A solution-processable star-shaped molecule for high-performance organic solar cells via alkyl chain engineering and solvent additive

Organic Electronics 14 (2013) 219–229

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A solution-processable star-shaped molecule for high-performance organic solar cells via alkyl chain engineering and solvent additive Jie Min a,⇑, Yuriy N. Luponosov c, Tayebeh Ameri a, Andreas Elschner d, Svetlana M. Peregudova e, Derya Baran a, Thomas Heumüller a, Ning Li a, Florian Machui a, Sergei Ponomarenko c, Christoph J. Brabec a,b a Institute of Materials for Electronics and Energy Technology (I-MEET), Friedrich-Alexander-University Erlangen-Nuremberg, Martensstraße 7, 91058 Erlangen, Germany b Bavarian Center for Applied Energy Research (ZAE Bayern), Haberstraße 2a, 91058 Erlangen, Germany c Enikolopov Institute of Synthetic Polymeric Materials of the Russian Academy of Sciences, Profsoyuznaya St. 70, Moscow 117393, Russia d Heraeus Precious Metals GmbH & Co. KG, Conductive Polymers Division (Clevios), Chempark Leverkusen Build. B202, D-51368 Leverkusen, Germany e Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova St. 28, Moscow, 119991, Russia

a r t i c l e

i n f o

Article history: Received 24 September 2012 Received in revised form 2 November 2012 Accepted 2 November 2012 Available online 23 November 2012 Keywords: D–p–A molecule Highest occupied molecular energy level Organic solar cells (OSCs) 4-Bromoanisole (BrAni)

a b s t r a c t A new star-shaped D–p–A molecule, tris{4-[50 0 -(1,1-dicyanobut-1-en-2-yl)-2,20 -bithiophen5-yl]phenyl}amine N(Ph-2T-DCN-Et)3, with high efficiency potential for photovoltaic applications was synthesized. As compared to its analogue S(TPA-bT-DCN), it showed stronger absorption in the region of 350–450 nm and a lower lying highest occupied molecular energy level (HOMO). Solution-processed organic solar cells (OSCs) based on a blend of N(Ph-2T-DCN-Et)3 and PC70BM resulted in a high PCE of 3.1% without any posttreatment. The PCE of N(Ph-2T-DCN-Et)3 based solar cells was further improved to 3.6% under simulated AM 1.5 by addition of a new additive 4-bromoanisole (BrAni). Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Organic solar cells (OSCs) have gained significant interest as a promising technology for renewable energy because of their predominant advantages such as low cost, light weight and large-area fabrication on flexible substrates [1–5]. Recently, the newly developed low bandgap polymers [6–12] and fullerene derivatives [13,14] have enabled the fabrication of very high power conversion efficiencies (PCEs) of >7% in the bulk heterojunction (BHJ) OSCs. Despite the higher PCEs reported for polymer BHJ solar cells compared with the solution-processed organic small molecule solar cells, the advantages of well-defined molecular structure, definite molecular weight, easy purification, easy mass-scale production, and high purity ⇑ Corresponding author. E-mail address: [email protected] (J. Min). 1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.orgel.2012.11.002

without batch to batch variations render small moleculebased OSCs a promising follow up technology [15–18] A better understanding of molecular structure–property relationships and more reproducible fabrication protocols are therefore anticipated [19]. The current research pursuits for small molecule-based OSCs concentrate on obtaining higher efficiency by designing and synthesizing new organic small molecules, including dendritic oligothiophenes, [20–22] star- or X-shaped molecules, [23–29] linear analogs with donor–acceptor–donor (D–A–D) or A–D–A structures, [19a,19d,30–35] fused polycyclic arene, [36] and other organic dyes [37–39]. Despite the fact that the highest PCEs (5–7%) of solution-processed small molecules BHJ OSCs have been reported, the relatively low PCE is still hindrance to their application. Star-shaped molecules have been developed as an interesting class of semiconducting materials and used in OSCs because of a number of advantages [25b,40]. By tailoring

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polymers or small molecules. Recently, Heeger et al. [19a] received the highest PCE for small molecule-based OSCs based on DTS(PTTh2)2:PC70BM from 4.5% to 6.7% by using an additive of 1,8-diiodooctane (DIO). However, to our knowledge, Only a small number of the reports published on additive effects used in small molecule-based OSCs [19a,19c]. Therefore, it is necessary to pay more attention to exploring and developing novel and innovative additives to improve the photovoltaic properties of small molecule-based OSCs. Here we designed and synthesized a new star-shaped D–p–A molecule N(Ph-2T-DCN-Et)3 by a novel synthetic way, as shown in Scheme 1. Compared with S(TPAbT-DCN) (Fig. 1a) with PCE of 1.4%, solution-processed OSCs based on a blend of N(Ph-2T-DCN-Et)3 and PC70BM resulted in a higher PCE of 3.1% without any post-treatment. In addition, we have demonstrated that 4-bromoanisole (BrAni) is an effective solvent additive for N(Ph-2TDCN-Et)3 based OSCs. The optimized OSCs based on N(Ph-2T-DCN-Et)3:PC70BM (1:2, wt.%) with 2 vol.% of BrAni, exhibits a high PCE of 3.6% with Voc of 0.96 V, Jsc of 7.6 mA/cm2, and FF of 50%, under the illumination of AM1.5G, 100 mW/cm2.

