Optimization of Lead Base Perovskite Solar Cell with ZnO and CuI as Electron Transport Material and Hole Transport Material Using SCAPS-1D

Perovskite solar cells (PSCs) research is substantially drawing attention because of the fast improvement in their power conversion efficiency (PCE), cheapness, possibility to tune the bandgap, low recombination rate, high open circuit voltage, excellent ambipolar charge carrier transport and strong and broad optical absorption. In this research, Zinc oxide as electron transport material (ETM) and copper iodide as hole transport material (HTM) have been optimized using SCAPS-1D simulation software. The thickness, bandgap, of ZnO (ETM) and CuI (HTM) was investigated. Results shows that the thickness and bandgap were found to strongly influence the PCE of perovskite solar cell. ZnO/CuI   was found to be a better replacement to TiO2/Cu2O for stability and low degradation rate. It was observed that the maximum efficiency is 22.04%, Voc of 0.84V, JSC of 32.83mA/cm2 and FF of 79.79% was obtained when the thickness of ETM and HTM layer of (CH3NH3PbI3) PSCs which was found to be optimum at 0.2μm for the final optimization

An experimental study of graphitic carbon nitride-based materials and selected metal-based semiconductors in organic solar cells combined with a computational study of perovskite solar cells.

Doctoral Degree. University of KwaZulu-Natal, Durban.Abstract available in PDF

Thin-film carbon nitride (C2N)-based solar cell optimization considering Zn1−xMgxO as a buffer layer

Carbon nitride (C2N), a two-dimensional material, is rapidly gaining popularity in the photovoltaic (PV) research community owing to its excellent properties, such as high thermal and chemical stability, non-toxic composition, and low fabrication cost over other thin-film solar cells. This study uses a detailed numerical investigation to explore the influence of C2N-based solar cells with zinc magnesium oxide (Zn1−xMgxO) as a buffer layer. The SCAPS-1D simulator is utilized to examine the performance of four Mg-doped buffer layers (x = 0.0625, 0.125, 0.1875, and 0.25) coupled with the C2N-based absorber layer. The influence of the absorber and buffer layers’ band alignment, quantum efficiency, thickness, doping density, defect density, and operating temperature are analyzed to improve the cell performance. Based on the simulations, increasing the buffer layer Mg concentration above x = 0.1875 reduces the device performance. Furthermore, it is found that increasing the absorber layer thickness is desirable for good device efficiency, whereas a doping density above 1015 cm−3 can degrade the cell performance. After optimization of the buffer layer thickness and doping density at 40 nm and 1018 cm−3 , the cell displayed its maximum performance. Among the four structures, C2N/Zn0.8125Mg0.1875O demonstrated the highest PCE of 19.01% with a significant improvement in open circuit voltage (Voc), short circuit density (Jsc), and fill factor (FF). The recorded results are in good agreement with the standard theoretical studies.Web of Science111art. no. 9

Synthesis and Characterization of Indium Phosphide Quantum Dots for Photoelectrochemical Applications

