Tandem solar cells, in which two solar cells with different absorption characteristics are linked to use a wider range of the solar spectrum, were fabricated with each layer processed from solution with the use of bulk heterojunction materials comprising semiconducting polymers and fullerene derivatives. A transparent titanium oxide (TiO(x)) layer separates and connects the front cell and the back cell. The TiO(x) layer serves as an electron transport and collecting layer for the first cell and as a stable foundation that enables the fabrication of the second cell to complete the tandem cell architecture. We use an inverted structure with the low band-gap polymer-fullerene composite as the charge-separating layer in the front cell and the high band-gap polymer composite as that in the back cell. Power-conversion efficiencies of more than 6% were achieved at illuminations of 200 milliwatts per square centimeter.
A simple, sensitive, and reproducible sensing technique is described for the unambiguous detection of unlabeled single-stranded DNA by surface-enhanced Raman spectroscopy (SERS). By self-assembling probe-tethered Ag nanoparticles to a moderately smooth Ag film using the complementary target species, electromagnetic “hot spots” are created which strongly enhance the Raman signal of the species present in the hot spot. These species can include a Raman label (a molecule with a very large Raman cross-section) that dominates the spectrum and generates highly reproducible signals. The self-assembly process does not take place in the absence of the target species. Consequently, a strong SERS signal is observed only in the presence of the target. The SERS signal was also absent in the presence of noncomplementary species. AFM analysis indicates that strong SERS signal intensity arises from only a few surface-bound nanoparticles which generate an enhancement factor ∼105−106 greater than the metal film alone. Notably, this nanoparticle−film DNA detection method does not require any chemical deposition of silver to read out the SERS spectrum from the surface-bound labels.
Tapping mode atomic force microscopy (AFM) was used to probe the surface and internal structures of bulk heterojunction poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) films. AFM images obtained from the surface of the P3HT:PCBM film and from the cross section reveal for the first time the nanoscale three-dimensional P3HT and PCBM networks. Two different measurement modes of conducting atomic force microscopy (C-AFM) were used to study the nanoscale charge transport in P3HT:PCBM films. In C-AFM, the local current map at a constant bias and the surface topography are obtained simultaneously. Hole and electron current images can be used to determine the composition of the phase segregation domains. Alternatively, local current−voltage curves can be measured that can be used to calculate charge carrier mobilities. Hole current images and mobilities were obtained using Pt-coated silicon probes, whereas Mg-coated silicon probes were used to measure nanoscale electron mobilities. Variations in electrical properties and surface topography were examined. Comparison against films of neat P3HT shows that blending P3HT with PCBM decreases the polymer hole mobility. In all cases, nanoscale hole and electron mobilities increase upon annealing, consistent with increased internal order. A direct correlation between the C-AFM-determined nanoscale properties and solar cell power conversion efficiencies is observed.
A simple method for ssDNA detection based on SERS signals and PNA slides is reported. Upon hybridization with ssDNA, the surface charge of the PNA slides changes from neutral to negative. Subsequent treatment with partly aggregated, positively charged silver nanoparticles results in selective electrostatic adsorption onto surfaces containing PNA/ssDNA duplexes. Addition of rhodamine6G gives rise to SERS signals characteristic of this dye, which are diagnostic of the hybridization event. Characterization by SERS maps and AFM reveals that the distribution of nanoparticles is random and that approximately 1 in 10 aggregate sites is responsible for the SERS spectra.
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