Impact of self-assembly on the photovoltaic properties of a small molecule oligothiophene donor

Solar Energy 195 (2020) 223–229

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Impact of self-assembly on the photovoltaic properties of a small molecule oligothiophene donor

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Amanpreet Kaur Hundala,1, Anubha Agarwalb,1, Mohammed A. Jameelc, Salman Alic, ⁎ ⁎ Jing-Yu Chenb, Navneet Kaurd, Lathe Jonesa, Jing-Liang Lib, , Steven J. Langfordc, Akhil Guptac, a

Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Science, RMIT University, GPO Box 2476, Melbourne, Victoria 3001 Australia Institute for Frontier Materials, Deakin University, Waurn Ponds, Victoria 3216 Australia c School of Science, Faculty of Science, Engineering and Technology, Swinburne University of Technology, Hawthorn, Victoria 3122 Australia d Department of Chemistry, Panjab University, Sector 14, Chandigarh 160014, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Barbituric acid Rhodanine Triphenylamine Organic solar cell Small molecule donor

The positive impact of self-assembly on the photovoltaic properties of a donor-π-acceptor oligothiophene molecule coded as CP3, which contains a barbituric acid as a terminal acceptor unit and triphenylamine as donor, enabling hydrogen-bonded spherulites to assemble in its films is described. The hydrogen-bonded supramolecular array of barbiturated-oligothiophene molecules were directly visualized by scanning electron microscopy. Bulk-heterojunction solar cells comprising CP3 and the fullerene PC61BM show a power conversion efficiency of 6.31%, which is markedly higher than a structural analogue CP4, which comprises a N-ethylrhodanine terminal acceptor unit incapable of participating in the same degree of self-assembly, supporting the impact of selfassembly in BHJ devices.

1. Introduction The development of new organic semiconducting materials plays a vital role in improving the power conversion efficiencies (PCEs) of bulkheterojunction (BHJ) devices. BHJ organic photovoltaic (OPV) devices using small molecules have attracted significant attention due to this diversity in donor and acceptor building blocks, which has facilitated the exploration of structure–property relationships, leading to design rules for improved OPV devices (Mishra et al., 2012; Hundal et al., 2019). However, the as-casted active layers of such materials provide low PCEs, presenting a significant hurdle for the commercial applications of organic solar cells. These semiconducting materials can be polymers or small molecules, though there are a number of advantages in using small molecules, such as well-defined molecular weight, high purity and the reproducibility of synthetic procedures without batch-tobatch variation (Sun et al., 2012; Wang et al., 2016; Zhang et al., 2015). These small molecules can be developed either as donors or acceptors, with a range of permutations possible, including donor-acceptor, acceptor-donor-acceptor and acceptor-acceptor-acceptor, to name a few, and based on the selection of appropriate building blocks. Several arrangements have generated small molecule donors, leading to impressive PCEs of around 10% and 12% for small molecule-based organic

and tandem solar cells, respectively (Kan et al., 2015; Li et al., 2017). In the optimization process of BHJ device fabrication, control over the nanostructure of the semiconducting donor and acceptor materials as well as their phase separation in the active layer is of prime importance as they are directly related to charge separation and charge transportation (Hoppe et al., 2006; Beaujuge et al., 2011). Thus, it remains critical to design semiconducting materials with good mobilities and excellent morphologies. Conventional D–A material combinations, such as the donor polymer P3HT and fullerene acceptor PC61BM, have been thoroughly studied to understand device design, the effect of blend morphology, and subsequent effects on device properties (Lu et al., 2015; Wang et al., 2019). It is understood from the current literature that the efficiency of a BHJ system depends on the precise molecular organisation of the donor and acceptor molecules. It has been reported that the presence of selfassembled structures such as nanowires provides an efficient hole transporting network, with a hole mobility around 100 times higher than those without nanowire structures (Lee et al., 2017; Kim et al., 2015). Moreover, there are several reports of solution-processed, polymer nanowire solar cells based on ‘regio-regular’ poly(3-alkylthiophene) with improved PCEs (Kim et al., 2011; Kim et al., 2010). Such reports suggest that the interconnected network of self-assembled

