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. 2023 Dec 4;15(49):57427–57433. doi: 10.1021/acsami.3c12914

Impact of Intermittent Deposition on Spontaneous Orientation Polarization of Organic Amorphous Films Revealed by Rotary Kelvin Probe

Masahiro Ohara †,*, Hokuto Hamada †, Noritaka Matsuura †, Yuya Tanaka ‡, Hisao Ishii †,§,∥,*
PMCID: PMC10901167  PMID: 38047501

Abstract

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The control of the molecular orientation and resultant polarization is essential for improving the performance of organic optoelectronic devices. Conventionally, the substrate temperature and deposition rate are tuned to control the molecular orientation of vapor-deposited films. In this study, we proposed a novel method, referred to as “intermittent deposition”, in which the polarization direction and magnitude are controlled by introducing intervals during physical vapor deposition. The rotary Kelvin probe measurement of the Alq3 and TPBi films clearly showed a time-dependent decrease in the surface potential owing to the surface relaxation of the molecular orientation immediately after deposition. Through a series of intermittent depositions, in which the deposition shutter is repeatedly opened and closed at certain intervals, a relaxed surface layer was built up, and we could control the polarization magnitude. For the Alq3 film, even the polarization direction was switched. The proposed new deposition method is applicable to general organic molecules, not limited to polar molecules, thereby potentially tuning the conduction properties of organic devices and fabricating novel devices.

Keywords: spontaneous orientation polarization, organic light-emitting diode, physical vapor deposition, molecular orientation, surface relaxation, rotary Kelvin probe

1. Introduction

Organic optoelectronic devices, including organic light-emitting diodes (OLEDs), typically employ asymmetrically structured molecules to form pinhole-free amorphous thin films with a uniform thickness. Controlling the molecular orientation is an essential technique for enhancing the device performance, such as carrier injection,1,2 transport,3,4 and light extraction efficiency.5−8 To control the molecular orientation of vacuum-vapor-deposited films, the substrate temperature and deposition rate are often adjusted.9

Here, let us consider the processes in physical vapor deposition in relation to the deposition parameters. Physical vapor deposition involves several elementary processes: (1) molecular adsorption or reflection at the surface, (2) molecular diffusion on a substrate, and (3) molecular collision to be desorped or aggregation to form a film. Both the deposition rate and substrate temperature are considered in adjusting the diffusion time of the molecules on the substrate surface. For instance, reducing the deposition rate yields sufficient time for the adsorbed molecules on the substrate to rearrange themselves, thereby achieving a more stable energy state before being covered by subsequently incoming molecules. Similarly, an increase in substrate temperature leads to an increase in the molecular diffusion rate, with more chances for rearrangement. However, the relaxation time scale that can be controlled by these parameters is limited to very short time scales. Considering the time for forming a monolayer, it is only a few seconds at most.

In contrast, a long relaxation time (several days or more) allows organic amorphous films to fully relax and even crystallize.10,11 Thus, we can expect that different orientations are achieved by utilizing molecular relaxation on an intermediate time scale (∼100 s). Specifically, the repeated cycles of opening and closing the deposition shutter at an intermediate interval are expected to be a new technique to modify the relaxation time and molecular orientation.

Molecules related to the use of OLEDs, which have attracted considerable attention in recent years, often have permanent dipole moments (PDMs) owing to their asymmetric structure. When these molecules are subjected to vacuum vapor deposition, they are often slightly oriented with a head and tail distinction, resulting in film polarization with a giant surface potential (GSP). This phenomenon is referred to as spontaneous orientation polarization (SOP) and is often reported for various OLED-related materials.12−17 As polarization induces interfacial charges that affect the charge injection and accumulation behavior in the device,13,18,19 several attempts have been focused on controlling the polarity and magnitude of SOP in addition to the molecular orientation itself.20,21 However, the detailed mechanism of such an anisotropic orientation is yet to be fully understood. In addition, the polarity of the SOP is not symmetric; most materials exhibit positive GSP, limiting the polarization control. To fully control the device performance, it is highly desired to control both the polarity and the magnitude in the vacuum vapor deposition process.

