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Conversion efficiency enhanced photovoltaic device with nanohole arrays in antireflection coating layer
Optics Communications 284 (2011) 4773–4777
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Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Conversion efficiency enhanced photovoltaic device with nanohole arrays in antireflection coating layer Ning Wang, Yong Zhu ⁎, Wei Wei, JianJun Chen, Ping Li, Yumei Wen Key Laboratory for Optoelectronic Technology & System (Chongqing University), Education Ministry of China, Chongqing, China 400030
a r t i c l e
i n f o
Article history: Received 1 November 2010 Received in revised form 25 May 2011 Accepted 25 May 2011 Available online 12 June 2011 Keywords: Nanohole arrays Photovoltaic device Antireflection coating layer Conversion efficiency
a b s t r a c t In this paper, the authors introduce an enhanced photovoltaic device with nanohole arrays only in its antireflection coating. These nanoholes can improve light trapping efficiency as well as photoelectric conversion efficiency of the device. The authors analyze the light absorption of the devices with nanohole arrays by Finite-Difference Time Domain method and calculate the photoelectric conversion efficiency. The results show that the nanohole arrays can improve the light trapping more efficiently than the Si3N4 antireflection coating, especially, in 400–600 nm spectral range. Nanohole arrays with different characteristic parameters were fabricated in the antireflection coating layer of a Φ200 μm Si detector by using focused-ion beam system. With the optimized nanohole arrays, the enhancements factor of the experimental sample's photoelectric conversion efficiency is ~ 16% within the 400–600 nm spectral range and ~ 10% within the 400– 1100 nm spectral range. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Silicon is a good choice for photovoltaic applications due to its low cost, abundance in nature, nontoxicity, long-term stability and wellestablished technology. Currently, commercial solar cells have a 200– 300 μm crystalline silicon (c-Si) active layer, and which accounts for ~40% of the total cost [1]. As the cost of thick c-Si active layer is too high for large-size implementation, thin film solar cells with several microns active layer deposited on cheap substrate were developed and widely used in the past ten years. However, thin active layer has low light absorption and this leads to low photoelectric conversion efficiency (PCE, usually b10%). Therefore, light absorption enhancement became a most concerned topic in the research field of photovoltaic cells. Nanostructure enhanced antireflection was firstly reported by Walheim et al. [2]. Surface plasmonic structures enhancement was studied by Stupca et al. [3] and Pillai et al. [4], silicon and argentum nanoparticles were coated on photovoltaic devices to enhance the light absorption. Song et al. [5] and Bermel et al. [6] fabricated periodic nanostructures on the surface or bottom c-Si substrate to improve the light trapping. Recently, nanorods [7] and nanowires [8,9] were analyzed and etched in the active layer of photovoltaic device to improve light absorption. Beside these nanostructures, nanohole arrays is an alternative choice, as nanoholes inside the active layer usually show better light absorption enhancement than nanorods do [10]. However, the improvement of light absorption does not lead to photoelectric conversion efficiency
⁎ Corresponding author. Tel./fax: + 86 23 65111019. E-mail address: [email protected] (Y. Zhu). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.05.063
enhancement consequently. Especially in the case of thin film solar devices, nanostructures in the active layer will change the recombination velocity and bulk lifetime of the layer, and somehow transform the nanoporous layer to a thin “dead layer”. As a result, the internal quantum efficiency falls significantly within short wavelength range (400– 600 nm) [11]. In addition, the technique to produce nanohole arrays in the active layer is not fully compatible with current photovoltaic device fabrication processes [12,13]. These two disadvantages restrict the practical implementation of nanostructures in the PV industry. To address these problems, in this paper, we propose an improved method to fabricate nanohole arrays only in the Si3N4 antireflection coating (AR) layer. The depths of nanoholes are equal to the thickness of the antireflection coating layer. Theoretical analyses as well as experimental results demonstrate its validity. This method could be implemented by simply etching nanoholes in the antireflection coating film, which does not change characteristics of the active silicon layer. It is comparatively simple and compatible with current photovoltaic device fabrication processes, which makes it a promising choice to improve the PCE of the cells in practical PV industry. 2. A quantitative analysis of photovoltaic device with nanohole arrays 2.1. Working principle Fig. 1 shows the structure profile of the proposed photovoltaic device. Similar to common photovoltaic devices, it contains a c-Si substrate, a gold (Au) metallic reflector on the rear side of the substrate and an antireflection layer on top of the c-Si substrate. The
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the absorption of the Si3N4 film and the Au layers are neglected. In the wavelength range above 500 nm, ripple patterns occur owing to the interference of light reflected at the top surface and the c-Si/Au interface, which is similar to the Fabry–Perot etalon. As expected, the bare c-Si device shows the worst absorption performance among these three surface structures. As shown in Fig. 2, in the range of 400– 1100 nm, the absorption of the device with surface nanoholes is better than that with the unpatterned AR coating, especially in the 400– 600 nm range, the absorption is improved significantly. In order to evaluate the enhancement of the PCE (η) due to the patterning of the AR layer, calculations are performed thanks to the following equations [15]: Jsc Voc ηf Pin
ð1Þ
Jsc = eNph ηc
ð2Þ
Voc = ðkT=eÞlnðJsc = Js0 + 1Þ
ð3Þ
η= Fig. 1. Structure profile of the photovoltaic device with surface nanohole array.
