An efficient hybrid solar and electromagnetic harvesting system for autonomous operation of small sensors

Journal Pre-proof An efficient hybrid solar and electromagnetic harvesting system for autonomous operation of small sensors Fayc¸al Meddour, Hichem Bencherif, Boukarkar Abdelheq, Mohamed Amir Abdi, Mounir Amir

PII:

S0030-4026(19)31921-7

DOI:

https://doi.org/10.1016/j.ijleo.2019.164022

Reference:

IJLEO 164022

To appear in:

Optik

Received Date:

28 October 2019

Accepted Date:

6 December 2019

Please cite this article as: { doi: https://doi.org/ This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Fayçal Meddour1,2,4,* [email protected], Hichem Bencherif1,3,4, Boukarkar Abdelheq5, Mohamed Amir Abdi1,6 and Mounir Amir7

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University of BATNA 2, Faculty of Technology, 05000 Batna 2, Algeria Laboratoire d’Electronique Avancée-LEA, 05000, Batna 2, Algéria 3 Laboratory of Metallic and Semiconductor Materials (LMSM) 1University of Biskra 4 Advanced Automatic and Systems Analysis Laboratory (LAAAS) 5 Ècole Supérieur Ali CHABATI, Algiers, Algeria 6 Laboratoire des Etudes Physico-Chimique des Matériaux-LEPCM, 05000, Batna 1, Algéria 7 University of mouloud mammeri of tizi-ouzou

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Highlights

Abstract

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a combination of solar cells and a rectenna is proposed to realize a solar and electromagnetic energy harvesting system. SHJ solar cell with a combination of AZO/Tio2 as antireflection coating and a diffraction grating morphology is proposed. SHJ solar cell carefully placed near the slot loop antenna, which is attached to the rectifier circuit, to realize the combined harvesting system Simulation and experimental results validate that the combined harvesting system is a promising candidate for autonomous operation of some sensors and small devices.

In this study, we propose a combination of solar cells and a rectenna to realize a solar and electromagnetic

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energy harvesting system. The proposed harvester consists of a circular slot antenna, a rectifier circuit which is designed with a reverse-biased Schottky diode, and SHJ solar cells with a combination of AZO/Tio2 as antireflection coating and a diffraction grating morphology. Each designed component has been characterized through simulations and measurements. First, the solar cells are designed and investigated separately. Then, they are carefully placed near the slot loop antenna, which is attached to the rectifier circuit, to realize the combined harvesting system. The heterojunction solar cells with an intrinsic thin layer (SHJ) have been designed based on light trapping mechanism, which reduces the reflection of solar radiation and interfacial 1

traps effects significantly. Solar cells’ simulations are obtained using an analytical model. The parameters of light trapping, as well as the effects of the position of the solar cells on board of the patch, are studied carefully. Simulation and experimental results validate that the combined harvesting system is a promising candidate for autonomous operation of some sensors and small devices. Indeed, the efficiency of the rectenna is enhanced greatly from 21% to 74% when combined with the solar cells.

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Keywords: Solar cells, rectenna, rectifier circuit, energy harvesting.

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1. Introduction

CRYSTALLINE silicon (C-Si) solar cells are of great importance in photovoltaic (PV) technologies since they

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are used in the fabrication of devices with high efficiency [1-4]. However, because of their high temperature

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of processing and growing consumption of material, ordinary c-Si solar cells are considered expensive. Hence, several low-cost solar cells design based on novel materials and thin-film geometries have been

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investigated [5-7]. The hydrogenated amorphous silicon (a-Si:H) is considered as a promising candidate for PV as it exhibits high absorption coefficients throughout the solar spectrum and full compatibility with the standard Si-production processes [8-10].

