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Perovskite solar cells (PSCs) have emerged as a promising renewable energy technology in recent years. However, their path towards industrial production and commercialization presents challenges that demand innovative solutions. This doctoral thesis is dedicated to addressing two pivotal issues in the development of PSCs: (1) the development of a fabrication method compatible with traditional semiconductor industry processes and (2) the exploration of approaches to improve device performance and stabilityThe present thesis separates these two issues into three parts. First, the fabrication method of MA-free perovskites via vacuum vapor deposition process is proposed. CsBr is added to FAPbI3giving FAxCs1-xPbI3-xBrx. to maintain the stable black perovskite phase. Furthermore, the effect of the thermal annealing process on perovskite films with different stoichiometric ratios was explored. It was found that thermal annealing enhances the crystallinity of both FAI-poor and stoichiometric films. For FAI-poor perovskite films, an increase in absorption as well as reduction in defect concentrations was achieved through annealing process. However, the opposite effect was observed for FAI-rich films. By optimizing the fabrication processes, a solar cell device with an efficiency of 16.6% was acquired. However, these vapor-deposited devices still exhibit lower performances compared to those prepared using solution processes, indicating the need for further improvements in perovskite layer composition and interfacial properties to enhance their efficiency. The second part of this dissertation demonstrates the concept of phase heterojunction (PHJ) solar cells by combining two polymorphs of the same material from the evaporation process (γ-CsPbI3) and solution process (β-CsPbI3). It was discovered that the photovoltaic parameters of these PHJ devices significantly surpass those of either single-phase device, resulting in a maximum power conversion efficiency of 20.1%. The enhancement comes from the following three factors: efficient passivation of the β-CsPbI3 by the larger bandgap γ-CsPbI3, an increase in the built-in potential of the PHJ devices enabled by the energetic alignment between the two phases, and enhanced absorption of light resulting from narrower band-gap β-CsPbI3 in the PHJ structure. The approach demonstrated here offers new possibilities for developing photovoltaic devices based on polymorphic materials. In the final part, a 1D-3D dimensional junction formed spontaneously by a two-step process is presented. It was found that the morphology, energy alignment, and defects of the buried interface were improved by creating 1D perovskites. In addition, strip-shaped PbI2 domains and voids are eliminated which ultimately enhances photovoltaic performance and stability. These improvements can be attributed to the passivation effect of the 1D perovskite and better energetic alignment. Furthermore, this dimensional junction strategy also shows potential for use in large-area devices.

This thesis consists of three main studies: magnetic field effects in thermally activated delayed fluorescent (TADF) organic light emitting diodes (OLEDs), magnetic field effects in bipolar and unipolar polythiophene (P3HT) devices and a study of hybrid organic/inorganic perovskite devices. Spin-dependent transport and recombination processes of spin-pair species have been detected by magnetic field effect (MFE) technique in carbon-based semi- conductor devices. Magneto-electroluminescence (MEL) and magneto-conductivity have been measured as a function of the applied magnetic field, B, in light emitting diodes. TADF materials have been used instead of simple fluorescent materials in OLEDs. We have observed very large magnetic response with TADF materials. The second study is magnetic field effects of regio-regular P3HT based OLED devices. P3HT is a well known semiconducting polymer, and its electrical properties such as magneto-conductance can be affected by an applied magnetic field. P3HT was chosen because it exhibits a sign change in magnetoresistance (MR) as the bias is increased. Unipolar and bipolar devices have been fabricated with different electrode materials to understand which model can be best to explain organic magnetoresistance effect, possibly depending on the operating regime of the device. Transport and luminescence spectroscopies were studied to isolate the different mechanisms and identify their fingerprints. The third study is on hybrid organic-inorganic perovskite devices. With the potential of achieving very high efficiencies and the very low production costs, perovskite solar cells have become commercially attractive. Scanning electron microscopy (SEM) images and absorption spectrum of the films were compared in single-step solution, two-step solution and solution-assisted vapor deposition techniques. Grain size, morphology and thickness parameters of perovskite films were studied within these techniques. Perovskite solar cells were fabricated and their efficiencies were measured.

