## 3 Results and Discussion  
The absorption spectrum of GNPs and the PL spectrum of Alq\({}_{3}\) are shown in Figure 2a. It is observed that the PL peak of Alq\({}_{3}\) and the absorption peak of GNPs are about 523 nm and 530 nm, respectively. The GNPs can enhance the characteristics of the device when the absorption peak of GNPs is very close to the PL peak of Alq\({}_{3}\) because of its SPRE [21]. In addition, Figure 2b,c show the TEM and high-resolution TEM images of GNPs, respectively. According to the normal and high-resolution TEM image in Figure 2b,c, the GNPs are mostly spherical and irregular in shape. Furthermore, it is also observed that the mean particle size of the GNPs in Figure 2b,c is calculated and found to be 10 nm when the absorption peak of the GNPs is 530 nm. It is well known that the absorption peak depends on the GNPs' size, shape, and the spacing between the GNPs [22]. Because the distance between the GNPs and excitons is an extremely important parameter, the distance was tuned by changing the thickness of the buffer layer to further confirm the relationship for coupling between the surface plasmon and excitons. The resulting J-V curves of the devices without GNPs and with different spacing distances between the emissions layer (EML) and GNPs are plotted in Figure 3a. It can be clearly seen that the J-V curves of devices with GNPs and without GNPs are almost identical. The almost identical phenomenon of the current density suggests that the GNPs doped into PEDOT:PSS do not affect the hole-injection characteristics. Figure 3b shows the L-V characteristics of the device without GNPs and with different spacing distances between EML and GNPs. It is observed that the luminance of the devices with GNPs is higher than that of the device without GNPs because of the SPRE of GNPs. Furthermore, the SPRE of GNPs will result in the surface plasmon-enhanced spontaneous emission rate. The luminance of the device with GNPs at a spacing distance of 6 nm is higher than that of devices without GNPs and with the other spacing distances because the GNPs at a spacing distance of 6 nm can considerably decrease the quenching of the fluorescence. The current efficiency-voltage characteristics of devices with a buffer layer of 6 nm and without GNPs are plotted in the inset of Figure 3b. Obviously, the current efficiency of the device with a buffer layer of 6 nm is significantly higher than that of the device without GNPs because the device with a buffer layer of 6 nm not only enhanced the spontaneous recombination of the excitons but also decreased the nonradiative decay process of the excitons. However, the power efficiency of the device with a buffer layer of 6 nm is 2.43 lm/watt at 3 V.  
Figure 2: (**a**) Absorption spectrum of the GNPs and PL spectrum of the Alq\({}_{3}\); (**b**) TEM image of the GNPs; (**c**) High-resolution TEM images of the GNPs. Figure 4 shows the plot of the enhancement factor and the Forster energy transfer efficiency as a function of the spacing distance. The enhancement factor is expressed as \(L_{2}/L_{1}\), where \(L_{2}\) is the maximum luminance of the device with GNPs and \(L_{1}\) is the maximum luminance of the device without  
Figure 3: (**a**) Current density-voltage curves; (**b**) luminance-voltage curves in log scale; and (**c**) current efficiencies curves of devices without GNPs and with different spacing distances. Inset of (**b**) shows the power efficiency-voltage characteristics of device. GNPs. It is observed that the enhancement factors are decreased with an increase in the thickness of the spacing layer in the condition of a thickness of spacing \(\geq\)6 nm because the GNPs are close to the excitons of the EML. Furthermore, the surface plasmon-enhanced spontaneous emission rate of the device is increased when the GNPs are close to the excitons of the EML.