Properties of mesoporous hybrid perovskite nanocrystals and its application in light-emitting diodes

The newly emerging metal halide perovskites have attracted scientific attention worldwide, and a lot of efforts have been devoted into the development of perovskites with optimized chemical composition [1–4]. Specifically, perovskite nanocrystals, also known as colloidal quantum dots, have been recognized as the next generation of promising optoelectronic materials, which combine virtually merits of quantum dots and halide perovskite. The perovskite nanocrystals feature favorable optical properties with respect to their bulk counterparts, such as large bandgap tunability, and efficient narrow-band emission [5–10]. As newcomer optoelectronic materials, the solution-processed semiconducting metal halide perovskites in terms of nanocrystals have been explored using different ways, including not only compositional bandgap engineering, but also quantum size-effects. Through halide compositional modulations, specially, the mixed halide systems (Cl/Br, Br/I), the bandgap energies and emission spectra are readily tunable over the entire visible spectral region [5, 11]. Postsynthetic chemical transformations of nanocrystals provide an avenue to compositional fine-tuning, and tunable optical properties of perovskite nanocrystals have been realized by anion exchange reactions [6, 12]. Moreover, the light emission can also been adjusted by tuning the average crystallite dimension in the film from tens of nanometers to a few micrometers [13]. To date, the best developed optoelectronic perovskite nanocrystal in terms of size, shape, and composition are all-inorganic perovskite CsPbX₃ (X = Cl, Br, I). In contrast, the potential of semiconducting organic–inorganic hybrid perovskites in the forms of nanocrystals, especially the FA-cation (FA: formamidinium) based perovskite nanocrystals, are rarely explored [14, 15].

As one of the most promising semiconductor materials, perovskite based optoelectronic devices as new runners have been brought to the forefront of research focus [16–18]. To achieve performed perovskite device, there are several main measures, such as structure engineering, layer modulation and composition optimization [19–21]. During device fabrication, the perovskite layer is commonly deposited by spin-coating process. Whereas, spin-coating technology that influenced by concentration, speed, time and materials, is critically important to the quality of the formed thin layer. As for pervskite concentration, there is limited research on it. Therefore, the study of effects of perovskite concentration on the device performance is significant, which could clarify the potential roles and find a new way to realized improved device performance.

Herein, we studied FA cation based perovskite nanocrystals and its application in light-emitting diodes (LEDs). In detail, we successfully synthesized concentrated organic–inorganic hybrid perovskite nanocrystals FAPbBr₃, and turn reader's attention to an interesting result: the hybrid perovskite nanocrystals after dilution treatment exhibits mesoporous structure , and tunable optical properties can be realized by adjusting dilution radio. The colloidal nanocrystals were synthesized using facile and inexpensive commercial precursors, exhibiting light emission dependence on the mesoporous structure. Furthermore, the effects of perovskite concentration on the device performance has been systematacially studied, which indicate that proper perovskite dilution could improve interlayer carrier transport and device performance.

2.1. Materials

2.2. Synthesis of mesoporous hybrid perovskite nanocrystals

2.3. Fabrication of perovskite LEDs

2.4. Characterizations

Controlled synthesis of materials benefits fundamental research and offers great promise and guidance for the practical applications [22, 23]. Herein, we first investigated the morphological evolution of the developed mesoporous hybrid perovskite FAPbBr₃ nanocrystals. The colloidal nanocrystals were obtained using facile and inexpensive commercial precursors. The solution-phase synthesis of monodispersed FAPbBr₃ nanocrystals follows the previous methods with some modifications [24]. The as-obtained FAPbBr₃ nanocrystals are highly concentrated (~40 mg ml⁻¹), followed by the dilution in THF with different concentration (30, 20, 10 mg ml⁻¹), which are denoted as Dilutions 1, 2, 3, respectively. It is interesting to find that the surface morphology of FAPbBr₃ nanocrystal is evolved with different dilution. As indicated in figure 1, the monodispersed FAPbBr₃ nanocrystals crystallize in cubic phase, and the average size of nanocrystal is about 40 nm. On close observation of figures 1(b)–(d), some holes appear on the surface of FAPbBr₃ nanocrystal when the products are subject to dilution treatment. The diluted samples exhibit bright emission under UV excitation, as shown in figure 1(e).

