Journal of Semiconductors, Volume. 46, Issue 7, 072801(2025)

Green perovskite CsPbBr3 light-emitting electrochemical cells with distributed Si nanowires-based electrodes for flexible applications

Viktoriia Mastalieva1, Anastasiya Yakubova1、*, Maria Baeva1, Vladimir Neplokh1,2,3, Dmitry M. Mitin1, Vladimir Fedorov1,2, Alexander Goltaev1, Alexey Mozharov1, Fedor Kochetkov1,2, Andrei S. Toikka1,4, Ramazan Kenesbay1, Ekaterina Vyacheslavova1, Alexander Vorobyev1,2, Kristina Novikova1,2, Dmitry Krasnikov5, Jianjun Tian6, Albert G. Nasibulin5, Alexander Gudovskikh1, Sergey Makarov4,7, and Ivan Mukhin1,2
Author Affiliations
  • 1Alferov University, Khlopina 8/3, 194021, St. Petersburg, Russia
  • 2Peter the Great St. Petersburg Polytechnic University, Polytechnicheskaya 29, 195251, St. Petersburg, Russia
  • 3Institute of Chemistry, St Petersburg State University, 7/9 Universitetskaya Emb., 199034, St. Petersburg, Russia
  • 4ITMO University, Kronverksky Pr. 49, bldg. A, 197101, St. Petersburg, Russia
  • 5Kemerovo State University, Krasnaya Str. 6, Kemerovo, 650000, Russia
  • 6University of Science and Technology Beijing, Beijing 100083, China
  • 7Qingdao Innovation and Development Base, Harbin Engineering University, Qingdao 266000, China
  • show less

    The emergence of cesium lead halide perovskite materials stable at air opened new prospects for the optoelectronic industry. In this work we present an approach to fabricating a flexible green perovskite light-emitting electrochemical cell (PeLEC) with a CsPbBr3 perovskite active layer using a highly-ordered silicon nanowire (Si NW) array as a distributed electrode integrated within a thin polydimethylsiloxane film (PDMS). Numerical simulations reveal that Si NWs-based distributed electrode aids the improvement of carrier injection into the perovskite layer with an increased thickness and, therefore, the enhancement of light-emitting performance. The X-ray diffraction study shows that the perovskite layer synthesized on the PDMS membrane with Si NWs has a similar crystal structure to the ones synthesized on planar Si wafers. We perform a comparative analysis of the light-emitting devices’ properties fabricated on rigid silicon substrates and flexible Si NW-based membranes released from substrates. Due to possible potential barriers in a flexible PeLEC between the bottom electrode (made of a network of single-walled carbon nanotube film) and Si NWs, the electroluminescence performance and I ? V properties of flexible devices deteriorated compared to rigid devices. The developed PeLECs pave the way for further development of inorganic flexible uniformly light-emitting devices with improved properties.

    Keywords

    Introduction

    The development of energy-efficient, bright, and flexible light-emitting devices has been a driving force in creating new flexible displays[1], e-skin[2], and bio-implantable optoelectronics[3]. Organic materials are widely used in flexible electronics, achieving distinguished brightness. However, the lifetime of devices strongly depends on complex processing and it’s not a trivial task to achieve high spectral purity and stability[46] due to their organic origin.

    Cesium lead halide perovskite CsPbX3 (X = Cl, Br, I) materials opened new prospects for the optoelectronic industry[79] owing to high charge carriers mobility, great spectral purity[10, 11], and composition-based tunable emission wavelength[1214]. Perovskite light-emitting electrochemical cells (PeLEC) provide the improved conductivity, lifetime, brightness and stability, compared to perovskite light-emitting diodes (PeLED) due to the use of ionic and polymer additives[15].

    However, mechanical and electrical stability is crucial for developing flexible devices both on organic and perovskite material[16]. In this regard, using electrodes insensitive to mechanical deformations is vital to provide stable light emitting devices[17]. In our previous work, we suggested the distributed high-density GaP nanowires (NWs) flexible electrodes, improving the perovskite layer's charge injection and transport processes[18, 19]. Nonetheless, the demonstrated electroluminescence (EL) signal was spatially nonuniform due to the nonuniform size distribution of epitaxially-grown NW arrays. Also, the complex doping process is crucial for conductivity. Therefore, searching the compromise between improved charge carrier injection and spatial nonuniformity is a relevant challenge for PeLEC device development.

