The Device Quantum Efficiency

TABLE 6.3. Spin-Speed-Dependent Quantum Efficiency and Emission Color 

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In most of the electrophosphorescence based OLEDs the device quantum efficiencies drop rapidly with increasing current density and consequently with the brightness due to triplet-triplet annihilation at high current densities. WOLED based on phosphorescent material had a maximum forward viewing power efficiency of 26 + 3 Im at low luminosity, decreasing to 11 1 Im W-1 at 1000 cd m 2 (Kamata et al 2002, D Andrade et al 2004). 

The external conversion efficiency can be significantly increased when the phase matching of the dye/ matrix layers to the EL devices is improved so that waveguiding losses are reduced. When the small air gap between the color conversion films and the EL device are eliminated, e.g., by depositing the color-conversion layer directly onto the device, quantum efficiencies for color conversion from blue to green of around 90% and for blue to red of around 40% are obtained [175]. 

Efficiency. Efficiency of a device can be reported in terms of an internal quantum efficiency (photons generated/electrons injected). The external quantum efficiency often reported is lower, since this counts only those photons that escape the device. Typically only a fraction of photons escape, due to refraction and waveguiding of light at the glass interface (65). The external efficiency can be increased through the use of shaped substrates (60). 

The hole current in this LED is space charge limited and the electron current is contact limited. There are many more holes than electrons in the device and all of the injected electrons recombine in the device. The measured external quantum efficiency of the device is about 0.5% al a current density of 0.1 A/cm. The recombination current calculated from the device model is in reasonable agreement with the observed quantum efficiency. The quantum efficiency of this device is limited by the asymmetric charge injection. Most of the injected holes traverse the structure without recombining because there are few electrons available to form excilons. 

Recent work with multi-layer polymer LEDs has achieved impressive results and highlights the importance of multi-layer structures [46]. Single-layer, two-layer and three-layer devices were fabricated using a soluble PPV-based polymer as the luminescent layer. The external quantum efficiencies of the single-layer, two-layer, and three-layer devices were 0.08%, 0.55%, and 1%, respectively, with luminous efficiencies of about 0.5 hn/W, 3 lm/W, and 6 lm/W. These results clearly demonstrate improvement in the recombination current because of the increase in quantum efficiency. The corresponding increase in luminous efficiency demonstrates that the improvement in recombination efficiency was achieved without a significant increase in the operating bias. 

The crystal quality of the InGaN QWs becomes poor mainly due to the lattice-constant mismatch and the difference of the thermal expansion coefficient between InN and GaN with increasing the In composition [4,5]. Therefore, in order to improve the external quantum efficiency (i/ext) of the InGaN-based LEDs and LDs, it is important to elucidate and optimize the effects of the various growth conditions for the InGaN active layer on the structural and optical properties. Recently, we reported a fabrication of efficient blue LEDs with InGaN/GaN triangular shaped QWs and obtained a substantial improvement of electrical and optical properties of the devices [6,7]. 

As a result, power efficiency is a function of the internal quantum efficiency, TjInt the light extraction, rjout. and the voltage, V. Thus, to improve device performance, advances in these three key areas are required. Examples of strategies used to maximize power efficiency are described below. 

Although one would prefer to know the internal quantum efficiency, it is only possible to measure the external quantum efficiency. Much of the light generated by an OLED is wave-guided out from the edges of the device. 

The external quantum efficiency of the EL from PS-based devices has been increased from low initial values of 0.001% [Ko9] to values close to 1% [Ni4, La6, Co5]. This, however, is still about one order of magnitude smaller than the maximum quantum efficiency of state-of-the-art LEDs based on III-V semiconductor heterostructures. 

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