Article
pubs.acs.org/JPCC

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 anddiindenoperylene (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 meansof X-ray diﬀraction and X-ray absorption, we show that the crystallinity and the molecularorientation of 6T is inﬂuenced by the preparation conditions and that the 6T ﬁlm
templates the growth of the subsequent acceptor layer. These structural changes areaccompanied 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 toincreased 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. Consequently, the origin of V oc in organic donor/acceptor
heterojunction solar cells has been the subject of intense
research in recent years. In analogy to inorganic semiconductors, this has led to the general understanding that the
open circuit voltage is vastly determined by a photovoltaic
energy gap ΔE. In a broad range of material combinations thisenergy has been identiﬁed as the donor/acceptor intermolecular energy gap.1−4 This is the energy diﬀerence between thehighest occupied molecular orbital (HOMO) of the donor andthe lowest unoccupied molecular orbital (LUMO) of theacceptor and is denoted as ED/A in the scope of this article.
(Note that several terms exist in the literature all referring tothis energy, under slightly diﬀerent conditions.1,5−9) To© 2014 American Chemical Society

determine ED/A, it is most common to measure the HOMO
levels of the donor and acceptor material separately and thenderive ED/A by calculating the acceptor LUMO from the
HOMO and the transport gap.8 In this case it is crucial that thetransport 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, intermolecularenergy gaps calculated from literature values of diﬀerent sources
have to be treated with caution. Additionally, ΔE may be
estimated by various optical3,4,7 and electrical techniques,3
some of which allow a more or less direct access to ΔE.4,7 For
Received: June 21, 2014
Revised: September 30, 2014
Published: October 27, 2014
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ArticleFigure 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.

example, ΔE can be extracted from the actual solar cell device
by linear extrapolation of the temperature dependence of Voc to0 K.4,7,9,11 Starting from the Shockley equation for the current−
voltage (j−V) characteristics of a solar cell under illumination(4)

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 inﬂuenceon 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 heterojunctions 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 sphericalbuckminster 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 thedevice characteristics.

The factor j00 introduced in eq 3 is speciﬁc for a certain solar
cell and provides a measure for the electronic coupling at theheterojunction.6,13,14
While ΔE sets an upper limit for the open circuit voltage at atemperature approaching 0 K, Voc is commonly found to be
reduced by approximately half a volt at room temperature fororganic solar cells.3,8 This can mainly be attributed to inevitableentropic losses, including radiative recombination and additional 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, aplanar heterojunction (PHJ) architecture is generally supposed
to exhibit less recombination losses than any bulk heterojunction (BHJ) device of the same material system.19,20 Apartfrom this purely geometric impact of the nanostructure, thelocal morphology at the donor/acceptor interface is expected to

EXPERIMENTAL SECTION
All ﬁlms were prepared on indium tin oxide (ITO) covered
glass slides purchased from Thin Film Devices (patterned forsolar 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. (Notethat 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 transferredthrough a nitrogen atmosphere to a second vacuum chamber (2j = j0 [exp(eV /nkBT ) − 1] − jsc(1)

Voc can be written as the voltage V measured at zero current (j= 0)eVoc = nkBT ln(jsc /j0 + 1)

(2)

Here jsc denotes the short circuit current, j0 is the reverse
saturation current, n is the ideality factor, and kB is Boltzmann’sconstant. By inserting the term12
j0 = j00 exp( −ΔE /nkBT )(3)

we receive a relation, where Voc is determined by ΔE and losses
caused by recombinationeVoc ≈ ΔE − nkBT ln(j00 /jsc )

■

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ArticleTable 1. Characteristic Values and Fit Results of the Solar Cell Devices Presented in Figure 1a
acceptor

6T growthjsc (mA/cm2)

Voc (V)FF (%)PCE (%)RsA (Ω cm2)nj0 (mA/cm2)

C60
C60
DIP
DIP

RT
HT
RT
HT

2.6
2.2
1.4
1.2

0.44
0.33
1.22
1.35

61
43
57
59

0.67
0.31
0.97
0.96

1.5
1.5
7.5
3.4

1.6
2.1
1.8
2.3

3.0 × 10−5
2.2 × 10−3
1.7 × 10−12
3.4 × 10−11

The 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 RsA, ideality factor n, and dark saturation current j0 are extracted by a ﬁt to the dark characteristics with the Shockley equation.
aFigure 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).

