Silicon Nanowire Heterojunction Solar Cells with an Al2O3 Passivation Film Fabricated by Atomic Layer Deposition
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Silicon nanowires (SiNWs) show a great potential for energy applications because of the optical confinement effect, which enables the fabrication of highly efficient and thin crystalline silicon (c-Si) solar cells. Since a 10-μm-long SiNW array can absorb sufficient solar light less than 1200 nm, the 10-μm-long SiNW was fabricated on Si wafer to eliminate the influence of the Si wafer. On the other hand, Surface passivation of the SiNWs is a crucial problem that needs to be solved to reduce surface recombination and enable the application of SiNWs to c-Si solar cells. In this study, aluminum oxide (Al2O3) was fabricated by atomic layer deposition for the passivation of dangling bonds. However, owing to a complete covering of the SiNWs with Al2O3, the carriers could not move to the external circuit. Therefore, chemical–mechanical polishing was performed to uniformly remove the oxide from the top of the SiNWs. A heterojunction solar cell with an efficiency of 1.6% was successfully fabricated using amorphous silicon (a-Si). The internal quantum efficiencies (IQE) of the SiNW and c-Si solar cells were discussed. In the wavelength region below 340 nm, the IQE of the SiNW solar cell is higher than that of the c-Si device, which results in an increase of the absorption of the SiNW cells, suggesting that SiNWs are promising for crystalline-silicon thinning.
KeywordsSilicon nanowire Passivation Chemical–mechanical polishing Atomic layer deposition Solar cell
External quantum efficiency
The current at forward bias
Internal quantum efficiency
The current at reverse bias
To form a contact between the SiNWs and a-Si, the Al2O3 present on the top of the SiNWs was removed by chemical–mechanical polishing (CMP) and etching. The influence of Al2O3 etching on the properties of the solar cells was investigated.
Fabrication of SiNW Arrays and Al2O3
A p-type Si (100) wafer (8–10 Ω cm, 550 μm) was immersed in hydrofluoric acid (HF) solution with AgNO3 to deposit silver particles. The Si wafer was chemically etched, using 4.8 M HF and 0.15 M H2O2 at room temperature, and subsequently added into an HNO3 solution to remove the silver films. Finally, the oxide layer present on the prepared SiNW array was removed using the HF solution. SiNWs with lengths of 10, 15, and 20 μm were fabricated by changing the etching time. Since the space between the SiNWs is large, silica particles with a diameter of about 80 nm (dispersed in an ethanol solution) were filled into the space between the wires. Then, 66-nm-thick Al2O3 was deposited by ALD to passivate the dangling bonds. Field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7001F) was applied to examine the structure of the prepared SiNW arrays.
Removal of Al2O3 on the Top of SiNWs
Fabrication of the Solar Cell Structure
Characteristics of a reference solar cell fabricated on the same wafer without any treatment
Ref solar cell
Results and Discussion
Characteristics of SiNW solar cells with Al2O3 removed by CMP
Surface passivation of SiNWs is crucial for their application in solar-cell devices. Al2O3 was fabricated by ALD to passivate the dangling bonds. Since ALD can deposit Al2O3 over the entire SiNWs, the carrier cannot move to the external circuit. In this study, an etching paste and the CMP technique were applied to etch Al2O3 from the top of the SiNWs. With the etching paste, SiNW solar cells with 0.14% efficiency were successfully obtained. However, since the SiNW array was aggregated by surface tension, the contact area between SiNWs and a-Si was small, leading to a low Isc. To further improve the efficiency, the etching thickness was increased, and the efficiency could be improved to 1.6% by increasing Isc. In the case of the EQE, the intensity of the SiNW solar cell is lower than that of the c-Si solar cell. Since reflectance in short wavelength region from 300 to 500 nm is drastically decreased, the EQE was improved. The IQEs of the SiNW and c-Si solar cells were discussed to eliminate the influence of the reflectance. In the wavelength region below 340 nm, the IQE of the SiNW device is higher than that of the c-Si solar cell, which results in an increase of the absorption of the SiNWs, suggesting that SiNWs are promising for crystalline-silicon thinning.
The work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA), Japan Science and Technology Agency (JST) and JSPS KAKENHI Grant Number 17 K14921.
Availability of Data and Materials
All data supporting the conclusions of this article are included within the article.
SK carried out the experiment and wrote the initial draft of the manuscript. KG contributed to the sample fabrication and sample measurement. YK supervised the work and finalized the manuscript. TS gave the final approval of the version of the manuscript to be published. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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