PHOTOPHYSICAL STUDIES IN TWO-DIMENSIONAL (2D) PEROVSKITE

Abstract

Over the last decade, organic-inorganic hybrid perovskites (OIHPs) have been at the forefront of the most tremendous advances in the field of new age semiconductors for low-cost optoelectronics. Ever since Miyasaka's first report in 2009 on the use of lead halide perovskite as a light-harvesting material, the field of halide perovskite has made remarkable progress, with the power conversion efficiency (PCE) exceeding 25%. In addition, substantial advancements have been made in the development of perovskite LEDs (PeLEDs), with external quantum efficiencies (EQEs) surpassing 23% for green and red emission wavelengths, and exceeding 20% for near-infrared (NIR) emission wavelengths. Notably, the highly tunable optical and electronic properties of perovskite such as low-temperature cost-effective facile solution processability, panchromatic absorption, higher charge carrier diffusion length, tunable bandgap, high defect tolerance, appropriate exciton binding energies, reliable colour purity along with excellent mechanical flexibility makes them suitable candidates for the next generation optoelectronic devices. Due to these exceptional opto-electronic properties, the additional applications of OIHPs beyond photovoltaics have been extensively explored for example light emitting diodes, neuromorphic computing and memristors, photoelectrochemical cells, fuel cells for H2 generation, supercapacitor and batteries for energy storage, field effect transistors, radiation detectors and many more. Despite the favourable opto-electronic properties of perovskite semiconductors, their intrinsic instability to moisture, heat and light, and toxicity of Pb are still remains the critical obstacles that limits their prospective market uptake. Moreover, perovskite is plagued by abundant intrinsic defects and anomalous grain boundaries making them unstable for a variety of applications. More recently, two-dimensional (2D)/ quasi-2D perovskite (number of inorganic layers n > 3), or mixed dimensional 2D/3D perovskite, has emerged as one of the most promising strategies for mitigating the chemical and intrinsic instability issues. The possibility of incorporating bulky organic cations in 2D perovskite relaxes the size restrictions imposed by the Goldschmidt tolerance factor, thereby expanding the range of structurally versatile low dimensional perovskites. Unlike traditional 3D perovskites with the chemical formula ABX3, where the inorganic or organic cations (A) occupy the gaps between lead halide octahedra (BX6)4–, 2D perovskites incorporate larger organic cations (L) as spacers. These bulky cations serve to isolate the layers of inorganic metal halide octahedra, forming quantum well superlattices. The hydrophobic long chain spacer cations can be effective in isolating the ionic lattice of inorganic perovskite octahedrons from the ambient water molecules. The organic spacer responsible for dielectric confinement in 2D perovskite consequently influence its electronic and structural properties. These 2D or quasi-2D perovskites have tunable photophysical properties including higher exciton binding energies,multiple quantum well structures, natural cascaded energy funneling, and band gap versatility, in addition to their rich structural diversity and excellent moisture stability. These properties are critical and have rendered 2D perovskites remarkably interesting for a variety of high-performance light-emitting and photovoltaic applications. Despite the aforementioned favourable optoelectronic properties, there are still many limitations that affect the performance of these 2D perovskite semiconductors and corresponding optoelectronic devices. For instance, for photovoltaic applications, the insulating nature of organic cations, long chain length, their stacking orientation and concentration can severely affect the movement of photogenerated charge carriers in 2D perovskites. Similarly random distribution of inorganic perovskite quantum wells with distinct well width, their parallel alignment to the substrate is also critical to facilitate efficient charge extraction. Subsequently, the primary reasons for low performance in quasi-2D perovskite LEDs include the random distribution of multiple quantum well layers due to self-assembly process, inefficient energy funneling due to polydisperse quantum well structures and imbalance charge injection. Additionally, the presence of high density of surface defect states such as halide anion vacancies/organic cation vacancies primarily contribute toward nonradiative recombination losses in 2D perovskites and are detrimental for