Xianwei Zeng

Hydrothermal synthesis of ultrasmall CuCrO2 nanocrystal alternatives to NiO nanoparticles in efficient p-type dye-sensitized solar cells

In this study, we present a facile hydrothermal synthesis of ultrasmall delafossite CuCrO2 nanocrystals, with a typical size of 15 nm × 5 nm, and a high surface area of 87.86 m2 g−1, thermally stable up to 400 °C. The screen-printed CuCrO2 films sintered at different temperatures and under different atmospheres present different optical transmittances, tunable BET surface areas, dye adsorbing amounts, and tunable hole transport rates and hole recombination kinetics, which allows the optimization of the performance of p-type dye sensitized solar cells (DSSCs) based on CuCrO2 photocathodes. At optimized conditions, an open circuit voltage of 102 mV, a short circuit density of 0.491 mA cm−2, a fill factor of 0.398 and an overall photoconversion efficiency of 0.0194% were finally achieved for a coumarin 343 dye sensitized CuCrO2 solar cell. This record is acceptably high in comparison to a DSSC based on an NiO photocathode prepared and tested in similar experimental conditions. The light harvesting properties, charge collection capability, and flat-band potential of CuCrO2 photocathodes and the NiO photocathode have been critically compared.

Remarkable photocurrent of p-type dye-sensitized solar cell achieved by size controlled CuGaO2 nanoplates

In this paper, we report the successful hydrothermal synthesis of CuGaO2 nanoplates with a critically small size and deposition methods to fabricate effective p-type semiconductive photocathodes in dye-sensitized solar cells (DSCs). Based on an efficient P1 dye and an iodide electrolyte, the optimal CuGaO2 photocathode has achieved remarkably high photocurrent density, up to 2.05 mA cm−2. To the best of our knowledge, this is the highest achieved by nanocrystalline p-type semiconductors besides NiO. The light harvesting, charge collection in CuGaO2 and NiO based DSCs have been compared. Owing to size control of the CuGaO2 nanoplates and subsequent mechanical pressing for photocathode film deposition, the light harvesting efficiency of the CuGaO2 photocathode is greatly increased to a comparable level to that of the NiO reference. Another noteworthy feature is the remarkably high charge collection efficiency of the CuGaO2 photocathode, which should benefit from the nature of delafossite oxides with a high conductivity leading to a much higher hole diffusion coefficient in the DSC system. The open-circuit voltage is 199.3 mV, about twice as high as that of the NiO reference, benefiting from the valence band position shifting from −5.15 eV for NiO to −5.29 eV for CuGaO2versus the vacuum level.

Enhanced Performance of p‐Type Dye‐Sensitized Solar Cells Based on Ultrasmall Mg‐Doped CuCrO2 Nanocrystals

Herein, we present ultrasmall delafossite-type Mg-doped CuCrO2 nanocrystals prepared by using hydrothermal synthesis and their first application as photocathodes in efficient p-type dye-sensitized solar cells. The short-circuit current density (Jsc ) is notably increased by approximately 27% owing to the decreased crystallite size and the enhanced optical transmittance associated with Mg doping of the CuCrO2 nanocrystalline sample. An open-circuit voltage (Voc ) of 201 mV, Jsc of 1.51 mA cm(-2) , fill factor of 0.449, and overall photoconversion efficiency of 0.132% have been achieved with the CuCr0.9 Mg 0.1 O2 dye photocathode sensitized with the P1 dye under optimized conditions. This efficiency is nearly three times higher than that of the NiO-based reference device, which is attributed to the largely improved Voc and Jsc . The augmentation of Voc and Jsc can be attributed to the lower valance band position and the faster hole diffusion coefficient of CuCr0.9 Mg 0.1 O2 compared to those of the NiO reference, respectively, which leads to a higher hole collection efficiency.

Fine tuning of fluorene-based dye structures for high-efficiency p-type dye-sensitized solar cells.

We report on an experimental study of three organic push-pull dyes (coded as zzx-op1, zzx-op1-2, and zzx-op1-3) featuring one, two, and three fluorene units as spacers between donors and acceptors for p-type dye-sensitized solar cells (p-DSSC). The results show increasing the number of spacer units leads to obvious increases of the absorption intensity between 300 nm and 420 nm, a subtle increase in hole driving force, and almost the same hole injection rate from dyes to NiO nanoparticles. Under optimized conditions, the zzx-op1-2 dye with two fluorene spacer units outperforms other two dyes in p-DSSC. It exhibits an unprecedented photocurrent density of 7.57 mA cm(-2) under full sun illumination (simulated AM 1.5G light illumination, 100 mW cm(-2)) when the I(-)/I3(-) redox couple and commercial NiO nanoparticles were used as an electrolyte and a semiconductor, respectively. The cells exhibited excellent long-term stability. Theoretical calculations, impedance spectroscopy, and transient photovoltage decay measurements reveal that the zzx-op1-2 exhibits lower photocurrent losses, longer hole lifetime, and higher photogenerated hole density than zzx-op1 and zzx-op1-3. A dye packing model was proposed to reveal the impact of dye aggregation on the overall photovoltaic performance. Our results suggest that the structural engineering of organic dyes is important to enhance the photovoltaic performance of p-DSSC.

