Realization of radial p-n junction silicon nanowire solar cell based on low-temperature and shallow phosphorus doping
© Dong et al.; licensee Springer. 2013
Received: 26 October 2013
Accepted: 16 December 2013
Published: 27 December 2013
A radial p-n junction solar cell based on vertically free-standing silicon nanowire (SiNW) array is realized using a novel low-temperature and shallow phosphorus doping technique. The SiNW arrays with excellent light trapping property were fabricated by metal-assisted chemical etching technique. The shallow phosphorus doping process was carried out in a hot wire chemical vapor disposition chamber with a low substrate temperature of 250°C and H2-diluted PH3 as the doping gas. Auger electron spectroscopy and Hall effect measurements prove the formation of a shallow p-n junction with P atom surface concentration of above 1020 cm−3 and a junction depth of less than 10 nm. A short circuit current density of 37.13 mA/cm2 is achieved for the radial p-n junction SiNW solar cell, which is enhanced by 7.75% compared with the axial p-n junction SiNW solar cell. The quantum efficiency spectra show that radial transport based on the shallow phosphorus doping of SiNW array improves the carrier collection property and then enhances the blue wavelength region response. The novel shallow doping technique provides great potential in the fabrication of high-efficiency SiNW solar cells.
Silicon nanowire (SiNW) array shows unique electrical and optical properties compared to bulk silicon [1–7], which makes it to be a potential candidate for photovoltaic applications [4, 8, 9]. The short circuit current density of SiNW solar cells can be greatly enhanced due to the effective light trapping property of the SiNW array structure and the decoupling of light absorption and the carrier collection based on a coaxial core/shell structure [10–12].
Different techniques were developed to prepare SiNW array, such as vapor–liquid-solid (VLS) method [10, 13], metal-assisted chemical etching (MACE) technique [14–16], and dry etching process [17–19]. Among these techniques, MACE technique is a relatively inexpensive and simple approach to fabricate SiNW arrays. And, it is compatible with the commercial manufacturing process of crystalline Si solar cells. Thus, lots of research works [14, 20–23] related to the SiNW solar cells based on the combination of MACE and traditional c-Si solar cell techniques have been carried out. Benefiting from the excellent light trapping property of SiNWs, the short circuit current density of the reported SiNW solar cells is improved compared with that of the traditional planar solar cells.
However, it is hard to form radial p-n junction structures on SiNW arrays fabricated by MACE technique using traditional high-temperature diffusion process. Because the thickness of the doping layer is much larger than the radius of the nanowire, the SiNW layer is often completely doped after the high-temperature diffusion doping process. Furthermore, the enlarged surface area of SiNW structure usually results in a fairly thick dead layer on the top part of the SiNW layer with excessive doping concentration, which may bring serious Shockley-Read-Hall (SRH) and Auger recombination [21, 22]. More research works are needed to achieve controllable shallow doping, which may enable the realization of radial p-n junction and reduce the carrier recombination in SiNW-based solar cells.
Hot-wire chemical vapor deposition (HWCVD), also referred to as catalytic chemical vapor deposition, has been studied extensively for the deposition of silicon-related thin films under low substrate temperatures. It was demonstrated that HWCVD has the advantages of conformal film deposition on small objects with high-aspect ratio structure [24, 25] and no ion bombardment, which make it suitable for fabrication of devices with SiNW structure. Moreover, HWCVD technique can be used to form shallow doping layer (less than 10 nm in depth) on single-crystal silicon surface at temperatures less than 350°C by catalytically generated phosphorous radicals from hot filaments [26–29].
In this paper, MACE and HWCVD techniques were used to fabricate radial p-n junction SiNW solar cells. Our aim is to realize a conformal radial p-n junction structure on the SiNW arrays. For better understanding of the behaviors of radial p-n junction SiNW solar cell, the differences in performance and optical response property between the radial p-n junction solar cell and the axial p-n junction solar cells were compared and discussed.
Polished (100) oriented silicon wafers (CZ, p-type, resistivity 1 to 5 Ω · cm) were used in this work. The silicon wafers were cleaned in ethanol and acetone for 10 and 5 min at room temperature in an ultrasonic machine, respectively, and then they were dipped into a HF solution for 1 min to remove the oxide layer. The cleaned silicon wafers were then immersed into an aqueous solution containing 5 mol L−1 HF and 0.02 mol L−1 AgNO3 to prepare SiNW array. The length of the SiNWs can be controlled by adjusting the etching time, the concentration of AgNO3, or the temperature of the solution. In this work, we tailored the length of SiNWs by tuning the etching time without changing other experimental parameters (the temperature was fixed at 30°C). After etching, the as-prepared samples were rinsed copiously in deionized water and then dipped into a NH3 · H2O:H2O2 (v/v, 3:1) solution for 5 min to remove the Ag layer which wrapped on the SiNW array after etching. Finally, the samples were cleaned with the standard RCA clean procedure.
