IIT Bombay
DESE
Photovoltaic energy conversion
Lecture #18
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion Slide 1
P-N junction energy bands IIT Bombay
DESE
ND - NA
ND
x
-NA
P-side N-side
Ec EF
Ei
EF
Ev
Fermi level
remains
EF Ec invariant at
Ei the
Ev equilibrium
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L12-Slide 2
IIT Bombay
Space charge region DESE
q ( p N d n N a )
Time, t=0 + - + - + - + - + - + - + - + -
+ - + - + - + - + - + - + - + -
Fixed charges - + - +
+ - + - + - + - + - + -
Mobile charges + - + - + - + - + - + - + - + -
N-side P-side
t>0 - +
+ - + - + - + - + - + - + -
+ - + - + - + - + - + - + -
+ - + - + - + - + - + - + -
+ - + - + - + - + - + - + -
10/17/24
Space charge region
Lecture 18: EN 601: Photovoltaic energy conversion L12-Slide 3
IIT Bombay
Junction at Thermal Equilibrium DESE
•Net current is zero, J=0
Ec
EF •Jn=0, Jn,drift+Jn,diff=0
Ei •Jp=0, Jp,drift+Jp,diff=0
Ev
Particle flow Current flow
Hole Diffusion Hole Diffusion
Hole Drift Hole Drift
Electron Diffusion Electron Diffusion
Electron Drift Electron Drift
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L12-Slide 4
IIT Bombay
P-N J forward bias
n DESE
• Forward bias is applied such that the potential barrier across the
junction is reduced, =V0-V(applied)
•Depletion region width decreases
Negligible voltage drop (neutral
Negligible voltage drop
region, high doping)
(ohmic contact)
P N
V
V Most of the voltage applied
P N x appears across the depletion
region
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L14-Slide 5
IIT Bombay
P-N J forward bias
n DESE
In
(V0-V)
Ec
EFp EFn
V
Ei
= EFn-EFp
E
Direction
v
of current
Hole Diffusion
Ip Hole Drift
Electron Diffusion
Electron Drift
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L14-Slide 6
IIT Bombay
P-N J under reverse bias
n DESE
•Reverse bias is applied such that the potential barrier across the
junction is increased, =V0 + V(applied)
• Depletion region width increases
In
Direction of current
V0 + V
EFp Hole Diffusion
Ec Hole Drift
EFn Electron Diffusion
Ei Electron Drift
Ev
Ip
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L14-Slide 7
IIT Bombay
Comparison of voltage bias DESE
P N P N P N
V=0 V>0, Vf V<0
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L14-Slide 8
IIT Bombay
Diode Current: Qualitative Solution DESE
• Total current crossing the diode is sum of the diffusion and drift
current
• Under equilibrium there is no current flow
• In forward bias current flow mainly due to diffusion of carriers, which
increases exponentially
•In reverse bias both electron and hole diffusion components are
negligible due to large barrier. Current is relatively small (due to
generation of carrier) and it independent of voltage.
At equilibrium
I = I (diffusion) – I (generation) = 0 when V = 0
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L15-Slide 9
Diode Current: Qualitative Solution IIT Bombay
DESE
• In forward bias, the probability that a carrier can diffuse across junction
is proportional to exp(qVf / kT)
• In reverse bias, the probability that a carrier can diffuse across
junction is proportional to exp(-qVr / kT)
Forward bias current, diffusion current = Igen* e(qVf / kT)
Current in the reverse bias, drift current = Iequi=Igen=I0
Total current = I (forward) – I (reverse)
( qV / kT ) I
I I gen e I gen Reverse
I (diffu.) saturation
qV / kT current
I I 0 (e 1) I (gen.)
V
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L15-
Slide 10
IIT Bombay
Solar cell DESE
The basic steps in the operation of a solar cell are:
• Generation of light-generated carriers;
• Collection of the light-generated carries to generate a current;
• Generation of a voltage across the solar cell; and
• Dissipation of power in the load and in parasitic resistances.
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L19-
Slide 11
IIT Bombay
PN Junction under illumination DESE
Time t=0
Ec
EF
Ec
Ei
Ev
Ei
Ev
Current flow
Hole Diffusion Ln W Lp
Hole Drift
Electron Diffusion
• Shining of light will generate
electron-hole pair throughout the
Electron Drift
semiconductor
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L19-
Slide 12
Junction under illumination IIT Bombay
DESE
P-type N-type
Carrier will die
Time t>0
+ Ec
Ei
-
Ev
Pile up of carriers, responsible Ln W Lp
ξ Carrier will die, will
for generation of forward voltage
not participate in
(Photovoltaic effect)
Direction of the light current
generated current
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L19-
Slide 13
IIT Bombay
Junction under illumination DESE
•Generation of voltage in P-N junction
radiation
P-type
+
- N-type
Ln W Lp
Direction of current flow under
illumination
•P-N behaves like a forward bias P-N
junction under illumination
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L19-
Slide 14
IIT Bombay
Solar Cell I-V Equation DESE
Total current for solar cell under illumination can be given as:
qV / kT
I total I 0 ( e 1) I L
Where IL is light generate current or photo current
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L20-
Slide 15
IIT Bombay
Solar Cell I-V Curve DESE
I • A P-N junction in the dark
I (diffu.)
consumes power, as it can be
operated in 1st or 3rd quadrant
V
I0 • Effect of solar radiation on the I-V
curve
• Under illumination solar cell can be
operated in the fourth quadrant
corresponding to delivering power
to the external circuit
• Current in the illuminated solar cell is negative, flows against the
conventional direction of a forward diode
10/17/24 Lecture 18: EN 601: Photovoltaic energy conversion L20-
Slide 16
IIT Bombay
Solar Cell I-V Curve DESE
I Solar cell parameters
Isc • Voc - open circuit voltage,
Pm
Im
• Isc - short circuit current,
• Pm - maximum power point
V • I , V – current and voltage
Vm Voc m m
Usual I-V plot of solar cell – at maximum power point
Current is shown on positive y - •
FF – Fill factor
axis
• η – Efficiency
• Rs – series resistance
10/17/24 • R – shunt resistance
Lecture 18: EN 601: Photovoltaic
sh energy conversion L20-
Slide 17
