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Cite this: DOI: 10.1039/c3nr05027k
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Self-organization and nanostructural control in thin film heterojunctions† Sebastiano Cataldo,a Camillo Sartorio,a Filippo Giannazzo,b Antonino Scandurrac and Bruno Pignataro*a In spite of more than two-decades of studies of molecular self-assembly, the achievement of low cost, easyto-implement and multi-parameter bottom-up approaches to address the supramolecular morphology in three-dimensional (3D) systems is still missing. In the particular case of molecular thin films, the 3D nanoscale morphology and function are crucial for both fundamental and applied research. Here we show how it is possible to tune the 3D film structure (domain size, branching, etc.) of thin film heterojunctions with nanoscale accuracy together with the modulation of their optoelectronic properties by employing an easy two-step approach. At first we prepared multi-planar heterojunctions with a programmed sequence of nanoscopic layers. In a second step, thermal stimuli have been employed to induce the formation of bulk heterojunctions with bicontinuous and interdigitated phases having a size below the exciton diffusion length. Importantly, the study of luminescence quenching of these systems
Received 19th September 2013 Accepted 18th November 2013
can be considered as a useful means for the accurate estimation of the exciton diffusion length of semiconductors in nanoscale blends. Finally, nearly a thousand times lower material consumption than
DOI: 10.1039/c3nr05027k
spin coating allows a drastic reduction of material wasting and a low-cost implementation, besides the
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considerable possibility of preparing thin film blends also by employing materials soluble in different solvents.
1. Introduction How can bulk nanostructured thin lms be precisely engineered in three dimensions (3D) with methods accessible by a traditional chemical laboratory? In spite of more than twodecades of studies dealing with the control of molecular selfassembly at bidimensional surfaces,1–7 different emerging applications need to control the 3D bulk morphology and functions of molecular thin lms on the nanoscale.8 In particular, the study of bi-component thin lms continues to introduce and develop innovative material properties with a wide range of applicability, driving research toward the exploration of several scientic and technological applications including emerging elds like optoelectronics,9 nanolithography,10 organic electronics,11,12 surface coating, membranes,13 diagnostics and so on.14–17 In this context, key properties are connected to the peculiar structure of the supramolecular phases, hence thin lm nano-engineering plays a fundamental role
a
Dipartimento di Fisica e Chimica, Universit` a degli Studi di Palermo, V.le delle Scienze, Ed.17 – 90100 Palermo, Italy. E-mail: [email protected]
b c
CNR-IMM, Strada VIII, 5 - 95121 Catania, Italy
CCR-SuperLab c/o STMicroelectronics, Strada VIII, 5 - 95121 Catania, Italy
† Electronic supplementary information (ESI) available: Experimental methods about realization of MHJs; surface free energy measurements, AFM, C-AFM, XPS and optical spectrophotometry additional discussion and gure. See DOI: 10.1039/c3nr05027k
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since the control of the structural order can greatly enhance the device properties.8,9,16,18–22 For instance, in order to achieve efficient charge transfer in organic and hybrid photovoltaics, the donor (D) and acceptor (A) materials need to be distributed inside the thin lms in vertical bicontinuous domains with size of the order of the exciton diffusion length (EDL; 3–20 nm)23–26 allowing for efficient exciton separation and balanced charge migration to the electrodes.21,27–29 Furthermore, phase-separated ordered blends of ferroelectric and semiconducting polymers showed nonvolatile two-level memory functionality30,31 and the control of self-organization in supramolecular ordered domains allows enhancement of charge mobility and eld effect performance in molecular thin lms and ambipolar transistors.32,33 Finally, nanoscale 3D order in molecular thin lms is of great interest in several other elds such as sensors, OLEDs, displays, advanced spectroscopies and the realization of calibration standards.8 Different bottom-up and top-down low cost solution approaches have been reported to control the nanoscale thin lm structure8 including the surface directed spinodal decomposition,34,35 entropic connement,36 phase-direction agents,37 use of block-copolymers,38 additive lithography,39,40 nano-imprinting lithography,41 polymer molecular weight control,9 blending ratio modulation42 and side chain functionalization.43 Nevertheless, the above methods need a couple of materials soluble in the same solvent, detrimental chemical functionalizations, undesirable additives and/or may lead to uncontrolled phase separation.
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Here we develop an out-of-equilibrium approach for the 3D structural control in bi-component organic thin lms (i.e. two semiconductive molecular systems), which allows control of the nanostructural features (dimension, vertical continuity, density, and branching) and simultaneously enabling their physical property modulation.
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2.
Results and discussion
2.1. From multilayer- to bulk-heterojunctions by a multiparameter approach The basic idea of our 3D nanopatterning strategy consists of a two-step process: (1) The preparation of a multilayer planar heterojunction (MHJ) made of alternating semiconductive D and A nanoscopic thin layers with a programmed architectural sequence (Scheme 1). (2) The promotion of interlayer-diffusion by external stimuli like thermal heating (Scheme 2). Accordingly, MHJs have been prepared by the Langmuir– Schaefer technique (LS) allowing precise control of the thickness of each deposited layer along with the deposition sequence. By exploiting such a strategy we deposited d layers of the D, followed by a layers of the A material and iterated the d n process for an n number of cycles nally obtaining a a thin lm architecture, as shown in Scheme 1. Next, MHJs have been thermally treated to induce the layer interdiffusion (Scheme 2). Our idea is based on the concept that by starting from a multilayered architecture with a periodic sequence (Scheme 2a), the system may evolve to form vertically bi-continuous complex nanostructures through non-equilibrium pathways arising from the phase separation of the materials inside the organic MHJ superstructure (Scheme 2b and c). Finally, the process is expected to lead to the thermodynamically favoured steady state. In the peculiar case of a system consisting of a solidsupported thin lm of two immiscible components, if the surface tension of the solid substrate is larger than that of the two lm components, segregation of a bilayer equilibrium
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structure, as that shown in Scheme 2d as an example, with the lowest surface free energy component at the top may be expected.7,44,45 In the case of partial miscibility, the nal thermodynamic shape would be importantly a result of the trade-off between the lowering of surface free energy, favouring segregation, and bulk free energy minimization driven by increasing mixing entropy and/or enthalpy variations.46–48 Thus, by a multi-parameter approach including optimising a suitable energy pulse, i.e. by changing temperature (thermalmotion/aggregation-speed) and/or annealing time, and varying the heterojunction architecture, i.e. by changing the thickness and sequence of layers, it becomes possible to tune the evolution dynamics, even by using multiple sequential steps, and stop the process at a certain point freezing the system in a desired out-of-equilibrium state (see Scheme 2c) like that with vertical bi-continuous phases having a functional dimension, density and interface area. Of course, the supramolecular organization of the monomolecular layer at the air–water interface can also inuence the morphological evolution of the thin lm and as a further parameter this will be a subject of future studies, for instance by modulating the lm pressure in the LS through. Instability phenomena at the propagation front of 2D layers spreading at solid surfaces have been already exploited to realize ordered superstructures made of sub-micrometric stripes and channels with controlled periodicity and lateral size.49 With respect to the 2D case, 3D thin lms exhibit one more degree of freedom that is crucially expected to increase bifurcations. Importantly, unlike other methods, this methodology employs very low quantities (tenths of milligrams for tens of square centimetres of the deposited lm) of materials that can be soluble in the same or even in different solvents, since each layer is deposited from the air–water interface, independently.
2.2. 3D functional modulation of donor/acceptor molecular thin lms In order to show the application of the above approach we used two photoactive couples (Fig. S1†) which already gave phase separation in thin lms:50,51
Schematic representation of the thin film MHJ preparation method. (a) Langmuir–Schaefer technique is used to transfer d layers of D (orange sticks) then a layers of A (blue spheres) from the air–water interface to the ITO/PET substrate; these alternate depositions may be iterated d n times to get the desired architecture; (b) examples of MHJs with different n architectures obtained by changing n, a and d. a
Scheme 1
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Scheme 2 Schematic representation of the annealing-induced interdiffusion process for a MHJ consisting of two immiscible components. The system would evolve from a multi-planar heterojunction (a) to a bulk heterojunction (c) by means of oscillating waves (b); a prolonged annealing would lead to an equilibrium state (d) characterized by the complete separation of the two components.
