Crystalline Organization of a Methanofullerene as Used for Plastic Solar‐Cell Applications

one of the best performing semiconducting polymers used in bulk heterojunction devices is PCDTBT, a polycarbazole derivative with solar-conversion efficiencies as high as 7.2%. Here we report the formation of bilayer ordering in PCDTBT, and postulate that this structural motif is a direct consequence of the polymer's molecular design. This bilayer motif is composed of a pair of backbones arranged side-to-side where the alkyl tails are on the outer side. This is in stark contrast to the monolayer ordering found in other conjugated polymers. The crystalline bilayer phase forms at elevated temperatures and persists after cooling to room temperature. The existence of bilayer ordering, along with its high-packing fraction of conjugated moieties, may guide the synthesis of new materials with improved optoelectronic properties.

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“…6b, the sharp peaks observed are from the crystallization of PCBM, consistent with previous results 26 . The scattering profiles along q z (Fig.…”

Section: Resultssupporting

confidence: 81%

Bilayer order in a polycarbazole-conjugated polymer

et al. 2012

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“…[19][20][21][22][23] It is well known that thermal annealing improves the power conversion efficiency of polymer-based BHJ photovoltaics, but the effects of annealing on carrier mobility in the individual BHJ components and the way annealing affects the degree of phase segregation also have been the subject of debate. 19,20 Finally, because BHJs have a complex, difficult-to-characterize nanoscale morphology, 15,[24][25][26][27][28] there have been no systematic studies investigating how the transit times for electrons and holes on the two components affect the general shape of the device current-voltage response under solar illumination.…”

Section: -13mentioning

confidence: 99%

Reappraising the Need for Bulk Heterojunctions in Polymer−Fullerene Photovoltaics: The Role of Carrier Transport in All-Solution-Processed P3HT/PCBM Bilayer Solar Cells

et al. 2009

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The most efficient organic solar cells produced to date are bulk heterojunction (BHJ) photovoltaic devices based on blends of semiconducting polymers such as poly(3-hexylthiophene-2,5-diyl) (P3HT) with fullerene derivatives such as [6,6]-penyl-C 61 -butyric-acid-methyl-ester (PCBM). The need for blending the two components is based on the idea that the exciton diffusion length in polymers like P3HT is only ∼10 nm, so that the polymer and fullerene components must be mixed on this length scale to efficiently split the excitons into charge carriers. In this paper, we show that the BHJ geometry is not necessary for high efficiency, and that all-solution-processed P3HT/PCBM bilayer solar cells can be nearly as efficient as BHJ solar cells fabricated from the same materials. We demonstrate that o-dichlorobenzene (ODCB) and dichloromethane serve nicely as a pair of orthogonal solvents from which sequential layers of P3HT and PCBM, respectively, can be spin-cast. Atomic force microscopy, various optical spectroscopies, and electron microscopy all demonstrate that the act of spin-coating the PCBM overlayer does not affect the morphology of the P3HT underlayer, so that our spin-cast P3HT/PCBM bilayers have a well-defined planar interface. Our fluorescence quenching experiments find that there is still significant exciton splitting in P3HT/PCBM bilayers even when the P3HT layer is quite thick. When we fabricated photovoltaic devices from these bilayers, we obtained photovoltaic power conversion efficiencies in excess of 3.5%. Part of the reason for this high efficiency is that we were able to separately optimize the roles of each component of the bilayer; for example, we found that thermal annealing has relatively little effect on the nature of P3HT layers spin-cast from ODCB, but that it significantly increases the crystallinity and thus the mobility of electrons through PCBM. Because the carriers in bilayer devices are generated at the planar P3HT/PCBM interface, we also were able to systematically vary the distance the carriers have to travel to be extracted at the electrodes by changing the layer thicknesses without altering the bulk mobility of either component or the nature of the interfaces. We found that devices have the best fill-factors when the transit times of electrons and holes through the two layers are roughly balanced. In particular, we found that the most efficient devices are made with P3HT layers that are about four times thicker than the PCBM layers, demonstrating that it is the conduction and the extraction of electrons through the fullerene that ultimately limit the performance of both bilayer and BHJ devices based on the P3HT/ PCBM material combination. Overall, we believe that polymer-fullerene bilayers provide several advantages over BHJ devices, including reduced carrier recombination and a much better degree of control over the properties of the individual components and interfaces during device fabrication.

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“…show one of these structures). Values near of these PCBM d-spacings have been reported [5]. In the structure shown in Figure 2d, one can notice that some areas are not very clear because the nanodomains are covered with the polymer.…”

mentioning

confidence: 88%

“…poly(3-hexylthiophene (P3HT). The P3HT [4] and fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) [5] crystallinities have been reported independently in blended film of PCBM:P3HT [4] and in PCBM film [5], respectively.…”

Section: Introductionmentioning

confidence: 99%

Methanofullerene elongated nanostructure formation for enhanced organic solar cells

et al. 2007

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Using transmission electron microscopy (TEM) and Z-contrast imaging we have demonstrated elongated nanostructure formation of fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) within an organic host through annealing. The annealing provides an enhanced mobility of the PCBM molecules and, with good initial dispersion, allows for the formation of exaggerated grain growth within the polymer host. We have assembled these nanostructures within the regioregular conjugated polymer poly(3-hexylthiophene) (P3HT). This PCBM elongated nanostructure formation maybe responsible for the very high efficiencies observed, at very low loadings of PCBM (1:0.6, polymer to PCBM), in annealed photovoltaics. Moreover, our high resolution TEM and electron energy 2 loss spectroscopy studies clearly show that the PCBM crystals remain crystalline and are unaffected by the 200-keV electron beam.

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Crystalline Organization of a Methanofullerene as Used for Plastic Solar‐Cell Applications

Cited by 204 publications

References 26 publications

Bilayer order in a polycarbazole-conjugated polymer

Bilayer order in a polycarbazole-conjugated polymer

Reappraising the Need for Bulk Heterojunctions in Polymer−Fullerene Photovoltaics: The Role of Carrier Transport in All-Solution-Processed P3HT/PCBM Bilayer Solar Cells

Methanofullerene elongated nanostructure formation for enhanced organic solar cells

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