Voc from a Morphology Point of View: the Influence of Molecular
Orientation on the Open Circuit Voltage of Organic Planar
Heterojunction Solar Cells
Ulrich Hörmann,*
,†
Christopher Lorch,
‡
Alexander Hinderhofer,
‡,§
Alexander Gerlach,
‡
Mark Gruber,
†
Julia Kraus,
†
Benedikt Sykora,
†
Stefan Grob,
†
Theresa Linderl,
†
Andreas Wilke,
∥
Andreas Opitz,
∥
Rickard Hansson,
⊥
Ana Sofia Anselmo,
⊥
Yusuke Ozawa,
§
Yasuo Nakayama,
§
Hisao Ishii,
#,§
Norbert Koch,
∥,∇
Ellen Moons,
⊥
Frank Schreiber,
‡
and Wolfgang Brütting*
,†
†
Institute of Physics, University of Augsburg, Universitätsstraße 1, 86135 Augsburg, Germany
‡
Institute of Applied Physics, University of Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
§
Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
∥
Department of Physics, Humboldt University of Berlin, Brook-Taylor-Straße 15, 12489 Berlin, Germany
⊥
Department of Engineering and Physics, Karlstad University, SE-65188 Karlstad, Sweden
#
Center for Frontier Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
∇
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH - BESSY II, Albert-Einstein-Straße 15, 12489 Berlin, Germany
•S
Supporting Information
ABSTRACT: The film morphology and device performance of planar heterojunction
solar cells based on the molecular donor material α-sexithiophene (6T) are investigated.
Planar heterojunctions of 6T with two different acceptor molecules, the C60 fullerene and
diindenoperylene (DIP), have been prepared. The growth temperature of the 6T bottom
layer has been varied between room temperature and 100 °C for each acceptor. By means
of X-ray diffraction and X-ray absorption, we show that the crystallinity and the molecular
orientation of 6T is influenced by the preparation conditions and that the 6T film
templates the growth of the subsequent acceptor layer. These structural changes are
accompanied by changes in the characteristic parameters of the corresponding
photovoltaic cells. This is most prominently observed as a shift of the open circuit
voltage (
Voc): In the case of 6T/C60 heterojunctions,
Voc decreases from 0.4 to 0.3 V,
approximately, if the growth temperature of 6T is increased from room temperature to 100
°C. By contrast,
Voc increases from about 1.2 V to almost 1.4 V in the case of 6T/DIP solar
cells under the same conditions. We attribute these changes upon substrate heating to
increased recombination in the C60 case while an orientation dependent intermolecular coupling seems to change the origin of
the photovoltaic gap in the DIP case.
■ INTRODUCTION
A high open circuit voltage (
Voc) is one of the key factors
governing the performance of a photovoltaic cell. Conse-
quently, the origin of
Voc in organic donor/acceptor
heterojunction solar cells has been the subject of intense
research in recent years. In analogy to inorganic semi-
conductors, this has led to the general understanding that the
open circuit voltage is vastly determined by a photovoltaic
energy gap Δ
E. In a broad range of material combinations this
energy has been identified as the donor/acceptor intermolec-
ular energy gap.1−4 This is the energy difference between the
highest occupied molecular orbital (HOMO) of the donor and
the lowest unoccupied molecular orbital (LUMO) of the
acceptor and is denoted as
ED/A in the scope of this article.