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(1) poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM); (2) poly(9,90 -dioctyluorene-co-benzothiadiazole) (F8BT) and the above mentioned P3HT. MHJ superlattices similar to those sketched in Scheme 1b with different architectures and controlled thickness have been prepared. Sequentially deposited layers retain their own structure and lie steadily at ambient temperature one on top of the other (see Fig. S4 and S5†). As shown in Fig. S2,† the overall lm thickness depends linearly on the MHJ layers’ number, every single layer contributing by a value of 6–7 nm for both D and A. Accordingly, MHJs of 12–16 layers are about 70–120 nm in thickness. Fig. 1 shows the AFM images of the same region of a 2 P3HT:PCBM 4 MHJ, showing the effects of the thermal 2 treatment. By annealing the sample for 10 minutes at 130 C, the morphology evolves through a loss of PCBM globular features which still appear surrounded by smoother regions (Fig. 1b). Also, the bright contrast features present in the restricted phase-lag image (Fig. 1e) indicate that the top surface at this stage consists of domains of the two different components due to the diffusion of P3HT at the topmost surface (lowest surface energy component; gP3HT ¼ 17.5 mJ m2, see ESI†) and as an opposite process, the sinking of PCBM (gPCBM ¼ 38 mJ m2)44 inside the lm. Note that plasma treated ITO has a gITO larger than both the components (about 65 mJ m2)52 and thus a thermodynamic equilibrium planar heterojunction (see Scheme 2d) with P3HT on the topmost and PCBM at the ITO interface is expected. In fact, by annealing the sample at 130 C, a morphological smoothing is observed, with the root mean square roughness decreasing from 20 nm (untreated sample, Fig. 1a) to 12 nm (10 min annealing, Fig. 1b), down to 8 nm (20 min, Fig. 1c). Moreover, the phase-lag image of the longest annealed sample clearly shows bright contrasting P3HT bers (see the zoomed area in Fig. 1f), which largely enrich the top surface. At this stage, the phase separation results in a high percentage of dark PCBM domains whose size is between 10 and 20 nm (see Fig. S7†) and ber diameters conveniently approaching the EDL of P3HT (about 5– 9 nm).25,28,53 The interdiffusion of layers during annealing is also supported by the AFM imaging of the thin-lm sections performed before and aer the thermal treatment (see Fig. S3†). Seemingly, we applied the above method to the P3HT:F8BT couple. Also in this case, the stronger the annealing treatment the smoother the surface and the phase-lag images show a quite uniform low contrast F8BT layer before annealing, whereas bright-contrasting plaques appear upon annealing due to the topmost segregation of P3HT (see Fig. S6†). Note that also in this case, P3HT is the lowest surface tension component (gF8BT ¼ 26.0 mJ m2; see ESI†) but, unlike the blend with PCBM, here P3HT does not form bers.51 The diffusion and segregation of P3HT to the topmost surface is corroborated for both the investigated D/A couples by
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2 4 P3HT:PCBM MHJ films on ITO/PET at different annealing times: (a and d) non-annealed; (b and e) 130 C, 2 10 min; (c and f) 130 C, 20 min; (a–c) height images; (d–f) phase-lag images. Z scale is 150 nm for (a) and 60 nm for (b and c). Fig. 1
AFM surface analysis of
X-ray Photoelectron Spectroscopy (XPS). The S2p spectral region 3 for a P3HT:PCBM 1 MHJ (see Fig. 2a) reveals the pres3 ence of a very low P3HT S2p signal before the annealing treatment indicating that the surface consists mainly of an homogeneous layer of PCBM. Indeed, the thickness of the three outermost PCBM layers (about 15–20 nm) exceeds the sampling depth of XPS which is about 9–10 nm.54 The detection of very small sulphur signals can be attributed to the presence of defective holes in the PCBM topmost layers. Aer the annealing treatment at 130 C for 10 min, the S2p spectrum shows a shi of about 0.23 eV toward lower binding energy with respect to the S2p peak of bare P3HT thin lms (data not shown). According to the literature,45,55 this agrees with an increased P3HT/PCBM chemical interaction and interface area. Moreover, upon annealing the atomic percentage of sulphur is twenty-fold increased (Fig. 2a). This is corroborated by the decreased C1s shake-up component of C60 (see Fig. S10 and the related text†). Finally, the spectra of untreated and annealed samples do not show components of oxidized sulphur in the thiophene groups, conrming that the aqueous sub-phase has not altered P3HT during the LS deposition process. The inter-diffusion of materials along with the segregation of P3HT at the top-layer is effectively represented by plotting the S/ C ratio versus different P3HT:PCBM MHJ architectures (Fig. 2b). Indeed, as the thickness of the topmost PCBM layer decreases, the S/C ratio becomes larger. The thermal reorganization of 2 3 1 architectures displays a large increase 1 and 2 3 2 in S/C ratio, especially for the 1 where P3HT is closer to 2 the surface. Similar behaviour has been observed for P3HT:F8BT (see Table S3†).
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On these bases it can be argued for both the material couples that our approach leads to the formation of a thin lm bulk hetero-junction (BHJ) with nanometric bicontinuous chargepercolation paths as sketched in Scheme 2c. Studies on the interdiffusion of P3HT and PCBM were done by some other groups. Chen et al.,56,57 Treat et al.58 and Collins et al.59 studied the annealing effect in a P3HT/PCBM bilayer, observing that it is mainly PCBM that diffuses inside the residual disordered regions of P3HT with a diffusion coefficient of about 1010 to 1011 cm2 s1.57,58 Moreover, a partial miscibility between the two materials was observed with a temperature dependent concentration of PCBM in P3HT of about 3–4% aer annealing at 120 C and up to 10% for treatment at 180 C.59 Nevertheless, these studies were carried out on bilayers built with the lowest-surface-tension P3HT already placed at the top layer. Indeed, by annealing the bilayers, SIMS depth proles56,57,59 showed that the interface between the two materials remains sharp and a scarce BHJ is formed, though a certain amount of PCBM diffuses into P3HT. In a different study, Heeger et al.60 built up P3HT/PCBM bilayer solar cells with a reverse layer sequence, i.e. by depositing PCBM as the topmost layer, and studied the evolution of the structure as a result of the thermal annealing. Aer the treatment a BHJ structure was obtained and neither morphological nor performance differences could be noticed in comparison with a usual spin-cast deposited BHJ solar cell. Also, by the thermal treatment of a conventional spin-cast BHJ, Chen et al.57 and in a more recent paper Clark et al.61 showed that P3HT accumulates at the air interface whilst PCBM enriches the buried substrate interface, leaving in the middle a bicontinuous BHJ structure. These researchers interpreted those results on the basis of a surface energy rationale. Properties of organic semiconductor thin-lms have been studied by scanning probe microscopy techniques, which are
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3 1 P3HT:PCBM MHJ before and after annealing at 130 C for 10 min; (b) XPS sulphur/carbon 3 3 ratio for different MHJ architectures; “Ann.” indicates annealed samples; 1 architecture, composed of only 3 layers of P3HT, is reported as 0 3 reference. (c) Conductive AFM of a 2 P3HT:PCBM MHJ as-deposited; (d) annealed at 130 C for 10 min; (e–g) I–V curves acquired in the 3 points indicated by white arrows. The dashed lines mark the bias at which the current maps were acquired (3 V); the cartoon insets show schematically how the data have been acquired. Fig. 2
(a) XPS spectra of the S2p region for a
