Synthesis and characterization of silicon nanowires using tin catalyst for solar
cells application
Minsung Jeon
⁎
, Koichi Kamisako
Department of Electronic and Information Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan
abstractarticle info
Article history:
Received 20 November 2008
Accepted 6 January 2009
Available online 8 January 20 09
Keywords:
Sn-catalyzed SiNWs
Reflectance
Solar cells
Phosphorous diffusion
VLS mechanism
Hydrogen radicals
Tin-catalyzed silicon nanowires were synthesized for solar cells application. Voluminous silicon nanowires
were fabricated on single crystalline silicon wafer. Optical reflectance and solar cell efficiency of the
synthesized silicon nanowires were explored. The reflectance of as-synthesized silicon nanowires was
obtained approximately 5% in the short wavelength region (λb 500 nm). A short circuit current of 2.3 mA/cm
2
and open circuit voltage of 520 mV for 1 cm
2
SiNWs solar cell was obtained.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
In recent years, nanowires including nanorods based solar cells
have attractive interest due to their characteristics and processing
benefits. The nanowires-enabled solar cells allow for decoupling the

circuit current (J
sc
) and open circuit voltage (V
oc
) of the fabricated cells
were 0.53 mA/cm
2
and 125 mV, respectively [5]. As mentioned above,
Au nanoparticles are well-used metal catalyst and it easily synthesizes
the SiNWs at low temperature. However, it is known to be a deep level
impurity in silicon. In contrast with Au, tin (Sn) appears to be the
favorable catalyst because the Sn–Si alloy has relatively low eutectic
temperature of 232 °C [16] and forms with extremely low content of
the elemental semiconductors.
In our previous work, we successfully synthesized large quantities
of SiNWs using various materials by the hydrogen radical-assisted
deposition method including hydrogen radicals pretreatment
[12,14,17–19]. Moreover, Sn-catalyzed SiNWs were easily controlled
by introducing hydrogen flow gas ratios [20]. In this letter, we
synthesized SiNWs using Sn nanoparticles on single crystalline silicon
(c-Si) wafer for solar cell application and their characteristics are
explored.
2. Experimental details
About 7 Ω cm boron-doped Cz silicon (100) wafers were used for
fabrication of solar cells. Sn metal thin film as catalyst was evaporated
on these c-Si wafers. The wafers were dipped in diluted hydrofluoric
(5% HF) solution to remove native oxide layer and rinsed in pure water.
Then, the wafers were immediately located into the evaporation
vacuum chamber. The Sn metal film was deposited in situ onto the Si
wafer by a thermal evaporation source at a rate of less than 1 nm/s.

3. Results and discussion
After synthesis of SiNWs on p-type c-Si wafer, phospho rous diffusion was
performed on the front surface using spin-on phosphors silicate g lass (PSG) coating.
The n the samples were diffused to fabricate p–n
+
junction in quartz tube for 30 min at
900 °C in N
2
ambient. After diffusion, the samples were dipped in 5% HF solution to
remove parasitic layer. Fig.1(a) ill ustrates a schematic of the SiNWs solar cell design in
this work. As shown in the Fig. 1(a), the silver (Ag) contacts with thickness of 180 nm
were evaporated on the surface of synthesized SiNWs without any transparent
conductive ox ide (TCO) film. To investigate the morphological property of SiNWs, a
FE-SEM observation was carried out after synthesis of SiNWs. Fig. 1(b) shows the top-
view (15° tilted) and cross-section (see inset of Fig. 1(b)) FE-SEM images of the SiNWs
fabricated on c-Si wafer.
As can be seen these figures, voluminous SiNWs were synthesized whisker-like.
The diameters of SiNWs on the bottom and top were approximately 60 nm and thinner
than 10 nm, respectively. Their lengths extended up to ~ 1.5 μm. Moreover, the SiNWs
were tapered and catalysts remained on the top of SiNWs. It indicates that the Sn-
catalyzed SiNWs are synthesized via VLS mechanism [8,9]. The detailed mechanisms
were well described elsewhere [14]. After diffusion, the surface morphologies were also
observed. Here, the doping level in the silicon nanowires, which is measured by
secondary ion-microprobe mass spectrometer, was estimated to be 1.4×10
20
atom/cm
3
base on planar Si wafer. Fig. 1(c) shows the FE-SEM image of the phosphorous-diffused
SiNWs. The shapes of SiNWs after formation of pn junction were slightly curved.
As mentioned above, we expect the SiNWs solar cells to show improved optical

contact between the SiNWs and neighboring SiNWs. Moreover, it might be the effect of
Fig. 1. (a) Schematic of the SiNWs solar cells. FE-SEM images of the (b) as-synthesized SiNWs and (c) phosphorous-diffused SiNWs. All the scale bars represent 2.5 μm.
Table 1
Synthesis conditions for Sn-catalyzed SiNWs fabricated on c-Si (100) wafer
Hydrogen radical treatment Synthesis of SiNWs
Hydrogen (H
2
) gas flow [sccm] 100 180
Silane (SiH
4
) gas flow [sccm] – 15
Microwave power [W]40 40
Working pressure [Torr] 0.5 0.5
Temperature [°C] 400 400
Time [min] 1 20
778 M. Jeon, K. Kamisako / Materials Letters 63 (2009) 777–779
the Schottky barrier between the evaporated Al contact and silicon wafer because of
insufficient annealing conditions. For low shunt resistance, it might be the leakage
current from the front metal contact, i.e., local shunting across the solar cell.
In addition, the poor results were caused by the unoptimized diameter distribution
and the shapes of the synthesized SiNWs. It is typically not suitable for the application
of electrical and optical devices. Furthermore, unremoved metal nanoparticles caused
the reduction of the carrier lifetime, i.e., increasing surface recombination velocity.
Therefore, the sizes and impurity removal must be controlled for further improvement.
Moreover, the improvement of the cell performance owing to the deposition of TCO
films, such as indium–tin oxide and fluorine doped tin oxide, on the surface of synthe-
sized SiNWs will be expected.
4. Conclusion
We have demonstrated tin-catalyzed silicon nanowires solar cells
fabricated by the hydrogen radical-assisted deposition method on a c-

Fig. 3. I–V characteristics of the fabricated SiNWs solar cell under dark and light
conditions.
Fig. 2. (a) Photograph of the fabricated SiNWs solar cell. (b) The difference of reflectance
between conventional planar c-Si solar cell and SiNWs solar cell.
779M. Jeon, K. Kamisako / Materials Letters 63 (2009) 777–779