Origin of the tunable carrier selectivity of atomic-layer-deposited TiOx nanolayers in crystalline silicon solar cells

Abstract

Titanium oxide (TiOx) nanolayers grown by atomic layer deposition are investigated with respect to their application as carrier selective contacts for crystalline silicon (c-Si) solar cells. Although TiOx is known to act as an electron contact, in this work the selectivity of TiOx layers is found to be widely tunable from electron to hole selective depending on deposition conditions, post-deposition treatments, and work function of the metal electrode used. Using TiOx and an intrinsic hydrogenated amorphous silicon buffer layer, solar cell test structure exhibiting open-circuit voltages (Voc) as high as 720 and 650 mV are shown for electron and hole selective contacts, respectively. Surface photovoltage and capacitance-voltage measurements reveal that carrier selectivity is correlated with the amount of c-Si band bending induced by TiOx, which are governed not only by the effective work function difference at the Si/TiOx interface, but also by the negative fixed charge present in the TiOx layer. This new finding is in contrast to the previous model for carrier transport where selectivity is determined only by the asymmetric band offsets at the Si/contact interface. It highlights the influence of induced band bending to produce carrier depletion/inversion conditions, and the importance of its selectivity effect in a c-Si absorber.

Atomic-Layer-Deposited TiOx Nanolayers Function as Efficient Hole-Selective Passivating Contacts in Silicon Solar Cells

“…It has been demonstrated that the hole selectivity of the t-ALD TiO x increases with an increase of the work-function of the capping material. 23,24 Our results suggest that the relatively large work function of the capping electrode is essential to sustain the band bending created by the fixed charge in the TiO x layer. Finally, the passivation of Si surface by TiO x layer works unpinning the Fermi-level position at the TiO x /Si interface, allowing it to move toward the valence band.…”

“…In our preliminary work, , it was shown that the t-ALD TiO x deposited under similar conditions exhibits distinct features in terms of induced band bending and surface passivation. First, it gives strong band bending of 800–900 meV for n -Si but less than 100 meV for p -Si.…”

Polysilicon contact structures for silicon solar cells using atomic layer deposited oxides and nitrides as ultra‐thin dielectric interlayers

“…Upon annealing, ALD SiO x , SiN x , AlO x , AlN x , and TiO x interlayers deposited on silicon can form negative or positive fixed charges that accumulate holes or electrons near the surface of the silicon wafer 40,47,58–62 . In dependence of the thickness of the interlayer and the annealing temperature, the band bending and the fixed charges were determined for the ALD layers before implementing them as interlayers in the polysilicon contact structures, see Figure 2.…”

“…Using plasma‐enhanced ALD, ultra‐thin SiO x , SiN x , AlO x , AlN x , and TiO x interlayers were deposited either on top of the conventional thermally‐grown SiO x or on hydrofluoric acid (HF) dipped silicon wafers at different deposition (substrate) temperatures in a FlexAL system from Oxford Instruments. As precursors, bis (diethylamino)silane (BDEAS) for SiO x 37–40 and SiN x, 41 trimethylaluminium (TMA) for AlO x 42–44 and AlN x, 45,46 and titanium isopropoxide (TTIP) for TiO x 47 were used together with oxygen (O 2 ), nitrogen (N 2 ), or nitrogen(N 2 )/hydrogen (H 2 ) plasma gases for SiO x , AlO x and TiO x, SiN x , and AlN x , respectively.…”

Activation and Deactivation of Silicon Surface Passivation by Niobium Oxide Films

“…Therefore, TiO 2 has been widely used in selective contacts with c-Si solar cells and the highest conversion efficiency reached 22.5%. [25] Recently, studies have found that it also can act as a hole-selective contact with c-Si by changing the interface composition of TiO 2 /c-Si, [26,27] indicating that band bending and the position of the Fermi level play a more important role in carrier selectivity than band offsets. In contrast to the commonly studied TiO 2 , Nb 2 O 5 features excellent optical transmittance, a wide bandgap (3.3-3.8 eV), higher carrier mobility (%26 cm 2 V À1 s À1 ), [28] as well as good chemical stability, which is expected to be a superior selective contact passivating material.…”