The surface morphologies and the optical properties of the samples were observed using scanning electron microscope (Hitachi S-4800, Tokyo, Japan) and spectrophotometer (Hitachi UV-4100), respectively. The P atom concentration profile was obtained from the Auger electron spectroscopy (ULVAC-PHI, PHI-700, Kanagawa, Japan) measurement with a sputtering rate of 4 nm/min. The carrier type and P concentration of low-temperature doping Si wafer were characterized by Hall effect measurement using an Accent HL5500 Hall System (Bio-Rad Laboratories, Hercules, CA, USA) at room temperature. The illuminated current density-voltage (J-V) characteristics of the fabricated solar cells were measured under AM1.5 (100 mW/cm2, at 25°C) illumination. The series resistances of the devices were estimated from the illuminated J-V curve using RS = dV/dJ| J = 0.
Results and discussion
Morphologies of SiNW structures
L , R avg , and AER of the SiNWs fabricated with different etching times
Figure 2b shows the SEM image of the SiNW array after ITO layer deposition. It is seen that the surface of the ITO layer is not smooth, and the thickness of the ITO layer is not uniform along the nanowire growth direction. However, it is good to see that the whole nanowire is covered by the ITO layer, as the inset of Figure 2b shows. It provides a possibility for radial transport and collection of the carriers. The ITO layer on the top of the SiNW array connects together after 4-min deposition, which is beneficial for the top electrode contact.
However, the surface area is much enhanced with the increase of the length of the SiNWs. Based on a simple geometric model of the SiNWs with an average diameter of 200 nm and an average distance of 100 nm between the adjacent nanowires, the surface area enhancement ratio is calculated to be 9.06, 21.15, 40.31, and 81.62 for SiNW arrays with an average length of 1, 2.5, 5, and 10 μm, respectively. Accompanied by the enlarged surface area as well as the junction area brought by the SiNW structure, the surface recombination will be aggravated. Actually, the minority carrier lifetimes of the samples measured after the same cleaning process decrease as the SiNW length increases. For longer SiNWs, the cleaning process needs to be modified to improve the minority carrier lifetime. And, it is difficult to realize a conformal deposition of the ITO layer on the long SiNW structure. Furthermore, some of the SiNWs bend down in long SiNW conditions, which is not good for the following solar cell fabrication process. Thus, in this paper, SiNW arrays with the length of 1 μm were used for the solar cell fabrication.
Distribution of P atom concentration under low-temperature doping
Solar cell performance
Parameters of the solar cells: V oc , J sc , FF, EFF, and R s
NW-R with Al BSF
The internal quantum efficiency (IQE) of these solar cells were derived also from the EQE characteristics via IQE = EQE/(1 − R), where R is adopted from Figure 3. There is only a slight difference between the IQE and EQE for the two SiNW-based solar cells due to the low reflectance in the whole wavelength range of the SiNW structure. If we compare the IQE values of the two axial p-n junction solar cells, we can see that the IQE of the NW-A solar cell is much lower in the short-wavelength region (400 < λ < 600 nm) than that of the P-A solar cell. This could be attributed to the formation of the dead layer and serious surface recombination brought by the SiNW structure. As for the NW-R solar cell, in spite of the equally serious surface recombination related to the SiNW structure, an IQE enhancement in the short-wavelength region can be seen. The IQE behavior further indicates that the radial p-n junction is beneficial for carrier transport and collection.
Here, we should point out that the average length of the SiNWs (1 μm) is much smaller compared with the thickness of the Si substrate. The NW-R solar cell is actually a semi-radial p-n junction solar cell. Even so, the contributions of the semi-radial transport to the improvements of blue wavelength response and short circuit current density are significant. It should be also noted that all solar cells were fabricated without any surface passivation in order to give an intuitionistic comparison. If optimized surface passivation and ITO coverage can be utilized, NW-R solar cell based on longer SiNW array with optimized diameter will show a more obvious EQE enhancement in a wider wavelength range.
Vertically free-standing SiNW arrays were fabricated on Si wafer surface using metal-assisted chemical etching technique. The excellent light trapping property of the SiNW structure provides a short circuit current density enhancement of 29.35% for the SiNW solar cell compared with that of the flat solar cell. The low-temperature doping technique of HWCVD was successfully applied to form shallow junction in the SiNW structures, and radial p-n junction is achieved on SiNW array. As a result, the short circuit current density of the radial p-n junction SiNW solar cell is further enhanced by 7.75% compared with that of the axial p-n junction SiNW solar cell. The quantum efficiency spectra indicate that radial transport based on the shallow doping of SiNWs enhances the blue wavelength region response.
This work was supported by the Beijing City Science and Technology Project (grant no. D121100001812003) and the National Basic Research Program of China (grant no. 2011CBA00705).
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