Characterization of organic bulk heterojunction and silicon nanowire solar cells
Katherine Song
Department of Electrical Engineering, Princeton University, Princeton, NJ 08544
Sponsor/host: Bob Street
Electronic Materials and Devices Laboratory, Palo Alto Research Center, Palo Alto, CA 94304
Organic bulk heterojunction (BHJ) solar cells
Effect of series resistance on photocurrent analysis
Silicon nanowire solar cells
•Organic solar cells attractive because of their low cost, low weight, and flexibility.
•BHJ solar cells: a polymer-fullerene blend spontaneously phase separates to form a
vertically-structured cell with nanoscale heterojunctions.
•Studied cells based on 2 different blends:
•poly(3-hexylthiophene) (P3HT) /
[6,6]-phenyl C61-butyric acid methyl ester (PCBM)
•Power conversion efficiency (PCE) ~ 4-5%.
•poly[N-9’-hepta-decanyl-2,7-carbazole-alt-5,5(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT) /
[6,6]-phenyl C70-butyric acid methyl ester (PC70BM)
•PCE ~ 6%.
•Cells fabricated by S. Cowan at the University of Santa Barbara.
•Typical structure shown in figure on right [1].
• Analysis of generation via photocurrent vs. voltage measurements key to understanding
recombination in organic solar cells.
• Experimentally measured photocurrent not necessarily reflective of internal photocurrent.
• PEXP(V) (normalized experimentally measured photocurrent) at a fixed illumination
evaluated by measuring voltage dropped across a load resistor of known value with
a lock-in amplifier:
•Concept: enhance light scattering and absorption by increasing surface area.
•Advantage: easily incorporated into existing a-Si solar cell fabrication process.
•Structure [2]:
55 nm ITO
15 a-Si P+
100 nm a-Si Intrinsic
1
70 nm a-Si N+
PCDTBT/PC70BM
Un-doped Si NW
0.8
PEXP(V)
0.6
0.4
0.2
0
100
5
0
•Photocurrent vs. voltage (DC measurement):
0
0.4
band
offset
0.6
1
1.2
1.4
Voltage (V)
-0.2
• Proposed model to correct for series resistance effects:
• Extract values for dark saturation current J0, ideality factor n, and series resistance
RS from measured dark current JD(V).
e(V  J D (V )RS ) 
J D (V ) J O exp



nkT
interface
band gap
acceptor
(fullerene)
0.8
Photocurrent (mA)
• Photoconductivity spectrum useful for extracting interface band gap and band offset
(difficult to determine from optical absorption measurements).
• Band offset provides energy to split an exciton into a separate electron and hole.
donor
(polymer)
10k
-0.2
Spectral response measurements
bulk
band gap
1k
-0.4
-0.6
-0.8
• Since experimentally-measured photocurrent eGPEXP(V) = JD(V) – JL(V),
increasing light intensity

Data:
• Changes in slope of spectrum indicate location of absorption bands.
• Bulk optical band gap ~ 1.9 eV for both types of cells.
• Interface band gap ~ 1.2 eV for P3HT/PCBM cells; ~ 1.4 eV for PCDTBT/PC70BM cells.
e(V  J D (V )RS ) 
eGPEXP(V ) J O exp
 J L (V )


nkT
-5
-1
0
1
Voltage (V)
0
• Applying the model to experimental data:

-0.05
1
10
JD (mA/cm2)
0.6
P3HT/PCBM
0.4
P(V)
1000
-0.1
1
0.1
0.01
0.001
0.6
0.2
0.8
1
Voltage (V)
-0.15
-0.2
increasing light intensity
band offset
10
0
10
8
1
0.1
0.01
-0.4
4
P3HT/PCBM
2
2.1
0.0001
1
1.5
2
2.5
0.1
0.2
0.3
0.4
0.5
Voltage (V)
Energy (eV)
0.5
0
internal
0.4
1.9
-0.3
-0.6
0
1.8
experimental
J0 = 2.025x10-12 A/cm2
RS = 18.4 Ω·cm2
n = 1.66
6
2
0.001
-0.25
-0.2
PCDTBT/PC70BM
a (mm-1)
Absorption coefficient (cm-1)
-2
•Close-up of power-producing quadrant:
e(V  J L (V )RS ) 
eGPINT (V  J L (V )RS ) J O exp
 J L (V )



nkT
PCDTBT/PC70BM
100
-3
•Shape of curve suggests significant series resistance.
eGPINT (VINT ) JO exp eVINT /nkT J L (V )
0.8
10000
-4
• “Corrected” internal photocurrent depends on internal voltage VINT = V – JL(V)RS

instead of external
voltage V.
1000000
100000
-1.2
Photocurrent (mA)
Experiment:
• Solar cell illuminated with monochromatic light source chopped at 230 Hz.
• Voltage dropped across a 9.1kΩ load resistor measured with lock-in amplifier.
• 100kΩ resistor used instead at low energies to amplify weak signal.
• 715, 850, and 1000 nm cutoff filters used to suppress scattered light at low energies.
• Spectrum normalized to incident power, measured by calibrated Si and Ge photodiodes.
• Optical absorption coefficient, α(ħω) obtained from photocurrent (IPC) spectrum:
IPC(ħω)=I0[1-exp(-α(ħω)d)]
I0: photoconductivity at complete absorption; determined by fitting spectrum to optical
absorption measurements (inset in graph below)
d: thickness of cell (~100nm)
-1
3
Energy (eV)
References:
1. Sung Heum Park, Anshuman Roy, Serge Beaupré, Shinuk Cho, Nelson Coates, Ji Sun Moon, Daniel Moses, Mario Leclerc, Kwanghee Lee & Alan J. Heeger, Nature Photonics 3, 297 - 302 (2009).
2. Figure and image from Sourobh Raychaudhuri.
0.6
0.8
Voltage (V)
1
1.2
0.6
0.7
0.8
0.9
1