title: Charge transfer in the weak driving force limit in blends of MDMO-PPV and dithienylthiazolo[5,4-d]thiazoles towards organic photovoltaics with high V(OC).
abstract: A series of three 5'-aryl-2,5-dithienylthiazolo[5,4-d]thiazole (DTTzTz) semiconducting molecules with different aryl substituents has been investigated as alternative acceptor materials in combination with the donor polymer poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) in order to evaluate the photoinduced charge transfer (CT) efficiency in the resulting blends, designed towards possible application in organic photovoltaics. Photoluminescence quenching together with polaron detection by electron paramagnetic resonance and photoinduced absorption (PIA) demonstrate an increasing charge transfer efficiency when the DTTzTz substituents are varied from thien-2-yl to 4-trifluoromethylphenyl and 4-cyanophenyl groups, correlating well with the increasing acceptor strength in this series of molecules. In line with this observation, there is a decrease in the effective optical bandgap relative to pure MDMO-PPV that becomes more pronounced along this series of acceptor compounds, reaching 0.12 eV in the blend with 4-CN-Ph-DTTzTz. Intermolecular interactions between the blend components lead to lower energy transitions which are found to contribute significantly to the device external quantum efficiency. The high V(OC) reached in devices based on MDMO-PPV:4-CN-Ph-DTTzTz blends meets the expectations for such a donor:acceptor combination. However, thermal activation of charge carrier recombination occurs because of the weak driving force for charge transfer, as shown by time-dependent PIA measurements, and this is suggested as a cause for the observed low photovoltaic performance.
authors.affiliation: University of Antwerp, Physics Department, Universiteitsplein 1, B-2610 Antwerpen, Belgium.
authors.forename: Nissy
authors.initials: N
authors.lastname: Nevil
authors.forename: Yun
authors.initials: Y
authors.lastname: Ling
authors.forename: Sarah
authors.initials: S
authors.lastname: Van Mierloo
authors.forename: Jurgen
authors.initials: J
authors.lastname: Kesters
authors.forename: Fortunato
authors.initials: F
authors.lastname: Piersimoni
authors.forename: Peter
authors.initials: P
authors.lastname: Adriaensens
authors.forename: Laurence
authors.initials: L
authors.lastname: Lutsen
authors.forename: Dirk
authors.initials: D
authors.lastname: Vanderzande
authors.forename: Jean
authors.initials: J
authors.lastname: Manca
authors.forename: Wouter
authors.initials: W
authors.lastname: Maes
authors.forename: Sabine
authors.initials: S
authors.lastname: Van Doorslaer
authors.forename: Etienne
authors.initials: E
authors.lastname: Goovaerts
mesh_terms: D011211:Electric Power Supplies; D049449:Luminescence; D015394:Molecular Structure; D009930:Organic Chemicals / Q000138:chemical synthesis / Q000737:chemistry*; D011145:Polyvinyls / Q000737:chemistry*; D012993:Solar Energy; D013844:Thiazoles / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D009930:Organic Chemicals; D011145:Polyvinyls; D013844:Thiazoles; C579007:dithienylthiazolo(5,4-d)thiazole; C579008:poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene)