Solution-Processed Small-Molecule Solar Cells with 7% Efficiency Alan J. Heeger Professor of Physics & Materials, UC Santa Barbara
Aug 31, 2014
Solution-Processed Small-Molecule Solar Cells with 7% Efficiency
Alan J. HeegerProfessor of Physics & Materials, UC Santa Barbara
Acknowledgements
Gui BazanGreg Welch
Yanming Sun and Weilin Leong (device fabrication and analysis)
Chris Takacs (High resolution phase contrast TEM)
Loren Kaake (Ultrafast spectroscopy)
This is a CEEM project with partial support from AFOSR
Why Organic Photovoltaics (OPV)?
Thin, flexible, light weight and rugged products
High throughput, roll-to-roll manufacturing
Potential for low cost and low carbon footprint
Unique semi-transparent modules with tunable colors
Remarkably interesting and challenging science --- nanoscale photovoltaics!
Roll-to-Roll Manufacturing
Bulk Heterojunction Solar Cells Fabricated from Small Molecules (Not Polymers)
New Direction
With polymers --- batch-to-batch variations in solubility, molecular weight, polydisperity and purity provide materials with considerably different processing properties.
Difficult, therefore, to evaluate the fundamental structure-performance relationships.
New Direction:Solution-Processed Small-Molecule Solar Cells
with 6.7% Efficiency
eVITO/MoOx/TTPSiBT:PC70BM/Al
-5.4 eV
-4.7 eV
-3.6 eV
-5.2 eV-4.3 eV
-6.1 eV
-4.3 eV
-3.0
-4.0
-5.0
-6.0
-7.0
DTS(PTTh2)2 PC70BM
a
b c
OMe
O
NS
N
NS N
SS
Si
SS S
N SN
300 400 500 600 700 800 900 10000.0
0.2
0.4
0.6
0.8
1.0
1.2
Nor
mal
ized
Abs
orpt
ion
Wavelength (nm)
Solution Film
Design and synthesis:G. Bazan and G. Welch
Dynamics and Time Scales
Mobile carrier sweep out by the internal
voltage (built-in electric field)
Ground state
Interfacial traps and
Interfacial excitons
Excited state and
charge transfer
hν
Recombination
t < 100 fs
t < 100 fs
Energy outns - s
HOMO
LUMO
HOMO
LUMO
Electron Acceptor
Electron Donor
The Initial Discovery (1992):Ultrafast Photo-induced electron transfer
*
*n
hn
e-
+
Ultrafast charge separation with quantum efficiency approaching Unity !
50 fs
Brabec et al Chem Phys Lett 2001
-60 -50 -40 -30 -20 -10 0 1010-1110-1010-910-810-710-610-510-4
Ids1/
2 mA
1/2 )
Ids
Vgs (V)
14.0
0.02.04.06.08.010.0
12.0
0 -10 -20 -30 -40 -50 -60
I ds (
A)
Vds (V)
-20 V-30 V
-40 V
-50 V
-60 V
0.0
-20.0
-40.0
-60.0
-80.0
-100.0 a b
OFET data obtained from pristine DTS(PTTh2)2. a) Output curves b) transfer curves (Vds=-60 V).
Hole mobility (saturation regime) 0.12 cm2/Vs
on/off ratio of 107.
-0.2 0.0 0.2 0.4 0.6 0.8 1.0-14-12-10-8-6-4-202
Cur
rent
den
sity
(mA
/cm
2 )
Voltage (V)
80:20 70:30 60:40 50:50
Relatively small amount of fullerene is required --- Optimum Donor-to-fullerene ratio is 7:3
Device science:Yanming Sun and Wei Lin Leong
Summary of device parameters of SM-BHJ solar cells with active layers cast from solutions with different blend ratios .
DTS(PTTh2)2/PC70BM
(weight ratios)a
Voc (V) Jsc (mA/cm2) FF (%) PCE (%)
80:2070:3060:4050:50
0.800.800.830.84
11.412.57.24.4
39.445.235.428.6
3.604.522.121.06
aActive layer thin films cast from 4% w/v CB solutions with varying weight ratio of the DTS(PTTh2)2:PC70BM components
7:3 G24:PC70BM (η=6.7%)
Top View
SideView
100 nm
7:3 G24:PC70BM (η=6.7%)
James Rogers, Ed Kramer and Gui Bazan
300 400 500 600 700 800 9000
10
20
30
40
50
60
IPC
E (%
)
wavelength (nm)
80:20 70:30 60:40 50:50
IPCE spectra of SM-BHJ solar cells based on DTS(PTTh2)2/PC70BM active layer with different blend ratios.
