Photocurrent Mapping in High-Efficiency Radial p–n Junction Silicon

ARTICLE pubs.acs.org/JPCC

Photocurrent Mapping in High-Efficiency Radial p
n Junction Silicon Nanowire Solar Cells Using Atomic Force Microscopy Jih-Shang Hwang,*,† Ming-Chun Kao,† Jian-Min Shiu,† Chieh-Ning Fan,† Shien-Chau Ye,† Wen-Shen Yu,† Hsiu-Mei Lin,‡ Tai-Yuan Lin,† Surojit Chattopadhyay,§ Li-Chyong Chen,|| and Kuei-Hsien Chen^ †

Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan Institute of Bioscience and Biotechnology, National Taiwan Ocean University, Keelung 202, Taiwan § Institute of Biophotonics, National Yang-Ming University, Taipei 112, Taiwan Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan ^ Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan

)

‡

ABSTRACT: Rapid formation of radial p
n junctions on electroless-etched silicon nanowires (SiNWs) was successfully demonstrated. With a low-cost objective, a homemade nonhazardous diffusion source of high phosphor concentration annealed at a small thermal budget was used. The SiNW solar cell, with Au electrodes, has shown a power conversion efficiency of 8.41%, which is higher by 30% compared with its planar counterpart. The SiNW solar cell incorporates an inherent antireflection property, reduced diffusion length requirement, and broad-band spectral quantum efficiency. The evidence of a successful radial p
n junction formation in the NWs has been revealed through the help of a conducting atomic force microscope (AFM) scanning for the photogenerated currents on the fractured surfaces of the NWs. The demonstrated radial junction fabrication technique is believed to reduce the cost of production and promote widespread use of them.

’ INTRODUCTION One way to address the approaching energy crisis is to involve sustainable and renewable energy sources, such as solar cells. Among them, the costly crystalline Si (c-Si) technology is the front runner with power conversion efficiencies (η) reaching ∼25%.1,2 Nevertheless, the current bottleneck to the widespread photovoltaic (PV) use lies not only with η but also on the production cost. For example, the cost of c-Si takes up to half of the cell production cost.3,4 To reduce the cost, solar cells employing amorphous and polycrystalline thin-film5 and nanotechnologies6,7 are widely investigated. Alternative techniques, such as the rapid thermal process (RTP), for high-throughput production of solar cells have received due attention.8
10 Additional concerns for the technology include elevated cost for handling and disposal of toxic and hazardous chemicals, such as the dopant sources, including PH3, POCl3, and B2H6, not to mention the explosive silane (SiH4) used for the thin-film technology. In light of the above, we suggest that the electroless-etched silicon nanowires (EE-SiNWs), with the process reported by Peng et al.,11,12 is a promising candidate for cost reduction of solar cells. Compared with plasma-etched or vapor
liquid
solid grown SiNWs, EE-SiNWs offer inherent wideband antireflection (AR),13,14 rendering the extra AR layer redundant, while conserving the crystallinity and carrier transport properties after the facile and low-cost etch processing. Even more, SiNW solar cells with radial p
n junctions are theoretically shown to be less sensitive to impurities than planar Si solar cells,15 implying redundancy of r 2011 American Chemical Society

high-purity materials. The EE-SiNWs can be fabricated on Si thin film deposited on glass,16 instead of c-Si, saving costs further. The idea of cost reduction through increased production throughput also applies to EE-SiNW solar cells. Usually, the emitter formation is one key time-consuming process in the production. For example, using POCl3 as the diffusion source, Peng et al. reported an EE-SiNW solar cell with an η of 9.31%,13 wherein a diffusion thermal budget of 930 C for 30 min was required. Similarly, Fang et al. reported an η of 11.37% using aligned, but slant SiNWs,17 with a thermal budget of 930 C for 50 min. By using a commercial spin-on dopant (SOD) at 1050 C for 30 min, Um et al. reported a 5.25% EE-SiNW solar cell.18 Similarly Jung et al. have reported an η of 7.19% in an EESiNW solar cell mixed with micrometer-sized (2 μm) rods, while the diffusion time was reduced to 5 min only.19 The radial p
n junctions in the NWs allow the use of a thick material to increase the optical absorption and simultaneously provide short collection lengths for the dissociated carriers in a direction normal to the light absorption. This results in an improved η and relaxes the costly requirement of Si purity.15 Efforts have been made on the formation of radial p
n junctions on EE-Si wires (η = 7.19%),19 plasma-etched Si pillar arrays (η = 8.7%),20 and VLS-grown Si wires (η = 2.3%),21 all with micrometer sizes. A recent attempt to Received: June 1, 2011 Revised: August 24, 2011 Published: October 06, 2011 21981

