- Email: [email protected]
Morphology-tunable assembly of periodically aligned Si nanowire and radial pn junction arrays for solar cell applications
Applied Surface Science 258 (2012) 6169–6176
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Morphology-tunable assembly of periodically aligned Si nanowire and radial pn junction arrays for solar cell applications Xiaocheng Li ∗ , Kun Liang, Beng Kang Tay ∗ , Edwin H.T. Teo School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
a r t i c l e
i n f o
Article history: Received 29 September 2011 Received in revised form 22 December 2011 Accepted 18 February 2012 Available online 14 March 2012 Keywords: Si nanowire Chemical etching Periodicity Solar cell Radial pn junction
a b s t r a c t Large-area periodically aligned Si nanowire (PASiNW) arrays have been fabricated on Si substrates via a templated catalytic chemical etching process. The diameter, length, packing density, and even the shape of Si nanowires (SiNWs) could be precisely controlled and tuned. A local coupling redox mechanism involving the reduction of H2 O2 on silver particles and the dissolution of Si is responsible for formation of SiNWs. With the as-prepared SiNWs as templates, three kinds of PASiNW radial pn junction structures were fabricated on Si substrates via a solid-state phosphorous diffusion strategy and their applications in solar cells were also explored. The PASiNW radial pn junction-based solar cell with big diameter and interspace shows the highest power conversion efficiency (PCE) of 4.10% among the three kinds of devices. Further optimization, including surface passivation and electrode contact, is still needed for the higher efficiency PASiNW radial pn junction-based solar cells in the future. © 2012 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional nanostructures are regarded as the potential building blocks for nanoscale optoelectronics devices [1]. SiNWs, a very important semiconductor nanostructure, exhibit excellent electronic, optical, chemical and thermal properties that depend on their dimensions and their growth directions [2–5]. For applications, SiNWs must be efficiently and economically integrated into various nano-device structures and their morphologies must be precisely controlled. For example, the SiNW arrays with big interspace allow for better accommodation of large volume changes during the repeated charge and discharge process, without fracture that occurs in bulk or micro-size Si materials, and thus was considered as the ideal anode material of lithium batteries [6]. PASiNW arrays with moderate packing density have proved to possess excellent solar energy harvesting ability [7,8]. For many of these applications, a facile and low-cost process is urgently required to fabricate large-area uniform SiNW arrays with controlled diameter, length, shape and packing density. Up to now, many methods, including laser ablation [9], chemical vapor deposition [10], molecular beam epitaxy [11], and Cl2 plasma reactive ion etching [12], have been developed to fabricate SiNWs. For these methods, either the rigorous experimental conditions, such as high temperature, hazardous Si precursors, and
∗ Corresponding authors. Tel.: +65 67904533; fax: +65 67920415. E-mail addresses: [email protected] (X. Li), [email protected] (B.K. Tay). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.091
complex instruments are often required, or the twisted SiNWs are obtained, which severely limited the potential applications of SiNWs. Recently, a simple method, termed metal-assisted chemical etching, has been developed to fabricate SiNW arrays by immersing the Si wafer into the mixture solution of HF and AgNO3 [13,14]. However, this wet etching process resulted in the difficulty in precisely controlling the position, diameter and interspace of SiNWs, thus producing disordered SiNW bundles rather than highly ordered SiNW arrays. The investigations on morphologies-tunable assembly of PASiNW arrays and their applications are relatively less available [15,16]. Furthermore, the formation mechanism of SiNW arrays and role of Ag particles are still not clear, and is a subject of debate. One believes that Ag particles protect the Si underneath from being etched away and etching occurs in the surrounding area, resulting in Ag capped SiNWs [13,17], while the other argues that Ag particles only catalytically etches away the silicon directly contacting with them [18]. The two mechanisms are completely different and in some sense contradictory each other. Therefore, it is needed to clarify the formation mechanism of SiNWs and exact role of Ag particles during wet etching process. With these in mind, in this study, we present a relatively simpler method of fabricating large-area PASiNW arrays on Si substrates via a templated catalytic chemical etching route. We demonstrate how to fabricate large-area uniform distributed SiNW arrays with tunable diameter, length, interspace, packing density, and even the shape of the SiNWs by adjusting the diameter of polystyrene (PS) spheres, plasma etching duration, chemical etching duration and arrangement of PS spheres. With the aid of the reduced PS mask,
6170
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
the current debate on the formation mechanism of SiNW arrays and role of Ag nanoparticles are clarified. Inspired by the excellent light harvesting performance of PASiNW radial pn junction and their advantages in solar cells, we also fabricate the PASiNW radial pn junction-based solar cell by annealing the phosphoric acid film coated on PASiNW arrays, and investigate the effect of the diameter, interspace on the efficiency of as-prepared solar cells. Although the high conversion efficiency of 9–11% has been reported [19,20], the reported SiNW-based solar cells actually based on planar pn junction geometry, not the real SiNW radial pn junctions nanostructure. The SiNW arrays constructed on Si wafer actually acted as the antireflection layer to suppress the high reflection loss of Si wafer. In our study, the power conversion efficiency of as-prepared PASiNW radial pn junction-based solar cell can reach ∼4.10% under illumination condition. The advantage of this geometry lies in its low requirement for the grade of raw material [8,21–23]. We hope our fabrication process and strategy could provide a promising solution to PASiNW radial pn junction-based solar cells with higher power conversion efficiency. 2. Experimental details 2.1. Substrate treatment P type Si (1 0 0) with resistivity of 1–30 cm was used as substrates for all experiments discussed here. The Si wafers were cut into 2 × 2 cm2 squares and pre-cleaned with Piranha solution (H2 SO4 /H2 O2 = 3:1, v:v) at 90 ◦ C and RCA solution (NH3 /H2 O2 /H2 O = 1:1:5, v:v) at 75 ◦ C for 1 h in sequence, to remove the surface contamination and obtain a hydrophilic surface. Further cleaning was conducted by rinsing them in deionized water for several times. 2.2. Fabrication of PASiNW arrays PS spheres (517 nm, 323 nm and 170 nm in diameter) with 10% suspension in water were purchased from Microparticles GmbH (Berlin, Germany). The fabrication process of SiNW arrays includes five steps, similar to our previous report [7]. The first step involves the creation of the initial mask, i.e., a self-assembled PS spheres monolayer. Two methods were used for creating PS spheres monolayer according to the different sizes of PS spheres. Monolayer of PS spheres with diameter of 517 nm was created with a custombuilt system by dropping 7 l of diluted suspension solution on Si substrates at spin rate of 600 rpm. Monolayer of PS spheres with diameter of 320 nm and 170 nm is created by a floatingtransferring technique as described elsewhere [24]. Then, a reactive ion etching (RIE) process was employed to reduce the diameter of PS spheres, and produced a non-close-packaged arrangement of diameter-reduced PS spheres. After deposition of a thin layer of silver film with thickness of ∼25 nm, the Si substrates were immersed into the mixture solution of HF acid and H2 O2 contained within a Teflon vessel to fabricate SiNW arrays. The concentration of HF acid and H2 O2 was 4.6 M and 0.44 M, respectively. Finally, the Si substrates were placed into toluene, APM (H2 O2 /NH4 OH/H2 O = 2:1:5) and HPM solution (HCl/H2 O2 /H2 O = 2:1:8) successively, to remove the residual PS spheres and silver particles, respectively. 2.3. Fabrication of PASiNW radial pn junction arrays and PASiNW radial pn junction-based solar cells The fabrication process of PASiNW radial pn junction arrays is schematically depicted in Scheme 1. It mainly involves two steps: (a) spin-coating the phosphoric acid onto the surface of assynthesized PASiNW arrays at speed of 2000 rpm; (b) annealing of the samples by a rapid thermal annealing (RTA) system in nitrogen
Scheme 1. Schematic diagram of the fabrication process of PASiNW radial pn junction arrays.
