Influence of Polymer-Blend Morphology on Charge Transport and Photocurrent Generation in Donor−Acceptor Polymer Blends

Abstract

Monte Carlo algorithms are used to simulate the morphologies adopted by polymer chains in a polymer-blend film in the limits where the chains are mutually attractive (homophilic regime) and mutually repulsive (heterophilic regime) and then to simulate the drift transport of charges through the polymer chains. In the homophilic regime, chains aggregate into tangled domains resulting in a relatively high percolation threshold, a high density of configurational trap states, and slow, dispersive charge transport. In the heterophilic regime at the same polymer volume fraction, chains self-organize into a lacework pattern resulting in a low percolation threshold and efficient, trap-free charge transport. For homophilic morphologies interchain hopping is rate-limiting and mobility is insensitive to chain length, whereas for heterophilic morphologies intrachain transport is important and mobility increases with increasing chain length. The morphologies are used in simulations of photocurrent quantum efficiency for donor-acceptor blend photodiodes, which show that the effects of morphology on charge pair generation and recombination compete with the effect on transport, such that the optimum blend composition is sensitive to both morphology and recombination rate. We conclude that it is essential to consider the connectivity of and morphology adopted by polymer chains in the optimization of materials for organic solar cells.

The effect of morphology upon mobility: Implications for bulk heterojunction solar cells with nonuniform blend morphology

“…1 shows that carrier mobility is improved when the domain size of the transporting component is increased, as would be expected. 12 When ␣ Ͼ 0.5, Fig. 1 shows that carrier mobility is reduced when the domain size of the nontransporting component is reduced.…”

“…This may be a serious deficiency in our knowledge, especially since the limited existing studies point to significant changes in charge transport when some aspects of morphology are changed. [12][13][14][15][16] In particular, Frost et al 12 showed that networks of self-repelling and self-attracting polymers show orders of magnitude difference in mobility and substantially different dispersive character.…”

Bicontinuous minimal surface nanostructures for polymer blend solar cells

“…13,14 The influence of polymer-blend morphologies consisting of coiled polymer chains on charge transport and photocurrent generation in polymer blends has also been studied using Monte Carlo simulation methods. 15 Drift-diffusion modelling has also been successfully used to model organic solar cells. [16][17][18] However, such models are unable to take full account of the effect of three-dimensional structures, which are of interest here, instead reducing the complex morphology to a basic homogenous description or a simplified two-dimensional structure.…”

Multi-Scale Modeling of Bulk Heterojunctions for Organic Photovoltaic Applications

“…The parameters in the transport equations can be estimated from quantum chemical calculations or experimental measurements (Coropceanu et al, 2007;Nelson et al, 2009;Westenhoff et al, 2006;. Such models of exciton and charge-carrier hopping between discrete sites have been used to study exciton and charge-carrier dynamics in BHJs, which can be simulated with a kinetic Monte Carlo algorithm (Frost et al, 2006;Groves et al, 2009;Meng et al, 2010;Watkins et al, 2005). These models can account for the molecular nature of the active layer, the morphology of the BHJ, and the anisotropy of exciton and charge transport in a straightforward fashion.…”

“…These models can account for the molecular nature of the active layer, the morphology of the BHJ, and the anisotropy of exciton and charge transport in a straightforward fashion. However, all implementations of such models so far (Frost et al, 2006;Groves et al, 2009;Meng et al, 2010;Watkins et al, 2005) have treated the dependence of the BHJ morphology on material properties and thermodynamic conditions in an approximate, albeit physically motivated, fashion: electron donor and acceptor sites were assumed to occupy sites on a cubic lattice and their interactions tuned to produce varying levels of phase separation after donor and acceptor sites were moved according to a Monte Carlo algorithm. Groves et.…”