Novel design of solar cell efficiency improvement ... - OSA Publishing

Novel design of solar cell efficiency improvement using an embedded electron accelerator on-chip Itsara Srithanachai,1 Surada Ueamanapong,1 Surasak Niemcharoen,1 and Preecha P Yupapin,2,* 1

Department of Electronics Engineering, Faculty of Engineering King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand 2 Nanoscale Science and Engineering Research Alliance (N’SERA), Faculty of Science King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand * [email protected]

Abstract: In this paper, we propose a novel design of an electron accelerator on-chip by using a small scale device known as a PANDA microring resonator, which can be embedded within the solar cell device, where the trapped electron can be accelerated and moved faster to the final destination. Therefore, the solar cell efficiency can be improved. In principle, a PANDA microring can generate the optical tweezers for hole tapping and transportation. The transported holes can be accelerated and moved via the optical waveguide to the solar cell device contact, where the effect of defects in silicon bulk can be solved. Therefore, this technique can be used to improve the solar cells performance. In practice, the accelerator unit can be embedded within the solar cell device, which allows the trapped holes moving to the required destination. This is claimed to be a novel technique by using a PANDA microring to accelerate the holes for solar cell performance improvement. Finally, this technique is the starting point of using a PANDA microring to enhance the performance of semiconductor device. ©2012 Optical Society of America OCIS codes: (040.5350) Photovoltaic; (350.6050) Solar energy.

References and links 1.

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Received 15 Mar 2012; revised 12 Apr 2012; accepted 13 Apr 2012; published 21 May 2012 4 June 2012 / Vol. 20, No. 12 / OPTICS EXPRESS 12640

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1. Introduction Energy has been an important issue of the world for half a century, where the most of energies are come from coal and oil. Moreover, the pollutions are increased due to the burning process of coal and oil [1–4]. Nuclear power plant is the other way to generate energy. This technique can produce the electrical energy using the heat of nuclear reaction, instead burning of coal and oil. On the other hand, the nuclear reaction is very dangerous because the accidents in nuclear reactions are distribution of radioactivity. Green energy is the better choice more than burning of coal, oil and nuclear reaction. The green energy can generate electrical energy from many ways such as water, wind and sunlight. Although, the green energy from wind and water can be used to substitute the energy from burning energy, however, the differences of depreciation of landscapes and weather to electrical generate from wind and water are less than the sunlight. A solar cell is the device for changing the sunlight to electrical energy. Although, the solar cells are important for green energy generation, on the other hand, the performance of the device is not high enough because there are many problems. The developments of solar cells are still continuously needed. Generally, the solar cell performance is caused by many problems such as the reflection of surface [5], silicon bulk defects [6,7]. To solve the reflection problem on the surface, there are many ways such as adjust the surface of solar cells angle to ability for obtaining most of sunlight [8,9]. Furthermore, the solar cell production is used high cost and not worth investment, where we found that the defect in silicon bulk is the important problem for solar cell performance decreasing because the electron and hole are recombined by trapping or defecting in silicon bulk. To solve this problem, several research works proposed the methods of modified bulk and heterojunction [10,11] and control hole transport in silicon bulk to contact [12]. Therefore, this work presents the use of a PANDA #162969 - $15.00 USD (C) 2012 OSA


