Characterization of a Benzothiadiazole Derivative
for Organic Electronic Applications
Ross Kerner, Louis Scudiero
Materials and Mechanical Engineering REU; Chemistry Department
Optical and Physical Properties
0.0
a)
400
% Mass
80
o
Temperature ( C)
Can the material be solution processed? Or thermal
vacuum deposition?
Film roughness affects cathode connection.
High mobility increases electrical performance.
Phases and morphology can greatly impact charge
Solid State Phases
mobility and absorbance coefficient.
15
0.4
b)
2
1
0
Film on ITO
10
0
N1s
Film on ITO
Binding Energy (eV)
S2s S2p
5
1.5
1.0
0.5
0.0
Binding Energy (eV)
400
500
600
700
Wavelength (nm)
0
• Thermal Gravimetric Analysis reveals
high thermal stability.
• Decomposition temperature (Td) begins
after 400oC.
• Powder crystallized near
melted near 205oC.
136oC
and
• Differential Scanning Calorimetry
reveals several exothermic phase
transitions before Td.
Peak
Pos.
Meas.
Atm%
Th.
Atm%
O 1s
533
6.3
6.2
C 1s
285
83
83
N 1s
399
6.0
6.2
S 2p
164
4.3
4.6
-0.5
In
Powder
ITO
8
6
4
2
0
1000
Binding Energy (eV)
1
100 200 300 400 500 600
700
3
18 16 14 12 10
Tmelt
0
600
O1s
0.6
0.0
C1s
20
Wavelength (nm)
2
85
500
400
25
0.8
0.2
Heat Flow (a.u.)
90
360
x 104
CPS
0.1
3
95
Important Properties
Charge Mobility
0.2
Td
100
75
Morphology
0.3
4
www.konarka.com
Toluene
Chloroform
Chlorobenzene
Dichlorobenzene
1.0
• Optical bandgap (Eg) of the film
was measured to be 1.81 eV.
Our goal is to design and characterize new semiconducting
organic materials that have high charge mobility, absorb strongly
across the solar spectrum, and possess the physical, optical and
electronic properties for applications in organic photovoltaics
(OPVs) or light emitting devices (OLEDs).
Processability
1.2
0.4
800
600
400
200
0
Binding Energy (eV)
UPS: He I hν = 21.22 eV
Work function (φ) = width of spectra – hν
ITO
Evac
φITO (measured) = -4.57 eV
DTBT-DTPA-TMeO/ITO
ΔE ≈ 0.9
eV
DTBT-DTPA-TMeO:
• HOMO = -5.3 eV
φ = 4.57 eV
3.66 eV
LUMO = 3.4 eV
• LUMO = HOMO + Eg = -3.4 eV
Eg = 1.9 eV
Ef
0.76 eV
XPS:
HOMO = 5.3 eV
Atomic ratios indicate that the chemical composition of the
original compound was preserved in the films
Experimental
Gold
Cathode
• Thin films of DTBT-DTPA-TMeO were solution processed by
spin coating on a glass or indium-tin oxide (ITO) coated glass
substrate with a 5-10 mg/mL chloroform solution at RPMs
between 1500 and 2500 in order to achieve desired
thicknesses
Energy Levels
Orbital energy levels are crucial for device design.
• Films were also successfully vacuum deposited
Atmospheric
Stability
Is the material stable under ambient conditions or
does it degrade quickly from exposure to O2, H2O,
UV radiation, etc.
• Patterned gold cathodes were vacuum deposited onto the
films at approximately 10-6 torr using a shadow mask.
• Device areas were measured to be 0.95±0.03 mm2 .
