Advanced Rear-Side Contact Schemes on i-PERC (Industrial Passivated Emitter and Rear Cell) Solar Cells

摘要

For many years, the photovoltaic industry has been using Al to form the back contact of crystalline silicon solar cells. The objective of this PhD is to reach a better understanding of the contact formation at the rear side of the Passivated Emitter and Rear Cell (PERC) type solar cell. This type of cell features at the rear side a stack of dielectrics as a passivation layer which is pre-opened in order to allow the subsequent metallization step. The way this dielectric stack is opened, by laser ablation, the deposition of the metal, with various techniques, and the high temperature step required for the formation of the Al-Si alloy which serves as a contact, will be investigated in this work.It will be demonstrated that Si dissolution in Al plays an important role in the formation of the contacts, giving on one hand the needed dopants to create an effective Back Surface Field (BSF) layer, and on the other hand forming a deep pyramidal shape in the contact region which might affect the reflection of the light inside in the cell, and increase the surface recombination velocity of the device.A novel method to characterize the local Al-Si alloy formation is introduced, allowing the in-situ observation of the high temperature step for the contact formation. By doing this, details of the process which cannot be taken into account once the device is finished, are studied and explained through hypotheses involving the phase diagram between both elements.The performance at cell level of the different combination of parameters affecting the formation of the BSF region has been evaluated. For this, changes in contact pitch, firing temperature and profile, Al thickness and composition are studied for the PERC cells.At the end, the rear reflectance loss mechanisms are also investigated by altering the dielectric layers used for passivation, together with the study of the effect of Si incorporation during the firing step, showing that this Si presence at the back side is the main responsible for the loss observed.