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Corresponding authors. Tel.: +61 3 5227 2440 (J.-L. Li), +61 402 131 118 (A. Gupta). E-mail addresses: [email protected] (J.-L. Li), [email protected] (A. Gupta). 1 A.K.H. and A.A. contributed equally to this work. https://doi.org/10.1016/j.solener.2019.11.073 Received 30 July 2019; Received in revised form 19 November 2019; Accepted 21 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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Fig. 2. Solution (S) and blend film (BF) spectra of CP3 (black curves) and CP4 (red curves); (1: 1 w/w CP3/CP4: PC61BM blend). Fig. 1. Molecular structures of CP3 and CP4.

domains can provide persistent percolation pathways for charge carriers, leading to a higher current. The formation of self-assembled structures can be facilitated through tuning solvent properties or additives (Lee et al., 2018), however, a pre-programmed approach using functionality, which can participate in intra- and inter-molecular hydrogen bonding, is worthy of examination (Yagai, 2006, 2015; González-Rodríguez and Schenning, 2011). Though a number of research groups have reported hydrogen-bonded small molecule semiconductors for organic solar cell applications (Wicklein et al., 2009; Bhaskar et al., 2011; Patra, et al., 2012; Kumar et al., 2013), there are limited examples that demonstrate a positive impact of self-organized nanostructures on performance (Hill et al., 2004; Maria et al., 2011). Given our understanding of the donor-acceptor (D–A) or D–π–Aformatted small molecule donors, we have designed and developed a variety of chromophores with controlled properties (Gupta et al., 2012; Kumar et al., 2013; Kadam et al., 2018; Rananaware et al., 2017). The potential in D–π–A materials are they: (1) can be synthesized and scaled-up using well-established chemical reactions; (2) offer an insight into structure-property relationships; (3) provide structural flexibility; (4) present a vast variety of building blocks to be explored so as to allow tuning of optoelectronic and photovoltaic properties. Here we report a D–A conjugated small molecule donor CP3, containing a barbituric acid as the terminal acceptor functionality capable of hydrogen bonding. We have also synthesised a structural analogue CP4, bearing a N-ethylrhodanine terminal acceptor unit, incapable of participating in self-assembly to the same degree. Drop-cast films of CP3 from tetrahydrofuran solution afforded spherulites – a self-assembled shape indicating strong

Fig. 3. SEM image of pristine film (drop-cast film in THF) of CP3 showing selfassembled spherulites.

hydrogen bonding – whereas its blend with the conventional acceptor, PC71BM, yielded a very fine and intermixed donor-acceptor surface when compared with its structural analogue CP4. The OPV devices with CP3 and CP4 as donor materials were fabricated, and the device outcome using CP3 distinctly outperformed the devices based on CP4 (CP3 = 6.31% vs CP4 = 3.74%), a result that not only validates the positive impact of self-assembly on the photovoltaic properties of the oligothiophene donor, but is amongst the highest and most encouraging