[ES] Hoy en día, existen desafíos tecnológicos y de ingeniería que se beneficiarían de las contribuciones de la nanociencia y la nanotecnología. A esta escala, las propiedades físicas y químicas de los sistemas han de cumplir con el respeto al medio ambiente (ahorro de energía, minimización de la contaminación, calentamiento global, etc.). Para estos fines, las nanopartículas basadas en puntos cuánticos de semiconductores II-VI "Quantum Dots" han sido las más estudiadas. Entre varios materiales, los puntos cuánticos de InP (InP-QDs) han despertado un gran interés debido a las características de baja toxicidad. Este prometedor elemento es el tema central de esta tesis. Para obtener partículas monodispersas en solución, la ruta de inyección en caliente presenta varias ventajas que la convierten en una técnica útil para controlar el tamaño de las nanopartículas. Este trabajo trata de la síntesis de puntos cuánticos de InP por el método de inyección en caliente para aplicaciones fotoelectroquímicas. Comenzamos nuestro trabajo optimizando la síntesis de InP QDs por el método de inyección en caliente mientras estudiamos los parámetros de la síntesis sobre las propiedades morfológicas, estructurales y especialmente las propiedades de fotoluminiscencia de los puntos cuánticos de InP. Inicialmente, la optimización de las condiciones de los puntos cuánticos se basó en la mejora de las propiedades ópticas, en particular la fotoluminiscencia. Cuando pasivamos los InP QDs con una envolvente de ZnS, la doble envolvente ZnS/ZnS, logra disminuir los defectos superficiales y esto resulta en la mejora de la fotoluminiscencia de los InP QDs. Además, la morfología superficial de estos QDs tiene una forma esférica más regular y homogénea. Por otro lado, las propiedades ópticas de los InP QDs dopados con vanadio no mostraron ninguna mejora en la fotoluminiscencia, mientras que si se observó una disminución en el tamaño de las nanopartículas. El segundo objetivo de esta tesis gira en torno a los QDs de InP depositados en nanotubos metálicos de dióxido de titanio (TiO2) por el método de recubrimiento por centrifugado con el fin de comparar la eficiencia fotoelectroquímica de los QDs de InP (núcleo), los QD de InP/ZnS de núcleo/corteza y los QD de InP/ZnS/ZnS de núcleo/corteza/corteza. Este estudio muestra un aumento en la fotocorriente casi 4 y 6 veces mayor que TiO2 / InP QDs. Esta medición tiene como objetivo observar el comportamiento dinámico del material y evaluar si las cargas se recombinan rápidamente en los nanotubos de TiO2 a partir de los puntos cuánticos. Se obtuvo una buena eficiencia en la respuesta de fotocorriente después del sistema de crecimiento del sistema núcleo/corteza/corteza debido a la pasivación de sitios de recombinación no radiativos, como los estados de trampas superficiales. Este resultado fue confirmado los estudios de simulación de los diferentes parámetros que caracterizan la célula solar basada en TiO2/InP, TiO2/InP/ZnS y TiO2/InP/ZnS/ZnS con el software SCAPS-1D. Según los cálculos numéricos, se ha obtenido un buen rendimiento de la mencionada célula con la adición de capa de ZnS. Los resultados de la simulación muestran que el InP fue capaz de utilizar todo el espectro de luz cuando se recubrió con la capa de ZnS en la parte superior.[CA] Avui dia, hi ha desafiaments tecnològics i d'enginyeria que es beneficiarien de les contribucions de la nanociència i la nanotecnologia. En aquesta escala, les propietats físiques i químiques dels sistemes han de complir amb el respecte al medi ambient (estalvi d'energia, minimització de la contaminació, escalfament global, etc.). Per a aquestes finalitats, les nanopartícules basades en punts quàntics de semiconductors II-VI "Quantum Dots" han estat les més estudiades. Entre diversos materials, els punts quàntics d'InP (InP-QDs) han despertat un gran interès a causa de les característiques de baixa toxicitat. Aquest prometedor element és el tema central d'aquesta tesi. Per obtenir partícules monodisperses en solució, la ruta d' injecció en calent presenta diversos avantatges que la converteixen en una tècnica útil per controlar la mida de les nanopartícules. Aquest treball tracta de la síntesi de punts quàntics d'InP pel mètode d'injecció en calent per a aplicacions fotoelectroquímiques. Comencem el nostre treball optimitzant la síntesi d'InP QDs pel mètode d'injecció en calent mentre estudiem els paràmetres de la síntesi sobre les propietats morfològiques, estructurals i especialment les propietats de fotoluminiscència dels punts quàntics d'InP. Inicialment, l' optimització de les condicions dels punts quàntics es va basar en la millora de les propietats òptiques, en particular la fotoluminiscència. Quan passivem els InP QDs amb una envolupant de ZnS, la doble envolupant ZnS/ZnS, aconsegueix disminuir