Low Temperature Plasma Processes: From Silicon Thin Films to Epitaxial Growth & Nanostructure for PV

In this plenary talk, Prof. Pere Roca i Cabarrocas presents an overview of low-temperature plasma processes for photovoltaic applications, drawing on 30 years of research. He begins by contextualizing the global energy challenge, noting that renewables currently provide only 1.4% of global energy, and emphasizes the need for terawatt-scale PV deployment. He reviews the dominance of crystalline silicon technology, its high efficiencies (record 26.7% vs. theoretical limit 29%), and the cost reductions that have made PV a commodity. However, he argues that the conventional silicon production chain (Siemens process, ingot pulling, wafer slicing) is energy-intensive and material-wasteful, limiting scalability to terawatt levels. He proposes thin-film technologies as a solution, highlighting amorphous and microcrystalline silicon deposited by plasma-enhanced CVD at low temperatures (200°C). He discusses polymorphous materials, tandem structures, and the potential to reach 20% efficiency. He then introduces his group’s work on low-temperature epitaxial growth of crystalline silicon on silicon substrates, achieving high-quality layers with good electronic properties, and even heteroepitaxy of germanium on gallium arsenide. He also presents silicon nanowire solar cells grown via plasma processes, which decouple light absorption and carrier collection, and demonstrates a tandem device combining amorphous and nanowire cells. He concludes that while crystalline silicon will remain dominant in the near term, alternative thin-film and nanostructured approaches may be needed for long-term terawatt-scale deployment.