the functional groups in the core and the arm, star molecules can be designed to realize a low band gap, strong and broad absorption, together with high mobility, resulting in improved PCE [24,28,29,41,42]. Valuable insight into the design of star molecular shaped molecules was given by the pioneering work from Roncali et al. [24], who introduced such fundamental concepts as the use of dicyanovinyl end groups, thiophene spacers as well as the vinylene linkages. Recently, Li et al. [28] synthesized a star-shaped D–p–A molecule (S(TPA-bT-DCN)) with triphenylamine (TPA) as core and donor unit, dicyanovinyl (DCN) as end group and acceptor unit, and 4,40 -Dihexyl2,20 -bithiophene (bT) as the p-bridge. BHJ OSC based in S(TPA-bT-DCN) as the electron donor and [6,6]-phenylC70-butyric acid methyl ester (PC70BM) as the electron acceptor showed PCE of 1.4%. By introducing the vinylene linkage between TPA and bithiophene in S(TPA-bT-DCN), they showed that the new molecule has red-shifted and broader absorption spectrum, as well as higher PCE (3.0%). Zhan et al. [29] reported a star-shaped small molecule by incorporating the acceptor units with TPA as the core, benzothiadiazole (BT) as the bridge, and oligothiophene as the arm. The BHJ OSC based on blend with this molecule as donor and PC70BM as acceptor yielded even higher PCE (4.3%). However, the influence of side chains and especially side chains position of small molecules on their photovoltaic properties is less reported [43]. There is a great need for investigating and understanding effect of the side chain on the OSCs performance. Except the design of the molecular structure, significant gains in OSCs efficiencies have been realized through an optimization of processing conditions using methods such as thermal [44] and solvent [45] annealing or the application of solvent additives [19,46]. Among these approaches, the addition of solvent additive is one of the simplest and most effective means of optimizing the morphology of OSCs: it can influence the size of the fullerene domains, and can enhance the crystallinity of the self-organized

(a) S

Br

1) Mg 2)C2H5COCl, Li2MnCl4

S

O

S S

N

S

3

98%

O

O S S

S

S S N

N Br

S

B O

O

S

Br

O

THF

2

O

O

O O

1) BuLi 2) IPTMDOB

-78 ... +23 oC

85%

(b) O

O O S S

benzene, reflux

1

74%

Fig. 1a presents the molecular structures of S(TPA-bTDCN) and N(Ph-2T-DCN-Et)3. In spite of the fact that their structures are similar, the synthetic routes are completely different. We synthesized N(Ph-2T-DCN-Et)3 by Knövenagel condensation reaction from novel ketone precursor 5 (Scheme 1), which is chemically more challenging as compared to the synthesis of S(TPA-bT-DCN) from the corresponding aldehyde precursor [28], and successfully avoided free protons at DCN groups which may negatively influence on the stability of such materials in the organic solar cells. Fig. 1b depicts the schematics of a conventional architecture devices and the chemical structure of additive

p-TosH, HO-CH2-CH2-OH

S S

THF 0 ... +23 oC

2. Results and discussion

3

4

Pd(PPh3)4 toluene/ethanol reflux

5

HCl THF, reflux S

S

88% S

S O

O

O

Scheme 1. Synthesis of N(Ph-2T-DCN-Et)3.

CN

pyridine, 105 0C

99%

Br

NC

71%

N(Ph-2T-DCN-Et)3

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(a)

C6H13 NC

NC

CN S S

C6H13 S C6H13 NC

CN S S

C6H13

N S

S

C6H13

N

CN

S

CN C6H13

S

S

CN CN

S

S NC

CN

CN

S(TPA-bT-DCN)

N(Ph-2T-DCN-Et) 3

(b) Ag Br

Ca N(Ph-2T-DCN-Et)3 :PC70 BM PEDOT:PSS ITO

CH3

O

BrAni (additive)

Fig. 1. (a) Chemical structures of S(TPA-bT-DCN) and N(Ph-2T-DCN-Et)3; (b) Schematic illustration of the N(Ph-2T-DCN-Et)3/PC70BM solar cell architecture, and chemical structure of BrAni.

BrAni. The conventional architecture devices use a PEDOT:PSS-bladed ITO to collect holes and a Ca/Ag cathode to collect electrons. In the following sections, we will discuss in detail the synthesis, optoelectronic and photovoltaic properties of N(Ph-2T-DCN-Et)3 based solar cells as a function of additive addition. 2.1. Synthesis Synthesis of N(Ph-2T-DCN-Et)3 consists of six consecutive reaction steps as outlined in Scheme 1. First, the pinacolineboronic derivative of 2-(2,20 -bithien-5-yl)- 2ethyl-1,3-dioxolane (3) was synthesized in three steps (Scheme 1a). For this purpose 1-(2,20 -bithien-5-yl)propan-1-one (1) was synthesized by the acylation of 2, 20 -bithien-5-yl magnesium bromide, prepared in situ from 5-bromo-2,20 -bithiophene and magnesium, with propanoyl chloride, using lithium manganese chloride as a catalyst, in 74% isolated yield. Reaction of compound 1 with ethylene glycol at the presence of p-toluenesulfuric acid as a catalyst gave the protected ketone 2 in 85% isolated yield. Subsequent lithiation of compound 2 followed

by the treatment with 2-isopropoxy-4,4,5,5-tetramethyl1,3,2-dioxaborolane (IPTMDOB) gave rise to compound 3 in 98% isolated yield. Then tris(4-bromophenyl)amine was reacted with compound 3 under Suzuki conditions to yield star-shaped compound 4 in 88% isolated yield (Scheme 1b). In the next step, solution of 4 in THF was treated with HCl to remove the protecting dioxolane groups, resulting in the poorly soluble star-shaped ketone 5, which precipitates from the reaction mixture and can be collected by filtration in 99% yield. Finally, N(Ph-2TDCN-Et)3 was obtained in 71% yield by Knövenagel condensation reaction of compound 5 with excess of malononitrile in pyridine, which was used both as a base and as a solvent. Microwave heating of the last reaction was found to decrease both the reaction time and the amount of by-products as compared to the conventional heating. Thus the novel synthetic scheme gives in general higher yields as compare to the known synthetic route [28]. Moreover, the synthetic approach elaborated in this work for the first time allowed obtaining the star-shaped molecule with DCN groups from a triketone precursor. The target compound N(Ph-2T-DCN-Et)3 was found to be soluble in

