Photonic crystal assisted ultra-thin silicon photovoltaic solar cell

Photonic crystal assisted ultra-thin silicon photovoltaic solar cell

Christian Seassala, Yeonsang Parka, Alain Faveb, Emmanuel Drouarda, Erwann Fourmondb, Anne Kaminskib, Mustapha Lemitib, Xavier Letartrea, Pierre Viktorovitcha

Université de Lyon, Institut des Nanotechnologies de Lyon-INL, UMR CNRS 5270 aEcole Centrale de Lyon, 36 Avenue Guy de Collongue, F-69134 Ecully, France;

bINSA de Lyon, Bât. Blaise Pascal, 7 avenue Capelle, F-69621 Villeurbanne, France

ABSTRACT

A new concept of ultra-thin film photovoltaic solar cell including a planar photonic crystal is proposed. The goal is to couple the incident light into broad resonances guided in the absorbing layer. To achieve this, a periodic lattice is patterned within the active layer, for example made of holes in amorphous silicon. By adjusting the pattern dimensions, the spectral position and quality factor of these resonances can be controlled so as to optimise the global absorption. Design details will be discussed in this communication.

Keywords: Photonic crystal, Photovoltaïc solar cells, Silicon

1. INTRODUCTION So as to optimize the efficiency of solar cells, a key issue is to increase sunlight collection, and photons lifetime inside the absorbing material. Classical strategies combine the integration of an anti-reflection layer and/or of a texturation of the top surface, and a reflector on the back side. These approaches are generally based on geometrical optics, and the textured surfaces are based on pyramids that exhibit a size larger than the optical wavelength. More recently, a few groups proposed to make profit of wavelength scale patterns in order to further control the photons in solar cells. In particular, Zeng et al. [1] proposed to implement a grating, or even a photonic crystal (PC) structure at the back side of the cell, in order to control the direction of the reflected light, and to increase the photon path. On the other hand, it has been reported that metallic nanoparticles positioned on top of a thin silicon-on-insulator waveguide can improve absorption or emission of light through the coupling between radiated light and guided modes, through surface plasmons [2-3]. In this paper, we propose to develop a photonic engineering scheme in the particular case of absorbing layers with thicknesses on the order of 100 nm to 1µm. A so significant decrease of the thickness is important for two main reasons. First, this could significantly reduce the cost of such devices. Second, the reduction of the thickness is a key issue in order to circumvent the problem of the very short photocarriers diffusion length in such standard absorbing layers as amorphous silicon (a:Si) or made of organic materials. With such thin layers, the optical path length is so small that the conversion efficiency is very limited, in particular for long wavelengths. Thus, we propose a photonic design in order to realize such ultra-thin silicon solar cells that could exhibit a high absorption and a reasonable efficiency. The main idea is to pattern a PC within the absorbing layer, and to couple the incoming light into slow light modes of such a structure. If it is carefully designed, such a PC can play at the same time the role of the sun light collector, and can control the photon lifetime. Therefore, there is no more need to integrate an anti-reflexion coating, and the exploitation of a back-side mirror may not be always necessary. In the next section of this paper, we introduce a simple 1D PC structure that exhibits slow light modes on a selected wavelength range. Its optical properties will be discussed, and we will show

Photonics for Solar Energy Systems II, edited by Andreas Gombert Proc. of SPIE Vol. 7002, 700207, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.781284

Proc. of SPIE Vol. 7002 700207-12008 SPIE Digital Library -- Subscriber Archive Copy

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how such a patterning can affect light absorption. Then, in section 3, we will present how the absorption can be increased on the whole solar spectrum, by optimizing the geometrical parameters of the structure. Lastly, in section 4, we will discuss on the possible influence of the patterning on the carrier transport properties, and on the global efficiency.

2. ABSORPTION IN A 1D PC RESONATOR We first consider the planar 1D PC schematically presented Fig. 1a. This structure is a periodic lattice of air slits patterned in a thin semiconductor layer. Let us first investigate the properties of a PC structure that is supposed to be transparent, and which refractive index is set to n=3.5. Fig. 1b display its band structure, for 95nm thick layer, a 260nm lattice parameter, and an air filling factor of 13%.

