Design of perylene diimides for organic solar cell: Effect of molecular steric hindrance and extended conjugation

Materials Chemistry and Physics xxx (2015) 1e9

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Design of perylene diimides for organic solar cell: Effect of molecular steric hindrance and extended conjugation Erika Kozma a, *, Dariusz Kotowski a, Marinella Catellani a, Silvia Luzzati a, Marco Cavazzini b, Alberto Bossi b, Simonetta Orlandi b, Fabio Bertini a a b

Istituto per lo Studio delle Macromolecole, Consiglio Nazionale delle Ricerche, 20133 Milano, Italy Istituto di Scienze e Tecnologie Molecolari, Consiglio Nazionale delle Ricerche, 20133 Milano, Italy

h i g h l i g h t s
New D-A-D n-type materials have been synthesized.
PDI-SF and PDI-BSF were used as acceptors in organic solar cells.
Performances of 1.32% were achieved in blend with P3HT in a BHJ conventional architecture.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 November 2014 Received in revised form 12 June 2015 Accepted 5 July 2015 Available online xxx

Core-substituted perylene diimides (PDI) are promising candidates as n-type semiconductor materials for organic photovoltaics. The chemical functionalization of perylene diimides in the bay positions is a versatile tool to obtain a series of electron acceptor materials with tunable electron affinity. These materials usually feature a donor-acceptor D-A structure in which the electron withdrawing PDI core is covalently linked with different electron donating chemical groups. The structural and electronic properties of the substituents define and modulate the optical/electrical properties of the semiconductor and the performance as photovoltaic material. In this work we designed two PDI molecules with D-A-D structure using spirobifluorene group as substituent directly linked to the perylene core (PDI-SF) and with insertion of a bithiophene moiety (PDI-BSF). In both molecules we found a reduced tendency to form aggregates in the solid state thanks to the cross-shaped rigid structure and strong steric hindrance of the spirobifluorene group. Additionally, in the case of PDI-BSF the presence of the bithiophene linker contributes significantly to extend the conjugation, resulting in a panchromatic absorption in the whole visible to NIR region. We present the synthesis of these materials and their characterisation in terms of absorption spectroscopy, cyclic voltammetry and computational calculations. Finally we show preliminary results of their use as active components in P3HT/PDIs bulk heterojunction solar cells. © 2015 Elsevier B.V. All rights reserved.

Keywords: Semiconductors Organic compounds Optical properties Electrochemical techniques

1. Introduction In the last decades the global warming and the increasing energy demand have motivated the search of sustainable energy generated from renewable sources. The photovoltaic technology, harvesting energy directly from endless solar irradiation, is one of the most feasible ways to produce sustainable and clean electrical power. Today's photovoltaic market, with a global installation of

* Corresponding author. E-mail address: [email protected] (E. Kozma).

38 GW in 2013 [1], is represented by two main device technologies: crystalline silicon solar cells and thin films inorganic solar cells [2]. Both PV technologies provide high photo-conversion performances, though coupled with high manufacturing cost, high-purity crystalline substrates, high installation costs and often use of toxic rare earth elements. Third-generation solar cells are addressing the economical and ecological problems of the inorganic solar cells, by using a variety of new materials including organic dyes, conductive polymers and conjugated molecules [3]. These organic materials can be assembled in photovoltaic devices using low cost and low temperature manufacturing techniques, such as inks printing or roll-to-roll

http://dx.doi.org/10.1016/j.matchemphys.2015.07.025 0254-0584/© 2015 Elsevier B.V. All rights reserved.

Please cite this article in press as: E. Kozma, et al., Design of perylene diimides for organic solar cell: Effect of molecular steric hindrance and extended conjugation, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.07.025

2

E. Kozma et al. / Materials Chemistry and Physics xxx (2015) 1e9

processing [4]. Organic solar cells combine inexpensive production costs compatible with flexible substrates and large area, with ecofriendly disposal processes at the device end of life. In bulk-heterojunction (BHJ) organic solar cells the sunlight conversion in electricity involves the photoinduced electron transfer process between two semiconducting materials with different ionization potential and electron affinity [5]. The active layer of BHJ devices is a bi-continuous interpenetrating network made of an electron-donor and an electron-acceptor material: a ptype conjugated polymer or molecule and usually an n-type fullerene derivative. A wide number of organic donor semiconductors have been explored for photovoltaic applications, but organic materials behaving as acceptor semiconductor and electron transporter are rare [6]. Fullerene derivatives are the most commonly used acceptors as they show outstanding electron affinity and charge transport, despite the weak harvesting of the sunlight and the high commercial cost [7]. Perylene diimide dyes (PDI) have been suggested as possible substitute of fullerenes in solar cells exhibiting excellent electron transport and good sunlight absorption. Furthermore, PDIs have extraordinary chemical and photochemical stability [8] along with the relevant possibility to easily tune both their electrical and electronic properties by chemical tailoring [9] as well as strongly modify and eventually hamper the solid state packing. Despite such promising properties, the photovoltaic parameters of bulk heterojunction solar cells containing PDI-based semiconductors are far below the theoretically expected values. Different papers in literature have been devoted to elucidate the charge photogeneration process in polymer/perylene solar cells [10,11]. It was found that the photovoltaic conversion is limited by the presence of PDI aggregates in the photoactive layer which are acting as exciton traps. Therefore, the design of new PDI n-type semiconductors should be oriented to create materials with reduced tendency to form domains in the solid state. PDI is a chemically versatile building block whose electronic properties and 3D morphology can be easily and specifically tailored either by introducing substituent groups at the imide positions [12] or at the perylene core positions [13]. Usually, the substitution of the core bay positions with chemical groups having high steric demand, hampers PDI intermolecular p-p stacking interactions, thus reducing the formation of large domains in the solid [14]. It was also indicated that bay-functionalization induces PDI core to twist from planarity [15] broadening and red shifting the molecule optical absorption [16]. To date, different design strategies for perylene based derivatives have been proposed and used as photoactive materials in organic solar cells. Small molecules or polymers with D-A-D/D-pA-p-D [14,17e19] or A-D-A/A-p-D-p-A [20e22] chemical structures (where A, D and p are respectively: a PDI electronwithdrawing moiety, an electron donor group and a p-conjugated bridge) exhibit promising photovoltaic performances in solar cell together with different donor materials such as P3HT [22e24], PBDTTT-C-T [20,21,25,26] or thiophene based oligomers [27,28]. Since the photovoltaic performances are influenced by many factors, it is difficult to predict whether a D-A-D structure is better than an A-D-A molecule. Nevertheless, a commonly accepted approach in the design of PDIs derivatives for BHJ solar cell application consists in limiting the strong tendency of planar PDI to form extended anisotropic crystallites and intermolecular aggregate states [10]. In both D-A-D and A-D-A cases, through rational design, significant dihedral angles between the perylene diimide plane and their bay substituents have been obtained, featuring non-planar structures, which suppress intermolecular p-p interactions and crystallization.

Recently a series of PDI derivatives with A-D-A structure have been prepared, characterized and tested as acceptor materials for solar cells [22]. For this type of structure, when the donor groups have low steric hindrance, the photovoltaic properties are heavily depreciated because the molecules still tend to self-aggregate and crystallize. On the other hand, the introduction of a bulky donor group, such as spirobifluorene, between two perylene dimiides is particularly favorable for the photovoltaic performances. Keeping this in mind, we report the design, synthesis and photophysical characterization of two new perylene diimide molecules, PDI-SF and PDI-BSF, featuring D-A-D and D-p-A-p-D chemical structure and containing the sterically demanding spirobifluorene moiety (Fig. 1). The three dimensional structure of the spirobifluorene group hampers the p-p intermolecular aggregation in the solid state: in PDI-SF molecule the spirobifluorene group is directly linked in the bay positions to the PDI core, while in PDI-BSF a bithiophene linker is inserted between perylene diimide and spirobifluorene. 2. Experimental 2.1. Materials and instruments Dibromo-N,N’-bis(10-nonadecyl)perylene-3,4,9,10tetracarboxylic acid diimide [29] and 2-bromo-9,9’-spirobifluorene [30] were obtained as described in literature. 9H-Fluoren-9-one, bromine, n-butyllithium, 2-bromobiphenyl, bis(neopentyl glycolato)diboron, 2,2’-bithiophene-5-boronic acid pinacol ester, Pd(dppf)Cl2, tetrakis(triphenylphosphine) palladium(0), N-bromosuccinimide were purchased from SigmaeAldrich and used as received without further purification. Toluene was distilled and used under an inert gas atmosphere. 1 H NMR spectra were recorded on a 400 MHz Bruker spectrometer operating at 11.7 T. Thermogravimetric analysis (TGA) was performed on a PerkineElmer TGA 7 instrument with a platinum pan using 2.5 mg of material as probe. Cyclic voltammetry measurements were performed at a scan rate of 50 mV/s in dichloromethane solution, under nitrogen atmosphere with a computer controlled Amel 2053 (with Amel 7800 interface) electrochemical workstation. Tetrabutylammonium tetrafluoroborate solution (0.1M) in acetonitrile was used as electrolyte in a three electrode single-compartment cell using a platinum electrode and an SCE standard reference electrode. Fc/Fcþ redox couple was used as internal standard. UVeVis absorption spectra were performed with a Perkin Elmer Lambda 9 spectrophotometer on chloroform solutions or spin coated films on quartz. Density functional theory (DFT) and TD-DFT calculations have been performed on the PDIs systems with the Gaussian03 computational package [31]. For computational purposes N-alkyl chains of the PDI moiety have been replaced by simple eCH3 groups. Ground state geometries have been optimized at the B3LYP/ 6-31G** theory level and vibrational analysis performed [32]. The first 10 singlet and triplet transitions have been calculated by the TDDFT B3LYP/6-311G* from the optimised ground state geometry, while the HOMO and LUMO energy levels are reported from the B3LYP/6-311G* calculations. Chemcraft suite was employed to render HOMO and LUMO orbitals. The current densityevoltage measurements were performed directly in the glove box where the cell was assembled and annealed, with a Keithley 2602 source meter, under a 1 sun, AM1.5G spectrum obtained from an ABET Technologies solar simulator. EQE spectral responses were recorded by dispersing a Xe lamp through a monochromator, using a Si solar cell with calibrated spectral response to measure the incident light power intensity at

Please cite this article in press as: E. Kozma, et al., Design of perylene diimides for organic solar cell: Effect of molecular steric hindrance and extended conjugation, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.07.025

E. Kozma et al. / Materials Chemistry and Physics xxx (2015) 1e9

3

Fig. 1. Chemical structures of PDI-SF (left) and PDI-BSF (right).

each wavelength. The devices were taken outside the glove box for the EQE measurements, after mounting them on a sealed cell to avoid moisture and oxygen exposure. 2.2. Synthetic procedures The synthetic routes to compounds PDI-SF and PDI-BSF are illustrated in Scheme 1. Similar to other our work, di-nonadecyl substituent has been chosen on the perylene unit to ensure good solubility. The starting material, dibromo-N,N’-bis(10-nonadecyl) perylene-3,4,9,10-tetracarboxylic acid diimide (1) is obtained as a mixture of 1,6-and 1,7-regioisomers (1:3 M ratio according to 1H

