Yuh Lang Lee

High-performance quantum dot-sensitized solar cells based on sensitization with CuInS2 quantum dots/CdS heterostructure

A high-performance quantum dot-sensitized solar cell (QDSSC) is reported, which consists of a TiO2/CuInS2-QDs/CdS/ZnS photoanode, a polysulfide electrolyte, and a CuS counter electrode. The sensitization process involves attaching presynthesized CuInS2 QDs (3.5 nm) to a TiO2 substrate with a bifunctional linker, followed by coating CdS with successive ionic layer adsorption and reaction (SILAR) and ZnS as the last SILAR layer for passivation. This process constructs a sensitizing layer that comprises CdS nanocrystals, closely packed around the earlier-linked CuInS2 QDs, which serve as the pillars of the layer. The CuS counter electrode, prepared via successive ionic solution coating and reaction, has a small charge transfer resistance in the polysulfide electrolyte. The QDSSC exhibits a short-circuit photocurrent (Jsc) of 16.9 mA cm−2, an open-circuit photovoltage (Voc) of 0.56 V, a fill factor of 0.45, and a conversion efficiency of 4.2% under one-sun illumination. The heterojunction between the CuInS2 QDs and CdS extends both the optical absorption and incident photon conversion efficiency (IPCE) spectra of the cell to a longer wavelength of approximately 800 nm, and provides an IPCE of nearly 80% at 510 nm. The high TiO2 surface coverage of the sensitizers suppresses recombination of the photogenerated electrons. This results in a longer lifetime for the electrons, and therefore, the high Voc value. The notably high Jsc and Voc values demonstrate that this sensitization strategy, which exploits the quantum confinement reduction and other synergistic effects of the CuInS2-QDs/CdS/ZnS heterostructure, can potentially outperform those of other QDSSCs.

In Situ Gelation of Electrolytes for Highly Efficient Gel‐State Dye‐Sensitized Solar Cells

However, the volatile nature of the organic solvents (e.g., acetonitrile and 3-methoxypropionitrile) used in the liquid electrolytes triggers the leakage and evaporation of the solvents under long-term use. Therefore, critical sealing techniques are required in assembling the liquid-type DSSCs. To solve these drawbacks, several attempts have been taken by replacing the liquid electrolytes with gel-type polymer electrolyte, room temperature ionic liquid, hole-transporting materials based on organic [ 3 ] or inorganic [ 4 ] semiconductors, and liquid electrolytes solidifi ed by cross-linked gelators. [ 5 , 6 ] However, the energy conversion effi ciencies of DSSCs using these materials are lower than that achieved by the liquid electrolyte, ascribed to the lower conductivity of charge in the solid and gel electrolytes compared to that in the liquid electrolyte. Among various alternatives for liquid electrolytes, polymer gel electrolyte (PGE) appears to have superior properties in terms of ionic conductivity and cell performance. For the PGE, the liquid electrolyte was trapped in a polymer matrix and, thereby, the solvent evaporation was inhibited. Although the polymer network may hinder the ion transfer, the ions can move freely in the cages of polymer network which is helpful to the ion conductivity of a PGE. For the practical application of PGEs, the high viscosity of gel-electrolytes makes it diffi cult for them to penetrate into the pores of mesoporous TiO 2 electrodes. Furthermore, the injection of a PGE should be operated at elevated temperature to decrease the viscosity of the gel-electrolyte, which is diffi cult to apply on module cells with much larger working area. To solve this problem, in situ polymerization of pre-penetrated monomers has been developed either by thermalor photopolymerization. However, in situ polymerization of a monomer is diffi cult to carry out in a solution containing I 3 − /I − redox couple because radical intermediates are always deactivated by iodine. Several systems had been developed to prevent the inhibition of free radical by inhibitors. [ 7 , 8 ] A gel-state DSSC with an effi ciency of 7.72% was reported based on the use of chemically

Photoactive p-type PbS as a counter electrode for quantum dot-sensitized solar cells

A photoactive PbS film synthesized by successive cycles of coating with ionic solutions and reaction can function as a performance-promoting counter electrode for quantum dot-sensitized solar cells (QDSSCs). The PbS film has a wide absorption spectrum that extends to the near infra-red region, making it capable of absorbing the long-wavelength light that penetrates the photoanode of a QDSSC. Under simulated one-sun illumination, this PbS film exhibits a p-type photovoltaic response in a polysulfide electrolyte, showing a quasi-Fermi level shift of +0.25 V. For QDSSCs consisting of a TiO2/CuInS2/CdS/ZnS photoanode and a polysulfide electrolyte, the PbS film outperforms Pt and CuS films as a counter electrode even though CuS has a much higher electrocatalytic activity in the polysulfide electrolyte than PbS. The photoactive characteristics of the PbS electrode increase the photocurrent of the resulting QDSSC. The p-type conductivity of the PbS forms a partial tandem junction between the PbS and the anode, increasing the photovoltage and the fill factor. Under one-sun illumination, a QDSSC assembled with the photoactive p-type PbS counter electrode achieves a maximum power conversion efficiency of 4.7%, which is more than 15% greater than that of a cell assembled with the highly electrocatalytically active CuS.

