Liguo Wei

An Insight into the Role of Oxygen Vacancy in Hydrogenated TiO2 Nanocrystals in the Performance of Dye-Sensitized Solar Cells

Hydrogenated titanium dioxide (H-TiO2) nanocrystals were successfully prepared via annealing TiO2 in H2/N2 mixed gas flow at elevated temperatures ranging from 300 to 600 °C. Electron paramagnetic resonance (EPR) spectra were used to determine the produced oxygen vacancy in H-TiO2. Variations in temperature were studied to investigate the concentration change of oxygen vacancy in H-TiO2. The H-TiO2 nanocrystals prepared at different temperatures were employed into photoanodes sensitized by N719 dye and found to have exceptional effect on the solar-to-electric energy conversion efficiency (η). Photoanodes with H-TiO2 nanocrystals hydrogenated at 300 °C show the highest short-circuit current density (Jsc) of 18.92 mA cm(-2) and photoelectrical conversion efficiency of 7.76% under standard AM 1.5 global solar irradiation, indicating a 27 and 28% enhancement in Jsc and η, respectively, in comparison to those with TiO2. The enhancement is attributed to high donor density, narrow band gap and positive shift of flat band energy (Vfb) of H-TiO2 that promote the driving force for electron injection. Intensity-modulated photocurrent spectroscopy (IMPS) accompanied by intensity-modulated photovoltage spectroscopy (IMVS) and other analyses were applied to shed more light on the fundamental mechanisms inside the charge transfer and transport in these systems.

Enhanced Near-Infrared to Visible Upconversion Nanoparticles of Ho3+-Yb3+-F– Tri-Doped TiO2 and Its Application in Dye-Sensitized Solar Cells with 37% Improvement in Power Conversion Efficiency

New near-infrared (NIR)-to-green upconversion nanoparticles of Ho(3+)-Yb(3+)-F(-) tridoped TiO2 (UC-F-TiO2) were designed and fabricated via the hydrosol-hydrothermal method. Under 980 nm NIR excitation, UC-F-TiO2 emit strong green upconversion fluorescence with three emission bands at 543, 644, and 751 nm and convert the NIR light in situ to the dye-sensitive visible light that could effectively reduce the distance between upconversion materials and sensitizers; thus, they minimize the loss of the converted light. Our results show that this UC-F-TiO2 offers excellent opportunities for the other types of solar cells applications, such as organic solar cells, c-Si solar cells, multijunction solar cells, and so on. When integrating the UC-F-TiO2 into dye-sensitized solar cells (DSSCs), superior total energy conversion efficiency was achieved. Under AM1.5G light, open-circuit voltage reached 0.77 ± 0.01 V, short-circuit current density reached 21.00 ± 0.69 mA cm(-2), which resulted in an impressive overall energy conversion efficiency of 9.91 ± 0.30%, a 37% enhancement compared to DSSCs with pristine TiO2 photoanode.

Enhance the performance of dye-sensitized solar cells by co-sensitization of 2,6-bis(iminoalkyl)pyridine and N719

Three organic dyes 2,6-bis(iminoalkyl)pyridines [2,6-(2,6-R2C6H2NCMe)2]C5H3N (R = methyl, ethyl, isopropyl) (named DM, DE and DP, respectively) were synthesized and assembled onto nanocrystalline TiO2 film to prepare 2,6-bis(iminoalkyl)pyridine/N719 co-sensitized photoelectrodes for dye-sensitized solar cell (DSSCs) applications, and their photoelectrochemical performances were studied. In each of the three composite electrodes, the aggregation of N719 was alleviated and its spectra response was enhanced by the 2,6-bis(iminoalkyl)pyridines in region of 400 to 750 nm, and the total resistance of DSSCs was decreased after co-sensitizing with 2,6-bis(iminoalkyl)pyridine, all of which facilitate to improve efficiency of DSSCs. The optimized cell which was co-sensitized with DM gave a short circuit current density of 16.57 mA cm−2, an open circuit voltage of 0.72 V and a fill factor of 0.59 corresponding to an overall conversion efficiency of 7.00% under standard global AM1.5 solar irradiation conditions, which is 28.91% higher than that for DSSCs only sensitized by N719 (5.43%). The incident photo-to-current conversion efficiency, electrochemical impedance spectroscopy, surface photovoltage spectroscopy and UV-visible adsorption spectra were used to analyze their photoelectrochemical performances, and X-ray single-crystal structural analysis was used to analyze the structures of 2,6-bis(iminoalkyl)pyridines.

