ChemComm COMMUNICATION Downloaded by Soochow University China on 20/04/2013 02:39:00. Published on 08 April 2013 on http://pubs.rsc.org | doi:10.1039/C3CC41321G Cite this: DOI: 10.1039/c3cc41321g Received 20th February 2013, Accepted 8th April 2013 DOI: 10.1039/c3cc41321g www.rsc.org/chemcomm Pan Zhang,a Chao Li,a Yaowen Li,*ab Xiaoming Yang,a Liwei Chen,b Bin Xu,c Wenjing Tianc and Yingfeng Tu*a was successfully synthesized, which could eﬀectively induce poly(3hexylthiophene) (P3HT) to form highly ordered bulk heterojunction structure without any external treatment. This ordered active layer exhibits good photovoltaic performance. Polymer solar cells (PSCs) with a bulk heterojunction (BHJ) active layer have been widely investigated.1 Of the many candidate materials, BHJ structures composed of poly(3-hexylthiophene) (P3HT) and [6,6]phenyl-C61-butyric acid methyl ester (PCBM) have been extensively investigated. The most outstanding advantage of this material pair is that an ordered BHJ morphology with interpenetrating nanoscale networks can be formed by external treatment methods, such as thermal annealing, mixture of solvents, additives, and solvent annealing, which is one key to improving BHJ device performance. However, the external treatment like thermal annealing usually possesses greater driving force for PCBM diﬀusion and aggregation, which can induce the degradation of nanoscale BHJ morphology. On the one hand, using the external treatment with annealing free approaches like cooling the saturated P3HT solution,2 mixture of solvents and additives it is diﬃcult to obtain optimized morphologies of blend films, because P3HT crystallization and phase separation of the two components occur in a single step, and the two processes can, therefore, interfere with each other.3 Therefore, a new approach without external treatment which can achieve well controlled morphology is used to synthesize novel acceptors with controlled molecular electronic structures and solid-state supramolecular structures. a Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, P. R. China. E-mail: [email protected], [email protected]; Fax: +86 512 65882130; Tel: +86 512 65882130 b i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou, 215123, P. R. China c Jilin University - State Key Laboratory of Supramolecular Structure and Materials, Changchun, 130012, P. R. China † Electronic supplementary information (ESI) available: Synthetic process and characterizations of PCBB-C8; electrochemical properties; thermal properties; SEM and AFM images and photovoltaic parameters. See DOI: 10.1039/c3cc41321g c View Journal A fullerene dyad with a tri(octyloxy)benzene moiety induced eﬃcient nanoscale active layer for the poly(3-hexylthiophene)-based bulk heterojunction solar cell applications† A bulk tri(octyloxy)benzene moiety grafted onto fullerene (PCBB-C8) This journal is View Article Online The Royal Society of Chemistry 2013 It is known that the large free region of P3HT in P3HT:PCBM blend films can be ‘‘frozen in’’ during spin-coating,4 because PCBM molecules disperse between the polymer chains and serve as defect sites, thus suppressing P3HT crystallization by destroying the stacking of polymer chains. So the P3HT crystallization requiring a free region is generally produced by the driving force obtained from external treatment.5,6 The reported approaches: thermal annealing or solvent annealing can provide the driving force to ‘‘heal’’ the disordered structure, with the mobile P3HT chains self-organizing into ordered structure accompanied by free diﬀusion and aggregation of PCBM molecules.7–9 Therefore, a fullerene derivative with a bulk flexible substituent and supramolecular structure, which can provide a C60-free region for extending the P3HT chains and shortening the molecular relaxation time, may facilitate P3HT crystallization in the absence of external treatment. Furthermore, it is reported that P3HT crystallizes more easily and rapidly than PCBM,10 implying that the rate of