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.