atmosphere at 850 ◦ C for 30 s followed by removal of remaining glass film in 2% HF solution. After radial pn junction structure was formed, a thin layer of aluminum film with thickness of ∼200 nm was deposited on the rear side of samples and annealed at 530 ◦ C to form a good ohmic contact, simultaneously remove the parasitic pn junction at rear side. Then a Ti/Pd/Ag (60/60/500 nm) multilayered film was deposited on the surface of PASiNW radial pn junction arrays to form the front electrode via a shadow mask evaporation process in electron-beam evaporation system. Finally, the samples were annealed at 200 ◦ C for 6 h in nitrogen atmosphere, and cut into 1 × 1 cm2 pieces for testing. 2.4. Characterization The morphologies of the samples were characterized by LEO 1550 field emission scanning electron microscope (FESEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS) and JEOL 2010 high-resolution transmission electron microscopy (HRTEM). The TEM samples were prepared by dispersing the as-synthesized SiNWs in ethanol within an ultrasonic bath and dropped onto a carbon-coated copper grid. Optical reflectance spectra were recorded by PerkinElmer LAMBDA 950 UV/Vis/NIR spectrophotometer. The PCE measurement of PASiNW radial pn junction-based solar cell was performed by using a solar simulator under Air Mass (AM) 1.5 G illumination with intensity of 100 mW/cm2 . 3. Results and discussion 3.1. Assembly of PASiNWs with controlled diameter, length and packing density In our experiments, self-assembled PS spheres monolayer was used as mask to fabricate PASiNW arrays. Fig. 1a shows a typical SEM image of the as-prepared monolayer of PS spheres with diameter of 517 nm on Si (1 0 0) substrates. The stacking defects and dislocations are related to the different crystalline arrangement of PS spheres. High-magnification SEM image indicates a hexagonalclose-packaged arrangement of PS spheres (see inset image of Fig. 1a). After RIE duration of 480 s, the diameter of PS spheres was reduced to ∼225 nm, and the close-packaged PS spheres arrangement was transformed into a non-close-packaged structure, as shown in Fig. 1b. The plasma etching rate was calculated to be ∼0.606 nm/s under the RIE conditions described earlier. After deposition of silver films with thickness of ∼25 nm, the samples were immersed into the mixture solution of HF and H2 O2 to fabricate PASiNW arrays. During the chemical etching process, large amounts of bubbles were observed on Si surfaces. Meanwhile, the color of the substrate surface changed from gray to straw yellow, and finally become to black, indicating excellent light absorption. Fig. 2a and b shows the SEM images of Si surfaces with chemical etching duration of 2 min at different magnifications. Low-magnification of SEM image, as shown in Fig. 2a, clearly demonstrates homogeneously distributed PASiNW arrays
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
Fig. 1. FESEM images of self-assembled monolayer of PS spheres with diameter of 517 nm on Si surface: (a) before and (b) after plasma etching duration of 480 s.
over large-area, indicating an effective method for mass production of PASiNWs. Tilted-view SEM image with a larger magnification, as shown in Fig. 2b, indicates that SiNWs with diameter of 220 nm are vertically aligned on Si surface, implying that the chemical etching process are preferred along the direction perpendicular to Si (1 0 0) surface. This can be further confirmed by the cross-sectional view of long SiNW arrays with length of ca. 4.75 m (see Fig. 2d). No SiNW bundles or clumps are observed on Si substrate; even for the samples with chemical etching duration of 8 min, demonstrating a good method for fabricating vertically aligned SiNW arrays. The diameter of the SiNWs is found to strongly dependent on, but slightly less than that of diameter-reduced PS spheres. Fig. 2c shows the diameter distribution of 100 individual SiNWs and 100 reduced PS spheres with nominal diameter of 517 nm. The Gaussian fit curves indicate that SiNWs have a mean diameter of 219.9 nm with a standard deviation of 8.4 nm, which closely matches the mean diameter of 223.6 nm with standard deviation 7.9 nm of reduced PS spheres. The density of SiNWs is calculated to be ∼3.74 × 108 /cm2 , same as that of the PS spheres monolayer. Fig. 2e shows the TEM image and corresponding selected-area electron diffraction (SAED) of a single SiNW. It is obvious that SiNW surface is not as smooth as that grown by CVD method. The etching traces as marked by arrows could be easily observed, especially at top sidewall of the SiNW. The diameter at the top side of SiNW is slightly smaller than that at the bottom side, with a gradual angle of ∼0.7◦ along the axial direction of SiNW. The reason might be that the top side suffers longer etching time in solution than that of bottom side. The SAED patterns, as shown in the inset image of Fig. 2e, indicate that SiNWs still
6171
remain single-crystalline structure after chemical etching process. Fig. 2f shows the relationship between the chemical etching duration and length of the SiNWs. It is obvious that the length of SiNWs increases approximately linearly with the chemical etching duration. The calculated formation rate of SiNWs is about 0.53 m/min. This knowledge will allow us to fabricate the desired length of SiNW arrays by adjusting the chemical etching duration. Besides the length and diameter, the packing density and interspace of SiNWs could also be precisely controlled and tuned through varying the nominal diameter of PS spheres and plasma etching duration. Figs. 3a, b and 4 show typical SEM images of SiNW arrays using the reduced PS spheres with diameters of ∼215, 153, and ∼103 nm as masks. Other parameters, including nominal diameter of employed PS spheres, RIE duration, density and diameter of as-synthesized SiNWs, are summarized in Table 1. Compared with that of Fig. 2, the packing density of SiNWs for the samples employing PS spheres with nominal diameters of 323 and 170 nm increases obviously, and reaches about 9.58 × 108 /cm2 and 3.46 × 109 /cm2 , respectively (see Figs. 3 and 4). The packing density of SiNWs in Fig. 3a and b is same because of same nominal diameter of PS spheres for both cases. From Table 1, it can be concluded that the packing density of SiNWs is finally determined by the nominal diameter of employed PS spheres. The bigger size of nominal diameter of PS spheres, the lower packing density of SiNWs. The diameter of SiNW depends on that of the reduced PS sphere. The longer plasma etching duration, the smaller diameter of PS spheres, consequently, the smaller diameter and bigger interspace of SiNWs. The vacancies as marked with arrows in Fig. 3a are actually the absence of SiNWs in these points, which is correlated with the stacking defects during the arrangement of PS spheres monolayer. This also implies that the area without covered by PS sphere will be etched away during the chemical etching process. The rough surface of SiNWs in Fig. 4 may be related with the small diameter and possible detachment of reduced PS spheres during chemical etching process. Actually, the shape of the SiNWs could also be changed. By altering the spinning speed of spin-coating and dosage of employed PS suspension, double-layer PS spheres could be created on Si surface. After a long RIE duration, the thickest area, shown as the triangular-shaped pattern in Fig. 5a, remained on Si substrate. By using these special diameter-reduced PS spheres as mask, the triangular-shaped SiNW arrays could be fabricated on Si surface via the aforementioned chemical etching process, as shown in Fig. 5b. 3.2. Formation mechanism of SiNW arrays Up to now, two schools of though dominate the debate on the formation mechanism of SiNWs due to the ambiguous detection of the position of silver particles before and after chemical etching process. To detect the position of Ag particles after chemical etching process, the samples were characterized as quickly as possible by SEM and EDS without any further treatments. Fig. 6a shows the cross-sectional view of as-prepared sample. It can be seen that some PS spheres still adhere on the top of SiNWs after chemical etching process. Some white nano-particles or rings are also observed at root of SiNWs. EDS indicates that these white nano-particles or rings consist of silver element (see inset image Fig. 6a). This implies that PS spheres protect the underlying Si from being covered by Ag during the metal deposition process, and consequently from being etched away during the chemical etching process, while the silicon underlying the Ag honeycomb is gradually etched away. As a result, the SiNW arrays are formed underlying the PS spheres. This further indicates that the formation process SiNWs actually is a silver-induced chemical etching of Si beneath it. The Ag nanoparticles continuously sink during chemical etching process and finally remain at the root of SiNW arrays.
6172
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
Fig. 2. FESEM and TEM images of SiNWs on Si (1 0 0) substrate: (a) overall view; (b) tilted-view of SiNW arrays with chemical etching duration of 2 min; (c) diameter distribution of SiNW and reduced PS spheres (the statistics number for both cases is 100); (d) cross-sectional view of SiNWs with chemical etching duration of 8 min; (e) TEM image of a single SiNW and corresponding SAED pattern and (f) relationship between the length of SiNWs and chemical etching duration.
This also excludes the possibility that the Ag particles fall into the interspace of SiNWs during or after the chemical etching process. In analogy with the pioneering studies of porous Si [25–27], a localized electrochemical mechanism as follows is proposed to describe the fabrication process of SiNWs: Cathode :
H2 O2 + 2H+ → 2H2 O + 2h+
(1)
2H+ + 2e− → H2 ↑ Anode :
(2) +
Si + 4h + 4HF → SiF4 + 4H
+
(3)
SiF4 + 2HF → H2 SiF6 Overallreaction :
(4)
Si + H2 O2 + 6HF → 2H2 O + H2 SiF6 + H2 ↑ (5)
In this mechanism, the nanometer sized silver particles/films act as the local cathode to mediate the reduction of H2 O2 and generate the hole (see Eq. (1)) [26]. Meanwhile, a hydrogen molecule forms on local cathode, which is consistent with the bubbles observed during chemical etching process. According to the schematic diagram in Fig. 6b, the redox potential of H2 O2 /H2 O reaction is ∼1.77 V, which is far higher than the valence band of Si. Therefore, the holes originated from the reduction of H2 O2 on Ag surface can be easily injected into the valence band of Si through the Ag/Si interface and delivered to the location where the chemical etching occurs [27]. These holes were finally consumed during the oxidation of local anode from Si0 to Si4+ , producing the soluble species SiF6 2− and going to solution in the form of H2 SiF4 (see Fig. 6c and Eqs. (3) and
(4)). This is the dissolution step of this mechanism which determines the growth rate of SiNWs. The cathode reaction is the key step of this mechanism because the holes consumed in anode reaction are originated from the reduction of H2 O2 . Since the holes are injected into the Si valence band through the Ag/Si interface, chemical reaction occurs only in the local vicinity of the Si, consequently, only the Si areas contacted with the silver particles/films were etched away. Surprisingly, gravity is not the major force to drive the Ag particles sink during the etching process. During the experiment, we find that the SiNW arrays are always vertically aligned on Si (1 0 0); even the substrates were tilted or inversely placed into Teflon vessel. We speculate that the counter-force produced by the emission of hydrogen bubbles and the localized electrostatic force between the negatively charged Ag particles and positive charged Si surface underlying the Ag particles may be the driving force for the sinkage of Ag particles.
