Toward a better understanding of conjugated polymer blends with non-spherical small molecules: coupling of molecular str
- PDF / 487,411 Bytes
- 11 Pages / 584.957 x 782.986 pts Page_size
- 54 Downloads / 144 Views
A major obstacle in the organic solar cell field is the inability to predict the relevant microstructural length scales that determine charge transport of the interpenetrating polymer/small molecule network based on the component chemical structures. This has led to a trial-and-error approach, which is extremely labor-intensive. This manuscript is our attempt to move toward forming a link between small molecule chemical structure and the morphological hierarchy of the blend. We focus on geometric motifs of small molecule organic semiconductors which have 2D, nonspherical 3D, and quasispherical 3D molecular orbital extent. We find that phase separation in these blends is a function of the molecular structure, and that the small molecule chemical structure is coupled to the crystallite orientation distribution of the polymer matrix. We further find that the ability of a molecule to form a network with a well-defined length scale of phase separation depends on the polymer persistence length.
I. INTRODUCTION
The organic photovoltaic (OPV) field has matured substantially since the initial discovery of ultrafast photoinduced electron transfer from a conjugated polymer to a fullerene derivative in a thin blend film.1 After the quick realization that efficient OPV devices must incorporate a large density of electron donor/ acceptor interfaces, the bulk heterojunction architecture (BHJ) was born.2 Though various combinations of conjugated polymer and small molecule devices have been explored to date, conjugated polymer/fullerene derivative blends have by far received the most attention, primarily due to their facile thin film processing.3–5 In OPV devices, photogenerated electronic excited states (excitons) spatially diffuse until they relax to the ground state or encounter a donor/acceptor interface, where electron–hole pair disassociation occurs, leading to holes localized on the donor(s) and electrons on the acceptor(s). If the spatially separated electron and hole can overcome their binding energy, both charges migrate away from the heterointerface, driven by a combination of electric and chemical potential gradients.6–8 It is a remarkable fact that a straight-forward means of creating a relatively efficient OPV device is by simply letting the solvent evaporate from a thin liquid layer of solution comprised of the donor/acceptor blend.
Contributing Editor: Moritz Riede a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.490
The nonequilibrium thermodynamics of solvent evaporation along with the hydrodynamic forces acting on the solution during thin film formation determine the initial state of the blend film. The resulting solid film contains an interpenetrating donor/acceptor network, whose mean length scale of phase separation is known to be a function of various processing variables, such as choice of solvent, thermal and solvent annealing, solvent additives, etc.4,9–13 As the extent of donor/acceptor phase separation varies, the volume spanned by charge-transporting paths changes for
Data Loading...
