Photovoltaic solar cells using poly(3,3-didodecylquaterthiophene)

ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 92 (2008) 558–563 www.elsevier.com/locate/solmat

Photovoltaic solar cells using poly(3,3-didodecylquaterthiophene) G. Wantza,, F. Lefevrea, M.T. Danga, D. Laliberte´b, P.L. Brunnerb, O.J. Dautelc a

Universite´ Bordeaux 1, Laboratoire IMS, UMR CNRS 5218, Ecole Nationale Supe´rieure de Chimie et de Physique de Bordeaux, 16 Av. Pey Berland, 33607 Pessac Cedex, France b Solaris-Chem,1 598 rue chaline, J7T 3E8, St Lazare, Que´bec, Canada c Laboratoire AM2N, Institut Charles Gerhardt Montpellier, UMR CNRS 5253, Ecole Nationale Supe´rieure de Chimie de Montpellier, 8 rue de l’Ecole Normale 34296 Montpellier Cedex 05, France Received 23 November 2007; received in revised form 6 December 2007; accepted 7 December 2007

Abstract The fabrication and the optimization of photovoltaic solar cells based on poly(3,3-didodecylquaterthiophene) (12-PQT) blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) are reported. On the one hand, the effect of the blend composition shows the necessity to increase the amount of fullerene derivative compared to conventional poly(3-hexylthiophene)-based systems. On the second hand, thermal annealing of devices is optimized and discussed. Overall, in this work, the highest power conversion efficiencies are shown in the range of 0.3–0.4% which represents a lower value than that reported with poly(3-hexylthiophene). Results are discussed in terms of charge carrier mobility and phase segregation in this bicontinuous donor-acceptor network. r 2007 Elsevier B.V. All rights reserved. Keywords: Poly(3, 3-didodecylquaterthiophene); Polymer solar cell; Mobility; 12-PQT

1. Introduction Polymer-based photovoltaic (PV) cells have attracted a great deal of attention because of their potential use for realizing low-cost, solution-processable, large area and flexible solar cells [1–4]. Recently, power conversion efficiencies approaching 5% have been reported for bulk heterojunction (BHJ) devices based on poly(3-hexylthiophene) blended with [6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) [5]. To date, the highest reported efficiencies have been demonstrated by the use of solution processed tandem cells with values approaching 6.5% [6]. In a single BHJ cell, the respective mobilities of charge carriers, holes in the polythiophene and electrons in the fullerene derivative, are critical parameters for high efficiency. Extraction and recombination of charge carriers are competing processes that are both governed by the mobility of charge carriers [7]. State-of-the-art hole mobility with solution-processed field effect transistors Corresponding author. 1

E-mail address: [email protected] (G. Wantz). www.solarischem.com

0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.12.006

(FET) have been reported with highly crystalline polythiophene derivatives with values from 0.18 to 0.6 cm2 V1 s1 [8–11]. Poly(3,3-didodecylquaterthiophene), 12-PQT, is one these high mobility materials (Fig. 1). To date, only a few attempts have been performed to evaluate the interest of such a high mobility polymer as electron-donating material in BHJ solar cells. Thompson et al. [12] have obtained power conversion efficiencies from 0.2% to 0.5% by tuning the composition of 12-PQT: PCBM blends without thermal annealing of devices since, in their case, thermal annealing did not affect device performances. Demadrille et al. have demonstrated similar values with a similar poly(quaterthiophene) derivative with shorter alkyl chains, poly(3,3-dioctylquaterthiophene), 8-PQT, used with PCBM without composition nor thermal treatment optimization [13]. Compared to P3HT, 12-PQT differs in term of alkyl chains in the fact that a better packing of polymer chains is possible inducing a better cristallinity, i.e. a higher hole mobility [14]. They also differ in terms of energy levels. 12-PQT exhibits HOMO and LUMO levels lying, respectively at 5.35 eV and 3.39 eV [12], compared to P3HT at, respectively, 5.1 eV and 2.9 eV. Such a lower ionization energy could also be

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C12H25 S *

S

S S

n

*

C12H25 Fig. 1. Chemical structure of 12-PQT.

of interest for enhanced chemical stability of the compound as well as device lifetime. In this sense, Koppe et al. [15] have evaluated the use of poly(terthiophene) derivatives in BHJ solar cells with efficiencies in the range of 0.6%. The fabrication of 12-PQT-based cells with emphasis on the effects of 12-PQT:PCBM composition as well as thermal annealing on device performances is presented here.

