Yield of Singlet Excitons in Organic Light-Emitting Devices: A Double Modulation Photoluminescence-Detected Magnetic Resonance Study

Double-modulation (DM) photoluminescence (PL) detected magnetic resonance (PLDMR) measurements on poly(2-methoxy-5-($2^{\prime }$-ethyl)-hexoxy-1,4-phenylene vinylene) are described. In these measurements, the laser excitation power is modulated at $1<f_L<100\mathrm{kHz}$, and the spin-$1/2$ PLDMR of the PL response at $f_L$ is monitored by microwave modulation at $f_M\ll f_L$. The frequency response of the DM-PLDMR is inconsistent with the assumption that the PLDMR is due to delayed PL from nongeminate polaron recombination, which is the basis for previous predictions of the singlet-exciton (SE)-to-triplet exciton (TE) ratio in organic light-emitting devices. Rather, the frequency response is consistent with the assumption that the PLDMR is due to variations in the rate of quenching of SEs by TEs and polarons.