Modulation of the molecular orientation at the bulk heterojunction

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Organic Electronic Devices

Modulation of the molecular orientation at the bulk heterojunction interface via tuning the small molecular donor – nonfullerene acceptor interactions Muhammad Abdullah Adil, Jianqi Zhang, Dan Deng, Zhen Wang, Yang Yang, Qiong Wu, and Zhixiang Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08608 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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ACS Applied Materials & Interfaces

Modulation of the molecular orientation at the bulk heterojunction interface via tuning the small molecular donor – nonfullerene acceptor interactions Muhammad Abdullah Adil †, ‡, Jianqi Zhang†,*, Dan Deng†, Zhen Wang†, Yang Yang†, Qiong Wu†, Zhixiang Wei†,* †

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence

in Nanoscience, National Center for Nanoscience and Technology, Chinese Academy of Sciences, Beijing 100190, China, ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

Keywords: molecular packing orientation, nonfullerene systems, interface of the bulk heterojunction layer, interfacial molecular interactions, side chain length

ACS Paragon Plus Environment

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Abstract: The interfacial molecular packing orientation of the nonfullerene systems at the donor-acceptor interface is considered as one of the key parameters in fabricating highperformance devices due to the anisotropic molecular characteristics of conjugated donors (D) and non-fullerene acceptors (A). However, regulating the interfacial molecular orientation for the nonfullerene systems is still scarcely studied. Herein, modulation of the interfacial molecular packing orientation of bulk heterojunction layer is successfully realized via tuning the D-A interactions. The results indicate that the molecule with relatively shorter alkyl side chain (2FC4C6) due to weak D-A interactions, is unable to influence the molecular orientation of the active layer, as compared to their longer alkyl side chain counterpart (2F-C6C8), which demonstrates strong D-A interactions and thus efficiently modulates the overall packing orientation. The power conversion efficiencies of 6.41% and 8.23% are obtained for the relatively short and long alkyl side chain donors with IDIC acceptor, respectively. Hence strong D-A interactions due to long enough alkyl side chain on a donor small molecule can modify the interfacial molecular packing orientation of the system, leading to a better performing device.


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1. Introduction The solution-processed organic solar cells (OSCs) have captivated immense attention as a promising cost-effective and convenient means of utilizing the solar energy as compared to their traditional counterparts, as they can be fabricated by simple solution-processing techniques which in turns enables the fabrication of the flexible large area devices, as well as producing relatively inexpensive and light-weight devices.1-3 Considering the active layer structure, the bulk heterojunction (BHJ) structures exhibit a prominent dominance over the layer by layer structures, as by blending the donor with an acceptor material the interfacial area can be significantly increased and ultimately promotes exciton separation due to the formation of interpenetrating networks.4 It has been observed that OPV device fabrication using solution processing techniques leads to a group of very complex morphological configurations, classified by domain size, purity and aspects of crystallinity. Among them, the crystallite orientation with respect to the electrodes is one of the most important factors.5-7 For photovoltaic devices, since the carrier transport in the vertical direction is essential, the face-on molecular orientation, with respect to the substrate, is preferred as it stimulates various photophysical processes such as inhibiting recombination and promoting photo-induced charge separation.8-9 Similarly considering the BHJ interface, an interaction between the donor and acceptor in the face-on orientation, with respect to each other at the interface, as well as the substrate is vastly desired as this enables an improved orbital overlapping between the donor and acceptor, ultimately resulting in enhanced electronic coupling and leading to better device performance.5, 8-9 Therefore, it is important to adjust the interfacial molecular orientation of the active layer components in order to promote the charge transfer efficiency of such π-conjugated photovoltaic devices. However, there still exists a


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scarcity of efficient methods to control and modify the donor-acceptor preferential orientation at the interfaces, hindering the fabrication high performing OSCs. Understanding the importance, special attention has now been given to effectively control or modify the interfacial molecular orientation of the active layer components while optimizing their phase separation structures. A few researchers have hence employed various methods such as solvent processing,5 thermal treatments,9 and even modifying the molecular structures of the active layer components,2 to influence the interfacial orientation and hence attain best results. However, most of these methods involve fullerene derivatives as acceptor materials and no attention has yet been paid to the non-fullerene systems. Recently, numerous n-type organic semiconductor acceptors have been successfully designed to replace the renowned fullerene and its derivatives,10-17 enabling a relatively higher suppression of non-radiative recombination losses,18 ultimately resulting in high open circuit voltages (Voc). Similarly, efforts have also been made to replace the polymer donors with the small molecules and hence fabricating the all-small-molecule nonfullerene solar cells, where both the donor and acceptor are π-conjugated materials, capable of reaching PCEs up to of just over 11%.19 Moreover, it has been demonstrated that side chains along with acting as solubilizing groups play a key role in determining the molecular orientation.1, 20-21 There are many results indicating that side chain parameters such as their shape, type, position, length, branching points, and bulkiness, demonstrate significant effects on the interaction between the donors and acceptors at the BHJ interface and ultimately on the performance of the solar cells.22-26 However, little to none success has been achieved by using this technique to modulate the interfacial molecular orientation, especially for the all-small-molecule nonfullerene systems.


