Application of poly-Si on oxide junctions as one or both polarities of high-efficiency IBC solar cells

Abstract

This work deals with the application of passivating contacts, specifically POLO contacts, which can increase the selectivity of the charge carriers at the metal contact and thus the efficiency due to their excellent passivating effect. These poly-Si based contacts, also called TOPCon, are just establishing themselves in industrial solar cell manufacturing. However, there are still some open questions regarding their operation and optimal fabrication.

The first part of the paper is focused on the so-called POLO2-IBC cell, with which Felix Hasse’s team, with the collaboration of the author of this thesis, was able to achieve a record efficiency of 26.1 % for p-type material in 2018. With an area of 4 cm² and very complex patterning processes, this back-contacted record cell is not an industrially relevant cell, but it demonstrates the great potential of POLO contacts. A distinctive feature of this cell is the continuous layers of thin oxide and overlying poly-Si that form the so-called „poly-Si on oxide“ (POLO) junctions. The electron-collecting and hole-collecting contacts, fabricated by doping via ion implantation, are separated only by narrow intrinsic POLO regions.

Extensive monitoring of the fabrication process and a simulation study show that the potential of this cell type with 26.1 % has not yet been fully exploited. Furthermore, the comparison with a high resistivity base material shows the significantly higher susceptibility of the passivation quality with decreasing doping concentration, so that despite higher intrinsic recombination, the used 1.3 Ohm cm material is identified as most suitable.

Separating the p+ and n+ contact through the undoped region proves to be an elegant solution, which, however, only works under certain conditions. Diffusion of dopants from the n+ and p+ regions into the intrinsic region improves the otherwise poor passivation quality there. At the same time, however, this increases the unwanted recombination current between the contacts, so the choice of the appropriate width is of immense importance. With the understanding of the working principle gained, less complex structuring is conceivable in the future.

However, in order to make the leap from the laboratory cell to industrial application, stability against the firing step for contact formation with common screen printing pastes is essential. Although the POLO contacts prove by their manufacturing process alone that they can withstand high temperatures with very good passivation quality, firing without a capping layer leads to deterioration of the passivation quality. In the second experimental part of this work, it is shown that this behavior is probably due to the two orders of magnitude higher heating and cooling rates in combination with thermal stresses. However, the degradation can be counteracted with the help of hydrogen rich dielectric layers. Nevertheless, it is shown, that too much hydrogen can also have a negative effect here. For typical firing temperatures of around 800 °C, a stack of Al2O3/SiNy layers yields the best results.

In the last section of this work, the use of a n+POLO contact in a screen-printed POLO-IBC cell can be successfully demonstrated with an efficiency of 23.92 %. The excellent passivation quality of 0.2 fA/cm², which can be obtained using Al2O3/SiNy stack to cap the n+POLO junction, plays an important role here. Overall, this work has helped to transfer passivating contacts from high-efficiency POLO2-IBC laboratory cells to the promising POLO-IBC cell concept suitable for industrial applications.