Background: Elongated nanostructures, such as nanowires, have attracted significant attention for application in silicon-based solar cells. The high aspect ratio and characteristic radial junction configuration can lead to higher device performance, by increasing light absorption and, at the same time, improving the collection efficiency of photo-generated charge carriers. This work investigates the performance of ultra-thin solar cells characterised by nanowire arrays on a crystalline silicon bulk. Results: Proof-of-concept devices on a p-type mono-crystalline silicon wafer were manufactured and compared to flat references, showing improved absorption of light, while the final 11.8% (best-device) efficiency was hindered by sub-optimal passivation of the nanowire array. A modelling analysis of the optical performance of the proposed solar cell architecture was also carried out. Results showed that nanowires act as resonators, amplifying interference resonances and exciting additional wave-guided modes. The optimisation of the array geometrical dimensions highlighted a strong dependence of absorption on the nanowire cross section, a weaker effect of the nanowire height and good resilience for angles of incidence of light up to 60°. Conclusion: The presence of a nanowire array increases the optical performance of ultra-thin crystalline silicon solar cells in a wide range of illumination conditions, by exciting resonances inside the absorber layer. However, passivation of nanowires is critical to further improve the efficiency of such devices.

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Doped hydrogenated silicon oxide layers (SiO_X:H) have recently been successfully integrated as front window layers, back reflector layers, intermediate reflector layers, passivation layers, and junction layers in thin-film silicon solar cells. Depending on the deposition conditions of the SiO_X:H layers, some devices suffer from a degradation in performance in time. In this paper, we demonstrate the responsible mechanism involved. It is demonstrated that oxidation of the p-Type doped (p-)SiO_X:H with a high crystallinity and, therefore, poor passivation of crystalline grains is responsible for this degradation. The oxidation of p-SiO_X:H is caused by the in-diffusion of water vapor from the ambient air. Stable p-SiO_X:H can be obtained if the material is processed at higher pressure. In addition, the degradation can be prevented if the cell is well encapsulated, like using dense n-Type (n-)SiO_X:H in the back reflector of the cell.

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Mixed-phase hydrogenated silicon oxide (SiOx:H) is applied to thin-film hydrogenated amorphous silicon germanium (a-SiGe:H) solar cells serving as both p-doped and n-doped layers. The bandgap of p-SiOx:H is adjusted to achieve a highly-transparent window layer while also providing a strong electric field. Bandgap grading of n-SiOx:H is designed to obtain a smooth transition of the energy band edge from the intrinsic to n-doped layer, without the need of an amorphous buffer layer. With the optimized optical and electrical structure, a high conversion efficiency of 9.41% has been achieved. Having eliminated other doped materials without sacrificing performance, the sole use of SiOx:H in the doped layers of a-SiGe:H cells opens up great flexibility in the design of high-efficiency multi-junction thin-film silicon-based solar cells.@en

The direct utilization of sunlight is a critical energy source in a sustainable future. One of the options is to convert the solar energy into electricity using thin-film silicon-based solar cells (TFSSCs). Solar cells in a triple-junction configuration have exhibited the highest energy conversion efficiencies within the thin-film silicon photovoltaic technology. Going further from the state-of-the-art device structures, this thesis works on the concept of quadruple-junction TFSSCs, and explores the potential and feasibility of such configuration. The initial experimental realization of quadruple-junction TFSSCs is demonstrated in Chapter 2. The fabricated thin-film a-SiOx:H/a-Si:H/nc-Si:H/nc-Si:H solar cells showed favorable fill factors (FF) and exceptionally high open-circuit voltages (VOC) up to 2.91 V, suggesting a high quality of the material depositions and of the process control. Optical simulations were used in the design of the device structure, to precisely control the thickness and optical absorption in the layers. This preliminary experiment indicated how improvements can be made by better light management. The spectral response of the component subcells is important information for the study of multi-junction solar cells, and the accurate measurement of such properties turns out to be challenging. Chapter 3 analyzes the mechanism of the spectral response measurement of multi-junction solar cells, by means of modeling the optoelectrical response of the subcells and their internal interactions. The formation of measurement artifacts, and their dependence on cell properties and measurement conditions, are elucidated. The analyses lead to comprehensive guidelines on how to conduct a trustworthy measurement and sensible data interpretation. Absorbing semiconductor materials with different bandgaps are desirable for multi-junction solar cells. Thin-film a-SiGex:H cells have been developed to accommodate an absorber material with an intermediate bandgap between that of a-Si:H and nc-Si:H. Chapter 4 reports the development of a-SiGex:H cells using mixed-phase SiOx:H materials in the doped layers. Bearing the band alignment in mind, the optimization of p- and n-type SiOx:H layers resulted in satisfying device performance. The use of SiOx:H p- and n-layers offers great flexibility when integrating the cell in a multi-junction solar cell. Chapter 5 describes the development of quadruple-junction TFSSCs using four different absorber materials. The thin-film wide-gap a-Si:H/narrow-gap a-Si:H/a-SiGex:H/nc-Si:H solar cells promotes reasonable spectral utilization because of the descending bandgap along the direction of light incidence. The tunnel recombination junctions between the subcells have been optimized to ensure effective interconnections thus the proper functioning of the multi-junction device. Advanced light management, which involved the use of modulated surface textured front electrode, was arranged for enhancing the optical performance. These investigations reveal the potential of quadruple-junction TFSSCs. Chapter 6 evaluates the benefit of multi-junction solar cells with different number of subcells. The gains and losses inherent in adding more subcells have been critically assessed from the optical and electrical points of view. The effects of optical reflection, parasitic absorption, tunnel recombination junctions, and filtered illumination in multi-junction cells on the performance were investigated. In general, all types of losses increase with the number of subcells. Among them, the filtered illumination in the subcells can play a significant role in case of a large number of subcells. These results show that such comprehensive analysis helps to judge whether it is reasonable to develop a multi-junction solar cell with a certain structure. @en