To enhance the efficiency of SHJ solar cells, a multilayer antireflection coating (ARC) approach is

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considered as the most appropriate solution. Indeed, when a multilayer structure is used, the optical losses caused by the incident solar radiation reflectance are reduced greatly [11-14]. In this work, a multilayer ARC technique and a grating design parameter analysis are considered. Besides, the presented analysis is compared with experimental data, which may serve as useful guidelines for further improvements in the design of amorphous/crystalline c-Si-based solar cells. Based on a defect density of 1011cm-2, Tucci et al. [15] have demonstrated in their experimental results that the maximum obtained 2

efficiency, for a planar structure, is about 16%. In the proposed work, to improve the mentioned-above efficiency, we have set two goals: First, based on the experimental results achieved by Tucci et al. [15], we have established a conventional analytical model [16]. The constructed results indicate that the solar cell’s current-voltage characteristics agree well with the experimental results for an interface state density of 1011 cm-2 and 1013 cm2

. Secondly, we have proposed a new model for the geometrical shape’s optimization to design and construct

a double layer antireflection coating (DLARC) and a triangular texture morphology targeting the

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achievement of high electrical and reliability performance of the solar cell. The proposed technique focuses

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on increasing the device’s immunity against interfacial defects. The proposed design offers an excellent opportunity for the selection of the material’s type, the thickness of each ARCS layer, as well as the angular

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parameters of the triangular morphology. The reflectance is reduced by destructively interfering the reflected light over a dedicated range of wavelengths. The proposed solar cell, with DLARC and a triangular texture

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morphology, has demonstrated electrical improvement and reliability for double layer (AZO / TiO2) coating

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design compared with the single layer antireflection coating SLARC (ZnO). What is more, the proposed solar cell has shown that, for adequate light trapping and DLARC film-coating parameters, the short-circuit

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current, and the efficiency are improved by about 19.47% and 21.56%, respectively. On the other hand, in the absence of light, the solar cells may not collect the energy efficiently. Therefore, in this study, we propose to combine the proposed solar cells with a rectenna (antenna and rectifier circuit) to harvest the energy regardless the intensity of light and therefore to increase the efficiency significantly. The rectenna collects

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the electromagnetic energy transmitted by the surrounding wireless devices, as explained in [17]. The collected energy can be used for a sustainable power supply targeting autonomous operation of some sensors and small devices.

In this paper, first, the design of the solar cells is explained in detail, and a comparison study with a conventional approach is given to highlight the importance of this work. After that, the solar cells are combined with a proposed rectenna to enhance the efficiency. Simulated results are carried out to validate the proposed method. Finally, a general conclusion of this work is given. 3

2. Methodology 2.1 Device Geometry Fig. 1 (a) shows the geometry of the proposed SHJ solar cell. It includes the following layers: a DLARC (AZO and TiO2), a triangular textured a-Si:H n-layer, a central region, a p/p+ c-Si junction, and an Al-based ohmic contact located at the bottom side. The fabricated DLARC can be obtained based on several methods, for instance, by reactive sputtering, thermal evaporation, and sol-gel methods [18-20]. The DLARC and the proposed cell’s texture morphology

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are depicted in Fig. 1 (b). We note that the considered ray propagation through the layers is demonstrated as

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well. According to fig. 1(b), for a triangular surface of the texture, the incident light can be trapped through the shape of the structure to establish an injection mechanism with a double light. Hence, the majority of the

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light is trapped into the solar cell.

(a)

(b)

Fig.1 (a) The proposed solar cell’s side view. (b) The DLARC (AZO/TiO2) and triangular texture

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morphology representation.

In Fig.1 (b), θiA represents the angle of the incident light on the plane A, ψ is the angle of the texture, H represents the triangle’s height, while d1 and d2 stand for the thicknesses of ARC1 and ARC2 layers, respectively. During the simulations, we have considered several angular parameters of the triangular morphology, including different refractive indices to decrease the reflectance loss of light incidence over a wide range of 4

wavelengths. Also, we have tuned the thicknesses of the ARC layers, accordingly. We have concluded that the reflected light’s total amount, via the different layers, depends strongly on the material of the ARC as well as the trapping geometry. 3

ANALYTICAL MODEL

3.1 REFLECTED INCIDENT LIGHT For a normal incidence penetrating light through DLARC structure, with different refractive indices and

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different thicknesses, and by considering the transmitter’s texture morphology, the matrix of the two stacked

i k 0 n j d j cos pj 1

1 ik 0 p j n j d j cos

j

j

(1)

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Mj

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layers can be reformulated as in [21]:

2 / ) , and dj and nj ( j = 1, 2) stand for each

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where k0 represents the propagation vector in the vacuum (k 0 layer’s thickness and the refractive index, respectively.

