Performance Enhancement of Thin-Film c-Si Solar Cell with Group III

2013 IEEE Jordan Conference on Applied Electrical Engineering and Computing Technologies (AEECT)

Performance Enhancement of Thin-Film c-Si Solar Cell with Group III-V Material Grating Structures Farhana N. Rahman1, Md. Ibrahim Khalil1, Taohid Latif1, M. Aynal Haque2 1 Department of Electrical Engineering & Computer Science, North South University, Bangladesh 2 Department of Electrical & Electronic Engineering, Bangladesh University of Engineering and Technology, Bangladesh E-mail:[email protected], [email protected], [email protected], [email protected] Abstract—Efficient light trapping structures in a solar cell improves the overall performance of the devices and substantially reduce its overall cost. Even though thin-film photovoltaic (PV) offers significant cost reduction by reducing the overall thickness compared to traditional bulky thick film Photovoltaics, the overall performance is not optimal due to lower absorption properties. Thus to improve the overall efficiency the absorption of the PV cell has to be improved. In this paper, to improve the optical absorption of the PV cells, we present a new diffraction grating structure known as assembly grating by implementing the commercially available group III-V materials. Through optimization of the spacing of grating separations, hence the lattice constants and the ratio of each grating heights, we found 39.70% ultimate efficiency under terrestrial ASTM-1.5, 1 sun condition with Indium Phosphide (InP) as the grating material over the range of 400-2000 nm. Proposed structures show around 14.61% higher efficiency than same lattice constant c-Si thin-film photovoltaics. Keywords—thin-film photovoltaic, multi-junction material, diffraction grating, assembly grating. I.

INTRODUCTION

Silicon is the material of choice for photovoltaic applications due to its low cost, abundance nature, nontoxicity, long-term stability, and well-established technology. Thus, cost-effective PV technologies play an important role for the deployment of large-scale photovoltaic devices. The promising way to reduce the overall cost of PV cell is to make it as thin as possible. However, the overall efficiency in a conventional PV is not satisfactory which is attributed to the poor absorption in the visible spectrum where maximum spectral irradiance is present. In contrast thick film PV devices have better absorption capabilities due to larger optical paths. Even though thick film devices show wider spectral absorbance, the overall photocurrent is reduced as the minority carrier diffusion length is several times the material thickness. So, to collect all the photo carriers, the film thickness is required to be within smaller ranges. Thus, light trapping has become an important aspect of increasing the efficiency of crystalline Si solar cell.

978-1-4799-2303-8/13/$31.00 ©2013 IEEE

Besides, to increase the efficiency of a solar cell, it is essential to minimize the surface reflection at the incident (front) surface. At the same time, absorption in the active layer where photons are absorbed has to be increased. This can be done by increasing the optical path length (which is the distance light travels) in a wide wavelength range, i.e. retain the light within a solar cell for a longer time. Normal incident light striking the surface of the solar cell passes through the anti-reflective coating and the assembly grating structure and is absorbed by the active region of the PV layer made of c-Si, thereby generating electric current. As the light reaches the bottom of the layer, it is reflected by the metallic back reflector back to the active region of the PV layer where it is reabsorbed and photons are multiply reflected inside the grooves of gratings which lead to an increase in the optical path length and hence, the absorbance. Combinations of materials, such as Silicon-dioxide (SiO2), titanium dioxide (TiO2), Silicon nitride (Si3N4) and some transparent conductive oxides are commonly used as anti-reflection coating materials for c-Si [1]. With the addition of an idealized back reflector, there is no light transmittance; and the light that would otherwise pass through the substrate is reflected back into the active region where it can be reabsorbed. A diffraction grating is a periodic arrangement of grooves or slits where diffraction of light occurs. The groove has a period similar to the wavelength of light. Each groove or slit behaves like a source of light, i.e., for the reflected or transmitted light. The photons are multiply reflected inside the grooves of gratings which lead to increase in absorbance. Diffraction gratings are characterized by three structural parameters - the grating period (Λ), the grating depth (h), and the duty-cycle (f), which is described as the ratio between the groove width and the period. The main idea, behind integrating a diffraction grating is to not only increase the path length of diffracted waves, but also to ensure that the waves remain in the silicon structure by means of total internal reflection (TIR). Total internal reflection occurs when a light wave travelling in a high index material, is obliquely incident on an interface between the high index and another lower index material. If the light wave is incident at or beyond a critical angle, it gets totally reflected back into the higher index medium. This TIR phenomenon follows from Sell’s law of refraction, in the case where the angle of refraction is 90


