Charge transfer in the weak driving force limit in blends of MDMO-PPV and dithienylthiazolo[5,4-d]thiazoles towards organic photovoltaics with high VOCElectronic supplementary information (ESI) available: Table S1: profilometer thickness measurements of spin-cast films; figures: optical absorption and PL spectra of the pure compounds in solution (S1) and in spin-cast films (S2); X-band LI-EPR spectra of 1 : 1 and 4 : 1 polymer : DTTzTz films (S3); time-dependent PIA in 1 : 1 blends of MDMO-PPV w

• Main Authors: Nevil, Nissy, Ling, Yun, Mierloo, Sarah Van, Kesters, Jurgen, Piersimoni, Fortunato, Adriaensens, Peter, Lutsen, Laurence, Vanderzande, Dirk, Manca, Jean, Maes, Wouter, Doorslaer, Sabine Van, Goovaerts, Etienne
• Format: Journal Article
• Language: English
• Published: 31.10.2012
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c2cp42399e

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. Efficient and bidirectional charge transfer is demonstrated spectroscopically in all organic blends with novel acceptor molecules for solar cell applications.