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By applying the Fresnel formulae and based on Fig. 1 (b), the reflection coefficient, describing the mutual

r12

n1 cos n1 cos

1 1

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optical effect between ARC1 and the air, is shown in [22] n2 cos n2 cos

(2)

2

2

Regarding the parameter r23, we consider an analog expression at the interface of ARC1/ARC2. The total

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reflection coefficient r34 is obtained by considering the shape of the structure at the interface of ARC2/a-Si:H (n region). Particularly, r34 = RARB when we assume that RA and RB are the reflection coefficients at the walls A and B, respectively. For a non-polarizing incident light ray and by taking into account its perpendicular and parallel components, we can express [23]:

r34

R || R 2

R A|| RB||

R A RB

(3)

2

5

n4 cos n4 cos

where for the surface A: R A||

and for the surface B: RB||

n4 cos n4 cos

n3 cos n3 cos

iA iA

n3 cos n3 cos

iB iB

tA

(4a)

n3 cos n3 cos

RA

tA

(4c)

tB

n3 cos n3 cos

RB

tB

n4 cos n4 cos

iA iA

n4 cos n4 cos

iB iB

(4b)

tA tA

(4d)

tB tB

In this part, the subscripts i and t stand for the incident and transmitted light beam, respectively. Based on the law of Snell-Descartes, which can be applied to this system, we get:

(5a) iA

tA

1

1 sin n23

1

iA

(5b)

sin

tB

1 sin n23

(5c) where n23 = n3/n2.

iB

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iB

sin

r 2.

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In the end, by employing the mentioned-above expressions, the reflectivity can be obtained as R

Taking into account the path of light in the different regions of the textured solar cell, the light absorption

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phenomenon in these regions is modeled by an effective absorption coefficient 𝛼̅ textured [24].

3.2

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𝛼̅ textured=αflat/ cos θtA Photocurrent

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In our proposed solar cell, the total density of the photocurrent can be obtained by solving Poisson’s equation and carrier continuity equations in all the regions of the device:

J ph

J ph, E

J ph, D

J ph, B

(6)

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where Jph,E is the current density of the light generated in the transmitter layer, Jph,D in the junction spacecharge region, and Jph,B in the base region, respectively. For the n-type a-Si:H region [25]:

𝐿𝑝 2 𝛼𝑎−𝑆𝑖:𝐻 2 −1

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𝐽𝑝ℎ, 𝐸 (𝜆) = [

𝑞𝜑(1−𝑅)𝛼𝑎−𝑆𝑖  𝐿𝑝

𝑆 𝑝 𝐿𝑝

𝑆 𝑝 𝐿𝑝

] × −𝛼𝑎−𝑆𝑖 𝐿𝑝 𝑒𝑥𝑝(−𝛼𝑎−𝑆𝑖 𝑊𝐸 ) + [

𝑊

𝑊

𝑆𝑝 𝐿𝑝

𝑊

𝑊

( 𝐷 )𝑠ℎ( 𝐿 𝐸 )+𝑐ℎ( 𝐿 𝐸 ) 𝑝 𝑝 𝑝

−

𝑊

𝑒𝑥𝑝(−𝛼𝑎−𝑆𝑖 𝑊𝐸 ) ( 𝐷 𝑐ℎ( 𝐿 𝐸 )+𝑠ℎ( 𝐿 𝐸 )) 𝑝 𝑝 𝑝 𝑆𝑝 𝐿𝑝

( 𝐷 +𝛼𝑎−𝑆𝑖 𝐿𝑝 ) 𝑝

𝑊

( 𝐷 )𝑠ℎ( 𝐿 𝐸 )+𝑐ℎ( 𝐿 𝐸 ) 𝑝 𝑝 𝑝

]

