Indium tin oxide (ITO) has been extensively used as a transparent conductive oxide (TCO) material due to its excellent optical and electrical properties. The ITO layer is applied on the front and rear sides of the front-back contact (FBC) SHJ cells. As the front TCO layer, ITO provides anti-reflective features and facilitates the lateral transport of electrons. Conversely, a well-engineered rear TCO layer facilitates favorable band alignment for charge carrier collection and enhances the reflection of near-infrared light. However, the consumption of indium for solar cell applications competes with the growing demand across other industries. Furthermore, indium is scarce and expensive, contributing to the increase in capital expenditure. As a result, research into the feasibility of eliminating the TCO layer has gained significant interest in the past few years. While the concept of TCO-free solar cells has been demonstrated in limited studies with efficiencies exceeding 20%, TCO-free FBC-SHJ solar cells employing copper- based metallization remain unexplored. This work extends the TCO-free contacts by integrating the copper-plated metallization on the front side of SHJ cells.

Direct contact between silver and silicon, following the elimination of the TCO layer, fails to withstand wet-chemical processing due to weak adhesion of silver to silicon. A thin titanium layer was introduced between the silicon and silver layers to improve adhesion. In addition, direct metal deposition onto the silicon surface induced sputtering-related defects, which degrade the passivation quality of the solar cells. Thus, the effect of annealing treatment on the surface passivation is studied. Furthermore, annealing plays a crucial role in decreasing the contact resistivity on the front side, which is essential for efficient electron extraction.

In this study, an efficiency of 22.62% was achieved for front TCO-free FBC-SHJ solar cells with copper-plating metallization, attributed to an open-circuit voltage (Voc) of 720 mV and a fill factor (FF) of 80%. It was observed that Voc and FF were influenced by the annealing duration and the thickness of the hydrogenated nanocrystalline silicon oxide layer, respectively. On the other hand, efficiencies of 19.24% are attained for rear TCO-free FBC-SHJ solar cells featuring a localized-area front ITO layer with Ti/Ag back contact, respectively. Electrical characterizations revealed negligible passivation quality degradation upon front TCO localization. However, the introduction of rear Ti interlayer decreased the short-circuit current density (Jsc) due to the lower near-infrared wavelengths reflectivity of titanium compared to silver. Combining the optimized front and rear TCO-free developments yielded a completely TCO-free contact featuring front copper-plated metallization with an efficiency of 21.19%. The device demonstrated good passivation quality, evidenced by Voc exceeding 710 mV and FF above 80%. Future development may consider the feasibility of depositing anti-reflective coating on the rear side to increase the Jsc further. In addition, the stability of TCO-free devices under prolonged light exposure remains a concern, as indicated by a 7% FF relative decrease after 100 hours of continuous illumination.

