Organic blends containing heterojunction structures at the interfacial phase have been applied extensively in organic optoelectronic devices to modify charge transfer, separation, and recombination processes. Scaling and controlling the transition domains at the hetero-interface are of crucial importance for deep insights into the involved physics and for architecturing the devices with improved performance. However, it is difficult to recognize and characterize these transition domains directly using the conventional microscopic techniques, in particular when different molecules are dissolved in the same solvent with equal solubility. In this work, we introduce a technique defined as laser-induced selective cross-linking to isolate the interfacial phase from other phases into a directly measurable practicity. Thus, the hetero-domains become visualized and directly measurable. Based on the insolubility of the selectively cross-linked molecules in organic solvents, a lift-off process may remove the uncross-linked or incompletely cross-linked molecules, so that the hetero-domain is more clearly visualized and more precisely measured. A transition domain in a scale of about 200 nm is resolved in the F8BT/PFB blend film between their respectively rich phases after the selective cross-linking of the F8BT molecules by a blue laser. Furthermore, hetero-crosslinking between F8BT and PFB molecules was also resolved by both microscopic and near-field spectroscopic investigations.
© 2013 Optical Society of America
Heterojunction structures have been extensively investigated and applied in organic photovoltaic [1–5] and light-emitting diodes or light-emitting transistors [6–8]. Charge separation or recombination may be enhanced due to the different electrical properties of the organic semiconductors that form the interfacial junctions [9,10]. Polymer blends containing bulk or bilayer heterojunction structures may enable highly efficient exciplex [11,12] or electroplex emissions . It is of crucial importance to understand the nature and manipulate the behaviors of the charge carriers at the heterointerface for the development of devices with high performance. A variety of methods have been demonstrated to characterize the electronic properties of organic interfaces. Schottky-gated heterostructures have been used to probe the conducting layer at the rubrene/PDIF-CN2 single crystal interface . Charge-transfer properties were investigated at the pentacene/PCBM interface using a field effect transistor device . Near-field probing [16,17] or ultrafast spectroscopy  have been employed to investigate the generation and transport properties of the charge carriers at the heterointerface.
The heterostructures that are located at the interface between different molecular phases actually have an effective range or width, where different molecules coexist or are entangled with each other to form an interpenetrating network of the donor and acceptor materials . This characterizes both the morphology of the active layer  and the charge transport properties, consequently determining dominantly the performance of the device. In this work, we demonstrate that the heterointerfacial domain becomes “visualized” using laser-induced cross-linking process, as illustrated in Fig. 1, so that this special charge transport region may be measured precisely. The interpenetrating region is modified differently in morphology from the basically homogeneous phases by the selective cross-linking and shows up as an isolated phase, which is measurable using conventional microscopic methods. Poly(9, 9’-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9’-dioctylfluorene-co- bis-N,N’-(4-butylphenyl)-bis-N,N'-phenyl-l,4-phenylenediamine) (PFB) have been utilized to construct the bulk heterojunction structures for demonstrating the effectiveness of this visualization technique.
2. Preparation of the blend film with hetero-interfacial phases
F8BT and PFB were dissolved in xylene separately with a concentration of 15 mg/ml before they were mixed with a volume-to-volume ratio of 1:1 to prepare the blend solution with a weight-to-weight ratio of 1:1. As specified by the manufacturer, PFB has a molecular weight of about 103000 and F8BT has an average molecular weight in the range from 10000 to 20000. The thin film of F8BT/PFB bulk heterojunction structures with a thickness of about 100 nm is produced by spin-coating the blend solution at a speed of 2000 rpm for 30 seconds onto a fused-silica substrate with an area of 10 × 10 mm2 and a thickness of 1 mm. The fabrication has been performed at a temperature of 25 °C and a humidity of 42%.
The phase-separation scheme between F8BT and PFB can be clearly resolved by the atomic force microscopic (AFM) image in Fig. 2(a), where the F8BT-rich phase has a mean size of about 700 nm in diameter and a maximum height of about 29 nm above the baseline. The absorption spectra of pure F8BT, pure PFB, and the blend film have been extensively investigated , where F8BT and PFB have absorption peaks at about 457 nm and 382 nm, respectively. Different phase-separation schemes and related physics in the F8BT/PFB blend film have been described in detail in [21,22].