In this study, we show that an interval during deposition is applied for novel molecular orientation control. For the real-time measurement of the surface potential during and immediately after deposition, a rotary Kelvin probe (RKP) has been developed, and the impact of the deposition interval on the molecular orientation of tris(8-hydroxyquinolinato)aluminum (Alq3) and 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) films was investigated. As a consequence, we revealed that the outermost surface layer exhibited orientation relaxation in several tens of seconds after closing the deposition shutter. By repeating intermittent deposition, we could modify the magnitude of polarization of the Alq3 and TPBi films. For Alq3 film, furthermore, we succeeded in switching the polarization direction, which created a potential valley in single-component films. This new technique enables arbitrary modification of the potential profile in the film to tune the conduction properties of the existing device and to achieve special device properties.

2. Rotary Kelvin Probe

The concept of RKP was first reported in the 1950s22 and modified in the 1970s.23 In the system, a lamp and a photodiode were used to synchronize a lock-in-amplifier with a rotating motion. Because an environmental light can decrease the GSP due to the cancellation by photocarriers,12 previous RKP systems cannot be applied in our study. Further, existing systems do not pay attention to the shape of the rotary electrode determining the thickness uniformity over the deposited film. Thus, we have developed a novel RKP to overcome the aforementioned issues, while simultaneously enabling vacuum deposition and surface potential measurement.24

Figure 1 shows the block diagram of our apparatus. The rotary electrode is connected to a stepper motor (Oriental Motor, PK523HPVA) that could rotate at 1200 rpm. The reference electrode, which is positioned approximately 0.5 mm below the sample substrate, functions as a variable capacitor with the sample substrate. Both electrodes have a fan-shaped design that ensures constant exposure time of the sample substrate in the radial direction. The rotary electrode is electrically connected via a carbon brush at the center of rotation to reduce the charging effect due to friction and maintain stable measurements. This system incorporates a vacuum-compatible stepper motor that integrates the entire unit into a vacuum chamber. The unit is then covered with a metal cover to protect the core unit from contamination due to sample deposition. The details of the system will be described elsewhere.25

Figure 1.

Figure 1

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Schematic diagram of the developed apparatus for simultaneous vacuum deposition and surface potential measurement: (a) main unit of the RKP with a rotary electrode and its surroundings; (b) measurement circuit; and (c) vacuum deposition system with a crucible and computer-controlled shutter.

3. Results and Discussion

3.1. Effect of Surface Relaxation on the Molecular Orientation

Let us start by showing the results measured by a conventional vibration KP. Figure 2a shows the observed surface potential Vsp of Alq3 films as a function of the thickness. For the film deposited at 0.2 Å/s (blue squares), the increase in the surface potential is proportional to the thickness, resulting in a GSP slope of 10.7 mV/nm. This linearity indicates that the positive and negative polarization charges of 0.30 mC/m2 are formed at the surface and bottom of the film, respectively. This situation resembles the potential profile in a capacitor with a constant electric field due to the positive and negative charges that result in a voltage drop proportional to the thickness. As the deposition rate is increased up to 4.0 Å/s (red diamonds), similar proportionality is observed, and the slope was increased to 36.9 mV/nm. Therefore, as the deposition rate increases, the surface potential difference increases. This tendency is consistent with the results in the literature.21,26

Figure 2.

Figure 2

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Surface potential of vacuum-deposited Alq3 films measured by conventional and rotary KPs: (a) thickness dependence of the surface potential of Alq3 films deposited at 0.2–4.0 Å/s measured by the vibration KP. (Inset) Details of the thin region. (b) Relationship between the incremental deposition step and GSP slope for each deposition step. (c) Time-dependent variations of the surface potential (blue) and film thickness (orange) of the Alq3 film measured by the RKP system. (d,e) The expansions of the results immediately after the start (d) and the end (e) of the deposition.

If we look closely at the thin thickness region as shown in the inset of Figure 2a, the Vsp at the first few points with the thickness of less than 20 nm does not show significant change proportional to the thickness but almost no variation. At first glance, this result suggests the extremely low orientation polarization for this thin-thickness region; the first few layers on the substrate have lower order, and orientation order may be formed by further deposition. Such tendency has often been reported in previous studies.12,26 Here, considering the relaxation time for the deposited molecules, it should be noted that the incremental thickness among the data points in the thin region (<20 nm) is extremely small, and there exists at least a couple of minutes interval between adjacent measurements. Sample deposition cannot be performed during the KP measurement using a vibration KP because the vibrating electrode covers the sample surface. Thus, the sample should be transferred between measurement and deposition chambers, leading to the surface relaxation under a long interval time.

