Metal Halide Perovskite Light-Emitting Transistor with Tunable Emission Based on Electrically Doped Semiconductor Nanocrystal-Based Microcavities

Abstract

Electroluminescence of metal halide perovskites has been widely reported via the fabrication and optimization of light-emitting diodes and light-emitting transistors. Light-emitting transistors are particularly interesting owing to the additional control of the gate voltage on the electroluminescence. In this work, the design of a microcavity, with a defect mode that can be tuned with an applied voltage, integrated with a metal halide light-emitting transistor is shown. The optical properties of the device have been simulated with the transfer matrix method, considering the wavelength-dependent refractive indexes of all the employed materials. The tunability of the microcavity has been obtained via the employment of doped semiconductor nanocrystalline films, which show a tunable plasma frequency and, thus, a tunable refractive index as a function of the applied voltage. Consequently, the tunability of the electroluminescence of the metal halide perovskite light-emitting transistor has been demonstrated.

1. Introduction

The electroluminescence of metal halide perovskites is attracting increasing attention owing to their narrowband emission, near-unity photoluminescence efficiency, and low-cost solution-based fabrication [1,2,3,4,5]. In addition to the manufacture of very efficient solar cells [6,7,8,9,10] and light-emitting diodes [11,12,13], the development of perovskite-based light-emitting transistors is of great interest [14,15,16,17,18,19].

To enhance the electroluminescence of light-emitting transistors, the integration of photonic crystals as gate dielectrics has been reported in organic light-emitting transistors. In this way, enhanced emission efficiency and enhanced emission directionality has been demonstrated [20,21]. Because of the similar wet chemistry fabrication techniques for organic semiconductors and metal halide perovskites, photonic crystals as gate dielectrics can also be implemented for metal halide perovskite light-emitting transistors. To further increase the enhancement, a combination of a metal halide perovskite light-emitting transistor with a microcavity has been proposed. In such a design, a photonic crystal is used as a gate dielectric and the other photonic crystal is fabricated onto the metal halide perovskite layer [22].

With a proper choice of materials, a microcavity can be tuned with an external stimulus in order to tune the light-emitting transistor emission. For example, the employment of metals or doped semiconductors [23,24,25] as components of the photonic crystal leads to a tunable photonic band gap with the application of an external voltage [26]. In this work, the design of a metal halide perovskite light-emitting transistor combined with a microcavity, which can be tuned with an external voltage, is presented. The transmission spectra are simulated with the transfer matrix method and the wavelength-dependent refractive indexes of all the materials have been employed.

2. Materials and Methods

The simulations of the light transmission spectra of the structures studied in this work have been performed with the transfer matrix method, which is well established for one-dimensional multilayer systems [27,28,29,30]. The studied system is glass/multilayer/air.

For silicon dioxide (SiO₂), the following Sellmeier equation has been employed [31,32]:

$$n_{{SiO}_2}^2(\lambda )-1=\frac{0.6961663{\lambda }^2}{{\lambda }^2-{0.0684043}^2}+\frac{0.4079426{\lambda }^2}{{\lambda }^2-{0.1162414}^2}+\frac{0.8974794{\lambda }^2}{{\lambda }^2-{9.896161}^2}$$

(1)

For titanium dioxide (TiO₂), the wavelength-dependent refractive index is given by [33]

$$n_{Ti{O}_2}(\lambda )={\left(4.99+\frac{1}{{96.6\lambda }^{1.1}}+\frac{1}{{4.60\lambda }^{1.95}}\right)}^{1/2}$$

(2)

The wavelength-dependent refractive of the active material MAPbI₃ is taken from Refs. [34,35]. The optical response of indium tin oxide (ITO) nanocrystalline films and fluorine indium co-doped cadmium oxide (FICO) nanocrystalline films has been simulated by employing the Drude model and the effective medium approximation [36]. The filling factor of the ITO and FICO nanocrystalline films is 0.65. For ITO, $N=2.49\times {10}^{26} {\mathrm{c}\mathrm{m}}^{-3}$, ${\epsilon }_{\infty }=4$, $m^{*}=0.4{ m}_e$, and $\Gamma =0.1132 \mathrm{e}\mathrm{V}$ [24]. For FICO, $N=1.68\times {10}^{27} {\mathrm{c}\mathrm{m}}^{-3}$, ${\epsilon }_{\infty }=5.6$, $m^{*}=0.43 m_e$, and $\Gamma =0.07 \mathrm{e}\mathrm{V}$ [24,37].

