Dirk Vanderzande

Observation of the subgap optical absorption in polymer-fullerene blend solar cells

This letter reports on highly sensitive optical absorption measurements on organic donor-acceptor solar cells, using Fourier-transform photocurrent spectroscopy (FTPS). The spectra cover an unprecedented dynamic range of eight to nine orders of magnitude making it possible to detect defect and disorder related sub-band gap transitions. Direct measurements on fully encapsulated solar cells with an active layer of poly[2-methoxy-5-(3′,7′-dimethyl-octyloxy)]-p-phenylene-vinylene:(6,6)-phenyl-C61-butyric-acid (1:4 weight ratio) enabled a study of the intrinsic defect generation due to UV illumination. Solar cell temperature annealing effects in poly(3-hexylthiophene):PCBM (1:2 weight ratio) cells and the induced morphological changes are related to the changes in the absorption spectrum, as determined with FTPS.

The ISOS-3 inter-laboratory collaboration focused on the stability of a variety of organic photovoltaic devices

Seven distinct sets (n ≥ 12) of state of the art organic photovoltaic devices were prepared by leading research laboratories in a collaboration planned at the Third International Summit on Organic Photovoltaic Stability (ISOS-3). All devices were shipped to RISO DTU and characterized simultaneously up to 1830 h in accordance with established ISOS-3 protocols under three distinct illumination conditions: accelerated full sun simulation; low level indoor fluorescent lighting; and dark storage with daily measurement under full sun simulation. Three nominally identical devices were used in each experiment both to provide an assessment of the homogeneity of the samples and to distribute samples for a variety of post soaking analytical measurements at six distinct laboratories enabling comparison at various stages in the degradation of the devices. Over 100 devices with more than 300 cells were used in the study. We present here design and fabrication details for the seven device sets, benefits and challenges associated with the unprecedented size of the collaboration, characterization protocols, and results both on individual device stability and uniformity of device sets, in the three illumination conditions.

Varying polymer crystallinity in nanofiber poly(3-alkylthiophene): PCBM solar cells: Influence on charge-transfer state energy and open-circuit voltage

The effect of poly(3-alkylthiophene) (P3AT) crystallinity in (nanofiber P3AT):PCBM photovoltaic devices on the energy of the charge-transfer state (ECT) and on the open-circuit voltage (Voc) is investigated for poly(3-butythiophene), poly(3-pentylthiophene) and poly(3-hexylhiophene). P3AT crystallinity, expressed as the crystalline nanofiber mass fraction f to the total P3AT mass in the spin-coating dispersion, is varied between ∼0.1 and ∼0.9 by temperature control. ECT, as obtained by Fourier-transform photocurrent spectroscopy decreased with f as ECT=ECT0−0.2f eV. Alkyl side-chain length only influences ECT0. Voc relates to ECT as Voc=ECT/q−0.6 V.

Charge dissociation in polymer:fullerene bulk heterojunction solar cells with enhanced permittivity

The dissociation efficiency of bound electron-hole pairs at the donor-acceptor interface in bulk heterojunction solar cells is partly limited due to the low dielectric constant of the polymer:fullerene blend. We investigate the photocurrent generation in blends consisting of a fullerene derivative and an oligo(oxyethylene) substituted poly(p-phenylene vinylene) (PPV) derivative with an enhanced relative permittivity of 4. It is demonstrated that in spite of the relatively low hole mobility of the glycol substituted PPV the increase in the spatially averaged permittivity leads to an enhanced charge dissociation of 72% for these polymer:fullerene blends.

