Charge Transport Properties in TiO₂ Network with Different Particle Sizes for Dye Sensitized Solar Cells

The charge transport properties in the TiO(2) nanoparticle networks with the different TiO(2) nanoparticle sizes were investigated by means of electrochemical impedance spectroscopy (EIS) with consideration of morphological aspects of mesoporous TiO(2) network including particle size (d(p)), coordination number (N(n)), neck diameter (d(n)), and effective surface area (S(e)). The morphological analysis of the network revealed that the particle size and surface area would be factors exerting an impact on the charge transport properties, while the coordination number and neck diameter seemed to be consistent with the nanoparticle size. As a result, the electron transport along with the TiO(2) network was predominantly affected by the particle size in terms of the mean free path; the bigger particle size provides both long travel distance and less collision chance with the boundary. Surface area seems to exert a strong influence on the recombination when it is in contact with an electrolyte, suggesting that pore size distribution determining penetration of an electrolyte has to be considered in terms of the effective surface area (S(e)). Due to the low transport resistance, high recombination resistance, and low chemical capacitance, the largest particle showed the longest diffusion length (L(n)). However, the highest efficiency observed in 15 nm TiO(2) nanoparticle photoanode indicated that the compensating characteristics of the morphological factors of the network for light harvesting efficiency (LHE) (surface area) and charge collection efficiency (η(c), particle size) should be balanced in designing a nanostructured network for high performance DSCs.