Investigation of Poly(Cyclopentadithiophenes) as Electron Donor Materials for Organic Solar Cells

Available online at www.sciencedirect.com

Energy Procedia 31 (2012) 1 – 10

E-MRS Spring Meeting 2011, Symposium S: Organic Photovoltaics: Science and Technology

Investigation of poly(cyclopentadithiophenes) as electron donor materials for organic solar cells Vladislav A. Kostyanovskiy,a Pavel A. Troshin,a* Getachew Adam,b,c N. Serdar Sariciftci,b and V. F. Razumova a

Institute forProblems of Chemical Physics of Russian Academy of Sciences, Semenov Prospect 1, 142432 Chernogolovka, Moscow region, Russia. b Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University Linz, Altenbergerstrasse 69, 4040 Linz, Austria c Addis Ababa University ,P.O.Box 1176, Addis Ababa, Ethiopia

Abstract We present the synthesis and investigation of poly(4,4-dialkylcyclopentadithiophene)-2,6-diyls (PACPDTs) as electron donor materials in organic bulk heterojunction solar cells in combination with the fullerene derivatives [60]PCBM and [70]PCBM. It was shown that the length of alkyl side chains attached to the polymer backbone affects significantly the active layer morphology and also influence to some extent on the photovoltaic performance. Spectroelectrochemical measurements performed for PACPDTs proved the potential of these polymers to be applied in electrochromic devices. © 2013 Authors. Published by Elsevier © 2012The Published by Elsevier Ltd.Ltd. Selection and/or peer-review under responsibility of S. E. Shaheen, D. Selection and peer-review under responsibility of the European Material Research Society (E-MRS) C. Olson, G. Dennler, A. J. Mozer, and J. M. Kroon. Keywords: cyclopentadithiophene; organic semiconductor; conjugated polymers; polymer solar cells; organic solar cells; electrochromic devices

1. Introduction Organic electronics has been rapidly developing during the last decade. Conjugated polymers form one of the most important groups of organic semiconductors. Depending on the molecular structure, conjugated polymers demonstrate semiconducting properties of both p- and n-types, which makes them

* Corresponding author. Tel/Fax: +7-496-522-1418. E-mail address: [email protected].

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the European Material Research Society (E-MRS) doi:10.1016/j.egypro.2012.11.158

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promising materials for organic solar cells, photodetectors, field-effect transistors and light-emitting diodes [1-6]. The use of the soluble polymer-based materials allows for the manufacture of flexible, lightweight and cheap organic solar cells. The active layer of such solar cells is represented by a composite of a conjugated polymer with appropriate n-type semiconductor material, which is typically some soluble fullerene derivative. The efficiency of the best organic solar batteries reaches 6-8% nowadays [7-12]. It might be possible to reach higher efficiencies by using new polymer-based materials with improved properties [13]. However, the guidelines for the design of well-performing conjugated polymers are not established up to date because of the complex morphological behaviour of the fullerene/polymer composites [14]. A family of conjugated poly(4,4-dialkylcyclopentadithiophene)-2,6-diyls (PACPDTs) bearing alkyl side chains of different lengths might be considered as good model polymer system for investigation in bulk heterojunction organic solar cells in combination with fullerene derivatives. PACPDTs have superior optical properties compared to the poly(3-alkylthiophenes) (PATs) that potentially allow for the generation of higher current densities and overall power conversion efficiencies in solar cells using these materials [ 15 - 16 ]. Many conjugated copolymers comprising cyclopentadithiophene units were synthesized and investigated in organic solar cells [17-22]. However, PACPDTs homopolymers were not investigated up to our best knowledge. In this work we performed a systematic investigation of PACPDTs homopolymers bearing side chains varied from hexyl to decyl as electron donor materials in organic solar cells. 2. Results and discussion 2.1. Synthesis and characterization of PACPDTs The key precursor for the synthesis of PACPDTs is cyclopentadithiophenone 4. Several methods for the preparation of 4 are described in the literature [23-25] The most efficient seems to be the route based on the coupling of 3-thiophenecarboxaldehyde and 3-thienyllithium with the subsequent double lithiation of the intermediate product 1 and quenching of the formed dianion with I2 that leads to 2 (Fig. 1). Oxidation of the alcohol 2 with chromic anhydride produces ketone 3 which undergoes facile cyclization under the Ullmann-type reaction conditions forming the target cyclopentadithiophenone 4. The reduction of cyclopentadithiophenone 4 with hydrazine and potassium hydroxide in ethylene glycol afforded cyclopentadithiophene 5. Alkylation of 5 with respective alkylbromides according to the standard procedure produced target monomers 6a-e. There are several methods reported in the literature for the polymerization of dialkylcyclopentadithiophenes [15,16,26]. The most efficient and simple technique is the oxidative polymerization of the monomer using iron (III) chloride which typically affrods PACPDTs in very high molecular weights [15-16]. Nickel-catalized Kumada-type reaction can be considered as a good alternative to the oxidative polymerization yielding the title PACPDTs in medium molecular weights [16]. Low molecular weight PACPDTs were prepared using Rieke method utilizing in-situ generated organozinc reagents [27]. We performed the synthesis of polymers 7a-e using the oxidative polymerization reaction. The reaction conditions and the procedures used for the purification of the polymers were reported previously [15].

