光伏元件eportfolio.lib.ksu.edu.tw/user/t/0/t094000004/repository...from s.o. kasap,...

261

第六章

光伏元件

262

目錄

6-1 太陽能光譜6-2 光伏元件的原理6-3 pn接面光伏I-V特性6-4 串聯電阻和等效電路6-5 溫度效應6-6 太陽能電池材料，元件和效率

263

6-1 太陽能光譜

264

0

Black body radiation at 6000 K

AM0

AM1.5

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.00

0.5

1.0

1.5

2.0

2.5

Wavelength (µm)

SpectralIntensity

dW cm-2 (µm)-1or

kW m-2 (µm)-1

The spectrum of the solar energy represented as spectralintensity (Iλ) vs wavelength above the earth's atmosphere(AM0 radiation) and at the earth's surface (AM1.5radiation). Black body radiation at 6000 K is shown forcomparison (After H.J. Möller, Semiconductors for SolarCells, Artech House Press, Boston, 1993, p.10) From S.O. Kasap, Optoelectronics and Photonics: Principles and Practices (PrenticeHall)

265

Direct Diffuse

(a) Illustration of the effect of the angle of incidence θ on the ray path length and the

definitions of AM0, AM1 and AM(secθ). The angle α between the sun beam and the horizon

is the solar latitude (b) Scattering reduces the intensity and gives rise to a diffused radiation

Atmosphere

AM0

AM1

θ

AM(secθ)

h0h

α

(a) (b)

α

Tilted PV deviceEarth

?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

267

6-2 光伏元件的原理

268

Neutraln-region

Neutralp-region

W

Eo

Voc

Medium λ

Long λ

Depletionregion

DiffusionDrift

Fingerelectrode

Backelectrode

n p

Le

The principle of operation of the solar cell (exaggerated features tohighlight principles)

Lh

Short λ


圖6.3 太陽能電池的工作原理 ( 跨大的圖形以放大工作原理 )。

269

Finger electrodes

p

n

Bus electrodefor current collection

Finger electrodes on the surface of a solar cellreduce the series resistance


圖6.4 太陽能電池表面上的文叉指型電極可降低串聯電阻。

LeLh W

Iph

x

EHPs

exp(−αx)

Photogenerated carriers within the volume Lh + W + Le give rise to a photocurrent Iph. Thevariation in the photegenerated EHP concentration with distance is also shown where α is thabsorption coefficient at the wavelength of interest.


圖6.5 在 Ln + W +Le 體積內光產生的載子造成光電流 Iph。光產生電子–電洞對濃度隨著距離的改變也圖示在這裡，是有興趣波長的吸收係數。

α270

273

6-3 pn接面光伏I-V特性

274

假如是光的強度，那麼短路電流 (short circuit current) 為

(1)IKII phsc −=−=

275

Iph

R

I

V V = 0Iph

I = Id − Iph

VId

Isc = –Iph

R(a) (b) (c)

(a) The solar cell connected to an external load R and the convention for the definitions ofpositive voltage and positive current. (b) The solar cell in short circuit. The current is thephotocurrent, Iph. (c) The solar cell driving an external load R. There is a voltage V and currentI in the circuit.

Light


278

V

I (mA)

Dark

Light

Twice the light

0.60.40.2

20

?0

0Iph

Voc

Typical I-V characteristics of a Si solar cell. The short circuit current is Iphand the open circuit voltage is Voc. The I-V curves for positive currentrequires an external bias voltage. Photovoltaic operation is always in thenegative current region.?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

圖6.7 一個太陽能電池的典型 I-V 特性，短路電流為 Iph，開路電壓為 Voc ；對於正電流的 I-V 曲綫需要一個外加偏壓，光伏應用都是用在負電流區。

279

V

I (mA)

0.60.40.2

–20

0

Voc

–10

Isc= –Iph

V′

The Load Line for R = 30 ž(I-V for the load)

I-V for a solar cell under anillumination of 600 Wm-2.

Operating PointSlope = – 1/R

PI′

(a) When a solar cell drives a load R, R has the same voltage as the solar cellbut the current through it is in the opposite direction to the convention thatcurrent flows from high to low potential. (b) The current I′ and voltage V′ inthe circuit of (a) can be found from a load line construction. Point P is theoperating point (I′, V′). The load line is for R = 30 ž .

