Co-plating of polysilicon-based passivating contacts in bifacial solar cells

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Towards the theoretical efficiency limit in silicon solar cells by means of a novel co-plated passivating contact approach

In the last years, PV manufacturing has seen a shift from Al-BSF (Back Surface Field) to PERC (Passivated Emitter and Rear Contact) solar cells to drive down the recombination current at the rear surface of the cells. Other shifts, e.g. transition from multicrystalline to monocrystalline material and from p-type to n-type base doping, are also taking place to minimize recombination currents in the bulk of the device. However, today the weakest point of crystalline silicon solar cells is still the recombination at the semiconductor/metal contact interface. To maximize the open circuit voltage of silicon solar cells and approach the thermodynamic efficiency limit, commonly known as Shockley-Queisser limit, a lot of the recent research in the field of crystalline silicon photovoltaics has focused on mitigating the recombination losses at that interface.

In crystalline silicon solar cells, this problem can be effectively tackled with the implementation of polysilicon-based passivating contacts. A polysilicon-based contact consists of the stack 'interfacial oxide/n-type or p-type polysilicon thin-film/metal' at the un-doped or doped semiconductor surface to be contacted. As a passivating contact, this structure must be designed to shield the minority carriers from the recombination sites at the metal contact while enabling a good transport and collection of the majority carriers to that contact. Thus, the ideal passivating contact would feature minimum minority carrier recombination as well as low specific contact resistivity.

Polysilicon-based passivating contacts show a lot of potential, but every effort must be taken to retain the excellent passivation quality during metallization. The standard metallization technique in the industry consists of screen-printing. However, screen-printing offers 2 main disadvantages: (a) it has a quite high Ag usage, and (b) it is not ideal for maintaining the polysilicon passivation quality in the metallized regions due to the etching of the poly-Si layer during the subsequent firing step of the contacts.

Therefore the main purpose of this research work is the development of a soft plating metallization process for polysilicon passivating contacts. On the one hand, the plating would limit the Ag consumption. On the other hand, it has the potential to minimize the metallization-induced damage compared to screen-printing and, therefore, to maintain the passivation quality while enabling the use of thinner polysilicon layers.

The research work realized during this internship will focus on different aspects of the plating of polysilicon-based contacts, among which:

  1. Improvement of the first step of the plating sequence, immersion plating of nickel, which accounts for the main losses in the passivation quality during the metallization process itself.
  2. Co-plating or, in other words, simultaneous plating of both n-type and p-type polysilicon contacts in bifacial solar cell structures.
  3. Quantification of the metallization performance of plated polysilicon contacts based on: (1) recombination losses by means of PL-QSSPC (Photo-Luminescence-Quasi Steady State Photoconductance Decay), (b) specific contact resistivity, and (c) metal adhesion.
  4. Material characterization (SEM, TEM, ECV, EDS...) of the polysilicon contact after metallization.

Type of project: Internship

Duration: 6-12 months

Required degree: Master of Engineering Technology, Master of Science, Master of Engineering Science

Required background: Energy, Nanoscience & Nanotechnology

Supervising scientist(s): For further information or for application, please contact: Maria Recaman Payo (Maria.RecamanPayo@imec.be)

Allowance only for students from a non-Belgian university

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