Investigation on Halide Perovskite Nanostructures and Thin Films for Optoelectronic and Solar Cell Applications

Introduction

Halide Perovskite Semiconductors

The term 'perovskite' refers to the mineral form of calcium titanate (CaTiO3), which was discovered in 1839 by the German mineralogist Gustav Rose and named after the Russian mineralogist Lev Perovski. Among the large group of halide perovskite materials, organic-inorganic methylammonium lead halide (CH3NH3PbX3) and all-inorganic cesium lead halide (CsPbX3) have shown great promise in solar cells and other optoelectronic applications.

Crystal Structure of Halide Perovskite

To form mixed halide perovskite or doping in perovskite, A, B and/or X sites can be replaced partially or completely with alternative elements/molecules of similar size. Above 330 K, the tetragonal phase is transferred to the cubic phase (space group Pmm), and below 160 K the tetragonal phase of CH3NH3PbI3 is converted to the orthorhombic phase (space group Pnma).

Band Structure of Halide Perovskite

CH3NH3PbBr3 crystals exhibit three different phases, orthorhombic (<145 K), tetragonal (145-237 K) and cubic (>237 K).9 Note that at room temperature, CH3NH3PbI3 shows the tetragonal phase while CH3NH3PbBr3 shows the percute phase. At room temperature, CsPbX3 possesses a thermodynamically preferred orthorhombic structure with space group Pnma.10 Note that different perovskite polymorphs have a strong influence on their optoelectronic properties.

Key Properties of Halide Perovskite

Tunable Light Absorption and Emission
Superior Charge Transport
Defects and Traps

Since perovskite is a direct band gap semiconductor, it shows strong photoluminescence (PL) emission due to the band edge excitonic recombination. Due to chemical or structural changes caused by ion migration, the trap density in halide perovskites can increase, leading to poor device performance.

Fabrication of Perovskite Films and Nanostructures

Fabrication of Perovskite Films
Fabrication of Perovskite Nanostructures

The SEM image of (b) the PbI2 film, (c) the annealed perovskite layer formed by an interdiffusion process, and (d) the annealed perovskite film spun from the premixed PbI2 and CH3NH3I solution. 1.7(a) shows the schematic illustration of the deposition of perovskite films by the dual-source vapor deposition process.

Applications of Metal Halide Perovskite

Perovskite Solar cells
Photodetectors .…
Light-Emitting Devices

In the polycrystalline film, grain boundaries are formed due to breaks in the crystal structure of the material. The responsivity of the hybrid device increases with the increase in applied bias due to the enhanced photogenerated carrier separation (see Fig.

Challenges in the Fabrication and Applications of Halide Perovskites

The toxicity of lead to humans and the environment has been a challenge for the development and future applications of perovskite-based devices. However, lead-free devices have poor environmental and storage stability compared to lead-based devices.117 Further investigation is required for lead-free perovskite.

Focus of the Present Thesis

Perovskite-based devices can degrade when exposed to thermal stress, light, oxygen, or moisture.116 Further research is required to gain deeper insights into degradation and ion migration in halogenated perovskites. Replacing lead with other metal atoms in high concentration in perovskites has been challenging and requires a thorough investigation.

Organization of the Thesis

This may be due to the high surface area of the Si NWs of sample NW2. The FFT pattern shown in the right inset confirms the good crystallinity of the NCs.

Controlled Fabrication of Perovskite Nanoparticles with High

Introduction

The size of the perovskite NPs was controlled by controlling: (a) the porosity of NWs during the etching process, and (b) the concentration of perovskite precursor solution. Effect of the size of the perovskite NPs on the PL emission energy and the PL quantum yield (QY) is thoroughly studied.

Experimental Details

Sample Preparation …

Growth of Mesoporous Si NWs …
Synthesis of CH 3 NH 3 PbI 3

Characterization Techniques

Our study revealed that the decrease in particle size, the quantum confinement of carriers in the perovskite NPs and the reabsorption of the photon emitted from Si NWs by the perovskite NPs followed by the PL emission are primarily responsible for the great enhancement of PL by the perovskite NPs. perovskite NPs on mesoporous Si NW template. The PL QY of the samples was measured with an integrating sphere (FM-SPHERE, Horiba) attached to the fluorimeter.

Results and Discussion

Morphology and Structural Analyses ….…

FESEM and TEM Analyses …
XRD Analysis

Diffuse Reflectance Spectroscopy
Steady-State Photoluminescence Study
Time-Resolved Photoluminescence Study

We also measured the PL quantum yield of the Pe NPs in different samples. Note that the average size of the Pe NPs is smallest when the perovskite concentration is lowest.

Conclusions

A comparison of the PL spectra of pristine heterostructure 1L-MoS2 and 1L-MoS2/CsPbBr3 NCs is shown in the inset of Fig. Note that the intensity of the carbon 1s peak is relatively higher in the Pe film, as expected. We have tuned the size (diameter 3-30 nm) and PL properties of CH3NH3PbI3.