powerful tools for nanoscale characterization. As an example, Kelvin Probe Force Microscopy (KPFM) has been applied to directly prove the charge transfer between perylene-dicarboximide clusters and P3HT under illumination conditions.62 Nevertheless, being KPFM a surface sensitive technique, it only provides information on the lateral charge transfer between neighboring acceptor and donor domains on the blend surface. Here, in order to investigate the formation of vertical bicontinuous charge-percolation, we employed Conductive Atomic Force Microscopy (C-AFM) to locally measure the vertical current transport through the organic lm from the nanometric tip contact to the ITO back contact (see cartoon insets of Fig. 2e–g). Fig. 2c shows a C-AFM image acquired with a tip/ITO bias of 3 V 3 (positive tip bias) for an untreated 2 P3HT:PCBM MHJ. 3 This sample presents a quite homogeneous current map, except for some dark spots (higher current) probably due to structural defects. Moreover, the asymmetric I–V curve shown in Fig. 2e is representative of diode-like behaviour as observed locally pointby-point on the surface. In fact, p- and n-type P3HT and PCBM63 form 3 p–n junctions connected in series. Aer thermal annealing, vertical conductive pathways are observed as indicated by the numerous nanometric high current dark domains of Fig. 2d with a diameter close to the EDL of the organic semiconductors (Fig. S9†). Furthermore, the local I–V curves of
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Fig. 2f and g recorded inside (PCBM) and outside (P3HT) black domains remind the behaviour of metal–semiconductor–metal devices with a Schottky barrier (see ESI† for more information). In the case of P3HT/F8BT mixtures, C-AFM measurements did not show signicant contrast probably because of the similarity of the two polymers in electron affinity (see ESI†). Since C-AFM requires a contact-mode operation, slight smoothening of the sample surface can occur. The corresponding height images acquired simultaneously with the current maps are shown in Fig. S8.† However, by properly modulating the tip–sample interaction, smoothening can be minimized and we note a good correspondence between the size distributions of the dark domains imaged by C-AFM and the PCBM domains imaged in tapping mode (Fig. S7 and S9†). Besides the possibility of obtaining bi-continuous pathways, the proposed methodology allows a tight control of the dynamics of the interdiffusion process. This permits us to nely regulate the amount of interface area between the two different phases and thus it should allow tuning the size and density of the percolative domains. This provides the important consequence that the opto-electronic properties of the BHJ can be nely modulated. In order to demonstrate the latter point, we performed uorescence quenching measurements recording emission spectra directly on freshly prepared thin lm MHJs (about
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80 nm thick). Fig. 3a and b show the trend of the quenching percentage versus the interface area for four different P3HT:F8BT and P3HT:PCBM MHJ architectures, respectively. These samples embody six layers for each component and UV-Vis spectra show similar absorbance values of the polymers. Although the samples have a similar thickness (about 80 nm), they have different interfacial areas. As expected, the quenching percentage increases with the interfacial area going from the 1 6 6 MHJ, for both the D–A pairs. In order 1 to the 1 6 to model the relationship between interface area and uorescence quenching, data from the two systems are best tted by using the following Hill-like function, Q f Sn/(kn + Sn), where Q is the quenching percentage, S is the interface area, k is the value of S at which Q has half of its maximum value and n is a factor which expresses the growth rate of the interface with the annealing. By using n ¼ 0.82 and k ¼ 0.23 0.01 for F8BT:P3HT as well as n ¼ 1 and k ¼ 4.71 0.26 for P3HT:PCBM, we obtained the curves in Fig. 3, which describe the growth of the interface area with respect to increasing Q percentages. In order to understand such behaviour, one has to consider that light absorption induces the exciton formation inside the D layers. If the exciton forms at distances from the A interface larger than the donor EDL, it may recombine leading to D uorescence. Otherwise, it diffuses through the D layer and once it reaches the interface with A, the D uorescence may quench either by electron transfer (e.g. P3HT:PCBM couple)53,64 or by a non-radiative recombination that results in Foster Resonant Energy Transfer (FRET, see also ESI†) which in turn may lead to uorescence at larger wavelengths (e.g. P3HT:F8BT).51 For both couples, quenching is observed to increase up to the plateau as the D layer thickness approximates the donor EDL (about 3–6 nm for P3HT; about 10–15 nm for F8BT).25,26,28,53,65 In particular, the donor EDL can be evaluated by substituting the value of S at the onset of the plateau, extracted from the tting, in the equation EDL z l N/(1 + S), where l is the thickness of a donor monolayer (6–7 nm) and N is the total number of deposited layer (12 in this case). In the case of P3HT and F8BT, we obtained EDL values respectively of 3.9
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0.3 nm and 12 2 nm, which are in agreement with those values reported in the literature (see above). Note that while in the case of P3HT:PCBM, the short P3HT EDL allows a good condence of S for the annealed samples, in the case of F8BT:P3HT the longer F8BT EDL yields S values for the annealed samples with an increased uncertainty in the plateau region above 6 cm2. By annealing the P3HT:PCBM and P3HT:F8BT MHJs respectively at 130 C for 10 minutes and at 75 C for 30 seconds, the corresponding quenching percentage increases (Fig. 3a and b blank symbols) by a different percentage depending on the employed MHJ starting architecture. Hence, the starting MHJ architecture is an important means to freeze the system with a programmable interface area. As a main result, the annealing process may lead, also for F8BT:P3HT, to vertical percolative paths with minimized domain size, i.e. it is possible to obtain BHJs showing a uorescence quenching comparable to or even larger than the MHJs with the highest interfaces. What above allows us to implement literature methods used to extract the EDL23–26 of organic semiconductors by exploiting properties like uorescence quenching, as originally done by Heeger and coworkers.66 Fluorescence signals of both these blends acquired at sequential annealing steps change with time (Fig. S12a and b†) in agreement with the above-discussed interdiffusion process. In particular, the quenching of the P3HT signal in the P3HT:PCBM blend indicates charge transfer whereas in P3HT:F8BT a decrease of the F8BT and the simultaneous increase of the P3HT signals conrm the FRET (see also the energy diagram and spectra in Fig. S1, S11 and S13, respectively†). Interestingly Fig. 4 shows that for different P3HT:F8BT MHJs, higher annealing temperatures allow the highest quenching percentages to be reached faster. At these temperatures, aer reaching a maximum, quenching is observed to decrease indicating that the energy transfer becomes less efficient because of the formation of separated phases with increasing size (see the bilayer structure in Scheme 2d). Similar trends have also been recorded for charge transfer in P3HT:PCBM (see Fig. S14†). On the other side, the behaviour at
Fig. 3 Quenching percentage vs. interface area of different architectures (see legends) before (black symbols) and after (white symbols) thermal annealing for (a) P3HT:F8BT (75 C; 30 s) and (b) P3HT:PCBM (130 C; 10 min).
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A comparison of different envisaged out-of-equilibrium systems, shapes, branching and interface areas achievable by combining different starting MHJ architectures with annealing temperatures and times.