3.3. Fabrication of PASiNW radial pn junction arrays for solar cell applications PASiNW radial pn junction structure has theoretically proved to be an effective geometry for suppressing the reflection loss, simultaneously, providing shorter collection lengths for excited carriers and facilitating the more efficient collection of photo-generated carriers versus planar Si pn junction wafer. This will enable the use of low-grade raw materials and thus reduce the manufacture cost
Table 1 Summary of structural parameters of fabricated PASiNWs. Nominal diameter of PS spheres (nm)
Plasma etching duration (s)
Diameter of reduced PS spheres (nm)
Diameter of SiNWs (nm)
Density of SiNWs (/cm2 )
Corresponding figure
517 323 323 170
480 180 280 115
225 215 153 103
220 212 150 101
3.74 × 108 9.58 × 108 9.58 × 108 3.46 × 109
Fig. 2a–d Fig. 3a Fig. 3b Fig. 4
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
6173
Fig. 3. FESEM images of PASiNW arrays fabricated on Si (1 0 0) substrates using reduced PS spheres with diameters of (a) ∼215 nm and (b) ∼153 nm as masks, respectively. The nominal diameter of employed PS spheres is 323 nm for both two cases.
of Si-based solar cells [8,21–23]. For SiNW radial pn junction-based solar cells, most of previous reports only focus on the theoretical simulation of radical pn junction structure [8,22,23], while the experimental reports on radial SiNW pn junction-based solar cells are relatively rare [28–31], even not mentioned the PASiNW radial pn junction-based solar cells. According to aforementioned results, we know that the diameter, length, interspace and packing density of PASiNWs could be fully controlled and tuned. This will pave the way for fabricating of large-area PASiNW radial pn junction arrays and exploring their possible applications in photovoltaic devices. In this study, three types of PASiNWs with diameter of ∼225, 215,
Fig. 4. FESEM image of PASiNW arrays fabricated on Si (1 0 0) substrates using reduced PS spheres with diameter of ∼103 nm as mask. The nominal diameter of PS spheres is 170 nm.
Fig. 5. (a) The schematic diagram of formation process of triangular shaped PS spheres; (b) FESEM image of triangular shaped SiNWs arrays on Si (1 0 0) surface. The nominal diameter of used PS sphere is 517 nm.
and 103 nm, defined as sample A, B and C, respectively, are selected as templates to fabricate PASiNW radial pn junction arrays. The interspace of PASiNWs for three samples is ∼290, 110, and 67 nm, respectively. The length of SiNWs is ∼1.1 m. Before fabrication of PASiNW radial pn junction-based solar cells, optical reflection loss of all three samples is measured because of their significant importance for the efficiency of solar cells [21,32,33]. As shown in Fig. 7, samples A, B and C give the low reflectance loss of 3.82, 2.87, and 2.49%, respectively, within the wavelength range of 200–1000 nm, which is lower than that of the traditional antireflection layers, such as the porous Si, SiO2 , Si3 N4 , MgF2 –ZnS double layers, demonstrating their huge potential in solar cells [34–38]. The reflectance loss of the samples is roughly reduced with the decreases of the diameter and interspace of SiNWs and reaches 2.49% for SiNWs with diameter of ∼103 nm and interspace of ∼67 nm. The SiNWs with small diameter and interspace shows the lower reflection loss at short wavelength region but higher reflection loss at long wavelength region, compared with SiNWs with big diameter and interspace. Previous simulation also indicates that incorporation of SiNWs with small diameter could significantly improve the absorption in high energy regime and correspondingly reduce the reflection loss of sample for incident light [21]. The low reflectance loss of as-prepared samples is mainly correlated with the huge specific surface area and the typical sub-wavelength structure of PASiNWs, which has proved to be a useful structure to harvest more sunlight [7,29,39]. In addition, the gradient morphology of single SiNW with graded refractive index profile is also believed to be an important reason for the low reflection loss of the samples [40,41].
6174
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
Fig. 6. Formation mechanism of SiNW arrays. (a) Cross-sectional view of SiNW arrays and Ag nanoparticles remained at the bottom of SiNW arrays; inset image is corresponding EDS pattern; (b) comparison of the Si edge energies and redox potential of H2 O2 /H2 O electrode and (c) schematic elucidations of the formation mechanism of SiNW arrays.
Fig. 7. Reflectance loss spectra recorded from pristine Si wafer, samples A, B and C, respectively. Sample A is covered by PASiNWs with diameter of ∼225 nm and interspace of ∼290 nm; sample B is covered by PASiNWs with diameter of ∼215 nm and interspace of ∼105 nm; sample C is covered by PASiNWs with diameter of ∼105 nm and interspace of ∼65 nm.
By using the antireflective PASiNW arrays as templates, we fabricate the PASiNW radial pn junction arrays via the aforementioned solid-state phosphorous diffusion method. Although it is difficult to precisely measure the thickness of phosphorous-doped layer on SiNW surfaces, according to the previous result, the thickness of phosphorous-doped Si layer is estimated to be ∼80 nm after RTA treatment at 850 ◦ C for 45 s [42]. Fig. 8 shows the current–voltage (J–V) characteristic curves of devices A, B and C under dark and illumination conditions, respectively. The optical image of the asprepared devices is shown in left-bottom of Fig. 8a where the black area consists of PASiNW radial pn junction arrays while the white block lines stand for the front Ti/Pd/Ag multilayered film electrode. As can be seen that, both devices A and B exhibit a clear diode behavior with a turn-on voltage of ∼0.3 V in darkness. No obvious reverse current leakage was observed, especially for device A, demonstrating the good quality of radial pn junction structure. Upon illumination, device A demonstrates obvious photoelectron behavior and yields a PCE of 4.10% with an open-circuit voltage (VOC ) of 0.529 V and short-circuit current density (JSC ) of 13.4 mA/cm2 , higher than that of pristine Si wafer as reported in our previous study [7]. Device B exhibits a relatively low PCE of 1.22% with a VOC of 0.434 V and JSC of 5.98 mA/cm2 , due to the slight current leakage. Unexpectedly, device C shows almost linear I–V curve under
