO film electrode has promising application in the field of DSSCs, and the optimization of porous film fabrication condition is efficient for the improvement of ZnO-based DSSC’s performances.

Dye-sensitized solar cells (DSSCs) were fabricated by using porous ZnO electrodes derived from home-made ZnO nanoparticles. Electrochemical impedance spectra and open-circuit photovoltage decay curves measurements were performed to investigate the photoelectrochemical characteristics of ZnO films annealed at different temperatures. The experimental results indicate that the effects of the bulk traps and the surface states within the ZnO films on the recombination processes of the photoinjected electrons in DSSCs depend on the annealing temperature. The DSSC based on the ZnO electrode annealed at 400 °C exhibits an optimal energy conversion efficiency of 3.92% under the illumination of one sun simulated sunlight because the farthest decrease in the effects of both bulk traps and surface states at this film can maintain a lower charge recombination probability. This result indicates that the ZnO film electrode has promising application in the field of DSSCs, and the optimization of porous film fabrication condition is efficient for the improvement of ZnO-based DSSC’s performances.

A highly asymmetric A2BC type zinc phthalocyanine (Zn-di-PcNcTh) has been designed and synthesized. The Zn-di-PcNcTh used a π electron rich thiophene ring in place of the benzenoid rings of phthalocyanine which acted as an electron donor, diphenylphenoxy substituents to retard aggregation and a carboxyl-naphthalene unit as an electron acceptor. The asymmetric phthalocyanine shows a strongly split Q-band and wide spectral absorption in the visible/near-IR light region, which can extend the spectral response region of graphitic carbon nitride (g-C3N4) from ∼450 nm to more than 800 nm. By using it as a sensitizer of 1.0 wt% Pt-loaded graphitic carbon nitride (g-C3N4), the experimental results indicate that Zn-di-PcNcTh-Pt/g-C3N4 shows a H2 production efficiency of 249 μmol h−1 with an impressive turnover number (TON) of 9960.8 h−1 under visible light (λ ≥ 420 nm) irradiation, much higher than that of pristine Pt/g-C3N4. Owing to the introduction of a highly bathochromic shift of 3,4-dicyanothiophene and the valuable “push–pull” effect from the thiophene (electron donor) to the carboxyl-naphthalene (electron acceptor) unit, Zn-di-PcNcTh/g-C3N4 gives an extremely high apparent quantum yield (AQY) of 2.44%, 3.05%, and 1.53% under 700, 730, and 800 nm monochromatic light irradiation, respectively, under optimized photocatalytic conditions.

A series of asymmetric zinc porphyrin (ZnPy) derivatives bearing different external substituents were synthesized and used to sensitize Pt-loaded graphitic carbon nitride (Pt/g-C3N4) for photocatalytic H2 production. Among them, ZnPy-1 has one benzoic acid and three phenyls as peripheral substituents, while ZnPy-2, ZnPy-3, and ZnPy-4 contain one benzoic acid and three pseudo-pyridines with different N-atom positions. The experimental results indicate that the pseudo-pyridine substitution for the phenyls in ZnPy-1 lead to enhanced photosensitization with an order of ZnPy-1 < ZnPy-2 < ZnPy-3 < ZnPy-4 under visible light (λ > 420 nm) irradiation. ZnPy-4-sensitized Pt/g-C3N4 (ZnPy-4-Pt/g-C3N4) exhibits the best average H2 production activity of 524 μmol h−1 with an extremely high turnover number (TON) of 11089 h−1, which is much higher than that (328 μmol h−1) of ZnPy-2-Pt/g-C3N4 with a TON of 6942 h−1. Also, ZnPy-4-Pt/g-C3N4 shows a much higher apparent quantum yield (AQY) of 32.3% than that (11.5%) of ZnPy-2-Pt/g-C3N4 under 420 nm monochromatic light irradiation. The different N-atom positions in the pseudo-pyridines result in different interactions of the ZnPy dyes with a sacrificial reagent, which then strongly influences the photoactivity for H2 production. The present results demonstrate the molecular engineering aspect of ZnPy dyes in which fine-tuning of molecular structures is crucial for improving the photocatalytic H2 production activity of dye-sensitized semiconductors.

