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Silicon light source plays a key role in silicon optoelectronics, but its realization is an extremely challenging task. Although there are long-term intensive efforts to this topic, the power conversion efficiency (PCE) of the silicon-based electroluminescence is still no more than 1%. In this present report, a highly efficient silicon light source has been achieved. The device structure is p-Si (5 Ωcm)/ SiO2(~2nm)/NPB/CBP: (ppy)2Ir(acac)/Bphen/Bphen: Cs2CO3/Sm/Au. The SiO2 passivated Si is the anode having a suitably high hole-injection ability, and CBP: (ppy)2Ir(acac) is a highly efficient phosphor doped organic material. The device turn-on voltage is 3.2 V. The maximum luminance efficiency and maximum luminous power efficiency reach 69 cd/A and 62 lm/W, respectively, corresponding to a maximum PCE of 12% and an external quantum efficiency of 17%.
©2008 Optical Society of America
1. Introduction
Silicon light source plays a key role in silicon optoelectronics and is of high worldwide interest. [1] The requirement of power conversion efficiency (PCE) from electrical power to optical power of a silicon light source for optical interconnections is of ≥10%. [2] Unfortunately, the PCEs of silicon light sources nowadays are still around 1% or lower. The main reason is that silicon is an indirect bandgap material and has an inherent low efficiency in light emission. Researchers try to overcome this obstacle and have been exploring many routes. Among them, silicon nanostructures, optically active center doping and direct bandgap silicon compounds etc. have been emphasized. [3–11] One of other routes is combining the highly efficient light emitting semiconductors with silicon, such as III-V or II-VI compounds with silicon. However, there is a lattice mismatch at the interface between these compound semiconductors and silicon. To avoid the obstacle people try another way, that is, to combine emissive organic materials to silicon, where silicon is usually an anode to inject holes as well as the substrate. The organic semiconductors are amorphous and there is no lattice mismatch at the interface between organic and silicon. The combination is simple, low cost, and generally compatible with CMOS technology. [12–14] Such silicon-based organic light emitting diodes (OLEDs) may act as a light source to implement inner-/inter-chips chips optical connections. They also have a great potential application for display, when driven by TFT active matrices.
Great effort has been taken to develop silicon-based OLEDs. To our knowledge, the reported PCEs for those devices are below 1% to date, [12, 15–21] including the very recent work. [21] Here we have realized a highly efficient silicon-based electroluminescence: a silicon-based phosphorescent OLED with a maximum PCE of 12% and maximum external quantum efficiency (EQE) of 17%, at 3.5 V. In this diode, an ultrathin SiO2 layer passivated p-silicon anode offers a suitable strong hole injection; [16, 18, 19] an electrical doping electron transport material offers a strong electron injection; [22] A phosphor is used as a highly efficient emitter [23]; a low work function and highly transparent metal bilayer is used as a cathode. [18]
2. Experimental results and discussion
We adopted a p-type silicon wafer passivated by an ultrathin SiO2 layer as both the anode and substrate of the device due to its efficient hole-injection property. The wafer is (100) oriented and has an electrical resistivity of about 5 Ωcm. It was cleaned routinely and the passivating SiO2 layer (~2 nm) was grown by low temperature (~400 °C) oxidation on its front side as reported in our previous papers [18, 19]. This SiO2 layer is indispensable to reduce the surface recombination current. Al Ohm contact was formed in the backside. The hole transport layer is NPB [N,N’-bis -(1-naphthl) -diphenyl-1,1’-biphenyl-4, 4’-diamine]. The emitter is (ppy)2Ir(acac) [bis(2-phenylpyridine) iridium(III) acetylacetonate], which is doped into the host material, CBP (4,4’-N,N’-dicarbazole-biphenyl). [23] This phosphor doped organic emitter has a nearly 100% internal quantum efficiency. To provide an enhanced electron injection, Cs2CO3 doped BPhen (4,7-diphenyl-1,10-phenanthroline) is