microring resonator to improve the solar cells performance, in which the trapped holes can be controlled and faster transport to the required destination. However, the use of a PANDA microring resonator is a new system, which has been proposed by the Suwanpayak et al [13]. By using the optical tweezer, the solar cell efficiency can be increased. The optical tweezers are generated by a PANDA microring, which can be used to accelerate and move electron and hole via the optical waveguide to the solar cell device contact. By using this technique, hole can transport to the contact without recombination in silicon bulk, therefore, the current of solar cell is increased. Moreover, the use of a PANDA microring is also founded in many applications such as photonic microdevice [14], hybrid transistor [15], therapeutic applications [16], and telephone networks [17]. In this paper, the trapping tool generation is reviewed and the new design system for particle accelerator described. Finally, simulation results using the commercial MATLAB is demonstrated, where all parameters were used closely to the practical fabricated device. 2. Particle trapping principle A novel system of the P-N diode using a PANDA microring resonator was proposed by the authors in reference [13]. By using a dark-bright soliton pulses propagating within a modified add/drop optical multiplexer (PANDA microring), the trapping tools can be formed, which can be used to trap molecules/atoms. Inthis work, the multiplexed signals with slightly different wavelengths of the dark solitons are controlled and amplified within the system. The dynamic behaviors of dark bright soliton interaction are also analyzed and described. Finally, the use of optical switching to form a P-N photodetector using the Gaussian control at the add port is discussed in detail. We are looking for a stationary dark soliton pulse, which is introduced into the add/drop optical filter system as shown in Fig. 1. The input optical field (Ein) and the add port optical field (Eadd) of the dark, bright soliton or Gaussian pulses are given by [18].

 z   T  Ein (t ) = A tanh   exp   − iω0 t  ,  2 LD    T0 

(1a)

 z   T  Ein (t ) = A sech   exp   − iω0t  ,  T0   2 LD  

(1b)

 z   Eadd (t ) = E0 exp   − iω0 t  ,  2 LD  

(1c)

A0 and z are the optical field amplitude and propagation distance, respectively. T = t − β1 z , where β1 and β 2 are the coefficients of the linear and second-order terms of

Here

Taylor expansion of the propagation constant. LD = T02 / B2 is the dispersion length of the soliton pulse.

T0 in Eq. is a soliton pulse propagation time at initial input (or soliton pulse

width), where t is the soliton phase shift time, and the frequency shift of the soliton is ω0 . The optical fields of the system within the device as shown in Fig. 1 are obtained and expressed in following forms.

 n n = n0 + n2 I = n0 =  2  Aeff 

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  P, 

(2)


E1 = − jκ i + τ 1 E4 ,

 jωT E2 = exp   2

Here Ei is the input field,

(3)

  −α L   exp   E1 ,   4 

(4)

E3 = τ 2 E2 − jκ 2 E1 ,

(5)

 jωT   −α L  E4 = exp   exp   E3 ,  2   4 

(6)

Et = τ t Ei − jκ1 E4 ,

(7)

Ed = τ 2 Ea − jκ 2 E2 ,

(8)

Ea is the add(control) field, Et is the through field, Ed is the drop

field, E1 … E4 are the fields in the ring at points 1…4, κ1 is the field coupling coefficient

κ2

between the input bus and ring,

is the field coupling coefficient between the ring and

output bus, L is the circumference of the ring, T is the time taken for one round trip(roundtrip time), and α is the power loss in the ring per unit length. We assume that this is the lossless Lneff 2 coupling, i.e., τ 1,2 = 1 − κ1,2 , T = . c The output intensities at the drop and through ports are given by 2

Ed = Et

Here

2

−κ1κ 2 A1,2 Φ1/2 1 − τ 1τ 2 AΦ =

Ei +

τ 2 − τ 1 AΦ Ea , 1 − τ 1τ 2 AΦ

−κ κ A Φ τ 2 − τ 1 AΦ Ei + 1 2 1,2 1/ 2 Ea , 1 − τ 1τ 2 AΦ 1 − τ 1τ 2 AΦ

(9)

(10)

 −α L   jωT  2 A1/2 = exp   the half-round-trip amplitude), A = A1,2 , Φ1/2 exp    4   2 

(the half-round-trip phase contribution), and Φ = Φ1/2 2 . To form the broad soliton spectrum output, two nonlinear ring resonators are introduced. The circulated roundtrip light fields of the right ring radii, Rr , are given in Eq. (11) and (12), respectively [19].

