Lundstrom ECE 305 S16
ECE-305: Spring 2016
Solar Cell Fundamentals
Professor Mark Lundstrom Electrical and Computer Engineering
Purdue University, West Lafayette, IN USA [email protected]
3/22/16
Pierret, Semiconductor Device Fundamentals (SDF) pp. 356-361
Solar cells
2
modern Si solar cell
Chapin, Pearson, Fuller, 1954
http://www.bell-labs.com/org/physicalsciences/timeline/span10.html#
solar cells today
3 SunPower http://us.sunpower.com
recombination and dark current
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minority carriers injected across junction
Fn FPqVA
VA +
ID
Lundstrom ECE 305 S16 Every time a minority electron recombines on the p-side, one electron flows in the external current.
generation and current
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minority carriers collected by junction
Fn FPqVA
IL < 0
hf > EG
Lundstrom ECE 305 S16
Every time a minority electron is generated and collected by the PN junction, one electron flows in the external current.
light and dark current
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VD
I mA( )ID = I0 e
qVA /kBT 1( )
0.7V
Lundstrom ECE 305 S16
IL < 0photocurrent
dark current
hf > EG
solar cell operation
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1) Light generates e-h pairs
EF
Lundstrom ECE 305 S16
solar cell operation
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2) PN junction collects e-h pairs
EF
3) Current flows through load IL < 0
RL
VL +
forward bias across PN junction develops
ID > 0
Lundstrom ECE 305 S16
solar cell operation
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4) Forward bias reduces current
FpFn qVD
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5) IV characteristic is a superposition
ITOT = I0 eqVD kBT 1( ) ISC
light-generated current
diode (dark) current
IV characteristic
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PD = ITOTVD < 0
VD
ID
ITOT = I0 eqVD kBT 1( ) ISC
ISC
Pout = ISCVD = 0
VOC
Pout = ITOTVOC = 0
Pout = ImpVmp = ISCVOCFF
= PoutPin
= ISCVOCFFPin
ID = I0 eqVD kBT 1( )
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solar cell efficiency
= PoutPin
= ISCVOCFFPin
1) Short circuit current 2) Open-circuit voltage 3) Fill factor
Lundstrom ECE 305 S16
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1) Maximum short circuit current
Example: Silicon Eg = 1.1eV. Only photons with a wavelength < 1.13 m will be absorbed.
solar spectrum (AM1.5G)
Pin = 100 mW cm2
< hcEG
JSC max = 44 mA cm2
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2) Open-circuit voltage
ITOT = I0 eqV /kBT 1( ) ISC
Lundstrom ECE 305 S16
ITOT = 0 = I0 eqVOC /kBT 1( ) ISC
I0 eqVOC /kBT 1( ) = ISC
VOC 0.7 V
VOC =kBTqln ISC
I0
n 0( ) = ni2 NA( ) eqVA kBT 1( )
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Increasing VOC
0
n x( )
Wp
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3) Efficiency
Lundstrom ECE 305 S16
= PoutPin
= ISCVOCFFPin
44 103A/cm2( ) 0.7 V 0.8
100 103 V/cm2
25%
VD
ID
ISC
VOC
ID = I0 eqVD kBT 1( )
High efficiency Si solar cell
Martin Green Group UNSW Zhao, et al., 1998 (24.5% at 1 sun) 16
370 400 m
= PoutPin
= ISCVOCFFPin
VOC =kBTqln ISC
I0
FF = 0.81
JSC = 41.5 mA/cm2 94%( )
VOC = 0.703
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JSC VOC Trade-off
1) Smaller bandgaps give higher short circuit current
2) Larger bandgaps give higher open-circuit voltage
3) For the given solar spectrum, an optimum bandgap exists. EG
JSC VOC
Lundstrom ECE 305 S16
Shockley-Queisser Limit
34%
VOC =kBTqln ISC
I0
I0 ni2
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solar cell summary
1) Light is absorbed and produces e-h pairs 2) PN junctions separate e-h pairs and collect the carriers. 3) Current flow in external circuit produces a FB voltage and
the FB diode current reduces the total current. 4) Power out is ISCVOC FF. 5) Unlike integrated circuit chips, where the value added
comes from the design/system, manufacturing costs are critical in PV.
Lundstrom ECE 305 S16
