The use of the focused ion beam technique to prepare cross-sectional transmission electron microscopy specimen of polymer solar cells deposited on glass

The use of the focused ion beam (FIB) technique for cross-sectional transmission electron microscopy (TEM) specimen preparation of polymer solar cells deposited on glass substrates is described. Ultra-thin sections were prepared using the ‘lift-out’ technique. Electron microscopy investigations of these specimen resulted in detailed morphological information of the devices (e.g. thickness and interface roughness of the layers). In comparison with standard sample preparation routes for TEM investigations the used technique is well suited for precise sectioning of hybrid structures

Thermally induced transient absorption of light by poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonic acid) (PEDOT:PSS) films: A way to probe charge-carrier thermalization processes

IR-induced transient absorptions in the millisecond and sub-picosecond time domains have been used to study the dynamics of charge carriers of a conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS). On the millisecond timescale, the transient absorption is ascribed to a thermal effect induced by absorbed IR light. The decay of the transient absorption is limited by the transport of heat from the polymer film to the substrate and corresponds to the decay kinetics of IR-induced changes in the resistivity of the material. Near 1.5 eV, the IR-induced absorption can be modeled in terms of an interband transition. The assignment of the optical transients in terms of carrier heating opens the possibility to study charge carrier thermalization processes using short laser pulses. Pump-probe spectroscopy on a sub-picosecond timescale shows that the initial thermalization of the excited charge carriers occurs with a time const. of less than 500 fs, i.e., faster than for noble metal

Morphology and thermal stability of the active layer in poly(p-phenylenevinylene)/methanofullerene plastic photovoltaic devices

The morphology of composite thin films consisting of a conjugated polymer (poly[2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-1,4-phenylenevinylene], MDMO-PPV) and methanofullerene ([6,6]-phenyl C61 butyric acid methyl ester, PCBM), which are used as the active layer in polymer photovoltaic devices, has been extensively studied using transmission electron microscopy (TEM) and selected-area electron diffraction (SAED). Composite MDMO-PPV:PCBM films have been prepared with PCBM concentrations varying from 20 to 90 wt %. PCBM-rich clusters are clearly observed in TEM bright-field mode when the PCBM concentration is increased to ca. 75 wt % in the composite film. The SAED analysis shows that these clusters consist of many PCBM nanocrystals with random crystallographic orientations. Furthermore, we show that these nanocrystals are also present in the homogeneous matrix at PCBM concentrations below 75 wt %. Annealing of the blend films has been performed at temperatures between 60 and 130 °C for different times. In all cases, but especially when the annealing temperature is above the glass transition temperature of MDMO-PPV (80 °C), PCBM molecules show high diffusion mobility, resulting in accelerated phase segregation and in the formation of large PCBM single crystals in the film. The observed phase segregation, even at temperatures as low as 60 °C, indicates that the thermal stability of MDMO-PPV:PCBM films will likely limit the long-term performance of solar cells based on these materials

Low-bandgap polymer photovoltaic cells

A-novel low-bandgap conjugated polymer (PTPTB, Eg = ~1.6 eV), consisting of alternating electron-rich N-dodecyl-2,5-bis(2'-thienyl)pyrrole (TPT) and electron-deficient 2,1,3-benzothiadiazole (B) units, as a donor material is studied together with a soluble fullerene derivative (PCBM) as acceptor to prepare bulk heterojunction photovoltaic cells. Photoinduced absorption (PIA) and fluorescence spectroscopy on blends of PTPTB and PCBM gave direct spectral evidence of the photogeneration of a charge-separated state. Preliminary results on photovoltaic cells prepared using thin PTPTB:PCBM films as an active layer, sandwiched between ITO/PEDOT:PSS and Al electrodes, showed promising characteristics

Crystalline organization of a methanofullerene as used for plastic solar-cell applications

