Crystallization-Induced 10-nm Structure Formation in P3HT/PCBM Blends

Abstract

The miscibility and aggregation of PCBM ([6,6]-phenyl-C 61 -butyric acid methyl ester) in a polymer matrix is of great importance for the development of fullerene-based organic photovoltaic cells (OPVs). In this study we have systematically investigated the loading of PCBM in regioregular and regiorandom P3HT (poly(3-hexylthiophene-2,5-diyl). Using optical microscopy, we demonstrate the partial miscibility of PCBM in thermally annealed P3HT films and relate it to the relative crystallinity of P3HT. The low polydispersity and the nearly 100% regioregularity of a self-synthesized P3HT allowed a detailed X-ray characterization as a function of PCBM content, revealing a superstructure of periodic amorphous and crystalline lamellar domains of fully chain extended polymer chains. PCBM dissolves in the amorphous interlamellar P3HT regions (partially indexmatching the X-ray scattering contrast) up to a threshold, above which PCBM aggregates start to form. These results show that crystallization of P3HT into 10-nm-wide lamellar domains sets the main length scale in P3HT/PCBM structure formation. PCBM is displaced into the amorphous intralamellar regions, swelling the lamellar stack. This structure formation by crystallization, which is intrinsic to most semicrystalline polymers, followed by the enrichment, segregation, and crystallization of PCBM provides an interdigitated structure, which is conceptually ideal for excitonic solar cells.

Effects of amorphous poly(3‐hexylthiophene) on active‐layer structure and solar cells performance

“…As shown in Figure 10 . The large aggregates are PCBM crystals, 25,56 which is supported by the WAXS data shown in Supporting Information Figure S4 where the sharp PCBM characteristic peaks are observed after 120-min annealing. The aggregation of PCBM leads to a greater phase separation scale, thus causing the upturns in low q range as shown in Figure 9.…”

“…Second, although amorphous P3HT and PCBM are miscible, the amount of PCBM existing in the interlamellar amorphous P3HT regions is limited due the restrain of P3HT tie chains. 24,25 There is a threshold concentration for PCBM to dissolve in the interlamellar amorphous P3HT regions, above which a phase separation occurs and the aggregates may be in a scale larger than the exciton diffusion length so that efficiency decreases accordingly. 2 Many efforts have been made to retard the phase separation in polymer/fullerene blends so as to enhance the stability of devices, including the crosslinking of polymer to fix the structure, 26,27 the chemical modification of fullerene to lower the diffusion rate, [28][29][30] and the incorporation of additives or functional groups to enhance the miscibility between polymer and fullerene.…”

“…Previous reports have shown that the addition of a significant amount of rra-P3HT into rr-P3HT/PCBM blends prevents the large PCBM aggregates. 24,25,35 However, the performance of such solar cells, especially the long-term stability, has not been systematically studied. In this work, we carefully examined the effect of rra-P3HT on the morphology, absorption, photoluminescence (PL), and solar cell performance of P3HT/PCBM blends.…”

“…It is possible that the P3HT nanofibers are formed by the 1D arrangement of the small crystallites. 25,50 In contrast to the dimension change of crystallites, the scale of phase separation domains n for as-cast films is large, 94.8 and 82.4 nm for B-95 and B-100, respectively. After thermal annealing, n greatly reduces to 40-50 nm and then increases again to 73.7 and 88.8 nm for B-95 and B-100, respectively, after 30 days.…”

“…Such a scattering profile has previously been interpreted to originate either from PCBM clusters 20,[46][47][48][49] or from the spatial arrangement of P3HT crystallites. 25,50 Since the scattering shoulder is absent in B-0 sample that contains PCBM clusters [Supporting Information Fig. S3(c)], we suggest that the shoulders are caused by P3HT crystallites, which appear in the profiles of B-95 and B-100 samples due to the increasing contrast between pure P3HT crystallites and PCBM/amorphous P3HT intermixed phases, in comparison to P-100 sample that contains P3HT crystallites but without the shoulder [Supporting Information Fig.…”