or pyrrole units

Solar Energy Materials & Solar Cells 94 (2010) 2318–2327

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Efficient bulk heterojunction solar cells based on low band gap bisazo dyes containing anthracene and/or pyrrole units J.A. Mikroyannidis a,n, D.V. Tsagkournos a, S.S. Sharma d,e, Anil Kumar d, Y.K. Vijay d, G.D. Sharma b,c,nn a

Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece Physics Department, Molecular Electronic and Optoelectronic Device Laboratory JNV University, Jodhpur, Rajasthan 342005, India c Jaipur Engineering College, Kukas, Jaipur, Rajasthan, India d Physics Department, Thin film and Membrane Science Laboratory University of Rajasthan, Jaipur, Rajasthan 302004, India e Government Women Engineering College, Ajmer, Rajasthan 305002, India b

a r t i c l e in f o

a b s t r a c t

Article history: Received 5 June 2010 Received in revised form 23 July 2010 Accepted 1 August 2010 Available online 9 September 2010

2,5-Bis(2-anthracenyldiazo)-1H-pyrrole (dye D1) and 2,20 -[1,4-phenylenebis(azo)]bis-1H-pyrrole (dye D2) were synthesized by four- and two-step reactions, respectively. Their absorption spectra were broad with thin film absorption onset at 829 and 738 nm corresponding to an optical band gap of 1.39 and 1.68 eV for D1 and D2, respectively. The solution processed bulk heterojunction (BHJ) photovoltaic devices fabricated from D1 and D2 as donor blended with PCBM as an acceptor showed power conversion efficiency (PCE) up to 2.13% and 1.59%, respectively. The PCE has been further improved up to 2.42% and 2.18%, after thermal annealing of the D1:PCBM and D2:PCBM layers, respectively. The higher PCE of the photovoltaic devices based on the D1:PCBM blend as compared to D2:PCBM is attributed to the higher hole mobility and lower band gap of D1 relative to D2. The combination of D1 with D2 and PCBM allows not only a broad absorption, but also tuning of the inter energy level leading to a higher short circuit current (Jsc) and open circuit voltage (Voc). The best performance device exhibited PCE of about 3.61% with the thermally annealed D1:D2:PCBM photoactive layer. Such a high PCE using small molecules is due to the increased values of both Jsc and Voc. & 2010 Elsevier B.V. All rights reserved.

Keywords: Bulk heterojunction solar cells Bisazopyrrole Bisazophenylene Anthracene Low band gap Ternary BHJ

1. Introduction Organic photovoltaic (PV) cells, which directly convert sunlight into electricity, have drawn considerable attention from both the academia and industry, because of their promise as low-cost, lightweight, renewable energy sources [1]. Bulk heterojunction (BHJ) PV cells based on solution-processable conjugated p-type semiconducting materials blended with a soluble fullerene derivative, [6,6]-phenyl C61-butyric acid methyl ester (PCBM), offer advantages of facile and large-area fabrication on flexible substrates by various printing or coating techniques or a roll-toroll process [2] affording high power conversion efficiencies (PCE) of greater than 6% recently [3]. However, the structural characteristics including molecular weight, polydispersity and regioregularity as well as the purity of the polymer greatly affect its functional properties, the performance and the stability [4] of the resulting devices. In addition to polymers, p-conjugated small molecules have been found attractive as alternative solution-

n

Corresponding author. Tel.: + 30 2610 997115; fax: +30 2610 997118. Corresponding author. Tel.:+ 91 0291 2720857; fax: + 91 0291 2720856. E-mail addresses: [email protected], [email protected] (J.A. Mikroyannidis), [email protected] (G.D. Sharma). nn

0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.08.001

processable p-type donor materials. The use of solution-processable small molecules for PV applications has drawn considerable attention [5], and achieved PCE of 1.7%–4.4%, in recent years. In particular, the use of soluble small molecules as donor material has been proposed [6–12]. Actually, molecular donors present some specific advantages in terms of structural definition, reproducibility of synthesis and purification. Although initial prototypes of molecular BHJ exhibited modest performances [7], considerable progress has been achieved in a short time [7–12]. Thus, a power conversion efficiency (PCE) of 4.40% has recently been reported for a cell based on a diketopyrrolopyrrole (DPP) donor and [6,6]-phenyl-C71-butyric-acid methyl ester (PC71BM) as acceptor [9]. These reports indicate that the organic small molecules are promising for use as high efficiency photovoltaic materials in the near future. The PCE mainly depends upon the light harvesting ability of the active materials, exciton dissociation at the donor–acceptor interface and change transport and collection at the opposing electrodes. However, the short exciton diffusion length and relatively low absorptivity in organic materials limit the thickness of the active material in photovoltaic cells. Materials that possess large absorptivity and broad absorption coverage (or having a complementary absorption to the currently used solar cell sensitizers) in the solar spectrum and high charge mobility are

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therefore essential for the improvement of overall PCE of the photovoltaic device. Azo dyes are well-known class of organic photoactive materials due to their excellent optical switching properties, good chemical stabilities and high solution process abilities [13,14]. These materials are widely used in heat transfer printing and textile industries [15,16], optical data storage [17], switching technologies [18] and photo-refractive polymer industries [19]. NIR-absorbing azo dyes, based on five-membered heterocyclic rings, have been made effectively by extending the conjugated N¼N bond from monoazo to bisazo dyes to enhance molecular p-resonance effects [20]. Various azo dyes have been used as sensitizers for dye-sensitized solar cells (DSSCs) [21,22]. However, the utilization of azo compounds for BHJ solar cells is limited [23]. A series of 2,2-dipyrrole monomers separated by aza-spacers have been recently synthesized using modified Schiff and azo-coupling reactions. These 2,2-dipyrroles linked with conjugated aza-spacers have been evaluated as precursors of narrow band gap conducting polymers [24]. Finally, anthracene containing compounds have been used for BHJ solar cells. Particularly, a diarylanthracene bearing two dihexyloxy-substituted benzene rings has been synthesized and used as donor for BHJ solar cells with PCBM [25]. Various anthracene containing copolymers and small molecules have been recently used for PV applications [26–31]. The present investigation describes the synthesis and characterization of two broadly absorbing p-conjugated bisazo dyes. In particular, a symmetrical 2,5-bisazopyrrole containing anthracene terminal units at both sides (dye D1) was successfully synthesized. In addition, a symmetrical 1,4-bisazophenylene bearing pyrrole terminal units at both sides (dye D2) was synthesized. Both dyes carried bisazo linkages which are expected to extend the conjugation, and consequently their absorption curve into the near infrared spectrum region. Moreover the presence of the anthracene and/or pyrrole units contributes to the broadening of their absorption. The absence of aliphatic chains increases the rigidity and thermal stability of the dyes, but reduces their solubility in common organic solvents. The solution processed BHJ photovoltaic devices fabricated from these materials as donors, blended with PCBM as acceptor showed a PCE up to 2.42%. The ternary BHJ photovoltaic device consisting of D1:D2:PCBM blend exhibited an enhanced PCE, relative to those of photovoltaic devices fabricated either with D1:PCBM or D2:PCBM as an active layer, which was attributed to the increased light harvesting and appropriate energy levels of both dyes.

