Energy levels of the most commonly used polymer-hole transport materials are summarized and shown in Fig. 1a, which can be seen that PVK has become the most popular one for blue perovskite due to its deep HOMO energy level.33,34 To further be suitable for the wide bandgap blue perovskite materials, atomically precise [Ag6PL6] (Ag6 hereafter) NCs were incorporated into the PVK HTL for PBA2Csn − 1PbnBr3n+1 blue PeLEDs to improve device performance. The ultraviolet-visible (UV-vis) absorption spectrum of Ag6 NCs dissolved in chlorobenzene (CB) with a band edge of 500 nm in Fig. 1b demonstrates that the Ag6 NCs were successfully synthesized. The inset of Fig. 1b shows the molecule structure of monolayer-protected Ag6 NCs which are composed of an octahedral Ag6 framework and the ligand. The unique structure of Ag6 NCs aids in improving Ag solubility in organic solvents, which expands the application of noble metal Ag with high conductivity and high mobility as a dopant in PeLEDs. The transmission electron microscopy (TEM) images of Ag6 NCs in CB are shown in Fig. S1, implying that Ag6 NCs have excellent monodispersity and the size distribution is around 2.4 nm. According to the ultraviolet photoelectron spectra (UPS) spectra and Tauc-plot shown in Fig. 1c and Fig. S2, the HOMO and lowest unoccupied molecular orbital (LUMO) energy levels of the Ag6 NCs are calculated to be -6.29 eV, and − 3.45 eV, respectively. As shown in Fig. 1d, with the increase of the Ag6 NCs content, the characteristic absorption intensity of the PVK in CB solvent at 400 nm related to the Ag6 NCs increased, indicating that the Ag6 NCs had been successfully mixed into PVK. Moreover, the scanning electron microscopy (SEM) and elemental mapping images of PVK films doped with 20% Ag6 NCs show that all elements (C, N, Ag, and S) are uniformly distributed in the film, where S and Ag are characteristic elements of Ag6 NCs, indicating that Ag6 NCs have been successfully doped into PVK film (Fig. S3). In addition, the X-ray photoelectron spectroscopy (XPS) spectra of PVK films without and with Ag6 NCs are shown in Fig. S4, a characteristic peak at 162.6 eV can be observed for the Ag6 NCs modified PVK films, which is related to S from the Ag6 NCs, indicating that Ag6 NCs have been successfully doped into PVK films. (Fig. S4).35,36
The blue PeLEDs are fabricated with the device structure of glass/ITO/PEDOT: PSS/Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl)] (TFB)/HTL/quasi-2D perovskite film /1, 3, 5-tris(N-phenylbenzimiazole-2-yl)benzene (TPBi)/(8-Quinolinolato)lithium (Liq)/Al, where PVK modified with Ag6 NCs is used as the HTL. The schematic structure and cross-sectional SEM images of the device are shown in Fig. 2a, and the optimized thicknesses of PEDOT: PSS, TFB/PVK with Ag6 NCs, quasi-2D perovskite of PBA2Csn − 1PbnBr3n+1, TPBi, and Liq/Al layers are about 35, 20, 15, 50, and 100 nm, respectively. The normalized PL spectra of the perovskite film and the normalized electroluminescence (EL) spectra of devices without and with 20% Ag6 NCs doping are shown in Fig. 2b. It is worth noting that the EL spectra are almost identical to the PL spectra, illustrating that luminescence derives from the perovskite ETL rather than other materials and the Ag6 NCs do not affect the EL spectra. The EL emission peaks located in the blue region are at 488 nm with a narrow full-width at a half-maximum around 28 nm, which is consistent with PL spectra for the quasi-2D perovskite film. The EL spectra intensity of optimal and control devices gradually increases as the bias voltage increases from 4 to 7 V, as shown in Fig. 2c and Fig. S5, respectively, while the peak position remains unchanged indicating the great spectral stability of the devices. The current density-voltage-luminance intensity (J-V-L) curves and the EQE-current density (EQE-J) curves of the control device and the optimal device based on 20% Ag6 NCs modified PVK are shown in Fig. 2d and e, respectively. The maximum luminance is obtained by the optimal device of 5554 cd/m2, higher than that of the control device (2442 cd/m2). The optimal device exhibits a maximum EQE of 12.02%, which is 1.3-folds higher than the control device (9.48%). In addition, further investigating the effect of the variation of Ag6 NCs concentration on the device performance, the EQE of those devices increases as the content of Ag6 NCs adds from 10–20% but drops sharply when the Ag6 NCs content increases continuously to 30% (Table 1). Moreover, the current density at the same voltage shows an increasing tendency and the turn-on voltages of those devices gradually decrease, indicating that the Ag6 NCs may increase carrier injection and transport ability to improve PeLEDs performance (Fig. S6). Device performance drop of the excessive Ag6 NCs addition may result from the reducing the quality of perovskite films, which can be confirmed by the root mean square (RMS) of AFM images changing in the perovskite films on the modified PVK films (Fig. S7).
