The influence of fullerene side chain functionalization on both the morphology and electro-optical properties of bulk-heterojunction polymer:fullerene solar cells is discussed through a systematic investigation of material blends consisting of the conjugated polymer poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) as donor and fullerene molecules with different side chains as the acceptor. The varying side chain of the fullerenes was found to induce morphological changes as confirmed by different analytical techniques such as Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), and Nuclear Magnetic Resonance (NMR). The fullerene with the shorter side chain, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), forms crystalline nanophases in the blend, while this is not the case for the alternative diphenylmethanofullerene acceptor, [6,6]-1,1-bis(4,4′-dodecyloxyphenyl)methanofullerene (DPM-12). The introduction of NMR allows us to estimate the fraction of crystalline fullerene. The morphological changes have a profound effect on the characteristics of charge transfer states (CT) formed at the polymer:fullerene interfaces. Crystallization of fullerene molecules shifts the energy of the CT state. This shift in energy is directly manifested in the open-circuit voltage of solar cells based on the fullerene acceptors under investigation.
The dithiocarbamate precursor route is a suitable way to synthesize poly(p-phenylene vinylene) derivatives in an efficient manner. It is demonstrated that this precursor route combines the straightforward monomer synthesis of the Gilch route with the superior polymer quality of the more complex sulfinyl route. To obtain the polymers, the bisdithiocarbamate MDMO monomer has been polymerized using either lithium bis(trimethylsilyl)amide (LHMDS) or potassium tert-butoxide (KtBuO). The addition of either base results in the formation of high molecular weight precursor polymer. It is shown that the polymerization mechanism follows a radical pathway. Furthermore it is demonstrated that the molecular structure of the polymer shows a certain degree of regioregularity when LHMDS is used. The thermal conversion of the precursor polymer into the conjugated system is studied by in situ UV-vis and FT-IR spectroscopy. A NMR study on 13 C-labeled MDMO-PPV reveals the presence of only a minimal amount of structural defects in the microstructure of the polymer, further confirming the excellent characteristics of the dithiocarbamate precursor route.
In this article, it is demonstrated that doctor blading of thin poly-3-hexylthiophene/phenyl-C 61 -butyric acid methyl ester (1/1) bulk-hetero junction films from toluene leads to an improved nanocrystallinity, when compared with their unannealed chlorobenzene processed counterparts. This difference in morphology was demonstrated by solid-state NMR and Rapid Heating Cooling Calorimetry (RHC), being useful complementary techniques to investigate the active layer morphology of photovoltaic devices. An increased PC 60 BM nanocrystallinity is indi-cated by several NMR relaxation decay times (T 1C , T 1H , and T 1qH ) and confirmed by an increase of the melting enthalpy in RHC experiments. An improved solar cell performance further strengthens this conclusion. V C 2011 Wiley Periodicals, Inc.
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