Beschreibung:
Abstract: Screen-printing metallisation is a cost-effective technique widely used in the production of high-efficiency solar cells on n-type silicon substrates. However, a limited doping concentration of boron (B) in p+ emitters poses serious challenges towards realising a metal emitter contact, which has not only low resistance to majority charge carriers but also offers low recombination of minority charge carriers. This work investigates a novel approach to form selective boron emitters (SE) on n-type Si substrates. In this concept, a Laser-Induced Forward Transfer Doping (DLIFT) process is first applied to form the highly doped regions just beneath the metal fingers, which is followed by a subsequent boron tribromide (BBr3) tube furnace diffusion to build a lightly-doped homogeneous emitter. Heavily doped B emitters formed by using DLIFT show sheet resistance in the range of Rsheet = 10 – 80 Ω/sq, surface carrier concentration Ns ≈ 0.5 – 1×1021 cm 3 and depth of 0.8 μm. The lightly-doped B emitters diffused by using a BBr3 tube furnace show low surface dopant concentration Ns < 1.1×1020 cm-3 and low emitter depth of 0.6 μm. The application of BBr3 diffusion after DLIFT is found to improve the implied open-circuit voltage (Voc) of DLIFT emitters in 6.7 mV compared BBr3 diffused emitters. The gain in implied Voc is accredited to a reduction of DLIFT process-induced defects and tensile stress, as compressive stress release of -200 MPa is measured by using micro Raman spectroscopy for the DLIFT samples after the BBr3 diffusion. The emitter saturation current density (j0e) simulated for DLIFT metallised emitters with Ns = 5.0×1020 cm 3 varies in a range between 993 fA/cm2 (Rsheet = 11.7 Ω/sq) and 261 fA/cm2 (Rsheet = 94.2 Ω/sq). The result implies that j0e of metallised emitters is significantly lower for DLIFT highly-doped selective emitters in comparison to the homogeneously formed lowly-doped emitters formed after BBr3 diffusion emitters, whenever Rsheet > 40 Ω/sq. Besides, simulations for DLIFT metallised emitters with Ns = 1.0×1021 cm-3 support that a high surface concentra-tion is beneficial for metallised emitters. The high doping concentration in DLIFT emitters leads to a more substantial shielding of minority carriers, which result in a higher lifetime for minority carriers. Application of the SE approach presented in this work offers an increase of 0.3% in solar cell efficiency in comparison to the homogeneous emitter formed by BBr3 diffusion based on device simulations. If the DLIFT induced defects in the silicon crystal lattice could be avoided or eliminated, j0e values in the range between 54.5 fA/cm2 at Rsheet = 9.5 Ω/sq and 18.4 fA/cm2 at Rsheet = 40 Ω/sq are theoretically achievable.<br>Besides, DLIFT process mechanisms and corresponding threshold laser fluences are investigated in this work. The DLIFT process starts with desorption of hydrogen (H) at a threshold fluence of 0.15±0.08 J/cm2. Then the layer surface in contact with the glass lifts off at a threshold fluence of 0.26±0.07 J/cm2. If the laser fluence increases of 0.48±0.14 J/cm2, then the silicon melts. Si evaporated from the layer accumulates at the glass-layer interface at a threshold fluence of 0.61±0.12 J/cm2. The Si evaporated exerts a stress on the layer that bulges it with a threshold fluence of 0.64±0.06 J/cm2 and moves away from the glass surface to spread on the Si wafer.<br>Therefore, DLIFT is a suitable and technologically flexible approach to provide highly doped boron regions with reasonable low j0e and low metal-semiconductor contact resistance for screen printed n-type Si solar cells