2. Experimental 2.1. Reagents and solvents 9-Nitroanthracene (1) was prepared from the nitration of anthracene by means of concentrated nitric acid in glacial acetic acid [32]. It was purified by recrystallization from methanol. 9-Aminoanthracene (2) was prepared from the reduction of 1, using hydrazine monohydrate in ethanol in the presence of a catalytic amount of Pd/C [32]. 1,4-Phenylenediamine was sublimed under vacuum. N,N-Dimethylformamide (DMF) and tetrahydrofuran (THF) were dried by distillation over CaH2. All other reagents and solvents were commercially purchased and were used as supplied. 2.2. Synthesis of dyes 2.2.1. 2,5-Bis(2-anthracenyldiazo)-1H-pyrrole (dye D1) A flask was charged with suspension of 2 (0.74 g, 3.83 mmol) in water (5 mL) and ethanol (10 mL). Hydrochloric acid (2 mL)

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was added to the suspension. The mixture was cooled and kept at 0–5 1C in an ice bath and diazotized by adding solution of NaNO2 (0.27 g, 3.94 mmol) in water (5 mL), followed by stirring for 0.5 h at 0–5 1C. The solution of the diazonium salt 3, thus, prepared was immediately used for the next coupling reaction. The solution of 3 was slowly added to a solution of pyrrole (0.13 g, 1.93 mmol) and pyridine (3 mL) in ethanol (30 mL) at 0–5 1C. The resulting mixture was stirred for 10 h, and then concentrated under reduced pressure. The precipitate was filtered, washed with water and dried to afford D1 (0.81 g, 89%). FT-IR (KBr, cm  1): 3350 (N–H stretching); 1592, 1502, 1448 (aromatic). 1 H NMR (DMSO-d6) ppm: 9.75 (s, 1H, NH); 8.39 (s, 2H, H10 of anthracene); 7.98 (m, 8H, H1, H4, H5, H8 of anthracene); 7.44 (m, 8H, H2, H3, H6, H7 of anthracene); 6.88 (s, 2H, pyrrole). Anal. Calcd. for C32H21N5: C, 80.82; H, 4.45; N, 14.73. Found: C, 80.24; H, 4.92; N, 14.58. 2.2.2. 2,20 -[1,4-phenylenebis(azo)]bis-1H-pyrrole (dye D2) A flask was charged with a suspension of 1,4-phenylenediamine (1.22 g, 11.28 mmol) in water (10 mL). Hydrochloric acid (2 mL) was added to the suspension. The mixture was cooled and kept at 0–5 1C in an ice bath and diazotized by adding solution of NaNO2 (1.60 g, 23.19 mmol) in water (5 mL), followed by stirring for 0.5 h at 0–5 1C. The solution of the bisdiazonium salt 4, thus, prepared was immediately used for the next coupling reaction. The solution of 4 was slowly added to solution of pyrrole (1.51 g, 22.56 mmol) and pyridine (3 mL) in ethanol (40 mL) at 0–5 1C. The resulting mixture was stirred for 10 h, and then concentrated under reduced pressure. The precipitate was filtered, washed with water and dried to afford D2 (2.20 g, 74%). FT-IR (KBr, cm  1): 3354 (N–H stretching); 1598, 1514, 1454 (aromatic). 1 H NMR (DMSO-d6) ppm: 9.04 (s, 2H, NH); 7.93 (m, 4H, phenylene); 6.24–6.12 (m, 4H, H3, H4 of pyrrole); 6.58 (m, 2H, H5 of pyrrole). Anal. Calcd. for C14H12N6: C, 63.62; H, 4.58; N, 31.80. Found: C, 63.12; H, 4.42; N, 31.28. 2.3. Characterization methods IR spectra were recorded on Perkin-Elmer 16PC FT-IR spectrometer with KBr pellets. 1H NMR (400 MHz) spectra were obtained, using a Brucker spectrometer. Chemical shifts (d values) are given in parts per million with tetramethylsilane as an internal standard. UV–vis spectra were recorded on a Beckman DU-640 spectrometer with spectrograde THF or DMF. Elemental analyses were carried out with a Carlo Erba model EA1108 analyzer. The electrochemical properties of both dyes were examined, using cyclic voltammetry (CV) (EDCA electrochemistry system). A glassy carbon electrode was used as working electrode. The dyes were coated on the glassy carbon electrode from a solution in DMF and THF for D1 (5 mg/mL) and D2 (5 mg/mL), respectively, and immersed in 0.1 mol/L Bu4NPF6 acetonitrile solution used as a supporting electrolyte. Cyclic voltammograms were recorded, using Ag/Ag + as reference electrode at a scan rate of 100 mV/s. 2.4. Device fabrication and characterization All devices were fabricated on indium tin oxide (ITO) glass substrates. The ITO coated glass substrates were cleaned with acetone in an ultrasonic bath. A thin layer of PEDOT:PSS (poly(3,4-ethylene-dioxythiophene):poly(styrene sulfonate)) (50 nm) was spin coated on it from PEDOT:PSS aqueous solution at 2000 rpm and dried subsequently at 100 1C for 30 min in air, and

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then transferred into a glove box. The active layer of the blend of D1 or D2 with PC60BM was spin coated onto the PEDOT:PSS layer, and then dried in ambient conditions. The concentration of D1:PCBM and D2:PCBM blend solution used in this study for spin coating was 10 mg/mL in DMF and THF, which were used as solvents of D1 and D2, respectively. The thickness of the active layer was approximately 80 nm. The blended active layers were annealed at 120 1C for 3 min before the deposition of the aluminum (Al) electrode. An Al top electrode was deposited in vacuum onto the active layer at a pressure  5  10  5 pa through a shadow mask defining an active area of 0.05 cm2. The structure of the device is ITO/PEDOT:PSS/D1 or D2:PCBM/Al. Current–voltage (J–V) characteristics of the devices were measured using a computer controlled Keithley 238 source meter in dark and under illumination intensity of about 100 mW/cm2 in ambient conditions. A xenon light source (Oriel, USA) was used to give simulated irradiance of 100 mW/cm2 (equivalent to an AM1.5 irradiation) at the surface of the device. The J–V characteristics measurements under illumination were carried out in a dark chamber (wooden box) with a window slit of 2 cm2 area for illumination.

3. Results and discussion 3.1. Synthesis and characterization Scheme 1 outlines the synthesis of the dyes D1 and D2. Specifically, 9-nitroanthracene (1) [32] and 9-aminoanthracene (2) [32] were synthesized according to the literature. The diazotization of 2 by means of NaNO2 and hydrochloric acid at temperatures 0–5 1C afforded the diazonium salt 3. The coupling of the latter with pyrrole in a mol ratio 2:1 gave the target bisazopyrrole D1. This reaction was carried out in ethanol in the presence of pyridine to neutralize the reaction mixture. In addition, 1,4-phenylenediamine reacted likewise with NaNO2/ HCl to afford the bisdiazonium salt 4. Finally, it was coupled with double equivalent of pyrrole to give the bisazophenylenediamine D2. The dyes were obtained as dark green-red solids in high yields (74–89%). Since both dyes lack aliphatic solubilising chains, they were moderately soluble in THF, chloroform and dichloromethane, while were readily soluble in DMF and dimethylacetamide. D1 showed lower solubility than D2 due to the more rigid structure of the former. A literature survey revealed that dye D1 has not been previously synthesized. In contrast, the synthesis [33] and utilization of D2 has been reported in certain Japanese patents. Dye D2 has been utilized for field-effect transistor [34], organic electroluminescent device