-0.2 0.0 0.2 0.4 0.6 0.8-14-12-10-8-6-4-202
Cur
rent
den
sity
(m
A/c
m2 )
Voltage (V)300 400 500 600 700 800 9000
10
20
30
40
50
60
70
IPC
E (%
)Wavelength (nm)
0% 0.2% 0.6% 1.0%
Performance of solar cells with a DTS(PTTh2)2:PC70BM active layer as a function of DIO content (as processing additive).
(a) Current-voltage curves (b) IPCE spectra.
Processing with 0.25% DIO (as additive) optimum: PCE = 6.7% FF = 0.59 Voc= 0.78V
Jsc= 14.4 mA/cm2
Each of these values sets a new record for small molecule OPV!
-60 -40 -20 0 20 40 6010-10
10-9
10-8
10-7
10-6
10-5
10-10
10-9
10-8
10-7
10-6
10-5
n-type mode
with 0.25% DIO w/o DIO
I ds (V
)
Vgs (V)
p-type mode
Transfer characteristics of bipolar field-effect transistors based on DTS(PTTh2)2/PC70BM blended films processed with and without DIO additives.
Hole mobility 6×10-3 cm2/Vs Electron mobility 2×10-3 cm2/Vs
Nearly balanced.
1 2 3 4 5 640
45
50
55
60
FF (%
)
Process run1 2 3 4 5 6
4.0
4.5
5.0
5.5
6.0
6.5
7.0
PCE
(%)
Process run
1 2 3 4 5 610
11
12
13
14
15
J sc (m
A/c
m2 )
Process run1 2 3 4 5 6
0.70
0.72
0.74
0.76
0.78
0.80
V oc (V
)
Process run
Performance distribution of SM-BHJ solar cells based on DTS(PTTh2)2/PC70BM active layers with 0.25% DIO (v/v).
Independent 6 batches of device fabrication were made and for each batch at least 2 cells were tested (totally 14 cells).
The average PCE is 6.2%.
DTS(PTTh2)2 BHJ on MoOx (no DIO )(163 nm per side)
(50 nm scale bar)
False Color Raw Image
TEM:Chris Takacs
In-plane stacking of DTS(PTTh2)2 at 0.31 Å-1. (approx. 2 nm d-spacing)
Regions of solid color indicate the spatial extent and direction of the crystal lattice planes.
Defocused --- Phase Contrast TEM
DZ = defocus lengthu = spatial frequency
= e-wavelength (2.5 pm)
)sin(2)()()( 2uZuEuAuCTF D
Phase Contrast TEM E-beam undergoes many small
angle deflections proportional to the density of carbon nuclei
PCBM has higher density of carbon nuclei
E-beam velocity is slower in PCBM than in Polymer
Result: Phase Contrast
Morphology of the BHJ Materials
Defocused (Phase Contrast) TEM imaging of the lattice planes
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
0.2
0.4
0.6
0.8
1
1.2x 10
-9
Pow
er S
pect
rum
(Arb
.)
q (Å-1)
Azimuthally integrated power spectrum from BHJ raw images:Blue is from no DIO device (pristine), Red is 0.25% DIO device (optimal) device, Green is 0.60% DIO device. The dashed line represents the theoretical |CTF|2 for a defocus of -1000 nm.
DTS(PTTh2)2 BHJ on MoOx (no DIO )(163 nm per side)
(50 nm scale bar)
False Color Raw Image
TEM:Chris Takacs
In-plane stacking of DTS(PTTh2)2 at 0.31 Å-1. (approx. 2 nm d-spacing)
Regions of solid color indicate the spatial extent and direction of the crystal lattice fringes.
DTS(PTTh2)2 BHJ on MoOx (0.25% DIO)(163 nm per side)
False Color Raw Image
TEM:Chris Takacs
In-plane stacking of DTS(PTTh2)2 at 0.31 Å-1. (approx. 2 nm d-spacing)
Regions of solid color indicate the spatial extent and direction of the crystal lattice fringes.
50 nm scale bar
Caution: Generation of impurity during synthesis
- Remove impurity via extraction with hexanes and column chromatography- Can avoid methyl transfer by lowering reaction temperature
Typically less than 1% impurity
Device Voc (V) Jsc (mA/cm2) FF (%) PCE (%)
G24:PC70BM 0.78 14.4 59.3 6.70
G24 (impurity):PC70BM 0.74 10.2 39.6 3.00
300 400 500 600 700 8000
10203040506070
IPCE
(%)
Wavelength (nm)
G24 G24 with impurity
0.0 0.2 0.4 0.6 0.8-16-14-12-10-8-6-4-202
G24 G24 with impurity
I(mA/
cm̂2)
Voltage(v)
PC84BM Traps in PCDTBT:PC84BM • Introduce first-order (monomolecular) recombination and decrease VOC
• Reduce mobility
• Reduce jSC and prevent fast sweep out
Result: All parameters adversely affected:VOC jSC FF
-0.5 0.0 0.5 1.0-8
-6
-4
-2
0
2
m84:m60 = 1:100
m84:m60 = 0:1
Cur
rent
den
sity
(mA
/cm
2 )
Voltage (V)
m84:m60 = 1:1,000
a.
