Ce travail a pour but d'implémenter des procédés d’implantation ionique pour des cellules solaires à hétérojonctions de silicium (SHJ) afin d'en simplifier le procédé de fabrication ou d’en augmenter les performances.Nous avons d'abord étudié le procédé pour réaliser le dopage des couches de silicium amorphe hydrogéné (a-Si:H). Par ce nouveau procédé, il est possible de réaliser des dopages localisés de manière simple, à travers des masques, ce qui peut permettre une diminution des coûts de fabrication de certains types de cellules SHJ comme les cellules à contacts interdigités à l'arrière. Les implantations de phosphore et de bore ont été étudiées, pour la réalisation de dopage respectivement de type n et p. Les comportements et les conclusions sont très différents pour ces deux types de dopage. Le phosphore étant plus lourd que le bore, il est possible de l'implanter dans des couches très minces sans endommager fortement l'interface avec le silicium cristallin, mais la création très importante de défauts dans le a-Si:H, résistant à des recuits post-implantation, conduit à de fortes dégradations des propriétés électriques du a-Si:H, et il n'a pas été possible d'atteindre des niveaux de conductivité suffisants. Au contraire, pour le bore, conformément à des résultats de la littérature, les atomes sont activés plus facilement par un recuit post-implantation grâce à la forte diminution de la concentration de défauts localisés. Cependant, le bore, implanté plus profondément, atteint e plus facilement l'interface, ce qui nécessite des recuits à plus haute température pour guérir les défauts d'interface. Néanmoins, pour des couches de a-Si:H de l'ordre de 25 nm, nous avons pu trouver des conditions technologiques permettant d'obtenir des propriétés comparables à celles obtenues par le procédé classique de dépôt de (p) a Si:H assisté par plasma, à savoir des valeurs élevées de conductivités du a-Si:H (10-4 Ω-1cm-1) et de passivation d’interface (i VOC > 700 mV).Une deuxième partie de ce travail est consacrée à l’étude d’une nouvelle cellule, dite à homo hétérojonction de silicium (HHJ) comprenant un homo-émetteur additionnel (p+) c-Si à l’hétéro-interface côté émetteur. Le but est d’améliorer la passivation de l’interface afin d’augmenter le rendement de la cellule. Des simulations numériques ont mis en évidence une augmentation de FF de la cellule HHJ, que nous avons pu attribuer à une meilleure passivation par effet de champ et à une diminution de la résistance globale du a-Si:H due à des modifications des courbures de bandes. Elles ont aussi montré la nécessité d’un homo-émetteur suffisamment mince et fortement dopé (5×1018 cm-3). De ce fait, nous avons utilisé le procédé d’implantation ionique pour développer des profils de bore adéquats et avons pu vérifier expérimentalement que l'incorporation de la couche de (p+) c-Si permet la diminution de la résistance de contact et l'amélioration de la passivation de l'interface (i) a-Si:H/(p+) c-Si par effet de champ lorsque la concentration de bore en surface n'est pas trop importante. Ces deux améliorations ont pu être concrétisées dans la réalisation de cellules présentant une amélioration du facteur de forme et de meilleurs rendements de conversion par rapport à des cellules SHJ de référence. Cette réalisation constitue la première preuve de concept pour les cellules de type HHJ. / This work aims at investigating the use of ion implantation to process silicon heterojunction solar cells (SHJ) in order to improve the ratio of cost to produced power (€/Wp) of the cells either by cost reduction due to manufacturing simplification or by increase of the cell performance.A first part of the work consists in doping hydrogenated amorphous silicon (a-Si:H) layers by ion implantation. Using hard masks, doping of localized regions required in cell architectures like interdigitated back contact cells can thus be easily achieved at lower cost. Both boron and phosphorus implantation have been studied for p- and n-type doping, respectively. These two types behave very differently. Phosphorous being heavier than boron, very shallow implantation can be achieved on thin a-Si:H layers onto crystalline wafers without damaging the interface. However very high defect densities are created in a-Si:H which cannot be annealed out by post-implantation annealing treatments. Therefore it was not possible to reach conductivity values suitable for solar cell applications. For B implantation, consistently with previous work, the activation of B atoms has been achieved upon annealing thanks to a decrease of localized bandgap states. Also, boron can penetrate deeper and reach high concentration at the a-Si:H/c-Si interface, which