NMR features) and it was used without further purification [29]. 2-(9,90 -Spirobifluorenyl)-boronic acid neopentyl glycol ester (2) was synthesized by Suzuki-Miyaura cross-coupling reaction from 2-bromo-9,9'-spirobifluorene, which was prepared through a twostep procedure as reported in the literature [30]. The perylene based derivatives were obtained via Stille or Suzuki coupling reaction, following the reported protocol. 2.2.1. 2-(9,90 -Spirobifluorenyl)-boronic acid neopentyl glycol ester (2) In a Schlenk flask placed under nitrogen atmosphere, 2-bromo9,9'-spirobifluorene (1.3 g, 3.29 mmol) (for the complete synthetic

Scheme 1. Synthetic procedure for the preparation of PDI-SF and PDI-BSF.

Please cite this article in press as: E. Kozma, et al., Design of perylene diimides for organic solar cell: Effect of molecular steric hindrance and extended conjugation, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.07.025

4

E. Kozma et al. / Materials Chemistry and Physics xxx (2015) 1e9

procedure of this compound see ESI 1), bis(neopentyl glycolato) diboron (0.99 g, 4.39 mmol), Pd(dppf)Cl2 (0.136 g, 0.165 mmol) and KOAc (0.968 g, 9.87 mmol) were suspended in deareated anhydrous DMSO (20 ml). After three freezeepumpethaw cycles, the mixture was heated at 80  C under vigorous stirring for 18 h. The dark solution was then cooled at room temperature, and toluene (50 ml) was added. The resulting organic solution was washed several times with water (10  20 ml), dried over MgSO4 and the solvent was removed under reduced pressure. The solid residue was dissolved in the minimum amount of CH2Cl2 and ethanol was added (50 ml). The precipitation of a pale brown solid was observed by concentrating the solution at reduced pressure to a quarter of the initial volume. The solid was collected by filtration and washed with cold ethanol, affording the title compound (1.12 g) as a pale yellow solid in 79% yield. M.p. 240e241  C. 1H NMR(CDCl3, 400 MHz): d ¼ 7.83e7.86 (5H, m, spirofl), 7.33e7.37 (3H, m, spirofl), 7.18 (1H, br s, spirofl), 7.07e7.11 (3H, m, spirofl), 6.68e6.72 (3H, m, spirofl), 3.63 (4H, s, CH2), 0.94 (6H, s, CH3). 2.2.2. 1, 7- bis[2-(9,90 -Spirobifluorenyl)]-N,N’-bis-(10-nonadecyl)perylene-3,4,9,10-bis(dicarboximide) (PDI-SF) A mixture of 1,7-dibromo-N,N’-bis(10-nonadecyl)perylene3,4,9,10-tetracarboxylic diimide (1) (0.50 g, 0.46 mmol) and spirofluorenyl boronic ester (2) (0.43 g, 1 mmol) were dissolved in dry toluene (10 ml) and 2M K2CO3 solution (2.5 ml). Pd[PPh3]4 (0.025 g, 0.021 mmol) was added and the reaction mixture was stirred at 100  C for 18 h. Shortly after the addition of the catalyst, the colour of the mixture starts to change. At the end of the reaction time, the solution was cooled to room temperature, extracted with CHCl3, washed with water, extracted and dried over MgSO4 anh. concentrated to a smaller volume and dropped into methanol. The precipitate was filtered, and further purified by column chromatography, using hexane/CHCl3 3:2 as eluent. 1,7-bis[2-9,90 (Spirofluorenyl)]-N,N’-bis-(10-nonadecyl)-perylene-3,4,9,10-bis(dicarboximide) was obtained as a deep violet solid in 48% yield (0.345 g). 1H NMR (TCE, 400 MHz): d ¼ 8.38 (2H, m, pery), 8.04 (2H, m, pery), 7.85 (4H, m, pery and spirofl), 7.60 (6H, m, spirofl), 7.36 (4H, m, spirofl), 7.07 (10H, m, spirofl), 6.62 (8H, m, spirofl), 5.07 (2H, m, -CH-N), 2.08 (4H, m, -CH2-), 1.78 (4H, m, -CH2), 1.16 (56H, m, -CH2-), 0.77 (12H, t, -CH3). 2.2.3. 1, 7-bis(50 -bromo-2,2’-bithiophen-5’-yl)-N,N’-bis-(10nonadecyl)-perylene-3,4,9,10-bis(dicarboximide) (3) A mixture of 1,7-dibromo-N,N’-bis(10-nonadecyl)perylene3,4,9,10-tetracarboxylic diimide (1) (0.50 g, 0.46 mmol) and 2,2’bithiophene-5-boronic acid pinacol ester (0.30 g, 1.03 mmol) were dissolved in dry toluene (10 ml) and 2M K2CO3 solution (2.5 ml). Pd [PPh3]4 (0.025 g, 0.021 mmol) was added and the reaction mixture was stirred at reflux for 72 h. The colour of the mixture changes into dark green. At the end of the reaction time, the solution was cooled to room temperature, extracted with CHCl3, washed with water, extracted and dried over MgSO4, concentrated to a smaller volume and dropped into methanol. The precipitate was filtered, and further purified by column chromatography, using hexane/CHCl3 3:2 as eluent (82% yield, 0.475 g). After the purification of the product, a bromination reaction has been carried out, using 2.2 Eq of N-bromsuccinimide (NBS). Briefly, the perylene-bithiophene derivative was dissolved in chloroform:acetic acid mixture (10:1 v/v), followed by the addition of NBS in one portion. The reaction mixture was stirred at r.t. for 24 h. After the completion of the reaction time, the solution was extracted with chloroform, dried, concentrated to a smaller volume and dropped into methanol. A black solid was obtained in 98% yield (0.524 g). 1 H NMR(TCE, 400 MHz): d ¼ 8.60 (2H, dd, pery), 8.25 (4H, dd, pery), 7.23 (4H, m, thiophene), 7.13 (4H, m, thiophene), 5.05 (2H, m,