(7)

where WE is the thickness of the emitter, Sp is the effective recombination velocity corresponding to this layer, and Lp and Dp are the minority carrier diffusion length and diffusion constant, respectively. For the space-charge zone [25]: 6

J ph, D 1 e

a Si WE

q 1 Re a SiW1

(8)

a SiW1

e

1 e

c Si W2

where W1 and W2 represent the edges of the space-region. For the p-type c-Si region, Jph,B(λ) is the sum of two terms as explained and given in [26]: J A ph , B

J ph, B

(9)

J R ph, B

where JAph,B is in relation with the direct absorption of the light, and it can be expressed as follows [26], 𝑞𝜑(1−𝑅)𝛼𝑐−𝑆𝑖 𝐿𝑛

(𝜆) = (

𝛼𝑐−𝑆𝑖 2 𝐿𝑛 2 −1

) 𝑒𝑥𝑝   (−𝛼𝑎−𝑆𝑖  (𝑊𝐸 + 𝑊1 ) − 𝛼𝑐−𝑆𝑖  𝑊2 ) × [𝛼𝑐−𝑆𝑖  𝐿𝑛 −

𝑆𝑘 𝐿𝑛 𝑊 𝑊 ( 𝐷 )(𝑐ℎ ( 𝐿 𝐵 )−𝑒𝑥𝑝(−𝛼𝑐−𝑆𝑖 𝑊𝐵 ))+𝑠ℎ ( 𝐿 𝐵 ) 𝑛 𝑆 𝐿𝑛 𝑊 𝑊 ( 𝐾 )𝑠ℎ ( 𝐿 𝐵 )+𝑐ℎ ( 𝐿 𝐵 ) 𝐷𝑛 𝑛 𝑛

𝑛

+

𝛼𝑐−𝑆𝑖 𝐿𝑛 𝑒𝑥𝑝  (−𝛼𝑐−𝑆𝑖 𝑊𝐵 ) 𝑆 𝐾 𝐿𝑛 𝑊 𝑊 ( 𝐷 )𝑠ℎ ( 𝐿 𝐵 )+𝑐ℎ ( 𝐿 𝐵 ) 𝑛 𝑛 𝑛

]

(10)

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𝑛

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, 𝐵

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𝐽 𝐴 𝑝ℎ

where Sk represents the effective recombination velocity at the backside of the solar cell, and WB is the

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thickness of the base. Based on the equation of the diffusion, which affects the reflected light, the photocurrent

𝐽𝑅 𝑝ℎ

(𝜆) = 𝑞𝑅𝑑 (1 − 𝑅) , 𝐵

𝑆𝑝𝑝+ 𝐿𝑛 𝑊 )(𝑐ℎ ( 𝐿 𝐵)−𝑒𝑥𝑝(−𝛼𝑐−𝑆𝑖 𝑊𝐵 )) 𝐷𝑛 𝑛 𝑆𝑝𝑝+ 𝐿𝑛 𝑊 𝑊 ( 𝐷 )𝑠ℎ ( 𝐿 𝐵 )+𝑐ℎ ( 𝐿 𝐵 ) 𝑛 𝑛 𝑛

(

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[𝛼𝑐−𝑆𝑖  𝐿𝑛 +

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JRph,B can be formulated as follows [26]:

𝐿 𝑛𝛼𝑐−𝑆𝑖 𝜑 𝑒𝑥𝑝( − 𝛼𝑐−𝑆𝑖 𝑊𝑐𝑒𝑙𝑙 ) × (𝛼𝑐−𝑆𝑖 2 𝐿𝑛 − 1) 𝑊

+

𝑠ℎ ( 𝐿 𝐵 )−𝛼𝑐−𝑆𝑖 𝐿𝑛 𝑒𝑥𝑝  (𝛼𝑐−𝑆𝑖 𝑊𝐵 ) 𝑛 𝑆𝑝𝑝+ 𝐿𝑛 𝑊 𝑊 ( 𝐷 )𝑠ℎ ( 𝐿 𝐵 )+𝑐ℎ ( 𝐿 𝐵 ) 𝑛 𝑛 𝑛

]

(13)

where Wcell is the thickness of the entire cell, Rd is the reflectance at the back region, and Spp+ is the velocity of

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the effective back-surface recombination.

