A new dithienosilole-based oligothiophene with methyldicyanovinyl groups for high performance solution-processed organic solar cells

Organic Electronics 15 (2014) 3800–3804

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Letter

A new dithienosilole-based oligothiophene with methyldicyanovinyl groups for high performance solution-processed organic solar cells Yuriy N. Luponosov a,⇑,1, Jie Min b,⇑,1, Tayebeh Ameri b, Christoph J. Brabec b,c, Sergei A. Ponomarenko a,d a

Enikolopov Institute of Synthetic Polymeric Materials of the Russian Academy of Sciences, Profsoyuznaya st. 70, Moscow 117393, Russia Institute of Materials for Electronics and Energy Technology (I-MEET), Friedrich-Alexander-University Erlangen-Nuremberg, Martensstraße 7, 91058 Erlangen, Germany c Bavarian Center for Applied Energy Research (ZAE Bayern), Haberstraße 2a, 91058 Erlangen, Germany d Chemistry Department, Moscow State University, Leninskie Gory 1-3, Moscow 119991, Russia b

a r t i c l e

i n f o

Article history: Received 11 August 2014 Received in revised form 5 September 2014 Accepted 5 September 2014 Available online 18 September 2014 Keywords: Oligothiophene Donor–acceptor oligomer Organic solar cell Small molecule Dicyanovinyl groups

a b s t r a c t A new linear dithienosilole-based oligothiophene end-capped with methyl and electronwithdrawing dicyanovinyl groups, DTS(Oct)2-(2T-DCV-Me)2, was prepared in good yield. This oligomer exhibited broad absorption spectra in bulk down to the near-IR region with the optical edge at 900 nm, resulting in an initially high power conversion efficiency of 5.44% in solution-processed organic solar cells using PC71BM as an acceptor. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Organic solar cell technology is a promising candidate for the solar energy conversion compared to its inorganic counterparts due to its low cost, light weight, and potential use in flexible devices [1]. Solution-processed bulk heterojunction organic solar cells (BHJ OSCs) possess potential for commercialization because of their high internal quantum efficiency and possibility to use large-scale printing techniques [2]. Today the most efficient organic solar cells (OSCs) based on polymer donor (D) and fullerene derivative acceptor (A) achieve power conversion efficiencies ⇑ Corresponding authors. E-mail addresses: [email protected] (Y.N. Luponosov), Min.Jie@ww. uni-erlangen.de (J. Min). 1 Yu. N. Luponosov and J. Min contributed equally. http://dx.doi.org/10.1016/j.orgel.2014.09.006 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

(PCEs) over 10% both in single and tandem solar cells [3]. However polymers suffer from difficult purifications, broad molecular weight distributions, and batch to batch variations. In a parallel effort, OSCs based on small molecules have attracted extensive attention due to easy mass-scale production, well-defined molecular structures, definite molecular weights, easily controlled high purity and well photovoltaic performance reproducibility [4]. Valuable insight into the design of oligomers for organic photovoltaics was given by the work of Bauerle et al., who have introduced the A–D–A strategy and used oligothiophene fragments as donor units and dicyanovinyl (DCV) groups as terminal acceptor units [5]. Chen et al. significantly extended design of donor–acceptor oligomers and used dithienosilole [6] or benzodithiophene [7] as central donor blocks and alkyl cyanoacetate [6,8] or rhodanine as acceptor groups [9]. Bazan et al. introduced D1–A–D2–A–D1