only difference is that a nanohole arrays structured Si3N4 antireflection film is adopted instead of a common unpatterned Si3N4 coating. To get the minimum reflection, two conditions must be met: 1) the light amplitudes reflected at both interfaces must be equal; that is, n0/ pffiffiffiffiffiffiffiffiffiffi nf = nf/ns or nf = nons , with n0, nf, and ns being the refractive indices of air, film, and c-Si substrate respectively; and 2) the optical path length must be chosen for the reflected wave to interfere destructively; that is, the film thickness must be 1/4 of a reference wavelength in the optical medium. Although condition 2) can be easily met, condition 1) poses a problem: the refractive index of Si3N4 film does not match the equation n0/nf = nf/ns. In this paper, the Si3N4 AR coating is patterned by nanohole arrays, and the nanohole arrays can change the effective refractive index of Si3N4 film, which makes the Si3N4 film's refractive index match the refractive indices for c-Si substrate and air and leads to a significant decrease of the reflectivity, therefore, improves the light trapping efficiency [2]. A quantitative analysis is carried out to obtain the PCE enhancement of the device with nanohole arrays. The Finite-Difference Time-Domain (FDTD) Method (Lumercial FDTD Solutions) [14] is used to numerically solve dielectric functions [15] of the proposed photovoltaic device and to calculate the absorption spectrum. Here, we suppose that the thickness h of the c-Si substrate is 2 μm, the thickness t of the Si3N4 film is 90 nm, the lattice constant a of the nanohole arrays is 500 nm, the filling fraction in air f, defined as f = πd 2/4a 2 where d is the diameter of the nanoholes [16], is 0.4 and the depth of the nanoholes is 90 nm. The parameters considered correspond to an optimized configuration, as will be seen further in Section 2.2. For comparison, we also calculate the absorption spectrum of c-Si PV device with and without the 90 nm thick unpatterned antireflection coating. Fig. 2 shows the calculated absorption spectrum of the c-Si photovoltaic device with various surface structures, in the calculation,
Fig. 2. Calculated absorption spectrum of silicon photovoltaic device with different surface structures.
, where Jsc is the short-circuit current density, Voc is the open-circuit voltage, ηf is the filling factor of the photovoltaic device and Pin is the total incident power under the AM1.5 solar spectrum. The constant e is the electron charge and k is the Boltzmann's constant. Nph is the total number of absorbed photons per unit area per unit time, which can be calculated by the overlap integral of the absorption spectrum of the structure and the solar spectrum. Js0 is the reverse bias saturation current and ηc is a phenomenological parameter representing carrier collection efficiency mainly affected by surface recombination and solar cell material quality. Typical values of these parameters were adopted in our calculation, i.e., Pin = 0.1 W/cm 2, ηf = 0.8, ηc = 0.85, Js0 = 1.5 × 10 −15 A/cm 2, and T = 300 K. According to Eqs. (1)–(3), the calculated PCE of the photovoltaic cell are 8.48% (without any antireflection coating), 10.71% (with Si3N4 coating), and 12.71% (with nanohole coating). It is thus clear that devices with surface nanohole arrays structures can reach higher photoelectric conversion efficiency than the ones with an unpatterned AR coating. 2.2. Optimization of the parameters of the nanohole arrays As the effective refractive index can be adjusted by changing the parameters of nanohole arrays [2], it is important to optimize those parameters to get the maximum efficiency. Our optimization is focused on two key parameters, the lattice constant a and the filling fraction f. When the lattice constant and the filling fraction are changed, the PCE is different as shown in Fig. 3a. For different lattice, the efficiency rises when the filling fraction increases from 0 to 0.4 then drops quickly when the filling fraction exceeds 0.4. For different filling fraction, the maximum efficiency occurs when the lattice constant is 500 nm. Therefore the optimized filling fraction is 0.4 and the optimized lattice constant is 500 nm. Devices with 1 μm, 2 μm and 3 μm thick c-Si active layers are considered here. We assume the filling fraction is 0.4 [10], and calculate the PCE for different lattice constants (i.e. 200, 300, 400, 500, 600, and 700 nm). Besides, the efficiency of devices with bare c-Si substrate and c-Si substrate with 90 nm Si3N4 coating are also listed as references. As shown in Fig. 3b, the optimum efficiency for 1 μm, 2 μm and 3 μm devices with nanohole arrays are 10.87%, 12.71% and 15.01% respectively. All of them are higher than bare c-Si substrate and c-Si substrate plus Si3N4 coating with the same c-Si layer thickness. Furthermore, for the different c-Si thicknesses h, the maximum efficiency always occurs at the same lattice constant of 500 nm. For the optimum nanostructure parameters (a = 500 nm and f = 0.4), we calculate the thickness dependence of the PCE for photovoltaic devices. As shown in Fig. 3c, the device with surface nanohole arrays shows higher efficiency than that with Si3N4 AR