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To get a closer look at the surface morphologies of samples, the corresponding high-resolution TEM images are displayed in figure 2. There are some little circles modified on the perovskite nanocrystal. From figures 2(b) and (c), we can see that lattice fringes break at the edge of the circle, indicating the little circles are amorphous holes. By comparing, the average size of little holes is slightly increased from 4 to 7 nm. it can be concluded that the effective area of the nanocrystal is correspondingly reduced as a function of increased dilution, such as Dilution 3. The effective area of perovskite nanocrystal is decided from the hole size and hole number. In detail, we picked 10 perovskite nanocrystals at random and tested all hole sizes on it, followed by the calculation of average effective area for different samples. Moreover, it is found that some smaller particles are attached on the nanocrystals, which can be confirmed to be metallic Pb metals, as reported in previous reports [25, 26].

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The dilution treatment on the perovskite nanocrystals results in mesoporous morphology, while the resultant mesoporous perovskites showed cubic crystal structure without emergency of secondary phases, as confirmed by XRD results in figure 3. The perovskite nanocrystals exhibit representative diffraction patterns, such as peaks around 15° and 30°, corresponding to crystal planes (l00) (l = 1, 2), respectively. However, the crystallinity is decreased especially for Dilution 3 (10 mg ml⁻¹). The basic crystal structure unit of perovskite is the inorganic corner-sharing PbBr₂ octahedra units surrounding organic cation FA⁺, as illustrated in figure 3(b). The dilution treatment may result in disintegration of the crystal structure partly, then the decreased crystal structure, as confirmed by XRD results. We selected Dilution 1 as an example, which was deposited on a glass slide using cotton swab as word 'GUST'. The word displays uniform green emission under UV excitation.

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The facile solution synthesis of FAPbBr₃ nanocrystals at room temperature, in fact, is a reversible reaction, as displayed below:

$$\begin{eqnarray*}&&{\rm{F}}{\rm{A}}{\rm{B}}{\rm{r}}\,+{\rm{P}}{\rm{b}}{\rm{B}}{{\rm{r}}}_{2}\,\leftrightarrow {\rm{F}}{\rm{A}}{\rm{P}}{\rm{b}}{\rm{B}}{{\rm{r}}}_{3}.\end{eqnarray*}$$

The mesoporous structure is evolved mainly from the decomposition of unstable FAPbBr₃ nanocrystals in diluted solution. As is suggested in the report, the underlying weak interaction between the organic cations and the surrounding halides due to the eight equivalent orientations of the organic cation along the body diagonals in unit cell, together with the chemically non-inertness of organic cations, result in the instability of FA cation based perovskite materials [27]. The concentrated original solution facilitates the stable existing of perovskite nanocrystals, whereas the subsequent dilution process promotes the reverse reaction, resulting in decomposition of FAPbBr₃ nanocrystals in some degree according to the dilution concentration, and thus the evolution of the final mesoporous structure. The presence of metallic Pb atoms in the mesoporous FAPbBr₃ nanocrystals indicates the unintended losses of Br atoms from PbBr₂ [26]. On the other hand, the halide vacancies have been recognized as defect centers in perovskite crystal [6, 28–30]. It has been indicated from the density functional theory that the present of halide vacancies could generate structural defects. Therefore, in consideration of above analysis, we can speculate that the presence of interstitial Pb atoms and Br vacancies (V_Br) could lead to structural defects in perovskite nanocrystals FAPbBr₃, which may be one of factors to result in the structural decomposition and the formation of mesoporous structure.

To investigate the optical properties of mesoporous perovskite nanocrystals, we performed the measurement of photoluminescence (PL) spectra. As shown in figure 4(a), it can be clearly seen that the light emission peaks are effectively tuned in the range from 544 to 520 nm when the perovskite concentration was decreased from 40 mg ml⁻¹ (original) to 10 mg ml⁻¹ (Dilution 3). The luminescence features narrow emission line widths of 18–30 nm, quantum yields of 52%–82%. It is worth pointing out that the formation of mesoporous structure does not lead to external emission from defects. Therefore, the optical properties are dependent on the evolution of mesoporous structure. After overall consideration of PL and TEM results, it could be concluded that the emission blueshift may be ascribed to the decrease of effective area of mesoporous nanocrystal. The evolution of light emission of mesoporous FAPbBr₃ nanocrystals is further investigated by tracing the PL spectra over time, as shown in figure 4(b).

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Dilution is a time-dependent process. As time goes by, due to the different degree of dilution, there will be a mixture of original samples and dilution samples with mesoporous structure at early dilution time. As shown in figure 4(b), a second peak appeared in PL curves at different dilution time, accompanying the main peak. From the results, it can be seen that there are two peaks located at 544 and 524 nm at the first 5 s. Following the next 5 s, the peak shifted to 520 nm while keeping 544 nm unchanged. At the time of 15 s, the emission peak of 509 nm was appeared, accompanying the main peak of 544 nm, the concentration of perovskite at this time is detected about 8 mg ml⁻¹. This indicates the transformation of mesoporous structure is time-dependent process. With extension of time, the final evolved nanocrystals have relatively stable morphology and optical properties when the reaction in the solution reaches equilibrium.

