Fabrication Of High-efficiency Non-Platinum Counter Electrodes For Dye-sensitized Solar Cells

As the depletion of fossil fuel, environmental problems and energy crisis are seriously restricting human society development. Solar energy is renewable and clean. Developing solar energy is the preferred approach to solve the problems and crisis. Dye-sensitized solar cells (DSSCs) are effective solar-to-electricity conversion devices with advantages in simple fabrication, high conversion efficiency, and available transparency over expensive silicon-based solar cells. Conventionally, precious metal platinum is employed as counter electrode for dye-sensitized solar cells. Platinum presents high electrical conductivity and superior electrocatalytic activity for I3-reduction process. However, high price and rare storage of this precious metal limit large-scale application of dye-sensitized solar cells in future. Therefore, research on low-cost non-Pt substitute is essential for industrialization of dye-sensitized solar cells. The main purpose of the works in the thesis is to develop novel non-platinum high-efficiency counter electrode by investigating the relationship between electrode structure and performance and electrochemical properties on electrode-electrolyte interface.1. Systematically investigating electrochemical performance of transition metal nitrides as counter electrode materials in DSSC. MoN, WN, and Fe2N are synthesized by nitridation process. The energy conversion efficiency of DSSCs with MoN, WN, Fe2N counter electrodes are5.57%,3.67%and2.65%, respectively. MoN presents the best performance in the three nitrides, comparable to platinum electrode (6.56%). Electrochemical measurements indicate that electrocatalytic activity of MoN and WN are better than platinum electrode, while Fe2N has relatively low activity. However, high diffusion resistance on MoN and WN electrodes is presented as the nanoparticle-aggregated electrode films can not afford effective ion diffusion passage. In addition, the thick electrode films (more than lOum) have strong adsorption effect on I3-/I-which further hinders the mass transport on counter electrode and decreases the final electrochemical performance. To further improve electrochemical performance of MoN, the high ion diffusion resistance in electrode film must be effectively solved.2. Microstructure regulation and composite fabrication based on MoN material.1) MoN-CNTs nanocomposites are prepared. In this composite, MoN nanoparticles are tightly attached to the surface of CNTs. This structure combines the superior electrocatalytic activity of MoN and effective transport network and porous structure of CNTs. The synergistic effect between MoN and CNTs effectively improve ion transfer process on counter electrode. The result demonstrates that ion diffusion resistance of MoN-CNTs nanocomposite is much lower than that of MoN and CNTs. Finally, DSSC employing MoN-CNTs counter electrode presents a conversion efficiency of6.74%, which is much higher than that of MoN (5.57%) and CNTs (5.79%).2) Porous MoN nanorods are prepared by nitridation of organic-inorganic Mo-compound nanowires. The porous MoN electrode inherits the superior intrinsic electrocatalytic activity of MoN, whilst it has larger porosity than MoN nanoparticles. The interconnected porous structure is favorable for ion diffusion. EIS results indicate that the diffusion resistance on MoN nanorod electrode is obviously lower than on MoN nanoparticle electrode. The DSSC using MoN nanorods has a conversion efficiency of7.29%, higher than that of MoN nanoparticle (6.48%), and comparable to conventional platinum electrode (7.42%).3. Making transparent non-platinum counter electrodes. Transparent counter electrode is an essential component for transparent DSSC, which should be good in both electrochemical performance and optical transparency.1) Nickel sulfide, cobalt sulfide and iron sulfide films are fabricated by solution process and subsequent calcination. Thickness of sulfides films are controlled to guarantee a high light transmission. As limitation from organics in the precursor, nickel sulfide film consists of nanograins with a diameter of only about10nm. Additionally, the organic layer will transform into carbon with high dispersion among nickel sulfide grains after the subsequent heat treatment, which is helpful for improving electrical conductivity of film. The result demonstrates that nickel sulfide with100nm thickness presents the optimized performance. The corresponding DSSC has a conversion efficiency of7.37%which is even better than conventional pyrolytic platinum electrode (7.20%). Meanwhile, the electrode exhibits good optical transparency, above80%. The DSSCs using cobalt sulfide and iron sulfide counter electrodes exhibit the highest conversion efficiency of5.98%and4.00%, respectively. Cobalt sulfide has relatively low transparency, only50%, while iron sulfide presents high light transmittance, above90%.2) Ni2N transparent electrodes are fabricated by magnetron sputtering method. Nickel nitrides are formed by nitrogen incorporating in nickel lattice during sputter process. The components and morphology of nickel nitrides film are obviously varied as different nitrogen atom is incorporated. When certain nitrogen is incorporated into nickel lattices, pyramidal Ni2N nanoparticles are obtained. This Ni2N structure has superior electrocatalytic activity, and low ion diffusion resistance as the thin film. The corresponding DSSC has a conversion efficiency of6.84%, which is higher than sputtered platinum electrode (6.42%). The electrode also exhibits a light transmittance as high as85%.In summary, electrocatalytic activity of metal nitrides are systematically investigated. Based on the results, we fabricate novel non-Pt counter electrode materials with better electrochemical performance by microstructure regulation or material compositing. Meanwhile, metal sulfides and nitrides electrodes exhibiting both high electrochemical performance and high transparency are fabricated. These works establish theoretical basis and practical guidance for developing low-cost and high-efficiency non-Pt counter electrodes for DSSCs.