degrees; since the argument of the sine function cannot exceed unity. It should be noted that this condition only holds for materials with identical permeability, which is true for most dielectrics. The assembly grating structure uses different combinations of different gratings within a period to achieve high absorption through the visible range where most of the spectral irradiance is concentrated. The optimum grating ratio1:2:3was found for different values of h1, h2 and h3 [2].

TABLE I: BAND GAP AND LATTICE CONSTANT OF THE MATERIALS. Materials Eg(eV) Lattice Constant(Ǻ) Crystalline Silicon (c-Si)

1.12

5.431

Indium Phosphide (InP)

1.35

5.869

Indium Gallium Arsenide (InGaAs)

1.2

5.868

Gallium Phosphide (GaP)

2.26

5.451

Gallium Arsenide (GaAs)

1.42

5.653

In fig. 2 we measured the optical absorbance of each materials and found InP and InGaAs are the potential candidate for the group III-V photovoltaic materials as it provides better absorption throughout the visible ranges.

w

1

InP InGaAs GaAs GaP c-Si

(a)

(b)

Fig.1. (a) Schematic of the cross-sectional view of the typical grating surface of a solar cell. The structure consists of a substrate and grating surface featured with width (w), depth (h) and period (Λ), (b) Assembly Grating Structure (h1:h2:h3 = 1:2:3 and w1=w2=w3) [2].

II.

DESIGN AND ANALYSIS

In this paper, we propose a thin-film c-Si solar cell of 500nm thickness using assembly of diffraction grating structures made of cost effective commercially available group III–V materials for light trapping, anti-reflective coating to reduce the surface reflections and Aluminium (Al) back reflector to reduce the transmittance through the photovoltaic cell. The proposed technique will reduce the thickness and weight of the PV cell as no extra layer of material is required for grating while maintaining good absorption characteristics and cell efficiency. As amount of material needed in manufacturing each solar cell is reduced, the cost per watt of output power is ultimately reduced. The thickness of the c-Si is kept same as the ref. 3 and then different multi-junction photovoltaic materials have been investigated on top of it. A wide spectrum of 400-2000 nm is chosen to harness the maximum possible solar energy. To perform the numerical analysis we have considered transverse electric field (TE), i.e. and periodic boundary conditions. The incident ray is directly fallen perpendicularly on the devices and it propagates through the devices. We evaluated the performances using the nanohub photonicsSHA2D simulation tools [4]. During the simulation crystalline silicon (c-Si) is considered as the active photovoltaic layer. Materials that have been investigated for diffraction grating structure are – Indium Phosphide (InP), Indium Gallium Arsenide (InGaAs), Gallium Arsenide (GaAs) and Gallium Phosphide (GaP). In the table below, the list of materials with their corresponding band gaps are mentioned. The materials used in the structures are based upon their corresponding band gaps and stacked from high to low order.

Absorptance

0.8 0.6 0.4 0.2 0 400

500

600

700

800

900

1000

1100

1200

Wavelength (nm)

Fig.2. Absorption profile of the materials listed in the Table-I

To make the optimization, the following design considerations are followed a. An active photovoltaic region made of c-Si and 500nm thickness. b. A top surface of multi-layered anti-reflective coating made of Silicon dioxide (SiO2) and silicon nitride (Si3N4). c. Assembly diffraction grating made of Indium Phosphide (InP). d. A metallic back reflector made of Aluminium as it provides zero transmittance. Then, the Ultimate Efficiency [5] of the Photovoltaic Cell is calculated using the following formula. λg

∫

300 nm

n=

I (λ) A(λ)

4000 nm

∫

300 nm

λ dλ λg

(1)

I (λ)dλ

where, A(λ) is the absorbance of the devices, I(λ) represents the spectral irradiances of ASTM-1.5, λg is the maximum limit of device absorption wavelength .To evaluate the ultimate efficiency, η we have used Simpsons 3/8 numerical integration methods.


