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Potential fluctuations on CZTSSe solar cells admittance
Frédérique Ducroquet, Louis Grenet, Raphaël Fillon, Henri Mariette
To cite this version:
Frédérique Ducroquet, Louis Grenet, Raphaël Fillon, Henri Mariette. Potential fluctuations on CZTSSe solar cells admittance. 29th International Conference on Defects in Semiconductors, Jul 2017, Matsue, Japan. �hal-01983597�
0,0E+00 5,0E-05 1,0E-04 1,5E-04 2,0E-04
100 1000 10000 100000 1000000 10000000 100000000
160K 300K
102 103 104 105 106 107 108
w (rad/s) 2.10-4
1.5 10-4
1.10-4
5.10-5
0 C(F/cm-2 )
230K
w/o potential fluctuations with potential fluctuations
x=0 x
xj
SCR
r
1,E-09 1,E-08 1,E-07 1,E-06 1,E-05 1,E-04
0,002 0,003 0,004 0,005 0,006 0,007 0,008
Ncompl
1019 5.1019 1020 5.1020
[Ncomplexe] (cm-3)
exp
102
10
1
10-1
10-2
10-3
0.002 0.004 0.006 0.008 1/T (K-1)
G(S/cm-2 )
Potential Fluctuations
on CZTSSe Solar Cells Admittance
University of Grenoble-Alpes (UGA), Minatec, Grenoble, France
a IMEP-LAHC, b CEA/LITEN, c CEA/INAC, Inst. Néel
F. Ducroquet a , L. Grenet b , R. Fillon b , H. Mariette c
Contact: Frédérique Ducroquet: [email protected]
2 – Solar cell structure
4 – Admittance modelling
small signal Poisson equation including the deep level ac response:
- Potential fluctuations explain the stretching of the capacitance step
6 – localized states transport
9 – Conclusions 1 – Context
• Influence of potential fluctuations: - on defect analysis
- on electrical transport
Objectives:
• Density Functional Theory 1 V Cu and Cu Zn - create acceptor levels
- are among defects with the lowest formation energy - may be stabilized by the formation of neutral defect
complexes: [Cu Zn +Zn Cu ], [V Cu +Zn Cu ], [Cu Zn +Sn Zn ]
• Low V oc on CZTSSe solar cells role of point defects and potential fluctuations suggested but not clearly established
Experiment:
Admittance spectroscopy @ 0V
from 140K to 300K, 100Hz to 1MHz
V
oc(V) J
sc(A.cm
-2) FF (%)
~ 0.33-0.38 ~ 0.03 50 ~ 5.3-6.6
• plane capacitor:
3• one deep acceptor level:
• potential fluctuations:
Refs: (1): S. Chen et al, Adv. Mat., 25(11), 1522-1539 (2013) , (2) L. Grenet et al. Solar Energy Mat. Solar Cells, 126:135–142, (3) M. Begulawa et
C.R. Crowell, Solid-State Electron. 17, 203 (1974), (4) B. Pistoulet et al, Phys. Rev. B, 30 5987 (1984), (5) D. Monroe, Phys. Rev. Lett. 54, 146 (1985).
0,0E+00 5,0E-05 1,0E-04 1,5E-04 2,0E-04
100 1000 10000 100000 1000000 10000000 100000000
140K
300K
102 103 104 105 106 107 108
w (rad/s) 2.10-4
1.5 10-4
1.10-4
5.10-5
0 C(F/cm-2 )
dCZTS
C
0
3 – Admittance measurement: C(w)
One capacitance step: response of a deep acceptor level at
E aexp =E v +(0.2±0.03)eV
not observed on G/w spectrum
can explain the strong increase of the series resistance at low temperature generally observed on CZTSSe solar cells.
At low temperature, geometrical capacitance value CZTSSe absorber fully depleted
Complex occupation function defined for each elemental energy :
r
( ) ( ) ( ) ( ) ( )
2
2
p x n x N x f x
x q dx
d
Ta
) ,
( , )
, (
) ,
) ( ,
( w
w
w
w w
x x x
x x
C
ac E( )
Truncated and asymmetric Gaussian distribution
4Hole concentration in valence band:
Admittance expression:
Ionized defect concentration:
h g E
g E
v v
E v
v
h v
w w v
v E E dE
dE E
f E
E cste
p
0
0
2
2
exp 0
) ( 1
) ,
( 1
) ( )
, ( )
( )
( )
, ( 1
) ) (
, ( )
,
( w
j x
x p x
f c
x n x
f x c
x
f
n p
) ,
~ ( )
,
~ (
) ,
( E ~ )
,
~ ( )
,
~ (
~
0
w w
w
w
w w
w
w
bulk bulk T
x x
x x
j Q x
C j
C j
G
Y
h g E
g E
ta ta
ta F
ta ta
h vta
w w ta
E dE E
kT E g E
cste N
0
2
2
exp 0
exp 1
5 – C(w) fit
1- E
afit(0.28eV) > E
aexpdue to PF
Conduction in disordered materials:
5-0,1 -0,05 0 0,05 0,1 0,15 0,2 0,25 0,3
1,E+18 1,E+20 1,E+22 1,E+24 1,E+26 1,E+28
E
vE
mTE
E
ta1012 1014 1016 1018 1020 1022
DOS
0.3
0.2
0.1
0
-0.1
Ener gy
At moderate temperatures, multi-
trapping relaxation process dominates:
hole transition between extended states below the mobility edge E
mand localized states in band tail
neutral complex defects can introduce a high density of states in the band tail:
hopping transport near TE, the Transport Energy, can occur at low temperature:
8 – dc conductivity
low frequency: dc conductivity
high frequency/low temperature:
universal power-law frequency response
7 – G( w )
hopping through localized states in band tail due to
high concentration of neutral complexes
• Two steps process 2 :
- Precursors deposition: ZnS by sputtering, Cu/Sn by electron beam evaporation
- Selenization: annealing under Se vapor
• Hopping process in the band tails has to be taken into account to model the ac transport properties
• At low temperature, potential fluctuations effects are enhanced in CZTSSe bulk due to free carrier
compensation mechanism.
• High density of neutral complexes can favor the dc charge carrier transport due to hopping process in the band tails
10 – Perspectives
- Continuous capacitance variation superimposed to the defect related step not reproduced by the model
associated with w
sconductance
increase dielectric relaxation effect
1,0E-01 1,0E+00 1,0E+01 1,0E+02 1,0E+03
100 1000 10000 100000 1000000 10000000 100000000
102 103 104 105 106 107 108
w (rad/s) 103
102
10
1
10-1 G(S/cm-2 )
140K
300K dc
ac ~ ws
s~ 0.85
kT TE E
B
TE
r
Fdc