PCBM diﬀusion and aggregation is a critical factor for determining the kinetic driving force for the formation of ordered BHJ morphology. Thus, a weak driving force for fullerene derivative diﬀusion and aggregation may induce a better nanoscale BHJ morphology. In this communication, we report design and synthesis of a tri(octyloxy)benzene–fullerene dyad (PCBB-C8, Fig. 1a) and unambiguously demonstrate that it possesses weak diﬀusion and aggregation ability, and it can induce P3HT to self-organize into P3HT crystallites in the absence of external treatment. The as-cast P3HT:PCBB-C8 blend film based PSCs exhibit high Jsc, suggesting that this selforganized highly ordered blend film can facilitate charge separation and transportation, which is a good perspective for the external treatment free solar cell fabrication. The chemical structure of PCBB-C8 is shown in Fig. 1a, and the detailed synthetic procedure and characterizations of compounds are described in ESI† (Scheme S1 and Fig. S1–S4). PCBB-C8 possesses good solubility in common organic solvents due to the long tri(octyloxy)benzene substituent. Diﬀerential scanning calorimetry (DSC) study shows no obvious endothermic peaks and phase transition, which indicates that PCBB-C8 is amorphous (Fig. S6, ESI†). The energy levels of PCBB-C8 and PCBM for comparison were investigated using cyclic voltammetry (CV). Similar lowest Chem. Commun. View Article Online Downloaded by Soochow University China on 20/04/2013 02:39:00. Published on 08 April 2013 on http://pubs.rsc.org | doi:10.1039/C3CC41321G Communication Fig. 1 (a) Chemical structure of PCBB-C8; (b) UV-vis absorption spectra of PCBM and PCBB-C8 in chloroform solution with diﬀerent concentrations; (c) normalized UV-vis absorption spectra of P3HT:PCBM and P3HT:PCBB-C8 blend films with and without thermal annealing; (d) photographs of spin-coated P3HT:PCBM and P3HT:PCBB-C8 blend films before and after thermal annealing. unoccupied molecular orbital (LUMO) energy levels of PCBB-C8 and PCBM ( 3.80 eV and 3.83 eV) are attributed to similar fullerene skeletons. Meanwhile, the two fullerene derivatives showed exactly the same UV-vis absorption spectra in shapes and intensity for the same dilute concentration in chloroform (Fig. 1b). All these results indicate that the tri(octyloxy)benzene substituent does not aﬀect the electronic structure of the molecule. To test for self-organizing properties of P3HT and PCBB-C8 in the solid state, we investigated the UV-vis absorption spectra of P3HT:fullerene derivative blend films (Fig. 1c). Generally, the thermally annealed P3HT:PCBM blend film shows much more flat and redshifted peaks at around 500 nm and 554 nm (P3HT p–p* transitions), and a clearer vibronic shoulder at 604 nm (P3HT inter-chain interaction), respectively, compared with that of blend films before thermal annealing. All of these features are the typical absorption peaks for the self-organized P3HT crystallites, which are induced by the thermal annealing process.5,11 Interestingly, the P3HT:PCBB-C8 blend film without the thermal annealing process exhibits a shape nearly identical to that of the thermally annealed P3HT:PCBM blend film, with an even larger red shift at around 500 nm, and higher vibronic shoulder intensity at 554 nm and 604 nm. Even after thermal annealing, the absorption spectrum of the P3HT:PCBB-C8 blend film shows no obvious diﬀerence. All these suggested that the P3HT:PCBB-C8 blend film has already formed highly crystalline P3HT before thermal annealing.12 This result can be further confirmed by the violet color emission of P3HT:PCBB-C8 blend films, which is even more violet than that of thermally annealed P3HT:PCBM blend films (Fig. 1d). To gain an understanding of the self-organized microstructures and confirm the crystallization of P3HT:PCBB-C8 blend films, out-ofplane X-ray diﬀraction (XRD) was used to determine the stacking mode. As shown in Fig. 2a, the P3HT:PCBM blend film without thermal annealing shows a broad and weak diﬀraction peak