Fan
Bai
ab,
Meicheng
Li
*ac,
Rui
Huang
a,
Yingfeng
Li
ac,
Mwenya
Trevor
a and
Kevin P.
Musselman
*d
aState Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China. E-mail: mcli@ncepu.edu.cn
bSchool of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
cSuzhou Institute, North China Electric Power University, Suzhou 215123, China
dDepartment of Physics, University of Cambridge, Cambridge CB3 0HE, UK. E-mail: kpdm2@cam.ac.uk
First published on 21st November 2013
A facile and low-cost one-step template-free approach is presented for the fabrication of tapered silicon nanowire (SiNW) arrays. A silver network catalyst is used to chemically etch silicon in a HF/H2O2 solution, where the solution is chosen to selectively dissolve the silver network during the etching process, resulting in the formation of tapered SiNWs. Notably, the filling ratio of the tapered SiNWs can be tuned simply by varying the pattern of the silver network. Surface reflection was strongly suppressed in the tapered SiNW arrays (only 400 nm in thickness), which were employed in SiNWs/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) heterojunction solar cells exhibiting a power conversion efficiency of 6.7%. The tapered SiNW arrays prepared by this one-step template-free method are expected to be efficient structures for a variety of photovoltaic devices.
A few methods for the fabrication of tapered SiNW arrays have been proposed to date. These include vapor–liquid–solid growth6–9 and dry etching4,10–14 techniques. However, these two vacuum-based approaches require expensive equipment, which limits their scalability and throughput. Recently a two-step wet etching processes to produce tapered SiNW arrays has been reported.15,16 SiNWs are formed by metal-assisted chemical etching, where silver nanoparticles applied from solution are used as the catalyst. The SiNWs are then immersed in a KOH solution to produce a tapered structure, where the tapering was reported to result from anisotropic silicon etch rates.15 Although the wet etching approach reduces the fabrication cost, the need for two etching processes introduces complexity. Furthermore, the control of the structural characteristics, particularly the filling ratio of the tapered SiNWs, is difficult to be realized using this process. Therefore, the development of a facile and low-cost fabrication approach for tapered SiNW arrays with tunable filling ratio is necessary.
In this work, a one-step template-free method is presented for the fabrication of tapered SiNW arrays with various filling ratios. A silver network catalyst is deposited onto the silicon surface using a simple sputter coater and tapered SiNW arrays are obtained through a one-step chemical etch in HF/H2O2 solution. By varying the sputtering conditions we control the initial surface coverage and hence the SiNW density. By careful selection of the etching solution we induce in situ oxidative dissolution of the silver network and the formation of tapered SiNWs. We demonstrate the ability to reduce surface reflection from silicon by introducing the tapered SiNW arrays and by controlling their density, and likewise demonstrate the ability to improve the performance of a SiNW/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) heterojunction solar cell by controlling these parameters.
In order to verify the performance of the tapered SiNWs, we fabricated prototype SiNW/PEDOT:PSS solar cells similar to those previously reported.18,19 A 150 nm thick silver layer was deposited by magnetron sputtering onto the backside of the Si substrate to form the rear contact. Highly conductive PEDOT:PSS solution was prepared by mixing commercial PEDOT:PSS (Clevios PH1000) and 5 wt% dimethyl sulfoxide (DMSO). A two thirds volume ratio of isopropyl alcohol was then added into the PEDOT:PSS solution to get good wettability. The PEDOT:PSS solution was spin coated onto the tapered SiNW arrays at 1000 rpm for 60 s and then annealed at 140 °C for 10 min on a hot plate in atmosphere to form SiNWs/PEDOT:PSS heterojunctions. Finally, a silver grid was deposited on the top of the PEDOT:PSS by magnetron sputtering with a shadow mask.
The morphological characteristics of the tapered SiNWs were observed by scanning electron microscopy (SEM) with a FEI Quanta 200F and by high resolution transmission electron microscopy (HRTEM) with a Tecnai G2 F20. The total reflection spectra were measured using a solar cell IPCE/QE measurement system with an integrating sphere. The current density versus voltage (J–V) characteristics of the solar cells were recorded using a Keithley 2400 sourcemeter under a solar simulator (XES-301S+EL-100 class AAA) at AM 1.5 condition.
The morphology of the tapered SiNW arrays in the present work is closely associated with that of the silver network catalyst. In the metal-assisted chemical etching process, silver particles serve as the catalysts to facilitate the generation of silicon oxide at the silver/silicon interface, then the silicon oxide is subsequently dissolved by the HF solution, leading to selective etching of the silicon.20 To get vertical arrays of isolated SiNWs, a high density of silver particles is therefore essential. High-density silver nanoparticle networks, such as those produced in this work, have a collective interaction21,22 that limits the movement of single particles into the bulk silicon in random downward directions. As a result, dense particles sink along the vertical direction, producing vertical array of SiNWs. If the standard reduction potential of oxidizing agents in the solution is greater than that of silver, silver particles can be oxidized to silver ions, causing a variation in the size and morphology of particles during their sinking process.23–25 In view of these points, we designed our silver catalyst and etching solution accordingly. The network-like configuration of silver enhances the collective interaction between catalysts, which promotes generation of vertical arrays of SiNWs. Meanwhile, the introduction of a suitable concentration H2O2 into the HF solution resulted in a gradual oxidation and shrinkage of the silver network during its sinking process. These synergetic effects resulted in the formation of tapered SiNW arrays.