In the end, we obtain the total density of the photocurrent density by integration of (6) on the considered range of the wavelength and by using the reflection coefficient R as explained in the section above. 3.3

Current-Voltage Behavior

By taking into account the equivalent circuit of the solar cell’s two-diode, the curve of the current-voltage is displayed in [27] 7

J (V )

J ph

J 0D e

V JRS Vt

1

J 0R e

V Vt

1

V

JRS

(14)

RSH

In (14), J0D stands for the current density of the reverse dark, and J0R represents the current density of the generation-recombination. The series resistance is represented by Rs, while the shunt resistance is denoted by Rsh. Based on the curve of J-V, the fill factor of the solar cell (FF) and the conversion efficiency (η) can be obtained based on the following two equations: Pm ( R) J SC ( R)VOC ( R)

(15)

( R)

Pm ( R) (16) Pi

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FF ( R)

where Pm stands for the maximum power point, JSC is the current density of the short circuit, and VOC

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represents the voltage of the open circuit. We note that these three parameters depend on the net reflection

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coefficient R. Finally, by integrating the spectral irradiance of the solar cell IRS(λ), we obtain the incident power Pi: IRS

d

(17)

Based on (17), the analytical expression of IRS(λ) can be written as follows:

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max

Pi

min

0.06977 7.0625 1 e

0.2605 0.15994

2.28411

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IRS

e

0.26053 0.22285

(18)

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Equation (18) can be regarded as the best fit for describing the behavior of the real solar spectral irradiance when 0.3μm ≤ λ≤ 2.5μm as explained in [28]. Based on the mentioned-above computations, the proposed solar cell’s main parameters, including, VOC, JSC,

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and η, have been normalized according to the values calculated for a fresh device (Di = 0). The goal of this study step is to demonstrate the immunity behavior of the performance of the optimized solar cell against the degradation effect because of the existence of an explicit interfacial defect. 4. Results and Discussions 4.1 Model validation

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The analytical model, as well as the experimental devices J-V curves are shown in Fig. 2. The proposed model agrees well with the experimental data given in [15]. Table I. The reference parameters of the SHJ solar cell (see Fig. 1)

ARC1 thickness, d1 (nm)

80

(n) a-Si:H doping Nd (cm )

1019

(p) c-Si doping Na (cm-3)

1015

(p+) c-Si doping Na+ (cm-3)

1019

Defect density, Di (cm-2)

1011, 1013

(n) a-Si:H thickness WE(nm)

44

(i) a-Si:H thickness Wi (nm)

17

(p) c-Si thickness WB (µm)

325

(p+) c-Si thickness WBSF

18

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(nm) 40

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35

20 15

5 0 0,0

Lines : Analytical model Symbols: experimental results [38] 13 -2 Planar SHJ (Di =10 cm ) 10 -2 Planar SHJ (Di =10 cm )

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25

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Current (mA)

30

10

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1.6

ARC1 refractive index, n1

0,1

0,2

0,3

0,4

0,5

0,6

Voltage (V)

Figure.2 I-V The conventional solar cell’s characteristics for Di=1011cm-2 ,1013cm-2

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Table II. Analytical model output parameters Figure of merits Jsc (mA/cm2) Voc (V) FF (%) (%)

Analytical model (Di=1011cm-2) Experimental results (Di=1011cm-2) [15] 38 0. 602 70.11 16.04%

38.2 0.602 69.5 16%

Table III. The parameters used in the calculations for heterostructure solar cell

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Experimental [15]

traps 80 30 1.84 2.3 50°

80 / 2 (ZnO) / / / / / / / 1011 15 10 348 5

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2×1019 1015 1019 1011 30 17 325 18

