In this paper, the pore filling of spiro-MeOTAD
(2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)9,
9'-spirobifluorene) in mesoporous TiO2 films is quantified for the first time using XPS depth profiling and UVVis
absorption spectroscopy. We show that spiro-OMeTAD can penetrate the entire depth of the film, and its
concentration is constant throughout the film. We determine that in a 2.5-•m-thick film, the volume of the
pores is 60-65% filled. The pores become less filled when thicker films are used. Such filling fraction is much
higher than the solution concentration because the excess solution on top of the film can act as a reservoir
during the spin coating process. Lastly, we demonstrate that by using a lower spin coating speed and higher
spiro-OMeTAD solution concentration, we can increase the filling fraction and consequently the efficiency of
the device.
We have fabricated bulk heterojunction photovoltaic (PV) cells using a perfluoropolyether (PFPE) elastomeric stamp to
control the morphology of the donor-acceptor interface within devices. Devices were fabricated using the Pattern
Replication In Non-wetting Templates (PRINT) process to have nanoscale control over the bulk heterojunction device
architecture. The low-surface energy, chemically resistant, variable modulus, fluoropolymer based molds used in
PRINT provide a route to patterning, with nanometer resolution, general polymeric donor materials such as
polythiophene and polyphenylenevinylene derivatives and 'hard' inorganic oxide structures typically used as acceptor
materials in hybrid organic solar cells such as TiO2, ZnO, and CdSe. This "top-down" approach allows for patterning
over large areas and for the functionalization of the donor/acceptor interface. Specifically, nanostructured anatase titania
with post-like features ranging from 30-100 nm in diameter and 30-65 nm in height was fabricated to form the ordered
bulk heterojunction of a titania-poly(3-hexylthiophene) (P3HT) PV-cell. Nanostructured devices showed a two-fold
improvement in both short-circuit current (Jsc) and power conversion efficiency (PCE) relative to reference bilayer cells.
Additionally, we will discuss devices fabricated with other organic and inorganic materials in order to investigate the
effect on cell performance of controlling the nanoscale architecture of the bulk heterojunction via patterning.
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