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Organic Photovoltaic Devices Based on Blends of Regioregular Poly(3-hexylthiophene) and Poly(9,9-dioctylfluorene-co-benzothiadiazole) Youngkyoo Kim,*,† Steffan Cook,‡ Stelios A. Choulis,† Jenny Nelson,† James R. Durrant,‡ and Donal D. C. Bradley† Center for Electronic Materials and Devices, Department of Physics, Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2BW, UK, and Center for Electronic Materials and Devices, Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, UK Received March 12, 2004. Revised Manuscript Received August 5, 2004
We have fabricated organic photovoltaic devices with blends of regioregular poly(3hexylthiophene) (P3HT) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as an electron donor and an electron acceptor, respectively. Several fabrication parameters such as blend composition, film thickness, solvent, and presence of LiF layer were varied in order to find the maximum device performance. The highest external quantum and power conversion efficiencies were achieved for the blend film with 60 wt % P3HT using p-xylene as a solvent. Insertion of a LiF layer further improved the power conversion efficiency from 0.02% to 0.13% under AM1.5 condition (1 Sun). To understand the relatively poor efficiency even in the optimized device, this polymer blend system was analyzed in relation to the following factors: charge separation efficiency, as measured by photoluminescence quantum efficiency; charge carrier mobility, measured by time-of-flight; and charge recombination dynamics, measured by transient absorption spectroscopy. The results showed that the electron mobility of F8BT is responsible mainly for the low efficiency in the presence of minor contribution of the charge separation efficiency.
Introduction structures1-4
Bulk heterojunction have been successfully applied in a variety of high-efficiency organic photovoltaic devices (solar cells) and photodiodes, following the observation of 100% quantum yield for the photoinduced electron transfer from electron-donating conjugated polymers to electron-accepting fullerene molecules at donor-acceptor interfaces. However, such perfect charge transfer in the bulk heterojunction structure is effective only at the interface, not in the bulk, which indicates that most of the excitons photogenerated in electron-donating polymers some way from the interface decay to their ground states without any chance for charge transfer to electron-accepting molecules. This leads to the importance of interface morphology in order to achieve high charge transfer yield. In this regard it has been recently reported that the device performance was greatly enhanced through control of the film morphology using different solvents5,6 and/or a compatibilizing polymer.7 This is attributed to improved * Corresponding author. Tel: +44-020-759-47562. Fax: +44-020758-13817. E-mail:
[email protected]. † Department of Physics. ‡ Department of Chemistry. (1) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. (2) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Maeseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. (3) Dittmer, J. J.; Marseglia, E. A.; Friend, R. H. Adv. Mater. 2000, 12, 1270. (4) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 15.
charge separation efficiency (ηCG), upon increasing the interfacial area, which directly contributes to the external quantum efficiency (EQE or ηEQE) given by eq 1
ηEQE(λ) ) Af(λ)‚ηCG(λ)‚ηCC(γ‚µ)
(1)
where Af, λ, ηCC, γ, and µ are the absorbance of the active layer, the wavelength of the incident light, the charge collection efficiency, a parameter related to the charge recombination, and the charge carrier mobility, respectively. In addition to maximizing the interfacial area, increasing charge carrier mobilities inside the bulk heterojunction structure is of fundamental importance because the charge collection efficiency is significantly affected by both the hole mobility of the electron donor and the electron mobility of the electron acceptor (see eq 1).8 This influence has been experimentally demonstrated with blends of electron-donating conjugated polymers and electron-accepting soluble fullerenes4,9 or inorganic nanocrystals10,11 that exhibit particularly high electron mobility. More recently further improvements (5) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841. (6) Snaith, H. J.; Arias, A. C.; Morteani, A. C.; Silva, C.; Friend, R. H. Nano Lett. 2002, 2, 1353. (7) Kim, Y.; Cook, S.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C. Adv. Funct. Mater. 2004, to be submitted for publication. (8) Nelson, J. Phys. Rev. B. 2003, 67, 155299. (9) Choulis, S. A.; Nelson, J.; Kim, Y.; Poplavskyy, D.; Kreouzis, T.; Durrant, J. R.; Bradley, D. D. C. Appl. Phys. Lett. 2003, 83, 3812.