Origin of Strong Cathodoluminescence and Fast Photoresponse from Embedded

Introduction

To overcome this problem, an alternative approach is to grow Per NPs/NCs confined on porous inorganic templates. The nature and origin of the CL emission from the Per group NPs in the NW group was systematically studied for NPs of different sizes and different energies of the incident electron beam. We investigated the air stability of Per NPs embedded in the middle of the NW array, and it showed higher performance compared to the bulk film.

Experimental Details

Sample Preparation

Growth of Mesoporous Si NWs
Synthesis of CH 3 NH 3 Br
Synthesis of CH 3 NH 3 PbBr 3 NPs

The origin of the enhanced optical absorption and high PL QY from Per NPs was investigated. After spin-coating, the samples were annealed on a hot plate at 85 °C for 15 min, resulting in the formation of Per NPs on the surface of the NWs. To compare the properties of Per NPs with their bulk counterpart, a 0.6 M precursor solution was deposited on a bare silicon wafer to form a perovskite film, referred to as a bulk Per film.

Results and Discussion

Morphology and Structural Analyses

FESEM and FETEM Analyses
XRD Analysis

Diffuse Reflectance Spectroscopy
Cathodoluminescence Study
Photoluminescence Study
Stability of Per NPs under High Humidity Ambient
Low-Temperature Photoluminescence Study
Performance of a Per NP/Si NW Heterojunction Photodetector

These images also clearly show the uniform decoration of the Per NPs on the NW surface. 3.7(a), it is evident that the PL intensity of Per NPs is dramatically higher than that of the Per film. 3.9(c) shows the variation of the integrated PL intensity with temperature for Per NPs and the Per film.

Conclusions

4.11(c) shows a comparison of the PL spectra of the CsPbBr3 NCs film and the 1L-MoS2/CsPbBr3 NCs film. The improved performance of the PD heterojunction is attributed to the efficient photogenerated electron transfer from CsPbBr3. Interestingly, as the substrate temperature (for PbI2 deposition) increased, the photovoltaic performance of the corresponding devices improved significantly, as shown in Fig.

Solid-State Synthesis of Stable and Color Tunable Perovskite Nanocrystals and

Introduction

Here, we report a facile, room-temperature, nearly solvent-free, large-scale solid-state synthesis of highly luminescent CsPbX3 NCs with emission across the visible region. The structural and optical properties of the dye-tunable all-inorganic perovskite composite and NCs were systematically studied. These highly luminescent perovskite NCs were used for color-tunable light-emitting devices using poly(methacrylate methacrylate) (PMMA) NC composite films in commercial UV LED chips, which show superior operational stability (>100 h).

Experimental Details

Sample Preparation and Device Fabrication

Solid-state Synthesis of CsPbX 3 NCs
Construction of Color-tunable LEDs

Computational Methodology

The spectral photoresponsivity of PD was measured using a Xenon lamp (Newport) with a manual monochromator (Newport) and a source meter (Keithley 2400). One of the primary focuses of this study is the charge transfer mechanism between MoS2 and CsPbBr3 NCs. To explore the possible charge transfer between the surface and the NCs system, we have obtained charge density distribution of the composite system while doing self-consistent electronic structure calculation.

Results and Discussion

Synthesis Strategy of CsPbX 3 NCs

FETEM Analysis
XRD and XPS Analyses

Absorbance and Photoluminescence Studies
Stability Study
Low-Temperature Photoluminescence Study
Applications in Light Emitting Diodes
Electronic Structure Calculation and Charge Transfer Mechanism in 1L-

The enhanced optical absorption of the 1L-MoS2/CsPbBr3 NCs hybrid film is used for the efficient photodetection by the heterojunction PD. The topography profiles of the pristine 1L-MoS2 and hybrid 1L-MoS2/CsPbBr3 are shown in Figs. Thus, the significant improvement in response time, i.e. rise and fall time of the PC, attributed to the efficient charge separation at the 1L -MoS2/CsPbBr3 heterojunction.

Conclusions

This may be due to the decrease in crystallinity and structural defects of NCs with high Mn substitution content. 5.6(d) shows a comparison of the PL intensity of the excitonic and Mn-related peaks of CsPbCl3 doped with 3% Mn. 6.9(b) shows the spectral response of the pristine and hybrid photodetector with PCBM as ETL.

Origin and Tunability of Dual Color Emission in Highly Stable Solid-State

Introduction

In this chapter, we investigated the origin and tunability of dual-color emission from solid-state synthesized, highly luminescent Mn-doped CsPbCl3 NCs with high Mn substitution. Low-temperature PL and excitation intensity-dependent PL studies reveal important insights into the origin of Mn-related orange-red PL in the dual-color emitting Mn-doped CsPbCl3 NCs. The implications of these results for a low-cost synthesis strategy for Mn-doped CsPbCl3 NCs with superior environmental stability for WLED are discussed.

