ARTICLE pubs.acs.org/JPCC
Efficient Light Harvesting by Well-Aligned In2O3 Nanopushpins as Antireflection Layer on Si Solar Cells Chun-Ying Huang,† Guan-Cheng Lin,§ Yeun-Jung Wu,§ Tai-Yuan Lin,*,§ Ying-Jay Yang,† and Yang-Fang Chen*,‡ †
Graduate Institute of Electronics Engineering, National Taiwan University, Taipei 10617, Taiwan Department of Physics, National Taiwan University, Taipei 10617, Taiwan § Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan ‡
ABSTRACT: Light harvesting from In2O3 nanopushpins as an antireflection layer has been investigated. We report on the growth of In2O3 nanopushpins on silicon solar cells by catalyst-free and improved two-zone chemical vapor deposition in order to provide a suitable growth environment with low temperature and high growth rate for practical applications. On the basis of spectral reflectance and cell responsivity measurements, we show that In2O3 nanopushpins provide efficient light trapping properties. A dramatic enhancement in short-circuit current (from 19.65 to 24.73 mA/cm2) with In2O3 nanopushpins than that of the bulk silicon wafer is observed. The solar cells incorporated with In2O3 nanopushpins therefore open an excellent alternative to serve as an antireflection layer for the enhancement of light harvesting and cell efficiency.
’ INTRODUCTION Although single and multicrystalline silicon (Si) solar cells have been the workhorse for this market, taking over 80% of total cell production, low efficiency and corresponding high costs still limit it to comprehensive success until now. One of the most important problems in Si solar cells is that approximately 35 40% of incoming light is reflected by the surface of solar cells.1 Over the past years, research into the use of subwavelength structures, such as Si nanowires (SiNWs) or Si nanotips, has demonstrated as promising candidates for antireflection (AR) coatings.2 6 A top-down process is usually implemented for fabricating these Si nanostructures. For example, wet-chemical etching techniques have been frequently used to prepare SiNWs, and Si nanotips were widely synthesized by reactive ion etching (RIE).2 6 However, after wet or dry etching, the surface of Si is destroyed and full of defects. The resulting surface recombination centers could further hinder Si solar cells from high efficiency. Besides, owing to the inherent nature of sharp structure at the end of Si nanotips, the tips may be destroyed during the thermal diffusion of phosphorus and thus the performance of antireflection is reduced.3,4 More recently, many approaches focus on bottom-up processes for next-generation AR coating, such as aligned ZnO nanorod arrays.7 10 Among various emerging methods of ZnO growth, the hydrothermal method is the most popular technique because of its low-temperature process and low cost. However, one major drawback of the reported approach is the slow growth rate. For instance, to produce barely ∼0.2 μm-long nanorods, it requires about three hours, which is not beneficial for mass production.8 Aside from long-term growth duration, complicated r 2011 American Chemical Society
fabrication processes, including solution spin-coating, seed layer baking (>1 h), substrate immersing, residual salt removal, and sample drying, are also impediments for the realization of practical applications.8,10 Indium oxide (In2O3), which is a wide band gap semiconductor of 3.6 eV with low resistivity and low free carrier absorption, has received considerable attention for its applications in many optoelectronic devices.11 13 It is worth noting that the reduction of absorption within the antireflection layer caused by free carriers decreases optical loss in the visible and near-infrared regions in solar cells.14 Till now, there have been a number of reports on the synthesis of In2O3 nanostructures by using various processes, such as carbothermal reduction, chemical vapor deposition (CVD), electrodeposion, laser ablation, sputtering, and thermal evaporation, etc.15 20 Considering the reduction of cost in mass production, CVD without a high vacuum system is a suitable method for solar cell applications. However, the synthesis of In2O3 nanostructures using CVD is usually under a high temperature condition (>900 °C),21,22 which may cause damage to the active layers of solar cells. For example, in silicon solar cells, the top finger contact will penetrate deep down to the p-n junction at a high firing temperature.1 In this study, a simple straightforward and economical technique was employed to fabricate well-aligned In2O3 nanopushpins serving as an AR coating layer. A two-zone CVD technique was used to operate the vapor generation, leading to the Received: February 21, 2011 Revised: June 1, 2011 Published: June 07, 2011 13083
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Figure 1. (a) Schematic diagram of a two-zone CVD system for In2O3 nanopushpins growth. (b) Schematic diagram of silicon solar cell structure with In2O3 nanopushpins.
