Approaches to improving pulsed EDMR spectroscopy in organic electronic devices using adiabatic pulses

Abstract

Exploiting organic electronic material in optoelectronic devices requires us to understand the generation, mobility, and recombination of charge carriers. In these materials, the quantum mechanical property of spin has a considerable effect on these processes. Due to weak spin-orbit coupling, spin is a good quantum number, and spin-dependent processes play a significant role in conductivity and recombination. Consequently, spin-based probes such as electron spin resonance are effective ways to understand the underlying electronic properties in these materials. The long-lived spin states also point toward applications in sensing, using either magnetoresistive or resonant effects to provide new functionality. A useful technique for this is electrical detection of magnetic resonance (EDMR), which uses electron spin resonance for spin manipulation and conductivity measurements to detect the resulting changes in transport properties. For example, the change in the current through an organic light-emitting diode (OLED) due to EDMR allows us to determine spin lifetimes and spin-dependent recombination rates of polaron pairs. A number of proposals have been shown that OLEDs can be used as magnetometers via phase-sensitive electron spin resonance approaches. Owing to the disorder inherent in these materials, a challenge applying these techniques to organic devices is the inhomogeneous broadening of polaron spin resonances due to interactions with nuclear spins in the organic materials. Previous work shows that adiabatic pulse schemes can improve both fidelity and sensitivity of EDMR in organic devices under ideal conditions. Here, I extend these results to include simulations with realistic spin lifetimes and recombination rates. I experimentally determine the spin and carrier lifetimes for MEH-PPV-based devices and use these parameters to model the impact of adiabatic pulse schemes using these parameters in a stochastic Liouville framework, expanding on previous work by explicitly including Redfield terms. I show theoretically that chirp pulses increase the fidelity of operation in a Hahn echo sequence compared to square pulses, and that the trajectories of spin pairs under adiabatic excitation schemes has a more complex impact on the resulting fidelity than two independent spins. This work is important for applications which exploit the properties of organic semiconductors such as spin-based sensing.