The purpose of our research is growing the coupled InGaAs quantum dots on the n-type substrate by molecular beam epitaxy in laboratory, and we choose 5,10 and 15 nanometers to be the thicknesses of GaAs spacer between the quantum dots layer. Due to the couple effect, we hope to realize the theorem of intermediate band proved by Luque and Marti. We measure the characteristic of samples by electroluminescence spectra, photoelectric current spectra, electrical absorption spectra and electro reflectance spectra in laboratory; moreover, we acquire the basic parameters of solar cell by AM1.5G for analyzing. From the basic parameters of solar cell, we know that the quantum dots can enhance the photocurrent by absorbing additional photons , however, the strain caused by quantum dots would decay the open voltage seriously, so that the efficiency always under the baseline. Each efficiency of 9-stack QDs are 4.3%(c494),5.1%(c519),5.3% (c520),and each efficiency of 9-stack Dwells are 3.9%(c524),4.2%(c525),4.7%(c526), and 10-stack QDs(5nm) is 2.9%(c514),and 12-stack QDs(10nm) is 4.48%(c538),and 12-stack QDs(15nm) is 5.89%. The break through of this paper is that the efficiency of c529¡]VOC=0.64V,JSC=11.97mA/cm2,FF=67%,£b=5.89%¡^is higher than GaAs¡]VOC =0.87 V, JSC =7.4 mA/cm2,FF=72.3%,£b=5.6%¡^,and we attribute this performance to its good quality of miniband, because the current can be enhanced a lot, and it will make up for the lose of open voltage and filling factor, so that the efficiency can be higher than GaAs baseline.

abstract: As existing solar cell technologies come closer to their theoretical efficiency, new concepts that overcome the Shockley-Queisser limit and exceed 50% efficiency need to be explored. New materials systems are often investigated to achieve this, but the use of existing solar cell materials in advanced concept approaches is compelling for multiple theoretical and practical reasons. In order to include advanced concept approaches into existing materials, nanostructures are used as they alter the physical properties of these materials. To explore advanced nanostructured concepts with existing materials such as III-V alloys, silicon and/or silicon/germanium and associated alloys, fundamental aspects of using these materials in advanced concept nanostructured solar cells must be understood. Chief among these is the determination and predication of optimum electronic band structures, including effects such as strain on the band structure, and the material's opto-electronic properties. Nanostructures have a large impact on band structure and electronic properties through quantum confinement. An additional large effect is the change in band structure due to elastic strain caused by lattice mismatch between the barrier and nanostructured (usually self-assembled QDs) materials. To develop a material model for advanced concept solar cells, the band structure is calculated for single as well as vertical array of quantum dots with the realistic effects such as strain, associated with the epitaxial growth of these materials. The results show significant effect of strain in band structure. More importantly, the band diagram of a vertical array of QDs with different spacer layer thickness show significant change in band offsets, especially for heavy and light hole valence bands when the spacer layer thickness is reduced. These results, ultimately, have significance to develop a material model for advance concept solar cells that use the QD nanostructures as absorbing medium. The band structure calculations serve as the basis for multiple other calculations. Chief among these is that the model allows the design of a practical QD advanced concept solar cell, which meets key design criteria such as a negligible valence band offset between the QD/barrier materials and close to optimum band gaps, resulting in the predication of optimum material combinations. / Dissertation/Thesis / Ph.D. Electrical Engineering 2011