× 10−7 mbar) where bathocuproine (BCP) and aluminum were
deposited to form the top contact (Figure 1a). 6T (SigmaAldrich), C60 (Creaphys), and DIP (3. Physikalisches Institut,
University of Stuttgart) were puriﬁed twice by temperature
gradient sublimation prior to use. BCP (Sigma-Aldrich) wasused 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 wererecorded at the ID10B (ESRF) beamline using a wavelength of
0.925 Å. Scanning force microscopy (AFM) images wererecorded in air with a Thermo Microscopes Autoprobe CPReserach 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-raysaround the C 1s-edge, the total electron yield was determinedby measuring the sample current. Ultraviolet photoelectron
spectroscopy (UPS) measurements were performed at beamline BL8B at the Ultraviolet Synchroton Orbital Radiation(UVSOR) facility, Institute for Molecular Science (IMS),
Okazaki, Japan. The photon energy, incident angle, andphotoelectron 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 rateof 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 sourcemeasurement unit. j−V curves under illumination wererecorded at an intensity of 100 mW/cm2 using an LOT-Orielsolar simulator equipped with an AM1.5G ﬁlter set. Temperature dependent measurements of Voc were recorded in acontinuous 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 areshown 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
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Articlewithin 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.

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 andto what is commonly found for thin ﬁlms of C60.31,32
Responsible for the intensity increase is probably also thechanged orientation of the C60 crystallites, similar to the eﬀects
observed for C60 thin ﬁlms on DIP.33 The appearance of thestrong C60 signal is accompanied by a structural change of theunderlying 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 sharperfeatures in the XRR pattern, indicating increased crystallinity of
the underlying thiophene ﬁlm.
This structural change does not happen with DIP as a coverlayer, where the underlying 6T appears to remain unchanged
even in the high temperature case. Yet, 6T seems to have atemplating eﬀect on DIP, which grows diﬀerently on RT andHT deposited 6T. This is indicated by the DIP (111) peakvisible 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 hightemperature 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 oflying 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. Thisimplies that only DIP in the upright standing orientation ispresent at the interface, when prepared on 6T ﬁlms consistingof 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 beextracted from the grazing incidence X-ray diﬀraction data
shown in Figure 2d by ﬁtting the observed peaks and applying
the Scherrer equation D = 2πKs/fwhm,36 where D is thecoherent crystallite size, Ks = 0.94 the Scherrer factor for
spherical domains, and fwhm is the full width at half-maximumof 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 areabout 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 fromthe β-phase of upright standing 6T molecules, previously
observed in the literature.37,38 The lateral coherent size of thesecrystallites is 15 nm in both cases. Unfortunately, the feature
marked as 6T* cannot be clearly assigned and might be asuperposition of contributions from the 6T (32−1) plane andthe β-phase.
To probe the molecular orientation directly at the donor/
acceptor interface, angular dependent NEXAFS spectra have■

