ABSTRACT
Herein we report a comparative morphological analysis of the perylene diimide (PDI)- and fullerene-based organic solar cells (OSCs) to identify the factors responsible for low performance of PDI-based devices. A PDI derivative, bis-PDI, and a fullerene derivative, PC70BM, are mixed with an efficient polymer donor, PffBT4T-2OD. The large disparity in power conversion efficiencies (PCEs) of OSCs composed of PffBT4T-2OD:bis-PDI (PCE = 5.18%) and PffBT4T-2OD:PC70BM (PCE = 10.19%) observed are attributed to differences in the nanostructural motif of bulk heterojunction (BHJ) morphologies of these blend systems. The X-ray scattering and surface energy characterizations revealed that the structurally dissimilar bis-PDI and PC70BM molecules determine the variation in blend film morphologies, and in particular, the molecular packing features of the donor PffBT4T-2OD polymer. In addition, high-resolution transmission electron microscopy (HRTEM) images explore the BHJ morphologies and presence of longer polymer fibrils in PffBT4T-2OD:bis-PDI system, justifying the unbalanced charge transport and high hole mobility. The low performance of PffBT4T-2OD:bis-PDI devices was further investigated by studying charge carrier recombination dynamics by using light-intensity-dependent and transient photovoltage (TPV) experiments. Furthermore, the temperature-dependent experiments showed the photovoltaic properties, including charge recombination losses, are strongly affected by energetic disorder present in bis-PDI-based system.
ABSTRACT
This study comprehensively explores the nanostructural properties of two diketopyrrolo[3,4-c]pyrrole-1,4-dione (DPP)-based small molecules with different alkyl side groups and their blends with the fullerene derivative PC71BM, using grazing-incidence wide-angle X-ray scattering synchrotron techniques. Preferentially relative face-on orientation within the larger and more ordered stacking phase of SM1 with its shorter side group (ethylhexyl) was observed in the majority of both pristine and blend thin films, whereas SM2 crystals showed strictly perpendicular orientation. These contrasting crystalline characteristics led to significant differences in the results, from which crystalline structure-performance property correlations are proposed. Thus, the results not only demonstrate important scientific insights into the relationship between molecular structure and crystalline formation but also provide molecular design directions that will facilitate further improvement to the morphology and performance of DPP-based small-molecule solar cells.
ABSTRACT
A series of small compound materials based on benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) with three different alkyl side chains were synthesized and used for organic photovoltaics. These small compounds had different alkyl branches (i.e., 2-ethylhexyl (EH), 2-butyloctyl (BO), and 2-hexyldecyl (HD)) attached to DPP units. Thin films made of these compounds were characterized and their solar cell parameters were measured in order to systematically analyze influences of the different side chains of compounds on the film microstructure, molecular packing, and hence, charge-transport and recombination properties. The relatively shorter side chains in the small molecules enabled more ordered packing structures with higher crystallinities, which resulted in higher carrier mobilities and less recombination factors; the small molecule with the EH branches exhibited the best semiconducting properties with a power conversion efficiency of up to 5.54% in solar cell devices. Our study suggested that tuning the alkyl chain length of semiconducting molecules is a powerful strategy for achieving high performance of organic photovoltaics.