Influence of polymer ionization potential on the open-circuit voltage of hybrid polymer/TiO2 solar cells

We report studies of the dependence of the open-circuit voltage (VOC) of polymer/titanium dioxide hybrid devices on the ionization potential of the polymer (IP). Once corrected for differences in photocarrier generation by the polymers, the measured VOC values vary linearly with the polymer IP, with a slope of 0.8±0.1. This behavior agrees with recent studies of polymer/fullerene photovoltaic devices and is consistent with the hypothesis that VOC of an organic donor-acceptor solar cell is limited by the energy difference between the highest occupied molecular orbital of the donor (in this case, the polymer) and the lowest unoccupied electronic level of the acceptor (in this case, the conduction band edge of the TiO2).We report studies of the dependence of the open-circuit voltage (VOC) of polymer/titanium dioxide hybrid devices on the ionization potential of the polymer (IP). Once corrected for differences in photocarrier generation by the polymers, the measured VOC values vary linearly with the polymer IP, with a slope of 0.8±0.1. This behavior agrees with recent studies of polymer/fullerene photovoltaic devices and is consistent with the hypothesis that VOC of an organic donor-acceptor solar cell is limited by the energy difference between the highest occupied molecular orbital of the donor (in this case, the polymer) and the lowest unoccupied electronic level of the acceptor (in this case, the conduction band edge of the TiO2).

Generation of specifically substituted pyridines and pyridones from 2(1h) pyrazinones and acetylenes : A FMO description

Abstract The title compounds were obtained from reaction of variously substituted 2(1H)pyrazinones with acetylenic derivatives. Experimental evidence points out to a two step mechanism : a Diels Alder cycloaddition followed by immediate decomposition of the adducts into the title products via two competitive retro Diels Alder reactions. The product distribution, which is shown to be highly dependent on the substitution pattern of the reactants, is accounted for by a simple FMO model.

Hysteresis-free electron currents in poly(p-phenylene vinylene) derivatives

The interpretation of electron currents in conjugated polymers is strongly hindered by the occurrence of hysteresis. We investigate the transport of electrons in electron-only devices based on derivatives of poly(p-phenylene vinylene) (PPV) for various hole-blocking bottom electrodes as well as purification of the polymer. The use of a variety of hole blocking bottom contacts, as metallic electrodes and n-type doped polymers, did not give any improvement in the observed hysteresis. By purification of the PPV, hysteresis free electron-only currents can be obtained. The deep traps responsible for hysteresis, with a concentration in the 10 16 cm -3 range, are not responsible for the trap-limited electron transport as observed in purified PPV.

Diels-Alder reactions of the heterodiene system in 2(1H)-pyrazinones

Abstract Variously substituted 2(1H-pyrazinones react with acetylenic derivatives to give specifically substituted pyridones or pyridines. The observed selectivity can be explained in terms of HO-LU interactions for the reagents and two competitive retro Diels-Alder reactions of the primary bicycloadducts. With cyanotosylate as dienophile no pyrimidone derivative but a new 2(1H)-pyrazinone is obtained.

Poly(3-alkylthiophene) nanofibers for optoelectronic devices

Semiconducting poly(3-alkylthiophene) nanofibers show remarkable optical and electrical properties, and because of their high aspect ratios they are perfectly suited to serve as organic quasi one-dimensional charge carriers. Hence, they offer interesting perspectives for next generation printable optoelectronic applications. This feature article provides an overview of the current state of the art regarding the preparation and characterization of poly(3-alkylthiophene) nanofibers, and a discussion on nanofiber-based optoelectronic applications, i.e. organic field-effect transistors and bulk heterojunction organic solar cells. In addition, current shortcomings and points of attention for future development are identified.

Description of the nanostructured morphology of [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) by XRD, DSC and solid‐state NMR

PCBM or [6,6]‐phenyl‐C61‐butyric acid methyl ester is nowadays still one of the most successful electron acceptors for plastic bulk heterojunction (BHJ) photovoltaic devices. In this study, a set of complementary techniques, i.e. solid‐state NMR, XRD and DSC, is proposed as a fast and sensitive tool to screen the morphology of PCBM specimens with different preparation histories. Based on proton NMR relaxation decay time values, an interval can be derived that situates the average crystal dimensions and which can further be refined on the basis of XRD patterns and DSC thermograms. Copyright