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New diketopyrrolopyrrole based D–A–D π-conjugated molecules: Synthesis, optical, electrochemical, morphological and photovoltaic properties
Materials Chemistry and Physics xxx (2014) 1e6
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New diketopyrrolopyrrole based DeAeD p-conjugated molecules: Synthesis, optical, electrochemical, morphological and photovoltaic properties Erika Kozma*, Dariusz Kotowski, Francesco Galeotti, Marinella Catellani, Silvia Luzzati, Fabio Bertini Istituto per lo Studio delle Macromolecole e Consiglio Nazionale delle Ricerche, via Bassini 15, 20133 Milano, Italy
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
New diketopyrrolopyrrole molecules with fused aromatic rings are presented. The influence of structural variation on the optical, electrochemical and film morphology are described. Preliminary photovoltaic devices of the as cast DPPs/PC71BM blends show similar performances with the literature data.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 April 2014 Received in revised form 22 May 2014 Accepted 19 June 2014 Available online xxx
Two solution processable donoreacceptor, p-conjugated molecules that consist of diketopyrrolopyrrole (DPP) central acceptor unit with dibenzofuran (DPP-DBF) or acenaphtene (DPP-ACN) donor substituents, were prepared by Suzuki coupling reaction. The optical, electrochemical and film forming properties of these DeAeD molecules were investigated and used as active materials in bulk heterojunction solar cells. The molecules were characterized by UVeVis spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), cyclic voltammetry (CV), atomic force microscopy (AFM). While the optical absorption and frontier orbital energy levels are less sensitive to the nature of the donor, the materials thermal properties, film morphology and the photovoltaic performances are significantly altered. © 2014 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Organic compounds Atomic force microscopy Differential scanning calorimetry
1. Introduction Symmetrical diketopyrrolopyrrole derivatives which are substituted with aryl units at the 3 and 6 positions are used as high performance organic pigments in inks, paints and plastic industry [1,2]. More recently, the strong electron-accepting property and the
* Corresponding author. Tel.: þ39 02 23699 739; fax: þ39 02 70636 400. E-mail addresses: [email protected], [email protected] (E. Kozma).
good solubility and processability of N,N'-dialkylated diketopyrrolopyrroles has opened new perspectives to broader applications as organic semiconductor materials, especially in organic photovoltaics (OPVs) [3e9]. OPV solar cells feature a bulk heterojunction 3D nanostructure, where a p-type organic semiconductor-donor material (conjugated polymers or small molecules) is blended with an ntype material, which are fullerene derivatives [10] or other acceptor materials [11,12]. Among other solar energy conversion devices, organic solar cells are attractive for their eco-friendly fabrication processing, mechanical flexibility and low cost. In the last years the
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development of new organic materials and non-conventional device architectures assured an extremely fast increasing of OPV solar cell performances. The research attention was focused more on the development of new donor which could fulfill the requirements to achieve high power conversion efficiencies (PCEs). Performances up to 10% were reached by using p-type conjugated polymers solar cells [13] or small molecules solar cells [14]. The use of polymeric p-type semiconductor or molecular semiconductor in OPV solar cells exhibits both advantages and limits. Donor polymers are usually easily processable but their preparations are sensible to the reproducibility and the electronic and morphological properties depend on their molecular weight. On the other hand, small molecule donor materials possess some advantages such as well-defined molecular structure and high purity along with the possibility of an easy modification of the electronic structure via chemical tailoring. One of the major limitations of the small molecule donors consists in their low molecular weight which has a strong impact on the charge transport properties and on the morphological properties. N,N'-dialkylated diketopyrrolopyrrole derivatives, characterized by strong electron-accepting properties along with good stabilities, can be exploited in the preparation of a wide series of donoreacceptor molecular structures with low band-gap (DeA, DeAeD or AeDeA) for application in OPV devices. The introduction of fused aromatic rings at the end of DPPs could facilitate favorable pep interactions which can generate molecular self-assembly through the ending groups [15]. Understanding the effects that specific chemical moieties have on material properties is thus essential to facilitate the development of good performing materials. In this work, the synthesis of new diketoppyrolopyrrole based molecules end-capped with dibenzofuran (DPP-DBF) and acenaphtene (DPP-ACN) is described and the effects of the structural variations on the electronic properties of the chromophores is discussed. 2. Experimental 2.1. Materials and instruments 3,6-di-(2-thienyl)-2,5-dihydropyrrolo-[3,4-c]pyrrole-1,4-dione and 1-bromo-2-ethylhexane were purchased from TCI Europe and 4-(dibenzofuranyl)boronic acid, acenaphtene-5-boronic acid, tetrakis(triphenylphosphine) palladium(0) were purchased from Aldrich. The solvents were dried and distilled and the reactions were done under inert gas atmosphere. 1 H NMR spectra were recorded on a 400 MHz Bruker spectrometer operating at 11.7 T. Differential Scanning Calorimetry (DSC) measurements were carried out on a PerkineElmer Pyris 1 instrument. The samples were placed in a sealed aluminum pan, heated to 250 C and held at this temperature for 3 min to cancel previous thermal history. The measurements were carried out from 50 to 200 C under nitrogen atmosphere using heating and cooling rates of 10 C min1. The annealed samples were prepared by cooling the molecules from 250 to 20 C under controlled conditions (10 C min1) and annealing them for long times (5e24 h) at 50 C. Thermogravimetric analysis (TGA) was performed on a PerkineElmer TGA 7 instrument with a platinum pan using 1 mg of material as probe. Before performing TGA run, the sample was held at 100 C for 1 h; the scans were carried out from 50 to 700 C at heating rate of 10 C min1 in a nitrogen atmosphere at a flow rate of 35 ml min1. The UVeVis spectra were performed with a Perkin Elmer Lambda 9 spectrophotometer on chloroform solutions or spin coated films on quartz.