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type of oligomers by using a central electron-rich core flanked by electron-poor units and terminated with p-conjugated end-caps [10]. Recently, small-molecule donor materials blended with fullerene acceptors have achieved efficiencies approaching those of their polymer counterparts 7–9% [11]. However, their efficiencies need to be further improved for commercial applications. New D–A organic small molecules with optimized properties are urgently needed, which require developing of new design concepts and synthesis of novel materials to maximize the photovoltaic parameters. To address this need, we have designed a new small molecule DTS(Oct)2-(2T-DCV-Me)2 based on an electron-reach dithienosilole (Scheme 1), flanked by bithiophene fragments and terminated with methyl DCV groups. At first sight the design of this molecule is similar to A– D–A oligomers described in works of Bauerle et al. [5] and especially in recent work of Wang et al. [12] However, there is a significant difference by the presence of terminal alkyl groups in DTS(Oct)2-(2T-DCV-Me)2, which requires a different synthetic route for its preparation. It should be noted that usually preparation of the oligomers with DCV groups are based on Knövenagel condensation of their aldehyde precursors with malononitrile [13], which leads to the presence of active protons [14] at DCV groups and may negatively influence the stability of such materials in OSCs. Moreover, such design concept limits the possibilities of fine-tuning of their properties by such powerful tool as terminal alkyl-chain engineering since the alkyl chains can be attached to b-positions of the thiophene rings only. These problems can be overcome by choosing a ketone precursor for Knövenagel condensation, which is, however, significantly hindered under the standard reaction conditions. Nevertheless, recently we were able to show that highly efficient and stable star-shaped small molecules for OSCs can be prepared in high yields by Knövenagel condensation of star-shaped triketones with malononitrile in pyridine under a microwave heating [15]. Here we for the first time expand this approach to design and synthesis of linear A-D-A oligothiophene with central dithienosilole block. 2. Results and discussion Synthesis of DTS(Oct)2-(2T-DCV-Me)2 consists of five consecutive reaction steps as outlined in Scheme 2. First, the protected ketone (2) was prepared by reaction of 5acetyl-2,20 -bithiophene [15a] with 2,2-dimethyl-1,3-propanediol at the presence of p-toluenesulfuric acid as a cat-

Scheme 1. Chemical structure of DTS(Oct)2-(2T-DCV-Me)2.

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Scheme 2. Syntheis of DTS(Oct)2-(2T-DCV-Me)2.

alyst in 93% isolated yield. Subsequent lithiation of compound 2 followed by the treatment with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (IPTMDOB) gave rise to its pinacolboronic ester (3) in 99% isolated yield. Then 4,40 -bis(octyl)-5,50 -dibromo-dithieno[3,20 0 b:2 ,3 -d]silole (4) was reacted with compound 3 under Suzuki conditions to yield oligomer precursor 5 in 82% yield. In the next step, solution of 5 in THF was treated with 1 M HCl to remove the protecting 5,5-dimethyl-1,3dioxane groups, resulting in the poorly soluble ketone 6 in 98% yield. Finally, Knövenagel condensation of compound 6 with malononitrile in pyridine under a microwave irradiation afforded a desired DTS(Oct)2-(2T-DCV-Me)2 in 80% isolated yield. Detailed synthetic procedures and characterizations can be found in the Supporting Information (SI). This novel oligomer demonstrates reasonable solubility in common organic solvents such as THF, chloroform, ochlorobenzene etc. For example, the solubility of DTS(Oct)2-(2T-DCV-Me)2 in chloroform was found to be 5 mg/ml (see SI for experimental details). The thermal properties were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA reveals a high thermal and thermaloxidative stability of DTS(Oct)2-(2T-DCV-Me)2 with an onset decomposition temperature (at 5% weight-loss (Td)) at approx. 400 °C (Fig. S14, SI). The DSC trace of the novel compound shows a single melting peak at 248 °C with DH = 86.77 J/g (Fig. S15, SI). Such high melting temperature and large value of the transition enthalpy allow us to conclude about a highly crystalline nature of this material [16]. The electrochemical properties of DTS(Oct)2-(2T-DCVMe)2 were investigated using cyclic voltammetry (CVA) (Fig. S16, SI). The standard formal reduction potential (ured) is 1.06 V vs. SCE, while the standard formal