the performance of both SCs and LEDs. This thesis mainly focuses on addressing the performance-limiting challenges of n-layered 2D perovskite films and optoelectronic devices. We have used the surface and bulk passivation strategies to improve the crystal orientation, phase distribution, morphology and curing various defect states in the 2D perovskite films. Our initial work involved the development of thin films of Ruddlesden Popper (RP) phase 2D perovskites, incorporating butyl amine (BA) organic spacers. We performed phase engineering and fabricated (BA)2(MA)n-1(Pb)nBr3n+1 films with varying n values and treated the resulting n=4 phase films with Lewis base molecules using an antisolvent-based surface passivation approach to enhance their radiative emission properties. Next, we investigated the potential of aromatic diamine organic spacers in structurally beneficial Dion-Jacobson (DJ) phase 2D perovskites. Through additive engineering, we were able to promote vertical crystal orientations, narrowed phase distribution, and passivate defect states. As a result of the enhanced morphology and defect passivation in the DJ phase 2D perovskite thin films, we were able to achieve high efficiency DJ phase 2D perovskite solar cells (PSCs). Following up this work, we utilized a low-dimensional interface engineering approach to combine high-efficiency 3D perovskite devices with improved humidity-resistant DJ phase 2D perovskite layers. The favourable energy band alignment, efficient carrier extraction, and effective defect passivation collectively contributed to achieving high efficiencies in 2D/3D PSCs of up to 21%, with minimal interfacial losses and high open-circuit voltages. In addition, we investigated the improved operational stability of the fabricated 2D/3D PSCs under various aging conditions. Lastly, we explored a series of n-layered 2D perovskite single crystals containing butyl amine (BA) organic spacers, which exhibited higher phase purity and lower defect density compared to polycrystalline thin film systems. We further investigated the potential of these 2D perovskite single crystals for X-ray detection and imaging applications. To achieve the aforementioned goals, this thesis is structured into the following 6 chapters: In chapter 1, firstly the statistics of global energy demand and supply is reviewed, importance of renewable energy sources is established. Then the development of unconventional semiconducting energy resources is classified. Afterwards, the brief introduction of organic -inorganic hybrid halide perovskite is described. In brief, the fundamental and basic structure of perovskite is explained, followed by an overview of the development of various optoelectronic device architectures based on halide perovskite. Additionally, the working mechanisms of these devices is also discussed along with their current state of the art relevance. Furthermore, the challenges of perovskite optoelectronic devices in terms of their inherent structural instability, disorders and defect states existence is explained in details. The following section delved into the significance of low dimensional 2D perovskites in addressing the chemical and intrinsic instability concerns for 3D perovskites and also provide a detailed explanation of the structural and optoelectronic properties of 2D perovskites. Lastly, a concise literature review of optoelectronic devices based on 2D perovskites is presented, highlighting the challenges and opportunities associated with 2D perovskite film and device fabrication. Moreover, a comprehensive discussion on strategies to improve the performance of these devices is also included. In chapter 2, we have shown the order of magnitude enhancement of radiative emission in butylamine (BA)-based quasi-2D perovskite (BA)2(MA)n−1PbnBr3n+1 after passivating with two different Lewis bases-a small organic molecule triphenylphosphine oxide (TPPO) and an insulating polymer polymethyl methacrylate (PMMA). The reduction in crystal grain size was observed after passivation, attributed to the complexation of the passivating molecules (PM) on the surface and nanocrystal pinning (A-NCP) phenomena. Both the steady-state and time-resolved photoluminescence study confirmed significant enhancement in fluorescence intensity and improved average lifetime (τavg. = 19.4 ns) after surface passivation. The interaction mechanism between the layered perovskite and PMs was probed with FTIR spectroscopy, XPS, and KPFM study. All these studies confirmed that the C=O group in PMMA and P=O group in TPPO decrease the acceptor-type defects (uncoordinated Pb2+ and Br vacancies) in these RP phase perovskites. Furthermore, the