Bimolecular charge carrier recombination has been clarified in bulk-heterojunction solar cells based on a blend of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene using the time-of-flight method. We show how bimolecular recombination influences the charge carrier transport, how it limits the efficiency of low-mobility solar cells, and how to estimate the bimolecular recombination coefficient. We found that bimolecular recombination in these efficient photovoltaic materials is orders of magnitude slower as compared to Langevin recombination expected for low-mobility materials. This effect is inherent to the nanomorphology of the bicontinuous interpenetrating network creating separate pathways for electrons and holes, and paves the way for the fabrication of bulk-heterojunction solar cells where bimolecular recombination is not the limiting factor.

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Thin-film field-effect transistors (FETs) have been prepared using poly(3-hexylthiophene)/ arachidic acid and quinquethiophene/arachidic acid Langmuir–Blodgett (LB) films with thicknesses ranging from a monolayer to some ten monolayers. The effect of the number of layers on the mobility and conductivity has been studied. This is to our knowledge the first demonstration of a LB FET utilizing organic semiconductors as the active material.

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The performance of organic field-effect transistors, OFETs, restricts their potential use to applications where low efficiency and short lifetimes are acceptable, although recently the field has evolved rapidly. [1,2] Proposed applications are radiofrequency information tags, e.g., price tags and inventory labels, sensors, and simple displays.[3±6] A simple, low-cost manufacturing process is desired, however such processes usually result in significantly lower device performance. The manufacture of state-of-the-art OFETs today [7] normally requires the device to be processed in an inert atmosphere and to be shielded from oxygen and humidity in the air.[8] The best organic transistors have almost reached the performance levels of amorphous silicon devices and the technology is entering the commercial stage.[9±13]We report on the manufacture and characteristics of a highperformance OFET. The special feature of the device is its ability to take advantage of ambient moisture by using a hygroscopic gate dielectric. (The device is hereafter referred to as hygroscopic insulator field-effect transistor, HIFET.) The all-polymer HIFET can be produced by a solution process using printing techniques onto flexible plastic substrates where all processing steps can be performed without requiring controlled atmosphere. We record a one-hundred-fold enhancement in measured currents at low drive voltages of only a few volts, with good current saturation and current modulation in ambient atmosphere. The static performance of the device characterized by current level and modulation at low voltages (less than 1 V) is greatly enhanced by moisture in the hygroscopic insulator. This study concentrates on the novel device characteristics and proposes an explanation for the device operation mechanism.The HIFET measured in room atmosphere shows excellent performance in terms of current level and modulation, as can be seen from the output curve in Figure 1A. The current saturates at less than 1 V on the drain electrode for a low constant gate voltage (V G varied within less than ±1 V), indicating a threshold voltage close to 0 V. The corresponding transfer curve in the saturated region is shown in Figure 1B, from which the turn on voltage (V to ) and the ON/OFF ratio can be estimated to be 0.4 V and~200, respectively. Here, V to is giv-