27 pius augustine solarcells

Dr. Pius Augustine, SH College, Kochi

Solar cells are the building block of photovoltaic
modules (solar panels)
Solar Cell or Photovoltaic cell
Old name: Solar battery
An electrical device that
converts the energy of light
directly into electricity by the
photovoltaic (PV) effect.

Photovoltaics discovered by Henry Becquerrel - 1839
First silicon based PN junction photovoltaic invented
by R. S. Ohl in 1940
Albert Einstein got Nobel Prize for his
explanation on Photoelectric effect.

American engineer who
is generally recognized
for patenting the
modern solar cell (US
Patent 2402662, "Light
sensitive device"

Silicon solar cells came into being in the
middle of 1900s
Efficiency was less than 6%

 Solar cells are bundled together to make
larger units called solar modules
 Solar modules are coupled into bigger
units known as solar panels
Solar modules and Solar panels

Solar cell is a junction (pn+ junction) with
sunlight shining on it.
n+ means heavily doped n region
n+ region will be thin so that light will
penetrate and reach the junction

We should have some basic understanding on the
following.
 Working of a pn junction – biasing etc
 Light aborption by a semiconductor
 Light absorption in a pn junciton

Conduction band
Valence band
Corelevels
Bands-Very closed energy levels.
Topmost filled band is VB and
empty band is CB (at 0K)
At operating temperature- a few
VB electrons will move to CB
Core levels are unperturbed.

Intrinsic
ne = nh
p –type
nh ≈ NA
-
n-type
ne ≈ ND
+
Extrinsic semiconductor

N-type -Addition of every impurity atom (donor) will
break a bond in Si and push one electron to CB
Number of electrons = number of Phosphorous atoms
introduced (donor atoms)
If it is cooled, impurities may be freezed out, because
insufficient thermal energy to break weak bonds of
Phosphorous.
P-type – for every B introduced, there will be one empty states
in the VB.

N type - EF is near CB – With a little thermal energy, there is
a probability for electrons to cross over to CB and
corresponding probability of states below Fermi level to be
unoccupied. (But VB is way down below)
In p –type, EF is close to VB.
Fermi Level (EF)
Highest filled energy level (EF)
at 0K. Tells how the states are
filled up

If energy is fermi energy,
probability that the state will be
occupied is ½.
If the energy is below EF –fermi
function is one.
If the energy is above EF (ie.
band gap) – no electron is
expected and fermi function is
zero

ne = ni
nh = ni
ne x nh = ni
2
Energy needed for electron to
reach CB = Band gap (EBG)
Probability that one of the bonds
is broken and an electron goes to
CB is
ni α exp (-EBG/kBT)
Intrinsic semiconductor

ne = ni
nh = ni
ne x nh = ni
2
ni α exp (-EBG/kBT)
In Si: EBG= 1.1 eV
kBT (at room temperature) = 0.025 eV
Probability for breakage is very small ≈1010/cm3. (very small
fraction of total number of Si atoms)
However, the number of atoms are very large
If temperature is increased …. ?? Higher probability
Intrinsic semiconductor

ne = ND
+
ne x nh = ni
2
nh = ni
2 /ND
+
Suppose we go for a moderate doping
of 1017 Phosphorous atoms per cm3
ni (at room temperature) = 1010cm-3
Number of holes
nh = ni
2/ND
+
= 1020/1017 = 103 cm-3
n-type semiconductor

nh = NA
-
ne x nh = ni
2.
ne = ni
2 /NA
-
Suppose we go for a moderate doping
of 1017 Boron atoms per cm3.
ni (at room temperature) = 1010cm-3.
Number of electrons
ne = ni
2 /NA
-
= 1020/1017 = 103 cm-3
p-type semiconductor

Just for concept -A p-type material
and n-type material are joined.
Visualize – two tanks containing
water at two different levels are
connected.
Readjusted to single level
Combined energy band diagram
of PN junction - single fermi level
[take it as grounded (reference
voltage is zero)]
EF V = 0
PN junction in equilibrium
Fermi level is constant

ne = ND
+
ne x nh = ni
2
nh = ni
2 /ND
+
nh = NA
-
ne x nh = ni
2
ne = ni
2 /NA
-

Imagine moving from left to right without knowing the
possibility of a junction
Will reach P-side, where Fermi level is close to VB
Similarly from right to left
Will reach n-side, where Fermi level is close to CB.
Junction connects CB of n-side to VB of p-side.

