The transition to renewable energy sources is essential to mitigate the impacts of climate change, with solar energy playing a pivotal role in this shift. Photovoltaic (PV) technology. However, the traditional single-junction cells are reaching their theoretical efficiency, which currently stands at 29.43%. Through the use of tandem solar cells, this efficiency limit can be surpassed. The tandem cell is being researched further to increase the power conversion efficiency (PCE). However, the conventional two-terminal (2T) and four-terminal (4T) tandem configurations face challenges related to current matching and optical losses, respectively.
This project addresses these challenges by focusing on the development and optimization of a three-terminal (3T) perovskite/silicon tandem module. The research aims to enhance the energy yield of 3T module under real-world conditions by developing a comprehensive 3T model within the Photovoltaic Material and Devices (PVMD) Toolbox, bridging the gap between cell-level and module-level optimization, which has been underexplored in existing studies.
The first step involved the development of the 3T model, which incorporates independent connections for each sub-cell and utilizes an interdigitated back contact (IBC) on the bottom cell as the third terminal. The 3T model is developed and simulated in MATLAB to be validated, showing close alignment of IV curves compared to the existing literature. The 3T model is then used to simulate a 72-cell module to be validated with a widely used electrical simulator called LTspice. The validation of the developed MATLAB 3T model against LTSpice simulations demonstrated a close match, with an RMSE result of 0.02% errors, confirming the model's accuracy in predicting the IV curves and energy yield for various operating conditions.
The second phase of the research involved detailed comparisons between the performances of 2T and 3T modules, each consisting of 72 cells, under both standard test conditions (STC) and real-world conditions in Delft as a sample. The simulations revealed that the 3T module provides less annual energy yield than 2T. However, the 3T module performs better than 2T at handling spectral irradiance variations and current mismatch situations under real-world conditions. At a certain hour, 3T outperforms 2T by yielding 220.75 W compared to 219.1 W. The loss analysis confirms that the 3T module produces less mismatch loss under real conditions than the 2T module. The simulations at four different locations also show that 3T has a certain number of times when 3T yields more energy than 2T. This shows the potential of 3T to outperform 2T by optimizing the perovskite bandgap energy and thickness.
The optimization of the 3T module focused on adjusting the perovskite layer's bandgap energy and thickness. The optimal bandgap energy is identified as 1.64 eV, and the ideal thickness is 600 nm for 2T, yielding 588.79 kWh. On the other hand, 3T has optimal perovskite bandgap energy and thickness of 1.82 eV and 600 nm, respectively, yielding lower energy of 583.24 kWh. Then, the modules are expanded into 144-cell modules, which results in 3T consistently yielding around 0.4% to 0.8% more energy at its optimum perovskite bandgap energy and thickness at 4 location samples compared to the 2T. For example, at Delft, the 3T yields more annual energy of 1172 kWh than 2T of 1167 kWh. This is due to the reduced end loss produced by 3T at a larger number of cells in the module. These results show that the 3T outperforms 2T at its respective optimum perovskite bandgap energy and thickness at every location sample and at a larger number of cell modules. Although the current 3T technology with IBC is still expensive, it is expected to become competitively priced in the future. These findings highlight the importance of ongoing optimization and development of 3T modules to fully unlock their potential in various environmental conditions.

Throughout the development of crystalline-silicon/perovskite tandem solar cells, the degradation rate of the perovskite top cells has become a limiting parameter. Currently, perovskite faces stability issues, mainly caused by corrosion and the light-soaking effect. This work investigates the degradation rate of the perovskite top cell, where a model is created to determine the parameters from the single diode model that change the IV-curve of a solar cell and lifetime energy yield simulations were performed to find the maximum tolerable degradation rate of perovskite in a tandem module to obtain a higher energy yield compared to a single-junction module Using experimental data from the King Abdullah University of Science and Technology (KAUST), where the external parameters from the measurements were provided, the changes over time in the ideality factor, saturation current, shunt resistance, and series resistance were estimated, using a numerical model. Linear trend lines through the fitting parameters over time were used and lifetime energy yield simulations were performed. For these simulations, a crystalline-silicon/perovskite tandem cell with an efficiency of 31.1% was compared to to a single-junction cell with an efficiency of 23.9%, where the single-junction cell’s optics are the same as the tandem cell’s bottom cell. To perform lifetime energy yield simulations, the PVMD toolbox is used.
Simulating the lifetime energy yield over 10 years, a perovskite degradation rate of 27.5%/year was found using the electrical parameters under STC, where the power output decreased from 4.95W to 0.24W per cell. Comparing 2T and 4T configurations, where the cell optics were kept the same, degradation rates of 21.2%/year and 9.5%/year were found for 2T and 4T, respectively. Looking at different geographical locations, degradation rates of 19%, 20%, 20.6% and 21.2% per year were found for Delft, Lagos, Lisbon and Shanghai, respectively. These differences were caused by current mismatch between the top and bottom cell, where the highest mismatch of 0.55 mA/cm2 was found in Shanghai.
To determine the maximum tolerable degradation rate of the perovskite top cell resulting in a higher lifetime energy yield for a tandem module compared to a single-junction module over 25 years, various degradation rates were analyzed. The perovskite degradation rate must be lower than 1.5%/year and 3.1%/year for 2T and 4T configurations, respectively, assuming no degradation in the silicon single-junction cell and the tandem cell’s bottom cell.

