Evaluation and Control of the Orientation of Small Molecules for Strongly Absorbing Organic Thin Films

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Article

Evaluation and Control of the Orientation of SmallMolecules for Strongly Absorbing Organic Thin Films

Christoph Schünemann, David Wynands, Klaus-Jochen Eichhorn, Manfred Stamm, Karl Leo, and Moritz RiedeJ. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp400604j • Publication Date (Web): 15 May 2013

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Evaluation and Control of the Orientation of Small

Molecules for Strongly Absorbing Organic Thin Films

Christoph Schünemann*1,2,3

, David Wynands2, Klaus-Jochen Eichhorn

2, Manfred Stamm

2,3,

Karl Leo1,3

, and Moritz Riede1

1 Institut für Angewandte Photophysik, Technische Universität Dresden, 01062 Dresden,

Germany, http://www.iapp.de

2 Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Str. 6, 01069 Dresden, Germany,

http://ipfdd.de

3 Technische Universität Dresden, 01062 Dresden, Germany

*Corresponding author:

Christoph Schünemann

Technische Universität Dresden

Institut für Energietechnik

Helmholtzstraße 14

01062 Dresden

Telefon: +49351-463 37619

Email: [email protected]

Alternative contact:

Prof. Karl Leo

Technische Universität Dresden

Institut für Angewandte Photophysik

01062 Dresden

Email: [email protected]

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Abstract

In the photoactive film of organic solar cells, the orientation of the absorber molecules is one

of the key parameters to achieve high absorption and high photocurrents as well as efficient

exciton and charge transport. However, most organic absorber small molecules, like zinc-

phthalocyanine (ZnPc) or diindenoperylene (DIP) grow more or less upright standing in

crystalline thin films. Considering absorption, this molecular alignment is unfavorable. In this

work we control the orientation of ZnPc and DIP in crystalline absorber films by varying the

substrate or organic underlayer appropriately. For this purpose, a precise evaluation of the

molecular orientation and packing is important. We find that a combination of the methods

variable angle spectroscopic ellipsometry (VASE) and grazing incidence X-ray diffraction

(GIXRD) can fulfil this requirement. The combination of these complementary methods

shows that the growth of DIP and ZnPc is nearly upright standing on weakly interacting

substrates, like glass or amorphous charge transport films. In contrast, on strongly interacting

metal sublayers and PTCDA templating layers, both molecules arrange in a strongly tilted

orientation (mean tilt angle 54°-71° with respect to the substrate normal), inducing a

significant enhancement of absorption (maximum extinction coefficient from 0.72 to 1.3 for

ZnPc and 0.14 to 0.4 for DIP). However, even when deposited on metal or PTCDA sublayers

not all ZnPc and DIP molecules in the film are oriented in the desired flat-lying fashion. This

highlights that classifying organic films into either solely flat lying structures or solely upright

standing structures, as often made in literature, is a too simplified picture.

Keywords: morphology; diindenoperylene; phthalocyanine; organic solar cells; molecular

design; molecular orientation; X-ray diffraction; ellipsometry; substrate influence; absorption

Introduction

Most organic small molecules exhibit an anisotropic shape. Consequently, the resulting

organic thin films are typically anisotropic as well, resulting in anisotropic optical and

electrical properties. Anisotropy in thin film absorption is highly relevant for organic solar

cell application. Depending on the molecular orientation within the film, i.e. on the direction

of the molecular transition dipole moment with respect to the incident light, the absorption of

organic thin films can vary by several orders of magnitude.1-3

Accordingly, for efficient

organic solar cells, the molecules of the absorber film are required to be arranged in the way

that the transition dipole moment is perpendicular to the incidence of light (see figure 1b and

e). With this preferred molecular arrangement, a film thickness of several tenths of

nanometers can be sufficient to absorb most of the sunlight for typical absorption coefficients.

Accordingly, the challenge of exciton separation at the donor-acceptor film interface limited

by the low exciton diffusion length of only several nanometers in organic films4 can be

overcome by choosing absorber films with preferred molecular orientation.

The desired molecular arrangement in the organic absorber film can be achieved by two ways:

tuning the molecular design5,6

or using substrates or sublayer with high surface energy. Here,

we concentrate on the latter approach. For evaporable small organic molecules, two different

types of crystalline thin film growth can be obtained: For thin film deposition on weakly

interacting substrates (WIS), a more or less upright standing growth of planar and nearly

linear extended small molecules (with respect to the substrate surface) is typical. Here, the

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formation of Van-der-Waals bonds with molecular neighbors is energetically preferred in

comparison to the molecule-substrate interaction. Contrary, thin film deposition on strongly

interacting substrates can induce a flat-lying growth of planar or linear extended molecules

caused by a stronger molecule-substrate interaction in comparison to the intermolecular

forces. This kind of film growth can be found for organic molecules deposited onto substrates

with large surface energy, e.g. metal surfaces.7-11

Additionally, the so-called weak molecular epitaxial growth can be used to achieve flat-lying

molecules in the absorber film as shown by Wang et al.12

Here, the knowledge of inorganic

thin film growth is transferred to organic materials: A crystalline templating layer can induce

an adapted molecular arrangement in the epitaxial film deposited on top. 3,4,9,10-perylene

tetracarboxylic dianhydride (PTCDA) typically forms films with flat-lying molecular

orientation also on WIS.12-14

The orientation of the π-system of the PTCDA film favors a flat-

lying growth of other planar molecules deposited on top of this PTCDA sublayer.