Novel highly asymmetric zinc tetraazaporphyrin (TAP) derivatives (Zn-tri-TAPNc and Zn-tri-PcNc) with one carboxyl and three tert-butyl peripheral substituent groups were synthesized. A highly asymmetric zinc phthalocyanine (ZnPc) derivative (Zn-tri-PcNc) has a benzo-annelated ring which contains tribenzonaphtho-condensed tetraazaporphyrin with the same peripheral substituents as Zn-tri-TAPNc. As a sensitizer for the TiO2-based dye-sensitized solar cell, Zn-tri-PcNc derived from the benzo-annelation of the TAP macrocycle showed improved light harvesting and electron injection efficiency, which can retard the charge recombination, resulting in a great improvement in the incident photon-to-current conversion efficiency (IPCE). The Zn-tri-PcNc-sensitized solar cell exhibited a higher conversion efficiency (2.89%) than the Zn-tri-TAPNc-sensitized one (1.20%) under AM 1.5G solar irradiation. The present results on the TAP macrocycle's benzo-annelation demonstrate that optimization of molecular structure via changing the peripheral substituent group's “push–pull” effect and enlarging the conjugated π-system is an effective approach to improve the performance of the tetraazaporphyrin-based dye-sensitized solar cell.

A zinc phthalocyanine (ZnPc) derivative (Zn-tri-PcNc-8) containing tri-benzonaphtho-condensed porphyrazine with one carboxylic and six diphenylphenoxy peripheral substitutions was designed and synthesized as a sensitizer for dye-sensitized solar cells (DSSCs). For the purpose of extending the absorption spectra while minimizing the formation of ZnPc molecular aggregates, bulky 2,6-diphenylphenoxy groups were used as electron donor moieties, and the carboxylic group as an anchoring group to graft the sensitizer onto the semiconductor. It was found that a TiO2-based solar cell sensitized by Zn-tri-PcNc-8 shows a maximum incident photon-to-current conversion efficiency in the red/near-IR light range (650–750 nm), and a solar cell sensitized at near room temperature (30 °C) for 48 h exhibits the best efficiency (3.01%). The efficiency was much higher than that (1.96%) for a solar cell sensitized by its analogue (Zn-tri-PcNc-2) having one carboxyl and three tert-butyl groups without chenodeoxycholic acid (CDCA), indicating that the introduction of six bulky diphenylphenoxy substitutions with large steric hindrance in the ZnPc macrocycle can effectively suppress the molecular aggregates, thus resulting in an improved conversion efficiency. The present results shed light on an effective solution to adjust the ZnPc property via chemical modification such as changing the “push–pull” effect and adding large steric hindrance substituents to further improve the efficiency of the phthalocyanine-sensitized solar cell.

Zinc phthalocyanine (ZnPc) derivatives with asymmetric (Zn-tri-PcNc-2) or symmetric (Zn-tetrad-Nc) structure, which possess wide spectral response in the visible/near-IR light region, are synthesized and utilized as a sensitizer of graphitic carbon nitride (g-C3N4) with 0.5 wt% Pt-loading for photocatalytic H2 production. The experimental results indicate that Zn-tri-PcNc-2 exhibits much better photosensitization on g-C3N4 than Zn-tetrad-Nc under visible/near-IR light although Zn-tetrad-Nc possesses wider and stronger optical absorption property than Zn-tri-PcNc-2. Zn-tri-PcNc-2-Pt/g-C3N4 exhibits an average H2 production rate of 132 μmol h−1, which is much better than that (26.1 μmol h−1) of Zn-tetrad-Nc-Pt/g-C3N4 under visible-light (λ ≥ 500 nm) irradiation. Moreover, Zn-tri-PcNc-2-Pt/g-C3N4 also shows much higher apparent quantum yield (AQY) than Zn-tetrad-Nc-Pt/g-C3N4 under red/near-IR light irradiation. Especially, Zn-tri-PcNc-2-Pt/g-C3N4 exhibits impressively higher AQY (1.07%) than that (0.22%) of the Zn-tetrad-Nc-Pt/g-C3N4 under 700 nm monochromatic light irradiation. The much better photoactivity of Zn-tri-PcNc-2-Pt/g-C3N4 than Zn-tetrad-Nc-Pt/g-C3N4 is caused by the asymmetric structure of Zn-tri-PcNc-2, which can result in the electronic orbital directionality of its excited state, much faster photogenerated electron transfer to g-C3N4, and higher red/near-IR light utilization efficiency as compared to Zn-tetrad-Nc-Pt/g-C3N4. The present results provide an important insight into the effects of molecular structure and optical absorption property of phthalocyanine derivatives on the photoactivity of the dye-sensitized semiconductor, and also guide us to further improve the solar energy conversion efficiency by optimizing the molecular structure and effectively utilizing the visible/near-IR light of sunlight.