10.1021/cm049585c CCC: $27.50 © 2004 American Chemical Society Published on Web 10/15/2004
Organic Photovoltaic Devices Based on Polymer Blends
Figure 1. Chemical structure of polymers (P3HT and F8BT) and cross-sectional view of organic photovoltaic device composed of glass, transparent electrode (ITO), PEDOT:PSS layer (BL), P3HT:F8BT layer (blend), LiF (optional), and metal electrode (Al).
in power conversion efficiency have been achieved using poly(3-hexylthiophene) (P3HT) or C70-PCBM for which the absorption spectrum extends to longer wavelengths.12-14 Unlike the above-mentioned fullerene based blends which show high efficiency, polymer/polymer bulk heterojunction structures still show rather poor power conversion efficiency (PCE) and EQE.6,15-17 Moreover, the factors limiting the photovoltaic performance of polymer/polymer solar cells have not yet been systematically analyzed. In this regard we have briefly reported on polymer/polymer bulk heterojunction solar cells with blends of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and regioregular P3HT.18 These polymers were chosen for study, in part, because F8BT and regioregular P3HT exhibit relatively high electron and hole mobilities, respectively.19-21 Here we report the effect of blend composition, choice of solvent, blend morphology, film thickness, and choice of electrode on P3HT:F8BT bulk heterojunction device characteristics. The limiting factors for efficiency are discussed in relation to the charge carrier mobility, charge separation efficiency, and charge recombination dynamics. Experimental Section A. Materials and Solutions. Regioregular P3HT was synthesized by Merck Chemicals Ltd. (see Figure 1). The weights and number average molecular weight were 4.4 × 104 and 2.8 × 104, respectively, leading to a polydispersity index of 1.6.22 Regioregularity of greater than 96% was measured by nuclear magnetic resonance spectroscopy. The concentration of residual metallic impurities (mostly Ni and Mg) was less than 10 µg/g. The F8BT polymer was synthesized by The (10) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (11) Sun, B.; Marx, E.; Greenham, N. C. Nano Lett. 2003, 3, 961. (12) Schilinsky, P.; Waldauf, C.; Brabec, C. J. Appl. Phys. Lett. 2002, 81, 3885. (13) Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. Adv. Funct. Mater. 2003, 13, 85. (14) Wienk, M. M.; Kroon, J. M.; Verhees W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed. 2003, 42, 3371. (15) Arias, A. C.; MacKenzie, J. D.; Stevenson, R.; Halls, J. J. M.; Inbasekaran, M.; Woo, E. P.; Richards, D.; Friend, R. H. Macromolecules 2001, 34, 6005. (16) Arias, A. C.; Corcoran, N.; Banach, M.; Friend, R. H.; MacKenzie, J. D.; Huck, W. T. S. Appl. Phys. Lett. 2002, 80, 1695. (17) Pacios, R.; Bradley, D. D. C. Synth. Met. 2002, 127, 261. (18) Kim, Y.; Choulis, S. A.; Cook, S.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C. Proceedings of the IEEE 3rd World Conference on Photovoltaic Energy Conversion, 2004, 1LN-C-01. (19) Campbell, A.; Bradley, D. D. C.; Antoniadis, H. Appl. Phys. Lett. 2001, 79, 2133. (20) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (21) Choulis, S. A.; Kim, Y.; Nelson, J.; Bradley, D. D. C.; Giles, M.; Shkunov, M.; McCulloch, I. Appl. Phys. Lett. 2004, in press.