requires higher temperature annealing compared to P implantation to recover a good interface passivation quality. Nevertheless, for a-Si:H layers of about 25 nm process conditions allowing similar properties to PECVD-doped (p) a-Si:H deposition (i.e. conductivity of 10-4 Ω-1cm-1 and interface passivation allowing i-VOC > 700 mV) have been obtained.A second study is dedicated to the study of a new cell concept, named silicon homo-heterojunction (HHJ) which comprise an additional homo-emitter (p+) c-Si at the emitter interface. The goal is to improve the interface passivation in order to increase the cell efficiency. Numerical simulations have evidenced an improved fill factor in this cell that is attributed to a field effect passivation improvement and a decrease in series resistance related to band bending changes in the a-Si:H layers. The need of sufficiently shallow and strongly doped (> 5×1018 cm-3) emitter has also been evidenced. Therefore, ion implantation has been used to develop suitable boron profiles and both the increase in fill factor and the decrease in contact resistances have been obtained when the boron surface concentration is not too high. These improvements have been validated by processing HHJ solar cells that exhibit a fill factor improvement and an improved efficiency compared to SHJ cells. This achievement is a first proof of concept of the HHJ architecture.

Organic electronic devices are sustainable alternatives to the conventional electronics, due to their advantages of low cost, mechanical flexibility and wide range of applications. With the myriad list of organic materials available today, the opportunities to imagine new innovative devices are immense. Organic electronic devices such as OLEDs (organic light emitting diode), OPVs (Organic photovoltaics) and OFETs (organic field effect transistors) are among the leading device categories. Although OLED’s have been a huge commercial success, other categories are not lagging. Radical thinking is necessary to improve on the current performances of these devices. One such thinking is to combine the versatile ZnO (Zinc Oxide) material to organic semiconductors. This can be achieved by exploiting the dual nature of ZnO’s semiconducting and piezoelectric property. Many devices have used ZnO in combination with organic semiconductors for applications ranging from sensors, photovoltaics, OFET’s, memory and many others. The goal of the work is to incorporate the piezoelectric nature of hydrothermally grown ZnO nanowires for Opto-electronic applications. Although the initial research work was done on incorporating the piezo effect of bulk grown ZnO nanowires in improving the efficiency of an OPV, we discovered a unique memory effect in this device by incorporating ZnO nanowires in an inverted organic photovoltaic architecture. The device switched between a rectifying response in dark to resistive behavior under illumination with a finite transition time and was reversible. Since then we decided to explore few of the opto-electronic applications of this technology. The synthesis and characterization of crystalline ZnO nanowires, nanoforest and planar ZnO nanofilm are reported along with the application of these ZnO nanostructures in optoelectronic devices. Noncentro symmetry of crystalline ZnO nanostructures makes it an excellent candidate to be used as piezo functional material and these nanostructures are characterized using electrochemical cell containing ZnO electrode as the working electrode. ZnO nanostructures like nanowires, nanoforest and planar nanofilm are similarly characterized for piezo property using electrochemical technique. Different devices require distinguishing physical and electrical properties of ZnO nanostructures, hence morphology, effect of pre-strain, surface area, surface coverage and thickness of these nanostructures were evaluated for its piezoresponse. It is shown that it was possible to obtain similar piezoresponse among different ZnO nanostructures in addition to taking advantage of the structural benefits among various categories of nanostructures as per requirement. The presented research can be used as the proof-of-the-concept that ZnO nanostructures can be designed and fabricated with a prestrain to adjust the piezo response of the material under external forces. Therefore, the structure with the prestrain can be employed in various electronic and optical devices where the piezo voltage can be used for adjusting the energy band bending at an interface.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第18381号 / エネ博第293号 / 新制||エネ||61(附属図書館) / 31239 / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 佐川 尚, 教授 八尾 健, 教授 萩原 理加 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DGAM

