May 27, 2004 Photovoltaics Laboratory Chalcogenide Solar Cells: Choosing the Window Colorado State...
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Transcript of May 27, 2004 Photovoltaics Laboratory Chalcogenide Solar Cells: Choosing the Window Colorado State...
May 27, 2004 Photovoltaics Laboratory
Chalcogenide Solar Cells: Choosing the Window
Colorado State University
Funding: US National Renewable Energy Laboratory (NREL) Japanese New Energy Development Organization (NEDO)
Special thanks to Markus Gloeckler for assistance with figures
Jim Sites
Markus Gloeckler, Alex Pudov, and Ana Kanevce (CSU) Falah Hasoon and Miguel Contreras (NREL)
Hans Schock (IPE) and Tokio Nakada (AGU)
Collaborators:
European Materials Research Society – Spring 2004
Approach
May 27, 2004 Photovoltaics Laboratory
(1) Device-physics approach to the selection of window layers for fabricating high-performance solar cells with CdTe and CIGS absorbers.
[Device physics no means the whole story, but may give useful direction even when material structure or other factors play a major role]
(2) Large range of possible band gaps will be considered.
(3) Attempt to be quantitative.
(4) Focus on two areas: (a) Window absorption: how much of an effect? (b) Conduction-band offset: what happens when it changes?
Choosing the Window: Outline
May 27, 2004 Photovoltaics Laboratory
(1) Photon considerations: Window absorption.
(2) Conduction-band offset problem I: Big spikes (and their “red-kink” precursor) that limit current.
(3) Conduction-band offset problem II: The cliff problem that limits voltage.
(4) How much slack does one get in choosing the window?
(5) Conclusions.
Short-Wavelength Current: CdS Windows on CdTe
May 27, 2004 Photovoltaics Laboratory
Granata, Sites, Contreras-Puente and Compaan, IEEE PVSC-25, 853 (1996)
Same current loss should apply for CI(G)S cells.
Short-Wavelength Collection
Current Loss with Alternative Windows
May 27, 2004 Photovoltaics Laboratory
Absorption spectra based on that of CdS, but shifted in energy.
Calculated Values
Fractional Current Loss
May 27, 2004 Photovoltaics Laboratory
For 100-nm window layer
Larger fraction with smaller current from larger-gap absorber.
Efficiency Contours
May 27, 2004 Photovoltaics Laboratory
Record CIGS Cell
Parameters for record CIGS cells EC effects neglected
100 nm window VOC = Eg – 550 meV Fill-factor = 80%
Choosing the Window: Outline
May 27, 2004 Photovoltaics Laboratory
(1) Photon considerations: Window absorption.
(2) Conduction-band offset problem I: Big spikes (and their “red-kink” precursor) that limit current.
(3) Conduction-band offset problem II: The cliff problem that limits voltage.
(4) How much slack does one get in choosing the window?
(5) Conclusions.
Sign Convention for EC
May 27, 2004 Photovoltaics Laboratory
EC < 0
position [m]0.1 0.2 0.3 0.4 0.5
-2
-1
0
1EC > 0
position [m]0.1 0.2 0.3 0.4 0.5
-2
-1
0
1
ZnOCdS
CIGS
ZnOCdS
CIGS
"spike" "cliff"
Smaller Gap Absorber
Larger Gap Absorber
Some consensus on EC magnitudes between theory, experiment, and numerical simulations of J-V curves
Spike can impede photoelectrons (may be bad) Cliff slows forward electrons in interfacial-recombination region (also may be bad)
Earlier “Red-Kink” (Solarex Cells)
May 27, 2004 Photovoltaics Laboratory
Also seen In cells from NREL, Boeing, and Siemens/Shell
Dark and Red-light J-V Curves
Producing a “Red” Spectrum
May 27, 2004 Photovoltaics Laboratory
Wavelength [nm]
200 400 600 800 10000
20
40
60
80
100 filter trans-
mittance [%]AM1.5
AM1.5 X filter
[mA/cm2/m]
Wavelength [nm]300 400 500 600 700 800 900 1000 1100
Tra
nsm
ittan
ce [%
]
0
20
40
60
80
100
600-nm high-pass filter Series of high-pass filters with different-wavelength cut-offs
Use a high-pass filter
Red kink with CdS occurs when no photons are above 2.4 eV
The Red Kink in CdS/CIS
May 27, 2004 Photovoltaics Laboratory
Position [m]0.2 0.4 0.6
Ene
rgy
[eV
]
-0.5
0.0
0.5
1.0
1.5
CdS
with "blue"photons
without "blue"photons
Ec
CISZnO
Voltage [V]
-0.5 0.0 0.5 1.0C
urre
nt D
ensi
ty [m
A/c
m2]
-40
-20
0
20
40
Well-behavedCollection
ImpededCollection
Red & DarkConverge
dark
white
red
NREL CdS/CIS J-VConduction Band at V = 0
(light/dark difference exaggerated)
CdS barrier impedes electron transport; blue photons may generate sufficient electron-hole pairs in CdS to alter trap occupation and mitigate the effect.
Can be a serious problem if no blue photons present.
Usually not a problem with white light, but small “kink” sometimes seen.