Reliability assessment against interfacial traps effect

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A-

Proposed a-Si:H/c with

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parameters Light trapping parameters AZO thickness, d1 (nm) TiO2 thickness, d2 (nm) AZO refractive index, n1 TiO2 refractive index, n2 Texture angle, ψ Texture height, H (nm) SHJ parameters (n) a-Si:H doping (cm-3) (p) c-Si doping (cm-3) (p+) c-Si doping (cm-3) Defect density, Di (cm-2) (n) a-Si:H thickness (nm) (i) a-Si:H thickness (nm) (p) c-Si thickness (μm) (p+) c-Si thickness (nm) 4.2 Solar cell study

Fig.3 FOMs variation as a function of interfacial traps density The variations of the FOMs of the solar cell versus the defect density in both cases, the typical solar cell, and the proposed geometry, are shown in figure 3. We note that the proposed SHJ solar cell provides an essential improvement in the immunity of the device. This enhancement is realized based on tuning the DLARC parameters and optimizing the triangular texture morphology. 10

11

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Light trapping parameters effect

(b)

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B-

Fig.4 Conversion efficiency and short circuit current as a function of antireflection coating thickness

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a) AZO thickness effect D2=40nm, b) TiO2 thickness effect D1=40nm

Fig. 4 demonstrates the impact of the thicknesses of the two layers, AZO and TiO2, on the efficiency of the

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solar cell, and the current of the short circuit as well. We note that the efficiency and the current of the short

4n ARC 1, 2

20

3 5 ; . 4n ARC 1, 2 4n ARC 1, 2

(19)

18

16

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Conversion efficiency (%)

;

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d1, 2

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circuit increase as the thicknesses d1 and d2 in Fig. 1 (b) have the form of

14

DLARC (AZO/TiO2) SLARC (ZnO)

12

10 0

20

40

60

80

100

Angle of texturisation (°)

Fig.5 Conversion efficiency versus the angle of texturization for both SLARC and DLARC. (for both d1=80, d2=30nm ) 12

The conversion efficiency of the solar cell versus the texture angle is depicted in Fig. 5. It is worth noting that the highest value of η is obtained for a texture angle of 50°.

50 45

35 30 25 20 11

15

-2

The Proposed design (Di =10 cm ) 11 -2 conventional design (Di =10 cm )

10

of

-2

Current density (mA/cm )

40

5 0 0,0

0,1

0,2

0,3

0,4

0,5

0,6

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Voltage (V)

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Fig.6 I-V characteristics comparison for both proposed and conventional designs

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The conventional and proposed devices’ J-V curves are plotted in Fig. 6, and the photovoltaic parameters obtained from each curve are displayed in Table 3. We note that the proposed triangular textured solar cell

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with the DLARC (AZO/TiO2) performs an improved short circuit current density compared with a conventional planar c-Si-based SHJ solar cells with a SLARC(ZnO). This improvement is mainly due to the

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superior light trapping ability (effective photon absorption) across the solar spectrum. Although there is an interfacial defect density of 1011 cm-2, the proposed design exhibits a fill-factor of 70.2% and a conversion efficiency of 19.45 %.

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Table.IV. Electrical output parameters of the experimental and the proposed solar cells Figure of merits Jsc (mA/cm2) Voc (V) FF (%) (%)

Proposed a-Si:H/Si (Di=1011cm-2)

Experimental (Di=1011cm-2) [15]

46 0. 601 70.2 19.45%

38.2 0.602 69.5 16%

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4.3 Application of the solar cell: TABLE.V. Photovoltaic Parameters JD1 408.7 I(mA/cm2) D1 (mA) 1634.8

JD2 4.3 I(mA/cm2) D2 (mA) 17.2

RS 1.7 RS 6.8

Rp 105 Rp 4*105

J0 367.1 I(mA/cm2) 0 (mA) 1468.4

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proposed solar cell

JPV 46 I(mA/cm2) PV (mA) 184

Fig.7 Circuit diagram of the hybrid RF solar harvester.