(Note that several terms exist in the literature all referring to
this energy, under slightly different conditions.1,5−9) To
determine
ED/A, it is most common to measure the HOMO
levels of the donor and acceptor material separately and then
derive
ED/A by calculating the acceptor LUMO from the
HOMO and the transport gap.8 In this case it is crucial that the
transport gap is precisely known and that the HOMO levels are
determined in a configuration as close to the device as possible
since vacuum level alignment cannot generally be assumed in
an organic multilayer device.10 In particular, intermolecular
energy gaps calculated from literature values of different sources
have to be treated with caution. Additionally, Δ
E may be
estimated by various optical3,4,7 and electrical techniques,3
some of which allow a more or less direct access to Δ
E.4,7 For
Received: June 21, 2014
Revised:
September 30, 2014
Published: October 27, 2014
Article
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© 2014 American Chemical Society
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J. Phys. Chem. C 2014, 118, 26462−26470
example, Δ
E can be extracted from the actual solar cell device
by linear extrapolation of the temperature dependence of
Voc to
0 K.4,7,9,11 Starting from the Shockley equation for the current−
voltage (
j−
V) characteristics of a solar cell under illumination
=
−
−
j j
eV nk T
j
[exp( /
) 1]
0
B
sc
(1)
Voc can be written as the voltage
V measured at zero current (
j
= 0)
=
+
eV
nk T
j j
ln( /
1)
oc
B
sc 0
(2)
Here
jsc denotes the short circuit current,
j0 is the reverse
saturation current,
n is the ideality factor, and
kB is Boltzmann’s
constant. By inserting the term12
=
−Δ
j
j
E nk T
exp(
/
)
0
00
B
(3)
we receive a relation, where
Voc is determined by Δ
E and losses
caused by recombination
≈ Δ −
eV
E
nk T
j j
ln( / )
oc
B
00 sc
(4)
The factor
j00 introduced in eq 3 is specific for a certain solar
cell and provides a measure for the electronic coupling at the
heterojunction.6,13,14
While Δ
E sets an upper limit for the open circuit voltage at a
temperature approaching 0 K,
Voc is commonly found to be
reduced by approximately half a volt at room temperature for
organic solar cells.3,8 This can mainly be attributed to inevitable
entropic losses, including radiative recombination and addi-
tional nonradiative recombination losses.2−4,7,15−18
The
magnitude of these losses strongly depends on the film
morphology of the active layer. For example a larger interface
area is expected to yield a larger recombination current. Thus, a
planar heterojunction (PHJ) architecture is generally supposed
to exhibit less recombination losses than any bulk hetero-
junction (BHJ) device of the same material system.19,20 Apart
from this purely geometric impact of the nanostructure, the
local morphology at the donor/acceptor interface is expected to
have an at least equally important impact on the recombination
current: It has been shown by quantum chemical calculations
that relative molecular orientation may have a strong influence
on the recombination rate of a given donor/acceptor pair.21,22
Additionally, it has recently been demonstrated that structural
disorder at the donor/acceptor interface of planar hetero-
junctions may affect the average electronic coupling and thus
the recombination rate.23,24
With a special focus on
Voc we investigate nominally planar
heterojunction solar cells based on the donor α-sexithiophene
(6T) and two differently shaped acceptors, the spherical
buckminster fullerene C60 and the rod-like diindenoperylene
(DIP; for structural formulas see Figure 1d).25−27 We combine
structural and energetic investigations with the analysis of solar
cell device properties in order to relate the thin-film
morphology to the open circuit voltage of the solar cell. The
intrinsic difference in symmetry of the two acceptors yields
additional insight on the effect of molecular orientation on the
device characteristics.
■ EXPERIMENTAL SECTION
All films were prepared on indium tin oxide (ITO) covered
glass slides purchased from Thin Film Devices (patterned for
solar cells) and Merck (unpatterned for all other samples). In
all cases the ITO substrate was spin coated with PEDOT:PSS
(Heraeus Clevios AI4083) and dried in air at 150 °C for 30 min
resulting in a 30 nm thick layer. 6T films were deposited by
vacuum thermal evaporation (2 × 10
−7
mbar) at a deposition
rate of 0.3 Å/s and a substrate temperature of 100 °C or room
temperature (RT), respectively. In the high temperature (HT)
case, the 6T film was always cooled to room temperature in
vacuum before further deposition of the acceptor layer. (Note
that throughout this article RT and HT denote the growth
condition of 6T and only 6T.) Subsequently C60 or DIP were
deposited at a rate of 0.5 Å/s. Solar cell devices were transferred
through a nitrogen atmosphere to a second vacuum chamber (2
Figure 1. (a) Device stack of the investigated solar cells. Current−voltage characteristics of 6T/C60 (solid lines) and 6T/DIP (dash dotted lines)
solar cells under illumination (b) and in the dark (c). Room temperature (RT) devices are shown in blue; devices with 6T (and only 6T) grown at a
substrate temperature of 100 °C (HT), in red. The fits to the dark characteristics by the Shockley equation are shown as green dashed lines. (d)
Structural formulas of the investigated materials.
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× 10−7
mbar) where bathocuproine (BCP) and aluminum were
deposited to form the top contact (Figure 1a). 6T (Sigma-
Aldrich), C60 (Creaphys), and DIP (3. Physikalisches Institut,
University of Stuttgart) were purified twice by temperature
gradient sublimation prior to use. BCP (Sigma-Aldrich) was
used as received. X-ray reflectivity (XRR) was measured ex-situ
with a GE/Seifert X-ray diffractometer (Cu Kα1). Grazing
incidence X-ray diffraction and MarCCD detector images were
recorded at the ID10B (ESRF) beamline using a wavelength of
0.925 Å. Scanning force microscopy (AFM) images were
recorded in air with a Thermo Microscopes Autoprobe CP-
Reserach device in tapping mode. Angular resolved near edge
X-ray absorption fine structure (NEXAFS) spectroscopy was
performed at beamline D1011 at the MAX-lab synchrotron
facility, Lund, Sweden. Using polarized monochromatic X-rays
around the C 1s-edge, the total electron yield was determined
by measuring the sample current. Ultraviolet photoelectron
spectroscopy (UPS) measurements were performed at beam-
line BL8B at the Ultraviolet Synchroton Orbital Radiation
(UVSOR) facility, Institute for Molecular Science (IMS),
Okazaki, Japan. The photon energy, incident angle, and
photoelectron emission angle were set to 30 eV, 45°, and 0°
(surface normal), respectively. For valence band measurements
a bias of +5 V was applied to the sample. In-situ prepared 6T
layers (10 nm) for UPS measurements were deposited at a rate
of 0.3−0.8 Å/s at a pressure of approximately 9 × 10
−7
mbar.