Synthesis and Characterization of CuAlO2 and AgAlO2 Delafossite Oxides through Low-Temperature Hydrothermal Methods

In this work, we present one-step low temperature hydrothermal synthesis of submicrometer particulate CuAlO2 and AgAlO2 delafossite oxides, which are two important p-type transparent conducting oxides. The synthesis parameters that affect the crystal formation processes and the product morphologies, including the selection of starting materials and their molar ratios, the pH value of precursors, the hydrothermal temperature, pressure, and reaction time, have been studied. CuAlO2 crystals have been synthesized from the starting materials of CuCl and NaAlO2 at 320-400 °C, and from Cu2O and Al2O3 at 340-400 °C, respectively. AgAlO2 crystals have been successfully synthesized at the low temperature of 190 °C, using AgNO3 and Al(NO3)3 as the starting materials and NaOH as the mineralizer. The detailed elemental compositions, thermal stability, optical properties, and synthesis mechanisms of CuAlO2 and AgAlO2 also have been studied. Noteworthy is the fact that both CuAlO2 and AgAlO2 can be stabilized up to 800 °C, and their optical transparency can reach 60%-85% in the visible range. Besides, it is believed the crystal formation mechanisms uncovered in the synthesis of CuAlO2 and AgAlO2 will prove insightful guildlines for the preparation of other delafossite oxides.

Boosting the Photocurrent Density of p-Type Solar Cells Based on Organometal Halide Perovskite-Sensitized Mesoporous NiO Photocathodes

The p-n tandem design of a sensitized solar cell is a novel concept holding the potential to overcome the efficiency limitation of conventional single-junction sensitized solar cells. Significant improvement of the photocurrent density (Jsc) of the p-type half-cell is a prerequisite for the realization of a highly efficient p-n tandem cell in the future. This study has demonstrated effective photocathodes based on novel organometal halide perovskite-sensitized mesoporous NiO in liquid-electrolyte-based p-type solar cells. An acceptably high Jsc up to 9.47 mA cm(-2) and efficiency up to 0.71% have been achieved on the basis of the CH3NH3PbI3/NiO solar cell at 100 mW cm(-2) light intensity, which are significantly higher than those of any previously reported liquid-electrolyte-based p-type solar cells based on sensitizers of organic dyes or inorganic quantum dots. The dense blocking layer made by spray pyrolysis of nickel acetylacetonate holds the key to determining the current flow direction of the solar cells. High hole injection efficiency at the perovskite/NiO interface and high hole collection efficiency through the mesoporous NiO network have been proved by time-resolved photoluminescence and transient photocurrent/photovoltage decay measurements. The limitation of these p-type solar cells primarily rests with the adverse light absorption by the NiO mesoporous film; the secondary limitation arises from the highly viscous ethyl acetate-based electrolyte, which is helpful for the solar cell stability but hinders fluent diffusion into the pore channels, giving rise to a nonlinear dependence of Jsc on the light intensity.

Selective laser sintering of TiO2 nanoparticle film on plastic conductive substrate for highly efficient flexible dye-sensitized solar cell application

In this paper, we report a novel selective laser sintering of TiO2 nanoparticle (Degussa P25) film on plastic conductive substrates for highly efficient flexible dye-sensitized solar cell (DSC) applications. The so-called “selective sintering” means that the absorbed laser energy can effectively promote the electrical contacts between the TiO2 nanoparticles, but does not cause damage to the plastic conductive substrate. The choice of the near-infrared (wavelength = 1064 nm) laser source is critical for the effectiveness of the laser sintering. The laser sintering technology can effectively decrease electron transport resistance and increase recombination resistance of the TiO2 nanoparticle film characterized by electrochemical impedance spectroscopy and transient photovoltage/photocurrent decay measurements, resulting in much improved charge collection efficiency. Thus, compared to the reference sample, the laser sintered film has achieved an improved short-circuit current density from 9.2 to 10.4 mA cm−2, fill factor from 0.71 to 0.77, and solar conversion efficiency from 4.5% to 5.7%. The fast and effective selective laser sintering technique has great potential to be integrated into scalable roll-to-roll manufacturing of highly efficient flexible DSCs.

Enhanced performance of p-type dye sensitized solar cells based on mesoporous Ni1−xMgxO ternary oxide films

A series of Ni1−xMgxO (x = 0–0.2) oxide mesoporous films with p-type semi-conductivity prepared by surfactant directed self-assembly method have been successfully applied as photocathodes in a p-type dye sensitized solar cell (DSC) system. By gradually increasing the Mg content from 0 to 20% in the ternary oxides, the effective light harvesting efficiency increases monotonically, which is associated with the increased dye absorbing amount and improved optical transmittance, meanwhile the flat-band potential gradually increases, implying a continuous negative shift of the valance band position of the p-type semiconductors. The latter is closely related to the charge injection from dye to semiconductor and the photovoltage (Voc) of the solar cell. The overall power conversion efficiency is optimized for the Ni0.9Mg0.1O photocathode, which is significantly improved by about 85% from pure NiO. The enhanced performance is attributed to 34.4% increased photocurrent density (Jsc), 22.5% increased Voc, and 13.0% increased fill factor. These improvements can be explained by increased light harvesting, enhanced charge collection, and flat-band potential positive shift. On further increasing the Mg content in the ternary oxide to Ni0.8Mg0.2O, the valance band position is too deep and hinders efficient hole injection. Jsc of the corresponding solar cell decreases largely. This work proves that Ni0.9Mg0.1O is a superior alternative to NiO as a photocathode material in p-type DSCs.

Facile Tuning of the Size and Shape of TiO2 Nanorods Enables Optimiznation of Charge Collection Kinteicsfor Highly Efficient Dye-Sensitized Solar Cells

novel, high-yield, and cost-effective hydrothermal method for the preparation of single crystal-like anatase TiO2 nanorods (NRs) with specific {101} exposed crystal planes and preferred [001] growth direction, which is governed by the “oriented attachment” mechanism