Three-terminal tandem solar cells enabled by back-contacted bottom cells featuring passivating, carrier-selective polysilicon based junctions

This thesis investigates back-contacted (IBC) bottom solar cells with passivating and carrier-selective POLO contacts with three terminals (3T-POLO-IBC cell). Such cells form the foundation of monolithic three-terminal tandem solar cells. This novel tandem solar cell enables the use of sub-cells with mismatched photocurrents. Thus, this tandem solar cell technology platform offers the flexibility with respect to subcell material selection, the ease of fabrication, and a robustness to spectral variations of incident light over the course of the day and year. Three building blocks of the 3T POLO IBC bottom solar cell, which are based on each other, are examined: First, the passivating and carrier-selective POLO contact. Second, the integration of POLO contacts on the rear side of a solar cell. Third, the principle of operation of a bottom cell with three terminals. In the first part, the process of charge carrier extraction at selective contacts to the photoabsorber is theoretically explored. The selectivity of a contact is defined on the basis of (reaction) kinetic considerations at the contact in terms of the rate ratio of desired processes to undesired processes. The extraction efficiency of charge carriers at the contact is derived as the ratio of the external voltage versus the internal voltage from a thermodynamic point of view. To emphasize the unifying nature of the definitions in this thesis, the existing literature definitions are calculated from the definitions in this thesis. The extraction efficiency is related to the selectivity coefficient of the contact and the limiting efficiency of a silicon solar cell with given contact selectivity is calculated accordingly. After the detailed theoretical investigation on selectivity, the properties of n+ and p+ POLO contacts are examined. Low saturation current densities between 2 fA/cm² and 18 fA/cm² and contact resistivities between 0.4mOhmcm² and 10mOhmcm² are found at the same time. It is shown that the efficient carrier transport of majority carriers is ensured by pinholes in the interfacial oxide. The resulting logarithmic selectivity coefficient of POLO contacts is determined to be above 15, which is one of the highest values measured. This makes POLO contacts predestined for solar cells with the highest efficiencies. POLO contacts are integrated on the rear side of a back-contact cell with POLO contacts for both polarities. Thereby, the p+ and n+ doped poly-Si on the backside of the solar cell form a parasitic graded p+n+ junction within the defect-rich poly-Si with a carrier lifetime of a few picoseconds. The arising recombination limits the achievable efficiency of the POLO-IBC cell to about 18%. For this reason, the parasitic junction is removed during the cell fabrication process by wet-chemically introducing a trench between the n+- and p+-doped poly-Si regions. The POLO-IBC cell with isolated n+- and p+ poly-Si regions achieves a certified efficiency of 24.25%. For the last part, a third POLO contact is added to the POLO-IBC cell and the 3T-IBC bottom cell is studied in detail using current-voltage measurements. First, the different realization options for a 3T tandem solar are sorted and the corresponding nomenclature is presented. Two different 3T IBC bottom cell architectures are identified. The first one – the unijunction bottom solar cell – contains a single minority carrier contact and two majority carrier contacts. The second one – the bipolar junction bottom solar cell – on the other hand, has two minority carrier contacts and a single majority carrier contact. Both 3T bottom cell architectures are fabricated based on a modified POLO-IBC fabrication process. The principles of operation and loss mechanisms are elucidated using J-V measurements on illuminated devices and by means of analytical modeling. The experiments show that the third contact of a 3T unijunction and bipolar junction bottom cell allows the collection or injection of additional minority or majority carriers from or into the bottom cell. Ideally, the power output of such a 3T bottom cell is nearly independent of the current density applied by the top cell. Therefore, no current matching of both subcells is required. However, the transport of majority carriers or minority carriers through the unijunction or bipolar junction bottom cell causes a loss, which, however, can be made negligible by a specific design of the bottom cell. The design rules are explained in detail. After the detailed investigations, a 3T unijunction bottom cell with a textured n+-POLO front contact with an efficiency of 20.3% and a simplified screen-printed PERC-like 3T bipolar junction bottom cell with 14.4% are developed. The latter is an attractive approach to utilize the dominant PERC technology in a low-cost tandem solar cell with maximum energy yield. Finally, the first 3T GaInP//POLO-IBC tandem cell demonstrator is fabricated with an efficiency of 27.3% and a net efficiency gain of 0.9% is demonstrated compared to the 2T operation of the 3T tandem cell.Die vorliegende Arbeit untersucht Rückkontakt-Bottomsolarzellen mit passivierenden und ladungsträger-selektiven POLO-Kontakten