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The Journal of Physical Chemistry C form radial p
n junctions on EE-SiNWs by coating amorphous p-Si on n-Si, followed by rapid thermal annealing (RTA),4 resulted in an η of only 0.5%, possibly due to the high defect density of the amorphous layer. A direct evidence for the existence of radial p
n junctions within the NWs is still rare. The capacitance analysis method developed by Gunawan et al.22 has been one of the few techniques capable of demonstrating the existence of the radial junctions in NWs. However, that requires the exact knowledge of the geometry of the NWs and, thus, is not suitable for the randomly distributed EE-SiNWs. In this paper, we demonstrate a rapid formation of radial p
n junctions in the crystalline EE-SiNWs by using RTA at an extremely low thermal budget of 980 C for 10 s only with a nonhazardous cost-effective (compared to commercial ones) homemade SOD of high phosphor concentration. The existence of the radial p
n junctions was directly demonstrated, for the first time, by using a current mapping mode in the conducting atomic force microscope (AFM) with a conducting diamond tip scanning on the laserilluminated cross-sectional surface of the intentionally fractured SiNWs. An EE-SiNW radial junction solar cell without intentional passivation was demonstrated with an η of 8.41% at this stage, which is already 30% higher relative to its planar Si counterpart. Further optimization of such solar cells could lead to cells with much higher efficiencies at lowered production costs due to their reduced requirement on crystal quality and inherent antireflection, as well as their fast, cheap, nonhazardous, and simple fabrication processes.

’ EXPERIMENTAL SECTION It is well-known that η relies on the purity of the silicon substrate in use. Therefore, it would not be appropriate to evaluate the solar cell processing techniques solely by comparing the reported η, because substrates of different qualities may have been used in different reports. In this paper, we took two pieces of substrates (2 cm  2 cm) cut from the same 250 μm thick p-type (100) Si wafer (E-light, 1
5 Ω 3 cm, one side polished), forming an EESiNW and a planar Si solar cell, with the diffusion processes optimized respectively, to avoid substrate quality dependence. For simplicity, both electrodes of the cells were formed by Au sputtering. The EE-SiNW solar cells, with radial p
n junctions, were prepared by RTA of an EE-SiNW substrate predeposited with the homemade SOD of high phosphor concentration, followed by simple formation of gold contacts on both sides. For preparation of the EE-SiNW substrate, an oxide-removed 2 cm  2 cm piece of the Si substrate, with the unpolished side covered by PTFE tape, was dipped into a mixture of silver nitrate (0.02 M, 30 mL) and HF (4 M, 50 mL) held at 50 C for electrolessetching of SiNWs.13 After the required etching time, the substrate was found covered with silver nanostructures, which were later removed by dipping the substrate in nitric acid solution, resulting in a dark antireflective EE-SiNWs substrate. The substrate, with the PTFE tape removed, was subsequently immersed in 10% HF and deionized (DI) water for a few seconds and nitrogen-dried to reduce the possibly of oxide formation. After the formation of the EE-SiNWs, a homemade SOD was prepared as the diffusion source for the junction formation. The SOD was made with a sol
gel process modified from the recipes of Kamil et al.23 Two solutions were prepared with the help of a micropipet. Solution A consisted of a mixture of tetraethoxysilane (TEOS) (0.306 mL) and isopropyl alcohol (IPA) (0.702 mL). Solution B consisted of a mixture of 85% phosphoric acid solution (H3PO4) (1.000 mL), IPA (0.902 mL), and DI water (0.080 mL).