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In this work, we have studied the interaction of small molecular donor and acceptor material by the virtue of two different alkyl side chain lengths, as they influenced the tuning of the interfacial molecular packing orientation of a narrow bandgap planar IDT based fused ringed nonfullerene electron acceptor with four n-hexyl side chains (IDIC), exhibiting a preferential face-on molecular packing orientation, as it interacts with an intrinsically edge-on small molecule donor, comprising of thiophene-substituted benzodithiophene (TBDT) as a core, 2(thiophen-2-yl)thieno [3,2-b]thiophene as p-bridges and end-capped with 4,7-difluoro-1Hindene-1,3(2H)-dione; abbreviated as BTID-2F, that our group has synthesized earlier,1 (Figure 1a). The donors, on the basis of the side chains attached, are termed as 2F-C6C8 and 2F-C4C6, where the carbon number indicates the length of the side chain. Hence by blending 2F-C6C8 and 2F-C4C6 with the IDIC small molecule individually and fabricating inverted structures revealed the system with longer alkyl side chain to produce superior performance. 2. Results and discussion 2.1 Optical and electrochemical properties Figure 1b represents the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of BTID-2F donors and IDIC acceptor, measured via cyclic voltammetry (Figure S1). The HOMO levels of 2F-C4C6 and 2F-C6C8 came out to be 5.19 eV and -5.24 eV, whereas the LUMO levels lie at -3.37 eV and -3.41 eV, respectively. The energy levels of the IDIC small molecule were obtained from the literature,27 exhibiting a value of -5.69 eV and -3.91 eV for HOMO and LUMO, respectively. Figure 1c and d exhibit the UV-vis absorption spectra of the pristine solution and films for the two BTID-2F donors and IDIC acceptor, along with the 2F-C6C8: IDIC and 2F-C4C6: IDIC blend films. An obvious redshift of about 100 nm and 45 nm was observed for the two donors


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and IDIC acceptor respectively, going from solution to films architectures, indicating good aggregation in the film morphology. The blend films for the 2F-C6C8: IDIC system demonstrated a complementary absorption range from 450 nm to 790 nm, whereas the 2F-C4C6: IDIC blend gave a complementary absorption range from 480 nm to 790 nm, favorable for the light-induced excitons generation. 2.2. Photovoltaic performance The photovoltaic performance of the two fabricated devices, having different alkyl side chain lengths, was measured under AM 1.5G irradiation (100 mW cm-2). Since the active layer only comprised of the small molecules, blends were made using chloroform (CF) as the solvent, along with 2-Chlorophenol (CP) as the additive, maintaining an optimal D:A ratio of 1:1. The corresponding device parameters are summarized in Table 1, from where it can be seen that when the 2F- C6C8 donor was blended with the IDIC acceptor, without the CP additive, the device gave a Voc of 0.896 V, a Jsc of 11.74 mA cm-2, a fill factor of 61.04%, and a PCE of 6.42%. However, the inclusion of CP additive to this blend resulted in an increase in the Jsc, raising it to 13.98 mA cm-2 and ultimately giving an overall PCE of 8.23%. Similarly blending the 2F-C4C6 with IDIC produced a Jsc of 12.05 mA cm-2 and 10.72 mA cm-2 for the systems with and without CP, resulting in an overall PCE of 6.21% and 5.17%, respectively. Hence irrespective of the length of the alkyl side chains, these results indicate that the inclusion of CP as an additive in the current systems helps in increasing the Jsc and hence the overall PCE of the two systems. Comparing the performance of the additive containing devices based upon the alkyl side chains length, again from table 1, the devices fabricated by using the donor with longer alkyl side chains (2F-C6C8) exhibit better performance in terms of Jsc and FF, as compared to its relatively shorter alkyl side chain counterpart (2F-C4C6), ultimately giving a PCE of 8.23% and


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6.21%, respectively. Figure 2a shows the J-V curves whereas Figure 2b demonstrates the external quantum efficiency (EQE) spectra of the two devices, where both the devices exhibit a wide EQE range from 300 nm to 800 nm. However, the 2F-C6C8 system demonstrated a better EQE performance overall, giving the best value of about 66% around 575 nm as compared to the 2F-C4C6 system which only managed to achieve about 52% EQE at the same wavelength. Furthermore, a Jsc value of 13.67 mA cm-2 and 11.87 mA cm-2 was calculated for 2F-C6C8 and 2F-C4C6 systems respectively from these curves. Figure S3a represents the J-V curves for the devices without additives. On the basis of these results, it might be said that by having longer alkyl side chain leads to a better interaction among the donor and acceptor, consequently resulting in better performance. Considering the 2F-C4C6 system, since it has much compact structure, having shorter alkyl side chains, while fabricating it would crystallize before the IDIC and hence the interaction among them would be significantly decreased. Thus despite being more crystalline and due to the poor interaction with the IDIC, the performance of the 2F-C4C6 system is inferior to that of the 2FC6C8 system. These results with more evidence are further discussed below. 2.3. Bimolecular recombination and charge carrier transportation The Jsc dependence on light illumination intensity is usually given by Jsc ∝ Iα,28 where ‘α’ close to unity represents extremely weak bimolecular recombination, that can be neglected.29-30 Figure 3a demonstrates the variation of Jsc as a function of illumination intensity for the 2F-C4C6 and 2F-C6C8 devices on a log-log scale, from where it can be seen that even though the 2F-C6C8 occupies a higher position in the plot as compared to 2F-C4C6 system, the α value of 1.00 and 0.98 respectively, are obtained for the two devices, indicating that the bimolecular recombination in both the devices is quite weak. Additional information about the charge recombination


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mechanism can be attained by examining the correlation of Voc dependence upon light illumination intensity. Generally, bimolecular recombination is the primary process occurring if the slope (s) of the ln of light intensity versus Voc curve is close to kT/q, where ‘k’ is the Boltzmann constant, ‘q’ is the charge, and ‘T’ is the absolute temperature. Similarly, if the ‘s’ is close to 2kT/q, the trap-induced charge recombination is the dominant process in active layers.31 Hence, by plotting the Voc values as a function of the ln of the light intensities (Figure3b), an ‘s’ value of 0.025 and 0.027 is obtained for the 2F-C6C8 and 2F-C4C6 systems, giving the ‘n’ value to be 0.97 and 1.05 respectively (where n = sq/kT) revealing a relatively higher bimolecular recombination at Voc for the 2F-C4C6 system. The effective voltage (Veff) versus photocurrent density (Jph) curves of the two devices are shown in Figure 3c. Jph is defined as Jph = JL − JD, where JL and JD are the current density under light illumination and in the dark, respectively. The Veff is defined as Veff = V0 − V, where V0 is the voltage at which Jph = 0 and V is the applied bias.32 Hence the charge collection and exciton dissociation efficiency can be evaluated by the Jph/Jsat values, where Jsat is the saturation current density and can be given by Jsat = qLGmax, where ‘Gmax’ is the maximum exciton generation rate, ‘L’ is the thickness of the active layer and ‘q’ is elementary charge at short-circuit conditions or maximal power output condition, respectively. From the curve, the Jph/Jsat values are 91.5% and 94%, for 2F-C4C6 and 2F-C6C8 blends, respectively. Hence the relatively longer alkyl side chains of the 2F-C6C8 donor enables the corresponding system to achieve higher Jph/Jsat value, indicating better exciton dissociation and charge transport as compared to the 2F-C4C6:IDIC blend. Similarly, by following the work done by Holmes et al.,33 the charge collection efficiency (ηCC) of 86% and 79.8% has been obtained for the 2F-C6C8 and 2F-C4C6 systems, respectively (Figure S2). Moreover, the electron and hole mobility for both devices is given in table S1, whereas their