At first, a c-Si of 500nm is investigated with an incident light wave perpendicular on the c-Si surfaces, which is propagating through the silicon. From fig.3 it can be observed that a significant portion of the light is lost due to reflectance and transmittances. Thus the devices suffer significantly a low efficiency of 7.5967%. 1

Absorbance Transmittance Reflectance

Absorptance

0.8 0.6

light through the solar cell. Hence, with the increased path length, the effective thickness of the active region is essentially increased, and thus allows more photons, with longer absorption depths, to be absorbed. The reflectance increases significantly from that of the case of bare c-Si. This happens because of the light that is reflected from the back surface and not yet absorbed within the active silicon region. This light, that would otherwise have been transmitted, now contributes to the overall reflective losses from the solar cell structure. These can be observed in fig.4 and 5. The cell now shows an efficiency of 25.0874%. Amount of Absorptance

1 0.4

0.8 Absorbance Transmittance Reflectance

Absorptance

0.2

0.6

0 400

500

600

700 800 900 Wavelength (nm)

1000

1100

1200

0.4

Fig.3. Absorbance, transmittance and reflectance of thin film crystalline silicon solar cell.

Secondly, the c-Si is investigated with two layers of anti-reflective coating made of SiO2 (66nm thickness) and Si3N4 (34nm thickness), then a back reflector made of aluminum (40nm thick). Two anti-reflective coating materials were used as the refractive index of materials varies for different wavelengths over the whole solar spectrum. Multiple layers of materials of increasing refractive index gently increase the index variation from air which will let maximum amount of incident light into the cell allowing more light that actually enters the active photovoltaic region. We found that the front surface reflectance is reduced due to the multi-layered antireflective coating and the back reflector reduces the transmittance from the active photovoltaic region. Incident Light

0.2 0 400

600

800

1000

1200

1400

1600

1800

2000

Wavelength (nm)

Fig.5. Absorpbance, Transmittance and Reflectance of thin film c-Si solar cell with AR Coating and aluminium back reflector. It is depicted that the transmittance is zero due to the use of back reflector.

Finally, c-Si with assembly diffraction grating made of InP (h1:h2:h3 = 66:132:200), AR (anti-reflective) coating and back reflector is investigated. Incident Light

Light

Reflected Light

AR Coating

SiO2 Si3N4

AR Coating

Assembly Grating made of InP

SiO2 Si3N4

Absorbed Light Absorbed Light

C-Si

Double Optical Path Length

No Transmitted Light

Aluminium back reflector

C-SI

No Transmitted Light

Aluminium back reflector

Light reflected back and forth continuously

Fig.4. Schematic of incident light on the surface of c-silicon with layered anti-reflection coating and metallic back reflector.

With an idealized back reflector, no light is transmitted through the devices and the optical path length is doubled but the absorption does not increase significantly due to low absorption profiles of c-Si. This is because many of the shorter wavelengths are absorbed within the first pass of

Fig.6. Schematic of a plane light wave incident on the surface of c-silicon with an added AR coating, back reflector and diffraction grating.

Fig. 7 shows that with the addition of the InP grating the absorption has increased further because the optical path


1

ASTM-1.5 and Thin Film c-Si with Grating

1.5 Spectral Irradiance W m-2 nm -1

length has further increased. However, there is also increased reflectance due to the out coupling of the diffracted or reflected light. This thin film c-Si with diffraction grating made of InP shows the maximum efficiency of 39.6999%.

ASTM-1.5 c-Si Thin Film with InP Grating c-Si Thin Film without Grating bare Thin Film c-Si

1

0.5

Absorptance

0.8 Absorbance Transmittance Reflectance

0.6

0 400

800

1000

1200

1400

1600

1800

2000

Wavelength (nm)

Fig.7. Absorptance, Transmittance and Reflectance of thin film c-Si with assembly diffraction grating made of InP, AR Coating and Back Reflector.