[35], electrophotographic photoreceptor [36], nonlinear optical material [37], but not for PV applications. The FT-IR and 1H NMR spectra of the dyes were consistent with their chemical structures. In particular, both dyes showed characteristic absorption bands around 3350 (N–H stretching of pyrrole) and 1600, 1500 and 1450 cm  1 (aromatic). It is well known that the N¼N frequency of aromatic azo compounds is difficult to be identified by an IR spectroscopy, because this frequency is weak and overlaps with other aromatic bands [38]. The 1H NMR spectra of the dyes displayed common signals at 9.75–9.04 (NH of pyrrole) and 6.12–6.88d (pyrrole). Dye D1 exhibited a characteristic upfield singlet at 8.39d assigned to H10 of anthracene. 3.2. Photophysical properties Fig. 1 presents the UV–vis spectra of the dyes in dilute (10  5 M) solution (DMF for D1 and THF for D2) and thin film. All photophysical characteristics of dyes are summarized in Table 1. Both dyes showed in solution two absorption maxima (la,max), one at short-wavelength ( 400 nm) and one at long-wavelength (  600 nm). The absorption curve of D1 was broader than that of D2. The thin film absorption spectrum of D1 was more complex and showed three la,max at 417, 560 and 800 nm. The thin film absorption spectrum of D2 displayed la,max at 448 nm, followed by a long tail which was extended up to  800 nm. The optical band gaps (Eopt g ) of the dyes were calculated from the thin film absorption onset, which is located at 829 and 738 nm correspondof 1.39 and 1.68 eV for D1 and D2, respectively. ing to Eopt g The presence of the two anthracene units in the molecule of of this dye as D1 should be responsible for the lower Eopt g of D1 is slightly lower than that compared to D2. The Eopt g (1.54 eV) of 2,5-bis(4-hexyloxyphenylazo)-1H-pyrrole, which has been prepared in our laboratory and used for BHJ solar cells [23]. It is known that azo dyes suffer isomerization reaction absorbing light in the visible and near UV range [39]. The situation is more complex in the bisazo compounds, which are capable of light-induced configurational transitions (photoisomerization). In general, the trans azo (–N¼ N–) isomer can absorb UV light and isomerize to the cis form. When the isomerization pump light is switched off, the cis isomers can return back to the trans form through the thermal isomerization process, because the cis isomers are unstable in natural conditions. The trans–cis photoisomerization of bisazo dyes D1 and D2 by irradiation with UV light was investigated. Fig. 2 shows the results obtained using UV–vis absorption spectroscopy for measuring cis–trans photoisomerization in solutions for the two dyes. Before irradiation with the UV light, the dye molecules are in energetically more

Scheme 1. Synthesis of dyes D1 and D2.

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Fig. 2. UV–vis absorption spectra of irradiated dye solutions. The solutions of the dyes were prepared, using as solvent DMF for D1 and THF for D2.

Fig. 1. UV–vis absorption spectra of dyes in solution (top) and thin film (bottom). The solutions of the dyes were prepared, using as solvent DMF for D1 and THF for D2.

Table 1 Optical and electrochemical properties of dyes.

3.3. Electrochemical properties

Dye

D1

D2

la,maxa In solution (nm) la,maxa In thin film (nm)

428, 608 417, 560, 800 829 1.39 0.49  1.02

396, 580 448 738 1.68 0.74  0.98

 5.05  3.63 1.42

 5.50  3.78 1.72

Thin film absorption onset (nm) b Eopt (eV) g Eox onset (V) Ered onset (V) HOMO (eV) LUMO (eV) c Eel g (eV)

D1 displayed more distinct decrease in absorbance than dye D2. Generally, the two dyes showed relatively low decrease in absorbance even after irradiation with UV light for 300 s.

a la,max: the absorption maxima from the UV–vis spectra in solution (DMF for D1 and THF for D2) or in thin film. b opt Eg : optical band gap determined from the absorption onset in thin film. c el Eg :electrochemical band gap determined from cyclic voltammetry.

stable trans state. Upon irradiating with UV light of wavelength 365 nm for various time periods, molecules are excited to cis state, which is evident by the decrease in absorbance value. Dye

Cyclic voltammetry data are employed to calculate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of both dyes from the onset oxidation and reduction potentials, respectively. The cyclic voltammograms of both materials are shown in Fig. 3. Both dyes showed reversible oxidation (p-doping/re-reduction) and irreversible (n-doping/re-oxidation) waves. The potentials have been measured with respect to the Ag/Ag + electrode. The HOMO and LUMO energy levels were estimated from the following expressions [40] EHOMO ¼ eðEox þ4:7Þ

ð1aÞ

ELUMO ¼ eðEred þ4:7Þ

ð1bÞ

where Eox and Ered are the onset oxidation and reduction potentials, with respect to Ag/Ag + potential (4.7 eV vs. vacuum), EHOMO and ELUMO are the HOMO and LUMO energy levels,

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Fig. 3. Cyclic voltammograms of D1 (a) and D2 (b).

Fig. 4. Current–voltage characteristics of the ITO/PEDOT:PSS/D1 or D2/Au devices, plotted in the form of ln[JL3/V2] vs. (V/L)1/2.

Fig. 5. Current–voltage (J–V) characteristics of the ITO/PEDOT:PSS/D1:PCBM/Al device in dark (a), under illumination of the as cast (b) and thermally annealed (c) thin film.

respectively. These values are summarized in Table 1. The electrochemical band gaps are comparable with the optical band gaps estimated from the thin film absorption onset.

current in the dark and using Eq. (2), the zero field mobility for D1 and D2 is about 3.8  10  5 and 2.2  10  5 cm2/Vs, respectively. The hole mobilities for both dyes are comparable to other small molecules reported in the literature [5](i.,k).

3.4. Charge carrier mobility of D1 and D2 The mobility of the charge carriers in the photoactive layer used in the device is also an important factor, which influences the performance of the organic solar cells. We have measured the J–V characteristics of the devices in dark at room temperature using ITO/PEDOT:PSS/D1 or D2/Au configuration, to estimate the hole mobility. The J–V data were analyzed using nonlinear square fitting of the modified Mott Gurney equation, for the space charge limited current model, described as [41] rffiffiffiffi! 9 V2 V ð2Þ J ¼ ee0 m 3 exp 0:89 b 8 L L where J is the current density, V is the applied voltage, L is the thickness of the active layer, m is the zero field mobility, e is the dielectric constant, eo is the permittivity of free space (8.85  10  12 F/M) and b is the field activation factor. The J–V characteristics of the devices were plotted in the form of ln(JL3/V2) vs. (V/L)1/2 and are shown in Fig. 4. The fitted curves using Eq. (2) have been also shown in this figure. From the analysis of the J–V

3.5. Photovoltaic properties of BHJ devices We have investigated both dyes for their utility as light absorber and electron donating materials with PCBM as an electron acceptor for BHJ photovoltaic devices. The J–V characteristics of the devices in dark and under illumination intensity of 100 mW/cm2 are shown in Figs. 5 and 6. The optimized photovoltaic parameters are listed in Table 2. The Voc of the device based on D1 is slightly lower than that of the device based on D2. In BHJ photovoltaic devices, the Voc is related to the energy difference between the LUMO ( 4.1 eV) of the acceptor (PCBM) and the HOMO of the donor [42]. This difference is lower for D1 as compared to D2, and it is attributed to the lower value of Voc for the device based on D1. The energy difference between the HOMO of D2 ( 5.4 eV) or D1 ( 5.2 eV) and the LUMO of PCBM is estimated to be 1.3 and 1.1 eV for D2:PCBM and D1:PCBM active layers, respectively, which are the theoretical limits for the Voc, using a BHJ layer. The low experimental value of Voc for both devices, as compared to the theoretical value, may be attributed

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Fig. 7. Photoluminescence (PL) spectra of D1, D2, D1:PCBM and D2:PCBM.

Fig. 6. Current–voltage (J–V) characteristics of the ITO/PEDOT:PSS/D2:PCBM/Al device in dark (a), under illumination of the as cast (b) and thermally annealed (c) thin film.

