Photovoltaic effect of spin-triplet emitters PtOEP and Ir(ppy) 3 in organic light emitting diode devices

Abstract

Photovoltaic effect has been studied on single- and multi-layer OLED devices based on phosphorescent PtOEP and Ir(ppy) 3 molecules, together with the electroluminescence (EL). The incident photon to current efficiency (IPCE) spectra of these PtOEP and Ir(ppy) 3 devices are similar to the absorption spectra of PtOEP and Ir(ppy) 3 molecules, respectively. This indicates that the photovoltaic effect is caused by the optical excitation of PtOEP and Ir(ppy) 3 molecules. The single-layer devices show weak EL luminance and low EL efficiency. Although the EL efficiency of the multi-layer Ir(ppy) 3 OLED device is high, the IPCE value is quite low, e.g. 0.013 % in the single-layer device at the absorption peak wavelength of 386 nm. The same is true for PtOEP OLED, e.g. 0.0425 % at the absorption peak wavelength of 371 nm. Such a low IPCE is considered to be due to low carrier mobility of PtOEP and Ir(ppy) 3 molecules. It was found that the multi-layer PtOEP and Ir(ppy) 3 OLED devices with emitting layer of guest-host system show much less efficient photovoltaic effect than the single-layer devices without guest-host.