3. Laser-induced selective cross-linking into the polymer blend films
Exposing the blend film to a steady-state laser beam at 457 nm with an average power of about 100 mW and a diameter of about 3 mm at FWHM actually excites the F8BT molecules selectively. This kind of laser radiation leads to the cross-linking between the F8BT molecular chains, which takes place not only for the exposure to the UV light as usually observed [23,24], but also for that to lasers within the absorption band of the molecules . It has been shown that the cross-linking process exhibits high spatial resolution. This has been verified experimentally by the so-called “interference cross-linking”, which was introduced using UV laser irradiation in our previous publication  and has been demonstrated for the successful nano-patterning into polymer films. It is understandable that exposing the F8BT/PFB blend film to the blue laser beam at 457 nm will induce cross-linking only between the F8BT molecules, instead of between the PFB molecules due to their extremely low absorption at 457 nm. As has also been investigated , the cross-linking between the F8BT molecules leads to reduction in both the thickness and refractive index of the film during the formation of a three-dimensional network. Meanwhile, the polymer film tends to become transparent and non-luminescent.
Figure 1 illustrates schematically how the F8BT- and PFB-rich phases, as well as the hetero-interfacial phase between them, evolve with the blend film exposed to the blue laser beam at 457 nm. Due to the strong interaction with the blue laser beam, the F8BT molecules experience strong cross-linking process within the F8BT-rich phase, the interfacial domain, and even within the PFB-rich phase for a small portion. This leads to a gradient of the morphological change across the F8BT-to-PFB phase because of the reduction in the F8BT concentration from the F8BT-rich through the interfacial to the PFB-rich phase. As a result, the morphology or the thickness changes most dramatically at the center of the F8BT-rich phase, whereas, almost no change can be produced within the PFB-rich phase. Consequently, a “saddle” scheme forms on either side of the center F8BT-rich phase, as is illustrated by the lower panel of Fig. 1. If seen from above or as will be shown in the AFM images in the following sections, a “dark” ring forms to separate a “bright” ring phase from the center F8BT-rich phase. The PFB-rich phase actually has played the role of a skeleton for the resultant outer “ring phase”. The modeling in Fig. 1 not only proposes the existence of a transition domain between F8BT- and PFB-rich phases, but also introduces a method to visualize the heterointerfacial domain, which shows up as a scalable or measurable practicality. The dashed white lines in Fig. 1 are actually used to indicate a gradient of the reduction in the F8BT concentration in the transition domain from the F8BT- to the PFB-rich phases.
4. Visualization and scaling of the hetero-interfacial domain
4.1 Formation and evolution of the interfacial ring-phase with the exposure time
A series of laser-induced selective cross-linking experiments were performed on the blend film at different exposure times. Figures 2(a)-2(e) show the atomic force microscopic images of the studied area with an exposure time of 0, 20, 40, 60, and 90 minutes, respectively. Before exposed to the blue laser beam, the F8BT-rich phase separated from the PFB-rich phase has a mean diameter of about 770 nm and a maximum height of about 36 nm, as shown in Fig. 2(a). With increasing the exposure time, a ring occurs on the outer shell of the F8BT-rich phase, which becomes clearer with increasing the exposure time. For an exposure time of 20 minutes, as shown in Fig. 2(b), the center phase area is reduced to less than 500 nm in diameter and the maximum height of the F8BT phase is reduced to smaller than 31 nm. Although the ring already forms, it is still connect to the center phase area. When the exposure time is increased to 40 minutes, both the area and the height of the center phase are reduced further to less than 420 nm in diameter and less than 26 nm in height, respectively, as shown in Fig. 2(c), so that the ring is already separated from the center phase and a dark circle is observed in between. After an exposure time of 60 minutes, the ring becomes isolated from the main body of the F8BT-rich phase and it can be measured in both the width and height, as shown in Fig. 2(d). The center area has now a diameter of about 390 nm and a height less than 22 nm. Actually, the cross-linking process becomes nearly