Manipulating the Morphology of P3HT–PCBM Bulk Heterojunction Blends with Solvent Vapor Annealing

Abstract

Using grazing incidence X-ray scattering, we observe the effects of solvent vapors upon the morphology of poly(3-hexylthiophene)−phenyl-C 61 -butyric acid methyl ester (P3HT−PCBM) bulk heterojunction thin film blends in real time; allowing us to observe morphological rearrangements that occur during this process as a function of solvent. We detail the swelling of the P3HT crystallites upon the introduction of solvent and the resulting changes in the P3HT crystallite morphology. We also demonstrate the ability for tetrahydrofuran vapor to induce crystallinity in PCBM domains. Additionally, we measure the nanoscale phase segregated domain size as a function of solvent vapor annealing and correlate this to the changes observed in the crystallite morphology of each component. Finally, we discuss the implications of the morphological changes induced by solvent vapor annealing on the device properties of BHJ solar cells.

Recent Approaches to Controlling the Nanoscale Morphology of Polymer-Based Bulk-Heterojunction Solar Cells

“…A P3HT:PCBM film prepared using dichlorobenzene (DCB) [boiling point (bp) = 178 °C] or chlorobenzene (CB) (bp = 132 °C) possesses a better-ordered structure and an improved crystallinity in comparison with a film prepared using the low-boiling solvent, chloroform (CF) (bp = 61 °C), as is shown (Figure 4) by their X-ray diffraction (XRD) intensities [81]. Recently, Verploegen et al [70] studied the differences between the spin-cast films produced with CF or CB as solvents using grazing incidence wide-angle X-ray scattering (GIWAXS) of the film ( Figure 5). They observed that the direction of the P3HT lamellae for the film cast using CB was different from the direction of the P3HT lamellae for the film cast using CF.…”

“…A previous result by Moon et al [85] showed a different role played by the CN additive on the morphology of a thin film consisting of (4,4-didodecyldithieno[3,2-b:20,30-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl (Si-PDTBT) donor and [6,6]-phenyl-C 70 -butyric acid methyl ester (PC 70 BM) acceptor. The additive suppressed the large-scale aggregation of PC 70 BM and promoted the correct morphology for high-performance BHJ devices [85]. The nanomorphology of the Si-PDTBT:PC 70 BM BHJ material without the use of a CN additive was indicative of oval-shaped PC 70 BM aggregate with a diameter >200 nm [85].…”

“…Protocols for controlling the formation of morphology with self-organized ordered structures of the polymer units within the active layer can be realized by appropriate solvent selection [63], slow drying of spin-coated films [64][65][66], thermal [67][68][69] and solvent annealing [70][71][72] treatment of the films, surface treatment of ITO [73], the application of an electric field [74], the use of chemical additives [75], additive-spraying techniques [76], and the use of nanowires (NWs) [77] and nanoparticles [78].…”

“…Grazing incidence wide-angle X-ray scattering (GIWAXS) images of a 1:1 P3HT:PCBM blend cast using chloroform (CF) and chlorobenzene (CB), respectively. Reprinted with permission from ACS [70].…”

“…The additive suppressed the large-scale aggregation of PC 70 BM and promoted the correct morphology for high-performance BHJ devices [85]. The nanomorphology of the Si-PDTBT:PC 70 BM BHJ material without the use of a CN additive was indicative of oval-shaped PC 70 BM aggregate with a diameter >200 nm [85]. The P3HT: [6,6]-phenyl-C 61 butyric acid methyl ester (PC 61 BM) film produced using 1,6-hexanedithiol (HDT) or 1,8-octanedithiol (ODT) as solvent additives possessed optimal phase segregation with a suitable domain size [86].…”