Conjugated polymer chains have many degrees of conformational freedom and interact weakly with each other, resulting in complex microstructures in the solid state. Understanding charge transport in such systems, which have amorphous and ordered phases exhibiting varying degrees of order, has proved difficult owing to the contribution of electronic processes at various length scales. The growing technological appeal of these semiconductors makes such fundamental knowledge extremely important for materials and process design. We propose a unified model of how charge carriers travel in conjugated polymer films. We show that in high-molecular-weight semiconducting polymers the limiting charge transport step is trapping caused by lattice disorder, and that short-range intermolecular aggregation is sufficient for efficient long-range charge transport. This generalization explains the seemingly contradicting high performance of recently reported, poorly ordered polymers and suggests molecular design strategies to further improve the performance of future generations of organic electronic materials.
X-ray diffraction studies of oriented poly(3-n-alkylthiophene) (P3AT's) solution-cast films show that these samples are characterized by well-organized lamellar structures whereby stacks of planar thiophene main chains are uniformly spaced by the alkyl side chains. In spite of considerable hydrocarbon chemical bonding and conformational disorder, a high degree of structural regularity is observed with appreciable three-dimensional ordering of the polymer chains. Analysis of the resultant scattering data in combination with model structure factor calculations leads to suggestive models in which an open packing of the alkyl chains is maintained with considerable side-chain disorder and side-chain mobility. There is evidence that this flexibility allows for an additional ordering process to occur at the interface formed by the alkyl side-chain end groups and that the liquid crystalline behavior is intimately related to the appearance of this structure.
The most promising device structure for organic photovoltaic devices presented to date is the ''bulk-heterojunction'' whereby a hole-conducting (electron-donating) conjugated polymer, such as poly(3-hexylthiophene) (P3HT), is blended with an electron-conducting (electron-accepting) smallmolecular compound, such as a fullerene derivative. The reported strong composition-and thermal-treatment dependence of the power conversion efficiency of such binaries suggests that phase behavior, processing conditions and the resulting microstructure play a dominant role in the performance of devices based on these systems. Here, we propose a simple rationale for selecting the optimum composition of such crystalline/crystalline polymer/small molecule blends. We find that these binary systems feature simple eutectic phase behavior, and that the optimum composition for device performance is slightly hypoeutectic when expressed in terms of the polymer component. In accord with classical understanding of eutectic solidification, these blends feature a finely phase-separated matrix surrounding primary crystals of the small-molecular species. The combination of large interfacial area and component connectivity yield a desired microstructure for use in bulk-heterojunctions.While significant advances have been made in recent years, [1][2][3][4] power conversion efficiencies of organic photovoltaic devices still lag behind those of conventional inorganic solar cells. These lower values are primarily due to reduced photocurrents, resulting not only from poor optical absorption in the red but also, in many cases, from a failure to convert absorbed photons into current with high efficiency. In devices comprising bulk-heterojunction binaries, deposited from solution as a single composite film, a major obstacle to enhanced performance is the laborious optimization of the ratio of components [5][6][7][8][9][10][11] and processing protocols [7][8][9][10][11][12][13][14][15][16] in order to achieve a blend microstructure that simultaneously maximizes exciton generation, exciton dissociation, and charge transport to electrodes. From the perspective of a binary blend of non-interacting donor and acceptor components, optimum photocurrent generation should result for the optimum compromise between (i) high light absorption, achieved by maximizing the volume fraction of the component with the stronger absorption in the visible (usually the polymer in a polymer/small molecule bulk-heterojunction device), (ii) efficient charge separation, realized by maximizing the donor-acceptor interface area, and (iii) balanced charge transport, accomplished by compensating any imbalance in mobility by the volume available for charge conduction, provided that both components form percolating structures. [17,18] However, this view does not allow for the effects of compositional changes on molecular order and microstructure, and hence on the optoelectronic material properties. In practice, photocurrent is maximized in many donor-acceptor systems at compositions quite...
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