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temperatures up to 180 1C. Devices were left to cool down to room temperature before testing. Contacts were taken using a prober (Karl Suss PM5). Current-voltage curves were recorded using a Keithley 4200 SCS, under an illumination of 100 mW/cm2 from a K.H.S. SolarCelltest575 solar simulator with AM1.5G filters. The thermal characteristics of 12-PQT were studied using a differential scanning colorimeter (TA-Q1000) with a scanning rate of 10 1C/min. Atomic force microscopy (AFM) measurements have been realized on a commercial optical deflection microscope (stand-alone configuration for a large sample, Dimension 3100 Veeco Instruments with a Nanoscope IIIa, Digital Instrument) operated in ambient conditions. Topographic images were taken in the tapping mode, and commercial silicon cantilever with a nominal radius of 5–10 nm and a spring constant in the range of 20–70 N/m were used.

2. Experimental procedure 3. Results and discussion Twelve-PQT was synthesized by a modified procedure of one-pot borylation/Suzuki reaction from Melucci et al. [16] from 2,2000 -dibromo-3,3000 -dialkyl-quaterthiophene. The molecular weight and distribution (Mn ¼ 13 160 and Mw/Mn ¼ 1.77, respectively) were determined by gel permeation chromatography (GPC) using polystyrene as a standard. PV solar cells were fabricated on ITO (Indium-Tin Oxide) covered glass substrates, purchased from Merck Display, with a resistivity of 12 O/square. ITO sheets were etched and cleaned in successive ultrasound bath of dionised water, Acetone, Ethanol and Isopropanol, followed by a UV-Ozone treatment of 10 min. A 30 nm-thick buffer layer of poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT-PSS, Baytron P), was deposited by spin-coating at 4000 rpm. This conducting polymer layer was annealed at 80 1C under rotary pump vacuum for 1 h. All procedures after this point were performed in an inert-atmosphere glovebox of nitrogen (O2 and H2Oo0.1 ppm). Then, the active layer (12-PQT: PCBM blends) was deposited by spin-coating from o-dichlorobenzene (o-DCB) solutions at 50 1C. Blends were prepared at various 12-PQT:PCBM compositions from 1:0.1 to 1:8 (weight ratio), respectively. It should be emphasized that 12-PQT is not fully soluble in o-DCB at room temperature. It first forms a dispersion of nanoparticules that become fully soluble and transparent at 50 1C. Cooling down the solution induces a gel-like system that can be reversibly solubilised by heating again at 50 1C. Immediately prior to deposition, solutions were passed through 0.45 mm PTFE syringe filters. The achieved layer thickness was monitored using an Alpha-step IQ profilometer and was kept at approximately 200710 nm by tuning concentration and spinning speed. Cathodes of aluminum (150 nm) were then thermally evaporated under secondary vacuum (7  107 mbar) through a shadow mask. Thermal annealing was performed after Al deposition on a temperature-controlled hotplate at various

Fig. 2 shows the evolution of PV parameters obtained a function of the 12-PQT:PCBM composition. Voc, Jsc, FF and power conversion efficiency (PCE) are, respectively, the open circuit voltage, the short-circuit current, the fill factor and the power conversion efficiency. PV performances increase as a function of the acceptor loading with a saturation at high PCBM loading starting at a 12PQT:PCBM weight ratio of approximately of 1:3. In this case, the PCBM proportion is significantly higher than that of conventional P3HT:PCBM-based systems with which best reported power conversion efficiencies have usually been obtained with a P3HT:PCBM weight ratio close to 1:1 or in slight favor of the polymer [17–19]. 12-PQT-based PV cells have not been extensively studied yet. Thompson et al. [12] recently found the same trends in term of donoracceptor ratio with the same blends of 12-PQT:PCBM. A similar composition dependence of PV parameters has already been reported by Andersson et al. [20] dealing with cells based on blends of a specific electron-donating copolymer of thiophene, fluorine and electron accepting groups with PCBM. In the one hand, within a pristine fullerene layer, the electron mobility is usually found lower by a few orders of magnitudes than the hole mobility in a pristine polymer, such as P3HT for example [21]. On the other hand, it has been demonstrated that the hole mobility in polymer:fullerene blends decreases with the acceptor loading while the electron mobility along the percolated fullerenes increases [21–24]. This leads to an optimum charge balance between holes and electrons enabling high efficiencies. Here, the optimum ratio deals with higher amounts of acceptor. The hole mobility in pristine 12-PQT is higher than the hole mobility in pristine P3HT. As a consequence, to be able to balance the charge carriers, a higher amount of acceptor is required in the system. It should also be noted that no significant morphology changes have been observed by AFM on the polymer

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Fig. 2. Relationship between photovoltaic parameters and 12-PQT:PCBM composition (plotted as PCBM weight fraction) without thermal annealing. Voc, Jsc, FF and PCE are, respectively, the open circuit voltage, the short-circuit current, the fill factor and the power conversion efficiency. The dashed lines are plotted as guidelines for the eye.