    In this work, we propose the integration of distributed electrodes based on highly doped and ordered Si NWs to the flexible PeLEC light emitting layer. The considered Si NW arrays have great uniformity thanks to the processing approach based on microsphere lithography combined with dry plasma etching. The integration of lead halide perovskites with Si NWs into flexible structure enables the development of more efficient, cost-effective, and versatile optoelectronic devices, such as solar cells[20], photodetectors[21], transistors[22], image sensors[23], and LEDs[24, 25]. Fig. 1(a) presents a schematic illustration of conventional planar PeLED structure (the left panel of Fig. 1(a)) compared to our PeLEC device (the right panel of Fig. 1(a)), illustrating simplicity and efficiency of developed architecture. To achieve a lateral conductivity of the distributed electrode, the NWs-based membrane was combined with a film of single-walled carbon nanotubes (SWCNTs). For simplicity, the top electrode to perovskite layer was also made from SWCNT film. The NW array morphology uniformity provides the homogeneous injection of charge carriers and, therefore, intensive light emission of perovskite LEC, confirmed by the numerical simulation and experimental results. We also compare the performance of the same perovskite LECs with the distributed Si NW-based electrodes before and after release from the rigid Si substrate. Our findings highlight the potential benefits and contributions of PeLECs with a Si NWs distributed electrode to the development of future flexible light-emitting devices.

    (Color online) (a) Schematic presentation of the conventional design of a flexible planar perovskite light-emitting device (on the left) and flexible perovskite device with a distributed Si NWs electrode (on the right). (b) Fabrication workflow of a flexible PeLEC with a distributed Si NW-based electrode. (c) Photo of the bent PeLEC membrane with a distributed Si NW-based electrode; scale bar is 1 cm.

    Figure 1.(Color online) (a) Schematic presentation of the conventional design of a flexible planar perovskite light-emitting device (on the left) and flexible perovskite device with a distributed Si NWs electrode (on the right). (b) Fabrication workflow of a flexible PeLEC with a distributed Si NW-based electrode. (c) Photo of the bent PeLEC membrane with a distributed Si NW-based electrode; scale bar is 1 cm.

    Materials and methods

    Numerical simulation

    To evaluate the efficiency of the light-emitting devices based on the hybrid n-Si NWs/CsPbBr3 structures, numerical simulation was conducted. We employed the drift-diffusion model in axial-symmetrical 2D geometry for Si/perovskite/SWCNT structure (electron-emitter/active material/hole-emitter). Non-ideality of electrical contacts was not taken into consideration. CsPbBr3 material properties were adopted from Refs. [2629], while Si properties were taken from Refs. [3033]. To estimate the radiative perovskite recombination rate, we used van Roosbroeck−Shockley's theory[34].

    Silicon NW array fabrication

    The Si NW arrays were fabricated using microsphere lithography and dry plasma etching in an SF6/O2 gas mixture at cryogenic temperature[35]. Cryogenic etching provides a deep anisotropic etching of Si. At a low temperature of approximately −100 °C, the side surface of the etched Si structure is passivated due to the formation of a SiOxFy compound. This non-volatile layer prevents further lateral etching, providing the fabrication of a high aspect ratio structure. Moreover, this process avoids the corrugation of the side surface compared to the Bosch process[35, 36].

    In this work, we used the n-type antimony doped silicon (100) wafers (the resistivity of 0.013−0.015 Ω·cm). The process began with plasma-chemical deposition (CVD) of a 500 nm thick SiO2 layer on a Si substrate using Oxford Instruments PlasmaLab 100 PECVD equipment (United Kingdom). Then, by the means of microsphere mask lithography, a template on the SiO2/Si surface was formed. For this purpose, a monolayer of 2 μm in diameter polystyrene spheres was deposited from a colloidal solution by spin-coating at 700 r/mim for 1 min.

    The fabrication of Si NWs through the microsphere mask included three dry etching steps. First, the diameter of polystyrene spheres was reduced by O2 plasma etching; second, the SiO2 layer was etched in CHF3 plasma through the mask of polystyrene spheres; finally, inductive-coupled plasma (ICP) cryogenic etching of Si through the formed hard SiO2 mask was performed. The following parameters providing a vertical etching profile were employed: SF6 flow of 50 sccm, O2 flow of 9 sccm, RF power of 30 W, ICP power of 1000 W, and temperature of −120 °C. The plasma etching process was carried out using the Oxford Plasmalab System 100 ICP380 (United Kingdom) setup. Fig. 2(a) shows typical scanning electron microscope (SEM) images of the etched Si NW arrays.

    Isometric SEM images of (a) an etched Si NW array; (b) a Si NWs array after PDMS coating; (c) the revealed top parts of Si NWs in the partially etched PDMS membrane. Inset in (a) shows a cross-section view SEM image of an as-fabricated Si NW array. The scale bars for all images are 2 µm.

    Figure 2.Isometric SEM images of (a) an etched Si NW array; (b) a Si NWs array after PDMS coating; (c) the revealed top parts of Si NWs in the partially etched PDMS membrane. Inset in (a) shows a cross-section view SEM image of an as-fabricated Si NW array. The scale bars for all images are 2 µm.