saturated at a 60-minute exposure time so that the ring features, as well as the size and height of the F8BT-rich phase stay almost unchanged when the exposure time is increased further from 60 to 90 minutes, as shown in Fig. 2(e). Figure 2(f) shows the variation of the mean diameter (D) of the F8BT-rich phase (the central area) with the exposure time (T). If looking at the schematic illustration in Fig. 1, we can understand a reduction in the size of the F8BT rich-phase domain. The experimental results in Fig. 2(f) verify such a reduction, although very rough measurements on the diameters of the center rich-phase domain have been performed. These measurements have been done on the AFM images of the selectively cross-linked blend films before the lift-off process. For each exposure time, an area of 5 × 5 μm2 was selected, which contains 30-40 “grains” of the F8BT rich-phase domains. For the case before exposure, as shown in Fig. 2(a), the diameter was determined simply by the identified edge of the “grains”. For the exposed samples, the diameter of each “grain” in the center of the rich-phase domain was measured using the identified boundary of the inner edge of the dark ring, as shown in the inset of Fig. 2(f). Then, an average value was calculated for each sample with different exposure times (0, 20, 40, 60, and 90 minutes) by dividing the sum of all measured diameters by the number of measured “grains”, respectively. Due to the contrast and clarity of the AFM images, this is only a rough evaluation on the “grain” sizes at different exposure times. Nevertheless, this measurement result reflects consistently the variation of the central F8BT rich phase during the selective cross-linking process. A more precise evaluation by plotting the profile of the cross-section is demonstrated in Fig. 3 in section 4.2. Furthermore, the possibility to perform liftoff on the cross-linked network introduces a more convincing method to determine the scale of the interfacial domain, as will be described in section 4.3.
4.2 Scaling the heterointerfacial domain
For scaling the hetero-interfacial domain, the ring-phase structures are measured using atomic force microscopy, as shown in Figs. 3(a) and 3(c), where Fig. 3(c) is an enlarged AFM image using a perspective view. An exposure time of 60 minutes was employed in the selective cross-linking process for the fabrication of the structures in Figs. 3(a) and 3(c). A plot of the cross-section at the phase-separation site marked by the green bar is shown in the panel ❶ of Fig. 3(e), where the position of the dark ring between the F8BT-rich and the interfacial phases is indicated by the downward arrows. The F8BT-rich phase is as high as 28 nm at the peak position, whereas, the interfacial domain shows a height lower than 13 nm. The studied phase in Fig. 3(c) is totally about 1.5 μm in diameter with the central F8BT-rich phase take about 900 nm. It is easy to measure the radial width of the interfacial phase, which ranges from 200 to 300 nm if scaled by the blue arrow-bars. Thus, the interfacial domain extends in region of about 300 nm according to Fig. 3(c). It should be noted that a big F8BT-rich phase domain has been chosen, therefore, both the diameter and the height of this phase domain are larger than the average values given in section 4.1.
4.3 Lift-off for high-contrast visualization and precise measurement of the cross-linked heterointerfacial domain
Although the ring phase is clearly resolved in Figs. 3(a) and 3(c), the contrast of the height modulation at the interface is still low for the precise evaluation on the scale of the heterointerfacial domain. One of the most important features of cross-linked molecules is that they become insoluble in their good solvents. Thus, the cross-linked F8BT molecules become insoluble in chloroform, implying a chance for liftoff to show the cross-linked network with high contrasts. In such a liftoff process, the blend films were immersed in chloroform for about 10 minutes before they were cleaned by flushing with fresh chloroform and dried by blowing with compressed air. This was also done in a chemical ventilating hood with a temperature of 25°C and a humidity of 42%.
Figures 3(b) and 3(d) show the AFM images of the selectively cross-linked blend film after the liftoff process, where an enlarged AFM images with perspective view is shown in Fig. 3(d). The ring phase is revealed much more clearly with the contrast enhanced by the dark ring between the interfacial and the F8BT-rich phases, which results from the removal of the PFB and the incompletely cross-linked F8BT molecules.
