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.
The other challenge with implementing polysilicon-based contacts on both sides of the solar cell is to limit the parasitic absorption of incoming photons in the polysilicon layer on the front side of solar cell. This issue needs to be tackled by applying a patterned polysilicon layer only under the front contacts
Therefore, the first purpose of this research work is the development of patterning step of polysilicon layers by means of laser or inkjet masking. The second purpose of this research work is the investigation 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 of polysilicon-based contacts, among which:
1. Patterning of polysilicon by inkjet masking and etching and/or laser.
2. Investigation of co-plated test samples, in other words, samples plated using simultaneous plating process for 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: (a) recombination losses by means of PL-QSSPC (Photo-Luminescence-Quasi Steady State Photoconductance Decay), (b) specific contact resistivity, and (c) metal adhesion.
Type of project: Internship, Combination of internship and thesis
Required degree: Master of Engineering Technology, Master of Engineering Science
Required background: Energy, Electrotechnics/Electrical Engineering
Supervising scientist(s): For further information or for application, please contact: Sukhvinder Singh (Sukhvinder.Singh@imec.be)
Imec allowance will be provided for students studying at a non-Belgian university.