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corresponding curves have been exhibited in figure S3 (h and i). It can be seen that the mobility performance for both the systems is quite similar, where the 2F-C6C8 system exhibits slightly better hole transport (µh) characteristics, whereas, the 2F-C4C6 system demonstrates slightly better electron mobility (µe) of the two blends. Thus the difference between the FF and the JSC between the two devices can be attributed to the higher charge dissociation and higher charge collection efficiencies of the relatively longer alkyl side chain donor in the blend system, leading to an enhanced device performance. 2.4. Film morphology and microstructure. In order to analyze the lateral morphologies, molecular orientation and the crystallinity of the active layers, the pristine long and short alkyl side chain donors, IDIC acceptor and their respective blends films were characterized on ZnO/ITO substrates using Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS).34 The 2D GIWAXS patterns for all the films are shown in Figure 4a-e, whereas their corresponding out-of-plane and in-plane curves are exhibited in Figure 4f, respectively. For the pristine donor molecules (Figure 4a, b), the (100), (200) and (300) diffraction peaks are present in the out-of-plane direction, while the (010) peak appears in the in-plane direction, indicating a preferential edge-on molecular packing orientation. Similarly, the evaluation of the alignment in the IDIC small molecule indicates a strong preferential face-on molecular packing orientation (Figure 4c). Interestingly, for the 2F-C4C6: IDIC blend, the (100) and (010) peaks are located at both, the out-of-plane and in-plane directions (see arrows in Figure 4d), respectively. Compared with the 2D GIWAXS patterns of pure 2F-C4C6 and IDIC, it can be concluded that within the blend the (100) peak in the out-of-plane direction and the (010) peak in the in-plane direction originates from the pure 2F-C4C6, whereas the (100) peak in the in-plane direction and the (010) peak in the


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out-of-plane direction comes from the pure IDIC. This indicates that the donor (2F-C4C6) and the acceptor (IDIC) within the blend have kept the same orientation as the pure film, giving their preferential orientations simultaneously. However, the 2D GIWAXS pattern of the 2F-C6C8: IDIC blend shows a different behavior. The scattering intensity of the (100) peaks are mainly distributed in the out-of-plane direction (see arrows in Figure 4e). Similarly, in terms of the scattering behavior of the pure materials, the donor (2F-C6C8) keeps the molecular orientation while the acceptor (IDIC) changes its orientation from face-on to edge-on. This indicates that the donor (2F-C6C8) and the acceptor (IDIC) have a strong interaction with each other and the donor can modulate the orientation of the acceptor. Considering the out-of-plane and in-plane curves in Figure 4f, it can be seen that the out-ofplane curve for 2F-C4C6 blend is actually a combination of the pristine 2F-C4C6 donor (Figure S4a) and IDIC curves, demonstrating (010) peak from IDIC and (100) peak from 2F-C4C6 respectively, at the same position as in the pristine materials, demonstrating a weak interaction among the two components. For the 2F-C6C8 curves, it can be seen that the (100) peak position of the blend is at a slightly different position as compared to the pristine 2F-C6C8 (Figure S4a). Similarly the (010) peak due to IDIC in the blend has a relatively lower intensity as compared to the pristine IDIC, indicating that there has been an interaction between the donor and acceptor in this case which led to a different curve as compared to the pristine components. The same behavior can be seen in the in-plane curves as well where the 2F-C4C6 system shows identical curves as the combination of pristine materials (Figure S4b), revealing a lack of D:A interaction, whereas, the 2F-C6C8 shows a different behavior indicating a relatively strong interaction between the donor and acceptor due to its relatively longer side chains.


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To further verify the strong interaction between the donor and the acceptor, quantitative analysis was made. The data is presented in Figure S4 and Table S2, where the (100) peaks of the pure 2F-C4C6, 2F-C6C8 and IDIC can be located at 0.34 Å-1, 0.31 Å-1 and 0.40 Å-1, representing a lamellar distance spacing of 18.5 Å, 20.3 Å and 15.7 Å, respectively. Considering the data from table S2, as mentioned earlier, the pure IDIC acceptor as well as the 2F-C4C6 donor demonstrates an identical peak position as compared to their blend, whereas the 2F-C6C8 system gives a slightly lower value of 0.29 Å-1 and a larger value of 0.44 Å-1 as compared to its pure counterpart and IDIC, respectively. Furthermore, both pristine donor materials exhibit an almost identical scattering intensity and CCL values, indicating a similar degree of crystallization in the pristine systems. However, the addition of IDIC to form the blends leads to a reduction in crystallinity for both systems. A comparison between the blends reveals a significant reduction in the crystallinity of the 2F-C6C8 blends, as both the scattering intensity as well as the CCL exhibited a lower value as a consequence of IDIC inclusion, in comparison to the 2F-C4C6 system. Azimuthal-cut profiles for the pristine IDIC and its corresponding blends have also been displayed in Figure S4c, from where it can be seen that IDIC in the 2F-C4C6 blends exhibit quite similar behavior to that of the pristine IDIC acceptor. However for 2F-C6C8 system, there is a relatively intense scattering peak at 90° position due to the change in the molecular orientation of the IDIC from face-on to edge-on. This again confirms the relatively strong interaction between the 2F-C6C8 donor and IDIC acceptor as a result of donor’s relatively longer alkyl side chains. From the Figure S4, it can be seen that there is not much difference between the two donors in their pristine form. However, once the formation of the blends and due to the modulation of the molecular packing orientation of the IDIC acceptor, the relatively longer alkyl side chain system