    Fabrication of Si NW-based distributed electrode

    The fabrication of the Si NW-based distributed electrodes involves several steps. First, a 5 nm thick layer of Yb was deposited on Si NW top parts by e-gun evaporation using Boc Edwards (UK) Auto 500 setup (vacuum of 3 × 10−5 mbar) to protect Si NWs during further O2/CF4 etching of polymer. For the Si NWs/polymer membrane fabrication, a commercial polydimethylsiloxane (PDMS) (Dow Corning Sylgard 184) was mixed in a standard base-to-curing agent ratio of 10 : 1. The mixture was dropped onto a Si NW array and distributed via the G-coating method (5000 r/mim for 60 min)[37, 38]. After coating, the sample was placed on a hot plate at 80 °C for 6 h. Fig. 2(b)) presents an SEM image of Si NWs covered with a PDMS layer.

    An O2/CF4 plasma etching was used to partially reveal the Si NW top parts above the PDMS surface (for further electrical contact) and maintain a uniform PDMS membrane thickness of approximately 3−4 μm. Finally, the protective Yb layer was removed selectively from NWs in hydrochloric acid (HCl) at a concentration of 1 : 10 for 2 min. The sample surface and thickness of the polymer layer were analyzed by SEM.

    For on-silicon rigid devices (not released from rigid Si substrates), the bottom electrode consisting of a 100 nm thick Al layer was deposited on the Si substrate backside by e-gun evaporation using a Boc Edwards (UK) Auto 500 setup at 3 × 10−5 mbar.

    Fabrication of CsPbBr3 emission layer

    A CsPbBr3 solution (molar concentration of 0.2 mmol/mL) was prepared in a dry nitrogen atmosphere by dissolving lead (II) bromide salt (PbBr2) (99.9% purity, Lankhit) and cesium bromide salt (CsBr) (99.9% purity, Lankhit) in anhydrous dimethylsulfoxide (DMSO) (99.8% purity, Sigma Aldrich) solvent and stirred overnight at 60 °C and 300 r/mim. The polyethylene oxide (PEO) (Sigma-Aldrich) solution (concentration 10 mg/mL) was prepared in an inert atmosphere of nitrogen by adding PEO powder (Mw = 1 000 000) in DMSO; to dissolve the polymer, the mixture was stirred overnight at 60 °C and 300 r/mim. The perovskite-polymer composite solution (CsPbBr3 : PEO/DMSO) was prepared in the ambient condition by mixing PEO and CsPbBr3 solutions in a 1 : 0.13 weight ratio of dry components. Then, the perovskite-polymer solution was mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (anhydrous, 99.99% trace metals basis, Sigma-Aldrich)/DMSO solution (concentration of 10 mg/mL) with a ratio of 1 : 0.01 by weight. This final solution was stirred for 1 h at 60 оC and 300 r/mim.

    Prior to perovskite-polymer composite spin-coating, the sample surface was treated in RF oxygen plasma (power of 400 W) for 1 min. To obtain a light-emitting layer, the composite perovskite thin film was spin-coated on the top of the revealed Si NWs and annealed at 60 or 200 °C on a hot plate (see the fabrication workflow in Fig. 1(b)).

    Fabrication of SWCNT transparent flexible electrodes for PeLEC device

    SWCNTs were synthesized by an aerosol (floating catalyst) chemical vapor deposition method using CO as a carbon source and ferrocene as a catalyst precursor[39, 40]. The nanotubes were collected downstream of the reactor on a paper filter in the form of randomly oriented SWCNTs thin film with a 90% transparency (at a 550 nm wavelength).

    After CsPbBr3 : PEO layer formation, a flexible top electrode, made of 90% transparent SWCNT film with 84 Ω/square sheet resistance[41], was applied using a dry transfer method[42]. To achieve a lateral conductivity of the distributed electrode, the NWs-based membrane was combined with a SWCNT film. To improve the electrical contact with Si NWs and perovskite layer, SWCNT films were densified with 99.9% isopropyl alcohol (IPA), not damaging the perovskite[43]. The densification process was performed using a drop-casting method, which effectively eliminates air gaps and improves the contact between the SWCNT and CsPbBr3 layer. Finally, gallium (Ga) metal drops were placed on the top of the SWCNT film for further contact with the external electrical circuit (see the fabrication workflow in Fig. 1(b)). Thus, the combination of the SWCNT dry transfer method, IPA treatment, and Ga droplets application provides the good top contact to CsPbBr3 layer, without perovskite layer damaging.