The installed capacity of photovoltaic (PV) technology has increased beyond expectations and is expected to grow even more. One of the most promising PV technologies currently dominating the PV market is the silicon heterojunction (SHJ) solar cells, which hold the highest c-Si single junction efficiency of 27.3% in an interdigitated-back-contacted design. To achieve such high efficiencies, an optimized (i)a-Si:H passivation layer is of great importance. This thesis aims to scale up the lab baseline SHJ technology by optimizing large-area c-Si surface passivation with hydrogenated intrinsic amorphous silicon (i)a-Si:H layers. In view of the development of high-efficiency perovskite/c-Si tandem solar cells, this thesis also covers the development of nanotexture on crystalline silicon (c-Si) wafers used for the bottom cells.
The research for optimization of large-area c-Si passivation with (i)a-Si:H layers first started with obtaining recipes that would deposit uniform (i)a-Si:H layers over M2-sized glasses. The deposition conditions of plasma-enhanced chemical vapor deposition (PECVD) were optimized and various promising recipes that delivered highly uniform (i)a-Si:H layers were obtained. No statistically significant correlation was found between the deposition parameters and the uniformity of the (i)a-Si:H layers.
The passivation quality of the (i)a-Si:H layers was assessed by symmetrically depositing the (i)a-Si:H layers on c-Si wafers and measuring the lifetime. For single-layer passivation strategy, (i)a-Si:H layers prepared with the highly hydrogen-diluted silane (SiH4) resulted in relatively low lifetime values, potentially due to epitaxial growth, while layers deposited with less hydrogen-diluted SiH4 or pure SiH4 resulted in better lifetime values. The best lifetime achieved with single-layer passivation was 8.23 ms. Furthermore, utilizing a bilayer concept that first deposits an ultra-thin (i)a-Si:H layer with pure SiH4 and then stacks on its top a highly hydrogen-diluted (i)a-Si:H layer, can enhance the passivation quality over their single-layer counterparts. With proper selection of the (i)a-Si:H layers that compose the bilayer and the optimum thickness combinations, a lifetime of 16.10 ms was achieved. Moreover, the passivation quality can be further boosted by applying a hydrogen plasma treatment (HPT) of 30 seconds on top of the optimized bilayer. HPT durations longer than 30 seconds resulted in a decrease in lifetime, possibly due to the oversaturation of hydrogen that leads to defect formation. A lifetime as high as 21.53 ms was obtained by combing the bilayer approach and a HPT.
Last but not least, the creation of nanotexture on c-Si wafers was done by anisotropic etching, using KOH, KsSiO3, and surface additive monoTEX 2.6. By varying texturing conditions, namely, temperature and duration, various surface morphologies of c-Si wafers were obtained. According to SEM characterizations, pyramidal features below 1 μm were successfully created mainly thanks to K2SiO3 slowing down the etching rate during the texturing process. The lowest reflectance was obtained by etching the c-Si wafers at 70 °C for 20 minutes. It was also found that K2SiO3 has an extremely slow dissolving rate and the most effective way of dissolving K2SiO3 is by adding KOH and K2SiO3 to DI water at room temperature and heating it to 80 °C while mixing.

Silicon heterojunction solar cells (SHJ) showed a record efficiency of 26.81%, approaching the theoretical limit of single-junction crystalline silicon (c-Si) solar cells. To further improve the efficiency, a wide bandgap perovskite top cell can be stacked on top of the SHJ bottom cell forming tandem solar cells, which utilize better the solar spectrum. Recently, a record efficiency of 33.70% was achieved for a monolithic two-terminal perovskite/SHJ tandem solar cell. Typically, a transparent conductive oxide (TCO) layer, functioning as the recombination junction, is used to connect the two sub-cells. However, tandem solar cells with this conventional TCO recombination junction often feature high reflection losses originating from the intermediate interfaces between the two sub-cells. Therefore, this master thesis focused on minimizing these intermediate reflection losses by substituting the TCO-based recombination junction with proposed TCO-free recombination junctions.

Firstly, comprehensive optical simulation studies that compared 2T tandem solar cells with various recombination junctions were performed. In the case of single-side-textured (front-side-flat) tandem configuration, as compared to the reference cell with tin-doped indium oxide (ITO) recombination junctions, the use of the more transparent tungsten-doped indium oxide (IWO) allowed an improved implied photocurrent density in the bottom cell (Jimp,bottom) from 18.30 mA/cm2 to 18.70 mA/cm2. Further, by using the TCO-free recombination junction composed of (p)nc-SiOx:H/(n)nc-SiOx:H or (p)nc-Si:H/(n)nc- Si:H, ranges of optimum thickness combinations were discovered, which allowed high Jimp,bottom values of 20.30 mA/cm2 or 19.80 mA/cm2, respectively. Both TCO-free recombination junctions demonstrated enhanced light coupling to the bottom cell thanks to the optimized interference effect at the intermediate interfaces between two sub-cells, minimizing the associated reflection losses. Furthermore, the designs of tandem solar cells featuring various recombination junctions were optimized to reach maximum matched tandem current density. For the reference cell with ITO recombination junction, a matched tandem current density of 19.40 mA/cm2 was obtained, while the use of TCO-free recombination junctions, for instance, 60 nm (p)nc-SiOx:H/ 70 nm (n)nc-SiOx:H or 30 nm (p)nc-Si:H/75 nm (n)nc-Si:H, demonstrated high Jimp,bottom values of 19.80 mA/cm2 and 19.80 mA/cm2, respectively. These results highlight the optical advantageous implementations of proposed TCO-free recombination junctions for monolith tandem solar cells. Similar observations but less significant improvement by using the proposed TCO-free recombination junctions were found in double-side-textured tandem solar cells. This is due to the already minimized reflection losses of the (p)nc-SiOx:H/(n)nc-SiOx:H or (p)nc-Si:H/(n)nc- Si:H configurations (1.3 mA/cm2 and 1.4 mA/cm2 respectively) as a result of the textured front surface.