At the junction Fermi level is from close to CB (with
very few electrons) to VB (with very few holes) and it
is called depletion region.
Movement of charge establishes a barrier potential
(V) and an electric field is created
Energy of electrons in the ‘n’ side will be lowered
by –e x V (W = qV)

Voltage is created in n-side is known as built in potential Vbi.
Slope (gradient) at the junction is proportional to electric field
ne = ND
+
nh = ni
2 /ND
+
nh = NA
-
ne = ni
2 /NA
-

There is a
probability for
electrons in n-
side to reach
holes in p-side.

PN Junction under forward bias
Solar cell is working under forward bias to give power
Since barrier is reduced, probability
for electrons in n-side to reach holes
in p-side increase exponentially.
Probability = exp(-q(Vbi-VA)/kBT)
When forward biased – equilibrium will be disturbed.
Forward bias lowers energy barrier or positive voltage on p-
side and pulls the bands down (Vbi – Vapplied)
Eg. (Vbi = 1 V and V applied is say ½ V)

Energy E = qV
Probability
= exp(-q(Vbi )/kBT x exp(qVA)/kBT)
= equilibrium probability x exp(qVA)/kBT)
Since kBT is small, if you apply a few tens of a volt,
then it will be a big number
Or It is much easier for electrons to cross the barrier.
A similar argument for holes as well.

There are excess electrons in the p side due to forward biasing
(which was not there in equilibrium condition)
What is the total concentration of this excess electrons or
charge in P side?
It can be determined using the equation Q = q ∫∆nh(x)dx.
And current is Q/t
What is the time (t)?
t is the average time for an electron
on the p side to recombine or diffuse
to the contact and recombine

So t is the average time for electrons or holes to recombine. Or
the average time to diffuse to the contact for recombining.
Every time electron and hole recombine – Will get current
through the diode.
The current will be flowing through the diode
Which is ID = Q-/te +Q+/th.
te is the time for electron from p side to attach to +ve terminal
of the battery and th for electrons from negative terminal
(electrode) to diffuse into the excess holes at the n-side.

Recombination leads to current
Ex: In GaAs recombination takes place through defects and
energy that is lost in dropping the electron down is emitted as
light (LED)

Recombination continue……

Pure P-side was originally in
balanced state ie. number of
holes = B ions.
Now when electron comes
down and nullify a hole, an
electrostatic imbalance is
created.
In order to eliminate this imbalance, it immediately
kick one electron from valence band to create a hole.

Net effect is electron-hole recombination.
Each time an electron-hole recombination takes place one
electron moves in the external circuit.
Now n-side is in trouble.
One electron moves through
the power supply and reaches
n-side which create imbalance
in n-side. Which combine with
a hole in the VB.

ID = I0 [exp(qVB/nkBT) – 1]
ID –current through the diode IO – reverse saturation current
VB – bias voltage across the diode k-Boltzmann’s constant
n-ideality factor (n = 1 for silicon)
T- temperature in Kelvin
q/kT ≈ 39
I is exponentially proportional to (qVA/kB) and -1
comes from inequilibrium created.
Schockley diode equation or ideal diode equation

If energy of incident light is greater than B.G, it can
promote electron from V.B to C.B.
Create a hole in VB and electron in CB.
λ should be short enough for it to happen.
Largest wavelength that can cause PV response is decided by
band gap of the material
Minimum wavelength range is determined by the
absorbance of the material

Note:
As wavelength decreases, absorbance increases,
or light is absorbed only on the surface of the
material.
In Solar cell this wavelength range depends
upon the solar spectrum
Solar cell works over a broad λ range of the spectrum

 Extends from UV to IR region (encloses Visible
region)

The flux of solar radiation energy that arrives at the outermost
layers of the atmosphere is called the total solar
irradiance (TSI).
It varies slightly in an
annual cycle (by about 3%)
because the earth revolves
around the sun on an elliptic
orbit.