Figure 2. Cross-sectional SEM image of as-grown In2O3 nanopushpins with growth time (a) 5 min, (b) 10 min, (c) 15 min. (d) EDX spectroscopy of In2O3 nanopushpins.
conditions of low temperature and high growth rate of In2O3 nanopushpins.23 On the basis of the reflection and external quantum efficiency (EQE) measurements, it is found that the characteristic of In2O3 nanopushpins offers a broad-band light harvesting from visible to near infared (IR). We have also demonstrated that Si solar cells incorporated with In2O3 nanopushpins as the AR coating show a large enhancement in the short-circuit current (Jsc) and power conversion efficiency, which is consistent with our expectation due to the effect of antireflection and light trapping. Our approach therefore opens a new route to enhance the solar cell efficiency with a low cost process for mass production.
two-zone furnace. To ensure that the probes directly touch the electrodes, before the synthetic process, a strip of Si wafer was placed on the bus electrode, at the probe point. Therefore, the position for contact is naked, and thus In2O3 nanopushpins on top of the electrodes do not affect the accuracy of measurement. The experimental setup for In2O3 nanopushpins growth is depicted schematically in Figure 1a. Five grams of high-purity indium (In) grains, serving as the starting materials, were placed in a quartz boat located inside zone I .Si solar cells were then placed downstream from the tube reactor separated from the starting materials by 6 in. The distance is vital to the shape of In2O3 and growth rate.23 Argon gas was introduced into the quartz tube from one end and flowed out from the other end. The temperature of zone I (starting materials) was set at 800 °C, and zone II was set at 400 °C. After the reaction, a layer of product was found deposited on Si solar cell. To study the effect of reaction time on the morphology of In2O3 nanopushpins and corresponding power conversion efficiency of solar cells, the synthesis was carried out for 5, 10, and 15 min, respectively. A schematic of our structure configuration is also shown in Figure 1b. It should be noted that, unlike previous report (the In2O3 nanopushpins were randomly lying parallel to the surface),24 our In2O3 nanopushpins vertically stand on the substrate, which is beneficial for antireflection.
’ EXPERIMENTAL SECTION To investigate the effect of In2O3 nanopushpins on Si-based solar cells, the solar cells were fabricated by a conventional solar cell process. The detailed fabrication process of Si solar cell was reported in our previous work.4 The single-crystalline Si wafers employed in this work were boron-doped (100) wafers with resistivities of 5 12 Ω cm. The thickness of wafers was 550 μm. The surfaces of the wafers were first cleaned in order to eliminate any impurities, based on a standard RCA cleaning. The emitter layer was formed by coating phosphorus on the front side and then thermal diffusion of phosphorus through rapid thermal process (RTP) at 850 °C for 4 min. Phosphosilicate glass (PSG) layers were removed by dipping them in BOE solution for 2 min. Immediately, rapid thermal oxidation (RTO) process under pure oxygen (150 sccm) at 700 °C for 1 min was used to form a SiO2 passivation layer. Ohmic contacts were made by depositing a Ti/Au (30 nm/300 nm) on the front side and then annealing at 500 °C for 3 min. It should be noted that, to prevent Al from being oxidized, metallization of back contact was carried out after each fabrication step, including In2O3 nanopushpins growth. In the present synthetic process, the In2O3 nanopushpins were fabricated in an improved CVD system, which contains a
’ RESULTS AND DISCUSSION To avoid failure of the device because of high temperature, the fabrication temperature is fixed at 400 °C, and only the growth time is changed. Parts a c of Figure 2 show the cross-sectional scanning electron microscopy (SEM) images of the vertically aligned In2O3 nanopushpins on Si substrates with growth time of 5, 10, and 15 min, respectively. Each In2O3 nanopushpin consists of a nanorod stem and a ball-like tip. As shown in Figure 2d, the energy dispersion X-ray (EDX) spectroscopy also shows the appearance of In and O peaks from nanorod stems as well as balllike tips. The diameter of each nanorod is between 100 and 13084
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Figure 3. XRD pattern of synthesized In2O3 nanopushpins on Si(100) substrate.
Figure 4. Reflectance measurements of In2O3 nanopushpins grown on solar cell devices with different growth times. The inset is a photograph of the fabricated device.