dark and illumination conditions and gives the lowest PCE of 0.74% among the three devices. The PCE performances of the as-prepared devices are closely related to their surface morphologies. For device A, the hydrophilic surface property and large interspace of ∼290 nm between SiNWs provides enough space to accommodate sufficient phosphoric acid and thus produce high quality radial pn junction arrays after RTA process. As a result, it gives a typical J–V curve with high VOC and JSC , as well as the high shunt resistance and low series resistance. For device C, the small interspace of ∼50 nm seriously impedes the penetration of phosphoric acid into the bottom of SiNWs, thus producing a non-uniform radial pn junction after RTA process. Consequently, device C presents a linear J–V curve with serious current leakage both in darkness and under illumination, see Fig. 8c. The interspace of SiNWs for device B is ∼105 nm, between that of device A and C, offer the capacity to accommodate a certain amount of phosphoric acid during spin-coating process. However, the phosphorous acid contained within the interspaces of PASiNWs arrays is still not sufficient to produce the high quality SiNW radial pn junction arrays during RTA process and finally leads to the J–V curve with low VOC and JSC under illumination condition. Therefore, the big interspace of SiNWs is critical for accommodating sufficient phosphorous acid and fabricating the high quality radial pn junction arrays in our experiment. The length of SiNWs also has a significant effect on efficiency of solar cells. Theoretically, long SiNWs have excellent sunlight absorption. Simulation results indicate that for SiNWs, the length of ∼2 m is deemed enough for efficient light absorption [43]. When the length exceeds 3 m, light absorption becomes almost saturated. In our experiment, SiNW arrays with length of ∼1.1 m exhibit a very low reflection loss of 3.82–2.49%, depending on the diameter and interspace of SiNWs, which fully reaches the requirement of light absorption for high efficient solar cells. Moreover, long SiNWs lead to the difficulties in fabricating high quality radial pn junction and good electrical contacts, thus the poor carrier collection. Accordingly, there exists an optimized SiNW length of ∼1–2 m compromised based on the trade-off of efficient light trapping and effective carrier collection needed for high efficient solar cells in experiments. SiNW radial pn junction structure has become one of most promising directions for high efficiency solar cells recently due to its short collection length for excited carrier and excellent light trapping ability. Results from Yang’s group indicated that the light trapping path length enhancement factor varies between 1.7 and 73 for PASiNWs, depending on the length of SiNWs [29]. Unfortunately, we can not exactly measure the enhancement factor of the PASiNW-based solar cells because of their Si wafer substrates. According to previous report, the light trapping ability of PASiNW arrays are still believed to have an important contribution for the high efficiency of fabricated devices in our study [29]. Besides,
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
6175
compared with the radial pn junction produced by oppositely doped film deposition on performed random SiNW arrays, the solid-state diffusion process employed in our experiment greatly reduces the defect on the interface of pn junction and thus produces the high quality pn junction. Even with these advantages, the PCE of fabricated PASiNW radial pn junction-based solar cell is still lower than that of planar Si wafer-based solar cells with SiNW arrays as antireflection layer [7,20], although it represents a viable path way toward high efficiency and low-cost Si-based solar cell. The enhanced surface defects resulted from the chemical etching process may be the possible reason, which leads to the severe surface and junction recombination. We did not perform any surface passivation, which is known to be important for high efficiency planar solar cells and should be even more critical for our PASiNW-based solar cells. Some surface passivation technologies, such as the silicon oxide and silicon nitride films, should be used to annihilate the surface defect and dangling bonds on surface of SiNWs in the future. Meanwhile the phosphorous diffusion process and the nanowire array geometries should also be further optimized to reduce the junction recombination. With these measurements, higher efficiency radial PASiNW-based solar cells could be expected. 4. Conclusion We fabricated large-area uniformly distributed PASiNW arrays with low-cost and high-throughput by using a templated silvercatalyzed chemical etching process. The diameter, length, packing density, and even the shape of SiNWs could be precisely controlled. With the aid of diameter-reduced PS masks, the controversy on the formation mechanism and role of Ag during the etching process was further understood and clarified. With the fabricated PASiNWs as template, PASiNW radial pn junction arrays were fabricated by annealing phosphoric acid film coated on sample surface. Under illumination, the PASiNW radial pn junction-based solar cell with large diameter of ∼225 nm and interspace of ∼290 nm shows the highest PCE of 4.10% among three as-prepared devices. The high conversion efficiency of PASiNW-based solar cell is attributed to the extremely low reflectance of periodically aligned nanowire structure and the radial pn junction geometry, as well as the pristine pn junction formation process. Acknowledgments The authors are grateful for the finance support of Temasek Laboratories @ NTU Research Seed Fund and AcRF Tier 2 (ARC 13/08) from MOE in Singapore. References
Fig. 8. I–V curves of as-prepared devices A, B and C, in sequence, under dark and AM 1.5 G illumination conditions. Devices A, B and C are prepared based on samples A, B and C, respectively, with tunable diameters and interspaces.
[1] M. Law, J. Goldberger, P. Yang, Annu. Rev. Mater. Res. 34 (2004) 83–122. [2] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353–389. [3] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289–1292. [4] Y. Cui, C.M. Lieber, Science 291 (2001) 851–853. [5] A.I. Hochbaum, R.K. Chen, R.D. Delgado, W.J. Liang, E.C. Garnett, M. Najarian, A. Majumdar, P.D. Yang, Nature 451 (2008) 163–165. [6] C.K. Chan, H. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nat. Nanotechnol. 3 (2008) 31–35. [7] X. Li, J. Li, T. Chen, B.K. Tay, J. Wang, H. Yu, Nanoscale Res. Lett. 5 (2010) 1721–1726. [8] J. Li, H. Yu, S.M. Wong, G. Zhang, X. Sun, P.G.Q. Lo, D.L. Kwong, Appl. Phys. Lett. 95 (2009) 033102. [9] A.M. Morales, C.M. Lieber, Science 279 (1998) 208–211. [10] A.I. Hochbaum, R. Fan, R. He, P. Yang, Nano Lett. 5 (2005) 457–460. [11] B. Fuhrmann, H.S. Leipner, H.R. Hoche, L. Schubert, P. Werner, U. Gosele, Nano Lett. 5 (2005) 2524–2527. [12] C.M. Hsu, S.T. Connor, M.X. Tang, Y. Cui, Appl. Phys. Lett. 93 (2008) 133109. [13] K. Peng, Y. Yan, S. Gao, J. Zhu, Adv. Funct. Mater. 13 (2003) 127–132. [14] Z. Huang, N. Geyer, P. Werner, J. de Boor, U. Gösele, Adv. Mater. 23 (2011) 285–308. [15] Z. Huang, H. Fang, J. Zhu, Adv. Mater. 19 (2007) 744–748.
6176
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
[16] K.Q. Peng, M.L. Zhang, A.J. Lu, N.B. Wong, R.Q. Zhang, S.T. Lee, Appl. Phys. Lett. 90 (2007) 163123. [17] T. Qiu, X.L. Wu, Y.F. Mei, G.J. Wan, P.K. Chu, G.G. Siu, J. Cryst. Growth 277 (2005) 143–148. [18] H. Fang, Y. Wu, J.H. Zhao, J. Zhu, Nanotechnology 17 (2006) 3768–3774. [19] K. Peng, Y. Xu, Y. Wu, Y. Yan, S.T. Lee, J. Zhu, Small 1 (2005) 1062–1067. [20] H. Fang, X. Li, S. Song, Y. Xu, J. Zhu, Nanotechnology 19 (2008) 255703 (6pp.). [21] L. Hu, G. Chen, Nano Lett. 7 (2007) 3249–3252. [22] B.M. Kayes, H.A. Atwater, N.S. Lewis, J. Appl. Phys. 97 (2005) 114302. [23] Y. Zhang, Wang, A. Mascarenhas, Nano Lett. 7 (2007) 1264–1269. [24] Y.J. Zhang, W. Li, K.J. Chen, J. Alloys Compd. 450 (2008) 512–516. [25] Y.L. Kawamura, K. Fukami, T. Sakka, Y.H. Ogata, Electrochem. Commun. 10 (2008) 346–349. [26] X. Li, P.W. Bohn, Appl. Phys. Lett. 77 (2000) 2572–2574. [27] S. Chattopadhyay, X. Li, P.W. Bohn, J. Appl. Phys. 91 (2002) 6134–6140. [28] E.C. Garnett, P. Yang, J. Am. Chem. Soc. 130 (2008) 9224–9225. [29] Y.-M. Chang, J. Shieh, J.-Y. Juang, J. Phys. Chem. C 115 (2011) 8983–8987. [30] B. Tian, X. Zheng, T.J. Kempa, Y. Fang, N. Yu, G. Yu, J. Huang, C.M. Lieber, Nature 449 (2007) 885–889. [31] L. Tsakalakos, J. Balch, J. Fronheiser, B.A. Korevaar, O. Sulima, J. Rand, Appl. Phys. Lett. 91 (2007) 233117.