    Processing of flexible membrane-based PeLEC

    For flexible membrane-based PeLEC processing, the PDMS-encapsulated Si NW array with the already formed perovskite film and SWCNT-based top electrical contact were released from the rigid silicon substrate. Before delamination, to protect the perovskite layer from mechanical and environmental impact, the sample was coated with a supporting layer of PDMS (average thickness of 500 μm) and annealed on the hot plate at 60 °C for 5 h. Additionally, this PDMS layer provides mechanical support to the membrane, enabling easier membrane release. The protected membrane with PeLED was mechanically removed from the Si substrate using a microtome blade. Bottom electrical contact to the Si NWs array was formed at the cut interface from SWCNTs pads via the previously described approach (see Fig. 1(b)). The photo of a released flexible PeLEC membrane with a distributed Si NW-based electrode is shown in Fig. 1(c).

    Structural and optoelectronic characterization

    The perovskite films structural characterization was performed using a Bruker Kappa Apex II diffractometer (Germany) equipped with a microfocus Incoatec ImS 1.0 Cu−Ka X-ray source (1.5418 Å) and 2D charge-coupled detector (Apex CCD). Perovskite film samples were mounted with the surface normal to the phi axis of rotation. X-ray diffraction (XRD) patterns were collected at a grazing incidence angle of 10° with an exposure time of 10 min. Samples were rotated a full 360° around the phi-axis to obtain uniform diffraction rings. The acquired 2D diffraction patterns were integrated into 2θ-intensity profiles using DIOPTAS software[26]. The thin film phase composition was evaluated using PROFEX software[27].

    To analyze film texturing, three-dimensional (3D) reciprocal space maps (RSMs) were acquired by performing a full 360° phi-scan with an angular step of 0.5°. The reciprocal space intensity distribution was reconstructed using RecSpaceQt software[28]. Structural information regarding CsPbBr3 and CsPb2Br5 phases was adopted from the Crystallography Open Database (COD).

    Photoluminescence (PL) spectra were acquired using a mercury lamp (i-line with a wavelength of 365 nm) and recorded using the Ocean Optics QE Pro optical fiber spectrometer (USA) coupled with a Carl Zeiss Axio Imager A2m (Germany) microscope operating in a fluorescent regime. The detection area was a spot of 2 μm in diameter. The fluorescent images of the samples were obtained using the previously mentioned microscope with a 100× objective EC Epiplan-NEOFLUAR (Carl Zeiss).

    The current−voltage characteristics (I−V curves) of the fabricated device were acquired using a Keithley 2401 source meter. Data on device EL was collected with a Telescopic Optical Probe 150 of CAS 120 Instrument Systems spectroradiometer.

    Results and discussion

    Numerical simulation of n-Si NWs/CsPbBr3 PeLEC system

    We performed numerical simulation of the band structure and optoelectronic properties of the considered hybrid perovskite/Si NWs system. The model consisted of n-doped Si NW, partially embedded into a perovskite layer (Fig. 3(a)). To simplify the model, the electrical contact between NWs and perovskite was considered to have Ohmic behavior, while the surface layer of perovskite was additionally doped to align the Fermi level with the conduction band minimum of Si. In the simulations, we varied the following geometrical parameters of the system: the length of Si NWs embedded into the perovskite layer (Hemb), NW diameter (DNW), perovskite thickness over NW (L), and NW surface density (P) (see Fig. 3(a)).

    (Color online) (a) Schematic view of the considered semiconductor system. (b) The calculated system band diagram at the Si NWs/CsPbBr3 interface (transverse coordinate). (c) The system’s calculated current density and radiative recombination quantum efficiency dependencies over the applied voltage (for the fixed Hemb = 500 nm, DNW = 100 nm, L = 1000 nm, P = 0.2 μm−2). Insert shows the distribution of radiative recombination rate in the cross-section of the PeLEC structure. The calculated maps of quantum efficiency on (d) NW surface density and NW diameter (for the fixed Hemb = 500 nm and L = 1000 nm) and (e) perovskite thickness and NW height embedded into the active perovskite layer (for the fixed DNW = 75 nm and P = 0.2 μm−2).

    Figure 3.(Color online) (a) Schematic view of the considered semiconductor system. (b) The calculated system band diagram at the Si NWs/CsPbBr3 interface (transverse coordinate). (c) The system’s calculated current density and radiative recombination quantum efficiency dependencies over the applied voltage (for the fixed Hemb = 500 nm, DNW = 100 nm, L = 1000 nm, P = 0.2 μm−2). Insert shows the distribution of radiative recombination rate in the cross-section of the PeLEC structure. The calculated maps of quantum efficiency on (d) NW surface density and NW diameter (for the fixed Hemb = 500 nm and L = 1000 nm) and (e) perovskite thickness and NW height embedded into the active perovskite layer (for the fixed DNW = 75 nm and P = 0.2 μm−2).