Chem. Mater., Vol. 16, No. 23, 2004 4813 Dow Chemical Company and carefully purified. Typical number and weight average molecular weights are 1.5 × 104 and 4.95 × 104, respectively.23 To study the blend composition dependence of device performance, blend solutions with various compositions (P3HT:F8BT ) 0:100, 20:80, 40:60, 60:40, 80:20, 100:0 by weight ratio) were prepared using p-xylene as a solvent at a solids content of 30 mg/mL. For studying the effect of solvent, four different blend solutions with a fixed blend composition (P3HT:F8BT ) 60:40) were prepared using chloroform (bp ) 61 °C), toluene (bp ) 110.6 °C), chlorobenzene (bp ) 132 °C), and p-xylene (bp ) 138 °C), respectively. For time-of-flight (TOF) mobility measurement, concentrated solutions (60-100 mg/mL) of pristine P3HT and F8BT as well as the P3HT:F8BT (60:40) blend were prepared in p-xylene. These solutions were vigorously stirred for more than 24 h at room temperature to maximize dissolution of the polymers, and optically clear solutions were obtained. B. Film and Device Fabrication. The pristine polymer and blend solutions were spin-coated onto a quartz substrate for ultraviolet-visible (UV-vis) absorption, photoluminescence (PL), PL quantum efficiency, and PL decay measurements. A thick pristine P3HT film (1.2 µm) and thin P3HT/F8BT (60:40 by weight) blend film (∼50 nm) spin-coated on glass slides were used for transient absorption decay measurements. A blend film of P3HT and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) (1:2 by weight) was also prepared for comparison of the transient absorption decay data. For photovoltaic device fabrication ITO coated glass (∼25 Ω/cm2) was cleaned with acetone and isopropyl alcohol several times and then dried with flowing nitrogen. On top of the cleaned ITO coated glass, a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (Baytron P VP AI 4083 grade, HC Stark) layer was spin-coated, followed by “soft-baking” at 110 °C for 15 min. Next the polymer blend films were spin-coated on the PEDOT:PSS layer with various spin speeds. The thickness of the polymer blend films ranged from ∼30 to ∼100 nm. All of the pristine polymer and blend films were subjected to “softbaking” at 50 °C for 30 min after spin-coating before subsequent processing steps were undertaken. Finally, the electrode, consisting of an optional ∼1 nm thick LiF layer and ∼150 nm thick aluminum layer, was deposited on the soft-baked polymer blend films by thermal evaporation at ∼3 × 10-6 Torr. The active area of all devices was 4.5 mm2 (see Figure 1). The structures used for TOF measurements were the same as those used for the photovoltaic devices except the PEDOT:PSS layer (i.e., ITO/polymer film (1.2∼1.6 µm thick)/Al). C. Measurements. The optical absorption coefficient and PL spectra of blend films were measured using an UV-vis absorption spectrometer (Unicam, Analytical Technology, Inc.) and spectrofluorimeter (FluoroMax-3, Jobin-Yvon), respectively. The PL quantum efficiency measurements were performed using the same spectrofluorimeter equipped with a calibrated integrating sphere.24 The PL decay times for thin films were measured using a time-resolved measurement system equipped with a streak camera (C4334, Hamamatsu), a stabilized picosecond light pulser (C4725, Hamamatsu), an imaging spectrograph (Chromex, Inc), and a laser diode (409 nm, 59 ps, 255 mW). TAS measurements were carried out in a nitrogen atmosphere using a nanosecond time resolution laser system equipped with a nitrogen laser (6 ps, GL-3300, Photon Technology International) as the excitation source, a laser diode (830 nm, Thorlabs) as the probe light source, a digital oscilloscope (TDS220, Tektronix), and a cryostat (Oxford Instruments).25 The temperature and probe beam size for transient absorption decay measurements were room temperature and 4 mm2, respectively. An atomic force microscope (AFM, Burleigh Instruments) and optical microscope (Zeiss, Axioplan) were used to examine the surface and bulk morphol(22) Kim, Y.; Giles, M.; Mcculloch, I.; Goulding, M.; Bradley, D. D. C. Private communications, 2002. (23) Grell, M.; Redecker, M.; Whitehead, K. S.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P.; Wu, W. Liq. Cryst. 1999, 26, 1403. (24) de Mello, J. C.; Wittmann, H. F.; Friend, R. H. Adv. Mater. 1997, 9, 230.