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common organic solvents such as THF, chloroform, dichloromethane, chlorobenzene, etc. Its purity and molecular structure were confirmed by GPC, 1H-, and 13C- spectroscopy, elemental analysis, and high resolution mass-spectroscopy measurements (see Section 4 and Supporting Information). 2.2. Optical and electrochemical characterization The thin film optical absorption spectra of S(TPA-bTDCN) [28] and N(Ph-2T-DCN-Et)3 are shown in Fig. 2a. The strong absorption peaks of these two molecules at ca. 517 nm are attributed to the intramolecular charge transfer (ICT) transition between the TPA-bithiophene donor unit and DCN acceptor unit [28]. However, the absorption spectrum of the N(Ph-2T-DCN-Et)3 film is significantly higher than that of S(TPA-bT-DCN) in the region of 350–450 nm because of the disappearance of torsional interactions of bithiophene by modifying the alkyl side chain positions. It’s indicating a more planar structure of N(Ph-2T-DCN-Et)3 than S(TPA-bT-DCN) due to reduced steric hindrance and eliminated torsional interactions between TPA and bithiophene. The absorption edge of N(Ph-2T-DCN-Et)3 film is at 654 nm, corresponding to a bandgap of 1.89 eV. Fig. 2b displays the UV–vis absorption spectra of N(Ph2T-DCN-Et)3/PC70BM (1:2, wt.%) films blended with and without BrAni in CB, and thickness of the films is kept at about 90 nm. It can be seen that the blend films with BrAni show stronger absorption peaks within the range of 350–620 nm compared to that for the blend film without BrAni, and these films blended with different concentrations of BrAni display the similar absorption spectra. The stronger absorptions could be ascribed to more ordered structure of N(Ph-2T-DCN-Et)3 with the treatment of BrAni, and it’s necessary to the increase of Jsc and EQE values. Compared with the optical absorption of the N(Ph-2TDCN-Et)3/PCBM film, that of the N(Ph-2T-DCN-Et)3/PC70BM film is enhanced owing to the additional absorption provided by PC70BM. The electrochemical properties of N(Ph-2T-DCN-Et)3 were investigated using cyclic voltammetry (CV) (Figs. S13 and S14, Supporting Information). The measurements

were carried out in the 1,2-dichlorobenzene:acetonitrile (4:1) mixture of solvents using 0.1 M Bu4NPF6 as supporting electrolyte. The onset reduction potential (ured) of N(Ph-2T-DCN-Et)3 is 0.99 V vs. Ag/Ag+, while the onset oxidation potential (uox) is 0.92 V vs. Ag/Ag+. From the value of ured and uox of the molecule, the LUMO and HOMO energy levels of N(Ph-2T-DCN-Et)3 were calculated to be 3.41 and 5.32 eV, respectively, according to the equations of LUMO = e(ured + 4.40) (eV) and HOMO = e(uox + 4.40) (eV) [47]. In comparison to the HOMO level of S (TPA-bT-DCN) (5.22 eV), [28] N(Ph-2T-DCN-Et)3 showed a lower HOMO energy level, which should result in improved Voc because the Voc is usually proportional to the difference between the LUMO level of acceptor and the HOMO level of donor. 2.3. Devices characteristics The BHJ solar cells were fabricated with a conventional architecture consisting of glass/ITO/PEDOT:PSS/N(Ph-2TDCN-Et)3:fullerene derivatives (1:2, CB, wt.%) /Ca(15 nm)/ Ag(80 nm). To achieve optimal performance of N(Ph-2TDCN-Et)3 based on OSCs, four different fullerene derivatives were investigated. The current–voltage characteristics of these devices are shown in Fig. 3, and the corresponding device parameters with optimal blending ratio and thickness of the active layer (80–90 nm) are compiled in Table 1. At the optimal donor/acceptor weight ratio, the OSC based on N(Ph-2T-DCN-Et)3:PCBM demonstrates a short-circuit current (Jsc) of 4.91 mA/cm2, an open-circuit voltage (Voc) of 0.96 V, a fill factor (FF) of 44.84%, and a PCE of 2.11%. In addition, we introduced PC70BM as replacement for PC60BM. As shown in Fig. 3, the Jsc of N(Ph-2T-DCN-Et)3:PC70BM based device increased from 4.91 to 7.00 mA/cm2 because of the stronger and broader absorption of PC70BM in the visible region. Compared to the performance of the S(TPA-bT-DCN):PC70BM based device (PCE of 1.4%), [28] the device using the N(Ph-2TDCN-Et)3:PC70BM blend showed higher Voc (0.96 V vs. 0.84 V), Jsc (7.00 mA/cm2 vs. 5.21 mA/cm2) and FF(45.36% vs. 30.80%), leading to a higher PCE of 3.1%. The higher value of the Voc for the device with N(Ph-2T-DCN-Et)3:PC70BM is attributed to the lower HOMO level of

Fig. 2. (a) UV–vis absorption spectra of S(TPA-bT-DCN) and N(Ph-2T-DCN-Et)3 in film state, and (b) Absorption spectra of the blend films of N(Ph-2T-DCNEt)3/PCBM (1:2, wt.%), N(Ph-2T-DCN-Et)3/PC70BM (1:2, wt.%) with and without BrAni (2 vol.%).

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Fig. 3. J–V characteristics for OSCs made of N(Ph-2T-DCN-Et)3 blended with different fullerene derivatives (1:2, wt.%) and N(Ph-2T-DCN-Et)3:PC70BM with 2 vol.% BrAni, under the illumination of AM1.5G, 100 mW/ cm2.

N(Ph-2T-DCN-Et)3. The increase in Jsc for the device based on N(Ph-2T-DCN-Et)3:PC70BM can be attributed to the stronger absorption of N(Ph-2T-DCN-Et)3, which causes an enhancement in the light-harvesting property. The benefits of adding an additive into the blend solutions of the donor and acceptor have been reported by different groups as a smart strategy to improve the photovoltaic performance of the OSCs significantly. Heeger and coworkers increased the PCE of OSC based on DTS(PTTh2)2 as donor from 4.5% to 6.7% by adding 1,8-diiodooctane (DIO) [19a]. Here we used a new solvent additive (BrAni) to modify the microstructure and cause effective photovoltaic properties of the OSCs based on the N(Ph2T-DCN-Et)3:PC70BM. By introducing a small amount of BrAni to an otherwise identical solution-processed active layer, improved photovoltaic properties were observed, and the values of Jsc, FF, and PCE were all improved as BrAni was added up to 4 vol.%. The highest PCE is achieved at an additive concentration of 2 vol.%, and the average PCE increases up to 3.6% if BrAni is added to the CB solution of N(Ph-2T-DCN-Et)3:PC70BM prior to the doctor-blading process. The J–V characteristics and performance parameters of the devices based on N(Ph-2T-DCN-Et)3 blended with four fullerenes are shown in Fig. 3 and summarized in Table 1, respectively. We can see that the photovoltaic

