Three solution processable n-type semiconducting perylene bisimides (PBI) with an unsymmetrical substitution pattern are evaluated in terms of their charge transport properties, morphology, and crystal structure. The nature of the substituents is varied from hydrophobic alkyl chains to hydrophilic oligoethyleneglycol (OEG) chains to control intermolecular interactions and to tune self-assembly properties of the compounds. A correlation of structure and morphology with charge transport properties is attempted. Bulk X-ray diffraction (XRD) data are indicative for a lamello-columnar packing motif in the case of PBI 1 and PBI 3 and a columnar hexagonal packing for PBI 2. Further, OEG chains induce liquid crystalline phases while the alkyl substituted compound is crystalline. In the amorphous state after film formation all three materials have a low electron mobility in the range of 10(-5) cm(2) V-1 s(-1). After annealing in the ordered state the mobility of the liquid crystalline compounds increases by 2 orders of magnitude up to 7 X 10(-3) cm(2) V-1 s(-1), while the mobility of the crystalline material decreases by a factor of 4.

Well-soluble fullerene-grafted copolymers PPCBMs with high contents of pendant phenyl-C-61-butyric acid methyl ester (PCBM) between 30 and 64 wt % are reported. Herein, the tailor-made precursor copolymers poly(4-methoxystyrene-stat-4-tert-butoxystyrene) obtained by reversible addition-fragmentation chain transfer (RAFT) polymerization are functionalized via an efficient polymer-analogous esterification. The synthesized acceptor copolymers retain the optical and electrochemical properties of the incorporated PCBM independent of their fullerene weight fraction. Their electron transport properties are studied by the space-charge limited current (SCLC) method. The maximum electron mobility mu(e) of 1 X 10(-4) cm(2) V-1 s(-1) is achieved for 37 wt % of incorporated PCBM. Below 50 wt % of PCBM, the acceptor polymers exhibit exceptional high charge carrier mobility compared to the corresponding blends of molecular PCBM and precursor copolymer. Detailed structural studies using AFM, TEM, and XRD are performed. We confirm amorphous morphology both in thin films and in bulk for the PPCBMs, which clearly indicates the absence of PCBM nanocrystals. Thus, an efficient charge carrier percolation is facilitated by the homogeneous distribution of PCBM in the copolymer. Additionally, the absence of nanocrystal formation was demonstrated during thermal annealing.

Herein, we analyze charge carrier mobility and morphology of the active blend layer in thin film organic solar cells and correlate them with device parameters. A low band gap donor-acceptor copolymer in combination with phenyl-C61-butyric acid methyl ester (PCBM) or two bis-adduct fullerenes, bis-PCBM and bis-o-quino-dimethane C60 (bis-oQDMC), is investigated. We study the charge transport of polymer: fullerene blends in hole-and electron-only devices using the space-charge limited current method. Lower electron mobilities are observed in both bis-adduct fullerene blends. Hole mobility, however, is decreased only in the blend containing bis-oQDMC. Both bis-adduct fullerene blends show very high open circuit voltage in solar cell devices, but poor photocurrent compared to the standard PCBM blend for an active layer thickness of 200 nm. Therefore, a higher short circuit current is feasible for the polymer: bis-PCBM blend by reducing the active layer thickness in order to compensate for the low electron mobility, which results in a PCE of 4.3%. For the polymer: bis-oQDMC blend, no such improvement is achieved due to an unfavorable morphology in this particular blend system. The results are supported by external quantum efficiency measurements, atomic force microscopy, transmission electron microscopy and UV/vis spectroscopy. Based on these results, the investigations presented herein give a more scientific basis for the optimization of solar cells.

ABSTRACTFour different perylene side‐chain semiconductor polymers, synthesized by a combination of “click” chemistry and nitroxide‐mediated radical polymerization, are compared in terms of their optical, electrochemical, and charge transport properties. The nature of the solubilizing side chains and the chromophoric π‐conjugation system of the pendant perylene moieties are systematically changed. Two poly(perylene bisimide)s with hydrophobic (PPBI 1) and hydrophilic substituents (PPBI 2) are compared with poly(perylene diester benzimidazole) (PPDEB) and poly(perylene diester imide) (PPDEI). Optical properties are investigated by UV/vis and photoluminescence spectroscopy, and charge transport is studied by organic field effect transistor and space‐charge‐limited current measurements. Cyclic voltammetry is used to estimate highest occupied molecular orbital and lowest unoccupied molecular orbital levels. The extended π‐conjugation system of PPDEB leads to a broader absorption in the visible region when compared with PPDEI and the PPBIs. Although absorption properties of PPDEB could be considerably improved by varying the perylene core, the charge carrier mobility could be drastically improved by tuning the substituents. Very high electron mobilities of 1 × 10−2 cm2 V−1 s−1 were achieved for PPBI 2 carrying oligoethyleneglycol substituents. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1480–1486