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oxidation potential (uox) is 0.86 V vs. SCE. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels were correspondingly calculated to be 3.34 and 5.26 eV, according to LUMO = e(ured + 4.40) (eV) and HOMO = e(uox + 4.40) (eV) [17]. The value of electrochemical band gap (Eg) was found to be 1.92 eV. The optical properties of DTS(Oct)2-(2T-DCV-Me)2 were investigated by UV–vis spectroscopy in a solution and a thin solid film (Fig. 1). In THF solution, the absorption peak at around 387 nm corresponds to the p–p⁄ transition of the conjugated backbone, whereas the lower energy absorption band peaking at 536 nm is ascribed to the intramolecular charge transfer transition. The optical band gap (Eopt g ) calculated from the edge of absorption (konset – 637 nm) was found to be 1.95 eV, which is in good agreement with the electrochemical band gap. The molar extinction coefficient (e) at maximum of absorption achieved rather high value - 80,000 M1cm1. In comparison to its absorption in solution, the molecular absorption band in film was significantly broadened and red-shifted, extending the absorption maximum from 556 nm to 637 nm and exhibiting a new long-wave peak [6] at 708 nm. It should be noted that the intensity of this peak was found to be depended on a film thickness and a solvent used for film casting. For example, the intensity of long-wave absorption peak in the film cast from chloroform was less as compared to the main absorption band (Fig. S18). The Eopt calculated g from the absorption edge of films cast from THF (900 nm) and chloroform (827 nm) was found to be 1.38 eV and 1.50 eV respectively. The reasons for this huge red-shift and the appearance of new long-wave shoulder in the film absorption should be due to pronounced intermolecular p–p interactions and an ordering of the oligomer molecules in the solid-state. Further interpretation of molecular packing for DTS(Oct)2-(2T-DCV-Me)2 deserves its thorough investigation by X-ray technique, which will be done in our following works. Thus, DTS(Oct)2-(2TDCV-Me)2 demonstrates efficient overlap of absorption with solar spectrum and the largest edge of absorption in bulk among dithienosilole-based oligomers.

The photovoltaic properties of DTS(Oct)2-(2T-DCVMe)2 were preliminarily investigated by fabricating of solution-processed bulk heterojunction (BHJ) OSCs in the conventional structure glass/ITO/PEDOT:PSS/DTS(Oct)2(2T-DCV-Me)2:PC71BM/Ca/Al. OSC devices that contain various weight ratios of D:A (from 1:0.5 to 1:4) in the active layers have been investigated and compared, as shown in Table 1 and Fig. S17. It was found that the OSC device with a D/A ratio of 1:0.8 and a thickness of the active layer of 90–100 nm gives the highest PCE = 5.44%, with an open-circuit voltage (Voc) of 0.85 V, a short-circuit current (Jsc) of 10.2 mA/cm2, and a fill factor (FF) of 62.74%, as shown in Fig. 2a. The external quantum efficiency (EQE) curve of the best device is shown in Fig. 2b. The integral current density calculated based on the EQE curve is 9.68 mA/cm2, indicating the good consistency of the Jsc value measured under simulated AM 1.5. The slightly lower calculated Jsc value is probably due to the degradation of the device performance during the J–V and EQE measurements in air. Fig. 2b and Fig. S18 illustrate that both DTS(Oct)2-(2T-DCV-Me)2 and PC71BM have contributions to the quantum efficiency of the device. However, EQE spectrum of active layer obtained from chloroform demonstrates the blue-shifted absorption edge at ca. 800 nm as compared to the absorption edge (827 nm) of pristine oligomer in film cast from chloroform (Fig. S18). This observation can be explained by some destruction in the ordering of the oligomer molecules in the blend with PC71BM. The hole mobility of DTS(Oct)2-(2T-DCV-Me)2 and its optimized blend with PC71BM was determined via the space charge limited current (SCLC) method with the device structure glass/ITO/PEDOT:PSS/active layer/MoO3/ Ag. The related average values of the hole mobilities of pristine and blended films, using three different thicknesses, were found to be 1.74  103 and 5.83  104 cm2 V1 s1, respectively (Fig. S19), which have the same order of magnitude as the hole mobilities found for some donor materials in highly efficient OSCs [7–9,15a]. In addition, this result suggests that in order to achieve high photovoltaic properties in small molecule BHJ OSCs, the hole and electron mobilities should be balanced, and both of them should be greater than 104 cm2 V1 s1. In order to investigate the morphology of the active layer blend films, atomic force morphology (AFM)

Table 1 The performance of the OSCs based on DTS(Oct)2-(2T-DCV-Me)2:PC71BM blends, under the illumination of AM 1.5 G at 100 mW cm–2.