abstract: This dissertation focuses on the structural and optical properties of III-V semiconductor materials. Transmission electron microscopy and atomic force microscopy are used to study at the nanometer scale, the structural properties of defects, interfaces, and surfaces. A correlation with optical properties has been performed using cathodoluminescence. The dissertation consists of four parts. The first part focuses on InAs quantum dots (QDs) embedded in a GaInP matrix for applications into intermediate band solar cells. The CuPt ordering of the group-III elements in Ga0.5In0.5P has been found to vary during growth of InAs QDs capped with GaAs. The degree of ordering depends on the deposition time of the QDs and on the thickness of the capping layer. The results indicate that disordered GaInP occurs in the presence of excess indium at the growth front. The second part focuses on the effects of low-angle off-axis GaN substrate orientation and growth rates on the surface morphology of Mg-doped GaN epilayers. Mg doping produces periodic steps and a tendency to cover pinholes associated with threading dislocations. With increasing miscut angle, the steps are observed to increase in height from single to double basal planes, with the coexistence of surfaces with different inclinations. The structural properties are correlated with the electronic properties of GaN epilayers, indicating step bunching reduces the p-type doping efficiency. It is also found that the slower growth rates can enhance step-flow growth and suppress step bunching. The third part focuses on the effects of inductively-coupled plasma etching on GaN epilayers. The results show that ion energy rather than ion density plays the key role in the etching process, in terms of structural and optical properties of the GaN films. Cathodoluminescence depth-profiling indicates that the band-edge emission of etched GaN is significantly quenched. The fourth part focuses on growth of Mg-doped GaN on trench patterns. Anisotropic growth and nonuniform acceptor incorporation in p-GaN films have been observed. The results indicate that growth along the sidewall has a faster growth rate and therefore a lower acceptor incorporation efficiency, compared to the region grown on the basal plane. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

The absorption of the sun's radiation and its efficient conversion to useful work by a photovoltaic solar cell is of interest to the community at large. Scientists and engineers are particularly interested in approaches that exceed the Shockley-Queisser limit of photovoltaic solar-energy conversion. The abstract notion of increasing the efficiency of photovoltaic solar cells by constructing a three-transition solar cell via an absorber with intermediate levels is well-established. Until now, proposed approaches to realize the three-transition solar cell do not render the efficiency gains that are theorized; therefore, researchers are experimenting to ascertain where the faults lie. In my opinion, it is unclear if the abstract efficiency gains are obtainable. Furthermore, it is difficult to determine whether three-transition absorbers are even incorporated in the existing three-transition solar cell prototypes. I assert that there are material systems derived from the technologically important compound semiconductors and their ternary alloys that more clearly determine the suitability of employing nanostructured absorbers to realize a three-transition solar cell. The author reports on a nanostructured absorber composed of InAs quantum dots completely enveloped in a GaAsSb matrix that is grown by molecular beam epitaxy. The material system, InAs/GaAs$_{0.88}$Sb$_{0.12}$, is identified as an absorber for a three transition solar cell. This material system will more easily determine the suitability of employing nanostructured absorbers because its quantum-dot heterojunctions have negligible valence-band discontinuities, which abate the difficulty of interpreting optical experimental results. A key tool used to identify the GaAs$_{1-x}$Sb$_{x}$ ($xapprox 0.12$) is a maximum-power iso-efficiency contour plot. This contour plot is only obtainable by first having analyzed the impact of both finite intermediate-band width and spectral selectivity on the optimized detailed-balance conversion efficiencies of the three-transition solar cell. Obtaining the contour plot is facilitated by employing a rapid and precise method to calculate particle flux (Appendix~ ef{ch:Rapid-Precise}). The author largely determines the electronic structure of the InAs/GaAs$_{1-x}$Sb$_{x}$ ($xapprox 0.12$) absorber that is grown by molecular beam epitaxy from optical experimental methods and in particular, from photoluminescent spectroscopy. The interpretation of the experimental photoluminescent spectrum is facilitated by having first studied the theoretical photoluminescent spectra of idealized three-transition absorbers.

