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Ultrathin epitaxial silicon solar cells with inverted nanopyramid arrays for efficient light trapping Alexandre Gaucher, Andrea Cattoni, Christophe Dupuis, Wanghua Chen, Romain Cariou, Martin Foldyna, Loïc Lalouat, Emmanuel Drouard, Christian Seassal, Pere Roca i Cabarrocas, and Stéphane Collin Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b01240 • Publication Date (Web): 15 Aug 2016 Downloaded from http://pubs.acs.org on August 16, 2016
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Ultrathin epitaxial silicon solar cells with inverted nanopyramid arrays for efficient light trapping Alexandre Gaucher1, Andrea Cattoni1, Christophe Dupuis1, Wanghua Chen2, Romain Cariou2, Martin Foldyna2, Loic Lalouat3, Emmanuel Drouard3, Christian Seassal3, Pere Roca i Cabarrocas2, Stéphane Collin*1 1
Centre de Nanosciences et de Nanotechnologies, CNRS, Univ. Paris-Sud, Université Paris-
Saclay, C2N – Marcoussis, 91460 Marcoussis, France 2
LPICM, CNRS, Ecole Polytechnique, Université Paris-Saclay, 91128 Palaiseau, France
3
Institut des Nanotechnologies de Lyon (INL), 69134 Ecully Cedex, France
ABSTRACT. Ultrathin c-Si solar cells have the potential to drastically reduce costs by saving raw material, while maintaining good efficiencies thanks to the excellent quality of monocrystalline silicon. However, efficient light trapping strategies must be implemented to achieve high short-circuit currents. We report on the fabrication of both planar and patterned ultrathin c-Si solar cells on glass using low temperature (T < 275 °C), low-cost and scalable techniques. Epitaxial c-Si layers are grown by PECVD at 160°C and transferred on a glass substrate by anodic bonding and mechanical cleavage. A silver back mirror is combined with a front texturation based on an inverted nanopyramid array fabricated by nanoimprint lithography and wet etching. We demonstrate a short-circuit current density of 25.3 mA/cm² for an
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equivalent thickness of only 2.75µm. External quantum efficiency (EQE) measurements are in very good agreement with FDTD simulations. We infer an optical path enhancement of 10 in the long wavelength range. A simple propagation model reveals that the low photon escape probability of 25% is the key factor in the light trapping mechanism. The main limitations of our current technology and the potential efficiencies achievable with contact optimization are discussed.
KEYWORDS. Solar cells, crystalline silicon, low-temperature epitaxy, light trapping, nanoimprint lithography.
Silicon is the dominant technology for photovoltaic solar cells. Efficiencies of 25.6% have been achieved at the cell level1, which is close to the 29.4% efficiency limit for crystalline silicon solar cells2. One of the main challenges facing current silicon technologies is the reduction of costs at the module and cell levels. A rationale way is to reduce the consumption of purified crystalline silicon. Indeed, the thickness of state-of-the-art solar cells (100-200µm) is far from the theoretical limits, and could be reduced by more than one order of magnitude with efficient light-trapping3. In wafer-based technologies, however, reducing the wafer thickness cannot be pursued below 100µm-thick absorbers because of handling issues and important kerf losses. In this sense, thin-film silicon-based technologies are a promising way to further reduce the silicon consumption. On the one hand, amorphous silicon (a-Si:H) and microcrystalline silicon (µc-Si:H) thin films benefit from a high absorptivity, but suffer from low overall efficiencies as compared to wafer-based crystalline solar cells1 due to poor transport properties. They can be improved by rapid thermal crystallization and lead to thin crystalline silicon on
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glass (CSG) solar cells. Although not monocrystalline, record modules have reached efficiencies of 10.5% and Jsc=29.7 mA/cm2 with silicon thicknesses below 2µm4,5,6. On the other hand, the emerging field of thin film crystalline silicon (c-Si) solar cells has the potential for reducing the silicon consumption while maintaining high efficiencies thanks to the excellent quality of monocrystalline silicon. Recent numerical studies have shown that state-of-the-art thick c-Si solar cell efficiencies could be achieved using only a few-µm thick c-Si layers7,8 when efficient light trapping strategies are put in place. Several techniques have been used to produce thin crystalline silicon layers such as epitaxial growth on porous silicon and transfer to a foreign substrate9,10, exfoliation of a c-Si foil from a commercial wafer11 or “epi-free” process based on