[32] J. Li, H. Yu, S.M. Wong, X. Li, G. Zhang, P.G.-Q. Lo, D.-L. Kwong, Appl. Phys. Lett. 95 (2009) 243113–243123. [33] X.C. Li, B.K. Tay, J. Nanosci. Nanotechnol. 11 (2011) 10539–10543. [34] V. Yerokhov, I. Melnyk, A. Tsisaruk, I. Semochko, Opto-electron. Rev. 8 (2000) 414–417. [35] B.S. Kim, D.H. Lee, S.H. Kim, G.H. An, K.J. Lee, N.V. Myung, Y.H. Choa, J. Am. Ceram. Soc. 92 (2009) 2415–2417. [36] K. Sahoo, M.-K. Lin, E.-Y. Chang, Y.-Y. Lu, C.-C. Chen, J.-H. Huang, C.-W. Chang, Nanoscale Res. Lett. 4 (2009) 680–683. [37] M. Cid, N. Stem, C. Brunetti, A.F. Beloto, C.A.S. Ramos, Surf. Coat. Technol. 106 (1998) 117–120. [38] P. Singh, S.N. Sharma, N.M. Ravindra, JOM 62 (2010) 15–24. [39] K.-Q. Peng, S.-T. Lee, Adv. Mater. 23 (2011) 198–215. [40] Y.F. Huang, S. Chattopadhyay, Y.J. Jen, C.Y. Peng, T.A. Liu, Y.K. Hsu, C.L. Pan, H.C. Lo, C.H. Hsu, Y.H. Chang, C.S. Lee, K.H. Chen, L.C. Chen, Nat. Nanotechnol. 2 (2007) 770–774. [41] J. Zhu, Z. Yu, G.F. Burkhard, C.-M. Hsu, S.T. Connor, Y. Xu, Q. Wang, M. McGehee, S. Fan, Y. Cui, Nano Lett. 9 (2008) 279–282. [42] S. Sivoththaman, W. Laureys, J. Nijs, R. Mertens, Appl. Phys. Lett. 71 (1997) 392–394. [43] J. Li, H. Yu, Y. Li, Nanoscale 3 (2011) 4888–4900.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Morphology-tunable assembly of periodically aligned Si nanowire and radial pn junction arrays for solar cell applications Xiaocheng Li ∗ , Kun Liang, Beng Kang Tay ∗ , Edwin H.T. Teo School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
a r t i c l e
i n f o
Article history: Received 29 September 2011 Received in revised form 22 December 2011 Accepted 18 February 2012 Available online 14 March 2012 Keywords: Si nanowire Chemical etching Periodicity Solar cell Radial pn junction
a b s t r a c t Large-area periodically aligned Si nanowire (PASiNW) arrays have been fabricated on Si substrates via a templated catalytic chemical etching process. The diameter, length, packing density, and even the shape of Si nanowires (SiNWs) could be precisely controlled and tuned. A local coupling redox mechanism involving the reduction of H2 O2 on silver particles and the dissolution of Si is responsible for formation of SiNWs. With the as-prepared SiNWs as templates, three kinds of PASiNW radial pn junction structures were fabricated on Si substrates via a solid-state phosphorous diffusion strategy and their applications in solar cells were also explored. The PASiNW radial pn junction-based solar cell with big diameter and interspace shows the highest power conversion efficiency (PCE) of 4.10% among the three kinds of devices. Further optimization, including surface passivation and electrode contact, is still needed for the higher efficiency PASiNW radial pn junction-based solar cells in the future. © 2012 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional nanostructures are regarded as the potential building blocks for nanoscale optoelectronics devices [1]. SiNWs, a very important semiconductor nanostructure, exhibit excellent electronic, optical, chemical and thermal properties that depend on their dimensions and their growth directions [2–5]. For applications, SiNWs must be efficiently and economically integrated into various nano-device structures and their morphologies must be precisely controlled. For example, the SiNW arrays with big interspace allow for better accommodation of large volume changes during the repeated charge and discharge process, without fracture that occurs in bulk or micro-size Si materials, and thus was considered as the ideal anode material of lithium batteries [6]. PASiNW arrays with moderate packing density have proved to possess excellent solar energy harvesting ability [7,8]. For many of these applications, a facile and low-cost process is urgently required to fabricate large-area uniform SiNW arrays with controlled diameter, length, shape and packing density. Up to now, many methods, including laser ablation [9], chemical vapor deposition [10], molecular beam epitaxy [11], and Cl2 plasma reactive ion etching [12], have been developed to fabricate SiNWs. For these methods, either the rigorous experimental conditions, such as high temperature, hazardous Si precursors, and
∗ Corresponding authors. Tel.: +65 67904533; fax: +65 67920415. E-mail addresses: [email protected] (X. Li), [email protected] (B.K. Tay). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.091
complex instruments are often required, or the twisted SiNWs are obtained, which severely limited the potential applications of SiNWs. Recently, a simple method, termed metal-assisted chemical etching, has been developed to fabricate SiNW arrays by immersing the Si wafer into the mixture solution of HF and AgNO3 [13,14]. However, this wet etching process resulted in the difficulty in precisely controlling the position, diameter and interspace of SiNWs, thus producing disordered SiNW bundles rather than highly ordered SiNW arrays. The investigations on morphologies-tunable assembly of PASiNW arrays and their applications are relatively less available [15,16]. Furthermore, the formation mechanism of SiNW arrays and role of Ag particles are still not clear, and is a subject of debate. One believes that Ag particles protect the Si underneath from being etched away and etching occurs in the surrounding area, resulting in Ag capped SiNWs [13,17], while the other argues that Ag particles only catalytically etches away the silicon directly contacting with them [18]. The two mechanisms are completely different and in some sense contradictory each other. Therefore, it is needed to clarify the formation mechanism of SiNWs and exact role of Ag particles during wet etching process. With these in mind, in this study, we present a relatively simpler method of fabricating large-area PASiNW arrays on Si substrates via a templated catalytic chemical etching route. We demonstrate how to fabricate large-area uniform distributed SiNW arrays with tunable diameter, length, interspace, packing density, and even the shape of the SiNWs by adjusting the diameter of polystyrene (PS) spheres, plasma etching duration, chemical etching duration and arrangement of PS spheres. With the aid of the reduced PS mask,
6170
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
the current debate on the formation mechanism of SiNW arrays and role of Ag nanoparticles are clarified. Inspired by the excellent light harvesting performance of PASiNW radial pn junction and their advantages in solar cells, we also fabricate the PASiNW radial pn junction-based solar cell by annealing the phosphoric acid film coated on PASiNW arrays, and investigate the effect of the diameter, interspace on the efficiency of as-prepared solar cells. Although the high conversion efficiency of 9–11% has been reported [19,20], the reported SiNW-based solar cells actually based on planar pn junction geometry, not the real SiNW radial pn junctions nanostructure. The SiNW arrays constructed on Si wafer actually acted as the antireflection layer to suppress the high reflection loss of Si wafer. In our study, the power conversion efficiency of as-prepared PASiNW radial pn junction-based solar cell can reach ∼4.10% under illumination condition. The advantage of this geometry lies in its low requirement for the grade of raw material [8,21–23]. We hope our fabrication process and strategy could provide a promising solution to PASiNW radial pn junction-based solar cells with higher power conversion efficiency. 2. Experimental details 2.1. Substrate treatment P type Si (1 0 0) with resistivity of 1–30 cm was used as substrates for all experiments discussed here. The Si wafers were cut into 2 × 2 cm2 squares and pre-cleaned with Piranha solution (H2 SO4 /H2 O2 = 3:1, v:v) at 90 ◦ C and RCA solution (NH3 /H2 O2 /H2 O = 1:1:5, v:v) at 75 ◦ C for 1 h in sequence, to remove the surface contamination and obtain a hydrophilic surface. Further cleaning was conducted by rinsing them in deionized water for several times. 2.2. Fabrication of PASiNW arrays PS spheres (517 nm, 323 nm and 170 nm in diameter) with 10% suspension in water were purchased from Microparticles GmbH (Berlin, Germany). The fabrication process of SiNW arrays includes five steps, similar to our previous report [7]. The first step involves the creation of the initial mask, i.e., a self-assembled PS spheres monolayer. Two methods were used for creating PS spheres monolayer according to the different sizes of PS spheres. Monolayer of PS spheres with diameter of 517 nm was created with a custombuilt system by dropping 7 l of diluted suspension solution on Si substrates at spin rate of 600 rpm. Monolayer of PS spheres with diameter of 320 nm and 170 nm is created by a floatingtransferring technique as described elsewhere [24]. Then, a reactive ion etching (RIE) process was employed to reduce the diameter of PS spheres, and produced a non-close-packaged arrangement of diameter-reduced PS spheres. After deposition of a thin layer of silver film with thickness of ∼25 nm, the Si substrates were immersed into the mixture solution of HF acid and H2 O2 contained within a Teflon vessel to fabricate SiNW arrays. The concentration of HF acid and H2 O2 was 4.6 M and 0.44 M, respectively. Finally, the Si substrates were placed into toluene, APM (H2 O2 /NH4 OH/H2 O = 2:1:5) and HPM solution (HCl/H2 O2 /H2 O = 2:1:8) successively, to remove the residual PS spheres and silver particles, respectively. 2.3. Fabrication of PASiNW radial pn junction arrays and PASiNW radial pn junction-based solar cells The fabrication process of PASiNW radial pn junction arrays is schematically depicted in Scheme 1. It mainly involves two steps: (a) spin-coating the phosphoric acid onto the surface of assynthesized PASiNW arrays at speed of 2000 rpm; (b) annealing of the samples by a rapid thermal annealing (RTA) system in nitrogen
Scheme 1. Schematic diagram of the fabrication process of PASiNW radial pn junction arrays.