DIP and C60 layers for UPS were deposited stepwise at a rate of
0.3−0.5 Å/s in the low 10
−5
mbar range. Current−voltage (
j−
V) characteristics were recorded with a Keithley 236 source-
measurement unit.
j−
V curves under illumination were
recorded at an intensity of 100 mW/cm2 using an LOT-Oriel
solar simulator equipped with an AM1.5G filter set. Temper-
ature dependent measurements of
Voc were recorded in a
continuous flow liquid nitrogen cryostat (CryoVac). The solar
cells were illuminated with a simulated AM1.5G spectrum at an
intensity of half a sun (C60 samples) or roughly one sun (DIP
samples).
Note that, even though characteristics of single cells are
shown in this article, each is representative for the respective
device type. Statistics on a minimum of five individually
prepared solar cells per type (each with at least two working
pixels) have shown that the presented
Voc values are accurate
Table 1. Characteristic Values and Fit Results of the Solar Cell Devices Presented in Figure 1
a
acceptor
6T growth
jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
Rs
A (Ω cm2)
n
j0 (mA/cm2)
C60
RT
2.6
0.44
61
0.67
1.5
1.6
3.0 × 10
−5
C60
HT
2.2
0.33
43
0.31
1.5
2.1
2.2 × 10
−3
DIP
RT
1.4
1.22
57
0.97
7.5
1.8
1.7 × 10
−12
DIP
HT
1.2
1.35
59
0.96
3.4
2.3
3.4 × 10
−11
aThe values of
jsc,
Voc, the fill factor (FF), and the power conversion efficiency (PCE) are extracted from the
j−
V characteristics under illumination.
Series resistance
Rs
A, ideality factor
n, and dark saturation current
j0 are extracted by a fit to the dark characteristics with the Shockley equation.
Figure 2. X-ray reflectivity spectra (a) of 6T, 6T/DIP, and 6T/C60 bilayers and reciprocal space maps (RT (b) and HT (c)) and GIXD sprectra (d)
of 6T/DIP bilayers. Room temperature (RT) 6T films are shown in blue, and 100 °C grown 6T (HT) films are shown in red in (a) and (d). The
reciprocal space maps are background corrected and stitched together from two different images. Standing 6T (DIP) is marked in yellow (green);
lying 6T (DIP), in red (blue) in (b) and (c).
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within 20 mV (with the exception of high temperature 6T/C60,
which is accurate within 30 mV). A statistical error of 10−12%
is estimated for the stated short circuit current values.
■ RESULTS
In order to get an understanding of how the film structure and
the molecular orientation affect the recombination and thus the
open circuit voltage in our small molecule solar cells, we start
off with the electrical device characterization and then look into
the morphology of the different devices by X-ray scattering, X-
ray absorption, and AFM measurements. The interface
energetics are studied by UPS and temperature dependent
Voc measurements.
Electrical Device Characterization. The device stack and
the
j−
V characteristics of PHJ solar cells of 6T/C60 and 6T/
DIP are shown in Figure 1. The characteristic quantities are
summarized in Table 1. Comparing the behavior of room
temperature (blue) and high temperature (red) 6T/C60 cells,
the curves under illumination (Figure 1b) show that
Voc is
reduced by about 0.1 V, when 6T is grown at a substrate
temperature of 100 °C. At the same time
jsc is decreased, which
can be attributed to the drastically reduced absorption of HT
grown 6T.28 Note that, while eq 2 indicates that a reduced
jsc
will also lead to a slightly reduced
Voc, the observed voltage loss
is far more drastic and cannot be solely explained by the lower
jsc. Instead, a fit of the exponential regime of the dark
j−
V curve
(Figure 1c) with the Shockley equation (eq 1) gives deeper
insight into the origin of the voltage loss. The parameters
extracted from the fits are listed in Table 1. Here the dark
saturation current
j0 is of special interest as it can be regarded as
a measure for charge carrier recombination. In the case of HT
grown 6T,
j0 is almost 2 orders of magnitude larger than in the
RT case. This implies that the observed
Voc loss can clearly be
associated with an increased recombination rate in the 6T/C60
device with the 6T layer grown at elevated temperature.