stability of the passivated film enhanced significantly, as confirmed by contact angle measurement. Our study establishes that uncoordinated Pb2+ passivation by a Lewis base provides a viable strategy for photoluminescence (PL) lifetime, intensity, and stability enhancement in quasi-2D perovskite films. In chapter 3, we have first discussed phase engineering in Dion Jacobson (DJ) phase 2D perovskite by using two different aromatic organic spacer amine groups and further explored the role of additive engineering in controlling perovskite crystallization, improving film morphology and in passivation of the defect states. Here, we have reported the eclipsed DJ phase 2D perovskite by using 1,5-naphthalene diammonium (NDA) and p-Xylylene diammonium (XDA) cations and subsequently treated the fabricated films with ammonium thiocyanate (NH4SCN) additive to further improve the film crystallinity, out of plane orientation and carrier mobility. We observe that 2 mol% NH4SCN surface treatment in DJ phase perovskite leads to better film morphology and improved crystallinity, as confirmed by XRD and SEM. Time-resolved photoluminescence spectroscopy (TRPL) and steady-state space-charge limited current (SCLC) mobility measurement reveal significant reduction of trap-assisted non-radiative recombination and improvement of carrier mobility in the thiocyanate passivated perovskite. Consequently, PCE of the NH4SCN treated (𝑁𝐷𝐴)(𝑀𝐴)3(𝑃𝑏)4(𝐼)13 and (𝑋𝐷𝐴)(𝑀𝐴)3(𝑃𝑏)4(𝐼)13 perovskite devices enhanced to 15.08% and 17.05% respectively. We have further studied the intensity-dependent J-V characteristics, which demonstrate reduction of ideality factor confirming the effective suppression of trap assisted non-radiative recombination, consistent with the transient PL results. The electrochemical impedance spectroscopy (EIS) confirms the improved charge carrier transport in the NH4SCN additive treated devices. Additionally, the NH4SCN-assisted unencapsulated device exhibits long-term operational stability under a variety of ageing situations, demonstrating the better resilience of the NH4SCN-treated quasi-2D phase to environmental stimuli. This study opens up the strategy for developing high-efficiency and stable 2D perovskite solar cells. In chapter 4, we have designed stable 2D/3D interfaces based on NDA and XDA cations as the building block for 2D perovskite. In this study, we utilized the 1,5 diammonium naphthalene iodide (NDAI) and XDAI bulky organic spacer cations in IPA antisolvent and used solution-based approach for the fabrication of 2D/3D heterostructures. The organic spacer induced 2D perovskite perform multifunctional roles in passivating the anionic iodide/uncoordinated Pb+2 vacancies in the 3D perovskite as well as facilitating charge carrier transfer by improving energy band alignment at the perovskite/HTL interface. As a result, the 2D/3D Perovskite solar cells (PSCs) have shown improved fill factor and open circuit voltage with the remarkably enhanced PCE in comparison to reference MAPbI3 PSCs. The increased PCE in 2D/3D PSCs is mainly attributed to the reduced defect density and suppressed non-radiative recombination losses. Moreover, the hydrophobic 2D capping layer endows the 2D/3D heterojunction perovskites with exceptional moisture, thermal and UV stability, highlighting the promise of highly stable and efficient 2D/3D PSCs. In chapter 5, we have fabricated the homologous series of phase pure 2D perovskite single crystals with lesser defect density by using a conventional slow cooling method. We further investigated the structural and optoelectronic properties of the series of (BA)2(MA)n-1(Pb)nI3n+1 2D perovskite single crystals. Afterwards, the radiation detection characteristics of these (BA)2(MA)n-1(Pb)nI3n+1 are intensively investigated, including their electronic properties, charge transport behavior, X-ray detection capabilities. Among (BA)2(MA)n-1(Pb)nI3n+1 crystal series, n=1 phase (BA)2PbI4 single crystal X-ray detectors exhibited a noteworthy detection performance with a sensitivity of 148 μCGy-1cm-2 at 10 V mm-1 applied electric field, an ultralow detection limit less than 241 nGy/s, a very low and stable dark current and a quick response time. Moreover, the fabricated devices have shown remarkably stable response for continuous X-ray exposure and ultrahigh storage stability of over 4 months, assessing the material robustness and stability. Finally, high-resolution X-ray imaging is demonstrated by using these 2D perovskite single crystal-based detectors. Finally, in chapter 6, we have summarized the conclusive findings from all previous research works and proposed the future advancement strategies.