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Cowan S.R.,University of California at Santa Barbara | Leong W.L.,University of California at Santa Barbara | Banerji N.,University of California at Santa Barbara | Dennler G.,IMRA Europe | Heeger A.J.,University of California at Santa Barbara
Advanced Functional Materials | Year: 2011

Small amounts of impurity, even one part in one thousand, in polymer bulk heterojunction solar cells can alter the electronic properties of the device, including reducing the open circuit voltage, the short circuit current and the fill factor. Steady state studies show a dramatic increase in the trap-assisted recombination rate when [6,6]-phenyl C 84 butyric acid methyl ester (PC 84BM) is introduced as a trap site in polymer bulk heterojunction solar cells made of a blend of the copolymer poly[N-9″-hepta-decanyl-2,7- carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′, 3′-benzothiadiazole) (PCDTBT) and the fullerene derivative [6,6]-phenyl C 61 butyric acid methyl ester (PC 60BM). The trap density dependent recombination studied here can be described as a combination of bimolecular and Shockley-Read-Hall recombination; the latter is dramatically enhanced by the addition of the PC 84BM traps. This study reveals the importance of impurities in limiting the efficiency of organic solar cell devices and gives insight into the mechanism of the trap-induced recombination loss. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Clarke T.M.,University of Wollongong | Peet J.,Konarka Technologies | Nattestad A.,University of Wollongong | Drolet N.,Konarka Technologies | And 4 more authors.
Organic Electronics: physics, materials, applications | Year: 2012

Organic photovoltaic devices based on the donor:acceptor blend of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′, 7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) have received considerable attention in recent years due to their high power conversion efficiencies and the ability to achieve close to 100% internal quantum efficiency. However, the highest efficiencies were all attained using active layers of less than 100 nm, which is not ideal for either maximised potential performance or commercial viability. Furthermore, more recent reports have documented significant charge carrier trapping in these devices. In this paper two charge extraction techniques (photo-CELIV and time-of-flight) have been used to investigate the mobility and recombination behaviour in a series of PCDTBT:PCBM devices. The results not only confirm significant charge carrier trapping in this system, but also reveal close to Langevin-type bimolecular recombination. The Langevin recombination causes a short charge carrier lifetime that results in a short drift length. The combination of these two characteristics (trapping and fast bimolecular recombination) has a detrimental effect on the charge extraction efficiency when active layers greater than ∼100 nm are used. This accounts for the pronounced decrease in fill factor with increasing active layer thickness that is typically observed in PCDTBT:PCBM devices. © 2012 Elsevier B.V. All rights reserved.

Boix P.P.,Jaume I University | Larramona G.,IMRA Europe | Jacob A.,IMRA Europe | Delatouche B.,IMRA Europe | And 2 more authors.
Journal of Physical Chemistry C | Year: 2012

All-solid semiconductor-sensitized solar cells lack models allowing their characterization in terms of the fundamental processes of charge transport and recombination. Nanostructured TiO 2/Sb 2S 3/CuSCN solar cells were characterized by impedance spectroscopy, and a model was proposed for this type of cells. One important feature resulting from this analysis was the hole transport diffusion, which could be assimilated to a series resistance affecting the cell fill factor. The other important feature was the recombination rate, which could be described in a similar manner as other cells using nanostructured TiO 2 electrodes and which had an important impact on the open circuit. A simulation of the current-voltage curves using such model allowed us to get an approximate quantification of the losses caused by each process and to evaluate the possible improvements on the performance of this kind of cell. © 2011 American Chemical Society.

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