    Fig. 3(b) shows the calculated energy bands at the interface of Si and perovskite. There is no potential barrier for electrons, which provides an efficient injection of electrons from Si to the perovskite layer. Fig. 3(c) presents the dependencies of current density and radiative recombination quantum efficiency on the applied electrical bias. Due to the assumption of the ideal contacts at all interfaces, the system’s knee voltage corresponds to the perovskite band gap value. The maximum radiative quantum efficiency reaches 18% for 2.1 V applied electrical bias, which is an appropriate result for PeLEC without any charge transport layers[44]. Overall, the relatively low device performance is caused by the simplified geometry of the PeLEC structure (the absence of hole and electron transport layers limiting the direct transfer of carriers). The further decrease of efficiency with voltage can be associated with the direct flow of the injected carriers through the active area without recombination (direct current flow to the opposite electrode). To support this statement, we numerically calculated (i) the total current passing through the PeLEC structure and (ii) corresponding radiative current responsible for light emission (see Fig. S1 in the Supporting materials). One can see that radiative current (Irad) is much smaller than total current (Itot), which is associated with the absence of potential barrier for the injected carriers (see Fig. 3(b)) and explains the relatively low PeLEC efficiency. Moreover, the law of total current increase with the applied voltage is superlinear, while the law of radiative current is close to be linear, which means that the device efficiency is not monotonic. The external quantum efficiency is proportional to the ratio of Irad/Itot and has non-monotonic behavior (see Fig. S1 in the Supporting materials).

    The insert in Fig. 3(c) shows the calculated rate of radiative recombination in the perovskite at the applied voltage of 2.1 V. One can see that the recombination process is favored near the interface with NW, while the radiative recombination rate is maximized near the base of NW.

    Fig. 3(d) presents the dependence of the quantum efficiency on the NW surface density and diameter. The maximum device performance is achieved for lower Si NWs diameters (<800 nm) and densities (<0.75 μm−2), which can be explained by suppressing the number of charge carriers passing through the perovskite layer without recombination due to increasing perovskite volume (i.e. the smaller NW diameter the lower its volume and the bigger the perovskite volume emitting the light). Decreasing the NW surface density leads to the effective growth of the hole-emitter surface, i.e., SWCNT/perovskite interface area. This provides more uniform holes’ injection to Si-based electron-emitter under applied bias, which results in a decrease in the fraction of charge carriers that directly transfer through the system without radiative recombination. It should be noted that despite the initial quantum efficiency increase, the further decrease of Si NW diameter (<500 nm) and surface density (<0.5 μm−2) leads to the decline in total flowing current and, as a result, the light emission power.

    To achieve a balance between mechanical and electrical properties, the NW morphology parameters, such as diameter and height, should be in the certain range. We also calculated dependencies of current density, passing through the entire PeLEC structure, on the NW parameters (Fig. S2(a) in the Supplementary materials). For the chosen NW array density (defined by the diameter of microspheres employed for Si etching), the optimal NW diameter corresponds to the range of (300−800 nm). Although, according to Fig. 3(d), decreasing the NWs diameter leads to the slight increase of quantum efficiency, the luminescence of the device is proportional to the efficiency multiplied by the current density (the higher current provides the higher luminance of light emitting structure). Therefore, for higher device performance, the NW diameter should be increased.

    Moreover, NWs with diameter lower than critical one drastically lose their conductivity due to significant influence of surface defects (since the part of the conductive NW channel is depleted[45]). Thus, 0.8 μm NW diameter is a balance between good conductivity, device performance and mechanical durability.

    At the same time, increasing both the height of Si NW embedded into the perovskite (Hemb) and the perovskite thickness (L) boosts device performance (see Fig. 3(e)). In this case, the increase in active perovskite volume leads to a more uniform distribution of current passing through the structure and, therefore, positively influences quantum efficiency. Our simplified modeling demonstrates the enhanced recombination with increasing thickness (L), but does not fully account for the decreased conductivity of thicker perovskite layers. The optimal performance is a result of balancing these competing effects: the increased active volume and more uniform current distribution, and charge transport efficiency and conductivity losses. It is desirable to have at least 1 μm of active perovskite layer above Si NWs to avoid contact between two electrodes (upper SWCNT and SiNWs) and charge tunneling. According to our calculation results, the optimal Si NWs height embedded into the perovskite layer, as well as L, are 1 μm (see Fig. 3(e) and Fig. S2(b)). Further increase of Hemb and L is not reasonable, due to the growth of perovskite layer resistance, negatively influencing the device performance.

    At the same time, according to our experimental results, the minimal height of Si NWs array allowing the encapsulation into the PDMS layer and its further release from the substrate is above 4 μm (due to viscosity of the polymer and its mechanical strength, also see Ref. [46]). Therefore, at least 4 μm NW height is required for a device fabrication. At the same time, it is not reasonable to further increase NWs height due to resistance growth.