Vladislav A. Kostyanovskiy et al. / Energy Procedia 31 (2012) 1 – 10

Fig. 1 Synthesis of PACPDTs

The soluble fractions of all polymers had molecular weights within the range of 7000 – 20000 g/mole with polydispersities between 1.7 and 2.7 as revealed from the GPC data (Table 1). The optical properties of the polymers were studied in the solution using absorption spectroscopy. It was demonstrated that all polymers absorb light intensively in the whole visible range. It is also noTable that the spectra of 7a-e in solution are significantly red-shifted compared to the solution-state absorption spectrum of P3HT (Fig. 2). The absorption edges and, consequently, the band gaps are virtually the same for all synthesized conjugated polymers (Table 1). This confirms that the solubilizing alkyl side chains do not affect electronic properties of the polymers 7a-e.
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 Fig. 2 Absorption spectra of polymers 7a-e in chlorobenzene

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Vladislav A. Kostyanovskiy et al. / Energy Procedia 31 (2012) 1 – 10

Electrochemical properties of the polymer films prepared on the conducting ITO electrodes were studied using cyclic voltammetry technique. It was shown that all polymers undergo reversible oxidation at 0.6-0.8 V and irreversible reduction in the potential range between -1.3 and -1.4 V (Table 1, Fig. 3). The band gaps of the polymers estimated from the onsets of the oxidation and reduction waves were around 1.8-2.0 eV which agrees with the values obtained from the optical absorption spectra. Table 1. Optical and electrochemical properties of the polymers 7a-e.

Polymer 7a

MW, g/mol

PDI

8500

λ max,

λ onset,

Eg(opt),

nm

nm

eV

2.3

534

~670

~1.84

E1/2(ox),

Ep(red).

V vs qAg/AgCl

V vs qAg/AgCl

0.81

-1.36

7b

7000

1.7

555

~700

~1.76

-

-

7c

13000

2.7

552

~700

~1.76

0.76

-1.29

7d

11000

2.2

567

~700

~1.76

0.63

-

7e

20000

2.3

545

~700

~1.76

0.82

-1.41

450

~560

1.9

P3HT

   
   


                       

 

 

 

 

   
  
 
    

Fig. 3. CVA curve for the polymer 7e (solid film on ITO, 0.1M TBAHFP6 in acetonitrile was used as electrolyte, scan rate 50 mV/s, quasi Ag/AgCl was used as reference electrode).

2.2. Photovoltaic devices based on PACPDTs The polymers 7a-e were investigated as electron acceptor materials in organic solar cells. A typical architecture of organic bulk heterojunction solar cell is shown in Fig. 4 (a).

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Vladislav A. Kostyanovskiy et al. / Energy Procedia 31 (2012) 1 – 10

a

b

Fig. 4. Schematic layout of (a) organic bulk heterojunction solar cells; (b) molecular structures of conventional fullerene-based materials.

