Crystalline silicon (c-Si) homojunction architectures like aluminum back-surface field (Al-BSF) and passivated emitter and rear cell (PERC) dominated the solar cell market for the last decades. Recently, carrier-selective passivating contact (CSPC) designs have shown that they can effectively reduce recombination losses and exhibit efficiencies above 25 % which approaches the Shockley-Queisser limit of single-junction solar cells. To overcome this intrinsic efficiency limit, research and industry have started to investigate multijunction devices which utilize two different absorber layers in order to reduce the spectral mismatch losses. In this project, the focus lies on the fabrication of a high temperature CSPC bottom cell with polycrystalline silicon oxide (poly-SiOx) for the application in a 2-terminal perovskite/c-Si tandem device. Poly-SiOx features a higher transparency due to a wider band gap which can decrease the free-carrier absorption (FCA) and induce larger band bending in comparison to the classic polycrystalline silicon (poly-Si) contacts.
To fabricate a c-Si solar cell with high temperature carrier-selective passivating contacts, the passivation of the contacts was optimised using symmetric lifetime samples. It was found that the optimal processing conditions for the dry thermal oxidation process and the high temperature annealing step are 675 °C for 3 minutes and 900 °C for 15 minutes, respectively. This resulted in an implied open-circuit voltage (iV_OC) for the n-doped and p-doped symmetric samples after hydrogenation of 745 mV and 674 mV, respectively. Those optimised processing conditions were adopted for the fabrication of a single-junction n-type front-side polished and p-type rear-side textured CSPC solar cell. This resulted in a solar cell with an efficiency of 16.67 %. By lowering the thermal budget of the recovery annealing conditions after the sputtering of the transparent conductive oxide (TCO), the passivation properties are restored more effectively. This resulted in an enhanced solar cell with a final efficiency of 18.76 %. Other techniques to improve the performance like the modification of the p-layer thickness, the incorporation of an additional annealing step after the a-Si deposition, or the introduction of indium tungsten oxide (IWO) as the TCO did not result in an improvement of the final solar cell efficiency. Optical simulations were performed in GenPro4 to investigate the optical performance of the solar cell. Finally, a 2-terminal perovskite/c-Si tandem device with a high temperature carrier-selective passivating c-Si bottom cell was fabricated in cooperation with TU Eindhoven. The final tandem solar cell exhibits an efficiency of 23.10 % with a fill factor of 74.00 %, a V_OC of 1.76 V and a J_SC of 17.81 mA/cm².

Royal IHC is known for building large dredging vessels which collect soil from the seabed. The vessel needs to remain in position, called dynamic positioning, which is done by large thrusters. These are mostly electrically powered thus the total amount of required power can range up to 50 MWs. Royal IHC wants to investigate the possibility of a DC vessel where the synchronous generator’s AC voltage is rectified via a passive rectifier. The question is how stable this is and what influences the voltage stability.

The thesis focuses on building a model of a DC powered propulsion system that allows analysis of voltage stability of the vessel. The model is generalized and consists of two parallel diesel generator sets with a passive rectifier. These generators are connected to a common DC bus with constant power loads which represent the loads of the vessel. The model is built in Matlab Simulink using the specialized power system toolbox and is verified with an RScad simulation. A measurement-based analysis tool is developed utilizing prony analysis. This allows analyzing the results of the simulation model and data from the field. Both cases are tested, the tool allows to get insight into the stability of the vessel.