Scheme 1. Synthetic protocol to generate CP3 and CP4. 224

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2.2. Synthetic details 5-((5′-(4-(Diphenylamino)phenyl)-[2,2′-bithiophen]-5-yl)methylene)pyrimidine-2,4,6(1H,3H,5H)-trione (CP3) and (E)-5-((5′-(4-(diphenylamino)phenyl)-[2,2′-bithiophen]-5-yl)methylene)-3-ethyl-2thioxothiazolidin-4-one (CP4) were synthesized in a manner reported previously (Gupta et al., 2012). CP3: A cherry solid; 1H NMR (400 MHz, DMSO‑d6) δ 11.25 (d, J = 5.8 Hz, 2H), 8.47 (s, 1H), 8.14 (d, J = 4.0 Hz, 1H), 7.69–7.60 (m, 4H), 7.51 (d, J = 3.9 Hz, 1H), 7.35 (t, J = 7.7 Hz, 4H), 7.14–7.05 (m, 6H), 6.97 (d, J = 8.6 Hz, 2H); 13C NMR (126 MHz, DMSO‑d6) δ 163.44, 163.24, 151.08, 150.23, 147.62, 147.38, 146.63, 145.82, 145.20, 134.68, 133.66, 129.69, 128.59, 126.69, 126.37, 124.64, 124.44, 124.20, 123.78, 122.37; HRMS (APCI): Calculated for C31H21N3O3S2 (M)+ 547.1019; found 547.1020. CP4: A red solid; 1H NMR (400 MHz, 1,1,2,2-tetrachloroethane-d2) δ 7.84 (s, 1H), 7.48 (d, J = 8.5 Hz, 2H), 7.36 (d, J = 4.0 Hz, 1H), 7.34–7.24 (m, 6H), 7.22 (d, J = 3.8 Hz, 1H), 7.18–7.04 (m, 8H), 4.18 (q, J = 7.1 Hz, 2H), 1.30 (t, J = 7.1 Hz, 3H); 13C NMR (126 MHz, 1,1,2,2-tetrachloroethane-d2) δ 191.83, 167.01, 147.77, 146.95, 145.62, 135.91, 135.52, 133.91, 129.31, 126.76, 126.61, 126.40, 125.11, 124.69, 124.61, 123.37, 123.22, 122.90, 119.98, 39.90, 12.20; HRMS (APCI): Calculated for C32H24N2OS4 (M)+ 580.0766; found 580.0767.

Fig. 4. The polarised light microscope (PLM) image of pristine film (drop-cast film in THF) of CP3 showing self-assembled spherulites.

efficiencies based on the D–π–A format. The molecular structures of CP3 and CP4 are represented in Fig. 1.

3. Results and discussion 2. Experimental details

Both CP3 and CP4 were designed based on the D–π–A format using triphenylamine as a common donor moiety. While designing CP3, we decided it should utilize the full donating and accepting strengths of the triphenylamine and barbituric acid functionalities, respectively. The rationale behind this idea lies in the fact that the alterations in the

2.1. Materials and methods All the details are given in the Supplementary Information.

Fig. 5. Proposed molecular array of hydrogen-bonded oligothiophenes. 225

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Fig. 6. AFM images of pristine films of CP3 (upper) and CP4 (lower). The RMS roughness of 3.25 nm and 1.25 nm was observed for CP3 and CP4 surfaces, respectively.

Fig. 7. SEM image of 1: 1 blend film of CP3: PC61BM.

synthesized from commercially available substrates via the Knoevenagel condensation reaction of 5′-(4-(diphenylamino)phenyl)[2,2′-bithiophene]-5-carbaldehyde with the active methylene groups of either barbituric acid or N-ethylrhodanine, respectively, (Scheme 1). All structures were confirmed by 1H and 13C NMR spectroscopies, and mass

properties of the terminal acceptor units can improve intramolecular charge transfer (ICT) transition, enhance absorption, and help align highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. This should in effect improve the overall performance of a BHJ device. Both CP3 and CP4 were 226

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Fig. 8. AFM images for unannealed blends of (a) CP3 and (b) CP4 with the specified weight ratios of 1:1 (D: A). The image (c) is the annealed blend surface (SVA annealing) of CP3 showing better phase-separated morphology when compared with the as-cast blend (image a). The RMS roughness of 1.05 nm and 4.65 nm were observed for the unannealed blend surfaces of CP3 and CP4, respectively, whereas the surface roughness of 1.71 nm was observed for the annealed blend surface of CP3.