A numerical model have been developed in order to describe and achieve deeper understanding of experimentally obtained I-V curves from Cu2O/ZnO p-n heterojunctions for potential use as future solar cell material. The model was created using the simulation software COMSOL Multiphysics® and their semiconductor module. To experimentally study the samples two approaches were taken: (1) macro-electrical measurements and (2) local I-V measurements using conductive AFM. The final model is one-dimensional, time dependent and with the ability to study photovoltaic effects of samples with different layer thickness at different voltage ramping speeds and different light irradiance. The model is also able to study the effects of using different contact materials by treating the contacts as ideal Schottky contacts. The dynamic behavior of a Cu_2O/ZnO heterojunction was studied by considering the systems response to a voltage step and the effect of changing the voltage ramping speed. The output from the step response, the current as a function of time, is varying a short time after a step has occurred before settling on to a steady value. The response also shows an overshoot of the current in the direction of the voltage step and the final steady value depends on whether the junction is conducting or not. The effects of this behavior on the shape of the I-V curves are witnessed when studying the different voltage ramping speeds. The voltage is ramped from 2 V to -2 V and back again for different speeds (V/s). The I-V curves have different shapes when sweeping the voltage in different directions and the magnitude increases with increasing speed. The photovoltaic effects were studied by applying different light irradiances. The behavior of the model agrees well with the theory for an ideal diode solar cell. An investigation was done of how the work function of the metal in contact with the Cu_2O affects the shape of the I-V curve under dark and illuminated conditions. The metal work function was changed from 4.5 eV to 6.5 eV in steps of 0.4 eV and does not affect the shape of the I-V curves much in dark after increasing it above 4.5 eV. The effects are more visible under illuminated conditions where a "step"-behavior appears for the lower values of the work function. Only one of the physical samples show a noticeable light effect. The macro-electrical measurement on this sample is compared with simulated results and are in qualitative agreement with each other. The agreement between the local electrical measurements and the simulated results is not as good with the current model.

Silicon phthalocyanines (R₂-SiPcs) are an emerging class of high-performance n-type or ambipolar organic semiconductors which have found application in organic electronic devices, including organic thin-film transistors (OTFTs), organic photovoltaics (OPVs) and organic light-emitting diodes (OLEDs). Owing to their tetravalent silicon metal centre, R₂-SiPcs can be substituted with a range of axial ligands including phenols, carboxylic acids, and silanes to tune their intermolecular interactions, optical properties, electronic properties and solubility. While early reports of R₂-SiPcs have demonstrated promising results, the relationship between their structure and performance in OTFTs is poorly understood. Additionally, many OTFTs with R₂-SiPcs as semiconductor only demonstrate n-type behaviour under inert atmospheres due to their shallow lowest unoccupied orbital level below -4.1 eV making them susceptible to electron trapping by moisture and oxygen. This thesis presents developments in both the understanding of how R₂-SiPc structure influences performance, device engineering and exploration of these materials in ammonia sensors. First, I develop of structure-property relationships for a catalogue of fifteen R₂-SiPcs integrated into OTFTs including eleven materials used in OTFTs for the first time. I then explore the influence of dielectric surface chemistry on the texture of R₂-SiPc films and their resulting performance in OTFTs using silane self-assembled monolayers and para-sexiphenyl to understand the weak epitaxial growth behaviour of this class of materials. Next, I report eight novel peripherally fluorinated and axially substituted silicon phthalocyanines (R₂-FₓSiPcs) to investigate the influence of peripheral and axial fluorination on air-stable electron transport and determine the threshold for achieving air-stable n-type OTFTs. Finally, I integrate R₂-FₓSiPcs into organic heterojunction ammonia gas sensors to understand the influence of peripheral fluorination on the majority charge carrier in this device architecture.

