The relative importance of domain size, domain purity and domain interfaces to the performance of bulk-heterojunction organic photovoltaics

Abstract

Groves, C. (2012) 'The relative importance of domain size, domain purity and domain interfaces to the performance of bulk-heterojunction organic photovoltaics.', Energy and environmental science., 5 (6). pp. 7657-7663. Further information on publisher's website:http://dx.doi.org/10.1039/C2EE21327CPublisher's copyright statement:Additional information: 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-profit 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 wayThe 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. performance due to the difficulty of demonstrating causal links between properties and performance. As a result, the relative importance of size and purity of domains, and the interfaces between, is not generally known. In this paper we use morphological and charge transport modeling techniques to show unambiguously the effect of these features on the performance of OPVs. Surprisingly we find that the commonly reported 'optimum' domain size of ~10nm is only of significant benefit in structures with pure domains, and further that sharpening the interface between domains is of greater benefit to performance. We also show how changing the interaction parameter of the blend components is a versatile technique to achieve sharper interfaces and higher OPV performance. More generally, we have demonstrated a combination of modeling techniques that are able to give an indication of answers to questions relevant to OPVs that would be difficult to achieve experimentally. ABSTRACTThe domain size, domain purity and interfacial width between domains for a bulk heterojunction are controllably altered through use of Cahn-Hilliard modeling and their relative effect on OPV performance is predicted using Monte Carlo modeling. It is found that locally-sharp, well-connected domains of only 4nm extent out perform morphologies with broadened interfaces and/or impure domains even when domain sizes were at the 'optimum' size of ~10nm. More generally, these data provide information on the most effective method to optimize the as-cast bulk heterojunction morphology depending upon initial domain purity and the nature of interfaces between domains. Further, it indicates why morphology optimization is more effective for some blends than others. It is shown that the quench depth of the blend can be used as a general technique to control the interfacial structure of the morphology and realize substantial increases in short circuit photocurrent.3

Suppression of geminate charge recombination in organic photovoltaic devices with a cascaded energy heterojunction

“…A Monte Carlo charge transport model similar to that reported elsewhere 29 was used to simulate  GS . Models of this type have been shown to agree quantitatively with J-V curves of polymer bilayer OPV devices with abrupt 30 and roughened 31 interfaces.…”

Simulating charge transport in organic semiconductors and devices: a review

“…However, it has latterly been argued that the Braun expression incorrectly assumes that separation and recombination follow exponential kinetics [109]. However, from a pragmatic point of view, it is difficult to be critical of the Braun model as analytical expressions which more correctly account for transport environment in OSCs [110][111][112] still do not encompass all of the potentially relevant physics [113,114]. Given that the geminate recombination model will (almost) inevitably not include physics relevant to the OPV in question, it is perhaps safest to consider the recombination rate in such models as a fitting parameter that is indicative of how good or bad GR is, but having no further quantitative meaning [73].…”

“…Later experiments and modelling by Yan et al [377] on non-annealed and annealed bilayer OPVs, such that the former had a sharper interface than the latter, supported the notion that interface morphology was important in determination of GR. Both Lyons et al [114] and Gagorik et al [382] showed that impure domains, where they lead to disconnected islands of donor material in the acceptor phase and vice versa, can act as charge recombination centres. This suggests that molecularly mixed phases reported in some in high performance OPVs [343,344] support ambipolar charge transport [388], since if complete charge transport networks for both electrons and holes were not present in the mixed phase, trapping and recombination would occur.…”

“…Hence for OPVs which rely on conventional exciton dissociation, nano-structured morphologies are required, and device models have been used extensively to understand which forms of morphology are most favourable. As well as bilayers [73,143,377], morphologies examined include interdigitated pillars [48,201,[378][379][380], gyroids [381], idealised BHJs [48, 147,201,380] and non-ideal BHJs [114,123,382,383]. It is shown that interdigitated pillars have highest performance, since the electric field is either parallel or perpendicular to the plane of the heterojunction, while perhaps unexpectedly, gyroid and idealised BHJ morphologies have very similar performance [381].…”

“…The traditional picture of an ideal BHJ morphology includes sharp interfaces between the donor and acceptor, and domains which are both pure and of a size which balances the competing needs of exciton dissociation and charge transport [147,385]. Lyons et al [114] used a KMC model to examine the relative impact of these morphological factors on OPV performance. Surprisingly, it was found that sharp interfaces between the donor and acceptor had a greater effect on GR than domain size.…”