Cyclic voltammetry measurements were performed under nitrogen atmosphere with a computer controlled Amel 2053 (with Amel 7800 interface) electrochemical workstation in a three electrode single-compartment cell using platinum electrodes and SCE as standard electrode, with Fc/Fcþ redox couple as internal standard with a tetrabutylammonium tetrafluoroborate solution (0.1 M) in anhydrous dichloromethane. AFM investigations were performed using a NT-MDT NTEGRA apparatus in tapping mode under ambient conditions. 2.2. Synthesis Scheme 1 outlines the synthesis of DPP-DBF and DPP-ACN. 2.2.1. 3,6-bis-(4-dibenzofuran-2-yl)-thiophen-2-yl)-2,5-di-(2ethyl-hexyl)-pyrrolo[3,4-c]pyrrole-1,4-dione (DPP-DBF) In an oven dried 50 ml schlenk, 3,6-bis-(5-bromothiophene-2yl)-2,5-di-(2-ethyl-hexyl)-pyrrolo[3,4-c]pyrrole-1,4-dione [16] (1) (0.340 g, 0.5 mmol) was mixed with 8 ml anhydrous toluene and 4 ml of 2.0 M potassium carbonate and the resulting mixture was degassed for 10 min. 4-(Dibenzofuranyl)boronic acid (0.238 g, 1.125 mmol), tetrakis(triphenylphosphine)palladium (7 mg) were added to the mixture and degassed for an additional 5 min. The reaction mixture was stirred and heated to 90 C under nitrogen overnight. The reaction mixture was allowed to cool down to room temperature, after which it was poured into 100 ml methanol and the stirred for 30 min. The precipitated solid was then collected by vacuum filtration and washed with several portions of methanol. The crude product was purified by column chromatography using chloroform as eluent. The solvent was removed, to obtain a dark purple powder (yield 63.5%). 1 H NMR (CDCl3): 9.14 (d, J ¼ 4.12 Hz, 2H, thiophene), 8.05 (d, J ¼ 4.12 Hz, 2H, thiophene), 8.00 (d, 2H, arom.), 7.94 (d, 2H, arom.), 7.82 (d, 2H, arom.), 7.67 (d, 2H, arom.), 7.52 (t, 2H, arom.), 7.41 (m, 4H, arom.), 4.18 (m, 4H, -NeCH2e), 2.05 (m, 2H, eCH), 1.44 (m, 10H, eCH2), 1.29 (m, 6H, eCH2), 0.98 (t, 6H, eCH3), 0.88 (m, 6H, eCH3). 13 C NMR (CDCl3) 164.62, 161.37, 155.93, 148.52, 137.85, 135.80, 132.28, 132.07, 130.48, 128.79, 128.71, 128.12, 127,54, 126.69, 126.46, 124.72, 124.51, 123.88, 107.92, 45.93, 39.28, 30.12, 28.23, 23.46, 23.04, 14.01, 10.43. Elem.Anal.Calcd. for C54H52N2O4S2: C, 75.67%; H, 6.11%; N, 3.27%; O, 7.47%; S, 7.48%. Found: C, 75.05%; H, 6.06%; N, 3.32%; O, 7.31%; S, 7.32%. 2.2.2. 3,6-bis-(5-acenaphten-2-yl)-thiophen-2-yl)-2,5-di-(2-ethylhexyl)-pyrrolo[3,4-c]pyrrole-1,4-dione (DPP-ACN) In an oven dried 50 ml schlenk, 3,6-bis-(5-bromothiophene-2yl)-2,5-di-(2-ethyl-hexyl)-pyrrolo[3,4-c]pyrrole-1,4-dione [16] (1) (0.340 g, 0.5 mmol) was mixed with 8 ml anhydrous toluene and 4 ml of 2.0 M potassium carbonate and the resulting mixture was degassed for 10 min. Acenaphthene-5-boronic acid (0.222 g, 1.125 mmol), tetrakis(triphenylphosphine)palladium (7 mg) were added to the mixture and degassed for an additional 5 min. The reaction mixture was stirred and heated at 90 C under nitrogen overnight. The reaction mixture was allowed to cool down to room temperature, after which it was poured into 100 ml methanol and the stirred for 30 min. The precipitated solid was then collected by vacuum filtration and washed with several portions of methanol. The crude product was purified by column chromatography using chloroform as eluent. The solvent was removed, to obtain a blue powder (yield 68%). 1 H NMR (CDCl3): 9.05 (d, J ¼ 4.12 Hz, 2H, thiophene), 8.01 (d, J ¼ 4.12 Hz, 2H, thiophene), 7.63 (d, 2H, arom.), 7.53 (t, 2H, arom.),
Please cite this article in press as: E. Kozma, et al., New diketopyrrolopyrrole based DeAeD p-conjugated molecules: Synthesis, optical, electrochemical, morphological and photovoltaic properties, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/ j.matchemphys.2014.06.048
E. Kozma et al. / Materials Chemistry and Physics xxx (2014) 1e6
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(i) Pd[PPh3]4, K2CO3 2M, THF and 4-(dibenzofuranyl)boronic acid (DPP-DBF) or acenaphtene-5-boronic acid (DPP-ACN) Scheme 1. Synthetic route toward DPP-DBF and DPP-ACN. (i) Pd[PPh3]4, K2CO3 2M, THF and 4-(dibenzofuranyl)boronic acid (DPP-DBF) or acenaphtene-5-boronic acid (DPP-ACN).
7.47 (d, 2H, arom.), 7.35 (m, 4H, arom.), 4.04 (m, 4H, -NeCH2e), 3.41 (m, 8H, eCH2-CH2-), 1.98 (m, 2H, eCH), 1.35 (m, 10H, eCH2), 1.28 (m, 6H, eCH2), 0.87 (t, 6H, eCH3), 0.82 (t, 6H, eCH3). 13C NMR (CDCl3) 159.84, 149.72, 139.81, 136.69, 133.42, 133.23, 130.24, 129.06, 128.89, 128.14, 127.80, 127.10, 126.84, 124.93, 124.78, 123.92, 108.25, 48.57, 48.43, 46.03, 39.32, 30.39, 28.55, 23.67, 23.24, 14.17, 10.59. Elem.Anal.Calcd. for C54H56N2O2S2: C, 78.22%; H, 6.81%; N, 3.38%; O, 3.86%; S, 7.73%. Found: C, 77.89%; H, 6.64%; N, 3.54%; O, 3.81%; S, 7.52%.