at q = 0.39 Å. It indicates that P3HT possesses low crystallinity or an amorphous phase, which is caused by the highly dispersed PCBM among the P3HT chains and restricted free regions for P3HT crystallization.9,13–15 For the P3HT:PCBM blend film with thermal annealing and the P3HT:PCBB-C8 blend film without thermal annealing, sharp diﬀraction peaks at q = 0.39 Å corresponding to Chem. Commun. ChemComm Fig. 2 (a) Left: the out-of-plane XRD spectra of P3HT:PCBM and P3HT:PCBB-C8 blend films without and with thermal annealing, right: the P3HT crystallite structure; (b) the speculative self-organized process of P3HT:PCBB-C8 blend films by the solution spin-coating method. the (100) orientation for the P3HT lamellar organized crystallites13 are observed. The slightly lower intensity and the same angle of diﬀraction peaks at q = 0.39 Å of the P3HT:PCBB-C8 blend film suggest slightly decreased crystallinity of P3HT and the same lamellar d-spacing between them. Analogous intensity and angle of diﬀraction peaks of the P3HT:PCBB-C8 blend film without and with thermal annealing further confirm that the crystallization of P3HT is mainly determined by the inducement of PCBB-C8 during the spin-coating process. Overall, the tri(octyloxy)benzene bulk substituent acts as a driving force for thermal annealing to induce the crystallization of P3HT. As illustrated in Fig. 2a (right), the calculated d-spacing value of crystallite arrangement of interdigitated P3HT side-chains from X-ray diﬀraction agrees well with that reported in the literature.14 Transmission electron microscopy (TEM) was used to further investigate their microstructure and nanoscale morphology. After thermal annealing the P3HT:PCBM blend film shows obvious changes in the long fibrillar P3HT crystallites (in the range of 40–65 nm) than in the state before annealing (Fig. 3a and b), which are broadly distributed in the image with an optimum morphology for high-performance BHJ PSCs.10 In spite of this, the thermally grown PCBM clusters as illustrated using in situ scanning electron microscopy (SEM) (Fig. S8a–d, ESI†) are unable to participate in the well ordered BHJ morphology, and may cause defects in the electronic properties.15 In this study, the P3HT:PCBB-C8 blend film without thermal annealing has relatively smaller P3HT crystal size (in the range of 20–35 nm) and good phase separation in the TEM image (Fig. 3c). No obvious PCBB-C8 aggregation (Fig. S8e–h, ESI†) and slightly Fig. 3 TEM images obtained by the blend films spin-coated with chlorobenzene in 10 mg mL 1 (a) P3HT:PCBM (w/w, 1 : 1) without thermal annealing; (b) P3HT:PCBM (w/w, 1 : 1) with thermal annealing at 150 1C for 10 min; (c) P3HT:PCBB-C8 (w/w, 1 : 1) without thermal annealing. This journal is c The Royal Society of Chemistry 2013 View Article Online ChemComm Communication Downloaded by Soochow University China on 20/04/2013 02:39:00. Published on 08 April 2013 on http://pubs.rsc.org | doi:10.1039/C3CC41321G Fig. 4 (a) Current–voltage characteristics of photovoltaic cells based on P3HT:PCBM and P3HT:PCBB-C8; (b) the external quantum eﬃciency (EQE) spectra of devices based on P3HT:PCBM and P3HT:PCBB-C8. Device fabrication conditions: ablend film without the thermal annealing process; bblend film with thermal annealing at 150 1C for 10 min. varied roughnesses in Atomic Force Microscopy (AFM) (Fig. S9e–h, ESI†) during the thermal annealing process indicate that the selforganized P3HT crystallites were induced by the free region (bulk tri(octyloxy)benzene moiety) of PCBB-C8 during the spin-coating process. Considering the previous results and the mechanism of P3HT crystallization, the speculative self-organized process of the P3HT:PCBB-C8 blend film by the solution spin-coating method is illustrated in Fig. 2b, and several conclusions can be derived. First, the bulk tri(octyloxy)benzene moiety of PCBB-C8 can decrease crystallization, diﬀusion and aggregation of PCBB-C8. Second, due to the flexible trioctyloxyl chains and weakly ordered