Table 2 Photovoltaic parameters of the BHJ devices fabricated with different active layers. Active layer

Short circuit current (Jsc) (mA/cm2)

Open circuit voltage (Voc) (V)

Fill factor (FF)

Power conversion efficiency (Z) (%)

D1:PCBM (as cast) D1:PCBM (annealed) D2:PCBM (as cast) D2:PCBM (annealed)

5.40

0.86

0.48

2.23

6.12

0.84

0.51

2.62

4.30

0.95

0.38

1.55

5.50

0.93

0.41

2.10

to charge carrier losses at the electrodes [43], energy level alignment of small molecule/electrode interface [44] and lowered effective band gap of the blends due to the formation of charge transfer complexes [45]. Since the HOMO level of D2 is not aligned with the work function of PEDOT:PSS ( 5.1 eV), it causes band bending at the anode interface and results in a voltage loss that is responsible for the lower value of Voc, as expected theoretically. The Jsc and the overall PCE for the device based on D1 is higher than that of D2 under similar conditions. The driving force for the efficient photoinduced charge transfer in the BHJ device is the energy difference between the LUMO levels of the donor and acceptor components employed in the device. The higher value of Jsc and PCE of the device based on D1:PCBM blend compared to D2:PCBM, is attributed to the larger difference in the LUMO levels of donor and acceptor, leading to a more efficient photoinduced charge transfer in the device based on D1. The lower band gap of D1 as well as the higher hole mobility and finally the enhanced absorption coefficient in longer wavelength region may also be responsible for this behavior. The HOMO level of D1 is almost the same with that of PEDOT:PSS, indicating that holes can be more easily transported from D1 to PEDOT:PSS than that of D2. The photoluminescence (PL) spectra of D1:PCBM and D2:PCBM films (Fig. 7) indicate that D1 and D2, which displayed peak at 790

and 735 nm, respectively, were significantly quenched by blending with PCBM. This means that an intermolecular photoinduced charge transfer (PICT) from the excited state of D1 and D2 to PCBM occurred. An analogous behavior has been observed in the case of a conjugated polymer and fullerene blend system [46]. We have observed that the PL quenching is higher for the D1:PCBM blend than that for the D2:PCBM. This indicates that the degree of photoinduced charge transfer is more efficient in former blend with respect to latter blend. This results in more free charge carriers in the device based on D1:PCBM, which are consequently responsible for the higher PCE. One important feature of the small molecules is their selfassembling properties during the film formation. Because the self-assembly of electroactive molecules has a strong effect on their optical and electronic properties, we have examined the morphology of the as cast and the thermally annealed D1:PCBM and D2:PCBM thin films. Fig. 8(a) shows the AFM topographic images of the D2:PCBM thin films with and without thermal treatment. Similar patterns have been observed for D1:PCBM thin films (as cast and thermally annealed). The surface of the D2:PCBM is quite smooth, showing an average root mean square (rms) roughness of 3.7 and 5.3 nm with and without thermally annealed films, respectively. Similar AFM images have also been observed for D1:PCBM showing average rms surface roughness about 4.1 and 5.6 nm for the as cast and thermally annealed films. Compared to thin films with and without thermal annealing, a considerable increase in the rms roughness after the thermal annealing was observed in both blends. This indicates that the domain size of annealed film increases the crystallinity of the blend, which is also responsible for the improved PCE of the photovoltaic devices based on thermally annealed films. XRD patterns (as shown in Fig. 8b) of the thin films were also used to determine the difference between the crystalline nature of the as cast and thermally annealed D2:PCBM blend. It can be seen from this figure that the diffraction pattern peak of D2:PCBM film is centered at 2y ¼22. 251, that corresponds to an inter-planar ˚ The thermal annealing increases this peak distance of 12.4 A. indicating a higher degree of crystallinity. This change in the film crystallinity after thermal annealing agrees with the change observed in the rms surface roughness of the thermally annealed film. Since most of the fullerene acceptors, such as PCBM, do not show any diffraction peak in the range of 2y values used [47], we assume that the change in the crystallinity of the blend film after thermal annealing is mainly attributed to the increase in crystallinity of the donor material (D1 or D2), which leads to an increase

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Fig. 8. (a) Roughness analysis using AFM and (b) XRD patterns of the as cast and thermally annealed D2:PCBM blend.

mobility is more balanced for the thermally annealed blend and is one of the factors that contribute to the increase of PCE.

Table 3 Hole and electron mobilities in the BHJ layers. Blend

Hole mobility (cm2/Vs)

Electron mobility (cm2/Vs)

1.2  10  5 8.6  10  5 0.95  10  5 5.6  10  5

3.4  10  4 3.9  10  4 3.2  10  4 3.6  10  4

3.6. Photovoltaic properties of D1:D2:PCBM blend D1:PCBM D1:PCBM D2:PCBM D2:PCBM

(as cast) (annealed) (as cast) (annealed)

in hole mobility and results in an enhancement in the PCE of the devices. In most of the organic BHJ photovoltaic devices, the large difference in the mobilities of electron and hole, results in charge accumulation near the collecting electrodes and inefficient charge collection, decreases the fill factor and PCE due to the charge recombination. To measure the carrier mobilities (electron and hole) in the blend film, the J–V characteristics of hole only (ITO/PEDOT:PSS/D1 or D2:PCBM/Au structure) and electron only (Al/D1 or D2:PCBM/Al structure) devices were fabricated using the as cast and thermally annealed blends. The hole and electron mobilities were estimated using the SCLC model as described earlier. The hole and electron mobilities for the as cast and thermally annealed blend films are listed in Table 3. It can be seen from this table that the electron mobilities do not change significantly. However, after thermal annealing, the hole mobility increases by a factor of about 7.2 and 6.0 for D1:PCBM and D2:PCBM active layers, respectively. The increase in the hole mobility is attributed to both the enhanced surface roughness and crystalline nature (as shown in AFM and XRD data) of the donor material (D1 or D2) in the blend, which increases the percolation pathways for holes in donor component after thermal annealing. Therefore, the difference in the electron and hole mobilities is lower for the device based on thermally annealed blend as compared to the as cast film. This indicates that the carrier

At present, current research in the field of organic photovoltaic device is to enhance the light harvesting property of the active layer and photocurrent generation in the photovoltaic device. For example, the use of cascade multilayer device structures in solar cells can enhance the device performance [48]. However, most of these multilayer devices have been fabricated, using complicated processes. A common strategy for forming thin films from two or more dissimilar soluble materials is to mix them together. This approach is used extensively in the preparation of polymer light emitting diodes [49], and recently for polymer solar cells [50,12b]. We attempted to prepare efficient BHJ solar cells incorporating three blended organic semiconductors. Three criteria must be considered for the third organic semiconductor: (i) its energy levels must have correct offset with respect to those of its blend counterparts, (ii) it can operate as either an electron acceptor and transport or electron donor and hole transport, and (iii) it should have high absorption coefficients in complementary absorption ranges with respect to those of its blend counterparts. The optical band gaps of D1 and D2 are 1.39 and 1.68 eV, respectively, and the dyes behave as electron donors, as discussed earlier. As it can be seen from the absorption spectra of D2 and D1, these two materials have complementary absorption spectra, and the blend of these two may absorb the photon within whole visible and near IR spectrum. Therefore, we have utilized blend of D2 and D1 with PCBM as a photoactive layer for BHJ photovoltaic devices. Fig. 9 displays the J–V curves, after combining the D1, D2 and PCBM in 1:1:1 ratio, and the photovoltaic parameters are summarized in Table 4. As it can be seen from Tables 2 and 4,

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Fig. 9. Current–voltage (J–V) characteristics under illumination of the ITO/ PEDOT:PSS/D1:D2:PCBM/Al device, which was fabricated with the as cast and thermally annealed thin film.