Fig. 1. Absorption spectrum of DTS(Oct)2-(2T-DCV-Me)2 in THF and film cast from THF.

DTS(Oct)2-(2T-DCVMe)2:PC71BM

Voc (V)

Jsc (mA cm2)

FF (%)

PCE(PCEa) (%)

1:0.5 1:0.8 1:1 1:1.5 1:2 1:2.5 1:3 1:4

0.88 0.85 0.82 0.84 0.80 0.80 0.76 0.78

8.05 10.2 10.1 9.02 9.62 8.51 5.13 4.49

53.32 62.74 55.95 57.18 50.98 44.14 49.42 39.10

3.78(3.37) 5.44(4.97) 4.63(4.38) 4.33(3.92) 3.92(3.72) 3.01(2.79) 1.93(1.82) 1.37(1.25)

a The average PCE is obtained from twelve cells, and the ranges of photovoltaic parameters of various devices are presented in Fig. S17.

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Fig. 2. (a) J–V curves and (b) EQE curves for DTS(Oct)2-(2T-DCV-Me)2:PC71BM (1:0.8, wt%) OSCs under the illumination of AM 1.5 G at 100 mW cm–2.

Fig. 3. AFM top images (5  5 lm) of DTS(Oct)2-(2T-DCV-Me)2:PC71BM blend film (1:0.8, wt%) (a: topography; b: phase).

investigations were carried out. Here, the samples were prepared in the same way as the photoactive layers for the solar cells. As shown in Fig. 3, it can be observed that the blend film of DTS(Oct)2-(2T-DCV-Me)2:PC71BM (1:0.8, wt%) is uniform with a root mean square (RMS) roughness of 2.27 nm. Moreover, as can be seen from the AFM phase image of the blend film, it also formed numbers of nanoscale domains. These small domains are beneficial to the charge transport and thus enhanced efficiency of the OSCs [15,18]. 3. Conclusions In conclusion, a new small molecule, DTS(Oct)2-(2TDCV-Me)2, with A-D-A framework was prepared in good yield by using a novel design, which also can be used for the preparation of various dicyanovinyl-based oligomers with dithienosilole and, probably, with other central fragments. This oligomer demonstrates high thermal and thermal-oxidative stability, and a strong absorption in thin film extending from 300 to 900 nm. The BHJ OSC based on DTS(Oct)2-(2T-DCV-Me)2:PC71BM (1:0.8, wt%) shows an initially high PCE of 5.44% without any special treatment needed. This preliminary work demonstrates that DTS(Oct)2-(2T-DCV-Me)2 is a promising photovoltaic small molecule-based donor material for future application of OSCs. We envisage that further optimization of both the blend absorption and morphology of an active layer and the chemical structure of oligomer would greatly improve the performance of devices.

Acknowledgements Authors thank P.V. Dmitryakov (NRC Kurchatov Institute) for DSC and TGA measurements, Dr. S.M. Peregudova (INEOS RAS) for CVA measurements and Dr. N.M. Surin (ISPM RAS) for optical spectroscopy measurements. This work was supported by the Presidium of Russian Academy of Sciences (Program P-8), the Program of President of Russian Federation for Support of Young Scientists (grant MK-6716.2013.3), Sonderforschungsbereich 953 ‘‘Synthetic Carbon Allotropes’’, the ‘‘Solar Technologies go Hybrid (SolTech)’’ and the China Scholarship Council (CSC).

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