mit drei Anschlüssen (3T-POLO-IBC-Bottomzelle). Sie bilden das Fundament monolithischer Tandemsolarzellen mit drei Anschlüssen. Diese neuartigen Tandemsolarzelle erlaubt die Verwendung von Subzellen, dessen Fotoströme fehlangepasst sind. Damit bietet diese Tandemsolarzellen-Technologie Flexibilität bei der Materialauswahl der Subzellen, einfache Herstellbarkeit und Robustheit gegenüber spektraler Änderung des einfallenden Lichts im Tages- und Jahresverlauf. Es werden drei aufeinander aufbauende Bausteine der 3T-POLO-IBC-Bottomsolarzelle untersucht: Erstens, der passivierende und ladungsträger-selektive POLO-Kontakt. Zweitens, die Integration von POLO-Kontakten auf der Rückseite der Solarzelle. Drittens, die Funktionsweise einer Bottomzelle mit drei Anschlüssen. Im ersten Teil wird der Prozess der Ladungsträgerextraktion an selektiven Kontakten zum Fotoabsorber theoretisch ergründet. Die Selektivität eines Kontaktes wird auf der Grundlage von (reaktions-) kinetischen Betrachtungen am Kontakt als das Ratenverhältnis gewollter Prozesse zu ungewollten Prozessen definiert. Die Extraktionseffizienz von Ladungsträgern am Kontakt wird als das Verhältnis der externen Spannung gegenüber der internen Spannung aus thermodynamischen Gesichtspunkten abgeleitet. Um den vereinheitlichenden Charakter der Definitionen in dieser Arbeit hervorzuheben, werden die bestehenden Literatur-Definitionen aus den Definitionen in dieser Arbeit berechnet. Die Selektivität und Extraktionseffizienz werden miteinander korreliert und daraus der Wirkungsgrad einer Solarzelle mit vorgegebener Kontaktselektivität errechnet. Nach der detaillierten theoretischen Untersuchung der Selektivität werden die Eigenschaften von n+ und p+ POLO-Kontakten untersucht. Es werden niedrige Sättigungsstromdichten zwischen 2 fA/cm² und 18 fA/cm² und gleichzeitig Kontaktwiderstände zwischen 0,4 mOhmcm² und 10 mOhmcm² ermittelt. Es wird gezeigt, dass der effiziente Ladungsträgertransport der Majoritäten durch Pinholes im Grenzflächenoxid sichergestellt wird. Der resultierende logarithmische Selektivitäts-Koeffizient von POLO-Kontakten wird auf über 15 bestimmt. Damit gehören POLO-Kontakte zu den Kontakten mit der höchsten Selektivität und sind für Solarzellen mit höchsten Effizienzen prädestiniert. Die POLO-Kontakte werden auf der Rückseite einer Rückkontaktzelle mit POLO-Kontakten für beide Polaritäten integriert. Dabei formt das p+ und n+ dotierte Poly-Si auf der Rückseite der Solarzelle einen parasitären, gradierten p+n+-Übergang im defektreichen Poly-Si mit einer Ladungsträgerlebensdauer von wenigen Pikosekunden. Die resultierende Rekombination limitiert die erreichbare Effizienz der POLO-IBC-Zelle auf etwa 18%. Aus diesem Grund wird der parasitäre Übergang während des Zellherstellungsprozesses entfernt, indem ein Graben zwischen die n+- und p+-dotierten Poly-Si-Regionen nasschemisch eingebracht wird. Die POLO-IBC-Zelle mit isolierten n+- und p+-Poly-Si-Gebieten erzielt einen zertifizierten Wirkungsgrad von 24,25%. Für den letzten Baustein wird die POLO-IBC-Zelle um einen dritten POLO-Kontakt ergänzt und die 3T-IBC-Bottomzelle mittels Strom-Spannungsmessungen im Detail untersucht. Zuerst werden die unterschiedlichen Realisierungsmöglichkeiten für eine 3T-Tandemsolar einsortiert und die dazugehörige Nomenklatur vorgestellt. Dabei werden zwei verschiedene 3T-IBC-Bottomzellen-Architekturen unterschieden. Eine Unijunction-Bottomsolarzelle enthält einen einzigen Minoritätsladungsträgerkontakt und zwei Majoritätsträgerkontakte. Eine Bipolar-Junction-Bottomsolarzelle hingegen hat zwei Minoritätsladungsträgerkontakte und einen einzigen Majoritätsladungsträgerkontakt. Beide 3T-Bottomzell-Architekturen werden auf Basis eines modifizierten Herstellungsprozesses für POLO-IBC-Solarzellen realisiert. Das Funktionsprinzip und die Verlustmechanismen werden mit Hilfe von J-V -Messungen an beleuchteten Bauelementen und mit Hilfe analytischer Modellierung untersucht. Die Experimente zeigen, dass der dritte Kontakt einer 3T-Unijunction- und Bipolar-Junction-Bottomzelle das Sammeln oder Injizieren von zusätzlichen Minoritäts- oder Majoritätsladungsträgern aus der oder in die Bottomzelle ermöglicht. Im Idealfall ist die Leistungsabgabe einer solchen 3T-Bottomzelle nahezu unabhängig von der Stromdichte, die von der Topzelle angelegt wird. Daher ist keine Stromanpassung beider Subzellen erforderlich. Allerdings verursacht der Transport von Majoritätsladungsträgern bzw. Minoritätsladungsträgern durch die Unijunction- bzw. Bipolar-Junction-Bottomzelle hindurch einen Verlust, welcher jedoch durch eine gezielte Auslegung der Bottomzelle vernachlässigbar klein ausfallen kann. Die Auslegung wird im Detail erläutert. Schließlich wird eine 3T-Unijunction-Bottomzelle mit einem texturierten n+-POLO-Frontkontakt mit einem Wirkungsgrad von 20,3% und eine vereinfachte siebgedruckte PERC-ähnliche 3T-Bipolar-Junction-Bottomzelle mit 14,4% entwickelt. Letztere ist ein attraktiver Ansatz, um die dominierende PERC-Technologie in einer kostengünstigen Tandemsolarzelle mit maximaler Energieausbeute zu nutzen. Abschließend wird der erste 3T-GaInP//POLO-IBC-Tandemzellen-Demonstrator mit einem Wirkungsgrad von 27,3% hergestellt und ein Netto-Wirkungsgradgewinn von 0,9% im Vergleich zum 2T-Betrieb der 3T-Tandemzelle demonstriert

