Laser Activation for Highly Boron-Doped Passivated Contacts

Abstract

Passivated, selective contacts in silicon solar cells consist of a double layer of highly doped polycrystalline silicon (poly Si) and thin interfacial silicon dioxide (SiO2). This design concept allows for the highest efficiencies. Here, we report on a selective laser activation process, resulting in highly doped p++-type poly Si on top of the SiO2. In this double-layer structure, the p++-poly Si layer serves as a layer for transporting the generated holes from the bulk to a metal contact and, therefore, needs to be highly conductive for holes. High boron-doping of the poly Si layers is one approach to establish the desired high conductivity. In a laser activation step, a laser pulse melts the poly Si layer, and subsequent rapid cooling of the Si melt enables electrically active boron concentrations exceeding the solid solubility limit. In addition to the high conductivity, the high active boron concentration in the poly Si layer allows maskless patterning of p++-poly Si/SiO2 layers by providing an etch stop layer in the Si etchant solution, which results in a locally structured p++-poly Si/SiO2 after the etching process. The challenge in the laser activation technique is not to destroy the thin SiO2, which necessitates fine tuning of the laser process. In order to find the optimal processing window, we test laser pulse energy densities (Hp) in a broad range of 0.7 J/cm2 ≤ Hp ≤ 5 J/cm2 on poly Si layers with two different thicknesses dpoly Si,1 = 155 nm and dpoly Si,2 = 264 nm. Finally, the processing window 2.8 J/cm2≤ Hp ≤ 4 J/cm2 leads to the highest sheet conductance (Gsh) without destroying the SiO2 for both poly Si layer thicknesses. For both tested poly Si layers, the majority of the symmetric lifetime samples processed using these Hp achieve a good passivation quality with a high implied open circuit voltage (iVOC) and a low saturation current density (J0). The best sample achieves iVOC = 722 mV and J0 = 6.7 fA/cm2 per side. This low surface recombination current density, together with the accompanying measurements of the doping profiles, suggests that the SiO2 is not damaged during the laser process. We also observe that the passivation quality is independent of the tested poly Si layer thicknesses. The findings of this study show that laser-activated p++-poly Si/SiO2 are not only suitable for integration into advanced passivated contact solar cells, but also offer the possibility of maskless patterning of these stacks, substantially simplifying such solar cell production.

Three-Step Process for Efficient Solar Cells with Boron-Doped Passivated Contacts

“…As detailed in Ref. [8], we found the linear dependence of the melt depth on H p in the laser processing of the poly-Si layers, which agrees with the same finding for the conventional laser processing of bulk c-Si [35,36]. This behavior enabled us to estimate the melt depth created by different H p [8].…”

“…In contrast to the reactivation/healing process as discussed here, our previous study [8] showed that laser energy densities H p > H T led to increased iV OC values (in the first laser activation process). The observed difference originates from the thermal annealing step in the present process sequence (after laser activation).…”

“…All fields were processed with the same laser pulse energy density H p = 2.1 J/cm 2 . This H p value, in our previous publication, was found to be the optimal laser processing setting for our poly-Si layer with a thickness of d poly-Si = 150 nm [8]. The irradiation was performed with the same pulse repetition rate of f = 16 kHz.…”

“…However, iV OC significantly decreases when H p exceeds the transition laser pulse energy density H T , i.e., for H p > H T . Our previous work introduced H T as the laser pulse energy density, which results in a melt depth equal to the poly-Si thickness [8].…”

“…In a previous publication, we introduced a laser activation process resulting in poly-Si layers that are supersaturated with electrically active boron atoms beyond the solid solubility limit [8]. Due to the significantly higher diffusion coefficient (D) of boron in molten Si compared to solid Si (D l ≈ 10 −4 cm 2 /s in liquid [9], D s ≈ 10 −11 cm 2 /s in solid at 1150 • C [10]), the majority of boron atoms, including the electrically deactivated atoms, diffuse into the molten Si after a laser pulse melts the lightly furnace-doped p + -poly-Si layer.…”