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Figure 2. UV-vis absorption (inset) spectra of P3HT/F8BT blend films and external quantum efficiency (EQE) spectra of the devices: P3HT:F8BT (weight ratio) ) 0:100 (a), 20:80 (b), 40:60 (c), 60:40 (d), 80:20 (e), and 100:0 (f). In the EQE measurements the monochromatic light intensity at 480 nm was 11.53 mW/cm2.
Kim et al.
Figure 3. Open circuit voltage (Voc) and fill factor (FF) of devices as a function of P3HT composition. Inset shows current density as a function of applied voltage: P3HT:F8BT (weight ratio) ) 0:100 (a), 20:80 (b), 40:60 (c), 60:40 (d), 80:20 (e), and 100:0 (f). All measurements were carried out under the illumination of a monochromatic light with the intensity of 11.53 mW/cm2 at 480 nm.
ogy of the polymer blend films. The photovoltaic characteristics of devices under monochromatic light exposure were measured using a system equipped with an electrometer (Keithley 237), a monochromator (Oriel CVI CM110), and a Xe lamp (150 W). The device performance under white light illumination (AM1.5) was measured using a home-built solar simulator based on a filtered Xe lamp with output intensity of 100 mW/cm2 (Costronics Electronics). The time-of-flight (TOF) mobilities were measured using a purpose-built system equipped with a Nd: YAG laser (Quantel Ultra GRM230, 355 or 532 nm, 7 ns pulse width) and a digital oscilloscope (TDS3052, Tektronix).
Experimental Results A. Effect of Varying Composition. As the P3HT content increases, the blend films show an increased absorption coefficient at wavelengths beyond 550 nm, owing to the P3HT absorption contribution (see inset to Figure 2). In particular, the absorption peak (ca. 620 nm) related to the intermolecular packing of P3HT chains20 is clearly observed for the blend film with 80 wt % P3HT. This situation is slightly different from our previous report using chloroform as solvent,18 and indicates the influence of solvent on blend film morphology. This specific absorption feature is evident in the EQE spectra: the device with 80 wt % P3HT shows higher EQE than that with 60 wt % P3HT in the long wavelength range between 580 and 650 nm. However, the highest EQE overall was achieved for the blend film with 60 wt % P3HT. The integrated photocurrent under white light illumination is higher for the blend film with 60 wt % P3HT than that with 80 wt % P3HT. Current-voltage (J-V) characteristics measured under monochromatic excitation (inset to Figure 3) show that the devices with blend films produce higher short circuit current than those made from pristine polymer films. This indicates that the present blend films do indeed generate photocurrents through charge separation at the donor-acceptor bulk heterojunction. As the P3HT content increases, the open circuit voltage de(25) Montanari, I.; Nogueira, A. F.; Nelson, J.; Durrant, J. R.; Winder, C.; Antonietta, M.; Sariciftchi, N. S.; Brabec, C. Appl. Phys. Lett. 2002, 81, 3001.
Figure 4. External quantum efficiency (EQE) and power conversion efficiency (PCE) of devices. EQE (filled square) and PCE (filled triangle) measurements were carried out under the illumination of a monochromatic light with the intensity of 11.53 mW/cm2 at 480 nm, while the EQE (filled circle) was measured under 0.06 mW/cm2 at 480 nm. Inset shows the incident light intensity (Iin) dependence of short circuit current density: P3HT:F8BT (weight ratio) ) 0:100 (a), 20:80 (b), 40: 60 (c), 60:40 (d), 80:20 (e), and 100:0 (f).
creases, reaching a minimum for the blend film with 80 wt % P3HT. This gradual decrease in the open circuit voltage is attributed to the fact that P3HT has lower ionization potential (Ip) than F8BT.18 The maximum fill factor is obtained for blend films with 60 and 80 wt % P3HT, whereas the blend film with 60 wt % P3HT shows higher short circuit current than that with 80 wt % P3HT. It is also noteworthy that the fill factor is slightly higher for the device with pristine P3HT than for devices with blend films that have a P3HT content lower than 40 wt %. The maximum EQE was achieved at 480 nm for the blend film with 60 wt % P3HT as shown in Figures 2 and 4. Reducing the light intensity by 3 orders of magnitude increased the EQE only slightly. At very low incident light intensity (
