Suppression of non-radiative recombination toward high efficiency perovskite light-emitting diodes

The organic-inorganic perovskite is a promising material for high performance light-emitting diodes (LEDs) owing to its tunable bandgap, high color purity, and photoluminescence quantum yield.^1–13 Previous work has achieved the external quantum efficiency (EQE) over 20% in anion-exchange quantum dot perovskite LEDs (PeLEDs).¹⁴ However, besides the rational tuning of perovskite film itself, the interface quenching and the charge injection balance also deserve systematic investigation.

Nickel oxide (NiO) is a promising hole transporting layer in PeLEDs because of its good stability, high hole mobility, and solution processability.^15,16,25,26 However, the NiO layer still caused adverse charge non-radiative recombination and restricted the efficiency of PeLEDs. A functional layer inserted between the perovskite and the metal oxide charge transport layers has been proved be an effective method to suppress the non-radiative recombination.^17,18 This work demonstrated that the recombination control is indispensable for obtaining high efficiency PeLEDs.

Here, a poly(9-vinlycarbazole) (PVK) layer was introduced onto a NiO surface before depositing the perovskite film. This method reduced the leakage current by one order of magnitude and effectively improved the radiative recombination of excitons. As a result, the photoluminescence quantum yield (PLQY) of perovskite film with this organic interlayer yielded a value of 54%, and the PeLED achieved a high current efficiency (η_c) of 34.2 cd/A and an EQE of 11.2%.

The NiO film (∼20 nm) was fabricated onto the pre-cleaned indium tin oxide (ITO) substrate according to the previous work.¹⁶ A 15 min oxygen plasma process was conducted on the NiO film. After that, PVK dissolved in chlorobenzene (4 mg/ml) was spin-coated onto the NiO film to form a PVK film (∼10 nm). Figures 1(a) and 1(b) demonstrate the surface morphology of NiO and NiO/PVK films on ITO substrates probed by atomic force microscopy (AFM). Pristine NiO displayed a root mean square (RMS) roughness value of ∼2.2 nm, which was larger than the PVK modified one (∼0.9 nm). The composition of perovskite precursor solution was a mixture of 0.2M Cs_0.9FA_0.2PbBr_3.1 (FA = formamidinium) precursor with 0.12M NMABr (NMA = 1-naphthylmethylamine)⁶ in dimethyl sulfoxide (DMSO). After the perovskite solution was deposited onto the NiO or NiO/PVK substrate, a short annealing was conducted. A detailed description is presented in the experimental section (supplementary material). The morphology of perovskite is shown in Figs. 1(c) and 1(d). The RMS roughness value was ∼3.8 nm for the perovskite layer on bare NiO. In comparison, the perovskite film on PVK covered NiO was smoother with a reduced RMS roughness value of ∼2.8 nm. According to scanning electron microscope (SEM) images in Figs. S1(a) and S1(b) (supplementary material), the perovskite film on a NiO/PVK layer demonstrated a compact film, which may be correlated with low surface tension of PVK.^19,20

The optical absorption spectra of perovskite films were shown in Fig. 2(a). Similar perovskite absorption peaks on both NiO and NiO/PVK were observed, which indicated that different substrates had a negligible effect on the perovskite bandgap. To further characterize the crystallinity of perovskite layers, θ-2θ X-ray diffraction (XRD) patterns were conducted, as exhibited in Fig. 2(b). Two clear XRD peaks at 15° and 31° corresponding to (100) and (200) plane reflection were observed, respectively. When the perovskite film grew on the NiO/PVK layer, diffraction peaks from (100) and (200) planes became stronger. This phenomenon is consistent with the previous studies that the hydrophobic PVK surface could improve the crystallinity of perovskite.²⁰ 