To confirm the above analysis, morphological evolution of the silver network during etching was investigated. The morphology of the as-deposited silver network is shown in Fig. 2(a). After etching the silicon sample in HF/H2O2 solution for 10 s, the original network was split into isolated nanoislands of a similar size, as shown in Fig. 2(b). When the etching duration was increased to 30 s, SiNW arrays were observed on the silicon substrate (Fig. 2(c)). From the inset in Fig. 2(c), it can be clearly seen that these silver nanoislands sink into the bulk silicon. Moreover, the size of silver nanoislands slightly decreased in comparison to that in Fig. 2(b). These results clearly indicate that the tapered nanowire structure follows from the in situ oxidation of the silver catalyst and that the morphology of the tapered SiNWs is strongly dependant on the original pattern of the silver network.
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Fig. 2 Planar-view SEM images of the silver network catalyst on a silicon substrate for different etching durations. (a) 0 s; (b) 10 s; (c) 30 s. |
Based on the above analysis, control over the filling ratio of the tapered SiNWs was realized by varying the original pattern of the silver network. The network in Fig. 3(a) is characterized by small particles and a high density of pores. On the contrary, in Fig. 3(b) the particle clusters are large and the voids more sparse. These different silver network catalyst configurations were achieved by varying sputtering times and sputtering currents precisely. Following the one-step chemical etching of these samples, dense and sparse tapered SiNW arrays were obtained, and their morphologies are shown in Fig. 3(c) and (d), respectively. The filling ratio of the dense and sparse SiNWs is about 5.48 × 108 mm−2 and 1.57 × 108 mm−2, respectively. From the insets in Fig. 3(c) and (d), it can be seen that the dense and sparse SiNW arrays have an approximate thickness of 400 nm. It is worth emphasizing that the control of the filling ratio demonstrated here for the tapered SiNWs doesn't require conventional templates or complicated lithography, yet it permits tailoring of the optical properties of the SiNW for application in a variety of solar cells.
The total reflection spectra of the dense and sparse tapered SiNWs were measured and are shown in Fig. 4. Compared to the polished silicon wafer, sparse SiNWs with a length of 400 nm reduce the average reflectance from 39.6% to 10.5% in the wavelength range of 300–1000 nm. The reflection is further suppressed by increasing the filling ratio of the tapered SiNWs. The average reflectance of the dense SiNWs is about 1.6%, which is 6 times lower than that of the sparse SiNWs and similar to the lowest values previously reported for tapered SiNWs made by other techniques.12,14,16 The difference in antireflective properties between the dense and sparse SiNWs is attributed to different effective refractive index (RI) mismatches at the air/Si interface15 and to enhanced multiple scattering effects in the dense SiNWs.5
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Fig. 4 Comparison of total reflection spectra of silicon wafer, sparse SiNWs and dense SiNWs; inset images show the photos of sparse SiNWs and dense SiNWs. |
To demonstrate their applicability to solar cells, tapered SiNWs were used to construct Si/PEDOT:PSS heterojunctions, as shown schematically in Fig. 5(a). In these devices, light is absorbed by the tapered SiNWs, the photogenerated holes move toward the PEDOT:PSS, and the photogenerated electrons move along the SiNWs toward the back electrode.26 For comparison, a planar Si/PEDOT:PSS solar cells with similar structure was fabricated as well. Fig. 5(b) shows the typical current density–voltage characteristic of the tapered SiNWs/PEDOT:PSS heterojunction solar cells and the planar Si/PEDOT:PSS solar cells under 1 sun illumination (AM 1.5). The corresponding photovoltaic parameters including the short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE) are given in the table (Fig. 5(c)). The PCE of the planar Si/PEDOT:PSS solar cells is 0.53%. Similar processing was used for the planar Si/PEDOT:PSS and SiNW/PEDOT:PSS cells, and the low efficiency of the planar cell may be attributable to the large surface reflection loss on the planar silicon wafer. For the sparse SiNWs solar cells, the highest PCE of 6.72% is obtained. Notably, the Jsc value for the sparse SiNW solar cells (25.15 mA cm−2) is similar to that of previously reported SiNW/PEDOT:PSS cells employing nanowires fabricated by alternative techniques,18,26–28 and exceeds that of the dense SiNW sample (9.8 mA cm−2) by a factor of 2.56, despite the better anti-reflective properties of the dense tapered SiNWs. The increased Jsc is attributed to more effective filling of PEDOT:PSS into the sparse SiNWs versus the dense SiNWs, as was reported previously for short versus long nanowires.18,19,29 Moreover, the external quantum efficiency (EQE) of these SiNWs solar cells was measured using a solar cell IPCE/QE measurement system. As we can see from Fig. 5(d), the EQE value of the sparse SiNWs is higher than that of the dense SiNWs in the wavelength range of 300–1100 nm, implying that the photogenerated carrier recombination in the sparse SiNWs is relatively weaker compared to the dense SiNWs.
These results indicate that the ability to control the filling ratio of tapered SiNWs demonstrated here is required to achieve a balance in the reduced reflection, the polymer infiltration and the carrier recombination, and hence to optimize the photovoltaic performance of the solar cells. Further enhancement in the PCE of these devices may result from optimizing the structural parameters of the tapered SiNWs, improving the contact between the SiNWs and PEDOT:PSS, increasing the conductivity of PEDOT:PSS, and so on.
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