Accordingly, the deposition of such a molecular epitaxial film on top of a PTCDA templating

layer can induce flat-lying growth demonstrated by Heutz et al. and Sullivan et al.15,16

The

fact that polar molecules like PTCDA are oriented flat-lying even on WIS can be explained by

the enhanced molecule-substrate interaction provided by its permanent quadrupole moment

induced by the functional carbonyl groups.

In this study, we systematically change the orientation of two different absorber small

molecules, the planar shaped zinc-phthalocyanine (ZnPc) and the linearly shaped

diindenoperylene (DIP) (molecular structure: see figure 2a and 3a). Both molecules typically

grow crystalline films with nearly upright standing molecular orientation for deposition on

WIS which is proven in detail in the first part of the results section. This molecular orientation

results in a low thin film absorption. Furthermore, to achieve different degree of crystallinity

and crystallite sizes, the substrate temperature Tsub is varied as well. The preferred molecular

tilting of the DIP and ZnPc molecules within the film is achieved by deposition onto metal

sublayers of Ag, Al, and Au. In addition, molecular epitaxial growth of ZnPc and DIP films

on PTCDA templating layers is also applied to gain the desired flat-lying orientation. All

substrate and sublayer variations are done for both kinds of molecules in order to verify if the

orientation control can generally applied to these molecule types. Some of the investigations

of the present work are already discussed in literature.1,4,9,11,14,17-20

However, measurement of

the general molecular orientation is challenging with only one method at hand and results can

consequently be ambiguous. Therefore, we introduce in this work a combination of GIXRD

and VASE to give detailed information about the molecular orientation. The combination of

both techniques is ideal to reveal details about optical anisotropy, crystallinity, and molecular

orientation at the same time. By applying both methods to films of the two chosen standard

molecules ZnPc and DIP grown on a broad variety of different substrates we give a

comprehensive view on how molecular orientation can be controlled. While VASE

measurements can provide precise results only for smooth organic films and substrate

surfaces, the film roughness is marginal for accurate GIXRD measurements. Accordingly,

some of the investigated DIP films with a high RMS-roughness of more than 10 nm (AFM

RMS-roughness) illustrate the limit of VASE applicability for thin film characterization.

Furthermore, we demonstrate that a simple distinction into films with upright standing and

flat-lying molecular orientation, as often made in literature,4,10,20-25

is not sufficient to describe

a realistic scenario of organic thin film growth.

At the end of the introduction part, we shortly want to review some relevant results in the

field of manipulating the orientation of small molecules in thin films. Yokoyama et al.

analyzed the molecular orientation of several small molecules with different structure by the

VASE method.6 They exhibited that molecules in crystalline organic films show the tendency

to be oriented standing upright on WIS. In contrast, in amorphous films small molecules

reveal in a more flat-lying orientation (not randomly oriented). Both ZnPc and DIP are often

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used to investigate changes in molecular orientation. Dürr et al. obtained a mixture of flat-

lying and upright standing DIP on SiO2 substrates20

and completely flat-lying DIP molecules

on Au substrates (method: X-ray diffraction).11

A contentious issue is the range of this

substrate effect on the arrangement of the molecules grown on top. Casu et al. investigated the

growth of DIP on a polycrystalline Au foil (method: photoemission electron microscopy) and

obtained a formation of flat-lying DIP molecules in the first monolayer.10

However, for the

following monolayers a mixture of flat-lying and upright standing DIP molecules is exhibited

showing a limited range of the substrate effect. In contrast, Lunt et al. found flat-lying DIP

molecules in films which are thicker than 200 nm (method X-ray diffraction) contradicting the

observation of Casu et al.4 For this purpose, Lunt et al. used a 0.5 nm thin PTCDA templating

layer. Yim et al. discussed the influence of H2Pc on PTCDA and calculated a flat-lying

growth on PTCDA sublayer and a standing upright herringbone arrangement on SiO2

substrates.9 Heutz et al. were able to confirm this hypothesis experimentally for H2Pc on a

PTCDA sublayer (method X-ray diffraction).18

Methods

Before thin film evaporation, the organic materials PTCDA (ABCR, Germany), BF-DPB

(N,N'-((diphenyl-N,N'-bis)9,9,-dimethyl-fluorene-2-yl)-benzidine, Sensient, USA) and

BPAPF (9,9-bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene, Lumtec, Taiwan) are

purified at least twice by vacuum gradient sublimation. DIP (University of Stuttgart,

Germany) is purified by carrier gas sublimation and vacuum sublimated zinc-phthalocyanine

(ZnPc) and C60 (both CreaPhys, Dresden, Germany) are used as received. All films are

deposited via thermal evaporation in a multichamber UHV system (Bestec, Germany) and a

custom-made single chamber vacuum system (K. J. Lesker, U.K.) at a base pressure of (10-7

to 10-8

) mbar. Quartz crystal microbalances are used to control the film thickness. For this

purpose, the following thin film densities are used, determined by profilometer measurements:

ρDIP= 1.35 g/cm³, ρZnPc = 1.34 g/cm³, ρC60 = 1.54 g/cm³, ρPTCDA = 1.30 g/cm³, ρBF-DPB =

1.21 g/cm³, and ρBPAPF = 1.21 g/cm³.