The slope of Vsp and thickness step Δd for each incremental deposition are plotted in Figure 2b. For Δd of >30 nm, a constant slope is observed. However, the slope rapidly decreases with decreasing Δd, which can be attributed to the change in the molecular orientation during a long relaxation time on the order of minutes. Assuming that the molecular orientation of the surface layer is relaxed to reduce the polarization, the small deposition step induced a significant reduction in the surface potential, whereas the larger deposition step has minimal impact because of the smaller surface/bulk ratio. Therefore, a smaller deposition step can induce a smaller Vsp slope at Δd of <30 nm. For the very small deposition step (Δd < 10 nm), a negative Vsp slope is observed, indicating the polarity switching of GSP.27

To precisely examine such a Δd-dependent effect, the real-time measurement of Vsp during deposition is necessary. To achieve this, RKP can measure the GSP without deposition interruptions. Figure 2c displays the surface potential (blue) and film thickness (orange) of the Alq3 film measured by using our RKP system as a function of time. In contrast to the results of the conventional KP, the surface potential can be observed as an almost continuous curve. In particular, the surface potential increases with increasing film thickness, confirming the GSP formation in the film. The potential gradient per unit film thickness was 51.5 mV/nm. After deposition, the cancellation of the potential by light irradiation was also observed, confirming changes in the surface potential during film deposition due to GSP. This cancellation is because the film’s negative and positive polarization charge can be compensated by the photoinduced holes and electrons, respectively.12

Focusing on the thin-film region shown in Figure 2d, the surface potential increases in proportion to the film thickness from the very thin region, in contrast to the results in Figure 2a. This indicates the molecules are anisotropically oriented to the same degree as in the bulk for the thin films with thickness of 10 nm or less, demonstrating the advantage of our RKP system. Zooming in on the potential change immediately after closing the shutter in Figure 2e, we measured a slight decay in potential. Although the relaxation process cannot be modeled in detail, the decay was fitted with an exponential function as a first-order approximation. As a result, the time constant was determined to be 78 s.

Next, the change in Vsp during the deposition interruption was measured to investigate possible surface relaxation as a function of time. Figure 3a shows the change in the surface potential during the intermittent deposition of Alq3 at 1 Å/s. The thickness changed with the step function, as shown by the orange curve. The opening (for 10 s) and closing (for 180 s) of the deposition shutter were repeated; the flat region in the orange line corresponds to “shutter closed”. In the deposition phase, the Vsp showed approximately a 40 mV step increase, while the Vsp exhibited ∼10 mV decay during the interruption before the next deposition. Here, the film is formed at a thickness of 10 Å at a time, and the 40 mV increase is in good agreement with the GSP slope in the bulk.

Figure 3.

Figure 3

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Time variation of the surface potential during intermittent deposition of Alq3. (a) Results for the deposition at 1 Å/s. (b) Relationship between the potential decrease during the interval and the deposition step at 1 Å/s. (c) Surface potential change of the film deposited at 4 Å/s, indicating the switching Vsp polarity.

We investigated the dependence of potential decay ΔVsp on deposition thickness Δd and found it to be about 10 mV, which is almost independent of Δd, as shown in Figure 3b. The experiment here was performed on a single sample. That is, the deposition steps were increased sequentially from the smallest to the largest, which means ΔVsp is also independent of the total thickness.

The situation can be interpreted as follows. When an incremental layer is deposited, the whole part of the layer is oriented as a bulk region. Subsequently, the outermost surface layer starts to relax with a decrease of 10 mV immediately after the shutter closes. Assuming that the thickness of the monolayer is roughly 1 nm and the bulk GSP is 40 mV/nm, the relaxation ratio is equivalent to approximately 25% of the monolayer. In reality, it is possible that the surface layer is partially antiparallel configurated, or it is also possible that the overall orientation of the surface layer is reduced by about 25%. At this time, the actual relaxation structure will not be determined without the use of other structural analysis methods. Anyhow, these results demonstrate the existence of surface relaxation at the very surface. This is direct evidence for the previous suggestion on the formation of a layered structure on the surface with a thickness of approximately a single molecular layer.28,29