The long-term average of the TSI is called the solar
constant S.
Its reference-value is 1366.1 W/m2
S has changed by less than 0.2 W/m2 (0.015%) over
the past 1000 years, otherwise the observed climate
variations would have been stronger

In photovoltaics, the solar constant quantifies
the amount of radiation energy that arrives from
the sun at the outermost layers of the
atmosphere.
Usually, both the seasonal variations due to the
Earth's orbit and the 11-year solar cycle can be
neglected in PV.

In photovoltaics to quantify the radiation, we usually
use either light intensity or photon flux
Light intensity- also called energy flux or
irradiance, which is energy per area per time,
typically [W/m2] or [mW/cm2]
Photon flux- which is the number of photons
per area per time [cm–2s–1].

Note:
It is convenient to quantify the photon flux Nph in the
equivalent units of electric current
Jph = q⋅Nph [mA/cm2]
Jph equals the electric current that a cell can deliver if it
converts every photon into a free electron–hole pair.

Both absorption and scattering depend strongly on
the path length of sunlight through the atmosphere,
i.e. on the sun’s elevation angle above the horizon.
At horizon- sun light passes through more air.
If the sun shines from 90 degrees above the horizon –
(zenith) the light goes by definition through the
optical air mass 1. The spectrum is then called 'AM1'.

If the sun is at an angle z from the zenith (or an
angle h from the horizon) the air mass is higher: to a
first approximation, assume that the earth is flat

The AM0 spectrum, integrated over its entire range, must
be equal to the solar constant S, and this is not necessarily
the case in real measurements due to daily variations and due
to uncertainties in calibration. Hence, the measured data needs
to be scaled.

Note:
The AM0 spectrum applies only to solar cells mounted
on satellites (when normalized to S = 1366.1 W/cm2)
or on space crafts
Solar cells for terrestrial applications face a rather
different solar spectrum, which cannot be
approximated by the AM0 spectrum or the black
body spectrum.

Terrestrial solar spectrum is variable, asthe sun
changes color and intensity all the time.
So, photovoltaic community has adopted standard
spectra for measuring the efficiency of solar cells.
Without standards, a cell that responds well to red
light would show a diminished performance when
tested under a bluish solar spectrum.

When light penetrates the earth atmosphere, the AM0
spectrum is changed due to the following main
mechanisms:
Absorption due to molecules and particles
in the earth atmosphere.
Scattering at molecules and particles in
the atmosphere.
Reflection from the earth surface.

Absorption – Sources of absorption in AM1.5d spectrum

 Most solar cells receive radiation not only directly from the
sun disc (direct radiation), but also from a large portion of
the sky (diffuse radiation).
 Backscattering.
 the reflection of sunlight at the ground influences the
appearance of the sky.
 The amount of backscattering depends on the broadband
average of the reflectance on the ground, called albedo.

 Eg: if the ground is covered by snow, sunlight
is strongly reflected back to the sky, where it
gets scattered again in all directions – so, to an
observer on the ground, the blue sky appears
approximately double as bright as without
snow.

 the air mass is 1.5 (where the sun is about 41° above the
horizon
 the cell has no optical concentration system, so the cell faces a
hemisphere including the surrounding ground (which is light
sandy soil)
 Spectrum that includes the blue sky and the surrounding
ground is called a global spectrum, so the spectrum is called
AM1.5g.

 the site is at sea level under standard pressure (1013.25
milibars);
 the atmospheric conditions are mostly from the U.S.
standard atmosphere,
 the total irradiance is 100 mW/cm2.

The standard direct (AM1.5d) solar spectrum includes
radiation coming only from small surroundings of the sun disc
(5.8° aperture angle) and is projected orthogonally onto the
cell. Calculations with the same atmospheric conditions as for
the 1.5g spectrum then yield a total irradiance of 90 mW/cm2

i. Absorption of light – generating e-h pairs
(excitons)
ii. Separation of charge carriers of opposite
types
iii. Separate extraction of those carriers to an
external circuit.
Operation of a photovoltaic (PV) cell - three basic attributes

 It is a bound state of an e- and electron hole (in
valence band), which are attracted to each
other by the electrostatic colomb force
 It is an electrically neutral quasi-particle that
exist in insulators, semiconductors and some
liquids
Exciton

 A photon is (when) absorbed by a semiconductor
excites an electron from the valence band into the
conduction band.
 Leaves a +ve charge electron hole in the valence
band
 Electrons which surround the hole so formed (in
V.B) repels the e- in the conduction band.
Exciton continue….