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device with nanohole arrays, ηSi3N4 is the PCE of the device with unpatterned Si3N4 film), when the thickness of the c-Si active layer is 3 μm and the same for both devices. The influence of the incident angle of light on the efficiency of a photovoltaic device is crucial for the device performance. Fig. 4 shows the cell efficiency of the 2 μm-thick c-Si photovoltaic device with three different surface structures as a function of the incident angle. The cell efficiency of c-Si photovoltaic device with a flat surface drops rapidly as the incident angle increases due to the increased reflection loss as shown in Fig. 4. In the device with nanohole arrays, however, the PCE is sustained at an incident angle in excess of 50°. More interestingly; the device exhibits a better performance at a slightly tilted incident angle compared with the normal incidence angle. 3. Experiments results and discussion Nanohole arrays in Si3N4 AR coating can be realized by lithography and etching. Several lithography technologies can be used for the nanoscale fabrication such as nano-imprint [17], extreme ultraviolet lithography (EUVL) [18] and interferometric lithography [19]. In addition the electron beam lithography (EBL) and the focused ion beam (FIB) system can also be used for the nanoscale fabrication. For large scale fabrication in the future, the nano-imprint lithography is a good candidate because of its low cost, high throughput, and uniformity. Meanwhile, to get quick comparative experimental results, a Φ200 μm Si detector that has the same I–V curve and photoelectric characteristics as common solar cells was selected as the experimental sample. Its c-Si active layer is 3 μm thick. The nanohole arrays were fabricated directly in the Si3N4 AR coating of sample by using a focusedion beam system (strata FIB 201, FEI Company, 30 keV Ga ions). The depths of the nanoholes were carefully controlled to make sure they are equal to the thickness of the sample's antireflection film (~90 nm). To get comparison of nanohole arrays with different lattice constants, we fabricated three types of nanohole arrays with different lattice constants in the AR coating of one sample. Fig. 5 gives SEM images of those three nanohole arrays, their design parameters are a = 400 nm, f = 0.4, area= 14 μm × 14 μm, a = 500 nm, f = 0.4, area = 40 μm × 40 μm and a = 600 nm, f = 0.4, area = 14 μm × 14 μm. Because of the fabrication error, the real fabrication parameters are a = 412 nm, d = 286 nm, a = 514 nm, d = 370 nm, a = 610 nm, d = 428 nm. Two experimental conditions should be carefully satisfied. Firstly, the spot size of the incident light should be smaller than the area of the nanohole arrays. Secondly, the spot size should remain unchanged while the light is moving from one nanohole arrays to another. Because the areas of these nanohole arrays are quite small, we built a specific set up to measure the characteristics of the device as shown in Fig. 6b. The detector sample with surface nanohole arrays is fixed on the microscope (Keyence VHX-600) workbench. A single mode fiber probe with core diameter 8.3 μm, which transfers the light from the
Fig. 3. Calculated PCE as a function of the lattice constant, filling fraction and the cSilicon of thickness. (a) PCE vs. lattice constant and filling fraction (b) PCE vs. lattice constant (f = 0.4) (c) PCE vs. thickness (f = 0.4, a = 500 nm).
coating and without any coating, whatever the thickness of the active layer is. Based on the calculation, to manufacture a photovoltaic device with the same efficiency, the thickness of the c-Si of the device with the nanohole arrays is 20%–30% less than that with Si3N4 AR coating, which indicates about 30% reduction of c-Si consumption cost. On the other hand, the PCE of device with surface nanohole arrays has 18.9% enhancement compared with device coated with Si3N4 (ηe = (ηnanohole − ηSi3N4) / ηSi3N4, where ηe is the PCE enhancement of the device with nanohole arrays, ηnanohole is the PCE of the
Fig. 4. PCE vs. incident angle.
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Fig. 6. Schematic diagram of the experiment (a) photo of the experimental sample (b) experiment set up.
Fig. 5. SEM images of the surface nanohole arrays (a) a = 400 nm, f = 0.4 (b) a = 500 nm, f = 0.4 (c) a = 600 nm, f = 0.4.
