Parameter optimization of laser-doped selective emitters for applications in silicon solar cells

The use of selective emitters in p-n junction solar cells is a well-known way to increase cell efficiency by 0.4 - 0.5% (absolute) with the addition of a few processing steps. In a selective emitter, the region directly below the metal-contact fingers is more heavily doped than the shallow p-n junction. This allows for enhanced carrier collection by shielding minority carriers from the contacts, thereby lowering recombination at the metal-semiconductor interface. In contrast to earlier expensive techniques involving fine-line lithography, laser processing provides an ideal way to create these selective emitters because of its ability to locally heat and dope the surface of the cell without any external patterning steps. In this study, Q-switched lasers of wavelengths 1064, 532, and 355 nm are used at a range of pulse energies to create selective emitters on a p-type FZ silicon wafer with a thin n+ dopant film deposited on the top surface of the wafer. In addition to the Q-switched lasers, a 1070 nm continuous wave laser is also used and both the pulse energy and pulse duration are varied. To determine the effect of the n+ dopant film, the thickness of the film is also varied and processed with all of the lasers. The results from these lasers and the different dopant layers are characterized electrically through current-voltage measurements and compared to determine the optimal processing wavelength and energy for the selective emitters which maximize diode performance while minimizing crystal lattice damage and series resistance.