Based on optical simulation studies conducted on 2T tandem solar cells, the electrical effectiveness of proposed TCO-free recombination junctions was examined by fabricating proof-of-concept single junction single-side-textured SHJ solar cells. First, we focused on the passivation optimization of the flat (100) c-Si surface as it is prone to detrimental epitaxial growth. An impressive minority carrier lifetime of 16.87 ms was achieved by combing (n)nc-Si: H and (i)a-Si: H bi-layer in a symmetrical configuration. Moreover, we also observed, in general, better conductivity when increasing thicknesses of doped nc- SiOx: H layers when they were deposited on glass or (i)a-Si: H coated glass substrates. Eventually, proof-of-concept single junction single-side-textured SHJ solar cells featuring the proposed TCO-free recombination junction were fabricated. According to the optical simulations, various optimum thickness combinations of (p)nc-SiOx: H and (n)nc-SiOx: H or (p)nc-Si: H and (n)nc-Si: H that composes the recombination junction were tested. Overall, the optically promising TCO-free recombination junctions in 2T tandem solar cells also delivered high FF values in proof-of-concept single-junction SHJ solar cells, demonstrating their potential to be implemented to fabricate high-efficiency monolithic 2T tandem solar cells.

The photovoltaic (PV) market is currently dominated by crystalline silicon (c-Si) technology, which accounts for 95% of the production. However, the record single-junction c-Si solar cell has achieved a record efficiency of 26.81% approaching the theoretical efficiency limit of 29.43%. In an effort to overcome this barrier, multi-junction devices are being investigated, and perovskite/Si tandem technology is attracting a lot of interest. This is because it is compatible with well-established c-Si technology and is comparatively less expensive. Recently, two-terminal (2T) monolithic tandem devices with a c-Si bottom cell and a perovskite top cell have achieved an impressive 33.7% efficiency. By utilizing a solution-based process it allows for fabricating high-quality perovskite layers. However, problems develop when the bottom cell’s light trapping features exceed the thickness of the perovskite layer. To ensure uniform coverage of high-quality perovskite layer, the light trapping features must be lowered to sub-micrometre (< 1 μm) height.

This thesis investigates three methods for creating sub-micron features on the c-Si bottom cell. The first two approaches include wet-chemical processing from the top-down and bottom-up techniques. Using a poly-Si etch solution, the top-down approach aims to lower the height of large surface features from 3-5 μm to less than 1 μm. This method resulted in substantial reflection losses and poor passivation due to surface-induced roughness, even though the peak height was lowered to 0.9 μm. In order to tackle this issue, an additive was added to the mixture, producing a morphology with a 1.1 μm lower peak height, compared to an initial peak height of 3 μm. Further, the passivation has seen a two-fold increase in minority carrier lifetime. This increase is attributed to the reduced surface roughness that was inspected through SEM images. However, the surface still contained nano-scale features whose origination could be related to the cleaning procedure followed or the (i)a-Si:H layer growth. The second method is a bottom-up approach and uses KOH, K2SiO3, and a surface additive to create sub-micron features on a flat Siwafer. Once the chemical concentrationswere optimized, the peak height was effectively lowered to less than 0.8 μm at an F-ratio of F = 2.0. But, this experiment is conducted at a temperature of 80◦ C. Many non-uniformities in pyramid distribution were observed as well as nano-scale roughness for F-ratio of F=0. This seemed to be resolved with better pyramid homogeneity and reduced nano-scale roughness by lowering the temperature to 70◦ C. While, maintaining a peak height below 0.8 μm at F=2.0. Moreover, this approach also exhibited lower reflection losses which is close to reflection observed from large pyramidal features (˜3μm).