The photovoltaic cells were fabricated using the following technique. First of all, 50-60 nm thick layers of the hole conductor PEDOT:PSS were spin-coated on the top of the ITO-covered glass slides at the spinning frequency of 3000 rpm. The resulting PEDOT:PSS films were dried at 150 оС within 15 minutes and then photoactive layer was spin-coated on the top at 700-900 rpm frequency. The active layer was represented by the blends of the fullerene derivatives [60]PCBM or [70]PCBM with the polymers 7ae. The devices were finalized by the thermal evaporation of thin aluminium electrodes (100 nm) on the top of the photoactive layer. The performance of the solar cells was evaluated by measuring their I-V curves in dark and under the simulated solar light irradiation (standard conditions: АМ 1.5 spectrum, 100 mW/cm2). The short circuit current density (ISC), open circuit voltage (VOC), fill factor (FF) and overall light power conversion efficiency (η) were calculated from the I-V curves. The optimal weight ratio between the fullerene derivative and the polymer was revealed experimentally by comparing the photovoltaic performances of the devices with 1:1, 1:2, 1:3 and 1:4 (w/w) polymer to fullerene ratios. It was found that all polymers performed better in 1:2 ratio with both [60]PCBM and [70]PCBM fullerene derivatives. The obtained results are presented in Table 2. Table 2. Photovoltaic properties of polymers 7a-e Blend composition

VOC, mV

ISC, mA/cm2

FF

η, %

7a/[70]PCBM 1:2

450

0.6

0.34

0.09

7b/[70]PCBM 1:2

400

0.7

0.34

0.10

7c/[70]PCBM 1:2

450

0.5

0.32

0.07

7d/[70]PCBM 1:2

450

0.7

0.34

0.11

7e/[70]PCBM 1:2

450

0.7

0.33

0.10

7e /[60]PCBM 1:2

425

0.8

0.41

0.14

It is seen from the Table 2 that all PACPDTs showed low photovoltaic performances. The polymer 7e demonstrated the highest power conversion efficiency in combination with [60]PCBM which reached 0.14% only. This result agrees with the available literature data. The solar cells based on a similar siliconsubstituted PACPDT homopolymer yielded power conversion efficiency of 0.02% [28]. The reason for the observed poor photovoltaic performance of PACPDTs might be in the unfavorable morphology of their composites with the fullerene derivatives. To verify this hypothesis we investigated

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fullerene/polymer composite topology using atomic force microscopy (AFM). The obtained images are shown in Fig. 5. It is clearly visible that the films are strongly non-homogeneous. This suggests that the phase separation between the polymer and the fullerene derivative produced huge (up to 300 nm) clusters of individual materials.

Fig. 5. AFM images for the blends of polymers 7a-e with the fullerene derivatives

Vladislav A. Kostyanovskiy et al. / Energy Procedia 31 (2012) 1 – 10

It is known that the exciton diffusion lengths in organic semiconductors do not exceed 10-20 nm [29]. As a consequence, the composites with the cluster sizes significantly larger than the exciton diffusion lengths show poor photovoltaic performances. Excitons photogenerated inside large clusters recombine before reaching the interface with the other material where charge separation might take place. The composites of polymers 7a-e exhibited the clusters with the average size of 200-350 nm. Therefore, the vast majority of excitons generated in these blends will recombine and give no contribution to the photocurrent generation. Taking into account these considerations, we believe that the strong phase separation in the blends of polymers 7a-e with the fullerene derivatives is a reason for their poor photovoltaic performance. The detailed analysis of the AFM images allowed us to find certain correlations between the cluster sizes in the films and and the length of the alkyl side chains (number of carbon atoms) attached to the polymer backbone (Fig. 6).

Fig. 6. The relation between the average cluster sizes in the blend films and the length of the alkyl side chains attached to the polymer backbone

Fig. 6 suggests that the most homogeneous films (the smallest cluster sizes) can be obtained for the polymers with n-octyl and 2-ethylhexyl side chains. This conclusion agrees well with the literature data. Indeed, the best performances in solar cells were obtained using conjugated polymers modified with noctyl and 2-ethylhexyl chains [ 30]. 2.3. Electrochromic devices based on PACPDTs The cyclopentadithiophene-based polymers are known to be promissing materials for electrochromic devices [31-34]. We investigated spectroelectrochemical properties of the polymers 7a-e. Figure 7 shows the evolution of the absorption spectrum of a thin film of the polymer 7e deposited on the ITO electrode when the electrochemical cell was biased between -0.6V and +1.3 V. Increase in the potential from 0.5 to 1.2 V results in the disappearing of the main absorption band at 550 nm and the appearance of a new band

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Vladislav A. Kostyanovskiy et al. / Energy Procedia 31 (2012) 1 – 10

in the near-infrared spectral region. Meanwhile, the film changes its colour from the dark blue to the light green and shows a dramatic enhancement in the transparency in the visible range. Such electrochromic behaviour was observed for all polymers 7a-e. Therefore, these polymers might be potentially useful for construction of the so-called “intelligent” windows or electronic paper.

