Morphological Characterization of a Low‐Bandgap Crystalline Polymer:PCBM Bulk Heterojunction Solar Cells

Abstract

]phenyl-C61-butyric acid methyl ester (PCBM) were investigated under different processing conditions. The surface morphologies and vertical segregation of the "As-Spun", "Pre-Annealed", and "Post-Annealed" fi lms were studied by scanning force microscopy, contact angle measurements, X-ray photoelectron spectroscopy, near-edge X-ray absorption fi ne structure spectroscopy, dynamic secondary ion mass spectrometry, and neutron refl ectivity. The results showed that PSBTBT was enriched at the cathode interface in the "As-Spun" fi lms and thermal annealing increased the segregation of PSBTBT to the free surface, while thermal annealing after deposition of the cathode increased the PCBM concentration at the cathode interface. Grazing-incidence X-ray diffraction and small-angle neutron scattering showed that the crystallization of PSBTBT and segregation of PCBM occurred during spin coating, and thermal annealing increased the ordering of PSBTBT and enhanced the segregation of the PCBM, forming domains ∼ 10 nm in size, leading to an improvement in photo voltaic performance.

The effect of mesomorphology upon the performance of nanoparticulate organic photovoltaic devices

“…For P3HT:PCBM BHJ OPVs the optimal blend ratio is 1:0.8, which corresponds to an optimal polymer fraction of 0.56 [27,28]. By contrast, the optimal composition reported for PSBTBT:PCBM BHJ OPVs is 1:1.5 and corresponds to an optimal polymer fraction of 0.40 [23]. Previous studies have shown that the device performance decreases significantly the farther the polymer fraction is from its optimal value [29,30].…”

“…Static water contact angles for P3HT (106 7 0.3°) [24], PSBTBT (104.3°) [23] and PCBM (50.6°) [23] have been reported. While contact angles for conducting polymers are highly dependent upon the level of doping of the polymer sample, these values indicate that the surface energy of PSBTBT is similar to that of P3HT.…”

“…Whereas P3HT is a largely amorphous polymer in the as-spun state and must be thermally treated to produce a more crystalline material [19,20], in contrast PSBTBT is a highly crystalline polymer [15,[21][22][23]. Upon blending with a fullerene, PSBTBT:fullerene blends display excellent charge transport properties, with little recombination occurring [23].…”

On the morphology of polymer‐based photovoltaics

“…Russell et al used a broad range of techniques to investigate the morphology of the PSBTBT/PCBM blends and the evolution of the morphology by thermal annealing. 101 The results showed that PSBTBT was enriched at the cathode interface in the films immediately after spin coating and thermally annealing the films led to an increase in the concentration of PSBTBT at the surface. However, if an electrode was evaporated onto the surface of the film initially, thermal annealing was found to increase the concentration of PCBM at the electrode (cathode) interface [ Fig.…”

Optical probes of chain packing structure and exciton dynamics in polythiophene films, composites, and nanostructures

“…However, unlike their ''hard'' semiconductor analogs, the ''soft'' nature of organic semiconductors-referring specifically to the response of the nuclear framework upon exciton generation-conspires to significantly limit charge mobility in these materials, and hence overall device efficiency. 9, 10 An issue of perhaps equal importance is the limitation on mobility imposed by morphological heterogeneities in polymerblend thin films, which has prompted a great deal of recent research aimed at controlled morphologies for long-range transport of charges. [11][12][13][14] In this mini-review, we discuss recent reports in the literature highlighting the connection between polymer architecture, morphology and supramolecular structure, and the fundamental photophysical processes behind charge generation and separation.…”