Fig. 4
the lowest temperatures is compatible with low molecular mobility at the D/A interfaces yielding slower diffusion especially in the MHJs with thicker layers, where larger masses are involved in the phase separation. This causes lower quenching values at shortest annealing time along with no maxima, at least
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in the experienced time scale. Accordingly, Fig. 4 demonstrates also the importance of the initial MHJ architecture in affecting the structural evolution speed (shown schematically in Fig. 4), which is faster when the D/A layers are thinner. Note that in the experienced time scale the observed quenching is not fully compatible with a complete phase separation at the nal kinetic step (see Scheme 2d) and a trade-off between the lowering of surface free energy, favouring the segregation at the top surface of the lowest surface tension component, and the bulk free energy minimization, including possible miscibility, has to be considered. Furthermore, in P3HT:PCBM blends, PCBM tends to crystallize under needle-like systems at the nal kinetic steps (data not shown).59,67 On the basis of data presented above for the couple of systems studied here, we actually show that opto-electronic properties like uorescence quenching and electron or energy transfer can be modulated in a wide range due to the possibility to regulate the interface area of the intermixed components (Fig. 3). For instance, as shown in Fig. 4, by choosing a suitable treatment time and temperature, it is possible to maximize the quenching percentage (see also Fig. S12 and S13†). On the other hand, this also involves the modulation of the thin lm structure. Indeed, since the highest quenching percentage is reached only if the size of material domains approaches the EDL,63 this is the evidence that by varying the annealing time we are actually modulating the phase separation of the BHJ. In particular, it has to be noted in Fig. 4 that the compromise between the temperature and the initial MHJ architecture is important in dening the speed of evolution of the structures along with their features and opto-electronic properties. Increasing annealing temperatures and using highly interfaced MHJs would lead faster to high interface areas (then high uorescence quenching and better charge/energy transfer properties), which may be reduced at longer times. On the other hand, low interface MHJs and annealing temperatures would result in wider structural features (lower quenching and opto-electronic properties) with minimized branching. It is noteworthy that the choice of the most suitable MHJ starting sequence and annealing conditions allows reaching quenching values and materials interface area comparable to or even larger than those of optimized spin-coated BHJs. In order to compare our method with the traditional spin-coating, we prepared spin coated P3HT:PCBM BHJs and we annealed these at the same time/temperature as the MHJs. The measured quenching percentage was about 81% before annealing and 72% aer. This value is lower than those obtained by different MHJs annealed under the same conditions (see comparison in Fig. 3b). Thus, as already reported68–70 in the case of spin-coated BHJs the thermal treatment, which is an unavoidable step to enhance polymer chain packing and then charge mobility of materials, seems to worsen the quenching efficiency and thus the charge transfer properties of the system. Importantly, these results suggest that MHJs are more effective as starting points to obtain BHJs with optimal structure and energy/charge transfer performance.
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3.
Conclusion
In conclusion, we dealt with 3D structure manipulation in engineered thin lms and developed a general method for the nanofabrication of 3D bulk heterojunctions and two-component thin lms. This allows controlling both nanoscale morphology and function of thin lms in 3D by individual or sequential dynamic adaptive steps. In principle, it could be employed for a wide range of materials including those soluble in different solvents. Importantly, in spite of spin-coating this method leads to improved BHJ properties and allows the use of very small amounts of materials (tens of micrograms for a 100 nm thick lm on a 1 cm2 substrate instead of tens of milligrams), thus presenting the practical advantage of easily testing the thin lm optoelectronic properties of materials synthesized in small amounts in a scientic laboratory. Moreover, this method can also be used to easily extract the EDL of materials in organicbased thin lms. Future perspective concerns the application of this method to material blends made of different kinds of nanostructures and molecular materials including carbon nanotubes, nanorods, inorganic nanostructures, other polymers, etc., with the aim of having ne control on different structurerelated properties and to develop devices, like for instance organic and hybrid solar cells, capacitors, memories, etc., with enhanced efficiency. This method is of interest for all of these applications where the device efficiency is improved by a nanoscale 3D ordering of thin lms and, even more importantly, constitutes a conceptual means to develop complex outof-equilibrium 3D lms with desired shapes and properties; hence it may represent a fundamental tool towards the complex system engineering.
4. Experimental section To implement the different architectures, poly(3-hexylthiophene-2,5-diyl) (P3HT regioregular, electronic grade, 99.995%, average Mn 15 000–45 000, Aldrich), [6,6]-phenyl C61 butyric acid methyl ester (PCBM, MW 910.88, Aldrich) and poly(9,90 dioctyluorene-co-benzo-thiadiazole) (F8BT, MW 21 500 with polydispersity 2.9, American Dye Source) were used (Fig. S1†). Each product was dissolved in chloroform (HPLC grade, Aldrich) at a concentration of 0.1 mg ml1. Langmuir–Schaefer (LS) lms were prepared by using a KSV minitrough Langmuir–Blodgett (LB) apparatus. Any solution was randomly spread (400 mL for P3HT, 800 mL for PCBM and 300 mL for F8BT) by using a microsyringe into ultrapure water (Millipore Milli-Q, 18.2 MU cm) used as the subphase and thermostatted at 20 C. Aer the solvent evaporation, the oating lm at the air–water interface was compressed by the use of two mobile barriers of Teon at a rate of 5 mm min1. The surface pressure was simultaneously monitored by using a Wilhelmy balance. The surface layer was transferred at a pressure value of 30 mN m1 by the Langmuir– Schaefer (LS) deposition technique, which consists of approaching the solid substrate parallel to the surface subphase. From the spreading of solution on the subphase to the complete deposition onto the solid substrate, our small apparatus allows preparation of up to nearly 100 cm2 of thin lm in about Nanoscale
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15 minutes. It has to be noted that a bigger LS apparatus allows preparation of much greater thin-lm areas in the same time and that higher temperatures lead to a time reduction due to the speeded up solvent evaporation. The LS transfer was performed on exible substrates of indium tin oxide coated polyethylene terephthalate (ITO/PET) of 1 cm2 in area (surface resistivity 45 U per sq, transmittance 550 nm > 79%, Aldrich) previously cleaned with ethanol and then treated with oxygen plasma (1 mbar, 10 W, 1 min). Aer undergoing these treatments, ITO surfaces greatly increased their hydrophilicity, with the water contact angle passing from 70 to a nal value of 10 . Surface free energy of P3HT and F8BT thin-lms consisting of three layers deposited by LS on PET/ITO foils has been measured by employing the sessile drop technique. Contact angles of ultrapure water (Millipore Milli-Q, 18.2 MU cm), glycerol (Aldrich, 99%) and tritolyl-phospate (Aldrich, 90%) have been measured at room temperature by using a Kernco G-II Stage apparatus by depositing 5 ml drops of samples on the surfaces of P3HT and F8BT (see ESI† for data and calculation). The surface morphology and the thin lm thickness have been investigated by using an AFM Nanoscope IIIa (Veeco Instruments Inc., Santa Barbara, California). Etched-silicon probes (RTESP-type, Veeco) having a nominal curvature of 10 nm were used. During the scanning, height and phase images were simultaneously recorded by collecting 512 512 points for each scan and maintaining the scan rate below 1 line per second. The thickness of thin lms has been measured by the scratching method, consisting of roughly pressing the AFM tip against the surface during three consecutive scans to completely remove the material. The lm thickness is then determined from the section analysis of the carved scratch (Fig. S2†). The local electrical properties of the heterojunctions were investigated by Conductive Atomic Force Microscopy (C-AFM) under ambient conditions (50% humidity and 25 C) by means of a Dimension Nanoscope V (Veeco Instruments Inc., Santa Barbara, California) equipped with a Tunnelling AFM (TUNA) module. The current maps were obtained in the contact regime using Pt/Ir coated tips (Veeco probes, SCM-PIC type). X-ray Photoelectron Spectroscopy (XPS) characterization was performed by using a Kratos AXIS-HS Spectrometer. Al Ka radiation of 1486.6 eV has been used under the conditions of 10 mA and 15 keV. Areas of 2 2 mm have been analyzed. The pass energy of 40 eV has been used both for survey and high resolution spectra. Absorption and uorescence spectra were recorded on freshly prepared thin lms of about 80 nm thickness, with a Specord S 600 spectrophotometer (Analytik Jena, Jena, Germany) and a Fluoromax-4 (HORIBA Jobin Yvon, Edison, USA) spectrouorimeter with a 150 W xenon arc lamp excitation source, respectively. The uorescence spectra of the LS lms were recorded by aligning the sample holder at an angle of 60 and normalizing with respect to the absorbance value of the sample at 470 nm, whereas the quenching data were calculated versus the uorescence of an equivalent sample of only P3HT (see ESI† for more details and extended descriptions).
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Acknowledgements
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The authors would like to acknowledge Giuseppe Francesco Indelli from Consorzio Catania Ricerche for the technical assistance. Italian MiUR is acknowledged for funding through FIRB - Futuro in Ricerca (RBFR08DUX6) and PON R&C 2007– 2013 (“TESEO” – PON02_00153-2939517 and “Plastic electronics for smart disposable systems” – PON02_003553416798).
