DOI: https://doi.org/10.29363/nanoge.inform.2019.001
Publication date: 8th January 2019
Most efficient organic photovoltaic (OPV) cells comprise a polymeric donor and a small-molecule acceptor blended in a so-called bulk heterojunction (BHJ) thin film architecture. The BHJ morphology is complex; it is now, for instance, well accepted that most BHJ blends used in OPVs are at least to a certain extent finely intermixed, rarely featuring entirely phase-pure domains. This renders elucidating the nature of molecularly mixed phases, and how these relate to device characteristics, a major challenge in the field. Recently, Ade et al. [1] have introduced the so-called domain-composition-variation concept, which accounts for the relative purity of domains as compared to a reference sample and can be experimentally determined by means of soft X-ray scattering experiments. Despite the fact that it represents a great advance because it allows for a phenomenological correlation between the domain purity and device characteristics, the lack of quantitative information about the absolute domain composition in real devices is still hindering a more refined understanding of how mixed phases impact the device function. Here, we present a new methodology based on fast scanning calorimetry that allows to readily determine the absolute composition of intermixed domains in BHJs processed using the same parameters as those employed for device fabrication (e.g. spin-casting a sub-200-nm thin film on PEDOT:PSS). The method exploits the well-known fact that the devitrification temperature, i.e. the Tg, of a finely intermixed blend in the glassy state is coherently affected by the composition of the blend. Thus, by conducting a careful characterization of the Tgof donor:acceptor thin film blends as well as to OPV cell replicas – by means of physical-ageing experiments [2] –, we are able to determine the composition of the intermixed domains of the OPV cells replicas. We demonstrate the applicability of our method for various donor:acceptor systems and discuss the impact of the knowledge gained on materials selection, device fabrication and long-term stability
J.M. thanks MINECO for the Ramón y Cajal contract and the Ikerbasque Foundation for the Ikerbasque Research Fellow program. Financial support from Fundación Iberdrola is also acknowledged. D.C. acknowledges the Basque Country Government (Ref. No. IT-654- 13 (GV)) and MINECO (Grant No. MAT2015-63704-P) for their financial support.
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