The dash-dotted lines in Figure 1 show the
j−
V character-
istics of 6T/DIP devices. Again a reduced
jsc is observed for the
cell with the HT grown 6T. In contrast to the 6T/C60 case,
despite the reduced photo current the open circuit voltage is
increased by 130 mV, from 1.22 to 1.35 V, by heating the
substrate during 6T deposition. The fit of the dark character-
istics with the Shockley equation (dashed line in Figure 1c,
parameters in Table 1) reveals that the observed difference in
Voc between the RT and HT devices appears not to be
correlated with recombination. In particular,
j0 is about one
order of magnitude larger for the high temperature grown 6T/
DIP device despite its higher
Voc.
Structural and Morphological Investigation. The
results of the X-ray reflectivity measurements are shown in
Figure 2a. The spectra of both neat 6T films show peaks
stemming from the (400), (600), (800), (10,00), and (12,00)
lattice planes of a standing-up low temperate phase of 6T.29 In
the room temperature case (blue curve) the (10,00) peak is
broadened by a contribution from the (41−1) peak, which can
be assigned to flat lying 6T molecules. This phase vanishes if
the film is grown at 100 °C (red curve), and only the purely
upright standing phase with an angle of ∼70° between the long
molecular axis and the surface is observed. Note that this is in
accordance with the reduced absorption observed for HT
grown 6T.28 The optical transition dipole moment of 6T is
oriented along the long molecular axis which is unfavorable for
the absorption of light impinging perpendicular to the
substrate.30
In the case of the 6T/C60 bilayers, C60 exhibits a small (111)
peak of the fcc phase, when deposited onto the room
temperature 6T. This peak drastically increases for C60 grown
on top of the high temperature 6T film, indicating remarkably
high crystallinity compared to the room temperature case and
to what is commonly found for thin films of C60.31,32
Responsible for the intensity increase is probably also the
changed orientation of the C60 crystallites, similar to the effects
observed for C60 thin films on DIP.33 The appearance of the
strong C60 signal is accompanied by a structural change of the
underlying 6T film. By comparison of the neat 6T(HT) film
and the 6T(HT)/C60 bilayer, one can see that the fullerene
seems to induce a ripening of the 6T which leads to sharper
features in the XRR pattern, indicating increased crystallinity of
the underlying thiophene film.
This structural change does not happen with DIP as a cover
layer, where the underlying 6T appears to remain unchanged
even in the high temperature case. Yet, 6T seems to have a
templating effect on DIP, which grows differently on RT and
HT deposited 6T. This is indicated by the DIP (111) peak
visible in the X-ray reflectivity data (Figure 2a) that belongs to
the flat lying λ-orientation of DIP. In films grown on 6T(RT)
this phase coexists with the upright standing σ-orientation
indicated by the DIP (001) and (002) peaks.25 These peaks
become far more pronounced, if DIP is grown on high
temperature 6T, while the (111) peak of DIP vanishes.
Reciprocal space maps of 6T(RT)/DIP and 6T(HT)/DIP
bilayers are shown in Figure 2b,c, respectively. The vanishing of
lying 6T and DIP domains for HT films is clearly illustrated by
the comparison of both images and confirms the results of the
X-ray reflectivity measurement. Yet, a small contribution of the
(100) peak of lying DIP is still visible even in the HT case. The
growth of lying DIP domains has, however, previously been
observed for increasing layer thickness34,35 and is therefore
expected to be present in the upper part of the film, only. This
implies that only DIP in the upright standing orientation is
present at the interface, when prepared on 6T films consisting
of solely upright standing molecules. On the contrary, if both
upright standing and flat lying domains exist in the 6T film,
domains of both orientations will also coexist in the DIP layer.
The lateral coherent crystallite sizes of 6T and DIP can be
extracted from the grazing incidence X-ray diffraction data
shown in Figure 2d by fitting the observed peaks and applying
the Scherrer equation
D = 2π
Ks/fwhm,36 where
D is the
coherent crystallite size,
Ks = 0.94 the Scherrer factor for
spherical domains, and fwhm is the full width at half-maximum
of the peak. For the RT sample this yields approximate
coherent crystallite sizes of 28 and 16 nm for lying and standing
6T, respectively, as well as 35 and 16 nm for lying and standing
DIP. In the HT case, no lying 6T is present. The average size of
crystallites of standing molecules is 18 nm for 6T and 19 nm for
DIP. Crystallites of lying DIP in the upper part of the film are
about 23 nm in size. Note that an additional feature denoted by
6T-β is visible in the reciprocal space maps and the grazing
incidence X-ray diffraction data. This presumably stems from
the β-phase of upright standing 6T molecules, previously
observed in the literature.37,38 The lateral coherent size of these
crystallites is 15 nm in both cases. Unfortunately, the feature
marked as 6T* cannot be clearly assigned and might be a
superposition of contributions from the 6T (32−1) plane and
the β-phase.