ARTICLE

The SOD was completed by mixing solution A and B together. A 1 mL portion of the SOD source was spin-coated (1000 rpm, 10 s) and dried (110 C, 10 min) on the SiNW substrate to form a uniform and thick doping layer. Dopant diffusion, into the EE-NWs, was achieved by high-temperature RTA using an Ulvac Mila-3000 RTA system. An optimized and small RTA thermal budget of 980 C for only 10 s with a temperature ramp of 28 C/s was used for the emitter formations in the EE-SiNW solar cell. The total annealing time was about 5 min, including the cooling (to room temperature) time. The substrate was subsequently dipped in HF and DI water, respectively, for a few seconds to remove the phosphorus glass covering on the EE-SiNWs. Note that the prepared SOD would soon solidify in about 3 min after formation due to the high concentration (estimated to be 2.94  1021/cm3) of phosphoric acid added in the SOD. A control solar cell was fabricated on the other 2 cm  2 cm piece of substrate using the same diffusion process, but at a different optimum RTA condition of 980 C for 5 s. After the phosphorus glass removal, both the EE-SiNW and the control planar solar cells were nitrogen-dried and finally completed by sputtering gold electrodes at an equivalent thickness of 1.44 μm on both sides, followed by contact annealing (600 C, 10 min) and mechanical edge isolation (resulting in a 1 cm  1 cm cell). For the emitter electrodes of both cells, a shadow mask with 20% metallization fractions (finger spacing = 1 mm) was applied during gold sputtering, whereas no shadow mask was used on the base electrodes. It should be noted that neither a passivation nor any AR layer was applied on the cells. The performance of the solar cells was characterized by the I
V curves measured using a Keithley 2400 source meter under AM 1.5G illumination from a 500 W solar simulator (Sciencetech, SS0.5KW). A photovoltaic reference cell (PVM201, PV Measurements Inc.) was used to calibrate the illumination intensity. The external quantum efficiency (EQE) spectra of the cells were characterized using a Newport 1000W halogen lamp and a grating monochromator (Acton Spectra Pro 2300i) with an SR540 optical chopper and a lock-in amplifier (SR-830) to avoid the environmental electrical and optical interference. A calibrated Newport 818-UV sensor was employed for the absolute spectral responsivity (in units of A/W) or EQE (in unit of percent) evaluation of the solar cells. The specular reflectance measurement on the cells was carried out on a Lambda 19 UV/vis/IR spectrometer (PerkinElmer). A Hitachi S4800 scanning electron microscopy (SEM) system was employed for the morphology of the EE-SiNWs at nanoscale. To confirm the formation of radial p
n junctions inside the EESiNWs, a conductive AFM (NTMDT, P47H) was employed to map the photogenerated currents in an area, at zero bias voltage, on the laser-illuminated fractured EE-SiNWs nearby the top gold electrode (schematic diagram shown in Figure 1A). An additional EE-SiNW solar cell was prepared for this purpose. We have used a 100 mW, 532 nm diode pumped solid-state (DPSS) continuous wave laser at an oblique incidence on the spare EE-SiNW solar cell during the scan. The EE-SiNWs were intentionally broken with a glass plate gently scratching through the nanowires. An AFM cantilever, with a conductive diamond tip (NTMDT DCP11), was used to scan over the area exposed after the scratch to detect the photogenerated currents.

’ RESULTS AND DISCUSSION We begin by demonstrating the existence of the radial p
n junctions within the NWs of the fabricated EE-SiNW solar cell 21982

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Figure 1. (A) Schematic diagram of a conductive AFM mapping the photocurrents generated from the fractured SiNWs with radial p
n junctions. (B) Typical SEM cross-sectional image of EE-SiNWs. (C) A top view of (D).

using a conducting AFM scanning over the fractured cross sections of the NWs (see the schematic diagram shown in Figure 1A). It can be understood that the zero-bias photocurrent can be detected on top of the EE-SiNW cross section only if a radial p
n junction exists inside the NWs. As shown in Figure 1A, a contact was made on the surface of the NWs (n-type), while the AFM tip was detecting (at zero bias between the tip and the contact) current across the cross section of the broken surfaces of NWs. If the NWs were diffused under high thermal budget, the broken surface, exposing the core, would also be n-type, resulting in zero photocurrent. The photocurrent would be observed only if the inner core of the NWs remained p-type, and a radial p
n junction existed in the circuit, consisting of the AFM tip/radial junction/contact, generating the unidirectional photocurrents. Note that a typical SEM cross section and a top view of the as-etched EE-SiNWs are also shown in Figure 1B, C, respectively, whose morphologies resemble those of previous reports. The AFM scanning results are summarized in Figure 2A
C. The surface morphology of the scratched area on the EESiNW solar cell (Figure 2A) looks similar to the NWs in the SEM image of Figure 1C, but there appears to be larger bundles in Figure 2A. To understand this, we investigated a scratched region of the EE-SiNW solar cell, by SEM, and found that there was still remnant silicon oxide around the NWs, making them wrapped up into bundles (Figure 2D). Energy-dispersive spectroscopy (EDS) analysis (inset, Figure 2D) indicates that this remnant oxide was from the annealed TEOS left behind from the diffusion source, in the tiny air gaps within the EE-SiNWs, even after the short hydrofluoric acid (HF) rinsing. Figure 2D looks similar to Figure 2A except that the AFM had limited z-axis scanning range, resulting in limited revelation of the NWs. Figure 2B shows a zero-biased photocurrent mapping of Figure 2A measured with the configuration of Figure 1A. The clear white dots within the bundled nanowires in Figure 2B provided solid evidence that the radial p
n junctions within the EE-SiNWs really existed. Figure 2C shows a current profile of Figure 2B. The currents were all unidirectional and positive, which means that the currents were flowing upward from the scanned cross sections to the AFM tip, due to the existence of the radial junctions, and were photogenerated.

