Figure 3 shows TEM images of CsPbBr1.2I1.8 QDs mixed with various concentration of TOPO. Obviously, the size distribution of QD particles appeared barely change, but condensation and compact as the concentration of TOPO added into the solution of CsPbBr1.2I1.8 QDs increases due to self-aggregation effect [22,23].
Figure 4(a) plots the photoluminescence (PL) spectra of CsPbBr1.2I1.8 QDs mixed with various concentration of TOPO. The PL peak position of all samples is around 652 nm. With the increase of the concentration of TOPO, the intensity of PL spectrum increases. The spectrum of sample of CsPbBr1.2I1.8 mixed with 0.4 mmole TOPO exhibits the strongest intensity and narrowest FWHM (28 nm), respectively. It may be attributed to the more condense of QDs as concentration of TOPO increases, as shown in Fig. 3. Another factor is the reduction of surface defects of CsPbBr1.2I1.8 QDs after passivation treatment with TOPO [24]. Figure 4(b) plots the absorbance of spectra of CsPbBr1.2I1.8 QDs mixed with various concentration of TOPO. The absorbance spectra are cut-off at ~650 nm for all samples. It consists with the PL spectra.
Figure 5(a)-(d) display top-view FESEM images of CsPbBr1.2I1.8 films mixed with various concentration of TOPO. As shown in Fig. 5(a), the morphology of TOPO-free CsPbBr1.2I1.8 film appears some holes and incomplete. However, a complete CsPbBr1.2I1.8 film was formed when the 0.2 mmole TOPO was added into QDs solution. It may be contributed to the self-aggregation effect of TOPO, as shown in Fig. 5(c) and (d) [22,23]. Although the coverage and compact are improved by using TOPO, the height difference on the surface of CsPbBr1.2I1.8:TOPO film is great owing to the reaction rate. In this work, Pb(NO3)2 was employed to improve the morphology and to smooth the CsPbBr1.2I1.8:TOPO perovskite QDs film, as shown in Fig. 5(e), because Pb(NO3)2 can suppress and slow down the growth of perovskite QD film [25,26]. The surface passivation and growth rate reduction can promote the crystalline quality of CsPbBr1.2I1.8 perovskite QDs films. As shown in Fig. 5(f), the peak locations of XRD patterns mostly same. All samples exhibit a typical cubic phase, the high pure α-phase is obtained with two main peaks at 14.6° and 29.5°, corresponding to the (100) and (200) planes of cubic phase, respectively [27,28]. Obviously, the CsPbBr1.2I1.8:TOPO (0.2 and 0.4 mmole) films show a decreased full width at half-maximum (FWHM) for the (200) peak in comparison with TOPO-free film. It suggests increased crystallite size and decreased surface defect [27,29], in well agreement with the PL spectra mentioned above. In addition, no peaks were newly formed, disappeared or shifted, implying that the perovskite crystal structure did not change during TOPO and Pb(NO3)2 passivation treatment.
Figure 6 shows the atomic force microscope (AFM) images of the CsPbBr1.2I1.8 QDs film. Ra is the average roughness and Rz is the difference between the highest point and the lowest point on the surface of film. For the CsPbBr1.2I1.8:TOPO (0.2 mmole) film without surface passivation using Pb(NO3)2, Ra and Rz are 15.2 and 123.8 nm, respectively. However, for the CsPbBr1.2I1.8:TOPO (0.2 mmole) film with surface passivation using Pb(NO3)2, Ra and Rz are 2.7 and 24.8 nm, respectively. The surface of the film with treatment of Pb(NO3)2 is relatively smooth.
Structure of CsPbBr1.2I1.8 perovskite QDs LED is shown in Fig 7. The function of ITO, PEDOT:PSS Poly-TPD, QDs, and TPBi is anode, hole transport layer, hole injection layer, active layer, and electron transport layer, respectively. Figure 8(a) sketches the band structure of CsPbBr1.2I1.8 perovskite QDs LED. Poly-TPD and TPBi layers can effectively confine carriers in the structure.
As shown in Fig. 8(b), the peak range of electroluminescence (EL) spectra of CsPbBr1.2I1.8 perovskite QDs LED is from 668 to 671 nm. The variation may be contributed to error of measurement or process. The EL spectrum has a red shift of 20 nm compared with the PL spectrum. It may be contributed to the heat caused by resistance of materials in the LED structures. As shown in Figure 8(c), the brightness of CsPbBr1.2I1.8 QDs LED doped with TOPO increases with the concentration of TOPO doping due to surface passivation and reducing non-radiative recombination in the perovskite active layer [24]. However, excessive TOPO will result in a decrease in brightness. The CsPbBr1.2I1.8 QDs LED doped with TOPO: 0.2 mmole shows a better luminous brightness with a luminosity of 437.2 cd/m2 (@9.5 V)
In the next phase, Pb(NO3)2 was employed to add into the solution of perovskite:TOPO to improve the morphology and uniformity of active layer according to the AFM images, as shown in Fig. 6. The current efficiency and luminosity is 0.175 cd/A and 502.7 cd/m2, respectively (@11 V). The improvement is caused by slow formation rate of perovskite active layer, such that reduction of leakage current [30-32]. As shown in Fig. 8(f), after 14-days burn-in test, the CsPbBr1.2I1.8 QDs LED with doped TOPO: 0.2 mmole and Pb(NO3)2 shows the best luminance owing to the surface passivation and smooth surface of the active layer.
Figure 9 shows CIE chromaticity diagram of CsPbBr1.2I1.8 with (TOPO, Pb(NO3)2). The inset is the photo with 10 V bias in the dark. The coordinates of CIE chromaticity diagram are (0.738, 0.2905). It is very close to the boundary CIE chromaticity diagram. The advantage of this work is that red-light color is very pure due to good uniformity of the perovskite active layer. It is suitable for the application of display technology.