Photoluminescence and electroluminescence study of violet-blue and green emitting polymers and their blend

The photoluminescence and electroluminescence of violet-blue (I) and green (H) emitting polymers and their blend were studied. This included the effects of excitation wavelength, concentration, and film thickness on the photoluminescence; also, double-layer LEDs (ITO/PEDOT/polymer/Ca/Al) were fabricated and compared with single-layer LEDs. For the first time a strong dependence of the PL spectra on excitation wavelength for a violet-blue polymer in solution was observed. When the applied voltage was increased, the EL spectra showed a blue shift, and the emission intensity for the observed higher energy transition increased at the expense of the lower energy transition. Forster energy transfer in the photoluminescence and electroluminescence of the polymer blend was also studied. The results indicate that the energy transfer was independent of excitation wavelength and applied voltage. The LED using the blend was found to be more efficient than either of the devices using polymers I or II, because polymer I provides excitons which migrate to the chain of polymer II and contribute to the total emission. Furthermore, polymer I acts as a diluent to reduce exciton quenching. The chromophore blend also aids the separation of PL output from absorption, thereby alleviating self-absorption and improving the device efficiency.