Light from the sun is an attractive source of energy for its renewability, supply, scalability, and cost. Silicon solar cells are the dominant technology of choice for harnessing solar energy in the form of electricity, but the designs are approaching their practical efficiency limits. New multijunction designs which use the tunable properties of the more expensive III-V semiconductors have historically been relegated to space applications where absolute power conversion efficiency, resilience to radiation, and weight are more important considerations than cost. Some of the more recent developments in the field of semiconductor materials are the so-called III-nitride materials which mainly use either indium, aluminum or gallium in combination with nitrogen. Indium gallium nitride (InGaN) is one of these III-nitride semiconductor alloys that can be tailored to span the vast majority of the solar spectrum. While InGaN growth traditionally requires expensive substrate materials such as sapphire, three-dimensional nanowire growth modes enable high quality lattice mismatched growth of InGaN directly on silicon without a metamorphic buffer layer. The absorption and electronic properties of InGaN can also be tuned by incorporating it into quantum confined regions in a GaN host material. This opens up a route towards cost-effective, high efficiency devices such as light emitted diodes and solar cells which can operate over a large range of wavelengths. The combination of the two material systems of InGaN/GaN and silicon can marry the low cost of silicon wafers with the desirable optoelectronic properties of III-nitride semiconductors. This thesis investigates the potential for highly nanostructured InGaN/GaN based devices using quantum-dot-in-nanowire designs as novel solar cells which can enable intermediate band absorption effects and multiple junctions within a single nanowire to absorb more of the solar spectrum and operating more efficiently. Such semiconductor nanostructures can in principle reach power conversion efficiencies of over 40\% on silicon, with a cost closer to conventional silicon solar cells as opposed to methods which use non-silicon substrates. In the primary strategy, the nanowires contain InGaN quantum dots which act as photon absorption/carrier generation centres to sequentially excite photons within the large band gap semiconductor. By using this intermediate band of states, large operating voltages between contacts can be maintained without sacrificing the collection of long wavelength solar photons. In this work, we characterize the properties of such nanowires and experimentally demonstrate sub-bandgap current generation in a large area InGaN/GaN dot-in-nanowire solar cell. Experimental characterization of InGaN / GaN quantum dots in nanowires as both LEDs and solar cells is performed to determine the nanowire material parameters to understand how they relate to the nanowire device performance. Multiple microscopy techniques are performed to determine the nanowire morphology and contact effectiveness. Optical characterization of bare and fabricated nanowires is used to determine the anti-reflection properties of nanowire arrays. Photoluminescence and electroluminescence spectroscopy are performed. Illuminated current-voltage characteristics and quantum efficiencies are determined. Specular and diffuse reflectivities are measured as a function of wavelength. Technology computer-aided design (TCAD) software is used to simulate the performance of the overall nanowire device. The contribution from quantum dots or quantum wells is simulated by solving for the carrier wavefunctions and density of states with the quantum structures. The discretized density of states from the quantum dots is modelled and used in a complete drift-diffusion device simulation to reproduce electroluminescence results. The carrier transport properties are modified to demonstrate effects on the overall device performance. An alternate design is also proposed which uses an InGaN nanowire subcell on top of a silicon bottom subcell. The dual-junction design allows a broader absorption of the solar spectrum, increasing the operating voltage through monolithically grown series-connected, current-matched subcells. The performance of such a cell is simulated through drift-diffusion simulations of a dual-junction InGaN/Si solar cell. The effects of switching to a nanowire subcell based on the nanowires studied in this thesis is discussed.

To cope with energy requirements the utilization of renewable energies, particularly the Sun supplies the biggest and abundant energy source in Earth. Photo-voltaic and solar cell are the well advance and burning technology and a field of hot research. Majority of research centers and universities are working in this field. 1G, 2G, 3G and next generation of photo-voltaic cells have been developed and still to improve its efficiency and to decrease it 0.2 $/W cost. Our work mainly based on the theoretical and physical analysis of thin-film Photovoltaic devices. We will explore different software used for the analysis of PV cells, and will analyse different simulation related to solar cells like open circuit voltage VOC, Short circuit current JSC, Fill Factor FF (%) and external Quantum efficiency (%) for thin film solar cell including CIGS, CIS, CGS, CdTe, SnS/CdS/ZnO etc. To have different analysis for different combination and different replacement for materials used in the solar cell fabrication. To cope with the PV cost and environmental hazards we have to find alternate solutions. / Ullah, H. (2015). Simulation studies of photovoltaic thin film devices [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/48800

