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Thickness‐Dependent Energy‐Level Alignment at the Organic–Organic Interface Induced by Templated Gap States

Planar organic heterostructures are widely explored and employed in photovoltaic cells, light‐emitting diodes, and bilayer field‐effect transistors. An important role for device performance plays the energy level alignment at the inorganic–organic and organic–organic interfaces. In this work, incremental ultraviolet photoelectron spectroscopy measurements and real‐time X‐ray scattering experiments are used to thoroughly investigate the thickness‐dependent electronic and structural properties of a perfluoropentacene (PFP)‐on‐[6]phenacene heterostructure. For both materials an incremental increase of the material work function (positive interface dipole) is found. For [6]phenacene, this can be assigned to a thickness‐dependent change of molecular arrangement evident from a change of the unit cell volume and a consequential alteration of the ionization energy. In the case of PFP the interface dipole stems from charge transfer from the substrate into unoccupied molecular orbitals resulting in an electrostatic potential on the surface. The magnitude of this potential can be correlated with an increased gap state density resulting from templated structural defects mediated by the bottom layer.


Introduction
The energy level alignment (ELA) at organic/organic interfaces is a key aspect affecting the performance of various organic

Structural Characterization
First, we discuss the structure of the different heterostructures, since this information is vital for the evaluation of the electronic properties in the next section. Figure 1a shows the real-time XRR data of the heterostructures where the bottom [6]phenacene (thickness 0 to 18 nm) layer was grown at a substrate temperature of T = i) 250, ii) 300, and iii) 350 K. After the growth of [6]phenacene was finished the system was given time to equilibrate to room temperature before the growth of the top PFP layer (thickness 18-36 nm) was started, so that all PFP films were deposited at the same substrate temperature on the [6]phenacene layer.
Evident from the more pronounced Kiessig oscillations (q z ≈ 0.05-0.15 Å −1 ) in (ii) and (iii) we can conclude that increasing the substrate temperature leads to overall smoother [6]phenacene films. Interestingly, with deposition of PFP on top of the low temperature sample Kiessig oscillations appear, meaning the overall roughness is decreasing and, after 18 nm, does not differ compared to the higher temperatures (see also Figure S1, Supporting Information). This behavior can be explained with PFP molecules diffusing down the [6]phenacene terraces where they accumulate and fill the space between grains. [12] Another feature are Laue fringes originating from the coherently scattering crystal size along the z-direction surrounding the Bragg reflection. Their appearance correlates with the length scale of crystalline coherence and indicates larger coherently scattering domains for higher temperatures.
Analyzing the Bragg peak position for the different thicknesses, we find a shift toward higher q z -values with higher film thickness for the bottom [6]phenacene layer. Converted into a real-space value, this is equivalent to the lattice spacing decreasing, meaning the molecular tilt angle Θ relative to the substrate is changing. A schematic representation can be found in Supporting Information. This appears to be a generally thickness-dependent effect which was further investigated by an electron density fit for the sample grown at room temperature ( Figure 1c and Supporting Information). We find the lattice spacing of the two first layers to be significantly larger (18.7 and 16.7 Å) than for the subsequent layers, for which the spacing stays constant. The same behavior is found also for the samples grown at higher and lower temperature, however, the final outof-plane lattice spacing is different (see Table 1). Since PFP also grows in a standing up configuration and due to its almost identical long axis length compared to [6]phenacene molecules, the observed Bragg peak does not visibly deviate with the start of PFP deposition, but rather stays at a constant q z position corresponding to a lattice spacing of ≈15.8 Å and implying that the top layer PFP out-of-plane structure is not thickness dependent. Figure 1b shows GIXD data of [6]phen/PFP heterostructures. All three heterostructures exhibit several in-plane Bragg reflections corresponding to nearly standing molecules and the short axis (ab-plane) of the unit cell parallel to the substrate plane. For [6]phenacene, the Bragg reflection positions after deposition of 18 nm are moderately temperature dependent, leading to the unit cell parameters summarized in Table 1 and showing good agreement with a recently published crystal structure. [13] Figure 1d (bottom) shows the thickness dependence of V UC taking also the out-of-plane data into account. We find an overall decrease of V UC between 6 and 12 nm and a weak decrease between 12 and 18 nm. V UC , shown in Table 1, differs only slightly between the different substrate temperatures for the films at the final thickness of 18 nm.
From the peak width of the in-plane reflections, the in-plane coherently scattering island size (D coh ) of the three films is estimated. For T = 350 K, D coh is increasing from 91.2 to 124.5 Å between 3 and 18 nm. The 300 and 250 K samples show only a negligible thickness dependence but depend on the temperature with D coh being 96.5 (300 K) and 75.9 Å (250 K) (Figure 1e, bottom). Since D coh depends on defects and strain which reduce the long-range crystalline order in the thin films, we can conclude that increasing the temperature leads to higher crystallinity. Interestingly, also PFP exhibits an increased crystallinity when the [6]phenacene layer beneath was grown at high temperature, an templating effect also found in PFP grown on diindenoperylene layers. [8a] We find D coh for PFP to be 160.9, 182.3, and 197.5 Å for the 250, 300, and 350 K sample, respectively ( Figure 1e, top). The Bragg reflection position is not affected by the bottom layer and thickness, as evident from Figure 1d, top), where the unit cell volume is plotted versus the thickness (see also Table 1).