adopted as the electron transport layer. [22] Here, no p-type doped hole transport layer has been adopted because an enhanced hole injection has been already provided by the passivated p-silicon anode. [19, 20] The transparent cathode is a Sm/Au stacked bilayer, possessing a low work function and air-stable property. [18, 20] The organic materials and the cathode metals were thermally evaporated successively on the front side of the p-type Si wafer in a chamber with a base pressure of 4×10-6 Torr. The typical deposition rate was ~1 Å/s, monitored by quartz crystal oscillators. The resulting silicon-based phosphorescent organic light emitting diode (Si-PhOLED) has a structure of p-Si/SiO2 2 nm/NPB 40 nm/CBP: (ppy)2Ir(acac) (10 wt%) 40 nm/ Bphen 25 nm/Bphen: Cs2CO3 (mass ratio of 1:1) 15 nm / Sm 5 nm/Au 15 nm, which is schematically shown in Fig. 1(a). The flat-band energy diagram of the device is shown in Fig. 2. A control device with the identical structure, organic materials, cathode, and parameters except for substituting Si with ITO and substituting backside Ohm-contact Al layer (50 nm) with a thick Al layer (150nm) as a mirror, referred to as ITO-PhOLED, was also fabricated. The ITO anode is routinely cleaned and then treated by O2 plasma. The evaporation process of organics and metals is exactly the same as that of Si-PhOLED.
The top-emission electroluminescence spectra for the Si-PhOLED at 1, 100 and 10000 cd/m2 are shown in Fig. 1(b). The current density, voltage and top-emission luminance of the Si-PhOLED and the ITO-PhOLED were measured by using two computer-controlled multimeters and a fluorescent spectrometer calibrated by a PR650 spectrometer in air at room temperature. Figure 1(c) shows the dependence of the electroluminescence intensity on the observation angle, indicating that little cavity effect occurs in the Si-PhOLED.
Fig. 1. (a) A schematic configuration of the Si-PhOLED (b) the normalized top-emission electroluminescence spectra for the Si-PhOLED at 1, 100 and 10000 cd/m2. For clarity, some curves have been shifted vertically. (c) The dependence of the electroluminescence intensity on the observation angle
Fig. 2. The schematic flat-band energy diagram for Si-PhOLED, and the dot line is for the guest material (ppy)2Ir(acac) within CBP.
The current density and luminance versus voltage characteristics for the Si-PhOLED and the ITO-PhOLED are depicted in Fig. 3. Luminance was measured from the top Sm/Au cathode. As can be seen in Fig. 3, at the same voltage, the luminance of Si-PhOLED is lower than that of the ITO-PhOLED, however, the current density of Si-PhOLED is much lower than that of the ITO-PhOLED in the low applied voltage region. One of possible reasons for the higher current density of ITO-PhOLED may be due to leakage current caused by the coarse surface of the ITO anode. The turn-on voltage at luminance of 1 cd/m2 is 3.2 V for the Si-PhOLED.
Fig. 3. (a) The current density versus voltage characteristics and (b) luminance versus voltage characteristics for the Si-PhOLED and the ITO-PhOLED.
Figure 4 shows the luminance efficiency and the luminous power efficiency versus voltage characteristics of the Si-PhOLED and the ITO-PhOLED. For the Si-PhOLED, the maximum luminance efficiency of 69 cd/A and maximum luminous power efficiency of 62 lm/W are both obtained at an applied voltage of 3.5 V, corresponding to a current density of 4.4 µA/cm2 and a luminance of 3.0 cd/m2. The corresponding maximum PCE is 12 % and the maximum EQE is of 17 %. The PCE and EQE as the functions of the applied voltage are shown as the inset in Fig. 4. Even at a luminance of 500 cd/m2, corresponding to current density of 2.1 mA/cm2 and applied voltage of 5.6 V, luminance efficiency and luminous power efficiency of the Si-PhOLED are still 24 cd/A and 14 lm/W, respectively. The corresponding PCE is 2.7 % and the EQE is 5.9 %. It is noted that, the efficiency drops monotonically with the increase of the current. It can be explained by the strong quenching effect between the triplet states in the phosphor. [24] For the ITO-PhOLED, the maximum luminance efficiency and maximum luminous power efficiency are 25 cd/A and 17 lm/W, 36% and 27% of the corresponding values of Si-PhOLED, respectively.