Experimental Details

Sample Preparation and Device Fabrication …. …

Solid-State Synthesis of Undoped and Mn-doped CsPbCl 3
Preparation of the NCs-PMMA film and fabrication of

Furthermore, it is demonstrated that Mn-doped CsPbCl3 NCs synthesized by the physical milling method exhibit superior stability due to the high Mn doping efficiencies achieved through the non-equilibrium synthesis conditions. Furthermore, white LED devices were constructed by using blue-emitting CsPbCl1.5Br1.5 NCs, green-emitting CsPbBr3 NCs, and orange-red emitting 10% Mn-doped CsPbCl3 NCs composite films with PMMA. Mn-doped CsPbCl3 NCs/PMMA solutions were drop-casted sequentially on top of the film.

Results and Discussion

Synthesis Strategy and Doping of NCs by Physical Milling

FETEM Analysis
XRD Analysis
XPS Analysis

Absorbance Study
Photoluminescence Study
Temperature-Dependent Photoluminescence Study
Stability Study
Application of Mn Doped NCs in the WLEDs

Interestingly, with the increase of Mn doping concentration, the absorption edges of the doped samples gradually blue shift, as shown in Figure 5.6(a), with the increase of Mn doping content from 0% to 30%, the relative intensity of the Mn-related emission is systematically increased with respect to the intensity of the excitonic PL emission. Note that the PL lifetime of the Mn-related state is on the order of milliseconds (ms), while the excitonic emission lifetime is on the order of nanoseconds (ns) (discussed later).

Conclusions

These devices show improved ambient stability compared to the case of PbI2 layer deposited at room temperature. Interestingly, the absorption of the Pe films was found to be higher for the PbI2 film deposited at higher substrate temperatures. The photovoltaic performance of Pe solar cells using PbI2 films deposited at different substrate temperatures is shown in Fig.

Plasmonic Hole-Transport-Layer Enabled Self-Powered Hybrid Perovskite

Introduction

Interestingly, these PDs can operate without any external bias due to built-in potential, although the stability and cost are the main issues for these devices, as various organic HTL and ETL accelerate the degradation of the device.8 Another important problem with perovskite photodetector is the poor performance of the device when manufactured under ambient conditions. For the deposition of the active Pe layer, we used a novel N2 gas-assisted rapid crystallization method to form a uniform Pe layer in ambient air. We investigate the mechanism of improved performance of the hybrid photodetector (with Ag NPs) showing high responsivity and high detectivity along with faster photoresponse than the pristine (without Ag NPs) device.

Experimental Details

Synthesis of CH 3 NH 3 I
Synthesis of Ag Nanoparticles
Device Fabrication

We found that conventional spin-coating in ambient atmosphere produces rough and non-uniform Pe film, and the multiple drop casting using anti-solvent results in non-uniform Pe layer and poor reproducibility of the device. To form a uniform Pe layer, we used dry N2 gas flow during the spin coating for rapid crystallization and uniformity of the film. Schematic of the fabrication process and device configuration of the hybrid photodetector is shown in Fig.

Results and Discussion

Optical Studies
Performance in Photodetection
Mechanism of Improved Photodetection

Magnified view of the photocurrent growth and decay of the Ag NP (Ag20) device. 6.6 (d), it is clear that the responsivity of the hybrid device is significantly enhanced by the incorporation of plasmonic Ag NPs. The enhanced responsivity of the hybrid device in the entire visible range is attributed to the enhanced carrier extraction and transport by the plasmonic Ag NPs.

Conclusions

The high roughness, together with the many holes in the Pe30 layer, is due to the higher roughness of the corresponding PbI2 layer deposited at Ts = 30°C. This high-quality Pe layer is responsible for the high optical absorption of the film (discussed later). Statistical distributions of (c) Jsc, (d) Voc, (e) FF and (d) the corresponding PCE of the different groups of perovskite solar cells.

Vacuum Deposited PbI 2 Film Grown at Elevated Temperature for Improved

Introduction

Thus, it is imperative to explore alternative green methods for Pe film deposition without toxic solvents. These PbI2 layers were dipped in CH3NH3I solution and annealed at 90ºC, resulting in the growth of Pe film. The Pe film with PbI2 film deposited at a substrate temperature of 150ºC exhibits pinhole-free and compact superior morphology.

Experimental Details

Synthesis of CH 3 NH 3 I
Growth of CH 3 NH 3 PbI 3 film and Fabrication of Solar Cells 154

Thus, one can look for a combined method, such as vapor-solution deposition, to grow high quality Pe film. Thus, 66% increase in PCE is attributed to improved quality Pe film with pinhole-free compact morphology and superior optical absorption in solar cells using the PbI2 layer deposited at elevated temperature of 150ºC. Different characterizations were done immediately after the fabrication to avoid the degradation of Pe film.

Results and Discussion

Morphology Analysis
Structural Analysis
Optical Studies
Performance of Perovskite Solar cells

Due to the smooth and compact nature of the large grain Pe film, the recombination of photogenerated carriers is reduced (discussed later). Thus, the enhancement of carrier separation and transport in the Pe150 film leads to superior performance of the corresponding solar cell. Note that the Jsc of solar cells depends mainly on the absorption of the active material in the solar cells.