(512e) Use of Mesoporous Titania Thin Films with Controlled Pore Orientation in Photovoltaic Applications

Mesoporous titania thin films with 2D Hexagonal Close Packed (HCP) cylindrical nanopores have been synthesized by an evaporation-induced self-assembly (EISA) technique with Pluronic surfactant F127 (with average structure (EO)₁₀₆(PO)₇₀(EO)₁₀₆ where EO is an ethylene oxide unit and PO is a propylene oxide unit) as the template. To provide vertical alignment of the pores, surface modification of substrates with crosslinked surfactant F127 has been used to provide a chemically neutral surface. Conditions are found giving vertically aligned, 2D HCP titania films with pores near 8-9 nm in diameter, which are precisely the structures expected to provide short carrier diffusion length and high hole conductivity required for an efficient bulk heterojunction solar cell. Anatase titania is a n-type semiconductor with the valence and conduction bands located at +3.1 and -0.1 eV relative to the Fermi level, thus giving a band gap of +3.2 eV. Therefore, titania readily absorbs UV light with a wavelength below 387 nm. Because of this, these titania films can be used as a window layer with a p-type semiconductor incorporated into the pores and at the top surface of the device to synthesize a photovoltaic cell.  The pores provide opportunities to increase the surface area for contact between the two semiconductors, to align organic semiconductors, and to induce quantum confinement effects.

Here, these titania films with hexagonal phase are infiltrated with a hole conducting polymer, P3HT, in order to create a p-n junction for an efficient hybrid solar cell, by spin coating followed by thermal annealing. This assembly is hypothesized to give better photovoltaic performance as compared to a disordered or bicontinuous cubic nanopore arrangements (K.M. Coakley et al. MRS Bull. 2005 30, 57). Confinement in cylindrical nanopores is expected to provide isolated, regioregular “wires” of conjugated polymers with tunable optoelectronic properties, which have been shown to improve hole conductivity over that in bicontinuous cubic structure [M.D. McGehee, MRS Bull. 2009, 34, 95].  The kinetics of infiltration into the pores show that maximum infiltration occurs within less than one hour in these films, and give materials with improved photovoltaic performance relative to planar TiO₂ / P3HT assemblies. These oritneted mesoporous titania films are also used to develop an inorganic solar cell by depositing CdTe at the top using the Close Spaced Sublimation (CSS) technique.  Conversion efficiencies of at least 5-7% are observed in photovoltaics prepared using mesoporous titania films, which are significantly enhanced relative to planar TiO₂/CdTe devices. These mesoporous titania films have a great potential in inorganic solar cell development and can potentially replace CdS window layer which is conventionally used in inorganic CdS-CdTe solar cells.