els defectes superficials i això resulta en la millora de la fotoluminiscència dels InP QDs. A més, la morfologia superficial d' aquests QDs té una forma esfèrica més regular i homogènia. D'altra banda, les propietats òptiques dels InP QDs dopats amb vanadi no van mostrar cap millora en la fotoluminiscència, mentre que si es va observar una disminució en la mida de les nanopartícules. El segon objectiu d'aquesta tesi gira al voltant dels QDs d'InP dipositats en nanotubs metàl·lics de diòxid de titani (TiO2) pel mètode de recobriment per centrifugat per tal de comparar l'eficiència fotoelectroquímica dels QDs d'InP (nucli), els QD d'InP/ZnS de nucli/cortesa i els QD d'InP/ZnS/ZnS de nucli/cortesa/cortesa. Aquest estudi mostra un augment en la fotocorrent gairebé 4 i 6 vegades més gran que TiO2 / InP QDs. Aquest mesurament té com a objectiu observar el comportament dinàmic del material i avaluar si les càrregues es recombinen ràpidament en els nanotubs de TiO2 a partir dels punts quàntics. Es va obtenir una bona eficiència en la resposta de fotocorrent després del sistema de creixement del sistema nucli/cortesa/cortesa a causa de la passivació de llocs de recombinació no radiatius, com els estats de trampes superficials. Aquest resultat va ser confirmat els estudis de simulació dels diferents paràmetres que caracteritzen la cèl·lula solar basada en TiO2/InP, TiO2/InP/ZnS i TiO2/InP/ZnS/ZnS amb el programari SCAPS-1D. Segons els càlculs numèrics, s' ha obtingut un bon rendiment de l' esmentada cèl·lula amb l' addició de capa de ZnS. Els resultats de la simulació mostren que l'InP va ser capaç d'utilitzar tot l'espectre de llum quan es va recobrir amb la capa de ZnS a la part superior.[EN] Today, there are modern technological and engineering challenges that would advantage from the contributions of the nanoscience community and nanotechnology. At this scale, the physical and chemical properties of the systems are highly dependent on respect for the environment (energy saving, pollution minimization, global warming etc¿). In this term, nanoparticles based on II-VI semiconductors "Quantum Dots" have been by far the most studied. Among several material, InP Quantum Dots has brought huge interest because of the low toxicity features. This promising element is the subject of this thesis. Hence, to obtain monodisperse particles in solution, the hot injection route has several advantages that make it a useful technique, such as controlling the size of the nanoparticles. This work deals with the synthesis of InP Quantum Dot by hot injection method as the basis for photoelectrochemical application. We started our work by optimizing the synthesis of InP QDs by the hot injection method while studying the synthesis parameters on the morphological, structural, and specially the photoluminescence properties of InP Quantum Dots. Initially, the optimization of the Quantum Dots conditions was based on the enhancement the optical properties in particular the photoluminescence. When we passivated the InP QDs by ZnS shell, ZnS/ZnS double shell we succeed to decrease the surface defects resulting on the enhancement of the InP QDs photoluminescence. Also, the surface morphology of these QDs has a more regular spherical form and is well dispersed. On the other hand, the optical properties of the InP QDs doped with Vanadium was shown no improvement in the photoluminescence while there's a decrease on the size of the nanoparticle. The second aim of this thesis revolves around InP QDs deposited on metallic titanium dioxide nanotubes TiO2 by spin coating method with a view to comparing the photoelectrochemical efficiency of core InP QDs, core/shell InP/ZnS QDs, and core/shell/shell InP/ZnS/ZnS QDs. This study shows an increase in the photocurrent almost 4 and 6 times higher than TiO2/InP QDs. Hence, this measurement aims to observe the dynamic behavior of the material and to assess whether the charges recombine rapidly into the TiO2 NTAs Nanotubes from the quantum dots. So, a good efficiency in the photocurrent response was obtained following the growth core/shell/shell system due to the successful passivation of non-radiative recombination sites such as surface trap states. This result was supported by a simulation study of the different parameters characterizes the solar cell based TiO2/InP, TiO2/InP/ZnS and TiO2/InP/ZnS/ZnS with software SCAPS-1D. According to this theoretical work, a good performance of the cell has obtained in the adding of ZnS layer. The simulation results show that the InP was able to successfully utilize the full spectrum of light when coated with ZnS layer on top.Harabi, I. (2023). Synthesis and Characterization of Indium Phosphide Quantum Dots for Photoelectrochemical Applications [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/19401

