Tuning the Dimensions of C₆₀‐Based Needlelike Crystals in Blended Thin Films

Abstract

A new ordered structure of the C60 derivative PCBM ([6‐6]‐phenyl C61‐butyric acid methyl ester) is obtained in thin films based on the blend PCBM:regioregular P3HT (poly(3‐hexylthiophene)). Rapid formation of needlelike crystalline PCBM structures of a few micrometers up to 100 μm in size is demonstrated by submitting the blended thin films to an appropriate thermal treatment. These structures can grow out to a 2D network of PCBM needles and, in specific cases, to spectacular PCBM fans. Key parameters to tune the dimensions and spatial distribution of the PCBM needles are blend ratio and annealing conditions. The as‐obtained blended films and crystals are probed using atomic force microscopy, transmission electron microscopy, selected area electron diffraction, optical microscopy, and confocal fluorescence microscopy. Based on the analytical results, the growth mechanism of the PCBM structures within the film is described in terms of diffusion of PCBM towards the PCBM crystals, leaving highly crystalline P3HT behind in the surrounding matrix.

Ultrafast Transient Optical Studies of Charge Pair Generation and Recombination in Poly-3-Hexylthiophene(P3ht):[6,6]Phenyl C61 Butyric Methyl Acid Ester (PCBM) Blend Films

“…The fitted value of L tends to decrease with increasing PCBM content and is larger for annealed than nonannealed films. This is consistent with the decreasing fraction of P3HT in the blend film as PCBM content increases and with the observation that PCBM molecules aggregate during thermal annealing to leave larger P3HT domains. , Both of these trends are supported by measurements of photoluminescence quenching reported previously, which consistently show (i) decreasing PL intensity with increasing PCBM content in nonannealed films, indicating that dispersed PCBM molecules act as quenching sites for excitons in P3HT, and (ii) weaker PL from nonannealed than annealed films, indicating smaller or less pure P3HT domains before annealing induced phase segregation. A much smaller value of L is needed to fit the 70% PCBM annealed blend data than the 30% or 50% annealed blends, in order to account for the faster kinetics at short times in the 70% PCBM case.…”

The Molecular Structure of Polymer−Fullerene Composite Solar Cells and Its Influence on Device Performance

“…In Figure 4d, we see further evidence of P3HT crystallization with an HRTEM image showing P3HT b-axis stacking which corresponds to the (002) reflection. 10 The d-spacing of 0.38 nm, as shown in 4e and 4f, matches well with values reported elsewhere in the literature. 2,16 PCBM crystallization was also investigated and the results are presented in Figure 5.…”

“…In Figure , gray scale images of a lamellae region (Figure b) reveal a periodicity of 1.7 nm which matches reasonably well with the 1.64 nm a -axis spacing of the (001) reflection, as observed in XRD studies of P3HT. , As the hexyl side chain is only 0.75 nm long, the d -spacing suggests there is no interdigitation of the side chains. In Figure d, we see further evidence of P3HT crystallization with an HRTEM image showing P3HT b -axis stacking which corresponds to the (002) reflection . The d -spacing of 0.38 nm, as shown in 4e and 4f, matches well with values reported elsewhere in the literature. , …”

The Influence of Poly(3-hexylthiophene) Regioregularity on Fullerene-Composite Solar Cell Performance

“…Zooming in on the homogeneous parts of the films of the 86 and 96% RR P3HT−PCBM blend samples, one can see bicontinuous networks of P3HT and PCBM under transmission electron microscopy (see Figure S5 in the Supporting Information). Similar needlelike PCBM crystals in blend films with P3HT had been observed by others and were believed to be due to the fast diffusion of PCBM toward the PCBM crystals, whose growth rate depends on blend composition and annealing conditions. , Here we have shown that in addition to the processing conditions of the film, the chemical nature of the polymer significantly affects the extent of growth of these PCBM crystals during thermal annealing.…”