Tunable Luminescence and Application in Dye-Sensitized Solar Cells of Zn(II)/Hg(II) Complexes: Methyl Substitution-Induced Supramolecular Structures Based on (E)-N-(6-Methoxypyridin-2-ylmethylene)arylamine Derivatives

Using Schiff-base ligands (E)-N-(6-methoxypyridin-2-yl)(CH═NAr) (where Ar = C6H5, L1; 2-MeC6H4, L2; 2,4,6-Me3C6H2, L3), six Zn(II)/Hg(II) complexes, namely, [ZnL1Cl2] (Zn1), [HgL1Cl2] (Hg1), [ZnL2Cl2] (Zn2), [HgL2Cl2] (Hg2), [ZnL3Cl2] (Zn3), and [HgL3Cl2] (Hg3) have been synthesized under solvothermal conditions. The structures of six complexes have been established by X-ray single-crystal analysis and further physically characterized by EA, FT-IR, (1)H NMR, and ESI-MS. The crystal structures of these complexes indicate that noncovalent interactions, such as hydrogen bonds, C-H···Cl, and π···π stacking, play essential roles in constructing the resulting supramolecular structures (1D for Hg3; 2D for Zn2, Hg2; 3D for Zn1, Hg1, and Zn3). Upon irradiation with UV light, the emission of complexes Zn1-Zn3 and Hg1-Hg3 could be finely tuned from green (480-540 nm) in the solid state to blue (402-425 nm) in acetonitrile solution. It showed that the ligand and metal cation can influence the structures and luminescence properties of complexes such as emission intensities and maximum wavelengths. Since these ligands and complexes could compensate for the absorption of N719 in the low-wavelength region of the visible spectrum and reduce charge recombination of the injected electron, the ligands L1-L3 and complexes Zn3/Hg3 were employed to prepare cosensitized dye-sensitized solar cells devices for investigating the influences of the electron-donating group and coordination on the DSSCs performance. Compared to DSSCs only being sensitized by N719, these prepared ligands and complexes chosen to cosensitize N719 in solar cell do enhanced its performance by 11-41%. In particular, a DSSC using L3 as cosensitizer displays better photovoltaic performance with a short circuit current density of 18.18 mA cm(-2), corresponding to a conversion efficiency of 7.25%. It is much higher than that for DSSCs only sensitized by N719 (5.14%).

Band edge movement in dye sensitized Sm-doped TiO2 solar cells: a study by variable temperature spectroelectrochemistry

Pure TiO2 and 8 at% Sm-doped TiO2 nanoparticles are prepared via a novel hydrolysis followed by a hydrothermal process at 473 K for 24 h and successfully used in the photoanode of dye sensitized solar cells (DSSCs). The performance of DSSCs based on 8 at% Sm-doped TiO2 is significantly better compared to DSSCs based on undoped TiO2. The Jsc is 14.53 mA cm−2 and η is 6.78%, which is 15% and 5% higher than that of DSSCs based on undoped TiO2, respectively. The results of a variable temperature spectroelectrochemistry study show that the conduction band edge of 8 at% Sm-doped TiO2 shifts positively. The lower conduction band position enhances the driving force of electrons and improves the electron injection efficiency from the lowest unoccupied molecular orbital (LUMO) of the dye to the conduction band (CB) of TiO2, and the narrower band gap expands the response in the visible region and increases the utilization percentage of sunlight. These all contribute to enhancing the performance of cells based on an 8 at% Sm-doped TiO2 photoanode. The as prepared Sm-doped TiO2 material is proven in detail to be a better photoanode material than pure TiO2.

Effect of different donor groups in bis(6-methoxylpyridin-2-yl) substituted co-sensitizer on the performance of N719 sensitized solar cells

Three bis(6-methoxylpyridin-2-yl) substituted pyridine-anchor co-sensitizers with different donor groups are synthesized by 1,2-diaminobenzene (named L4), 1,2-diaminocyclohexane (named L5) and 1,4-butanediamine (named L6) with the aim to obtain rigid, semi-rigid and flexible co-sensitizer molecules, respectively. Their adaptability in N719 sensitized solar cells as co-sensitizers and the effects of their molecular rigidity, conjugation and co-planarity on the performance of DSSCs are studied. The results show that these three co-sensitizers are suitable to use in N719 sensitized solar cells. However, due to the different donor groups in the molecular structure, the co-sensitization performance of the rigid co-sensitizer with a large conjugate system is better than that of semi-rigid and flexible co-sensitizers. A short circuit current density of 13.27 mA cm−2, an open circuit voltage of 0.73 V and a fill factor of 0.63 corresponding to an overall conversion efficiency of 6.16% under AM 1.5G solar irradiation were achieved when rigid L4 was used as the co-sensitizer, which is 30% higher than that for DSSCs only sensitized by N719 (5.37%) under the same conditions. Mechanistic investigations are carried out by various spectral and electrochemical characterizations.

Reduced graphene oxide modified TiO2 semiconductor materials for dye-sensitized solar cells

Reduced graphene oxide (rGO) was prepared by reduction of graphene oxide (GO) under mild conditions and rGO/TiO2 composite semiconductor materials were prepared by mixing rGO into TiO2 paste and were used to deposit the photoanode films of dye sensitized solar cells (DSSCs). After preparation, the successful synthesis of rGO was confirmed using X-ray diffraction (XRD) and Raman spectroscopy analysis. The effect of rGO content on the performance of dye-sensitized solar cells was also investigated. After the addition of rGO, the photoanodes displayed enhanced dye adsorption properties with lower internal resistances, faster electron transport and lower charge recombination rate, which resulted in a high current density. At the optimum rGO concentration, the DSSC exhibited a Jsc of 15.23 mA cm−2, a Voc of 0.71 V, and a FF of 0.62 with the energy conversion efficiency (η) of 6.69%, indicating a increase in Jsc and η respectively with respect to that of a DSSC based on an unmodified TiO2 photoanode, which gives a Jsc of 13.56 mA cm−2, a Voc of 0.70 V, and a FF of 0.63 with a η of 5.97%. However, the addition of excess rGO weakened the crystallization of particles on the surface of the photoelectrodes, which led to the enhancement of charge recombination, the reduction of dye adsorption and the decrease of photoelectric conversion efficiency of DSSCs. The rGO modified TiO2 semiconductor materials could really enhance the efficiency of DSSCs after the optimal amount of rGO was successfully determined.