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exhibits a relatively intense overall edge-on peak. These results thus reveal a relatively stronger interaction between the 2F-C6C8 donor and IDIC acceptor, as compared to its short alkyl chain counterpart. It has been reported that the molecular orientation at the donor-acceptor interface can be revealed by measuring surface free energy (SFE) of the materials. The SFE of the pristine films was calculated through the contact-angle measurements, by employing H2O (θwater) and glycerol (θglycerol) as the probe liquids. The SFE is usually expressed as the sum of London dispersion (γd) and polar (γp) components of their interfacial energies.35 Table 2 thus contains the observed contact angles for the thin films, along with their corresponding SFE, γd, and γp values (originating from polarizability and permanent dipole, respectively).36-37 It can be seen that the pristine 2F-C6C8 donor films exhibit a higher SFE, and more importantly, a higher γd value as compared to the pristine 2F-C4C6 films, whereas, a reverse trend is observed for the γp values. It has been demonstrated that the acceptors with a large γd would increase the exposed πconjugated framework at the acceptor-donor interface, which can promote the formation of desirable charge-separated states.2 Thus these result are consistent with the GIWAXS results showing a face-to-face π-π stacking at the interface. Hence by analyzing the GIWAXS patterns and SFE results, we have proposed a model shown in Figure 4g. It is quite clear that once the edge-on oriented 2F-C4C6 donor comes in the vicinity of the face-on oriented IDIC acceptor at the bulk heterojunction interface, for both systems, with and without CP (Figure S3 (d and f)), they are unable to influence the molecular packing orientation of each other, as they both show their own preferential orientation even after being blended to form the active layers. This behavior is preferentially due to the relatively shorter alkyl side chains of the 2F-C4C6 donor as they tend the donor to have a more compact and


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hence crystalline structure. Thus at the interface, both of the active layer components fail to influence the packing orientation of each other and ultimately result in poor performance. On the contrary, a very strong interaction between the 2F-C6C8 donor the IDIC acceptor blend in the presence of CP at the bulk heterojunction interface has been portrayed in the Figure 4e, resulting in a strong influence on the molecular packing orientation of each other and consequently leading to a superior performance (Figure 4g). The morphological structure of the two active layers was evaluated using the Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) as can be seen from Figure 5 (a–d). Considering the TEM analysis (Figure 5a and b) at a magnification of 100 nm, the morphologies of the active layers formed from 2F-C4C6 and 2F-C6C8 donors appear to be quite identical. Even at a higher magnification of 50nm (Figure S5), it is very difficult to find any morphological differences among the two active layers. The only minor difference is that the TEM images for the 2F-C4C6 donor appear to be slightly sharper, indicating a slightly more crystalline morphology than the 2F-C6C8 active layers. This might be due to the compact structure of the 2F-C4C6 molecule as active layers formed from relatively shorter alkyl side chain donor result in relatively more crystalline morphologies, exhibiting relatively sharper domains. Coming to the AFM images (Figure 5c and d) the crystalline behavior of the 2F-C4C6 blends becomes quite apparent as very distinct domains can be seen in the image. Further, by comparing the AFM images of the pristine 2F-C4C6 donor (Figure S6a) with its blend (Figure 5c) as well as with the pristine 2F-C6C8 donor (Figure S6b), the more distinct domains in the 2F-C4C6 blends can be attributed to the relatively non-homogenous nature of the domains in the pristine 2F-C4C6 molecule. Hence, the presence of relatively short alkyl side chains in the pristine 2F-C4C6 not only leads to a compact structure, but also inclines the donor to form relatively random crystal


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domains as compared to the 2F-C6C8 system. Furthermore, by analyzing the 2F-C4C6 pristine and blend images, it can be seen that even in the blend configuration, the 2F-C4C6 donor has maintained its pre-blend attributes. Hence, the addition of IDIC to this system yields a relatively poor interaction between the donor and acceptor, resulting in a blend morphology that is quite similar to that of the pristine donor, ultimately resulting in poor device performance. On the contrary, by comparing the pristine and blend systems of the 2F-C6C8 donor molecule (Figure 5d and S6b), it can be seen that the addition of IDIC, due to a relatively strong donor-acceptor interactions, leads to a smooth and continuous film morphology. Thus, the 2F-C6C8 system, owing to their long alkyl side chains, can properly interact with the IDIC acceptor before crystallizing and exhibit high device performance, as indicated by the GIWAXS patterns. Hence by looking at the Figure 5c and d, it becomes obvious that the relatively long alkyl side chains of the 2F-C6C8 donor are responsible for a relatively strong interaction between the 2F-C6C8 donor and IDIC acceptor, ultimately leading to a better performing device. 2.5. Effect of additive. In order to determine the effect of the additive, we characterized both donor systems, with and without CP, using Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS). The 2D GIWAXS patterns for all the films are shown in Figure S3 (d-g), from where it can be seen that it is the addition of CP that influences the change in the molecular orientation of IDIC from faceon to edge-on for 2F-C6C8 system (Figure S3 (e and g)). In order to determine the reason, we checked the solubility of the three materials in CF and CP, respectively. The IDIC acceptor, even at a concentration of 100 mg/ml had no problems in forming a solution in both CP and CF at the room temperature, whereas the 2F-C4C6 and 2F-C6C8 donors only formed a solution at a significantly reduced concentration of 15 mg/ml and 20 mg/ml in CF, whereas 25 mg/ml and 35


