Post on 28-Jan-2016
Solar Cells, Sluggish Capacitance, and a Puzzling
Observation
Tim GfroererDavidson College, Davidson, NC
with Mark WanlassNational Renewable Energy Lab, CO
~ Supported by Bechtel Bettis, Inc. and the American Chemical Society – Petroleum Research
Fund ~
Experiments by . . .
Kiril Simov (Davidson ’05)
Patten Priestley (Davidson ’03)and Malu Fairley (Spelman ’03)
Outline
• Semiconductors, defects, and solar cells• Diode capacitance and the DLTS
experiment• Our measurements and an unusual result• A new model for minority carrier
trapping/escape during DLTS
Semiconductors
PeriodicPotentialPhyslet
r
V(r) Energy levels
Spacing decreasing
n=3
n=2
n=1
a
a
--
f ree atoms atomic crystal
5.6 5.7 5.8 5.9 6.0 6.10.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
I nAs
GaAs
SevereMismatch
InPSubstrate
Band
gap
(eV)
Lattice parameter (Angstroms)
InGaAs Bandgap vs. Alloy Composition
Bandgapvs. LatticePhyslet
Semiconductor Defects
Lattice-Mismatch Applet
Defect Level Physlet(from the forthcoming Physlet Quantum Physics: An Interactive Introduction to Quantum Theory
by Mario Belloni et al., due out this Fall
Solar Cell Operation
Conduction Band
Valence Band
PHOTONEN
ER
GY
ELECTRON
E-Field
E-Field
HOLE
E-Field
E-Field
+ +
++
---
-
-
CURRENT
ABSORPTION
When a photon is absorbed, an electron is excited into the conduction band, leaving a hole behind in the valence band. An internal electric field sweeps the electrons and holes away, creating electricity.
Defect-Related Trappingand Recombination
Conduction Band
Valence Band
EN
ER
GY Defect Level
-
+
PHONONS
PHONONS
But electrons can recombine with holes by hopping through defect levels and releasing phonons (heat). This loss mechanism reduces the efficiency of a solar cell.
Defect-Related Transition Probabilities
P ~ 10-
3
P ~ (0.5)10 ~ 10-3
P ~ 10-5
P ~ 10-1
P ~ (0.5)16 ~ 10-5
P ~ (0.5)4 ~ 10-1
The probability P of transitions involving phonon emission depends on the number of phonons required, which is determined by the position of the defect level in the gap.
-
+ + +
- -
p/n Junction Formation
NP+
++
+++++++
+
--
- -
-
+++++++
+++++
Depletion Layer
+-
+
+
-
-
+
+
+
-
-
-
+
+
+
-
-
-
+
Bias-Dependent Depletion
+
+-
- NP+
++
+++++++
+
-
-
-
+++++++
+++++
Depletion Layer
+-
+-
- +
+
+
-
-
+ -
-
With Bias
Diode Capacitance
No bias
Reverse bias
d1 Vbuilt-in
Vbuilt-in+Vapplied
d2
C = Q/V
~ 0A/d
Reverse bias increases the separation between the layers where free charge is added or taken away.
EN
ER
GY
Defect characterization via DLTS
+
+-
- NP+
++
+++++++
+
-
-
-
+++++++
+++++
Depletion Layer With Bias
+-
+-
- +
+
+
-
-
+
Temporary Reduced Bias
Depletion Layer With Bias
Depletion Layer With Bias
-
-
+
+ -
-
Temporary Reduced Bias
+
+
Typical DLTS Measurements
0.0 0.1 0.2 0.3 0.4 0.5
e-6
e-5
e-4
e-3
e-2
e-1
e0
T = 200K T = 180K T = 160K T = 140K
Capaci
tance
Change (
a.u
.)