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PAPER
Cite this: DOI: 10.1039/c3nr05027k
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Self-organization and nanostructural control in thin film heterojunctions† Sebastiano Cataldo,a Camillo Sartorio,a Filippo Giannazzo,b Antonino Scandurrac and Bruno Pignataro*a In spite of more than two-decades of studies of molecular self-assembly, the achievement of low cost, easyto-implement and multi-parameter bottom-up approaches to address the supramolecular morphology in three-dimensional (3D) systems is still missing. In the particular case of molecular thin films, the 3D nanoscale morphology and function are crucial for both fundamental and applied research. Here we show how it is possible to tune the 3D film structure (domain size, branching, etc.) of thin film heterojunctions with nanoscale accuracy together with the modulation of their optoelectronic properties by employing an easy two-step approach. At first we prepared multi-planar heterojunctions with a programmed sequence of nanoscopic layers. In a second step, thermal stimuli have been employed to induce the formation of bulk heterojunctions with bicontinuous and interdigitated phases having a size below the exciton diffusion length. Importantly, the study of luminescence quenching of these systems
Received 19th September 2013 Accepted 18th November 2013
can be considered as a useful means for the accurate estimation of the exciton diffusion length of semiconductors in nanoscale blends. Finally, nearly a thousand times lower material consumption than
DOI: 10.1039/c3nr05027k
spin coating allows a drastic reduction of material wasting and a low-cost implementation, besides the
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considerable possibility of preparing thin film blends also by employing materials soluble in different solvents.
1. Introduction How can bulk nanostructured thin lms be precisely engineered in three dimensions (3D) with methods accessible by a traditional chemical laboratory? In spite of more than twodecades of studies dealing with the control of molecular selfassembly at bidimensional surfaces,1–7 different emerging applications need to control the 3D bulk morphology and functions of molecular thin lms on the nanoscale.8 In particular, the study of bi-component thin lms continues to introduce and develop innovative material properties with a wide range of applicability, driving research toward the exploration of several scientic and technological applications including emerging elds like optoelectronics,9 nanolithography,10 organic electronics,11,12 surface coating, membranes,13 diagnostics and so on.14–17 In this context, key properties are connected to the peculiar structure of the supramolecular phases, hence thin lm nano-engineering plays a fundamental role
a
Dipartimento di Fisica e Chimica, Universit` a degli Studi di Palermo, V.le delle Scienze, Ed.17 – 90100 Palermo, Italy. E-mail: [email protected]
b c
CNR-IMM, Strada VIII, 5 - 95121 Catania, Italy
CCR-SuperLab c/o STMicroelectronics, Strada VIII, 5 - 95121 Catania, Italy
† Electronic supplementary information (ESI) available: Experimental methods about realization of MHJs; surface free energy measurements, AFM, C-AFM, XPS and optical spectrophotometry additional discussion and gure. See DOI: 10.1039/c3nr05027k
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since the control of the structural order can greatly enhance the device properties.8,9,16,18–22 For instance, in order to achieve efficient charge transfer in organic and hybrid photovoltaics, the donor (D) and acceptor (A) materials need to be distributed inside the thin lms in vertical bicontinuous domains with size of the order of the exciton diffusion length (EDL; 3–20 nm)23–26 allowing for efficient exciton separation and balanced charge migration to the electrodes.21,27–29 Furthermore, phase-separated ordered blends of ferroelectric and semiconducting polymers showed nonvolatile two-level memory functionality30,31 and the control of self-organization in supramolecular ordered domains allows enhancement of charge mobility and eld effect performance in molecular thin lms and ambipolar transistors.32,33 Finally, nanoscale 3D order in molecular thin lms is of great interest in several other elds such as sensors, OLEDs, displays, advanced spectroscopies and the realization of calibration standards.8 Different bottom-up and top-down low cost solution approaches have been reported to control the nanoscale thin lm structure8 including the surface directed spinodal decomposition,34,35 entropic connement,36 phase-direction agents,37 use of block-copolymers,38 additive lithography,39,40 nano-imprinting lithography,41 polymer molecular weight control,9 blending ratio modulation42 and side chain functionalization.43 Nevertheless, the above methods need a couple of materials soluble in the same solvent, detrimental chemical functionalizations, undesirable additives and/or may lead to uncontrolled phase separation.
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Here we develop an out-of-equilibrium approach for the 3D structural control in bi-component organic thin lms (i.e. two semiconductive molecular systems), which allows control of the nanostructural features (dimension, vertical continuity, density, and branching) and simultaneously enabling their physical property modulation.
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2.
Results and discussion
2.1. From multilayer- to bulk-heterojunctions by a multiparameter approach The basic idea of our 3D nanopatterning strategy consists of a two-step process: (1) The preparation of a multilayer planar heterojunction (MHJ) made of alternating semiconductive D and A nanoscopic thin layers with a programmed architectural sequence (Scheme 1). (2) The promotion of interlayer-diffusion by external stimuli like thermal heating (Scheme 2). Accordingly, MHJs have been prepared by the Langmuir– Schaefer technique (LS) allowing precise control of the thickness of each deposited layer along with the deposition sequence. By exploiting such a strategy we deposited d layers of the D, followed by a layers of the A material and iterated the d n process for an n number of cycles nally obtaining a a thin lm architecture, as shown in Scheme 1. Next, MHJs have been thermally treated to induce the layer interdiffusion (Scheme 2). Our idea is based on the concept that by starting from a multilayered architecture with a periodic sequence (Scheme 2a), the system may evolve to form vertically bi-continuous complex nanostructures through non-equilibrium pathways arising from the phase separation of the materials inside the organic MHJ superstructure (Scheme 2b and c). Finally, the process is expected to lead to the thermodynamically favoured steady state. In the peculiar case of a system consisting of a solidsupported thin lm of two immiscible components, if the surface tension of the solid substrate is larger than that of the two lm components, segregation of a bilayer equilibrium
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structure, as that shown in Scheme 2d as an example, with the lowest surface free energy component at the top may be expected.7,44,45 In the case of partial miscibility, the nal thermodynamic shape would be importantly a result of the trade-off between the lowering of surface free energy, favouring segregation, and bulk free energy minimization driven by increasing mixing entropy and/or enthalpy variations.46–48 Thus, by a multi-parameter approach including optimising a suitable energy pulse, i.e. by changing temperature (thermalmotion/aggregation-speed) and/or annealing time, and varying the heterojunction architecture, i.e. by changing the thickness and sequence of layers, it becomes possible to tune the evolution dynamics, even by using multiple sequential steps, and stop the process at a certain point freezing the system in a desired out-of-equilibrium state (see Scheme 2c) like that with vertical bi-continuous phases having a functional dimension, density and interface area. Of course, the supramolecular organization of the monomolecular layer at the air–water interface can also inuence the morphological evolution of the thin lm and as a further parameter this will be a subject of future studies, for instance by modulating the lm pressure in the LS through. Instability phenomena at the propagation front of 2D layers spreading at solid surfaces have been already exploited to realize ordered superstructures made of sub-micrometric stripes and channels with controlled periodicity and lateral size.49 With respect to the 2D case, 3D thin lms exhibit one more degree of freedom that is crucially expected to increase bifurcations. Importantly, unlike other methods, this methodology employs very low quantities (tenths of milligrams for tens of square centimetres of the deposited lm) of materials that can be soluble in the same or even in different solvents, since each layer is deposited from the air–water interface, independently.
2.2. 3D functional modulation of donor/acceptor molecular thin lms In order to show the application of the above approach we used two photoactive couples (Fig. S1†) which already gave phase separation in thin lms:50,51
Schematic representation of the thin film MHJ preparation method. (a) Langmuir–Schaefer technique is used to transfer d layers of D (orange sticks) then a layers of A (blue spheres) from the air–water interface to the ITO/PET substrate; these alternate depositions may be iterated d n times to get the desired architecture; (b) examples of MHJs with different n architectures obtained by changing n, a and d. a
Scheme 1
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Scheme 2 Schematic representation of the annealing-induced interdiffusion process for a MHJ consisting of two immiscible components. The system would evolve from a multi-planar heterojunction (a) to a bulk heterojunction (c) by means of oscillating waves (b); a prolonged annealing would lead to an equilibrium state (d) characterized by the complete separation of the two components.