Energy level alignment, electron injection, and charge recombination characteristics in CdS/CdSe cosensitized TiO2 photoelectrode

The band-edge levels of CdS-, CdSe-, and CdS/CdSe-sensitized TiO2 electrodes were determined by ultraviolet photoelectron spectroscopy (UPS) to explore the reason leading to the high performance of the TiO2/CdS/CdSe electrode. The obtained UPS results show the stepwise energy level in the TiO2/CdS/CdSe electrode, indicating energy level alignment occurrence between CdS and CdSe in the TiO2/CdS/CdSe. Time-resolved photoluminescence and open-circuit photovoltage decay experiments reveal that the photogenerated electrons in the TiO2/CdS/CdSe have higher injection efficiency, but lower recombination rate to the electrolyte, attributable to the stepwise structure of band-edge levels constructed by the effect of the energy level alignment.The band-edge levels of CdS-, CdSe-, and CdS/CdSe-sensitized TiO2 electrodes were determined by ultraviolet photoelectron spectroscopy (UPS) to explore the reason leading to the high performance of the TiO2/CdS/CdSe electrode. The obtained UPS results show the stepwise energy level in the TiO2/CdS/CdSe electrode, indicating energy level alignment occurrence between CdS and CdSe in the TiO2/CdS/CdSe. Time-resolved photoluminescence and open-circuit photovoltage decay experiments reveal that the photogenerated electrons in the TiO2/CdS/CdSe have higher injection efficiency, but lower recombination rate to the electrolyte, attributable to the stepwise structure of band-edge levels constructed by the effect of the energy level alignment.

Preparation of highly efficient gel-state dye-sensitized solar cells using polymer gel electrolytes based on poly(acrylonitrile-co-vinyl acetate)

A highly efficient gel-state electrolyte was fabricated by using poly(acrylonitrile -co-vinyl acetate) (PAN–VA) as the gelator of an 3-methoxypropionitrile (MPN)-based liquid electrolyte and was applied in dye-sensitized solar cells (DSSCs). The VA segaments act to dissolve the copolymer into the electrolyte, forming a gel-state structure. The electric conductivity of the gel-state electrolyte is comparable to that of the liquid electrolyte, attributed to the enhancement effect of the AN segments to the dissociation of LiI and 1-propyl-2,3-dimethylimidazolium iodide (DMPII). This effect also leads to a slightly downward shift of the TiO2 conduction band edge toward positive potentials. The energy conversion efficiency of the DSSC achieved by using this gel-electrolyte is 8.34%, which is 97% the value of the liquid-state cell (8.63%).

Hierarchically structured superhydrophobic coatings fabricated by successive langmuir-blodgett deposition of micro-/nano-sized particles and surface silanization

The present study demonstrates the creation of a stable, superhydrophobic surface by coupling of successive Langmuir-Blodgett (LB) depositions of micro-xa0and nano-sized (1.5xa0µm/50xa0nm, 1.0xa0µm/50xa0nm, and 0.5xa0µm/50xa0nm) silica particles on a glass substrate with the formation of a self-assembled monolayer of dodecyltrichlorosilane on the surface of the particulate film. Particulate films, in which one layer of 50xa0nm particles was deposited over one to five sublayers of larger micro-sized particles, with hierarchical surface roughness and superhydrophobicity, were successfully fabricated. Furthermore, the present two-scale (micro-xa0and nano-sized particles) approach is superior to the previous one-scale (micro-sized particles) approach in that both higher advancing contact angle and lower contact angle hysteresis can be realized. Experimental results revealed that the superhydrophobicity exhibited by as-fabricated particulate films with different sublayer particle diameters increases in the order of 0.5xa0µm>1.0xa0µm>1.5xa0µm. However, no clear trend between sublayer number and surface superhydrophobicity could be discerned. An explanation of superhydrophobicity based on the surface roughness introduced by two-scale particles is also proposed.

