• Energy Research

Solution-based processing is a rapidly growing area in the electronics and photonics field due to the possibility of reducing fabrication costs of materials for solar cells, transistors, memory and many other devices. Moreover thanks to its intrinsic nature it provides the possibility to perform conformal processing on structured surfaces. Most of the solution-processing work has so far been devoted to organic materials, but in this work an approach focused on nanostructured silicon is presented. The idea consists in the immersion of a silicon wafer, with Si nanowires grown on top, in a chemical bath containing dopant precursors molecules diluted in a solvent. The molecules deposit from the liquid all over the exposed surfaces and work as a dopant source for the Si nanowires during successive thermal annealing. Doping levels of 1e19 cm-3 are controllably obtained without structural damage and hetero-interfaces creation. The Si-NWs array used presents density of 2e10 cm-2, average length of 500 nm and diameters up to 70 nm. The doped Si-NWs are then integrated in complete solar cells which have been electrically characterized. It is found that the molecular doping method applied to the SiNW arrays provides higher short circuit current and fill factor than the reference samples.

When fabricating Si-based devices, many process steps require the use of expensive, high-power consumption, environmentally unfriendly, operator-unsafe machines, and processes. Among the many involved process steps, the ones needed to fabricate the metallurgical junction make use of conventional doping methods, which do not always represent optimal solutions. The high costs of the processing equipment and the use of hazardous materials, not to count the structural damage produced, intrinsically limit future developments towards nm-scaled and low cost approaches. Recently a chemistry-based method has been proposed to form the junction on Si, the so-called molecular doping. In this approach, the samples to be doped are subjected to a silylation process, during which a layer of dopant-containing molecules is deposited in a liquid bath kept at boiling temperature. After the coating, the samples are annealed to decompose the molecule and release the dopants inside the target. The peculiarity of using a liquid source allows for avoiding the structural damage. The entire doping procedure is simple and cost-effective, and it is based on the use of ester molecules, which are less harmful than the standard materials. In this work, we present experimental results on this chemistry-based technique, demonstrating its efficiency in creating the junction and demonstrate its feasibility in the fabrication of solar cells prototypes. Moreover, with respect to the literature, we show for the first time the effects of the protective layer presence over the dopant source molecules in the final solar cells electrical properties. As a proof of concept, we have numerically investigated the Si-based solar cell using the SCPAS-1D simulator. The finding claims that, the proposed samples have a good match in terms of the performance of the devices compared to the conventional Si-solar cells. Henceforth, the proposed work can provide a guideline to achieve less expensive, more environmentally friendly techniques for molecular doping in Si without affecting its performance in the metallurgical junction.