aggregation of PCBB-C8, PCBB-C8 provides the C60-free region and shortens P3HT relaxation time for P3HT crystallization during the spin-coating process. The weak PCBB-C8 aggregation may be deemed as supramolecular C60 nano/micro-architectures using the intermolecular forces introduced by C60 (p–p) and octyloxyl chain interactions (van der Waals, vdW).16 Third, this free region is smaller than that provided by the thermal annealing P3HT:PCBM blend film, which may minimize the self-organized P3HT crystallized scale. We tested how this self-organized P3HT:PCBB-C8 blend film induced by PCBB-C8 impacted the properties of photovoltaic devices. The BHJ PSCs were fabricated under typical P3HT:PCBM device fabrication conditions.11 Fig. 4a shows the current density– voltage (J–V) characteristics of the devices, and the corresponding photovoltaic parameters are given in Table S1 (ESI†). The P3HT:PCBM based device exhibits significantly improved photovoltaic performance after thermal annealing due to the nanoscaled P3HT crystallization and good phase separation17 which have also been demonstrated by our previous results. It is interesting to find that Jsc of the P3HT:PCBB-C8 blend film without the thermal annealing process based device is significantly improved from 1.99 mA cm 2 to 8.70 mA cm 2 compared with the P3HT:PCBM based device under the same device fabrication conditions, and is even similar to that of P3HT:PCBM with the thermal annealing process. Therefore, these results firmly identify our speculation that substituting PCBB-C8 for PCBM as the acceptor material can induce P3HT to self-organize with good phase separation, higher nanoscale crystallites and improved microstructure, which can facilitate the charge separation and transportation. However, it possesses deteriorated FF compared with that of the thermally annealed P3HT:PCBM device. Even after the thermal This journal is c The Royal Society of Chemistry 2013 annealing process (Fig. S10 and Table S1, ESI†), the FF cannot be still improved. The deteriorated FF may be influenced by the unbalanced (holes vs. electrons) and decreasing charge transport and thus a higher exciton recombination rate, which is determined by the relatively higher series resistance (Rs) and lower shunt resistance (Rsh) of the devices (Table S1, ESI†). PCBB-C8 with a nonconducting long tri(octyloxy)benzene moiety given a low content of C60 in the P3HT:PCBB-C8 blend film may lead to the relatively low electron mobility of the blend film. External quantum eﬃciency (EQE) spectra of the above devices are shown in Fig. 4b. Although the P3HT:PCBB-C8 based device shows a Jsc value similar to that of the P3HT:PCBM based device (with thermal annealing), the photon harvesting ability increasing at around 510 nm is attributed to the contribution of better self-organized shorter nanoscale P3HT crystallites and interpenetrating microstructure of higher interface area induced by PCBB-C8 during the spin-coating process. In summary, we have successfully synthesized bulk moiety grafted fullerene PCBB-C8. It can provide a free region for P3HT crystallization and induce the two components to form good phase separation without any external treatment. The self-organized highly ordered P3HT:PCBB-C8 blend film gives an eﬃcient charge separation and transportation, therefore leading to high Jsc of P3HT based BHJ PSCs. Further investigations into the use of these high C60 content fullerenes as acceptor materials and the external treatment free technology for large area flexible PSC fabrication are currently ongoing. Notes and references ´chet, J. Am. Chem. Soc., 2011, 133, 1 P. M. Beaujuge and J. M. C. Fre 20009–20029. 2 S. Berson, R. De Bettignies, S. Bailly and S. Guillerez, Adv. Funct. Mater., 2007, 17, 1377–1384. 3 Y. Yao, J. Hou, Z. Xu, G. Li and Y. Yang, Adv. Funct. Mater., 2008, 18, 1783–1789. 4 J. M. Hutchinson, Prog. Polym. Sci., 1995, 20, 703–760. 5 J. Jo, S.-S. Kim, S.-I. Na, B.-K. Yu and D.-Y. 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