The panels ❷ and ❸ in Fig. 3(e) show the profiles of the cross-section on two typical sites marked out by green bars in Fig. 3(d). The F8BT-rich phase is reduced significantly in its height and becomes strongly undulated due to the removal of the uncrosslinked PFB and incompletely cross-linked F8BT molecules underneath the sufficiently cross-linked shell, as compared with the measurement results in Figs. 3(a) and 3(c). The reduction in the height of the F8BT-rich phase results in the high contrast and much better visualization of the interfacial domain.
The interfacial phase has a mean width of about 200 nm and a height of 14 to 23 nm if measured using the blue-arrow bars in panels ❷ and ❸ in Fig. 3(e). The width value is obviously smaller than that obtained from Fig. 3(c), whereas, the height is even larger. Therefore, the liftoff process enhanced contrast of the nano-ring structures and enables better visualization or more precise measurements on the heterointerfacial domain.
In principle, only the cross-linked F8BT molecules are left on the substrate after the liftoff process. However, within the interfacial phase the F8BT and PFB molecular chains may be entangled with each other, so that the cross-linked F8BT molecular chains may hold the PFB molecules. This can also be understood by considering that the cross-linked F8BT molecules set nodes to the PFB molecular chains, which influences the formation and evolution of excitons and charges on/along the PFB molecular chains, consequently, leading to the degradation of the PFB emission. This can be verified by the spectroscopic investigation shown in Fig. 4, where the PL spectra have been measured using a very weak UV exciting laser at 404 nm after the blend film was illuminated by an intensive blue laser at 457 nm for 0, 3 and 6 minutes, respectively. The UV laser beam is smaller in diameter than the blue one on their overlap and is so weak that its excitation doesn’t cause PFB emission to degrade. As shown in Fig. 4, the intensive illumination by the blue laser makes the F8BT molecules cross-linked so that both the exciplex emission at about 630 nm  and the F8BT emission centered at about 540 nm  degrade quickly to about 20% and 70% the original intensity (for 0-minute illumination), respectively, implying more sensitive response of exciplex to laser radiation. The inset of Fig. 4 shows the enlarged the PL spectra in the range from 412 to 490 nm to show the evolution dynamics of the PFB emission centered at 450 nm.
It is understandable that the cross-linking and the degradation of the F8BT molecules should have reduced the energy transfer from PFB to F8BT after the photo-excitation, so that the PFB molecules become “liberated” and their emission should have recovered to become stronger with the illumination time by the intensive blue laser at 457 nm. However, as shown by the inset of Fig. 4, the PFB emission is also reduced with the crosslinking of F8BT molecules, as indicated by the green arrow. The black arrow in the inset of Fig. 4 shows clearly that the reduction in the PFB PL spectrum is not due to the degradation of the main PL spectrum composed of F8BT and exciplex emission. Therefore, some cross-linking can be taken as the inter-crosslinking between F8BT and PFB molecules. Furthermore, this mechanism favors the formation of the strongly cross-linked phase at the heterointerface. This kind of hetero-crosslinking process is more convincingly and more directly verified by the near-field spectroscopic characterization in section 5, where the blue-shifted PFB emission is observed in the lift-off blend film, implying reduced intermolecular interactions due to crosslinking.
Additionally, the F8BT molecules located deeper in the blend film was exposed less to the blue laser beam due to the strong absorption by molecules on the outer shell, implying possibly incomplete cross-linking. This portion of the molecules may be dissolved into chloroform in the liftoff process, leading to the inhomogeneous sinking of the cross-linking shell in the F8BT-rich phase. However, in the interfacial and the PFB-rich phases, the F8BT molecules may be more completely cross-linked due to the transparency of PFB molecules to the blue laser at 457 nm. This also favors the formation of “harder” interfaces. Thus, the lift-off process enables a much clearer visualization of the interfacial phase, which actually verifies further the existence and characterizes the scale of the heterointerfacial domain.
