Interdigitated back contacts solar cell based on thin crystalline silicon substrates

• Autores: Chen Jin
• Directores de la Tesis: Isidro Martin Garcia (dir. tes.), Moisés Garín Escrivá (codir. tes.)
• Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2018
• Idioma: español
• Tribunal Calificador de la Tesis: Carlos del Cañizo Nadal (presid.), Cristóbal Voz Sánchez (secret.), Hele Savin (voc.)
• Programa de doctorado: Programa de Doctorado en Ingeniería Electrónica por la Universidad de las Illes Balears y la Universidad Politécnica de Catalunya
• Materias:
  • Ciencias tecnológicas
    • Tecnología electrónica
      • Dispositivos fotoeléctricos
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• Resumen

  • JinThis thesis contributes to the fabrication technology of c-Si solar cells on thin substrates based on Interdigitated Back-Contacted (IBC) structures. The potential of this structure to obtain high efficiencies is well-known. However, important challenges should be addressed to adapt it to thin c-Si substrates, such as the manufacturing of the thin c-Si substrate itself, light absorption enhancement, device structure design, surface passivation, etc. Focused on these challenges, experiments and simulations have been carried out, including innovative thin c-Si substrate fabrication method Millefeuille process, novel IBC solar cell structures combining laser doping and silicon heterojunction technologies and thin IBC solar cell performance prediction through simulation. Finally, a 30 µm thick c-Si solar cell is fabricated by thinning down a finished device applying a silicon etching technique that combines dry and wet etching. Considering the Millefeuille process, based on the technological know-how the impact of both modulated profile and periodicity of silicon pores on the generated thin layer quality is explored and the results are visualized by SEM images. Furthermore, the solid-void transformation evolution during the high temperature annealing reveals the pore status at 35, 60 and 90 minutes, allowing a deeper understanding of the practical silicon atomic surface diffusion and the shape evolution. In order to find a viable and promising device structure that can be used in case of thin silicon substrates, a hybrid p-type solar cell structure is reported. In this case, emitter is based on silicon heterojunction technology while the base contacts are created by laser processing Al2O3/SiCx films. Special attention of the compatibility of both technologies has been paid in the proposed fabrication process including emitter region re-passivation and contact metallization. This work provides a new approach for achieving low-temperature high efficiency c-Si solar cells, as well as a novel pathway compatible to the fabrication of IBC devices based on thin c-Si substrate.In parallel with experimental progress, the simulation on thin c-Si IBC solar cell is carried out for performance study and prediction involving two typical rear surface doping structures: fully- and locally-doped. Simulation results of fully-doped structure reveal an efficiency potential of 16-17 % for thin c-Si IBC solar cell based on substrates of 10-15 µm without changing the technology developed for thick ones. Regarding the locally-doped structure, its performance is less tolerant to the degradation of front surface passivation. Additionally, a strong reduction of short-circuit current related to stronger requirements in the effective diffusion length is also deduced. Finally, a reduction of saturation current density, probably related to a change in the distribution of current that flow parallel to the rear surface, is also observed when the device is slimmed down. Next, a thin IBC c-Si solar cell efficiency potential is explored through rear contacts pitch study and the highest conversion efficiency is expected when contact pitches are minimum in the range of study. Finally, efforts are paid to get a thin c-Si solar cell through thinning down an already finished device of thick substrate. A silicon etching process based on RIE and wet chemical etching is proposed. Different experiments demonstrate that the front surface can be successfully repassivated after etching process. Additionally, random pyramids are created on that surface and the optical response of thin c-Si substrates is measured revealing a potential photogenerated current in the range of 40 mA/cm2 for 30 µm-thick substrates. Applying all these techniques to a final device, a 12.1 % efficiency is achieved and the front surface recombination velocity is deduced to be 1500 cm/s by comparing EQE with simulation results.