INTRODUCTION TO CHARGE CARRIER TRANSPORT IN ORGANIC SOLAR CELLSIncreasing energy consumption and rising energy prices in the world forces to look for energy alternatives, one of the most promising being the photovoltaic solar energy conversion. Various concepts and device architectures of organic solar cells have been actively studied for more than 30 years. [1][2][3][4][5][6][7][8] Efficiencies, routinely exceeding 4-5% have been reached in thin-film organic solar cells today. From purely academic point of view, the research of organic solar cells is interesting due to novel photophysical phenomena, whereas technologically low fabrication costs due to roll-toroll printing possibilities drive the economic point of view. There are four main important processes which might limit the power conversion efficiency of photovoltaic devices: 9 1. Light absorption in the film. 2. Free charge carrier generation. 3. Charge transport to the opposite electrodes and extraction by the electrodes. 4. Carrier recombination.When the photoexcitation (an exciton) is created after the photon energy is absorbed in the material, mobile charge carriers must be created by splitting the exciton into a free electron and hole. Therefore, donor and acceptor blends are used in the organic photovoltaics to facilitate photoinduced charge transfer. If the exciton reaches the donor-acceptor interface, the electron can then be transferred to the material with lower lying Lowest Unoccupied Molecular Orbit (LUMO) if I D * − E A − Coulomb < 0, where I D * is the ionization potential of the excited donor, E A the electron affinity of the acceptor, and Coulomb summarizes all the electrostatic interactions including the exciton binding energy and all polarizations. The important parameters here are the exciton diffusion length and the distance between the donor and acceptor phases.Furthermore, both these charge carriers must be transported to the opposite electrodes and reach them prior to recombination. If after photoinduced charge transfer the electron and hole are still bound by the Coulomb potential, then typically for low mobility materials, they cannot escape from each others attraction and will finally recombine. However, the excitons can be split into free electrons and holes when the carrier dissipation distance is larger than the Coulomb radius. To fulfill this condition the Coulomb field must be screened or charge carrier hopping distance must be larger than the Coulomb radius (for low mobility materials it is unrealistic). 10 In this case mobile charge carriers can be transported to the contacts either by carrier diffusion or electric field induced drift. In order to have unity quantum efficiency for charge extraction, one needs to fulfill the condition that the charge carrier transit time t tr is much smaller than the carrier lifetime τ (t tr τ). The carrier transit time t tr = d/µE is determined by the charge carrier mobility µ, sample thickness d, and the electric field E inside the film. If the photocurrent is governed by the carrier dri...

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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Time-dependent mobility and recombination of the photoinducted charge carriers in conjugated polymer/fullerene bulk heterojunction solar cells. Physical Review Letters, 72 035217-1-035217-10. Time-dependent mobility and recombination of the photoinducted charge carriers in conjugated polymer/fullerene bulk heterojunction solar cells