Ce travail s’intéresse aux cellules solaires multi-transitions. Deux semiconducteurs à niveaux subbandgap : un highly mismatched alloy, le GaAsPN, et un absorbeur à boites quantiques. Les états subbandgap permettent de modifier l’énergie de gap ou de créer une bande intermédiaire au milieu du gap. En premier lieu, une introduction de la cellule solaire par l’étude de luminescence est présentée. Des liens entre luminescence et propriétés électriques sont établis, et les limites thermodynamiques de l’efficacité des dispositifs multi-transitions sont explicitées. Enfin, une méthode optique de caractérisation des cellules solaires est démontrée. La première partie expérimentale de la thèse est dédiée au développement d’une top cell en GaAsPN en accord de maille avec une bottom cell en Silicium. Des simulations numériques ont mis en évidence les difficultés à surmonter pour ce type de matériau. La dynamique des porteurs a été étudiée par photoluminescence en régime permanent et résolue en temps. Ces mesures ont mis en évidence que les absorbeurs crûs souffraient d’états fortement localisés, majoritairement dus à des clusters d’azote. Ces états nous ont permis en revanche d’étudier les propriétés de bande intermédiaire de cet alliage. Enfin, une méthode optique de caractérisation, adaptée aux IBSCs et à la mise en évidence des deux mécanismes clés de ce concept (two-step two-photon absorption et la préservation de la tension). Cette méthode a été appliquée à deux candidats pour les IBSCs, un absorbeur à multi-puits quantiques et un à boîtes quantiques. Les résultats montrent que l’absorbeur à boîtes quantiques présente un comportement compatible avec les IBSCs. / This thesis deals with the multi-transition solar cells by studying two subband gap localised states materials: one highly mismatched alloy, GaAsPN, and one multi-stacked quantum dots heterostructure. These subband gap states give the possibility to tune the band gap energy or create two photon transitions inside a single the absorber. In a first part, a radiance based introduction of the solar cell is presented. Links between radiances and electrical properties are pointed out. From this analysis, the thermodynamic limits of the single and multiple transition solar cells are derived and key mechanisms for multi-transition solar cells are identified. A universal optical characterisation method for probing electrical properties of solar cells is displayed. The first experimental part of this thesis was dedicated to the development of a GaAsPN based pin top cell lattice matched with a Silicon bottom cell. Numerical simulations have been carried out. Carrier dynamics has been studied by steady-state and time-resolved photoluminescence, with the conclusion that the GaAsPN we grew still suffer from multiple strongly localised states below the band gap, mainly due to N-clusters. Finally, we have taken advantages of the strong carrier localisation for a use as an intermediate band solar cell. Eventually, a quantitative optical characterisation method was developed in order to evaluate the potential of an absorber as an IBSC. The two key processes, the two-step two-photon absorption and the voltage preservation, can be widely investigate through it. This method has been applied to two IBSC candidates, a MQW and a MSQD absorbers. The MSQD cell have shown IB compatibility.

[pt] A célula solar de banda intermediária (IBSC) é um dispositivo de terceira geração alternativo à célula solar de junção única e permite atingir maior eficiência mantendo a simplicidade de ter apenas uma junção pn, garantindo baixo custo e baixa complexidade de fabricação. Nesta tese, um extenso trabalho experimental é apresentado, utilizando as técnicas de microscopia de força atômica, microscopia eletrônica de transmissão, catodoluminescência e fotoluminescência, além de extenso trabalho teórico baseado em simulações realizadas com os programas nextnano e SCAPS. Através dos dados obtidos, é discutida a escolha do InGaP para a matriz da célula solar e do InAs para os pontos quânticos; a inclusão das field damping layers, que minimizam o efeito negativo do campo elétrico sobre os pontos quânticos; o desordenamento do InGaP bulk; como pontos quânticos pequenos e com cap layers de menor espessura alteram a tendência de ordenamento das camadas subsequentes de InGaP; a inclusão de uma camada de GaP para garantir a qualidade das interfaces durante o crescimento da estrutura; e a otimização dos pontos quânticos para atingir a energia ideal teórica para a banda intermediária. Cinco estruturas completas de células solares de referência e de banda intermediária baseadas nas discussões apresentadas são então propostas para crescimento futuro. Estas estruturas de IBSC devem apresentar figuras de mérito interessantes, como VOC entre 1,32 eV e 1,44 eV (1; 2), aumento entre 5 por cento e 50 por cento na ISC (3) e baixos efeitos resistivos, garantindo FF alto e eficiências superiores à das células solares de referência. / [en] The intermediate band solar cell (IBSC), an alternative to the single junction solar cell, is a third generation device that achieves greater efficiency while maintaining the simplicity of having only one pn junction, guaranteeing low cost and low complexity to manufacture. In this thesis, an extensive experimental work is presented, using atomic force microscopy, transmission electron microscopy, cathodoluminescence and photoluminescence techniques, in addition to an extensive theoretical work based in simulations performed with nextnano and SCAPS softwares. Through the obtained data, the choice of InGaP for the solar cell matrix and InAs for the quantum dots; the inclusion of field damping layers to minimize the negative effect of the electric field on the quantum dots; the disordering of bulk InGaP; the way small quantum dots with thinner cap layers alter the ordering tendency of subsequent layers of InGaP; the inclusion of a GaP layer to ensure the interfaces’ quality during the structure s growth; and the quantum dots optimization to reach the intermediate band ideal theoretical energy are discussed. Five complete structures for reference and intermediate band solar cells based in the presented discussions are then proposed for future growth. These IBSC structures should present interesting figures of merit, such as a VOC ranging between 1,32 eV and 1,44 eV (1; 2), an increase between 5 per cent and 50 per cent in ISC (3) and low resistance effects, ensuring a high FF and efficiencies superior to the reference solar cells.