As in the case of all the planar PC structures, the modes located over the light line may be coupled to the radiative continuum, i.e. the incident light. This structure exhibits two band-edge modes, located over the Γ-point, and that corresponds to wavelengths of 640 and 715nm. They correspond to low group-velocity modes, which are waveguided resonances that may exhibit a high density of optical modes. For symmetry reasons, only the first one can be coupled with the light incoming in the vertical direction, and therefore, only this mode will be considered.

(b) Fig. 1: Schematic view of the 1D PC (for a 260nm lattice parameter, a 13% air filling factor and a 95nm thickness) (a), band

diagram and electromagnetic field of the resonant mode at a/λ=0.4 (b)

Fig. 2 exhibits the reflectivity spectrum of this structure, as calculated by the Rigorous Coupled Wave Analysis (RCWA) method, in the case where the electric field is polarized in the direction of the slits. On this spectrum a reflectivity peak appears at 640nm; this is a typical signature of the slow light mode [4]. The linewidth of this resonance indicated its quality factor (Q0), and therefore gives information about the photon lifetime. In this case the linewidth is 2nm, which corresponds to Q~300.

Proc. of SPIE Vol. 7002 700207-2

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thickness, n=3.5 in air)

Now we consider the same PC structure, with absorbing semiconductor strips. Since our goal is to benefit from the increase of photon lifetime in order to enhance the absorption, it is necessary to match the photon lifetime τ0, only related to the optical losses (i.e. related to the Q0 factor of the transparent PC structure) with the photon lifetime τa, related to the absorption losses in the absorbing medium we consider. Such a matching is generally considered for resonant cavity enhanced photodetectors [5]. Introducing the absorption coefficient a and the quality factor, the matching condition τa= τ0 leads to a simple relation:

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Therefore, in order to optimize the absorption thanks to resonance mentioned above (Q=300), the absorption coefficient should be around 1150cm-1, or an extinction factor of 7x10-4. The absorption of the PC structure is then calculated by RCWA, for various values of the extinction coefficient, ranging from 0.004 to 0.04 (see Fig. 3a). These numerical results show that light is optimally absorbed if the extinction coefficient is 1.2x10-3, quite close to the value predicted by our very simple model. Fig. 3b exhibits the absorption, reflectance and transmission spectra of this last PC structure.

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Fig. 3: Calculated absorption at the resonance of the PC structure, for different values of the extinction coefficient, and calculated absorption, reflectance and transmission spectra for the optimum value of k, i.e. 1.2x10-3


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Here, we demonstrated that absorption can be controlled using such a resonant mode in a PC structure. We also clearly showed that the PC structure should be design in such a way that its resonances Q-factors are adapted to the extinction coefficient at the considered wavelength. In the example discussed above, the absorption is increased in a very narrow wavelength range, in relation with the relatively high Q-factor of the structure. Now we will show that this wavelength range can be controlled by changing the geometrical parameters of the PC structure.

Fig. 4 shows the reflection spectrum obtained for a transparent PC structure which air-filling factor has been increased up to 30%. This modification strongly affects the interaction of the resonant mode with the radiative continuum, and therefore the Q-factor. So as to maintain the resonant mode around 640nm, we also had to adjust the lattice parameter, which was increased up to 315nm, and the thickness, which was set to 100nm. The Q-factor is low about 35; the peak absorption should then be maximized for an absorption coefficient of about 104 cm-1 (k=0.06), quite close to the values corresponding to amorphous silicon (a:Si) around 640nm. Using RCWA simulation, we found the optimum peak absorption of 75%, at the resonant mode wavelength, for an extinction coefficient of 0.2 (see Fig. 4); once again the simulated value is of the same order as the one predicted by the simple time constants model. In the case of this last structure, the global absorption is significantly increased in a wider wavelength range.