A Comprehensive Review on Current Performance, Challenges and Progress in Thin-Film Solar Cells

Due to the recent surge in silicon demand for solar modules, thin-film photovoltaic (PV) modules have a potential to penetrate the market in significant numbers. As an alternate candidate, thin film technologies in PVs have the ability to achieve better performance. The competing thin-film PV technologies have the flexibility to adapt to any sort of curvature compared to rigid solar cells (SCs). Due to the peculiar characteristics of newer solar materials, stability issues, reflection losses, advancements in electrode materials and dopant materials with a photoactive layer are current challenges driving the industrial-academic voyage of development of solar materials for the betterment of Photo-conversion Efficiency (PCE). Based on the photoactive materials used over time, SC evolution was broadly classified into first, second and third generation SCs. In this review, the basic working mechanisms, various materials used, drawbacks and stability issues of different SCs are discussed extensively. Thin film SCs tend to absorb certain elastic deformations due to their flexible nature and to a certain extent. According to the NREL efficiency chart, multi-junctional SCs exhibit enhanced efficiency as compared to the other SCs. Among the third-generation SCs, the perovskite/Si tandem architecture shows a maximum efficiency of approximately 29%. Thin film flexible SCs find application in various sectors such as automobile, defense and/or energy storage device

Polymer Films for Photovoltaic Applications

Reprints of Polymers Special Issue entitled "Polymer films for photovoltaic applications", which covers all fields related to polymer films for photovoltaic applications, but special attention will be given to the following aspects:- The synthesis and suitable modification of polymer structure, to obtain polymer thin films for PV devices;- The influence of film deposition (thermal vacuum evaporation (TVE), chemical vapor deposition (CVD), spin coating, spray, etc.) on the properties of polymer films;- The thermo-optical properties of polymer thin films and blends of polymer films, as potential parts of PV systems;- The influence of doping or protonation of polymer thin films and blend polymer films on their properties;- Polymer thin films as active layers in PV solar cells—correlation of chemical structure and PV properties;- BHJ solar cells with polymer blends films—the choice of blend film composition to obtain the best PV parameters

Polymer Materials in Sensors, Actuators and Energy Conversion

Polymer-based materials applications in sensors, actuators, and energy conversion play a key role in recently developing areas of smart materials and electronic devices. These areas cover the synthesis, structures, and properties of polymers and composites, including energy-harvesting devices and energy-storage devices for electromechanical (electrical to mechanical energy conversion) and magneto-mechanical (magnetic to mechanical energy conversion), light-emitting devices, and electrically driving sensors. Therefore, the modulation of polymer-based materials and devices for controlling the detection, actuation, and energy with functionalized relative device can be achieved with the present reprint, comprising 12 chapters.This reprint is principally concerned with the topic of materials of materials, especially polymers. The contents not only involve essential information but also possess many novel academic applications in the fields. This Special Issue's title is "Polymer Materials in Sensors, Actuators and Energy Conversion" and covers the research field of polymers .Finally, I am very proud of my dear wife Winnie, son Vincent, and daughter Ruby. I thank them for supporting me in finishing the reprint. The reprint, involving 2 reviews and 10 regular papers, has been accomplished, and I am deeply thankful to all the authors for their assistance in producing a reprint with considerable number of chapters. I also hope that readers can achieve some useful understanding of polymer materials in sensors, actuators, and energy conversion, and that that they will be employed by scientists and researchers

Elaboration et caractérisation de films minces de pérovskites halogénées inorganiques : stabilisation de dispositifs photovoltaïques par ajout de la phase 2D Cs2PbCl2I2

