Vertical phase separation in spin-coated films of a low bandgap polyfluorene/PCBM blend - Effects of specific substrate interaction

We report on the effect of the substrate on the vertical phase separation in spin-coated thin films of poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-4',7'-di-2-thienyl-2',1',3'-benz othiadiazole] (APFO-3) blended with [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM). Compositional depth profiles of the films are measured by dynamic secondary ion mass spectrometry (SIMS). We found that changing the substrate from silicon to gold affects the composition profile near the substrate interface. This is caused by a specific interaction between the polymer (APFO-3) and the gold surface, as confirmed by X-ray photoelectron spectroscopy (XPS). The composition profile in the area away from the substrate interface, as well as the enrichment of the free surface with APFO-3, remain however unaffected by the choice of substrate. The vertical composition was also analysed for APFO-3:PCBM films spin-coated on indium tin oxide (ITO) coated with a thin layer of (3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). (c) 2006 Elsevier B.V. All rights reserved

Influence of solvent mixing on the morphology and performance of solar cells based on polyfluorene copolymer/fullerene blends

The influence of the solvent on the morphol. and performance of polymer solar cells was studied in devices based on blends of the polyfluorene copolymer, poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4\u27,7\u27-di-2-thienyl-2\u27, 1\u27,3\u27-benzothiadiazole)), and [6,6]-phenyl-C61-butyric acid Me ester. The blends are spin-coated from CHCl3 or from CHCl3 mixed with small amts. of xylene, toluene, or chlorobenzene. The devices are characterized under monochromatic light and AM1.5 solar illumination. An enhancement of the photocurrent d. was obsd. in diodes made from CHCl3 mixed with chlorobenzene, and a decreased photocurrent d. is obsd. in diodes made from CHCl3 mixed with xylene or toluene, compared to diodes made from neat CHCl3. The open-circuit voltages are almost the same for all diodes. The surfaces of the active layers were imaged with AFM. Height images indicate that a finer and more uniform distribution of domains corresponds to the diodes with enhanced photocurrent that are made from CHCl3 mixed with chlorobenzene, while a structure with larger domains is assocd. with the lower photocurrents in the diodes made from CHCl3 mixed with xylene or toluene. The influence of the morphol. on the excited-state dynamics and charge generation was studied using time-resolved spectroscopy. Fast formation of bound charge pairs followed by their conversion into free charge carriers was resolved, and excitation-intensity-dependent non-geminate recombination of free charges was obsd. A significant enhancement in free-charge-carrier generation was obsd. on introducing chlorobenzene into CHCl3. Imaging photocurrent generation from the solar cells with a light-pulse technique shows an inhomogeneous photocurrent distribution, which is related to the undulations in the thickness of the active layer. Thicker parts of the diodes yield higher photocurrent values. [on SciFinder (R)

Interlayer for Modified Cathode in Highly Efficient Inverted ITO-Free Organic Solar Cells

Inverted polymer solar cells with a bottom metal cathode modified by a conjugated polymer interlayer show considerable improvement of photocurrent and fill factor, which is due to hole blocking at the interlayer, and a modified surface energy which affects the nanostructure in the TQ1/[70]PCBM blend

Voltage-Induced Formation of Accumulation Layers at Electrode Interfaces in Organic Solar Cells

This work reports on organic bulk heterojunction solar cells based on poly(3-hexylthiophene) (P3HT) blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) in a configuration with so-called interdigital nanoelectrodes, i.e., vertical electrodes on substrates structured in the submicrometer range. In this setup, both electrodes are in place prior to the deposition of the photoactive blend solution and therefore allow for the application of a voltage during drying of the blend. A strong correlation is observed between the photovoltaic performance of these devices and the voltage that is applied during film formation. Even the polarity of the solar cells can be controlled with this method. It is suggested that this is a consequence of a strong segregation of donor and acceptor phases at the electrode interfaces induced by the applied voltage. Further experiments on planar solar cell geometries, including a solvent-vapor treatment and the introduction of an additional layer of pure P3HT, as well as numerical simulations, are presented. All results obtained are consistent with the suggested hypothesis