Figure 2. Results of cAFM current mapping (A
C) and an SEM image of the fractured SiNWs of the EE-SiNW solar cell (D). The morphology (A) and the cAFM current mapping image (B) on the fractured SiNWs evidenced the photocurrents generated from radial p
n junctions. Profiles of (A) and (B) (specified by the dotted lines) are shown in (C). An EDS spectrum proving the existence of remanent oxide is shown in the inset of (D).

One may wonder if radial junctions existed on all the NWs. To answer this question, we estimated the diffusion profile (planar case) through “point defect coupled diffusion” using “A TCAD lab.”24 The diffusion depth was estimated to be 50 nm. As the SiNWs are nonuniform, the “diameters” varied from 30 to 300 nm. Hence, only NWs with larger diameters (>100 nm) would have the radial junctions. We can estimate the occurrence ratio of these radial junctions with a simple calculation. Assuming 21983

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Figure 3. Performance of solar cells made with planar Si (A) and with EE-SiNWs (B). The corresponding solar cell parameters are both indicated. A cross-sectional SEM image revealing the gold contact above an EE-SiNW solar cell is shown in the inset of (B).

a spike (Figure 2C), as the tip is scanning along a line, represents the existence of a junction, we estimate the density of NWs having the radial junction to be 0.70 junctions/μm. Likewise, from Figure 1C, we can roughly count the average number of NWs that intersect with any imaginary horizontal line to be 1.6 NWs/μm. From the above, we can estimate that radial junctions occur only on 44% of the NWs. This is the occurrence ratio of the p
n junctions and is reasonable considering the 50 nm diffusion depth and the polydispersed (30
300 nm) NWs. The performance of the planar and the EE-SiNW solar cells is shown in Figure 3A,B, respectively, along with their corresponding solar cell characteristic parameters. The inset of Figure 3B reveals that the NWs, though bundled after annealing, are intact. The gold contact had almost formed a continuous film above the EE-SiNWs, which would have helped the efficient collection of the photogenerated carriers. The η of the EE-SiNWs reaches 8.41%, which is about 30% higher than that of its planar counterpart (η = 6.45%). Likewise, the short-circuit current density (Jsc) of the EE-SiNW solar cell (24.75 mA/cm2) is 33% larger than that of the planar one, while the two cells have similar fill factors (FF) and open-circuit voltages (Voc). By carefully repeating the experiment five times, we have confirmed an averaged enhancement factor in η to be 26.5%. The enhanced η may be attributed to the improved AR, in combination with the efficient carrier collection in the radial junction NWs that are passivated by the remnant oxide. The details will be discussed in the following. Figure 4A shows the EQE spectra of both solar cells with the respective specular reflectances shown in the inset. Clearly, the

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Figure 4. (A) External quantum efficiencies of an EE-SiNW solar cell and a planar solar cell, with the inset showing the corresponding reflectances. The discontinuity in the spectrum was due to a grating change. (B) The estimated internal quantum efficiencies of both cells.

planar cell had a poor EQE, compared to the EE-SiNWs cell, for wavelengths shorter than 780 nm due to the nonpassivated front surface causing serious carrier recombination on the surface of the cell. Such carrier recombination was less deleterious for the surface-passivated EE-SiNW solar cell (Figure 2D). According to the report by Leguijt et al.,25 low-temperature surface passivation on Si can be achieved, though not perfectly, by deposition of TEOS oxide, followed by low-temperature (400 C) annealing. The 980 C-grown remnant oxide, in our EE-SiNW solar cell, may have remedied, to a certain extent, the surface recombination problem. To estimate the enhancement in photocurrent if the light incident into an EE-SiNWs cell was all absorbed and converted, we use the following formalism. In general, if light transmission is negligible, Jsc for a solar cell can be estimated by the integration below26,27 J sc ¼ q