150 nm, and the lengths of the rods increase from ∼200 nm to ∼2000 nm as the growth time is increased. Each nanorod exhibits a ball-like structure and is well-aligned on Si substrate. Figure 3 shows an X-ray diffraction (XRD) pattern of In2O3 nanopushpins grown on Si substrate at 400 °C. Clearly, all the peaks correspond to the pure cubic phase of In2O3 with lattice parameter a = 10.11 Å (JCPDS card no. 06-0416), indicating that the pure phase of In2O3 was obtained and no other impurity peaks were detected. The reflectance spectrum was measured for nearly normal light incidence (5° offset) to determine the AR performance of In2O3 nanopushpins in the 350 950-nm range with a standard UV vis spectrometer (JASCO ARN-733) and an integrating sphere, as shown in Figure 4. In this measurement, the noise level is ∼0.002%. Different gratings were used for various light sources, depending on the measured regime (visible or near IR). Thus, due to the replacement of gratings, an unusual peak occurs near 850 nm. A flat solar cell with n+ emitter layer and surface passivation was used to serve as the reference sample. Apparently, In2O3 nanopushpins can significantly reduce reflection over the whole range of solar spectrum. A longer length of
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Figure 5. Current density voltage characteristics of silicon solar cell with In2O3 nanopushpins under AM1.5G illumination. Another solar cell without In2O3 nanopushpins is also shown for comparison.
stem can greatly enhance the AR effect. Since In2O3 is a wide bandgap material, the observed AR characteristics from visible to near IR can be correlated with the light trapping of structural properties instead of bandgap absorption. It is obvious that, for the growth duration of 15 min, the nanopushpins with the highest length have the lowest average reflection (∼10%), which is comparable to that of ZnO nanostructure.8 10 The inset of Figure 4 shows a photograph of a fabricated device, and a bare Si wafer is shown for comparison as well. We found that the longer inclined angles of the rods may increase the reflection as the growth time beyond 30 min. Therefore, the deposition of longer growth period was not considered in the current work. In Figure 5, the Si solar cells with In2O3 nanopushpins was characterized under the air mass 1.5 global (AM 1.5G) illumination condition and also compared to a standard one without any AR coating. The comparison highlights the fact that In2O3 nanopushpins are effective in increasing the Jsc from 19.65 to 24.73 mA/cm2 and power conversion efficiency from 6.77 to 8.58%. The improvement in conversion efficiency of the In2O3 nanopushpins is also supported by the external quantum efficiency (EQE) measurement, as shown in Figure 6a. Compared to the standard one, the EQE of Si solar cell with In2O3 nanopushpins exhibits an enhanced photoresponse from visible to near IR. This behavior can be used to support the fact that the enhancement in the Jsc of the Si solar cell with In2O3 nanopushpins compared with the standard one is due to the good antireflection property of nanopushpins. Interestingly, below a wavelength of ∼370 nm, the EQE of the Si solar cell with In2O3 nanopushpins is slightly smaller than that of the standard one. To interpret the above result, let us recall the fact that the bandgap of In2O3 is around 3.6 eV, corresponding to ∼350 nm in wavelength. Therefore, the slight deterioration of the EQE of Si solar cell with In2O3 nanopushpins around 370 nm is mainly caused by the In2O3 AR layer. However, in comparison to other wide bandgap materials as antireflection layers, such as ZnO (3.3 eV), In2O3 with a larger bandgap (3.6 eV) can offer a wider transparent window near the boundary of ultraviolet and visible region. If ZnO is synthesized by the hydrothermal method, it contains large amount of defects, and hence its band tailing states may hinder the blue light.24 In addition, it deserves to be mentioned that, a seed layer does not exist below the In2O3 nanopushpins. Hence, it is likely that UV light, not totally absorbed in the 13085
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Figure 6. (a) External quantum efficiency (EQE) spectra for silicon solar cell with and without In2O3 nanopushpins. Inset: Absorption of In2O3 films deposited on glass substrates. (b) EQE spectra for the different lengths of In2O3 nanopushpins in the UV region.
