For the continued growth of PV capacity, it will be necessary to store vast amounts of energy to overcome seasonal differences in energy generation. To this end, the conversion of electricity to liquid or gas fuels could play an important role in the future energy system. There are numerous ways to create fuels from solar energy, the possibilities range from completely monolithically integrated photoelectrochemical (PEC) devices to configurations where the PV is completely decoupled from the electrochemical reaction. This thesis is part of the DISCO project, where the aim is to create multi-junction PV cells that can generate a high voltage and integrate these into PEC devices. The aim of this thesis project is to optimize the photovoltaic performance of a triple-junction device based on crystalline silicon (c-Si), nanocrystalline silicon (nc-Si:H) and amorphous silicon (a-Si:H).

The bottom junction of this triple-junction device is a wafer-based silicon heterojunction (SHJ) cell. The used wafer should have a smooth surface texture, since previous research has shown that growing a nc-Si:H absorber layer on the regular random pyramid surface texture results in the formation of cracks. To this end, three different proposed smooth surface textures were compared amongst each other. These surface textures were a smooth pyramid surface texture, a hexagonal surface texture created with photolithography, and a texture of crater like surface features obtained by etching a sacrificial layer. It was found that from these three textures, the best performing SHJ cells could be made from wafers with a smooth pyramid surface texture. With respect to the other surface textures, the smooth pyramid surface texturing approach leads to the smallest fraction of exposed <100> crystal orientations, which are more challenging to passivate. The performance of the SHJ cell is extremely sensitive to the passivation quality of the intrinsic amorphous silicon layers on both side of the wafer. By optimizing this passivation layer and the doped layers, SHJ bottom cell efficiencies of over 18% were achieved.

The middle junction of this triple-junction device is based on a nc-Si:H absorber layer. By varying the crystallinity of this i-nc-Si:H absorber layer, it was found that optimal photovoltaic performance is achieved for a crystalline volume fraction between 50% and 65%. Furthermore, the thickness of this absorber layer can be varied to improve the current matching between the different subcells in the triple-junction device. To this end, the effect of a varying absorber layer thickness on the single-junction PV performance was investigated. It was found that the absorber layer thickness is a trade-off between on the one hand Voc and FF, and on the other hand Jsc. For example, doubling the absorber layer thickness from 2.5 µm to 5.0 µm results in a Jsc increase from 18.2 mA/cm² to 20.8 mA/cm², whereas the Voc and the FF respectively decrease from 506 mV and 0.65 to 481 mV and 0.59.
Once the bottom and middle junctions were optimized, different material combinations in the tunnel recombination junctions of the multi-junction device were investigated. It was found that p-nc-SiO_x and n-nc-SiO_x with appropriate doping levels and contact layers were the best performing materials in both tunnel recombination junctions. When the Voc of the tandem cells was compared to the sum of the single-junction cells, the optimized tunnel recombination junctions resulted in a Voc loss of 7 mV in the top tandem (nc-Si:H/a-Si:H), and 51 mV in the bottom tandem (SHJ/nc-Si:H).
By making use of all the performed optimizations described above, a triple-junction device based on c-Si/nc-Si:H/a-Si:H was processed. The final device had a PV efficiency of 13.63%, with a FF of 0.755, a Voc of 1982 mV and a Jsc of 9.11 mA/cm². To the best of our knowledge, this is a world-record efficiency for this type of solar cell configuration. If it is managed in future research to match the current over the different subcells more evenly, this device has the potential to achieve a current density over 12 mA/cm².