For the far majority of current photovoltaic devices, photons with an energy below 1.1eV are not utilized. Adding a Plasma Enhanced Chemical Vapor Deposition (PECVD) processed germanium bottom cell to a multi-junction device has the potential to provide a low-cost boost in conversion efficiency by utilizing a part of this spectral range below 1.1eV. For this thesis, 89 amorphous/nano-crystalline hydrogenated germanium (a-/nc-Ge:H) films were PECVD processed with the objective of creating a device quality material that is stable, intrinsic and has a high photoresponse. These characteristics are required to enable p-i-n bottom cell integration in a multi-junction device and prevent post-deposition oxidation and carbonation. To this end, the following three approaches were applied: I. Boron doping to increase the activation energy; II. Hydrogen plasma treatment (HPT) to decrease the defect density of the material with hydrogen passivation; III. A decreased electrode gap to improve the photoresponse. These approaches were integrated in the sample preparation steps. Subsequent film characterization was carried out by using vibrational analysis, elemental analysis, and opto-electrical analysis.
The influence of boron doping was considered insignificant and the influence of hydrogen plasma treatment was considered inconclusive. Furthermore, a decreased electrode gap did not result in a higher photoresponse. Reported photo-/dark conductivity ratios were in the range of 1-7. However, the decreased electrode gap resulted in lower deposition rates and higher refractive indices of 4.9-5.0 (compared to 4.1-4.9 for a 20mm electrode gap), indicating denser films. A substantial fraction of the films was processed at an increased temperature of 275°C. It was found that increasing the processing temperature from 200°C to 275°C resulted in considerably denser films with refractive indices of up to 5.2, leading to higher activation energy (up to 314meV) and very low post-deposition oxidation and carbonation. Therefore, by increasing the processing temperature, it was accomplished to produce denser, more stable films, which have a lower bandgap energy and are more intrinsic.

Carrier-selective passivating contacts (CSPC) have been proven to be effective in suppressing recombination losses at metal-silicon interface in high-efficiency crystalline silicon solar cells. The poly-Si-based CSPCs consisting of SiOX/poly-Si stacks are used in this thesis due to their compatibility with high thermal budgets. CSPCs with thin poly-Si layers are preferred since thicker poly-Si layers induce significant parasitic absorption at device level. The bifacial solar cell design could provide a higher energy yield than monofacial solar cells and largely reduce the consumption of metals such as silver, indium due to the contribution of reflected light. The objective of this thesis is to fabricate double-side textured copper (Cu)-plated bifacial poly-Si solar cells. It is achieved by developing thin poly-Si contacts, addressing the TCO sputtering-induced passivation damage, and optimizing the Cu-plating processes.
Firstly, the passivation of the poly-Si symmetric samples and solar cells was optimized. 16 nm-thick n+ poly-Si and 16 nm-thick p+ poly-Si symmetric samples with intrinsic layer grown through PECVD demonstrated good passivation quality indicated by their iVOC of 713 mV and 665 mV respectively. To further improve the passivation quality of the p+ poly-Si, the layer thickness and the doping gas flow ratios were varied. Best passivation was achieved for a solar cell precursor with 42 nm-thick p+ poly-Si layers. The optimum doping gas flow ratio was found to be SiH4-B2H6=20/15 sccm. The highest iVOC achieved was 692 mV.
TCO sputtering caused a severe passivation drop of ~90 mV in solar cell precursors. Post-deposition annealing treatments and 2-step TCO sputtering techniques were used to reduce such a passivation loss. However, it was challenging to reproducibly obtain solar cell precursors with good passivation quality before metallization step. The initial solar cell with double side TCO use showed cell parameters were: VOC 398 mV, FF 56.22%, JSC 30.55 mA/cm2 and η 6.85% (from n-side illumination).
To maintain a good passivation quality of the solar cell precursor, an 8 nm-thick MoOX buffer layer was introduced on top of the p+ poly-Si layer. This approach effectively reduced the passivation drop from 91 mV to 12 mV. However, the MoOX layer was observed to strongly react with the solution which was used for silver seed layer removal in Cu-plating metallization procedure. Different approaches were tested to obtain a well-plated p+ poly-Si side. As for the n+ poly-Si side, a TCO-free design was deployed. To ensure a good adhesion of the plated Cu fingers, a Ti/Ag (8 nm/192 nm) seed layer was employed before Cu-plating step. Moreover, an additional SiOX layer, which acts as the anti-reflection coating, was deposited on the n-side of a complete solar cell. With these adjustments, the solar cell performance was improved to: VOC 610 mV, FF 64.98%, JSC 36.95 mA/cm2 and η 14.64% (from n-side illumination).
Furthermore, with reducing the Ti thickness in the metal seed layer to 2 nm. The solar cell performance was further improved to VOC 611 mV, FF 69.58%, JSC 36.16 mA/cm2 and η 15.38% (from n-side illumination). The bifaciality factor is 89%.