To probe the molecular orientation directly at the donor/
acceptor interface, angular dependent NEXAFS spectra have
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been recorded for samples similar to the solar cell devices but
processed on Si/PEDOT instead of ITO/PEDOT. Since
NEXAFS spectroscopy is a surface sensitive technique, only
thin acceptor layers of nominally 5 nm have been used in order
to retain enough signal from the underlying 6T layer. Note that
NEXAFS yields an average orientation angle and cannot
distinguish different molecular orientations present in the
probed volume. NEXAFS spectra of the investigated materials
and bilayers are shown in Figure 3a. For clarity only spectra
measured for 30°, 55°, and 90° angles of incidence (w.r.t the
substrate plane) are shown (additionally, 40° and 70° have
been measured and used for determining the angle of molecular
orientation α, see the Supporting Information). The neat film
spectra of RT and HT grown 6T (Figure 3a, left) show a clear
dependence of the signal intensity on the angle of the incident
X-rays (dichroism), where the intensities of the π* resonances
are strongest at normal incidence. This indicates that the
molecules are oriented with the π* orbitals (preferentially)
parallel, and hence the conjugated plane perpendicular, to the
substrate (standing molecules).39−41 Clearly, the dichroism is
stronger for the HT case compared to the RT film. This implies
a larger average molecular orientation angle (more upright
standing molecules) for 6T grown at an elevated temperature
and is in accordance with the absence of lying 6T molecules
observed by XRR. A similar trend is visible for the 6T/DIP and
6T/C60 bilayers. In these cases, however, the angular
dependence of the NEXAFS spectrum is a superposition of
that of the thin acceptor layer and a contribution from the 6T
underneath. This becomes directly visible in the 6T/C60
spectra, where the apparent angular dependence is strongly
reduced by the isotropic absorption of C60. Therefore, these
bilayer spectra have been deconvoluted by a best fit of the π*
region with a linear combination of the neat component spectra
(neat DIP and C60 see far right in Figure 3a). These fits are
shown as the black dashed lines in Figure 3. The relative
contributions of the individual components then provide
information on their respective molecular orientation.39 The
general result is in agreement with the bulk analysis presented
above and confirms a mixture of lying and standing 6T at the
interface for RT and only standing 6T for HT samples. Yet, 6T
grown at 100 °C shows a weak tendency toward a slightly larger
molecular angle at the free surface and at the interface to C60,
but toward a slightly smaller angle at the 6T(HT)/DIP
interface compared to the bulk orientation. The same general
trends are visible for RT samples but the extracted average
molecular orientation angle is lower than the bulk value of
standing 6T and may thus be interpreted as a superposition of
contributions from standing and lying 6T molecules. Note that
because of the uncertainties introduced by the deconvolution
the results should be regarded as trends rather than absolute
angles. Nevertheless, the differences between RT and HT are
clear. For details refer to the Supporting Information.
The surface morphology corresponding to the X-ray
measurements is displayed in the AFM images shown in
Figure 3 (bottom). Samples with room temperature grown 6T
layers are presented in Figure 3b above, and those with high
temperature 6T are presented in Figure 3c under the height
scale bar. All samples with 6T grown at room temperature show
comparatively smooth surfaces with maximal height differences
of about 70 nm at nominal thicknesses of 60 (6T) and 120 nm
(6T/C60 and 6T/DIP). This changes drastically if 6T is grown
at elevated temperatures. While a comparatively large area still
Figure 3. NEXAFS spectra for three different angles of incidence (a) and AFM images (b, c) of neat films and bilayers of 6T, DIP, and C60. NEXAFS
spectra of neat DIP and C60 (not dichroic, violet line) are shown in the right most graph in (a). Height profiles are shown as insets in the heated 6T
AFM images, and the respective scale is given in nanometers at the right edge of the image. RT and HT refer to the growth conditions of the 6T film
which was grown at either room temperature or at 100 °C, respectively.
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seems to be smooth, pillars as high as 250 nm (see inset in the
left image of Figure 3c) appear in the neat 6T(HT) film.
Simultaneously, the lateral island size increases from roughly
100 nm to about 400 nm. DIP grown on top of such a film
(center image in Figure 3c) more or less preserves the present
6T topography and forms small grains on top of the 6T islands.
