Three-step process for efficient solar cells with boron-doped passivated contacts

Abstract

Crystalline silicon (c-Si) solar cells with passivation stacks consisting of a polycrystalline silicon (poly-Si) layer and a thin interfacial silicon dioxide (SiO2) layer show high conversion efficiencies. Since the poly-Si layer in this structure acts as a carrier transport layer, high doping of the poly-Si layer is crucial for high conductivity and the efficient transport of charge carriers from the bulk to a metal contact. In this respect, conventional furnace-based high-temperature doping methods are limited by the solid solubility of the dopants in silicon. This limitation particularly affects p-type doping using boron. Previously, we showed that laser activation overcomes this limitation by melting the poly-Si layer, resulting in an active concentration beyond the solubility limit after crystallization. High electrically active boron concentrations ensure low contact resistivity at the (contact) metal/semiconductor interface and allow for the maskless patterning of the poly-Si layer by providing an etch-stop layer in an alkaline solution. However, the high doping concentration degrades during long high-temperature annealing steps. Here, we performed a test of the stability of such a high doping concentration under thermal stress. The active boron concentration shows only a minor reduction during SiNx:H deposition at a moderate temperature and a fast-firing step at a high temperature and with a short exposure time. However, for an annealing time 𝑡anneal = 30 min and an annealing temperature 600 °C ≤ 𝑇anneal ≤ 1000 °C, the high conductivity is significantly reduced, whereas a high passivation quality requires annealing in this range. We resolve this dilemma by introducing a second, healing laser reactivation step, which re-establishes the original high conductivity of the boron-doped poly-Si and does not degrade the passivation. After a thermal annealing temperature 𝑇anneal = 985 °C, the reactivated layers show high sheet conductance (Gsh) with Gsh = 24 mS sq and high passivation quality, with the implied open-circuit voltage (iVOC) reaching iVOC = 715 mV. Therefore, our novel three-step process consisting of laser activation, thermal annealing, and laser reactivation/healing is suitable for fabricating highly efficient solar cells with p++-poly-Si/SiO2 contact passivation layers.