enhances the current density of BHJ devices. Thin films of CP3 were prepared by drop-casting THF solution onto a clean silicon substrate. Fig. 3 shows a typical scanning electron microscopy (SEM) image, indicating that CP3 self-assembles into regular spherulites with an average diameter of 100 nm ± 5 nm. We also conducted the polarized light microscope (PLM) analysis to observe self-assembly of CP3. The PLM analysis clearly indicated the formation of self-assembled spherulites, hence verifying the SEM results (see Fig. 4). Based on these images we propose an ordered hydrogen-bonded ribbon array (see Fig. 5). The AFM image of the pristine film of CP3 formed from chlorobenzene solution suggested a crystalline surface (see Fig. 6), which is an indication of strong intermolecular interaction such as through hydrogen bonding, achieving crystalline film surfaces (roughness of CP3 film = 3.25 nm). Such crystalline surfaces may help efficient charge separation, thus improving the current density and overall device performance. In contrast, the pristine film surface of CP4 was lacking in such features and appeared as a much smoother and flat

spectrometry [See Supplementary Information (SI)]. Both materials were soluble in routinely used solvents, though we observed that CP3 possessed higher solubility in polar aprotic solvents (i.e. tetrahydrofuran) compared to CP4. This is logical given the design of CP3, as the free hydrogen atoms of the barbituric acid functionality can interact with the oxygen atoms of polar aprotic solvents. This observation is critical, as the solvent plays a vital role in the self-assembly of a target chromophore (Chen et al., 2018). The optical absorption spectra of CP3 and CP4 are given in Fig. 2. The solution absorption spectrum of CP3 was red-shifted by 17 nm compared with CP4 (CP3 λmax = 509 nm vs CP4 λmax = 492 nm). The blended film spectrum of CP3 indicated broader light absorption properties than CP4 (red-shift of 10 nm). CP3 is expected to exhibit better light-harvesting properties than CP4 due to the larger and a stronger barbituric acid acceptor functionality, enhancing charge transfer transition in the spectrum of the blend film with CP3 absorbing most of the spectrum over the visible range, a characteristic that usually 227