On the other hand, if C60 is deposited onto a 6T film grown at
100 °C (right image of Figure 3c), the number and size of the
pillars increases and small grains of C60 (∼65 nm) are visible on
top of a larger island structure that resembles the previously
observed 6T structure. It seems that C60 induces a roughening
of the 6T film.
The morphological results derived above are visually
summarized in the schematics in Figure 4. Note that we
cannot exclude that C60 partially “rolls” off of the large pillars
and leaves fractions of the 6T uncovered. Also, it cannot clearly
be distinguished between a scenario, where a roughening is
caused by C60, but not by DIP, and a scenario, where
roughening has to be regarded as degradation of the 6T film,
which is suppressed by DIP but not by C60.
Interface Energetics. Even if we cannot deliberately
choose the molecular orientation of 6T, we switch from a
coexistence of lying and standing molecules to a standing only
configuration by growing the 6T layer at an elevated
temperature. This might potentially affect the interface
energetics at the heterojunction.42−44 The UPS spectra of
6T/DIP (a) and 6T/C60 (b) are shown in Figure 5. Films with
room temperature grown 6T are shown in blue, and those with
high temperature grown 6T, in red. Vertical lines mark the
HOMO level onsets of the respective films. As can be seen
from the energy difference Δ
EH in Figure 5a the HOMO−
HOMO offsets for 6T/DIP are identical (Δ
EH = 0.75 eV),
regardless of the temperature at which the 6T was grown. The
same is true for 6T/C60 (Figure 5b) but a peculiarity occurs in
this case for the HT grown 6T/C60 heterointerface: upon
deposition of a submonolayer of C60 (0.3 nm) the whole
spectrum, and with that the 6T HOMO onset, shifts to higher
binding energies. The same shift Δ (compared to the room
temperature film) is observed for the 5 nm thick bulk C60 film.
Thus, the relative HOMO positions of 6T and C60 (Δ
EH = 1.65
eV) are not affected. Note that for 5 nm of C60 on high
temperature 6T a small contribution of the underlying 6T is
still present. Yet, both the baseline and the slope of the C60
HOMO are clearly visible; thus, the determined onset energy is
not expected to suffer from a major impact of the 6T signal.
The identical HOMO−HOMO offsets found for heated and
unheated films imply that the intermolecular gaps
ED/A remain
unaffected by the growth conditions and thus by the observed
morphological changes for all investigated systems.
On the other hand, in accordance with eq 4, Δ
E can be
extracted from electrical device characterization by measure-
ment of the open circuit voltage for different temperatures and
a linear extrapolation to 0 K.4,7,9,11 This is shown in Figure 6 for
solar cell devices prepared identically to the films presented
above. Except for the RT 6T/C60 device, the
Voc(
T) curves
show a deviation from linearity at low temperatures. This is
usually accompanied by the occurrence of severely s-shaped
j−
V curves.45,46 For HT 6T/C60 this is likely due to the reduced
mobility induced by the molecular orientation unfavorable for
charge transport perpendicular to the substrate.38 In the 6T/
DIP case, this is expected to be caused by relatively large
electron injection barriers. Both effects have a larger impact at
lower temperatures.
For the 6T/C60 solar cells (lower data in Figure 6), both
preparation conditions yield the same photovoltaic gap Δ
E ≈
0.93 eV, within the uncertainty of the extrapolation although
the
Voc’s at 300 K differ by about 100 meV (Table 1). Thus,
enhanced recombination, as discussed before, seems to be the
main origin for the lower
Voc of the HT 6T/C60 device. In the
case of the 6T/DIP devices (upper data in Figure 6), it is
clearly visible that the extracted photovoltaic gap is larger for
the HT device (red triangles, 2.06 eV) than for the RT cell
(blue triangles, 1.90 eV). In fact, the difference in Δ
E of 160
Figure 4. Schematic drawing of the morphology derived from XRD,
NEXAFS, and AFM measurements. Growing 6T (orange rods) on a
substrate heated to 100 °C (HT) (right) leads to an increased order of
both the 6T and the acceptor top layer, which is always grown at room
temperature. In the 6T(HT) case C60 (green spheres) induces
additional crystallization and roughening of the 6T. DIP (green rods)
grows purely upright standing on HT grown 6T. DIP grown on room
temperature grown 6T(RT) is templated by the 6T orientation but an
overgrowth of the different domains is expected.
Figure 5. UPS valence region spectra of room temperature (RT)
(blue) and 100 °C (HT) (red) grown 6T and DIP (a) or C60 (b)
grown on top at room temperature. The HOMO level onsets (marked
by short vertical lines) are identical within the experimental error. Δ
EH
equals 0.75 eV for 6T/DIP and 1.65 eV for 6T/C60.
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meV matches well with the observed
Voc difference of 130 mV
(Table 1) and seemingly contradicts the UPS results from
which no difference of the donor/acceptor gap was concluded.