window layer, gets more chance to reach the front side of solar cell. In previous works with thick seed layer,8,25 it was found that the reflection and EQE dramatically decrease in the UV region, due to the bandgap absorption. By contrast, in our approach without a seed layer, only slight changes in EQE with different lengths of In2O3 nanopushpins were observed in the UV region, as shown in Figure 6b. It is worth noting that open-circuit voltage (Voc) exhibits a slight enhancement. This is because in principle the additional light trapping due to In2O3 nanopushpins on the top of cell can increase the generation of electron hole pairs and slightly change the Fermi energy distribution in the Si p-n junction. Another observation of the I V characteristics obtained from the slope around the Voc indicates that, after the growth of In2O3 nanopushpins on the front of solar cells, the fill factor was almost the same (∼61%). It implies that In2O3 nanopushpins on the top side of device will not affect the conductance of electrons in the n+ emitter layer and produce residual series resistance. It may be worth pointing out, in passing, that the difference in reflectivities between the bare Si surface and the In2O3 nanopushpins is roughly 25 30% in the visible wavelength range, but the difference in EQE is only about 10% in this range. Excluding the degradation of the contacts (due to the same FF), the growth of In2O3 nanopushpins may somehow affect the internal quantum efficiency (IQE). Especially, the parasitic absorption of In2O3 nanopushpins may lead to the difference between reflectivity and EQE. Let us us recall the fact that even if one used high-quality transparent conducting oxide (TCO), such as indium tin oxide (ITO) or AZO (ZnO:Al),26 28 the transmittance merely reached 80 90%. Thus, to estimate the magnitude of the parasitic absorption, the In2O3 films (the growth time is controlled for 15 min) were prepared on glass substrates under the same growth condition as that on the real device. The inset of Figure 6a shows the optical absorption of In2O3 films in the wavelength range from 250 900 nm. The average absorptance of the sample is ∼12% in the visible region, which means that although In2O3 is a wide bandgap material, the absorption of nanopushpins in the visible region can not be totally ignored. The parasitic absorption of In2O3 reduces surface Fresnel reflection and thus increases the difference in reflectivities between the Si surface and the In2O3 nanopushpins. Therefore, the main reason causing the difference between reflectivity and EQE can be attributed to the parasitic absorption of In2O3 nanopushpins.
’ CONCLUSIONS In conclusion, catalyst-free synthesis of well-aligned In2O3 nanopushpins was deposited on top of Si solar cell by a two-zone CVD. We have demonstrated that In2O3 nanopushpins can significantly reduce surface Fresnel reflection over the whole spectral range, revealed by reflection as well as EQE measurements. This fact reflects a large enhancement of Jsc and a slight increase in Voc, measured under AM1.5G illumination. Because of the low cost fabrication process and high growth rate, we therefore believe that In2O3 nanopushpins provide an excellent alternative to serve as the AR layer for the practical application in solar cells manufacture. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (Y.-F.C.);
[email protected] (T.-Y.L.).
’ ACKNOWLEDGMENT This work was supported by the National Science Council and the Ministry of Education of the Republic of China under Contract Nos. NSC-96-2628-M-002-011-MY3 and NSC-98-2112-M-019003-MY3. We thank Prof. Jr-Hau He for providing reflectance measurement technique. ’ REFERENCES (1) Nelson, J. The Physics of Solar cells; Imperial College Press: London, 2003. (2) Peng, K.; Wang, X.; Lee, S. T. Appl. Phys. Lett. 2008, 92, 163103. (3) Huang, C. Y.; Yang, Y. J.; Chen, J. Y.; Wang, C. H.; Chen, Y. F.; Hong, L. S.; Liu, C. S.; Wu, C. Y. Appl. Phys. Lett. 2010, 97, 013503. (4) Huang, C. Y.; Wang, D. Y.; Wang, C. H.; Chen, Y. T.; Wang, Y. T.; Jiang, Y. T.; Yang, Y. J.; Chen, C. C.; Chen, Y. F. ACS Nano 2010, 4, 5849. (5) Huang, C. Y.; Wang, D. Y.; Wang, C. H.; Wang, Y. T.; Jiang, Y. T.; Yang, Y. J.; Chen, C. C.; Chen, Y. F. J. Phy. D: Appl. Phys 2011, 44, 085103. (6) Kalita, G.; Adhikari, S.; Aryal, H. R.; Afre, R.; Soga, T.; Sharon, M.; Koichi, W.; Umeno, M. J. Phy. D: Appl. Phys. 2009, 42, 115104. (7) Lee, Y. J.; Ruby, D. S.; Peters, D. W.; McKenzie, B. B.; Hsu, J. W. P. Nano Lett. 2008, 8, 1501. (8) Chen, J. Y.; Sun, K. W. Sol. Energy Mater. Sol. Cells 2009, 94, 930. 13086
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