source to the surface of the detector (Fig. 6a), is fixed on a microdisplacement platform (Melles Griot nanoMax-HS 17MAX600/R). As the numerical aperture of the single mode fiber is 0.13, to get a spot smaller than 14 μm, the gap between surface of the fiber probe and the detector sample should be less than 53 μm. Firstly, the microdisplacement platform was finely adjusted in vertical direction, and the gap between was fixed to 50 μm. Subsequently, the platform was moved horizontally to make sure the light spot locates in the middle of one nanohole array. After testing one nanohole array, the platform was moved horizontally again to let the light spot illuminate another array. During the horizontal movement, the vertical position of the fiber probe should remain unchanged. Therefore the light spot size
and intensity were all kept constant. The three nanohole arrays were tested one by one. For comparison, the performance of the device without nanohole arrays was also measured. The photoelectric characteristics of the sample device were measured by a high performance digital multimeter (Keithley 2002 Multimeter). According to Eq. (1) and Section 2.2, to get the PCE enhancement of the device with nanohole arrays, we analyze the performances of the three types nanohole arrays by monitoring their open-circuit voltage Voc and short-circuit current Isc.(ηe = (VocnIscn/VocsIscs)-1, where Vocn, Vocs are the open-circuit voltage of the device with nanohole arrays and with unpatterned Si3N4 film respectively, and Iscn, Iscs are the short-circuit current of the device with nanohole arrays and with unpatterned Si3N4 film respectively). Fig. 7a and b give the measurement results of nanohole arrays. As references, the results of common Si3N4 (AR) without any nanoholes are also listed at the left top corner of the figure. The sample device was excited by lasers at 488 nm (275 mW/cm2, Coherent Sapphire 488–200) and 532 nm (87.5 mW/cm2, Melles Griot 85-GCB-020) to measure the power enhancement in the wavelength range 400–600 nm. We also measured the response between 400 nm and 1100 nm by using solar simulator (PET Solar Simulator SS100) whose spectral energy distribution is similar to the AM1.5 solar spectrum. In both figures, the maximum enhancement occurs when lattice constant is 500 nm, which agree with our simulation result. The enhancement of Isc excited by 532 nm laser is 15.67%, and excited by
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value, as shown in Fig. 4. 2) Because of the fabrication processes, the depth of the nanohole is not fabricated accurately and may destroy the c-Si active layer, which may lead to the decrease of the PCE. 4. Conclusions In this paper, the photoelectric conversion efficiency enhancement of the photovoltaic device with nanohole arrays only in its antireflection coating was analyzed. An optimum nanohole arrays with lattice constant of 500 nm and filling fraction of 0.4 are calculated. To get quick comparative experimental results, a Φ200 μm Si detector was selected as an experimental sample. Three types of nanohole arrays, which have different lattice constants, were fabricated on that sample by the focused-ion beam method. Experimental results showed that the PCE of the sample is enhanced by ~ 16% in the 400– 600 nm range and by ~ 10% in the 400–1100 nm range. As predicted by the simulation, the nanohole arrays show better performance in short wavelength range (400–600 nm). Although the PCE enhancement did not reach the simulation value, experiments demonstrated the feasibility of our method. As only the Si3N4 antireflection coating is patterned with nanoscale structure, the side effect of dead layer induced by nanoporous in the active layer is avoided. In addition, the fabrication could be implemented through lithography and etching, and which is compatible with current photovoltaic device fabrication process. Therefore, this method is a promising choice in practical photovoltaic device industry. Acknowledgments Authors would like to thank the financial support of this work via National Natural Science Foundation of China (No. 50830202 and No. 60707010).
Fig. 7. Enhancement in open-circuit voltage and short circuit current for different nanohole arrays structure excited by different light source: (a) Voltage response (b) current response.
488 nm laser is about 13.53%. On the other hand, the increases of Voc are only 2.61% (532 nm) and 2.48% (488 nm). So the enhancement factor of the photoelectric conversion is ~ 16%. It can be seen that the enhancement was higher in the 400–600 nm spectral range. When excited by the solar simulator, the enhancement of Voc is 2.20% and that of Isc is 7.68%, which result in an efficiency enhancement factor of ~ 10% in the wavelength range of 400–1100 nm. It's clear that the improvement is dominated by current rather than voltage. Insensitive to the incident angle is another important optical property of the devices with nanohole arrays. To verify that, the device with nanohole arrays sample is fixed on the microscope bench and the micro-displacement platform is tilted ±10° off the vertical incidence position during the measurement as shown in Fig. 5b. It was found out that the PCE was almost the same as the one with a vertical incidence, which is better than the device without any coating. Contrast to the simulation results, it can be seen that the measured enhancement is smaller than the simulation result. There are several reasons leading to the differences: 1) the lattice constant of nanohole arrays of the fabricated sample is not consistent with the simulation exactly, and the diameter of the nanohole is larger than the simulation
References [1] K.R. Catchpole, A. Polman, Opt. Express 16 (2008) 21793. [2] Stefan Walheim, Erik Schäffer, Jürgen Mlynek, Ullrich Steiner, Science 283 (1999) 520. [3] M. Stupca, M. Alsalhi, T. Al Saud, A. Almuhanna, M.H. Nayfeh, Appl. Phys. Lett. 91 (2007) 063107. [4] S. Pillai, K.R. Catchpole, T. Trupke, M.A. Green, J. Appl. Phys. 101 (2007) 093105. [5] Song Young Min, Yu Jea Su, Lee Yong Tak, Opt. Lett. 35 (2010) 276. [6] P. Bermel, C. Luo, L. Zeng, L.C. Kimerling, J.D. Joannopoulos, Opt. Express 15 (2007) 16986. [7] L. Hu, G. Chen, Nano Lett. 7 (2007) 3249. [8] V. Sivakov, G. Andrä, A. Gawlik, A. Berger, J. Plentz, F. Falk, S.H. Christiansen, Nano Lett. 9 (2009) 1544. [9] L. Tsakalakos, J. Balch, J. Fronheiser, B.A. Korevaar, Appl. Phys. Lett. 91 (2007) 233117. [10] Han Sanġ Eon, G. Chen, Nano Lett. 10 (2010) 1012. [11] Hao-Chih Yuan, Vernon E. Yost, Matthew R. Page, Paul Stradins, Daniel L. Meier, Howard M. Branz, Appl. Phys. Lett. 95 (2009) 123501. [12] M.H. Klühr, A. Sauermann, C.A. Elsner, K.H. Thein, S.K. Dertinger, Adv. Mater. 18 (2006) 3135. [13] R.B. Wehrspohn, S.L. Schweizer, V. Sandoghdar, Phys. Status Solidi A. 204 (2007) 3708. [14] M. Qiu, S. He, J. Appl. Phys. 87 (2000) 8268. [15] E.D. Palik, Handbook of optical constants of solids, 3rd ed, Academic, Orlando, FL, 1985. [16] C. Lin, M.L. Povinelli, Opt. Express 17 (2009) 19371. [17] S.Y. Chou, P.R. Krauss, P.J. Renstrom, Science 272 (1996) 85. [18] V. Auzelyte, H. Sigg, B. Schmitt, H.H. Solak, Nanotechnology 21 (2010) 215302. [19] S.S. Sankha, H.S. Harun, R. Jörg, D. Christian, Microelectronic Engineering 87 (2010) 854.