recrystallization of amorphous material deposited on a foreign substrate12. Different light trapping strategies have been described in the literature such as the use of photonic crystals13, diffraction gratings14-16, plasmonic scatterers17,18, Mie resonators19 or aperiodic structures20. Numerical issues can arise from absence of periodicity or from the different length scales (nanoscale texturation and thicknesses up to several tens of microns). In such cases, propagation matrix models have been proposed recently as an alternative to conventional electromagnetic numerical methods21,22. Crystalline silicon solar cells with absorbing layer thickness of 43µm have shown very promising results with efficiencies of 19%23 and 20.6%24 respectively. Several authors have focused on further reducing the absorber thickness below 10µm25-29. Interestingly, best short-circuit currents have been reported using wavelength scale structuration such as nanopyramids29 or nanocones25,27. Branham et al.29 recently reported an efficiency of 15.7% for a 10µm-thick crystalline silicon absorbing layer. However, as for refs.25-29, commercial silicon on
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insulator (SOI) wafers were etched chemically to provide ultrathin silicon layer. Moreover, expensive and high temperature processes were used in these proof-of-concept studies. The present work aims at producing ultrathin (2 to 10µm thick) crystalline silicon solar cells with low cost, low temperature and scalable processes. It focuses on the fabrication of ultrathin c-Si layers transferred on glass and on the implementation of efficient light-trapping techniques to increase the short-circuit current. In particular, we chose an inverted pyramid array with a wavelength-scale pitch on the front side of the cells, combined with a silver back mirror. In the following, we describe the fabrication process and device characterization. A record shortcircuit current density of Jsc=25.3mA/cm2 is measured for 3µm-thick monocrystalline solar cells. We then provide an in-depth optical analysis based on external quantum efficiency (EQE) measurements and numerical FDTD calculations. A simple propagation model is also used in order to reveal the key factors of the light-trapping mechanism: the high optical path enhancement of 10 is mainly due to the back mirror and the low photon escape probability (25%). We finally discuss the main limitations of our current technology and the potential efficiencies achievable with further optimizations. 3µm-thick c-Si epitaxial layers were grown by plasma enhanced chemical vapor deposition (PECVD) at low temperature30,31 (T700nm, and results in short-circuit current losses of 4.4mA/cm² and 4mA/cm², respectively. The original purpose of the ZnO:Al back contact layer is to reduce the absorption in the Ag mirror, which is effectively the case in Fig. 4(b). However, this beneficial effect is counter-balanced by parasitic absorption. Moreover, it is likely that it increases the series resistance, hence reducing the fill factor of the cell. Overall, the optimization of both front and back TCO layers provides room for improvement and increase in Jsc.
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We propose to further analyze the light-trapping mechanism with a simple model based on the propagation of diffracted waves in the patterned cell. It is schematically represented in Fig. 5(a), where the top and back passivation and TCO layers are not shown for the sake of clarity.
(a)
(b)
Rext
2
Pesc 1.5
(p,q)
d0
θ deff
Rf
deff/d
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1 Inverted pyramids grating Flat surface Lambertian scatterers
Rb 0.5 400
500
600
700 800 900 Wavelength (nm)
1000
1100
Figure 5. (a) Schematic of the simplified solar cell. (b) Normalized mean effective thickness as a function of the wavelength for various structurations of the absorber: inverted pyramid array (blue), flat surface (dashed black), and lambertian scatterers (dashed red). The incoming photon flux can be reflected on the front surface (coefficient Rext), or coupled to a set of diffracted waves of order (p,q) in the silicon layer. The resulting angular distribution of photons is specific to the inverted pyramid array. It depends on the diffracted angles θ(p,q) and the diffracted efficiencies DE(p,q). θ(p,q) is defined as the angle between the diffracted wave (p,q) and the direction normal to the plane. The diffracted efficiencies DE(p,q) are proportional to the energy (or number of photons) in each diffracted wave 41. For the sake of simplicity, they are normalized so that the sum of the diffraction efficiencies of propagating waves at a given wavelength is 1. They are determined numerically by FDTD simulations.