Er 1 =

j 1 − γ κ 0 E1 α

1 − 1− γ 1 − κ0 e

− L1 − jκ n L1 2

,

(11)

α

Er 2 =

j 1 − γ κ 0 E1 e

− L1 − jκ n L1 2

α

1− 1− γ 1− κ0 e

− L1 − jκ n L1 2

,

(12)

Thus, the output circulated light field, E0 , for the right ring is given by

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(1 − κ 0 ) − (1 − γ ) e

1− γ

E0 =

α

− L1 − jκ n L1 2

,

α

1 − 1 − γ 1 − κ0 e

− L1 − jκ n L1 2

(13)

Similarly, the output circulated light field, E0 L , for the left ring at the left side of the add/drop optical multiplexing system is given by

E0 L

α − L2 − jκ n L2  1 − γ 1 − κ − 1 − γ e ( 3) 2  3 ( 3) = E3  α − L2 − jκ n L2 2  1 − 1 − 1 − e γ κ 3 3 

Where κ 3 is the intensity coupling coefficient,

κn =

the attenuation coefficient, wavelength light field and,

2π

x1 = (1 − γ 1 )

,

(14)

γ 3 is the fractional coupler intensity loss, α

is

is the wave propagation number, λ is the input

λ

L2 = 2π RL , RL is the radius of left ring. E1 , E3 and E4 are defined by given

From Eq. (11)-(14), the circulated light fields, 1/ 2

  ,  

x2 = (1 − γ 2 ) , y1 = (1 − κ1 ) , and y2 = (1 − κ 2 ) . 1/ 2

1/ 2

1/ 2

jx1 κ1 Ei1 + jx1 x2 y1 κ 2 E0 L Ei 2 e

E1 =

−

αL 22

− jκ n

L 2

,

α

1 − x1 x2 y1 y2 E0 E0 L e

E3 = x2 y2 E0 E1e E4 = x2 y2 E0 E1e

−

αL 22

−

αL 22

− jκ n

L 2

− jκ n

L 2

− L − jκ n L 2

+ jx2 κ 2 Ei 2 ,

+ jx2 κ 2 Ei 2 e

−

αL 22

− jκ n

(16)

L 2

Thus, from Eq. (11)-(17), the output optical field of the through port

(

,

)

∗

Similarly, the output optical field of the drop port

αL 22

− jκ 2

L 2

,

(18)

2

(19)

( Et 2 ) is given by

Et 2 = x2 y2 Ei 2 + jx2 κ 2 E0 E1e

−

αL 22

− jκ n

L 2

,

(20)

( Pt 2 ) is expressed by Pt 2 = ( Et 2 ) ⋅ ( Et 2 ) = Et1 , *

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−

( Pt1 ) is written by

Pt1 = ( Et1 ) ⋅ ( Et1 ) = Et1 ,

The power output of the drop port

(17)

( Et1 ) is expressed by

Et1 = x1 y1 Ei1 + jx1 x2 y2 κ1 E0 E0 L E1 − x1 x2 κ1κ 2 E0 L Ei 2 e The power output of the through port

(15)

2

(21)


An optical tweezer is recognized as a promising tool for molecule/atom trapping, which is basically depended on the laser wavelength control, where in this work the tweezer is formed by using a PANDA microring resonator. The optical tweezer generation can be designed and embedded within the device based on silicon, in which an electron can be injected to the contact without loss, and the transport time can be controlled before reaching the contacts. The optical tweezers can be varied and adjusted via the add port of a PANDA ring. Figure 2 shows the optical tweezer for electron and hole trapping and transportation via the optical waveguide. The trapping tool size (d) is required to tune between these two conditions, where (i) d > electron size, this case an electron can escape from the trap in the transport process to the contact, (ii) d < electron size, this case an electron cannot be trapped when the size of the trap too small. Therefore, the trap size is required to fit the electron/hole size (0.22 nm) [20].

