Since the past decade, low-cost solution-processable organic photovoltaics (OPVs), based on bulk heterojunction (BHJ) architectures have experienced a continuous evolution. 1 Despite being outperformed by other material systems, 2 the donoracceptor blend of regioregular poly(3-hexylthiophene) (rr-P3HT) and phenyl-C 61 -butyric acid methyl ester (PCBM), remains fundamental as an OPV benchmark.Kayunkid et al. 3 recently reported that P3HT crystallizes with a monoclinic (a = 16 to 17 Å (depending on the processing conditions), b = 7.8 Å, c = 7.8 Å, γ = 86.5°) unit cell, with space group P2 1 /c and with two chains per cell. P3HT self-organizes into lamellar structures. 4 A lamella is made of stacked backbones in the direction of the alkyl side chains (a direction, Figure 1a-c). In turn adjacent lamellae are stacked (stacking distance = b/2) perpendicular to the parallel/cofacial conjugated backbones (b direction, Figure 1a-c). In a schematic view, the π-stacking direction is always orthogonal to the alkyl-stacking direction and can be parallel to the sample substrate, with the alkyl-stacking direction perpendicular (edge-on, Figure 1a) to the sample substrate or with the alkyl-stacking direction parallel to the sample substrate (face-on, b). The arrangement with the backbone perpendicular to the substrate, with both the π-stacking and alkyl-stacking directions parallel to the substrate, is far less likely, being reported only in nanoimprinted samples 4e,f (Figure 1c).

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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

The bis and tris adducts of [6,6]phenyl-C(61)-butyric acid methyl ester (PCBM) offer lower reduction potentials than PCBM and are therefore expected to offer larger open-circuit voltages and more efficient energy conversion when blended with conjugated polymers in photovoltaic devices in place of PCBM. However, poor photovoltaic device performances are commonly observed when PCBM is replaced with higher-adduct fullerenes. In this work, we use transmission electron microscopy (TEM), steady-state and ultrafast time-resolved photoluminescence spectroscopy (PL), and differential scanning calorimetry (DSC) to probe the microstructural properties of blend films of poly(3-hexylthiophene-2,5-diyl) (P3HT) with the bis and tris adducts of PCBM. TEM and PL indicate that, in as-spun blend films, fullerenes become less soluble in P3HT as the number of adducts increases. PL indicates that upon annealing crystallization leads to phase separation in P3HT:PCBM samples only. DSC studies indicate that the interactions between P3HT and the fullerene become weaker with higher-adduct fullerenes and that all systems exhibit eutectic phase behavior with a eutectic composition being shifted to higher molar fullerene content for higher-adduct fullerenes. We propose two different mechanisms of microstructure development for PCBM and higher-adduct fullerenes. P3HT:PCBM blends, phase segregation is the result of crystallization of either one or both components and is facilitated by thermal treatments. In contrast, for blends containing higher adducts, the phase separation is due to a partial demixing of the amorphous phases. We rationalize the lower photocurrent generation by the higher-adduct fullerene blends in terms of film microstructure.

We use grazing-incidence wide-angle X-ray scattering (GIWAXS) and molecular modeling to understand the difference in crystallization of several cyclopentadithiophene−benzothiadiazole polymer derivatives. We observe using GIWAXS that when the carbon bridging atom is substituted by a silicon atom, the π−π stacking distance is decreased while the lamellar stacking distance is increased. Using molecular modeling, we calculate the potential energy surfaces of an ordered array of oligomers as a function of π−π stacking and lamellar stacking distances and find two local minima for both the carbon and silicon analogues. This finding is consistent with the GIWAXS observations. We suggest that it may be possible to crystallize the carbon and silicon versions in the same crystal structure by varying the processing conditions. We derive new potential parameters from quantum chemical calculations for side chains motions and implement those within a new force field for molecular dynamics. We find that the side chains are more flexible in the case of the silicon bridging atom. We propose that the flexibility enhancement may influence both thermodynamics and kinetics of crystallization and may result in crystallization of the polymer in the first or the second energetically favored crystal structures. This interpretation is supported by the finding of only one minimum in potential energy for longer, less bulky, and, thus, more flexible side chains for the carbon analogue.