Poly-si based passivating contacts for high-efficiency silicon solar cells

Abstract: Crystalline silicon (Si) solar cells are currently dominating the global photovoltaic market. The conversion efficiency of these cells is predominantly limited by recombina-tive losses at the metal-semiconductor interface. Passivating and carrier selective con-tacts address this issue by spatially decoupling charge carrier generation in the Si ab-sorber and carrier separation at the electrodes thus enabling efficiencies closer to the theoretical limit of 29.4% [1]. In the context of high-efficiency Si solar cells, the con-cept of a tunnel-oxide passivating contact (TOPCon) was first introduced by Feldmann et al. in 2013 based on the development of bipolar junction transistor (BJT) in the field of microelectronics in the 1980ies. TOPCon consists of an ultra-thin interfacial oxide SiOx grown on the Si surface for interface passivation which is capped with a highly doped polycrystalline Si layer (hereinafter poly-Si) leading to carrier selectivity.
The ultrathin interfacial oxide layer of 1 – 2 nm thickness is a vital part of the con-tact and was examined in more detail in the scope of this thesis to gain deeper insights into the interplay between oxide properties, high-temperature annealing, poly-Si doping and subsequent hydrogenation. To this end, the stoichiometry of differently prepared interfacial oxides in the as-grown state was investigated by means of X-ray photoelec-tron spectroscopy (XPS). The findings were correlated with the thermal stability of the oxide layers integrated in TOPCon which experience further structural modification during the high-temperature annealing step. More specifically, it was observed that a more stoichiometric interfacial oxide is a more effective diffusion barrier for dopants and, thereby enhanced the thermal stability of TOPCon structures with respect to the passivation quality. Furthermore, stoichiometric changes in the oxide layer upon sub-sequent contact formation (thermal annealing) were analyzed using XPS after selective etch back of the doped poly-Si layer. It was found that the interfacial oxide becomes more stoichiometric during annealing. In addition, a slight increase in oxide thickness of 0.2 – 0.6 nm was observed. This tentatively could be explained by a large amount of unbound interstitial oxygen in the interface region indicated by decrease in the oxygen excess fraction by 2.5 - 3%abs.
Even though a certain level of surface passivation and field-effect passivation is established after contact formation, a final hydrogenation step is essential to achieve a very high level of surface passivation, especially on textured surfaces. This thesis therefore studied the hydrogenation mechanism to improve the understanding of the underlying processes. For this purpose, different hydrogen sources were compared regarding their ability to chemically passivate defects at the Si/SiOx interface and presumably in the doped poly-Si layer as well as their thermal stability in a low and high temperature range. To this end, hotplate annealing series were performed on textured n-type TOPCon structures and Al2O3/SiNx multi-layer stacks were exposed to fast-firing processes. Very distinct activation characteristics were detected and for Al2O3 an activation energy EA = 0.51 ± 0.05 eV for the passivation of defects in n-type TOPCon structures was extracted from its Arrhenius activation behavior. It was also observed that the Al2O3 capping layers enable a higher level of surface passivation and higher thermal stability compared to SiNx. Implemented into multi-layer stacks, Al2O3 acts as a hydrogen diffusion barrier und prevents effusion from the structure that deteriorates the passivation quality irreversibly in n-type TOPCon. However, p-type TOPCon revealed an enhanced stability upon firing up to 850°C with a single SiNx layer and did not require an additional Al2O3 capping.
In industrial production, the poly-Si layer was commonly fabricated using low-pressure chemical vapor deposition (LPCVD) up to now. This thesis discusses the successful realization of TOPCon – both poly-Si and nitrous SiOxNy – using industry-relevant plasma-enhanced chemical vapor deposition (PECVD) equipment used for SiNx or Al2O3 deposition in existing production lines. In the scope of this work, it was shown that batch-type low-frequency direct plasma PECVD allows for a damage-free deposition of doped a-Si on the interfacial oxide. Special focus was put onto the development of an in situ doped p-type contact as well as the fundamental understanding of the junction’s working principle and its limiting aspects. It was shown that an intrinsic a-Si layer between the highly doped a-Si(p) layer and interfacial oxide as well as careful control of boron diffusion across poly-Si/SiOx/c-Si junctions are essential for a good performance. Here, an additional native SiOx interlayer between a-Si(i) and a-Si(p) can be beneficial at high annealing temperatures above 950°C and dopant diffusion into the bulk can be reduced by increasing the interfacial oxide thickness (and stoichiometry). The optimal annealing temperature is higher compared to n-type TOPCon and strongly depends on the interfacial oxide properties. Both, high passivation quality with recombination current densities (J0 < 7 fA/cm²) and low contact resistivities (ρc < 5 mΩcm²) were obtained on test structures with planar surface. Finally, the developed boron and phosphorus-doped layers were successfully integrated into proof-of-concept both-sided contacted TOPCon solar cells yielding a very high Voc of up to 719.5 mV and 22.8% conversion efficiency

Object Details

• Location: Deutsche Nationalbibliothek Frankfurt am Main
• Extent: Online-Ressource
• Language: Englisch
• Notes: Universität Freiburg, Dissertation, 2022

Classification and Topics

• Classification: Elektrotechnik, Elektronik
• Keyword: Tōkyō Kōgaku Kikai Kabushiki Kaisha

Contributors, Places and Time

• Event: Veröffentlichung
• (where): Freiburg
• (who): Universität
• (when): 2022
• Creator: Polzin, Jana Isabelle
• Contributor: Glunz, Stefan ; Glunz, Stefan ; Haug, Franz-Josef ; Albert-Ludwigs-Universität Freiburg. Institut für Mikrosystemtechnik ; Fraunhofer-Institut für Solare Energiesysteme ; Albert-Ludwigs-Universität Freiburg. Fakultät für Angewandte Wissenschaften

Further information

• DOI: 10.6094/UNIFR/228739
• URN: urn:nbn:de:bsz:25-freidok-2287390
• Rights: Open Access; Der Zugriff auf das Objekt ist unbeschränkt möglich.
• Last update: 25.03.2025, 1:51 PM CET