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Conversion efficiency enhanced photovoltaic device with nanohole arrays in antireflection coating layer Ning Wang, Yong Zhu ⁎, Wei Wei, JianJun Chen, Ping Li, Yumei Wen Key Laboratory for Optoelectronic Technology & System (Chongqing University), Education Ministry of China, Chongqing, China 400030
a r t i c l e
i n f o
Article history: Received 1 November 2010 Received in revised form 25 May 2011 Accepted 25 May 2011 Available online 12 June 2011 Keywords: Nanohole arrays Photovoltaic device Antireflection coating layer Conversion efficiency
a b s t r a c t In this paper, the authors introduce an enhanced photovoltaic device with nanohole arrays only in its antireflection coating. These nanoholes can improve light trapping efficiency as well as photoelectric conversion efficiency of the device. The authors analyze the light absorption of the devices with nanohole arrays by Finite-Difference Time Domain method and calculate the photoelectric conversion efficiency. The results show that the nanohole arrays can improve the light trapping more efficiently than the Si3N4 antireflection coating, especially, in 400–600 nm spectral range. Nanohole arrays with different characteristic parameters were fabricated in the antireflection coating layer of a Φ200 μm Si detector by using focused-ion beam system. With the optimized nanohole arrays, the enhancements factor of the experimental sample's photoelectric conversion efficiency is ~ 16% within the 400–600 nm spectral range and ~ 10% within the 400– 1100 nm spectral range. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Silicon is a good choice for photovoltaic applications due to its low cost, abundance in nature, nontoxicity, long-term stability and wellestablished technology. Currently, commercial solar cells have a 200– 300 μm crystalline silicon (c-Si) active layer, and which accounts for ~40% of the total cost [1]. As the cost of thick c-Si active layer is too high for large-size implementation, thin film solar cells with several microns active layer deposited on cheap substrate were developed and widely used in the past ten years. However, thin active layer has low light absorption and this leads to low photoelectric conversion efficiency (PCE, usually b10%). Therefore, light absorption enhancement became a most concerned topic in the research field of photovoltaic cells. Nanostructure enhanced antireflection was firstly reported by Walheim et al. [2]. Surface plasmonic structures enhancement was studied by Stupca et al. [3] and Pillai et al. [4], silicon and argentum nanoparticles were coated on photovoltaic devices to enhance the light absorption. Song et al. [5] and Bermel et al. [6] fabricated periodic nanostructures on the surface or bottom c-Si substrate to improve the light trapping. Recently, nanorods [7] and nanowires [8,9] were analyzed and etched in the active layer of photovoltaic device to improve light absorption. Beside these nanostructures, nanohole arrays is an alternative choice, as nanoholes inside the active layer usually show better light absorption enhancement than nanorods do [10]. However, the improvement of light absorption does not lead to photoelectric conversion efficiency
⁎ Corresponding author. Tel./fax: + 86 23 65111019. E-mail address: [email protected] (Y. Zhu). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.05.063
enhancement consequently. Especially in the case of thin film solar devices, nanostructures in the active layer will change the recombination velocity and bulk lifetime of the layer, and somehow transform the nanoporous layer to a thin “dead layer”. As a result, the internal quantum efficiency falls significantly within short wavelength range (400– 600 nm) [11]. In addition, the technique to produce nanohole arrays in the active layer is not fully compatible with current photovoltaic device fabrication processes [12,13]. These two disadvantages restrict the practical implementation of nanostructures in the PV industry. To address these problems, in this paper, we propose an improved method to fabricate nanohole arrays only in the Si3N4 antireflection coating (AR) layer. The depths of nanoholes are equal to the thickness of the antireflection coating layer. Theoretical analyses as well as experimental results demonstrate its validity. This method could be implemented by simply etching nanoholes in the antireflection coating film, which does not change characteristics of the active silicon layer. It is comparatively simple and compatible with current photovoltaic device fabrication processes, which makes it a promising choice to improve the PCE of the cells in practical PV industry. 2. A quantitative analysis of photovoltaic device with nanohole arrays 2.1. Working principle Fig. 1 shows the structure profile of the proposed photovoltaic device. Similar to common photovoltaic devices, it contains a c-Si substrate, a gold (Au) metallic reflector on the rear side of the substrate and an antireflection layer on top of the c-Si substrate. The
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the absorption of the Si3N4 film and the Au layers are neglected. In the wavelength range above 500 nm, ripple patterns occur owing to the interference of light reflected at the top surface and the c-Si/Au interface, which is similar to the Fabry–Perot etalon. As expected, the bare c-Si device shows the worst absorption performance among these three surface structures. As shown in Fig. 2, in the range of 400– 1100 nm, the absorption of the device with surface nanoholes is better than that with the unpatterned AR coating, especially in the 400– 600 nm range, the absorption is improved significantly. In order to evaluate the enhancement of the PCE (η) due to the patterning of the AR layer, calculations are performed thanks to the following equations [15]: Jsc Voc ηf Pin
ð1Þ
Jsc = eNph ηc
ð2Þ
Voc = ðkT=eÞlnðJsc = Js0 + 1Þ
ð3Þ
η= Fig. 1. Structure profile of the photovoltaic device with surface nanohole array.
