Effect of hydrogen plasma treatment on the passivation performance of TiOx on crystalline silicon prepared by atomic layer deposition

Abstract

The authors report on the effect of hydrogen plasma treatment (HPT) on the passivation performance of titanium oxide (TiOx) on crystalline silicon (c-Si) fabricated by atomic layer deposition. Recently, TiOx has gathered attention as an electron-selective contact material for silicon heterojunction (SHJ) solar cells due to its preferable work function and band lineup. Moreover, TiOx has excellent light transmission properties due to its large bandgap energy. In order to improve the power conversion efficiency of SHJ solar cells with TiOx, it is necessary to enhance the passivation performance. The effective carrier lifetime representing the passivation performance is enhanced by HPT and amounts to 407.2 μs after HPT at 200 °C for 1 min. This value is twice as high as after forming gas annealing, which is the standard method to enhance the passivation performance of TiOx/c-Si heterostructures. Nuclear reaction analysis clarifies that the hydrogen concentration (CH) at the TiOx/c-Si interface of the HPT-processed sample is higher than that of an as-deposited sample and that the peak position of the CH distribution is shifted closer to the TiOx/c-Si heterointerface after HPT. Moreover, thermal desorption spectroscopy shows that Si–H and Si–H2 hydrogen bonds increase with HPT. These results indicate that the atomic hydrogen produced by the hydrogen plasma diffuses toward the TiOx/c-Si interface and terminate the local dangling bonds, which is responsible for the improved passivation performance.

Dopant‐free passivating contacts for crystalline silicon solar cells: Progress and prospects

“…Miyagawa et al investigated the HPT effect on the passivation quality of the ALD-TiO x /c-Si structure. 203 They found that HPT at 200 C for only 1 min significantly improves τ eff to 400 μs, which is more than twice higher than the FGA-treated samples (Figure 8F), indicating that HPT may replace FGA to obtain better passivation quality under certain conditions. HPT reduces the dangling bonds at the TiO x /c-Si interface, as evidenced by the increased Si-O and hydrogen-Si bonds (Si-H, Si-H 2 ) from XPS and thermal desorption spectroscopy (TDS) results.…”

“…With HPT, hydrogen atoms can diffuse to the c ‐Si surface more efficiently and quickly, thus saturating the dangling bonds and decreasing J 0c of the passivating contact. Miyagawa et al investigated the HPT effect on the passivation quality of the ALD‐TiO x / c ‐Si structure 203 . They found that HPT at 200 °C for only 1 min significantly improves τ eff to 400 μ s, which is more than twice higher than the FGA‐treated samples (Figure 8F), indicating that HPT may replace FGA to obtain better passivation quality under certain conditions.…”

“…(F) τ eff as a function of HPT process time. Reproduced with permission 203 . Copyright 2020, American Vacuum Society…”

“…Reproduced with permission. 203 Copyright 2020, American Vacuum Society which likely originates from the plasma damage and suppressed diffusion of hydrogen.…”

Improved Performance of Titanium Oxide/Silicon Oxide Electron‐Selective Contacts by Implementation of Magnesium Interlayers

“…The passivation performance of the TiO x has been improved by postdeposition annealing, [12][13][14][15][16][17] inserting silicon oxide interlayer, [12][13][14][15][16][17] and hydrogen plasma treatment. [11,[18][19][20] The origin of the improved passivation performance is plausibly caused by the formation of Si─O─Ti bonds, [16,21,22] hydrogenation of dangling bonds on the Si surface, [11,[18][19][20] and enhanced band bending by trapped electrons in TiO x . [23] Electron transport at contacts/TiO x /c-Si heterointerface is one of the issues to be solved due to relatively high contact resistivity over 100 mΩ cm 2 .…”