Norihiro Ikeno

Interface engineering for the passivation of c-Si with O3-based atomic layer deposited AlOx for solar cell application

We have investigated the effects of deposition temperature and post-annealing on the passivation performance of AlOx films deposited by O3-based atomic layer deposition for crystalline Si. We found that the dramatic enhancement in the passivation performance of room-temperature deposited AlOx films by post-annealing is due to the phase transformation of aluminum silicate to mullite in an AlOx interlayer and the resulting self-aligned AlOx/SiOx interface. This result is interesting for the fabrication of high-performance silicon solar cells with AlOx passivation layers.

Investigation on antireflection coating for high resistance to potential-induced degradation

This paper is focusing on a relationship between potential induced degradation (PID) and characteristics of anti-reflection coating (ARC). The module, which has an ARC deposited by plasma-enhanced chemical vapor deposition (PE-CVD) from Shimadzu Corporation, indicated high resistance to PID with keeping conventional refractive index. This ARC had a property of high conductivity and low oxygen concentration.

Photoelectron spectroscopic study of band alignment of polymer/ZnO photovoltaic device structure

Using x-ray photoelectron spectroscopy, we investigated the band alignment of a Ag/poly(3-hexylthiophene-2,5-diyl) (P3HT)/ZnO photovoltaic structure. At the P3HT/ZnO interface, a band bending of P3HT and a short surface depletion layer of ZnO were observed. The offset between the highest occupied molecular orbital of P3HT and the conduction band minimum of ZnO at the interface contributed to the open circuit voltage (Voc) was estimated to be approximately 1.5 ± 0.1 eV, which was bigger than that of the electrically measured effective Voc of P3HT/ZnO photovoltaic devices, meaning that the P3HT/ZnO photovoltaic structure has the potential to provide improved photovoltaic properties.

Plasma-enhanced chemical-vapor deposition of silicon nitride film for high resistance to potential-induced degradation

The antireflection coating (ARC) on crystalline silicon solar cells plays an important role in preventing potential-induced degradation (PID). In a previous work, we reported that the module, which has an ARC prepared by plasma-enhanced chemical-vapor deposition (PE-CVD) using a hollow cathode, indicated high resistance to PID with a constant conventional refractive index (RI). In this work, we report further investigation of the high-PID-resistant ARC. The results indicate that the high-PID resistant ARC had high conductivity, high Si–H bond density, and low N–H bond density. Furthermore, both higher PID resistance and higher conversion efficiency are achieved using an ARC of double or triple layers comprising stacked silicon nitride layers of different RI than those of a conventional single-layer ARC.

Combinatorial Synthesis Study of Passivation Layers for Solar Cell Applications

We investigated the new materials applicable for the field effect passivation layer in crystalline Si solar cells, ZrO2-Al2O3 and ZrO2-Y2O3 binary systems, by using combinatorial synthesis method. As-deposited samples indicated hysteresis curves and flat band-voltage (VFB) shifts at capacitance-voltage (C-V) measurements. After oxygen gas annealing (OGA) at 700 oC for 5min, an improvement of the hysteresis and a positive shift of VFB were observed. OGA process influenced defects density related to decreasing oxygen vacancy. OGA processed ZrO2 incorporated with 20 % Al2O3 and 15 % Y2O3 structures showed the maximized negative fixed charge of -5.8 × 1012 cm-2 and -7.8 × 1012 cm-2 in each system, respectively, suggesting that the ZrO2 based alloy systems were revealed to be the promising material for the passivation in the solar cell application.