0.0 10-4 10-3 10-2 10-1 1000.5
0.6
0.7
0.8
0.9
m84
:m60
= 0:1m
84:m
60 = 1:10,000
m84:m60 = 1:1,000m84:m60 = 1:100m84:m60 = 1:0
(estimated)
Ope
n ci
rcui
t vol
tage
(V)
PC84
BM Concentration
d.
SS
N
C8H17 C8H17
n
N NSa OMe
O
Dynamics and Time Scales
Mobile carrier sweep out by the internal
voltage (built-in electric field)
Ground state
Interfacial traps and
Interfacial excitons
Excited state and
charge transfer
hν
Recombination
t < 100 fs
t < 100 fs
Energy outns - s
80
60
40
20
0
CT (
fs)
0.60.40.20.0DIO (% v/v)
-1.0
-0.5
0.0
0.5
1.0
A
(au)
1.51.00.50.0-0.5time (ps)
DIO (% v/v) 0.25 0.16 0.08 0.00
( a ) ( b )
Ultrafast charge separation is enhanced by DiO additive
Continuum of interface states can enable the ultrafast charge transfer
e- center of mass
donor acceptore/h pair
exciton
CT states
CT continuum
CT exciton
CS stateE
Looking for a model General properties of an attractive
potential in a continuum Bound states Scattering states
Loss of carriers at early times due to bimolecular recombination at very high carrier densities
Still lower pump power ------ approx. 2.7x1017 electrons per cm
COMPETITION between SWEEP–OUT and RECOMBINATION
Mobile carrier sweep out by the internal
voltage (built-in electric field)
Ground state
Interfacial traps and
Interfacial excitons
RecombinationTime scale ???
Energy outns - s
0 5 10 15 20
10-3
10-2
10-1
100
Time (s)
J / V
int (m
-1
cm
-2)
0.7V0.6V0.5V0.4V0.2V0.0V-0.5V-1.0V
PCDTBT:PC70BMT = 300K
OMe
O
PCDTBT
PC70BM
Vint = VBI – V
Photoconductance =
Current / Internal voltage
Transient photocurrent in operating solar cell:Competition between Sweep-out and Recombination
When sweep-out is faster than recombination --- high efficiency.
Device Jsc (mA cm-2) Voc (V) FF (%) PCE (%)
70:30 13.39 0.78 62.7 6.50 80:20 9.81 0.78 54.1 4.12 85:15 5.51 0.77 39.6 1.68 90:10 3.55 0.76 37.2 1.0 95:05 0.20 0.73 26.1 0.04 100:0 / 0.50 26.3 /
-0.2 0.0 0.2 0.4 0.6 0.8-14-12-10-8-6-4-202
Cur
rent
den
sity
(m
A/c
m2 )
Voltage (V)
70:30 80:20 85:15 90:10 95:05
300 400 500 600 700 800
10
20
30
40
50
60
70
IPC
E (%
)
Wavelength (nm)
70:30 80:20 85:15 90:10 95:05
Solution-Processed Small-Molecule Solar Cells with 6.7% Efficiency
A Promising New Direction !
Nature Materials (submitted)
G. Bazan, G. Welch, Y. Sun, W-L Leong, C.J. Takacs and AJH
“Plastic” Solar Cells
Effic
ienc
y (%
)
20001995
NREL
NREL
NREL
NREL
United SolarUnited Solar12
8
4
0
16
20
2005UCSB
Cambridge
NREL
U. Linz SiemensKonarka
Konarka
Siemens
2010
12
8
4
0
16
20
OPV single junction
Year
Konarka
PlextronicsUCSB
Solarmer
Efficiency
Konarka
Time evolution of efficiency of “plastic”
solar cells9.3 % Japan;9.2% SCUT (inverted
structure)
“Plastic” Solar Cells
Low $ cost manufacturing
Low energy cost manufacturing
Low carbon “footprint” manufacturing
TechnologyEnergy for production
(MJ.Wp-1)
CO2 footprint
(gr.CO2-eq.Wp-1)
Energy payback
time(years)
mc-Si 24.9 1293 1.95CdTe 9.5 542 0.75CIS 34.6 2231 2.71
Flex OPV 2.4 132 0.19A. L. Roes et al, Progress in Photovoltaics 17, 372 (2009)
Semi-transparent BIPV based on OPV
BIPV for new constructions BAPV for retro-fit existing buildings
Power Plastic: Blending Functionality with Aesthetics
Questions ???