Degradation of PV modules reduces their operational lifetime, resulting in an increased levelised cost of electricity (LCOE) and shortening the useful lifetime of valuable materials. One of the leading cause of degradation is moisture. Understanding how this moisture diffuses through PV materials and in different conditions can help prolong modules’ lifetime. This thesis investigates moisture ingress in PV modules, specifically looking at alternative, and potentially more accurate, ways of modelling it.

An existing moisture ingress model provides the foundation for this research. It is optimised to ensure it delivers accurate results and adapted to reduce the computational time. Models typically assume Fickian diffusion to simulate moisture ingress; this assumption does not always hold, and deviation between these simulations and experimental results has been reported. Therefore, two mechanisms that are not in the model are explored: non-Fickian diffusion and material degradation. To simulate non-Fickian diffusion, a dual-transport method is used; the study finds the approach to deliver more accurate results for EVA, but not for PET. Optimal non-Fickian factors of NF: 32 and NF: 40 are found, corresponding to units of dimension (0.625 × 0.625) × 10−4 m2 and (0.5 × 0.5) × 10−4 m2, respectively, which is smaller than that found in literature. To simulate material degradation, an adapted version of the diffusion coefficient equation is proposed, incorporating a degradation constant based on the materials’ properties. Degradation constants γEVA = 0.04 and γPET = 0.005 are found following a statistical analysis. This constant is able to match the simulation closer to the experimental data.

These findings are then used to analyse the behaviour of other PV materials (TPO and ionomer encapsulants, and TPT and PA backsheets) and behaviour in different climates. The simulations find very slow moisture ingress for ionomer under non-Fickian diffusion and a strong deviation from Fickian diffusion. In EVA/PET simulations, non-Fickian behaviour is found to deviate more from Fickian behaviour in warmer climates. Using an acceleration factor, Af , to relate accelerated tests and realistic-operation, the degradation constants for EVA and PET are converted for real-world simulations. They are also found for other PV materials, based on their material properties. The approach shows promising results for the TPO/PET and ionomer/PET simulations, showing degradation in proportion with their material properties. However, simulations that include EVA appear to strongly limit moisture diffusion, indicating a revision of the EVA degradation constant should be made. TPO/PET degradation simulations show minimal degradation over 20 years in different climates, but more material degradation in colder
climates is found.

Degradation of PV modules reduces their operational lifetime, resulting in an increased levelised cost of electricity (LCOE) and shortening the useful lifetime of valuable materials. One of the leading cause of degradation is moisture. Understanding how this moisture diffuses through PV materials and in different conditions can help prolong modules’ lifetime. This thesis investigates moisture ingress in PV modules, specifically looking at non-Fickian diffusion and material degradation as alternative, and potentially more accurate, ways of modelling it. To simulate
non-Fickian diffusion, a dual-transport method is used; the study finds the approach to deliver more accurate results for EVA, but not for PET. To simulate material degradation, an adapted version of the diffusion coefficient equation is proposed, incorporating a degradation constant based on the materials’ properties. The findings are then used to analyse the behaviour of other PV materials and behaviour in different climates. The simulations find very slow moisture ingress for ionomer under non-Fickian diffusion and a strong deviation from Fickian diffusion. In EVA/PET simulations, non-Fickian behaviour is found to deviate more from Fickian behaviour in warmer climates. Degradation constants are found for the other PV materials. The approach shows promising results for the TPO/PET and ionomer/PET simulations, showing degradation in proportion with their material properties. However, simulations that include EVA appear to strongly limit moisture diffusion, indicating a revision of the EVA degradation constant should be made. TPO/PET degradation simulations in show minimal degradation over 20 years in different climates, but more material degradation in colder climates is found.