Out-of-plane GIXRD measurements are performed using a Bruker D8 Discover

diffractometer at the Fraunhofer CNT (Dresden, Germany). The device setup and the

measurement routine can be found elsewhere.26,27

For specular XRD (θ-2θ-scans)

measurements, an X-ray powder diffractometer URD6 (Seifert FPM, Germany) designed in

Bragg-Brentano geometry is used. All X-ray patterns are not background corrected.

Crystallite size t and microscopic strain s are calculated from the shape of the Bragg

reflections of the specular XRD scan according to the procedure described in reference 28

. The

topography of the thin films is investigated using an AFM Nanoscope IIIa (Digital

Instrument, now Veeco Instruments Inc.) in tapping mode at ambient conditions. Absorption

properties are determined from reflection and transmission measurements using a UV-3100

spectrometer (Shimadzu). The uniaxial anisotropic optical constants n (refractive index) and k

(absorption coefficient) of the samples are measured by VASE using an M2000 UI

ellipsometer (J.A.Woollam Co., Inc.), covering the wavelength range of 245 to 1680 nm at the

Leibniz-Institut für Polymerforschung Dresden e.V. (Germany). This method detects the

polarization change of the reflected light for different incident angles. In order to derive

precise results we use interference enhanced substrates, i.e. silicon wafer with an additional

970 nm thick film of SiO2.29

The quality of the fit is estimated by the uniqueness of the fitted

parameters and the mean squared error MSE according to reference.29

Detailed measurement

and fitting procedure are analogous to those described in reference.29

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Results

a) DIRECTION OF MOLECULAR TRANSITION DIPOLE MOMENT

The alignment of the transition dipole moment µ of organic absorber molecules towards the

electric field E of the incident photon is highly relevant to achieve high absorption. For a

planar wave (incident light), the electric field is aligned perpendicular to the propagation

direction k. For common solar cell applications, the case of illumination perpendicular to the

film surface is most relevant. To maximize the light absorption, the direction of the electric

field needs to be parallel to the transition dipole moment µ of the organic molecules as

depicted in figure 1b and e. For most planar molecules, µ is aligned within the molecular

plane. In the simple case of DIP, it is aligned parallel to the molecular long axis1 as illustrated

in figure 1a-c. Accordingly, organic films with upright standing (denotation σ-orientation)

DIP molecules only show very weak light absorption in case of perpendicular illumination

since µ is perpendicular to E (figure 1a). In contrast, for DIP molecules which are oriented

flat-lying (denotation λ-orientation) on the substrate, µ is aligned parallel to E and thus the

probability of light absorption is maximized (figure 1b). Figure 1c represents intermediate

absorption since the DIP molecules are tilted with respect to the substrate. Accordingly, for

improving the absorption and thus the photocurrent of organic solar cells comprising DIP, the

molecular orientation needs to be converted to a flat-lying structure.

In contrast to DIP, ZnPc exhibits two similar molecular long axes. Due to this symmetry type

the transition dipole moment is degenerated and thus can be oriented in two different

directions in the ZnPc plane30

as depicted in figure 1d-f. For upright standing ZnPc molecules

(tilt angle with respect to the substrate surface α=0°), µ can be aligned either perpendicular or

parallel to the electric field of the photon. In this case, the out-of-plane component of µ

(perpendicular to the substrate surface) equals the in-plane component (parallel to the

substrate surface) as depicted in figure 1d. Accordingly, films with upright standing ZnPc

molecules show sufficient absorption contrary to films with upright standing DIP molecules

(compare figure 1a and d). However, for flat-lying ZnPc molecules (α=90°) the out-of-plane

component of µ is zero (figure 1e). Hence, µ is aligned parallel to the substrate surface leading

to maximized thin film absorption. Thus, for both ZnPc and DIP a flat-lying molecular

orientation within the organic film is preferred to achieve maximum thin film absorption. In

figure 1f intermediate light absorption for tilted ZnPc molecules is shown. For a tilt angle of

α=55° the in-plane component of the extinction coefficient kin-plane is 2.5 times larger than the

out-of-plane component kout-of-plane.

Figure 1: Impact of molecular orientation on light absorption in case of a)-c) DIP and d)-f) ZnPc molecules. The

propagation direction k of the incident light is perpendicular to the surface while the electric field Exy and

magnetic field Bxy are within the surface plane. µ represents the transition dipole moment of the molecule. a), b),

and c) illustrate three different orientation of DIP (µ is parallel to the molecular long axis of DIP) in a thin film

with hypothetical crystalline structures. d), e), and f) depict different molecular orientations of ZnPc. Due to the

degeneracy of µ for ZnPc, µ can be oriented in two directions with respect to the molecular plane. kin-plane and

kout-of-plane represent the extinction coeffcient as e.g. derived from VASE for perpendicular and parallel

illumination, respectively.

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b) DEPOSITION ONTO WEAKLY INTERACTING SUBSTRATES

Upright standing ZnPc and DIP molecules can be achieved by thin film deposition onto

different kind of WIS, like SiO2, glass, ITO covered glass, amorphous hole transport films

BPAPF or BF-DPB, and polycrystalline C60 sublayers. Moreover, Tsub and the film thickness

are varied to obtain different degrees of crystallinity and crystallite size in the organic film.28

Figures 2a and 3a depict the absorption spectra of a DIP and a ZnPc film deposited on a glass

substrate. ZnPc or DIP films deposited onto MoO3, BPAPF, or BF-DPB films as well as onto

silicon wafers or SiO2 are found to be substrate independent in absorption (not shown here).

