Morphological Correlations in Model Poly(Alkylthiophene)-Fullerene Thin Film OPV Devices
Thin film bulk heterojunction (BHJ) systems exploiting fullerenes dispersed in conjugated polymers are popular material candidates for organic photovoltaics (OPVs). The device efficiency is dependent on a number of factors including the inherent morphology of the BHJs. To improve device efficiency and make OPVs commercially viable, it is essential to be able to understand and consequently control the morphology of BHJ films. We are studying model systems composed of mixtures of different poly(3-alkyl thiophene)s and fullerenes. BHJ morphologies of these mixtures in full device configuration have been explored using a variety of techniques and correlated to their device performance. Composition depth profile analysis using neutron reflectivity has shown that fullerene segregates to both the top (Al) and bottom (PEDOT:PSS) electrodes. The segregated excess fullerene at the PEDOT:PSS electrode has been shown to be inversely related to the fill factor (FF), whilst the excess at the Al electrode is linearly related to the short-circuit current. Grazing incidence SANS has shown that the lateral phase domain morphology varies through the BHJ thickness. At the Al electrode interface, domains of tens of nanometers are observed but everywhere else in the film domain sizes are over hundreds of nanometers. WAXS measurements have shown that devices with the highest efficiency have the lowest overall total crystallinity and the crystals are for the most part randomly oriented relative to the electrode surfaces, with a small degree of orientation of the (100) planes of the poly(thiophenes) parallel to the substrate. Using these design criteria, we are exploring the use of additives to control crystallization of the poly(thiophene). Given we observe a reduction in FF with increased fullerene at the PEDOT:PSS we are exploring the methods to modify the electrode interface to promote poly(alkylthiophene) segregation and hence increase FF.
Abstracts of Papers of the American Chemical Society
Ded, Nabanakur; bucknall, David G.; Karim, Alamgir; Gong, Xiong; Skoda, Max; and Rodgers, Sarah, "Morphological Correlations in Model Poly(Alkylthiophene)-Fullerene Thin Film OPV Devices" (2014). Polymer Science Faculty Research. 815.