Les pérovskites hybrides halogénées (ABX3) sont utilisées depuis cinq ans comme couches absorbantes pour de nouvelles cellules solaires à bas coût combinant les avantages des matériaux organiques (molécule A) et inorganiques (métal B et halogène X). Très récemment, des cellules solaires à boîtes quantiques à bases de pérovskites purement inorganiques ont également montré des efficacités prometteuses, ce qui en fait une alternative potentiellement stable et efficace à leurs cousins hybrides.Le but de cette thèse de doctorat est d'étudier et de mieux comprendre les instabilités structurelles et thermodynamiques de ces pérovskites halogénées, avec un focus sur la pérovskite purement inorganique CsPbI3.Dans un premier temps les propriétés vibrationnelles et électroniques des différentes phases de CsPbI3 sont étudiées grâce à différentes techniques ab-initio, dont la plupart sont basées sur la théorie de la fonctionnelle de la densité (DFT) et son approche en réponse linéaire (DFPT). Alors que la phase γ noire, cruciale pour les applications photovoltaïques, se comporte de manière harmonique autour de l'équilibre, pour les trois autres phases nos calculs de phonons froids révèlent une instabilité de double puits au centre de la zone de Brillouin. Nos calculs montrent également que le terme d'entropie d'ordre-désordre lié à ce double puits est crucial pour empêcher la formation de la phase pérovskitoïde jaune. Nous analysons ensuite en détail les changements structurels et l’effet Rashba dynamique le long de trajectoires de dynamique moléculaire à la lumière de ces résultats.La seconde partie de la thèse porte sur la stabilité thermodynamique de la pérovskite hybride MAPbI3. Notre étude expérimentale par ellipsométrie apporte une meilleure compréhension de la décomposition chimique de MAPbI3 en ses deux précurseurs, l’iodure de méthylamonium et l'iodure de plomb, que nous avons prédite grâce à des calculs de diagrammes de stabilité DFT et que nous confirmons par diffraction des rayons X. Enfin, nous démontrons que la pérovskite hybride MAPbI3 se comporte davantage comme les composés inorganiques (grande constante diélectrique, faible énergie de liaison des excitons) que comme les matériaux organiques (faible constante diélectrique, forte énergie de liaison d'exciton). / Hybrid halide perovskites (ABX3) have emerged over the past five years as absorber layers for novel high-efficiency low-cost solar cells combining the advantages of organic (molecule A) and inorganic (metal B, halogen X) materials. Very recently, fully inorganic perovskite quantum dots also shown promising efficiencies, making them a potentially stable and efficient alternative to their hybrid cousins.The aim of this PhD thesis is to study and better understand both the structural and thermodynamic instabilities of these halide perovskites, with a specific focus on purely inorganic CsPbI3 structures.We first use various ab-initio techniques, the majority of which are based on Density Functional Theory (DFT) and its linear-response approach (DFPT), to investigate the vibrational and electronic properties of the different phases of CsPbI3. While the black γ-phase, crucial for photovoltaic applications, is shown to behave harmonically around equilibrium, for the other three phases frozen phonon calculations reveal a Brillouin zone center double-well instability. We also show that avoiding the order-disorder entropy term arising from these double-well instabilities is key in order to prevent the formation of the yellow perovskitoid phase, and evidence a Rashba effect when using the symmetry breaking structures obtained through frozen phonon calculations. We then analyze the structural changes and the dynamical Rashba splitting along molecular dynamics trajectories in the light of our findings.In a second phase, we investigate the thermodynamical stability of hybrid perovskite MAPbI3. Our experimental ellipsometry-based study brings better understanding of the chemical decomposition of MAPbI3 into its two precursors, methylammonium and lead iodides, which we predicted using DFT stability diagram calculations and which we confirm by X-Ray diffraction. Last, we prove that hybrid perovskite structure MAPbI3 behaves more like inorganic compounds (high dielectric constant, low exciton binding energy) than like organic materials (low dielectric constant, high exciton binding energy).

