Energy level alignment at the anode of poly(3-hexylthiophene)/fullerene-based solar cells

Interfacial free energy driven nanophase separation in poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester thin films

The nanostructure of thermally annealed thin films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends on hydrophobic and hydrophilic substrates was studied to unravel the relationship between the substrate properties and the phase structure of polymer blends in confined geometry. Indeed, the nature of the employed substrates was found to affect the extent of phase separation, the PCBM aggregation state and the texture of the whole system. In particular, annealing below the melting temperature of the polymer yielded the formation of PCBM nanometric crystallites on the hydrophobic substrates, while mostly amorphous microscopic aggregates were formed on the hydrophilic ones. Moreover, while an enhanced in-plane orientation of P3HT lamellae was promoted on hydrophobic substrates, a markedly tilted geometry was produced on the hydrophilic ones. The observed effects were interpreted in terms of a simple model connecting the interface free energy for the blend films to the different polymer chain mobility and diffusion velocity of PCBM molecules on the different substrates.

Non-chemical approach toward 2D self-assemblies of Ag nanoparticles via cold plasma treatment of substrates

The nano-modification of selected substrates by means of atmospheric cold plasma treatment was exploited for the two-dimensional (2D) self-assembling of silver nanoparticles (Ag NPs). Such a useful combination of the cold plasma treatment of substrate surface and an immediate easy deposition of Ag NPs creating the 2D self-assemblies on the substrates is published for the first time, to the best of our knowledge. Except for the cold plasma treatment, mainly the following parameters influenced the resulting NP assemblies: the choice of solvent mixture, concentration of Ag NP dispersions, and the deposition technique. The 2D self-assemblies of Ag NPs, providing the same work function as a Ag electrode, were formed on the cold plasma-treated substrates when a drop-casting technique was employed. The possibility of an easy preparation of the Ag NP 2D self-assemblies on substrates without using any chemical agents and/or evaporating chamber could be exploited, e.g. in photovoltaic and light-emitting diode devices.

Photoemission spectroscopy and atomic force microscopy investigation of vapor-phase codeposited silver/poly(3-hexylthiophene) composites

Nanocomposite matrices of silver/poly(3-hexylthiophene) (P3HT) were prepared in ultrahigh vacuum through vapor-phase codeposition. Change in microstructure, chemical nature, and electronic properties with increasing filler (Ag) content were investigated using in situ XPS and UPS, and ambient AFM. At least two chemical binding states occur between Ag nanoparticles and sulfur in P3HT at the immediate contact layer, but no evidence of interaction between Ag and carbon (in P3HT) was found. AFM images reveal a change in Ag nanoparticles size with concentration which modifies the microstructure and the average roughness of the surface. Under codeposition, P3HT largely retains its conjugated structures, which is evidenced by the similar XPS and UPS spectra to those of P3HT films deposited on other substrates. We demonstrate here that the magnitude of the barrier height for hole injection (epsilon(v)(F)) and the position of the highest occupied band edge (HOB) with respect to the Fermi level of Ag may be controlled and changed by adjusting the metal (Ag) content in the composite. Furthermore, UPS reveals distinct features related to the C 2p (sigma states) in the 5-12 eV regions, indicating the presence of ordered P3HT, which is different from solution processed films.