Influence of nanomorphology, texture and crystallinity on the device efficiency of organic bulk heterojunction solar cells studied by energy-filtered transmission electron microscopy
The power conversion efficiency (PCE) of solution-processed bulk heterojunction (BHJ) solar cells has seen a dramatic increase in the last decades using new donor and acceptor materials as well as by the development of new processing conditions. By mixing an electron donor with an electron acceptor a certain nanomorphology of the two phases is evolving depending on the composite materials, the applied solvent and the post-processing conditions like solvent vapor (SVA) or thermal annealing (TA). While the crystallinity and texture of the donor/acceptor components determine the efficiency of exciton generation and mobility, the nanomorpholgy of the BHJ dictates the charge separation and transport properties. Therefore an in-depth understanding of the relationship between post-processing, morphology, crystallinity and device performance is of high importance to be able to further improve the device performance. Here we report on a high-efficiency photovoltaic system composed of a small molecule (DRCN5T) and a fullerene derivative (PC71BM) as electron acceptor. We investigated the influence of four different solvent vapor atmospheres on the nanomorphology, crystallinity, texture and efficiency of our organic solar cell system. We studied the nanomorpholgy evolution as a function of the solvent vapor atmosphere and the annealing time using energy-filtered transmission electron microscopy (EFTEM) imaging techniques. Thereby, we figured out that the choice of the solvent for SVA and especially its donor solubility has a high impact on the evolving nanomorpholgy and the speed of phase separation and crystal growth. The crystallinity and texture of the annealed and as-casted samples was investigated by energy-filtered electron diffraction (EF-ED) measurements where an energy selecting slit was positioned around the zero-loss peak in order to enhance the contrast of elastic scattering and Bragg diffraction. The crystallinity was further studied by aquiring high resolution TEM (HRTEM) images which show nicely the orientation and size of single DRCN5T crystallites. The nanomorpholgy and crystallinity were then further correlated to the device performance. Currently we are performing in situ EF-ED studies on the dose dependency of the electron beam on the deterioration behaviour of the small molecule crystallites. By that we can adjust a dose condition that is well suited to study beam sensitive organic crystals by lowering the beam damage of electron beam irradiation.
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|Extended Abstract||EFTEM elemental maps of carbon for different solvent vapor atmospheres (CHCl3, THF, CS2 and C3H6O2) and various annealing times. Dark areas correspond to sulfur-rich domains and brigth areas comprise carbon-rich domains.||2 MB||Download|