MAGNETIC-RESONANCE IN FILMS AND PHOTODIODES BASED ON POLY-(PHENYL-PHENYLENE-VINYLENE)

Films of poly-(2-phenyl-1,4-phenylene-vinylene) (PPPV) and photodiodes with PPPV as an active layer were studied by optically (ODMR) and ele ctrically (EDMR) detected electron-spin resonance (ESR). Two different signals were observed in ODMR: enhancement of the photoluminescence ( PL) at g=2.01 due to recombination of the photogenerated polarons (s=1 /2), and a half-field enhancement signal, attributed to the fusion of triplet excitons. Both processes lead to the formation of singlet exci tons. The spectral dependence of the s=1/2 signal follows the low ener gy part of the PL spectrum, indicating that delayed recombination of d istant polarons is influenced by ESR, whereas the cw PL contains both prompt and delayed contributions. The linewidth and the intensity of t he ODMR signal depend on the PL excitation intensity. Both effects are due to a decrease of the recombination lifetime of the polaron pairs at higher intensities. The relative decrease of the short-circuit phot ocurrent I-SC through a PPPV photodiode by ESR saturation is due to re combination of nonthermalized, nongeminate excess charge polarons in t he active layer of the device. This effect is at least two orders of m agnitude stronger than the enhancement of total PL at the same tempera ture. This feature is found to be common for conjugated polymers inves tigated so far, and reflects the fact that the total photogenerated I- SC is spin dependent, whereas ODMR selects only the nongeminate portio n of recombining species in the sample. (C) 1996 American Institute of Physics.