Comparative analysis of magnetic resonance in the polaron pair recombination and the triplet exciton-polaron quenching models

TL;DR

This study compares magnetic resonance models in conjugated materials, revealing distinct lineshape behaviors and power dependencies, which can help experimentally distinguish between the polaron pair recombination and triplet exciton-polaron quenching mechanisms.

Contribution

It provides a detailed theoretical comparison of the PPR and TPQ models, highlighting their different resonance lineshapes, power dependencies, and underlying spin dynamics, aiding experimental discrimination.

Findings

PPR model shows lineshape evolution from positive to negative with increasing microwave power.

TPQ model exhibits a monotonic saturation with positive resonance lineshape.

Resonance amplitude scales as P_L in PPR and as P_L^2 to P_L^3 in TPQ.

Abstract

We present a comparative theoretical study of magnetic resonance within the polaron pair recombination (PPR) and the triplet exciton-polaron quenching (TPQ) models. Both models have been invoked to interpret the photoluminescence detected magnetic resonance (PLDMR) in $\pi$-conjugated materials. We show that resonance lineshapes calculated within the two models differ dramatically in several regards. First, in the PPR model, the lineshape exhibits unusual behavior upon increasing the microwave power: it evolves from fully positive at weak power to fully negative at strong power. In contrast, in the TPQ model, the PLDMR is completely positive, showing a monotonic saturation. Second, the two models predict different dependencies of the resonance signal on the photoexcitation power, $P_L$. At low $P_L$, the resonance amplitude $\Delta I/I$ is $\propto P_L$ in the PPR model, while it is $\propto P_L^2$ crossing over to $P_L^3$ in the TPQ model. On the physical level, the differences stem from different underlying spin dynamics. Most prominently, a negative resonance within the PPR model has its origin in the microwave-induced spin-Dicke effect, leading to the resonant quenching of photoluminescence. The spin-Dicke effect results from the spin-selective recombination, resulting in a highly correlated precession of the on-resonance pair-partners under the strong microwave power. This effect is not relevant to TPQ, where the majority of triplets are off-resonance due to the strong zero-field splitting. The analytical evaluation of lineshapes for the two models is enabled by expressing these shapes via the eigenvalues of a complex Hamiltonian. This bypasses the necessity of solving the much larger complex system of stochastic Liouville equations. Our findings pave the way towards a reliable discrimination between the two mechanisms via cw PLDMR.