Table 1
Electronic performance of perovskite LEDs with 0–30% Ag6 NCs doping in PVK.
HTL | FWHM (nm) | Von (V) | Lmax (cd/m2) | EQEmax (%) |
PVK | 28 | 3.21 | 2442 | 9.48% |
10% Ag6 NCs | 29 | 2.96 | 3265 | 10.40% |
20% Ag6 NCs | 29 | 2.84 | 5554 | 12.02% |
30% Ag6 NCs | 29 | 2.98 | 4240 | 11.22% |
To elucidate the effect mechanism of Ag6 NCs additives on device performance, the energy levels of the PVK without and with different concentrations of Ag6 NCs films are explored by the UPS measurement and the Tauc-plots, and the corresponding results are shown in Fig. 3a and Fig. S8, respectively. It presents a decrease in the HOMO of PVK, from − 5.8 eV to -6.0 eV after the incorporation of Ag6 NCs, indicating that the Ag6 NCs can adjust the energy level of PVK. To investigate the influence of this change on the device performance enhancement, the conduction band minimum (CBM) and VBM of perovskite film are calculated by the absorption spectrum and UPS data (Fig. S9). As shown in Fig. 3b, it can be seen that the control device possesses a 0.15 eV potential barrier between PVK HTL and perovskite EML compared with the potential well between electron transport layer (ETL) and perovskite EML, which will cause inefficient hole injection and further affecting device efficiency. The 20% Ag6 NCs doped into PVK can modify the HOMO energy level of PVK from − 5.8 eV to -5.94 eV approaching the EML VBM, which will achieve barrier-free hole injection to enhance the injection efficiency, reduce the annihilation of carriers at the interface between EML and HTL.37–39 The energy level alignment changing may from two reasons, the surface potential changing or the stacking changes of the carbazole from the PVK by the effect of the Ag6 NCs. The diffraction of x-rays (XRD) was measured for the ultrathin film PVK modified without and with Ag6 NCs to explore the carbazole stack changes. As shown in Fig. S10, the grazing incidence XRD (GI-XRD) pattern gives detailed data about the PVK films with no change after the Ag6 NCs dopant suggesting the Ag6 NCs do not influence the carbazole stack of the PVK.40,41 The KPFM measurement shows that the surface potential of the PVK films modified by Ag6 NCs has changed significantly, indicating that the band change may be due to the formation of dipoles between Ag6 NCs and PVK (Fig. S11).42–48 The effect of the Ag6 NCs on the PVK film carrier transport is also further investigated, and the electrical conductivity (σ) of capacitor-like devices with the structure of ITO/HTL/Au is shown in Fig. 3c.49 With the contents of Ag6 NCs increasing from 10–30%, the calculated conductivities are 3.19×10− 3 S cm− 1, 4.78×10− 3 S cm− 1, and 6.54×10− 3 S cm− 1, which are both higher than the pure PVK (2.2×10− 3 S cm− 1), respectively, indicating that the electrical conductivity of the HTL strengthens gradually by doping Ag6 NCs due to the metallic properties of Ag6 NCs molecular states. The improved conductivity may be due to the increase in hole mobility, which is further demonstrated by the subsequent space-charge-limited-current region (SCLC) test. The hole mobility is calculated from the devices with the structure of ITO/PEDOT: PSS/HTL/Au (Fig. 3d).50 The hole mobility of PVK with 10%, 20%, and 30% Ag6 NCs films are 1.05×10− 4 cm2 V− 1 s− 1, 2.34×10− 4 cm2 V− 1 s− 1, and 3.57×10− 4 cm2 V− 1 s− 1, which is higher than 2.5×10− 5 cm2 V− 1 s− 1 of the pure PVK films indicating that the Ag6 NCs enhance the hole mobility. Summarizing the discussion above, the Ag6 NCs as the dopant can enhance the hole injection and the transport by the modified HOMO energy level of PVK and its metallic properties, thereby enhancing device performance.
