external quantum efficiency
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External Quantum Efficiency (EQE)
Measurement
Prasad Gandhi
Advisor: Dr. Brendan O’Connor
9/23/2011 1
Solar cell technologies
Silicon
GaAs
CuInGaSeCdTe
α-Si
+ organic semiconductors9/23/2011 2
Can organic solar cells make an impact?
Organic solar cell potential:-Low materials cost-Mechanical flexibility-Compatible with low-cost processing-Simpler installation
* Source: DOE
Utility Scale System cost ($/W)
(Business as usual)
DOE Goal:$1/W ~ 0.05-0.06 $/kWh(competitive with wholesale rates of electricity)
Efficiency ~ 10%Requires module: $50/m2
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National Renewable Energy Laboratory, 2011
Best Si
BestOrganic
25.0%
8.3%
Solar cell efficiency trends
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The generation of electricity from light using organic molecules (made primarily from Carbon)
Organic solar cells?
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Organic solar cells?
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Organic solar cell processes:
1. Photon absorbed, (A~100%)
2. Excited states diffuse, (ED~10%)
3. Charged formed at Donor
Acceptor interface, (CT ~100%)
4. Charge diffuses out, (cc~100%)
x
glass
electrode
electrode
~100 nm
External Quantum Efficiency
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Bulk heterojunction organic solar cell
+
-
Transparentsubstrate
Transparent Electrode (ITO)/ PEDOT:PSS
Active Layer (BHJ)P3HT:PCBM
Electrode (LiF/Al)
h
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Key parameters and equations
p jsc Voc FF
Current - voltage curve Quantum efficiency
Solar cell characteristics
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• Provides information on current a solar cell will produce when illuminated by a particular wavelength of light.
• EQE integrated over the solar spectrum predicts the photocurrent that a solar cell will produce when exposed to the solar spectrum.
Why measure EQE?
Quantum efficiency
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EQE Testing Equipment Setup
EQUIPMENT-•Solar Simulator & Power Supply•Monochromator•Photodetector•Organic Solar Cell (OSC) •Optical Chopper and controller -•Lock-In Amplifier•Computer
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External Quantum Efficiency Setup
MonochromatorLamp
Photo Diode
Controller
Optical Chopper
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Fig a) Front side Fig b) Backside
Getting the number of photons
Initial calibration of light at given wavelength measured with a calibrated photo-diode, this gives us the number of photons
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Performance relative to reference commercial “solar cell”
EQEP
E
hc
e
sNhc
sNe
W
AR
)/()/)(/(
)/(
Fig. Responsivity Curve of Hamamatsu photo diode
9/23/2011 14selectron ofNumber photons ofNumber charge, elementary e
light of Speed constant,Planck ,Wavelength ty,Responsivi
EP NN
chR
Performance relative to reference commercial “solar cell”
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Responsivity of HamamatsuPhoto-detector
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Fig b) Responsivity curve by HamamatsuFig a) Responsivity curve experimentally obtained
What can we learn, what do we need to do to improve our EQE?
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• Optimization of layer thickness(LT) of solar cells. Increase in LT increases absorption of photons but can reduce the mobility of charge carriers.
• Controlling the active layer growth rate results in an increased hole mobility and balanced charge transport
• Reduce degradation rate in performance of solar cell by amount of PCBM in layer