ee580 – solar cells todd j. kaiser

EE580 – Solar CellsTodd J. Kaiser

• Lecture 08

• Solar Cell Characterization

1Montana State University: Solar Cells Lecture 8: Characterization

Solar Cell Operation

External Load

+-

nEmitter

pBase

Rear Contact

Front Contact

Antireflection coating

Absorption of photon creates an electron hole pair. If they are within a diffusion length of the depletion region the electric field separates them.

The electron after passing through the load recombines with the hole completing the circuit


Solar Cell Electrical Model

• PV is modeled as a current source because it supplies a constant current over a wide range of voltages

• It has p-n junction diode that supplies a potential• It has internal resistors that impede the flow of

the electrons


Circuit Diagram

IRsh

Rs

RLoad

SolarInput

Front Contact

Rear Contact

RecombinationOhmic Flow

CurrentSource

V

I

ExternalLoad


Electrical Losses• Series Resistance

(Resistance of Hole & Electron Motion)

– Bulk Resistance of Semiconductor Materials

– Bulk Resistance of Metallic Contacts and Interconnects

– Contact Resistance

• Parallel Resistance or Shunt Resistance

(Recombination of Hole and Electron)

– PN junction Leakage

– Leakage around edge of Junction

– Foreign Impurities & Crystal Defects


Power & IV Curve• Power (Watts) is the rate at which energy (Joules) is

supplied by a source or consumed by a load… It is a rate not a quantity

• The power output by a source is the product of the current supplied and the voltage at which the current was supplied

• Power output = Source voltage x Source current

– P=V x I (Watts = Joules/second) = (Volts)x(Amperes)

• By changing the resistance of the load different currents and corresponding voltages can be measured and plotted


Solar Cell I-V Characteristics

Voltage

Current

Dark

Light

Twice the Light = Twice the Current

Current from Absorption of Photons

LkT

qV

IeII

10


Operating Point

V

I (mA)

0.60.40.20

Voc

100

Isc = Iph

The load line forR = 3 (I-V for the load)

I-V for a solar cellunder an illuminationof 700 W m-2

Operating point

Slope = 1/R

P

I

I

I

R

V

I

(a) (b)

0.50.30.1

V

200


Volt (V) 0 0.1 0.3 0.5 0.54 0.57

Current (A) 1.3 1.3 1.3 1.2 0.75 0

Power (W) 0 0.13 0.39 0.6 0.4 0

Voltage (V)

IV Plot

Current Power

Short Circuit Current

Open Circuit Voltage

Maximum Power

Operating Point

9

ocsc

mpmp

VI

VIFF

Vmp

Imp

Montana State University: Solar Cells Lecture 8: Characterization

Resistance EffectsISC

Current

Voltage VOC

Increasing RS

ISC

Current

Voltage VOC

decreasing RP

Both reduce the area of the maximum power rectangle therefore reducing the efficiency and fill factor

Ideal case RS = 0 and RP = ∞


Loss Mechanisms in Solar CellsLoss

Optical Electrical

ReflectionShadowingUnabsorbed Radiation

Ohmic Recombination

Solar Cell MaterialBaseEmitter

Contact MaterialFingerCollection BusJunctionMetal – Solar Cell

Emitter RegionSolar Cell MaterialSurface

Base RegionSolar Cell MaterialSurface

Space Charge Region


Photovoltaic Effect

Absorption of Light

Creation of extra electron

hole pairs (EHP)

Voltage

Current

Power = V x I

Excitation of electrons

Separation of holes and electrons by Electric Field

Movement of charge by Electric Field


Linking Cells• Solar cells are not usually used individually because they

do not output sufficient voltage and power to meet typical electrical demands

• The amount of voltage and current they output can be increased by combining cells together with wires to produce larger area solar modules

• Cells can be connected in a number of ways– Strings – where cells are connected in series– Blocks 2 or more strings connected together in parallel– Joining 2 or more blocks together


Solar Cell Panels

Voltage

Cur

rent

Voltage

Cur

rent

VoltageC

urre

nt

14

Series connections increase the voltage output

Parallel connections increase the current output

Blocks increase both current and voltage output


Calculating Voltage and Current• Series connections are made by connecting one cell’s n-

type contact to the p-type of the next cell• Parallel connections are made by joining each cells n-

type contacts together and p-type contacts together• Series connections the voltages add• Parallel connections the current add• Series connections the current flow is equal to the

current from the cell generating the smallest current (limited by poorest cell)

• Parallel connections the voltage is the average of the cells or string in parallel


Example: Cells Series Connected

• The voltage across terminals 12 is the sum of the voltages

• V12 = VA + VB + VC = 0.58 + 0.54 + 0.61 =1.73(V)

• The current through the cells is restricted by the smallest current produce by any of the cells

• I12 = 0.25 (A)

16

Cell AV = 0.58 VI = 0.28 A

Cell BV = 0.54 VI = 0.31 A

Cell CV = 0.61 VI = 0.25 A

1 2


Example: Cells Parallel Connected

• The voltage across terminals 34 is the average of the voltages

• V34 = (VA + VB + VC )/3 = (0.58 + 0.54 + 0.61)/3 = 0.58(V)

• The current at the terminals 34 is the sum of the currents in each cell

• I34 = (IA + IB +IC) = (0.28 + 0.31 + 0.25) = 0.84(A)

17

Cell AV = 0.58 VI = 0.28 A

Cell BV = 0.54 VI = 0.31 A

Cell CV = 0.61 VI = 0.25 A

3

4


Example: Block Connected

• The voltage across terminals 56 given by the series voltage already calculated:

• V56 = VA + VB + VC = 0.58 + 0.54 + 0.61 =1.73(V)

• The current at the terminals 56 is the sum of the currents in each string already calculated

• I56 = 3(Istring) = 3(0.25) = 0.75(A)

18

A5

6

B

C

A

B

C

A

B

C


Summary Linking Cells

• Linking modules or batteries is similar to connecting PV cells– Series Connections

• Voltages are added in series connections

• The current is restricted to the smallest current

– Parallel connections• The currents are added in parallel connections

• The voltages are averaged from each string

• Solar Cells and Modules are Matched to improve the power generated


ee580 – solar cells todd j. kaiser

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