To prove that the topography and molecular arrangement of ZnPc and DIP films is the same

among all chosen WIS, AFM, specular XRD, and GIXRD measurements are performed.

Topography measurements AFM images of ZnPc and DIP films of varied thicknesses deposited onto different WIS and at

different Tsub (see figures 9 and 10 in the supplemental part) show the same results

independent of substrate choice. In general it is observed that the grain sizes of the DIP film

surface are typically larger than that of ZnPc. For ZnPc films, the lateral extension of the

grains is slightly increasing for larger film thicknesses starting at an average of 40 nm for a

thickness of 50 nm up to 60 nm for 200 nm thin ZnPc films. Substrate heating during ZnPc

deposition induces a change of the spherically shaped grains to a more worm-like structure as

discussed previously in Reference 28

. For DIP films, the topography is found to be strongly

substrate dependent. The differences are more pronounced for the deposition onto heated

substrates and vary from spherical over polygonal grains to terrace-like growth at elevated

Tsub.

Figure 2: DIP on weakly interacting substrates: a) Absorption spectrum of a DIP film evaporated on glass, b) n

and k values of a 100 nm thin DIP film deposited onto a SiO2 substrate at Tsub=100°C (derived by VASE

measurements, MSE=30) including the mean tilt angle of the DIP molecules (estimated by the anisotropy of k),

and c) GIXRD pattern of DIP films deposited onto different weakly interacting substrates and sublayers (Bragg

reflections are specified by the Miller indices hkl) and orientation of the DIP molecules in the obtained 00l

Bragg reflections.

X-ray diffraction measurements In addition, GIXRD measurements are performed to investigate if the weak interaction

between WIS and organic molecules leads to the expected upright standing orientation. As

depicted in figure 3c, only two Bragg reflections are obtained for all ZnPc films deposited on

WIS which belong to the 100/200 or 200/400 reflection of the triclinic/monoclinic ZnPc

phase.28

The fact that only h00 Bragg reflections are obtained in GIXRD shows that all ZnPc

molecules present in crystallites are oriented in the same manner, namely in upright standing

orientation. Figure 3c shows a scheme of this molecular alignment within the detected h00

crystallites. This molecular orientation is also found in literature for most phthalocyanine

films.31-33

In a previous study, we evaluated the degree of crystallinity and crystallite size of

ZnPc films on different WIS by the shape of Bragg reflections from specular XRD (θ-2θ-

scan) measurements.28

We found different degrees of crystallinity while the crystallite size (in

direction perpendicular to the substrate surface) remained constant in the range of the ZnPc

film thickness for all chosen WIS. ZnPc films deposited onto heated substrates exhibit a

higher degree of crystallinity. This indicates that at least for ZnPc film deposition on unheated

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substrates, not all ZnPc molecules are present in h00 crystallites but some fraction might

arrange in amorphous or liquid-crystalline domains.

For the GIXRD pattern of DIP films deposited onto different WIS (figure 2c), only 00l

reflections of the σ-phase are obtained.34

Thus, the DIP molecules in the detected 001

crystallites are arranged in a herringbone structure with nearly upright standing orientation as

depicted in figure 2c. Analogous to ZnPc, specular XRD scans are performed to gain

information about the lateral size of the crystallites and additional information about the

crystallinity of the films. The Bragg reflection intensity and thus the degree of crystallinity is

found to depend on both substrate type and Tsub (see figure 11 in the supplemental part).

Comparable to ZnPc films, the deposition of DIP onto heated substrates leads to a higher

degree of crystallinity. Thus, for unheated substrates a certain fraction of DIP molecules

might not be arranged in the 00l crystallites but form amorphous or liquid-crystalline

domains. The lateral crystallite size determined from the shape of the XRD Bragg reflections

is in the range of the film thickness for all DIP samples, similar to ZnPc (see table in figure 11

of the supplemental part).

VASE measurements From X-ray diffraction measurements, only the orientation of ZnPc and DIP molecules

arranged in crystallites can be evaluated. To obtain information about the mean tilt angle of

all molecules within the film, the evaluation of the anisotropic optical constants as determined

by VASE measurements is a suitable method.29

For this kind of investigation, the type of the

organic thin film order is unimportant, i.e. amorphous, liquid-crystalline, and crystalline

arrangements of the molecules are detected and treated equally. The wavelength dependent n

and k values of a 100 nm thin DIP and ZnPc film deposited onto SiO2 are depicted in figure

4b and 5b, respectively. Here, kin-plane represents the extinction coefficient for light incidence

with the electrical field vector in direction of the substrate plane as e.g. for light incidence

perpendicular to the substrate. This kind of illumination corresponds to the one applied for

common absorption measurements in transmission and reflection geometry as well as for

solar cell characterization. In contrast, kout-of-plane describes the extinction coefficient for light

with the electric field vector perpendicular to the substrate plane, thus for the p-polarized part

of the light propagating at highly tilted angle of incidence. For the case that kout-of-plane ≠kin-

plane, the film is called uniaxially anisotropic as found for ZnPc or DIP films deposited onto

SiO2. All films investigated in this work are found to be biaxial isotropic, i.e. rotating the

sample leads to equal ellipsometric results.

