Low-cost, deterministic quasi-periodic photonic structures for light trapping in thin film silicon solar cells

Transcription

1 Low-cost, deterministic quasi-periodic photonic structures for light trapping in thin film silicon solar cells The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation As Published Publisher Xing Sheng et al. Low-cost, Deterministic Quasi-periodic Photonic Structures for light trapping in thin film silicon solar cells. Photovoltaic Specialists Conference (PVSC), th IEEE IEEE. Institute of Electrical and Electronics Engineers Version Final published version Accessed Tue Sep 04 07:28:36 EDT 2018 Citable Link Terms of Use Detailed Terms Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.

2 LOW-COST, DETERMINISTIC QUASI-PERIODIC PHOTONIC STRUCTURES FOR LIGHT TRAPPING IN THIN FILM SILICON SOLAR CELLS Xing Sheng 1, Jifeng Liu 2, Jurgen Michel 2, Anuradha M. Agarwal 2, and Lionel C. Kimerling 1, 2 1 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, Materials Processing Center, Massachusetts Institute of Technology, Cambridge, MA, ABSTRACT Light trapping has been an important issue for thin film silicon solar cells because of the low absorption coefficient in the near infrared range. In this paper, we present a photonic structure which combines anodic aluminum oxide (AAO) and a distributed Bragg reflector (DBR) in the backside of thin film silicon. Simulation results show that this quasi-periodic AAO structure has a stronger light trapping effect than perfectly periodic diffractive gratings. As a proof of concept, we incorporated the backside structure into thick silicon PV cells. By comparing the measured photoconductance of silicon with different backside structures, the enhancement of light absorption near the band edge of silicon is demonstrated for our proposed light trapping structure. INTRODUCTION Currently, most commercial solar cells are based on silicon wafers. Cost reduction of silicon wafer based solar cells is challenging because it is dominated by the starting material. Thin film silicon solar cells based on inexpensive substrates are designed to reduce the silicon consumption by 100 fold so that the material cost becomes negligible. However, as films become thinner, the absorption of photons with longer wavelengths is reduced. This problem is severe especially for silicon because of its indirect bandgap, thus the power conversion efficiency is decreased as well. To overcome this limit, several light trapping schemes were proposed to increase the optical absorption in thin film silicon. For example, Al or Ag is used for the backside contact as well as a reflective mirror [1]. Another example is to utilize a textured transparent conductive oxide prepared by sputtering and a subsequent etching step to generate a rough surface that scatters light into the Si plane [2]. Recently, we designed a textured photonic crystal (TPC) back reflector, which combines a one-dimensional reflection grating and a distributed Bragg reflector (DBR) [3]. The DBR is a multilayer stack of alternating SiO 2 and Si, which forms a one-dimensional photonic crystal with nearly 100% reflectivity in the nearinfrared range. Meanwhile, the grating can diffract the incident light into oblique angles, thus total internal reflection occurs at the front surface of silicon if the diffractive angle is larger than the critical angle between silicon and air interface. By integrating the TPC structure into 5µm thick monocrystalline silicon solar cells, a significant enhancement of the external quantum efficiency (EQE) was observed in a wavelength range from 600 nm to 1000 nm, leading to a 19 % increase in the cell efficiency [4]. However, this method has its own limitation. Since photolithography and other cleanroom facilities are required to fabricate the subwavelength gratings, the TPC structure cannot be scaled to large area applications. To reduce the fabrication cost while maintaining the same light trapping performance, in this paper we propose a low cost and easily-controllable Deterministic Quasiperiodic Photonic Structure (DQPS) for light trapping in thin film silicon solar cells. In the DQPS, the perfect periodic grating is replaced by a quasi-periodic array, which is fabricated by self-assembled anodic aluminum oxide (AAO) and subsequent DBR deposition. Using simulations, we investigate and compare the light trapping effects of the DQPS and previous TPC work. Finally, we present the experimental confirmation of the absorption enhancement by photoconductance measurements based on thick silicon wafers. DESIGN AND SIMULATION Since only a single-step anodization is used to obtain the AAO structure instead of a two-step anodization process [5], a relatively disordered pore distribution is generated. To compare the light trapping effect of our quasi-periodic structure with the perfectly periodic gratings, the absorption spectra of thin film silicon solar cells based on different back structures are simulated through Finite- Difference-Time-Domain (FDTD) calculations. Both the TPC and DQPS structures used in FDTD simulations are illustrated in Fig. 1. Perfectly periodic grating structure (TPC) can be fabricated by interference lithography, while single-step aluminum anodization forms a quasi-periodic distribution with disorder to a certain extent. Silicon is filled into the pores by PECVD to increase index contrast. The front silicon layer is 2 µm thick. The DBR layer in the backside is assumed to have 100% reflectivity in the red and near infrared range /09/$ IEEE