Table 4 Photovoltaic parameters of the BHJ devices fabricated with ternary active layers. Active layer

D2:D1:PCBM (as cast) D2:D1:PCBM (annealed)

Short circuit current (Jsc) (mA/cm2)

Open circuit voltage (Voc) (V)

Fill factor (FF)

Power conversion efficiency (Z) (%)

6.80

0.88

0.53

3.17

7.60

0.85

0.56

3.61

Fig. 10. Energy levels of electrodes and organic semiconductors (D1, D2 and PCBM) used in ternary blend.

the Jsc is higher for the photovoltaic device based D1:D2:PCBM as compared to both D1:PCBM and D2:PCBM. This is attributed to the enhanced light harvesting of the D1:D2:PCBM blend throughout the entire visible region. We expected the broad absorption spectrum of the ternary blend to generate a large number of the excitons and thus, a larger photocurrent. Fig. 10 summarizes the HOMO and LUMO levels of D1, D2 and PCBM, determined by CV. The HOMO level of D2 is located between the HOMOs of D1 and PCBM. The same applies for the LUMO levels. As a consequence, photoinduced charge transfer is energetically allowed between

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D1 and PCBM, between the D2 and PCBM, as well as between D1 and D2. Furthermore, the transfer of any kind of charge located on D2–D1/PCBM matrix is energetically favored. Such design of a blend enables both donors, D1 and D2, to contribute in charge generation, which is similar to a tandem cell. We have also measured the PL spectra of the ternary blend and found that its PL emission was further quenched as compared to that of the D1:PCBM and D2:PCBM blends. This PL quenching arose as a consequence of the ultra fast photoinduced charge transfer from D1 to D2 and from D2 to PCBM. Thus, D2 acts as both an electron acceptor and electron donor, when blended with D1 and PCBM. Moreover the difference in the energy levels (HOMO and LUMO) of D1 and D2 (as shown in Fig. 10) can accelerate the carrier transfer [51], resulting in an increased extraction of the charge carriers. Furthermore, we expected that wider energy difference between the LUMO energy level of the acceptor and the HOMO energy level of the donor at the D1/D2 and D2/PCBM junctions would lead to higher value of the Voc. Similar to the BHJ films, we have also investigated the J–V characteristics of the device, using the thermally annealed ternary blend, as shown in Fig. 9. The photovoltaic parameters are listed in Table 4. The overall PCE has been further improved up to 3.61%, which is mainly attributed to the increase in Jsc. We assume that the increase in both the crystalline nature of D1 and D2 and hole mobility is responsible for the improvement in Jsc and overall PCE.

4. Conclusions Two symmetrical bisazo dyes, D1 and D2, of low band gap containing anthracene and/or pyrrole units were synthesized. These dyes were partially soluble in THF, chloroform and dichloromethane, while being readily soluble in DMF and dimethylacetamide. They showed broad absorption spectra with optical band gaps of 1.39 and 1.68 eV for D1 and D2, respectively. The HOMO and LUMO levels of both D1 and D2 indicated that they are suitable for donor component, when blended with the PCBM acceptor. The BHJ devices were fabricated by spin coating the blends of D1 or D2 with PCBM (1:1 ratio), and we have characterized these devices through measuring the J–V characteristics under illumination. The overall PCE of the device based on D1:PCBM (2.23%) is higher than that of D2:PCBM (1.55%), which is attributed to both higher hole mobility for D1, as compared to D2, and also more efficient photoinduced charge transfer at the D/A interfaces. The PCE of the devices with the thermally annealed blends has been further enhanced (2.1% and 2.62% for D2:PCBM and D1:PCBM, respectively), which has been attributed to the increased crystallinity of the blends and increased hole mobility in the donor phase. Finally, we have also fabricated BHJ photovoltaic devices with ternary D1:D2:PCBM blends, exhibiting PCEs 3.15% and 3.61% with the as cast and thermally annealed blends, respectively. We believe that the improvement in the PCEs with ternary blend is attributed to the fine tuning of the energy levels that resulted in higher values of Jsc and Voc.

Acknowledgement We are also grateful to the Scientists in IUAC, New Delhi for recording the AFM images, XRD patterns and PL spectra of the samples. References [1] (a) H. Hoppe, N.S. Sariciftci, Morphology of polymer/fullerene bulk heterojunction solar cells, J. Mater. Chem. 16 (2006) 45–61;

2326

[2]

[3]

[4] [5]

[6]

J.A. Mikroyannidis et al. / Solar Energy Materials & Solar Cells 94 (2010) 2318–2327

