As global temperatures rise and energy demands increase, the need for clean, renewable
energy sources is more critical than ever. Solar energy is one of the key solutions, with the
majority of solar panels currently on the market being made from crystalline silicon. However, emerging photovoltaic (PV) technologies such as perovskite solar cells have already demonstrated efficiencies comparable to those of silicon solar cells, making them a promising contender to achieve even higher efficiencies.

Most of the layers in perovskite solar cells are deposited via spincoating, which is a fast and easy process but can only be done on laboratory-scale. However, deposition through thermal evaporation offers significant advantages, enabling fabrication of nanometer-thin films and facilitating large-scale fabrication needed for future industrialization of perovskite solar cell. Therefore, this research aims to develop perovskite solar cells entirely through thermal evaporation.

The reported number of hole transport materials deposited through thermal evaporation is limited. Recently, fully thermally evaporated perovskite solar cells have been created using the hole transport materials MoOx and TaTm, and these hole transport materials will be studies in this thesis.

The MoOx and TaTm were used as single and double hole transport layer to replace the
reference layer of spincoated PTAA. It was found that the MoOx in direct contact with the pervovskite resulted in a chemical reaction, which negatively affected the energy alignment. The MoOx also showed poor charge carrier selectivity, resulting in high interfacial recombination. Great hole extraction from the perovskite was observed for TaTm, however, a misalignment of the band energy with the electrode hindered the hole collection. Improved hole transfer was found with MoOx and TaTm being used a double hole transport layer. Here, the TaTm functions as a passivation layer between the MoOx and perovskite, while effectively blocking the electrons. In turn, the MoOx improved the energy alignment from the TaTm to the electrode to improve the hole collection.

A thickness optimization of the hole transport layers was also performed. For MoOx as
single hole transport layer, it was found that number of oxygen vacancies decreased with
thickness, leading to less recombination. No change was observed for TaTm as single hole
transport layer when varying the thickness. However, as a double hole transport layer with MoOx, increasing the thickness of TaTm led to an increase in Voc . Ultimately, a thin layer of 2 nm MoOx with a 5-nm thick TaTm showed the most promising results, demonstrating a final efficiency of 4.73%.

As the energy demand keeps rising and the need for clean, affordable energy increases, the switch to completely sustainable energy production comes closer and closer. Solar cells are expected to play a big role in this energy transition. However, the technology of the most used type of solar cell, crystalline silicon, is nearing its limit. This calls for other novel technologies that could surpass it. One of these promising technologies is the perovskite / c-Si tandem cell. This type of cell has shown promising results in recent years, but for it to be commercialized and challenge the current technologies that dominate the market, some steps need to be taken. This thesis focuses on hysteresis, a behaviour commonly shown in perovskite cells, resulting in an uncertainty when measuring the characteristics of these cells. The most common explanation for this behaviour has been due to ion migration under certain conditions. This report, however, looks at hysteresis from a different perspective. It investigates the effect of defects on the hysteresis behaviour, continuing on previous research on this mechanism. It is argued that the defects trap charge near the interfaces, affecting the charge extraction and with that the amount of recombination in the cell. This hypothesis is researched by modelling a state-of-the-art perovskite / c-Si tandem cell in TCAD Sentaurus. This research investigates multiple important parameters to these defects, such as defect density, distribution, energy level, and capture cross-section. With these variables, the hysteresis behaviour as a result of the defects is explored. These results are then used to compare the hysteresis behaviour of the tandem cell with a similar single-junction cell, after which the difference between the two is analysed. It is found the two types of cells have similar hysteresis behaviours, but not equal. With the main deviation finding its origin in the current-limiting that can be displayed by tandem cells.