In high performance LED devices, it is important to achieve balanced injection of electrons and holes into the emitting layer for effective radiative recombination. Besides high film quality of emitting layer, reducing the exciton diffusion to adjacent layers and decreasing the non-radiative recombination at the interface are also essential.²¹ The photoluminescence (PL) intensity was proportional to the radiative recombination ratio. A strong PL intensity for a perovskite film usually promised high efficiency PeLEDs. As shown in Fig. 2(c), the perovskite films demonstrated a green light emission peak on the NiO substrate with an FWHM of ∼21 nm and there is a slight red shift (∼2 nm) in the presence of PVK layer. The PL quantum yield is dramatically enhanced from 23% (without PVK) to 54% (with PVK). As shown in the inset of Fig. 2(c), the perovskite layer on NiO/PVK under ultraviolet light excitation displayed a uniform and pure green PL emission. Time-resolved PL decay of the perovskite layers on bare NiO and NiO/PVK films is plotted in Fig. 2(d). These curves were fitted based on the tri-exponential decay model. Perovskite films on the NiO/PVK and NiO layer displayed an average PL lifetime of ∼95.4 ns and ∼47.9 ns, respectively, which reveals that the non-radiative recombination was dramatically suppressed in the presence of PVK. The extended PL lifetime is consistent with the PLQY measurement.

To further investigate the uniformity of radiative recombination of perovskite films, confocal laser scanning microscopy was conducted to measure the dimensional distribution of time-dependent PL intensity, as depicted in Figs. 3(a) and 3(b). There were many dark regions in the confocal PL image [Fig. 3(a)] revealing that a large portion of injected charge carriers recombined through the non-radiative pathway in the NiO/perovskite film. Namely, there was a high density of quenching sites in perovskite films deposited on bare NiO substrates. After inserting a PVK layer, dark regions were dramatically reduced, as shown in Fig. 3(b). Additionally, fluctuation of time-dependent PL intensities [Fig. 3(c)] was estimated by monitoring five individual points as marked in Figs. 3(a) and 3(b). A faster dropping tendency of PL intensity was observed when the perovskite contacted with NiO. On the contrary, the PL intensity could remain nearly constant as a function of time for the perovskite on PVK/NiO, which should be correlated with suppressed non-radiative recombination at the interface. One reason for the enhancement of radiative recombination is owing to the higher quality of perovskite film. The other important reason is that the PVK layer blocked the channel of non-radiative recombination. Excitons formed in the perovskite region are most likely to dissociate at the interface between the perovskite and NiO.²² During this process, excitons could be quenched. However, when there was a PVK layer between the perovskite and NiO, the transfer process can be dramatically restricted since the energy of exciton (∼2.3 eV) formed in the perovskite is much smaller than that of PVK (∼3 eV).²³ These excitons would be confined in the perovskite region. It enhanced the probability of radiative recombination in the emitting layer.

The device configuration and energy band of PeLED were demonstrated in Fig. 4(a). 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi, 40 nm), lithium fluoride (LiF, 1 nm), and aluminum (Al, 100 nm) were sequentially evaporated onto the perovskite with shadow masks. The NiO/PVK bilayer served as hole transport layers. The perovskite film (∼60 nm) worked for the emitting layer. The TPBi was an electron transport layer. In a working device, holes were injected from the valance band of NiO to the highest occupied molecular orbital (HOMO) of PVK. Meanwhile, electrons were injected from the lowest unoccupied molecular orbital (LUMO) of TPBi. Finally, both electrons and holes were confined in the perovskite film to form excitons. Once radiative recombination occurred, photons were then emitted.