3. Results

In Figure 1, a sketch of the MAPbI₃ metal halide perovskite light-emitting transistor, coupled with a microcavity, is shown. In such a structure, the thickness of the MAPbI₃ layer is 40 nm, the thickness of the ITO layers is 100 nm, the thickness of the FICO nanocrystalline layers is 112.7 nm, the thickness of the SiO₂ layers is 144.15 nm, and the thickness of the TiO₂ layers is 73.5 nm. The design of the device follows the sequence (from bottom to top) of ITO/(TiO₂/FICO)₄/SiO₂/MAPbI₃/SiO₂/ITO/(FICO/TiO₂)₄/ITO. In Figure 1, the white spaces in the photonic crystal represent the repetition of the TiO₂/FICO unit cell.

The photonic crystal below the MAPbI₃ layer, made by TiO₂ and FICO nanocrystals, functions as a gate dielectric. Instead, the photonic crystal above the MAPbI₃ layer is made by TiO₂ and FICO nanoparticle layers sandwiched between two transparent electrodes of ITO and functions as electro-optic switch.

The application of an external voltage between the two ITO electrodes leads to a change in the carrier density of the doped semiconductor nanocrystals, i.e., FICO nanocrystals, resulting in a change in the dielectric function of the FICO nanoparticle layers and, thus, to a change in the effective refractive index of the photonic crystal. Such an effective refractive index change gives rise to a shift of the photonic band gap of the photonic crystal [26,38]. Referring to the structure sketched in Figure 1, the shift of the photonic band gap of the photonic crystal above the MAPbI₃ layer results in a modification of the light transmission spectrum of the light-emitting transistor coupled with the microcavity. In Figure 2, the transmission spectrum of the light-emitting transistor coupled with the microcavity is shown for two different carrier densities of the FICO nanocrystals: the solid black curve corresponds to the structure with a FICO carrier density of 1.68 × 10²⁷ charges/m³, while the dotted/dashed red curve corresponds to a FICO carrier density of 3.68 × 10²⁷ charges/m³.

With such a change in carrier density of FICO nanocrystals, it is possible to almost completely suppress the defect mode of the microcavity. This is mainly due to the asymmetry generated between the two photonic crystals of the microcavities, i.e., the one below the MAPbI₃ layer and the one above the MAPbI₃ layer. To highlight the possibility of tuning the defect mode of the microcavity, in Figure 3, the transmission spectrum of the light-emitting transistor coupled with the microcavity is shown for six different carrier densities of the FICO nanocrystals: 1.68 × 10²⁷ charges/m³ (solid black curve), 1.88 × 10²⁷ charges/m³ (dotted/dashed red curve), 2.08 × 10²⁷ charges/m³ (dashed dark red curve), 2.28 × 10²⁷ charges/m³ (solid brown curve), 2.48 × 10²⁷ charges/m³ (dotted/dashed dark green curve), and 2.68 × 10²⁷ charges/m³ (dashed green curve). The arrow in the figure highlights the increase in carrier density.

A shift of about 20 nm is shown. With a simple linear fit, it is possible to determine that 7.2 × 10²⁵ charges/m³ are needed for a shift of 1 nm of the defect mode. In a similar photonic structure, i.e., a photonic crystal made with indium tin oxide and titanium dioxide layers, a shift of 23 nm has been achieved with an external voltage of 10 V [38]. Considering the electroluminescence (EL) of a light-emitting transistor based on MAPbI₃ (the experimental data were taken from Ref. [16]), it is possible to finely tune the EL by changing the FICO nanocrystal carrier density (Figure 4). As in Figure 3, the arrow underlines the increase in carrier density. Taking into account the aforementioned result for a similar photonic structure reported in Ref. [38], the EL shift (in nm) over the external voltage (in V) can be estimated for this transistor and the value is about 2 nm/V.

An extension of this work could take into account an improvement in the performance, in terms of electroluminescence, of the light-emitting transistor through direct contact between a plasmonic material and the emitting material in the transistor, i.e., metal halide perovskite in this work. To study this possible improvement in transistor performance, very precise microscopic theories have been proposed for similar devices [39,40]. Such microscopic theories would allow the device and its characteristics to be studied with great accuracy.

4. Conclusions

In this work, the tunability, via the application of an external voltage, of the defect mode of the microcavity combined with a metal halide perovskite light-emitting transistor has been studied. In this way, the electroluminescence of the metal halide perovskite active layer can be modulated with an electric field. This tunability can be very interesting for lighting applications with tunable electrically stimulated light emitters. Since the integration of a microcavity in a light-emitting transistor can lead to a possible electrically injected metal halide perovskite laser, the device presented in this work can be interesting also for the realization of tunable lasers.