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computed absorption spectra of CP3 and CP4 are shown in Figs. S5 and S6, respectively. The promising optoelectronic properties of CP3 and CP4, difference in their molecular structures and the self-assembly feature of CP3 motivated us to evaluate their photovoltaic performance in BHJ-OPV devices. We prepared the active films by spin-coating the chlorobenzene solutions of 1:1 mixtures of donors (hydrogen-bonded oligothiophene CP3 and CP4) and acceptor (PC61BM), respectively. The logic behind using a high boiling point solvent lies in the fact that its use not only generates smoother films but is advantageous to avoid the formation of large-scale lumps on active surfaces. The definite influence of PC61BM on the molecular organization of CP3 was visible given the highly amorphous nature of the former. The SEM image of the drop-cast film (CP3: PC61BM (1:1 w/w)) suggested miscibility of two phases, primarily due to the discrete nature of the self-assembled structure (spherulites), and implied that there exists phase-separation between the oligothiophene donor and PC61BM (Fig. 7). The latter was also confirmed using AFM analysis – conducted in tapping mode – where a uniform blend indicated the high-quality intermixing of donor and acceptor domains (Fig. 8(a)). Such a well-plaited surface was further confirmed by transmission electron microscopy (TEM) analysis where a finer and fibrous texture was observed (Fig. S7, SI). This usually results in relatively higher values of current-density and fill factor. The SEM, AFM and TEM analyses did suggest a certain degree of structural organization of CP3 in the PC61BM matrix. The AFM image of the as-cast blend of CP4 and PC61BM lacked in such characteristics and a highly amorphous surface was observed (Fig. 8(b)). The different levels of phase-separation were apparently shown by morphology observation of the film surfaces by AFM. The as-cast blend surfaces showed RMS roughness of 1.05 nm and 4.65 nm for CP3 and CP4, respectively. The BHJ devices fabricated using the as-cast film of CP3:PC61BM and CP4:PC61BM showed a clear difference in PCE (4.30% vs 3.31%, Fig. 9), a result that corroborates morphological differences. In our previous studies on D–A-formatted small molecule donors, we have observed that thermal annealing of the as-cast active films does not affect the performance to a noticeable extent (Hangarge et al., 2017; Gupta et al., 2015). We made a similar observation in the present study where thermal annealing was ineffective in improving the device performance. We believe that in case of CP3 blend, the morphology of active surface is already matured through solution casting as a result of self-assembly of CP3, whereas in case of CP4 blend, it is primarily attributed to the low film quality upon annealing. However, as an alternative method to reorganize molecular packing, we applied the solvent vapor annealing (SVA) treatment to the active surfaces. Recent reports showed that the rapid SVA treatment was particularly useful in achieving high FF and PCE in molecular OPVs (Sun et al., 2014; Wessendorf et al., 2014). Moreover, the SVA treated films evolve into larger domains that are inter-connected to form networks throughout the entire active layer. Such networks are beneficial to fast charge transport. Solvent selection rules for the SVA treatment were identified based on the literature reports (Sun et al., 2014) and our expansive understanding of film characteristics. Based on the solvent selection criteria, the SVA treatment was undertaken using tetrahydrofuran (THF; 30 s) and the SVA treated CP3 blend film did exhibit improved performance. We believe that a noticeable change in the blend surface morphology (Fig. 8(c)) is the main reason for the improved performance (PCE = 6.31% vs 4.30%; enhancement of ~50%). Meanwhile, SVA with THF was not very effective in improving the device performance of CP4:PC61BM (~12% enhancement of PCE using SVA; Fig. 9). This may be associated with the quality of the as-cast and highly amorphous blend, together with the use of the rhodanine acceptor functionality, that is unlikely to self-assemble. The current–voltage (J–V) curves of the CP3:PC61BM devices fabricated without SVA and with SVA treatment illustrated an increase in short-circuit current density (Jsc) from 10.20 to 13.21 mA cm−2, with a moderate increase in the open circuit voltage (Voc) from 0.91 to 0.94 V by SVA (Fig. 9). A still

Fig. 9. Current–voltage curves for the optimized devices based on CP3 and CP4 in blends with PC61BM (1:1 wt ratio) under simulated sunlight (AM 1.5, 1000 W/m2). Device structure is: ITO/PEDOT:PSS (38 nm)/active layer (~75 nm)/Ca (20 nm)/Al (100 nm) where the active layers were spun on top of the films of PEDOT: PSS using chlorobenzene. [Note: AC stands for as-cast, whereas SVA means solvent vapour annealing].

Fig. 10. Comparative IPCE spectra of the best performing devices described in Fig. 9.

with a lower surface roughness of 1.25 nm (see Fig. 6). We also conducted the X-ray diffraction analysis where the pristine surface of CP3 was found to be crystalline when compared with the neat surface of CP4 (see Fig S1, SI). Furthermore, the differential scanning calorimetry (DSC) analysis (Fig. S2, SI) supported the higher crystallinity of CP3 as the melting point of CP3 was higher when compared with the melting point of CP4 (CP3 = 293–294 °C vs CP4 = 215–216 °C). The ionization potentials – corresponding to the HOMO energies – of CP3 and CP4 were approximated using photoelectron spectroscopy in air (PESA) [see Fig. S3, SI]. The HOMO energy levels of CP3 and CP4 were estimated to be 5.31 eV and 5.41 eV, respectively. The higher value of the HOMO energy in case of CP3 was consistent with the use of a strong acceptor unit in a given D–A design (Gupta et al., 2012). The overall band gap values of 1.59 eV and 1.77 eV were calculated for CP3 and CP4, respectively. The alignment of energy levels (as shown in Fig S4, SI) suggests that both materials exhibit energy levels complementing those of the conventional PC61BM acceptor in order to fabricate OPV devices. The theoretical density distribution using the Gaussian 09 suite of programs (Frisch et al., 2009) and the B3LYP/631G(d) level of theory suggested that the HOMO densities were populated over the donor triphenylamine group whereas the LUMO densities resided over the acceptor units (Fig. S5, SI). A drop in the optical bandgap was also observed for CP3 in DFT calculations, a result that provides strong support to the experimental finding, implying that the utilization of a large tautomerisable acceptor unit such as barbituric acid plays a crucial role for tuning the optoelectronic properties of a given D–A target when compared with the smaller acceptor units such as rhodanine. The molecular electrostatic potential (MEP) maps and the 228