DTBT-DTPA-TMeO
Carrier Mobilities
Single carrier diodes were constructed to measure the hole mobility
Space Charge Limited Current (SCLC) model[1]:
ITO
Glass
Hole only Diode
where J is the current density, ϵo is the permittivity of free space, ϵr is the
permittivity of the material (approximated from capacitance measurements), μ
is the zero field charge mobility, V is the applied voltage, and d is film thickness
The SCLC model can be further improved by expressing μ as a function of electric
field strength and , a coefficient similar to the Poole-Frenkel effect[2]:
2
3.4 eV
Current Density (A/m )
• Relatively Low
Conductivity
• Incomplete coverage
of solar spectrum
• Chemical Stability
Ultraviolet and X-Ray Photoelectron
Spectroscopy
CPS
Limitations:
• UV-Vis spectra of a) film
solution processed on clean
glass and b) from solutions.
The spectra display two
strong absorbance peaks
(~380 and 550 nm). The
peaks are red shifted in the
film.
Absorbance
Semiconducting organic materials are attractive for their cost, light
weight, highly tunable optical and electrical properties, and compatibility
with high throughput, roll-to-roll processing techniques. Despite these
advantages, organic electronics are not yet marketable due to several
factors that inorganic electronic technology is not limited by.
Absorbance
Organic Electronic Materials
Molecule of Interest
• Mobility was found by
fitting the model to the
dark current-voltage
(I-V) curve
Atomic Force Microscopy
(a)
(b)
(c)
• ϵr ≈ 3
• μ0 =
3.9(±3.8)x10-6
cm2/V•s
ITO
4.6 eV
Gold
5.3 eV 5.1 eV
Hole only Diode
(ITO positively biased)
10
1
Measured Current
PF-SCLC Model
SCLC Model
0.1
0.01
≈ -1.2x10-4 (m/V)1/2
1E-3
-1
0
1
2
3
4
5
6
7
8
Volts
Conclusion and Future Work
Tertmethoxy Di-triphenylamine Di-thiophene-benzothiadiazole
(DTBT-DTPA-TMeO: H42C54O4N4S3 molecular mass = 906 amu)
(d)
(e)
Thin films of DTBT-DTPA-TMeO can be solution processed and deposited
by thermal vacuum deposition. The optical, physical and electronic properties
investigated in this study by TGA/DSC, AFM, XPS/UPS and charge mobility
analysis indicate that this small molecule is a promising candidate for electronic
applications. Furthermore, the HOMO and LUMO energy levels are consistent
with many other similar materials used as electron donor layers in OPVs.
(f)
• Utilizes the novel donor-acceptor-donor (D-A-D) molecular model to
achieve desirable orbital energy levels.
• It is a small molecule version of D-A-D copolymers designed for
photovoltaic applications.
Work is in progress to further characterize a thermally deposited film in
order to optimize the mobility and absorbance of the material.
• Benzothiadiazole core is electron accepting and the thiophenetriphenylamine groups are electron donating.
• Both the ground state and excited state molecular geometries are
relatively planar.
Bare ITO (a). Surface morphology of ~40 nm thick DTBT-DTPA-TMeO solution processed thin film (b)
un-annealed and (consecutively) annealed at (c) 70oC and (d) 110oC for 15-30 minutes each. Surface
morphology of vacuum deposited film (e) un-annealed and (f) annealed at 110oC for 30 min.
• Film roughness did not change significantly with successive annealing at
increasing temperatures for intermediate periods of time
Ground State
Excited State
Theoretical Bandgap (Eg) = 1.933 eV
• Large features evolve on the surface when annealed above
annealed near 100oC for an extended period of time
• Overall film quality was suitable for device construction
100oC
or when
Future work will focus on integrating DTBT-DTPA-TMeO into a solution
processed OPV as well as a vacuum deposited OPV to finally determine the
parameters of a fully operational device.
Acknowledgements and References
Thank you Yilin Li for donating the compound.
This work was supported by the National Science Foundation’s REU program under grant number
DMR-1062898.
[1]M. A. Lampert and P. Mark, Current Injection in Solids (Academic, New York, 1970)
[2]P. W. M. Blom,M. J. M. de Jong, and M. G. van Munster, Phys. Rev. 55 656 (1997).