L'objectif des travaux de thèse est de développer un matériau photovoltaïque de 3e génération à base de pérovskites halogénées inorganiques qui soit plus stable et plus efficace. Dans le chapitre 1, un état de l'art sur les pérovskites halogénées est présenté, en mettant l'accent sur les phases 100% inorganiques présentant des propriétés intéressantes pour des applications photovoltaïques. Différentes voies de stabilisation sont discutées, notamment par la substitution cationique et la réduction de la dimension structurale. Le chapitre 2 décrit les méthodes de synthèse utilisées pour l'obtention des matériaux sous forme de poudres, monocristaux et films minces, ainsi que les techniques de caractérisation (micro-) structurales et physiques employées pour déterminer leurs propriétés. Dans le chapitre 3, la phase de type Ruddlesden-Popper Cs2PbCl2I2, mise au jour en 2018 par Li et ses collègues, fait l'objet d'une étude spécifique en raison de sa structure " 2D " originale. L'obtention sous la forme pulvérulente a été facilitée et les préparations de films minces et de monocristaux ont été développées au cours de ces travaux. Par la suite, des substitutions cationiques, sur le site du plomb, ont également permis de modifier les propriétés microstructurales et optoélectroniques. Dans le chapitre 4, les (trans-)formations de films minces de pérovskites dites "3D", de compositions CsPbI3 et CsPbI2Br, ont été analysées en détails par diffraction des rayons X in-situ. La phase noire a été mise en évidence juste après la première étape de spin-coating, à l'ambiante. Deux transitions de phases ont pu être caractérisée à la montée (jusqu'à 320°C) et à la descente. La présence de la phase "2D" permet d'accroître la stabilité de la phase noire de la forme 3D. Enfin au chapitre 5, l'optimisation réalisée sur des dépôts de couches minces d'un mélange de formes 3D et 2D "tout inorganique" est présentée. Un procédé de dépôt sous flux d'azote permet d''obtenir des films minces plus homogènes et compacts. Différents dopages visant à remplacer le plomb dans les films et améliorer la morphologie pour obtenir de meilleures performances ont été réalisés. La stabilité des films ainsi obtenus est discutée. Ces composés, intégrés dans des dispositifs photovoltaïques préparés au laboratoire XLIM de Limoges, ne permettent pas d'augmenter les rendements de conversion des cellules 3D seules mais montrent leur impact sur la stabilité de la phase "3D" noire. Enfin, une étude de la texture des films et des micro-contraintes permet de décrire les effets stabilisateurs de la phase noire.The objective of the thesis is to develop an efficient and stable 3rd generation photovoltaic material based on all- inorganic halide perovskite with 2D phase addition. In Chapter 1, a state of the art on halide perovskite is presented, with an emphasis on 100% inorganic phases exhibiting interesting properties for photovoltaic applications. Different stabilization routes are discussed, in particular by cationic substitution and reduction of the structural dimension. Chapter 2 describes the synthetic methods used to obtain materials in the form of powders, single crystals and thin films, as well as the (micro-)structural and physical characterization techniques used to determine their properties. In Chapter 3, the Ruddlesden-Popper Cs2PbCl2I2 phase, described in 2018 by Li and colleagues, is specifically investigated due to its "2D" structure. Obtaining this phase in the powder form was facilitated and preparations of thin films and single crystals were developed during this work. Subsequently, cationic substitutions, at the lead site, also made it possible to modify the microstructural and optoelectronic properties. The integration of this new "2D" structure, derived from perovskite structure, into a solar cell has been studied. In chapter 4, the thin films (trans-)formations of so-called "3D" perovskites, with stoichiometries CsPbI3 and CsPbI2Br, were analyzed in detail by in-situ X-ray diffraction. The black phase was highlighted just after the first spin-coating step, at room temperature. Two phase transitions could be characterized on the rise (up to 320 ° C) and on the way down. The presence of the "2D" phase increases the stability of the black phase of the 3D form. A "flash" heat treatment in air improves film quality and the influence of the "2D" phase on thin film stresses is discussed. Finally in chapter 5, the optimization carried out on thin film deposition of a mixture of 3D and 2D all-inorganic mix is presented. A nitrogen flow deposition process results in more homogeneous and compact thin films. Various dopings aiming at replacing lead in films and improving morphology to obtain better performance have been carried out. The stability of the films thus obtained is discussed. These compounds, integrated into photovoltaic devices prepared at the XLIM laboratory in Limoges, do not make it possible to increase the conversion yields of 3D cells alone but show a great impact on the stability of the black "3D" phase. A study of film texturing and micro-strains allows us to complete the information on thin layers of perovskites