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mg/ml in CP, respectively. This indicated the CP to be a relatively better solvent for the two donors as compared to CF, demonstrating even better solubility for 2F-C6C8 among the two donors. Hence, it can be claimed that the addition of CP, a high boing point additive, to the blends, provides a longer solution-to-dry-film window, which in turn enables a better interaction among the active layer components, as compared to the blends without CP. Similarly, due to the lower solubility of the 2F-C4C6 donor in CP, it crystallizes much earlier than the IDIC acceptor and thus exhibits poor overall interaction, ultimately resulting in poor device performance. The relatively higher solubility of the 2F-C6C8 system, on the other hand, provides enough time for the long alkyl chains of the donor to interact, and thus influence the orientation of the IDIC acceptor. It can therefore be concluded that for the current systems, both relatively long alkyl side chain as well as CP as an additive are required to fabricate best performing devices. 3. Conclusions In this work we have blended relatively long and small alkyl side chain bearing donors (2FC6C8 and 2F-C4C6 respectively), preferentially edge-on oriented small molecules, with a face-on oriented IDIC acceptor, to investigate the effect of the different side chain lengths upon the molecular packing orientation of the active layers. In terms of the photovoltaic performance, the longer alkyl side chain containing 2F-C6C8: IDIC system, in the presence of CP as additive, demonstrated a better performance by generating a Jsc and FF of 13.98 mA cm-2 and 65.20% resp., giving a PCE of 8.23%, as compared to its relatively short alkyl side chain counterpart that produced a Jsc, FF and a PCE of 13.66 mA cm-2, 53.53% and 6.41% respectively. Hence the photovoltaic results indicated that the longer alkyl side chain small molecule to be the superior donor. Furthermore, the TEM and especially AFM images revealed the relatively higher crystallinity in the 2F-C4C6 donor due to its shorter alkyl side chains, which resulted in its early


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crystallization and thus improper interaction with the IDIC acceptor at the BHJ interface, leading to poor performance. On the contrary, the longer side chains on the 2F-C6C8 donor enables it to interact efficiently with the acceptor and hence exhibit superior performance. This is further verified by considering the 2D-GIWAXS patterns of the two donors, where the short alkyl side chain 2F-C4C6 donor, even with the presence of the CP, were unable to interact with IDIC and thus couldn’t influence the molecular packing orientation at the BHJ interface, as their blend produced separate and discrete edge-on and face-on orientations which were due to the pristine donor and acceptor respectively. The 2F-C6C8:IDIC blend, on the other hand, demonstrated a clear influence on molecular packing orientation only in the presence of CP, as the donor and acceptor were able to strongly interact with each other at the interface due to the longer side chains of the donor. Hence, for our system, the combination of a long alkyl side chain bearing donor with a high boiling point additive led to the modulation of the preferential face-on molecular orientation of the acceptor to edge-on, leading to a system with edge-on orientation overall, where the active layer components were facing each other. Thus, these experiments and results reveal that the alkyl side chains, under suitable conditions, play a key role in influencing the molecular packing orientation within the active layer in the favorable direction. Thus by carefully designing a system, having long enough alkyl side chains can help increase the interaction between the donor and acceptor molecules at the bulk heterojunction interface, as well as influence the final molecular orientation of the active layer. 4. Experimental Section 4.1 Method The UV-vis absorption spectra were attained by using a JASCO V-570 spectrophotometer. Tecnai G2 F20 U-TWIN TEM was employed to obtain the Transmission electron microscopy


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(TEM) by spin coating the two active layers films under the similar condition for solar cell devices but on ITO/PEDOT:PSS substrates, which were later on immersed in water to peel off the active layer and eventually the floating layers were transferred to TEM grid. The Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS) measurements were conducted on a XenocsSAXS/WAXS system with X-ray wavelength of 1.5418 Å. Pilatus 300K was used as a 2D detector. And the gaps of the detector were erased with a virtual detector function. 4.2 Solar cell fabrication and measurement The inverted devices were fabricated by maintaining a Glass/ITO/ZnO/Active layer/MoOx/Ag architecture. The ITO coated glass substrates were prepared by washing them with deionized water, twice with ethanol, and then with isopropyl alcohol for 20 min sequentially in an ultrasonic bath. Nitrogen gas was then used to dry the substrates followed by UV/ ozone treatment for 5 min. A very thin layer of ZnO precursor solution, prepared earlier1, was then spin-coated at 5,000 R.P.M onto the ITO surface, followed by thermal annealing at 200°C for 30 min. The substrates were then transferred into a nitrogen-filled glove box. The active layer blends were prepared by dissolving the 2F-C4C6 and 2F-C6C8 donors with the IDIC acceptor individually, in a 1:1 D:A ratio, retaining an 8 mg mL-1 concentration with respect to the donor, in chloroform. The blend mixtures were stirred at 55°C for 30 mins, followed by an addition of 0.4% CP and 30 min more stirring. After the complete dissolution, the active layers were spincoated at 2000 RPM from chloroform solutions of blends. Ultimately, a 20 nm layer MoOx, followed by a 160 nm layer of Ag was evaporated through a mask under high vacuum (1 x10-4 Pa), maintaining an active area of 0.04 cm2 All the photovoltaic measurements were performed in air, at room temperature. Keithley 2420 Source-Measure Unit was employed to determine the current density-voltage (J–V) curves.


17


Page 18 of 28

Similarly, the photocurrent was measured under illumination using the Oriel Newport 150WSolar simulator (AM 1.5G). The EQE measurements were performed by using the Oriel Newport System (Model 66902).