-CH-N), 2.13 (4H, m, -CH2-), 1.80 (4H, m, -CH2-), 1.20 (56H, m, -CH2), 0.75 (12H, t, -CH3). 2.2.4. 1, 7-bis(5’-[2-(9,90 -Spirofluorenyl)]-2,2’-bithiophene-5’-yl)N,N’-bis-(10-nonadecyl)-perylene-3,4,9,10-bis(dicarboximide) (PDI-BSF) A mixture of compound 3 (0.524 g, 0.37 mmol) and spirobifluorenyl boronic ester (2) (0.332 g, 0.77 mmol) were dissolved in dry toluene (12 ml) and 2M K2CO3 solution (3 ml). Pd[PPh3]4 (0.026 g, 0.022 mmol) was added and the reaction mixture was stirred at 100  C for 18 h. At the end of the reaction time, the solution was cooled to room temperature, extracted with CHCl3, washed with water, extracted and dried over MgSO4, concentrated to a smaller volume and dropped into methanol. The precipitate was filtered, and further purified by column chromatography, using hexane/CHCl3 3:2 as eluent. 1,7-bis(5’-[2-(9,90 -Spirofluorenyl)]-2,2’-bithiophene-5’-yl)N,N’-bis-(10-nonadecyl)-perylene-3,4,9,10-bis(dicarboximide) was obtained as a black solid in 64% yield (0.45 g). 1H NMR (TCE, 400 MHz): d ¼ 8.32 (2H, m, pery), 8.14 (2H, m, pery), 7.70 (4H, m, pery and spirofl), 7.55 (6H, m, spirofl), 7.36 (4H, m, spirofl), 7.20 (4H, m, thiophene), 7.15 (4H, m, thiophene), 7.10 (10H, m, spirofl), 6.65 (8H, m, spirofl), 5.10 (2H, m, -CH-N), 2.12 (4H, m, -CH2-), 1.70 (4H, m, -CH2-), 1.12 (56H, m, -CH2-), 0.78 (12H, t, -CH3). 3. Results and discussion Both PDI-SF and PDI-BSF molecules exhibit high thermal stability, the temperatures representing 5% weight loss under inert atmosphere being 422  C (PDI-SF) and 407  C (PDI-BSF) (see ESI2). In Fig. 2 (left) are reported the extinction coefficient absorption spectra of the two molecules in chloroform solution. The extinction coefficients are: for PDI-SF ε (433 nm) ¼ 15760 M1 cm1 and εmax (572 nm) ¼ 25540 M1 cm1; for PDI-BSF εmax (385 nm) ¼ 95600 M1 cm1, ε (500 nm) ¼ 28800 M1cm1 and ε (626 nm) ¼ 18500 M1cm1. The maximum extinction coefficients for PDI-SF and PDI-BSF are 2.5  104 M1cm1 and 9.5  104 M1 cm1 respectively. PDI-SF solution spectrum shows major absorptions between 300 nm and 660 nm, with three peaks centered at 320, 433 and 572 nm: the first due to spirobifluorene and the other two to perylene diimide moieties. The lowest energy transition peak is assigned to a perylene S0eS1 transition with a dipole moment perpendicular to the long molecular axis [17]. Despite the poor resolution, this band maintains some of the perylene diimide typical vibronic structure. Unfunctionalized perylene diimide molecules usually absorb at ca. 530 nm; the red shifted absorption in PDI-SF and the shoulder at ca. 540 nm are typical signature of localized HOMO to LUMO p-p* transition of a twisted and less rigid perylene core bearing substituents in the bay-positions [16]. The band at 433 nm is ascribable to a S0eS2 transition, with a dipole moment perpendicular to the long molecular axis [17]. The absorption features of PDI-BSF are rather different in comparison with PDI-SF, due to the presence of the bithiophene linker. PDI-BSF displays a broad and panchromatic absorption in the whole visible spectrum up to NIR region between 300 and 800 nm. Four absorption bands are visible: the spirobifluorene band at 320 nm, the strong and intense peak at 390 nm assigned to the perturbed bithiophene-spirobifluorene absorption [33], the band around 500 nm and the unstructured broad band located at ca. 630 nm assigned respectively to the perylene S0eS2 and S0eS1 transitions. These two bands appear to be red shifted when compared to PDI-SF. These findings show that PDI-BSF D-p-A-p-D structure, compared with PDI-SF D-A-D one, has a more extended

Please cite this article in press as: E. Kozma, et al., Design of perylene diimides for organic solar cell: Effect of molecular steric hindrance and extended conjugation, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.07.025

E. Kozma et al. / Materials Chemistry and Physics xxx (2015) 1e9

5

Fig. 2. Left-Extinction coefficient absorption spectra of PDI-SF (open circle) and PDI-BSF (full triangle) in chloroform solution; Right-Fluorescence spectra of PDI-SF (open circles), PDI-BSF (full triangles) in chloroform solution at the same molar concentration (below 1  106 M). Excitation wavelength: 489 nm.

