Nanomorphologies in Conjugated Polymer Solutions and Films for Application in Optoelectronics, Resolved by Multiscale Computation

Conducting conjugated polymers, which provide flexibility as polymers as well as conductivity as metals, have nowadays become an essential solution-processable material for fabricating polymer light-emitting diodes (PLEDs) and plastic solar cells. In addition to the possibility of producing large-area thin films at room temperature, an appealing feature of exploiting long-chain organic semiconductors lies in the capability to fine-tune the optoelectronic behavior of solution-cast films by exploiting a broad variety of solvents or hybrid solvents in preparing the precursor solutions, later fabricated into dry thin films via spin coating or ink-jet printing. To improve the solubility in usual organic solvents, the polymers are often modified by grafting flexible alkyl or alkoxy side chains to the phenyl backbone, rendering the polymer chemical amphiphilicity. The semiflexible backbone and chemical amphiphilicity, in turn, give rise to a vast swath of single-chain and aggregation morphologies as different types of solvents are used to cast the polymer thin films, through mechanisms—generally referred to as the memory effect (Nguyen et al. 1999)—as schematically illustrated in Figure 1. Clearly, understanding how the above-mentioned material properties evolve during a practical processing is of paramount importance, yet this central goal remains challenging to conventional experimental protocols. Computation simulations, therefore, provide an important alternative by which in-depth information may be readily extracted that complement our knowledge from experimental characterizations, and thereby facilitates the pursuit of gaining practical controls over the molecular states of solution-cast thin films. This monograph aims to provide a comprehensive review of recently developed multiscale computation schemes that have been dedicated to resolving fundamental material properties in conjugated polymer solutions and films; prospects on emerging opportunities as well as challenges for upcoming applications in the area of organic optoelectronics are also remarked. Utilizing a standard, widely studied, conjugated polymer—poly(2-methoxy5-(2’-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV; see sketches in Figure 3)—as a representative example, we introduce the fundamentals and protocols of constructing self-