Extending the Potential of Thin-film Optoelectronics via Optical Engineering

Optoelectronics based on nanomaterials have become a research focus in recent years, and this technology bridges the fields of solid state physics, electrical engineering and materials science. The rapid development in optoelectronic devices in the last century has both benefited from and spurred advancements in the science and engineering of pho- ton detection and manipulation, image sensing, high-efficiency and high-power-density light emission, displays, communications and renewable energy harvesting. A particu- larly promising material class for optoelectronics is colloidal nanomaterials, due to their functionality, cost -efficiency and even new physics, thanks to their exotic properties in the areas of light-matter interaction, low-dimensionality, and solution-processability which dramatically reduces the time and cost required to fabricate thin film devices, and at the same time provides wide compatibility with existing materials interfaces and device structures. This thesis focuses on exploring and assessing the capabilities of lead sulfide quantum dot-based solar cells and photodetectors. The discussion involves advances in techniques such as implementing novel photonic structures, designing and building novel characterization systems and methods, and coupling to external optical structures and components. This thesis comprises three sections. The first section focuses on the design and adap- tion of photonic structures to tailor the function and response of photovoltaics and other absorption-based optoelectronics for specific applications. in the first part, we introduce consideration of complete multi-layer thin film interference effects into the design of so- ii lar cells. By numerical calculation and optimization of the film thicknesses as well as the precise fabrication control, devices with specific target colors or optical transparency levels were achieved. In the second part, we investigate the presence of 2D photonic crystal bands in absorbing materials that can be readily incorporated into nanomaterial thin films through nanostructuring of the material. We carried out simulations and the- oretical analyses and proposed a method to realize simultaneous selectivity in the device reflection, transmission and absorption spectra that are critical for optoelectronic appli- cations. The next section focuses on designing and building a multi-modal microscopy sys- tem for thin-film optoelectronic devices, accompanied with analyses and explanation of complex experimental data. The goal of the system was to provide simultaneous 2D spatial measurements of, including but not limited to, photoluminescence spectra, time- resolved photocurrent and photovoltage responses, and a rich variety of all the possible combinations of these measurements and their associated derived quantities, collected with micrometer resolution. The multi-dimensional data helped us understand the in- tercorrelation between local defective regions in films and the entire device behavior, as well as a more comprehensive profile of mutual relationships between solar cell figures of merit. In the last section, we discuss a new implementation of miniature solar concentrator arrays for lead sulfide quantum dot solar cells. First, we design and analyze the effects of a medium concentration ratio lens-type concentrator made from polydimethylsiloxane, a flexible organosilicon polymer. The concentrators were designed and optimized with the aid of ray-tracing simulation tools to achieve the best compatibility with colloidal nanomaterial-based solar cells. Experimentally, we produced an integrated concentrator system delivering 20-fold current and power enhancements close to the theoretical pre- dictions , and also used our concentrator measurements to explain the rarely-explored carrier dynamics critical to high-power operation of thin film solar cells. Next, we de- iii sign a wide-acceptance-angle dielectric solar concentrator that can be adapted to many types of high- efficiency small-area solar cells. The design was generated using rigorous optical models that define the behaviors of light rays, and was verified with ray-tracing optical simulations to yield results for the full annual 2D time-resolved collectible power for the resulting system. Finally, we discuss strategies for further extending the possi- bilities of nanomaterial-based optoelectronics for future challenges in energy production and related applications