conjugation of the perylene diimide with the aromatic substituents. This is leading to a wider absorption spectral region and a reduced band gap energy, resulting in a broader light harvesting, with absorption onsets almost reaching 800 nm. The fluorescence emission spectra (Fig. 2, right) are also red shifted passing from PDI-SF to PDI-BSF, with a spectrum peaked at 634 nm (PDI-SF) and 813 nm (PDI-BSF). As compared to PDI-SF, the PDI-BSF molecule displays a sharp drop of the fluorescence intensity. Such fluorescence quenching can be consistent with an efficient intra-molecular electron charge transfer (ICT) between the donor bithiophene-spirobifluorene moieties and the electron acceptor perylene core by excitation of the perylene chromophore [17]. The electron density distributions of the HOMO and LUMO levels (Fig. 4) are indeed revealing that the lowest energy optical transition has a more pronounced ICT character for PDI-BSF than for PDI-SF. The positive effect imparted by the bulky spirobifluorene in reducing solid state aggregation phenomena is highlighted by comparing the absorption spectra in solution and in solid state (see Fig. 3). For both PDI-SF and PDI-BSF, the film and the solution spectra in Fig. 3 are very similar with only minor broadening and shift towards lower energy in the solid. Therefore, there are no significant optical evidence of important close packed intermolecular p-stacking interaction. The cyclic voltammetry (CV) measurements provided specific information about the LUMO energy levels (Fig. 4, Table 1) of the

PDIs. As the oxidation potential were not attainable from these experiments, the HOMO energy levels were estimated from the difference between the optical band gap and the LUMO energy determined by electrochemical measurements. The reduction processes of the two materials differ by ca 0.1 eV being less negative in PDI-BSF possibly accounting for the partial participation of the thiophene groups to the electronic stabilization of the charge. The presence of a return peak in the reduction event indicates a chemically reversible process that is very important in the design of active materials for OPV. The HOMO levels, calculated subtracting the optical gap from the LUMO energy level point, indicate a difference of ca. 0.25 eV with PDI-SF being more positive; therefore the electronic distributions in the HOMO orbital levels might localize on different part of the molecule (more localized on the bithiophene in the case of PDI-BSF). The HOMO and LUMO energies of PDI-SF and PDI-BSF are lower compared with those of poly-3-hexylthiophene (P3HT), a common electron-donor material used in photovoltaic devices. The difference between the orbital energy levels of the PDI molecules and P3HT are appropriate for an efficient exciton dissociation at the D/A interfaces. To further understand the effect of PDI substituent groups on the electronic properties, theoretical calculations were performed. Ground state geometry of PDI-SF and PDI-BSF were optimized by DFT calculation at the B3LYP/6-31G** theory level which was shown to provide good consistency with experimental X-ray

Fig. 3. Absorption spectra of PDI-SF (left) and PDI-BSF (right) in 1 x 106 M chloroform solution (dash line) and in thin film (solid line).

Please cite this article in press as: E. Kozma, et al., Design of perylene diimides for organic solar cell: Effect of molecular steric hindrance and extended conjugation, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.07.025

6

E. Kozma et al. / Materials Chemistry and Physics xxx (2015) 1e9

Fig. 4. Cyclic voltammograms of PDI-SF and PDI-BSF in dichloromethane solution.

Table 1 Electrochemical characteristics, experimental and calculated HOMO-LUMO values and optical energy band gaps for PDI-SF and PDI-BSF. Sample

E0red (V)

E1/2 red (V)

Epred (V)

HOMO (eV)

LUMO (eV)

Eopt (eV) g

HOMOa (eV)

LUMOa (eV)

DHLb (eV)

PDI-SF PDI-BSF

0.63 0.53

0.70 0.59

0.76 0.68

5.63 5.38

3.70 3.81

1.93 1.57

5.53 5.18

3.25 3.35

2.28 1.83

p opt E0red-onset reduction potential; E1/2 red-halfway reduction potential; Ered-peak reduction potential; Eg -optical energy band gap (calculated from the onset of the absorption in opt solid state); ELUMO ¼ -e(E1/2 þ ELUMO. redþ4.4eV) and EHOMO ¼ Eg a B3LYP/6-31G** energies. b B3LYP/6-31G** HOMO-LUMO energy gaps, DHL ¼ HOMOa-LUMOa.

Fig. 5. Electron density distributions and energies for the frontier molecular orbitals of PDI-SF and PDI-BSF, obtained from DFT calculations.

Please cite this article in press as: E. Kozma, et al., Design of perylene diimides for organic solar cell: Effect of molecular steric hindrance and extended conjugation, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.07.025

E. Kozma et al. / Materials Chemistry and Physics xxx (2015) 1e9

7

Table 2 Photovoltaic performances of P3HT/PDI-based organic solar cells. Active layer (1:1 w:w)

Annealing conditions

VOC (V)

FF

JSC (mA/cm2)

PCE (%)