only difference is that a nanohole arrays structured Si3N4 antireflection film is adopted instead of a common unpatterned Si3N4 coating. To get the minimum reflection, two conditions must be met: 1) the light amplitudes reflected at both interfaces must be equal; that is, n0/ pffiffiffiffiffiffiffiffiffiffi nf = nf/ns or nf = nons , with n0, nf, and ns being the refractive indices of air, film, and c-Si substrate respectively; and 2) the optical path length must be chosen for the reflected wave to interfere destructively; that is, the film thickness must be 1/4 of a reference wavelength in the optical medium. Although condition 2) can be easily met, condition 1) poses a problem: the refractive index of Si3N4 film does not match the equation n0/nf = nf/ns. In this paper, the Si3N4 AR coating is patterned by nanohole arrays, and the nanohole arrays can change the effective refractive index of Si3N4 film, which makes the Si3N4 film's refractive index match the refractive indices for c-Si substrate and air and leads to a significant decrease of the reflectivity, therefore, improves the light trapping efficiency [2]. A quantitative analysis is carried out to obtain the PCE enhancement of the device with nanohole arrays. The Finite-Difference Time-Domain (FDTD) Method (Lumercial FDTD Solutions) [14] is used to numerically solve dielectric functions [15] of the proposed photovoltaic device and to calculate the absorption spectrum. Here, we suppose that the thickness h of the c-Si substrate is 2 μm, the thickness t of the Si3N4 film is 90 nm, the lattice constant a of the nanohole arrays is 500 nm, the filling fraction in air f, defined as f = πd 2/4a 2 where d is the diameter of the nanoholes [16], is 0.4 and the depth of the nanoholes is 90 nm. The parameters considered correspond to an optimized configuration, as will be seen further in Section 2.2. For comparison, we also calculate the absorption spectrum of c-Si PV device with and without the 90 nm thick unpatterned antireflection coating. Fig. 2 shows the calculated absorption spectrum of the c-Si photovoltaic device with various surface structures, in the calculation,
Fig. 2. Calculated absorption spectrum of silicon photovoltaic device with different surface structures.
, where Jsc is the short-circuit current density, Voc is the open-circuit voltage, ηf is the filling factor of the photovoltaic device and Pin is the total incident power under the AM1.5 solar spectrum. The constant e is the electron charge and k is the Boltzmann's constant. Nph is the total number of absorbed photons per unit area per unit time, which can be calculated by the overlap integral of the absorption spectrum of the structure and the solar spectrum. Js0 is the reverse bias saturation current and ηc is a phenomenological parameter representing carrier collection efficiency mainly affected by surface recombination and solar cell material quality. Typical values of these parameters were adopted in our calculation, i.e., Pin = 0.1 W/cm 2, ηf = 0.8, ηc = 0.85, Js0 = 1.5 × 10 −15 A/cm 2, and T = 300 K. According to Eqs. (1)–(3), the calculated PCE of the photovoltaic cell are 8.48% (without any antireflection coating), 10.71% (with Si3N4 coating), and 12.71% (with nanohole coating). It is thus clear that devices with surface nanohole arrays structures can reach higher photoelectric conversion efficiency than the ones with an unpatterned AR coating. 2.2. Optimization of the parameters of the nanohole arrays As the effective refractive index can be adjusted by changing the parameters of nanohole arrays [2], it is important to optimize those parameters to get the maximum efficiency. Our optimization is focused on two key parameters, the lattice constant a and the filling fraction f. When the lattice constant and the filling fraction are changed, the PCE is different as shown in Fig. 3a. For different lattice, the efficiency rises when the filling fraction increases from 0 to 0.4 then drops quickly when the filling fraction exceeds 0.4. For different filling fraction, the maximum efficiency occurs when the lattice constant is 500 nm. Therefore the optimized filling fraction is 0.4 and the optimized lattice constant is 500 nm. Devices with 1 μm, 2 μm and 3 μm thick c-Si active layers are considered here. We assume the filling fraction is 0.4 [10], and calculate the PCE for different lattice constants (i.e. 200, 300, 400, 500, 600, and 700 nm). Besides, the efficiency of devices with bare c-Si substrate and c-Si substrate with 90 nm Si3N4 coating are also listed as references. As shown in Fig. 3b, the optimum efficiency for 1 μm, 2 μm and 3 μm devices with nanohole arrays are 10.87%, 12.71% and 15.01% respectively. All of them are higher than bare c-Si substrate and c-Si substrate plus Si3N4 coating with the same c-Si layer thickness. Furthermore, for the different c-Si thicknesses h, the maximum efficiency always occurs at the same lattice constant of 500 nm. For the optimum nanostructure parameters (a = 500 nm and f = 0.4), we calculate the thickness dependence of the PCE for photovoltaic devices. As shown in Fig. 3c, the device with surface nanohole arrays shows higher efficiency than that with Si3N4 AR