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During propagation in the cell, photons can be reflected either on the back mirror (coefficient Rb) or on the front interface (coefficient Rf). Both coefficients are averaged values for the angular distribution of photons, and Pesc = 1-Rf is the average probability that photons reaching the front interface escape the cell. In the following, we assume perfect anti-reflection effect for incoming photons (Rext = 0), and perfect reflection on the back mirror (Rb = 1). Then, the optical path enhancement F induced by the nanopyramid array can have two origins: an increased effective thickness due to a mean diffraction angle θ>0, and light trapping in the absorber due to reflection on the front interface Rf >0. We first investigate the effect of the diffraction angles. For a round trip propagation at angle θ, the path length is 2 , where the effective thickness () = ⁄(). Lambertian scattering is a reference case, and corresponds to a uniform occupation of the density of optical states (DOS) in the absorber3. Its angular distribution () = 2 cos() sin ()
results
in
a
mean
effective
thickness
"! () cos() sin() = 2. In the case of inverted nanopyramid arrays, the = 2 #
mean effective thickness can be calculated as the sum the effective thicknesses of propagating diffracted orders weighted by their diffraction efficiency:
(2)
(&,() ) ∗ +, (&,() = ∑(&,() %
The result is plotted in Fig. 5(b), and compared to the flat interface and lambertian cases. Most of the incoming photons are diffracted by the nanopyramid array between θ = 33° and θ = 42°, with a weak wavelength dependence (see Supplementary Information). As a result, the normalized effective thickness ⁄ displays very few variations on the whole wavelength range, with values between 1.2 and 1.35, and an average of 1.25.
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If we assume no light trapping through reflection on the front surface (Rf =0), we expect double pass absorption according to Eq. (1), with an optical path enhancement / = 2 ⁄ = 2 ∗ 1.25 and d = deq = 2.75µm. The result is shown in Fig. 6 (dashed green line). The EQE measurement still largely exceeds double-pass absorption: the absorption enhancement due to the increased effective thickness is very weak and is not sufficient to explain the efficient light trapping evidenced by experimental measurements.
1 0.9 0.8 0.7 0.6 EQE, A
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0.5 0.4 0.3 0.2 0.1 0 400
EQE Double pass abs. with F = 2 Double pass abs. with F = 2*1.25 Yablonovitch abs. with F = 10 500
600
700 800 900 Wavelength (nm)
1000
1100
Figure 6. Comparison of the external quantum efficiency of solar cells with nanopyramid arrays with different models: double-pass absorption with F = 2 (flat cell) and F = 2*1.25 (effective thickness induced by diffraction), and propagation model with F = 10. The propagation model is plotted on the whole wavelength range (dotted red curve), but its domain of validity is highlighted in solid red. Reflection of upward photons at the front surface of the cell is thus a key in the light trapping mechanism. The light path enhancement factor F in our patterned solar cell can be estimated by using a simple propagation model for light trapping of diffracted waves3,42 (see supporting
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information). For weakly absorbing medium (long wavelength range), the absorption efficiency is given by: 0(1) =
(3)
23 5 6
234
As can be seen in Fig. 6, this model correctly fits EQE measurements in the 850nm-1000nm wavelength range, and leads to a high enhancement factor of F=10.
Moreover, it can be shown that the light path enhancement factor F can be simply expressed by the normalized effective thickness and the probability of escape (see supporting information):
/=2
(4)
788 9
:7;