E2 Z

EQEðEÞ 3 bs ðEÞ dE

E1

¼q

E2 Z

IQEðEÞ 3 ð1
R hemi ðEÞÞ 3 bs ðEÞ dE

E1

where EQE and IQE denote the external and internal quantum efficiencies, bs(E) is the number of incident photons of energy in the range of E to E + dE (or incident spectral photon flux density), Rhemi(E) denotes the spectral hemispherical reflectance of the cell, and E1 and E2 represent the lower and upper bound photon energy of the light source. If the IQE(E) is the same for 21984

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both the planar and the EE-SiNW cells, then the enhancement factor can be simply estimated by J sc;EE-SiNWs Jsc;planar-Si ZE2 IQEðEÞ 3 ð1
1:0  20%Þ 3 bs ðEÞ dE ¼

E1

ZE2

¼ 1:575

IQEðEÞ 3 ð1
ð0:365  80% þ 1:0  20%ÞÞ 3 bs ðEÞ dE E1

This means a 57.5% enhancement in Jsc, or in conversion efficiency (if Voc and the FF are the same), should be expected. For the above calculation, we assumed the R of the Si substrate to be a constant of 36.5% (an average specular reflectivity from 300 to 1000 nm), and the R of the EE-SiNWs to be ∼zero over all wavelengths, considering a 20% metallization factor (F) front grid, underneath which no photocurrent can be generated. Our experimental results show an enhancement of 33% only, indicating that there is still ∼24.5% (=57.5
33%) Jsc loss in the EESiNW cell relative to the ideal case. The difference in the observed and theoretical estimations of Jsc enhancements may be attributed to the oversimplified assumptions. First, the hemispherical reflectance should be measured using an integrating sphere with a calibrated standard to take into consideration the effect of light scattering, since the surfaces of the cells were not perfectly flat.26 Second, unlike the above assumption, there should be some photocurrent contribution even beneath the metallic front grid edge due to the diffused backscattered light from the unpolished back side, as the substrate thickness is only 250 μm. For a simple qualitative understanding of the PV behavior in the two cells, we can evaluate an estimated IQE, as shown in Figure 4B, by simply dividing EQE with (1
Rspec(λ)), where Rspec(λ) represents the specular reflectance (inset, Figure 4A). Because UV light is absorbed mostly at the surface, the decrease in IQE, in the UV region (Figure 4B), indicates the occurrence of surface recombination. Because the planar cell was not passivated, it has serious decay in IQE at λ < 780 nm, whereas the EE-SiNWs cell has a quite different behavior, showing a plateau in the IQE from 480 to 850 nm with a slight decay in the UV region near 400 nm. Nevertheless, from Figure 4B, it is clear that the IQE of the EESiNW cell is about 30% less than that of the planar cell within 700
1000 nm. The reason for such a decay in the IQE of a typical Si solar cell, in the near IR, is the decrease of minority carrier diffusion length as well as the increased interface recombination at the back contact. However, the two cells, under consideration, had the same back contact and crystal quality, and hence similar diffusion lengths were expected, leading to similar values of IQE in the near IR. We will show later that the lower IQE for the EE-SiNW cell (Figure 4B), within 700
1000 nm, may be attributed to the surface recombination due to the imperfect passivation on the NW surfaces. The plateau feature near the visible region in the IQE of the EE-SiNW solar cell (Figure 4B) can also be explained by the existence of radial p
n junctions with moderate surface passivation. The generation rate G(λ, x) and collection probability fc(x) of photogenerated minority carriers are two major factors determining the spectral quantum efficiency of a solar cell.28 G(λ, x) gives the rate of electron
hole pairs being generated at some position x due to the absorption of photons with a

Figure 5. Schematic diagrams showing the junction regions of a planar solar cell (A) and an EE-SiNW solar cell (B). Calculated generation rates G(λ)’s (normalized) and collection probabilities fc(x)’s along paths indicated in (A) and (B) are shown in (C).