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(1) poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM); (2) poly(9,90 -dioctyluorene-co-benzothiadiazole) (F8BT) and the above mentioned P3HT. MHJ superlattices similar to those sketched in Scheme 1b with different architectures and controlled thickness have been prepared. Sequentially deposited layers retain their own structure and lie steadily at ambient temperature one on top of the other (see Fig. S4 and S5†). As shown in Fig. S2,† the overall lm thickness depends linearly on the MHJ layers’ number, every single layer contributing by a value of 6–7 nm for both D and A. Accordingly, MHJs of 12–16 layers are about 70–120 nm in thickness. Fig. 1 shows the AFM images of the same region of a 2 P3HT:PCBM 4 MHJ, showing the effects of the thermal 2 treatment. By annealing the sample for 10 minutes at 130 C, the morphology evolves through a loss of PCBM globular features which still appear surrounded by smoother regions (Fig. 1b). Also, the bright contrast features present in the restricted phase-lag image (Fig. 1e) indicate that the top surface at this stage consists of domains of the two different components due to the diffusion of P3HT at the topmost surface (lowest surface energy component; gP3HT ¼ 17.5 mJ m2, see ESI†) and as an opposite process, the sinking of PCBM (gPCBM ¼ 38 mJ m2)44 inside the lm. Note that plasma treated ITO has a gITO larger than both the components (about 65 mJ m2)52 and thus a thermodynamic equilibrium planar heterojunction (see Scheme 2d) with P3HT on the topmost and PCBM at the ITO interface is expected. In fact, by annealing the sample at 130 C, a morphological smoothing is observed, with the root mean square roughness decreasing from 20 nm (untreated sample, Fig. 1a) to 12 nm (10 min annealing, Fig. 1b), down to 8 nm (20 min, Fig. 1c). Moreover, the phase-lag image of the longest annealed sample clearly shows bright contrasting P3HT bers (see the zoomed area in Fig. 1f), which largely enrich the top surface. At this stage, the phase separation results in a high percentage of dark PCBM domains whose size is between 10 and 20 nm (see Fig. S7†) and ber diameters conveniently approaching the EDL of P3HT (about 5– 9 nm).25,28,53 The interdiffusion of layers during annealing is also supported by the AFM imaging of the thin-lm sections performed before and aer the thermal treatment (see Fig. S3†). Seemingly, we applied the above method to the P3HT:F8BT couple. Also in this case, the stronger the annealing treatment the smoother the surface and the phase-lag images show a quite uniform low contrast F8BT layer before annealing, whereas bright-contrasting plaques appear upon annealing due to the topmost segregation of P3HT (see Fig. S6†). Note that also in this case, P3HT is the lowest surface tension component (gF8BT ¼ 26.0 mJ m2; see ESI†) but, unlike the blend with PCBM, here P3HT does not form bers.51 The diffusion and segregation of P3HT to the topmost surface is corroborated for both the investigated D/A couples by
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2 4 P3HT:PCBM MHJ films on ITO/PET at different annealing times: (a and d) non-annealed; (b and e) 130 C, 2 10 min; (c and f) 130 C, 20 min; (a–c) height images; (d–f) phase-lag images. Z scale is 150 nm for (a) and 60 nm for (b and c). Fig. 1
AFM surface analysis of
X-ray Photoelectron Spectroscopy (XPS). The S2p spectral region 3 for a P3HT:PCBM 1 MHJ (see Fig. 2a) reveals the pres3 ence of a very low P3HT S2p signal before the annealing treatment indicating that the surface consists mainly of an homogeneous layer of PCBM. Indeed, the thickness of the three outermost PCBM layers (about 15–20 nm) exceeds the sampling depth of XPS which is about 9–10 nm.54 The detection of very small sulphur signals can be attributed to the presence of defective holes in the PCBM topmost layers. Aer the annealing treatment at 130 C for 10 min, the S2p spectrum shows a shi of about 0.23 eV toward lower binding energy with respect to the S2p peak of bare P3HT thin lms (data not shown). According to the literature,45,55 this agrees with an increased P3HT/PCBM chemical interaction and interface area. Moreover, upon annealing the atomic percentage of sulphur is twenty-fold increased (Fig. 2a). This is corroborated by the decreased C1s shake-up component of C60 (see Fig. S10 and the related text†). Finally, the spectra of untreated and annealed samples do not show components of oxidized sulphur in the thiophene groups, conrming that the aqueous sub-phase has not altered P3HT during the LS deposition process. The inter-diffusion of materials along with the segregation of P3HT at the top-layer is effectively represented by plotting the S/ C ratio versus different P3HT:PCBM MHJ architectures (Fig. 2b). Indeed, as the thickness of the topmost PCBM layer decreases, the S/C ratio becomes larger. The thermal reorganization of 2 3 1 architectures displays a large increase 1 and 2 3 2 in S/C ratio, especially for the 1 where P3HT is closer to 2 the surface. Similar behaviour has been observed for P3HT:F8BT (see Table S3†).
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On these bases it can be argued for both the material couples that our approach leads to the formation of a thin lm bulk hetero-junction (BHJ) with nanometric bicontinuous chargepercolation paths as sketched in Scheme 2c. Studies on the interdiffusion of P3HT and PCBM were done by some other groups. Chen et al.,56,57 Treat et al.58 and Collins et al.59 studied the annealing effect in a P3HT/PCBM bilayer, observing that it is mainly PCBM that diffuses inside the residual disordered regions of P3HT with a diffusion coefficient of about 1010 to 1011 cm2 s1.57,58 Moreover, a partial miscibility between the two materials was observed with a temperature dependent concentration of PCBM in P3HT of about 3–4% aer annealing at 120 C and up to 10% for treatment at 180 C.59 Nevertheless, these studies were carried out on bilayers built with the lowest-surface-tension P3HT already placed at the top layer. Indeed, by annealing the bilayers, SIMS depth proles56,57,59 showed that the interface between the two materials remains sharp and a scarce BHJ is formed, though a certain amount of PCBM diffuses into P3HT. In a different study, Heeger et al.60 built up P3HT/PCBM bilayer solar cells with a reverse layer sequence, i.e. by depositing PCBM as the topmost layer, and studied the evolution of the structure as a result of the thermal annealing. Aer the treatment a BHJ structure was obtained and neither morphological nor performance differences could be noticed in comparison with a usual spin-cast deposited BHJ solar cell. Also, by the thermal treatment of a conventional spin-cast BHJ, Chen et al.57 and in a more recent paper Clark et al.61 showed that P3HT accumulates at the air interface whilst PCBM enriches the buried substrate interface, leaving in the middle a bicontinuous BHJ structure. These researchers interpreted those results on the basis of a surface energy rationale. Properties of organic semiconductor thin-lms have been studied by scanning probe microscopy techniques, which are
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3 1 P3HT:PCBM MHJ before and after annealing at 130 C for 10 min; (b) XPS sulphur/carbon 3 3 ratio for different MHJ architectures; “Ann.” indicates annealed samples; 1 architecture, composed of only 3 layers of P3HT, is reported as 0 3 reference. (c) Conductive AFM of a 2 P3HT:PCBM MHJ as-deposited; (d) annealed at 130 C for 10 min; (e–g) I–V curves acquired in the 3 points indicated by white arrows. The dashed lines mark the bias at which the current maps were acquired (3 V); the cartoon insets show schematically how the data have been acquired. Fig. 2
(a) XPS spectra of the S2p region for a