    Hence, the Si NWs-based distributed electrode (compared to the planar one) provides not only the structure's mechanical flexibility but also helps to improve the carrier injection into the perovskite layer with an increased thickness, which, in turn, enhances the light-emitting performance.

    According to the results of numerical calculations, the NW height and diameter were chosen as 4.2 and 0.8 μm, respectively, while the period of the array was 2 µm (defined by the diameter of the employed polystyrene spheres)—see SEM images in Fig. 2(a). The resulting height of exposed Si NWs was in the range of 0.5−1 µm across the whole sample surface (Fig. 2(c)). The thickness of the perovskite light-emitting layer was chosen to be above 2 µm.

    Impact of CsPbBr3:PEO processing parameters on film crystallization

    To investigate the influence of the spin-coating parameters on the resulting perovskite layer thickness, perovskite films were spin-coated on Si NW arrays at 600, 800, 1000, and 1200 r/mim. Due to the non-planar Si NWs array surface and composite perovskite-polymer solution's relatively high viscosity, the rotation speed (600−1200 r/mim) during spin-coating does not significantly affect the thickness of the acquired layer, which is approximately 2 µm. Fig. 4(a) shows typical cross-sectional SEM images of samples with similar film morphology and thickness obtained at lower and higher spinning velocities, namely 600 or 1200 r/mim (the annealing temperature of 60 °C was the same). Additionally, Fig. S3 in the Supplementary materials presents cross-section SEM images of perovskite films obtained at 800 and 1000 r/mim, demonstrating films of the same thickness. Since the perovskite film thickness was quite similar for all samples, both sample’s PL spectra (for the samples, coated at 600 and 1200 r/mim and annealed at 60 °C) exhibited a similar single peak curve shape with a PL signal maximum at ~525 and 528 nm for 600 and 1200 r/mim, respectively, and the full width at half maximum (FWHM) of PL response—18 and 20 nm for 600 and 1200 r/mim, respectively, see Fig. 4(b).

    (Color online) (a) Cross-sectional SEM images of perovskite layers, spin-coated at 600 and 1200 r/mim on Si NWs membranes (presented in artificial colors for clarity); the insets show the corresponding top-view PL images. (b) PL spectra of CsPbBr3 layers obtained with different coating speeds. (c) Isometric SEM images of CsPbBr3 layers spin-coated on Si NWs membranes at 600 r/mim and annealed at 60 and 200 °C; the insets show the corresponding top-view PL images. (d) PL spectra of CsPbBr3 layers obtained with different temperatures of annealing. (e) XRD patterns for a series of CsPbBr3 perovskite films spin-coated on Si NW membranes and planar Si substrate. The calculated positions of the Bragg reflections related to the CsPbBr3 (COD CIF file ID: 4510745)[47] and Cs4PbBr6 (COD CIF file ID: 4002857)[48] phases are shown by black and green vertical marks, correspondingly.

    Figure 4.(Color online) (a) Cross-sectional SEM images of perovskite layers, spin-coated at 600 and 1200 r/mim on Si NWs membranes (presented in artificial colors for clarity); the insets show the corresponding top-view PL images. (b) PL spectra of CsPbBr3 layers obtained with different coating speeds. (c) Isometric SEM images of CsPbBr3 layers spin-coated on Si NWs membranes at 600 r/mim and annealed at 60 and 200 °C; the insets show the corresponding top-view PL images. (d) PL spectra of CsPbBr3 layers obtained with different temperatures of annealing. (e) XRD patterns for a series of CsPbBr3 perovskite films spin-coated on Si NW membranes and planar Si substrate. The calculated positions of the Bragg reflections related to the CsPbBr3 (COD CIF file ID: 4510745)[47] and Cs4PbBr6 (COD CIF file ID: 4002857)[48] phases are shown by black and green vertical marks, correspondingly.

    Additionally, perovskite solution was spin coated on Si substrate using a wide speed range up to 5000 r/mim (keeping other parameters the same) and 60 oC annealing temperature. The thickness of the spin-coated layers was estimated using cross-section SEM imaging. As shown in Fig. S4 in the Supplementary materials, higher spin-coating speed results in the decrease of CsPbBr3 film and, therefore, the drop of photoluminescence (Fig. S5 in the Supplementary materials). We suggest to use Si NWs as distributed electrodes, embedded into luminescence perovskite layer, to achieve better conductivity for thick films. According to our calculations, increasing the perovskite layer thickness allows to achieve higher device performance. Thus, we suggest that the chosen spin-coating speed from 600 to 1200 r/mim corresponds to the optimal processing window for achieving the desired balance between film thickness, luminescence performance, and charge transport efficiency in our devices.