The uniaxial anisotropy of the DIP film deposited onto SiO2 substrate is very pronounced

since ∆n(λ=1200nm) = nin-plane - nout-of-plane = 1.70-2.14 =-0.44 is large. Furthermore, the

absorption for perpendicular light incidence kin-plane is much lower than the extinction

coefficient for parallel light incidence kout-of-plane (see figure 2 and table I). However, since this

film exhibits a high RMS-roughness (10 nm estimated by AFM), the fit model cannot

resemble the sample in an optimal way, which is also reflected in a rather high MSE value of

30. Accordingly, the n and k curves of the DIP films depicted in figure 2b should only be

considered estimate values to highlight the strong anisotropic growth of DIP on WIS. A mean

tilt angle of the DIP molecules of α=26° is approximated from kout-of-plane and kin-plane at

their maximum of 470 nm, 505 nm, and 550 nm. This implies that the DIP molecules are

nearly upright standing resulting in a low in-plane absorption since µ is aligned nearly

perpendicular to the electric field of the incident electromagnetic wave (see figure 1a). The

small molecular tilt angle of DIP derived from VASE is in agreement with the nearly upright

standing molecular orientation in the DIP crystallites found by GIXRD (figure 2c) and

confirms that most DIP molecules are present in 00l crystallites.

In comparison, the n and k spectra of the ZnPc film in figure 3b indicate that its optical

properties are less uniaxial anisotropic than the DIP film. Here, the maximum of kin-plane is

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smaller than kout-of-plane but much larger than for DIP: kin-plane = 0.72 at 610 nm for ZnPc while

kin-plane = 0.23 for DIP. Accordingly, the in-plane absorption of the ZnPc film is larger in

comparison to the DIP film which is confirmed by the absorption spectra in figure 2a and

figure 3a. This difference between ZnPc and DIP is caused by the degeneration of the

transition dipole moment in ZnPc. Hence, for upright standing ZnPc molecules µ can be

oriented parallel or perpendicular to the substrate surface (figure 1d) which implies sufficient

thin film absorption even for the case of upright standing molecules. For ZnPc deposition on

WIS, the mean tilt angle of the molecules is estimated to be in the range of 0° with respect to

the substrate normal (estimated from kin-plane and kout-of-plane at 610 nm). Accordingly, this tilt

angle corresponds to upright standing formation of ZnPc molecules within the film and

confirms the orientation of ZnPc in h00 crystallites found by GIXRD (see figure 3c).

Figure 3: ZnPc on weakly interacting substrates: a) Absorption spectrum of a ZnPc film evaporated on glass, b)

n and k values of a 100 nm thin ZnPc film deposited onto a SiO2 substrate at Tsub=30°C (derived from VASE

measurements, MSE=17) including the mean tilt angle of the ZnPc molecules (estimated by the anisotropy of k),

and c) GIXRD pattern of ZnPc films onto different weakly interacting substrates and sublayers (Bragg

reflections are specified by the Miller indices hkl) and orientation of the ZnPc molecules in the obtained h00

Bragg reflections of the triclinic or monoclinic ZnPc phase.

c) DEPOSITION ONTO STRONGLY INTERACTING METAL

SUBSTRATES

To change this undesired molecular orientation obtained for DIP and ZnPc film deposition on

WIS, different metal sublayers are predeposited on the WIS. In literature, Au substrates are

known to change the molecular orientation of DIP molecules to flat-lying.2,11,35

Accordingly,

those substrates are called strongly interacting substrates since the molecule-substrate

interaction is usually larger than the intermolecular interaction in order to cause the desired

flat-lying orientation. Here, Al, Ag, and Au sublayers deposited onto glass or SiO2 covered

silicon wafers are used.

DIP films on metal sublayers

GIXRD pattern as well as the anisotropic optical constants n and k of DIP deposited onto

metal sublayers are depicted in figure 4. Five new DIP Bragg reflections (hkl = 110, 111, 020,

120, 121) are obtained in the GIXRD pattern corresponding to the same β-phase34

as the 00l

reflections obtained for DIP on WIS (figure 2c). Thus, only the orientation of the DIP

molecules changes while the crystallographic phase of the crystallites remains the same. In

literature, the interpretation of the XRD results sometimes leads to confusion because new

orientations of molecules are denoted as “phases”.10,11,20,36-39

Hence, instead of σ- or λ-

orientation, the terminology σ- or λ-phase is used which is an incorrect notation from the

crystallographic point of view. In these articles the distinction between σ- or λ-orientation was

made only by the difference of the Bragg reflection position in the XRD pattern: small 2θ

values corresponded to σ- while large one belong to λ-orientation. A detailed discussion of

this interpretation can be found in the supplemental part. In this work, we are able to evaluate

the accurate molecular orientation of DIP because all obtained Bragg reflection can be

assigned to the known crystallographic β-phase.34

DIP molecules causing the five Bragg

reflections (110, 111, 020, 120, 121) in figure 4 are oriented in the desired flat-lying

orientation as depicted in figure 12 (see Supplementary).

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Comparing the GIXRD pattern of the DIP films deposited onto Al, Ag, and Au sublayers,

significant differences are apparent. For DIP on Al, pronounced Bragg reflections

corresponding to the σ-orientation and only weak reflections of the λ-orientation are obtained.

In contrast, for DIP on Au and Ag, no Bragg reflections of σ-oriented DIP molecules are

found. Instead, five intense Bragg reflections which belong to different crystallites with flat-

lying DIP molecules (λ-orientation) are obtained. Accordingly, the films on Ag and Au

exhibit no crystalline arrangement of upright standing DIP molecules while DIP on Al seems

to consist of mainly upright standing DIP molecules. Thus, the desired molecular orientation

to achieve high thin film absorption seems to be achieved using Au and Ag sublayers.