We fabricated and studied quadruple-junction wide-gap a-Si:H/narrow-gap a-Si:H/a-SiGex:H/nc-Si:H thin-film silicon solar cells. It is among the first attempts in thin-film photovoltaics to make a two-terminal solar cell with four different absorber materials. Several tunnel recombination junctions were tested, and the n-SiOx:H/p-SiOx:H structure was proven to be a generic solution for the three pairs of neighboring subcells. The proposed combination of absorbers led to a more reasonable spectral utilization than the counterpart containing two nc-Si:H subcells. Besides, the use of high-mobility transparent conductive oxide and modulated surface texture significantly enhances the total light absorption in the absorber layers. This work paved the way toward high-efficiency quadruple-junction cells, and a practical estimation of the achievable efficiency was given.

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The benefit of two-terminal multijunction solar cells in regard to the number
of junctions (subcells) is critically evaluated. The optical and electrical losses
inherent in the construction of multijunction cells are analyzed using information
from thin-film silicon photovoltaics as a representative case. Although
the multijunction approach generally reduces the thermalization and nonabsorption losses, several types of losses rise with the number of subcells.
Optical reflection and parasitic absorption are slightly increased by adding
supporting layers and interfaces. The output voltages decline because of the
tunnel recombination junctions, and more importantly of the illumination
filtered and reduced by the top subcell(s). The loss mechanisms consume
the potential gains in efficiency of multijunction cells. For thin-film silicon,
the triple-junction is confirmed to be the best performing structure. More
generally, only when each component subcell shows a high ratio between the
output voltage and the bandgap of the absorber material, a multijunction cell
with a large number of subcells can be beneficial. Finally, the high voltage
and low current density of multijunction cells with a large number of subcells
make them difficult to optimize and manufacture, vulnerable to any changes
in the solar spectrum, and thus less practical for the ordinary terrestrial
applications.@en

Accurate measurements on the spectral response of two-terminal multi-junction solar cells demand the application of appropriate bias light and bias voltage. Typically, fine-tuning the bias condition is a time- and effort-consuming process. In this work, a model with realistic input was developed for simulating the spectral response of multi-junction solar cells obtained in certain measurement conditions. As such spectral response can be anticipated in the measurement with the assigned opto-electrical bias configuration, the model expedites the process of finding the optimal bias condition, as well as the understanding of the causes of measurement artifacts. The model can be further extended to simulate the devices made with different PV technologies, so it facilitates the reliable characterization of novel multi-junction architectures.@en

Multijunction solar cells promise higher power-conversion efficiency than the single-junction. With respect to two-terminal devices, an accurate measurement of the spectral response requires a delicate adjustment of the light- and voltage-biasing; otherwise it can result in artifacts in the data and thus misinterpretation of the cell properties. In this paper, the formation of measurement artifacts is analyzed by modeling the measurement process, that is, how the current–voltage characteristics of the component subcells evolve with the photoresponse to the incident spectrum. This enables the examination on the operation conditions of the subcells, offering additional information for the study of artifacts. In particular, the influence of shunt resistance, bias-light intensity, and bias voltage on the measurement is examined. Having observed the dynamics and vulnerability of the measurement, the proper ways to configure and interpret a measurement are discussed in depth. As a practical example, simulations of the measurements on a quadruple-junction thin-film silicon solar cell demonstrate that the modeling can be used to interpret eventual irregularities in the measured spectral response. The application of such tool is especially meaningful taking account of the diverse and rapid development of novel hybrid multijunction solar cells, in which the role of reliable characterizations is essential.@en