A voltage is developed across the electrodes in the absence of
external load called open circuit potential
If a device is connected – closed circuit – current will flow
through the device. (Short circuit current)
Depletion
region
Electrodes Electrodes
n+ pShort λ
Medium λ
Long λ
+ -
E
Short wavelength is
absorbed by n+ region
as the absorption
coefficient is large and
penetration depth will
be small.

Absence of external load – the
current flowing is called photo
current (IPH) or short circuit current
(ISC)
e-h pairs are created within W as well as n and p regions due
to absorption of light
Holes will be accelerated towards p side and electrons towards
n side.
Minority carriers - diffuse only within diffusion width – Lh
and Le respectively

Total width of the region from
which current is generated (e-h
pairs produced) is Lh+W+Le
(depletion region +diffusion
region)
Isc = -Iph in the absence of external load
Iph depends on the intensity (Iop) of solar light shining on the
cell
Iph = k Iop.

 External load is connected
 Photocurrent will cause a voltage drop across R ie. V = IR
 This voltage will oppose the inbuilt voltage and finally
forward current will start flowing.
 ID – forward bias current
 ID = ISO [exp (eV/KBT -1)]
 ISO - reverse saturation current
 Net Current I = ID – Iph.
 = ISO [exp (eV/KBT -1)] – Iph

1. Constant current source which generate current due
to the incident light
2. PN junction which is under forward bias with
forward bias photocurrent ISO [exp (eV/KBT -1)]
3. Currents through above to oppose each other
4. External resistance R

I-V characteristics is a graphical representation
of the behavior of solar cell.
Gives information required to configure a solar
cell system, so that it can operate as close to
its optimal peak power point as possible.
I-V characteristics of Solar Cell

Describes solar cell energy conversion ability,
efficiency and output performance of the
cell.
Intensity of solar radiation (Insolation) that hits
the cell controll the current (I), while increase
in temperatrue of solar cell reduce its voltage
(V)

Dark Current = ISO [exp (eV/KBT -1)]
On shining light
Net Current I = ID – Iph = ISO [exp (eV/KBT -1)] –Iph.
Graph is shifted down (not starting from zero), because of -IPh

Curve passes through 4th
quadrant.
ie. device can deliver power
If Iop is even higher, graph
will be shifted further down.

Voc – maximum voltage obtainable at the load under
open circuit condition
Isc – Maximum current through the load under short
circuit condition
Power delivered is proportional to Area
(Area can be increase by increasing the
product Isc x Voc.
Note: by choosing proper load resistor, optut
power can be as high as 0.8 (Isc x Voc)
I-V characteristics of Solar Cell - Analysis

When the graph reaches X – axis, Current is zero
and the voltage is called open circuit voltage (Voc)
I = ID – Iph.
= ISO [exp (eVoc/KBT -1)] –Iph = 0
Rearrange this expression for Voc and
neglecting (-1),
Voc ≈ (KBT/e) ln ( Iph /Iso)

Two ways to increase Voc
1. If Iph is increased, Voc will increase
2. For a material with higher band gap, ni will be
lowered and ISO will decrease, Voc will increase
D – diffusion coefficients and L
-Diffusion lengths and
concentration of p and n regionsISO

Power = I * V = ISO [exp (eVoc/KBT -1)] – Iph * V
For Maximum Power dp/dV = 0
On solving, Vm = Voc – (KT/e) ln [1+ (eVm/KBT)]
Recursive relation (Vm on both sides)- solve for Vmax (Vm)
Vm can be substituted for getting Pmax.
Also Im – maximum current at Pmax.
Area within VmIm from I-V graph will give
maximum power
Maximum Power from solar cell?

For a material with higher band gap, Voc will
increase - Favorable
As the band gap increase the region of solar spectrum
being absorbed decrease (lower wavelength) (BG =
hc/λ) - Unfavorable
So, tradeoff – in choosing material -to get Pmax
Maximum Power from solar cell- Higher band gap material – Trade off??

When Solar cell is open ciruited
(not connected to load)
current will be minimum (zero)
voltage across the cell is
maximum known as open
circuit voltage Voc.
I-V Curve
When the solar cell is short circuited (+ve and –ve lead connected together)
voltage across the cell is minimum (zero), but I is maximum known as short
circuit current Isc.
ie. Max voltage is available from a solar cell, at open circuit and maximum
current at closed circuit.