Designs for PnP GaAs/InxGa1-xAs/GaAs heterojunction bipolar transistors (HBTs) are proposed and simulated with the aid of commercial software. Band diagrams, Gummel plots and common emitter characteristics are shown for the specific case of x=1, x=0.7, and x linearly graded from 0.75 to 0.7. Of the three designs, it is found that the linearly graded case has the lowest leakage current and the highest current gain. IV curves for all four possible classes of InAs/GaAs heterojunction (nN, nP, pN, pP) are calculated. A pN heterojunction is fabricated and characterized. In spite of the 7% lattice mismatch between InAs and GaAs, the diode has an ideality factor of 1.26 over three decades in the forward direction. In the reverse direction, the leakage current grows exponentially with the magnitude of the bias, and shows an effective ideality factor of 3.17, in stark disagreement with simulation. IV curves are taken over a temperature range of 105 K to 405 and activation energies are extracted. For benchmarking the device processing and the characterization apparatus, a conventional GaAs homojunction diode was fabricated and characterized, showing current rectification ratio of 109 between plus one volt and minus one volt. Because the PnP material for the optimal HBT design was not available, an Npn GaAs/InAs/InAs HBT structure was processed, characterized, and analyzed. The Npn device fails in both theory and in practice; however, by making a real structure, valuable lessons were learned for crystal growth, mask design, processing, and metal contacts. / Master of Science

Organic phototransistors (OPTs) can enable essential applications, such as nonvolatile memory, artificial synapses, and photosensors in next-generation optical communication and wearable electronics. Among these applications, nonvolatile OPT memories are particularly promising, as they can retain captured visual information for extended periods, making them valuable for data storage, image and video processing applications. The capability of storing multi-bit information, which provides a low-cost way to increase the memory density per unit cell area, is one of the most critical challenges of memory products. In this work, we explore different solution-processible electrets to obtain highly sensitive phototransistor memory devices. Different planar heterojunctions, including small molecule/small molecule and small molecule/polymer, are used to fabricate OPT memories. Additionally, we explore the feasibility of producing polymer/polymer planar heterojunctions through printing processes. Firstly, OPT memories that can be programmed with white light and erased by applying a negative voltage are fabricated with a planar heterojunction of a nonconductive nanographene layer and a semiconducting layer of 2,9-didecyldinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (C10-DNTT). We systematically study the optical and memory characteristics of devices with an 8 nm nanographene (NG) layer. The photosensitivity of such devices can be as high as 3.4×105. The memory also shows quite good endurance and data-storing stability; an endurance of 100 write-read-erase-read (WRER) cycles and 1.5×105 s retention time are obtained. The thickness of the NG layer has a considerable influence on the performance of fabricated devices. The results suggest that devices with a thicker NG layer are more sensitive to weak light. In comparison, devices with a relatively thin NG layer are found to be promising for multi-bit photo memory devices. Secondly, we fabricate OPT memories by replacing the nanographene layer with a commercially available semiconducting polymer, namely Poly(2,5-bis(2-octyldodecyl)-3,6-di(pyridin-2-yl)-pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-2,2’-bithiophene) (PDBPyBT). This polymer possesses a narrow bandgap and exhibits a broad range of light absorption, spanning from ultraviolet (UV) to red light wavelengths. As a result, the fabricated devices are capable of responding to a broad spectrum of light colors. The light response of these devices is investigated in terms of their reaction to different colors of light. Also, devices with varying thicknesses of the PDBPyBT layer are fabricated and studied. The results indicate that all of the fabricated devices demonstrate multi-bit programming properties, and the devices incorporating a thin, ribbon-structured PDBPyBT layer are particularly well-suited for applications as light dosimeters. Moreover, the results