P3HT:PDI-SF

non ann. 150  C, 5 min non ann. 150  C, 5 min

0.75 0.75 0.71 0.71

0.41 0.56 0.37 0.53

1.77 3.13 0.81 2.81

0.54 1.32 0.21 1.06

P3HT:PDI-BSF

parameters previously reported [32,34]. The HOMO and LUMO energy levels are presented in Table 1. The optimized geometry of PDI-SF and PDI-BSF show a significant twist of the PDI core of respectively 24.5 and 26.6 if measured between the mean planes containing the naphthalene subunit (ESI 3). The PDI core also undergoes a bow like distortion with an angles of 173.6 and 170.0 as measured from the two imide N atoms and the central benzene ring of the PDI. In PDI-SF, the plane including the spiro unit and the ring of the PDI whose it is connected, forms an angle of about 52e53 . In the case of PDI-BSF, this dihedral angle (connecting now a thiophene residue) is smaller (46e49 ) due to the less steric demands and allowing for a better conjugation. As shown in Fig. 5, the orbital wavefunction probability of the LUMOs are, in both materials, predominantly localized only on the PDI core. PDI-BSF shows a small but present participation to the density of the bond thiophene therefore, in part supporting the electrochemical measurements which indicate a lowering of the LUMO level in PDI-BSF. More different is the pictorial representation of the electron density in the HOMO. In fact, whereas PDI-SF display an almost localized density on the PDI core partially

extending to the spirobifluorene moiety, PDI-BSF shows an inverted density being the HOMO predominantly localized on the bithiophene groups with a small contribution of the PDI core, which are in line with the experimental observations derived from the UVeVis absorption and fluorescence spectra and the electrochemical characterization. Due to their excellent solution processability and their suitable energy levels with P3HT, we used the two PDI molecules as acceptor materials in bulk heterojunction solar cells. The devices were fabricated using a conventional architecture glass/ITO/PEDOTPSS/P3HT:PDIs/Ca/Al. Solutions of P3HT:PDIs (1:1 w/w) were prepared by using chlorobenzene as solvent with concentrations of 20 mg/ml. After the deposition of the active layer thermal annealing has been performed prior to the cathode deposition. Finally, Ca(10 nm)/Al(100 nm) cathodes were deposited by thermal evaporation through a 6 mm2 shadow mask in a vacuum chamber at a pressure of 2  106mbar. The photovoltaic performances including the open circuit voltage (VOC), the fill factor (FF) and the short-circuit current density (JSC), obtained under illumination of AM 1.5 G with an intensity of 100 mW cm2 are summarized in Table 2.

Fig. 6. Current density-voltage characteristics (A, B) and external quantum efficiencies (C, D) of P3HT:PDI-SF and P3HT:PDI-BSF as cast (blue lines) and annealed at 150  C/5 min (red lines). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Please cite this article in press as: E. Kozma, et al., Design of perylene diimides for organic solar cell: Effect of molecular steric hindrance and extended conjugation, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.07.025

8

E. Kozma et al. / Materials Chemistry and Physics xxx (2015) 1e9

Representative current density-voltage (J-V) curves and external quantum efficiencies of P3HT:PDI-SF and P3HT:PDI-BSF solar cells are depicted in Fig. 6. The non annealed P3HT:PDIs devices show a poor power conversion efficiencies (PCEs) of 0.54% for P3HT:PDI-SF and 0.21% for P3HT:PDI-BSF. These efficiencies are in good agreement with previous reported results for the not optimized device [35]. In order to improve the solar cell efficiencies thermal treatments have been carried out at 150 C for 5 min (Table 2). High VOC values are observed in both cases and their values are not susceptible to the thermal treatments. Minor differences in the LUMO levels are reflected in the VOC values; PDI-SF with slightly higher LUMO level affords the larger VOC. While the JSC values are low for the not annealed devices, after thermal treatment at 150  C, these values are doubled for P3HT:PDI-SF and more than tripled for P3HT:PDI-BSF. After the devices thermal treatment the fill factors increased of 35e40% suggesting the formation of a more organized structure at nanoscale levels with improved charge separation and more effective transport pathways induced by the thermal annealing [36]. The EQE spectra of the photovoltaic devices show that the photocurrent generation starts at 680 nm for P3HT:PDI-SF and around 800 nm for P3HT:PDI-BSF, showing a spectral pattern that varies consistently with the absorption spectra of the PDIs molecules. After thermal treatment, the maximum EQE value reaches 27% for PDI-SF and 23% for PDI-BSF devices. The PDI-BSF device shows a moderately enhanced photocurrent in the longer wavelength region around 700 nm, i.e. it has a better overlap to the solar radiation spectrum, owing to the absorption of PDI-BSF molecule in this region. This feature, which is in principle an advantage as it should improve the Jsc, is unfortunately counter-balanced by the observed lower photon-to current conversion efficiencies in the PDI-BSF based device. These preliminary results shows better performances when compared to other BHJ made with P3HT and PDIs derivatives with other bulky substituents in bay position [14,24]. Therefore we can already infer that spirobifluorene based PDIs are a class of promising acceptor molecules for photovoltaics. The solar cell optimization along with the charge mobility and the optical and morphological properties of the PDIs photoactive blends will be presented and discussed in another paper. 4. Conclusions In summary, two new Donor-Acceptor-Donor perylene based diimide molecules with bay substituted spirobifluorene group, linked directly to the perylene core or through a bithiophene linker, have been designed and characterized. The strong steric hindrance of the spirobifluorene moiety induces a significant twist and a bow like distortion of the perylene diimide core as shown by theoretical calculations. This reduces the p-p intermolecular interactions between the perylene diimide planes, diminishing the aggregation in the solid state. Both PDI based molecules were evaluated as acceptor molecules in conventional BHJ solar cells using P3HT as donor. In preliminary results, efficiencies of 1.32% were obtained for P3HT:PDI-SF solar cells upon thermal annealing of the active layers. Acknowledgements This work has been supported by Accordo Quadro between Regione Lombardia and CNR e Cluster Project ‘Energy’ n 17348, by the Fondazione Cariplo (project “PLENOS”, Ref 2011-0349) and the E.U. Marie Curie Reintegration project grant “DAMASCO” FP7PEOPLE-2010-RG-268229.