300e420 nm and 450e650 nm. The strongest absorption band at 450e650 nm is ascribed to the intramolecular charge transfer (ICT) band generated by DPP and the fused aromatic rings. The other absorption bands can be resulted from pep* transitions of the
3. Results and discussion The molecular structures of the novel DPP derivatives were designed on the basis of D-A-D framework with one electronwithdrawing DPP core and electron donating dibenzofuran (DPPDBF) and acenaphtene (DPP-ACN). Scheme 1 depicts the synthesis and structures of the compounds studied here. Compound 1 was synthetized according to the literature [16], using a two step approach: first 3,6-di-(2thienyl)-2,5-dihydropyrrolo-[3,4-c]pyrrole-1,4-dione was treated with 1-bromo-2-ethylhexane at 130 C in DMF in the presence of K2CO3, followed by a dibromination reaction with N-bromosuccinimide in CHCl3, giving a red colored crystalline solid in moderate yield (40%). The dibrominated precursor was then treated with 2.2 equiv of the appropriate fused aromatic boronic acid, giving DPPDBF and DPP-ACN. Both molecules are soluble in common organic solvents, such as chloroform, dichloromethane, THF, toluene. The effect of DPP core substitution with different fused aromatic rings on optical properties, thermal properties, and film morphology was studied by UVeVis absorption spectroscopy, cyclic voltammetry, differential scanning calorimetry and atomic force microscopy. The electronic properties of the compounds have been analyzed by UVeVis absorption spectroscopy and cyclic voltammetry. The UVeVis data recorded in chloroform solutions and thin films are depicted in Fig. 1. In solution, the UVeVis absorption spectra displays mainly two primary absorption bands in the ranges of
Fig. 1. UVeVis absorption spectra of DPP-DBF and DPP-ACN in chloroform solution (a) and thin films (b) as cast (solid line) and upon thermal treatment (80 C for 5 min, dashed line).
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ending fused aromatic rings and their conjugation with the thienyl linker. Both compounds have similar solution absorption profiles. Compared to the absorption in solution, the charge transfer band of the as cast film and annealed film of both compounds is broadened and significantly red shifted, indicating strong molecular interactions and more planar conformation of the conjugated backbone in the solid state [17]. The red-shift in film state of al DPPs as compared to the solution state suggests that pep stacking due to the intermolecular interaction in the solid state is more favorable than in solution. Upon annealing, DPP-DBF has similar absorption profiles as well as the position of vibronic peaks, in comparison with the as cast films. In contrast, DPP-ACN exhibit a sharper and red shifted vibronic peak, which could be an indication of a different molecular packing and a higher crystallinity. The optical band gaps (Eg) of DPP-DBF and DPP-ACN estimated from absorption onset wavelengths (Eg ¼ 1240/lonset) of the oligomer films are 1.80 and 1.81 eV, respectively. The electrochemical properties of the compounds in anhydrous dichloromethane were examined using cyclic voltammetry (SI-2). The oxidation and reduction potentials of the DPP based molecules, obtained in dichloromethane solution using SCE as reference are summarized in Table 1. The HOMO and LUMO energy levels were calculated from the electrochemical potentials and the optical band gap. In donoreacceptor type small molecules based on pushepull structure, the LUMO energy level is controlled by the electronacceptor unit, while the HOMO energy level is governed by the electron rich moiety. Similar results have been observed in DPPbased donoreacceptor polymers in which the LUMO was determined by the DPP unit and the HOMO was dependent on the donor moiety [18]. Since DPP-DBF and DPP-ACN have the same electron deficient unit (DPP), the LUMO level changes very slightly with the nature of the fused aromatic ring. The HOMO energy level in this kind of molecules is dictated by the electron donating dibenzofuran and acenaphtene, respectively. In this particular case, since the electron donating power of these fused aromatic rings is quite similar, also the HOMO energy level are identical. As shown by these optical and electrochemical results, the nature of the fused aromatic ring has little influence on the energy level of the frontier orbitals of the molecule and hence on the band gap of the resulting material. In order to get better insights of these materials, thermal and morphological studies were carried out. The thermal stability of the novel diketopyrrolopyrrole molecules was determined by TGA considering the onset of thermal decomposition, i.e. the temperature corresponding to initial 5% of weight loss (Td). TGA traces reported in Fig. 2a show that DPP-DBF and DPP-ACN are thermally stable in inert atmosphere with Td of 364 and 337 C, respectively. This is comparable to the thermal stability of other reported DPP-based materials [7,19e21]. The thermal behavior of the DPP-based molecules was evaluated by DSC analysis. To erase the effect of thermal history, the cooling and second heating scan loop of each compound was used to determine thermal transition temperatures (Fig. 2b). The cooling run of DPP-DBF shows a sharp exothermic transition centered at Table 1 Electrochemical characteristics, HOMOeLUMO values and energy band gap for DPPDBF and DPP-ACN. Sample
E0ox (V)
E0red (V)
HOMO (eV)
LUMO (eV)
Eopt g (eV)
DPP-DBF DPP-ACN
0.86 0.85
0.63 0.70
5.36 5.35
3.56 3.54
1.80 1.81
E0ox, onset oxidation potential; E0red, onset reduction potential; E0ox and E0red were opt determined by cyclic voltammetry to SCE; LUMO ¼ HOMO-Eopt g ; Eg , optical energy band gap (calculated from the onset of the absorption in thin film).
