Dong-Xing Kou

Effect of Crystallization of Cu2ZnSnSxSe4–x Counter Electrode on the Performance for Efficient Dye-Sensitized Solar Cells

Cu2ZnSnSxSe4-x (CZTSSe) counter electrodes (CEs) in dye-sensitized solar cells (DSSCs) are commonly developed with porous structures, but their high surface area could also retard electron transport processes owing to the abundant grain boundaries. Herein, we employed a convenient solution method and a rapid heating process to prepare well crystalline CZTSSe CEs in DSSCs. The influence of crystallization of CZTSSe film on DSSCs performances was discussed in depth. The thermogravimetric analysis, phase morphology, conductivity, and electrochemical characteristics of CZTSSe films were performed. It is found that the rapid heating process is beneficial to the formation of well crystalline film with large grains. As the porosity and grain boundaries in the bulk film are dramatically reduced with the enhanced crystallization, the charge transport process is gradually improved. Using cyclic voltammogram and electrochemical impedance spectroscopy measurements, we propose that the accelerating charge transport is of great importance to the photovoltaic performances of DSSCs due to their superior electrocatalytic activities. As the highest cell efficiency was achieved, well crystalline CZTSSe is an efficient CE catalytic material.

Solution-deposited pure selenide CIGSe solar cells from elemental Cu, In, Ga, and Se

A novel, robust and low-toxicity solution route to deposit CIGSe thin films for solar cell applications is proposed. The solvent mixture of 1,2-ethanedithiol and 1,2-ethylenediamine is employed for the first time to simultaneously dissolve elemental Cu, In, Ga, and Se, forming the CIGSe precursor solution. With this solution-processed CIGSe thin film solar cell, a power conversion efficiency of 9.5% has been achieved.

p-type Li, Cu-codoped NiOx hole-transporting layer for efficient planar perovskite solar cells

p-type inorganic hole transport materials of Li, Cu-codoped NiOx films were deposited using a simple solution-based process. The as-prepared films were used as hole selective contacts for lead halide perovskite solar cell. An enhanced power conversion efficiency of 14.53% has been achieved due to the improved electrical conductivity and optical transmittance of the Li, Cu-codoped NiOx electrode interlayer.

Cu2ZnSnSe4 nanocrystals capped with S2− by ligand exchange: utilizing energy level alignment for efficiently reducing carrier rec ombination

In this work, we employed a convenient one-step synthesis method for synthesizing Cu2ZnSnSe4 (CZTSe) nanocrystals (NCs) in an excess selenium environment. This excess selenium situation enhanced the reaction of metal acetylacetonates with selenium, resulting in the burst nucleation of NCs at relatively low temperatures. The phase morphology and surface and optoelectronic properties of NCs before and after ligand exchange were discussed in depth. It was found that pure tetragonal-phase structure CZTSe NCs with approximately 1.7-eV bandgap could be synthesized. The removal of large organic molecules on CZTSe NCs after ligand exchange by S2− decreased the resistivity. The bandgap of the films after ligand exchange by 550°C selenization was also decreased due to better crystallinity. For potential application in CZTSe solar cells, we constructed an energy level diagram to explain the mutual effect between the absorption layer and CdS layer. Using cyclic voltammetry (CV) measurement, we found that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of CZTSe films shifted down after ligand exchange. After energy level alignment at the CdS/CZTSe interface, a type I band alignment structure was more conveniently formed after ligand exchange. This structure acted as the barrier against injection electrons from ZnO to the CZTSe layer, and recombination would subsequently be depressed.

Engineering of interface band bending and defects elimination via a Ag-graded active layer for efficient (Cu,Ag)2ZnSn(S,Se)4 solar cells

Although the substitution of Cu by Ag to suppress CuZn defects offers several advantages in overcoming the large open-circuit voltage (Voc) deficit for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells, an excellent performance has not been achieved to date primarily due to the Fermi level pinning at the CdS/absorber interface and large recombination at the absorber/Mo interface. Herein, we developed a composition grading strategy to achieve a V-shaped Ag-graded structure with a higher Ag content on both the back and front surfaces of the (Cu,Ag)2ZnSn(S,Se)4 (CAZTSSe) layer. The key advantages of this Ag-graded structure are as follows: the higher content towards the CdS/absorber interface can create weak n-type donor defects and retard Fermi level pinning, whereas the lower content at the interlayer maintains the conductivity and light absorption; moreover, the other higher content towards Mo back contact can effectively suppress the recombination and improve the utilization of long-wave incident light. By appropriately adjusting the Ag gradient, we demonstrated a significant increase in Voc, and an unexpected conversion efficiency of 11.2% was achieved. This is the highest efficiency achieved to date for Ag-substituted CZTSSe solar cells, and the result supports a new aspect that synthesis of a composition-graded CAZTSSe absorber has great potential for future research.