¨ (b) S. Gunes, H. Neugebauer, N.S. Sariciftci, Conjugated polymer-based organic solar cells, Chem. Rev. 107 (2007) 1324–1338; (c) B.C. Thompson, J.M. Fre´chet, Polymer–fullerene composite solar cells, Angew. Chem., Int. Ed. 47 (2008) 58–77; (d) R. Po, M. Maggini, N. Camaioni, Polymer solar cells: recent approaches and achievements, J. Phys. Chem. C 114 (2010) 695–706. (a) F.C. Krebs, Fabrication and processing of polymer solar cells: a review of printing and coating techniques, Sol. Energy Mater. Sol. Cells 93 (2009) 394–412; (b) F.C. Krebs, S.A. Gevorgyan, J. Alstrup, A roll-to-roll process to flexible polymer solar cells: model studies, manufacture and operational stability studies, J. Mater. Chem. 19 (2009) 5442–5451; (c) F.C. Krebs, Polymer solar cell modules prepared using roll-to-roll methods: knife-over-edge coating, slot-die coating and screen printing, Sol. Energy Mater. Sol. Cells 93 (2009) 465–475; ¨ (d) F.C. Krebs, M. Jorgensen, K. Norrman, O. Hagemann, J. Alstrup, T.D. Nielsen, J. Fyenbo, K. Larsen, J. Kristensen, A complete process for production of flexible large area polymer solar cells entirely using screen printing—first public demonstration, Sol. Energy Mater. Sol. Cells 93 (2009) 422–441. (a) G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Singlequantum-dot-based DNA nanosensor, Nat. Mater. 4 (2005) 826–831; (b) Y. Kim, S. Cook, S.M. Tuladhar, S.A. Choulis, J. Nelson, J.R. Durrant, D.D.C. Bradley, M. Giles, I. Mcculloch, C.S. Ha, M. Ree, A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells, Nat. Mater. 5 (2006) 197–203; (c) J. Hou, H.Y. Chen, S. Zhang, G. Li, Y. Yang, Synthesis, characterization, and photovoltaic properties of a low band gap polymer based on silolecontaining polythiophenes and 2,1,3-benzothiadiazole, J. Am. Chem. Soc. 130 (2008) 16144–16145; (d) Y. Liang, D. Feng, Y. Wu, S. Tsai, G. Li, C. Ray, L. Yu, Highly efficient solar cell polymers developed via fine-tuning of structural and electronic properties, J. Am. Chem. Soc. 131 (2009) 7792–7799; (e) Y. Liang, Y. Wu, D. Feng, S. Tsai, H. Son, G. Li, L. Yu, Development of new semiconducting polymers for high performance solar cells, J. Am. Chem. Soc. 131 (2009) 56–57; (f) S.H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J.S. Moon, D. Moses, M. Leclerc, K. Lee, A.J. Heeger, Bulk heterojunction solar cells with internal quantum efficiency approaching 100%, Nat. Photonics 3 (2009) 297–303. ¨ M. Jorgensen, K. Norrman, F.C. Krebs, Stability/degradation of polymer solar cells, Sol. Energy Mater. Sol. Cells 92 (2008) 686–714. (a) J. Lu, P.F. Xia, P.K. Lo, Y. Tao, M.S. Wong, Synthesis and properties of multitriarylamine-substituted carbazole-based dendrimers with an oligothiophene core for potential applications in organic solar cells and light-emitting diodes, Chem. Mater. 18 (2006) 6194–6203; (b) S. Roquet, A. Cravino, P. Leriche, O. Ale´vˆeque, P. Fre re, J. Roncali, Triphenylamine-thienylenevinylene hybrid systems with internal charge transfer as donor materials for heterojunction solar cells, J. Am. Chem. Soc. 128 (2006) 3459–3466; (c) A. Cravino, S. Roquet, P. Leriche, O. Ale´vˆeque, P. Fre re, J. Roncali, A star-shaped triphenylamine p-conjugated system with internal charge-transfer as donor material for hetero-junction solar cells, Chem. Commun. (2006) 1416–1418; ¨ (d) K. Schulze, C. Uhrich, R. Schuppel, K. Leo, M. Pfeiffer, E. Brier, E. Reinold, P. B¨auerle, Efficient vacuum-deposited organic solar cells based on a new lowbandgap oligothiophene and fullerene C60, Adv. Mater. 18 (2006) 2872–2875; (e) F. Silvestri, M.D. Irwin, L. Beverina, A. Facchetti, G.A. Pagani, T.J. Marks, Efficient squaraine-based solution processable bulk-heterojunction solar cells, J. Am. Chem. Soc. 130 (2008) 17640–17641; (f) P.F. Xia, J.P. Lu, C.H. Kwok, H. Fukutani, M.S. Wong, Y. Tao, Synthesis and properties of monodisperse multi-triarylamine-substituted oligothiophenes and 4,7-bis(20 -oligothienyl)-2,1,3-benzothiadiazoles for organic solar cell applications, J. Polym. Sci., Part A: Polym. Chem. 47 (2008) 137–148; (g) M. Velusamy, J.-H. Huang, Y.C. Hsu, H.H. Chou, K.C. Ho, P.L. Wu, W.H. Chang, J.T. Lin, C.W. Chu, Dibenzo[f,h]thieno[3,4-b]quinoxaline-based small molecules for efficient bulk-heterojunction solar cells, Org. Lett. 11 (2009) 4898–4901; (h) J.A. Mikroyannidis, M.M. Stylianakis, P. Suresh, P. Balraju, G.D. Sharma, Low band gap vinylene compounds with triphenylamine and benzothiadiazole segments for use in photovoltaic cells, Org. Electron. 10 (2009) 1320–1333; (i) Y. Liu, X. Wan, B. Yin, J. Zhou, G. Long, S. Yin, Y. Chen, Efficient solutionprocessed bulk heterojunction solar cells based on donor–acceptor oligothiophene, J. Mater. Chem. 20 (2010) 2464–2468; (j) W. Zhang, S.C. Tse, J. Lu, Y. Tao, M.S. Wong, Solution processable donor– acceptor oligothiophenes for bulk heterojunction solar cells, J. Mater. Chem. 20 (2010) 2182–2189; (k) Y. Yang, J. Zhang, Y. Zhou, G. Zhao, C. He, Y. Li, M. Andersson, O. Inganas, F. Zhang, Solution-processable organic molecule with triphenylamine core and two benzothiadiazole-thiophene arms for photovoltaic application, J. Phys. Chem. C 114 (2010) 3701–3706; (l) X. Zhao, C. Piliego, B.S. Kim, D.A. Poulsen, B. Ma, D.A. Unruh, J.M.J. Frechet, Solution-processable crystalline platinum-acetylide oligomers with broadband absorption for photovoltaic cells, Chem. Mater. 22 (2010) 2325–2332. (a) J. Roncali, P. Leriche, A. Cravino, From one- to three-dimensional organic semiconductors: in search of the organic silicon? Adv. Mater. 19 (2007) 2045–2060; (b) M.T. Lloyd, J.E. Anthony, G. Malliaras, Photovoltaics from soluble small molecules, Mater. Today 10 (2007) 34–41;

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15] [16]

[17]

[18]

[19]

[20]