Crystalline organization of a methano-fullerene, [6,6]-phenyl C-61 butyric acid methyl ester (PCBM), as achieved in various thin-film deposition techniques, is reported. Mechanically stable, and thus self-supporting, thin films obtained via fast solvent evaporation techniques are found to be composed of densely and homogeneously distributed PCBM nanocrystals with various crystallographic orientations, as shown schematically in the Figure.</p

Substitution and preparation effects on the molecular-scale morphology of PPV films

The morphology of spin-cast films of poly(p-phenylenevinylene) (PPV) derivatives is studied as a function of the substitution pattern of the conjugated backbone. Moreover, the influence of concentration in the casting solution, annealing, the choice of solvent, and the role of defects are addressed. By using a recently developed scanning-probe technique, we are able to visualize individual polymer chains and aggregates on the surface of spin-cast films. We find that a symmetric substitution pattern strongly promotes interchain aggregation in the surface layer, whereas an unsymmetric pattern in some cases leads to intrachain or self-aggregation. The nature of these intrachain aggregates is further investigated using molecular dynamics simulations. The observed molecular morphologies can in most cases be qualitatively related to macroscopic electrooptical properties. Therefore, our results strongly suggest that the surface morphology may be regarded as indicative of the morphology of the entire film

In-situ compositional and structural analysis of plastic solar cells

Bulk-heterojunction photovoltaic cells consisting of a photoactive layer of poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] and a C60 deriv., 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene, (PCBM), sandwiched between an indium tin oxide anode covered with poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), and an Al cathode, were analyzed using TEM and cryogenic Rutherford backscattering spectrometry (RBS) to assess the structural and elemental compn. of these devices. Sample cross sections of fully processed photovoltaic cells were prepd. with the focused ion beam technique. TEM of these samples revealed the individual layers and their interfaces. RBS shows that during prepn., diffusion of In into the PEDOT:PSS occurs while the diffusion of Al into the polymer layers is negligible. An iodinated C60 deriv. (I-PCBM) was used to det. the concn. profile of this deriv. in the vertical direction of a 100 nm thick active laye

Supramolecular hydrogen-bonded oligo(p-phenylene vinylene) polymers

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Control of film morphology by folding hydrogen-bonded oligo(p-phenylenevinylen) polymers in solution

The film morphology of -conjugated oligomers has been controlled by self-assembly in solution. To this end supramolecular hydrogen-bonded systems of oligo(p-phenylenevinylene) (OPV) carrying ureido-s-triazine hydrogen-bonding groups are used. Neutron scattering experiments in dodecane solutions show that columnar stacks are formed. Films with thicknesses on the order of 100 nm are made that have a supramolecular organization resembling the organization present in solution. Uniform rodlike morphological domains range over several hundreds of nanometers as shown by atomic force microscopy. The rodlike morphology of the OPVs was also preserved when blended with a C60 derivative, producing stable photovoltaic devices

Compositional dependence of the performance of poly(p-phylene vinylene): methanofullerene bulk-heterojunction solar cells

The dependence of the performance of OC1C10-PPV:PCBM (poly(2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylene vinylene):methanofullerene [6,6]-phenyl C61-butyric acid methyl ester)-based bulk heterojunction solar cells on their composition has been investigated. With regard to charge transport, we demonstrate that the electron mobility gradually increases on increasing the PCBM weight ratio, up to 80 wt.-%, and subsequently saturates to its bulk value. Surprisingly, the hole mobility in the PPV phase shows an identical behavior and saturates beyond 67 wt.-% PCBM, a value which is more than two orders of magnitude higher than that of the pure polymer. The experimental electron and hole mobilities were used to study the photocurrent generation of OC1C10-PPV:PCBM bulk-heterojunction (BHJ) solar cells. From numerical calculations, it is shown that for PCBM concentrations exceeding 80 wt.-% reduced light absorption is responsible for the loss of device performance. From 80 to 67 wt.-%, the decrease in power conversion efficiency is mainly due to a decreased separation efficiency of bound electron-hole (e-h) pairs. Below 67 wt.-%, the performance loss is governed by a combination of a reduced generation rate of e-h pairs and a strong decrease in hole transport