Fig. 2. (a) TGA curves of DPP-DBF and DPP-ACN under inert atmosphere. (b) DSC scans of DPP-DBF and DPP-ACN from cooling (dashed line) and second heating (solid line) cycle, and heating curve of DPP-ACN after annealing at 50 C for 24 h.
168 C, while the subsequent heating scan displays an endothermic peak at 204 C with a heat flow of 42 J g1 due to the crystal melting, which is a typical behavior for DPP based molecules [5]. In contrast, DSC scans carried out in the temperature range 50e250 C at 10 C min1 of DPP-ACN do not show any thermal event. Furthermore, DSC runs carried out at lower scanning rate (2 C min1) showed the same featureless profile. As discussed later, the DPP-ACN surface morphology changes substantially after thermal treatment (AFM measurements). For a better understanding of these features, we investigated the DPP-ACN thermal behavior after prolonged annealing. Indeed, the DSC heating scan of the annealed sample (50 C for 24 h) shows a cold crystallization peak at 131 C followed by an endothermic event centered at 160 C with an enthalpy of fusion of 28 J g1. The characteristics observed in DPP-ACN thermograms are typically correlated to a low crystallization ability. We investigated the morphology of DPP-DBF and DPP-ACN thin films (spin-coating of 20 mg ml1 toluene solutions on silicon substrates), using tapping mode AFM. Fig. 3 reports representative scans of the two materials, before and after thermal annealing. Both DPPs exhibit cluster structure with very small domains and substantially smooth surface, with a root-mean-square (RMS) roughness of 0.71 and 0.28 nm. After annealing at 80 C, the DPPDBF grains size slightly grows up and RMS increases to 2.94 nm. This behavior is well-matched with the crystalline nature of this material, as observed by thermal analysis (Fig. 2b). On the other hand, in DPP-ACN film, the surface morphology changes significantly after annealing. In fact, the thermal annealing promotes the formation of lamellar grains of 2e400 nm of lateral dimension, and the RMS roughness increases to 3.51 nm. The tendency of DPP-ACN to form large crystalline domains when heated at 80 C agrees well with the crystallization features pointed out by
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Fig. 3. Tapping mode AFM images of DPP-DBF and DPP-ACN thin films as cast (top) and after thermal annealing (80 C for 5 min, bottom). Below each image, the corresponding cross section profile along the white line is reported.
DSC (Fig. 2b), and accounts for the observed evolution of the absorption spectrum of the corresponding films upon heating (Fig. 1b). Photovoltaic properties of DPP-DBF and DPP-ACN were investigated in bulk heterojunction solar cells, with PC71BM as acceptor, having ITO/PEDOT:PSS/DPPs:PC71BM/Al structure. Preliminary PCEs of 0.23% and respectively 0.52% were obtained for the as cast devices (SI-3), which are in concordance with the literature data for non optimized devices [22,23]. DPP-ACN exhibits higher external quantum efficiency and better photovoltaic parameters, which correlates well to its better performance. The VOC, which depends on the difference between the LUMO of the acceptor and HOMO of the donor, has high values in both cases (0.78 V and 0.81 V,
respectively) due to the deep HOMO level of DPP-DBF and DPPACN, while the JSC and FF are low. The latest parameters are strongly morphological dependent and therefore the optimization of the photovoltaic parameters has to be focused on finding the most favorable condition for better performances. 4. Conclusions In summary, we designed, synthesized and characterized novel diketopyrrolopyrrole based molecules, substituted with different fused aromatic rings. Optical and electrochemical data show that the nature of the fused aromatic ring has little effect on the material's energy level,
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but strongly affects the thermal, morphological and photovoltaic properties. This suggests that besides appropriate light harvesting properties and energy levels, the ability of the DPP-based molecules to organize in the solid state must be taken into account for designing new efficient materials. Work is in progress to elucidate the effect of thermal annealing on the active layer morphology and optimization of the photovoltaic performances. Acknowledgments This work has been supported by Accordo Quadro between Regione Lombardia and CNR e Cluster Project ‘Energy’ n 17348, by Regione Lombardia, by the E.U. Marie Curie Reintegration Grant “DAMASCO” FP7-PEOPLE-2010-RG-268229 and the Fondazione Cariplo Project “PLENOS”, Ref. 2011-0349. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2014.06.048. References [1] A. Iqbal, L. Cassar, A.C. Rochat, J. Pfenniger, O.J. Wallquist, Coat. Technol. 60 (1988) 37. [2] Z. Hao, A. Iqbal, Chem. Soc. Rev. 26 (1997) 203. [3] S. Qu, H. Tian, Chem. Commun. 48 (2012) 3039.