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第20405号 / 工博第4342号 / 新制||工||1673(附属図書館) / 京都大学大学院工学研究科高分子化学専攻 / (主査)教授 大北 英生, 教授 赤木 和夫, 教授 木村 俊作 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Hybride Metall-Halogen Perowskit basierte Solarzellen haben im letzten Jahrzehnt eine noch nie dagewesene Entwicklung hinsichtlich ihrer Effizienzsteigerung erzielt. Dies ging mit einem rapiden Anstieg des Interesses der Forschungsgemeinschaft einher. Ein wichtiger Faktor für die Realisierung von Forschungsergebnissen ist der Übergang von Laborskalen zu industriellen Dimensionen, was grundlegende Anpassungen im Herstellungsprozess notwenig macht. Hier hat sich die physikalische Gasphasenabscheidung für andere Materialklassen bereits als gute Lösung bei der Herstellung von qualitativ hochwertigen Dünnschichten erwiesen. In dieser Arbeit wird die Vakuumabscheidung von FA₁ ₋ ₓCsₓPbI₃ ₋ ₓBrₓ für die Anwendung in Solarzellen durch Dreiquellenkoverdampfung erforscht. Durch eine Kombination aus optischen und Röntgen-basierten Messverfahren konnte eine Veränderung der Aufnahme des organischen Halogensalzes, Formamidiniumiodid (FAI), in die Perowkitverbindung in Abhängigkeit vom Kammerdruck festgestellt werden. Dadurch tritt eine Stöchiometrieänderung auf, welche sich in einer Bandlückenverschiebung niederschlägt. Außerdem wurde eine Veränderung der Kristallitorientierung beobachtet. Diese Ergebnisse motivieren eine genauere Untersuchung des Verdampfungsverhaltens des organischen Halogensalzes. Mit Hilfe von Massenspektrometriemessungen und einer detaillierten Erfassung der Prozessparameter konnte eine Zersetzung von FAI während der Verdampfung festgestellt werden. Desweiteren wurden weitere Besonderheiten im Abscheideverhalten, wie zum Beispiel Verdampfunsgrenzen und Veränderungen des Toolingfaktors beobachtet. Die Ergebnisse leisten einen wesentlichen Beitrag zum tieferen Verständnis der Vakuumabscheidung von organischinorganischen Metall-Halogen Perowskiten.:Kurzfassung iv Abstract v List of publications vi 1. Introduction 1 2. Theoretical background 5 2.1. Basics of thin-_lm solar cells . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Status of current thin-_lm solar cell technologies . . . . . . . . . . . . 7 2.3. The Perovskite system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.4. Organic-inorganic mixed-halide metal perovskites . . . . . . . . . . . 14 2.5. Deposition methods of metal halide perovskite . . . . . . . . . . . . . 18 2.6. Vacuum deposition of perovskite . . . . . . . . . . . . . . . . . . . . . . 21 3. Experimental methods 27 3.1. Physical vapor deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.2. Vacuum deposition of perovskite layers . . . . . . . . . . . . . . . . . . 29 3.3. Analytical techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 4. Pressure dependent triple-source co-evaporation of methylammoniumfree perovskite 39 4.1. Precursor puri_cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 4.2. Film deposition procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 42 4.3. Crystallographic characterisation . . . . . . . . . . . . . . . . . . . . . . 43 4.4. Optical characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 4.5. Incorporation of FAI into thin-_lms . . . . . . . . . . . . . . . . . . . . . 50 4.6. Photovoltaic devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 5. Evaporation properties of formamidinium iodide 57 5.1. Degradation reactions of formamidinium iodide . . . . . . . . . . . . 57 5.2. Evaporation behaviour of formamidinium iodide . . . . . . . . . . . . 58 5.3. Theoretical considerations during the deposition process . . . . . . 61 5.4. Tooling behaviour of Formamidinium iodide . . . . . . . . . . . . . . . 63 5.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 6. Summary & Outlook 71 A. Appendix 75 List of Figures 81 List of Tables 82 List of Abbreviations 86 Bibliography 86 / In the last decade, hybrid metal halide perovskite-based solar cells have enjoyed an unprecedented surge in development within the research community. The next steps to further improve this technology will involve the transition from the laboratory to commercial-scale production, which will require adjustments in their fabrication processes. Here, physical vapour deposition has proven to be a good option for the fabrication of high-quality thin films for perovskites and other materials, like organic semiconductors. In this work, triple-source co-evaporation deposition of FA₁ ₋ ₓCsₓPbI₃ ₋ ₓBrₓ for the production of thin films for solar cell applications is investigated. With a combination of optical and X-ray-based measurement methods, a decrease in the incorporation of the organic halide salt formamidinium iodide (FAI) was found with increasing background pressure. This decrease results in a change in stoichiometry of the compound and, with it, a shift of the band gap. Furthermore, a change in crystallite orientation was observed. These findings motivate the examination of the evaporation behaviour of formamidinium iodide in more detail. With mass spectrometry measurements and detailed tracking of the process parameters, a degradation of FAI during evaporation was found. Furthermore, several effects of the deposition behaviour, evaporation limits, and tooling shifts were observed. These findings will be substantial for the deeper understanding of vacuum deposition of organic-inorganic metal halide perovskites, and will be significant in the expansion of perovskite-based solar technology.:Kurzfassung iv Abstract v List of publications vi 1. Introduction 1 2. Theoretical background 5 2.1. Basics of thin-_lm solar cells . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Status of current thin-_lm solar cell technologies . . . . . . . . . . . . 7 2.3. The Perovskite system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.4. Organic-inorganic mixed-halide metal perovskites . . . . . . . . . . . 14 2.5. Deposition methods of metal halide perovskite . . . . . . . . . . . . . 18 2.6. Vacuum deposition of perovskite . . . . . . . . . . . . . . . . . . . . . . 21 3. Experimental methods 27 3.1. Physical vapor deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.2. Vacuum deposition of perovskite layers . . . . . . . . . . . . . . . . . . 29 3.3. Analytical techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 4. Pressure dependent triple-source co-evaporation of methylammoniumfree perovskite 39 4.1. Precursor puri_cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 4.2. Film deposition procedure . . . . . . . . . . . . . . . . . . . . . . . . . . 42 4.3. Crystallographic characterisation . . . . . . . . . . . . . . . . . . . . . . 43 4.4. Optical characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 4.5. Incorporation of FAI into thin-_lms . . . . . . . . . . . . . . . . . . . . . 50 4.6. Photovoltaic devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.7. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 5. Evaporation properties of formamidinium iodide 57 5.1. Degradation reactions of formamidinium iodide . . . . . . . . . . . . 57 5.2. Evaporation behaviour of formamidinium iodide . . . . . . . . . . . . 58 5.3. Theoretical considerations during the deposition process . . . . . . 61 5.4. Tooling behaviour of Formamidinium iodide . . . . . . . . . . . . . . . 63 5.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 6. Summary & Outlook 71 A. Appendix 75 List of Figures 81 List of Tables 82 List of Abbreviations 86 Bibliography 86

The rapid development in perovskite solar cells (PSC) has generated a tremendous interest in the photovoltaic community. The power conversion efficiency (PCE) of these devices has increased from 3.8% in 2009 to a recent certified efficiency of over 20% which is mainly the product of the remarkable properties of the perovskite absorber material. One of the most important advances occurred with the replacement of the liquid electrolyte with a solid state hole conductor which enhanced PCE values and improved the device stability. Spiro-OMeTAD (2,2',7,7'-tetrakis(N,N'-di-p-methoxyphenylamine)- 9,9'-spirobifluorene) is the most common hole transport material in perovskite solar cells. Nevertheless, the poor conductivity, low charge transport and expensive synthetic procedure and purification have limited its commercialisation. Triphenylamines (TPA) like Spiro-OMeTAD are commonly employed due to the easy oxidation of the nitrogen centre and good charge transport. Other triarylamines have similar properties to Spiro-OMeTAD but are easier to synthesise. The aim of this doctoral thesis is to investigate different types of hole transport materials in perovskite solar cells. Three different series of triphenylamine-based HTM were designed, synthesised, characterised and studied their function in perovskite solar cells. A series of five diacetylide-triphenylamine (DATPA) derivatives (Chapter 3) with different alkyl chain length in the para position was successfully synthesised through a five step synthesis procedure. A range of characterisation techniques was carried out on the molecules including; optical, electrochemical, thermal and computational methods. The results show that the new HTMs have desirable optical and electrochemical properties, with absorption in the UV, a reversible redox property and a suitable highest occupied molecular orbital (HOMO) energy level for hole transport. Perovskite solar cell device performances were studied and discussed in detail. This project studied the effect of varying the alkyl chain length on structurally similar triarylamine-based hole transport materials on their thermal, optical, electrochemical and charge transport properties as well as their molecular packing and solar cell parameters, thus providing insightful information on the design of hole transport materials in the future. The methoxy derivative showed the best semiconductive properties with the highest charge mobility, better interfacial charge transfer properties and highest PCE value (5.63%). The use of p-type semiconducting polymers are advantageous over small molecules because of their simple deposition, low cost and reproducibility. Styrenic triarylamines (Chapter 4) were prepared by the Hartwig-Buchwald coupling followed by their radical polymerization. All monomers and polymers were fully characterised through electrochemical, spectroscopic and computational techniques showing suitable HOMO energy levels and desirable optoelectrochemical properties. The properties and performance of these monomers and polymers as HTMs in perovskite solar cells were compared in terms of their structure. Despite the lower efficiencies, the polymers showed superior reproducibility on each of the device parameters in comparison with the monomers and spiro-OMeTAD. Finally, star-shaped structures combine the advantages of both small molecules, like well-defined structures and physical properties, and polymers such as good thermal stability. Two star-shaped triarylamine-based molecules (Chapter 5) were synthesised, fully characterised and their function as hole-transport materials in perovskite solar cells studied. These materials afford a PCE of 13.63% and high reproducibility and device stability. In total this work provided three series of triarylamine-based hole transport materials for perovskite solar cells application and enabled a comparison of the pros and cons of different design structures: small-molecule, polymeric and star-shaped.

Solar energy as the most abundant source of energy is clean, non-pollutant, and completely renewable, which provides energy security, independence, and reliability. Organic-inorganic hybrid perovskite solar cells (PSCs) revolutionized the photovoltaics field not only by showing high efficiency of above 22% in just a few years but also by providing cheap and facile fabrication methods. In this dissertation, fabrication of PSCs in both ambient air conditions and environmentally controlled N2-filled glove-box are studied. Several characterization methods such as SEM, XRD, EDS, Profilometry, four-point probe measurement, EQE, and current-voltage measurements were employed to examine the quality of thin films and the performance of the PSCs. A few issues with the use of equipment for the fabrication of thin films are addressed, and the solutions are provided. It is suggested to fabricate PSCs in ambient air conditions entirely, to reduce the production cost. So, in this part, the preparation of the solutions, the fabrication of thin films, and the storage of materials were performed in ambient air conditions regardless of their humidity sensitivity. Thus, for the first part, the fabrication of PSCs in ambient air conditions with relative humidity above ~36% with and without moisture sensitive material, i.e., Li-TFSI are provided. Perovskite materials including MAPbI3 and mixed cation MAyFA(1-y)PbIxBr(1-x) compositions are investigated. Many solution-process parameters such as the spin-coating speed for deposition of the hole transporting layer (HTL), preparation of the HTL solution, impact of air and light on the HTL conductivity, and the effect of repetitive measurement of PSCs are investigated. The results show that the higher spin speed of PbI2 is critical for high-quality PbI2 film formation. The author also found that exposure of samples to air and light are both crucial for fabrication of solar cells with larger current density and better fill factor. The aging characteristics of the PSCs in air and vacuum environments are also investigated. Each performance parameter of air-stored samples shows a drastic change compared with that of the vacuum-stored samples, and both moisture and oxygen in air are found to influence the PSCs performances. These results are essential towards the fabrication of low-cost, high-efficiency PSCs in ambient air conditions. In the second part, the research is focused on the fabrication of high-efficiency PSCs using the glove-box. Both single-step and two-step spin-coating methods with perovskite precursors such as MAyFA(1-y)PbIxBr(1-x) and Cesium-doped mixed cation perovskite with a final formula of Cs0.07MA0.1581FA0.7719Pb1I2.49Br0.51 were considered. The effect of several materials and process parameters on the performance of PSCs are investigated. A new solution which consists of titanium dioxide (TiO2), hydrochloric acid (HCl), and anhydrous ethanol is introduced and optimized for fabrication of quick, pinhole-free, and efficient hole-blocking layer using the spin-coating method. Highly reproducible PSCs with an average power conversion efficiency (PCE) of 15.4% are fabricated using this solution by spin-coating method compared to the conventional solution utilizing both spin-coating with an average PCE of 10.6% and spray pyrolysis with an average PCE of 13.78%. Moreover, a thin layer of silver is introduced as an interlayer between the HTL and the back contact. Interestingly, it improved the current density and, finally the PCEs of devices by improving the adhesion of the back electrode onto the organic HTL and increasing the light reflection in the PSC. Finally, a highly reproducible fabrication procedure for cesium-doped PSCs using the anti-solvent method with an average PCE of 16.5%, and a maximum PCE of ~17.5% is provided.