Comprehending and Mitigating Backside Recombination in Cu(In,Ga)Se2 Solar Cells

The aim of this thesis is to comprehend and mitigate backside recombination in Cu(In,Ga)Se2 solar cells. The record Cu(In,Ga)Se2 solar cell has the highest efficiency reaching up to 23.35%[1] at the time of writing this thesis. To achieve such an efficiency level, CsF post-deposition treatments are used to improve the absorber quality in the bulk and on the front surfaces. A good quality absorber and front interface is necessary to make a high efficiency solar cell. Meanwhile, the interface between the metal back contact (molybdenum) and the absorber must be passivated to avoid non-radiative recombination losses due to high backside recombination. Traditionally, this is accomplished by constructing a Ga gradient that creates a higher conduction band gradient towards the backside. Unfortunately, the Ga gradient also introduces an inhomogeneous absorber, leading to higher non-radiative and radiative losses in the solar cells, limiting further improvement in power conversion efficiency. To solve this issue, a functional hole selective transport structure is proposed in this work. One of the critical issues of using hole selective transport layers for Cu(In,Ga)Se2 is that they must withstand the harsh growth condition of Cu(In,Ga)Se2, e.g. high temperature and Se pressure. It means that the introduced hole selective layers should have a good thermal stability to avoid massive diffusion. In this work, we developed a thermally stable hole transport layer which shows comparable passivation effects and transport of holes to the Ga gradient, but with a homogeneous absorber. Since the Ga gradient is not required with the hole selective transport structure, the absorber thickness can be reduced to less than 1.0 μm, thereby lowering manufacturing costs and making Cu(In,Ga)Se2 more cost-competitive with other solar cell technologies. Firstly, to gain a better understanding of how backside recombination affects the quasi-Fermi level splitting or open circuit voltage of solar cells, this study investigates the traditional backside passivation strategies of Ga gradient and metal oxide dielectric layers. The results confirm that reducing backside recombination can improve quasi-Fermi level splitting by at least 40 meV, even with a short minority carrier lifetime of only dozens of nanoseconds. These findings are supported by SCAPS simulations, which also demonstrate similar results. Secondly, after gaining an understanding of the quasi-Fermi level splitting losses caused by backside recombination, this study investigates several candidate hole selective transport layers, which are supposed to mitigate the backside recombination and transport holes simultaneously. Some single layers prove to be thermally unstable due to the harsh growth conditions of Cu(In,Ga)Se2, which causes a negative impact on quasi Fermi-level splitting or open-circuit voltage on solar cells. Others show good thermal stability but provide negligible passivation. To address this issue, the study proposes a combination of a hole transport layer with a metal oxide stabilizer, CuGaSe2/In2O3, which significantly improves thermal stability and provides a good passivation effect that enhances quasi-Fermi level splitting by approximately 80 meV. Beside the good passivation effect, we found that the hole transport properties depend on the excess Cu of the hole selective transport layer. The Cu annealing of CuGaSe2/In2O3 can remove the current blocking effects, which improves the FF from ~40% to ~77%. Thirdly, backside recombination can also impact the optical diode factor, and thus fill factor of solar cells. The optical diode factor (ODF) discussed in this thesis is based on injection level dependent metastable defects that transition from donors to acceptor, which additionally shifts down the Fermi level of the holes, thus leading to a higher ODF. Both experiments and simulation found that the higher backside recombination and doping density can lower the optical diode factor. In general, a lower optical diode factor is desired to achieve a higher fill factor. However, it has been found that a lower optical diode factor resulting from higher backside recombination is unfavorable due to significant losses in quasi-Fermi level splitting. Conversely, improving the doping density has been found to be preferable, as it enhances quasi-Fermi level splitting while simultaneously lowering the optical diode factor. This thesis presents a thorough investigation of the impact of backside recombination on the quasi-Fermi level splitting of Cu(In,Ga)Se2 solar cells. Using this understanding, a novel hole selective transport structure is proposed that enables the construction of a high-efficiency solar cell with a homogeneous absorber, making a significant shift in the paradigm of Cu(In,Ga)Se2 solar cells. With this shift, the non-radiative and radiative loss of solar cells due to inhomogeneity, e.g. Ga profile, can be removed. Additionally, the results presented in this thesis shed light on the relationship between the optical diode factor, backside recombination, and doping level, providing a direction for further optimization of Cu(In,Ga)Se2 solar cells to achieve even higher power conversion efficiency.7. Affordable and clean energ