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moderate fill factor (51% after SVA treatment) can be influenced with the change in the device architecture, which could be considered for the future development of small molecule donors based on the D-π-A format. For current–voltage (J–V) curves and detailed device parameters, see Fig. 9 and Table S1, The improvement in the CP3-based device is mainly due to an increase in the Jsc, a finding that is consistent with the broader blend film absorption of CP3 and PC61BM. A similar pattern was observed for the incident photon-to-current conversion efficiency (IPCE) measurement, where a much broader and stronger response was observed for CP3based devices when compared with CP4-based devices (Fig. 10). The IPCE maxima of 52% (at 570 nm) and 38% (at 500 nm) were observed for the CP3- and CP4-based devices, respectively. The SVA treatment of the CP3 blend improved the IPCE to 63% whereas a moderate improvement (~13%) was observed for CP4-based blend after SVA treatment. The CP3-based IPCE spectrum suggested better light-harvesting and a superior blending of donor and acceptor domains when compared with its structural analogue CP4. The X-ray diffraction (XRD) measurements of the blend surfaces of CP3 and CP4 were also carried out. The presence of peaks in CP3-based blend does suggest higher crystallinity (Fig. S8), a result that corroborates with the AFM and SEM analyses. The crystallinity of the blend surface grew upon SVA treatment, a result that validates the superior morphology and improved performance of CP3-based annealed blend. Similarly, we observed a slight change in the colour of CP3-based blend film upon annealing by THF vapour. Such a change was reflected by the change in absorption profile (Fig. S9, SI). There was a slight red-shift of the absorption maximum from 541 nm to 556 nm. This absorption enhancement is directly related to increased photocurrent, as suggested by the IPCE plot (Fig. 10). Furthermore, the space charge limited current (SCLC) method was applied to obtain information about the charge transportation in the devices (for details, see SI). The hole-only devices were fabricated and the hole mobilities of the order 10−4 cm2 V−1 s−1 were observed for CP3- and CP4-based devices [μh (CP3) = 9.8*10−4 cm2 V−1 s−1; μh (CP4) = 1.2*10−4 cm2 V−1 s−1]. In CP3:PC61BM, the improvement in the hole mobility is certainly advantageous for achieving higher Jsc and FF values and reflects in PCE. We believe that a subtle structural change (in CP3) that has made the target donor self-assemble is an important factor for the notable difference in the device characteristics. Moreover, this structural change has refined molecular conjugation, induced crystallinity, aligned energy levels and improved blend morphology when compared with the structural analogue. All these factors are critical for the design and development of the next generation of small molecule donors for OPV applications. We believe that other acceptor functionalities which are capable to participate in self-assembly will be worthy of further work.

