Definition of Base resistance
One of the common strategies for high-efficiency crystalline silicon solar cells for the rear surface is a combination of a dielectric passivation with a point like contact to the base. In similar devices, this trade off between ohmic losses and surface passivation defines the optimum distance between contacts or the pitch. For a specific rear point like contact, base resistance (Rbase) is commonly calculated earlier and not checked in finished devices. Since common techniques for measuring series resistance lead to a distinctive value that includes all ohmic losses.
Rbase can be determined based on the presence of the capacitor formed by the metal, dielectric or semiconductor structure that covers the majority of the rear surface. Rbase is measured on finished devices where a more accurate value of the contacted area is gathered which is valuable information for further device optimization.
High efficiency solar cells require excellent surface passivation with decent properties for carrier transport. For the rear surface of double side contacted solar cells where the base contact is located, these requirements are often obtained by the combination of a dielectric passivating film with the local contacts which are defined in a point like pattern. This configuration was applied already in the high efficiency concepts that was developed in the 90’s where the contacts were defined by photolithography. In the last years, this idea has been revisited again because of the introduction of laser techniques that allows the definition of the point-like pattern in a very efficient and cost effective way.
Generated current normally flows in a direction perpendicular to the cell surface from the bulk of the cell and after that laterally through the doped top layer till it is collected at a top surface contact.
The resistance of the base and the current of the base are assumed to be constant. The resistance to the flow of the current through the bulk component of the cell, or the “bulk resistance”, Rb, is defined as:
Rb= ρl / A= ρb W / A
While taking into consideration the material thickness,where:
l = length of the resistive or conducting path
ρb= bulk resistivity (which is inverse of conductivity) of the bulk cell material (range from 0.5 – 5.0 Ω cm for a typical silicon solar cell)
A = area of the cell
w = width of the bulk region of cell.