Characterization of organic bulk heterojunction and silicon nanowire solar cellsKatherine Song
Department of Electrical Engineering, Princeton University, Princeton, NJ 08544
Sponsor/host: Bob StreetElectronic Materials and Devices Laboratory, Palo Alto Research Center, Palo Alto, CA 94304
Effect of series resistance on photocurrent analysis
•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:
•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).
•Since experimentally-measured photocurrent eGPEXP(V) = JD(V) – JL(V),
• “Corrected” internal photocurrent depends on internal voltage VINT = V – JL(V)RS instead of external voltage V.
•Applying the model to experimental data:
Organic bulk heterojunction (BHJ) 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].
Silicon nanowire solar cells
•Concept: enhance light scattering and absorption by increasing surface area.•Advantage: easily incorporated into existing a-Si solar cell fabrication process.•Structure [2]:
•Photocurrent vs. voltage (DC measurement):
•Shape of curve suggests significant series resistance.
•Close-up of power-producing quadrant:
Spectral response measurements
• 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.
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)
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.
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.
0.5 1 1.5 2 2.5 30.0001
0.001
0.01
0.1
1
10
100
1000
10000
100000
1000000
Energy (eV)
Ab
so
rpti
on
co
eff
icie
nt
(cm
-1)
P3HT/PCBM
PCDTBT/PC70BM
1.8 1.85 1.9 1.95 2 2.05 2.10
20000
40000
60000
80000
100000
Energy (eV)
(-1)
amm
PCDTBT/PC70BM
P3HT/PCBM
0.4 0.6 0.8 1 1.2-0.6
-0.4
-0.2
1.11022302462516E-16
0.2
0.4
0.6
0.8
1
Voltage (V)
P(V
)
J0 = 2.025x10-12 A/cm2
RS = 18.4 Ω·cm2
n = 1.66
experimental
internal
0.6 0.8 10.001
0.01
0.1
1
10
Voltage (V)
JD
(m
A/c
m2
)
0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4-0.2
0
0.2
0.4
0.6
0.8
1
Voltage (V)
PE
XP
(V)
100 1k 5
PCDTBT/PC70BM
0
10k
55 nm ITO
15 a-Si P+
100 nm a-Si Intrinsic
70 nm a-Si N+
Un-doped Si NW
€
eGPINT (VINT )=JO exp eVINT /nkT( )− JL (V )
€
eGPINT(V − JL(V )RS )= JO expe(V − JL(V )RS )
nkT
⎛
⎝ ⎜
⎞
⎠ ⎟− JL(V )
€
JD(V )= JO expe(V − JD(V )RS )
nkT
⎛
⎝ ⎜
⎞
⎠ ⎟
€
eGPEXP(V )= JO expe(V − JD(V )RS )
nkT
⎛
⎝ ⎜
⎞
⎠ ⎟− JL(V )
band offset
-5 -4 -3 -2 -1 0 1-0.0012
-0.001
-0.0008
-0.0006
-0.0004
-0.0002
2.16840434497101E-19
Voltage (V)
Ph
oto
curr
ent (
mA
)
increasing light intensity
donor (polymer) acceptor
(fullerene)
band offset
interface band gap
bulkband gap
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-0.0003
-0.00025
-0.0002
-0.00015
-0.0001
-0.00005
5.42101086242752E-20
Voltage (V)
Ph
oto
curr
ent (
mA
)
increasing light intensity