Lecture 12 - Minimization of Recombination Losses.pdf

Minimization of Recombination Losses in PV cell

1. Minimization of Recombination
Losses
Engr. Dr. Adnan DaudKhan

2. Minimization of recombination losses
•When illuminated, carrier generation occurs throughout the volume
of solar cell.
•Recombination of generated carrier is possible at the surface, in the
depletion region and in the bulk of the semiconductor.
•All possible recombinationsshould be minimized in order to obtain
high I
L
.
2

3. Surface Passivation
•In the crystalline materials, surface represents sudden discontinuity of the crystal
arrangement, which gives rise to dangling or unfinished bonds.
•These bonds act as a good recombination center.
•In terms of energy band diagram, the dangling bonds at the surface give rise to energy
states in the middle of the bandgap, which act as a good recombination center.
•Thus, in order to avoid recombination, both front and rear surfaces should be passivated
i.e., the recombinativeproperties of the surface should be nullified.
•This is known as surface passivation.
•At the front surface, the surface passivation is achieved by deposition of a dielectric layer
such that it passivates the unfinished bonds at the surface (removing the energy states
from the middle of the bandgap).
•SiO
2
and Si
3
N
4
layers are used to passifythe crystalline Si surface.
•Both layers have high energy gap, which prevents carriers reaching the surface deposited
dielectric layer.
3

4. •In another method of surface passivation, high level doping, in a low
doped semiconductor, of similar impurities (for instance Al in P-type
Si or P in N-type Si) is done.
•This is typically done at the rear side (usually P-type) for crystalline Si
solar cells.
•In this way, a P
+
-P junction is obtained (P
+
represents heavy doping
10
18
cm
-3
or higher).
•It gives rise to an E-field at the junction in the direction from P to P
+
side.
•This junction represents potential energy barrier to the minority
electrons to flow to the rear surface.
•The E-field repels electrons back towards pnjunction, and reduces
the recombination at the surface.
•This field is known as back surface field (BSF).
4

5. 5
N
+
P P
+
Add P
+
layer at the back side.
Electrons repel near
the surface.
Electrons love to travel
down the hill and will
be collected by the
emitter contact.
Holes will be collected by the
base contact.
Now, by introducing the P
+
layer, an
upward slope is created in the band
diagram due to which the electrons
will be repelled from going near the
surface by the field which is created
by the P
+
layer.
The best material for BSF is Al,
which repels the electrons from
reaching the back surface.
Generated e-h pairs
in different regions.
–
–
–
+
+
+
+
+
+
E
So our problem was, electron can go
to the surface and recombine with the
hole.Surface
states
Due to diffusion process,
this electron will try to
move to the P
+
region.

6. •The effectiveness of the surface recombination is given in terms of
surface recombination velocity (SRV).
•It is defined as the rate of recombination at the surface divided by the
excess carrier concentration at the surface.
•Mathematically,
•Where S
n
and S
p
are SRVs for electrons and holes, respectively.
•The lower is the SRV, the better is the surface passivation.
•An SRV value of about 10cm/s corresponds to a well-passivated
surface, while an SRV of value more than 10
4
cm/s corresponds to a
poorly passivated surface.
6
and
sur sur
n p
sur sur
R R
S S
n p
 
 

7. 7
+4
+4+4
+4
+4
+4
+4
+4+4
Consider a Si crystal
surface
If we moved to the surface, these
Si atoms have not fulfilled all the
bonding requirements.
Theses unfinished bonds, breaks the symmetry of the lattice and results in the formation of energy states in
the bandgap region of the semiconductor.
These are called surface states.

8. 8
Ec
Ev
Surface states
These surface states are very good centers for recombination.

9. Design For High V
oc

10. Design For High V
oc
•V
oc
ofasolarcellismainlygovernedbyRecombination.
•HigherV
oc
canbeensuredbyminimizingRecombination.
•V
oc
isdefinedasthevoltageatwhichI
sc
becomesequalandoppositetoforwardbiasdiffusioncurrent(i.e.,
I
sc
=I
d
,sonetcurrentI=I
sc
–I
d
=0).
•Forwardbiasdiffusioncurrentdependson
(1)RecombinationinBulk
(2)Recombinationatthesurfaceofthesolarcell
•AnyincreaseinrecombinationgivesrisetoincreaseinreversesaturationcurrentI
o
.
I = I
0
( e
qV/kT
–1) –I
L
•ThusincreaseinI
0
duetoincreaseinRecombinationresultsinincreaseinforwardbiasdiffusioncurrent.
Then,asperdefinitionofV
oc
,increasedrecombinationinsolarcelldecreasesV
oc
.