atmosphere at 850 ◦ C for 30 s followed by removal of remaining glass film in 2% HF solution. After radial pn junction structure was formed, a thin layer of aluminum film with thickness of ∼200 nm was deposited on the rear side of samples and annealed at 530 ◦ C to form a good ohmic contact, simultaneously remove the parasitic pn junction at rear side. Then a Ti/Pd/Ag (60/60/500 nm) multilayered film was deposited on the surface of PASiNW radial pn junction arrays to form the front electrode via a shadow mask evaporation process in electron-beam evaporation system. Finally, the samples were annealed at 200 ◦ C for 6 h in nitrogen atmosphere, and cut into 1 × 1 cm2 pieces for testing. 2.4. Characterization The morphologies of the samples were characterized by LEO 1550 field emission scanning electron microscope (FESEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS) and JEOL 2010 high-resolution transmission electron microscopy (HRTEM). The TEM samples were prepared by dispersing the as-synthesized SiNWs in ethanol within an ultrasonic bath and dropped onto a carbon-coated copper grid. Optical reflectance spectra were recorded by PerkinElmer LAMBDA 950 UV/Vis/NIR spectrophotometer. The PCE measurement of PASiNW radial pn junction-based solar cell was performed by using a solar simulator under Air Mass (AM) 1.5 G illumination with intensity of 100 mW/cm2 . 3. Results and discussion 3.1. Assembly of PASiNWs with controlled diameter, length and packing density In our experiments, self-assembled PS spheres monolayer was used as mask to fabricate PASiNW arrays. Fig. 1a shows a typical SEM image of the as-prepared monolayer of PS spheres with diameter of 517 nm on Si (1 0 0) substrates. The stacking defects and dislocations are related to the different crystalline arrangement of PS spheres. High-magnification SEM image indicates a hexagonalclose-packaged arrangement of PS spheres (see inset image of Fig. 1a). After RIE duration of 480 s, the diameter of PS spheres was reduced to ∼225 nm, and the close-packaged PS spheres arrangement was transformed into a non-close-packaged structure, as shown in Fig. 1b. The plasma etching rate was calculated to be ∼0.606 nm/s under the RIE conditions described earlier. After deposition of silver films with thickness of ∼25 nm, the samples were immersed into the mixture solution of HF and H2 O2 to fabricate PASiNW arrays. During the chemical etching process, large amounts of bubbles were observed on Si surfaces. Meanwhile, the color of the substrate surface changed from gray to straw yellow, and finally become to black, indicating excellent light absorption. Fig. 2a and b shows the SEM images of Si surfaces with chemical etching duration of 2 min at different magnifications. Low-magnification of SEM image, as shown in Fig. 2a, clearly demonstrates homogeneously distributed PASiNW arrays
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
Fig. 1. FESEM images of self-assembled monolayer of PS spheres with diameter of 517 nm on Si surface: (a) before and (b) after plasma etching duration of 480 s.
over large-area, indicating an effective method for mass production of PASiNWs. Tilted-view SEM image with a larger magnification, as shown in Fig. 2b, indicates that SiNWs with diameter of 220 nm are vertically aligned on Si surface, implying that the chemical etching process are preferred along the direction perpendicular to Si (1 0 0) surface. This can be further confirmed by the cross-sectional view of long SiNW arrays with length of ca. 4.75 m (see Fig. 2d). No SiNW bundles or clumps are observed on Si substrate; even for the samples with chemical etching duration of 8 min, demonstrating a good method for fabricating vertically aligned SiNW arrays. The diameter of the SiNWs is found to strongly dependent on, but slightly less than that of diameter-reduced PS spheres. Fig. 2c shows the diameter distribution of 100 individual SiNWs and 100 reduced PS spheres with nominal diameter of 517 nm. The Gaussian fit curves indicate that SiNWs have a mean diameter of 219.9 nm with a standard deviation of 8.4 nm, which closely matches the mean diameter of 223.6 nm with standard deviation 7.9 nm of reduced PS spheres. The density of SiNWs is calculated to be ∼3.74 × 108 /cm2 , same as that of the PS spheres monolayer. Fig. 2e shows the TEM image and corresponding selected-area electron diffraction (SAED) of a single SiNW. It is obvious that SiNW surface is not as smooth as that grown by CVD method. The etching traces as marked by arrows could be easily observed, especially at top sidewall of the SiNW. The diameter at the top side of SiNW is slightly smaller than that at the bottom side, with a gradual angle of ∼0.7◦ along the axial direction of SiNW. The reason might be that the top side suffers longer etching time in solution than that of bottom side. The SAED patterns, as shown in the inset image of Fig. 2e, indicate that SiNWs still
6171
remain single-crystalline structure after chemical etching process. Fig. 2f shows the relationship between the chemical etching duration and length of the SiNWs. It is obvious that the length of SiNWs increases approximately linearly with the chemical etching duration. The calculated formation rate of SiNWs is about 0.53 m/min. This knowledge will allow us to fabricate the desired length of SiNW arrays by adjusting the chemical etching duration. Besides the length and diameter, the packing density and interspace of SiNWs could also be precisely controlled and tuned through varying the nominal diameter of PS spheres and plasma etching duration. Figs. 3a, b and 4 show typical SEM images of SiNW arrays using the reduced PS spheres with diameters of ∼215, 153, and ∼103 nm as masks. Other parameters, including nominal diameter of employed PS spheres, RIE duration, density and diameter of as-synthesized SiNWs, are summarized in Table 1. Compared with that of Fig. 2, the packing density of SiNWs for the samples employing PS spheres with nominal diameters of 323 and 170 nm increases obviously, and reaches about 9.58 × 108 /cm2 and 3.46 × 109 /cm2 , respectively (see Figs. 3 and 4). The packing density of SiNWs in Fig. 3a and b is same because of same nominal diameter of PS spheres for both cases. From Table 1, it can be concluded that the packing density of SiNWs is finally determined by the nominal diameter of employed PS spheres. The bigger size of nominal diameter of PS spheres, the lower packing density of SiNWs. The diameter of SiNW depends on that of the reduced PS sphere. The longer plasma etching duration, the smaller diameter of PS spheres, consequently, the smaller diameter and bigger interspace of SiNWs. The vacancies as marked with arrows in Fig. 3a are actually the absence of SiNWs in these points, which is correlated with the stacking defects during the arrangement of PS spheres monolayer. This also implies that the area without covered by PS sphere will be etched away during the chemical etching process. The rough surface of SiNWs in Fig. 4 may be related with the small diameter and possible detachment of reduced PS spheres during chemical etching process. Actually, the shape of the SiNWs could also be changed. By altering the spinning speed of spin-coating and dosage of employed PS suspension, double-layer PS spheres could be created on Si surface. After a long RIE duration, the thickest area, shown as the triangular-shaped pattern in Fig. 5a, remained on Si substrate. By using these special diameter-reduced PS spheres as mask, the triangular-shaped SiNW arrays could be fabricated on Si surface via the aforementioned chemical etching process, as shown in Fig. 5b. 3.2. Formation mechanism of SiNW arrays Up to now, two schools of though dominate the debate on the formation mechanism of SiNWs due to the ambiguous detection of the position of silver particles before and after chemical etching process. To detect the position of Ag particles after chemical etching process, the samples were characterized as quickly as possible by SEM and EDS without any further treatments. Fig. 6a shows the cross-sectional view of as-prepared sample. It can be seen that some PS spheres still adhere on the top of SiNWs after chemical etching process. Some white nano-particles or rings are also observed at root of SiNWs. EDS indicates that these white nano-particles or rings consist of silver element (see inset image Fig. 6a). This implies that PS spheres protect the underlying Si from being covered by Ag during the metal deposition process, and consequently from being etched away during the chemical etching process, while the silicon underlying the Ag honeycomb is gradually etched away. As a result, the SiNW arrays are formed underlying the PS spheres. This further indicates that the formation process SiNWs actually is a silver-induced chemical etching of Si beneath it. The Ag nanoparticles continuously sink during chemical etching process and finally remain at the root of SiNW arrays.
6172
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
Fig. 2. FESEM and TEM images of SiNWs on Si (1 0 0) substrate: (a) overall view; (b) tilted-view of SiNW arrays with chemical etching duration of 2 min; (c) diameter distribution of SiNW and reduced PS spheres (the statistics number for both cases is 100); (d) cross-sectional view of SiNWs with chemical etching duration of 8 min; (e) TEM image of a single SiNW and corresponding SAED pattern and (f) relationship between the length of SiNWs and chemical etching duration.
This also excludes the possibility that the Ag particles fall into the interspace of SiNWs during or after the chemical etching process. In analogy with the pioneering studies of porous Si [25–27], a localized electrochemical mechanism as follows is proposed to describe the fabrication process of SiNWs: Cathode :
H2 O2 + 2H+ → 2H2 O + 2h+
(1)
2H+ + 2e− → H2 ↑ Anode :
(2) +
Si + 4h + 4HF → SiF4 + 4H
+
(3)
SiF4 + 2HF → H2 SiF6 Overallreaction :
(4)
Si + H2 O2 + 6HF → 2H2 O + H2 SiF6 + H2 ↑ (5)
In this mechanism, the nanometer sized silver particles/films act as the local cathode to mediate the reduction of H2 O2 and generate the hole (see Eq. (1)) [26]. Meanwhile, a hydrogen molecule forms on local cathode, which is consistent with the bubbles observed during chemical etching process. According to the schematic diagram in Fig. 6b, the redox potential of H2 O2 /H2 O reaction is ∼1.77 V, which is far higher than the valence band of Si. Therefore, the holes originated from the reduction of H2 O2 on Ag surface can be easily injected into the valence band of Si through the Ag/Si interface and delivered to the location where the chemical etching occurs [27]. These holes were finally consumed during the oxidation of local anode from Si0 to Si4+ , producing the soluble species SiF6 2− and going to solution in the form of H2 SiF4 (see Fig. 6c and Eqs. (3) and
(4)). This is the dissolution step of this mechanism which determines the growth rate of SiNWs. The cathode reaction is the key step of this mechanism because the holes consumed in anode reaction are originated from the reduction of H2 O2 . Since the holes are injected into the Si valence band through the Ag/Si interface, chemical reaction occurs only in the local vicinity of the Si, consequently, only the Si areas contacted with the silver particles/films were etched away. Surprisingly, gravity is not the major force to drive the Ag particles sink during the etching process. During the experiment, we find that the SiNW arrays are always vertically aligned on Si (1 0 0); even the substrates were tilted or inversely placed into Teflon vessel. We speculate that the counter-force produced by the emission of hydrogen bubbles and the localized electrostatic force between the negatively charged Ag particles and positive charged Si surface underlying the Ag particles may be the driving force for the sinkage of Ag particles.
