J. Wildeman

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.

Abstract 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 λ max ∼ 850 nm and λ max ∼ 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.

Charge transport models developed for disordered organic semiconductors predict a non-Arrhenius temperature dependence $\mathrm{ln}(\ensuremath{\mu})\ensuremath{\propto}1/{T}^{2}$ for the mobility $\ensuremath{\mu}$. We demonstrate that in space-charge limited diodes the hole mobility (${\ensuremath{\mu}}_{h}$) of a large variety of organic semiconductors shows a universal Arrhenius temperature dependence ${\ensuremath{\mu}}_{h}(T)={\ensuremath{\mu}}_{0}\mathrm{exp}(\ensuremath{-}\ensuremath{\Delta}/kT)$ 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 ${\ensuremath{\mu}}_{0}$ of $30--40\text{ }\text{ }{\mathrm{cm}}^{2}/\mathrm{V}\text{ }\mathrm{s}$. As a result, we can predict the full temperature dependence of their charge transport properties with only the mobility at one temperature known.

Time-resolved luminescence spectroscopy has been used to investigate exciton diffusion in thin films of poly($p$-phenylene vinylene) (PPV)--based derivatives. Due to chemical modifications the PPV derivatives differ by three orders of magnitude in charge carrier mobility as a result of a reduced energetic disorder. From the photoluminescence decay curves of PPV/fullerene heterostructures, the exciton diffusion coefficient was found to increase by one order of magnitude with decreasing disorder. This increase in the diffusion coefficient is compensated by a decrease of the exciton lifetime, leading to an exciton diffusion length of $5--6\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for the various PPV derivatives.