The effect of the selenization process on grain size and performance of CuIn(Sx,Se1−x)2 counter electrodes

CuIn(Sx,Se1−x)2 thin films were fabricated using S2− capped CuInS2 nanoparticles and a post-selenizing process. The different thermal annealing technologies in selenization resulted in obvious distinction in CuIn(Sx,Se1−x)2 grain size and degree of crystallinity. The nanoparticle-derived CuIn(Sx,Se1−x)2 thin films were used as counter electrodes in dye-sensitized solar cells and detailed analyses of the counter electrodes using current–voltage (J–V), cyclic voltammetry (CV), as well as electrochemical impedance spectroscopy (EIS) were conducted to investigate the influence of the grain size and crystallinity on the catalytic and electrical conductivity properties of the CuIn(Sx,Se1−x)2 thin films.

Application of quaternary Cu2ZnSnS4 quantum dot-sensitized solar cells based on the hydrolysis approach

The band-tunable quaternary alloys have more excellent photoelectric properties and stability than their binary or ternary components, but their application as sensitizers in quantum dot-sensitized solar cells (QDSSCs) has seldom been reported. The key feature of this problem is that the fairly small-sized quaternary quantum dots (QDs) require the use of 1-dodecanethiol (DDT) to suppress the growth of QDs, which cannot be displaced by a bifunctional molecular linker. Herein, we developed a novel synthesis and functionalization strategy for presynthesized Cu2ZnSnS4 (CZTS) QDs by utilizing mercapto-acetic acid octyl ester as the capping ligand. Unlike the commonly used ligand exchange approach, the long alkyl chains are removed via a hydrolysis procedure at pH > 7. Benefitting from the broad absorption spectral range, good loading ability and the improvement of the electron transport process after ligand hydrolysis, the constructed “green” CZTS QDSSCs finally achieved an impressive conversion efficiency of 3.29% with a high short-circuit current of 17.48 mA cm−2 without further modification. This efficiency is the first reported value for CZTS QDSSCs so far, which is comparable to most efficiencies for single species sensitizers of around 3%, and demonstrates that it is possible to obtain comparable or even better photovoltaic performance than the toxic cadmium or rare indium QDs.

ZnO nanotubes supported molecularly imprinted polymers arrays as sensing materials for electrochemical detection of dopamine

In this study, ZnO nanotubes (ZNTs) were prepared onto fluorine-doped tin oxide (FTO) glass and used as supports for MIPs arrays fabrication. Due to the imprinted cavities are always located at both inner and outer surface of ZNTs, these ZNTs supported MIPs arrays have good accessibility towards template and can be used as sensing materials for chemical sensors with high sensitivity, excellent selectivity and fast response. Using K3[Fe(CN)6] as electron probe, the fabricated electrochemical sensor shows two linear dynamic ranges (0.02-5μM and 10-800μM) towards dopamine. This proposed electrochemical sensor has been applied for dopamine determination with satisfied recoveries and precision. More complex human urine samples also confirmed that the proposed method has good accuracy for dopamine determination in real biological samples. These results suggest potential applicability of the proposed method and sensor in important molecule analysis.

Elemental Precursor Solution Processed (Cu1–xAgx)2ZnSn(S,Se)4 Photovoltaic Devices with over 10% Efficiency

The partial substitution of Cu+ with Ag+ into the host lattice of Cu2ZnSn(S,Se)4 thin films can reduce the open-circuit voltage deficit (Voc,deficit) of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. In this paper, elemental Cu, Ag, Zn, Sn, S, and Se powders were dissolved in solvent mixture of 1,2-ethanedithiol (edtH2) and 1,2-ethylenediamine (en) and used for the formation of (Cu1-xAgx)2ZnSn(S,Se)4 (CAZTSSe) thin films with different Ag/(Ag + Cu) ratios. The key feature of this approach is that the impurity atoms can be absolutely excluded. Further results indicate that the variations of grain size, band gap, and depletion width of the CAZTSSe layer are generally determined by Ag substitution content. Benefiting from the Voc enhancement (∼50 mV), the power conversion efficiency is successfully increased from 7.39% (x = 0) to 10.36% (x = 3%), which is the highest efficiency of Ag substituted devices so far.

Synthesis of metastable wurtzite CuInS2 nanocrystals and films from aqueous solution

For the first time, metastable wurtzite CuInS2 nanocrystals and films were successfully synthesized from aqueous solution under atmospheric conditions. The CuInS2 nanocrystals were prepared by stirring and reflux of a metal salt and sodium sulfide in aqueous solution containing thioglycolic acid as a stabilizer. The good absorption in the visible light region may find interesting application in solar cells. The I–V curve indicated that the wurtzite CuInS2 NCs favored the generation of photoinduced carrier and electronic transmission. The proposed synthesis strategy may be used as a general process for the synthesis of hydrophilic chalcogenide semiconductor NCs and is scalable for commercial production with minimal environmental impact.