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

Abstract

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. We use a Monte Carlo model to predict the effect of composition, domain size, and energetic disorder upon the mobility of carriers in an organic donor-acceptor blend. These simulations show that, for the changes in local morphology expected within the thickness of a typical bulk heterojunction photovoltaic device, changes in mobility of more than an order of magnitude are expected. The impact of nonuniform mobility upon space-charge-limited diode and photovoltaic ͑PV͒ device performance is examined using a drift-diffusion model. The current passing through a space-charge-limited diode is shown to depend upon the position of the layers with differing mobility. Accurate modeling of the current in such devices can only be achieved using a drift-diffusion model incorporating nonuniform mobility. Inserting a 20 nm thick layer in which the mobility is less by one order of magnitude than in the rest of the 70 nm thick PV device reduced the device efficiency by more than 20%. Therefore it seems vital to exert a high degree of control over the morphology throughout the entire blend PV device, otherwise potential PV performance may be lost.

Multi-Scale Modeling of Bulk Heterojunctions for Organic Photovoltaic Applications

“…Once the coarsened morphology was generated, a drift-diffusion model was used to electrically model the device. When energetic disorder of isolated or smaller domains/sites was used to reduce charge mobility locally, the effects of local morphology on electrical properties could be modelled (Groves et al, 2009). Charge mobility was shown to be the most important determinant of efficiency using this model.…”

“…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.…”

Simulating charge transport in organic semiconductors and devices: a review

“…Many organic electronic devices are either spatially heterogeneous [117,118] or multi-layered [119] and so it is important to consider how this might be included in a DD model. Transport of charges across a heterojunction is particularly important for OLED operation, as discussed further in section 4.3.…”

“…Groves et al [117] and Koster [124] used KMC and ME methods respectively to examine the impact of donor-acceptor morphology in BHJ OPV blends. The morphologies were generated using a simulated annealing approach [140] which ignores molecular detail but nonetheless results in an isotropic, phaseseparated structure.…”