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In this letter, we report the antireflection and light absorption enhancement by forming sub-wavelength nano-patterned Si structures via nano-sphere lithography technique. It is found that the surface reflection can be significantly suppressed in a wide spectral range (400-1000 nm) and the weighted mean reflection is less than 5%. Meanwhile, the broad band optical absorption enhancement is achieved consequently. Heterojunction solar cells are prepared by depositing ultrathin amorphous Si film on the nano-patterned Si structures, the short circuit current density increases to 37.2 mA/cm2 and the power conversion efficiency is obviously improved compared to the reference cell on flat Si substrate.
©2011 Optical Society of America
1. Introduction
Si plays an important role in today’s solar cells due to its abundant in the earth’s crust and the mature fabrication process. Due to the high index (3.4) of crystalline Si (c-Si), the incident light is reflected at the front surface which induces the efficiency loss [1–4]. Therefore, antireflection techniques, such as wet-etching process and the antireflection coatings, have been used to reduce the reflection loss in order to improve the cell performance. However, the chemical wet etching techniques are difficult to apply for the next generation solar cells with ultrathin Si wafer (~100 μm or less) [5–7]. It is desired to develop new techniques to reduce surface reflection and enhance the absorption of the thin films.
So far, several approaches have been proposed to have an efficient light management, in which, semiconductor nanostructures have attracted much attention since they can increase the effective absorption path of the incident light [4,8–10]. For example, S. J. Jang et al. reported the antireflection behaviors of porous Si nano-columnar structures with graded refractive index layers and the 7.5% reflectivity was obtained [8]. The light-trapping of Si nano-wires were also studied and 96% peak absorption was achieved [9]. J. Zhu et al. fabricated amorphous Si nano-cones by using SiO2 nanospheres and reported the optical absorption enhancement effect due to the suppressed refection [10]. In our previous work, we used modified nano-sphere lithography technique to get sub-wavelength periodically patterned Si structures. The nano-sphere lithography technique is a low-cost, simple process and the shape and size of the pattern can be easily modulated by selecting the appropriate size of the sphere and etching conditions [11]. The depth-dependent antireflection has been reported and the enhanced photo- and electro-luminescence characteristics have been observed [12,13].
In this letter, we report the formation of nano-patterned Si structures by using polystyrene nanosphere lithography method. The reflection can be suppressed to less than 10% and the optical absorption exceeds 90% in a wide spectral range (400-1000 nm) for nano-patterned samples. Moreover, by depositing amorphous Si (a-Si) on the nano-patterned Si substrate to obtain a-Si/c-Si heterojunction solar cells, the short circuit current and power conversion efficiency are obviously increased compared to that of reference sample on flat Si wafer.
2. Experimental
The periodically patterned Si nanostructures were prepared on (100) p-type Si wafer with the resistance of 1.5-3 Ω·cm. A monolayer of polystyrene (PS) nanospheres with a diameter of 220 nm was coated on the Si wafer by self-assembly technique. This monolayer of PS nanospheres acted as a mask and the surface of Si wafer was etched in conventional reaction ion etching (RIE) system. During the etching process, the flow meter of CF4 gas is 30 sccm and the r.f. power is about 20 W. The etching depth can be controlled by the etching time as revealed in our previous work [12]. After RIE process, PS nanospheres were then removed in tetrahydrofuran (THF) solution. The surface morphology was characterized by atomic force microscopy and cross-sectional transmission electron microscopy [12,13]. The reflection and transmission spectra were measured by using Shimadzu UV-3600 spectrometer.
The test cell device was prepared by successively depositing ultrathin intrinsic and n-type amorphous Si (a-Si) film (total thickness ~20 nm) on the nano-patterned Si substrate in conventional plasma enhanced chemical vapor deposition (PECVD, 13.56 MHz) system. Subsequently, transparent indium tin oxide (ITO) was sputtered on the front surface and Al electrode was evaporated as back contact. The reference cell was simultaneously prepared on the flat Si wafer. Figure 1(a) shows the schematic diagram of the cross-sectional structure of nano-patterned solar cells. The cell area is about 0.28 cm2. Figure 1(b) is cross-sectional TEM image of etched Si substrate which shows the nano-cone shape. The lateral size of nano-cone is about 220 nm which is in well agreement with the diameter of PS nanosphere. The depth is about 148 nm after 12 min etching. The depth can be controlled by changing the etching time as revealed in our previous work [12]. Figure 1(c) is the SEM micrograph of nano-patterned sample after deposition of intrinsic and P-doped amorphous Si films. It is shown that the sample keeps the sub-wavelength periodic structures of the substrate after the film deposition due to the conformal deposition process in CVD system [14].
Fig. 1 (a) Schematic diagram of the cross-sectional structure of nano-patterned solar cells. (b) Cross-sectional TEM image of etched Si substrate. (c) SEM micrograph of nano-patterned sample after deposition of intrinsic and P-doped amorphous Si films.