3 J = eηc s( λ) a( λ) dλ sc (1) Fig. 1. Grating configurations used in FDTD simulation. The material in blue region is alumina (n=1.72), while the material in green is silicon (n=3.5) With the refractive indices of different materials given in the literature [6], we calculate absorption of the silicon layers with different backside structures as a function of wavelength (see Fig. 2.). At wavelengths between 0.6 µm and 1.0 µm, both TPC (blue color) and DQPS (red color) have higher absorption compared to the reference silicon layer without any backside structure. The light trapping effect is more remarkable for longer wavelength where the silicon absorption coefficient decreases dramatically. For incident light above 0.9 µm, the absorption of 2 µm-thick bare silicon is negligible, while both TPC and DQPS show strong absorption peaks, and the peak intensity of DQPS is even higher. The oscillations are due to thin film Fabry Perrot oscillations. where e is the electron charge, η c is carrier collection efficiency which depends on the surface recombination and material quality (here we simply assume it is 100% for monocrystalline silicon). The total number of absorbed photons can be calculated by integrating over the product of the absorption spectrum a (λ) and solar spectrum s (λ). Calculated results are summarized in Table 1. In an ideal situation, the DQPS provides 53% absorption enhancement under AM1.5G illumination, which indicates a 53% enhancement of cell efficiency, even greater than a 49% enhancement obtained from the perfectly periodic TPC. Table 1. Simulated short-circuit current density of solar cells with different backside structures and enhancement factor J sc (ma/cm 2 ) Relative enhancement Bare 2 µm Si TPC % DQPS % EXPERIMENTAL RESULTS AND DISCUSSION The process steps in the fabrication of DQPS are shown in Fig. 3. A 200nm thick aluminum film is deposited on silicon substrates by e-beam evaporation. Then the Al thin film is anodized in a 4 wt% phosphoric acid at a voltage of 140 V until the aluminum film is completely oxidized to aluminum oxide. During the anodization, the temperature is kept at 5 ºC to prevent electrical breakdown. Afterwards, the samples are dipped in 5 wt% phosphoric acid for about 2 hours at room temperature to widen the pore size and remove the barrier layer. Under this condition, a structure with a pore diameter of about 200nm and a pore spacing of 300nm is formed, consistent with literature reports [5]. The DBR consisting of 5 pairs of Si/SiO 2, which can achieve 99.9% reflectivity in the near infrared range [7], is fabricated by Plasma Enhancement Chemical Vapor Deposition (PECVD). Fig. 2. Simulated absorption spectra for 2 µm thick silicon with different backside structures To predict the solar cell performances, the short-circuit current density can be calculated by /09/$ IEEE

4 Fig. 3. A schematic of the fabrication process and light trapping effect of DQPS. In order to measure the effects of the backside structures we use photoconductance measurements. A lock-in amplifier is used to precisely collect the photoconductances of difference samples under illumination. Light of the wavelength range of interest between 700 nm and 1200nm was generated by a white light source in combination with a monochromator. To demonstrate the light trapping effect of the DQPS as a proof of concept, we implement the DQPS on the backside of a 300 µm thick, double side polished silicon wafer. Thick silicon wafers are chosen because of the simplicity of process. However, similar responses will be obtained for different backside structures at short wavelengths because of complete absorption by the thick active silicon layer. The photoconductivity of semiconductors is directly related to the excess carrier density generated by photon absorption: σ = n eµ (2) Therefore, light trapping effects can be demonstrated by an enhanced photoconductive response. The measured photoconductive spectral responses for DQPS and a reference sample without backside structure at wavelengths between 0.7 µm and 1.2 µm are shown in Fig. 4. Compared to the reference sample, an increase in the photoresponse can be clearly observed above 1000 nm for the DQPS sample, in agreement with our calculations. This result experimentally confirms that DQSP structures can be applied for optical path length enhancement. Fig. 4. Photoconductive spectral responses of a 300 um thick silicon sample with and without DQPS on the backside of the wafers. CONCLUSIONS In this paper, we propose a novel quasi-periodic photonic light trapping structure, which can be easily fabricated through aluminum anodization and DBR deposition. Numerical calculations indicate that the DQPS is capable of improving the solar cell efficiency by more than 50 percent for thin film crystalline silicon. Moreover, photoconductance measurements based on thick silicon wafers clearly confirm the enhancement of photon collection near the silicon bandedge, in good agreement with the simulation results. Considering a thin-film silicon cell with a thickness of several microns, the efficiency increase would be much more significant. Furthermore, the fabrication process of this DQPS can be readily scaled to large areas for practical applications with significant cost reduction. These preliminary results provide a path to achieve low cost and strong efficiency enhancement for thin film silicon solar cells by using our novel DQPS structure. ACKNOWLEDGMENT This work was supported by Robert Bosch LLC through MIT Energy Initiative. REFERENCES [1] S. Hegedus et al., Improving performance of superstrate p-i-n a-si solar cells by optimization of n/tco/metal back contacts, 26th IEEE Photovoltaic Specialists Conference 1997, pp [2] J. Muller et al., TCO and light trapping in silicon thin film solar cells, Solar Energy 77, 917 (2004) /09/$ IEEE

5 [3] L. Zeng et al., Efficiency enhancement in Si solar cells by textured photonic crystal back reflector, Appl. Phys. Lett. 89, (2006). [4] L. Zeng et al., Demonstration of enhanced absorption in thin film Si solar cells with textured photonic crystal back reflector, Appl. Phys. Lett. 93, (2008). [5] H. Masuda et al., Self-Ordering of Cell Configuration of Anodic Porous Alumina with Large-Size Pores in Phosphoric Acid Solution, Jpn. J. Appl. Phys. 37, L1340 (1998). [6] E. Palik et al., Handbook of optical constants of solids, Academic Press (1998). [7] Y. Fink et al., A Dielectric Omnidirectional Reflector, Science 282, 1679 (1998) /09/$ IEEE