Fig. 1. A PANDA microring resonator

Fig. 2. Optical tweezer for hole trapping, where (a) trapping potential well, (b) an optical tweezer for hole trapping

3. Results and discussion By using the proposed design, the optical tweezer can be used to trap and move the electron/hole via the optical waveguide. A PANDA microring resonator can generate the optical tweezer by add/drop, the particles in a bottle will be trapped and moved by the input tweezers, the suitable tweezers can be controlled and generated by the controlled port signals. In our application, the improvement solar cell performance by using PANDA microring is proposed. Normally, the characteristics of solar cells study by J-V characteristics, the J-V characteristics of solar cells depend on factors such as (i) series and shunt resistance, (ii) the front and back contacts and (iii) the main junction. The main junctions in the important factor for controls the current of solar cells because of P-N junction can generate the electron-hole pair. However, the diffusion of carriers to contact has a problem in silicon bulks such as the

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recombination of hole. This problem can be solved by increasing the trapped hole speed to the contact via optical waveguide, without trapping within a silicon bulk. From Fig. 3, the holes are trapped in the junction and moved to the contact via optical waveguide. The ring radii are Radd = 15 µm, RR = 3 µm and RL = 3 µm, in which the evidence of the fabricated device was reported by the authors in reference [21].

Aeff is 300

µm , κ = κ1 = κ2 = κ3 = 0.5, the waveguide losses coefficient, α is 0.1 and coupling loss, γ is 0.01 and n0 is 1.37. The simulation results are obtained for four different center wavelengths 2

2

of tweezers generated, where the dynamical movements are (a) E1 , (b) (d)

2

2

E2 , , (c) \ E3 , ,

2

E4 , (e) through port and (f) drop port signals as shown in Fig. 4.

Figure 5 shows the optical tweezer that can be adjusted to fit the electron/hole size from 0.25 to 2 nm, which can be used for electron/hole transportation at the through port and drop port. The sizes of optical tweezers are important for trapping electron/hole moving to device contact. The current of solar cells compare between fabrication technique and using PANDA mircroring resonator for accelerator electron/hole, the current of solar cells is calculated by using Eq. (22).

I L = qA ( Ln + W + L p ) G

(22)

I L is the light current, A is the area, Ln is the diffusion length of electron, Lp is the diffusion length of hole and W is the depletion width and G is the generation rate. Where

The current of the device after using a trapping tool and technique of a PANDA microring resonator system is increased by 5 orders [22–25], which is shown in Fig. 6.

Fig. 3. Solar cell model using a PANDA microring, where (a) hole trapping and moving via optical waveguide, (b) diode model with PANDA microring.

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Fig. 4. Results of dynamic optical tweezers generated at different center wavelengths, where (a) \E1\2, (b) \E2\2, (c) \E3\2, (d) \E4\2, (e) through port and (f) drop port signals.

Fig. 5. Trapping potential well of optical tweezers with various trapping sizes, where (a) 2 nm, (b) 1 nm, (c) 0.5 nm, and (d) 0.25 nm.

Fig. 6. Light current of the diode using PANDA microring resonator.

4. Conclusions We have proposed a new method of electron/hole accelerator, which can be used to accelerate electron/hole within the solar cell device. By using optical tweezers and optical waveguide embedded within the solar cell device, the hole/electron speed can be increased about 5 orders. Silicon is the applied material in this study, in which an electron can move to the metal contact via an optical waveguide without recombination in the silicon bulk. The obtained results have shown that the PANDA microring resonator can be used to generate the optical tweezers, which can be adjusted for electron/hole trapping. This design can also be used for the applications such as capacitor, photodetector and transistor.

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Acknowledgments The authors would like to thank Thai Microelectronics Center (TMEC), National Electronics and Computer Technology Center (NECTEC), Thailand and Thailand Graduate Institute of Science and Technology (TGIST), Institute of Nanoscale Science and Engineering Research Alliance and Hybrid Computing Research Laboratory, King Mongkut’s Institute of Technology Ladkrabang (KMITL), Bangkok 10520, Thailand.

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