While organic semiconductors used in polymer:fullerene photovoltaics are generally not intentionally doped, significant levels of unintentional doping have previously been reported in the literature. Here, we explain the differences in photocurrent collection between standard (transparent anode) and inverted (transparent cathode) low band-gap polymer:fullerene solar cells in terms of unintentional p-type doping. Using capacitance/voltage measurements, we find that the devices exhibit doping levels of order 10 16 cm −3 , resulting in space-charge regions ~100 nm thick at short circuit. As a result, low field regions form in devices thicker than 100 nm. Because more of the light is absorbed in the low field region in standard than in inverted architectures, the losses due to inefficient charge collection are greater in standard architectures. Using optical modelling, we show that the observed trends in photocurrent with device architecture and thickness can be explained if only charge carriers photogenerated in the depletion region contribute to the photocurrent.

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New classes of liquid‐crystalline semiconductor polymers based on perylene diester benzimidazole and perylene diester imide mesogens are reported. Two highly soluble side‐chain polymers, poly(perylene diester benzimidazole acrylate) (PPDB) and poly(perylene diester imide acrylate) (PPDI) are synthesized by nitroxide‐mediated radical polymerization (NMRP). PPDB shows n‐type semiconductor performance with electron mobilities of 3.2 × 10 −4 cm 2 V −1 s −1 obtained in a diode configuration by fitting the space‐charge‐limited currents (SCLC) according to the Mott–Gurney equation. Interestingly, PPDI performs preferentially as a p‐type material with a hole mobility of 1.5 × 10 −4 cm 2 V −1 s −1 , which is attributed to the less electron‐deficient perylene core of PPDI compared to PPDB. Optical properties are investigated by UV‐vis and fluorescence spectroscopy. The extended π‐conjugation system due to the benzimidazole unit of PPDB leads to a considerably broader absorption in the visible region compared to PPDI. HOMO and LUMO levels of the polymers are also determined by cyclic voltammetry; the resulting energy band‐gaps are 1.86 eV for PPDB and 2.16 eV for PPDI. Thermal behavior and liquid crystallinity are studied by differential scanning calorimetry, polarized optical microscopy, and X‐ray diffraction measurements. The results indicate liquid‐crystalline order of the polymers over a broad temperature range. These thermal, electrical, and optical properties make the perylene side‐chain polymers attractive materials for organic photovoltaics.

The synthesis, characterization and thermotropic properties of novel asymmetrically substituted discotic molecules, perylene diester benzimidazoles (PDBIs), are presented. PDBIs were designed with an imidazole unit at 3,4 positions and a bisester moiety at 9,10 positions of the perylene tetracarboxylic acid core. By attaching linear or branched aliphatic substituents at the ester moiety and two alkyl or alkoxy substituents at the benzimidazole unit, sufficient solubility and the flexibility to obtain mesophases was guaranteed. Thermotropic behaviour, which is strongly influenced by the nature of the respective substituents at the diester and benzimidazole moiety, was investigated using differential scanning calorimetry (DSC), polarization optical microscopy (POM) and X-ray diffraction measurements (XRD). All PDBIs under investigation self-organize into liquid crystalline columnar hexagonal phases (Colh), among them PDBI-3 even at room temperature. Also the formation of a room temperature columnar plastic phase (Colhp) and the formation of a lamellar phase was observed. Due to extension of the π-conjugation system, the absorption of these well soluble discogens is significantly extended to longer wavelengths in the visible regime up to 680 nm.

Tailor-made synthesis and structure-property relationship of several swallow-tail N-substituted perylene bisimide (PBI) dyes are presented. PBI derivatives were synthesized by two distinct synthetic approaches, the details being evaluated herein. All the PBIs carry either alkyl swallow-tail or oligoethylenglycolether (OEG) swallow-tail moieties as N-substituents, and many of them are unsymmetrically substituted. We avoided substitution at bay positions of the perylene core to maintain the planarity and strong pi-pi interactions, which favor intermolecular order and charge carrier transport. The thermotropic behavior, which is strongly influenced by the nature of the substituents was investigated using differential scanning calorimetry (DSC), polarization optical microscopy (POM), and X-ray diffraction measurements (XRD). The introduction of OEG swallow-tail units facilitates thermotropic liquid crystalline behavior in most cases and the unsymmetrical substitution allowed the tuning of the mesophase-width. The mesophases exhibit characteristic columnar hexagonal (Col(h)) packing arising from pi-pi interactions between cofacially orientated perylene molecules. Thus, the inherent tendency of PBI molecules for crystallization could be effectively suppressed by incorporating flexible OEG swallow-tail units only at imide positions. This molecular design was crucial to obtain liquid crystallinity and intracolumnar long-range order. The substituents did not influence the electronic energy levels such as HOMO and LUMO.