Fig. 4 : Calculated reflection spectrum for the transparent PC structure (with a 315nm lattice parameter, a 30% air filling

factor and a 100nm thickness, n=3.5 in air), and absorption spectrum calculated for the optimum value of the extinction coefficient

Using a design quite close to last structure discussed above, we considered the real dispersion of a:Si over the 400-800nm wavelength range. Fig. 5 displays the characteristics (absorption and reflection) of a patterned a:Si layer, with the same geometrical parameter as in the last structure, except for the air filling factor, which is set to 50%, in order to account for the high refractive index of this material around 600nm. These characteristics are compared to the simulated absorption in the case of an unpatterned layer. At the resonant wavelength (around 625nm), patterning of the a:Si layer increases the absorption from 19 to 72%. In the whole 400-800nm spectral range, the absorption is increased from 31%, for the reference layer to 49%, in the case of the patterned structure.

It is important to note that at lower wavelength, in the 400-500nm range, the absorption of the PC is also somehow increased compared to the reference. In this spectral range, away from the resonance, this enhancement could explained by the lower reflectivity of the PC structure, compared to the case of the unpatterned membrane (see the reflection spectra in Fig. 5).


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Fig. 5: Calculated absorption and reflection spectra for a unpatterned 100nm thick a:Si layer, and for a similar layer

patterned as a 1D PC (315nm lattice parameter, a 50% air filling factor). The dispersion of a:Si is taken into account and the surrounding medium is air

These designs are based on very thin layers, where only one resonant mode can be used in the range we considered. In order to increase further the absorption, one has to play simultaneously with different slow light modes. This will be discussed in the next section.

3. DESIGN OF A MULTIMODE 1D PC SOLAR CELL A simple way to introduce more resonant modes into a PC structure is to increase its thickness. So as to maximize the integrated absorption in the case of a solar cell based on amorphous silicon, we integrated an optimization loop in an open access RCWA code [6], and we optimized the main parameters of the 1D PC. The maximum thickness was set to 1µm while the lattice parameters and filling factor were left free. The optimization criterion is the maximization of the integrated absorption in a given range, here 0.2-1.2µm. Fig. 6a shows a schematic view of the investigated structure. Here, we considered a more realistic situation where the PC is located on a glass substrate. Moreover, the air slits are filled with SiO2. The optimized absorption spectrum is displayed in Fig. 6b. This spectrum was found for a 1µm silicon thickness, a 490nm lattice parameter, and a 51% air filling factor. Absorption was also calculated for a reference unpatterned layer, as also shown in Fig. 6b. In the low wavelength range, the absorption is globally enhanced on the patterned structure, but no clear features appear. This is due to the strong optical resonance damping: at these wavelengths, the absorption coefficient of amorphous silicon is indeed very high (typically 106 cm-1). For wavelengths beyond 600nm, clear resonance peaks appear; they most probably combine vertical Fabry-Perot resonances and slow light modes. If we consider the whole range from 0.2 to 1.2µm the integrated absorption reaches 60% for TE polarization, i.e. for the same TE polarisation as in section 2. Comparing this value with the one calculated for the unpatterned layer, we can conclude that the integrated absorption was increased by a factor of 1.4 by patterning the PC on the silicon layer. More importantly, the increase can be much higher around wavelength corresponding to resonant modes, typically around 800nm. Lastly, the integrated absorption of TM polarized incident light reaches 59%. Therefore, even if the 1D PC structure is strongly polarization dependant, the negative impact of this property appears quite limited as far as the absorption integrated over a very wide wavelength range is considered.


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In this part, we showed that a simple PC structure may increase the absorption of a silicon layer over the whole solar spectrum. In the next section, we will investigate a more original photonic engineering where a PC structure is combined with a back side reflector.

