Although non-fullerene small molecular acceptors (NF-SMAs) are dominating current research in organic solar cells (OSCs), measurements of thermodynamics drivers and kinetic factors determining their morphological stability are lacking. Here, we delineate and measure such factors in crystallizable NF-SMA blends and discuss four model systems with respect to their meta-stability and degree of vitrification. We determine for the first time the amorphous-amorphous phase diagram in an NF-SMA system and show that its deep quench depth can result in severe burn-in degradation. We estimate the relative phase behavior of four other materials systems. Additionally, we derive room-temperature diffusion coefficients and conclude that the morphology needs to be stabilized by vitrification corresponding to diffusion constants below 10−22 cm2/s. Our results show that to achieve stability via rational molecular design, the thermodynamics, glass transition temperature, diffusion properties, and related structure-function relations need to be more extensively studied and understood. In recent years, the performance of organic solar cells (OSCs) has greatly improved with the development of novel non-fullerene small molecular acceptors (NF-SMA). The rapid increase in power conversion efficiency, now surpassing 15%, highlights an immediate and increasing need to understand the longevity and lifetime of NF-OSCs. However, the field relies mainly on a laborious trial-and-error approach to select polymer:NF-SMA pairs with desirable device stability. Here, we provide a structure-property relation that explains the morphological stability and burn-in degradation due to excessive demixing or crystallization. The framework presented in our study shows that a specific balance of interactions between polymer and NF-SMA can offer a short-term solution against excessive demixing. Long-term morphological stability that also suppresses crystallization can only be achieved by freezing in the initial quenched morphology through the use of polymers and/or NF-SMAs with low flexibility. This research provides a structure-property relation that sheds light on morphological stability of NF-OSCs by using the thermodynamic and the kinetic perspectives. We show that NF-OSCs can suffer from excessive amorphous-amorphous phase separation in the blends and crystallization of NF-SMA. The former instability channel can be eliminated in systems with an optimal miscibility, whereas the excessive phase separation in low miscibility systems and NF-SMA crystallization need to be suppressed through the utilization of polymers or NF-SMAs with low flexibility.
|Original language||English (US)|
|Number of pages||21|
|State||Published - 2019|
Bibliographical noteKAUST Repository Item: Exported on 2021-07-08
Acknowledged KAUST grant number(s): N000141712204
Acknowledgements: NCSU gratefully acknowledges the support of ONR grant N000141712204 and KAUST's Center Partnership Fund (No. 3321). UNC supported by NSF grant (CBET-1639429). I.M. and A.W. acknowledge funding from EPSRC Project EP/M005143/1 and EC FP7 Project SC2 (610115). X-ray data were acquired at beamlines 220.127.116.11 and 7.3.3 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under contract no. DE-AC02-05CH11231. SIMS was performed at the Analytical Instrumentation Facility (AIF) at NCSU, which is partially supported by the State of North Carolina and the National Science Foundation. The DSC instrument was purchased with UNC-GA ROI funds. C. Zhu, A. Hexemer, and C. Wang of the ALS (LBNL) provided instrument maintenance. Professor Enrique Gomez and Josh Litofsky are acknowledged for providing the initial FH program code. The authors acknowledge L. Ye for providing thoughtful and critical comments. H.A. conceived the scientific framework with the help of M.G. M.G. designed experimental protocols, coordinated the experimental work, performed the SIMS/DSC measurements, and analyzed the SIMS/DSC data. M.G. made the GIWAXS and R-SoXS samples, and H.H. analyzed the GIWAXS and R-SoXS data. M.G. fabricated P3HT:EH-IDTBR, FTAZ:ITIC, and PTB7-Th:EH-IDTBR solar cell devices and performed the subsequent stability tests. H.H. fabricated and tested the FTAZ:EH-IDTBR devices. Z.P. performed and analyzed the temperature-dependent UV-vis data. M.G. H.H. and H.A. wrote the manuscript. J.J.R. and A.W. synthesized the FTAZ and P3HT polymers, respectively. H.H. I.A. J.H.C. and S.J.S. performed the GIWAXS and R-SoXS measurements. All authors provided comments on the manuscript and contributed to the editing. H.A. directed the study. The authors declare no competing interests.