[4] B. Walker, A.B. Tamayo, X.D. Dang, P. Zalar, J.H. Seo, A. Garcia, M. Tantiwiwat, T.-Q. Nguyen, Adv. Funct. Mater. 19 (2009) 3063. [5] A.B. Tamayo, T.-Q. Nguyen, J. Phys. Chem. C 2 (2008) 11545. [6] J. Zhang, C. He, Z.G. Zhang, D. Deng, M. Zhang, Y. Li, Philos. Trans. A Math. Phys. Eng. Sci. 372 (2014) 20130009. [7] J. Liu, B. Walker, A. Tamayo, Y. Zhang, T.-Q. Nguyen, Adv. Funct. Mater. 23 (2013) 47. [8] Y.S. Choi, W.H. Jo, Org. Electron 14 (2013) 1621. [9] Y. Lin, L. Ma, Y. Li, Y. Liu, D. Zhou, X. Zhan, Adv. Energy Mater. 3 (2013) 1166. [10] A.J. Heeger, Adv. Mater. 26 (2014) 10. [11] A.F. Eftaiha, J.-P. Sun, I.G. Hill, G.C. Welch, J. Mater, Chem. A 2 (2014) 1201. [12] E. Kozma, M. Catellani, Dyes Pigm. 98 (2013) 160. [13] M.A. Green, K. Emergy, Y. Hishikawa, W. Warta, E.D. Dunlop, Prog. Photovoltaics 20 (2012) 12. [14] Y. Liu, C.C. Chen, Z. Hong, J. Gao, Y.M. Yang, H. Zhou, L. Dou, G. Li, Y. Yang, Sci. Rep. 3 (2013) 3356. [15] O.P. Lee, A.T. Yiu, P.M. Beaujuge, C.H. Woo, T.W. Holcombe, J.E. Millstone, chet, Adv. Mater. 23 (2011) 5359. J.D. Douglas, M.S. Chen, M.J. Fre tol, M. Allain, R. Po, [16] E. Ripaud, D. Demeter, T. Rousseau, E. Boucard-Ce P. Leriche, J. Roncali, Dyes Pigm. 95 (2012) 126. [17] J.-W. Mun, I. Cho, D. Lee, W.S. Yoon, O.K. Kwon, C. Lee, S.Y. Park, Org. Electron 14 (2013) 2341. [18] Y. Zou, D. Gendron, R. Neagu-Plesu, M. Leclerc, Macromolecules 42 (2009) 6361. [19] P. Sonar, G.-M. Ng, T.T. Lin, A. Dodabalapur, Z.-K. Chen, J. Mater. Chem. 20 (2010) 3626. [20] W. Shin, T. Yasuda, G. Watanabe, Y.S. Yang, C. Adachi, Chem. Mater. 25 (2013) 2549. [21] H. Bai, P. Cheng, Y. Wang, L. Ma, Y. Li, D. Zhu, X. Zhan, J. Mater. Chem. A 2 (2014) 778. [22] H. Shi, Z. Gu, X. Gu, H. Pan, X. Hu, C. Fan, M. Shi, H. Chen, Synth. Met. 183 (2014) 66. [23] Y. Kim, C.E. Song, A. Cho, J. Kim, Y. Eom, J. Ahn, S.J. Moon, E. Lim, Mater. Chem. Phys. 143 (2014) 825.
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Materials science communication
New diketopyrrolopyrrole based DeAeD p-conjugated molecules: Synthesis, optical, electrochemical, morphological and photovoltaic properties Erika Kozma*, Dariusz Kotowski, Francesco Galeotti, Marinella Catellani, Silvia Luzzati, Fabio Bertini Istituto per lo Studio delle Macromolecole e Consiglio Nazionale delle Ricerche, via Bassini 15, 20133 Milano, Italy
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
New diketopyrrolopyrrole molecules with fused aromatic rings are presented. The influence of structural variation on the optical, electrochemical and film morphology are described. Preliminary photovoltaic devices of the as cast DPPs/PC71BM blends show similar performances with the literature data.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 April 2014 Received in revised form 22 May 2014 Accepted 19 June 2014 Available online xxx
Two solution processable donoreacceptor, p-conjugated molecules that consist of diketopyrrolopyrrole (DPP) central acceptor unit with dibenzofuran (DPP-DBF) or acenaphtene (DPP-ACN) donor substituents, were prepared by Suzuki coupling reaction. The optical, electrochemical and film forming properties of these DeAeD molecules were investigated and used as active materials in bulk heterojunction solar cells. The molecules were characterized by UVeVis spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), cyclic voltammetry (CV), atomic force microscopy (AFM). While the optical absorption and frontier orbital energy levels are less sensitive to the nature of the donor, the materials thermal properties, film morphology and the photovoltaic performances are significantly altered. © 2014 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Organic compounds Atomic force microscopy Differential scanning calorimetry
1. Introduction Symmetrical diketopyrrolopyrrole derivatives which are substituted with aryl units at the 3 and 6 positions are used as high performance organic pigments in inks, paints and plastic industry [1,2]. More recently, the strong electron-accepting property and the
* Corresponding author. Tel.: þ39 02 23699 739; fax: þ39 02 70636 400. E-mail addresses: [email protected], [email protected] (E. Kozma).
good solubility and processability of N,N'-dialkylated diketopyrrolopyrroles has opened new perspectives to broader applications as organic semiconductor materials, especially in organic photovoltaics (OPVs) [3e9]. OPV solar cells feature a bulk heterojunction 3D nanostructure, where a p-type organic semiconductor-donor material (conjugated polymers or small molecules) is blended with an ntype material, which are fullerene derivatives [10] or other acceptor materials [11,12]. Among other solar energy conversion devices, organic solar cells are attractive for their eco-friendly fabrication processing, mechanical flexibility and low cost. In the last years the
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development of new organic materials and non-conventional device architectures assured an extremely fast increasing of OPV solar cell performances. The research attention was focused more on the development of new donor which could fulfill the requirements to achieve high power conversion efficiencies (PCEs). Performances up to 10% were reached by using p-type conjugated polymers solar cells [13] or small molecules solar cells [14]. The use of polymeric p-type semiconductor or molecular semiconductor in OPV solar cells exhibits both advantages and limits. Donor polymers are usually easily processable but their preparations are sensible to the reproducibility and the electronic and morphological properties depend on their molecular weight. On the other hand, small molecule donor materials possess some advantages such as well-defined molecular structure and high purity along with the possibility of an easy modification of the electronic structure via chemical tailoring. One of the major limitations of the small molecule donors consists in their low molecular weight which has a strong impact on the charge transport properties and on the morphological properties. N,N'-dialkylated diketopyrrolopyrrole derivatives, characterized by strong electron-accepting properties along with good stabilities, can be exploited in the preparation of a wide series of donoreacceptor molecular structures with low band-gap (DeA, DeAeD or AeDeA) for application in OPV devices. The introduction of fused aromatic rings at the end of DPPs could facilitate favorable pep interactions which can generate molecular self-assembly through the ending groups [15]. Understanding the effects that specific chemical moieties have on material properties is thus essential to facilitate the development of good performing materials. In this work, the synthesis of new diketoppyrolopyrrole based molecules end-capped with dibenzofuran (DPP-DBF) and acenaphtene (DPP-ACN) is described and the effects of the structural variations on the electronic properties of the chromophores is discussed. 2. Experimental 2.1. Materials and instruments 3,6-di-(2-thienyl)-2,5-dihydropyrrolo-[3,4-c]pyrrole-1,4-dione and 1-bromo-2-ethylhexane were purchased from TCI Europe and 4-(dibenzofuranyl)boronic acid, acenaphtene-5-boronic acid, tetrakis(triphenylphosphine) palladium(0) were purchased from Aldrich. The solvents were dried and distilled and the reactions were done under inert gas atmosphere. 1 H NMR spectra were recorded on a 400 MHz Bruker spectrometer operating at 11.7 T. Differential Scanning Calorimetry (DSC) measurements were carried out on a PerkineElmer Pyris 1 instrument. The samples were placed in a sealed aluminum pan, heated to 250 C and held at this temperature for 3 min to cancel previous thermal history. The measurements were carried out from 50 to 200 C under nitrogen atmosphere using heating and cooling rates of 10 C min1. The annealed samples were prepared by cooling the molecules from 250 to 20 C under controlled conditions (10 C min1) and annealing them for long times (5e24 h) at 50 C. Thermogravimetric analysis (TGA) was performed on a PerkineElmer TGA 7 instrument with a platinum pan using 1 mg of material as probe. Before performing TGA run, the sample was held at 100 C for 1 h; the scans were carried out from 50 to 700 C at heating rate of 10 C min1 in a nitrogen atmosphere at a flow rate of 35 ml min1. The UVeVis spectra were performed with a Perkin Elmer Lambda 9 spectrophotometer on chloroform solutions or spin coated films on quartz.