Associated content Supporting Information. The Supporting Information contains additional characterization data; Figures showing cyclic voltammograms, additional JV curves for devices fabricated without additives, as well as at different light intensities, AFM images and TEM images; GIWAXS data for in-plane and out-ofplane for pristine components; supplementary table for GIWAXS analysis (PDF)

Author information Corresponding Author *Email: [email protected]. *Email: [email protected]. ORCID Zhixiang Wei: 0000-0001-6188-3634 Jianqi Zhang: 0000-0002-3549-1482 Muhammad Abdullah Adil: 0000-0002-7658-5370 Notes The authors declare no competing financial interest. Acknowledgment


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Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This research was supported by the Ministry of Science and Technology of China (No 2016YFA0200700), the National Natural Science Foundation of China (Grant Nos 21534003, 91427302, 51773047, 21604017 and 21504066) and the ‘Strategic Priority Research Program’ of the Chinese Academy of Sciences (Grant No XDA09040200). References (1) Deng, D.; Zhang, Y.; Zhang, J.; Wang, Z.; Zhu, L.; Fang, J.; Xia, B.; Wang, Z.; Lu, K.; Ma, W.; Wei, Z. Fluorination-Enabled Optimal Morphology Leads to over 11% Efficiency for Inverted Small-Molecule Organic Solar Cells. Nat. Commun. 2016, 7, 13740. (2) Jinnai, S.; Ie, Y.; Karakawa, M.; Aernouts, T.; Nakajima, Y.; Mori, S.; Aso, Y. ElectronAccepting Pi-Conjugated Systems for Organic Photovoltaics: Influence of Structural Modification on Molecular Orientation at Donor-Acceptor Interfaces. Chem. Mater. 2016, 28 (6), 1705-1713. (3) Lin, Y.; Zhan, X. Non-Fullerene Acceptors for Organic Photovoltaics: An Emerging Horizon. Mater. Horiz. 2014, 1 (5), 470-488. (4) Xia, B.; Yuan, L.; Zhang, J.; Wang, Z.; Fang, J.; Zhao, Y.; Deng, D.; Ma, W.; Lu, K.; Wei, Z. Evolution of Morphology and Open-Circuit Voltage in Alloy-Energy Transfer Coexisting Ternary Organic Solar Cells. J. Mater. Chem. A 2017, 5 (20), 9859-9866. (5) Tumbleston, J. R.; Collins, B. A.; Yang, L.; Stuart, A. C.; Gann, E.; Ma, W.; You, W.; Ade, H. The Influence of Molecular Orientation on Organic Bulk Heterojunction Solar Cells. Nat. Photonics 2014, 8 (5), 385-391. (6) Hoppe, H.; Sariciftci, N. S. Morphology of Polymer/Fullerene Bulk Heterojunction Solar Cells. J. Mater. Chem. 2006, 16 (1), 45-61. (7) Salleo, A.; Kline, R. J.; DeLongchamp, D. M.; Chabinyc, M. L. Microstructural Characterization and Charge Transport in Thin Films of Conjugated Polymers. Adv. Mater. 2010, 22 (34), 3812-3838. (8) Zhou, K.; Zhang, R.; Liu, J.; Li, M.; Yu, X.; Xing, R.; Han, Y. Donor/Acceptor Molecular Orientation-Dependent Photovoltaic Performance in All-Polymer Solar Cells. ACS Appl. Mater. Interfaces 2015, 7 (45), 25352-61. (9) Zhang, R.; Yang, H.; Zhou, K.; Zhang, J.; Yu, X.; Liu, J.; Han, Y. Molecular Orientation and Phase Separation by Controlling Chain Segment and Molecule Movement in P3ht/N2200 Blends. Macromolecules 2016, 49 (18), 6987-6996. (10) Li, M.; Liu, Y.; Ni, W.; Liu, F.; Feng, H.; Zhang, Y.; Liu, T.; Zhang, H.; Wan, X.; Kan, B.; Zhang, Q.; Russell, T. P.; Chen, Y. A Simple Small Molecule as an Acceptor for Fullerene-Free Organic Solar Cells with Efficiency near 8%. J. Mater. Chem. A 2016, 4 (27), 10409-10413. (11) Bin, H.; Zhang, Z.-G.; Gao, L.; Chen, S.; Zhong, L.; Xue, L.; Yang, C.; Li, Y. Non-Fullerene Polymer Solar Cells Based on Alkylthio and Fluorine Substituted 2d-Conjugated Polymers Reach 9.5% Efficiency. J. Am. Chem. Soc. 2016, 138 (13), 4657-4664. (12) Lin, Y.; Zhao, F.; He, Q.; Huo, L.; Wu, Y.; Parker, T. C.; Ma, W.; Sun, Y.; Wang, C.; Zhu, D.; Heeger, A. J.; Marder, S. R.; Zhan, X. High-Performance Electron Acceptor with Thienyl Side Chains for Organic Photovoltaics. J. Am. Chem. Soc. 2016, 138 (14), 4955-4961. (13) Mishra, A.; Keshtov, M. L.; Looser, A.; Singhal, R.; Stolte, M.; Wuerthner, F.; Baeuerle, P.;