    Then, we investigated the influence of annealing temperature (Tann) on the quality of perovskite layer spin-coated at 600 r/mim on Si NW arrays. Annealing at Tann = 60 °C, corresponding to typical annealing temperatures for PeLECs[49], produces dendritic perovskite polycrystals of about tens of microns in linear size (see the left panel in Fig. 4(c)). In the case of annealing at Tann = 200 °C, which slightly exceeds the boiling temperature of DMSO (189 °C), higher rate of DMSO evaporation causes less inhomogeneity in crystal size distribution in the resulting film: see "smaller" ("diamond"-shape grains ~1 µm linear size) and "bigger" ("dendritic stars" ~ up to 10 µm in linear size) perovskite crystals in the right panel of Fig. 4(c). Furthermore, the PL spectrum of the sample annealed at 200 °C shows a double peak optical response (see Fig. 4(d)).

    CsPbBr3 perovskites crystals have two main symmetries: low-temperature (orthorhombic) and high-temperature (cubic). Cubic symmetry occurs above 130 °C, however it is unstable and changes to orthorhombic symmetry at lower temperatures (<88 °C)[50]. As reported in Ref. [51], cubic CsPbBr3 has an extremely asymmetry in the I−V curves and abnormal hysteresis behaviors. Moreover, the significant effect of ion migration leads to decreasing luminance and photovoltaic characteristics. At the same time, orthorhombic CsPbBr3 exhibits excellent stability, which is essential for stable operation of the device. Therefore, it is not reasonable to vary temperature between 60 and 200 °C to achieve cubic symmetry. At the same time, it is essential to study different nucleation mechanisms to choose the best for device fabrication.

    Fig. 4(e) presents the XRD patterns for a series of perovskite films spin-coated on Si NWs and planar Si substrates from an equimolar solution. The pattern decomposition using Le Bail refinement revealed that all the perovskite films exhibited a polycrystalline orthorhombic (Pnma) CsPbBr3 structure[28]. Remarkably, additional Bragg reflections, attributed to the Cs-rich Cs2PbBr6 phase, were observed (shown by diamond marks in Fig. 4(e)). Notably, the Cs-rich phase-related reflections vanished in the sample annealed at 200 °C. To investigate the distribution of crystalline grain orientation and size, three-dimensional reciprocal space maps (RSMs) were obtained. The corresponding two-dimensional RSM cross-sections, intersecting the reciprocal space origin and oriented normally to the thin film plane, are shown in Fig. S6 in the Supplementary materials.

    Despite the similar XRD patterns, a significant difference was observed in the intensity distribution of the diffraction rings among the studied samples. The 2D-RSMs obtained from the CsPbBr3 layer deposited on the Si wafer show solid diffraction rings with almost uniform intensity, indicating the isotropically oriented crystalline grains. In contrast, the perovskite film deposited on the Si NWs exhibited spotty diffraction rings, suggesting possible grain coarsening and formation of larger crystallites. Moreover, grain coarsening was more pronounced in the layer annealed at 60 °C compared to the structure annealed at 200 °C. These observations lead to the conclusion that the formation of CsPbBr3 layers on the Si NWs results in different grain sizes (compared to planar substrate) but does not lead to the appearance of preferential orientation or stabilization of unwanted metal bromide salts or Pb- or Cs-rich perovskite phases.

    Thus, to avoid the double emission peak, and achieve lower FWHM and more uniform perovskite coverage, going forward, for flexible PeLEC fabrication we employed composite perovskite-polymer layers, spin-coated on Si NW membranes at 600 r/mim and annealed at 60 °C.

    PeLEC-on-Si and flexible PeLEC characteristics study

    Fig. 5(a) shows a typical cross-sectional SEM image of the fabricated PeLEC-on-Si before the membrane release from the bulk silicon substrate: the thickness of the perovskite layer above Si NWs tops is around 2 µm. For measurements of I−V curves and EL characteristics, negative polarity was applied to the back Al contact of the rigid PeLEC-on-Si, while positive polarity was applied to Ga droplets on the top SWCNT electrode (see the scheme in Fig. 1); the same polarity was used in the case of the flexible PeLEC after release from the rigid Si substrate. Figs. 5(b) and 5(c) present optical images of the same PeLEC device in operation under applied electrical bias before and after releasing from Si substrate, respectively. As one can see, the PeLEC functionality did not suffer qualitatively during the delamination process.

    (Color online) (a) Сross-sectional SEM image of PeLEC-on-Si device (in artificial colors for clarity). Photo of (b) PeLEC-on-Si and (c) flexible PeLEC during EL measurements (acquired at 5 and 7.3 V applied voltage, respectively). (d) Luminance−voltage (red curve) and current density−voltage (black curve) dependencies for PeLEC-on-Si device, (e) EL spectra of PeLEC-on-Si and flexible PeLEC devices. (f) PeLEC-on-Si and flexible PeLEC I−V curves comparison.