powerful tools for nanoscale characterization. As an example, Kelvin Probe Force Microscopy (KPFM) has been applied to directly prove the charge transfer between perylene-dicarboximide clusters and P3HT under illumination conditions.62 Nevertheless, being KPFM a surface sensitive technique, it only provides information on the lateral charge transfer between neighboring acceptor and donor domains on the blend surface. Here, in order to investigate the formation of vertical bicontinuous charge-percolation, we employed Conductive Atomic Force Microscopy (C-AFM) to locally measure the vertical current transport through the organic lm from the nanometric tip contact to the ITO back contact (see cartoon insets of Fig. 2e–g). Fig. 2c shows a C-AFM image acquired with a tip/ITO bias of 3 V 3 (positive tip bias) for an untreated 2 P3HT:PCBM MHJ. 3 This sample presents a quite homogeneous current map, except for some dark spots (higher current) probably due to structural defects. Moreover, the asymmetric I–V curve shown in Fig. 2e is representative of diode-like behaviour as observed locally pointby-point on the surface. In fact, p- and n-type P3HT and PCBM63 form 3 p–n junctions connected in series. Aer thermal annealing, vertical conductive pathways are observed as indicated by the numerous nanometric high current dark domains of Fig. 2d with a diameter close to the EDL of the organic semiconductors (Fig. S9†). Furthermore, the local I–V curves of
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Fig. 2f and g recorded inside (PCBM) and outside (P3HT) black domains remind the behaviour of metal–semiconductor–metal devices with a Schottky barrier (see ESI† for more information). In the case of P3HT/F8BT mixtures, C-AFM measurements did not show signicant contrast probably because of the similarity of the two polymers in electron affinity (see ESI†). Since C-AFM requires a contact-mode operation, slight smoothening of the sample surface can occur. The corresponding height images acquired simultaneously with the current maps are shown in Fig. S8.† However, by properly modulating the tip–sample interaction, smoothening can be minimized and we note a good correspondence between the size distributions of the dark domains imaged by C-AFM and the PCBM domains imaged in tapping mode (Fig. S7 and S9†). Besides the possibility of obtaining bi-continuous pathways, the proposed methodology allows a tight control of the dynamics of the interdiffusion process. This permits us to nely regulate the amount of interface area between the two different phases and thus it should allow tuning the size and density of the percolative domains. This provides the important consequence that the opto-electronic properties of the BHJ can be nely modulated. In order to demonstrate the latter point, we performed uorescence quenching measurements recording emission spectra directly on freshly prepared thin lm MHJs (about
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80 nm thick). Fig. 3a and b show the trend of the quenching percentage versus the interface area for four different P3HT:F8BT and P3HT:PCBM MHJ architectures, respectively. These samples embody six layers for each component and UV-Vis spectra show similar absorbance values of the polymers. Although the samples have a similar thickness (about 80 nm), they have different interfacial areas. As expected, the quenching percentage increases with the interfacial area going from the 1 6 6 MHJ, for both the D–A pairs. In order 1 to the 1 6 to model the relationship between interface area and uorescence quenching, data from the two systems are best tted by using the following Hill-like function, Q f Sn/(kn + Sn), where Q is the quenching percentage, S is the interface area, k is the value of S at which Q has half of its maximum value and n is a factor which expresses the growth rate of the interface with the annealing. By using n ¼ 0.82 and k ¼ 0.23 0.01 for F8BT:P3HT as well as n ¼ 1 and k ¼ 4.71 0.26 for P3HT:PCBM, we obtained the curves in Fig. 3, which describe the growth of the interface area with respect to increasing Q percentages. In order to understand such behaviour, one has to consider that light absorption induces the exciton formation inside the D layers. If the exciton forms at distances from the A interface larger than the donor EDL, it may recombine leading to D uorescence. Otherwise, it diffuses through the D layer and once it reaches the interface with A, the D uorescence may quench either by electron transfer (e.g. P3HT:PCBM couple)53,64 or by a non-radiative recombination that results in Foster Resonant Energy Transfer (FRET, see also ESI†) which in turn may lead to uorescence at larger wavelengths (e.g. P3HT:F8BT).51 For both couples, quenching is observed to increase up to the plateau as the D layer thickness approximates the donor EDL (about 3–6 nm for P3HT; about 10–15 nm for F8BT).25,26,28,53,65 In particular, the donor EDL can be evaluated by substituting the value of S at the onset of the plateau, extracted from the tting, in the equation EDL z l N/(1 + S), where l is the thickness of a donor monolayer (6–7 nm) and N is the total number of deposited layer (12 in this case). In the case of P3HT and F8BT, we obtained EDL values respectively of 3.9
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0.3 nm and 12 2 nm, which are in agreement with those values reported in the literature (see above). Note that while in the case of P3HT:PCBM, the short P3HT EDL allows a good condence of S for the annealed samples, in the case of F8BT:P3HT the longer F8BT EDL yields S values for the annealed samples with an increased uncertainty in the plateau region above 6 cm2. By annealing the P3HT:PCBM and P3HT:F8BT MHJs respectively at 130 C for 10 minutes and at 75 C for 30 seconds, the corresponding quenching percentage increases (Fig. 3a and b blank symbols) by a different percentage depending on the employed MHJ starting architecture. Hence, the starting MHJ architecture is an important means to freeze the system with a programmable interface area. As a main result, the annealing process may lead, also for F8BT:P3HT, to vertical percolative paths with minimized domain size, i.e. it is possible to obtain BHJs showing a uorescence quenching comparable to or even larger than the MHJs with the highest interfaces. What above allows us to implement literature methods used to extract the EDL23–26 of organic semiconductors by exploiting properties like uorescence quenching, as originally done by Heeger and coworkers.66 Fluorescence signals of both these blends acquired at sequential annealing steps change with time (Fig. S12a and b†) in agreement with the above-discussed interdiffusion process. In particular, the quenching of the P3HT signal in the P3HT:PCBM blend indicates charge transfer whereas in P3HT:F8BT a decrease of the F8BT and the simultaneous increase of the P3HT signals conrm the FRET (see also the energy diagram and spectra in Fig. S1, S11 and S13, respectively†). Interestingly Fig. 4 shows that for different P3HT:F8BT MHJs, higher annealing temperatures allow the highest quenching percentages to be reached faster. At these temperatures, aer reaching a maximum, quenching is observed to decrease indicating that the energy transfer becomes less efficient because of the formation of separated phases with increasing size (see the bilayer structure in Scheme 2d). Similar trends have also been recorded for charge transfer in P3HT:PCBM (see Fig. S14†). On the other side, the behaviour at
Fig. 3 Quenching percentage vs. interface area of different architectures (see legends) before (black symbols) and after (white symbols) thermal annealing for (a) P3HT:F8BT (75 C; 30 s) and (b) P3HT:PCBM (130 C; 10 min).
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A comparison of different envisaged out-of-equilibrium systems, shapes, branching and interface areas achievable by combining different starting MHJ architectures with annealing temperatures and times.