At Isc or Voc Cell does not generate any electrical
power (P=VxI, of which one is zero)
MPP (Maximum Power Point)
Some point in between Isc and Voc, at which
solar cell generates maximum power
[(Imp, Vmp)point)
Ideal operation of a photovoltaic cell (solar cell)
is defined to be at MPP.

Experimental study shows
Vmp ~ (0.8-0.9) VOC
Imp ~ (0.85 -0.95) ISC
Note: Since solar cell voltage and current
both depend on temperature, the actual
power will vary with change in ambient
temperature

1. Fill Factor of Solar Cell (FF)-Power
Extraction Efficiency
2. Efficiency (η)
Figure of Merit of Solar Cell

A2
A1
Voltage (V)
CurrentDensityFill Factor of Solar Cell

Power Extraction Efficiency of solar cell
Measure of the quality of the solar cell
Calculated by comparing the maximum
power (Imp x Vmp) to the theoretical pwer
at the output (Isc x Voc)
FF = Imp x Vmp = Area A2
Isc x Voc Area A1
Fill Factor of Solar Cell

FF = Imp x Vmp = Area A2
Isc x Voc Area A1
Fill Factor of Solar Cell
Larger Fill Factor is desirable
I-V sweep (more square like)
Typical FF ranges from 0.5 to 0.82.
Often represented as % (multiply by 100)

 Power conversion efficiency (η) is
the measure of the quality of a
solar cell
 Solar cell is a wide area device to
capture as much solar energy as
possible
Note

Ratio of the electrical power output (Pout)
compared to the solar power (Pin) into the
cell.
Efficiency η = Pout/Pin
Since solar cell can be operated upto its
maximum power output to get maximum
η (generally Pout is taken as Pmax)
ηmax = Pmax/Pin
Efficiency (η)

Pin is taken as the product of the irradiance
of the incident light measured in W/m2 or
in Suns (1000 W/m2), with the surface
area of the solar cell (m2)
Note: Since I-V parameters are affected by
ambient conditions, cells should be
compared under some test conditions.
Efficiency (η) ηmax = Pmax/Pin

Common solar cell materials
Si, CdTe, CdS, CuInSe2 (Eg = 0.7 eV), CuInGaSe2(CuInXGa1-2Se2)
Efficiency η = 10 to 20 % (depending on the processing route
used to make it and the quality of the films grown).
Si based solar cells η = 22%
Polycrystalline Si η = 15 %
Amorphous Si η = 10 %
(cost is too low, because can be deposited on a glass slide)

Most solar cells convert 10-20% of energy they
receive into electricity.
A typical single junction silicon solar cell has a
theoretical maximum efficiency of about 30
% known as Shockley Quieisser limit.
How efficient are solar cells?

This is because of the waste of light falling on it
either due to insufficient frequency or
energy or wastage of excess energy.
The very best cutting edge laboratory cell can
manage 46% under ideal conditions
Real world domestic solar panel achieve
efficiency η=15%
How efficient are solar cells?

Solar cells are named after the semiconducting
materials they are made up of.
Some are useful for solar cells action with sunlight
reaching earth’s surface, while some are optimized
for space applications.
Solar cells can be single junction or multiple
junction types-based on different charge
separation mechanisms.
Classification of solar cells

Ist, 2nd or 3rd generation solar cells
Ist – use crystalline silicon (poly or
monocrystalline) – Hard structure
2nd – Thin film solar cells (flexible)
amorphous silicon, Cadmium Telluride
(Cd-Te), Copper Indium Gallium
Disulphide (CIGS)
Classification

Ist – use crystalline silicon (poly or
monocrystalline) – Hard structure
2nd – Thin film solar cells (flexible)
amorphous silicon, Cadmium Telluride
(Cd-Te), Copper Indium Gallium
Disulphide (CIGS)
ClassificationIst, 2nd or 3rd generation solar cells

3rd generation -New category of photovoltaics –
based on thinfilms (still in research)
GaAs thin film used for single crystalline thin film
solar cells. Though it is highly expensive,
showed record efficiency for a single junction
solar cells ~ 28%.
Hence used in space based solar power
applications-where money is not an issue.
Ist, 2nd or 3rd generation solar cells

A heterojunciton is the interface that occurs
between two layers or regions of dissimilar
crystalline semiconductors having unequal band
gaps
Homojunction - equal band gaps junction.
Combination of multiple heterojunctions together
in a device is called heterostructure.
Hetero junction Solar Cells

Hetero junctions are generally manufactured
using MBE (molecular beam epitaxy) or CVD
(chemical vapor deposition) technique.
In MBE or CVD, thickness of the grown film
can be precisely controlled.
Perfectly lattice matched interface can be
grown.
Continue….