2.5 2 1.5 Heat Flow (W/g)

blends surface as a function of the 12-PQT:PCBM ratio (not presented here). Thermal annealing of polythiophene:fullerene blends is a well-known technique to improve performances of solar cells through the tuning of the phase segregation between both materials at the nanometer scale [25,26]. Thompson et al. [12] reported that any thermal annealing of 12-PQT:PBCM films above 50 1C resulted in a significant decrease in device performance. As a consequence, they presented only results on unannealed cells. Demadrille et al. did not carry out any thermal treatment on their 8-PQT-based devices [13]. However, coatings of 12-PQT are affected by thermal treatments. Ong et al. [8] have shown that annealing a pristine 12-PQT film at temperature of 145 1C induces a highly crystalline film with extensive nanosized-crystal domains. Here, the thermal transitions of 12-PQT were investigated by differential scanning calorimetry (DSC). Fig. 3 shows the spectra recorded at heating/cooling rate of 10 1C and 12-PQT exhibits at least two discrete endotherms on heating at 95 1C and at 110 1C, corresponding, respectively, to crystalto-liquid crystal and liquid crystal-to-isotropic phase transitions. A small endotherm transition at 126 1C is also observed but not explained. These results differ significantly from those of Zhao et al. [27] which report two

1 0.5 0 -0.5 -1 -50

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50 100 Temperature (°C)

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Fig. 3. DSC thermogram of 12-PQT obtained from one-pot borylation/ Suzuki polymerization.

endotherms at 110 and 140 1C. Differences can find justification in the lower molecular weight obtained from the one-pot borylation/Suzuki polymerization over the

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oxidative coupling polymerization. Temperature transitions were reversible on temperature cycling and three exotherms are observed at 86, 77, and 68 1C, respectively. The exotherms are dependant of the cooling rate and crystallization processing can move from 53 to 81 1C by changing cooling rate from 40 to 1 1C/min, respectively (not shown). The calorimetry data suggested that the microstructure of the materials should be strongly affected by thermal annealing through the control of crystalline domains as observed by Zhao et al. [27]. PV cells based on 12-PQT:PCBM blends have been made with thermal post-treatments at various temperatures. Fig. 4 shows the results concerning a device based a 12-PQT:PCBM composition of 1:2. PV performances were found dependent on the annealing temperature with a maximum power conversion efficiency obtained at 100 1C. The fill factor is not significantly temperature-dependent here, but both, the short circuit current and the opencircuit voltage are also found maximum when device are treated at 100 1C. This specific temperature is in the range of transitions observed by DSC. Fig. 5 shows AFM microscopic pictures of blends: untreated, treated at 50, 100 and 130 1C. From the untreated one to the 100 1C one, significant phase segregation appears with increasing domain sizes in the range of 20–60 nm. To get a better

insight about this domain sizes, TEM studies should be done [12]. One should note that samples treated at 130 1C exhibit the formation of large crystalline domains, in the range of 10–25 mm long. It is believed that these domains are made of PCBM crystals, since similar shapes of systems have already been reported, at the same temperature, in Refs. [28,29]. At this stage, for optimum PV performance, the best 12-PQT:PCBM weight ratio is approximately 1:3 and thermal annealing is advised at 100 1C. Fig. 6 shows the current density-voltage characteristic of an optimized PV cell in the dark and under solar simulator AM1.5 illumination. This particular device is one of the most representative optimized cell and gave an open circuit voltage of 0.65 V, a short circuit current of 1.4 mA/cm2, a fill factor of 0.37 and a PCE of 0.33%. Such operating values, however, are still very low compared to conventional P3HT-PCBM-based PV cells with which a PCE of 3–4% is easily accessible. The limited fill factor as well as the limited short-circuit current are responsible for such a low PCE. One possible explanation lies in the anisotropic nature of charge transport in polythiophenes. 12-PQT is known as a high mobility system in organic FET where the transport occurs in the substrate plane directions at the interface with the gate dielectric material. Bulk mobilities

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Fig. 4. Relationship between photovoltaic parameters and annealing temperature. Finalized devices were annealed after cathode deposition, on a hot plate. Cooling to room temperature was performed slowly. Voc, Jsc, FF and PCE, respectively the open circuit voltage, the short-circuit current, the fill factor and the power conversion efficiency. 12-PQT: PCBM weight ratio is 1:2. The dashed lines are plotted as guidelines for the eye.