Fig. 4. (a) The luminance efficiency versus voltage and (b) the luminous power efficiency versus voltage characteristics for the Si-PhOLED and the ITO-PhOLED. The inset is the PCE and EQE for the Si-PhOLED as functions of driving voltage.
In the Si-PhOLED, the high PCE is attributed to four key factors. A strong hole injection is offered by the ultrathin SiO2 layer passivated p-silicon anode; a strong electron injection is offered by the Bphen: Cs2CO3 material as the efficient electron transport layer. Nearly a 100% internal quantum efficiency is provided by a highly efficient emitter, a phosphor doped organic, CBP: (ppy)2Ir(acac). An air-stable and highly transparent cathode having has a transparency of ~60%, is carried out by the Sm/Au bilayer, which offers a quite good light outcoupling efficiency. Strong hole and electron injections provide a low operating voltage and the efficient emitter provides highly convert efficiency from excitons to photons. [25] If we substituted AlQ [tris-(8-hydroxyquinoline) aluminum, a commonly used electron transport and emissive material] for Bphen: Cs2CO3 as the electron transport material, the maximum luminous power efficiency of the resulting device was about 23 lm/W, only 37% of the value for the Si-PhOLED; If we substituted AlQ for CBP: (ppy)2Ir(acac) as the emissive material; the maximum luminous power efficiency of this resulting device was 1.2 lm/W, only 2% of the value for the Si-PhOLED. Ref. 23 reported a phosphorescent organic light emitting diode with a structure of ITO/ CuPc (copper phthalocyanine) 12 nm/ NPB 40 nm/ CBP: (ppy)2Ir(acac) 20 nm/ BCP (2,9-dimethyl-4,7-diphenylphenatroline) 5 nm/ AlQ 40 nm/LiF/Al. It is a bottom-emission device with a transparent ITO anode and a reflective cathode, where neither the electron transport layer nor the hole transport layer was doped. The maximum luminous power efficiency of that device is 37 lm/W. This result implies that a higher efficiency of the Si-PhOLED may benefit from the simultaneous enhancements of the hole injection by the passivated p-Si anode and the electron injection by the Bphen: Cs2CO3 layer.
To our knowledge, the Si-PhOLED has the highest efficiency among the reported silicon-based electroluminescence devices. Here, the “silicon-based” means that silicon is not only a substrate but also has some other functions, such as being an anode. Those electroluminescence devices with combining III-V chips to silicon wafers by bonding technology, where silicon wafers are only substrates, are not included in the comparison objects.
3. Conclusion
We have demonstrated highly efficient silicon-based electroluminescence from the Si-PhOLED by combination of a phosphorescent material CBP: (ppy)2Ir(acac), an electrically doped electron transport layer Bphen: Cs2CO3, a low work function and transparent cathode Sm/Au, with a passivated silicon anode. This Si-PhOLED achieves a maximum PCE of 12% and an maximum EQE of 17% at an applied voltage of 3.5 V. As far as we know, the PCE of the Si-PhOLED is much higher than those of reported silicon-based OLEDs. The Si-PhOLED may be used as a silicon light source in silicon optoelectronics or other relative area in near future.
Acknowledgement
This work was supported by the National Natural Science Foundation of China, Grant No. 10674012, 50732001, and 10574008, Key Laboratory of Advanced Display and System Applications (Shanghai University and Ministry of Education, China), and the National Basic Research Program of China (973 Project), Grant No. 2007CB613402.
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