5. Near-field spectroscopic mapping of the crosslinking network
Figure 5 shows the near-field spectroscopic mapping of the emission from the crosslinking network between F8BT and PFB using a comparison between the schemes before (a) and after (b) the crosslinking process, and after the liftoff process (c), where the pure blue color corresponds to the PL spectrum of F8BT and the pure red to that of PFB molecules. A confocal microscope (Alpha 300S) from WITec GmbH has been employed to perform the spectroscopic mapping using an excitation laser at 355 nm. The spectroscopic mapping image in Figs. 5(a) and 5(b) basically agrees with the microscopic images in Fig. 2.
Before the laser-induced selective cross-linking process, high contrast may be observed in the phase-separation scheme in Fig. 5(a). The right panel of Fig. 5(a) gives the PL spectra at 3 typical sites: the F8BT-rich phase (A), PFB-rich phase (B), and the intermediate position between these two phases (C), which have been normalized to the peak of the F8BT PL spectrum. Clearly, the F8BT emission dominates the PL spectra and energy transfer from PFB to F8BT molecules results in the weak emission from PFB, where the F8BT and PFB emission is peaked at about 538 and 450 nm, respectively. After the crosslinking process, the F8BT emission is strongly quenched so that the contrast of the mapping image in Fig. 5(b) is reduced significantly. The PL spectra on three typical sites are shown on the right panel of Fig. 5(b), which are located within the F8BT-rich (D), the PFB-rich (E), and the interfacial (F) phases. The PFB emission now dominates the PL spectrum of the selectively cross-linked blend film, which is peaked at about 445 nm, indicating sufficient crosslinking of F8BT molecules. However, as the liftoff process has removed the uncross-linked PFB molecules and the incompletely cross-linked F8BT molecules, the F8BT emission dominates the PL spectrum again with the emission from PFB 40% as intensive as that from F8BT, as shown in Fig. 5(c). This verifies convincingly the co-existing of PFB and F8BT in the remaining cross-linking network. The PL spectra on three typical sites within F8BT-rich (G), PFB-rich (H), and the interfacial (I) phases show the blue shift of the PFB emission from 445 to 425 nm and F8BT emission from 538 to 525 nm.
The interfacial cross-linked nanorings were not resolved either in Fig. 5(b) or in Fig. 5(c). This is because of the 200-nm resolution limit of the confocal microscope and the sub-200-nm nanoring width. In particular, the liftoff process caused the F8BT-rich phase to become undulated due to the removal of the uncross-linked PFB and the incompletely cross-linked F8BT molecules underneath the sufficiently cross-linked top shell, where the undulation scale is also in the order of 200-300 nm. Therefore, strongly disordered mapping image is shown in Fig. 5(c).
Thus, the spectroscopic mapping data in Fig. 5 verify that the direct crosslinking takes place between F8BT molecules due to the selectively strong excitation by the blue laser at 457 nm. However, the PFB molecules still remain in the cross-linked network even after the liftoff process with blue-shift PL spectrum, implying insolubility of the remaining PFB molecules in chloroform with reduced intermolecular interactions. Therefore, the near-field spectroscopic results in Fig. 5 supplied direct evidence that the strong laser radiation has not only cross-linked the F8BT molecules, but also induced equivalent cross-linking between F8BT and PFB molecules. The PFB-involved crosslinking of the F8BT molecules has actually reduced both the PFB-PFB and F8BT-F8BT interactions, leading to the blue-shift of both the PFB and F8BT PL spectra. This extends the concept and the physical mechanisms of the laser-induced selective cross-linking process.
In summary, the heterointerfacial domain is visualized through laser-induced selective cross-linking in the F8BT/PFB blend film, which consequently verifies a transition region within the heterojunction structures. Thus, the dimension of this region can be determined directly using conventional microscopic techniques. The heterointerfacial phase in the PFB/F8BT blend film is measured to be about 200 nm in its full width. This method applies generally to thin film structures of organic semiconductor blends and enables precise probe of the electronic properties and the involved mechanisms in devices based on heterojunctions.
The authors acknowledge the 973 Program (2013CB922404) and the National Natural Science Foundation of China (11074018, 11274031) for the financial support.
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