We have systematically investigated the effects of surface modification of titania (TiO 2 ) in hybrid TiO 2 /regioregular poly(3-hexylthiophene) (P3HT) photovoltaic cells.By employing a series of para-substituted benzoic acids with varying dipoles and a series of multiply-substituted benzene carboxylic acids, the energy offset at the TiO 2 /polymer interface and thus the open circuit voltage of devices can be tuned systematically by 0.25 V. Transient photovoltage measurements showed that the recombination kinetics were dominated by charge carrier concentration in these devices and were closely associated with the dark current. The saturated photocurrent of TiO 2 /P3HT devices exhibits more than a two-fold enhancement when molecular modifiers with large electron affinity were employed. The ability of modifiers to accept charge from polymers, as revealed in photoluminescence quenching measurement with blends of polymers, was shown to be correlated to the enhancement in device photocurrent.A planar geometry photoluminescence quenching measurement showed that TiO 2 substrates modified by these same molecules that accept charge quenched more excitons in regioregular P3HT than bare TiO 2 surfaces. An exciton diffusion length in P3HT as large as 6.5 -8.5 nm it was found that all of the excitons that were quenched were accountable as extracted photocurrent. EQE was effectively increased from 5% to 10 -14% with certain surface modifiers; consequently exciton harvesting was more than doubled. The use of Ruthenium (II) sensitizing dyes with good exciton harvesting property coupled with suppression of the recombination kinetics improved the efficiency of optimized bilayer TiO 2 /P3HT devices from 0.34 % to 0.6 % under AM 1.5 solar illuminations. The implication of this work is directly relevant to the design of nanostructured bulk heterojunction inorganic-organic cells, in which efficient exciton harvesting and control of the recombination kinetics are key to achieving high efficiency. _____________________________ a) Electronic

Articles you may be interested inTerahertz mobility measurements on poly-3-hexylthiophene films: Device comparison, molecular weight, and film processing effectsWe have investigated the transport properties in the direction perpendicular to the substrate of regioregular poly͑3-hexyl-thiophene͒ of different molecular weights ͑MW͒ in a diode geometry. In these devices, which exhibit space-charge-limited behavior, we find that the mobility values at room temperature increase from 1.33ϫ 10 −5 cm 2 / V s to 3.30ϫ 10 −4 cm 2 / V s as the MW is increased from 2.9 to 31.1 kg/ mol. The mobility is found to be field independent for high MW films, but field dependent for the low MW films. The current-voltage characteristics of the diodes are also studied as a function of temperature from 160 K to 300 K. The activation energy for carrier transport, extracted from the Arrhenius plot, is found to decrease gradually from 143 meV to 126 meV as the MW is increased.

We demonstrate that the hole mobility in regioregular poly(3‐hexylthiophene) can be enhanced by a factor of 20 by infiltrating it into straight nanopores of anodic alumina. Optical characterization shows that the polymer chains are partially aligned in the charge‐transport direction.

Fabrication of bulk heterojunctions with well-ordered arrays of organic and inorganic semiconductors is a promising route to increasing the efficiency of polymer photovoltaic cells. In such structures, almost all excitons formed are close enough to the organic-inorganic interface to be dissociated by electron transfer, all charge carriers have an uninterrupted pathway to the electrodes, and polymer chains are aligned to increase their charge carrier mobility. Furthermore, ordered structures are interesting because they are relatively easy to model. Studies of ordered cells are likely to lead to better design rules for making efficient photovoltaic cells.

We demonstrate a method for embossing titania sol−gel precursor with poly(methyl methacrylate) (PMMA) molds to make thin films of titania that have dense arrays of 35−65 nm diameter pores, whose features are 1 order of magnitude smaller than those previously demonstrated for sol−gel molding. We show that the high modulus of PMMA is necessary to preserve small features with high aspect ratios on the mold for nanopatterning. The molds are prepared by thermally infiltrating PMMA into anodic alumina templates, whose pore dimensions and depths are adjustable by varying anodization conditions. The difficulties associated with mold release from a master are avoided by wet etching the template. These titania films, and others made with other semiconductors, could be useful for photovoltaic, photocatalytic, and sensing applications where nanostructuring of surfaces with controlled dimensions are essential.Nanostructured semiconductor materials have drawn a lot of attention because they exhibit interesting optical, electronic, and catalytic properties. Semiconductor films with a dense array of pores at the 10-50 nm length scale are attractive for photovoltaics, 1-4 photocatalytics, 5,6 and sensing applications. 7 For these applications, it is desirable to pattern semiconductors in a relatively simple and cheap process. Although soft lithography 8 and nanoimprint lithography 9 are promising ways to nanopattern materials, they both have limitations and may not be suitable for patterning certain materials and specifications. Soft lithography, which usually utilizes elastomer poly(dimethylsiloxane) (PDMS) molds, has been extensively used to pattern photoresists, biological macromolecules, and semiconducting polymers. 8,10 The resolution achievable with PDMS however has often been limited to >100 nm due to its relatively low compression modulus of ∼2 MPa. 11 The use of a harder version of PDMS (h-PDMS) with a compression modulus of ∼9 MPa has increased the resolution to about 50 nm, 11,12 but only for features that are not densely spaced together and of high aspect ratio. This points to the need for an even harder mold as patterning is pushed toward increasingly smaller scale. Nanoimprint lithography, which has formidable resolution approaching 5 nm, 13 uses a hard Si or SiO 2 mold to achieve high aspect ratio features. Nanoimprinted materials include photoresists, polymers, silicon, and sol-gel materials, 9,14-16 but some other materials cannot be patterned by nanoimprint lithography because the mold adheres to the film and cannot be removed. For instance, nanostructuring of sol-gel inorganics potentially useful in many low cost applications can be very challenging with either soft lithography or nanoimprint lithography. Here, we demonstrate molding of solgel precursors to make nanostructured metal oxide films, whose features are 1 order of magnitude smaller than those previously demonstrated for sol-gel molding. 15,17 We show that embossing with a hard polymer mold can be an attractive alternative to pattern nanoporo...