Supplemental Materials
Small Molecular Organic Photovoltaic Cells with Exciton
Blocking Layer at Anode Interface for Improved Device
Performance
Masaya Hirade1,2 and Chihaya Adachi1,2,3*
1
Center for Future Chemistry, Kyushu University, 744 Motooka, Nishi, Fukuoka
819-0395, Japan
2
Life Beans Center Kyushu, Bio Electromechanical Autonomous Nano-Systems
(BEANS) Laboratory, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
3
I2CNER, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan
*adachi@cstf.kyushu-u.ac.jp
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Experimental methods
First, indiun-tin-oxide (ITO) coated glass substrates were cleaned using a solvent
and UV/O3 treatment. After that, a poly(3,4-ethylenedioxythiophene):poly(4-styrene
sulfonate) (PEDOT:PSS, Clevios, PVP AI 4083) layer was formed on the ITO substrate
by spin coating (3000 rpm, 60 s.) and annealed for 10 min. at 200°C in air, and a 20
nm-thick PEDOT:PSS layer was obtained. After annealing, these substrates were
transferred into a vacuum deposition chamber immediately and organic layers were
deposited.
We
used
a
device
structure
of
ITO/PEDOT:PSS/tris[4-(5-phenyl
thiophen-2-yl)phenyl]amine (TPTPA, X nm)/tetraphenyldibenzoperiflanthene (DBP, Y
nm)/C60 (50 nm)/bathocuproine (BCP, 10 nm)/Al (100 nm). TPTPA, DBP and BCP
were purified by the thermal gradient vacuum sublimation method1 and C60 (MRT Co.
Ltd.) was used as received. The base pressure in the organic chamber was around 1×10-5
Pa and the deposition rate for each material was 0.03 nm/s. After the organic material
deposition, the substrates were transferred into a glove box and a shadow mask with 1
mm diameter circular openings was attached on top of the organic layers; the substrates
were then transferred into a metal chamber and finally a cathode layer was formed. The
base pressure in the metal chamber was around 1×10-6 Pa. After device fabrication, the
devices were transferred into a glove box and stored in a small measurement chamber
2
which allows measurement under vacuum. The current density-voltage (J-V)
characteristics were measured under AM 1.5 G illumination at 100 mW/cm2 supplied by
a solar simulator (OTENTO-SUN II, Bunko-Keiki Co.) in combination with a
semiconductor parameter analyzer (B1500A, Agilent Co.). The incident photon to
current efficiency was measured with a source meter (R6243, Advantest Co.) under
monochromatic light from a Xe arc lamp in conjunction with a monochrometer
(SM-10P, Bunko-Keiki Co.). All photovoltaic measurements were conducted under
vacuum conditions. The absorption and photoluminescence (PL) spectra of the materials
deposited on a quartz substrate were measured using a UV/VIS/NIR photometer
(950-PKA, PerkinElmer Lambda Co.), and a PL spectrometer (Fluoro MAX-4, Horiba
Co.). The morphological analysis was conducted using an X-ray diffractometer (XRD,
Ultima IV, Rigaku Co.) and an atomic force microscope (AFM, JSPM-5400, Jeol Co.).
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Figure S1. J-V characteristics of hole-only device of ITO/MoOX (3 nm)/DBP (100
nm)/MoOX (3 nm)/Al (100 nm) structure. Solid and dashed lines show J∝V2 and J∝V,
respectively. Inset: device structure of DBP hole-only device.
Figure S2. (a) Out-of-plane and (b) in-plane XRD patterns of ITO, ITO/PEDOT:PSS,
ITO/PEDOT:PSS/DBP (20 nm), ITO/PEDOT:PSS/TPTPA (10 nm), and
ITO/PEDOT:PSS/TPTPA (10 nm)/DBP (20 nm) layers.
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Figure S3. Absorption spectra of quartz/PEDOT:PSS/DBP
quartz/PEDOT:PSS/TPTPA (10 nm)/DBP (20 nm) layers.
(20
nm)
and
Figures S4(a)–(d) show the AFM images of various thicknesses of the TPTPA
layers deposited on a PEDOT:PSS layer. The upper images show the topographical
images and their cross-sectional images, and the lower figures show their phase images.
In the phase images of the 1 nm and 3 nm-thick TPTPA layers, the light and dark areas
correspond to the PEDOT:PSS and TPTPA layers, respectively. From these images,
while the 1 nm-thick TPTPA layer virtually forms an island structure, the 3 nm layer
shows a nearly continuous layer, and when the thickness exceeds 10 nm, the TPTPA
layer completely covers the underlayer.
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Figure S4. AFM images of various thicknesses (X nm) of TPTPA layers deposited on
PEDOT:PSS layers. Upper and lower figures show the topographical and phase images
of the layers, and the middle figures show cross-sectional profiles of the topographical
images. (a) X = 0 nm (PEDOT:PSS sole layer), (b) X = 1 nm, (c) X = 3 nm, and (d) X =
10 nm.
PL quenching method
In this study, we used a 4,4',4''-tris[3-methylphenyl(phenyl)-amino]triphenyl
amine (m-MTDATA) layer as an exciton quencher. The m-MTDATA layer has a larger
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energy gap of Eg~3.1 eV, (estimated from the absorption edge) than that of a DBP layer
and its highest occupied molecular orbital (HOMO) is located at a rather shallow level
of 5.1 eV (estimated by AC-2, Riken Keiki Co.). Because m-MTDATA and DBP form
the charge transfer complex, the excitons generated by light absorption in the DBP layer
migrate and reach the DBP/m-MTDATA interface, and a successive charge transfer
with a non-radiative decay occurs. Based on this phenomenon, we can estimate exciton
diffusion length (LD). Here, we define the exciton diffusion efficiency (ED) as below:
 ED  1 
PL1
PL 2
(2)
Here, PL1 and PL2 are the PL intensities of a DBP layer with and without an
m-MTDATA interface. Meanwhile, ED can be derived from LD and the film thickness
(d) as follows:
ED 
LD 1  exp(2 d LD ) 
d 1  exp(2 d LD ) 
(3)
Using eq. (2) and eq. (3), we can estimate the LD of a DBP layer. Here, we prepared a
quartz/TPTPA (10 nm)/DBP (X nm) film and a quartz/TPTPA (10 nm)/m-MTDATA (10
nm)/DBP (X nm) film and compared the PL intensities of the two films. Here, we
changed thickness of TPTPA layer from 5 nm to 20 nm. To prevent exciton quenching
by the quartz substrate, a TPTPA layer was used as an exciton blocking layer. Figure S5
shows the DBP layer thickness dependence on the PL1/PL2 values, and the solid lines
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show the theoretically calculated values for LD=6, 9, and 12 nm, respectively.
Figure S5. DBP layer thickness dependence on PL1/PL2 values and calculated results
based on eq. (2) and eq. (3) (LD=6, 9, and 12 nm, respectively). Inset: energy diagram
for m-MTDATA and DBP and charge transfer process.
Reference
1. Forrest, S. R. Chem. Rev. 97, 1793 (1997).
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