highlight that both the C10-DNTT and the PDBPyBT layer function as photo exciton generation and charge-trapping layers. Last, we seek to fabricate cost-effective organic multilayer devices through a solution-processing approach, eliminating the need for orthogonal solvents. We observe a crosslinking effect in the thin films caused by thermal annealing without using any crosslinker. Remarkably, this effect is found to be universal for several commercial semiconducting polymers investigated in our study. Following annealing at 200 ºC or higher temperatures, the thin films exhibit enhanced stability against the original solvent. Various analytical techniques are employed to examine the thin films to gain insights into the microstructural changes. Our results suggest that the observed crosslinking effect is predominantly attributed to a physical transformation, whereby the films became more crystalline after annealing at relatively high temperatures. To further explore the feasibility of fabricating multilayer devices, we simulate the construction of multilayer devices by top-gate-bottom-contact (TGBC) devices using the same solvent for the polymer dielectric layer and the semiconducting layer. We also fabricated planar polymer/polymer heterojunction via this method. Encouragingly, this approach demonstrated that thermal annealing could work as a straightforward and promising method for producing cost-effective organic multilayer devices, e.g., fully solution-processed diodes, functional transistors, and solar cells.:Abstract iii Contents vii 1 Introduction 1 1.1 Motivation 1 1.2 Organic semiconductor 2 1.2.1 Atom orbitals and molecular orbitals 2 1.2.2 Energy levels in solid 5 1.2.3 Fermi level 6 1.2.4 Band bending 7 1.2.5 From orbital to states 8 1.2.6 Organic semiconductor materials 9 1.2.7 Nanographene 10 1.2.8 Charge carrier transport in organic semiconductors 11 1.3 Organic field-effect transistors (OFET) 11 1.3.1 OFET architectures 12 1.3.2 OFET operation principle 12 1.3.3 OFET performance parameters 14 1.3.4 OFET memory 17 1.4 Optical electronics 20 1.4.1 Exciton pair generation. 20 1.4.2 Photoelectronic devices 21 1.4.3 Phototransistor devices 22 1.5 Phototransistor memories 23 1.5.1 Working mechanism of phototransistor memories 23 1.5.2 Phototransistor memory architecture 24 1.5.3 State-of-the-art organic phototransistor memory 25 1.6 Objective and outline 27 2 Materials and methods 29 2.1 Materials 29 2.2 Device fabrication 30 2.2.1 Substrate cleaning 30 2.2.2 Solution shearing 30 2.2.3 Thermal vapor deposition 31 2.3 Characterization 31 2.3.1 Thin film characterization 31 2.3.2 Current voltage characteristics 35 2.3.3 Capacitance 36 3 C10-DNTT/NG planar heterojunction phototransistor memories 37 3.1 Introduction 37 3.2 Thin films 39 3.2.1 Film and device fabrication 39 3.2.2 Characterization of thin films 39 3.3 Transfer characteristics under light 41 3.3.1 Writing process 41 3.3.2 Erasing process 48 3.3.3 C10-DNTT-only devices 51 3.4 Summary of working principle 52 3.5 Output characteristics and evaluation of the optical properties 52 3.6 Memory properties of NG-based OPT memory devices 55 3.7 Devices with different NG thicknesses 56 3.7.1 The impact of NG thickness 56 3.7.2 Devices fabricated from 0.05 mg ml-1 NG solution 60 3.8 Conclusion 64 4 C10-DNTT/PDBPyBT heterojunction phototransistor memories 67 4.1 Introduction 67 4.2 Device Architecture 68 4.3 Physical characterization of PDBPyBT and C10-DNTT thin films 69 4.4 Performance of devices with a thick PDBPyBT layer 72 4.4.1 Erasing and programming process 72 4.4.2 Response to different colors of light 78 4.5 Variation of PDBPyBT thickness 80 4.5.1 Transfer characteristics 80 4.5.2 Morphology of C10-DNTT 85 4.5.3 Output characteristics 86 4.5.4 Multi-level programming test 86 4.6 Comparison of the devices 92 4.7 Summary 93 5 Organic multilayer devices fabricated via thermal annealing 95 5.1 Introduction 95 5.2 Film Fabrication 97 5.3 Study on thin films 97 5.3.1 Thickness changes 97 5.3.2 Characterization of the thin films 99 5.3.3 Impact of re-annealing 107 5.3.4 Other semiconducting polymers 108 5.4 Discussion of the working mechanism 110 5.5 Impact of thermal annealing on devices’ performance 111 5.5.1 BGTC devices fabrication 111 5.5.2 TGBC devices fabrication 113 5.6 Planar heterojunction devices via solution processing 116 5.7 Conclusion 117 6 Conclusions and outlook 119 6.1 Conclusions 119 6.2 Outlook 120 Bibliography 123 List of Figures 143 List of Tables 155 List of abbreviations 157 Appendix A 159 Appendix B 165 B1.1 Introduction 165 B1.2 Devices with a 7 nm shear coated Al2O3 dielectric 166 B1.2.1 Normal-sized channel devices 166 B1.2.2 Ultra-wide channel devices 167 B1.3 Devices with a 30 nm ALD Al2O3 dielectric 169 B1.3.1 Normal-sized channel devices 169 B1.3.2 Ultra-wide channel devices 170 B1.4 Ferroelectric organic phototransistor devices 172 B1.4.1 Dielectric layer 172 B1.4.2 Devices with 10 nm HZO 173 B1.4.3 Devices with 30 nm HZO 175 Conclusion 176 Publications 177 Acknowledgment 179