Im ersten Teil der vorliegenden Arbeit wird der Einfluss der Abscheideparamter von In2S3 Dünnfilmen, die mittels thermischem Verdampfen hergestellt wurden, auf ihre physikalischen Eigenschaften untersucht. Es zeigte sich, dass die Abscheideparameter einen starken Einfluss auf die Oberflächenmorphologie und die strukturellen Eigenschaften haben. Durch eine Optimierung der Herstellungsparameter konnten β-In2S3 Dünnfilme in (103) Orientierung hergestellt werden. Epitaktisches Wachstum von In2S3 Schichten mit jeweils zwei bzw. vier Rotationsdomainen wurden auf c- und a-Saphir erreicht. Die fundamentale optische Bandlücke wurde für alle Dünnfilme zu 2.1 eV bestimmt. Eine starke persistente Photoleitung, welche auf tiefe Defekte innerhalb der Bandlücke zurückgeführt werden konnte, wurde unabhängig von den Abscheideparametern und dem gewählten Substrat beobachtet. Prototypen für Solarzellen wurden aus n-In2S3 und p-Zinkkobaltoxid (ZCO) hergestellt und zeigen ein hohes Sperrverhältniss und photovoltaische Aktivität, welche jedoch durch Absorption im ZCO limitiert wird. Im zweiten Teil der Arbeit wurden In2S3:V Dünnfilme ohne bzw. mit Saat- und Pufferschichten hergestellt, um deren physikalische Eigenschaften zu untersuchen bzw. um Zwischenbandsolarzellen herzustellen. Ein großer Dotierbereich von bis zu 11.4 at-% V, wurde durch einen kombinatorischer Ansatz erziehlt. Für Dünnfilme ohne Saatschicht wurde die Löslichkeitsgrenze von Vanadium in In2S3 zu 3.2 at-% V (auf Saphirsubstraten) bzw. 5.4 at-% V (auf Glassubstraten) bestimmt. Durch die Verwendung einer Saatschicht konnte die In2S3 β-Phase stabilisiert und darüber hinaus die Ausbildung von Fremdphasen unterdrückt werden. In2S3:V Dünnfilme mit über 5.8 at-% V auf Saphirsubstraten zeigten bei Raumtemperatur p-Typ Leitfähigkeit. Für Temperaturen unterhalb einer kritischen Temperatur ergab sich ein Wechsel von p- zu n-Leitung. Darüber hinaus sank die Mobilität dieser Schichten unterhalb der kritischen Temperatur signifikant ab. Die Ladungsträgerdichte blieb jedoch über den gesamte Temperaturbereich hinweg konstant und war mit Werten im Bereich von 1022 cm−3 zudem sehr hoch. Diese elektrischen Eigenschaften sind sehr untypisch für einen gewöhnlichen Halbleiter. Sie konnten jedoch im Rahmen dieser Arbeit durch das Modell der Zwischenbandsolarzelle beschieben werden. Als Schlussfolgerung dessen, wurde die Vanadiumkonzentration, bei der sich das Zwischenband ausbildet zu 3.2 at-% V bestimmt. Da sich herausstellte, das In2S3:V bei Raumtemperatur p-Typ ist, konnten keine Zwischenbandsolarzellen mit p-ZCO hergestellt werden.:1 Introduction 1 2 Theoretical background 3 2.1 Indium sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2 The physics of solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3 The concept of intermediate band solar cells . . . . . . . . . . . . . . . 8 2.4 Indium sulfide as intermediate band material . . . . . . . . . . . . . . . 11 2.5 Charge transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.6 Electronic defect states . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3 Methods 17 3.1 Growth and structuring techniques . . . . . . . . . . . . . . . . . . . . 17 3.1.1 Thermal evaporation . . . . . . . . . . . . . . . . . . . . . . . . 17 3.1.2 Pulsed laser deposition . . . . . . . . . . . . . . . . . . . . . . . 19 3.1.3 Sputter deposition . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.1.4 Photolithography . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.2 characterization techniques . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.2.1 X-ray diffraction measurement . . . . . . . . . . . . . . . . . . . 22 3.2.2 Hall effect measurement . . . . . . . . . . . . . . . . . . . . . . 23 3.2.3 Current-voltage measurement . . . . . . . . . . . . . . . . . . . 25 3.2.4 Temperature-dependent current-voltage measurement . . . . . . 26 3.2.5 Resistance measurement . . . . . . . . . . . . . . . . . . . . . . 26 3.2.6 Spectroscopic ellipsometry . . . . . . . . . . . . . . . . . . . . . 26 3.2.7 Energy dispersive X-ray spectroscopy . . . . . . . . . . . . . . . 27 3.2.8 Transmittance and reflection spectroscopy . . . . . . . . . . . . 27 4 Physical properties of undoped In2S3 . . . . . . . . . .29 4.1 Impact of the growth parameters on the composition . . . . . . . . . . 