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600nm

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600nm

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Fig. 5. Tapping mode AFM microscopy images of 12-PQT:PCBM coatings thermally annealed at various temperature: (a) as prepared, (b) 50 1C, (c) 100 1C and (4) 130 1C. Images shown here represent the derivative of topographic picture to enhance the contrast and facilitate the phase segregation visualization. Note that the scale bar is 600 nm for cases (a), (b) or (c); and 10 mm in case (d).

in 12-PQT should be different. Thompson et al. [12] reported that the bulk mobility is in the order of magnitude of 104 cm2 V1 s1, similar to the one of P3HT. As a consequence, no significant improvement of PV performance, directly related to mobility concepts, is expected using 12-PQT instead of P3HT. Obviously, mobility is not the only critical parameter for high efficiency organic solar cells. To understand more in details why 12-PQT do not lead to similar performances as P3HT, phase segregation considerations should be discussed through complementary studies.

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Fig. 6. J–V characteristics of an ITO/PEDOT-PSS/12-PQT:PCBM/Al device illuminated with 100 mW cm2 solar simulator AM1.5 (circles). The dark curve is also shown (squares). This particular device is one the most representative optimized cell and gave an open circuit voltage of 0.65 V, a short circuit current of 1.4 m A/cm2, a fill factor of 0.37 and a power conversion efficiency (PCE) of 0.33%.

In this study, the optimization of the manufacturing process of polymer PV solar cells based on poly(3,3didodecylquaterthiophene) (12-PQT) blended with 6,6]phenyl-C61 butyric acid methyl ester (PCBM) was investigated. The optimum performances have been obtained with 12-PQT:PCBM weight ratio in excess of PCBM and with thermal annealing at 100 1C. Optimized PCEs in the order of 0.3–0.4% have been reported. As a consequence, this high mobility electron donating polymer, extensively used in organic field effect transistors, is not

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found competitive compared to the well known poly (3-hexylthiophene) for use in low cost solar cells. Acknowledgments We would like to acknowledge American Dye Source (Montre´al, Canada) for supplying part of the materials used in this study. This work was financed by the French Re´gion Aquitaine, the ANR and the FEDER within the AFFOR and the NANORGYSOL programs. In addition, we are grateful to Adrien Schombourger for technical assistance and Dr. Laurence Vignau and Dr. Lionel Hirsch for discussions. Finally, we are thankful to Prof. Jun Gao from Queen’s University (Kingston, Ontario) for having initiating this study and for fruitful discussions. References [1] S. Gu¨nes, H. Neugebauer, N.S. Sariciftci, Chem. Rev. 107 (2007) 1324. [2] C. Lungenschmied, G. Dennler, H. Neugebauer, N.S. Sariciftci, M. Glatthaar, T. Meyer, A. Meyer, Sol. Energy Mater. Sol. Cells 91 (2007) 379. [3] F.C. Krebs, H. Spanggard, T. Mkjaer, M. Biancardo, J. Alstrup, Mater. Sci. Eng. B 138 (2007) 106. [4] E. Bundgaard, F.C. Krebs, Sol. Energy Mater. Sol. Cells 91 (2007) 954. [5] W. Ma, C.Y. Yang, X. Gong, K.W. Lee, A.J. Heeger, Adv. Funct. Mater. 15 (2005) 1617. [6] Y.K. Jin, K. Lee, N.E. Coates, D. Moses, T. Nguyen, Q.M. Dante, A.J. Heeger, Science 317 (2007) 222. [7] M.M. Mandoc, L.J.A. Koster, P.W.M. Blom, Appl. Phys. Lett. 90 (2007) 133504. [8] B.S. Ong, Y. Wu, P. Liu, S. Gardner, Adv. Mater. 17 (2005) 1141. [9] Y. Wu, P. Liu, B.S. Ong, T. Srikumar, N. Zhao, G. Bolton, S. Zhu, Appl. Phys. Lett. 86 (2005) 142102.

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