Electroluminescence (EL) spectra of PeLEDs with and without the PVK interlayer are shown in Fig. 4(b). The EL emission peak of NiO/PVK-based device was located at 512 nm corresponding to the Commission Internationale de l’Eclairage (CIE) color coordinate at (0.07, 0.71) in Fig. S2 (supplementary material). The FWHM of the emission peak was 21 nm, which was lined with the PL peak width. The NiO-based one was located at 510 nm. Figure 4(c) demonstrates the current density-voltage (J-V) and luminance-voltage curves of PeLEDs on the bare NiO and NiO/PVK. The current was raised quickly with the increasing voltage when the perovskite was directly deposited on the NiO layer because the poor contact led to a high leakage current level.²⁴ Once the voltage came up to 6.5 V, the luminance reached the highest value of 978 cd/m². However, when a PVK layer was inserted between the NiO and the perovskite, the leakage current was dramatically reduced by one order of magnitude. The device showed a leakage current as low as ∼10⁻⁴ mA/cm². The inset image in Fig. 4(b) is a photograph of a device based on the NiO/PVK structure under the operation condition, which displayed a bright and uniform emission region. The angular distribution of luminance intensity was demonstrated in Fig. S3 (supplementary material), which showed the Lambertian emission. The NiO/perovskite device reached a maximum luminance of 2736 cd/m² at 6.5 V. A maximum current efficiency of 34.2 cd/A was achieved at a current density of 0.05 mA/cm² (voltage of 3.5 V), which corresponded an EQE of 11.2%. The efficiency roll-off of the device was rather flat. As for the NiO/perovskite reference device, the maximum of EQE was only around 3.07% and efficiency roll-off is obvious.

The NiO/PVK injection scheme combined the advantages of high hole mobility of NiO and deep energy level of PVK, where an effective “ladder structure” hole injection approach could be realized.²¹ To testify that, PeLEDs with only PVK hole transport layers (ITO/PVK/perovskite/TPBi/LiF/Al) were fabricated, as shown in Fig. S4 (supplementary material). The J-V curve exhibited a lower leakage current compared with the NiO/perovskite device, which further proved the effective suppress of interface recombination by PVK. The highest luminance and the EQE for the PVK/perovskite device were 1872 cd/m² and 7.05%, respectively. Both of these parameters were lower than those of the NiO/PVK-based perovskite device. Especially, with the increase in voltage, the EQE curve of PeLED without the NiO layer showed a fast roll-off tendency due to the hole injection barrier from ITO to PVK. To prove the hole injection condition, the surface potential between the NiO and the PVK layer was measured by scanning Kelvin probe microscopy (SKPM), as shown in Fig. S5 (supplementary material). The PVK surface on NiO demonstrated a high potential compared with the pristine NiO surface, which revealed that PVK displayed a deeper energy band for hole injection. Therefore, the bilayer NiO/PVK structure could dramatically enhance the hole injection rate. The luminance stability of devices based on NiO and NiO/PVK films is demonstrated in Fig. S6. The working stability is improved after inserting a PVK layer between NiO and perovskite.

In conclusion, a PeLED with an EQE of ∼11% was achieved via the insertion of PVK layers that enhanced the radiative recombination of excitons and suppressed the leakage current caused by the direct contact of NiO and perovskite. In addition, compared with the bare NiO or PVK layer, this bilayer NiO/PVK structure achieved more favorable charge injection for efficient electroluminescence. This scheme for controlling exciton recombination by the simple interface engineering could be a promising method for high efficiency PeLED devices.

See supplementary material for the experimental section; SEM images of perovskite film; the CIE color coordinate of PeLEDs; angular distribution of luminance intensity for PeLED; J-V curves, luminance-voltage curves, EQE, and η_c of PeLED with a structure of ITO/PVK/perovskite/TPBi/LiF/Al; the SKPM image of NiO film partially covered with PVK on the ITO substrate; the luminance stability of the devices; and characterization of film and devices.

This work was supported by the National Key Research and Development Program of China (No. 2016YFA0202402), the National Natural Science Foundation of China (Nos. 61674108, and 11811520119), Jiangsu High Educational Natural Science Foundation (18KJA430012), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the 111 Program, and the Collaborative Innovation Center of Suzhou Nano Science and Technology. Yuqiang Liu and Yuan Liu thank the support from the Chinese Scholars Council (Nos. 201706920073 and 201506920047).