Highly efficient gel-state dye-sensitized solar cells prepared using poly(acrylonitrile-co-vinyl acetate) based polymer electrolytes

Poly(acrylonitrile-co-vinyl acetate) (PAN-VA) is utilized as a gelation agent to prepare gel-state electrolytes for dye-sensitized solar cell (DSSC) applications. Based on the synergistic effect of PAN-VA and TiO(2) fillers in the electrolyte, the gel-state DSSC can achieve a conversion efficiency higher than that of a liquid counterpart. The high performance of the gel-electrolyte is attributed to the in situ gelation property of the gel-electrolyte, the contribution of the PAN-VA to the charge transfer, as well as the enhancement effect of TiO(2) fillers on the charge transfer at the Pt-electrolyte interface. The experimental results show that the efficiencies of the gel-state cells have little dependence on the conductivity of the electrolytes with various contents of PAN-VA, but are closely related to the penetration situation of the electrolyte in the TiO(2) film. For PAN-VA concentrations ≤15 wt%, the electrolyte can be easily injected at room temperature based on its in situ gelation property. For higher PAN-VA concentrations, good penetration of the high viscous electrolyte can be achieved by elevating the operation temperature. By utilizing a heteroleptic ruthenium dye (coded CYC-B11), gel-state DSSCs with an efficiency of above 10% are obtained. Acceleration tests show that the cell is stable under one-sun illumination at 60 °C.

A hybrid PVDF-HFP/nanoparticle gel electrolyte for dye-sensitized solar cell applications

Graphite and TiO(2) nanoparticles are used as fillers to prepare a polymer gel electrolyte (PGE) based on I(-)/I(3)(-) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) for dye-sensitized solar cell (DSSC) applications. Graphite nanoparticles (GNP) were proved to be a more efficient filler than TiO(2) in enhancing the charge conductivity of the PGE, decreasing the activation energy for charge transport and inhibiting the charge recombination at the TiO(2)/electrolyte interface. The energy conversion efficiency of a DSSC fabricated using a PGE containing 0.25xa0wt% of GNP can be increased from 4.69% (without filler) to 6.04%, close to that of a liquid system obtained in this work.

Efficient water splitting over Na1−xKxTaO3 photocatalysts with cubic perovskite structure

This study improved the photocatalytic activity of NaTaO3 by replacing some Na ions in the 12-coordinate sites with larger K ions. Na1−xKxTaO3 photocatalysts of x = 0–0.2 were synthesized by the sol–gel method. K-doping at x = 0.05 resulted in rectifying the distorted perovskite NaTaO3 to a pseudo-cubic phase as well as significantly promoting the photocatalytic activity. The 180° bond angle of Ta–O–Ta in the pseudo-cubic phase may facilitate the separation of photogenerated charges for effective water splitting. Photoluminescence spectroscopic analysis confirmed that the flattened Ta–O–Ta linkage with K-doping suppresses the recombination of photogenerated charges. Further K-doping (with x > 0.05) leads to impurity formation, which bends the Ta–O–Ta linkage and creates defect states, lowering the photocatalytic activity of the K-doped NaTaO3. This study demonstrates that an appropriate ion replacement to tune the crystal structure can significantly promote electron transport in photocatalysts and thus their activity.

Sensitivity Enhancement for Quantitative Electrochemical Determination of a Trace Amount of Accelerator in Copper Plating Solutions

An accelerator is an indispensable organic additive for the bottom-up filling of copper electroplating in nano- or micro-scale features. However, its effective concentration is too low to be easily determined and controlled. Herein, a new electrochemical analysis method based on self-assembly monolayers of thiol molecules on a gold electrode was developed to accurately determine a trace amount of accelerator. The accelerator employed in copper plating solutions is bis-(3-sulfopropyl) disulfide (SPS), which is the most common accelerator for the filling of vias and trenches of interconnects. The SPS concentration in copper plating solutions ranged from 0.3 to 9.0 ppm. Following selective chemisorption of SPS onto the gold electrode, the SPS-modified gold electrode was transferred into a specific electrolyte composed of CuSO 4 , H 2 SO 4 , polyethylene glycol and chloride ions to run cyclic voltammetry (CV) for copper deposition and stripping. A specific peak current of copper reduction formed in the CV, and its peak area depended on the SPS concentration in the copper plating solution. A good linear calibration line was obtained by using this electrochemical analysis method, which can determine a trace amount of SPS in a concentration range of 0.3-1.0 ppm, which is a significant challenge for traditional instruments.