A power conversion efficiency of 4.2% (AM1.5, 1000W∕m2) is measured for an organic solar cell based on an active layer of an alternating copolymer, containing a fluorene and a benzothiadiazole unit with two neighboring thiophene rings, and a fullerene derivative. Using optical modeling, the absorption profile in the active layer of the solar cell is calculated and used to calculate the maximum short circuit current. The calculated currents are compared with measured currents from current-voltage measurements for various film thicknesses. From this the internal quantum efficiency is estimated to be 75% at the maximum for the best device.

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The degradation of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylene vinylene] (MDMO-PPV) during the processing of hybrid organic/inorganic bulk-heterojunction solar cells with zinc oxide (ZnO) from a molecular precursor as acceptor is reported. Upon addition of diethylzinc, the absorption spectrum of MDMO-PPV shifts to the blue, and hole transport through the polymer deteriorates dramatically, indicating a reduction of the conjugation length of the polymer backbone. To prevent polymer degradation through the breaking of trans vinyl bonds, regioregular poly(3-hexylthiophene) (P3HT) is introduced as the electron donor. This system of P3HT and precursor ZnO reveals an unchanged UV-vis absorption profile and zero-field hole mobility with respect to the pristine polymer as well as an improved photovoltaic performance with an estimated power conversion efficiency of 1.4% (AM1.5 global reference spectrum, 1 kW/m 2 ).

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.

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.

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...