Trap-Free Transport in Ordered and Disordered TiO2 Nanostructures

• Published in: Nano letters 2014-05, Vol.14 (5), p.2305-2309
• Main Authors: Villanueva-Cab, Julio, Jang, Song-Rim, Halverson, Adam F, Zhu, Kai, Frank, Arthur J
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Diffusion in nanoscale solids; Diffusion in solids; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Low-dimensional structures (superlattices, quantum well structures, multilayers): structure, and nonelectronic properties; Materials science; Nanoscale materials and structures: fabrication and characterization; Nanotubes; Physics; Specific phase transitions; Structural transitions in nanoscale materials; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Transport properties of condensed matter (nonelectronic)
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl4046087

Understanding the influence of different film structures on electron diffusion in nanoporous metal oxide films has been challenging. Because of the rate-limiting role that traps play in controlling the transport properties, the structural effects of different film architectures are largely obscured or reduced. We describe a general approach to probe the impact of structural order and disorder on the charge-carrier dynamics without the interference of transport-limiting traps. As an illustration of this approach, we explore the consequences of trap-free diffusion in vertically aligned nanotube structures and random nanoparticle networks in sensitized titanium dioxide solar cells. Values of the electron diffusion coefficients in the nanotubes approached those observed for the single crystal and were up to 2 orders of magnitude greater than those measured for nanoparticle films with various average crystallites sizes. Transport measurements together with modeling show that electron scattering at grain boundaries in particle networks limits trap-free diffusion. In presence of traps, transport was 103–105 times slower in nanoparticle films than in the single crystal. Understanding the link between structure and carrier dynamics is important for systematically altering and eventually controlling the electronic properties of nanoscaled materials.