1. To enhance short wavelength response –
shallow junction is desirable
2. To minimize series resistance of top layer,
higher doping is required, which in turn cause
increased absorption in that layer and reduce
diffusion length.
3. Recombination loss of the incident radiation on
the surface
Drawbacks of Homo Junction Solar Cells

Drawbacks are considerably reduced in III-V
semiconductors like GaAs, AlGaAs etc heterojuncitons.
For Heterojunction solar cells:
AlxGa1-xAs/GaAs heterostructure is generally used
Band gap of AlGaAs > Band gap of GaAs
Photons with energy less than band gap of AlGaAs
will not be absorbed – ie. transparent and reach
GaAs.
ie. short wavelength response is improved.
Advantages of Hetero Junction Solar Cells – continue…

Electrons generated in GaAs layer will be confined by
pGaAlAs/p-GaAs heterojunction
Reduce surface recombination and enhance η
Thickness of AlGaAs may be increased without affecting its
light transmission characteristics
Increased thickness reduce sheet resistance which is in
parallel to GaAs resistance (Rparallel < individual).

Thick AlGaAs layer also improves the radiation
tolerance of solar cell.
Efficiency may be increased even further by
attaching two or more p-n junctions having
different band gaps.
Lower energy pohtons which are not absorbed by
one cell will be absorbed by the other of lower B.G.

Practical Heterojunction Solar cell - First type
A PN junction formed of P type and n type GaAs and
a thin layer of AlGaAs is deposited over it, which
helps in passivating the surface defect states
AlGsAs is having higher band gap than GaAs
So AlGaAs will be transparent for incident radiation,
which will reach GaAs
Gives higher efficiency

Practical Heterojunction Solar cell - Second type
PN junction between n-AlGaAs (B.G = 2 eV) and p-GaAs (B.G
= 1.4 eV)
AlGaAs due to high band gap act as a window for radiation to
reach the junction
n- layer need not be thin (which is the requirement in
homojunction cells)
Because of the difference in the band gaps of p and n material,
e-h pairs produced can be easily separated and get current

Behavior of a semiconductor junction depends crucially on the alignment
of the energy bands at the interface.
Semiconductor interface can be organized into 3 types of hetero
junctions
Hetero junction Solar Cells- Continue

One solar cell on top of the other
2 pn junctions
Materials will be chosen such that B.G of
first will be greater than that of second.
(BG1 > BG2)
Shorter wavelength will be absorbed in the
first PN junction and longer wavelength
will be absorbed in second.
P
N
P
N
B G1
B G2
hc/λ
Issues: Processing complexity and cost?

NREL – National Renewable Energy Lab
1976 2020
Solar cell research 1976 to 2020

Silicon solar cells – blue part of the graph
Showed higher efficiency compared to other materials over the
years until GaAs based heterojunction solar cells came into
the picture
Si solar cells showed record efficiency of 15% to 24%
Crystalline silicon is the mainstream technology (blue
squares)
Highest efficiency (~46 %) violet squares with dot inside –
with multijunction (combination of band gaps) solar cells. –
expensive, but high η

Perovskite solar cells – use a perovskite
structured material as active layer.
η=5% (beginning of 2009) to 20% (in 2014)
Very rapidly advancing technology and hot
topic in solar cell field.
New Research In Solar Cells

Reports say that, perovskite solar cells are extremely
cheap to scale up making them a very attractive
option for commercialization.
Liquid inks, dyes (dye sensitized solar cells, DSSC),
Quantum dots (QDSC) – use low band gap
semiconductor nanoparticles fabricated with
crystallite sizes small enough to form quantum
dots.
Organic/polymer solar cells – built from thin films of
organic semiconductor.
New Research In Solar Cells

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27 pius augustine solarcells

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