Control of blend morphology at the microscopic scale is critical for optimizing the power conversion efficiency of plastic solar cells based on blends of conjugated polymer with fullerene derivatives. In the case of bulk heterojunctions of regioregular poly(3-hexylthiophene) (P3HT) and a soluble fullerene derivative ([6,6]-phenyl C61-butyric acid methyl ester, PCBM), both blend morphology and photovoltaic device performance are influenced by various treatments, including choice of solvent, rate of drying, thermal annealing and vapour annealing. Although the protocols differ significantly, the maximum power conversion efficiency values reported for the various techniques are comparable (4-5%). In this paper, we demonstrate that these techniques all lead to a common arrangement of the components, which consists of a vertically and laterally phase-separated blend of crystalline P3HT and PCBM. We propose a morphology evolution that consists of an initial crystallization of P3HT chains, followed by diffusion of PCBM molecules to nucleation sites, at which aggregates of PCBM then grow.

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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The effect of the addition of 1,8‐octanedithiol (ODT) during processing on the microstructure of blend films of poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]dithiophene)‐alt‐4,7(2,1,3‐benzothiadiazole)] (PCPDTBT) and [6,6]‐phenyl‐C71 butyric acid methyl ester ([70]PCBM) is studied. Grazing incidence X‐ray diffraction and absorption spectroscopy show that the crystalline order of PCPDTBT increases when ODT is introduced in the solution phase either to neat polymer systems or to blends with [70]PCBM. The increased crystalline order is accompanied by less dispersive hole transport in the polymer, and leads to a more efficient formation of a percolating fullerene network within the blend. This contributes to an increase in photocurrent generation. However, the bimolecular recombination rate as determined from photovoltage transients increases upon addition of ODT, limiting the power conversion efficiency to values well below those expected from the energy levels of PCPDTBT. We propose some explanations for this increase in bimolecular recombination, based also on variable angle spectroscopic ellipsometry measurements. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Ternary blends of poly(3‐hexylthiophene): [6,6]‐phenyl C61‐butyric acid methyl ester (P3HT:PC61BM) and the insulating bulk polymers high‐density polyethylene (HDPE), isotactic‐ and atactic polystyrene (i‐PS, a‐PS), are investigated. Addition of up to ≈50 wt% of the electronically inert, semicrystalline HDPE and i‐PS to the organic semiconducting system does not significantly degrade the performance of photovoltaic devices fabricated with these ternary blends.