EFFECTS OF INTERNAL FIELDS IN QUANTUM DOTS

In this work we study the effect of built in electrostatic fields in Quantum Dots. Built-in electrostatic fields in Zincblende quantum dots originate mainly from--(1) the fundamental crystal atomicity and the interfaces between two dissimilar materials, (2) the strain relaxation, and (3) the piezoelectric polarization. We also study the geometric dependence of built in fields on 3 shapes namely Box, Dome and Pyramid. The main objectives are 3 fold they are (1) Explore the nature and the role of crystal atomicity at the interfaces and built-in fields (strain-field, and piezoelectric polarization) in determining the energy spectrum and the wave functions. (2) To identify the shift in the one-particle energy states, symmetry-lowering and non-degeneracy in the first excited state and strong band-mixing in the overall conduction band electronic states. (3) Finally geometric dependence of the above-mentioned phenomena. We discuss the importance atomistic effects and the need for 3 dimensional atomistic simulator NEMO 3D. We also discuss the effect of built in fields in HEMT (High Electron Mobility Transistor)

Strained Si heterojunction bioploar transistors

This dissertation addresses the world’s first demonstration of strained Si Heterojunction Bipolar Transistors (sSi HBTs). The conventional SiGe Heterojunction Bipolar Transistor (SiGe HBT), which was introduced as a commercial product in 1999 (after its first demonstration in 1988), has become an established device for high-speed applications. This is due to its excellent RF performance and compatibility with CMOS processing. It has enabled silicon-based technology to penetrate the rapidly growing market for wide bandwidth and wireless telecommunications once reserved for more expensive III–V technologies. SiGe HBTs is realised by the pseudomorphic growth of SiGe on a Si substrate, which allows engineering of the base region to improve performance. In this way the base has a smaller energy band gap than the emitter, which increases the gain. The energy band gap of SiGe reduces with increasing Ge composition, but the maximum Ge composition is limited by the amount of strain that can be accommodated within a given base layer thickness. Therefore, a new innovation is necessary to overcome this limitation and meet the continuous demand for high speed devices. Growing the SiGe base layer over a relaxed SiGe layer (Strain Relaxed Buffer) can increase the amount of Ge that can be incorporated in the base, hence, increasing the device performance. In this thesis, experimental data is presented to demonstrate the realisation of sSi HBTs. The performance of this novel device has been also investigated and explained using TCAD tool.EThOS - Electronic Theses Online ServiceEngineering and Physical Sciences Research CouncilGBUnited Kingdo