Fig. 4
the lowest temperatures is compatible with low molecular mobility at the D/A interfaces yielding slower diffusion especially in the MHJs with thicker layers, where larger masses are involved in the phase separation. This causes lower quenching values at shortest annealing time along with no maxima, at least
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in the experienced time scale. Accordingly, Fig. 4 demonstrates also the importance of the initial MHJ architecture in affecting the structural evolution speed (shown schematically in Fig. 4), which is faster when the D/A layers are thinner. Note that in the experienced time scale the observed quenching is not fully compatible with a complete phase separation at the nal kinetic step (see Scheme 2d) and a trade-off between the lowering of surface free energy, favouring the segregation at the top surface of the lowest surface tension component, and the bulk free energy minimization, including possible miscibility, has to be considered. Furthermore, in P3HT:PCBM blends, PCBM tends to crystallize under needle-like systems at the nal kinetic steps (data not shown).59,67 On the basis of data presented above for the couple of systems studied here, we actually show that opto-electronic properties like uorescence quenching and electron or energy transfer can be modulated in a wide range due to the possibility to regulate the interface area of the intermixed components (Fig. 3). For instance, as shown in Fig. 4, by choosing a suitable treatment time and temperature, it is possible to maximize the quenching percentage (see also Fig. S12 and S13†). On the other hand, this also involves the modulation of the thin lm structure. Indeed, since the highest quenching percentage is reached only if the size of material domains approaches the EDL,63 this is the evidence that by varying the annealing time we are actually modulating the phase separation of the BHJ. In particular, it has to be noted in Fig. 4 that the compromise between the temperature and the initial MHJ architecture is important in dening the speed of evolution of the structures along with their features and opto-electronic properties. Increasing annealing temperatures and using highly interfaced MHJs would lead faster to high interface areas (then high uorescence quenching and better charge/energy transfer properties), which may be reduced at longer times. On the other hand, low interface MHJs and annealing temperatures would result in wider structural features (lower quenching and opto-electronic properties) with minimized branching. It is noteworthy that the choice of the most suitable MHJ starting sequence and annealing conditions allows reaching quenching values and materials interface area comparable to or even larger than those of optimized spin-coated BHJs. In order to compare our method with the traditional spin-coating, we prepared spin coated P3HT:PCBM BHJs and we annealed these at the same time/temperature as the MHJs. The measured quenching percentage was about 81% before annealing and 72% aer. This value is lower than those obtained by different MHJs annealed under the same conditions (see comparison in Fig. 3b). Thus, as already reported68–70 in the case of spin-coated BHJs the thermal treatment, which is an unavoidable step to enhance polymer chain packing and then charge mobility of materials, seems to worsen the quenching efficiency and thus the charge transfer properties of the system. Importantly, these results suggest that MHJs are more effective as starting points to obtain BHJs with optimal structure and energy/charge transfer performance.
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3.
Conclusion
In conclusion, we dealt with 3D structure manipulation in engineered thin lms and developed a general method for the nanofabrication of 3D bulk heterojunctions and two-component thin lms. This allows controlling both nanoscale morphology and function of thin lms in 3D by individual or sequential dynamic adaptive steps. In principle, it could be employed for a wide range of materials including those soluble in different solvents. Importantly, in spite of spin-coating this method leads to improved BHJ properties and allows the use of very small amounts of materials (tens of micrograms for a 100 nm thick lm on a 1 cm2 substrate instead of tens of milligrams), thus presenting the practical advantage of easily testing the thin lm optoelectronic properties of materials synthesized in small amounts in a scientic laboratory. Moreover, this method can also be used to easily extract the EDL of materials in organicbased thin lms. Future perspective concerns the application of this method to material blends made of different kinds of nanostructures and molecular materials including carbon nanotubes, nanorods, inorganic nanostructures, other polymers, etc., with the aim of having ne control on different structurerelated properties and to develop devices, like for instance organic and hybrid solar cells, capacitors, memories, etc., with enhanced efficiency. This method is of interest for all of these applications where the device efficiency is improved by a nanoscale 3D ordering of thin lms and, even more importantly, constitutes a conceptual means to develop complex outof-equilibrium 3D lms with desired shapes and properties; hence it may represent a fundamental tool towards the complex system engineering.
4. Experimental section To implement the different architectures, poly(3-hexylthiophene-2,5-diyl) (P3HT regioregular, electronic grade, 99.995%, average Mn 15 000–45 000, Aldrich), [6,6]-phenyl C61 butyric acid methyl ester (PCBM, MW 910.88, Aldrich) and poly(9,90 dioctyluorene-co-benzo-thiadiazole) (F8BT, MW 21 500 with polydispersity 2.9, American Dye Source) were used (Fig. S1†). Each product was dissolved in chloroform (HPLC grade, Aldrich) at a concentration of 0.1 mg ml1. Langmuir–Schaefer (LS) lms were prepared by using a KSV minitrough Langmuir–Blodgett (LB) apparatus. Any solution was randomly spread (400 mL for P3HT, 800 mL for PCBM and 300 mL for F8BT) by using a microsyringe into ultrapure water (Millipore Milli-Q, 18.2 MU cm) used as the subphase and thermostatted at 20 C. Aer the solvent evaporation, the oating lm at the air–water interface was compressed by the use of two mobile barriers of Teon at a rate of 5 mm min1. The surface pressure was simultaneously monitored by using a Wilhelmy balance. The surface layer was transferred at a pressure value of 30 mN m1 by the Langmuir– Schaefer (LS) deposition technique, which consists of approaching the solid substrate parallel to the surface subphase. From the spreading of solution on the subphase to the complete deposition onto the solid substrate, our small apparatus allows preparation of up to nearly 100 cm2 of thin lm in about Nanoscale
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15 minutes. It has to be noted that a bigger LS apparatus allows preparation of much greater thin-lm areas in the same time and that higher temperatures lead to a time reduction due to the speeded up solvent evaporation. The LS transfer was performed on exible substrates of indium tin oxide coated polyethylene terephthalate (ITO/PET) of 1 cm2 in area (surface resistivity 45 U per sq, transmittance 550 nm > 79%, Aldrich) previously cleaned with ethanol and then treated with oxygen plasma (1 mbar, 10 W, 1 min). Aer undergoing these treatments, ITO surfaces greatly increased their hydrophilicity, with the water contact angle passing from 70 to a nal value of 10 . Surface free energy of P3HT and F8BT thin-lms consisting of three layers deposited by LS on PET/ITO foils has been measured by employing the sessile drop technique. Contact angles of ultrapure water (Millipore Milli-Q, 18.2 MU cm), glycerol (Aldrich, 99%) and tritolyl-phospate (Aldrich, 90%) have been measured at room temperature by using a Kernco G-II Stage apparatus by depositing 5 ml drops of samples on the surfaces of P3HT and F8BT (see ESI† for data and calculation). The surface morphology and the thin lm thickness have been investigated by using an AFM Nanoscope IIIa (Veeco Instruments Inc., Santa Barbara, California). Etched-silicon probes (RTESP-type, Veeco) having a nominal curvature of 10 nm were used. During the scanning, height and phase images were simultaneously recorded by collecting 512 512 points for each scan and maintaining the scan rate below 1 line per second. The thickness of thin lms has been measured by the scratching method, consisting of roughly pressing the AFM tip against the surface during three consecutive scans to completely remove the material. The lm thickness is then determined from the section analysis of the carved scratch (Fig. S2†). The local electrical properties of the heterojunctions were investigated by Conductive Atomic Force Microscopy (C-AFM) under ambient conditions (50% humidity and 25 C) by means of a Dimension Nanoscope V (Veeco Instruments Inc., Santa Barbara, California) equipped with a Tunnelling AFM (TUNA) module. The current maps were obtained in the contact regime using Pt/Ir coated tips (Veeco probes, SCM-PIC type). X-ray Photoelectron Spectroscopy (XPS) characterization was performed by using a Kratos AXIS-HS Spectrometer. Al Ka radiation of 1486.6 eV has been used under the conditions of 10 mA and 15 keV. Areas of 2 2 mm have been analyzed. The pass energy of 40 eV has been used both for survey and high resolution spectra. Absorption and uorescence spectra were recorded on freshly prepared thin lms of about 80 nm thickness, with a Specord S 600 spectrophotometer (Analytik Jena, Jena, Germany) and a Fluoromax-4 (HORIBA Jobin Yvon, Edison, USA) spectrouorimeter with a 150 W xenon arc lamp excitation source, respectively. The uorescence spectra of the LS lms were recorded by aligning the sample holder at an angle of 60 and normalizing with respect to the absorbance value of the sample at 470 nm, whereas the quenching data were calculated versus the uorescence of an equivalent sample of only P3HT (see ESI† for more details and extended descriptions).
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Acknowledgements
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The authors would like to acknowledge Giuseppe Francesco Indelli from Consorzio Catania Ricerche for the technical assistance. Italian MiUR is acknowledged for funding through FIRB - Futuro in Ricerca (RBFR08DUX6) and PON R&C 2007– 2013 (“TESEO” – PON02_00153-2939517 and “Plastic electronics for smart disposable systems” – PON02_003553416798).
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