95cd/A (at 17.2V), 6.96cd/A (at 20.8V), 7.52cd/A (at selleck chemicals llc 21.4V), and 5.78cd/A (at 26.2V), respectively. The maximum luminance and power efficiency of four devices (shown in Table 1) became lower when increasing the doping concentration. The doping phosphor dye was not only the emission center but also traps for free carriers in emission layer, which could be predicted from the device energy level diagram shown in Figure 4. More free carriers would be trapped in the devices when adding more phosphor dye in the blended host layer, which would affect the current density, numbers of emission elements, and finally the whole device efficiency. This point could be proved by the current density curve of four devices at high driving voltages shown in Figure 2. The current density of device A (1wt% doping concentration) was 12.

8mA/cm2 and decreased to 1.2mA/cm2 for device D (8wt% doping concentration) at 20V forward bias, which were listed in Table 1. The roll-off of external quantum efficiency of electroluminescent device at large current density was due to triplet-triplet (T-T) annihilation for most phosphorescent organic device. Endo et al. analyzed T-T annihilation for Ir(ppy)3 doped CBP host devices, and a best fit of the model to the data was obtained [24]. Almost all organic devices indicated a gradual decrease in efficiency at the high current density (according to high voltage). The quantum efficiency of light emission (��) can be calculated from the following equation:�Ǧ�0=J04J(1+8JJ0?1).(1)In (1), ��0 is the quantum efficiency without triplet-triplet annihilation, and J0 in (2) is the ��onset�� current density at �� = ��0/2:J0=4qdkTT��2.

(2)In which q is the electron charge, d is the thickness of the exciton formation zone, �� is the phosphorescent life time, and kTT is the T-T annihilation rate constant. As the current density (driving bias) increased, the efficiency annihilation became more obvious as shown in (1). For comparing, the turn-on voltage (Von ), maximum lumiance (Lmax ), current density at 20V (J20V), maximum power efficiency (��pmax ), and current efficiency (��lmax ) were list in Table 1. Stable green light emission, wide emission spectra, and high efficiency of 9.95cd/A could be achieved with the 1wt% doping concentration polymer device with blended host materials.Figure 5The power efficiency and current efficiency of four doping devices at different biases.

Table 1Some detailed characteristics of four organic devices with different doping concentrations, including turn-on voltage, maximum luminance, current density at 20V, maximum current, and power efficiency.4. ConclusionsDifferent doping concentration polymer organic devices with iridium complex doped in PFO:PBD blended Anacetrapib host material were fabricated using spun coat process. The EL spectra of devices indicated stable green light emission from (tpbi)2Ir(acac) with two main peaks at 522nm and 554nm and a wide FWHM of 116nm from 504nm to 620nm.