Cyclic voltammetry measurements were performed under nitrogen atmosphere with a computer controlled Amel 2053 (with Amel 7800 interface) electrochemical workstation in a three electrode single-compartment cell using platinum electrodes and SCE as standard electrode, with Fc/Fcþ redox couple as internal standard with a tetrabutylammonium tetrafluoroborate solution (0.1 M) in anhydrous dichloromethane. AFM investigations were performed using a NT-MDT NTEGRA apparatus in tapping mode under ambient conditions. 2.2. Synthesis Scheme 1 outlines the synthesis of DPP-DBF and DPP-ACN. 2.2.1. 3,6-bis-(4-dibenzofuran-2-yl)-thiophen-2-yl)-2,5-di-(2ethyl-hexyl)-pyrrolo[3,4-c]pyrrole-1,4-dione (DPP-DBF) In an oven dried 50 ml schlenk, 3,6-bis-(5-bromothiophene-2yl)-2,5-di-(2-ethyl-hexyl)-pyrrolo[3,4-c]pyrrole-1,4-dione [16] (1) (0.340 g, 0.5 mmol) was mixed with 8 ml anhydrous toluene and 4 ml of 2.0 M potassium carbonate and the resulting mixture was degassed for 10 min. 4-(Dibenzofuranyl)boronic acid (0.238 g, 1.125 mmol), tetrakis(triphenylphosphine)palladium (7 mg) were added to the mixture and degassed for an additional 5 min. The reaction mixture was stirred and heated to 90 C under nitrogen overnight. The reaction mixture was allowed to cool down to room temperature, after which it was poured into 100 ml methanol and the stirred for 30 min. The precipitated solid was then collected by vacuum filtration and washed with several portions of methanol. The crude product was purified by column chromatography using chloroform as eluent. The solvent was removed, to obtain a dark purple powder (yield 63.5%). 1 H NMR (CDCl3): 9.14 (d, J ¼ 4.12 Hz, 2H, thiophene), 8.05 (d, J ¼ 4.12 Hz, 2H, thiophene), 8.00 (d, 2H, arom.), 7.94 (d, 2H, arom.), 7.82 (d, 2H, arom.), 7.67 (d, 2H, arom.), 7.52 (t, 2H, arom.), 7.41 (m, 4H, arom.), 4.18 (m, 4H, -NeCH2e), 2.05 (m, 2H, eCH), 1.44 (m, 10H, eCH2), 1.29 (m, 6H, eCH2), 0.98 (t, 6H, eCH3), 0.88 (m, 6H, eCH3). 13 C NMR (CDCl3) 164.62, 161.37, 155.93, 148.52, 137.85, 135.80, 132.28, 132.07, 130.48, 128.79, 128.71, 128.12, 127,54, 126.69, 126.46, 124.72, 124.51, 123.88, 107.92, 45.93, 39.28, 30.12, 28.23, 23.46, 23.04, 14.01, 10.43. Elem.Anal.Calcd. for C54H52N2O4S2: C, 75.67%; H, 6.11%; N, 3.27%; O, 7.47%; S, 7.48%. Found: C, 75.05%; H, 6.06%; N, 3.32%; O, 7.31%; S, 7.32%. 2.2.2. 3,6-bis-(5-acenaphten-2-yl)-thiophen-2-yl)-2,5-di-(2-ethylhexyl)-pyrrolo[3,4-c]pyrrole-1,4-dione (DPP-ACN) In an oven dried 50 ml schlenk, 3,6-bis-(5-bromothiophene-2yl)-2,5-di-(2-ethyl-hexyl)-pyrrolo[3,4-c]pyrrole-1,4-dione [16] (1) (0.340 g, 0.5 mmol) was mixed with 8 ml anhydrous toluene and 4 ml of 2.0 M potassium carbonate and the resulting mixture was degassed for 10 min. Acenaphthene-5-boronic acid (0.222 g, 1.125 mmol), tetrakis(triphenylphosphine)palladium (7 mg) were added to the mixture and degassed for an additional 5 min. The reaction mixture was stirred and heated at 90 C under nitrogen overnight. The reaction mixture was allowed to cool down to room temperature, after which it was poured into 100 ml methanol and the stirred for 30 min. The precipitated solid was then collected by vacuum filtration and washed with several portions of methanol. The crude product was purified by column chromatography using chloroform as eluent. The solvent was removed, to obtain a blue powder (yield 68%). 1 H NMR (CDCl3): 9.05 (d, J ¼ 4.12 Hz, 2H, thiophene), 8.01 (d, J ¼ 4.12 Hz, 2H, thiophene), 7.63 (d, 2H, arom.), 7.53 (t, 2H, arom.),
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(i) Pd[PPh3]4, K2CO3 2M, THF and 4-(dibenzofuranyl)boronic acid (DPP-DBF) or acenaphtene-5-boronic acid (DPP-ACN) Scheme 1. Synthetic route toward DPP-DBF and DPP-ACN. (i) Pd[PPh3]4, K2CO3 2M, THF and 4-(dibenzofuranyl)boronic acid (DPP-DBF) or acenaphtene-5-boronic acid (DPP-ACN).