Fig. 4. EQE spectra for OSCs devices of N(Ph-2T-DCN-Et)3 blended with different fullerene derivatives (1:2, wt.%) and N(Ph-2T-DCN-Et)3:PC70BM with BrAni used.

parameters Jsc (calculated from the EQE spectrum) and FF have significantly improved up to 7.60 mA/cm2 and 50%, respectively, resulting in an overall PCE of 3.6%. The higher Jsc for N(Ph-2T-DCN-Et)3:PC70BM based device is attributed to stronger absorption of the N(Ph-2T-DCN-Et)3:PC70 BM blend processed with the BrAni additive, as shown in Fig. 2b. The slightly higher FF could be also due to the improved microstructure of the system in presence of the additive and reduced recombination, respectively. Compared to the performance of the N(Ph-2T-DCNEt)3:PCBM or PC70BM based devices as shown in Fig. 3, the devices using the bis-PCBM and ICBA as acceptors showed a higher Voc (0.98 V and 1.02 V) because of their higher LUMO levels. However, the Jsc and FF of these devices based on them were significantly lower. The photovoltaic performances of these devices, incorporating N(Ph-2T-DCN-Et)3:PCBM, bis-PCBM, ICBA, or PC70BM (1:2, wt.%) films and the effects of concentration of BrAni on the photovoltaic properties of OSCs, are shown in Fig. S15 (Supporting Information). To check the contributions from the absorption of active layer to the photocurrent, we measured the EQE of the devices. The EQE spectra of the devices based on the various blends are shown in Fig. 4. OCSs based on N(Ph2T-DCN-Et)3:PCBM, N(Ph-2T-DCN-Et)3:bis-PCBM, and N(Ph-2T-DCN- Et)3:ICBA systems display the lower EQE values, which is consistent with their low Jsc, under the

Table 1 Photovoltaic properties of OSCs based on N(Ph-2T-DCN-Et)3:fullerene derivatives (1:2, wt.%) with and without additive.

a b

Active layer

Additive (vol.%)

Voc (V)

Jsc(a) (mAcm2)

Jsc(b) (mAcm2)

FF (%)

PCE(b) (%)

N(Ph-2T-DCN-Et)3:PCBM N(Ph-2T-DCN-Et)3:bis-PCBM N(Ph-2T-DCN-Et)3:ICBA N(Ph-2T-DCN-Et)3:PC70BM N(Ph-2T-DCN-Et)3:PC70BM N(Ph-2T-DCN-Et)3:PC70BM N(Ph-2T-DCN-Et)3:PC70BM S(TPA-bT-DCN):PC70BM [28]

Without Without Without Without BrAni(1%) BrAni(2%) BrAni(4%)

0.96 0.98 1.02 0.96 0.96 0.96 0.94 0.84

5.69 1.54 1.23 7.39 7.62 7.81 7.77 5.21

4.91 1.61 1.06 7.00 7.35 7.60 7.51

44.84 27.72 27.90 45.36 45.40 50.00 47.00 30.80

2.11 0.44 0.30 3.08 3.20 3.60 3.32 1.40a

Measured under the illumination of AM1.5G, 100 mW/cm2. Calculated from EQE spectra.

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illumination of AM1.5G, 100 mW/cm2. The N(Ph-2T-DCNEt)3:PC70BM device processed with 2 vol.% of BrAni shows significantly enhanced EQE relative to that without additive. The changing tendency of EQEs of N(Ph-2T-DCNEt)3:PC70BM devices with additive is consistent with the strong absorption intensity displayed in Fig. 2b. This result indicates that the N(Ph-2T-DCN-Et)3:PC70BM blend with BrAni is more beneficial for light harvesting than the N(Ph-2T-DCN-Et)3:PC70BM blend without the additive, and it might possess a more favorable morphology for higher Jsc.

nected regions and domains compared with images of the BHJ film without using the processing additive. Although surface AFM images cannot be directly correlated to the device performance, a microstructure coarsening as a function of the process additive would be consistent with the device performance (compare Fig. 5d with e). The changes between Fig. 5d with e are not dramatic, but distinct, further data is required to understand the impact of the additive addition on the microstructure.

2.4. Atomic force microscopy (AFM)

Photoluminescence (PL) quenching provides direct evidence for exciton dissociation, and the degree of the PL quenching reflects the efficiency of the exciton charge separation, which influences the Jsc value of the OSCs, based on the blend composites. In order to understand the origin of the effect of the different acceptors on the photovoltaic performance of N(Ph-2T-DCN-Et)3-based OSCs, we measured the PL spectra of the pristine N(Ph2T-DCN-Et)3 and the PL quenching for N(Ph-2T-DCNEt)3: fullerenes blend films with different ratios, at 488 nm excitation. The absorption of each corresponding film at this wavelength is used to normalize the obtained PL intensity. As shown in Fig. 6a, pristine N(Ph-2T-DCNEt)3 shows a strong emission peaking at 770 nm, which is strongly quenched by 2 orders of magnitude upon adding just 10 wt.% PCBM or PC70BM (1:0.1 composition). This significant reduction in the PL intensity is attributed to an efficient photoinduced charge generation between N(Ph-2T-DCN-Et)3 and PCBM or PC70BM. It is further apparent that the PL of N(Ph-2T-DCN-Et)3 was only quenched by 1 order of magnitude upon adding 10 wt.% bis-PCBM or ICBA. This visualizes that there is less electron

The active layers morphology forming on interpenetrating network with phase-separated domains in the active layer provide not only interfaces for charge separation of photo-generated excitons but also percolation pathways for charge carrier transport to the respective electrodes, which can critically affect the FF and PCE of the devices [14]. Hence we studied the surface morphology of the N(Ph-2T-DCN-Et)3:fullerene derivatives blend films prepared with or without BrAni by atomic force microscopy (AFM). Fig. 5a shows the AFM topography images of the N(Ph-2T-DCN-Et)3:PCBM films in the OSCs, and the root mean square (RMS) roughness of the film is 0.47 nm. The N(Ph-2T-DCN-Et)3:bis-PCBM and N(Ph-2T-DCN-Et)3:ICBA films (Fig. 3b and c) shows smooth surfaces without any remarkable features at the resolution of the AFM, but the domain size grows slightly in the N(Ph-2T-DCN-Et)3: bis-PCBM film. Fig. 5d and e display AFM images of N(Ph-2T-DCN-Et)3:PC70BM (1:2, wt.%) films with and without BrAni. AFM images of the N(Ph-2T-DCN-Et)3:PC70BM film processed with the BrAni exhibit larger intercon-