In this thesis, research on a-Si:H Schottky diodes and a-Si:H/c-Si heterojunctions is presented with the focus on the capacitance spectroscopy and information on electronic properties that can be derived from this technique. Last years a-Si:H/c-Si heterojunctions (HJ) have received growing attention as an approach which combines wafer and thin film technologies due to their low material consumption and low temperature processing. HJ solar cells benefit from lower fabrication temperatures thus reduced costs, possibilities of large-scale deposition, better temperature coefficient and lower silicon consumption. The most recent record efficiency belongs to Panasonic with 24.7% for a cell of 100 cm² was obtained. The aim of this thesis is to provide a critical study of the capacitance spectroscopy as a technique that can provide information on both subjects: DOS in a-Si:H and band offset values in a-Si:H/c-Si heterojunctions.The first part of the manuscript is devoted to capacitance spectroscopy in a-Si:H Schottky diodes. The interest is concentrated on the simplified treatment of the temperature and frequency dependence of the capacitance that allows one to extract the density of states at the Fermi level in a-Si:H. We focus on the study of the reliability and validity of this approach applied to a-Si:H Schottky barriers with various magnitudes and shapes of the DOS. Several structures representing n-type and undoped hydrogenated amorphous silicon Schottky diodes are modeled with the help of numerical simulation softwares. We show that the reliability of the studied treatment drastically depends on the approximations used to obtain the explicit analytical expression of the capacitance in such an amorphous semiconductor.In the second part of the chapter, we study the possibility of fitting experimental capacitance data by numerical calculations with the input a-Si:H parameters obtained from other experimental techniques. We conclude that the simplified treatment of the experimentally obtained capacitance data together with numerical modeling can be a valuable tool to assess some important parameters of the material if one considers the results of numerical modeling and performs some adjustments. The second part is dedicated to capacitance spectroscopy of a-Si:H/c-Si heterojunctions with special emphasis on the influence of a strong inversion layer in c-Si at the interface. Firstly, we focus on the study of the frequency dependent low temperature range of capacitance-temperature dependencies of a-Si:H/c-Si heterojunctions. The theoretical analysis of the capacitance steps in calculated capacitance-temperature dependencies is presented by means of numerical modeling. It is shown that two steps can occur in the low temperature range, one being attributed to the activation of the response of the gap states in a-Si:H to the small signal modulation, the other one being related to the response of holes in the strong inversion layer in c-Si at the interface. The experimental behavior of C-T curves is discussed. The quasi-static regime of the capacitance is studied as well. We show that the depletion approximation fails to reproduce the experimental data obtained for (p) a-Si:H/(n) c-Si heterojunctions. Due to the existence of the strong inversion layer, the depletion approximation overestimates the potential drop in the depleted region in crystalline silicon and thus underestimates the capacitance and its increase with temperature. A complete analytical calculation of the heterojunction capacitance taking into account the hole inversion layer is developed. It is shown that within the complete analytical approach the inversion layer brings significant changes to the capacitance for large values of the valence band offset. The experimentally obtained C-T curves show a good agreement with the complete analytical calculation and the presence of the inversion layer in the studied samples is thus confirmed.

