Hydrogenation of Phosphorus-Doped Polycrystalline Silicon Films for Passivating Contact Solar Cells

Abstract

We characterize and discuss the impact of hydrogenation on the performance of phosphorus-doped polycrystalline silicon (poly-Si) films for passivating contact solar cells. Combining various characterization techniques including transmission electron microscopy, energy-dispersive X-ray spectroscopy, low-temperature photoluminescence spectroscopy, quasi-steady-state photoconductance, and Fourier-transform infrared spectroscopy, we demonstrate that the hydrogen content inside the doped poly-Si layers can be manipulated to improve the quality of the passivating contact structures. After the hydrogenation process of poly-Si layers fabricated under different conditions, the effective lifetime and the implied open circuit voltage are improved for all investigated samples (up to 4.75 ms and 728 mV on 1 Ω cm n-type Si substrates). Notably, samples with very low initial passivation qualities show a dramatic improvement from 350 μs to 2.7 ms and from 668 to 722 mV.

Hydrogenation Mechanisms of Poly‐Si/SiOx Passivating Contacts by Different Capping Layers

“…However, when capping layers and postdeposition annealing treatments are used, the passivating‐contact performance improves significantly, as reported by many groups . It has been shown that, without postdeposition annealing, there is no passivation improvement, and also that either annealing in N 2 or FGA shows the same performance …”

“…Hydrogenation treatments in c‐Si solar cell technology are often performed via forming gas annealing (FGA) of samples in a mixture of H 2 and inert gas, by annealing the samples without any capping layers in a mixture of water vapor and N 2 , or by depositing hydrogen‐rich capping layers such as AlO x :H or SiN x :H and annealing them in N 2 or FGA. Previously, we have noticed that in the case of low‐resistivity substrates (1 Ω cm), FGA of poly‐Si/SiO x passivating contacts resulted in little performance improvement, whereas others have reported a noticeable improvement . However, when capping layers and postdeposition annealing treatments are used, the passivating‐contact performance improves significantly, as reported by many groups .…”

“…In all cases, the spectra are nearly identical. There is a sharp peak at ≈1125 nm attributed to the band–band emission from the c‐Si substrate and a broad peak in the range 1300–1500 nm attributed to the radiative defects inside the poly‐Si films, as reported by Nguyen et al In contrast, it is known that the poly‐Si films contain both amorphous and crystalline phases, and the hydrogenated a‐Si (a‐Si:H) phase will emit a distinct peak located at ≈950 nm . Surprisingly, there is no PL peak from the a‐Si:H phase after the AlO x :H deposition and subsequent FGA (Figure A), albeit there being a big improvement in the passivation quality (Figure ).…”

“…Previously, we have noticed that in the case of low‐resistivity substrates (1 Ω cm), FGA of poly‐Si/SiO x passivating contacts resulted in little performance improvement, whereas others have reported a noticeable improvement . However, when capping layers and postdeposition annealing treatments are used, the passivating‐contact performance improves significantly, as reported by many groups . It has been shown that, without postdeposition annealing, there is no passivation improvement, and also that either annealing in N 2 or FGA shows the same performance …”

“…In general, poly‐Si films contain a high density of defects, which may affect the passivating‐contact performance and thus the conversion efficiency of solar cells. This stems from the fact that poly‐Si films, formed by recrystallizing amorphous Si (a‐Si) films via a high‐temperature processing step, often contain both amorphous and crystalline phases . In the a‐Si phase, each Si atom is surrounded by four other Si atoms whose bonds are stretched, twisted, or broken (known as dangling bonds).…”