(c) J. Roncali, Molecular bulk heterojunctions: an emerging approach to organic solar cells, Acc. Chem. Res. 42 (2009) 1719–1730. S. Roquet, R. de Bettignies, P. Leriche, J. Roncali, Three-dimensional tetra(oligothienyl)silanes as donor material for organic solar cells, J. Mater. Chem. 16 (2006) 3040–3045. (a) M.T. Lloyd, A.C. Mayer, S. Subramanian, D.A. Mourey, D.J. Herman, A.V. Bapat, J.E. Anthony, A.V. Bapat, J.E. Anthony, G.G. Malliaras, Efficient solution-processed photovoltaic cells based on an anthradithiophene/ fullerene blend, J. Am. Chem. Soc. 129 (2007) 9144–9149; (b) C. He, Q. He, Y. Yi, G. Wu, C. Yang, F. Bai, Z. Shuai, L. Wang, Y. Li, Improving the efficiency of solution processable organic photovoltaic devices by a star-shaped molecular geometry, J. Mater. Chem. 18 (2008) 4085–4090; (c) F. Lincker, N. Delbosc, S. Bailly, R. De Bettignies, M. Billon, A. Ron, R. Demadrille, Fluorenone-based molecules for bulk-heterojunction solar cells: synthesis, characterization, and photovoltaic properties, Adv. Funct. Mater. 18 (2008) 3444–3453; ¨ (d) N.M. Kronenberg, M. Deppish, F. Wurthner, H.W.A. Ledemann, K. Deing, K. Meerholz, Bulk heterojunction organic solar cells based on merocyanine colorants, Chem. Commun. 48 (2008) 6489–6491. B. Walker, A.B. Tamayo, X.-D. Dang, P. Zalar, J.H. Seo, A. Garcia, M. Tantiwiwat, T.-Q. Nguyen, Nanoscale phase separation and high photovoltaic efficiency in solution-processed, small-molecule bulk heterojunction solar cells, Adv. Funct. Mater. 19 (2009) 3063–3069. A.B. Tamayo, B. Walker, T.-Q. Nguyen, A low band gap, solution processable oligothiophene with a diketopyrrolopyrrole core for use in organic solar cells, J. Phys. Chem. C 112 (2008) 11545–11551. ¨ ¨ ¨ H. Burckst ummer, N.M. Kronenberg, M. Gsanger, M. Stolte, K. Meerholz, ¨ F. Wurthner, Tailored merocyanine dyes for solution-processed BHJ solar cells, J. Mater. Chem. 20 (2010) 240–243. (a) T. Rousseau, A. Cravino, T. Bura, G. Ulrich, R. Ziessel, J. Roncali, BODIPY derivatives as donor materials for bulk heterojunction solar cells, Chem. Commun. 13 (2009) 1673–1675; (b) T. Rousseau, A. Cravino, T. Bura, G. Ulrich, R. Ziessel, J. Roncali, Multidonor molecular bulk heterojunction solar cells: improving conversion efficiency by synergistic dye combinations, J. Mater. Chem. 19 (2009) 2298–2300; (c) J.A. Mikroyannidis, A.N. Kabanakis, A. Kumar, S.S. Sharma, Y.K. Vijay, G.D. Sharma, Synthesis of a low-band-gap small molecule based on acenaphthoquinoxaline for efficient bulk heterojunction solar cells, Langmuir 26 (2010) 12909–12916; (d) J.A. Mikroyannidis, P. Suresh, G.D. Sharma, Synthesis of a perylene bisimide with acetonaphthopyrazine dicarbonitrile terminal moieties for photovoltaic applications, Synth. Met. 160 (2010) 932–938; (e) J.A. Mikroyannidis, P. Suresh, G.D. Sharma, Synthesis of benzoselenadiazole-based small molecule and phenylenevinylene copolymer and their application for efficient bulk heterojunction solar cells, Org. Electron. 11 (2010) 311–321; (f) G.D. Sharma, P. Suresh, J.A. Mikroyannidis, M.M. Stylianakis, Efficient bulk heterojunction devices based on phenylenevinylene small molecule and perylene-pyrene bisimide, J. Mater. Chem. 20 (2010) 561–567; (g) J.A. Mikroyannidis, M.M. Stylianakis, P. Suresh, P. Balraju, G.D. Sharma, Low band gap vinylene compounds with triphenylamine and benzothiadiazole segments for use in photovoltaic cells, Org. Electron. 10 (2009) 1320–1333; (h) G.D. Sharma, P. Balraju, J.A. Mikroyannidis, M.M. Stylianakis, Bulk heterojunction organic photovoltaic devices based on low band gap small molecule BTD-TNP and perylene-anthracene diimide, Sol. Energy Mater. Sol. Cells 93 (2009) 2025–2028. A. Vig, K. Sirbiladze, H.J. Nagy, P. Aranyosi, I. Rusznak, P. Sallay, The light stability of azo dyes and dyeings V. The impact of the atmosphere on the light stability of dyeings with heterobifunctional reactive azo dyes, Dyes Pigm. 71 (2006) 199–205. M.M.M. Raposo, A.M.R.C. Sousa, A.M.C. Fonseca, G. Kirsch, Thienylpyrrole azo dyes: synthesis, solvatochromic and electrochemical properties, Tetrahedron 61 (2005) 8249–8256. A.T. Slark, P.M. Hadgett, The effect of specific interactions on dye transport in polymers above the glass transition, Polymer 40 (1999) 4001–4011. G. Hallas, J.H. Choi, Synthesis and properties of novel aziridinyl azo dyes from 2-aminothiophenes—Part 2: application of some disperse dyes to polyester fibers, Dyes Pigm. 40 (1998) 119–129. M.S. Ho, A. Natansohn, P. Rochon, Azo polymers for reversible optical storage. 7. The effect of the size of the photochromic groups, Macromolecules 28 (1995) 6124–6127. Y. Nabeshima, A. Shishido, A. Kanazawa, T. Shiono, T. Ikeda, T. Hiyama, Synthesis of novel liquid-crystalline thiophene derivatives and evaluation of their photoresponsive behavior, Chem. Mater. 9 (1997) 1480–1487. G. Iftime, F.L. Labarthet, A. Natansohn, P. Rochon, K. Murti, Main chain-containing azo-tetraphenyldiaminobiphenyl photorefractive polymers, Chem. Mater. 14 (2002) 168–174. (a) P. Gregory, Aromatic disazo infrared-absorbing dyes, EP 280434, 1988, CAN 110:116654; (b) H. Umehara, M. Yanagimachi, Y. Taniguchi, S. Hirose, Optical recording medium using azo compound, JP 08108625, 1996, CAN 125:127897; (c) T. Misawa, H. Takuma, Optical recording medium using azo compound, JP 08108625, 1996, CAN 125:127897;

J.A. Mikroyannidis et al. / Solar Energy Materials & Solar Cells 94 (2010) 2318–2327

[21] [22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33] [34] [35]

[36] [37]

[38]

(d) A.T. Slark, J.E. Fox, The permeability of a diazothiophene dye solute in polymer matrixes above the glass transition, Polymer 38 (1997) 2989–2995; (e) S. Matsumoto, T. Yagisawa, S. Matsumoto, K. Shimabukuro, Near-infrared radiation-absorbing polyazo compounds, JP 11269136, 1999, CAN 131:272843; (f) R. Hattori, Presensitized lithographic plate containing azo compound giving visible images, JP 11119415, 1999, CAN 130:345070; (g) S. Yoshida, Y. Nakamura, S. Takesawa, Electrophotographic toner with improved photofixability containing polyazo compound, developer therefrom, and image-forming method, JP 2005099419, 2005, CAN 142:400497. K.R. Millington, K.W. Fincher, A.L. King, Mordant dyes as sensitizers in dyesensitized solar cells, Sol. Energy Mater. Sol. Cells 91 (2007) 1618–1630. ¨ zsoy, I. Durucasu, S. Icli, Optical H. Dincalp, S. Yanuz, O. Hakli, C. Zafer, C. O and photovoltaic properties of salicylaldimine-based azo ligands, J. Photochem. Photobiol. A: Chem. 210 (2010) 8–16. J.A. Mikroyannidis, A. Kabanakis, D. Tsagkournos, P. Balraju, G.D. Sharma, Bulk heterojunction solar cells based on a low band gap soluble bisazopyrrole and the corresponding BF2-azopyrrole complex, J. Mater. Chem. 20 (2010) 6464–6471. C. Pozo-Gonzalo, J.A. Pomposo, J. Rodriguez, E.Y. Schmidt, A.M. Vasiltsov, N.Y. Zorina, A.V. Ivanov, B.A. Trofimov, A.I. Mikhaleva, A.B. Zaitsev, Synthesis and electrochemical study of narrow band gap conducting polymers based on 2,20 -dipyrroles linked with conjugated aza-spacers, Synth. Met. 157 (2007) 60–65. L. Valentini, D. Bagnis, A. Marrocchi, M. Seri, A. Taticchi, J.M. Kenny, Novel anthracene-core molecule for the development of efficient pcbm-based solar cells, Chem. Mater. 20 (2008) 32–34. G.D. Sharma, P. Balraju, J.A. Mikroyannidis, M.M. Stylianakis, Bulk heterojunction organic photovoltaic devices based on low band gap small molecule BTD-TNP and perylene-anthracene diimide, Sol. Energy Mater. Sol. Cells 93 (2009) 2025–2028. P.D. Vellis, J.A. Mikroyannidis, D. Bagnis, L. Valentini, New anthracene-containing phenylene- or thienylene-vinylene copolymers: synthesis, characterization, photophysics and photovoltaics, J. Appl. Polym. Sci. 113 (2009) 1173–1181. A. Marrocchi, F. Silvestri, M. Seri, A. Facchetti, A. Tatticchia, T.J. Marks, Conjugated anthracene derivatives as donor materials for bulk heterojunction solar cells: olefinic versus acetylenic spacers, Chem. Commun. (2009) 1380–1382. S. Gunes, A. Wild, E. Cevik, A. Pivrikas, U.S. Schubert, D.A.M. Egbe, Effect of shifting of aromatic rings on charge carrier mobility and photovoltaic response of anthracene and thiophene-containing MEH-PPE-PPVs, Sol. Energy Mater. Sol. Cells 94 (2010) 484–491. D.A.M. Egbe, S. Turk, S. Rathgeber, F. Kuhnlenz, R. Jadhav, A. Wild, E. Birckner, G. Adam, A. Pivrikas, V. Cimrova, G. Knor, N.S. Sariciftci, H. Hoppe, Anthracene based conjugated polymers: correlation between p-p-stacking ability, photophysical properties, charge carrier mobility, and photovoltaic performance, Macromolecules 43 (2010) 1261–1269. J.A. Mikroyannidis, P.D. Vellis, S.H. Yang, C.S. Hsu, Synthesis, electroluminescence, and photovoltaic cells of new vinylene-copolymers with 4-(anthracene-10-yl)2,6-diphenylpyridine segments, J Appl. Polym. Sci. 115 (2010) 731–739. H. Adams, R.A. Bawa, K.G. McMillan, S. Jones, Asymmetric control in Diels–Alder cycloadditions of chiral 9-aminoanthracenes by relay of stereochemical information, Tetrahedron: Asymmetry 18 (2007) 1003–10012. T. Yokomichi, S. Tada, K. Seki, Bis(pyrrolyldiazo)phenylene compounds and their manufacture, Jpn. Kokai Tokkyo Koho (1995) JP 07076572A 19950320. T. Yokomichi, Field-effect transistor containing azo-containing conjugated polymer, Jpn. Kokai Tokkyo Koho (1995) JP 07022670 A 19950124. H. Nishino, Organic electroluminescent device having azo-containing polymer charge-transporting layer, Jpn. Kokai Tokkyo Koho (1994) JP 06346051 A 19941220. S. Tada, Azo-containing photoconducting polymer and electrophotographic photoreceptor using it, Jpn. Kokai Tokkyo Koho (1994) JP 06329792 A 19941129. T. Yokomichi, Nonlinear optical material consisting of azo group-having polymer with high third harmonic generation, Jpn. Kokai Tokkyo Koho (1995) JP 07020514 A 19950124. L.J. Bellamy, The infra-red spectra of complex molecules, Chapman and Hall (London), John Wiley & Sons, New York, 1975, p.304.

