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The origin of the enhanced performance of bulk heterojunction solar cells based on slowly dried films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester is investigated, combining charge transport measurements with numerical device simulations. Slow drying leads to a 33-fold enhancement of the hole mobility up to 5.0×10-7 m2 V-1 s-1 in the P3HT phase of the blend, thereby balancing the transport of electrons and holes in the blend. The resulting reduction of space-charge accumulation enables the use of thick films (∼300 nm), absorbing most of the incoming photons, without losses in the fill factor and short-circuit current of the device. [ABSTRACT FROM AUTHOR]

An increase in the workfunction of the metal top electrode leads to a reduction of the open-circuit voltage, short-circuit current, and power conversion efficiency of organic bulk-heterojunction solar cells. It has been demonstrated that the photocurrent obtained from an active layer comprised of a blend of poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylene vinylene) (OC1C10-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), with lithium fluoride topped aluminum, silver, gold, or palladium electrodes, shows a universal behavior when scaled against the effective voltage across the device. Indeed, model calculations confirm that the dependence of the photocurrent on the effective voltage is responsible for the observed variation in performance of each different electrode. Consequently, for any given metal, only the device’s open-circuit voltage is required in order to be able to predict the remaining solar cell parameters. [ABSTRACT FROM AUTHOR]