19


Page 20 of 28

Sharma, G. D. Unprecedented Low Energy Losses in Organic Solar Cells with High External Quantum Efficiencies by Employing Non-Fullerene Electron Acceptors. J. Mater. Chem. A 2017, 5 (28), 14887-14897. (14) Bin, H.; Yang, Y.; Zhang, Z.-G.; Ye, L.; Ghasem, M.; Chen, S.; Zhang, Y.; Zhang, C.; Sun, C.; Xue, L.; Yang, C.; Ade, H.; Li, Y. 9.73% Efficiency Nonfullerene All Organic Small Molecule Solar Cells with Absorption-Complementary Donor and Acceptor. J. Am. Chem. Soc. 2017, 139 (14), 5085-5094. (15) Li, S.; Liu, W.; Shi, M.; Mai, J.; Lau, T.-K.; Wan, J.; Lu, X.; Li, C.-Z.; Chen, H. A Spirobifluorene and Diketopyrrolopyrrole Moieties Based Non-Fullerene Acceptor for Efficient and Thermally Stable Polymer Solar Cells with High Open-Circuit Voltage. Energy Environ. Sci. 2016, 9 (2), 604-610. (16) Zhang, H.; Yao, H.; Hou, J.; Zhu, J.; Zhang, J.; Li, W.; Yu, R.; Gao, B.; Zhang, S.; Hou, J. Over 14% Efficiency in Organic Solar Cells Enabled by Chlorinated Nonfullerene SmallMolecule Acceptors. Adv. Mater. 2018, 30, 1800613. (17) Zheng, Z.; Hu, Q.; Zhang, S.; Zhang, D.; Wang, J.; Xie, S.; Wang, R.; Qin, Y.; Li, W.; Hong, L.; Liang, N.; Liu, F.; Zhang, Y.; Wei, Z.; Tang, Z.; Russell, T. P.; Hou, J.; Zhou, H. A Highly Efficient Non-Fullerene Organic Solar Cell with a Fill Factor over 0.80 Enabled by a Fine-Tuned Hole-Transporting Layer. Adv. Mater. 2018, doi:10.1002/adma.201801801. (18) Baran, D.; Kirchartz, T.; Wheeler, S.; Dimitrov, S.; Abdelsamie, M.; Gorman, J.; Ashraf, R. S.; Holliday, S.; Wadsworth, A.; Gasparini, N.; Kaienburg, P.; Yan, H.; Amassian, A.; Brabec, C. J.; Durrant, J. R.; McCulloch, I. Reduced Voltage Losses Yield 10% Efficient Fullerene Free Organic Solar Cells with > 1 V Open Circuit Voltages. Energy Environ. Sci. 2016, 9 (12), 37833793. (19) Yang, L.; Zhang, S.; He, C.; Zhang, J.; Yang, Y.; Zhu, J.; Cui, Y.; Zhao, W.; Zhang, H.; Zhang, Y.; Wei, Z.; Hou, J. Modulating Molecular Orientation Enables Efficient Nonfullerene Small-Molecule Organic Solar Cells. Chem. Mater. 2018, 30 (6), 2129-2134. (20) Duan, C.; Willems, R. E. M.; van Franeker, J. J.; Bruijnaers, B. J.; Wienk, M. M.; Janssen, R. A. J. Effect of Side Chain Length on the Charge Transport, Morphology, and Photovoltaic Performance of Conjugated Polymers in Bulk Heterojunction Solar Cells. J. Mater. Chem. A 2016, 4 (5), 1855-1866. (21) Mei, J.; Bao, Z. Side Chain Engineering in Solution-Processable Conjugated Polymers. Chem. Mater. 2013, 26 (1), 604-615. (22) Meager, I.; Ashraf, R. S.; Mollinger, S.; Schroeder, B. C.; Bronstein, H.; Beatrup, D.; Vezie, M. S.; Kirchartz, T.; Salleo, A.; Nelson, J.; McCulloch, I. Photocurrent Enhancement from Diketopyrrolopyrrole Polymer Solar Cells through Alkyl-Chain Branching Point Manipulation. J. Am. Chem. Soc. 2013, 135 (31), 11537-40. (23) Gao, J.; Dou, L.; Chen, W.; Chen, C.-C.; Guo, X.; You, J.; Bob, B.; Chang, W.-H.; Strzalka, J.; Wang, C.; Li, G.; Yang, Y. Improving Structural Order for a High-Performance Diketopyrrolopyrrole-Based Polymer Solar Cell with a Thick Active Layer. Adv. Energy Mater. 2014, 4 (5), 1300739. (24) Graham, K. R.; Cabanetos, C.; Jahnke, J. P.; Idso, M. N.; El Labban, A.; Ngongang Ndjawa, G. O.; Heumueller, T.; Vandewal, K.; Salleo, A.; Chmelka, B. F.; Amassian, A.; Beaujuge, P. M.; McGehee, M. D. Importance of the Donor:Fullerene Intermolecular Arrangement for HighEfficiency Organic Photovoltaics. J. Am. Chem. Soc. 2014, 136 (27), 9608-18. (25) Osaka, I.; Saito, M.; Koganezawa, T.; Takimiya, K. Thiophene-Thiazolothiazole Copolymers: Significant Impact of Side Chain Composition on Backbone Orientation and Solar


20

Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cell Performances. Adv. Mater. 2014, 26 (2), 331-8. (26) Dang, D.; Chen, W.; Himmelberger, S.; Tao, Q.; Lundin, A.; Yang, R.; Zhu, W.; Salleo, A.; Müller, C.; Wang, E. Enhanced Photovoltaic Performance of Indacenodithiophene-Quinoxaline Copolymers by Side-Chain Modulation. Adv. Energy Mater. 2014, 4 (15), 1400680. (27) Yang, L.; Zhang, S.; He, C.; Zhang, J.; Yao, H.; Yang, Y.; Zhang, Y.; Zhao, W.; Hou, J. New Wide Band Gap Donor for Efficient Fullerene-Free All-Small Molecule Organic Solar Cells. J. Am. Chem. Soc. 2017, 139 (5), 1958-1966. (28) Kyaw, A. K. K.; Wang, D. H.; Gupta, V.; Leong, W. L.; Ke, L.; Bazan, G. C.; Heeger, A. J. Intensity Dependence of Current-Voltage Characteristics and Recombination in High-Efficiency Solution-Processed Small-Molecule Solar Cells. Acs Nano 2013, 7 (5), 4569-4577. (29) Subramaniyan, S.; Xin, H.; Kim, F. S.; Shoaee, S.; Durrant, J. R.; Jenekhe, S. A. Effects of Side Chains on Thiazolothiazole-Based Copolymer Semiconductors for High Performance Solar Cells. Adv. Energy Mater. 2011, 1 (5), 854-860. (30) Liang, Y.; Wu, Y.; Feng, D.; Tsai, S.-T.; Son, H.-J.; Li, G.; Yu, L. Development of New Semiconducting Polymers for High Performance Solar Cells. J. Am. Chem. Soc. 2009, 131 (1), 56-57. (31) Cowan, S. R.; Roy, A.; Heeger, A. J. Recombination in Polymer-Fullerene Bulk Heterojunction Solar Cells. Phys. Rev. B 2010, 82 (24), 245207. (32) Lobez, J. M.; Andrew, T. L.; Bulovic, V.; Swager, T. M. Improving the Performance of P3htFullerene Solar Cells with Side-Chain-Functionalized Poly(Thiophene) Additives: A New Paradigm for Polymer Design. Acs Nano 2012, 6 (4), 3044-3056. (33) Pandey, R.; Holmes, R. J. Characterizing the Charge Collection Efficiency in Bulk Heterojunction Organic Photovoltaic Cells. Appl. Phys. Lett. 2012, 100 (8), 083303. (34) Muller-Buschbaum, P. The Active Layer Morphology of Organic Solar Cells Probed with Grazing Incidence Scattering Techniques. Adv. Mater. 2014, 26 (46), 7692-709. (35) Kaelble, D. H.; Uy, K. C. A Reinterpretation of Organic Liquid-Polytetrafluoroethylene Surface Interactions. J. Adhes. 1970, 2, 50-60. (36) Ma, K. X.; Chung, T. S.; Good, R. J. Surface Energy of Thermotropic Liquid Crystalline Polyesters and Polyesteramide. J. Polym. Sci. B. Polym. Phys. 1998, 36 (13), 2327-2337. (37) Ruckenstein, E.; Gourisankar, S. V. Surface Restructuring of Polymeric Solids and Its Effect on the Stability of the Polymer Water Interface. J. Colloid Interface Sci. 1986, 109 (2), 557-566.