UPS Data [6]Phenacene
In order to obtain a thorough understanding of the electronic properties of the entire heterostructure we next discuss the ELA of [6]phenacene on the SiO 2 -substrate. Figure 2 shows the UPS data for the different substrate temperatures during growth dependent on film thickness.
However, we find the ionization energy (IE) of [6]phenacene to be thickness dependent. The IE is determined by adding the respective binding energy to the work function (WF), measured from the SECO onset (also indicated by short vertical lines and plotted in Figure 3, top). As can be seen from Figure 3, the value of the IE is mainly governed by the thickness dependence of the WF. It was shown that the precise molecular orientation in ordered molecular assemblies can have a strong impact on the electrostatic surface potential and is influencing the WF and therefore the IE. [10c] In the present case the thickness dependence of the IE can be assigned to the structural change [6]phenacene is undergoing with increasing film thickness (see Figure 1d). This argument is further strengthened when comparing the respective trends for unit cell volume and WF. In both cases the change is most significant for film thicknesses between 3 and 9 nm and comparatively constant for film thicknesses above. The change of IE ranges from 106 meV for the low and room temperature sample to 91 meV for the high temperature sample respectively. Since the substrate Fermi energy (E F ) lies approximately in the middle of the energy gap given by the [6]phenacene HOMO onset (E Bind ≈ 1.5 eV below E F ) and the estimated LUMO onset position (≈1.6 eV above E F , obtained by adding the optical band gap of [6]phenacene (≈3.1 eV) to E F ), any effects of gap states on the ELA at the [6]phenacene/SiO 2 interface are unlikely and vacuum level alignment is expected. Since the gap states density is known to reach only a few hundred meV into the gap, there is a too large energy difference to allow for efficient electron transfer between substrate and organic material. [4a,4b]  First, we note a temperature dependent initial interface dipole (ID, i.e., difference between WF values) between [6]phenacene and the first 3 nm thick layer of PFP, with values of +100 meV for the low temperature and +30 meV for the higher temperatures. The PFP shows a thickness dependent shift of the WF (Figure 3b, top). However, in comparison to [6]phenacene the trend appears to be more linear with a comparably constant interface dipole when adding an additional material. Furthermore, the observed ID shows a temperature dependence with the total ID between 3 and 18 nm being 137 meV for PFP grown on the low temperature [6]phenacene and 115 and 110 meV for the 300 and 350 K samples respectively. Examining E Bind more closely, we find that, contrary to [6]phenacene, the binding energy does not stay constant, but is decreasing with increasing thickness (Figure 5, bottom).  In addition, this behavior is also temperature dependent with E Bind changing from 2.54 to 2.40 eV (Δ = 140 meV) for low temperature, from 2.57 to 2.46 eV (Δ = 110 meV) for room temperature and from 2.60 to 2.50 eV (Δ = 100 meV) for the high temperature sample. Calculating the IE for PFP we find a rigid shift for E Bind and WF and consequentially, the IE stays constant at an average value of 6.83 ± 0.03 eV for all thicknesses and temperatures. This result is further confirmed by the X-ray data, where we found a thickness independent V UC for PFP and would therefore not expect the IE to change.

UPS Data PFP on [6]Phenacene
The ELA of PFP on [6]phenacene can be rationalized considering structural parameters. As we have seen in Figure 1e, top), D coh of PFP shows a template effect when grown on top of [6] phenacene, with a higher crystal quality for elevated substrate temperatures. As was previously shown, an increase in the density of structural disorder is directly correlated with an increase in the gap state density. So far, due to experimental restrictions, only a direct measurement of HOMO gap states is possible. The direct detection of gap states reaching out from the LUMO level, which is principally accessible with inverse photoelectron spectroscopy, is still elusive. However, the impact of an altered LUMO gap state density on the ELA can be studied using a conventional UPS setup. This is apparent when adding PFP's band gap of 2.5 eV to the observed E Bind . [15] One finds that the E F of the substrate lies well within the PFP LUMO distribution, with therefore accessible "empty" electronic states. To establish the thermodynamic equilibrium, electrons are transferred from the SiO 2 into those states in the PFP layer and change the measured WF accordingly. The amount of transferred charges and the absolute value of the ID is now directly dependent on the number of accessible states beneath the substrate E F (i.e., the density of gap states) which, in turn, can be controlled by changing the crystallinity of the sample. [4e,16] Applying this model to the data presented here, we find qualitative agreement between the density of defects and the observed ID.

