Semiconducting polymer nanoparticles have attracted much attention in the recent years owing to their potential application in the fields of organic electronics and biotechnologies. Among these applications, application in the polymer solar cell is the most attractive. Polymer solar cells possess a phase separated morphology composed of electron donating polymers as well as electron accepting fullerene derivatives. Replacing the phase-separated domains with small nanoparticles composed of active layer materials have been identified as one of the methods for controlling and optimizing morphology. Using nanoparticles enables the tuning of sizes as well as internal aggregate structure of the particles.
Unfortunately, the use of nanoparticles in solar cells is challenging in view of the particle synthesis approach. The NPs for use in solar cell applications should be very small, 10 nm ideally and less than 20 nm at most. Many NP synthesizing methods use surfactants to control the particle size and their agglomeration. Surfactants on the particle surfaces have usually adverse effects on the device performance of solar cells.
Reprecipitation is a promising particle synthesis method for solar cell applications since it can create relatively small NPs without using a surfactant. However, the reproducibility of particle size is low, particularly for NPs smaller than 50 nm. Moreover, the formed NPs easily agglomerate due to the lack of a surfactant.
Masaru Nagai and colleagues from the Nanjing Tech University in China, developed a two-step reprecipitation method for preparing semiconducting polymer nanoparticles in a bid to overcome the problem with typical reprecipitation. Their research work is published in journal, Colloid Polymer Science.
The reproducibility problem is inherent in the particle formation mechanism of reprecipitation. The injected polymer solution into a poor solvent is split into small droplets, of which size determines the NP size. The diffusion of the poor solvent into the injected solution begins immediately after the solvent contacts the solution. Thus, the solution splitting process and the particle formation process compete.
The authors solved the reproducibility problem by dividing the solution splitting and the particle formation processes. The authors first prepared uniform droplets by mixing a polymer solution with de-ionized water and formed the particles by adding poor solvent. They regulated the number of polymer confined to a single droplet and tuned the size of the nanoparticles by changing the preliminary polymer concentration. The synthesis yield and size reproducibility are higher than with conventional reprecipitation, even for very small particles below 30 nm.
Another notable advantage of this method is the ability to control the agglomeration of NPs. The research team observed that the prepared nanoparticles had a large negative zeta potential, therefore, indicating longtime stability without using surfactant. This large negative potential was as a result of electrostatic charges caused by the friction between the deionized water and the droplets.
Using this method, the authors systematically synthesized very small NPs (10 to 65 nm) of poly(3-hexylthiophene) (P3HT) and comprehensively investigated the effect of NP size on various physical properties, including photophysical properties, and crystallinity. The absorption peak of the nanoparticle suspension was observed to systematically red-shift as the size of nanoparticles increased. By analyzing the absorbance peak intensity ratio, the researchers observed that the aggregate ratio increased with particle size. Even though the authors observed a size-dependent redshift in the photoluminescence spectra, all the spectra converged consistent with thin film formation. The effect of size was therefore subdued in the film state. When compared to those in the thin films, the (100) crystalline peaks lower in the nanoparticle films. This indicated that the nanoparticle films possessed more isotropic crystalline structures. The crystalline lattice gaps were unchanged irrespective of particle size, and were nearly the same as those of thin films.
Masaru Nagai, Jun Huang, Dong Cui, Zhoulu Wang, Wei Huang. Two-step reprecipitation method with size and zeta potential controllability for synthesizing semiconducting polymer nanoparticles. Colloid Polymer Science, volume (2017) 295, pages 1153–1164.
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