However, it is not clear why DIP films deposited on Al grow nearly similar to DIP on WIS.

One possible explanation might be given by an oxidation of the topmost Al layers due to the

residual partial pressure of oxygen within the vacuum system which might lead to a formation

of a thin amorphous aluminum oxide layer inducing similar growth as for DIP deposited on

SiO2. In contrast, Ag and Au are noble metals and thus do not exhibit a native oxide layer. A

drawback for application might be the rough topography of DIP films grown on Ag and Au

(see Figure 9 in supplemental part for AFM images). RMS-roughness of 20 nm and height

scale of 150 nm for a nominally 50 nm thin film might cause problems in solar cell

applications. Additionally, for such high RMS-roughness a precise determination of the

optical constants by VASE is not possible which is reflected by the high MSE of 24. Thus, the

estimated n and k spectra depicted in figure 4b can only be a rough approximation, especially

considering the out-of-plane component of absorption. However, in contrast to DIP deposited

on WIS, kin-plane is larger than kout-of-plane for DIP on Au (compare figure 2b). Hence, the

absorption of the DIP film for perpendicular illumination is significantly enhanced and the

mean tilt angle of the DIP molecules of α=62° (see inset in figure 4b) is much larger than for

DIP on WIS (α =26°). This demonstrates that the DIP molecules are oriented more tilted

towards the substrate although a complete flat-lying orientation of all molecules is not

achieved.

Figure 4: Morphology of DIP films deposited onto metal sublayers: a) GIXRD pattern of DIP deposited onto a

50 nm thin Ag, Al, or Au sublayer at Tsub=30°C (incident angle ω = 0.20°) and b) n and k values of a DIP film

deposited onto a 100 nm thin Au sublayer (derived from VASE, MSE=24) including an illustration of the

estimated mean tilt angle of the DIP molecules within the film. σ and λ mark Bragg reflections originating from

DIP molecules oriented perpendicular or parallel to the substrate surface, respectively.

ZnPc films on metal sublayers

Analogous morphological investigations are made for ZnPc films deposited onto the same

metallic sublayers. The results of the GIXRD pattern in figure 5a are comparable to the

observations made for the DIP films. While ZnPc deposited onto an Al sublayer exhibits only

Bragg reflections corresponding to the σ-orientation, ZnPc on Ag and Au sublayers exhibits

four Bragg reflections at larger 2θ angles belonging to the λ-orientation no matter if the

crystallographic phase is monoclinic or triclinic (see figure 12 of the supplemental part).

Accordingly, the growth mode of ZnPc and DIP molecules on Ag, Au, and Al are similar. A

major advantage of ZnPc in contrast to DIP is the relatively smooth surface topography when

being deposited onto Au or Ag (figure 10 in the supplemental part). Thus, the optical

constants derived from VASE are more reliable than for DIP and the fit of the ellipsometric

parameters of ZnPc results in a low MSE of 13. Comparing the extinction coefficients of

ZnPc on Au (figure 5b) and of ZnPc on SiO2 (figure 3b), strong changes in absorption are

obtained. For ZnPc on SiO2 the upright standing ZnPc molecules exhibit an in-plane

absorption kin-plane smaller than the out-of-plane component kout-of-plane.

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In contrast, for ZnPc on Au kin-plane is larger than kout-of-plane (see table I), indicating a change of

the molecular orientation to more flat-lying. More precisely, the mean tilt angle of the ZnPc

molecules changes from 0° in ZnPc films deposited on WIS (SiO2) to 54° for films

on an Au sublayer. Thus, the ZnPc molecules are more tilted towards the substrate surface.

However, flat-lying ZnPc molecules would exhibit a molecular tilt angle of α=90° which is

still not reached. The desired enhancement in absorption is reached because the maximum

absorption for perpendicular light incidence is nearly doubled for ZnPc on Au (kin-plane,max =

1.3) in comparison to ZnPc on SiO2 (kin-plane,max = 0.75).

Figure 5: Morphology of ZnPc films deposited onto different metal sublayers: a) GIXRD patterns of ZnPc

deposited onto a 50 nm thin Ag, Al, or Au film at Tsub=30°C (incident angle ω = 0.20°) and b) n and k spectrum

of ZnPc deposited onto a 100 nm thin Au film (derived from VASE, MSE=13) including an illustration of the

estimated mean molecular tilt angle of ZnPc. σ and λ denote Bragg reflections originating from ZnPc molecules

with upright standing and flat-lying orientation, respectively. The four Bragg reflections of the σ-orientation

between 2θ=23°...30° correspond to 0-11/111, -1-11/112, 0-12/113, -1-12/114 or -114 of the triclinic/

monoclinic ZnPc phase.

d) DEPOSITION ONTO PTCDA TEMPLATING LAYERS

Another opportunity to change the molecular orientation of organic molecules is given by

weak epitaxial growth.12,14,40-43

Here, we use a thin PTCDA film as a crystalline templating

layer. Subsequently, DIP and ZnPc films are deposited on top of this thin PTCDA sublayer.