Perovskite/Silicon tandem cells are gradually becoming more promising in the field of photovoltaic technology since they have already surpassed the theoretical power conversion efficiency (PCE) limit of single junction (SJ) silicon cells (~30%), while they can be commercially compatible. Several architectures of Perovskite/silicon devices were proposed. The most known of them are four terminals tandem (4TT) and two terminals tandem (2TT). Which are well researched and have already surpassed the highest efficiency achieved by SJ silicon cells. On the other hand, 3TT is less known architecture which have received less attention in research than 2TT and 4TT but seems to combine the advantages of both and eliminate their disadvantages. Although of the few research done on 3TT, they revealed great potential of achieving high efficiency. Yet it is still practically challenging to design and fabricate stable 3TT devices that bring 30% PCE’s goal to reality. To continue unveiling the potential of 3TT perovskite/Si device, more research needs to be done in areas that haven’t been investigated before. And that was the target of this project.
The 3TT device structure proposed in this work is a perovskite top cell on a c-Si IBC bottom cell. And the focus of the project was to investigate and optimize the design parameters of the perovskite top cell as well as the IBC bottom cell to improve the overall efficiency of the tandem device using 2D modelling. To achieve these Objectives, a 2-D simulation model template was built for the mentioned tandem device with the help of Sentaurus TCAD. Further, the model template has been validated. Subsequently, the validated model was used to conduct a comparison between the proposed 3TT device and an identical 4TT device to examine the compactivity of the model with other tandem configurations. This has shown that the 3TT model has performed as good as the 4TT model. Both models showed capability of achieving efficiency beyond 30%.
Then, the performance of the 3TT model was examined under different thicknesses of Perovskite layer. This revealed that the 3TT device has the highest efficiency when the perovskite layer has a thickness between 0.5 - 1.2 µm where efficiency higher than 30% can be achieved.
The next step was to investigate and redesign the IBC bottom cell to improve the 3TT performance. First, the device performance under different pitch values in the IBC bottom cell was examined. It was found that the lower the pitch is, the higher the performance becomes. It was also found that the pitch value doesn’t influence only the bottom cell but also the top cell. Which makes the performance of 3TT device more sensitive to pitch values than the SJ IBC cell. Second, the rear emitter to base ratio in the IBC bottom cell was investigated. Which showed that the emitter/base ratio has less influence on the 3TT behavior when the pitch value is within or less than the range of the diffusion length of the bulk material. But the influence of the ratio grows up when the pitch is higher. The 3TT device has its peak performance when the ratio is between 2:1 to 4:1.
After that, the tandem model was redesigned to corporate all the outcomes that was found in previous steps. This resulted in several scenarios based on best parameters to design 3TT models with the best outcome. Four scenarios have been suggested where each of them can achieve PCE higher than 31.5 %.
Finally, we have studied the effect of surface recombination on the performance of the 3TT device. Which revealed that most of surface recombination losses are coming from the interfaces between perovskite layer and ETL or HTL layers. Therefore, if these surface recombination losses are suppressed, that will lead to a PCE higher than 33 %.

Shading on photovoltaic modules is practically inevitable, especially in urban environments. The shadows cast by neighbouring objects on the solar panel force shaded solar cells to operate under reverse bias. In this case, instead of generating power, the shaded solar cell dissipates power, which is converted into heat and may induce the formation of hot-spots. Many attempts have been made to improve the shade tolerance photovoltaic modules. In this work, we focus on solar cells with low breakdown characteristics to build shade tolerant photovoltaic modules. These types of solar cells allow the current flow at low reverse bias voltages (around −4 V). The main design challenge is to maintain high conversion efficiencies while achieving low breakdown voltages.
In order to design shade tolerant photovoltaic modules, the carrier transport mechanisms in the solar cell under reverse bias conditions are firstly investigated. A robust simulation template is created in Sentaurus TCAD to perform a parametric evaluation, including both the structural and operating parameters, of the device I-V characteristics. The silicon heterojunction interdigitated back contact solar cell with a silicon oxide passivation layer is among the most promising cell structures to achieve both the low breakdown voltage and the high efficiency. Band-to-band tunneling happens between the heavily doped p+ and n+ regions at the rear side, which allow charge carriers to recombine without entering the bulk of the solar cell. We analyze the effect of the tunneling mass, the gap distance and the dopants penetration length on the forward and reverse I-V curves. Simulations suggest that it is possible to design high efficiency solar cells with breakdown voltages as low as −1.2 V. In addition, device performances under different temperature and irradiance conditions are analysed for the purpose of further investigations on the system level.
While this research study is mainly focused on the performance of solar cells, the results presented in this thesis facilitate comprehensive system level energy yield analyses of shade tolerant photovoltaic modules with low breakdown voltage solar cells.