Fig. 8 shows the overall absorption comparisons of bare cSi, c-Si with AR (anti-reflective) coating, with back reflector, both the AR (anti-reflective) coating and back reflectors and c-Si with assembly grating structure made with InP. It can be seen that the grating structure exhibits the maximum absorption. 1

with InP Grating Witout Grating with Back Reflector only with AR coating only c-Si Thin Film

Absorptance

0.8 0.6

1200

1400

1600

1800

The structure was also investigated by varying the number of gratings in a period and was found that as the number of gratings increases, the efficiency also increases, which are shown in table II and III. From the table we can conclude that, through optimization of the grating heights, we can achieve superior absorption efficiency.

TABLE II: THEORETICAL EFFICIENCY UNDER TERRESTRIAL ASTM 1.5 UNDER 1 SUN CONDITION Structure of thin film c-Si solar cell

Efficiency η(%)

Crystalline Silicon Only with AR coating only With Back Reflector only with AR Coating and Back Reflector with assembly diffraction grating made of InP, AR Coating and Back Reflector.

7.5967 10.8594 20.0934 25.0874 39.6997

TABLE III: THEORETICAL EFFICIENCY UNDER TERRESTRIAL AM 1.5 UNDER 1 SUN CONDITION WITH VARYING NUMBER OF GRATINGS

0.4 0.2 0 400

1000

Fig.9.Absorptance of the InP assembly grating structure in thin film c-Si in comparison with terrestrial ASTM 1.5 under 1 sun condition.

0.2 600

800

Wavelength(nm)

0.4

0 400

600

600

800

1000

1200

1400

1600

1800

2000

Number of gratings within a fixed period with depth ratios two, h1:h2 = 1:2 three, h1:h2:h3 = 1:2:3 four, h1:h2:h3:h4= 1:2:3:4

Efficiency (%) 38.9739 39.6999 39.9569

Wavelength (nm)

Fig.8. Comparison of absorbance among thin film bare c-Si, c-Si without and with grating

Fig. 9 shows the comparison of absorbance of the InP assembly grating structure with terrestrial ASTM 1.5G under 1 sun condition [6]. It can be seen that it absorbs the maximum energy over 400-2000 nm of solar spectrum.

III.

CONCLUSION

In this paper we proposed and demonstrated the design of a thin-film crystalline silicon solar cell with multijunction group III-V material grating structures. Through optimization of the grating heights we found that, the assembly grating structure has significant photon absorption capabilities in the initial range of 400-900 nm and moderate absorption in the 900-2000 nm range thus having overall high absorption in a wide solar spectrum. Although only the normal incident was considered, we would also take into account the angular sensitivity of the fallen light on the cell for future research. Furthermore, we have calculated the theoretical conversion efficiency of the thin film crystalline Silicon with Indium Phosphide as the grating material and


compared with the other three structures. The theoretical efficiency of 39.6999% indicates that the use of cheap multi-junction materials in diffraction grating can be actualized in the future. REFERENCES [1]

[2]

[3]

[4]

[5]

[6]

Müller, J., Rech, B., Springer, J., & Vanecek, M. (2004). TCO and light trapping in silicon thin film solar cells. Solar Energy, 77(6), 917-930. Elsevier. Yang, L., Xuan, Y., Han, Y., & Tan, J. (2012). “Investigation on the performance enhancement of silicon solar cells with an assembly grating structure”. Energy Conversion and Management, 54(1), 3037. Elsevier Ltd. Mutitu, J. G., Shi, S., Chen, C., Creazzo, T., Barnett, A., Honsberg, C., & Prather, D. W. (2008). “Thin film silicon solar cell design based on photonic crystal and diffractive grating structures”. Optics Express, 16(19), 15238-15248. Ni, X., Liu, Z., Gu, F.,Pacheco, M. G., Borneman, J., and Kildishev, A., V,“PhotonicsSHA-2D: Modeling of Single-Period Multilayer Optical Gratings and Metamaterials,” https://nanohub.org/resources/sha2d. M. I. Khalil, A. Rahman, A. M. Chowdhury, & G. -K. Chang, “Optical Absorption Enhancement in Solar Cell Employing Plasmonic Nanowire as the Core of C-Si Nanowire",Conference on Lasers and Electro-Optics (CLEO) June(9-14),2013, San Jose, California, USA. Paper-CF2E.7 ASTM, “Reference Solar Spectral Irradiance: Air Mass 1.5 Spectra [Online] (2011). Retrieved February 4, 2013 from http://rredc.nreal.gov/solar/spectra/am1.5