2.5. Photoluminescence (PL) quenching

Fig. 5. Contact mode AFM surface scans (5  5 lm2) of films without thermally annealed of: (a) N(Ph-2T-DCN-Et)3:PCBM (RMS = 0.47 nm), (b) N(Ph-2TDCN- Et)3:bis-PCBM (RMS = 0.64 nm), (c) N(Ph-2T-DCN-Et)3:ICBA(RMS = 0.50 nm), (d) N(Ph-2T-DCN-Et)3:PC70BM (RMS = 0.46 nm), and (e) N(Ph-2T-DCNEt)3:PC70BM (BrAni, 2 vol.%; RMS = 0.67 nm).

J. Min et al. / Organic Electronics 14 (2013) 219–229

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Fig. 6. (a) Photoluminescence spectra of pristine N(Ph-2T-DCN-Et)3 film and N(Ph-2T-DCN-Et)3 blended in fullerene derivatives with different ratios. (b) PL spectra of N(Ph-2T-DCN-Et)3:PC70BM (1:2, wt.%) blend films with and without the additive BrAni.

transfer efficiency between N(Ph-2T-DCN-Et)3:bis-PCBM or N(Ph-2T-DCN-Et)3:ICBA, which certainly explains the low Jsc of the devices. Interestingly, the PL intensity of N(Ph-2T-DCN-Et)3: PC70BM blend film increases slightly with adding 2 vol.% of BrAni, as seen in Fig. 6b, suggesting that the interface area between N(Ph-2T-DCN-Et)3 and PC70BM decreases because of the different solubility of N(Ph-2T-DCN-Et)3 and PC70BM in BrAni. This is consistent with the AFM data, and confirms that the growth of pure domains closer to the exciton diffusion length, allows more efficient extraction of the photogenerated charge carriers. 2.6. Transient photovoltage (TPV) TPV [48] is used to get information about charge carrier lifetime of a the solar cell at different light intensities. The samples were connected to the terminal of an oscilloscope with the input impedance of 1 MX and continuously illuminated with a white light LED to control Voc. The small perturbation is created by a 405 nm diode laser, which is adjusted to keep the height of the photovoltage peak smaller than 15 mV resulting in a voltage transient with an amplitude DV0 << Voc. Within the small perturbation regime, the transients exhibit single exponential decay, consistent with a pseudo-first-order rate equation of the form: [49,50].

DDV dDn Dn / ¼ keff ¼  dt dt sD n

ð1Þ

where DV is the photovoltage, t is the time, Dn is the change in the density of photogenerated carriers density due to the perturbation pulse, keff is the pseudo-first order rate constant and sDn is the corresponding carrier lifetime. The sDn was determined from photovoltage transients recorded at different light bias from 0.1 to 1 suns, corresponding to Voc values of 0.90–0.98 V. Fig. 7 shows the photovoltage transients for N(Ph-2TDCN-Et)3:PC70BM (1:2, wt.%) devices prepared with and without BrAni. The TPV data exhibits higher lifetimes for the N(Ph-2T-DCN-Et)3:PC70BM devices with additive compared to the N(Ph-2T-DCN-Et)3:PC70BM devices without the additive. This confirms a significantly slower

Fig. 7. Small perturbation charge carrier lifetime measured by transient photovoltage technique (TPV) for the N(Ph-2T-DCN-Et)3:PC70BM-based devices with and without the additive BrAni.

non-geminate recombination. This is probably related to the increased phase separation for the device with additive, as seen under the AFM. The charge carriers are confined in their corresponding phase with reduced recombination probability. Such morphology with good percolation in between each phase may also be indicated by our charge transport studies discussed in the following section, which show higher mobility for devices with the additive BrAni. 2.7. Charge transport Charge carrier mobility is one of the major concerns in designing organic photovoltaic materials and in fabricating OSCs. High charge carrier mobility is preferred for efficient transportation and photocurrent collection of the photoinduced charge carriers. We measured the hole only and electron only mobility of N(Ph-2T-DCN-Et)3:PC70BM blend films with or without BrAni by the space-charge limited current (SCLC) method, [50] to investigate the effect of the additive on the hole and electron mobility. For the hole-only and electron-only devices, SCLC is described by:

J SCL ¼

  9 V2 0:89  b pffiffiffiffi pffiffiffi V e0 er l 3in exp 8 L L

ð2Þ

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Table 2 Hole and electron mobilities determined from the SCLC measurements. N(Ph-2T-DCN-Et)3:PC70BM blend films

lh (cm2 V1 s1) le (cm2 V1 s1)

