We present experimental and theoretical studies of a nanopatterned photonic crystal formed between the bulk heterojunction blend,
poly-3-hexylthiophene:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and nanocrystalline zinc oxide (nc-ZnO). The nanopattern is fabricated using the Pattern Replication in Non-wetting
Templates (PRINT) technique. We summarize the fabrication method and show how it can be used to make a highly ordered hexagonal array of photovoltaic P3HT:PCBM posts. We also discuss theoretical studies of optical absorption for the nanopattern design that result in a 22% enhancement over a conventional planar cell. Spectroscopic ellipsometry is also used to determine the optical constants of solar cell materials that are used in the optical model. Finally, we
calculate the local exciton creation profile within the photoactive nanopattern to relate the nanostructured geometry to electrical performance.
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.
The Pattern Replication In Non-wetting Templates (PRINT) technique has been extended to patterning of isolated
features as well as embossed films of sub-500 nm "hard" inorganic oxides and nanocrystalline semiconductors and "soft"
semiconducting polymers including TiO2, SnO2, ZnO, ITO, BaTiO3, CdSe, poly(3-hexylthiophene) (P3HT), Poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV), and other polythiophene derivatives. The
low surface energy, chemically resistant, air permeable elastomeric perflouropolyether (PFPE) based molds allow for
numerous materials to be patterned on a variety of substrates including glass, transparent conductive oxides, and thin
films of conducting polymer for a wide range of electronic and optical applications. Additionally, PRINT has been
employed to pattern features with aspect ratios greater than 1, deposit a second layer of features on top of an initial layer
without pattern destruction, and replicate sub-100 nm sized features for photovoltaics applications. Materials and
patterns generated in this work were characterized using a variety of techniques including: Scanning Electron
Microscopy (SEM), Transmission Electron Microscopy (TEM), and X-ray Diffraction (XRD).
Synthetic polypeptides containing Disperse Red 1 (4-[N-ethyl, N- hydroxyethylamino]-4'-nitroazobenzene) nonlinear optical side chains were synthesized. The extent of side chain modification was ca. 10% and higher modifications were expected to produce crystallization of the polar NLO molecule and polymer insolubility. These polymers displayed a lyotropic liquid crystalline phase above a concentration of 15 - 20% (w/w) in dichloromethane. Corona poling (1.0 (mu) A/-5 kV) was used to align the Disperse Red 1 molecules of modified poly[L-glutamic acid] or modified poly[(gamma) -methyl-L-glutamate] thin films (1 - 2 micrometers ) that were produced by spin coating onto ITO covered glass substrates. Decay of the second harmonic (SH) signal was the slowest for films aligned by corona-onset poling at elevated temperatures (COPET) and the fastest for films aligned at room temperature. The SH intensities at 532 nm were comparable to the 1 mm thick, y-cut quartz standard and the (Is2(omega)/Iq2(omega))1/2 values were measured as a function of the film temperature during corona poling. The biexponential decay model was used to describe the trends in SH decay.
Second harmonic generation by NLO-active organic chromophores is used to monitor the poling dynamics and temporal stability of three kinds on `guest-host' materials: (1) helical polypeptides, (2) random coil polymers, and (3) smectic thermotropic liquid crystals. In (1) sidechain NLO-active chromophores reorient without changing the polymer backbone, whereas in (2) there is a two stage reorientation, sidechains followed by backbone. For the liquid crystal, (3) the mean field gives rise to a threshold poling potential and the polar chromophore orientation appears to be stabilized by the apolar but anisotropic mesogen orientational order.