This thesis explores how the carrier dynamics within semiconductor nanocrystals can be directly engineered through specific core-shell design. Emphasis is placed on how material characteristics, such as strain or alloying at a core-shell interface, can influence the exciton energies and the recombination dynamics within semiconductor nanocrystals. This study synthesises type-II heterojunction ZnTe/ZnSe core-shell nanocrystals via a diethyl zinc-free synthesis method, producing small size distributions and quantum yields as high as 12%. It was found that the 7% lattice mismatch between the core and shell materials places limitations on the range of structures in which coherent growth is achieved. By developing compositional and strained atomistic core-shell models a variety of physical and optical properties could be simulated and has led to a clear picture of the core-shell architecture to be built. This characterisation provides evidence that the low bulk modulus ZnTe cores are compressed by the higher bulk modulus smaller lattice constant ZnSe shells. Further studies show how strain is manifested in structures with 'sharp' core-shell interfaces and how intentional alloying the interface can influence the growth and exciton energies. A (2-6)-band effective mass model was able to distinguish between the as-grown 'sharp' and 'alloyed' interfaces which indicated that strain accentuates the redshift of the excitonic state whilst reduced strain within an alloyed interface sees a reduced redshift. Single nanocrystal spectroscopy investigations of brightly emitting single graded alloyed nanocrystals and of a size series of commercially available CdSe/ZnS nanocrystals showed almost no fluorescence intermittency (nearly 'non-blinking'). These investigations also identified trion recombination as the main mechanism within the blinking 'off' state. Ultimately this thesis adds to the growing understanding of how specific core-shell architectures manipulate the electronic structure and develops techniques to identify specific material characteristics and how these characteristics influence the physical and optical properties within semiconductor nanocrystals.

Les travaux développés dans cette thèse sont dédiés à l’étude des propriétés électroniques de diodes Schottky de silicium amorphe hydrogéné (a-Si:H) et d'hétérojonctions entre silicium amorphe hydrogéné et silicium cristallin, a-Si:H/c-Si au moyen de spectroscopies de capacité de jonctions.Lors de la fabrication des cellules solaires à haut rendement plusieurs paramètres d’une hétérojonction a-Si:H/c-Si doivent être considérés. Premièrement, la densité d’états dans le gap du a-Si:H est d’une grande importance car il s’agit de défauts qui favorisent le piégeage et la recombinaison de porteurs. Deuxièmement, la détermination des désaccords des bandes entre la couche amorphe et la couche cristalline est indispensable puisque ceux-ci contrôlent le transport à travers la jonction et déterminent la courbure des bandes dans c-Si, ce qui va notamment influencer la recombinaison des porteurs sous lumière, donc la tension de circuit ouvert des cellules. Cette thèse a pour but d’étudier la spectroscopie de capacité comme technique d'analyse de paramètres clés pour les dispositifs à hétérojonctions de silicium : la densité d’états dans le a-Si:H et les désaccords des bandes entre a-Si:H et c-Si.La première partie est dédiée à l’étude de la capacité de diodes Schottky. Nous nous concentrons sur un traitement simplifié de la capacité en fonction de la température et de la fréquence reposant sur une expression analytique obtenue par une résolution approchée de l'équation de Poisson. Ce traitement permet en principe d’extraire la densité d’états au niveau de Fermi dans le a-Si:H et la fréquence de saut des électrons depuis un état localisé au niveau de Fermi vers la bande de conduction. En appliquant ce traitement simplifié à la capacité calculée sans approximation à l'aide de deux logiciels de simulation numérique, nous montrons que sa fiabilité et sa validité dépendent fortement de la distribution des états localisés dans la bande interdite du a-Si:H et de la position du niveau de Fermi. Puis nous abordons l’étude de la capacité des hétérojonctions entre a-Si:H de type p et c-Si de type n, et nous mettons particulièrement en avant l’existence d'une couche d’inversion forte à l’interface dans le c-Si, formant un gaz bidimensionnel de trous. Dans une première partie, nous présentons une étude par simulation numérique de la dépendance de la capacité en fonction de la température, pour laquelle un ou deux échelons peuvent être mis en évidence à basse température. Leur analyse montre qu’un des ces échelons est attribué à l’activation de la réponse de la charge dans le a-Si:H, alors que l’autre, présentant une énergie d'activation plus grande, est lié à la modulation de la concentration des trous dans la couche d’inversion forte, lorsque celle-ci existe. On présente ensuite une discussion de résultats expérimentaux. Le