Morphology of PTCDA templating layers

For this purpose, PTCDA films are themselves characterized first. The strong absorption of

the 50 nm thin PTCDA film in figure 6a is assumed to originate from its flat-lying molecular

orientation within the film. The GIXRD pattern in figure 6b exhibits two peaks at 2θ=24.8°

corresponding to flat-lying PTCDA molecules and 27.5° belonging to a slightly tilted but

nearly flat-lying orientation of the monoclinic β-PTCDA phase.44

Since AFM measurements

reveal a smooth PTCDA surface (RMS-roughness 2.3 nm), accurate VASE measurements of

the 100 nm thin PTCDA film measured by GIXRD are possible (figure 6c). The film is found

to be strongly uniaxial anisotropic indicated by the difference between kin-plane = 1.22 and kout-

of-plane = 0.16 (at 480 nm) and additionally by ∆n(λ = 1200nm) = nin-plane - n out-of-plane = +0.53

being even larger than for DIP on SiO2 (figure 2b: ∆n(λ = 1200nm) = 0.40). Since µ is

aligned parallel to the long axis of PTCDA (due to similar molecular structure of DIP and

PTCDA), the mean tilt angle of µ is equal to the mean tilt angle of the PTCDA molecules in

the thin film and estimated to be 75° (see inset in figure 6c). Hence, the PTCDA molecules

are nearly flat-lying over the whole thickness of 100 nm and thus fulfill the requirements for

using PTCDA as thin templating layer.

Figure 6: Morphology of PTCDA films deposited onto quartz or silicon substrate at Tsub=30°C: a) Absorption

measurement in transmission, b) GIXRD pattern (incident angle ω = 0.19°), and c) n and k values (derived from

VASE, MSE=15) including an illustration of the estimated mean tilt angle of the PTCDA molecules. The peak

of the GIXRD pattern at 2θ=24.8° corresponds to the 051 Bragg reflection (flat-lying PTCDA molecules) and

the second at 2θ=27.8° to the -102 Bragg reflection (nearly flat-lying orientation) of the monoclinic β-phase.44

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DIP films on PTCDA templating layers

Figure 7a-c provides detailed insight into the morphology of epitaxial grown DIP films

deposited onto thin PTCDA templating layers. At first, a 0.5 nm to 2 nm thin sublayer of

PTCDA is evaporated on a WIS, i.e. quartz, glass or ITO substrate. As shown in figure 7a, the

absorption of a 50 nm thin DIP film on 0.5 nm PTCDA shows an enhancement in absorption

in comparison to DIP deposited onto glass. This is a strong evidence of molecular

reorganization within the DIP film. GIXRD measurements (figure 7b) confirm this

assumption: For a 30 nm thin DIP film deposited onto 25 nm PTCDA, no σ-orientation of

DIP molecules is obtained. Instead, we find several Bragg reflections corresponding to the λ-

orientation as obtained for DIP films deposited on Au or Ag sublayers (compare to figure 4a).

Thus, the DIP film growth is strongly affected by the PTCDA sublayer and thus confirms the

working principle of weak epitaxial growth. Lunt et al.4 showed that DIP is able to grow in λ-

orientation even for very thin templating layers of 0.5 nm PTCDA on quartz glass substrates.

However, as depicted in figure 7b, a reproduction of this thin film stack results only in a

mixture of Bragg reflections corresponding to σ- and λ-orientation of DIP. The reason for this

different observation might originate in a non-closed PTCDA sublayer formation in our

samples. Furthermore, small differences in the film thickness can also explain the differences

because the deposition of an only 0.5 nm thin PCTDA sublayer is challenging and Lunt et al.

did not mention how they estimated their film thickness.

Additionally, the growth of a 100 nm thin DIP film onto a 2 nm thin PTCDA templating layer

is investigated to prove if the templating of PTCDA is a long range order effect. Again, only a

mixture of Bragg reflections for σ- and λ-orientation is obtained which might be caused by a

non closed PTCDA sublayer for such low thicknesses. For this film, VASE measurements are

performed. Due to the high film roughness of DIP grown on PTCDA (AFM image see figure

9 in the supplemental part), the quality of the ellipsometric parameter fit is rather poor

(MSE=47). Thus, the optical constants shown in figure 7c can only be estimates and are not

accurate. However, in first approximation n and k of DIP deposited on a PTCDA sublayer are

comparable to DIP on Au (compare figure 4): kin-plane is larger than kout-of-plane which is

contrary to the upright standing DIP on SiO2 (figure 2b) and the mean tilt angle of the DIP

molecules is estimated to be α=71° (see inset in figure 7c). Also the anisotropy of n (∆n(λ =

1200nm) = nin-plane - n out-of-plane = 1.85 – 1.60 = +0.25) of the film is comparable to DIP on Au

(∆n(λ = 1200nm) = 1.80 – 1.54 = +0.26).

ZnPc films on PTCDA templating layers

Finally, the templating effect of PTCDA is applied for ZnPc films as well. The GIXRD

pattern of ZnPc deposited onto a 2 nm thin PTCDA templating layer in figure 7e exhibit the

same four Bragg reflections originated by λ-oriented molecules as observed for ZnPc

deposited onto Au or Ag sublayers (see figure 5). Hence, weak epitaxial growth can be

applied for both ZnPc and DIP. However, a fraction of the ZnPc molecules remain upright

standing in the film since the 100/200 ZnPc Bragg reflection is obtained as well. Thus, a

mixture of upright standing and flat-lying molecules is detected. The enhancement in

absorption is shown in figure 7d. Note that different film thicknesses of ZnPc are used. Since

the ZnPc film on PTCDA grows rather smooth (AFM images in figure 10 of the supplemental

part) in comparison to DIP on PTCDA, reliable optical constants can be gained from VASE

measurements (MSE=21). The fitted n and k spectra for ZnPc deposited onto PTCDA in

figure 7f are comparable to ZnPc films grown on an Au sublayer (figure 5c). Accordingly, the

mean tilt angle of the ZnPc molecules of 55° (see inset in figure 7f) is similar to the one for

ZnPc on Au. However, the tilt angle for flat-lying ZnPc molecules of 90° is again not reached.

