Molecular insights into photostability of fluorinated organic photovoltaic blends: role of fullerene electron affinity and donor–acceptor miscibility† Colin P. Brook, a Goutam Paul, b Vinila Nellissery Viswanathan, c Sandeep Satyanarayana, c Kumar M. Panidhara, c Bryon W. Larson, d Andrew J. Ferguson, d Amlan J. Pal, b Praveen C. Ramamurthy, c Steven H. Strauss, a Olga V. Boltalina a and Wade A. Braunecker * d In this work, the photostability of certain organic photovoltaic (OPV) active layers was demonstrated to improve by as much as a factor of five under white light illumination in air with the use of 1,7-bis- trifluoromethylfullerene (C 60 (CF 3 ) 2 ) as the acceptor in place of PC 60 BM. However, the results were highly dependent on the structure and functionality within the donor material. Twelve combinations of active layer blends were studied, comprised of six different high-performance donor polymers (two fluorinated and four non-fluorinated donors) and two fullerene acceptors (PC 60 BM and C 60 (CF 3 ) 2 ). The relative rates of irreversible photobleaching of the active layer blends were found to correlate well with the electron affinity of the fullerene when the polymer and fullerene were well blended, but a full rationalization of the photobleaching data requires consideration of both the electron affinity of the fullerene as well as the relative miscibility of the polymer–fullerene components in the blend. Miscibility of those components was probed using a combination of time-resolved photoluminescence (TRPL) measurements and scanning tunneling microscopy (STM) imaging. The presence of fluorinated aromatic units in the donor materials tend to promote more intimate mixing with C 60 (CF 3 ) 2 as compared to PC 60 BM. The full results of these photobleaching studies and measurements of donor–acceptor miscibility, considered alongside additional photoconductance measurements and preliminary device work, provide new molecular optimization insights for improving the long-term stability of OPV active layers. 1. Introduction The potential for organic photovoltaics (OPVs) to offer an inexpensive source of renewable energy in the form of light- weight and exible modules 1,2 has been driving extensive research efforts in the eld. 3–5 With reports of single junction 6,7 and tandem cells 8 now exceeding 17% power conversion effi- ciencies (PCEs), along with recent advances in large area printing of OPV modules, 1,9 the technology is becoming increasingly commercially viable. However, while encapsulation of the active layer can extend the operational lifetimes of certain OPV systems from days to years, 10,11 the intrinsic instability of the active layer remains a pertinent obstacle to the wide-spread real-world application of OPV technology. 9,10,12 While there are multiple known pathways that contribute to degradation, 12–15 thermal instability of the active layer morphology and irrevers- ible photobleaching of the active layer components are typically the most detrimental. The most favorable active layer architecture in high perfor- mance devices is the bulk heterojunction (BHJ), which exists as a metastable state consisting of interpenetrating domains of an electron donating material (typically a small molecule or conjugated polymer) and an electron accepting material (typi- cally a fullerene derivative, although efficient non-fullerene acceptors are becoming more common 5 ). The BHJ architec- ture both maximizes the amount of donor–acceptor contact for charge generation and separation, while producing morphol- ogies that facilitate charge transport to the electrodes. 16 However, because it is a metastable state, the BHJ is intrinsically susceptible to degradation under operating conditions through phase separation over time. While less common, some blends a Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA b School of Physical Sciences, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India c Department of Materials Engineering, Indian Institute of Science, Bangalore, Karnataka, 560012, India d National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, USA. E-mail: Wade.Braunecker@nrel.gov † Electronic supplementary information (ESI) available: Polymer characterization, experimental details regarding lm preparation, STM/STS analysis, TRPL and TRMC measurements, and OPV device work. See DOI: 10.1039/d0se00971g Cite this: Sustainable Energy Fuels, 2020, 4, 5721 Received 3rd July 2020 Accepted 18th September 2020 DOI: 10.1039/d0se00971g rsc.li/sustainable-energy This journal is © The Royal Society of Chemistry 2020 Sustainable Energy Fuels, 2020, 4, 5721–5731 | 5721 Sustainable Energy & Fuels PAPER Published on 25 September 2020. Downloaded by Indian Institute of Science on 12/30/2021 11:43:08 AM. View Article Online View Journal | View Issue