ICME Overview of Polymer Solar Cell Active Layer
When compared to silicon solar cell technologies, organic solar cells are low cost and flexible, making them easier to integrate into consumer products. The main drawbacks of organic solar cells are relatively low efficiency and durability. Multi-scale modeling of solar cell devices will help us better understand the physics and chemistry of device function and manufacturing processes to improve overall device performance. (2)
There are many ways to manufacture organic solar cells, with spin coating being one of the most common, especially in the lab because of its simplicity (1). Processing is incredibly important for organic solar cell function because of the strong dependence of performance on morphology. (4) The goal of this project will be to use multi-scale modeling to investigate the process of a solution consisting of organic donor and acceptor undergoing phase separation and crystallization while evaporation of the solvent takes place; the entire system will spin constantly, thinning the layer with increasing angular velocity.
For modeling purposes, this project will use poly(3-hexylthiophene) (P3HT) as a donor and phenyl-C61-butyric acid methyl ester (PCBM) as an acceptor in a solvent of ortho-dichlorobenzene (DCB). Reasons for using these molecules in solar cells are P3HT’s large absorption spectrum, PCBM’s excellent charge stabilization, complementary band energies for P3HT and PCBM to facilitate excited electron transfer, and both molecules relatively high electron mobility; C60 is functionalized with phenyl-C61-butyric methyl ester group mainly to assist in solvation. (2)
At the smallest length scale (<5 nm), electronic structure methods such as DFT and perturbation theory will be used to study single molecules and small polymers. The link to the macroscale ISV continuum from this scale will involve the absorption spectrums and electronic states of P3HT and PCBM by themselves. The link to the experiments at the next length scale will be the optimized geometries from this length scale.
At the next length scale (5-10nm), electronic structure methods and molecular dynamics will be used to study crystal nucleation and growth in P3HT and PCBM by themselves. The link to the macroscale ISV continuum from this scale will describe crystal formation and growth. The link to the experiments at the next length scale will be the energies of crystal formation and crystal structure.
At the next length scale (10-20nm), molecular dynamics will be used on larger systems of P3HT and PCBM to study grain structures. The link to the macroscale ISV continuum from this scale will involve grain morphology and crystal properties (e.g. elastic moduli). The link to the experiments at the next length scale will be the crystal structures.
At the next length scale (20-50nm), molecular dynamics and field theory will be used on combined systems of P3HT and PCBM to study phase separation and morphology at the donor-acceptor interface and interfaces with the anode and cathode. The link to the macroscale ISV continuum from this scale will involve the interfacial morphologies. The link to the next length scale is the mobility of the molecules between grains and at interfaces.
At the next length scale (50-100s nm) dislocation dynamics between grains and interfaces will be studied using Micro-3D. The link to the macroscale ISV continuum from this scale will involve the dislocation motion. The link to the next length scale is the hardening rules that affect void and crack nucleation.
The steps at the micrometer to 500 micrometer level describe further crack development following the same template as a typical ICME approach. These steps apply the organic solar cells and will have an important effect as the popularity of flexible devices increases.
Solvent evaporation is a driving force in phase separation and crystallization because of the effects on concentration profiles in the film. In addition to evaporation, the system is also undergoing a constant rotation. Solvent evaporation and force modeling on a solution can be studied using computational fluid dynamics. Phase data for crystal formation can be obtained from calculations at lower length scales and experimental data. Phase data and CFD results could be used to investigate the effects of evaporation on crystallization.
Taking these steps and the bridges between experiments feed into the macroscale ISV continuum that seeks to describe the large-scale properties using finite element analysis. Finite element analysis takes into account results at lower length scales and homogenizes them over a given sized element to make up the properties of the full-scale system. The properties of the elements can be derived from first principle equations or empirical relations.
Organic solar cells are an excellent topic to study using multi-scale modeling because of strong dependence of device performance on morphology. Computational approaches can be used to study nearly every area that affects device performance, such absorption spectrums, electronic properties, charge transport and charge transfer. (2) Multi-scale approaches promise to provide great insight into function and design of devices of the future.