7.47 (d, 2H, arom.), 7.35 (m, 4H, arom.), 4.04 (m, 4H, -NeCH2e), 3.41 (m, 8H, eCH2-CH2-), 1.98 (m, 2H, eCH), 1.35 (m, 10H, eCH2), 1.28 (m, 6H, eCH2), 0.87 (t, 6H, eCH3), 0.82 (t, 6H, eCH3). 13C NMR (CDCl3) 159.84, 149.72, 139.81, 136.69, 133.42, 133.23, 130.24, 129.06, 128.89, 128.14, 127.80, 127.10, 126.84, 124.93, 124.78, 123.92, 108.25, 48.57, 48.43, 46.03, 39.32, 30.39, 28.55, 23.67, 23.24, 14.17, 10.59. Elem.Anal.Calcd. for C54H56N2O2S2: C, 78.22%; H, 6.81%; N, 3.38%; O, 3.86%; S, 7.73%. Found: C, 77.89%; H, 6.64%; N, 3.54%; O, 3.81%; S, 7.52%.
300e420 nm and 450e650 nm. The strongest absorption band at 450e650 nm is ascribed to the intramolecular charge transfer (ICT) band generated by DPP and the fused aromatic rings. The other absorption bands can be resulted from pep* transitions of the
3. Results and discussion The molecular structures of the novel DPP derivatives were designed on the basis of D-A-D framework with one electronwithdrawing DPP core and electron donating dibenzofuran (DPPDBF) and acenaphtene (DPP-ACN). Scheme 1 depicts the synthesis and structures of the compounds studied here. Compound 1 was synthetized according to the literature [16], using a two step approach: first 3,6-di-(2thienyl)-2,5-dihydropyrrolo-[3,4-c]pyrrole-1,4-dione was treated with 1-bromo-2-ethylhexane at 130 C in DMF in the presence of K2CO3, followed by a dibromination reaction with N-bromosuccinimide in CHCl3, giving a red colored crystalline solid in moderate yield (40%). The dibrominated precursor was then treated with 2.2 equiv of the appropriate fused aromatic boronic acid, giving DPPDBF and DPP-ACN. Both molecules are soluble in common organic solvents, such as chloroform, dichloromethane, THF, toluene. The effect of DPP core substitution with different fused aromatic rings on optical properties, thermal properties, and film morphology was studied by UVeVis absorption spectroscopy, cyclic voltammetry, differential scanning calorimetry and atomic force microscopy. The electronic properties of the compounds have been analyzed by UVeVis absorption spectroscopy and cyclic voltammetry. The UVeVis data recorded in chloroform solutions and thin films are depicted in Fig. 1. In solution, the UVeVis absorption spectra displays mainly two primary absorption bands in the ranges of
Fig. 1. UVeVis absorption spectra of DPP-DBF and DPP-ACN in chloroform solution (a) and thin films (b) as cast (solid line) and upon thermal treatment (80 C for 5 min, dashed line).
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ending fused aromatic rings and their conjugation with the thienyl linker. Both compounds have similar solution absorption profiles. Compared to the absorption in solution, the charge transfer band of the as cast film and annealed film of both compounds is broadened and significantly red shifted, indicating strong molecular interactions and more planar conformation of the conjugated backbone in the solid state [17]. The red-shift in film state of al DPPs as compared to the solution state suggests that pep stacking due to the intermolecular interaction in the solid state is more favorable than in solution. Upon annealing, DPP-DBF has similar absorption profiles as well as the position of vibronic peaks, in comparison with the as cast films. In contrast, DPP-ACN exhibit a sharper and red shifted vibronic peak, which could be an indication of a different molecular packing and a higher crystallinity. The optical band gaps (Eg) of DPP-DBF and DPP-ACN estimated from absorption onset wavelengths (Eg ¼ 1240/lonset) of the oligomer films are 1.80 and 1.81 eV, respectively. The electrochemical properties of the compounds in anhydrous dichloromethane were examined using cyclic voltammetry (SI-2). The oxidation and reduction potentials of the DPP based molecules, obtained in dichloromethane solution using SCE as reference are summarized in Table 1. The HOMO and LUMO energy levels were calculated from the electrochemical potentials and the optical band gap. In donoreacceptor type small molecules based on pushepull structure, the LUMO energy level is controlled by the electronacceptor unit, while the HOMO energy level is governed by the electron rich moiety. Similar results have been observed in DPPbased donoreacceptor polymers in which the LUMO was determined by the DPP unit and the HOMO was dependent on the donor moiety [18]. Since DPP-DBF and DPP-ACN have the same electron deficient unit (DPP), the LUMO level changes very slightly with the nature of the fused aromatic ring. The HOMO energy level in this kind of molecules is dictated by the electron donating dibenzofuran and acenaphtene, respectively. In this particular case, since the electron donating power of these fused aromatic rings is quite similar, also the HOMO energy level are identical. As shown by these optical and electrochemical results, the nature of the fused aromatic ring has little influence on the energy level of the frontier orbitals of the molecule and hence on the band gap of the resulting material. In order to get better insights of these materials, thermal and morphological studies were carried out. The thermal stability of the novel diketopyrrolopyrrole molecules was determined by TGA considering the onset of thermal decomposition, i.e. the temperature corresponding to initial 5% of weight loss (Td). TGA traces reported in Fig. 2a show that DPP-DBF and DPP-ACN are thermally stable in inert atmosphere with Td of 364 and 337 C, respectively. This is comparable to the thermal stability of other reported DPP-based materials [7,19e21]. The thermal behavior of the DPP-based molecules was evaluated by DSC analysis. To erase the effect of thermal history, the cooling and second heating scan loop of each compound was used to determine thermal transition temperatures (Fig. 2b). The cooling run of DPP-DBF shows a sharp exothermic transition centered at Table 1 Electrochemical characteristics, HOMOeLUMO values and energy band gap for DPPDBF and DPP-ACN. Sample
E0ox (V)
E0red (V)
HOMO (eV)
LUMO (eV)
Eopt g (eV)
DPP-DBF DPP-ACN
0.86 0.85
0.63 0.70
5.36 5.35
3.56 3.54
1.80 1.81
E0ox, onset oxidation potential; E0red, onset reduction potential; E0ox and E0red were opt determined by cyclic voltammetry to SCE; LUMO ¼ HOMO-Eopt g ; Eg , optical energy band gap (calculated from the onset of the absorption in thin film).