3. Results and discussion
The antireflection effect of the nano-patterned substrates was characterized by the calibrated process over a broad wavelength range (400-1000 nm). It is found that the measured reflection spectra are almost the same whether using the integrating sphere or not, which indicates that the antireflection is angle-independent for our samples. Figure 2 shows the reflection spectra for the front surface of nano-patterned substrate after 12 min etching. The result for flat Si wafer is also presented for comparison. The polished surface of flat Si exhibits the high reflection (>40%) in the measurement range. The reflection is obviously suppressed for nano-patterned substrates as shown in Fig. 2. The reflection is reduced to less than 10% in the whole measurement range after 12 min etching and it is further reduced to 5% in the visible light region (400-800nm) after 16min etching [12]. The mean reflection can be calculated by weighting the reflection ratio of the substrate to the incident AM1.5 light in the 400-1000 nm spectral range. As shown in Fig. 3 , the weighted mean reflection is gradually reduced by increasing the etching time and it can be less than 5% after 12 min etching.
Fig. 2 Reflection spectra for the front surface of flat substrate and nano-patterned substrate after 12 min etching.
The absorption spectra of nano-patterned (12 min etching) and flat Si wafer are can be calculated based on the reflection and transmission measurements. As given in Fig. 4 , it is found that the absorption of nano-patterned Si substrate is significantly enhanced in a wide spectral range (400-1000 nm) which covers the most of the useful spectrum for Si-based solar cells. The absorption of nano-patterned sample is above 90% while that of the flat one is less than 60%. The mean absorption is also weighted with AM1.5 solar spectrum. It is shown that the weighted absorption is about 56% for flat Si substrate but increase to 92% for 12 min etched nano-patterned substrate.
Recently, several reports have been published on the suppression of light reflection by forming textured surface [7,15]. Our results suggest that the nano-patterned Si structures formed by the present approach exhibit the good antireflection and absorption enhancement characterization because it provides grading refraction index at the air/Si interface [12]. Meanwhile, the optical absorption can be enhanced by using sub-wavelength nano-structures. Since the feature size of surface pattern is smaller than wavelength, the incident electromagnetic wave can be coupled with the whole surface sub-wavelength structures which can trap the light to enhance the light harvesting in a wide spectral range [6,7,16]. Usually, the antireflection behaviors were achieved by introducing the Si nanostructures with high aspect-ratio. More recently, J. Li et al. discussed the light-trapping capability of nanostructures with low aspect ratio theoretically [7]. Their study indicates that the reflection can still be suppressed especially in the high energy region and light absorption can be significantly enhanced even with low aspect-ratio surface nano-texturing. In our case, the aspect-ratio is not so high according to the TEM observation which is about 0.67. However, the good antireflection and absorption enhancement properties reflect their possible application in the future solar cells without introducing a high level of surface defects and low carrier collection efficiency as in the high aspect-ratio structures [7,17].
The antireflection and optical absorption enhancement should improve the power conversion efficiency of solar cells. We fabricated the a-Si/c-Si heterojunction solar cells by successively depositing amorphous Si and n-type amorphous Si thin films on the nano-patterned Si substrate. The reference cell on flat Si substrate was also prepared under the same conditions for comparison. After film deposition, ITO and Al were evaporated on the front and back side as electrodes. The electrical properties of cells are characterized by current density (J) – voltage (V) measurements under a solar simulation with AM1.5 global spectrum (100 mW/cm2). The nano-patterned cell has the power conversion of 8.3%, which is significantly improved compared with the flat one. In order to further investigate the contribution of nano-patterned structures on the improvement of the cell performance, the external quantum efficiency (EQE) was measured for nano-patterned and flat cells and the results are given in Fig. 5 . It is shown that the EQE for nano-patterned cells is higher than that of flat one in almost the whole spectra range, especially in the visible light range. As a consequence, the short circuit current density Jsc, calculated based on EQE results, is obviously increased from 22.2 mA/cm2 for flat one to 32.1 mA/cm2 for nano-patterned one, which is close to the best result reported on the heterojunction solar cell by using p-type c-Si wafer [18,19]. The improvement of EQE and short-circuit current density clearly proves that more incident photons are absorbed by the Si materials and generated carriers are efficiently collected due to the nano-patterned structures.
Fig. 5 External quantum efficiency of heterojunction solar cells on flat and nano-patterned substrates, respectively.
It is worth pointing out that the cell fabrication conditions in the present work are not optimized, the cell performance, especially the open circuit voltage and fill factor, can be further improved by optimization of the deposition parameters of the i- and n-layers and the electrode contacts to reduce the series resistance and interface states.
4. Conclusion
In summary, the sub-wavelength nano-patterned Si structures are fabricated by using nano-sphere lithography technique which exhibits the good antireflection and optical absorption enhancement characteristics in a wide spectral range. The mean reflection can be reduced to less than 5% and the absorption can exceed 90%. The initial results of a-Si/c-Si heterojunction solar cell demonstrate that optical improvement by using nano-patterned Si substrate results in the significant increase of EQE and the short circuit current due to the enhanced light absorption and the current collection.
Acknowledgements
This work was supported by NSFC (Nos. 61036001, 10874070), “973” project (2007CB613401), NSF of Jiangsu Province (BK2010010) and the Fundamental Research Funds for the Central Universities (1112021001).
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