Compensated CdS
Kink Depends on CdS Thickness (Simulation)
May 27, 2004 Photovoltaics Laboratory
Position [m]0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Ene
rgy
[eV
]
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
variations in CdS thickness
w. light
r. light, dark
Voltage [V]-0.6 -0.3 0.0 0.3 0.6 0.9
Cur
rent
Den
sity
[mA
/cm
2 ]
-40
-20
0
20
40
NA = 1017 [cm-3]
n = 6 x 1016 [cm-3]
redwhite
10075 50 25
CdS parameters
noCdS
thickness (nm):
Weaker kink with thinner CdS. (Also seen experimentally)
More generally: strength of kink varies with the carrier densities of CdS and TCO, and with the CdS defect density.
Conduction Band. Impact of barrier increases with CdS thickness.
Kink Disappears at Higher Eg (NREL Cells)
May 27, 2004 Photovoltaics Laboratory
22.9% GaEg = 1.14 eV
Voltage [V]
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Cur
rent
Den
sity
[mA
/cm
2 ]
-40
-20
0
20
40
darkredwhite
17.8% GaEg = 1.12 eV
Voltage [V]-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Cur
rent
Den
sity
[mA
/cm
2 ]
-40
-20
0
20
40
darkredwhite
time
no GaEg = 1eV
Voltage [V]
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Cur
rent
Den
sity
[mA
/cm
2]
-40
-20
0
20
40
darkredwhite
time
35.8% GaEg = 1.19 eV
Voltage [V]
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Cur
rent
Den
sity
[mA
/cm
2 ]
-40
-20
0
20
40
darkredwhite
62.4% GaEg = 1.45 eV
Voltage [V]
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
Cur
rent
Den
sity
[mA
/cm
2 ]
-40
-20
0
20
40
darkredwhite
Eg = 1.11 eV
Eg = 1.40 eVEg = 1.22 eV
Conduction-band offset decreases; changes from spike to cliff
Choosing the Window: Outline
May 27, 2004 Photovoltaics Laboratory
(1) Photon considerations: Window absorption.
(2) Conduction-band offset problem I: Big spikes (and their “red-kink” precursor) that limit current.
(3) Conduction-band offset problem II: The cliff problem that limits voltage.
(4) How much slack does one get in choosing the window?
(5) Conclusions.
Effect of Interfacial Recombination on VOC
May 27, 2004 Photovoltaics Laboratory
CdS Window
Vary EC by expanding Eg (simulated)
See Poster P3.9 (Gloeckler)
Lack of spike allows significant interfacial recombination
Effect of EC at constant Eg discussed by several groups
CdS or Alternative Windows?
May 27, 2004 Photovoltaics Laboratory
Vary the window and hence the offset
But, kink can return!
May 27, 2004 Photovoltaics Laboratory
In(OH,S)C1591-23Bonly n-ZnO
Voltage [V]
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
Cu
rre
nt
De
nsi
ty [
mA
/cm
2]
-40
-20
0
20
40
dark0.01 sun0.1 sun1 sun
Voltage [V]
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
Cu
rre
nt
De
nsi
ty [
mA
/cm
2 ]-40
-20
0
20
40
darkwhite lightred light
CSU Photovoltaic Lab
whi
te li
ght
CdSULS345-8-3Standard ZnO
Voltage [V]
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
Cu
rren
t D
ensi
ty [
mA
/cm
2 ]
-40
-20
0
20
40
dark0.01 sun0.1 sun1 sun
Voltage [V]
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
Cu
rren
t D
ensi
ty [
mA
/cm
2 ]
-40
-20
0
20
40
darkwhite lightred light
whi
te li
ght
CSU Photovoltaic Lab
CdS Window (IPE) InS(O,OH) Window (IPE)
CIGS Absorber (Eg = 1.15 eV)
“Red” Cut-off 2.4 eV “Red” Cut-off 2.8 eV
See Poster P3.8 (Pudov)
Note: ZnS(O,OH) from AGU yields similar curves
Good Superposition
Choosing the Window: Outline
May 27, 2004 Photovoltaics Laboratory
(1) Photon considerations: Window absorption.
(2) Conduction-band offset problem I: Big spikes (and their “red-kink” precursor) that limit current.
(3) Conduction-band offset problem II: The cliff problem that limits voltage.
(4) How much slack does one get in choosing the window?
(5) Conclusions.
Choosing the Window Material
May 27, 2004 Photovoltaics Laboratory
Big Spike
Small Spike or Cliff
Offset Values from Zhang,Wei, and Zunger, JAP 83, 3192 (1998)
Match absorber and window materials so EC is in optimal range
Conclusions
May 27, 2004 Photovoltaics Laboratory
(1) From a device-physics perspective, the optimal choice of window material for chalcogenide solar cells varies with the band gap of the absorber.
(2) A general problem for CdS windows is low blue response.
(3) A problem for CdS on low-gap absorbers (CIS) is a big spike that impedes current. Mitigated by thin, high-carrier-density, or photoconductive CdS.
(4) A problem for CdS on high-gap absorbers (CdTe or CGS) is the lack of a barrier to inhibit interfacial recombination.
(5) At room temperature, a single window material is optimal over an approximate 300-meV range of absorber band gap.