Within the PVMD group at TU Delft, the study of perovskite/c-Si tandem solar cells is being researched. A comprehensive study is performed through advanced optical and electrical simulations to understand the tandem’s characteristics and operative behavior. We use GenPro4 and Sentaurus TCAD to perform the optical and electrical simulations, respectively. From the previous research, the developed J-V simulations revealed an indication of extra current collection from the tandem’s supporting layers. This extra current density makes the tandem’s short circuit current density J_SC higher than the current-matched J_ph of each perovskite and c-Si absorber layer. Because only J-V curves that could be simulated and not the EQE curves, it remained a mystery of which supporting layers in the tandem that contribute to the extra current collection. In this thesis project, a new EQE simulation methodology was developed to help to solve this mystery of extra current collection from the supporting layers. By performing the EQE simulation, we find that SnO2, TiO2, and Spiro-OMeTAD layers contribute to a total potential extra current collection of 2.50 mA/cm2 to 2.79 mA/cm2. A further study of the extra current collection is also conducted by modifying the simulation method that considers the absorbed photons’ energy in respect to the material’s energy bandgap. Under this further study, only absorbed photons with energy higher than the material’s energy bandgap can generate electron-hole pairs. After considering the absorbed photons’ energy in the simulation, only SnO2 and TiO2 layers that contribute to the significantly lower extra current collection of 0.07 mA/cm2.

The need for more efficient, cheaper, easily producible solar cells is growing as this combination of attributes can decrease the levelized cost of energy (LCOE). A two­terminal (2T) Perovskite and galliuminidium­gallium­selenide (Pvk/CIGS) tandem cell is an excellent candidate, as it can have a power conversion efficiency (PCE) of +30%, can be flexible (making it suitable for roll­-to­-roll production), and
the deposition of a Pvk solar cell (PSC) on top of established techniques like CIGS will only add one extra step to the production process, thereby improving the PCE significantly

This thesis is part of LAFLEX­-2T project which aims to design and engineer a highly efficient flexible 2T thin­-film solar device based on the Pvk/CIGS tandem configuration. The aim of this thesis is to develop an opto­-electrical model of the tandem configuration which replicates the inner physics of a reference solar cell. This can be used to give insights on the current losses occurring in the cell and on the limitations in the charge carrier transport towards the electrodes. Based on this analysis, a route of improvements is proposed with could result in more efficient solar cells.

The simulation template uses optical and electrical simulations based on GENPRO4 and TCAD Sentaurus, respectively. An extensive model for Pvk/CIGS tandem cells is presented and validated using experimentally obtained J­-V curve measurements. It was found that charge transport in the tunnel recombination junction (TRJ) depends on direct energy and in­direct energy transfer, in terms of two tunneling mechanisms: band to band tunneling (B2BT) and trap assisted tunneling (TAT). For TAT, a non­-local model facilitated by trap states is successfully implemented.
Simulation results reveal that the transport in the TRJ of the reference solar cell is based on TAT. The loss analysis points out that reflectance losses are responsible for a loss of 10.92 mA/cm². Similarly, losses due to parasitic absorption are equal to 5.32 mA/cm². The CIGS bottom was identified as the current limiting layer. The dominant recombination mechanisms in the Pvk and CIGS absorber layers are Shockley-­Read­-Hall (SRH) and surface recombination calculated as 4.71 mA/cm² and 1.59 mA/cm² for top Pvk and bottom CIGS, respectively. An increase in losses in the absorber layers in maximum power point (MPP) compared to short circuit (SC) conditions exposed charge transport issues in the collecting path of charge carriers.
After assessing the loss mechanisms, a road­map for efficiency improvements is proposed. After implementation, an increase in the PCE of the reference tandem cell was observed from 10.63% to 26.69%.

Solar cells can play a key role in the transition towards a sustainable future. This transition is one of the major challenges our society faces during the coming decades. Development of high-efficiency photovoltaic solutions at reasonable costs will help accelerating the transformation of our energy system. In this respect, perovskite solar cells are very promising due to their outstanding opto-electronical properties and low-cost fabrication. Obtaining a complete understanding of the device physics and charge transfer mechanisms inside perovskites is crucial for further device improvements. This thesis focuses on the notorious hysteresis in the current-voltage characteristics of perovskite solar cells. So far, this remarkable phenomenon has usually been explained using ion migration, despite the lack of clear experimental evidence. We implement a simulation platform of perovskite solar cells to analyse the charge transfer mechanisms among energy states including those with energy within the forbidden bandgap. We evaluate transient behaviour and identify the limiting physical mechanisms. To explain anomalous hysteresis in perovskite solar cells we use a novel approach in which charge accumulates near the material interfaces due to defects with relatively low capture cross-sections. Defects in lead halide perovskites create shallow sub-gap energy states, that act as charge carrier traps. Near the interfaces, this leads to accumulation of trapped charge carriers, effectively screening the electric field inside the perovskite layer. This reduces the device performance. A slow release of trapped charge due to low capture cross-sections results in hysteresis in the current-voltage curve at commonly used scan rates. TCAD Sentaurus is used as a platform to simulate J-V scans of a planar non-inverted architecture based on the archetypal perovskite MAPbI3, with TiO2 as electron transport layer and spiro-OMeTAD as hole transport layer. This thesis presents a systematic study of different trap distributions, both in the spatial and energetic domain. The capture cross-sections, densities, energy levels and locations of traps are varied and also the effect of scan rate is analysed. This work analyses both tail state defects and deep defects, based on reported values in literature. It is found that defects near the ETL/perovskite interface potentially cause anomalous hysteresis in the current-voltage curve. These defects have their transition energy around 0.25 eV and are possibly attributed to iodine interstitials.

