Bimolecular recombination in organic semiconductors is known to follow the Langevin expression, i.e., the rate of recombination depends on the sum of the mobilities of both carriers. We show that this does not hold for polymer/fullerene bulk heterojunction solar cells. The voltage dependence of the photocurrent reveals that the recombination rate in these blends is determined by the slowest charge carrier only, as a consequence of the confinement of both types of carriers to two different phases.

We report here the synthesis and characteristics of a new C 84 adduct ([84]PCBM), realized via a diazoalkane addition reaction. [84]PCBM was obtained as a mixture containing three major isomers.[84]PCBM was tested in a fullerene/poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) bulk heterojunction solar cell as the first C 84 derivative to be applied in device fabrication. Spin coating the active layer blend from 1-chloronaphthalene (the very best fullerene solvent) instead of ortho-dichlorobenzene was necessary to obtain the more efficient photovoltaic device. The PV results indicate that the hole mobility of MDMO-PPV may not be increased upon blending with [84]PCBM. This explains the relatively low I SC of the device as due to the buildup of space charge. The V OC of the device is ∼500 mV lower than that of the one with [60]PCBM, while [84]PCBM has a 350 mV higher electron affinity than [60]PCBM. This loss surpasses the linear relation between the donor HOMOacceptor LUMO energy gap and the V OC in this type of device. A maximum power conversion efficiency of 0.25% was reached for the MDMO-PPV:[84]PCBM cells.

The hole transport in various poly(p‐phenylene vinylene) (PPV) derivatives has been investigated in field‐effect transistors (FETs) and light‐emitting diodes (LEDs) as a function of temperature and applied bias. The discrepancy between the experimental hole mobilities extracted from FETs and LEDs based on a single disordered polymeric semiconductor originates from the strong dependence of the hole mobility on the charge carrier density. The microscopic charge transport parameters are directly related to the chemical composition of the analysed polymers. By chemically modifying the PPV, the hole mobility in both FETs and LEDs can be changed by orders of magnitude. For highly disordered PPVs it is demonstrated that the exponential density of states (DOS), which is used to describe the charge transport in FETs, is a good approximation of the tail states of the Gaussian DOS, which describes the charge transport in LEDs. Increase of the directional order in the PPV film enhances the mobility but also induces a strong anisotropy in the charge transport, thereby obscuring a direct comparison between sandwich and field‐effect devices. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The use of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) in combination with ZnO as middle electrode in solution-processed organic tandem solar cells requires a pH modification of the PEDOT:PSS dispersion. We demonstrate that this neutralization leads to a reduced work function of PEDOT:PSS, which does not affect the performance of polythiophene:fullerene solar cells, but results in a lower open-circuit voltage of devices based on a polyfluorene derivative with a higher ionization potential. The introduction of a thin layer of a perfluorinated ionomer recovers the anode work function and gives an open-circuit voltage of 1.92 V for a double junction polyfluorene-based solar cell.

The charge transport in white light-emitting polymers was investigated using hole-only devices of a series of copolymers. The series consists of three spirobifluorene copolymers with a common backbone. The polymers contain an increasing number of dyes that are included into the backbone to achieve white light emission. Because the dyes have different band gaps than the backbone, it is expected that they will hamper the charge transport in the polymer since they will result in electron traps or hole traps, or both. In this experiment we demonstrate that the dyes in the polymers under investigation have no influence on the hole transport. We therefore conclude that the dyes have their HOMO levels aligned and are thus likely to function as electron traps.

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