2327

[39] H. Rau, in: Photochemistry and Photophysics, J. F. Rabek (Ed.), (Chemical Rubber, Boca Raton) (1990) vol. II, pp. 119–141. [40] (a) Q.J. Sun, H.Q. Wang, C.H. Yang, Y.F. Li, Synthesis and electroluminescence of novel copolymers containing crown ether spacers, J. Mater. Chem. 13 (2003) 800–806; (b) L. Huo, J. Hou, H.Y. Chen, S. Zhang, Y. Jiang, T.L. Chen, Y. Yang, Bandgap and molecular level control of the low band gap polymers based on 3, 6-dithiophen-2yl-2,5-dihydropyrrolo [3,4-c]pyrrole-1,4-dione toward highly efficient polymer solar cells, Macromolecules 42 (2009) 6564–6571. [41] (a) A.R. Murphy, J.M.J. Frechet, Organic semiconducting oligomers for use in thin film transistors, Chem. Rev. 107 (2007) 1066–1096; (b) C. Kanimozhi, P. Balraju, G.D. Sharma, S. Patil, Synthesis of diketopyrrolopyrrole-containing copolymers: a study of their optical and photovoltaic properties, J. Phys. Chem. B 114 (2010) 3095–3103. [42] (a) N.S. Baek, S.K. Hau, H.L. Yip, O. Acton, K.S. Cehn, A.K.Y. Jen, High performance amorphous metalated p-conjugated polymers for field-effect transistors and polymer solar cells, Chem. Mater. 20 (2008) 5734–5736; (b) V.D. Mihailetchi, P.W.M. Blom, J.C. Hummelen, M.T. Rispens, Cathode dependence of the open-circuit voltage of polymer:fullerene bulk heterojunction solar cells, J. Appl. Phys. 94 (2003) 6849–6854. [43] G.G. Malliaras, J.R. Salem, P.J. Brock, J.C. Scott, Photovoltaic measurement of the built-in potential in organic light emitting diodes and photodiodes, J. Appl. Phys. 84 (1998) 1583–1587. [44] Y.D. Park, J.H. Cho, D.H. Kim, Y. Jang, H.S. Lee, K. Ihm, T.H. Kang, K. Cho, Energy-level alignment at interfaces between gold and poly(3-hexylthiophene) films with two different molecular structures, Electrochem. SolidState Lett. 9 (2006) G317–G319. [45] K. Vandewal, A. Gadisa, W.D. Oosterbaan, S. Bertho, F. Banishoeib, I. Van Serven, L. Lutsen, T.J. Cleji, D. Vanderzande, J.V. Manca, The relation between open-circuit voltage and the onset of photocurrent generation by chargetransfer absorption in polymer:fullerene bulk heterojunction solar cells, Adv. Funct. Mater. 18 (2008) 2064–2070. [46] (a) C. He, Q. He, X. Yang, G. Wu, C. Yang, F. Bai, Z. Shuai, L. Wang, Y. Li, Synthesis and photovoltaic properties of a solution-processable organic molecule containing triphenylamine and DCM moieties, J. Phys. Chem. C 111 (2007) 8661–8666; (b) K. Li, J. Qu, B. Xu, Y. Zhou, L. Liu, P. Peng, W. Tian, Synthesis and photovoltaic properties of novel solution-processable triphenylamine-based dendrimers with sulfonyldibenzene cores, New J. Chem. 33 (2009) 2120–2127; (c) N.S. Sariciflci, L. Smilowitz, A.J. Heeger, W. Wudl, Photoinduced electron transfer from a conducting polymer to buckminsterfullerene, Science 258 (1992) 1474–1476. [47] M. Chikamatsu, S. Nagamatsu, Y. Yoshida, K. Satio, K. Yase, Solution processed n-type organic thin film transistors with high field mobility, Appl. Phys. Lett. 87 (2005) 203504/1–203504/3. [48] (a) S. Sista, Y. Yao, Y. Yang, M.L. Tang, Z. Bao, Enhancement in open circuit voltage through a cascade-type energy band structure, Appl. Phys. Lett. 91 (2007) 223508/1–223508/3; (b) J. Dai, X. Jiang, H. Wang, D. Yan., Organic photovoltaic cells with near infrared absorption spectrum, Appl. Phys. Lett. 91 (2007) 253503/1–253503/3; (c) C. Zhang, S.W. Tong, C. Jiang, E.T. Kang, D.S.H. Chan, C. Zhu, Simple tandem organic photovoltaic cells for improved energy conversion efficiency, Appl. Phys. Lett. 92 (2008) 083310/1–083310/3. [49] (a) A.C. Morteani, A.S. Dhoot, J.S. Kim, C. Silva, N.C. Greenham, C. Murphy, E. Moons, S. Cina, J.H. Burroughes, R.H. Friend, Barrier-free electron–hole capture in polymer blend heterojunction light-emitting diodes, Adv. Mater. 15 (2003) 1708–1712; (b) J. Huang, G. Li, E. Wu, Q. Wu, Y. Yang, Achieving high-efficiency polymer white-light-emitting devices, Adv. Mater. 18 (2006) 114–117. [50] (a) H. Kim, M. Shin, Y. Kim, Distinct annealing temperature in polymer:fullerene:polymer ternary blend solar cells, J. Phys. Chem. C 113 (2009) 1620–1623; (b) M. Koppe, H.J. Egelhaaf, G. Dennler, M.C. Scharber, C.J. Brabec, P. Schilinsky, C.N. Hoth, Adv. Funct. Mater. 10.1002/afm.200901473. [51] (a) J. Dai, X. Jiang, H. Wang, D. Yan, Organic photovoltaic cells with near infrared absorption spectrum, Appl. Phys. Lett. 91 (2007) 253503/1–253503/3; (b) T. Tominaga, K. Hayashi, N. Toshima, Accelerated hole transfer by doublelayered metallophthalocyanine thin film for effective electroluminescence, Appl. Phys. Lett. 70 (1997) 762–763.