Tradeoffbetween Narrowing Optical Band Gap and Enhancing Electrical Conductivity of the Metal Nanoparticles-Modified Titanium Oxide Films

The n-type semiconducting titanium oxide thin films are well-known as electron transporting interlayer in photovoltaic cells. The favorable characteristics of interlayers in photovoltaics are high optical transmittance (T%), wide band gap energy (Eg) and high electrical conductivity (σ). Modifying titanium oxide films with metal nanoparticles would increase electrical conductivity but reduce optical band gap energy. We developed the sol-gel derived titanium suboxide (TiOx) films modified with silver (Ag) or gold (Au) or copper (Cu) nanoparticles (NPs). This study explores a tradeoff between narrowing optical band gap and enhancing electrical conductivity of nanostructured TiOx films by controlling the Auor Agor Cu-NPs loading concentrations (mol%) in titania. The Auand Cu-NPs loading concentration of 4 mol% should meet a tradeoff which yields the higher T%, wider Eg and higher σ compared to those of pure TiOx films. In addition, since the pure Cu is not thermodynamically stable in ambience as compared to Au and Ag, the stability of as-obtained colloidal CuNPs is also examined. A careful examination of the time evolution of surface plasmon resonance (SPR) bands of CuNPs indicates that their stability is only up to 4 h.