Characterization of Charge Trapping Phenomena in GaN-based HEMTs

This dissertation reports on charge-trapping phenomena and related parasitic effects in AlGaN/GaN high electron mobility transistors. By means of static and pulsed I-V measurements and deep-level transient spectroscopy, the main charge-trapping mechanisms affecting the dynamic performance of GaN-based HEMTs devoted to microwave and power switching applications have been comprehensively characterized, identifying the nature and the localization of the deep-levels responsible for the electrically active trap-states. A high-voltage measurement system capable for double-pulsed ID-VD, ID-VG and drain-current transient spectroscopy has been successfully designed and implemented. A characterization methodology, including the analysis of static I-V measurements, pulsed I-V measurements, and deep-level transient spectroscopy, has been developed to investigate the impact of voltage, current, and temperature on the parasitic effects of charge-trapping (threshold voltage instabilities, dynamic on-resistance increase, and transconductance reduction), and on trapping/detrapping kinetics. Experimental results gathered on transistor structures are supported by complementary capacitance deep-level transient spectroscopy (C-DLTS) performed on 2-terminal diode (FATFET) structures. Two main case-studies have been investigated. Schottky-gated AlGaN/GaN HEMTs grown on silicon carbide substrate employing iron and/or carbon doped buffers devoted to microwave applications, and MIS-gated double-heterostructure AlGaN/GaN/AlGaN HEMTs grown on silicon substrate devoted to power switching applications. The devices under test have been exposed to the complete set of current-voltage regimes experienced during the real life operations, including off-state, semi-on-state, and on-state. The main novel results are reported in the following: • Identification of a charge-trapping mechanism promoted by hot-electrons. This mechanism is critical in semi-on-state, with the combination of relatively high electric-field and relatively high drain-source current. • Identification of a positive temperature-dependent charge-trapping mechanism localized in the buffer-layer, potentially promoted by the vertical drain to substrate potential. This mechanism is critical in high drain-voltage off-state bias in high temperature operations. • Identification of deep-levels and charge-trapping related to the presence of doping compensation agents (iron and carbon) within the GaN buffer layer. • Identification of charge-trapping mechanism ascribed to the SiNX and/or Al2O3 insulating layers of MIS-gated HEMTs. This mechanism is promoted in the on-state with positive gate-voltage and positive gate leakage current. • Identification of a potential charge-trapping mechanism ascribed to reverse gate leakage current in Schottky-gate HEMTs exposed to high-voltage off-state. • The characterization of surface-traps in ungated and unpassivated devices by means of drain-current transient spectroscopy reveals a non-exponential and weakly thermally-activated detrapping behaviour. • Preliminary synthesis of a degradation mechanism characterized by the generation of defect-states, the worsening of parasitic charge-trapping effects, and the degradation of rf performance of AlGaN/GaN HEMTs devoted to microwave operations. The evidence of this degradation mechanism is appreciable only by means of rf or pulsed I-V measurements: no apparent degradation is found by means of DC analysis

Advanced AlGaN/GaN HEMT technology, design, fabrication and characterization

Nowadays, the microelectronics technology is based on the mature and very well established silicon (Si) technology. However, Si exhibits some important limitations regarding its voltage blocking capability, operation temperature and switching frequency. In this sense, Gallium Nitride (GaN)-based high electron mobility transistors (HEMTs) devices have the potential to make this change possible. The unique combination of the high-breakdown field, the high-channel electron mobility of the two dimensional electron gas (2DEG), and high-temperature of operation has attracted enormous interest from social, academia and industry and in this context this PhD dissertation has been made. This thesis has focused on improving the device performance through the advanced design, fabrication and characterization of AlGaN/GaN HEMTs, primarily grown on Si templates. The first milestone of this PhD dissertation has been the establishment of a know-how on GaN HEMT technology from several points of view: the device design, the device modeling, the process fabrication and the advanced characterization primarily using devices fabricated at Centre de Recherche sur l'Hétéro-Epitaxie (CRHEA-CNRS) (France) in the framework of a collaborative project. In this project, the main workhorse of this dissertation was the explorative analysis performed on the AlGaN/GaN HEMTs by innovative electrical and physical characterization methods. A relevant objective of this thesis was also to merge the nanotechnology approach with the conventional characterization techniques at the device scale to understand the device performance. A number of physical characterization techniques have been imaginatively used during this PhD determine the main physical parameters of our devices such as the morphology, the composition, the threading dislocations density, the nanoscale conductive pattern and others. The conductive atomic force microscopy (CAFM) tool have been widely described and used to understand the conduction mechanisms through the AlGaN/GaN Ohmic contact by performing simultaneously topography and electrical conductivity measurements. As it occurs with the most of the electronic switches, the gate stack is maybe the critical part of the device in terms of performance and longtime reliability. For this reason, how the AlGaN/GaN HEMT gate contact affects the overall HEMT behaviour by means of advanced characterization and modeling has been intensively investigated. It is worth mentioning that the high-temperature characterization is also a cornerstone of this PhD. It has been reported the elevated temperature impact on the forward and the reverse leakage currents for analogous Schottky gate HEMTs grown on different substrates: Si, sapphire and free-standing GaN (FS-GaN). The HEMT' forward-current temperature coefficients (T^a) as well as the thermal activation energies have been determined in the range of 25-300 ºC. Besides, the impact of the elevated temperature on the Ohmic and gate contacts has also been investigated. The main results of the gold-free AlGaN/GaN HEMTs high-voltage devices fabricated with a 4 inch Si CMOS compatible technology at the clean room of the CNM in the framework of the industrial contract with ON semiconductor were presented. We have shown that the fabricated devices are in the state-of-the-art (gold-free Ohmic and Schottky contacts) taking into account their power device figure-of-merit ((VB^2)/Ron) of 4.05×10^8 W/cm^2. Basically, two different families of AlGaN/GaN-on-Si MIS-HEMTs devices were fabricated on commercial 4 inch wafers: (i) using a thin ALD HfO2 (deposited on the CNM clean room) and (ii) thin in-situ grown Si3N4, as a gate insulator (grown by the vendor). The scientific impact of this PhD in terms of science indicators is of 17 journal papers (8 as first author) and 10 contributions at international conferences