As mentioned above, we conclude that the reasons for the introduction of Ag6 NCs to improve device performance are mainly divided into the following two aspects. As shown in Fig. 4a, the holes are rapidly transported through the acceleration of the Ag6 NCs, thus compensating for the low hole mobility of PVK, forming a more balanced carrier transport. Meanwhile, the energy level of the PVK adjusts by the dipole that forms between the PVK and Ag6 NCs, which is confirmed by the density functional theory (DFT) calculations. Figure 4b shows the electrostatic potentials (ESP) of the Ag6 NCs and the PVK. The ESP for the complex model formed between the Ag6 NCs and PVK is shown in Fig. 4c, and it can be seen that the Ag6 NCs and PVK possess the respective negative and positive ESPs which interact with each other and form the dipole. Moreover, the HOMO energy level of modified PVK achieves barrier-free hole injection between HTL and EML, which will increase hole injection efficiency and promote the injection balance of electrons and holes. The accumulation of holes at the interface between EML and HTL will be suppressed which can be further manifested by the increase in device performance.
To explore the connection between the device performance enhancement and the unique structure of the Ag6 NCs, ligands of Ag6 NCs and the Ag nanoparticles (NPs) as the Ag6 core analogs are used as additives for PVK to analyze the effect of various parts from Ag6 NCs on device performance. The effect of the ligand and NPs on energy level alignment and the hole transport ability were investigated, respectively. Figure 5a and b show the UPS and hole mobility results of the PVK films with 5%, 10%, and 15% ligands doping. It can be seen that the HOMO energy level of those films is 5.82 eV, 5.85 eV, and 5.87 eV, and the conductivity of those films are 2.30×10− 3 S cm− 1, 2.45×10− 3 S cm− 1, and 2.61×10− 3 S cm− 1, which shows insignificant change with PVK film indicating that may be mainly attributed to the fact that the ligands have little effect on the energy level alignment and hole mobility of PVK. The devices with different concentration ligands modified PVK HTL are fabricated with the same structure as the control device and the J-V-L and EQE-J curves of these devices are shown in Fig. S12. The current density curves of these devices at the same voltage and turn-on voltage are almost indistinguishable from the control devices, confirming that the ligands do not affect the hole injection and transport. Meanwhile, Fig. 5c shows the UPS spectra of the films with the different concentration Ag NPs indicating the Ag NPs with no ability to modify the suitable energy level alignment of the PVK like Ag6 NCs. However, the conductivity of PVK films modified with 10%, 20%, and 30% Ag NPs are 3.19×10− 3 S cm− 1, 4.87×10− 3 S cm− 1, and 6.54×10− 3 S cm− 1, respectively, which is higher than the PVK films suggesting the Ag NPs strengthen the conductivity of PVK. The devices with Ag NPs modified PVK HTL exhibits an increasing trend of current density at the same voltage with Ag NPs doping ratio adding but little change in the turn-on voltage with the doping ratio various, which confirms the Ag NPs possess the ability to enhance hole transport (Fig. S13). Moreover, the 10% Ag NPs modified device shows a higher EQE than the control device which may be attributed to the improved hole transport. However, the EQE sharply dropped with the Ag NPs doping contents further increasing from 10–30%. Further investigating the reason for EQE dropping, the AFM images of the PVK films with different concentrations of Ag NPs were measured and the responding results are shown in Fig. S14. The RMSs of PVK films are increased with the increase of Ag NPs doping content, indicating that doping Ag NPs reduced the quality of PVK films which is also proved by the leakage current increases with the concentration adding. Based on the above discussion, we confirmed the unique structure of the Ag6 NCs is an important reason that it can improve device performance by enhancing the energy level alignment and mobility of HTL as an additive.