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Optical and electrical study of core-shell silicon nanowires for solar applications

Zhenhua Li, Jian Wang, Navab Singh, and Sungjoo Lee

Open Access Open Access

Abstract

In this work, we report a CMOS comparable fabrication process of core-shell SiNW solar cell from single-crystalline p-type Si(100) test wafers. Optical lithography defined plasma etching was used to form highly ordered vertical SiNW arrays, which display a drastic reduction in optical reflectance over a wide range of wavelengths. BF2 and P ion implantations were employed for producing a sharp and shallow radial p-n junction. Under AM 1.5G illumination, the device demonstrates a short circuit current density (Jsc) of 14.2 mA/cm2, an open circuit voltage (Voc) of 0.485 V and a fill factor (FF) of 42.9%, giving a power conversion efficiency (PCE) of 2.95%. The Jsc observed is 52% higher than that in the control device with planar Si p-n junction, indicating significant enhancement in carrier generation and collection efficiency from the core-shell structure. Impact of series resistance (Rs) is also studied, highlighting potential improvement of PCE to 4.40% in the absence of Rs. With top contact optimized, PCE could further increase to 6.29%.

©2011 Optical Society of America

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Figures (7)
Fig. 1
Fig. 1 Schematic demonstration of fabrication process of core-shell SiNW solar cell. (a) Starting p-type Si test wafer. (b) BSF formation by BF2 implant. (c) DUV lithography patterning and resist trimming. (d) SiNW fabrication by SF6 based plasma etching. (e) BF2 implant to increase core dopant concentration. (f) Phosphorous shell implant. (g) Metal contact formation. (h) Illustration of four-rotational ion implantations for BF2 core implant (Left) and phosphorous shell implant (Right). Each stage consists of four sub ion implant steps, with rotation of 0°, 90°, 180° and 270° respectively and a vertical tilt of 7° for every implant. BF2 core implant was done with dose of 2.5 x 1013 cm−2 and energy of 80 keV; phosphorous shell implant was done with dose of 1015 cm−2 and energy of 7 keV.

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Fig. 2
Fig. 2 (a) 45° tilt Scanning Electron Microscope (SEM) image of resist nano-hemispheres on Si surface after lithography patterning and resist trimming. (b) 45° tilt SEM image of SiNW array formed by plasma etch. (c) Transmission Electron Microscope (TEM) image of SiNW device cross-section near the top surface where metal grid is deposited. The dark outline indicates the border of a nanowire under the grayish metal layer. (d) Enlarged view at the metal-Si interface of a nanowire. (e) 45° tilted top view of complete SiNW device (left) and planar Si control device (right) under visible light. Dark scale bars in (a)-(c) represent 1 um.

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Fig. 3
Fig. 3 (a) Reflectance data of SiNW surface and planar Si surface, measured using integrating sphere. (b) Reflected spectral irrandiance of SiNW surface comparing with that of planar Si surface; the inset shows incident spectral irradiance under standard AM 1.5G illumination.

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Fig. 4
Fig. 4 (a) Simulated boron profile in a nanowire after BF2 core implant (rotation: 0°, 90°, 180°, 270°; dose: 2.5 x 1013 cm−2, energy: 80 keV, tilt: 7° for each rotation) and 1 hour drive-in at 1000 °C. (b) Simulated phosphorous profile in a nanowire after P shell implant (rotation: 0°, 90°, 180°, 270°; dose: 1015 cm−2, energy: 7 keV, tilt: 7° for each rotation). The color gradient depicts distribution of different dopant concentrations in the vertical cross-section of the wire. Junction depth (at which both dopant concentrations are approximately equal) is estimated to be 50 nm. (c) A schematic illustration of the radial p-n junction in a nanowire, indicating the estimated junction depth and depletion width d.

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Fig. 5
Fig. 5 (a) I-V characteristic of core-shell SiNW solar cell in dark and AM 1.5G illumination. (b) Comparison of I-V characteristic between core-shell SiNW and planar Si solar cell under AM 1.5G illumination. (c) Comparison of dark I-V characteristics between core-shell SiNW and planar Si solar cell in reverse bias region. (d) Semi-log plot of dark current in forward bias region. (e) Local ideality factor as a function of voltage in forward bias region.

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Fig. 6
Fig. 6 Evaluation of series resistance using multiple intensity method in (a) core-shell SiNW solar cell and (b) planar Si solar cell. E represents incident illumination on the surface of the device.

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Fig. 7
Fig. 7 I-V curves before and after eliminating the effect of Rs for (a) core-shell SiNW solar cell and (b) planar Si solar cell.

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Tables (1)

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Table 1 I-V Characterization of Planar Si and Core-Shell SiNW Solar Cell

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Equations (4)

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V b i = k T q ln ( N A N D n i 2 ) ,
d = 2 ε q N A + N D N A N D ( V b i − V ) ,
I = I 0 exp [ q ( V c e l l + I R s ) n k T ] − I L ,
V c e l l = V a p p − I R s ,
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