■ DISCUSSION
As shown above the open circuit voltage of 6T/C60 solar cells is
strongly reduced, if 6T is grown at 100 °C substrate
temperature (Figure 6). At the same time a steeper slope of
the temperature dependence of
Voc of the high temperature
device and identical gaps independent of the preparations
conditions have been observed. This clearly shows that severe
recombination losses at the rough 6T(HT)/C60 interface are
indeed responsible for the observed
Voc loss. While it is evident
that a larger interfacial area will enhance recombination, this is
not likely to be the only cause of the voltage loss in this case. In
fact the increase of the interfacial area estimated by AFM is well
below 20%. It has been shown that the mutual molecular
orientation may have a large impact on the recombination
probability, too. In particular, Brédas et al. have shown by
simulations that for the donor/acceptor pair pentacene/C60 the
recombination process is far more efficient in the face-on
geometry than in the edge-on configuration.21 It seems very
likely that the same is true for the 6T/C60 system. Just like
pentacene, 6T is a rod-shaped molecule with the π-system
parallel to the long molecular axis. Both configurations are
expected to be present in both devices (cf. Figure 4a). The
morphological investigation, however, strongly suggests that the
structural disorder at the junction is significantly larger for the
room temperature grown interface than for the high temper-
ature sample. Increased disorder at the donor/acceptor
interface has recently been shown to yield reduced average
electronic coupling and thus larger open circuit voltages for
planar squaraine/C60 heterojunctions and might also be
responsible for the
Voc shift observed here.23,24 This is
strengthened by the change of the crystallite orientation in
the ordered C60 film observed by the X-ray diffraction
measurements, which also might potentially influence the
donor/acceptor coupling.21,22 Still, as was shown for pentacene
and C60,47 we cannot exclude additional intermixing of 6T and
C60 at the exposed terraces in the HT device that might
drastically increase the bulk heterojunction character of the HT
6T/C60 cell and thus significantly strengthen the role of the
face-on configuration. Additionally, in such a case trap-assisted
recombination would probably be significantly enhanced and
might become dominant.48,49
By contrast, for the system of 6T and DIP an increase of the
open circuit voltage is observed (Figure 6). A fit of the dark
j−
V characteristics suggests that this is not likely to be caused by
different recombination rates. This is confirmed by the linear
extrapolation of the temperature dependence of the open
circuit voltage: the data yield almost identical slopes but
different photovoltaic gaps (Δ
E(RT) = 1.90 eV and Δ
E(HT) =
2.06 eV).
In accordance with ref 8, the presented UPS data yield a
donor/acceptor gap of
ED/A = 1.8 eV for both 6T/DIP devices,
if a DIP transport gap of 2.55 eV8 is assumed. As shown in
Figure 6 this is close to the Δ
E value from the temperature
dependent analysis of the
Voc for the RT sample. Surprisingly,
however, it is distinctly different from the value extracted for
the HT cell.
Despite the fact that a linear extrapolation of the temperature
dependence of
Voc has been shown to yield reliable values for
the intermolecular gap
ED/A for a broad range of material
systems,9,11 it is known that this method does not result in
ED/A
under certain, extreme circumstances. Instead it has been
predicted to yield the optical gap
Eopt of the absorber, if the
absorption of the charge transfer state is extremely weak, and,
in particular, if the energy of the CT state comes close to the
optical gap.7,17,18,50 These special conditions seem to be fulfilled
for the HT 6T/DIP device since in this case the Δ
E value
extracted by the extrapolation method is remarkably close to
the optical gap of
Eopt =2.1 eV of DIP.51 This indicates that for
the HT 6T/DIP device driven at typical operating temperatures
the coupling between donor and acceptor molecules is too low
to be relevant for the open circuit voltage. Instead, the optical
gap of DIP takes the role of Δ
E and seems to determine the
Voc
for this particular solar cell.18,52
The identification of the photovoltaic gap Δ
E with the
donor/acceptor gap (Δ
E =
ED/A) for the room temperature
device but with the optical gap (Δ
E =
Eopt) for the high
temperature cell corresponds to the morphological config-
urations found for the different preparation conditions: The
presence of the (face-on) lying/lying 6T/DIP configuration in
the RT device is expected to yield significantly enhanced
electronic coupling21,22
at the donor/acceptor interface
compared to the (edge-on) standing/standing configuration
prevailing in the HT morphology.
With respect to the different photovoltaic gaps Δ
E identified
for the two devices, the similar slopes of the linear
Voc(
T)
regimes in Figure 6 indicate that the recombination losses are
similar for the room temperature and the high temperature 6T/
DIP cells.