This relaxation phenomenon is important in discussing the carrier injection properties of OLEDs. A schematic illustration and a schematic energy level diagram of the Alq3 layer with the top and bottom electrode are shown in Figure 4a,b. As shown in Figure 2d, a Vsp slope is generated even with one monolayer thickness, suggesting the polarization charge is located very close to the organic on bottom electrode interface. On the other hand, at the top electrode on organic interface, the outermost surface layer of the organic layer loses its polarization before depositing the overlayer electrode, suggesting that the interface can be regarded as a polarized organic layer/unpolarized (or less polarized) layer/electrode interface. In the former interface, the distance between the polarization charge at the organic layer and the electrode is almost zero, leading to no potential drop across the interface. In the latter interface, the finite distance between them can induce a nonzero potential drop, possibly assisting the carrier injection.

Figure 4.

Figure 4

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Schematic illustration (a) and energy level diagram (b) of the Alq3 layer with the top and bottom electrode.

The observed feature of the Vsp relaxation is highly dependent on the deposition rate. Figure 3c shows the time variations of the surface potential and film thickness during intermittent deposition at 4 Å/s. As in the case of 1 Å/s, a positive potential shift is observed immediately after the first to third depositions (1400–1800 s), but the direction of the potential shift is reversed after the fourth and fifth depositions (after 1800 s). This polarity reversal of the GSP “growth” with the thickness change suggests a reversal of the head-to-tail direction of the Alq3 molecule (with respect to the PDM direction) during the relaxation time. As of this moment, it is difficult to identify the detailed mechanism of this inversion phenomenon, but it is possible to envision a situation in which the orientation change is propagated by intermittent deposition. Creating one layer of film at different deposition rates, such as 1 and 4 Å/s, causes the film to relax at different configurations. The adsorption and diffusion behaviors of the molecules on top of the film will also change. These repetitions gradually changed the orientation of the film. These details would need to be studied in combination with simulation method.30,31

These data indicate that the interruption time during deposition can control the polarity as well as the magnitude of the GSP. In the experiment using vibrational KP, the GSP was inverted even at a deposition rate of 1 Å/s. However, in the experiment using the RKP system, the GSP was inverted only under the condition of a deposition rate of 4 Å/s. This may be ascribed to the different experimental conditions.

In addition, we performed an investigation for TPBi, which is a typical electron-transport material in OLEDs. Figure 5 shows the correlation between GSP slope and deposition steps measured by the vibration KP. As in the case of Alq3, the GSP slope increases with increasing Δd. The difference from the Alq3 case is that the GSP slope did not show significantly negative when the deposition step was reduced, and the GSP slope was slightly negative around Δd = 5 nm. This difference in trend may be attributed to the shape of the molecule. Alq3 is a bulky molecule with a ball shape, whereas TPBi is a disk-shaped molecule. Such a difference in the aspect ratio of the molecular geometry may have led to a difference in the surface diffusion behavior in TPBi due to the greater interaction at the surface. The GSP decay was also measured in real-time by our RKP [Figure 5(inset)]. These results confirmed the effect of intermittent deposition on the GSP decay, which suggests that polarization control by intermittent deposition should be applicable to EL materials in general.

Figure 5.

Figure 5

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Relationship between the incremental deposition step and GSP slope of TPBi for each deposition step measured by the vibration KP. (Inset) Time variation of surface potential during intermittent deposition of TPBi measured by our RKP.

3.2. Demonstration of the Formation of Potential Valley

Now, we can switch the GSP polarity; it is possible to construct an arbitrary potential profile, including positive and negative potential slopes, within a single Alq3 layer. Thus, we attempted to demonstrate the formation of a “V”-shaped potential as follows. Figure 6 shows the Vsp variations during the combination of intermittent and continuous deposition sequences. As shown in Figure 6a, the intermittent deposition induced the decrease of Vsp, and then the Vsp was switched to increase after changing to continuous deposition. The Vsp change was plotted as a function of the thickness in Figure 6b. The intermittent and continuous parts give GSP slopes of −12 and 16 mV/nm, respectively. The potential profile demonstrates the successful formation of the V-shaped potential in a single Alq3 film. If we look closely at the point of the deposition mode switch, the thickness to start the positive GSP growth was delayed by approximately 5 nm from the thickness where the deposition mode was changed, suggesting the presence of an unpolarized transition layer in which the polarization is almost changed from negative to positive. In this film, positive polarization charge exists close to the Alq3/indium tin oxide (ITO) interface, whereas the counter negative charge is located near the Alq3/Alq3 switching interface. After the stack of the unpolarized Alq3 layer, the negative charge and positive charges are located at the unpolarized/polarized Alq3 interface and film surface, respectively. Except for such transition regions, we can control the polarity and magnitude of the Alq3 polarization and generate a potential profile as we need. Such an interface can be regarded as a “homojunction with potential hill or valley”, which can function as a carrier accumulator and repeller and so on, leading to achieving novel electric properties. This intermittent deposition technique can be applied to other OLED materials, for further improvement of the OLEDs can be expected. In addition, this intermittent deposition technique can be applied to nonpolar organic molecules for various organic semiconductors for organic photovoltaic cells, transistors, and so forth.