Without BrAni

With BrAni

1.52  104 2.25  103

3.49  104 2.42  103

where JSCL is the current density, e0 is the permittivity of free-space, er is the relative dielectric constant of the active layer, l is the charge carrier mobility, L is the thickness of the device and Vin is the voltage dropped across the sample [51]. The field dependent SCLC expression yielded a reasonably good fit to the measured J–V curves of single-carrier devices. In Table 2 the charge-carrier mobilities found from the single carrier devices are summarized. Fig. 8a shows the J–V characteristics of hole-only devices of N(Ph-2T-DCN-Et)3:PC70BM blends without and with 2 vol.% of BrAni. The hole-only mobilities obtained for N(Ph-2T-DCN-Et)3:PC70BM without and with 2 vol.% of BrAni blend films are 1.52  104, and 3.49  104 cm2 V1 s1, respectively, as shown in Fig. 8b. Moreover, the electron-only mobilities obtained for these blend films are 2.25  103, and 2.42  103 cm2 V1 s1, respectively. Therefore, N(Ph-2T-DCN-Et)3:PC70BM blend film processed with 2 vol.% of BrAni shows higher electron and hole mobilities than N(Ph-2T-DCN-Et)3:PC70BM blend film without the additive, which agrees with the higher Jsc values of the OSCs with BrAni as additive. 3. Conclusions In summary, a new solution-processable small molecule N(Ph-2T-DCN-Et)3 with star-shaped D–p–A structure has been synthesized using a novel synthetic method, and it has been characterized to investigate the effect of the alkyl side chain on the photovoltaic properties of the OSCs. Compared with S(TPA-bT-DCN): PC70BM-based OSCs, the performance of N(Ph-2T-DCN-Et)3:PC70BM based device was enhanced from 1.4% to 3.1%, producing higher values of Jsc, Voc and FF. The underlying reason for this is thought to be the increased absorption of N(Ph-2T-DCN-Et)3 molecule, eliminated torsional interactions and reduced the HOMO level. In addition, we have demonstrated that BrAni is an effective solvent additive for N(Ph-2T-DCN-Et)3 based OSCs. We carried out device optimization of the OSCs based on N(Ph-2T-DCN-Et)3 as donor and PC70BM as acceptor by using different concentrations of BrAni. The optimized OSCs based on N(Ph-2T-DCN-Et)3:PC70BM (1:2, wt.%) with 2 vol.% of BrAni, without any post-treatment, exhibits a high PCE of 3.6% with Voc of 0.96 V, Jsc of 7.6 mA/cm2, and FF of 50%, under the illumination of AM1.5G, 100 mW/cm2. This improved performance is related to morphology optimization as verified by AFM, photoluminescence quenching, transient photovoltage and charge transport experiments. It was found that N(Ph2T-DCN-Et)3:PC70BM blend film processed with 2 vol.% of BrAni exhibits larger interconnected regions and domains, reduce the recombination and extend the carrier lifetime.

4. Experimental section 4.1. General GPC analysis was performed by means of a Shimadzu LC10AVP series chromatograph (Japan) equipped with an RID-10AVP refractometer and SPD-M10AVP diode matrix as detectors and a Phenomenex column (USA) with a size of 7.8  300 mm2 filled with the Phenogel sorbent with a pour size of 500 Å; THF was used as the eluent. Glassware was dried in a drybox at 150 °C for 2 h, assembled while hot, and cooled in a stream of argon. For thin layer chromatography, ‘‘Sorbfil’’ (Russia) plates were used. In the case of column chromatography, silica gel 60 (‘‘Merck’’) was taken. 1 H-NMR spectra were recorded at a ‘‘Bruker WP-250 SY ‘‘spectrometer, working at a frequency of 250.13 MHz and utilizing CDCl3 signal (7.25 ppm) as the internal standard. 13 C spectra were recorded using a ‘‘Bruker Avance II 300’’ spectrometer at 75 MHz. In the case of 1H NMR spectroscopy, the compounds to be analyzed were taken in the form of 1% solutions in CDCl3. In the case of 13C NMR spectroscopy, the compounds to be analyzed were taken in the form of 5% solutions in CDCl3. The spectra were then processed on the computer using the ACD Labs software. High resolution mass spectra (HR MS) were measured on a Bruker micro TOF II instrument using electrospray ionization (ESI). The measurements were done in a positive ion mode (interface capillary voltage – 4500 V) or in a negative ion mode (3200 V); mass range from m/z 50 to m/z 3000 Da; external or internal calibration was done with Electrospray Calibrant Solution (Fluka). A syringe injection was used for solutions in acetonitrile, methanol, or water (flow rate 3 lL/min). Nitrogen was applied as a dry gas; interface temperature was set at 180 °C. Elemental analysis of C, H, N elements was carried out using CHN automatic analyzer CE 1106 (Italy). The settling titration using BaCl2 was applied to analyze sulphur. Experimental error is 0.30–0.50%. The Knövenagel condensation was carried out in the microwave ‘‘Discovery’’, (CEM corporation, using a standard method with the open vessel option, 50 W. 4.2. Materials n-Butyl lithium (1.6 M solution in hexane), magnesium, isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (IPTMDOB), tetrakis(triphenylphosphine) palladium(0) Pd(PPh3)4 were obtained from Sigma–Aldrich Co. and used without further. THF, benzene, pyridine and ethylene glycol were dried and purified according to the known techniques and then used as solvents. 5-bromo-2,20 -bithiophene was obtained from Heraeus Precious Metals GmbH & Co. Li2MnCl4 was obtained as described in Ref. [52]. 4.2.1. 1-(2,20 -Bithien-5-yl)propan-1-one (1) A solution of 5-bromo-2,20 -bithiophene (17.3 g, 70.6 mmol) in 170 mL of THF was added dropwise to a suspension of magnesium (1.73 g, 72.0 mmol) in 15 mL of THF. The Grignard reagent was refluxed for 2 h, then cooled to room temperature and added dropwise to solution of propanoyl chloride (6.53 g, 70.6 mmol) and freshly

J. Min et al. / Organic Electronics 14 (2013) 219–229

227

Fig. 8. Hole only mobility (a) and electron only mobility (b) of N(Ph-2T-DCN-Et)3:PC70BM blends without and with 2 vol.% of BrAni.

