EE580 – Solar Cells Todd J. Kaiser • Lecture 08 • Solar Cell Characterization

Montana State University: Solar Cells Lecture 8: Characterization 1

Solar Cell Operation

n Emitter p Base Rear 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.

Front Contact External Load Montana State University: Solar Cells Lecture 8: Characterization + The electron after passing through the load recombines with the hole completing the circuit 2

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 Montana State University: Solar Cells Lecture 8: Characterization 3

Solar Input Recombination I Current Source

Circuit Diagram

Front Contact Rs Ohmic Flow R sh V R Load I Rear Contact Montana State University: Solar Cells Lecture 8: Characterization External Load 4

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 Montana State University: Solar Cells Lecture 8: Characterization 5

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 Montana State University: Solar Cells Lecture 8: Characterization 6

Solar Cell I-V Characteristics

Current

I



I

0  

e qV kT

 1   

I L

Dark Current from Absorption of Photons Light Twice the Light = Twice the Current Montana State University: Solar Cells Lecture 8: Characterization Voltage 7

Operating Point

I I R

(a)

V I

0  100

I

sc = 

I

ph  200

I

(mA)

I

 0.1

0.2

0.3

Slope = - 1/

R

0.4

V

 0.5

V

oc 0.6

V I-V

for a solar cell under an illumination of 700 W m -2

P

Operating point The load line for

R

= 3  (

I-V

for the load)

(b)

Montana State University: Solar Cells Lecture 8: Characterization 8

Short Circuit Current I mp

FF



I mp V mp I sc V oc

IV Plot

Operating Point Current Maximum Power Power Voltage (V)

Volt (V)

Current (A) Power (W)

0

1.3

0

0.1

1.3

0.13

0.3

1.3

0.39

0.5

1.2

0.6

Montana State University: Solar Cells Lecture 8: Characterization V mp

0.54

0.75

0.4

0.57

0 0 Open Circuit Voltage 9

I SC Current

Resistance Effects

I SC Current Increasing R S decreasing R P Voltage V OC Voltage Ideal case R S = 0 and R P = ∞ Both reduce the area of the maximum power rectangle therefore reducing the efficiency and fill factor Montana State University: Solar Cells Lecture 8: Characterization V OC 10

Loss Mechanisms in Solar Cells

Loss Optical Electrical Ohmic Recombination Reflection Shadowing Unabsorbed Radiation Solar Cell Material Base Emitter Contact Material Finger Collection Bus Junction Metal – Solar Cell Montana State University: Solar Cells Lecture 8: Characterization Emitter Region Solar Cell Material Surface Base Region Solar Cell Material Surface Space Charge Region 11

Photovoltaic Effect

Separation of holes and electrons by Electric Field Voltage Absorption of Light Excitation of electrons Creation of extra electron hole pairs (EHP) Movement of charge by Electric Field Montana State University: Solar Cells Lecture 8: Characterization Current Power = V x I 12

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 Montana State University: Solar Cells Lecture 8: Characterization 13

Solar Cell Panels

Voltage Voltage Parallel connections increase the current output Series connections increase the voltage output Montana State University: Solar Cells Lecture 8: Characterization Voltage Blocks increase both current and voltage output 14

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 Montana State University: Solar Cells Lecture 8: Characterization 15

Example: Cells Series Connected

1 Cell A V = 0.58 V I = 0.28 A Cell B V = 0.54 V I = 0.31 A Cell C V = 0.61 V I = 0.25 A • The voltage across terminals 12 is the sum of the voltages • V 12 = V A + V B + V C = 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 • I 12 = 0.25 (A) Montana State University: Solar Cells Lecture 8: Characterization 16 2

Example: Cells Parallel Connected

3 Cell A V = 0.58 V I = 0.28 A Cell B V = 0.54 V I = 0.31 A Cell C V = 0.61 V I = 0.25 A • The voltage across terminals 34 is the average of the voltages • V 34 = (V A + V B + V C )/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 • I 34 = (I A + I B +I C ) = (0.28 + 0.31 + 0.25) = 0.84(A) 4 Montana State University: Solar Cells Lecture 8: Characterization 17

Example: Block Connected

5 A A A B B B C C C 6 • The voltage across terminals 56 given by the series voltage already calculated: • V 56 = V A + V B + V C = 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 • I 56 = 3(I string ) = 3(0.25) = 0.75(A) Montana State University: Solar Cells Lecture 8: Characterization 18

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 Montana State University: Solar Cells Lecture 8: Characterization 19