that self-assembly of molecular arrays is a valid strategy to afford improved OPV device performance. Declaration of Competing Interest There are no conflicts to declare. Acknowledgements A.K.H., A.A. and A.G. acknowledge the vast variety of facilities at Deakin, Swinburne and RMIT Universities, and CSIRO Manufacturing, Clayton, Victoria Australia. A.G. acknowledges the assistance of Dr J. Subbiah at the University of Melbourne, Parkville, Australia. M.J. and A.G. acknowledge the supercomputer support from Swinburne University of Technology, Hawthorn, Victoria, Australia. The CSIRO Division of Manufacturing, Clayton, Victoria Australia is acknowledged for providing support through a Visiting Scientist position for A.G. J. Li acknowledges the Australian Research Council for support through a Future Fellowship project (FT130100057). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.11.073. References Beaujuge, P.M., et al., 2011. J. Am. Chem. Soc. 133, 20009. Bhaskar, R., et al., 2011. J. Phys. Chem. C 115, 14369. Chen, J.-Y., et al., 2018. Chem. Eur. J. 24, 14668. Di Maria, F., et al., 2011. J. Am. Chem. Soc. 133, 8654. Frisch, M.J., et al., 2009. Gaussian 09, Revision C.01. Gaussian Inc., Wallingford CT. González-Rodríguez, D., Schenning, A.P.H.J., 2011. Chem. Mater. 23, 310. Gupta, A., et al., 2015. Dyes Pigm. 119, 122. (a) Gupta, A., et al., 2012. Chem. Commun. 48, 1889; (b) Kumar, R.J., et al., 2013. Chem. Commun. 49, 6552. Hangarge, R.V., et al., 2017. Dyes Pigm. 137, 126. Hill, J.P., et al., 2004. Science 304, 1481. Hoppe, H., et al., 2006. J. Mater. Chem. 16, 45. Hundal, A.K., et al., 2019. Dyes Pigm. 170, 107661. Kadam, G., et al., 2018. Mater. Chem. Front. 2, 1090. Kan, B., et al., 2015. J. Am. Chem. Soc. 137, 3886. Kim, J.-H., et al., 2010. J. Mater. Chem. 20, 7398. Kim, J.S., et al., 2011. Adv. Funct. Mater. 21, 480. Kim, M., et al., 2015. Adv. Energy Mater. 5, 1401317. Kumar, R.J., et al., 2013. Beilstein J. Org. Chem. 9, 1102. Lee, H., et al., 2018. Adv. Mater. 30, 1800453. Lee, J., Sin, D.H., Moon, B., Shin, J., Kim, H.G., Kim, M., Cho, K., 2017. Energy Environ. Sci. 10, 247. Li, M., et al., 2017. Nat. Photon. 11, 85. Lu, L., Zheng, T., Wu, Q., Schneider, A.M., Zhao, D., Yu, L., 2015. Chem. Rev. 115, 12666. Mishra, A., et al., 2012. Angew. Chem. Int. Ed. 51, 2020. Patra, D., et al., 2012. Polymer 53, 1219. Rananaware, A., et al., 2017. Mater. Chem. Front. 1, 2511. Sun, Y.M., et al., 2012. Nat. Mater. 11, 44. Sun, K., et al., 2014. J. Mater. Chem. A 2, 9048. Wang, J.L., et al., 2016. J. Am. Chem. Soc. 138, 7687. Wang, J., et al., 2019. ACS Appl. Mater. Interfaces. https://doi.org/10.1021/acsami. 9b08007. Wessendorf, C.D., et al., 2014. Adv. Energy Mater. 4, 1400266. Wicklein, A., et al., 2009. ACS Nano 3, 1107. Yagai, S., 2006. J. Photochem. Photobiol. C 7, 164. Yagai, S., 2015. Bull. Chem. Soc. Jpn. 88, 28. Zhang, Q., et al., 2015. Nat. Photon. 9, 35.

4. Conclusions We have demonstrated that hydrogen-bonded, supramolecular arrays of barbiturated oligothiophene CP3 organize into spherulites. This organization pathway was somewhat maintained in the presence of an amorphous, soluble fullerene derivative, PC61BM, affording reasonable morphology that can realize the PCE of 6.31% after SVA treatment in BHJ solar cells. This performance is encouraging and competitive among those based on hydrogen-bonding small molecule donor materials based on D–π–A format. Hence, the present study demonstrates

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