X-ray evaluation of electronic and chemical properties and film structures in SiN passivation layer on crystalline Si solar cells

We investigated the electronic chemical properties and film structures in SiN passivation layers on crystalline Si by X-ray photoelectron spectroscopy (XPS) and X-ray reflectometry (XRR). From XPS analyses, a N-rich component (N–Si–N) and oxygen mixing were observed at the interface. The longest lifetime was obtained after post deposition annealing at 600 °C, where the N-rich layer was the thinnest with small peak height. Therefore, longer lifetime could be obtained as the N concentration at the interface decreased. The film consisted of three SiN layers and one SiO2 layer, as confirmed by XRR profile fitting. From the density profiles, a longer lifetime was obtained when the density gradient in the oxygen mixing layer was small. Results of XPS and XRR suggested that N at the interface significantly affected the lifetime. We believe it is important to understand the electronic chemical properties at the passivation interface to realize high-efficiency Si solar cells.

Relationship between passivation properties and band alignment in O3-based atomic-layer-deposited AlOx on crystalline Si for photovoltaic applications

The passivation properties and band structures in aluminum oxide (AlOx) deposited by ozone-based atomic layer deposition (ALD) at room temperature on p-type crystalline silicon were investigated by X-ray photoelectron spectroscopy (XPS). The effective carrier lifetime depends on the thickness of AlOx films, since the field effects induced in the films by fixed charges depend on film thickness. The fixed charges are different by two orders of magnitude between films with thicknesses of 10 and 30 nm. At the 30-nm-thick AlOx/Si interface, the completely accumulated band bending of the Si surface was observed. On the other hand, a thin depletion layer was formed at the 10-nm-thick AlOx/Si interface. From the time-dependent XPS measurements, a hole trap was observed toward AlOx, in which trapping centers existed.

Study on surface passivation by YZO/AlO x stacking double layer for crystalline Si solar cells

We investigated stacking double layer structure, the Y₂O₃-ZrO₂ composite film (YZO) on AlO_x, for the field effect passivation with high negative fixed charge densities on p-type Si. The composition spread YZO films were deposited at room temperature by using combinatorial sputtering technique. The fixed charge densities were extracted from the flat band voltage shift in the capacitance-voltage characteristics. The as-deposited ZrO₂ film incorporated with 15% Y₂O₃ stacking on the ALD AlO_x structure showed the highest negative fixed charge of -1.9 × 10¹² cm^-2. The field effect passivation can be controlled by the negative fixed charges in the YZO film depending on the composition and dipole uniformly formed at AlO_x/Si interface. After annealing in the oxygen atmosphere, passivation properties deteriorated caused by the Al diffusion at the YZO/AlO_x interface.

Room-temperature photoluminescence evaluation of small-angle grain boundaries in multicrystalline silicon

The photoluminescence properties of small-angle grain boundaries (SA-GBs) with various misorientation angles were evaluated before and after Fe contamination. Comparison of SA-GBs with the same misorientation angle showed that the Da1 band at 0.78 eV remain the same before and after the Fe contamination. At the SA-GBs with the misorientation angle of 5?, a strong emission existed at 0.87 eV, which has been reported as an oxygen-precipitation-related peak. The emission of this high-energy region became weaker and shifted toward lower energy for a smaller misorientation angle. The high-energy emission became weak after the Fe contamination. It is considered that the Fe contamination affected the PL spectra originating from oxygen precipitates.

Detailed study of the effects of interface properties of ozone-based atomic layer deposited AlOx on the surface passivation of crystalline silicon

The effects of interface properties such as a negative fixed charge density and an interface trap density on the surface passivation of crystalline Si by O3-based batch ALD AlOx were studied. High-quality surface passivation with Smax of ?10 cm/s was obtained from the AlOx samples deposited at 200 ?C after annealing. This feature is attributed to the excellent field effect passivation by the high negative fixed charge density of ??5 ? 1012 cm?2 and chemical passivation, which reduces the interface trap density to ?1 ? 1011 eV?1 cm?2. The annealed AlOx samples deposited at 200 ?C also show high thermal stability during firing at 850 ?C. Additionally, we found that the formation of a thin SiOx interlayer is essential for the formation of a high negative fixed charge density that induces strong field effect passivation, and that defect passivation at the Si/SiOx interface by diffused hydrogen from AlOx layers is the origin of chemical passivation.