Conclusion
We presented a detailed investigation of the thickness-and temperature-dependent structural and electronic properties of a [6] phen/PFP heterostructure grown on SiO 2 , using in situ X-ray scattering and ultralow background UPS. We find the molecular arrangement of the bottom [6]phenacene layer to be strongly depending on the film thickness, but only slightly impacted by a different growth temperature, additionally, the crystallinity is enhanced for elevated temperatures. A fact that is adopted by the PFP film grown on top via a templating effect. The unit cell parameters of PFP are not changing with increasing thickness and do not show a dependence on the bottom layer's substrate temperature during growth.
Two key findings arise regarding the electronic properties of the heterostructures, which are summarized in Figure 6. We find the ionization energy of [6]phenacene to be thickness dependent and correlated with the structural rearrangement the molecules undergo with continuing growth, while no HOMO shift is observed. In contrast, the ELA of PFP depends on the density of gap states that can be attributed to structural imperfections apparent from different sizes of coherently scattering domains. An increased density of gap states due to an increase of structural defects enables a larger number of electrons to transfer from the substrate to unoccupied gap states lying beneath the substrate E F . Electronically, the bottom [6] phenacene layer thereby simply acts as charge transport layer, however, due to the structural templating effect is ultimately responsible for the magnitude of the level shift of PFP. Our findings show that the spatial distribution of gap states is a key factor affecting the ELA at organic-organic interfaces as well as interfaces between an inert inorganic and an organic material. Moreover, the thickness dependence of the energy shift indicates that an accurate control of the molecular arrangement in thin organic semiconductor films may represent another key factor for controlling the efficiency and functionality of different organic devices.

Experimental Section
The X-ray experiments were conducted at beamline I07 of the Diamond Light Source in Didcot, UK using a Pilatus 100K detector and a monochromatic X-ray beam at 13 keV (wavelength 0.95385 Å). Slits directly in front of the detector were used to mimic a point detector where needed. An incidence angle of α i = 0.1° was chosen for the grazing incidence X-ray diffraction measurements. PFP (99.9% purity) was purchased from Kanto Denka Kogyo Co, [6]phenacene was bought from NARD Co Ltd. (Japan, purity 99.9%). For the UPS measurements, the samples were prepared within the UPS set-up and measurement was conducted without breaking the vacuum (p < 1 × 10 −8 mbar). Before installation, the silicon substrates (SiO 2 ) with a native oxide layer of 2.0 nm were cleaned in an ultrasonic bath with acetone, 2-propanol, and demineralized water successively. Before each sample preparation the substrates were heated to over 700 K overnight to desorb residues. The deposition rates were kept at 2 Å min −1 , which was monitored using a water-cooled quartz crystal microbalance during growth. The molecules were evaporated from thermally shielded Knudsen cells. The SiO 2 substrates were mounted on a molybdenum sample holder with options of resistive heating and cooling by liquid nitrogen as required. The substrate temperature was monitored using a thermocouple attached to the sample holder in close proximity to the substrates. UPS was highly surface sensitive with a penetration depth of only a few nanometers, which meant that only the top few molecular layers contributed to the measured data. The substrate WF was similar (variation only 10 meV) for the different samples and measured to be 4.07 eV for the low temperature and 4.07 and 4.06 eV for the higher temperature samples. Due to experimental restrictions the UPS measurements cannot be conducted in real-time during growth. The data was therefore measured after a new 3 nm thick layer was grown, but without breaking the vacuum. Before each UPS measurement the temperature was equilibrated to room temperature to rule out temperature effects on the UPS data. [14] After the measurement and before starting growth of the next 3 nm layer, the sample was again cooled down or heated up to the desired temperature. The UPS measurements were performed using an ultralow-background, high-sensitivity UPS apparatus with a hemispherical electron analyzer (MBS A-1) and a monochromatic HeI (energy: 21.2 eV) radiation source. All UPS spectra were measured at normal emission with an acceptance angle of ±18°. The He light was incident at 45° with respect to the sample surface. A bias of −5 V was applied to the sample in order to detect the secondary cutoff. The binding energy scale is referred to the Fermi level measured on a metal substrate. Ionization energy values were obtained as IE = E HOMO + Φ = E HOMO + hν -E cutoff , where Φ is the work function of the sample, defined by the energy separation of the vacuum level from the Fermi level. E HOMO is the HOMO edge position and E cutoff is the cut-off position.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.