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2015.07.025. References [1] Global Market Outlook for Photovoltaics, EPIA, 2014-2018. Report, http:// www.epia.org/fileadmin/user_upload/Publications/EPIA_Global_Market_ Outlook_for_Photovoltaics_2014-2018_-_Medium_Res.pdf. [2] B. Parida, S. Iniyan, R. Goic, A review of solar photovoltaic technologies, Renew. Sust. Energ. Rev. 15 (2011) 1625e1636. [3] (a) S. Günes, H. Neugebauer, N.S. Sariciftci, Conjugated polymer-based organic solar cells, Chem. Rev. 107 (2007) 1324e1338; (b) J.D. Myers, J. Xue, Organic semiconductors and their applications in photovoltaic devices, Polym. Rev. 52 (2012) 1e37; €uerle, Small molecule organic semiconductors on the move: (c) A. Mishra, P. Ba promises for future solar energy technology, Angew. Chem. Int. Ed. 51 (2012) 2020e2067. [4] F.K. Krebs, Fabrication and processing of polymer solar cells: a review of printing and coating techniques, Sol. En. Mater. Sol. Cells 93 (2009) 394e412. [5] A.J. Heeger, 25th anniversary article: bulk heterojunction solar cells: understanding the mechanism of operation, Adv. Mater. 26 (2014) 10e28. [6] X. Zhao, X. Zhan, Electron transporting semiconducting polymers in organic electronics, Chem. Soc. Rev. 40 (2011) 3728e3743. [7] C.-Z. Li, H.-L. Yip, A.K.-Y. Jen, Functional fullerenes for organic photovoltaics, J. Mat. Chem. 22 (2012) 4161e4177. [8] L. Perrin, P. Hudhomme, Synthesis, electrochemical and optical absorption properties of new Perylene-3,4:9,10-bis(dicarboximide) and Perylene-3,4: 9,10-bis(benzimidazole) derivatives, Eur. J. Org. Chem. 28 (2011) 5427e5440. [9] (a) E. Kozma, M. Catellani, Perylene diimides based materials for organic solar cells, Dyes Pigments 98 (2013) 160e179; (b) C. Li, H. Wonneberger, Perylene imides for organic photovoltaics: yesterday, today, and tomorrow, Adv. Mater. 24 (2012) 613e636. [10] I.A. Howard, F. Laquai, P.E. Keivanidis, R.H. Friend, N.C. Greenham, Perylene Tetracarboxydiimide as an electron acceptor in organic solar cells: a study of charge generation and recombination, J. Phys. Chem. C 113 (2009) 21225e21232. [11] S. Shoaee, Z. An, X. Zhang, S. Barlow, S.R. Marder, W. Duffy, et al., Charge photogeneration in polythiophene-perylene diimide blend films, Chem. Commun. (2009) 5445e5447. €uerle, Cyclic carboxylic imide structures as [12] J.L. Segura, H. Herrera, P. Ba structure elements of high stability. Novel developments in perylene dye chemistry, J. Mater. Chem. 22 (2012) 8717e8733. € ller, N. Kocher, D. Stalke, [13] (a) F. Würtner, V. Stepanenko, Z. Chen, C.R. Saha-Mo Preparation and characterization of regioisomerically pure 1,7-disubstituted perylene bisimide dyes, J. Org. Chem. 69 (2004) 7933e7939; (b) Z. Chen, U. Baumeister, C. Tschierske, F. Würtner, Effect of core twisting on self-assembly and optical properties of perylene bisimide dyes in solution and columnar liquid crystalline phases, Chem. Eur. J. 13 (2007) 450e465. [14] E. Kozma, D. Kotowski, S. Luzzati, M. Catellani, F. Bertini, A. Famulari, Synthesis and characterization of new electron acceptor perylene diimide molecules for photovoltaic applications, Dyes Pigments 99 (2013) 329e338. [15] F. Würthner, A. Sautter, C. Thalacker, Substituted Diazadibenzoperylenes: new functional building blocks for supramolecular chemistry, Angew. Chem. Int. Ed. 39 (2000) 1243e1245. [16] F. Würthner, C. Thalacker, S. Diele, C. Tschierske, Fluorescent j-type aggregates and thermotropic columnar mesophases of perylene bisimide dyes, Chem. Eur. J. 7 (2001) 2245e2253. [17] S. Chen, Y. Liu, W. Qiu, X. Sun, Y. Ma, D. Zhu, Oligothiophene-functionalized perylene bisimide system: synthesis, characterization, and electrochemical polymerization properties, Chem. Mater. 17 (2005) 2208e2215. [18] X. Zhan, Z. Tan, B. Domercq, Z. An, X. Zhang, S. Barlow, Y. Li, D. Zhu, B. Kippelen, S.R. Marder, A high-mobility electron-transport polymer with broad absorption and its use in field-effect transistors and all-polymer solar cells, J. Am. Chem. Soc. 129 (2007) 7246e7247. [19] Y. Zhou, Q. Yan, Y.-Q. Zheng, J.-Y. Wang, D. Zhao, J. Pei, New polymer acceptors for organic solar cells: the effect of regio-regularity and device configuration, J. Mater. Chem. A 1 (2013) 6609e6613. [20] Z. Lu, B. Jiang, X. Zhang, A. Tang, L. Chen, C. Zhan, C. Yao, Perylene diimide based non-fullerene solar cells with 4.34% efficiency through engineering surface donor/acceptor compositions, Chem. Mater. 26 (2014) 2907e2914. [21] X. Zhang, Z. Lu, L. Ye, C. Zhan, J. Hou, S. Zhang, et al., A potential perylene diimide dimer-based acceptor material for Highly efficient solution-processed non-fullerene organic solar cells with 4.03% efficiency, Adv. Mater. 25 (2013) 5791e5797. [22] Q. Yan, Y. Zhou, Y.-Q. Zheng, J. Pei, D. Zhao, Towards rational design of electron acceptors for photovoltaics: a study based on perylenediimide derivatives, Chem. Sci. 4 (2013) 4389e4394. [23] B. Jiang, X. Zhang, C. Zhan, Z. Lu, J. Huang, X. Ding, et al., Benzodithiophene bridged dimeric perylene diimide amphipjiles as efficient solution-processed non-fullerene small molecule, Polym. Chem. 4 (2013) 4631e4638. [24] E. Kozma, D. Kotowski, S. Luzzati, M. Catellani, F. Bertini, A. Famulari, G. Raos,