3.3. Fabrication of PASiNW radial pn junction arrays for solar cell applications PASiNW radial pn junction structure has theoretically proved to be an effective geometry for suppressing the reflection loss, simultaneously, providing shorter collection lengths for excited carriers and facilitating the more efficient collection of photo-generated carriers versus planar Si pn junction wafer. This will enable the use of low-grade raw materials and thus reduce the manufacture cost
Table 1 Summary of structural parameters of fabricated PASiNWs. Nominal diameter of PS spheres (nm)
Plasma etching duration (s)
Diameter of reduced PS spheres (nm)
Diameter of SiNWs (nm)
Density of SiNWs (/cm2 )
Corresponding figure
517 323 323 170
480 180 280 115
225 215 153 103
220 212 150 101
3.74 × 108 9.58 × 108 9.58 × 108 3.46 × 109
Fig. 2a–d Fig. 3a Fig. 3b Fig. 4
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
6173
Fig. 3. FESEM images of PASiNW arrays fabricated on Si (1 0 0) substrates using reduced PS spheres with diameters of (a) ∼215 nm and (b) ∼153 nm as masks, respectively. The nominal diameter of employed PS spheres is 323 nm for both two cases.
of Si-based solar cells [8,21–23]. For SiNW radial pn junction-based solar cells, most of previous reports only focus on the theoretical simulation of radical pn junction structure [8,22,23], while the experimental reports on radial SiNW pn junction-based solar cells are relatively rare [28–31], even not mentioned the PASiNW radial pn junction-based solar cells. According to aforementioned results, we know that the diameter, length, interspace and packing density of PASiNWs could be fully controlled and tuned. This will pave the way for fabricating of large-area PASiNW radial pn junction arrays and exploring their possible applications in photovoltaic devices. In this study, three types of PASiNWs with diameter of ∼225, 215,
Fig. 4. FESEM image of PASiNW arrays fabricated on Si (1 0 0) substrates using reduced PS spheres with diameter of ∼103 nm as mask. The nominal diameter of PS spheres is 170 nm.
Fig. 5. (a) The schematic diagram of formation process of triangular shaped PS spheres; (b) FESEM image of triangular shaped SiNWs arrays on Si (1 0 0) surface. The nominal diameter of used PS sphere is 517 nm.
and 103 nm, defined as sample A, B and C, respectively, are selected as templates to fabricate PASiNW radial pn junction arrays. The interspace of PASiNWs for three samples is ∼290, 110, and 67 nm, respectively. The length of SiNWs is ∼1.1 m. Before fabrication of PASiNW radial pn junction-based solar cells, optical reflection loss of all three samples is measured because of their significant importance for the efficiency of solar cells [21,32,33]. As shown in Fig. 7, samples A, B and C give the low reflectance loss of 3.82, 2.87, and 2.49%, respectively, within the wavelength range of 200–1000 nm, which is lower than that of the traditional antireflection layers, such as the porous Si, SiO2 , Si3 N4 , MgF2 –ZnS double layers, demonstrating their huge potential in solar cells [34–38]. The reflectance loss of the samples is roughly reduced with the decreases of the diameter and interspace of SiNWs and reaches 2.49% for SiNWs with diameter of ∼103 nm and interspace of ∼67 nm. The SiNWs with small diameter and interspace shows the lower reflection loss at short wavelength region but higher reflection loss at long wavelength region, compared with SiNWs with big diameter and interspace. Previous simulation also indicates that incorporation of SiNWs with small diameter could significantly improve the absorption in high energy regime and correspondingly reduce the reflection loss of sample for incident light [21]. The low reflectance loss of as-prepared samples is mainly correlated with the huge specific surface area and the typical sub-wavelength structure of PASiNWs, which has proved to be a useful structure to harvest more sunlight [7,29,39]. In addition, the gradient morphology of single SiNW with graded refractive index profile is also believed to be an important reason for the low reflection loss of the samples [40,41].
6174
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
Fig. 6. Formation mechanism of SiNW arrays. (a) Cross-sectional view of SiNW arrays and Ag nanoparticles remained at the bottom of SiNW arrays; inset image is corresponding EDS pattern; (b) comparison of the Si edge energies and redox potential of H2 O2 /H2 O electrode and (c) schematic elucidations of the formation mechanism of SiNW arrays.
Fig. 7. Reflectance loss spectra recorded from pristine Si wafer, samples A, B and C, respectively. Sample A is covered by PASiNWs with diameter of ∼225 nm and interspace of ∼290 nm; sample B is covered by PASiNWs with diameter of ∼215 nm and interspace of ∼105 nm; sample C is covered by PASiNWs with diameter of ∼105 nm and interspace of ∼65 nm.
By using the antireflective PASiNW arrays as templates, we fabricate the PASiNW radial pn junction arrays via the aforementioned solid-state phosphorous diffusion method. Although it is difficult to precisely measure the thickness of phosphorous-doped layer on SiNW surfaces, according to the previous result, the thickness of phosphorous-doped Si layer is estimated to be ∼80 nm after RTA treatment at 850 ◦ C for 45 s [42]. Fig. 8 shows the current–voltage (J–V) characteristic curves of devices A, B and C under dark and illumination conditions, respectively. The optical image of the asprepared devices is shown in left-bottom of Fig. 8a where the black area consists of PASiNW radial pn junction arrays while the white block lines stand for the front Ti/Pd/Ag multilayered film electrode. As can be seen that, both devices A and B exhibit a clear diode behavior with a turn-on voltage of ∼0.3 V in darkness. No obvious reverse current leakage was observed, especially for device A, demonstrating the good quality of radial pn junction structure. Upon illumination, device A demonstrates obvious photoelectron behavior and yields a PCE of 4.10% with an open-circuit voltage (VOC ) of 0.529 V and short-circuit current density (JSC ) of 13.4 mA/cm2 , higher than that of pristine Si wafer as reported in our previous study [7]. Device B exhibits a relatively low PCE of 1.22% with a VOC of 0.434 V and JSC of 5.98 mA/cm2 , due to the slight current leakage. Unexpectedly, device C shows almost linear I–V curve under
dark and illumination conditions and gives the lowest PCE of 0.74% among the three devices. The PCE performances of the as-prepared devices are closely related to their surface morphologies. For device A, the hydrophilic surface property and large interspace of ∼290 nm between SiNWs provides enough space to accommodate sufficient phosphoric acid and thus produce high quality radial pn junction arrays after RTA process. As a result, it gives a typical J–V curve with high VOC and JSC , as well as the high shunt resistance and low series resistance. For device C, the small interspace of ∼50 nm seriously impedes the penetration of phosphoric acid into the bottom of SiNWs, thus producing a non-uniform radial pn junction after RTA process. Consequently, device C presents a linear J–V curve with serious current leakage both in darkness and under illumination, see Fig. 8c. The interspace of SiNWs for device B is ∼105 nm, between that of device A and C, offer the capacity to accommodate a certain amount of phosphoric acid during spin-coating process. However, the phosphorous acid contained within the interspaces of PASiNWs arrays is still not sufficient to produce the high quality SiNW radial pn junction arrays during RTA process and finally leads to the J–V curve with low VOC and JSC under illumination condition. Therefore, the big interspace of SiNWs is critical for accommodating sufficient phosphorous acid and fabricating the high quality radial pn junction arrays in our experiment. The length of SiNWs also has a significant effect on efficiency of solar cells. Theoretically, long SiNWs have excellent sunlight absorption. Simulation results indicate that for SiNWs, the length of ∼2 m is deemed enough for efficient light absorption [43]. When the length exceeds 3 m, light absorption becomes almost saturated. In our experiment, SiNW arrays with length of ∼1.1 m exhibit a very low reflection loss of 3.82–2.49%, depending on the diameter and interspace of SiNWs, which fully reaches the requirement of light absorption for high efficient solar cells. Moreover, long SiNWs lead to the difficulties in fabricating high quality radial pn junction and good electrical contacts, thus the poor carrier collection. Accordingly, there exists an optimized SiNW length of ∼1–2 m compromised based on the trade-off of efficient light trapping and effective carrier collection needed for high efficient solar cells in experiments. SiNW radial pn junction structure has become one of most promising directions for high efficiency solar cells recently due to its short collection length for excited carrier and excellent light trapping ability. Results from Yang’s group indicated that the light trapping path length enhancement factor varies between 1.7 and 73 for PASiNWs, depending on the length of SiNWs [29]. Unfortunately, we can not exactly measure the enhancement factor of the PASiNW-based solar cells because of their Si wafer substrates. According to previous report, the light trapping ability of PASiNW arrays are still believed to have an important contribution for the high efficiency of fabricated devices in our study [29]. Besides,
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
6175
compared with the radial pn junction produced by oppositely doped film deposition on performed random SiNW arrays, the solid-state diffusion process employed in our experiment greatly reduces the defect on the interface of pn junction and thus produces the high quality pn junction. Even with these advantages, the PCE of fabricated PASiNW radial pn junction-based solar cell is still lower than that of planar Si wafer-based solar cells with SiNW arrays as antireflection layer [7,20], although it represents a viable path way toward high efficiency and low-cost Si-based solar cell. The enhanced surface defects resulted from the chemical etching process may be the possible reason, which leads to the severe surface and junction recombination. We did not perform any surface passivation, which is known to be important for high efficiency planar solar cells and should be even more critical for our PASiNW-based solar cells. Some surface passivation technologies, such as the silicon oxide and silicon nitride films, should be used to annihilate the surface defect and dangling bonds on surface of SiNWs in the future. Meanwhile the phosphorous diffusion process and the nanowire array geometries should also be further optimized to reduce the junction recombination. With these measurements, higher efficiency radial PASiNW-based solar cells could be expected. 4. Conclusion We fabricated large-area uniformly distributed PASiNW arrays with low-cost and high-throughput by using a templated silvercatalyzed chemical etching process. The diameter, length, packing density, and even the shape of SiNWs could be precisely controlled. With the aid of diameter-reduced PS masks, the controversy on the formation mechanism and role of Ag during the etching process was further understood and clarified. With the fabricated PASiNWs as template, PASiNW radial pn junction arrays were fabricated by annealing phosphoric acid film coated on sample surface. Under illumination, the PASiNW radial pn junction-based solar cell with large diameter of ∼225 nm and interspace of ∼290 nm shows the highest PCE of 4.10% among three as-prepared devices. The high conversion efficiency of PASiNW-based solar cell is attributed to the extremely low reflectance of periodically aligned nanowire structure and the radial pn junction geometry, as well as the pristine pn junction formation process. Acknowledgments The authors are grateful for the finance support of Temasek Laboratories @ NTU Research Seed Fund and AcRF Tier 2 (ARC 13/08) from MOE in Singapore. References
Fig. 8. I–V curves of as-prepared devices A, B and C, in sequence, under dark and AM 1.5 G illumination conditions. Devices A, B and C are prepared based on samples A, B and C, respectively, with tunable diameters and interspaces.