LightI

R

V

I

(a) (b)


填充因子 (fill factor) FF是太陽能電池的重要指標，定義成

(4)ocsc

mm

VIVI

=FF

280

假設，重新整理上面的方程式，我們得到 Voc 為

(5)

eTnkV Boc />>

⎟⎟⎠

⎞⎜⎜⎝

⎛=

o

phBoc I

Ie

TnkV ln

285

286

6-4 串聯電阻和等效電路

287

Neutraln-region

Neutralp-region

Fingerelectrode

Backelectrode

Depletionregion

RL

Rs

Rp

Series and shunt resistances and various fates of photegenerated EHPs.?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

圖6.9 串聯與分流電阻和光產生電子–電洞對的不同際遇。

289

I (mA)

V0

00.2 0.4 0.6

5

10

Vo c

Isc

Rs = 0

Rs = 20 Ω

Rs = 50 Ω

Iph

The series resistance broadens the I-V curve and reduces the maximumavailable power and hence the overall efficiency of the solar cell. The exampleis a Si solar cell with n ≈ 1.5 and Io ≈ 3 × 10-6 mA. Illumination is such thatthe photocurrent Iph = 10 mA.?1999 S.O. Kasap, Optoelectronics (Prentice Hall)圖6.11 串聯電阻會使I-V曲綫變寬，並降低最大可能功率，因此使得太陽能電池的整體效率降低。圖中的例子是矽製太陽能電池，且，照光使得光電流。5.1≈n mA 103 6−×≈oI mA 10=phI

291

0.60.40.20246

5

15

Voltage (V)Power (mW)

Current (mA)

20

10

1 cell

2 cells in parallel

Current vs. Voltage and Power vs. Current characteristics of one cell and twocells in parallel. The two parallel devices have Rs/2 and 2Iph. ?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

圖6.12 一個電池和兩個並聯電池的電流對電壓與功率對電流的特性。兩個並聯元件有和。2/sR phI2

293

A

IphV

Iph

Id

B

Rs

RL

I/2

Id

Iph

I

RsI/2

Two identical solar cells in parallel under the same illumination anddriving a load RL.?1999 S.O. Kasap, Optoelectronics (Prentice Hall)圖6.13 在相同照光強度之下的兩個太陽能電池並聯且推動一個負載 RL 。

294

6-5 溫度效應

297

6-6 太陽能電池材料，元件和效率

298

100% Incident radiation

× 0.74 Insufficient photon energyhυ < Eg

× 0.59

Excessive photon energyNear surface EHP recombinationhυ > Eg

× 0.95 Collection efficiency of photons

× 0.6 Voc ≈ (0.6Eg)/(ekB)

× 0.85

η ≈ 21%

FF ≈ 0.85

Overall efficiency

Accounting for various losses of energy in a high efficiency Sisolar cell. Adapted from C. Hu and R. M. White, Solar Cells(McGraw-Hill Inc, New York, 1983, Figure 3.17, p. 61).?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

圖6.14 在一個高效率矽製太陽能電池中，不同能量損失的計算。

299

LightOxide

np

Inverted pyramid textured surface substantially reduces reflectionlosses and increases absorption probability in the device

Le


圖6.15 倒金字塔的織狀表面可大量的降低反射損失，並增加元件的吸收機率。

301

p-AlGaAs window (< 0.02 µm

p-GaAs

n-GaAs

Passivated GaAs surface

AlGaAs window layer on GaAs passivates the surface statesand thereby increases the low wavelength photogenerationefficiency?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

圖6.16 在GaAs上面的AlGaAs視窗層覆蓋表面狀態，因此會增加短波長的光產生效率。

302

2 eV

1.4 eV

Ec

Ev

Ec

Ev

(a)

(b)

n p

GaAsAlGaAs

A heterojunction solar cell between two different bandgapsemiconductors (GaAs and AlGaAs)?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

圖6.17 兩不同能帶間隙半導體材料間的異質接面太陽能電池。

np

Cell 1 (Eg1) Cell 2 (Eg2 < Eg1)

n p

Connecting region.

A tandem cell. Cell 1 has a wider bandgap and absorbs energeticphotons with hυ > Eg1. Cell 2 absorbs photons that pass cell 1 andhave hυ > Eg2.


圖6.18 一種串聯式的電池，電池1有寬的能帶間隙可吸收的高能光子，電池2吸收通過電池1的光子。

1gEh >υ

2gEh >υ303

第六章目錄6-1 太陽能光譜6-2 光伏元件的原理6-3 pn接面光伏I-V特性6-4 串聯電阻和等效電路6-5 溫度效應6-6 太陽能電池材料，元件和效率

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