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device with nanohole arrays, ηSi3N4 is the PCE of the device with unpatterned Si3N4 film), when the thickness of the c-Si active layer is 3 μm and the same for both devices. The influence of the incident angle of light on the efficiency of a photovoltaic device is crucial for the device performance. Fig. 4 shows the cell efficiency of the 2 μm-thick c-Si photovoltaic device with three different surface structures as a function of the incident angle. The cell efficiency of c-Si photovoltaic device with a flat surface drops rapidly as the incident angle increases due to the increased reflection loss as shown in Fig. 4. In the device with nanohole arrays, however, the PCE is sustained at an incident angle in excess of 50°. More interestingly; the device exhibits a better performance at a slightly tilted incident angle compared with the normal incidence angle. 3. Experiments results and discussion Nanohole arrays in Si3N4 AR coating can be realized by lithography and etching. Several lithography technologies can be used for the nanoscale fabrication such as nano-imprint [17], extreme ultraviolet lithography (EUVL) [18] and interferometric lithography [19]. In addition the electron beam lithography (EBL) and the focused ion beam (FIB) system can also be used for the nanoscale fabrication. For large scale fabrication in the future, the nano-imprint lithography is a good candidate because of its low cost, high throughput, and uniformity. Meanwhile, to get quick comparative experimental results, a Φ200 μm Si detector that has the same I–V curve and photoelectric characteristics as common solar cells was selected as the experimental sample. Its c-Si active layer is 3 μm thick. The nanohole arrays were fabricated directly in the Si3N4 AR coating of sample by using a focusedion beam system (strata FIB 201, FEI Company, 30 keV Ga ions). The depths of the nanoholes were carefully controlled to make sure they are equal to the thickness of the sample's antireflection film (~90 nm). To get comparison of nanohole arrays with different lattice constants, we fabricated three types of nanohole arrays with different lattice constants in the AR coating of one sample. Fig. 5 gives SEM images of those three nanohole arrays, their design parameters are a = 400 nm, f = 0.4, area= 14 μm × 14 μm, a = 500 nm, f = 0.4, area = 40 μm × 40 μm and a = 600 nm, f = 0.4, area = 14 μm × 14 μm. Because of the fabrication error, the real fabrication parameters are a = 412 nm, d = 286 nm, a = 514 nm, d = 370 nm, a = 610 nm, d = 428 nm. Two experimental conditions should be carefully satisfied. Firstly, the spot size of the incident light should be smaller than the area of the nanohole arrays. Secondly, the spot size should remain unchanged while the light is moving from one nanohole arrays to another. Because the areas of these nanohole arrays are quite small, we built a specific set up to measure the characteristics of the device as shown in Fig. 6b. The detector sample with surface nanohole arrays is fixed on the microscope (Keyence VHX-600) workbench. A single mode fiber probe with core diameter 8.3 μm, which transfers the light from the
Fig. 3. Calculated PCE as a function of the lattice constant, filling fraction and the cSilicon of thickness. (a) PCE vs. lattice constant and filling fraction (b) PCE vs. lattice constant (f = 0.4) (c) PCE vs. thickness (f = 0.4, a = 500 nm).
coating and without any coating, whatever the thickness of the active layer is. Based on the calculation, to manufacture a photovoltaic device with the same efficiency, the thickness of the c-Si of the device with the nanohole arrays is 20%–30% less than that with Si3N4 AR coating, which indicates about 30% reduction of c-Si consumption cost. On the other hand, the PCE of device with surface nanohole arrays has 18.9% enhancement compared with device coated with Si3N4 (ηe = (ηnanohole − ηSi3N4) / ηSi3N4, where ηe is the PCE enhancement of the device with nanohole arrays, ηnanohole is the PCE of the
Fig. 4. PCE vs. incident angle.
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Fig. 6. Schematic diagram of the experiment (a) photo of the experimental sample (b) experiment set up.
