Visualizing Degradation at the Nanoscale from Morphological and Energetic Perspectives in Organic Photovoltaics

Organic photovoltaics (OPVs) have received tremendous interest in recent decades due to their flexibility, ease of processing and cost effectiveness [1-3]. With the advent of non-fullerene acceptors (NFA), the efficiency of these solar cells has broken previous records and has currently achieved power conversion efficiency (PCE) &gt; 18% [4]. OPVs is now confronting the next bottleneck which is its instability. Multiple pathways for degradation of organic solar cells have been reported [5], the most common route for the active layer being photodegradation and morphological degradation. The stability of NFA-based OPVs is contingent on the morphology and its evolution throughout the lifetime of the device. Since interfacial/mixed phase energetics are crucial for charge generation [6], it is imperative to maintain the energetics of Donor (D), Acceptor (A) rich region and mixed region in the blends during device aging and operation. Moreover, changes in the energetics caused by the morphological evolution due to de-mixing of D and A materials in the mixed region can have a vital role on degradation of device performance. A definitive morphological picture of the BHJ during degradation studies, down to a length scale of a few molecules, is therefore urgently needed, as this can shed light on the energetic landscape and its evolution within the BHJ. In this work, we take a deeper dive into the morphological aspect of degradation utilizing scanning tunneling microscopy/spectroscopy (STM/S), a unique and quite effective tool for characterizing morphology and associated energetics. We study the morphology of stable and unstable systems and the evolution of their domain and mixed phase energetics. We focus on PM6-based blends with ITIC-4F, Y6 and PC₇₁BM. Along with investigating the properties of the neat materials, we visualize and identify D and A-rich phases within the various BHJs and compare energetics within BHJ to neat D and A materials to correlate with its miscibility properties. In addition, we compare the histogram of Local Densities of States (LDOS) of the PM6 and A-rich phases in their neat and blend forms to gain insight into the energetic disorder as these materials are mixed in the blends. Finally, we visualize morphological degradation of PM6:Y6 and evaluate changes of LDOS within PM6, Y6 and mixed domains within the BHJ due to thermal annealing which is in correlation with changes observed in device performance parameters.<br/><br/>Reference:<br/>1. B.E. Logan, M. Elimelech, Nat. Commun. 488 (2012) 313–319.<br/>2. Y. Zhang, J. Wang, X. Wang, Renew. Sustain. Energy Rev. 32 (2014) 255–270.<br/>3. H. Fu, C. Li, P. Bi, X. Hao, F. Liu, Y. Li, Z. Wang, Y. Sun, Adv. Funct. Mater<i>.</i> (2019), 29, 1807006.<br/>4. Q. Liu, Y. Jiang, K. Jin, J. Qin, J. Xu, W. Li, J. Xiong, J. Liu, Z. Xiao, K. Sun, S. Yang, X. Zhang, L. Ding, Chinese Science Bulletin (2020), 65, 272-275.<br/>5. M. Jørgensen, K. Norrman, S. A. Gevorgyan, T. Tromholt, B. Andreasen and F. C. Krebs, Adv. Mater., 2012, 24, 580–612.<br/>6. H. Xin<i> et al.</i>, Polymer Nanowire/Fullerene Bulk Heterojunction Solar Cells: How Nanostructure Determines Photovoltaic Properties. ACS Nano, 4, 1861-1872 (2010).