A moisture ingress model is developed using the FEM software COMSOL. The model uses Fick’s second law of diffusion to model the diffusion of moisture in the module. The model also reflects the temperature dependent nature of parameters affecting moisture ingress by using an Arrhenius equation. The model is validated with experimental and simulated data from literature showing a good agreement. A brief comparison with alternative models such as analytical models and a simplified numerical model is also made. The model allows to simulate the moisture ingress into different PV modules in different conditions. The moisture ingress in 11 different locations with different climatic conditions is simulated. The moisture ingress is also simulated using 4 different encapsulants and 4 different backsheets. Finally, the model is adapted to simulate the moisture ingress into impermeable backsheet modules.\\

These results are used to find a relation between ambient conditions and the results delivered by the COMSOL model. A simplified relationship is found that holds for the different climates and encapsulants. It is found that the effective relative humidity in the environment is the key parameter in determining the amount of water that will be in the module once it reaches equilibrium. The time that it takes for a module to reach its moisture equilibrium content is determined by the temperature. The presence of these simplified relations can help in estimating the moisture ingress behaviour of a model without the need of carrying out a full FEM simulation. However, the dynamics of the system when using different backsheets does not follow the same simplified relations.\\

The degradation caused by water in the module is also studied. An analytical model is used to predict the degradation observed during damp heat tests. Due to the properties of the analytical model a different approach has to be followed for real life conditions. The degradation model is used to compare the expected degradation under different conditions. This shows that the expected degradation is larger in hot and humid climates while it is minimized in colder climates. The general degradation trend observed for the different climates is: Tropical > Arid > Temperate > Continental > Polar.

Increasing the energy yield per unit area of Photovoltaic (PV) modules is one of the main challenges the PV technology is currently facing. In search of ways to address this issue, solar tracking systems are a favorable solution, which according to literature can enhance the energy yield by up to 45%. Another way is to opt for high-efficiency solar cells. Tandem cells have emerged as a promising technology, achieving efficiencies of over 30%. The integration of these tandem cells with sun-tracking techniques suggests high-performing solar modules. The present work develops a solar tracking model to simulate the performance of tracking PV systems equipped with tandem modules. The model will be incorporated into the PVMD toolbox, a PV modeling software developed within the PVMD group. This software can predict the energy yield of PV systems using self-consistent models for each aspect of the energy conversion.

The current version of the toolbox makes it impractical to include solar tracking due to the time-consuming nature of ray tracing used to compute the irradiance. Ray tracing generates sensitivity values that illustrate how sensitive is the module to incoming irradiance from any direction in the skydome. Initially, this work focuses on substituting ray tracing with an alternative faster approach to express sensitivity based on view factors. The view factor and ray tracing method are compared with respect to computational time and extent of agreement. It was found that the view factor can significantly reduce the computational time from over 12 minutes, as required in ray tracing, to a few milliseconds for a single module orientation. Additionally, the view factor method generates sensitivity values closely matching those from ray tracing. For instance, a mean RMSE of 1.2% between the two methods is achieved, for an albedo of 0.2 and module tilt of 30 degrees. Sun tracking aims to locate the module orientation that maximizes the in-plane irradiance. Directly calculating the irradiance for every orientation to identify the optimal, is not a viable option, as it requires substantial
time. Thus, sun tracking was expressed as an optimization problem and algorithms were employed to address it. Based on the prevailing sky conditions three optimization case studies were defined on an hourly basis: sunny, cloudy, and intermediate hours. Multiple algorithms were compared across the three cases with selected criteria the convergence to the optimum and runtime. Matlab’s surrogate solver and an author-developed algorithm were selected, as a satisfying solution, compromising those two criteria.

Finally, energy yield simulations were performed on perovskite-silicon tandem modules mounted on a dual-axis tracking system. Four locations were selected, representing different real-world conditions: Stockholm, Athens, Bombay and Bogota. Results show the module’s tilt dynamic adaptability to sky conditions: increas- ing nearly to the sun’s zenith when direct light dominates, and lowering when diffuse light is prevalent. Furthermore, the seasonal fluctuations of the energy gain of tracking systems are explored, with locations further from the equator such as Stockholm exhibiting the highest variability of 19% in winter to 36.9% in summer. In addition, the annual energy gained among the locations was found to span between 24.8% (Bogota) and 34.1% (Bombay). An important finding is the direct proportionality in gains from absorbed irradiance to DC and AC yields, illustrating a 1:1:1 ratio. Then, the effect of tracking technology on mismatch losses of tandem modules was examined. Results indicated that tracking has little impact on both the current and power mismatch. For example, the power mismatch losses slightly increased from 1.10-1.46% in static PV systems to 1.29-1.77% for tracking topologies in the locations examined. Moreover, the tandem’s annual energy gain is compared to silicon heterojunction modules. The analysis showed similar gains across locations for both cell technologies.