wavelength of λ. The fc(x) can be seen as an assessment of minority carriers generated at a position x, diffusing into the depletion region, and then driven by the internal field and collected at the electrode. Schematic diagrams showing the junction regions of a planar and an EE-SiNW solar cell are provided in Figure 5A,B. Various paths for collection probability evaluation are also indicated. Assuming that the absorption of UV, vis, and IR light into the depth of the cell directly influences G(λ, x), we can simply calculate the normalized G(λ, x)’s and the fc(x)’s along the different paths (Figure 5C), assuming a list of parameters given in Table 1. Note that our calculation is based on the knowledge of the planar Si solar cell,28 and we assume a high surface recombination velocity on both the surface of the planar cell and the front end of the EE-SiNW cell, and a low surface recombination velocity on the sidewalls of the NWs. Our calculations ignored any nanoscale effects for the SiNWs. Figure 5C shows that the overlapped fc(x)’s along the three paths (shell, core, and depletion region) of the SiNWs feature an extended high fc(x) along the NWs, especially for photons in the visible range. It means that the SiNWs could benefit from both wide visible range absorption and quick collection of the photogenerated minority carriers, which would be responsible for the plateau feature in the estimated IQE from 480 to 850 nm in Figure 4B. The fast collection of the carriers also relaxes the 21985

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’ AUTHOR INFORMATION

Table 1. Parameters Used to Calculate Figure 5C parameter

symbol

values

Corresponding Author

*E-mail: [email protected].

diameter of NWs

RNW

120 nm

Si surface

x0

0

start of depletion region

x1

20 nm

end of depletion region

x2

40 nm

end of NWs

x3

4040 nm

diffusion length of electron

Le

30 μm

diffusion length of hole

Lh

1 μm

λUV λvis

’ REFERENCES

a representative wavelength for UV light a representative wavelength for visible light

350 nm 600 nm

a representative wavelength for IR light

λIR

850 nm

absorption coeff for λUV

α(λUV)

1.07  106/cma

absorption coeff for λvis

α(λvis)

5200/cma

absorption coeff for λIR

α(λIR)

420/cma

(1) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W. Prog. Photovoltaics 2010, 18, 346. (2) Wenham, S. R.; Green, M. A. Prog. Photovoltaics 1996, 4, 3. (3) Green, M. A. Sol. Energy 2004, 76, 3. (4) Garnett, E. C.; Yang, P. D. J. Am. Chem. Soc. 2008, 130, 9224. (5) Fthenakis, V. Renewable Sustainable Energy Rev. 2009, 13, 2746. (6) Kamat, P. V.; Schatz, G. C. J. Phys. Chem. C 2009, 113, 15473. (7) Tsakalakos, L. Mater. Sci. Eng., R 2008, 62, 175. (8) Noel, S.; Lautenschlager, H.; Muller, J. C. Semicond. Sci. Technol. 2000, 15, 322. (9) Peters, S.; Ballif, C.; Borchert, D.; Schindler, R.; Warta, W.; Willeke, G. Semicond. Sci. Technol. 2002, 17, 677. (10) Rohatgi, A.; Ebong, A.; Yelundur, V.; Ristow, A. Prog. Photovoltaics 2000, 8, 515. (11) Peng, K. Q.; Yan, Y. J.; Gao, S. P.; Zhu, J. Adv. Mater. 2002, 14, 1164. (12) Peng, K. Q.; Lee, S. T. Adv. Mater. 2011, 23, 198. (13) Peng, K. Q.; Xu, Y.; Wu, Y.; Yan, Y. J.; Lee, S. T.; Zhu, J. Small 2005, 1, 1062. (14) Jung, J. Y.; Guo, Z.; Jee, S. W.; Um, H. D.; Park, K. T.; Lee, J. H. Opt. Express 2010, 18, A286. (15) Kayes, B. M.; Atwater, H. A.; Lewis, N. S. J. Appl. Phys. 2005, 97, 114302. (16) Sivakov, V.; Andra, G.; Gawlik, A.; Berger, A.; Plentz, J.; Falk, F.; Christiansen, S. H. Nano Lett. 2009, 9, 1549. (17) Fang, H.; Li, X. D.; Song, S.; Xu, Y.; Zhu, J. Nanotechnology 2008, 19, 255703. (18) Um, H. D.; Jung, J. Y.; Seo, H. S.; Park, K. T.; Jee, S. W.; Moiz, S. A.; Lee, J. H. Jpn. J. Appl. Phys. 2010, 49, 04DN02. (19) Jung, J. Y.; Guo, Z.; Jee, S. W.; Um, H. D.; Park, K. T.; Hyun, M. S.; Yang, J. M.; Lee, J. H. Nanotechnology 2010, 21, 445303. (20) Yoon, H. P.; Yuwen, Y. A.; Kendrick, C. E.; Barber, G. D.; Podraza, N. J.; Redwing, J. M.; Mallouk, T. E.; Wronski, C. R.; Mayer, T. S. Appl. Phys. Lett. 2010, 96, 213503. (21) Kendrick, C. E.; Yoon, H. P.; Yuwen, Y. A.; Barber, G. D.; Shen, H. T.; Mallouk, T. E.; Dickey, E. C.; Mayer, T. S.; Redwing, J. M. Appl. Phys. Lett. 2010, 97, 143108. (22) Gunawan, O.; Wang, K.; Fallahazad, B.; Zhang, Y.; Tutuc, E.; Guha, S. Prog. Photovoltaics 2011, 19, 307. (23) Kamil, S. A.; Ibrahim, K.; Aziz, A. A. AIP Conf. Proc. 2008, 1017, 124. (24) Klimeck, G.; Vasileska, D. a TCAD Lab; 2010, DOI: 10254/ nanohub-r5682.2. (25) Leguijt, C.; Lolgen, P.; Eikelboom, J. A.; Weeber, A. W.; Schuurmans, F. M.; Sinke, W. C.; Alkemade, P. F. A.; Sarro, P. M.; Maree, C. H. M.; Verhoef, L. A. Sol. Energy Mater. Sol. Cells 1996, 40, 297. (26) Nelson, J. The Physics of Solar Cells; Imperial Colledge Press: London, 2003. (27) Basore, P. A. Extended spectral analysis of internal quantum efficiency. Proceedings of the 23rd IEEE Photovoltaic Specialists Conference, Louisville, KY, 1993; pp 147
152. (28) Green, M. A. Solar cells: Operating Principles, Technology, and System Applications; Prentice-Hall: Englewood Cliffs, NJ, 1982.

