Silicon heterojunction (SHJ) solar cell technology is one of the most promising PV technologies because of its high power conversion efficiencies, simple fabrication process and low manufacturing cost. The current world record efficiency of SHJ solar cell reaches up to 26.81%. SHJ solar cell produces the highest VOC over 750 mV among the c-Si solar cell technologies because of the excellent surface passivation provided by the few nanometers (i)a-Si:H layers. The objective of this thesis is to design and optimize the (i)a-Si:H bilayers that not only ensure excellent c-Si surface passivation quality but also allow less-resistive transport of charge carriers in SHJ solar cells. First, a database focusing on the microstructure properties of (i)a-Si:H layers was established. The database was compiled by systematically varying the PECVD deposition parameters of the pressure, power, and hydrogen dilution ratio. By means of FTIR (Fourier Transform Infrared Spectroscopy) characterizations, higher power and pressure were found to correspond with an increased absorption strength of high stretching modes (HSM), which indicated a higher R* and hydrogen content. These conditions led to the formation of a film that is rich in voids and hydrogen. Conversely, a denser a-Si:H film could be achieved by increasing the hydrogen dilution ratio, where the low stretching modes (LSM) were increased and became dominant. Overall, the built database showcases the possibilities to deposit (i)a-Si:H layers with microstructure factor spanning from 0.193 to 0.805. Subsequently, passivation optimization of the (i)a-Si:H bilayer on textured (111)-orientated c-Si wafers has been conducted. The optimal passivation is achieved by the combination of a 1 nm underdense i1 layer and a 9 nm dense i2 layer. The void-rich i1 layer with R* = 0.732 and the dense i2 layer with R* = 0.205 improved the passivation greatly from 2 ms to 7.5 ms, as compared to unoptimized STD1 layer (R* = 0.261) and STD2 layer (R* = 0.314). Despite various passivation qualities enabled by various bilayer structures, after hydrogen plasma treatment (HPT), the lifetime of all increased drastically to 17 ms, accompanied by increases FTIR-characterized in both H content and R*, which may indicate that passivation could be saturated after HPT. However, the passivation trend after VHF treatment was not clear. This could potentially be the instability in the PECVD deposition tool. Finally, (i)a-Si:H bilayers were implemented into FBC-SHJ solar cells, solar cells endowed with a bilayer consisting of X1 layer (R* = 0.382) and D2 layer (R* = 0.205) exhibited an average Voc of 715 mV. Combined with a denser D2 film, RS is reduced, helping to counteract the elevated series resistance associated with the incorporation of the X1 layer, contributing to the enhancement of the fill factor from 78.65% to 80.53%, although at the expense if a slightly lower VOC. The best-performing cell X1 + D2 with treatments gives achieved the highest efficiency of 23.23% (22.33 % on average) and the highest VOC of 717mV compared with an average of 22.24% and 711 mV of the ‘non-optimized’ standard cells fabricated. It is also observed that after treatments, the FF increases while VOC slightly reduces. However, the observed lack of a clear efficiency improvement when using the optimized bilayer as compared to the ‘non-optimized’ standard bilayer could be attributed to the instability of the PECVD tool, which primarily affects the lifetime of precursors, coupled with the relatively large error bars present in the results.

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.

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.

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.

Silicon heterojunction (SHJ) solar cells have exhibited efficiencies well above 25%. To further boost the efficiencies of c-Si-based solar cells, high-bandgap perovskite cells are stacked on top achieving a record efficiency of 29.52%. However, as most of the high-quality perovskite films are solution-processed, the front surface of the bottom device should be flat. Therefore, in this work SHJ bottom c-Si cells featuring front-side-flat and rear-side-textured morphology, which delivers high VOC together with excellent near-infrared response, have been optimized as bottom cells for tandem configurations. Firstly, RF-PECVD deposition conditions of a (i)a-Si: H monolayer for symmetric <100> flat c-Si surfaces were optimized. The optimized (i)a-Si:H monolayer ( 10-nm-thick) was obtained using pure SiH4, which results in rather moderate passivation performances (teff = 1.2ms, i-VOC = 701 mV). To improve further the passivation quality of monolayer (i)a-Si:H on flat <100> surface, other passivation approaches aiming at incorporating more H without promoting detrimental epitaxial growth have been investigated. With a bilayer deposition approach, which features firstly a less H-containing (i)a-Si:H to prevent epitaxial growth and then a second H-rich (i)a-Si:H layer, the passivation properties were slightly enhanced to τeff=1.4 ms and i-VOC=704 mV. Subsequently, by combining the bilayer approach with a post HPT, τeff of 2.0 ms and an i-VOC of 714 mV were achieved. Finally, by combining the bilayer approach with an intermediate HPT, the optimal passivation sample was deposited, with τeff of 2.4 ms and an i-VOC of 720 mV on the flat <100> surface. To gain a better understanding of the correlation between passivation qualities and the microstructure properties of (i)a-Si:H on flat <100> surface, the layers have been characterized mainly via Fourier-transform infrared spectroscopy (FTIR). From the analysis, it can be concluded that the passivation layer that contains sufficient H and a higher fraction of monohydrides is beneficial for achieving a better passivation quality. For the two-terminal tandem solar cells, bottom cells with (n)-contact on top are preferred due to the optical advantage of the perovskite top cells with the p-i-n configuration. Therefore, a first tandem cell with (n)a-Si:H has been fabricated in collaboration with TU Eindhoven resulting in 22.2% efficiency. Starting from this first fabricated tandem cell, its main optical limitations have been identified by performing advanced optical simulations using GenPro4, and the main strategies to overcome these optical drawbacks have been defined. By optimizing the front anti-reflection layers (MgF2 and ITO) thicknesses (at 100 nm and 20 nm, respectively), and reducing C60 thickness from 20 to 10 nm, front reflections, and parasitic absorption can be minimized. Thus a gain of implied photocurrent density of 1.8 mA/cm2 for the tandem cell was obtained. Further, by implementing (n)nc-SiOx:H doped layer in the SHJ bottom cell, instead of standard (n)a-Si:H layer the reflection between the top and bottom cell is also reduced, and enhanced light incorporation into the bottom cell is obtained. By adopting all the above optimizations and also adjusting the perovskite layer from 473 nm to 530 nm, a total improvement of 2.7 mA/cm2 in implied photocurrent density with respect to the initial 22.2% tandem cell can be achieved. After having identified different optically optimized SHJ bottom cells for tandem applications, both rear junction and front junction single-side-textured SHJ solar cells were fabricated. Firstly, the passivation quality of (i)a-Si:H/(n)-layer and (i)a-Si:H/(p)-layer on different (i)a-Si:H were investigated. Then RJ solar cells with three different (n)-type layers [(n)nc-SiOx:H;(n)nc-Si:H;(n)a-Si:H)] have been fabricated with optimal thicknesses individuated from the tandem optical simulations. Furthermore, a tunnel recombination junction SHJ solar cell with a layer stack of (n)nc-Si:H/(p)nc-SiOx:H/(p)nc-Si:H has been fabricated and measured as well. In conclusion, various doped contacts (both n- and p-type) were successfully implemented into SHJ solar cells, which delivered VOCs range from 700 to 714 mV and FFs range from 77.8% to 80.9%. Therefore, different well-functioning SHJ solar cells have been developed and are ready to be implemented as bottom cells for high-efficiency tandem devices.