High conversion efficiencies and low production costs have placed Silicon heterojunction (SHJ) solar cells as one of the top contenders in the photovoltaic market right now. When combined with the interdigitated-back-contacted (IBC) architecture, SHJ-IBC cells set world record efficiencies for silicon cells: in 2024, the record stands at 27.3% efficiency. However, the IBC architecture introduces complex processing steps. The goal of this project is the simplification of processing steps of SHJ-IBC solar cells, obtained with the use of hard masks for the direct deposition of patterned layers.

The first part of the project is focused on the development of hard masks. First, silicon based hard masks have been fabricated using wet and dry etching methods. Their fabrication process has been optimized until high quality masks with low defect rates have been achieved. In addition to these masks, quartz mask samples provided by a third party have been tested. Deposition of 50 and 100nm of (n) nc-Si:H have been made with all masks to evaluate their potential based on their resolution, quality, reusability and price. While quartz masks have demonstrated to be very promising and industry orientated, the best trade-off of qualities in a research environment has been found in dry etched silicon masks.

In the core of the project, dry hard masks have been used to substitute two photolithography steps in the fabrication flowchart of IBC-SHJ solar cells in order to achieve a significant process simplification. The use of hard masks has brought relevant impacts to the SHJ-IBC cell fabrication, like the removal of wet etching steps that can potentially bring uncertainty to process flowchart and the reduction of the processing time to half of what it was before the use of hard masks. Regarding the depositions made through the hard masks, the challenges of obtaining a precise alignment and accurate feature sizes been assessed. The implication of those challenges in the performance of the solar cells has been then evaluated.

Lastly, the thesis introduces a simplified fabrication process of SHJ-IBC solar cells with the implementation of hard masks. With the new fabrication process, efficiencies of up to 5.47% have been demonstrated. Through the introduction of the SHJ-IBC flowchart based on hard mask patterning, the project contributes to the ongoing effort to bring SHJ-IBC cells closer to the industry.

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.