Rachel Kilbride1,Matthew Bidwell2,Emma Spooner1,Robert Dalgliesh3,Philipp Gutfreund4,James Tellam3,Nicola Gasparini2,Iain McCulloch5,David Lidzey1,Richard Jones6,Andrew Parnell1
University of Sheffield1,Imperial College London2,ISIS Pulsed Neutron and Muon Source3,Institut Laue-Langevin4,University of Oxford5,The University of Manchester6
Rachel Kilbride1,Matthew Bidwell2,Emma Spooner1,Robert Dalgliesh3,Philipp Gutfreund4,James Tellam3,Nicola Gasparini2,Iain McCulloch5,David Lidzey1,Richard Jones6,Andrew Parnell1
University of Sheffield1,Imperial College London2,ISIS Pulsed Neutron and Muon Source3,Institut Laue-Langevin4,University of Oxford5,The University of Manchester6
Over the past decade, the development of small molecule non-fullerene acceptors (NFAs) as the photoactive components in organic photovoltaic (OPV) devices has pushed efficiencies close to the fabled 20%<sup>[1,2]</sup>, an efficiency threshold considered essential for commercially viable OPV technologies. Unfortunately, increases in OPV device efficiency have not been in step with increased long-term operational stability and devices typically demonstrate an initial ‘<i>burn-in</i>’ period of rapid performance loss. The origin of this performance loss remains poorly understood and mitigation strategies are yet to be developed. In this work, the stability of a well-studied polymer : fullerene system (PTB7-Th : PC<sub>71</sub>BM) is compared with a series of polymer: NFA systems based on PTB7-Th and the IDTBR family of NFAs. Polymer : IDTBR systems demonstrate superior stability, characterised by an unusual fill-factor-driven efficiency enhancement or ‘<i>burn-up</i>’ improvement period within 24h of fabrication. The morphological origins of this phenomenon are explored across a broad range of length scales (Å – nm - μm) using grazing incidence wide angle scattering (GIWAXS), small angle neutron scattering (SANS) and neutron reflectivity. Neutron scattering measurements of polymer : NFA blend films are made possible by the synthesis of deuterium-labelled NFA analogues (d-NFAs), providing unparalleled sensitivity and insight into the morphological stability and phase separation of the bulk-heterojunction photoactive blend. It is found that subtle changes in the degree of phase separation and NFA crystallinity during device ageing under inert conditions are closely linked to changes in PV efficiency. The findings described here provide a more thorough understanding of the morphological stability of the bulk heterojunction blend, essential for improving the operational lifetime and commercial viability of OPV technologies.<br/>[1] L. Hong, H. Yao, Z. Wu, Y. Cui, T. Zhang, Y. Xu, R. Yu, Q. Liao, B. Gao, K. Xian, H. Y. Woo, Z. Ge, J. Hou, <i>Adv. Mater.</i> <b>2019</b>, <i>31</i>, 1903441.<br/>[2] P. Bi, S. Zhang, Z. Chen, Y. Xu, Y. Cui, T. Zhang, J. Ren, J. Qin, L. Hong, X. Hao, J. Hou, <i>Joule</i> <b>2021</b>, <i>5</i>, 2408.