régime quasi-statique de la capacité fait ainsi l’objet d’une discussion. Nous mettons en relief le fait que l’approximation de la zone de déplétion ne permet pas de reproduire cette augmentation de la capacité en fonction de la température. Du fait de l’existence de la couche d’inversion forte, la chute de potentiel dans la zone de déplétion du c-Si est plus faible que la valeur déterminée par le calcul attribuant toute la chute de potentiel à la zone de déplétion. Par conséquent, cette approximation conduit à sous-estimer la capacité ainsi que son augmentation avec la température. Nous présentons alors un calcul analytique complet qui tient compte à la fois de la distribution particulière du potentiel dans le a-Si:H, et des trous dans le c-Si dont la contribution à la concentration totale de charges n'est pas négligeable dans la couche d’inversion forte. Le calcul analytique complet permet de bien reproduire les résultats expérimentaux de capacité en fonction de la température; ceci confirme la présence de la couche d’inversion forte dans les échantillons étudiés. / In this thesis, research on a-Si:H Schottky diodes and a-Si:H/c-Si heterojunctions is presented with the focus on the capacitance spectroscopy and information on electronic properties that can be derived from this technique. Last years a-Si:H/c-Si heterojunctions (HJ) have received growing attention as an approach which combines wafer and thin film technologies due to their low material consumption and low temperature processing. HJ solar cells benefit from lower fabrication temperatures thus reduced costs, possibilities of large-scale deposition, better temperature coefficient and lower silicon consumption. The most recent record efficiency belongs to Panasonic with 24.7% for a cell of 100 cm² was obtained. The aim of this thesis is to provide a critical study of the capacitance spectroscopy as a technique that can provide information on both subjects: DOS in a-Si:H and band offset values in a-Si:H/c-Si heterojunctions.The first part of the manuscript is devoted to capacitance spectroscopy in a-Si:H Schottky diodes. The interest is concentrated on the simplified treatment of the temperature and frequency dependence of the capacitance that allows one to extract the density of states at the Fermi level in a-Si:H. We focus on the study of the reliability and validity of this approach applied to a-Si:H Schottky barriers with various magnitudes and shapes of the DOS. Several structures representing n-type and undoped hydrogenated amorphous silicon Schottky diodes are modeled with the help of numerical simulation softwares. We show that the reliability of the studied treatment drastically depends on the approximations used to obtain the explicit analytical expression of the capacitance in such an amorphous semiconductor.In the second part of the chapter, we study the possibility of fitting experimental capacitance data by numerical calculations with the input a-Si:H parameters obtained from other experimental techniques. We conclude that the simplified treatment of the experimentally obtained capacitance data together with numerical modeling can be a valuable tool to assess some important parameters of the material if one considers the results of numerical modeling and performs some adjustments. The second part is dedicated to capacitance spectroscopy of a-Si:H/c-Si heterojunctions with special emphasis on the influence of a strong inversion layer in c-Si at the interface. Firstly, we focus on the study of the frequency dependent low temperature range of capacitance-temperature dependencies of a-Si:H/c-Si heterojunctions. The theoretical analysis of the capacitance steps in calculated capacitance-temperature dependencies is presented by means of numerical modeling. It is shown that two steps can occur in the low temperature range, one being attributed to the activation of the response of the gap states in a-Si:H to the small signal modulation, the other one being related to the response of holes in the strong inversion layer in c-Si at the interface. The experimental behavior of C-T curves is discussed. The quasi-static regime of the capacitance is studied as well. We show that the depletion approximation fails to reproduce the experimental data obtained for (p) a-Si:H/(n) c-Si heterojunctions. Due to the existence of the strong inversion layer, the depletion approximation overestimates the potential drop in the depleted region in crystalline silicon and thus underestimates the capacitance and its increase with temperature. A complete analytical calculation of the heterojunction capacitance taking into account the hole inversion layer is developed. It is shown that within the complete analytical approach the inversion layer brings significant changes to the capacitance for large values of the valence band offset. The experimentally obtained C-T curves show a good agreement with the complete analytical calculation and the presence of the inversion layer in the studied samples is thus confirmed.