31 4.2 Desorption mechanisms and their influence on the growth rates . . . . . 33 4.3 Surface morphological properties . . . . . . . . . . . . . . . . . . . . . 35 4.4 Structural properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.5 Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4.5.1 Dielectric function and absorption coefficient of In2S3 . . . . . . 43 4.5.2 Impact of the growth parameter . . . . . . . . . . . . . . . . . . 48 4.5.3 Impact of the composition . . . . . . . . . . . . . . . . . . . . . 49 4.5.4 Impact of the substrate crystallinity . . . . . . . . . . . . . . . . 51 4.6 Electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.6.1 Persistent photoconductivity . . . . . . . . . . . . . . . . . . . . 52 4.6.2 Temperature dependent resistivity and Hall effect measurements 63 4.7 Device characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4.7.1 Impact of the growth parameter . . . . . . . . . . . . . . . . . . 70 4.7.2 Impact of the substrate crystallinity . . . . . . . . . . . . . . . . 79 4.8 Solar cell performance . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.8.1 Impact of the growth parameter . . . . . . . . . . . . . . . . . . 83 4.8.2 Impact of the substrate crystallinity . . . . . . . . . . . . . . . . 88 5 Physical properties of vanadium-doped In2S3. . . . . . . . . .91 5.1 Vanadium incorporation into the In2S3 thin films . . . . . . . . . . . . 93 5.2 Surface morphological properties . . . . . . . . . . . . . . . . . . . . . 95 5.3 Structural properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 5.4 Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 5.5 Electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 5.6 Device characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 6 Summary and Outlook . . . . . . . . . .125 List of Abbreviations. . . . . . . . . . 131 List of Symbols. . . . . . . . . . 133 Bibliography . . . . . . . . . .137 List of Own and Contributed Articles . . . . . . . . . .149 Appendix . . . . . . . . . .151 Publikationsliste nach Promotionsordnung § 11(3). . . . . . . . . . 161 Zusammenfassung nach Promotionsordnung § 11(4) . . . . . . . . . .163 / In the first part of the presented work the influence of the growth parameter of In2S3 thin films, grown by physical vapor deposition, on their physical properties is investigated. The deposition parameters were found to have a strong influence on the surface morphology and the structural properties. By choosing appropriate deposition parameters β-phase In2S3 with a pure (103) orientation was achieved. Epitaxial growth with 2 and 4 rotational domains could be induced using c- and a-plane sapphire, respectively. The fundamental optical bandgap was determined to be direct with an energy of 2.1 eV for all In2S3 thin films. A strong persistent photoconductivity, which was attributed to deep defects within the bandgap, was observed for all In2S3 thin films independent of the preparation conditions and independent of the kind of substrate. Solar cells of n-In2S3/p-zinc-cobalt-oxide (ZCO) exhibit high current rectifications and photovoltaic activity but suffer from absorption in the ZCO layer. To study the physical properties of In2S3:V thin films and to implement intermediate band solar cells (IBSC) In2S3:V thin films without and with seed and buffer layers were fabricated, respectively. Using a combinatorial material synthesis approach doping concentrations of up to 11.4 at-% V were achieved. Thin films without seed layers exhibit a solubility limit of vanadium of 3.2 at-% V and 5.4 at-% V for thin films on sapphire and glass substrates, respectively. The In2S3:V β-phase could be stabilized and the formation of secondary phases suppresed by inserting a seed layer. A change of the type of the charge carriers from p-type at room temperature to n-type at low temperatures was observed for thin films with doping concentrations above 5.8 at-% V on sapphire substrates. Furthermore, the mobility decreases significantly below the critical temperature. Contrarily, a very high charge carrier concentration was observed independent of the temperature. This behavior, which is untypical for conventional semiconductors, could be described using the intermediate band (IB) model. According to the results of this work and the IB model, one can conclude, that above a vanadium concentration 3.2 at-% V an IB has formed. Due to the p-type conductivity of In2S3:V thin films at room temperature, rectifying IBSCs could not be implemented using p-type ZCO. Therefore, it should be replaced by an n-type material in future investigations.:1 Introduction 1 2 Theoretical background 3 2.1 Indium sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2 The physics of solar cells . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3 The concept of intermediate band solar cells . . . . . . . . . . . . . . . 8 2.4 Indium sulfide as intermediate band material . . . . . . . . . . . . . . . 11 2.5 Charge transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.6 Electronic defect states . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3 Methods 17 3.1 Growth and structuring techniques . . . . . . . . . . . . . . . . . . . . 17 3.1.1 Thermal evaporation . . . . . . . . . . . . . . . . . . . . . . . . 17 3.1.2 Pulsed laser deposition . . . . . . . . . . . . . . . . . . . . . . . 19 3.1.3 Sputter deposition . . . . . . . . . . . . . . . . . . . . . . . . . 20 3.1.4 Photolithography . . . . . . . . . . . . . . . . . . . . . . . . . . 21 3.2 characterization techniques . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.2.1 X-ray diffraction measurement . . . . . . . . . . . . . . . . . . . 22 3.2.2 Hall effect measurement . . . . . . . . . . . . . . . . . . . . . . 23 3.2.3 Current-voltage measurement . . . . . . . . . . . . . . . . . . . 25 3.2.4 Temperature-dependent current-voltage measurement . . . . . . 26 3.2.5 Resistance measurement . . . . . . . . . . . . . . . . . . . . . . 26 3.2.6 Spectroscopic ellipsometry . . . . . . . . . . . . . . . . . . . . . 26 3.2.7 Energy dispersive X-ray spectroscopy . . . . . . . . . . . . . . . 27 3.2.8 Transmittance and reflection spectroscopy . . . . . . . . . . . . 27 4 Physical properties of undoped In2S3 . . . . . . . . . .29 4.1 Impact of the growth parameters on the composition . . . . . . . . . . 31 4.2 Desorption mechanisms and their influence on the growth rates . . . . . 33 4.3 Surface morphological properties . . . . . . . . . . . . . . . . . . . . . 35 4.4 Structural properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.5 Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4.5.1 Dielectric function and absorption coefficient of In2S3 . . . . . . 43 4.5.2 Impact of the growth parameter . . . . . . . . . . . . . . . . . . 48 4.5.3 Impact of the composition . . . . . . . . . . . . . . . . . . . . . 49 4.5.4 Impact of the substrate crystallinity . . . . . . . . . . . . . . . . 51 4.6 Electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.6.1 Persistent photoconductivity . . . . . . . . . . . . . . . . . . . . 52 4.6.2 Temperature dependent resistivity and Hall effect measurements 63 4.7 Device characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4.7.1 Impact of the growth parameter . . . . . . . . . . . . . . . . . . 70 4.7.2 Impact of the substrate crystallinity . . . . . . . . . . . . . . . . 79 4.8 Solar cell performance . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.8.1 Impact of the growth parameter . . . . . . . . . . . . . . . . . . 83 4.8.2 Impact of the substrate crystallinity . . . . . . . . . . . . . . . . 88 5 Physical properties of vanadium-doped In2S3. . . . . . . . . .91 5.1 Vanadium incorporation into the In2S3 thin films . . . . . . . . . . . . 93 5.2 Surface morphological properties . . . . . . . . . . . . . . . . . . . . . 95 5.3 Structural properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 5.4 Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 5.5 Electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 5.6 Device characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 6 Summary and Outlook . . . . . . . . . .125 List of Abbreviations. . . . . . . . . . 131 List of Symbols. . . . . . . . . . 133 Bibliography . . . . . . . . . .137 List of Own and Contributed Articles . . . . . . . . . .149 Appendix . . . . . . . . . .151 Publikationsliste nach Promotionsordnung § 11(3). . . . . . . . . . 161 Zusammenfassung nach Promotionsordnung § 11(4) . . . . . . . . . .163