Download Increase of internal quantum efficiency in small

Increase of internal quantum eciency in small molecular
oligothiophene:C60 mixed heterojunction solar cells by substrate
heating Supplemental Material
D. Wynands,∗ M. Levichkova, K. Leo, and M. Riede
Institut für Angewandte Photophysik, Technische Universität Dresden, 01062 Dresden, Germany
C. Uhrich, G. Schwartz, D. Hildebrandt, and M. Pfeier
Heliatek GmbH, 01139 Dresden, Germany
(Dated: August 18, 2010)
I.
ADDITIONAL DEVICES
In addition to the devices presented in the paper we investigated two additional devices (D
and E) with a mixed layer thickness of xD =xE =20 nm. The structure is the same as of devices
3 0
2 5
n o n h e a
D
A
h e a te d
E
B
C
C u r r e n t d e n s ity ( m A /c m
2
)
2 0
1 5
1 0
te d d e
(2 0 n m
(3 0 n m
d e v ic e
(2 0 n m
(3 0 n m
(4 0 n m
v ic e s :
)
)
s :
)
)
)
5
0
-5
-1 0
-1 5
-0 .5 0
-0 .2 5
0 .0 0
0 .2 5
0 .5 0
0 .7 5
1 .0 0
V o lta g e ( V )
FIG. 1: Current-voltage characteristics of devices A to E. For performance parameters see Table I.
∗
Electronic address:
[email protected];
URL:
http://www.iapp.de
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TABLE I: Performance parameters of devices A to E.
device Voc (V) jsc (mAcm−2 ) FF I (mWcm−2 ) ηPCE (%)
A
0.88
7.3
0.422
126.0
2.1±0.1
B
0.89
10.9
0.607
119.3
4.9±0.2
C
0.88
11.6
0.570
117.7
4.9±0.2
D
0.89
7.1
0.503
128.2
2.5±0.1
E
0.90
9.7
0.640
125.9
4.4±0.2
A to C: ITO / n-C60 (5)/ C60 (15)/ DCV6T:C60 (x, Tsub )/ BPAPF (5)/ p-BPAPF (10)/
p-Di-NPD (30)/ NDP9 (1)/ Al (100). Device D was produced without substrate heating
(Tsub =30 ◦ C), whereas device E was heated to a substrate temperature of approximately
90 ◦ C during the deposition of the mixed layer. Mixing ratio of the DCV6T:C60 HJ is 2:1
by volume. Mixed layer thickness of devices A to E is xA =30 nm, xB =30 nm, xC =40 nm,
xD =20 nm, xE =20 nm, respectively.
Fig. 1 presents the current-voltage characteristics of all investigated devices. Performance
parameters are summarized in Table I. Due to to the smaller mixed layer thickness of the
additional devices D and E they exhibit smaller short circuit currents (D: 7.1 mAcm−2 , E: 9.7
mAcm−2 ) than corresponding devices, but are characterized by a slightly higher ll factor
(D: 0.503, E: 0.640). The enhanced power conversion eciency (PCE) of device E compared
to device D underlines the positive eect of substrate heating.
In Fig. 2 IQE spectra of the heated devices B, C, and E are shown. As stated in the paper
all devices exhibit nearly equal IQE despite their dierence in mixed layer thickness. This
shows that ηcc has no observable limiting eect on the IQE upon the investigated interval.
We therefore assume that the mixed layer morphology achieved by substrate heating is
considerably optimized.
II.
ADDITIONAL CHECK OF POWER CONVERSION EFFICIENCY
To conrm the determined PCE of device B and C we conducted outdoor measurements
on the balcony of our institute (51 ◦ 3'N, 13 ◦ 44' 24"E) on 09.08.2009. An aperture of
2.985 mm2 was used. Intensity of sun irradiation was 99 mWcm−2 determined by an outdoor
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E n e rg y (e V )
4
1 .0
3 .5
2 .5
h e a te d d e
E (2
B (3
C (4
0 .8
A b s o rb a n c e (O D )
3
v ic
0 n
0 n
0 n
2
1 .5
e s :
m )
m )
m )
0 .6
0 .4
0 .2
0 .0
3 0 0
4 0 0
5 0 0
6 0 0
7 0 0
W a v e le n g th ( n m )
FIG. 2: Internal quantum eciency of devices B, C and E.
1 5
d e v ic e
B
)
2
C u r r e n t d e n s ity ( m A /c m
/
C
/
1 0
V o c
js c
F F
5
= 0 .8 6 / 0 .8 6 V
= 8 .3 / 9 .2 m A /c m
2
= 0 .6 5 8 / 0 .6 2 2
ηP C E = 4 . 8 / 5 . 0 %
0
-5
-1 0
-0 .5 0
-0 .2 5
0 .0 0
0 .2 5
0 .5 0
0 .7 5
1 .0 0
V o lt a g e ( V )
FIG. 3: Current-voltage characteristics of devices B and C in outdoor measurement. Perfomance
parameters are given in the inset. Intensity is determined by an outdoor reference cell.
reference cell (Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany). In Fig. 3
the corresponding current-voltage curves are presented. Temperature of the assembly platform was 32.8 ◦ C. Nevertheless, a higher temperature of the devices during the measurement
has to be expected. We estimate the device temperature to be around 45 ◦ C since the device
could not be cooled directly. By this reason a higher ll factor and a lower open circuit
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voltage are observed as compared to the reported measurements under sun simulator. However, PCEs of 4.8 % and 5.0 % are determined for device B and C, respectively. Thus, the
reported eciencies for these devices are conrmed.
III.
OPTICAL MODEL
To separate the absorption of active absorber layers from absorption within transport
layers and metal contact, we use a transfer-matrix-formalism to model the device optically
[1]. Values of the optical constants n and k were derived by tting transmission and reection
spectra of thin lm layers of the respective materials [2]. However, the accuracy of these
values is limited, in that they do not account for the eect of dopant admixture into Di-NPD
and C60 . Optical constants of BPAPF were measured with dopant, although a thin neat layer
of BPAPF is also used in the device. The mixed layer optical constants were determined on
a heated quartz glas substrate. That means, we can not account for the inuence of the C60
underlayer or a possible spatially-dependence of the mixed layer optical constants with this
method. In Fig. 4 the calculated absorption prole of devices B, C, and D are presented to
show that the thinlm optics of device B and C is optimized for maximum absorption in
the mixed active layers.
IV.
[1] E. Centurioni.
Applied Optics
REFERENCES
, 44(35):75327539, 2005.
[2] T. Fritz, J. Hahn, and H. Boettcher.
, 170(2):249257, 1989.
Thin Solid Films
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s im u la te d a b s o r p tio n p r o file s o f d e v ic e s B , C , E
2 .5
)
3
0 .0
h
· 1
( 1
x
f l u
a
b
0 .5
s o
r b
1 .0
e
d
0
2
1 .5
C
p
B
8
o
/ s m
t o
2 .0
E
n
-
in c o m in g lig h t
T
)
: C
6
0
4 0
0
2 5
5 0
7 5
m
(
n
6
1 0 0
i x
m
2 0
d
6
C
0
C
D
- C
l
A
n
V
0
6
3 0
1 2 5
x - s ta c k p o s itio n ( n m )
FIG. 4: Absorbed photon ux within device B, C and E as calculated by optical modelling. In
device B and C, the absorption prole exhibits a maximum in the mixed layer showing that the
choosen transport layer thickness is optimal for maximized absorption. In device E the maximum
is not reached, meaning that a slightly higher hole transport layer thickness would be needed for
optimized absorption.