prepared Li2MnCl4 [52] (1.07 mmol) in 50 mL of THF at 0 °C. After addition of the Grignard reagent the cooling bath was removed and stirring was continued for 1 h. After completion of the reaction it was poured into 400 mL of distilled water and extracted three times with freshly distilled diethyl ether. The solvent was evaporated in vacuum and the residue was dried at 1 Torr to give the crude product in 93% reaction yield (according to 1H NMR). It was purified by distillation in vacuum (0.19 mBar, 134 °C) to give pure compound 1 (12.00 g, 74%) as a white solid, mp 96 °C. 1H NMR (250 MHz, CDCl3, d, ppm): 1.23 (t, 3H, J = 7.3 Hz), 2.90 (m, 2H, M = 4, J = 7.3 Hz), 7.05 (dd, 1H, J1 = 3.7, J2 = 1.1 Hz), 7.15 (d, 1H, J = 4.3 Hz), 7.28–7.33 (overlapping peaks, 2H), 7.58 (d, 1H, J = 4.3 Hz). 13C NMR (75 MHz, CDCl3): d [ppm] 8.59, 32.16, 124.05, 125.47, 126.30, 128.16, 132.34, 136.38, 141.98, 145.11, 193.49. Calcd (%) for C11H10OS2: C, 59.43; H, 4.53; S, 28.84. Found: C, 59.25; H, 4.54; S, 28.74. HRESIMS: found m/z 245.0062; calculated for [M + Na]+ 245.0065. 4.2.2. 2-(2,20 -Bithien-5-yl)-2-ethyl-1,3-dioxolane (2) Compound 1 (11.0 g, 49.5 mmol) was dissolved in dry benzene (220 mL). After complete dissolution p-toluenesulfonic acid (1.88 g, 9.9 mmol) and ethylene glycol (110 mL, 1.97 mol) were added. Then the mixture was stirred at reflux for 25 h using a Dean–Starck water separator. After that the saturated aqueous sodium bicarbonate solution was added and the mixture was extracted three times with toluene (300 mL). The combined organic phases were dried over sodium sulfate and filtered. The solvent was evaporated in vacuum and the residue was dried at 1 Torr to give 12.3 g of crude product in 89% reaction yield (according to 1H NMR). This crude product was purified by column chromatography on silica gel (eluent toluene) followed by recrystallization from hexane to give pure product (11.2 g, 85%) as a colorless liquid. 1H NMR (250 MHz, CDCl3, d, ppm): 0.96 (t, 3H, J = 7.3 Hz), 2.02 (m, 2H, M = 4, J = 7.3 Hz), 3.97–4.07 (overlapping peaks, 4H), 6.89 (d, 1H, J = 3.7 Hz), 6.97 (dd, 1H, J1 = 3.7, J2 = 1.2 Hz), 7.01 (d, 1H, J = 3.7 Hz), 7.12 (d, 1H, J = 3.7 Hz), 7.18 (d, 1H, J = 4.3 Hz). 13C NMR (75 MHz, CDCl3): d [ppm] 8.01, 33.51, 65.09, 109.26, 123.37, 123.54, 124.29, 125.06, 127.74, 136.92, 137.40, 145.53. Calcd (%) for C13H14O2S2: C, 58.62; H, 5.30; S, 24.07. Found: C, 58.79; H, 5.41; S, 24.11.

4.2.3. 2-[50 -(2-Ethyl-1,3-dioxolan-2-yl)-2,20 -bithien-5-yl]4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3) 1.6 M solution of butyllithium (25.34 mL, 40.5 mmol) in hexane was added dropwise to a solution of compound 2 (10.8 g, 40.5 mmol) in 300 mL of THF 78 °C. After the reaction mixture was stirred for 60 min at 78 °C, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (8.27 mL, 40.5 mmol) was added in one portion. The reaction mixture was stirred for 1 h at 78 °C, then the cooling bath was removed, and the stirring was continued for 1 h. After completion of the reaction, 400 mL of freshly distilled diethyl ether and 200 mL of distilled water and 40 mL of 1M HCl were added to the reaction mixture. The organic phase was separated, washed with water, and dried over sodium sulfate and filtered. The solvent was evaporated to give 15.77 g (99%) of the product (purity was also 99% according to GPC and 1H NMR) as a blue solid, mp 95 °C. The product was used in the subsequent synthesis without further purification. 1H NMR (250 MHz, CDCl3, d, ppm): 0.96 (t, 3H, J = 7.3 Hz), 1.34 (s, 12H), 2.01 (m, 2H, M = 4, J = 7.3 Hz), 3.97–4.05 (overlapping peaks, 4H), 6.89 (d, 1H, J = 3.7 Hz), 7.06 (d, 1H, J = 3.7 Hz), 7.17 (d, 1H, J = 3.7 Hz), 7.49 (d, 1H, J = 3.7 Hz). 13C NMR (75 MHz, CDCl3): d [ppm] 7.97, 24.73, 33.50, 65.10, 84.14, 109.23, 124.05, 124.74, 125.20, 136.79, 137.90, 144.13, 146.26. Calcd (%) for C19H25BO4S2: C, 58.17; H, 6.42; S, 16.34. Found: C, 58.21; H, 6.44; S, 16.24. HRESIMS: found m/z 393.1355; calculated for [M + H]+ 393.1364. 4.2.4. Tris{4-[50 -(2-ethyl-1,3-dioxolan-2-yl)-2,20 -bithien-5yl]phenyl}amine (4) In an inert atmosphere, degassed solutions of tris(4-bromophenyl)amine (1,4 g, 2,90 mmol) and compound 3 (4.1 g, 10,5 mmol) in toluene/ethanol mixture (70/7 mL) and 2 M solution of aq. Na2CO3 (15 mL) were added to Pd(PPh3)4 (360 mg, 0.31 mmol). The reaction mixture was stirred under reflux for 12 h, then it was cooled to room temperature and poured into 100 mL of water and 200 mL of toluene. The organic phase was separated, washed with water, dried over sodium sulfate and filtered. The solvent was evaporated in vacuum and the residue was dried at 1 Torr. The product was purified by column chromatography on silica gel (eluent toluene) to give pure compound 4 (2.65 g, 88%) as yellow solid, mp 110 °C. 1H NMR (250 MHz, CDCl3): d [ppm] 0.97 (9H, t, J = 7.3 Hz),