Fig. 2. (a) TGA curves of DPP-DBF and DPP-ACN under inert atmosphere. (b) DSC scans of DPP-DBF and DPP-ACN from cooling (dashed line) and second heating (solid line) cycle, and heating curve of DPP-ACN after annealing at 50 C for 24 h.
168 C, while the subsequent heating scan displays an endothermic peak at 204 C with a heat flow of 42 J g1 due to the crystal melting, which is a typical behavior for DPP based molecules [5]. In contrast, DSC scans carried out in the temperature range 50e250 C at 10 C min1 of DPP-ACN do not show any thermal event. Furthermore, DSC runs carried out at lower scanning rate (2 C min1) showed the same featureless profile. As discussed later, the DPP-ACN surface morphology changes substantially after thermal treatment (AFM measurements). For a better understanding of these features, we investigated the DPP-ACN thermal behavior after prolonged annealing. Indeed, the DSC heating scan of the annealed sample (50 C for 24 h) shows a cold crystallization peak at 131 C followed by an endothermic event centered at 160 C with an enthalpy of fusion of 28 J g1. The characteristics observed in DPP-ACN thermograms are typically correlated to a low crystallization ability. We investigated the morphology of DPP-DBF and DPP-ACN thin films (spin-coating of 20 mg ml1 toluene solutions on silicon substrates), using tapping mode AFM. Fig. 3 reports representative scans of the two materials, before and after thermal annealing. Both DPPs exhibit cluster structure with very small domains and substantially smooth surface, with a root-mean-square (RMS) roughness of 0.71 and 0.28 nm. After annealing at 80 C, the DPPDBF grains size slightly grows up and RMS increases to 2.94 nm. This behavior is well-matched with the crystalline nature of this material, as observed by thermal analysis (Fig. 2b). On the other hand, in DPP-ACN film, the surface morphology changes significantly after annealing. In fact, the thermal annealing promotes the formation of lamellar grains of 2e400 nm of lateral dimension, and the RMS roughness increases to 3.51 nm. The tendency of DPP-ACN to form large crystalline domains when heated at 80 C agrees well with the crystallization features pointed out by
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Fig. 3. Tapping mode AFM images of DPP-DBF and DPP-ACN thin films as cast (top) and after thermal annealing (80 C for 5 min, bottom). Below each image, the corresponding cross section profile along the white line is reported.
DSC (Fig. 2b), and accounts for the observed evolution of the absorption spectrum of the corresponding films upon heating (Fig. 1b). Photovoltaic properties of DPP-DBF and DPP-ACN were investigated in bulk heterojunction solar cells, with PC71BM as acceptor, having ITO/PEDOT:PSS/DPPs:PC71BM/Al structure. Preliminary PCEs of 0.23% and respectively 0.52% were obtained for the as cast devices (SI-3), which are in concordance with the literature data for non optimized devices [22,23]. DPP-ACN exhibits higher external quantum efficiency and better photovoltaic parameters, which correlates well to its better performance. The VOC, which depends on the difference between the LUMO of the acceptor and HOMO of the donor, has high values in both cases (0.78 V and 0.81 V,
respectively) due to the deep HOMO level of DPP-DBF and DPPACN, while the JSC and FF are low. The latest parameters are strongly morphological dependent and therefore the optimization of the photovoltaic parameters has to be focused on finding the most favorable condition for better performances. 4. Conclusions In summary, we designed, synthesized and characterized novel diketopyrrolopyrrole based molecules, substituted with different fused aromatic rings. Optical and electrochemical data show that the nature of the fused aromatic ring has little effect on the material's energy level,
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but strongly affects the thermal, morphological and photovoltaic properties. This suggests that besides appropriate light harvesting properties and energy levels, the ability of the DPP-based molecules to organize in the solid state must be taken into account for designing new efficient materials. Work is in progress to elucidate the effect of thermal annealing on the active layer morphology and optimization of the photovoltaic performances. Acknowledgments This work has been supported by Accordo Quadro between Regione Lombardia and CNR e Cluster Project ‘Energy’ n 17348, by Regione Lombardia, by the E.U. Marie Curie Reintegration Grant “DAMASCO” FP7-PEOPLE-2010-RG-268229 and the Fondazione Cariplo Project “PLENOS”, Ref. 2011-0349. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2014.06.048. References [1] A. Iqbal, L. Cassar, A.C. Rochat, J. Pfenniger, O.J. Wallquist, Coat. Technol. 60 (1988) 37. [2] Z. Hao, A. Iqbal, Chem. Soc. Rev. 26 (1997) 203. [3] S. Qu, H. Tian, Chem. Commun. 48 (2012) 3039.
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