Time (ms)
Pulsetowardzerobias
Return to steady-state reverse bias
free carriers
trapped carriers
DLTS Experimental Setup
Computer with LabVIEW
Temp Controller
Pulse GeneratorCryostat with sample
Digital Scope(Tektronix)
(1)(2)
(3)
(4)
(5)
Oxford77K
Agilent
Capacitance meter (Boonton)
Device Structure and Band Diagram
m (S) InP N = 3x10 cmD
18 -3
0.1 m (S) InP N = 1x10 cmD
19 -3
0.5 m (S) In Ga As N = 3x10 cm
0.53 0.47
D
16 -3
0.05 m (Zn) In Ga As N = 1x10 cm
0.53 0.47
A
19 -3
0.05 m (Zn) InP N = 2x10 cmA
18 -3
0.05 m (Zn) In Ga As N = 1x10 cm
0.53 0.47
A
19 -3
{ + + + - -- -+
Quasi EF,n
Conduction band
Valence bandp+/n Junction
-Quasi EF,p
Energy
Position
Depletionregion
W
Transient Capacitance: Escape
0.0 0.1 0.2 0.3 0.4 0.5
e-8
e-6
e-4
e-2
Steady-State Bias = -1.1VPulse = +0.1V
Cap
acita
nce
Cha
nge
(a.u
.)
Time (ms)
T = 130K T = 140K T = 145K T = 150K T = 160K
70 80 90 100e2
e4
e6
e8
e10
e12
esc
= 110 s
and SS Bias = -0.1Vand SS Bias = -1.1Vand SS Bias = -2.1V
Average Ea = 0.29 eV
Esc
ape
Rat
e (
s-1)
1 / kT (eV-1)
Pulse = +0.1V relative to SS
Filling Pulse Dependence: Capture
-200 0 200 400 600 800
e-7
e-6
e-5
e-4
e-3
e-2
e-1
T = 77K
Pulse Length: 10 s 30 s 100 s 200 s
Ca
pa
cita
nce
Ch
an
ge
(a
.u.)
Time (s)0 200 400 600
e-8
e-7
e-6
e-5
e-4
e-3
e-2
cap
= 113 +/- 2 s
Steady-State bias = -0.3VPulse: +0.2V (relative to SS)
C0 -
C
trap
s (a.
u.)
Pulse Length (s)
T = 77K
Proposed Model
+ + + - -- -+
Quasi EF,n
Conduction band
Valence band
Traps
p+/n Junction
-Quasi EF,p
Energy
Position
Depletionregion
d
W
Testing the Model
0.0 0.2 0.4 0.6e-10
e-8
e-6
e-4
e-2
Cap
acita
nce
chan
ge
C0
(a.u
.)
Time (ms)
Bias = -0.1V Bias = -1.1V Bias = -2.1V Bias = -3.1V
Pulse = +0.1VT = 77K
0 1 2 3
e-1
e0
e1
e2
e3
d W C
0 (a.u.)
Thi
ckne
ss (
nm)
Reverse Bias (V)
Variable-Bandgap Lattice-Mismatched Stuctures
Undoped InAsyP1-y, 30 nm
Undoped InxGa1-xAs, 1.5 μm
Undoped InAsyP1-y buffer, 1 μm
Undoped InAsyP1-y step-grade region:0.3 μm/step (~ -0.2% LMM/step), n
steps
Undoped InP substrate
Radiative Recombination
Conduction Band
Valence Band
PHOTON
EN
ER
GY
-
+
light in = heat + light out
radiative efficiency = light out / light in
heatlight in
light out
0.0 0.1 0.2 0.3 0.4 0.5 0.6100
104
108
1012
1016
Den
sity
of S
tate
s (c
m-3eV
-1)
Energy (eV)
Defect-Related Density of States
Valence Band
Conduction Band
EN
ER
GY
The distribution of defect levels within the bandgap can be represented by a density of states (DOS) function as shown above.
0
20
40
60
80
100
EV
EC
Energy
Log(
DO
S)
Eg = 0.80 eV
10231019 1021 1025
Ra
dia
tive
Eff
icie
ncy
(%
)
e-h Pair Generation and Recombination (cm-3s-1)
Radiative Efficiency Measurements
heat
light
1018 1020 1022 1024
0
20
40
60
80
100
EC
EV
EC
EV
Eg = 0.68 eV
Energy
Log(
DO
S)
Energy
Log(
DO
S)
Ra
dia
tive
Eff
icie
ncy
(%
)
e-h Pair Generation and Recombination (cm-3s-1)
Four Conclusions• 0.29eV hole trap is observed in n-type
InGaAs under reverse bias• Temperature-dependent capture and
escape rates are symmetrical• Rates level off at cold temperatures due to
tunneling• Device modeling points to defect states
near the p+/n junction
Two References• T.H. Gfroerer et al., APL 80, 4570 (2003).• T.H. Gfroerer et al., IPRM (2005).