Kinetics of Photogenerated Carbon Dangling Bonds in Organic Photovoltaic Thin Films—An EPR Study

Recent developments in bulk heterojunction (BHJ) organic solar cells based on non-fullerene acceptors (NFA) have rekindled interest in single junction BHJ organic photovoltaic (OPV) solar cells with &gt;19% efficiency reported by multiple groups. The low cost, lightweight, and flexible OPVs are promising as a renewable energy source, and as photodetectors in chemical and biological sensors. However, long-term stability issues continue to be a significant obstacle for OPV commercialization.<br/>Oxygen, moisture, and photon irradiation are known to be the major sources of degradation in BHJ OPVs. Proper encapsulation can prevent extrinsic degradation caused by ambient oxygen and water, but intrinsic degradation resulting from thermal processes and radiation still occurs. Multiple studies have investigated photogenerated defect formation in OPVs. The presence of carbon dangling bond (CBD) defects in photodegraded OPVs was suggested theoretically and identified using electron paramagnetic resonance (EPR) spectroscopy and optoelectronic measurements, but the mechanisms underlying the intrinsic photodegradation, particularly the early kinetics, still need to be completely understood.<br/>We report the investigation of early kinetics of spin-active photogenerated CDB defect formation and annealing in oxygen- and moisture-free environments using X-band EPR spectroscopy. Specifically, BHJ blends of poly[[4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-2,6- diyl]-2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]] (PBDB-T):PCBM and PBDB-T:ITIC are examined under irradiation at 300 nm to monitor the time evolution of CDBs for over 200 hours. The presence of CDBs in the samples generated in an oxygen- and water-free environment is confirmed by the obtained g-values. For the PBDB-T:PCBM blend, the spin count increases steadily until ~40 hours of irradiation, whereas for the PBDB-T:ITIC blend, it increases until ~60 hours. Subsequently, there is a notable decrease in the rate of defect formation. Further, the CDB defect count follows a power-law <i>t^β</i> with irradiation time t, where <i>β</i> ∼0.55-0.58. Theoretical analysis reveals a monomolecular defect creation model consistent with the expected theoretical value <i>β</i>= 1/2.<br/>Our findings deepen the understanding of early degradation kinetics in BHJ OPV films. This research is pivotal in developing enhanced materials that mitigate defect formation, thus advancing the stability of OPV systems essential for applications in large-scale energy production and biochemical optical sensing.<br/><br/>Acknowledgments<br/>This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. The research was performed at the Ames National Laboratory, which is operated for the U.S. DOE by Iowa State University under contract DE-AC02-07CH11358. We acknowledge the use of computational resources at the National Energy Research Scientific Supercomputing Center (NERSC) which is supported by the Office of Science of the U.S. DOE under contract no. DE-AC02-05CH11231.