The last method includes the use of lithography to produce 2D periodic inverted nano pyramids. This process flow showed that patterns with critical dimensions close to 100 nm may be transferred using high-throughput nano-imprinting lithography. By using a much rigid soft mold that did not showcase any pattern distortion. However, the limitations in mask opening created issues with the nano-pyramid formation. This further requires the optimization of suitable hard mask layers. To sum up, these methods demonstrate the possibility of producing effective sub-micron features, which is promising for perovskite/Si tandem technological advancements.

Silicon heterojunction (SHJ) solar cells are one of the most promising PV technologies nowadays due to high photoconversion efficiencies and low manufacturing costs. However, standard front/back-contacted (FBC) solar cells feature a front metal grid that brings shading to the front side of the devices, which limits the conversion efficiency of solar cells. One solution is to move metal contacts fully to the back by introducing interdigitated-back-contacted (IBC) architecture. In 2018, IBC-SHJ solar cells achieved a photoconversion efficiency of 26.7% holding a record for single-junction c-Si cells. However, the standard fabrication process of IBC solar cells comes with increased complexity and manufacturing costs. The focus of this research is on developing and optimizing a simple and industry-appealing fabrication process for high-efficiency IBC-SHJ solar cells.

The first part of the project investigates the simple processing of IBC-SHJ solar cells featuring tunneling recombination junction (TRJ). The first tunneling IBC devices are fabricated successfully. However, shunting is observed from the J-V curves of fabricated devices, which can be related to the internal shunting of tunneling IBC architecture and high lateral conductivity of p-type nc-Si:H-based blanket layer. Hence, alternative contact stacks are designed for application in the previously developed flowchart with the aim to minimize the lateral conductivity of the blanket layer and keep the fabrication process simple.

The proposed hole collection contact stack is firstly implemented in the front junction (FJ) FBC solar cells. The FJ FBC devices reach Voc of 704 mV and FF of 79.29% . Rear junction (RJ) FBC solar cells are fabricated featuring a novel electron collection contact stack that is developed within this thesis. Voc of 715 mV and FF of 82.24% are obtained in RJ devices. The results of RJ FBC solar cells with a newly developed contact stack show excellent results in terms of FF, making it a promising candidate for further application in IBC devices. The introduction of the optimized device design enabled the successful fabrication of IBC solar cells with the simple process developed for tunneling IBC devices while ensuring no shunting occurs. Fabricated devices represent a proof-of-concept of the novel configuration, providing a promising starting point for future development.

Interdigitated back-contacted solar cell (IBC) is a successful high-efficiency solar cell concept. Without a metal grid at the front side, the metal shading loss is eliminated. However, the fabrication process of an IBC cell is much more complicated than that of a FBC cell. Within the PVMD group, two poly-Si carrier-selective passivating contacts (CSPCs) IBC cell fabrication methods were developed, namely Self-aligned and Etch-back method. These two methods require respectively to pattern the IBC rear side twice and three times to define the emitter and BSF area. In this project, a novel poly-Si/SHJ hybrid IBC solar cell design using tunneling recombination junction (TRJ) is proposed, which only requires one pattern step to define the emitter and BSF area, thus, significantly simplifies the flowcharts of IBC cells.
The main objective of this thesis is to demonstrate the novel poly-Si/SHJ hybrid TRJ IBC cell concept by fabricating such high-efficiency hybrid IBC cells. With the proposed hybrid IBC cell design, the BSF layers are deposited on the full rear side after the emitter patterning. Thus, a TRJ is introduced at the emitter ((p+)poly-Si). The carrier tunneling efficiency across the TRJ layers should be guaranteed. The BSF layers passivation quality is also crucial for hybrid IBC cell performance, and the shunting due to the full area deposited BSF layers should be limited as well.
The TRJ proof-of-concept was firstly demonstrated in FBC cells due to their easier fabrication processes. And different materials combinations of (i)a-Si:H, (n)a-Si:H and (n)nc-Si:H were used to form TRJ with (p+)poly-SiOx. Firstly, it was found that the existence of (i)a-Si:H is detrimental to the device performance, especially for the cell FF. Secondly, for cells without (i)a-Si:H layer, the cell performance was improved by replacing (n)a-Si:H with more conductive and low activation energy (n)nc-Si:H layer. At last, the TRJ with dual-n-layer, (p+)poly-SiOx/(n)a-Si:H/(n)nc-Si:H, was found to be most promising. And the cell FF decreases with (n)a-Si:H layer thickness. However, instead of only depositing (n)nc-Si:H, the (n)a-Si:H was kept for its better passivation ability than (n)nc-Si:H, as it is directly deposited on the c-Si surface at BSF in hybrid IBC cells.
Then the hybrid design with dual-n-layer was demonstrated and optimized regarding the passivation quality, TRJ efficiency and the Rshunt in IBC cells. We firstly demonstrated that poly-Si delivers better performance than poly-SiOx due to its lower resistivity. With 18 nm(at textured BSF)(n)n-Si:H, the (n)a-Si:H layer thickness optimizes at 3 nm in poly-Si/SHJ hybrid cells. The pitch width was also found to have an influence on cell external parameters as the number of fingers decreases with pitch width. The cell FF increases and the Jsc decreases with pitch widening. The best cell obtained in this project has 3/18 nm (n)a-Si:H/(n)nc-Si:H and a medium pitch width (650 μm). It has a Voc of 665 mV, a Jsc of 39.36 mA/cm2, a FF of 74.33% and an efficiency of 19.45%.

