Efficiency external quantum

External quantum efficiency (EQE) is the ratio of the number of charge carriers produced by the cell to the number of photons incident on the cell (Lianos, 2011). Incident photon to current conversion efficiency (IPCE) (Antoniadou et al, 2010 BriUet et al, 2010 Tode et al, 2010 Varghese and Grimes, 2008 Yu ef a/., 2010), is one way to express EQE, which is defined as  

---

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). 

Typical light output versus current (L—I) and efficiency curves for double heterostmcture TS AlGaAs LEDs lamps are shown in Eigure 8. The ir LED (Eig. 8a) is typically used for wireless communications appHcations. As a result, the light output is measured in radiometric units (mW) and the efficiencies of interest are the external quantum efficiency (rj y. = C y., photons out/electrons in) and power efficiency. As a result of the direct band gap... 

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. 

Figure 13-13. (a) Currem-vollage data from MEH-PPV-bascd OLEDs willi Au anodes and various cathodes plotted according to E4. (13.5) (b) the external quantum efficiency for the diodes with Al and Ca cathodes The solid line represents the maximum efficiency of 2%. Reproduced with permission from 11511. Copy light 1998 by the American Physical Society. 

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]. 

The incident monochromatic photon-to-current conversion efficiency (IPCE), also called external quantum efficiency, is defined as the number of electrons generated by light in the external circuit divided by the number of incident photons as a function of excitation wavelength. It is expressed in Equation (7).29 In most cases, the photoaction spectrum overlaps with the absorption spectrum of the sensitizer adsorbed on the semiconductor surface. A high IPCE is a prerequisite for high-power photovoltaic applications, which depends on the sensitizer photon absorption, excited state electron injection, and electron transport to the terminals ... 

In the case of InP-GalnAsP buried ridge structure lasers, continuous threshold currents of 8.2 mA at 300 K have been achieved (Kazmierski et al., 1989). This value compares favorably with the thresholds around 10 mA when proton implantation is used for insulation of the same structure. High maximum output power above 15 mW CW per facet and good maximum external quantum efficiency of 0.25 were found on the plasma hydrogenated structures. 

Figure 7.5. The absolute external quantum efficiencies of CIGS/CdS and CIGS/CdZnS devices made with identical absorbers. [Reproduced with permission from Ref. 40. Copyright 2006 American Institute of Physics.]... 

The luminance reaches 100 cd/m2 at 2.5 V with EL efficiency of 2.5 cd/ A. The corresponding external quantum efficiency is about 2% ph/el. At —10 V bias, the photosensitivity at 430 nm is around 90 mA/W, corresponding to a quantum yield of 20% el/ph [135], The carrier collection efficiency at zero bias was relatively low in the order of 10-3 ph/el. The photosensitivity showed a field dependence with activation energy of 10 2 eV [135], This value is consistent with the trap distribution measured in the PPV-based conjugated polymers [136,137],... 

FIGURE 3.23 External quantum efficiencies of PtOEP/CBP and PtOEP/Alq3 devices as a function of current with and without a BCP blocking layer (left). Emission spectra of CBP-based electroluminescent devices with and without a BCP exciton blocking layer (right). (From O Brien, D.F., Baldo, M.A., Thompson, M.E., and Forrest, S.R., Appl. Phys. Lett., 74, 442, 1999. With permission.)... 

Y. Su, H. Huang, C. Li, C. Chien, Y. Tao, P. Chou, S. Datta, and R. Liu, Highly efficient red electrophosphorescent devices based on iridium isoquinoline complexes remarkable external quantum efficiency over a wide range of current, Adv. Mater., 15 884-888 (2003). 

FIGURE 7.1 A two-layer vapor-deposited OLED first demonstrated by Tang et al. [4], The diamine acts as the hole transporting layer, Alq3 acts as the electron transporting or emitting layer. The external quantum efficiency was 1%. 

In the vapor-deposited OLED community, a number of approaches have been employed to produce white light emission. White OLEDs have been demonstrated based on multilayer structures, e.g., stacked backlights [153,168], multidoping of single-layer structures [145], phosphorescent monomer-excimer emission layers [169] and on doping of phosphorescent materials into separate bands within the emission zone, called a tri-junction [170]. The trijunction device has produced the highest white OLED efficiency of 16% external quantum efficiency demonstrated thus far [171]. 

I Schnitzer, E Yablonovitch, C Caneau, TJ Gmitter, and A Scherer, 30% External quantum efficiency from surface textured, thin-film light-emitting diodes, Appl. Phys. Lett., 63 2174—2176, 1993. 

There are three principal efficiency measurements in OLED external quantum efficiency, luminous efficiency, and power efficiency [13]. 

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. 

While the external quantum efficiency of PS reported for solid-state contacts is usually low, wet contacts are found to give high EL efficiencies at low applied bias under anodic [Vi2, Itl, Ge2, Ha7] as well as cathodic conditions [Bsl]. An example of bright EL from a micro PS sample in acetic acid under anodic bias is shown... 

An optical microcavity produced by the latter process has been applied to tune the emission from erbium-doped PS [Zh6], Erbium compounds like Er203 are known to exhibit a narrow emission band at 1.54 pm, which is useful for optical telecommunications. Several methods have been used to incorporate erbium in PS. A simple and economical way is cathodic electrochemical doping. External quantum efficiencies of up to 0.01% have been shown from erbium-doped PS films under electrical excitation [Lo2]. The emission band, however, is much broader than observed for Er203. This drawback can be circumvented by the use of an optical cavity formed by PS multilayers. In this case the band is narrowed and the intensity is increased because emission is only allowed into optical cavity modes [Lo3]. 

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. 

---