HONH3Cl optimized CH3NH3PbI3 films for improving performance of planar heterojunction perovskite solar cells via a one-step route

Planar heterojunction perovskite solar cells (PHJ-PSCs) constructed with one-step precursor solution spin-coating deposition (OPSSD) usually give an extremely low performance mainly due to the poor morphology and low crystallinity of the perovskite films. In this work, by incorporating a suitable HONH3Cl additive in the perovskite precursor solution, a high quality perovskite film with improved morphology and crystallinity was obtained. The UV-vis measurement of the CH3NH3I solutions without and with HONH3Cl demonstrates that the improved quality of the perovskite film can be easily attributed to a combined effect of N2, I2, H2O and CH3NH3Cl originating from the oxidation of CH3NH3I triggered by the HONH3Cl additive, which can manipulate the crystallization process of the perovskite. Accordingly, the improved performance for the HONH3Cl-induced PHJ-PSCs can also be demonstrated. At the optimized molar ratio of 1 : 1 : 0.1 for PbI2 : CH3NH3I : HONH3Cl, the PHJ-PSCs exhibit an average power conversion efficiency (PCE) of 10.61 ± 0.51%, which is much higher than that of pristine 1 : 1 : 0 based cells without additive (7.21 ± 0.61%), and the best performing HONH3Cl-induced device can yield a PCE as high as 11.12% with a Jsc of 18.42 mA cm-2, Voc of 0.95 V and FF of 0.63. Introducing suitable HONH3Cl as an additive into the perovskite precursor solution is really an effective route to enhance the performance of the PHJ-PSCs via OPSSD.

Variable temperature spectroelectrochemistry study of silver-doped TiO2 and its influence on the performance of dye sensitized solar cells

Ag-doped TiO2 nanoparticles are prepared and used as semiconductor materials of photoanodes to improve the performance of dye sensitized solar cells (DSSCs). The results of variable temperature spectroelectrochemistry study show that the conduction band edge of Ag-doped TiO2 shifts positively, which enhances the driving force of electrons and improves the electron injection efficiency from the LUMO of the dye to the conduction band of TiO2. The deposition of Ag not only benefits efficient charge transfer but also could minimize the charge recombination process, resulting in a significant photocurrent enhancement. At the optimum Ag concentration of 1.2 at%, the DSSC exhibited a Jsc of 19.75 mA cm−2, a Voc of 0.73 V, and a FF of 0.57 with an energy conversion efficiency (η) of 7.34%, indicating a 57% and 30% increase in Jsc and η respectively than that of DSSC based on an undoped TiO2 photoanode, which gives a Jsc of 12.61 mA cm−2, a Voc of 0.75 V, and a FF of 0.59 with a η of 5.64%.

Luminescence properties of a Zn(II) supramolecular framework: easily tunable optical properties by variation of the alkyl substitution of (E)-N-(pyridine-2-ylethylidyne)arylamine ligands

A series of different alkyl substituted of Zn(II) complexes, [ZnL1Cl2] (Zn1), [ZnL2Cl2] (Zn2), [ZnL3Cl2] (Zn3), [ZnL4Cl2] (Zn4), [ZnL5Cl2] (Zn5) ((E)-N-(pyridine-2-yl)(CMeNPhR), where R = H, L1; 2-CH3, L2; 2,6-(CH3)2, L3; 2,4,6-(CH3)3, L4; 2-OCH3, L5) have been synthesized and characterized by single crystal X-ray diffraction, 1H NMR, FT-IR, and EA. The X-ray diffraction analyses revealed that although are all constructed by C–H⋯Cl/π hydrogen bonds and π⋯π interactions, the dimensions of these supramolecular frameworks complexes Zn1–Zn5 are quite different. Complexes Zn1 and Zn5 feature 3D 5-connected {46·64} and {44·66} topology structures, respectively, while complexes Zn2 and Zn3 feature 2D supramolecular layer with {63} topology structures. Complex Zn4 exhibits two different one-dimensional helix-shaped chains. Obviously, these results show that steric hindrance has a great impact on the final structures of the supramolecular architectures. Based on these varied structures caused by different alkyl substitutions, the emission maximum wavelengths of complexes Zn1–Zn5 can be tuned in a large range of 400–514 nm. The λem shift in the red direction after the substitution of alkyl is attributed to the HOMO–LUMO energy gap of complexes being effectively decreased due to the electron-donating ability of alkyl. These results are confirmed by the density functional theory calculations.