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2.02 (6H, m, M = 4, J = 7.3 Hz), 3.96–4.08 (12H, overlapped peaks), 6.90 (3H, d, J = 3.7 Hz), 7.03 (3H, d, J = 3.7 Hz), 7.08 (3H, d, J = 3.7 Hz), 7.10–7.17 (9H, overlapped peaks), 7.47 (6H, d, J = 8.5 Hz). 13C NMR (75 MHz, CDCl3): d [ppm] 8.02, 33.52, 65.12, 109.29, 123.10, 123.14, 124.41, 124.46, 125.17, 126.52, 128.99, 136.15, 137.02, 142.69, 145.47, 146.43. Calcd (%) for C57H51NO6S6: C, 65.93; H, 4.95; N, 1.35; S, 18.53. Found: C, 65.98; H, 5.04; N, 1.32; S, 18.39. HRESIMS: found m/z 1037.2020; calculated for [M]+ 1037.2035. 4.2.5. 1,10 ,100 -[Nitrilotris(4,1-phenylene-2,20 -bithiene-50 ,5diyl)]tripropan-1-one (5) 1M HCl (6.93 mL) was added to a solution of compound 4 (2.4 g, 2.3 mmol) in THF (80 mL) and then the reaction mixture was stirred for 4 h at reflux. During the reaction the product was gradually formed orange precipitate. After completion of the reaction the organic phase was separated using diethyl ether, washed with water and filtered off to give pure compound 5 (2.05 g, 98%) as orange crystals, mp 217 °C. 1H NMR (250 MHz, CDCl3): d [ppm] 1.24 (9H, t, J = 7.3 Hz), 2.87 (6H, m, M = 4, J = 7.3 Hz), 7.12– 7.18 (9H, overlapped peaks), 7.20 (3H, d, J = 3.7 Hz), 7.27 (3H, d, J = 4.3 Hz), 7.50 (6H, d, J = 8.5 Hz), 7.59 (3H, d, J = 4.3 Hz). Calcd (%) for C51H39NO3S6: C, 67.59; H, 4.34; N, 1.55; S, 21.23. Found: C, 67.49; H, 4.47; N, 1.47; S, 21.19. HRESIMS: found m/z 906.1297; calculated for [M+H]+ 906.1327. 4.2.6. Tris{4-[50 -(1,1-dicyanobut-1-en-2-yl)-2,20 -bithien-5yl]phenyl}amine – N(Ph-2T-DCN-Et)3 Compound 5 (1.85 g, 2.0 mmol), malononitrile (1.2 g, 18.0 mmol) and dry pyridine (15 mL) were placed in a reaction vessel and stirred under argon atmosphere for 8 h at 105 °C using the microwave heating. After completeness of the reaction the pyridine was evaporated in vacuum and the residue was dried at 1 Torr. This crude product was purified by column chromatography on silica gel (eluent dichloromethane). Further purification included precipitation of the product from its THF solution with toluene and hexane to give pure product as a black solid (1.52 g, 71%). 1 H NMR (250 MHz, CDCl3, Me4Si): d [ppm] 1.37 (9H, t, J = 7.3 Hz), 2.92 (6H, m, M = 4, J = 7.3 Hz), 7.15 (6H, d, J = 8.5 Hz), 7.22 (3H, d, J = 3.7 Hz), 7.27 (3H, d, J = 4.3 Hz), 7.35 (3H, d, J = 3.7 Hz), 7.52 (6H, d, J = 8.5 Hz), 7.96 (3H, d, J = 4.3 Hz). 13C NMR (75 MHz, CDCl3): d [ppm] 14.59, 30.82, 113.67, 114.64, 123.84, 124.55, 124.79, 126.88, 127.67, 128.45, 133.74, 134.70, 135.30, 146.27, 146.85, 146.88, 167.36. Calcd (%) for C60H39N7S6: C, 68.61; H, 3.74; N, 9.33; S, 18.32. Found: C, 68.57; H, 3.89; N, 9.27; S, 18.27. HRESIMS: found m/z 1049.1534; calculated for M+ 1049.1586. 4.3. Fabrication and characterization of the OSCs All the devices were fabricated in the normal architecture (see Fig. 1b). Photovoltaic devices were fabricated by doctor-blading on indium tin oxide (ITO)-covered glass substrates (from Osram). These substrates were cleaned in toluene, water, acetone, and isopropyl alcohol. After drying, the substrates were bladed with 50 nm PEDOT:PSS (HC

Starck, PEDOT PH-4083). Photovoltaic layers, consisting of N(Ph-2T-DCN-Et)3 and different acceptor in 1:2 wt.% ratios with or without the additive were dissolved at different concentrations in chlorobenzene (CB) and bladed on top of PEDOT:PSS layer. Finally, a calcium/silver top electrode of 15/80 nm thickness was evaporated. The typical active area of the investigated devices was 10.4 mm2. The current–voltage characteristics of the solar cells were measured under AM 1.5G irradiation on an OrielSol 1A Solar simulator (100 mW/cm2). Most of device performances mentioned in this paper are corrected to the EQE of the particular device. The EQE was detected with cary 500 Scan UV–vis–NIR Spectrophotometer under monochromatic illumination, which was calibrated with a mono-crystalline silicon diode. Absorption profiles were recorded with a Perkin Elmer Lambda-35 absorption spectrometer from 350 to 1100. Electrochemical properties were studied by cyclic voltammetry (CVA). The measurements were carried out in the 1,2-dichlorobenzene: acetonitrile (4:1) mixture of solvents using 0.1 M Bu4NPF6 as supporting electrolyte. The glassy carbon electrode was used as a work electrode. Potentials were measured relative to a saturated calomel electrode. AFM measurements were performed with a Nanosurf Easy Scan 2 in contact mode. Photoluminescence (PL) data were collected using a Perkin–Elmer LS55 Fluorescence Spectrometer. Unless otherwise stated, the PL excitation wavelength was set to 488 nm. For TPV measurements, devices were directly connected to an oscilloscope in open-circuit conditions (1 MX). The device was illuminated with white light to set the desired Voc, at this point equilibrium between charge formation, due to the illumination with light, and charge recombination was reached [49,50]. Single carrier devices were fabricated and the dark current–voltage characteristics measured and analyzed in the space charge limited (SCL) regime following the Ref. [50]. The structure of hole only devices was Glass/ITO/PEDOT:PSS/Active layer/PEDOT:PSS/Ag (100 nm). For the electron only devices, the structure was Glass/ITO/AZO/Active layer/Ca (15 m)/Ag (80 nm), where both Ca and Ag were evaporated. The reported mobility data are average values over the two cells of each sample at a given film composition. Acknowledgements High resolution mass spectra were recorded in the Department of Structural Studies of Zelinsky Institute of Organic Chemistry (Moscow, Russia). Authors would like to thank Dr. Peter Dmitryakov (NBICS center of Kurchatov Institute, Moscow, Russia) for DSC and TGA measurements. The authors gratefully acknowledge the support of the Cluster of Excellence ‘‘Engineering of Advanced Materials’’ at the University of Erlangen-Nuremberg, which is funded by the German Research Foundation (DFG) within the framework of its ‘‘Excellence Initiative’’. This work has been partially funded by the DFG project, Grant No. BR 4031/ 2-1, the Russian Ministry of Education and Science (GK No. 11.519.11.6020), The Program of President of Russian Federation for Support of Young Scientists (Grant MR1567.2011.3) and the China Scholarship Council (CSC).

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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2012.11.002.

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