Optical interference effects are important for the total absorption as well as the profile of the exciton generation rate in polymer:fullerene bulk heterojunction solar cells. For solar cells with an active layer of poly͓2-methoxy-5-͑3Ј ,7Ј-dimethyloctyloxy͒-1,4-phenylenevinylene͔ as electron donor and ͓6,6͔-phenyl C 61 butyric acid methyl ester as electron acceptor, the total exciton generation rate can be directly extracted from the saturated photocurrent. It is demonstrated that for solar cells with an active layer thickness smaller than 250 nm, a constant exciton generation profile, based on this extracted total rate, gives identical electrical characteristics as compared to exciton generation profiles from an optical model. For thicker cells interference effects have to be taken into account, since a uniform generation profile leads to an overestimation of recombination losses and space-charge formation.

Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. The effect of the molecular weight of poly[9,9-didecanefluorene-alt-(bis-thienylene) benzothiadiazole] (PF10TBT) on the photovoltaic performance of fullerene-based bulk heterojunction solar cells is investigated. An increase in molecular weight of two orders of magnitude results in a 30% increase of the short-circuit current and a rise of the fill factor from 0.45 to 0.63. Electron and hole transport are found to be virtually unaffected by changing molecular weight, which means that space-charge effects do not play a role in low molecular weight devices. Using optical modeling and numerical device simulations, we demonstrate that at low molecular weight the efficiency is mainly limited by a short lifetime of bound electron-hole pairs. This short lifetime prohibits efficient dissociation and is attributed to a deficiency in phase separation for low molecular weights.

Lowering of the optical band gap of conjugated polymers in bulk heterojunction solar cells not only leads to an increased absorption but also to an increase of the optimal active layer thickness due to interference effects at longer wavelengths. The increased carrier densities due to the enhanced absorption and thicker active layers make low band gap solar cells more sensitive to formation of space charges and recombination. By systematically red shifting the optical parameters of poly[2-methoxy-5-(3 0 ,7and 6,6-phenyl C 61 -butyric acid methyl ester, we simulate the effect of a reduced band gap on the solar cell efficiencies. We show that especially the fill factor of low band gap cells is very sensitive to the balance of the charge transport. For a low band gap cell with an active layer thickness of 250 nm, the fill factor of 50% for balanced transport is reduced to less than 40% by an imbalance of only one order of magnitude. The efficiency of organic polymer:fullerene bulk heterojunction solar cell performance has been steadily increasing in the last years, going from 2.5% efficiency in 2001 1 to 3.5% in 2003, 2 up to 5.5% in 2007, 3,4 and recently an efficiency of 7.4% has been reported. 5 Part of the improvement originated from the refinement of existing production techniques to optimize the active layer morphology.6-8 The majority of the improvement, however, came from the development of low band gap polymers. 3,4,[9][10][11][12][13][14][15][16][17] By decreasing the bandgap of the donor, the amount of absorbed photon flux increases due to an enhanced overlap with the solar spectrum. A lowering of the polymer band gap can either be achieved by a lowering of the lowest unoccupied molecular orbital (LUMO) or by a raise of the highest occupied molecular orbital (HOMO). The lowering of the LUMO is limited by the energy offset needed for electron transfer to the acceptor, typically 0.4 eV. A raise of the HOMO on the other hand will lead to a decrease of the open circuit voltage V oc . A theoretical study by Koster et al. 18 showed that a decrease of the bandgap from 2.1 to 1.5 eV, by lowering the LUMO and keeping the HOMO in place, is expected to result in an increase in power conversion efficiency g from 3.5% to over 8%. This enhanced performance mainly originated from an increased short circuit current density J sc due to an increased absorption. However, an increased absorption because of a better spectral overlap with the solar sprectrum also leads to an increased concentration of electrons and holes in the solar cells. This enhancement of the carrier density then gives rise to an increase of the bimolecular recombination, which is proportional to the product of the electron and hole density. Furthermore, the higher carrier densities make the solar cells also more sensitive for the built-up of space charge, caused by an imbalanced charge transport. Space charge built-up is also dependent on the absorption profiles in the solar cell, which were not included in these calculations. To obtain more in...

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We present the model calculations to explore the potential of polymer:fullerene tandem solar cells. As an approach we use a combined optical and electrical device model, where the absorption profiles are used as starting point for the numerical current-voltage calculations. With this model a maximum power efficiency of 11.7% for single cells has been achieved as a reference. For tandem structures with a ZnO/poly(3,4-ethylenedioxythiophene)/poly(styrenesulphonic acid) middle electrode an ultimate efficiency of 14.1% has been calculated. In the optimum configuration the subcell with the narrowest band gap is placed closest to the incoming light. Consequently, tandem structures are expected to enhance the performance of optimized single cells by about 20%.