    Figure 5.(Color online) (a) Сross-sectional SEM image of PeLEC-on-Si device (in artificial colors for clarity). Photo of (b) PeLEC-on-Si and (c) flexible PeLEC during EL measurements (acquired at 5 and 7.3 V applied voltage, respectively). (d) Luminance−voltage (red curve) and current density−voltage (black curve) dependencies for PeLEC-on-Si device, (e) EL spectra of PeLEC-on-Si and flexible PeLEC devices. (f) PeLEC-on-Si and flexible PeLEC I−V curves comparison.

    Fig. 5(d) presents a typical I−V curve of the PeLEC-on-Si. The diode knee voltage is 5 V, and an EL signal also appears at this voltage. The spectral peak position for PeLEC-on-Si (Fig. 5(e)) is 525 nm (which corresponds well to the CsPbBr3 band gap value of ~2.4 eV and acquired PL signal maximum), the FWHM of EL intensity peak is 18 nm (equal to the corresponding value of PL signal), the maximal spectral radiance is 15 nW/(cm2·sr). Fig. S7 in the Supplementary materials demonstrates the homogeneity of EL within one PeLEC-on-Si sample (3 separate pixels analyzed). All three light-emitting areas have identical EL peaks at 525 nm.

    Similarly, the flexible PeLEC device (obtained from the same PeLEC-on-Si device) was investigated functionally after releasing from the Si substrate and applying the bottom flexible SWCNT contact. The flexible PeLEC’s I−V curve is shown in Fig. 5(f). The flexible PeLEC has a higher knee voltage (7.3 V vs. 5 V, see Fig. 5(f)) and lower spectral radiance (12 nW/(cm2·sr) vs 15 nW/(cm2·sr) (see Fig. 5(e)), when compared to the rigid PeLEC-on-Si. The appearance of a parasitic potential barrier between the NWs and rear SWCNT pad may explain the higher knee voltage of the flexible PeLEC. The acquired I−V curves of the Si NWs/SWCNT pad interface are given in the Supplementary materials (Fig. S8); the latter shows that the potential barrier for the Si NWs/SWCNT pad interface is around 2.3 V, which corresponds well to the difference in flexible PeLEC and PeLEC-on-Si knee voltages. According to Fig. 5(e), the EL signal of the flexible device experiences slight redshift and splitting. We suggest that the nonuniform mechanical tension and/or degradation of CsPbBr3[52, 53] during the releasing process can be responsible for that. However, the EL intensity is uniform over the sample and comparable with the device on the rigid substrate before delamination.

    Conclusion

    We have demonstrated perovskite PeLEC devices on Si substrates and flexible PDMS membranes with distributed electrodes based on the highly ordered uniform arrays of Si NWs. Numerical simulation has shown that a flexible Si NWs-based distributed electrode helps to improve the carrier injection to the perovskite layer with an increased thickness, which, in turn, enhances the light-emitting performance. It has been experimentally shown that spin-coating speed does not significantly influence the perovskite layer morphology for our device NWs-based geometry. At the same time, the annealing temperature modified the morphology from dendrites to uniaxial grains with size distribution while being increased from 60 to 200 oC. The performed XRD analysis has revealed that perovskite films crystallized at different temperatures, and substrate morphology had no significant difference in the crystal structure, while corresponding PL responses slightly differed. As a result of all optimization procedures, we have developed high-quality samples where the highly-ordered Si NW array, fabricated employing microsphere mask etching, provides uniform light emission over the PeLEC pixels both on rigid and flexible substrates. This paves the way for further developing bright and uniform flexible light-emitting devices.

    Appendix A. Supplementary material

    Supplementary materials to this article can be found online at https://doi.org/10.1088/1674-4926/24120010.

    [27] N Doebelin, R Kleeberg. Profex: A graphical user interface for the Rietveld refinement program BGMN. J Appl Crystallogr, 48, 1573(2015).

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    Viktoriia Mastalieva, Anastasiya Yakubova, Maria Baeva, Vladimir Neplokh, Dmitry M. Mitin, Vladimir Fedorov, Alexander Goltaev, Alexey Mozharov, Fedor Kochetkov, Andrei S. Toikka, Ramazan Kenesbay, Ekaterina Vyacheslavova, Alexander Vorobyev, Kristina Novikova, Dmitry Krasnikov, Jianjun Tian, Albert G. Nasibulin, Alexander Gudovskikh, Sergey Makarov, Ivan Mukhin. Green perovskite CsPbBr3 light-emitting electrochemical cells with distributed Si nanowires-based electrodes for flexible applications[J]. Journal of Semiconductors, 2025, 46(7): 072801

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    Paper Information

    Category: Research Articles

    Received: Dec. 6, 2024

    Accepted: --

    Published Online: Aug. 27, 2025

    The Author Email: Anastasiya Yakubova (AYakubova)

    DOI:10.1088/1674-4926/24120010

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