Currently organic-inorganic hybrid perovskite solar cells (PSCs) is one kind of promising photovoltaic technology due to low production cost, easy fabrication method and high power conversion efficiency. Charge transport layers are found to be critical for device performance and stability. A traditional electron transport layer (ETL), such as TiO2 (Titanium dioxide), is not very efficient for charge extraction at the interface. Compared with TiO2, SnO2 (Tin (IV) Oxide) possesses several advantages such as higher mobility and better energy level alignment. In addition, PSCs with planar structure can be processed at lower temperature compared to PSCs with other structures. In this thesis, planar-structure perovskite solar cells with SnO2 as the electron transport layer are fabricated. The one-step spin-coating method is employed for the fabrication. Several issues are studied such as annealing the samples in ambient air or glovebox, different concentration of solution used for the samples, the impact of using filter for solutions on samples. Finally, a reproducible fabrication procedure for planer-structure perovskite solar cells with an average power conversion efficiency of 16.8%, and a maximum power conversion efficiency of 18.1% is provided.

Semiconductor nanostructures have attracted considerable research interest due to their unique physical and chemical properties at nanoscale which open new frontiers for applications in electronics and sensing. Zinc oxide nanostructures with a wide range of applications, especially in optoelectronic devices and bio sensing, have been the focus of research over the past few decades. However ZnO nanostructures have failed to penetrate the market as they were expected to, a few years ago. The two main reasons widely recognized as bottleneck for ZnO nanostructures are (1) Synthesis technique which is fast, economical, and environmentally benign which would allow the growth on arbitrary substrates and (2) Difficulty in producing stable p-type doping. The main objective of this research work is to address these two bottlenecks and find a solution that is inexpensive, environmentally benign and CMOS compatible. To achieve this, we developed a Sonochemical method to synthesize 1D ZnO Nanorods, core-shell nanorods, 2D nanowalls and nanoflakes on arbitrary substrates which is a rapid, inexpensive, CMOS compatible and environmentally benign method and allows us to grow ZnO nanostructures on any arbitrary substrate at ambient conditions while most other popular methods used are either very slow or involve extreme conditions such as high temperatures and low pressure. A stable, reproducible p-type doping in ZnO is one of the most sought out application in the field of optoelectronics. Here in this project, we doped ZnO nanostructures using sonochemical method to achieve a stable and reproducible doping in ZnO. We have fabricated a homogeneous ZnO radial p-n junction by growing a p-type shell around an n-type core in a controlled way using the sonochemical synthesis method to realize ZnO homogeneous core-shell radial p-n junction for UV detection. ZnO has a wide range of applications from sensing to energy harvesting. In this work, we demonstrate the successful fabrication of an electrochemical immunosensor using ZnO nanoflakes to detect Cortisol and compare their performance with that of ZnO nanorods. We have explored the use of ZnO nanorods in energy harvesting in the form of Dye Sensitized Solar Cells (DSSC) and Perovskite Solar Cells.