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

9/23/2011 3

National Renewable Energy Laboratory, 2011

Best Si

BestOrganic

25.0%

8.3%

Solar cell efficiency trends

9/23/2011 4

The generation of electricity from light using organic molecules (made primarily from Carbon)

Organic solar cells?

9/23/2011 5

Organic solar cells?

9/23/2011 6

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

9/23/2011 7

Bulk heterojunction organic solar cell

+

-

Transparentsubstrate

Transparent Electrode (ITO)/ PEDOT:PSS

Active Layer (BHJ)P3HT:PCBM

Electrode (LiF/Al)

h

9/23/2011 8

Key parameters and equations

p jsc Voc FF

Current - voltage curve Quantum efficiency

Solar cell characteristics

9/23/2011 9

• 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

9/23/2011 10

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?

9/23/2011 17

• 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