Figure 7: Morphology of a) - c) DIP and d) - f) ZnPc films deposited onto PTCDA templating layers: a) & d)

Absorption measurements in transmission (using an integrating sphere), b) & e) GIXRD pattern (incident angle

ω=0.20°), and c) & f) n and k spectrum (derived from VASE, MSE=47 for DIP and 21 for ZnPc) including an

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illustration of the estimated mean tilt angle of the molecules. In the X-ray pattern, Bragg reflection

corresponding to PTCDA, C60, and ITO are detected.

Sample GIXRD VASE

kin-plane, max kin-plane, max /

kout-of-plane, max

mean tilt

Angle α

ZnPc on WIS only σ 0.72 0.5 0±3°

ZnPc on Au only λ 1.29 2.3 54±2°

ZnPc on PTCDA σ and λ 1.04 2.6 55±2°

DIP on WIS only σ 0.14 0.1 26±7°

DIP on Au only λ 0.41 1.5 ≈ 62±9°

DIP on PTCDA σ and λ 0.32 1.8 ≈ 71±8°

Table I: Overview about the results of the molecular orientation dependency in ZnPc and DIP films on different

substrates/sublayers found by GIXRD and VASE measurements. λ stands for Bragg reflections corresponding to

flat-lying molecules and σ-orientation for upright standing (with respect to the substrate surface). kin-plane,max

represents the maximum in-plane (illumination perpendicular to the substrate surface) absorption (extinction

coefficient) of the film derived from VASE. The ratio of kin-plane,max to k-out-of-plane,max describesthe difference of in-

plane and out-of-plane absorption and thus the anisotropy of the film. The mean tilt angle α of all molecules in

the film is calculated by kin-plane,max and k-out-of-plane,max while the error of the tilt angles are estimated by the fit

quality.29

For α=0° all molecules are upright standing while in the case of α=90° all molecules would arrange

flat-lying.

Summary

We demonstrate how powerful the combination of the complementary spectroscopic methods

(VASE) and scattering methods (GIXRD) is to determine the molecular orientation in

crystallites and the whole organic film. Using the small molecules ZnPc and DIP we examine

that a simple distinction between flat-lying and upright standing molecules, often made in

literature, is too simplistic. In more detail, upright standing molecular orientation is found in

ZnPc films deposited on commonly used WIS and nearly upright standing molecular

orientation for DIP films on WIS (α ≈ 26°). Deposition of ZnPc and DIP on strongly

interacting Au and Ag metal substrates induces a more tilted molecular orientation of DIP (α

≈ 62°) and ZnPc (α =54°) films. Weak epitaxial growth of ZnPc and DIP films on thin

PTCDA templating layers facilitate similar growth and mean molecular orientation (see table

I). Thus, varying the substrate both GIXRD and VASE reveal a clear change of mean

molecular orientation causing strong differences in the in-plane (normal) absorption of the

thin films as summarized in table I. However, no completely flat-lying molecular orientation

is achieved in any organic film albeit this has often been stated for small molecule film

deposition on metal or PTCDA sublayers. Reasons for a maximum mean tilt angle of α ≈ 71°

(for DIP on PTCDA) might be given by a mixture of crystalline, amorphous or liquid-

crystalline regions. Liquid crystalline and amorphous region, probably exhibiting another

molecular orientation than crystalline regions, cannot be detected by GIXRD. In contrast,

VASE measurements provide an average molecular orientation of all these regions also

amorphous or liquid crystalline domains. In addition, we also observed a coexistence of

upright standing and flat-lying crystallites in ZnPc and DIP pristine films deposited on

PTCDA sublayers.

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Acknowledgments

This work is financially supported by the Bundesministerium für Bildung und Forschung

(BMBF) within the project 13N9872, and by the Free State of Saxony, and the European

Union via the European Regional Development Fund (ERDF) under SAB project number

71070. The authors also thank Reinhard Scholz (IAPP, TU Dresden, Germany) for discussion,

Stephan Hirschmann (University of Stuttgart, Germany) for supplying DIP material, and Dr.

Lutz Wilde (Fraunhofer CNT Dresden, Germany) for performing GIXRD measurements.

Supporting Information Available

In the supporting information misleading notations in the context of molecular orientations

used in literature, i.e. σ- or λ-orientation instead of σ- or λ-phase, are discussed.

Complementary to this discussion we visualized the detailed molecular orientation and

stacking of σ- or λ-oriented DIP and ZnPc molecules (all belonging to the β-phase) we

obtained from Bragg reflections of the thin films. For a better understanding the path of

rays for specular XRD and grazing incidence XRD (GIXRD) is illustrated as well.

Additional to the GIXRD pattern of DIP films on WIS in figure 2, we present specular

XRD pattern of such films. Finally, the influence of substrate choice on thin film

topography is pointed out by several AFM images of both DIP and ZnPc films. Especially

for DIP, the substrate type and deposition conditions have strong impact on thin film

growth. This information is available free of charge via the Internet at http://pubs.acs.org.

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Documents

Evaluation and Control of the Orientation of Small Molecules for Strongly Absorbing Organic Thin Films