The Application Of Metal Sulfides And Their Carbon Modification In The Counter Electrodes Of Dye-sensitized Solar Cells

Due to their low cost, easy fabrication and excellent performance, dye-sensitized solar cells (DSSCs) have attracted widely academic and industrial attentions. The counter electrode (CE) is an important component of DSSC. Their price and performance directly affect the cost and photovoltaic efficiency of the cells. Conventionally, noble metal platinum is employed as CE. Platinum electrode possesses high electrocatlytic activity for I3- reduction, superior conductivity and chemical stability. However, rare storage and high price of platinum impede large-scale production of DSSCs. Therefore, the exploration of non-Pt and efficient electrode materials is significant for industrialization of DSSCs. The main work in this thesis is to develop several efficient and non-Pt electrode materials, and investigate their electrocatalytic properties. The content is as follows:(1) A non-template method is proposed to prepare nickel sulfide with different phase, and the electrochemical properties of these nickel sulfides as the counter electrode materials for DSSCs are investigated systematically. Via adjusting the molar ratio of Ni/S in the reactions, hexagonal NiS and cubic NiS2 hollow spheres were synthesized. The specific surface areas of NiS and NiS2 hollow spheres are obtained quantitatively by BET test. The results show that the specific surface area of NiS2 is nearly twice as large as that of NiS. The electrochemical analyses demonstrate that NiS2 electrode exhibits superior electrocatalytic activity, higher electric conductivity and lower diffusion resistance of I3- ions. As a consequence, the DSSC with NiS2 CE generates higher power conversion efficiency (7.13%) than that with NiS CE (6.49%).(2) Hollow spherical NiS/NiS2 composite is synthesized by a one-step solvothermal route. The electrochemical analyses show that NiS/NiS2 electrode possesses superior electrocatalytic activity in the I3-/I- electrolyte. Finally, the DSSC based on the hollow spherical NiS/NiS2 CE achieves a satisfactory power conversion efficiency of 7.66%, which outperform that of using Pt CE (7.01%).(3) In order to prevent NiS nanoparticles aggregation, a reduced graphene oxide/NiS composite is established. On the one hand, NiS nanoparticles are dispersed on the surface of the reduced graphene oxide (RGO). On the other hand, RGO can offer an efficient channel for electron transferred. In addition, two-dimensional RGO has a huge surface area, which is better to provide more catalytic activity position for I3- ions reduction. The power conversion efficiency of the cell based on NiS/RGO CE is 7.67%, higher than that of using pristine NiS CE (6.25%) or RGO CE (2.92%).(4) In order to improve the conductivity of Bi2S3 nanoparticles, a carbon-modified Bi2S3 composite (Bi2S3-C) is developed. Since the aggregated Bi2S3 nanopariticles cannot afford effective transport channel, the electron transfer resistance on Bi2S3 electrode is much bigger than that on the platinum electrode. But after introducing the amorphous carbon, the transfer resistance on the Bi2S3-C electrode is low. As a "bridge", amorphous carbon can provide an effective passage for electron transport. In addition, Bi2S3-C microspheres have interconnected porous, which is favorable for I3- ion diffusion. EIS tests indicate that the diffusion resistance on Bi2S3-C electrode is lower than on Bi2S3 electrode. As a result, the DSSC fabricated with Bi2S3-C CE obtains a high conversion efficiency of 6.72% by the optimization of the carbon content in the composite, which increases by more than 20% compared with that of bare Bi2S3 CE, and comparable to conventional platinum electrode (6.75%). This suggests that Bi2S3-C CE is a potential alternative to expensive Pt CE for the low-cost DSSC.