Investigation of transport properties of
ZnO/PbS heterojunction solar cells
 Y. Cheng, M. C. D. Whitaker, R. Makkia, S. Cocklin, V. R. Whiteside, L. A.
Bumm, and I. R. Sellers, E. Adcock-Smith, K. P. Roberts, P. Harikumar
Yang Cheng 4/21/2016

Homer L. Dodge Department of Physics & Astronomy, University of Oklahoma, 440 W Brooks Street,
Norman, OK 73019

Department of Chemistry and Biochemistry, University of Tulsa, 800 South Tucker Drive Tulsa, OK 74104

Department of Physics and Engineering Physics, University of Tulsa, 600 S. College Ave. Tulsa, OK 74104
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
Introduction
• CQDs could potentially offer efficient solar cells using low cost, solution based
techniques.
• Confinement effects existed in CQDs lead to discrete energy levels which could
widen the band gap, allowing the optical and electrical properties to be finely
tuned by varying the QD size.
• Through Multi-exciton generation (MEG) [1] [2] process and multijunction
structure [3] CQDs solar cell has the potential to go beyond Shockley-Queisser
limit.
• To further increase the efficiencies, it will be beneficial to investigate
transport properties.
1. Sambur, et al. Science 330.6000 (2010): 63‐66.
2. Semonin, et al. Science 334.6062 (2011): 1530‐1533.
3. Choi, Joshua J., et al. Advanced Materials 23.28 (2011): 3144‐3148.
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
Photo-generated carriers extraction
• Device structure
Glass/ITO/ZnO/PbS/Au
EQE (%)
Glass
EQE (%)
ITO
ZnO NP’s
100
10-1
10
10-2
Absorbance
of PbS solution
PbS QD’s
1
400
Au
500
600
700
800
900
1000
Absorbance (arb.u)
100
10-3
1100
Wavelength (nm)
Absorbance and EQE measurements
clearly show photo-generated carriers
from PbS QDs.
,
Au
ITO ZnO
PbS
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
Surface states and Interfaces
• CQDs
• ITO/ZnO
,
•
ZnO/CQDs
•
CQD/contact
Au
ITO
ZnO
PbS
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
0
-5
Current Density (mA/cm2)
Current Density (mA/cm2)
Device performance
0.00
-0.01
-0.02
-0.03
-0.04
-0.05
-0.4
-0.2
0.0
Voltage（ V）
-10
-0.4
-0.2
0.0
0.2
0.4
Voltage（ V）
Leakage current, shunt paths which also result in low fill factor
Low Jsc: Series Resistance
Shunt path
Cross over:
Other barriers create another diode in series shifts the voltag
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
Trade offs
• Thicker PbS layer will increase the total absorption of photons, but
the extraction of the photo-generated carriers is limited by the
diffusion length.
• Doping the ZnO will change the
band alignment of ZnO/PbS, which
in return influence the electron transport ,
from the CQD layer to ZnO.
• High doping concentration of ZnO
will increase depletion width in
CQD layer. However, doping will also
introduce defects into ZnO layer which
decreases the diffusion length
Au
ITO
ZnO
PbS
Mott Schottky and Impedance Analysis
Mott‐Schottky Analysis
Impedance Spectroscopy
https://en.wikipedia.org/wiki/Electrical_impedance
http://www.stallinga.org/ElectricalCharacterization/2terminal/index.html
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
Mott Schottky and Impedance Analysis
Capacitance (F/cm2)
0.16
80Hz
100Hz
250Hz
500Hz
1000Hz
0.15
Bulk
Capacitance
0.14
Depletion
Capacitance
0.13
0.12
0.11
-0.4
-0.2
0.0
0.2
0.4
Voltage (V)
•
Both Depletion capacitance and bulk capacitance are observed in the C-V
plot.
•
However, increasing the sweeping frequency will lead to the reduction of the
capacitance.
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
Mott Schottky and Impedance Analysis
1/C2 (1/(F/cm2))
80
70
80Hz
100Hz
250Hz
500Hz
1000Hz
60
50
•
40
30
-0.4
-0.2
0.0
0.2
0.4
80 Hz
0.896 V
100 Hz
0.856 V
250 Hz
0.866 V
500 Hz
0.993 V
1000 Hz
1.164 V
The built in voltage
extracted from
plot of
the1/C
different frequencies
Voltage (V)
The built in voltage is way too large than expected based on the reported Fermi
level difference [1].
This could be existence of other capacitance from the interfaces or traps which
shifts the depletion capacitance.
1.
Pattantyus‐Abraham, Andras G., et al. ACS nano 4.6 (2010): 3374‐3380
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
Mott Schottky and Impedance Analysis
Current Density (mA/cm2)
Forward Bias
0.5
,
PbS
Au
0.0
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
Reverse Bias
Voltage（ V）
,
PbS
Kramer, I. J., & Sargent, E. H. Chemical reviews, 114(1), 863‐882.
Au
Mott Schottky and Impedance Analysis
8.0x104
Z''(  /cm2)
6.0x104
0V_Bias
4.0x104
-0.5V_Bias
2.0x104
0.25V_Bias
0.0
0.0
3.0x104
6.0x104
9.0x104
1.2x105
Z'(/cm2)
Increasing the forward Bias will shrink the Nyquist cole-cole plot.
However, when reverse bias the sample with higher voltage will also reduce the
impedance at lower frequency.
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
Mott Schottky and Impedance Analysis
‐0.5V
0V
0.25V
0.5V
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
Mott Schottky and Impedance Analysis
C2
C1
R2
R1
Rs
Diode2
Diode1
Rs
C1
R1
C2
R2
‐0.5V
362.13
9.2972
70619
67.924
367.47
0V
850.17
10.675
121220
90.005
534.18
0.25V
333.37
12.137
25657
89.671
299.29
0.5V
376.98
12.805
7176.9
63.26
328.84
Photovoltaics Materials & Device Group, University of Oklahoma: http://www.nhn.ou.edu/~sellers/group/index.html
Future goals
•
Extract more information (eg. Carrier lifetime) from the fitting to
investigate the transport mechanism.
•
Impedance spectroscopy under illumination will also be studied to
probe the nature of the second diode behavior.
•
Influence of different optimization methods on the C-V and
impedance will be investigated comprehensively.
Acknowledgement
This work is supported by NASA Oklahoma EPSCoR grant: NNX15AM75A and
NNX13AN01A