Note that it is still possible to simulate the measured
temperature dependences of
Voc of both 6T/DIP solar cells
with a common donor/acceptor gap of
ED/A = 1.8 eV from UPS
and identical optical gaps of
Eopt = 2.1 eV but with extremely
different electronic coupling. The linear extrapolation then
discriminates the two different energies for the two devices (see
the Supporting Information).
■ CONCLUSION
We have investigated the impact of morphology on the open
circuit voltage of organic planar heterojunction solar cells. It
Figure 6. Temperature dependence of the open circuit voltage of 6T/
C60 and 6T/DIP solar cells. The dashed lines represent extrapolations
of the linear regimes (opaque). Blue and red data colors denote the
growth condition of the respective 6T film which was at room
temperature (RT) and 100 °C (HT) substrate temperature,
respectively. The acceptor layer has in all cases been grown at room
temperature.
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was confirmed that the influence of the morphology on
Voc can
be remarkable and that its origin is beyond simple topo-
graphical changes. For the system of 6T and C60, morphology
can drastically change the rate of recombination. Beyond the
magnitude of the interface area this can be attributed to
interfacial disorder and the mutual orientation of the donor and
acceptor molecules, where the recombination rate depends on
the strength of the electronic coupling.21,23
In the case of the 6T/DIP heterojunction two anisotropic
molecules are involved. The morphological transition from a
coexistence of standing and lying 6T and DIP molecules in the
room temperature case to only standing molecules if 6T is
grown at high temperature is accompanied by a considerable
increase of the open circuit voltage. A linear extrapolation of
the temperature dependent open circuit voltage suggests that
different photovoltaic gaps are responsible for the observed
Voc
difference. If, however, the well established method of
determining
ED/A by means of photoelectron spectroscopy is
to be trusted, the increase of Δ
E for the high temperature
sample is not caused by an increased intermolecular gap.
Instead, comparison with the optical gap of DIP shows that
Eopt
becomes dominant over
ED/A. This has been predicted for cases
with extremely weak CT absorption7,17,18,50,52 but to our
knowledge not been reported for real devices. For these
extreme conditions the interpretation of Δ
E has to be
reconsidered and the outcome of a linear extrapolation of
Voc(
T) toward
T = 0 K should be taken with care.
For the room temperature 6T/DIP solar cell, the energies of
the photovoltaic gap match for both methods. This is attributed
to a significantly larger electronic coupling expected for the
lying/lying 6T/DIP configuration than for the standing/
standing orientation.
While different possible causes were proposed, combined
structural and electronic simulations would help to identify the
precise origin of the different electronic coupling of the four
devices on a molecular level. Nevertheless, the implications of
our findings are 2-fold. First, mutual orientation of donor and
acceptor molecules clearly has an influence on the open circuit
voltage of a solar cell. Apart from energetic changes,53 one
reason for this is orientation dependent electronic coupling.
Second, the identification of the photovoltaic gap retrieved
by linear extrapolation of the open circuit voltage with the
optical gap of the absorber in one case and with the
intermolecular gap in another case shows that the interpreta-
tion of this method is not always straightforward and merely
has to be regarded as an effective energy gap in certain cases.
This might be especially true for mixtures of crystallites with
different, but defined, molecular orientations.
■ ASSOCIATED CONTENT
•S
Supporting Information
Molecular orientation from NEXAFS. Simulation of the
Voc(
T)
behavior of the 6T/DIP system. This material is available free
of charge via the Internet at http://pubs.acs.org.
■ AUTHOR INFORMATION
Corresponding Authors
*E-mail: ulrich.hoermann@physik.uni-augsburg.de.
*E-mail: wolfgang.bruetting@physik.uni-augsburg.de.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This work was supported by the German Research Foundation
(DFG) within the priority program SPP 1355 “Elementary
Processes of Organic Solar Cells”, by the Bavarian State
Ministry of Science, Research and the Arts within the
collaborative research network “Solar Technologies go Hybrid”,
and the Landesstiftung Baden-Württemberg. The UPS works in
UVSOR were done under the Joint Studies Program [23-551]
of IMS. U.H. and S.G. thank the Bavarian Research Foundation
(BFS), and C.L. thanks the Carl-Zeiss-Stiftung for Ph.D.
scholarships. U.H. acknowledges the Japan Society for the
Promotion of Science (JSPS), and A.O. acknowledges the
Röntgen-Ångström-Cluster for financial support. E.M. thanks
the Göran Gustafsson Foundation for Research in Natural
Sciences and Medicine. We thank Takuya Hosokai, Takeshi
Watanabe and Alexei Vorobiev for helping with the X-ray
measurements at the ID10B and gratefully acknowledge the
technical expertise and advice of Alexei Preobrajenski, beamline
manager of D1011, MAX-lab.
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