a

The absorption coefficients were taken from: Palik, E. D. Handbook of Optical Constants of Solids; Academic Press: New York, 1985.

requirement for high crystalline quality.15 In Figure 5C, we have assumed a good passivation at the surface of the SiNWs. Instead, if we had a poor surface passivation of the SiNWs, the collection probability along the n-shell of the NWs, or fc,NW, shell, would no longer overlap with the other two fc(x)’s. The fc, NW,shell between x0 and x3 (a region that would absorb most of the light from UV to IR) (Figure 5C) would be reduced, and hence the IQE, since the minority carriers were located between the depletion region and the poorly passivated surface. The lower IQE for the EE-SiNW cell (Figure 4B) within 700
1000 nm can thus be ascribed to the imperfect passivation on the surface of the NWs. Consequently, improving the surface passivation becomes crucial for achieving high-efficiency solar cells at low costs, and the demonstrated silica wrapping of the NWs may provide us with an important starting point. There may be resistive power loss as the photogenerated carriers move through longer NWs, and an optimized design, involving diffusion depth, NW dimension, and the top metal contact grid pattern, has to be arrived at.

’ CONCLUSIONS We have successfully attained rapid formation of radial p
n junctions in the electroless-etched silicon nanowires with a homemade nonhazardous diffusion source of high phosphor concentration for cost reduction in solar cells. The fabricated solar cell comprising the nanowire features inherent antireflection, reduced diffusion length requirement, and broad band spectral quantum efficiency. The Si nanowire radial p
n junction solar cell, using Au as the two electrodes, has shown a conversion efficiency of 8.41%, which is 30% enhanced compared with its planar counterpart. The nanowires in the solar cells seemed bundled and are passivated by the residual silica formed from the annealed TEOS in the diffusion source. Furthermore, evidence of successful formation of radial p
n junctions in the nanowires has, for the first time, been revealed through the help of a conducting AFM scanning on the fractured surface of the nanowires. We believe that this kind of solar cell, with high efficiency and low cost, could remove the economic viability problem associated with the photovoltaic market.

’ ACKNOWLEDGMENT The authors are grateful for the financial support from the National Science Council and Academia Sinica, Taiwan.

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dx.doi.org/10.1021/jp205151d |J. Phys. Chem. C 2011, 115, 21981–21986