Distributed Modeling Approach for Electrical and Thermal Analysis of High-Frequency Transistors

The research conducted in this dissertation is focused on developing modeling approaches for analyzing high-frequency transistors and present solutions for optimizing the device output power and gain. First, a literature review of different transistor types utilized in high-frequency regions is conducted and gallium nitride high electron mobility transistor is identified as the promising device for these bands. Different structural configurations and operating modes of these transistors are explained, and their applications are discussed. Equivalent circuit models and physics-based models are also introduced and their limitations for analyzing the small-signal and large-signal behavior of these devices are explained. Next, a model is developed to investigate the thermal properties of different semiconductor substrates. Heat dissipation issues associated with some substrate materials, such as sapphire, silicon, and silicon carbide are identified, and thinning the substrates is proposed as a preliminary solution for addressing them. This leads to a comprehensive and universal approach to increase the heat dissipation capabilities of any substrate material and 2X-3X improvement is achieved according to this novel technique. Moreover, for analyzing the electrical behavior of these devices, a small-signal model is developed to examine the operation of transistors in the linear regions. This model is obtained based on an equivalent circuit which includes the distributed effects of the device at higher frequency bands. In other words, the wave propagation effects and phase velocity mismatches are considered when developing the model. The obtained results from the developed simulation tool are then compared with the measurements and excellent agreement is achieved between the two cases, which serves as the proof for validation. Additionally, this model is extended to predict and analyze the nonlinear behavior of these transistors and the developed tool is validated according to the obtained large-signal analysis results from measurement. Based on the developed modeling approach, a novel fabrication technique is also proposed which ensures the high-frequency operability of current devices with the available fabrication technologies, without forfeiting the gain and output power. The technical details regarding this approach and a sample configuration of the electrode model for the transistor based on the proposed design are also provided

Nanoscale Ferroic Materials—Ferroelectric, Piezoelectric, Magnetic, and Multiferroic Materials

Ferroic materials, including ferroelectric, piezoelectric, magnetic, and multiferroic materials, are receiving great scientific attention due to their rich physical properties. They have shown their great advantages in diverse fields of application, such as information storage, sensor/actuator/transducers, energy harvesters/storage, and even environmental pollution control. At present, ferroic nanostructures have been widely acknowledged to advance and improve currently existing electronic devices as well as to develop future ones. This Special Issue covers the characterization of crystal and microstructure, the design and tailoring of ferro/piezo/dielectric, magnetic, and multiferroic properties, and the presentation of related applications. These papers present various kinds of nanomaterials, such as ferroelectric/piezoelectric thin films, dielectric storage thin film, dielectric gate layer, and magnonic metamaterials. These nanomaterials are expected to have applications in ferroelectric non-volatile memory, ferroelectric tunneling junction memory, energy-storage pulsed-power capacitors, metal oxide semiconductor field-effect-transistor devices, humidity sensors, environmental pollutant remediation, and spin-wave devices. The purpose of this Special Issue is to communicate the recent developments in research on nanoscale ferroic materials