Structure and crystallinity of water dispersible photoactive nanoparticles for organic solar cells

Abstract

Water based inks would be a strong advantage for large scale production of organic photovoltaics. Formation of water dispersible nanoparticles produced by the Landfester method is a promising route to achieve such inks. We provide new insights into key ink properties for poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) nanoparticles such as internal structure and crystallinity of the dispersed nanoparticles and the previously unreported drastic changes that occur when the ink is cast into a film. We observe through Transmission Electron Microscopy (TEM) and small angle X-ray scattering (SAXS) that the nanoparticles in dispersion are spherical with nano domains of P3HT -partly crystalline. When wet processed and dried into films the nanoparticles lose their spherical shape and become flattened to oblate shapes with a large aspect ratio. Most particles are observed to have a diameter 13 times of the particle height. After casting to a film the crystal domains adopt a preferred orientation with the majority of the nano crystals (68%) with faceon orientation to the substrate. We propose that low substrate surface energy is responsible for particle deformation and texturing. Broader contextPolymer solar cells have demonstrated power conversion efficiencies comparable to what can be achieved with other thin-film technologies, but some issues with respect to environmental impact remain to be addressed. Photoactive nano-particles in aqueous dispersion have been envisioned as an effective way to do away with the use of toxic and environmental harmful solvents in the coating process of polymer solar cells. Preforming the donor/acceptor blend in nanoparticles that are dispersed in water allows for use of only aqueous or alcohol-based inks for coating, while at the same time providing a means of controlling the nano-structure of the heterojunction. We show that the nano-particles are very sensitive to the type of coating method and substrate surface energy, and that an otherwise optimal nano-structure may be compromised during coating by phase separation. Our results indicate that focusing the development effort on other types of stabilizing surfactants may provide the means to control phase separation and thus the formation of optimal nano-structure in roll-to-roll coated, environmental friendly polymer solar cells. Figure 3, Particle deformation and reorientation of crystal domains. A) Schematic of face-on and edge-on orientation of P3HT. B) AFM measurements of individual P3HT:PCBM 1:1 nanoparticles spin cast on a mica substrate.The fitted trend line finds the particle diameter is about 13 times the particle height. C) Azimuthal distribution of P3HT 100 crystallinity normalized to the highest degree of crystallinity. The overall texture (angular distribution of integrated diffraction intensity) does not change as a function of annealing. Before annealing 68% of the crystallinity is aligned edge-on with the substrate (80-100 degrees azimuthal angle. D) P3HT 100 average domain size as funct...

Unique p–n Heterostructured Water‐Borne Nanoparticles Exhibiting Impressive Charge‐Separation Ability

“…Schwarz et al [10] and Pedersen et al [11] found that they were phase separated within the core of the surfactant micelle and formed p-n heterostructured NPs, which is thus the nanoscale version of heterojunction materials. Schwarz et al [10] and Pedersen et al [11] found that they were phase separated within the core of the surfactant micelle and formed p-n heterostructured NPs, which is thus the nanoscale version of heterojunction materials.…”

“…In the synthesis of water-borne NPs, sodium dodecyl sulfate (SDS) is typically used as as urfactant, in which both p-and ntype componentso fc olloidal micelles are enclosed. Schwarz et al [10] and Pedersen et al [11] found that they were phase separated within the core of the surfactant micelle and formed p-n heterostructured NPs, which is thus the nanoscale version of heterojunction materials. Studies on p-n bulk morphology showedt hat the domain sizes of the two components need to be kept and/or controlled at the nanoscale.…”

Environmentally friendly preparation of nanoparticles for organic photovoltaics

“…4e-h). Compared to the reported P3HT:PCBM NPs 51 , which require lower annealing temperature to form coalescent film, the slightly harsh annealing performed to To further probe the morphological changes and thermomechanical behaviour of PTNT:PC 71 BM NP films, DMTA measurements 39 were performed on thin films cast from NP-xylene (Fig. 5) and NP-chloroform (see Fig.…”

Optimisation of purification techniques for the preparation of large-volume aqueous solar nanoparticle inks for organic photovoltaics

“…In principle, it should be possible to control the wetting properties of ASNP inks by tailoring the free-SDS surfactant concentration. However, to date it has been difficult to determine the SDS content of aqueous solar nanoparticle inks, even though it is known that the SDS content has an effect on the performance of resulting NP-OPVs [ 13 , 18 – 19 ].…”