11. Requirements For High V
oc
•Therequirementsforhighopencircuitvoltageistominimizetheforwardbiasdiffusioncurrent.
•Theforwardbiascurrentdependson
•Thenumberofminoritycarriersatthedepletionregionedge
•Howquicklytheymoveawayfromtheedge&
•Howquicklytheyrecombine.
Theseparametersarediscussedbelow:
(1)Point–1:
•Thenumberofminoritycarriersattheedgeofdepletionregionisduetoinjection.
•Theinjectedminoritycarrierfrommajoritysidedependsontheequilibriumminoritycarrier
concentrationmultipliedbyanexponentialfactorwhichdependsonforwardbiasvoltageand
temperaturei.e.,P
n
=P
no
e
qV/kT
.
•Thusminimizingminoritycarrierinjectionimpliesthattheminorityequilibriumconcentrationshould
bereduced.
•Itcanbeobtainedbyhighdopingatbothsidesofjunction.

12. (2)Point –2:
•The injected minority carriers move away and disappear by
recombination.
•If the minority carrier diffusion length is large, then the carriers will
travel longer distance before recombining.
•But if it is small, they will disappear soon, vacating space for other
minority carriers to get injected due to diffusion process, which will
increase the forward bias current and reduce Voc.
Thus, for high Voc, minority carrier diffusion length should be high
.
It
depends on:
(1)Typeofmaterial&
(2)Amountofdoping
12

13. •Higher doping reduces the minority carrier diffusion length (&
minority carrier lifetime) & the V
oc.
Thus doping at either side of the
junction should be low.
•The variation in the minority carrier lifetime as a function of doping
for Si is shown in the below figure.
13

14. (3) Point –3:
•The crystallographic defects in the bulk material and at the surface
cause carrier recombination, which reduces the Voc.
•Thus, for high V
oc
, both bulk & surface of the solar cell must be
passivated (in order to avoid recombination due to crystallographic
defects).
14

15. (4) Point –4:
•The solar cell volume should be small, which means the thickness of
the solar cell should be small.
•If light trapping scheme is used properly, no radiation would be lost
due to smaller thickness.
•In the case of small volume, there is lower recombination as
compared to large volume for same material parameters.
•Lower recombination would result in higher Voc.
•But now, since all radiation is absorbed in lower volume, at any given
time, there will be large carrier concentration in the bulk and near the
surface.
•Therefore, surface should be properly passivated.
15

16. •Itcanbeseenfrompoint(1)&(2)thatforhighV
oc
thereareconflicting
requirementsofsubstratedoping.
•Highdopingdecreasesequilibriumminoritycarrierconcentration
requiredforhighV
oc
,butatthesametimeincreasesrecombination
whichreducesV
oc
(italsoreducesI
sc
).
•Thus,therehastobeanoptimumdopingofthesubstrateateitherside
ofthejunction.
•ForSisolarcells,N-sideisthin.Itisusuallydopedheavilyinorderto
reduceresistivelossesbutP-sideisdopedlightlyinordertoachieve
highV
oc
.

17. •Variation in Voc, Iscand efficiency for a Si solar cell as a function of P-
side doping is shown with and without BSF.
17

18. Design for High FF

19. Design for High FF
•ForHighI
L
MaximumAbsorptionrequired
•ForHighI
L
&V
oc
MinimumRecombinationrequired
•ForHighηMaximumAbsorption,MinimumRecombination&MinimumResistive
losses
•Theresistivelossesaretheresultofseriesandshuntresistancesofthesolarcell.
BothofthemaffecttheFFofthecell.
•TheR
s
shouldbeaslowaspossible&R
sh
shouldbeashighaspossible(ideallyR
sh
of
thePNjunctionshouldbeverylarge).
•Afaultysolarcellcanresultinpartialshuntingofthejunction,whichlowersthe
shuntresistanceofthecell.
•TheR
sh
isnotadesignparameter.ItismainlytheR
s
thatisinourhandsadesign
parameter.

