Visualization of Nanoscale Multi-Orientational Ordering in Thin Films of Polymer/Non-Fullerene Acceptor Blend

We have studied the composition and nanoscale multi-orientational molecular ordering in a bulk heterojunction (BHJ) blend films of the well-known donor polymer PTB7-Th and narrow bandgap non-fullerene acceptor (NFA) IEICO-4F. Blend films cast from chlorobenzene (CB) with 3 vol% of 1-chloronaphthalene (CN) show a dramatic morphological instability transitioning from fine-scale nano-domains to large micron-sized phase-separated domains when either the spin-coating speed or spin-coating duration is reduced below a certain threshold. Interestingly, we observe a decrease in the oscillator strength corresponding to IEICO-4F for films with a coarse morphology compared to films with a fine-scale morphology, which we explain using structural studies. From micro-Raman spectroscopic studies, we find the predominant localization of IEICO-4F within the large circular microscopic domains, while the surrounding matrix is rich in PTB7-Th. Moreover, for these films we observe a capping layer of PTB7-Th at the centre of domain. The molecular orientation within PTB7-Th:IEICO-4F blend films has also been visualized by combining cryo-electron microscopy (EM) with X-ray scattering studies. We find that molecules are oriented differently at the edge and at the center of these domains, a feature that has not been observed previously in spin-coated films containing NFAs. Finally, we find that while varying the processing conditions drastically affects the morphology, the corresponding change in device performance is relatively low, which is in contrast to a general notion of organic photovoltaic systems such as those based on polymer/fullerene blends.