Crystalline silicon (c-Si) solar cells that are well-established technologies have already achieved a power conversion efficiency (PCE) as high as 26.7% by using the interdigitated-back-contacted (IBC) configuration. However, this efficiency is still limited by the spectral mismatch between t¬he absorption characteristics of c-Si and the AM 1.5 spectrum. To better utilize especially the low wavelength irradiance and to exceed the single-junction silicon solar cell theoretical PCE limit (29.43%), the perovskite/c-Si tandem solar cell concept was proposed. This thesis project aims to develop a high efficiency single-side textured front-back contacted (FBC) silicon heterojunction (SHJ) solar cell as the bottom cells for 2-terminal (2-T) perovskite/c-Si tandem solar cells. Therefore, we firstly focus on the optimizations of the deposition parameters (power and precursor gases) of hydrogenated intrinsic amorphous silicon ((i)a-Si:H) passivation layer and the subsequent application of hydrogen plasma treatment (HPT) on the (i)a-Si:H layer. An optimized 8.8 nm-thick single (i)a-Si:H together with hydrogen plasma treatment (HPT) achieve an effective lifetime (τeff) of 1.3 ms, implied open-circuit voltage (iVOC) of 704.3 mV on symmetrical passivated double-side-flat <100> oriented crystalline silicon (c-Si) wafer. To further improve the passivation, a bilayer structure of (i)a-Si:H with a total thickness of 10 nm is optimized to boost further the τeff to 8.3 ms and iVOC to 733.5 mV. Afterwards, we implement the optimized (i)a-Si:H layers into the front/back-contacted (FBC) rear junction solar cells with single-side textured morphology, which is designed to be used as the substrate for future integration of solution-processed perovskite top cell. For solar cells investigated, we keep the textured rear side always the same with optimized contact stacks consisting of p-type hydrogenated nanocrystalline silicon oxide ((p)-nc-SiOX:H) and p-type hydrogenated nanocrystalline silicon ((p)-nc-Si:H, while varying the layer stacks on the flat front side. Specifically cell performances are studied with either n-type hydrogenated nanocrystalline silicon ((n)-nc-Si:H) or n-type hydrogenated amorphous silicon ((n)-a-Si:H) with varied optimized (i)a-Si:H layers. We observe enhanced passivation qualities of solar cell precusors when adding the (n)-nc-Si:H/(n)-a-Si:H layer on top of the (i)a-Si:H, which enables the potential for realizing high-efficiency solar cells. During the fabrication of FBC-SHJ solar cells, we also find that a thinner (i)a-Si:H on the front side is critical to improve the device efficiency thanks to the effective reduction of both parasitic absorption and carrier transport losses. Accordingly the gain in JSC and FF dominate the improvement of PCE. Lastly, we present rear junction FBC-SHJ solar cell with n optimized stack of (n)nc-Si:H and (i)a-Si:H, with VOC of 711 mV, JSC of 34.82 mA/cm2, FF of 79.64% and PCE of 19.72%. This optimized solar cell is also considered to be ready for its application in tandem device fabrication.