In 1971 Goodman and Rose predicted the occurrence of a fundamental electrostatic limit for the photocurrent in semiconductors at high light intensities. Blends of conjugated polymers and fullerenes are an ideal model system to observe this space-charge limit experimentally, since they combine an unbalanced charge transport, long lifetimes, high charge carrier generation efficiencies, and low mobility of the slowest charge carrier. The experimental photocurrents reveal all the characteristics of a space-charge limited photocurrent: a one-half power dependence on voltage, a three-quarter power dependence on light intensity, and a one-half power scaling of the voltage at which the photocurrent switches into full saturation with light intensity.

A solution‐processed polymer tandem cell fabricated by stacking two single cells in series is demonstrated. The two bulk‐heterojunction subcells have complementary absorption maxima at λmax ∼ 850 nm and λmax ∼ 550 nm, respectively. A composite middle electrode is applied that serves both as a charge‐recombination center and as a protecting layer for the first cell during spin‐coating of the second cell. The subcells are electronically coupled in series, which leads to a high open‐circuit voltage of 1.4 V, equal to the sum of each subcell. The layer thickness of the first (bottom) cell is tuned to maximize the optical absorption of the second (top) cell. The performance of the tandem cell is presently limited by the relatively low photocurrent generation in the small‐bandgap polymer of the top cell. The combination of our tandem architecture with more efficient small‐bandgap materials will enable the realization of highly efficient organic solar cells in the near future.

Universal arrhenius temperature activated charge transport in diodes from disordered organic semiconductors Craciun, N. I.; Wildeman, J.; Blom, P. W. M. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Charge transport models developed for disordered organic semiconductors predict a non-Arrhenius temperature dependence ln / 1=T 2 for the mobility . We demonstrate that in space-charge limited diodes the hole mobility ( h ) of a large variety of organic semiconductors shows a universal Arrhenius temperature dependence h T 0 expÿ=kT at low fields, due to the presence of extrinsic carriers from the Ohmic contact. The transport in a range of organic semiconductors, with a variation in room temperature mobility of more than 6 orders of magnitude, is characterized by a universal mobility 0 of 30-40 cm 2 =V s. As a result, we can predict the full temperature dependence of their charge transport properties with only the mobility at one temperature known.

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The effects of aggregate formation on the photophysical properties of oligomers of MEH-PPV were studied in bulk solution to better understand the effects of aggregation on the emission properties of the polymer. Nanoaggregates of oligomers from 3 to 17 repeat units in length were formed using a solvent reprecipitation method. The spectra are not readily modeled using the classical dipole-dipole coupling picture of interchain interactions. A strong dependence of the photophysics on the oligomer chain length is also observed. Shortchain oligomers produce nanoaggregates with absorption and emission spectra essentially identical to those of the monomer. Long-chain oligomers form aggregates having more strongly perturbed absorption and fluorescence spectra and decreased emission yields. In these aggregates, the size of the 0-0 band relative to that of the vibronic replicates is a sensitive function of aggregate size and solvent precipitation conditions. Their fluorescence lifetimes are also strongly wavelength dependent. These trends are explained in terms of a core-shell model that postulates the existence of "single-chain-like" and "aggregate-like" emitters within a single aggregate.