20. Design for High FF
TheR
s
ofasolarcellismadeofseveralcomponents,whichincludes
(1)ResistanceoftheBase,R
b
.
(2)ResistanceoftheEmitter,R
e
(thinnersideofthejunctionisreferredasemitter).
(3)ResistanceofMetalcontactatfrontandrearsurface,R
bf
.
(4)ResistanceofMetal-semiconductorinterfaceorcontactresistanceR
c
.

21. •ThusR
s
canbewrittenas
R
s
= R
b
+ R
e
+ R
bf
+ R
c
•Rearsideistypically,completelycoveredbymetal,duetowhichtherearside
metalhasverysmall&negligibleresistance.
•Butatthefront,sincemetalreflectslight,metalcoverageshouldbeassmallas
possibleinordertoallowlighttoenterthecell.
•Asinglecontactatthefrontsurfacewillbeveryresistive,ascurrentisrequiredto
becollectedfromalargearea.Thereforemetalatthefrontisdistributedin
thickerandthinnermetallines.
•ThickerMetallineiscalledBusbar.whereasthinnerMetallinesarecalled
Fingers.
•Busbarscollectthecurrentfromthefingersanddeliversittotheexternalcircuit.
•FingersandbusbaroffrontmetalcontactcollectivelyareknownasMetalGrid.
21

22. Design for High FF
•Amongthevariouscomponentsthatcontributetotheseries
resistance,R
e
&R
bf
arethemostdominatingcomponents.
•Aproperdesignforloweringtheresistanceofthesetwocomponents
playsanimportantroleintheperformanceofthecell.
•Now,letuslookatthevariousresistivecomponents.

23. BaseResistance:
•Thebaseisusuallyuniformlydoped.Therefore,thecurrentfromthe
baseisassumedtobeconstant.
•Ifthebaseresistivityρ
b
isknownthenR
b
canbeestimatedas
R
b
=ρ
b
W
b
/A
whereW
b
isthewidthofthebasewhichisalmostequaltothewidthofthecell
W,asthicknessofemitterisquitesmall(junctionshouldbeclosetothesurface)
andAistheareaofthecell.
•Powerlossinthebaseduetosolarcellcurrentcanbeestimated
usingI
2
Rlaw.
23
This is actually the relationship
between the resistance and resistivity
i.e., R = ρl/A, where lis the length of
the material and A is the cross
sectional area.

24. Sheet resistance
•Sheet resistance (also known as surface resistance or surface
resistivity) is a common electrical property used to characterize thin
films of conducting and semiconducting materials.
•It is a measure of the lateral resistance through a thin square of
material, i.e. the resistance between opposite sides of a square.
•We know the expression for resistance i.e.,
R = ρL/A
A is the area of the bar
L is the length of the bar
24
Current is flowing in the
direction of length

25. •But in case of thin sheet of
material with a thickness “t”,
width “W”, and a length “L”.
•The current is flowing in the
direction of length.
•Now lets rewrite the equation of
resistance for this particular
material.
25
Current
This term is called a sheet resistance R
s
.
This kind of resistance is used for thin films where the precise
thickness is not known.
Here the thickness is not
in controlled i.e., precise
thickness is not known.

26. ??????
The unit of the sheet resistance is (ohm per square).
This square means that both L and W are equal.
So both will cancel each other i.e.,
??????
?
?
So
??????
(resistance = sheet resistance for a thin sheet)
??????
26
??????
□
W

27. Emitter Resistance:
•Unlikebase,thecurrentinemitterflowsinthelateraldirection.
•Theemitterofacellisformedbydiffusionathightemperature.
•Thereforedopingoftheemitterisnotuniform.Alsotheprecisethicknessoftheemitterisnotknown.
•Duetothisreason,itisnoteasytoestimatetheresistanceoftheemitter.
•Anewparametercalledsheetresistivityρ
sh
isdefinedtorepresenttheresistivebehaviorofemitter.
•Theρ
sh
isnothingbutSCresistivitydividebythethicknessoftheemitterW
e
•SotheemitterresistanceReis
Re=ρ
sh
ifL
1
=L
2
??????
??
=
??????
??????
?