Can Ferroelectricity Improve Organic Solar Cells?

Blends of semiconducting (SC) and ferroelectric (FE) polymers have been proposed for applications in resistive memories and organic photovoltaics (OPV). For OPV, the rationale is that the local electric field associated with the dipoles in a blend could aid exciton dissociation, thus improving power conversion efficiency. However, FE polymers either require solvents or processing steps that are incompatible with those required for SC polymers. To overcome this limitation, SC (poly(3-hexylthiophene)) and FE (poly(vinylidene fluoride-trifluoroethylene)) components are incorporated into a block copolymer and thus a path to a facile fabrication of smooth thin films from suitably chosen solvents is achieved. In this work, the photophysical properties and device performance of organic solar cells containing the aforementioned block copolymer consisting of poly(vinylidene fluoride-trifluoroethylene): P(VDF-TrFE), poly(3-hexylthiophene): P3HT and the electron acceptor phenyl-C₆₁ -butyric acid methyl ester: [60]PCBM are explored. A decrease in photovoltaic performance is observed in blends of the copolymer with P3HT:[60]PCBM, which is attributed to a less favorable nanomorphology upon addition of the copolymer. The role of lithium fluoride (the cathode modification layer) is also clarified in devices containing the copolymer, and it is demonstrated that ferroelectric compensation prevents the ferroelectricity of the copolymer from improving photovoltaic performance in SC-FE blends.

Morphology Tuning of ZnO/P3HT/P3HT- b-PEO Hybrid Films Deposited via Spray or Spin Coating

Hybrid films of zinc oxide (ZnO) and poly(3-hexylthiophen-2,5-diyl) (P3HT) show promising characteristics for application in hybrid bulk-heterojunction solar cells (HBSCs). However, the incompatibility of ZnO and P3HT may lead to a reduced interface area, thus reducing the probability of exciton separation and consequently lowering solar cell efficiencies. Here, a diblock copolymer P3HT- b-poly(ethylene oxide) (PEO) is introduced to improve the interface between ZnO and P3HT. ZnO is synthesized via a block copolymer assisted sol-gel approach, and the used zinc precursor is directly incorporated into the PEO blocks. Thus, the possibility of aggregation is reduced for both the inorganic and the organic components, and a good intermixing is ascertained. Two deposition methods, namely, spray and spin coating, are compared with respect to the resulting film structure, which is investigated with scanning electron microscopy and time-of-flight grazing-incidence small-angle neutron scattering measurements. Both the surface and inner morphologies reveal that the spin coated samples possess smaller and less diverse domain sizes than the sprayed films. Due to the advantage of spray coating in large-scale production, the morphology of the sprayed samples is tailored more meticulously by changing the weight fraction of ZnO in the films. The sprayed hybrid films show smaller domains and less aggregation with decreasing the amount of ZnO. This reveals that both the deposition method and composition of the ZnO/P3HT/P3HT- b-PEO hybrid films play an important role for tailoring the film morphology and thus for improving the performance of HBSCs in future application.

Tuning of the Morphology and Optoelectronic Properties of ZnO/P3HT/P3HT- b-PEO Hybrid Films via Spray Deposition Method

The self-assembly of amphiphilic diblock copolymers yields the possibility of using them as a template for tailoring the film morphologies of sol-gel chemistry-derived inorganic electron transport materials, such as mesoporous ZnO and TiO₂. However, additional steps including etching and backfilling are required for the common bulk heterojunction fabrication process when using insulating diblock copolymers. Here, we use the conducting diblock copolymer poly(3-hexylthiophene)- block-poly(ethylene oxide) (P3HT- b-PEO) in which P3HT acts as charge carrier transport material and light absorber, whereas PEO serves as a template for ZnO synthesis. The initial solution is subsequently spray-coated to obtain the hybrid film. Scanning electron microscopy and grazing-incidence small-angle X-ray scattering measurements reveal a significant change in the morphology of the hybrid films during deposition. Optoelectronic properties illustrate the improved charge separation and charge transfer process. Both the amount of the diblock copolymer and the annealing temperature play an important role in tuning the morphology and the optoelectronic properties. Hybrid films being sprayed from a solution with the ratio of ω_ZnO, ω_P3HT, and ω_{P3HT- b-PEO} of 2:1:1 and subsequent annealing at 80 °C show the most promising morphology combined with an optimal photoluminescence quenching. Thus, the presented simple, reagent- and energy-saving fabrication method provides a promising approach for a large-scale preparation of bulk heterojunction P3HT/ZnO films on flexible substrates.

Enhancing the thermal stability of the bulk-heterojunction photovoltaics based on P3HT/PCBM by incorporating diblock amphipathic P3HT–PEO at D/A interface

A diblock amphipathic copolymer P3HT–PEO was rationally designed and easily synthesized.

Fabrication of a multi-charge generable poly(phenyl isocyanide)-block-poly(3-hexylthiophene) rod–rod conjugated copolymer

A facile construction of diverse polymeric nanostructures was reported by simple quaternization reaction and UV irradiation starting from the same rod-rod conjugated PPI(-DMAENBA)-b-P3HT) diblock copolymers, which were prepared by sequential living copolymerization of PI and 3HT in one-pot.

Controlled aggregation of quantum dot dispersions by added amine bilinkers and effects on hybrid polymer film properties

Nanocrystal aggregation triggered by added bilinker prior to P3HT/ZnO film formation results in increased film thickness and light absorption.

Enhanced performance for organic bulk heterojunction solar cells by cooperative assembly of ter(ethylene oxide) pendants

A ter(ethylene oxide) functionalized donor and acceptor are explored to manipulate the self-assembly morphology of the photoactive layer in polymer solar cells.

Stabilizing liquid drops in nonequilibrium shapes by the interfacial jamming of nanoparticles

Nanoparticles assemble at the interface between two fluids into disordered, liquid-like arrays where the nanoparticles can diffuse laterally at the interface. Using nanoparticles dispersed in water and amine end-capped polymers in oil, nanoparticle surfactants are generated in situ at the interface overcoming the inherent weak forces governing the interfacial adsorption of nanoparticles. When the shape of the liquid domain is deformed by an external field, the surface area increases and more nanoparticles adsorb to the interface. Upon releasing the field, the interfacial area decreases, jamming the nanoparticle surfactants and arresting further shape change. The jammed nanoparticles remain disordered and liquid-like, enabling multiple, consecutive deformation and jamming events. Further stabilization is realized by replacing monofunctional ligands with difunctional versions that cross-link the assemblies. The ability to generate and stabilize liquids with a prescribed shape poses opportunities for reactive liquid systems, packaging, delivery, and storage.

Controlling the interaction of light with polymer semiconductors

In this study, a generally applicable strategy is described to manipulate the optical properties of a wide range of polymer semiconductors in the solid state. Blending these materials with a non-conjugated, polar polymer matrix is found to be the processing key to a drastic change and red-shift of the absorption characteristics.