4. INFLUENCE OF THE PATTERNING ON THE ELECTRICAL PROPERTIES OF THE SOLAR CELLS

In order to confirm the feasibility of a crystalline Si solar cell using PC and to evaluate the influence of surface recombinations on its photovoltaic efficiency, we realised 2D numerical simulation of such structure. As simulation


parameters, we select the surface recombination rate and carrier lifetime. For these numerical simulations of PC solar cell, the software package DESSIS, part of the modelling tools pack ISE TCAD 8.5 is used. It incorporates advanced physical models and robust numerical methods for the 2D simulation of semiconductor devices [7-8]. Because of the periodicity of the structure, a reference cell element was chosen for the simulation. A general view of the simulated structure is shown in Fig. 7. The solar cell thickness is 500 nm and the PC structure is as following: the corrugation thickness is 250 nm, its periodicity is 350 nm and the air filling factor is about 30%. Doping levels remain constant: 100 nm thick n+ emitter and p-type crystalline Si base. We consider transparent metallic contacts (no shadow losses) which cover all the top surface of the cell and complete absorption of the solar spectra thanks to the presence of PC. The surface of Si holes is covered by dielectric layer to reduce surface recombinations.

Fig 7: Simulated PC-based solar cell structure

If we consider a “perfect” solar cell, with no surface recombination and a 100 µs carrier lifetime, the final efficiency will be around 20%.However, we have to take into account the fact that if surface recombination can be limited on the Silicon surface using appropriate passivation treatment (SiNx, SiOx, a-Si ...etc.), it is more realistic to take the value of 106cm.s-1 for the surface recombination velocity at the Si-metal interface. Then efficiency will be limited to 10%. Moreover, if surface passivation into the holes is not optimised (surface recombination velocity value is then 105 cm.s-1), this efficiency will drop to 6%.

These preliminary simulation results show that it is possible to fabricate solar cell using PC with satisfactory efficiency, subject to an efficient surface passivation. In particular, a key point will be the definition of a collect structure with a low metallic contact surface. However, simulation of this structure has to be completed considering such parameters as thickness and doping of emitter, doping of Si absorbing layer and optimisation of metallic contact area.

5. CONCLUSION In this paper, we have presented the potential of planar photonic crystals patterned in an absorbing material for the realisation of a new class of photovoltaic solar cells. Constraints on the design of a PC structure in order to exploit surface addressable slow light modes that maximize light absorption have been discussed. In particular, we illustrated the importance of the matching between time constants related to the optical losses of the PC guided resonant mode (Q-factor) and the absorption losses of the unpatterned material (absorption coefficient at the resonant wavelength). A first example of such a PC solar cell design based on a:Si is a 1µm thick structure that combines many slow light modes, and that enables the absorption of about 60% of incident light. Lastly, first electrical simulation were performed in order to take into account the negative impact of the PC patterning on the surface recombinations, and finally on the efficiency. In a crystalline silicon solar cell, an efficiency between 6 and 20% is expected.

In future work, we will further optimize the photonic design, with the possible exploitation of a 2D PC structure, and of a multi-layered structure below the patterned absorbing layer. The electrical properties and global efficiency will then be calculated, depending on the real impact of the pattering on the surface recombination velocity. First prototype of such new solar cells will be fabricated and tested in the near future.

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ACKNOWLEDGEMENTS

This work was partly performed in the frame of the ANR SPARCS project. Yeonsang Park’s grant was financed by the French Embassy in Korea. Badhise Ben Bakir is acknowledged for his contribution on the photonic band structure simulation.

REFERENCES

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[2] Howard R. Stuart and Dennis G. Hall, “Absorption enhancement in silicon-on-insulator waveguides using metal island films”, Appl. Phys. Lett. 69, 2327 (1996)

[3] S. Pillai, K. R. Catchpole, T. Trupke, G. Zhang, J. Zhao, and M. A. Green, “Enhanced emission from Si-based light-emitting diodes using surface plasmons », Appl. Phys. Lett. 88, 161102 (2006)

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[5] M. Selim Ünlü and S. Strite, “Resonant cavity enhanced photonic devices”, J. Appl. Phys. 78, 607 (1995) [6] http://camfr.sourceforge.net/ [7] P.P. Altermatt, G. Heiser, T. Kiesewetter, K.R. McIntosh, C.B. Honsberg, S.R. Wenham, M.A. Green, 26th IEEE

PVSC, Anaheim, CA, 1997, pp. 179–182. [8] O. Nichiporuk, A. Kaminski, M. Lemiti, A. Fave,V. Skryshevsky: “Optimisation of interdigitated back contacts

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Photonic crystal assisted ultra-thin silicon photovoltaic solar cell

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