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(')-ethyl)-hexoxy-1,4-phenylene vinylene) are described. In these measurements, the laser excitation power is modulated at 1<f(L)<100 kHz, and the spin-1/2 PLDMR of the PL response at f(L) is monitored by microwave modulation at f(M)<<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.