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Recently, the need for a sustainable power supply targeting autonomous operation of some sensors and small devices has drawn much attention [29-31]. As an application, the proposed solar cell can be combined with a rectenna to realize a solar and electromagnetic energy harvesting system. Indeed, the rectenna (antenna + rectifier) collects the ambient RF (radio frequency) electromagnetic power and converts it into DC power. In

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the presence of light, the solar cell can convert the solar energy into DC supply power. Although the rectenna harvests energy regardless of the intensity of light; the amount of the collected DC energy is found to be small and insufficient to drive some devices [32]. On the other hand, the systems relying only on the energy collected by solar cells will fail in the absence of light. Hence, the combination of both of them (rectenna + solar cell) will enhance the performance of the wireless systems greatly.

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Fig.8 The proposed antenna with the embedded solar cell (unit: mm).

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The rectenna proposed in [32] consists of a circular slot loop fed by a coplanar waveguide etched on the same surface. The substrate used for the fabrication is of type ARLON AD1000 with a relative permittivity of

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10.35, a loss tangent of 0.0023, and a thickness of 0.762 mm. The rectenna exhibits an efficiency of only

74% without affecting much the antenna characteristics.

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21% at 2.45 GHz. Therefore, we propose to integrate the designed solar cell to improve the efficiency up to

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Fig. 8 shows the proposed antenna with the embedded solar cell. The solar cell with an area of 4 cm2 (2 cm x 2 cm) is positioned appropriately near the slot loop to occupy less overall size. The position of the solar cell

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is chosen after validation by simulations. High-Frequency Structure Simulator (HFSS) software is used to perform different simulations. The solar cell is modeled according to its physical geometry including the

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seven layers for more accurate results. Fig. 9 depicts the antenna reflection coefficient |S11| before and after inserting the solar cell. It is noticed that the two plots overlap which validates that the position of the solar cell doesn’t affect the antenna reflection coefficient. We would also like to mention that the antenna

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resonates around 2.45 GHz as reported previously in [32].

Fig. 9 The simulated reflection coefficient |S11| with and without the solar cell. 15

The antenna radiation patterns with and without the solar cell are shown in Fig. 10. Again, the solar cell’s position is optimal as the antenna preserves stable and similar pattern shape in both cases (with and without

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the solar cell).

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Fig. 10 The simulated antenna radiation patterns with and without the solar cell. (a) φ =0°, (b) φ =90° (solid black line: without the solar cell, dashed red line: with solar cell).

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The antenna working bandwidth evaluated at 10 dB of return loss is about 150 MHz (from 2.38 GHz to 2.53 GHz). According to Fig. 11(a), there is a slight increase in the antenna total gain when the solar cell is

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introduced. Figs. 8, 9, and 10 demonstrate that the proposed combination of the solar cell with rectenna system doesn’t affect much the performances of the antenna in terms of the reflection coefficient, radiation

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patterns, and total gain across the whole working bandwidth centered at 2.45 GHz.

Fig. 11 (a) The simulated antenna total gain with and without the solar cell. (b) The simulated efficiency with and without the solar cells versus RF input power. 16

The main improvement in this study can be illustrated by the figure 11(b) where the efficiency of the proposed combined system (rectenna + solar cells) is more than three time of magnitude than the conventional structure (without solar cell). This result reflects the high performance of our proposed design where this improvement is the main goal of researchers in this field. It is worthy to note that the circuit is numerically simulated under ADS to extract the results presented in figure 11(b). Conclusion

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In this paper, we have proposed high-performance solar cells based on a multilayer antireflection coating (ARC) and light trapping engineering. It can be seen from the obtained results that the efficiency of 16 %

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with planar solar cell with the SLARC (ZnO) was increased to 19.45% with textured solar cell with DLARC (AZO/TiO2) coating. The strong light trapping capability provided by the proposed triangular texture

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morphology and the more effective anti-reflective performance of the DLARC design, reduces the

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reflectivity in a wider wavelength region and also enhances the immunity against interfacial traps. The proposed solar cells are combined with a rectenna to form a solar and electromagnetic harvesting system. The

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solar cells convert the solar energy into DC power; while the rectenna harvests the surrounding electromagnetic power and transforms it into DC power as well. The total efficiency of the combined system

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is significantly improved more than three times the original design. The proposed system can operate regardless of the light intensity to feed some sensors and small devices autonomously.

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Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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