Figure 6.

Figure 6

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Formation of potential valleys in the Alq3 film by combining intermittent and continuous deposition. (a) Surface potential and thickness as a function of time. (b) Surface potential as a function of the thickness.

4. Conclusions

In this study, the relaxation processes of the molecular orientation on the surface of organic semiconductor amorphous films were investigated to understand the mechanism of SOP. A novel RKP system was used to simultaneously measure the surface potential during and after deposition, thereby allowing a detailed analysis of the surface potential changes over time. The obtained results showed that the molecules on the outermost surface layer rearranged themselves to a more electrostatically stable orientation within a few hundred seconds. As the GSP varies as a function of time, control of the deposition interval is essential. We have demonstrated the control of the polarization of the Alq3 and TPBi film by changing not only the deposition rate but also the deposition interval. For the Alq3 film, furthermore, even the orientation direction was inverted, and the polarization was switched. Overall, intermittent deposition is expected to be applicable even for nonpolar molecules as an effective method for controlling molecular orientation.

5. Methods

5.1. Material

Alq3 (device grade) was generously provided by Nippon Steel & Sumikin Chemical Co., Ltd., and used without further purification. TPBi (sublimed, purity <99.5%) was purchased from Luminescence Technology Corp.

5.2. Surface Potential Measurements by Vibration KP

Surface potential measurements were performed using a vibration KP (KP technology, UHVKP020) in a high vacuum system with a base pressure of P of <4 × 10–4 Pa. The Alq3 and TPBi films were incrementally formed on the ITO substrate at the deposition rate range of 0.2–4.0 Å/s, and the formed film was directly transferred from the evaporation chamber to the measurement chamber without exposure to air. Surface potential measurements were conducted at each deposition step.

5.3. Surface Potential Measurements by RKP

Alq3 was deposited on an ITO substrate and the surface potential was measured in real-time using an RKP. First, as a reference sample, an 800 Å thick Alq3 film was continuously deposited on an ITO substrate at a deposition rate of 1 Å/s. Next, Alq3 was deposited on the ITO substrate in steps of 10 Å by using a deposition shutter with a relaxation time of 180 s, and then 160 Å thick of Alq3 was deposited continuously on the substrate. The same experiment was conducted at different deposition rates of 1 and 4 Å/s. For TPBi, a 10 Å step deposition was performed on the ITO substrate.

5.4. Film Formation with Built-In Potential Valley by Intermittent Deposition

A film with built-in potential “valleys” in the film was fabricated by depositing 10 Å of Alq3 on the ITO substrate, repeating the process of relaxing the film for about 100 s several dozen times, and finally depositing the film continuously for about 400 Å.

Prior to each experiment, the ITO substrates were ultrasonically cleaned twice with acetone and 2-propanol without UV ozone treatment. The pressure in the vacuum chamber P was <4 × 10–4 Pa. The film thickness was calibrated from the data measured by a quartz crystal microbalancer using a stylus step meter (Kosaka Laboratory ET-4000A).

Acknowledgments

We thank Nippon Steel Chemical & Material Co. Ltd. for providing the Alq3 molecules. This research was supported by a Japan Society for the Promotion of Science (JSPS) KAKENHI Grant-in-Aid for Scientific Research (grant nos. 20H02810 and 21K05208) and Grant-in-Aid for JSPS Fellows (grant no. 22J21883). M.O. would also like to thank the Frontier Science Program for Graduate Students of Chiba University for its financial support.

The authors declare no competing financial interest.

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