To explore the optoelectronic properties of mesoporous perovskits, we fabricated perovskite LEDs for further study. As shown in the schematic diagram in figure 5(a), the typical device configuration consists of spin-coated hole injection/transport layer, perovskite emission layer, and evaporated electron injection/transport layer. The detailed device performance based on different samples are summarized in a table, as displayed in figure 5(b). In detail, figure 6 shows the characterization of perovskite LEDs, such as current density, luminance, current efficiency (CE) and external quantum efficiency (EQE). In figure 6(a), the turn on voltage of different devices are varied between 3.5 and 3.8 V, indicating the mesoporous structure makes little influence on I–V performance. The maximum luminance and CE climb up and then decline as the perovskite concentration gradually decreased. For example, device with original perovksites (40 mg ml⁻¹) show maximum luminance and CE of 13675 cd m⁻² and 5.9 cd A⁻¹, respectively. As the perovskite concentration decreased to 30 mg ml⁻¹ (Dilution 1), the luminance and CE are increased to 23370 cd m⁻² and 6.7 cd A⁻¹, respectively, which represents 1.7- and 1.1-fold improvement correspondingly. Compared to the Dilution 1, after further dilution treatment, the device performance is subject to continuous declination. Especially for Dilution 3, the luminance and CE decreased to 8784 cd m⁻² and 2.7 cd A⁻¹, respectively, about 1.5- and 2.2-fold drop compared to the original. Therefore, the best performance is achieved by Dilution 1, the EQE reached up to 2.0, while excessive dilution, such as Dilution 3 (10 mg ml⁻¹) with EQE of 0.8, lead to performance degradation obviously.

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The different performance between varied samples verified the perovskite concentration is one of key factors for achieving excellent perovskite LEDs. Compared to the device performance, the best performance is achieved by Dilution 1, which could be ascribed to several reasons as below: (1) the over-concentrated perovskite nanocrystals, such as the original (40 mg m⁻¹), may have too many ligands and defects on the surface, which would influence interlayer carrier injection and transport and thus the device performance. (2) The proper dilution of perovskite nanocrystals, such as Dilution 1, could not only effectively improve carrier transport properties but also enhance the quantum yield of emission. It can be inferred from the fact that the presence of interstitial Pb atoms or Br vacancies in the traditional perovskite nanocrystals is suitably suppressed in the mesoporous perovskites, due to the structural decomposition at defects position during dilution treatment. Therefore, the defects are properly reduced after decomposition of mesoporous perovskite, which may be beneficial for the improved carrier injection and transport. (3) The excessive dilution of perovskite nanocrystals to low concentration, cause the formation of metallic Pb atoms and halide vacancies in the products, followed the decreased quantum yield and device performance.

In summary, we prepared FA cation based hybrid perovskite nanocrystals FAPbBr₃, and investigate the related morphological evolution and optical properties under different dilution ratio. The diluted perovskite nanocrystals exhibite mesoporous structure, with some holes modified on the surface of nanocrystal. Optical properties demonstrate the blueshift of light emission, indicating the fine-tuning of optical properties could be realized by controlling the mesoporous structure. To evaluate the mesoporous perovskite nanocrystals, we fabricated perovskite LEDs and tested the device performance. The best performance is achieved by Dilution 1, which shows maximum luminance and CE of 23370 cd m⁻² and 6.7 cd A⁻¹, respectively. The results indicate the proper dilution of perovskite nanocrystals could improve interlayer carrier injection and transport and then the device performance, while the excessive dilution may cause increased defects in products such as metallic Pb atoms or halide vacancies. Therefore, proper perovskite concentration is one of key factors to realize excellent device performance.

This research is supported by International Science and Technology Cooperation Project of Guangdong Province (No. 2019A050510002), Research Fund of Guangdong-Hong Kong-Macao Joint Laboratory for Intelligent Micro-Nano Optoelectronic Technology (No. 2020B1212030010), Open Fund of Guangdong Provincial Key Laboratory of Information Photonics Technology (Guangdong University of Technology) (No. GKPT20-09), and Fujian Provincial Key Laboratory of Advanced Materials.

All data that support the findings of this study are included within the article (and any supplementary files).

Bingfeng Fan and Lei Hu contributed equally to this work.