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.

Although front and back contact silicon heterojunction solar cells exhibit promising external parameters, they are limited by the front highly absorptive doped layers. Due to their opto-electronic properties, research and development groups have demonstrated that transition metal oxides (TMO) are potential alternatives for doped layers in solar cells. In particular, molybdenum oxide (MoOx) is a good candidate for the p-contact layers. This thesis presents an investigation of the introduction of MoOx in the front contact of solar cells. To this purpose, the evolution of the passivation is evaluated along the fabrication process together with temperature sensitivity. The application of the layer negatively influences the passivation quality. Moreover, the passivation quality decreases more after annealing and transparent conductive oxide (TCO) deposition. We mitigate such issues by treating the surface of the passivation layer before MoOx application. The optimized contact stack shows implied open-circuit voltage above 715 mV and lifetime 1.57 ms. Finally, the contact stack is implemented in solar cells demonstrators. The best solar cell exhibits a short circuit current density (JSC) equal to 35.64 mA/cm2, an open-circuit voltage (VOC) of 690 mV, a fill factor (FF) of 77.29% and a total efficiency (η) of 19.01%.

Carrier-selective passivating contacts (CSPC) are one of the most promising silicon solar cell contact structures. These contacts enable the passivation of the silicon surface as well as a high carrier selectivity. So far excellent results have been demonstrated with the use of poly-Si in stack with SiO2. In this work, we optimize an alternative stack, based on carbon alloyed poly-Si (poly-SiCx). Alloying with carbon provides wet-chemical stability, the material becomes resilient to blistering and the bandgap can be tuned depending on the carbon content. This optimized poly-SiCx is implemented as the rear contact of a solar cell. We investigate doped poly-Si alloyed layers with different carbon contents. Structure characterization reveals that carbon incorporation hinders the crystallization of the material. In addition, alloying with carbon decreases the absorption coefficient of the material in the as-deposited state (amorphous), but following the thermal treatment higher carbon contents lead to a higher absorption coefficient. Finally, we observe that higher carbon alloying reduces the conductivity of the poly-SiCx by several orders of magnitude, but the optimized material demonstrates a dark conductivity of 0.006 S/cm, which is 3 orders of magnitude higher than the amorphous one. We optimize the passivation quality of (p)poly-SiCx in symmetrical samples by inserting an (i)a-Si interlayer between the SiO2 and the doped layer using different deposition methods (PECVD/LPCVD). With the optimization of the gas flow ratio, high-temperature annealing and hydrogenation, we obtain a high passivation quality (iVoc of 709 mV and Jo of 10.5 fA/cm2) for the optimum LPCVD deposited interlayer, while the optimum PECVD deposited interlayer demonstrates a signi cantly lower passivation quality (iVoc of 684 mV and Jo of 34.7 fA/cm2). The hydrogenation is performed with the deposition of a SiNx capping layer followed by Forming Gas Annealing at 400°C for 30 minutes. The potential of the (p)poly-SiCx passivating contact is investigated by the application on the rear side of a solar cell. We fabricate initially double-side polished cells with the champion cell achieving an active area efficiency (ηact ) of 15.68%, featuring a Voc of 692 mV, FF of 74.7% and EQE current (Jsc,EQE) of 30.35 mA/cm2. Finally, we also investigate the potential on front side textured cells, obtaining the champion cell with an ηact of 16.80%, maintaining the FF at 74.7%, a lower Voc of 677 mV and a Jsc,EQE at 33.22 mA/cm2 . For all cells the parasitic absorption at the front (n)poly-SiCx contact remains high, due to the non-optimized thickness of the aforementioned contact.