     
 




  
 



 



 



  

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Fig. 7. Electrochromic behaviour of the polymer 7e.

3. Conclusions In summary, we have synthesized a series of conjugated cyclopentadithiophene-based homopolymers 7a-e and investigated their optical and electrochemical properties. The polymers 7a-e were blended with the fullerene derivatives. The topology of the resulting composite thin films was revealed using AFM measurements. A correlation between the molecular structure of the polymers and the morphology of their composites with the fullerene derivatives was established. The prepared fullerene/polymer composites were evaluated as active layer materials in organic bulk heterojunction solar cells. The performed spectroelectrochemical studies suggested the application of the polymers 7a-e in electrochromic devices. Acknowledgement We gratefully acknowledge Dr. S. A. Ponomarenko (ISPM RAS, Moscow, Russia) for the GPC data. This work was supported by the Russian Foundation for Basic Research (grant 10-03-00443) and the Russian Ministry for Science and Education (contract № 02.740.11.0749).

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Appendix A. Synthetic procedure and spectroscopic characteristics for PACPDTs A.1. Oxidative polymerization of 4,4-dialkylcyclopentadithiophenes Monomer 6a-e (1 g, 2.5 mmol) was dissolved in a freshly distilled chloroform (5 cm3), the resulting solution was purged with argon for 15 minutes and then added dropwise to a suspension of FeCl3 (1.62 g, 10 mmol) in dried chloroform (50 cm3). The mixture was vigorously stirred for 24 hours under an argon atmosphere. The oxidized polymer was precipitated into methanol, filtered and dried. The filter cake was redissolved in chloroform (30 ml) and then hydrazine hydrate (2 mL) was added. The resulting mixture was heated at reflux within 24 h. The polymer was precipitated in methanol, collected by filtration and purified by column chromatography on silica. The fraction eluted with toluene was concentrated at the rotary evaporator and further precipitated with methanol. The precipitate was collected and dried in vacuum. The title polymers 7a-e were obtained in 15-25% yield (soluble in toluene fraction only). 7a. Mw=8.5 kg/mol, PDI =2.3. 1H NMR (CDCl3): 7.00 (s, 2H, aromatic), 6.90 (s, chain termini), 1.86 (m, 4H, CH2), 1.19 (m, 16H, CH2), 0.86 (m, 6H, CH3) ppm. λmax=534 nm, λonset= 670 nm. 7b. Mw=7.0 kg/mol, PDI=1.7. 1H NMR (CDCl3): 7.03 (s, 2H, aromatic), 6.85 (s, chain termini), 1.85 (m, 4H, CH2), 1.20 (m, 20H, CH2), 0.87. (m, 6H, CH3) ppm. λmax = 555 nm, λonset= 700 nm. 7c. Mw=13.0. kg/mol, PDI =2.7. 1H NMR (CDCl3): 7.01 (s, 2H, aromatic), 6.86 (s, chain termini), 1.88 (m, 4H, CH2), 1.21(m, 24H, CH2), 0.88 (m, 6H, CH3) ppm. λmax=552 nm, λonset=700 nm. 7d Mw=11.0 kg/mol, PDI=2.2. 1H NMR (CDCl3): 7.02 (s, 2H, aromatic), 6.81 (s, chain termini), 1.83 (m, 4H, CH2), 1.15(m, 20H, CH2), 0.80 (m, 6H, CH3) ppm. λmax= 567 nm, λonset=700nm. 7e. Mw=20.0 kg/mol, PDI =2.3. 1H NMR (CDCl3): 6.95 (s, 2H, aromatic), 6.87 (s, chain termini), 1.84 (m, 4H, CH2), 1.22(m, 28H, CH2), 0.85 ppm (m, 6H, CH3). λmax= 555 nm, λonset= 700nm.