RESULTS
In order to get an understanding of how the ﬁlm structure andthe molecular orientation aﬀect the recombination and thus theopen circuit voltage in our small molecule solar cells, we startoﬀ with the electrical device characterization and then look into
the morphology of the diﬀerent devices by X-ray scattering, Xray absorption, and AFM measurements. The interface
energetics are studied by UPS and temperature dependent
Voc measurements.
Electrical Device Characterization. The device stack andthe 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 substratetemperature 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 asa 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 characteristics of 6T/DIP devices. Again a reduced jsc is observed for thecell with the HT grown 6T. In contrast to the 6T/C60 case,
despite the reduced photo current the open circuit voltage isincreased by 130 mV, from 1.22 to 1.35 V, by heating thesubstrate during 6T deposition. The ﬁt of the dark characteristics 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 becorrelated with recombination. In particular, j0 is about oneorder 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 canbe 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 longmolecular axis and the surface is observed. Note that this is inaccordance with the reduced absorption observed for HT
grown 6T.28 The optical transition dipole moment of 6T isoriented along the long molecular axis which is unfavorable forthe absorption of light impinging perpendicular to thesubstrate.30
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ArticleFigure 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.

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 areshown 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 theinterface for RT and only standing 6T for HT samples. Yet, 6Tgrown 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 generaltrends are visible for RT samples but the extracted averagemolecular orientation angle is lower than the bulk value ofstanding 6T and may thus be interpreted as a superposition ofcontributions 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 hightemperature 6T are presented in Figure 3c under the heightscale bar. All samples with 6T grown at room temperature show
comparatively smooth surfaces with maximal height diﬀerencesof 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

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 orderto retain enough signal from the underlying 6T layer. Note that
NEXAFS yields an average orientation angle and cannot
distinguish diﬀerent molecular orientations present in theprobed 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 thesubstrate plane) are shown (additionally, 40° and 70° havebeen measured and used for determining the angle of molecularorientation 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 themolecules are oriented with the π* orbitals (preferentially)parallel, and hence the conjugated plane perpendicular, to thesubstrate (standing molecules).39−41 Clearly, the dichroism isstronger for the HT case compared to the RT ﬁlm. This implies
a larger average molecular orientation angle (more uprightstanding molecules) for 6T grown at an elevated temperatureand 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 angulardependence of the NEXAFS spectrum is a superposition ofthat 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, these26466

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Articleseems to be smooth, pillars as high as 250 nm (see inset in theleft 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 thepillars 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 rougheningof the 6T ﬁlm.
The morphological results derived above are visuallysummarized in the schematics in Figure 4. Note that weFigure 4. Schematic drawing of the morphology derived from XRD,
NEXAFS, and AFM measurements. Growing 6T (orange rods) on asubstrate heated to 100 °C (HT) (right) leads to an increased order ofboth 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 anovergrowth 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. ΔEH
equals 0.75 eV for 6T/DIP and 1.65 eV for 6T/C60.

cannot exclude that C60 partially “rolls” oﬀ of the large pillars
and leaves fractions of the 6T uncovered. Also, it cannot clearlybe distinguished between a scenario, where a roughening is
caused by C60, but not by DIP, and a scenario, whereroughening 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 elevatedtemperature. 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 withroom temperature grown 6T are shown in blue, and those withhigh temperature grown 6T, in red. Vertical lines mark the
HOMO level onsets of the respective ﬁlms. As can be seenfrom the energy diﬀerence ΔEH in Figure 5a the HOMO−
HOMO oﬀsets for 6T/DIP are identical (ΔEH = 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 inthis 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 (ΔEH = 1.65eV) are not aﬀected. Note that for 5 nm of C60 on hightemperature 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 andunheated 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, ΔE can beextracted from electrical device characterization by measurement of the open circuit voltage for diﬀerent temperatures anda linear extrapolation to 0 K.4,7,9,11 This is shown in Figure 6 forsolar 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 forcharge 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 atlower temperatures.
For the 6T/C60 solar cells (lower data in Figure 6), bothpreparation conditions yield the same photovoltaic gap ΔE ≈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 themain origin for the lower Voc of the HT 6T/C60 device. In thecase of the 6T/DIP devices (upper data in Figure 6), it isclearly 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 ΔE of 160
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The Journal of Physical Chemistry C