[1] M. Law, J. Goldberger, P. Yang, Annu. Rev. Mater. Res. 34 (2004) 83–122. [2] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353–389. [3] Y. Cui, Q. Wei, H. Park, C.M. Lieber, Science 293 (2001) 1289–1292. [4] Y. Cui, C.M. Lieber, Science 291 (2001) 851–853. [5] A.I. Hochbaum, R.K. Chen, R.D. Delgado, W.J. Liang, E.C. Garnett, M. Najarian, A. Majumdar, P.D. Yang, Nature 451 (2008) 163–165. [6] C.K. Chan, H. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nat. Nanotechnol. 3 (2008) 31–35. [7] X. Li, J. Li, T. Chen, B.K. Tay, J. Wang, H. Yu, Nanoscale Res. Lett. 5 (2010) 1721–1726. [8] J. Li, H. Yu, S.M. Wong, G. Zhang, X. Sun, P.G.Q. Lo, D.L. Kwong, Appl. Phys. Lett. 95 (2009) 033102. [9] A.M. Morales, C.M. Lieber, Science 279 (1998) 208–211. [10] A.I. Hochbaum, R. Fan, R. He, P. Yang, Nano Lett. 5 (2005) 457–460. [11] B. Fuhrmann, H.S. Leipner, H.R. Hoche, L. Schubert, P. Werner, U. Gosele, Nano Lett. 5 (2005) 2524–2527. [12] C.M. Hsu, S.T. Connor, M.X. Tang, Y. Cui, Appl. Phys. Lett. 93 (2008) 133109. [13] K. Peng, Y. Yan, S. Gao, J. Zhu, Adv. Funct. Mater. 13 (2003) 127–132. [14] Z. Huang, N. Geyer, P. Werner, J. de Boor, U. Gösele, Adv. Mater. 23 (2011) 285–308. [15] Z. Huang, H. Fang, J. Zhu, Adv. Mater. 19 (2007) 744–748.
6176
X. Li et al. / Applied Surface Science 258 (2012) 6169–6176
[16] K.Q. Peng, M.L. Zhang, A.J. Lu, N.B. Wong, R.Q. Zhang, S.T. Lee, Appl. Phys. Lett. 90 (2007) 163123. [17] T. Qiu, X.L. Wu, Y.F. Mei, G.J. Wan, P.K. Chu, G.G. Siu, J. Cryst. Growth 277 (2005) 143–148. [18] H. Fang, Y. Wu, J.H. Zhao, J. Zhu, Nanotechnology 17 (2006) 3768–3774. [19] K. Peng, Y. Xu, Y. Wu, Y. Yan, S.T. Lee, J. Zhu, Small 1 (2005) 1062–1067. [20] H. Fang, X. Li, S. Song, Y. Xu, J. Zhu, Nanotechnology 19 (2008) 255703 (6pp.). [21] L. Hu, G. Chen, Nano Lett. 7 (2007) 3249–3252. [22] B.M. Kayes, H.A. Atwater, N.S. Lewis, J. Appl. Phys. 97 (2005) 114302. [23] Y. Zhang, Wang, A. Mascarenhas, Nano Lett. 7 (2007) 1264–1269. [24] Y.J. Zhang, W. Li, K.J. Chen, J. Alloys Compd. 450 (2008) 512–516. [25] Y.L. Kawamura, K. Fukami, T. Sakka, Y.H. Ogata, Electrochem. Commun. 10 (2008) 346–349. [26] X. Li, P.W. Bohn, Appl. Phys. Lett. 77 (2000) 2572–2574. [27] S. Chattopadhyay, X. Li, P.W. Bohn, J. Appl. Phys. 91 (2002) 6134–6140. [28] E.C. Garnett, P. Yang, J. Am. Chem. Soc. 130 (2008) 9224–9225. [29] Y.-M. Chang, J. Shieh, J.-Y. Juang, J. Phys. Chem. C 115 (2011) 8983–8987. [30] B. Tian, X. Zheng, T.J. Kempa, Y. Fang, N. Yu, G. Yu, J. Huang, C.M. Lieber, Nature 449 (2007) 885–889. [31] L. Tsakalakos, J. Balch, J. Fronheiser, B.A. Korevaar, O. Sulima, J. Rand, Appl. Phys. Lett. 91 (2007) 233117.
[32] J. Li, H. Yu, S.M. Wong, X. Li, G. Zhang, P.G.-Q. Lo, D.-L. Kwong, Appl. Phys. Lett. 95 (2009) 243113–243123. [33] X.C. Li, B.K. Tay, J. Nanosci. Nanotechnol. 11 (2011) 10539–10543. [34] V. Yerokhov, I. Melnyk, A. Tsisaruk, I. Semochko, Opto-electron. Rev. 8 (2000) 414–417. [35] B.S. Kim, D.H. Lee, S.H. Kim, G.H. An, K.J. Lee, N.V. Myung, Y.H. Choa, J. Am. Ceram. Soc. 92 (2009) 2415–2417. [36] K. Sahoo, M.-K. Lin, E.-Y. Chang, Y.-Y. Lu, C.-C. Chen, J.-H. Huang, C.-W. Chang, Nanoscale Res. Lett. 4 (2009) 680–683. [37] M. Cid, N. Stem, C. Brunetti, A.F. Beloto, C.A.S. Ramos, Surf. Coat. Technol. 106 (1998) 117–120. [38] P. Singh, S.N. Sharma, N.M. Ravindra, JOM 62 (2010) 15–24. [39] K.-Q. Peng, S.-T. Lee, Adv. Mater. 23 (2011) 198–215. [40] Y.F. Huang, S. Chattopadhyay, Y.J. Jen, C.Y. Peng, T.A. Liu, Y.K. Hsu, C.L. Pan, H.C. Lo, C.H. Hsu, Y.H. Chang, C.S. Lee, K.H. Chen, L.C. Chen, Nat. Nanotechnol. 2 (2007) 770–774. [41] J. Zhu, Z. Yu, G.F. Burkhard, C.-M. Hsu, S.T. Connor, Y. Xu, Q. Wang, M. McGehee, S. Fan, Y. Cui, Nano Lett. 9 (2008) 279–282. [42] S. Sivoththaman, W. Laureys, J. Nijs, R. Mertens, Appl. Phys. Lett. 71 (1997) 392–394. [43] J. Li, H. Yu, Y. Li, Nanoscale 3 (2011) 4888–4900.