21


a)

b)

-3

Energy level (eV)

-3.37

c)

-3.41

-4

-3.91

-5

-5.19

-5.24

-5.69 2F-C4C6

-6

2F-C6C8

IDIC

d) 0.8 0.6

FILMS

1.0

2F-C4C6 2F-C6C8 IDIC 2F-C4C6:IDIC 2F-C6C8:IDIC

Absorbance (a.u.)

1.0

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 28

0.4 0.2 0.0 300

SOLUTIONS

0.8

2F-C4C6 2F-C6C8 IDIC

0.6 0.4 0.2 0.0

400

500

600

700

800

900

300

Wavelength (nm)

400

500

600

700

Wavelength (nm)

Figure 1. (a) Small molecule donor - BTID-2F with two different alkyl side chains and small molecule acceptor – IDIC. (b) Energy levels of the donors and acceptor; UV-vis absorption spectra of (c) films of the pristine components and the two blends and (d) pristine solutions.


22

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a)

b) 10 70 5

Optomized Performance

60

2F-C6C8 2F-C4C6

50

0

EQE (%)

Current Density (mA cm -2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-5

40 2F- C6C8 2F- C4C6

30 20

-10

10 -15 -1.0

-0.5

0.0

0.5

1.0

0 300

400

Voltage (V)

500

600

700

800

Wavelength (nm)

Figure 2. (a) Optimized J-V curves for the two devices. (b) EQE spectra of the devices in a.


23


a)

c)

b) 0.90

2F-C4C6 2F-C6C8

0.88

10

0.86 0.84

α = 1.00 α = 0.98

n= 0.97 n= 1.05

0.82

1 10

2

Light Intensity mW/cm

d)

100

2F-C4C6 2F-C6C8

J ph (mA cm -2 )

-2

2F-C4C6 2F-C6C8

V oc (V)

10

Jsc (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

91.5% 94.0% 1

0.80 7

20

55 2

Light intensity (mW/cm )

0.01

0.1

1

Veff (V)

e)

Figure 3. (a) Light intensity vs Jsc plot (symbols) along with their linear fits (b) Light intensity vs Voc curves (symbols) along with their linear fits, (c) Jph vs. Veff curves for the for the two devices.


24

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Figure 4. GIWAXS images of (a) pure 2F-C4C6, (b) pure 2F-C6C8, (c) pristine IDIC films, (d) 2F-C4C6: IDIC blend (e) 2F-C6C8: IDIC blend, (f) Corresponding in and out of plane curves for the two blends and pristine IDIC (g) proposed molecular packing orientation of the two active layers.


25


Figure 5.

Page 26 of 28

(a-b) TEM images (c-d) AFM Phase Images of 2F-C4C6 and 2F-C6C8 blends,

respectively.


26

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Table 1. Optimized photovoltaic performance of the two donor blends with IDIC retaining the inverted device structure (device structure: ITO/ZnO/active layer/MoOx/Ag). The average device performance with their standard deviation of four devices is provided in parentheses.

System

Additive

2F-C4C6 : IDIC

--

2F-C6C8 : IDIC

0.4% CP -0.4% CP

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

0.84 (0.83 ± 0.006) 0.90 (0.89 ± 0.008) 0.89 (0.88 ± 0.007) 0.90 (0.89 ±0.003)

10.72 (10.70 ± 0.053) 12.05 (11.73 ± 0.058) 11.74 (11.22 ± 0.041) 13.98 (13.41 ± 0.038)

57.21 (57.16 ±0.006) 57.20 (57.18 ± 0.005) 61.04 (59.91 ± 0.007) 65.20 (64.81 ± 0.005)

5.17 (5.10 ± 0.026) 6.21 (6.13 ± 0.035) 6.42 (6.30 ± 0.041) 8.23 (8.05 ± 0.032)

Table 2. Surface free energy (SFE) characteristics of the pristine films

System

θ water

θ glycerol

(deg)

IDIC

γd

γp

(deg)

SFE (mJ cm-2)

(mJ cm )

(mJ cm-2)

91.6

77.6

26.28

22.96

3.32

2F-C4C6

101.0

89.1

18.53

16.33

2.20

2F-C6C8

102.0

89.4

18.94

17.18

1.76

-2


27


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TABLE OF CONTENTS


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Modulation of the molecular orientation at the bulk heterojunction

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