Silicon heterojunction solar cells employing transition metal oxides as carrier selective contact are of particular interest due to the potential of reducing parasitic absorption while featuring optimal electrical properties. Recently, a record efficiency of 23.5% was achieved by employing molybdenum oxide (MoOx) as carrier selective contact. MoOx exhibits advantageous properties with respect to the p-doped standard amorphous silicon contacts due to its lower parasitic absorption and better thermal stability. However, achieving an efficient carrier collection is challenging and not well understood yet. In this work, transport of charge is studied from drift diffusion and atomistic approach by means of numerical simulations. Two different state-of-art computational tools are employed: TCAD Sentaurus for drift diffusion simulations, and VASP for ab initio simulations. Through drift diffusion simulations, the contact formation of molybdenum oxide as carrier selective contact is consistently explored including quantum confinement and transport based in mid-gap energy states. The work function of MoOx is shown to be the core for an efficient charge collection. Thanks to experimental results, it is revealed relevant phenomenon at MoOx/intrinsic amorphous silicon (i-a-Si:H) interface which includes silicon oxide formation and charge accumulated. Therefore, a special focus at interface is here presented, in order to study the inner physics of the detrimental effects and how to avoid them. Altogether, drift diffusion simulations reveal that MoOx thickness is an essential parameter because it strongly determines the work function and hence the efficiency of the solar cell. All the knowledge acquired is used to provide guidelines on the fabrication of these type of solar cells.
Looking at MoOx/a-Si:H interface, ab initio simulations are employed to study interface properties. Accordingly, such interface is analysed using both materials in their crystalline matrix. It is demonstrated that oxygen deficiency tunes the MoOx work function, a statement which is key for the proper contact formation and consistent with drift diffusion results. Finally, the charge arrangement at interface reveals the creation of an interface dipole together with silicon dioxide interlayer which is coherent with drift diffusion simulations analysis.

Interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells have demonstrated 26.6\% world record efficiency by combining outstanding passivation of Si thin film layers with the absence of front shading contact in the crystalline silicon (c-Si) absorber bulk. Furthermore, this type of solar cell merges advantages of c-Si with Si thin film technology that allows for adjustment and tuning of bandgap and Fermi energy in deposited layers, and thus, modifying material properties that also impact the carrier collection. As heterointerfaces tailor to band offset and potential barriers for collecting carriers, the transport is described by thermionic emission and tunneling mechanisms. Moreover, Si thin film layers typically exhibit a defective matrix with characteristic traps and charge distribution that affect solar cell external parameters. Such phenomena involves the so called trap assisted tunneling (TAT) together with trap recombination mechanisms in a complex physical system.

In this work, the effect of TAT mechanisms on IBC-SHJ is studied by means of numerical simulations based on TCAD Sentaurus. Firstly, the TAT model is implemented as non-local process. It was found that TAT exhibits a negligible effect on electron collection, but dominates transport mechanisms in case of hole contact. Such an effect depends on band alignment at the doped layer/transparent conductive oxide (TCO) interface, where band-to-band tunneling in combination with TAT describe the transport of holes. In particular, TCO carrier concentration in combination with activation energy of deposited layer allows the collection by either TAT or band-to-band tunneling. In more detail, the density of traps described typically as dangling bond improves the transport of collecting carriers. Thus, the collection of carriers is evaluated in terms of energy and trap concentration, demonstrating that traps with an energy level of 0.5 to 0.7eV over the valence band enhances the FF, and thus, the solar cell performance.

Finally, the complete model is calibrated by comparing measured with simulated external parameters from a reference solar cell with and without TAT mechanisms. From this analysis, it is demonstrated that solar cell external parameters deploys more consistent values for Shockley-Read-Hall (SRH) recombination associated to bulk lifetime and surface recombination velocity using TAT model.