Please cite this article in press as: E. Kozma, et al., Design of perylene diimides for organic solar cell: Effect of molecular steric hindrance and extended conjugation, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.07.025

E. Kozma et al. / Materials Chemistry and Physics xxx (2015) 1e9

[25]

[26]

[27]

[28]

[29]

[30]

[31]

Improving the efficiency of P3HT: perylene diimide solar cells via baysubstitution with fused aromatic rings, RSC Adv. 3 (2013) 9185e9188. Y. Lin, Y. Wang, J. Wang, J. Hou, Y. Li, D. Zhu, et al., A Star-Shaped perylene diimide electron acceptor for high-performance organic solar cells, Adv. Mater. 26 (2014) 5137e5142. W. Jiang, L. Ye, X. Li, C. Xiao, F. Tan, W. Zhao, et al., Bay-linked perylene bisimides as promising non-fullerene acceptors for organic solar cells, Chem. Comm. 50 (2014) 1024e1026. A. Sharenko, D. Gehrig, F. Laquai, T.-Q. Nguyen, The effect of solvent additive on the charge generation and photovoltaic performance of a solutionprocessed small molecule:perylene diimide bulk heterojunction solar cell, Chem. Mater. 26 (2014) 4109e4118. A. Sharenko, C.M. Proctor, T.S. van del Poll, Z. Henson, T.-Q. Nguyen, G.C. Bazan, A high-performing solution-processed small molecule: perylene diimide bulk heterojunction solar cell, Adv. Mater. 25 (2013) 4403e4406. (a) L.D. Wescott, D.L. Mattern, Donor-s-acceptor molecules incorporating a nonadecyl-swallowtailed perylenediimide acceptor, J. Org. Chem. 68 (2003) 10058; (b) P. Rajasingh, R. Cohen, E. Shirman, L.J.W. Shimon, B. Rybtchinski, Selective bromination of perylene diimides under mild conditions, J. Org. Chem. 72 (2007) 5973. (a) S.O. Jeon, K.S. Yook, C.W. Joo, H.S. Son, J.Y. Lee, A high triplet energy phosphine oxide derivative as a host and exciton blocking material for blue phosphorescent organic light-emitting diodes, Thin Solid Films 518 (2010) 3716e3720; (b) X. Zhang, J.-B. Han, P.F. Li, X. Ji, Z. Zhang, Improved, highly efficient, and green synthesis of bromofluorenones and Nitrofluorenones in water, Synth. Commun. 39 (2009) 3804e3815. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara,

[32]

[33]

[34]

[35]

[36]

9

K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision C.01, Gaussian, Inc., Wallingford CT, 2004. (a) M.C. Ruiz Delgado, E.-G. Kim, D.A. da Silva Filho, J.L. Bredas, Tuning the charge-transport parameters of perylene diimide single crystals via end and/ or core functionalization: a density functional theory investigation, J. Am. Chem. Soc. 132 (2010) 3375e3387; (b) J. Vura-Weis, M.A. Ratner, M.R. Wasielewski, Geometry and electronic coupling in perylenediimide stacks: mapping structurecharge transport relationships, J. Am. Chem. Soc. 132 (2010) 1738e1739. J. Huang, H. Fu, Y. Wu, S. Chen, F. Shen, X. Zhao, , et al.L. Yunqi, Size effects of oligothiophene on the dynamics of electron transfer in p-conjugated oligothiophene-perylene bisimide dyads, J. Phys. Chem. C 112 (2008) 2689e2696. J. Huang, H. Fu, Y. Wu, S. Chen, F. Shen, X. Zhao, et al., Size effects of oligothiophene on the dynamics of electron transfer in p-conjugated oligothiophene-perylene bisimide dyads, J. Phys. Chem. C 112 (2008) 2689e2696. W.S. Shin, H.H. Jeong, M.K. Kim, S.H. Jin, M.R. Kim, J.K. Lee, et al., Effects of functional groups at perylene diimide derivatives on organic photovoltaic device application, J. Mater. Chem. 16 (2006) 384e390. R.A. Marsh, J.M. Hodgkiss, S.A. Seifried, R.H. Friend, Effect of annealing on P3HT: PCBM charge transfer and nanoscale morphology probed by ultrafast spectroscopy, Nano Lett. 10 (2012) 923e930.

Please cite this article in press as: E. Kozma, et al., Design of perylene diimides for organic solar cell: Effect of molecular steric hindrance and extended conjugation, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.07.025