When designing the front surface of solar cells, it is important to achieve the following:
- Low resistivity contact at the metal-silicon interface.
- Low recombination at the metal-silicon interface.
- Low lateral series resistance within the emitter.
- High light absorption and broad spectral response.
- Reduced recombination in the emitter.
The requirements of 1,2 and 3 above incentivise a heavy diffusion at the front surface. An increase in the surface concentration of dopants decreases the Schottky barrier width at the contact, lowering the contact resistivity. Similarly, a heavy, deep diffusion underneath the contacts depletes the concentration of minority carriers within the vicinity of the recombination active metal-silicon interface, therefore suppressing contact recombination. Furthermore, as the amount of dopants increases, the sheet resistance decreases, which means that there is less resistive losses for carriers which diffuse laterally within the diffused region towards the contact.
However, an increase in dopants at the surfaces incident to light introduces band-gap narrowing effects, which reduces absorption in the shorter, blue wavelengths. Furthermore, increased doping increases Auger recombination in the emitter, which reduces the overall cell Voc. Hence, there are competing aims between achieving low resistive losses, metal-silicon recombination and spectral response and voltage.
Selective emitter solar cells
Selective emitter solar cells are characterised by localised regions of heavy doping underneath the metal contacts. This effectively decouples the requirement of heavy diffusion in the vicinity of the contacts, and light diffusion in the light-incident surfaces.The heavily diffused regions are limited to within the immediate vicinity of the contact, and the rest of the emitter is more lightly diffused to enable good light absorption. The heavy diffusion is tolerated because they only cover a small surface fraction and are optically shaded by the front contacts – hence the effects of increased Auger recombination and poor light absorption are significantly minimised. A schematic of a typical selective emitter configuration, as it was implemented in the PERL solar cell structure, is shown in Figure 1 below.
Figure 1 Schematic of a PERL cell, showing a selective emitter configuration with heavily diffused n+ regions underneath the metal contacts, surrounded by a more light diffused n-type emitter.
Selective emitters can be fabricated in numerous ways, including laser doping, etch back processing, or by diffusion through a mask. In a laser doping process, a spin-on-dopant source is coated over the front dielectric, which is then locally ablated with a laser. The laser ablates the dielectric and melts the silicon, incorporating the spin on dopants into the liquid crystal melt. Upon re-solidification, the dopants have been incorporated locally into the lased regions, creating the heavily doped regions. These areas are then metallised, typically in a plating step. An early example of this technology was the BP solar Saturn Cells and the Suntech Pluto cells.
Whilst it is common to think of selective emitter solar cells as front and rear contact solar cells, the principle of select localised regions of heavy doping can also apply to all-back contact solar cells.
In the animation below we show the how an etch back can be used to form a selective emitter.