Articledrastically increase the bulk heterojunction character of the HT
6T/C60 cell and thus signiﬁcantly strengthen the role of theface-on conﬁguration. Additionally, in such a case trap-assisted
recombination would probably be signiﬁcantly enhanced andmight become dominant.48,49
By contrast, for the system of 6T and DIP an increase of theopen 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 (ΔE(RT) = 1.90 eV and ΔE(HT) =2.06 eV).
In accordance with ref 8, the presented UPS data yield adonor/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 ΔE value from the temperaturedependent 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 temperaturedependence of Voc has been shown to yield reliable values forthe 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 theabsorption of the charge transfer state is extremely weak, and,in particular, if the energy of the CT state comes close to theoptical gap.7,17,18,50 These special conditions seem to be fulﬁlled
for the HT 6T/DIP device since in this case the ΔE 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 opticalgap of DIP takes the role of ΔE and seems to determine the Voc
for this particular solar cell.18,52
The identiﬁcation of the photovoltaic gap ΔE with thedonor/acceptor gap (ΔE = ED/A) for the room temperaturedevice but with the optical gap (ΔE = Eopt) for the hightemperature cell corresponds to the morphological conﬁgurations 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 ΔE 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 cellswith 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 thendiscriminates the two diﬀerent energies for the two devices (seethe Supporting Information).

Figure 6. Temperature dependence of the open circuit voltage of 6T/
C60 and 6T/DIP solar cells. The dashed lines represent extrapolationsof the linear regimes (opaque). Blue and red data colors denote thegrowth 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.

meV matches well with the observed Voc diﬀerence of 130 mV(Table 1) and seemingly contradicts the UPS results fromwhich no diﬀerence of the donor/acceptor gap was concluded.

■

DISCUSSIONAs shown above the open circuit voltage of 6T/C60 solar cells isstrongly reduced, if 6T is grown at 100 °C substratetemperature (Figure 6). At the same time a steeper slope ofthe temperature dependence of Voc of the high temperaturedevice 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 areindeed 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. Infact the increase of the interfacial area estimated by AFM is well
below 20%. It has been shown that the mutual molecularorientation 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 therecombination 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 likepentacene, 6T is a rod-shaped molecule with the π-systemparallel 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 thestructural disorder at the junction is signiﬁcantly larger for theroom temperature grown interface than for the high temperature sample. Increased disorder at the donor/acceptor
interface has recently been shown to yield reduced average
electronic coupling and thus larger open circuit voltages forplanar 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 thedonor/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■

CONCLUSION
We have investigated the impact of morphology on the open
circuit voltage of organic planar heterojunction solar cells. It
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The Journal of Physical Chemistry C
was conﬁrmed that the inﬂuence of the morphology on Voc canbe remarkable and that its origin is beyond simple topographical changes. For the system of 6T and C60, morphology
can drastically change the rate of recombination. Beyond themagnitude of the interface area this can be attributed tointerfacial disorder and the mutual orientation of the donor andacceptor 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 theroom temperature case to only standing molecules if 6T isgrown 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 ofdetermining ED/A by means of photoelectron spectroscopy is
to be trusted, the increase of ΔE for the high temperaturesample 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 caseswith extremely weak CT absorption7,17,18,50,52 but to ourknowledge not been reported for real devices. For these
extreme conditions the interpretation of ΔE has to bereconsidered 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 ofthe 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 theprecise 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 andacceptor 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 retrievedby linear extrapolation of the open circuit voltage with theoptical gap of the absorber in one case and with theintermolecular gap in another case shows that the interpretation 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.

■

ACKNOWLEDGMENTS■

REFERENCES

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 thecollaborative 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 theRöntgen-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 thetechnical expertise and advice of Alexei Preobrajenski, beamline
manager of D1011, MAX-lab.

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ASSOCIATED CONTENT

S Supporting Information*

Molecular orientation from NEXAFS. Simulation of the Voc(T)behavior of the 6T/DIP system. This material is available freeof charge via the Internet at http://pubs.acs.org.

■■

ArticleAUTHOR 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.
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