Fig. 5. SEM images of the surface nanohole arrays (a) a = 400 nm, f = 0.4 (b) a = 500 nm, f = 0.4 (c) a = 600 nm, f = 0.4.
source to the surface of the detector (Fig. 6a), is fixed on a microdisplacement platform (Melles Griot nanoMax-HS 17MAX600/R). As the numerical aperture of the single mode fiber is 0.13, to get a spot smaller than 14 μm, the gap between surface of the fiber probe and the detector sample should be less than 53 μm. Firstly, the microdisplacement platform was finely adjusted in vertical direction, and the gap between was fixed to 50 μm. Subsequently, the platform was moved horizontally to make sure the light spot locates in the middle of one nanohole array. After testing one nanohole array, the platform was moved horizontally again to let the light spot illuminate another array. During the horizontal movement, the vertical position of the fiber probe should remain unchanged. Therefore the light spot size
and intensity were all kept constant. The three nanohole arrays were tested one by one. For comparison, the performance of the device without nanohole arrays was also measured. The photoelectric characteristics of the sample device were measured by a high performance digital multimeter (Keithley 2002 Multimeter). According to Eq. (1) and Section 2.2, to get the PCE enhancement of the device with nanohole arrays, we analyze the performances of the three types nanohole arrays by monitoring their open-circuit voltage Voc and short-circuit current Isc.(ηe = (VocnIscn/VocsIscs)-1, where Vocn, Vocs are the open-circuit voltage of the device with nanohole arrays and with unpatterned Si3N4 film respectively, and Iscn, Iscs are the short-circuit current of the device with nanohole arrays and with unpatterned Si3N4 film respectively). Fig. 7a and b give the measurement results of nanohole arrays. As references, the results of common Si3N4 (AR) without any nanoholes are also listed at the left top corner of the figure. The sample device was excited by lasers at 488 nm (275 mW/cm2, Coherent Sapphire 488–200) and 532 nm (87.5 mW/cm2, Melles Griot 85-GCB-020) to measure the power enhancement in the wavelength range 400–600 nm. We also measured the response between 400 nm and 1100 nm by using solar simulator (PET Solar Simulator SS100) whose spectral energy distribution is similar to the AM1.5 solar spectrum. In both figures, the maximum enhancement occurs when lattice constant is 500 nm, which agree with our simulation result. The enhancement of Isc excited by 532 nm laser is 15.67%, and excited by
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value, as shown in Fig. 4. 2) Because of the fabrication processes, the depth of the nanohole is not fabricated accurately and may destroy the c-Si active layer, which may lead to the decrease of the PCE. 4. Conclusions In this paper, the photoelectric conversion efficiency enhancement of the photovoltaic device with nanohole arrays only in its antireflection coating was analyzed. An optimum nanohole arrays with lattice constant of 500 nm and filling fraction of 0.4 are calculated. To get quick comparative experimental results, a Φ200 μm Si detector was selected as an experimental sample. Three types of nanohole arrays, which have different lattice constants, were fabricated on that sample by the focused-ion beam method. Experimental results showed that the PCE of the sample is enhanced by ~ 16% in the 400– 600 nm range and by ~ 10% in the 400–1100 nm range. As predicted by the simulation, the nanohole arrays show better performance in short wavelength range (400–600 nm). Although the PCE enhancement did not reach the simulation value, experiments demonstrated the feasibility of our method. As only the Si3N4 antireflection coating is patterned with nanoscale structure, the side effect of dead layer induced by nanoporous in the active layer is avoided. In addition, the fabrication could be implemented through lithography and etching, and which is compatible with current photovoltaic device fabrication process. Therefore, this method is a promising choice in practical photovoltaic device industry. Acknowledgments Authors would like to thank the financial support of this work via National Natural Science Foundation of China (No. 50830202 and No. 60707010).
Fig. 7. Enhancement in open-circuit voltage and short circuit current for different nanohole arrays structure excited by different light source: (a) Voltage response (b) current response.
488 nm laser is about 13.53%. On the other hand, the increases of Voc are only 2.61% (532 nm) and 2.48% (488 nm). So the enhancement factor of the photoelectric conversion is ~ 16%. It can be seen that the enhancement was higher in the 400–600 nm spectral range. When excited by the solar simulator, the enhancement of Voc is 2.20% and that of Isc is 7.68%, which result in an efficiency enhancement factor of ~ 10% in the wavelength range of 400–1100 nm. It's clear that the improvement is dominated by current rather than voltage. Insensitive to the incident angle is another important optical property of the devices with nanohole arrays. To verify that, the device with nanohole arrays sample is fixed on the microscope bench and the micro-displacement platform is tilted ±10° off the vertical incidence position during the measurement as shown in Fig. 5b. It was found out that the PCE was almost the same as the one with a vertical incidence, which is better than the device without any coating. Contrast to the simulation results, it can be seen that the measured enhancement is smaller than the simulation result. There are several reasons leading to the differences: 1) the lattice constant of nanohole arrays of the fabricated sample is not consistent with the simulation exactly, and the diameter of the nanohole is larger than the simulation
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