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J. Phys. Chem. C 2009, 113, 1620–1623
Distinct Annealing Temperature in Polymer:Fullerene:Polymer Ternary Blend Solar Cells Hwajeong Kim,†,‡ Minjung Shin,† and Youngkyoo Kim*,† Organic Nanoelectronics Laboratory, Department of Chemical Engineering, Kyungpook National UniVersity, Daegu 702-701, Republic of Korea, and Institute of Biomedical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom ReceiVed: October 30, 2008; ReVised Manuscript ReceiVed: NoVember 26, 2008
A distinct thermal annealing temperature exhibiting a maximum efficiency was observed for polymer solar cells with a ternary blend film that consists of one soluble fullerene and two kinds of conjugated polymer (electron-donating polymer and electron-accepting polymer). The distinct sharp shape trend of device performance with annealing temperature has been assigned primarily to the influence of the electron-accepting polymer [poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT)], since the glass transition temperature of F8BT was exactly the same as with the best annealing temperature. The performance of the best ternary blend solar cell made in this work was poorer than the well-optimized binary blend with the higher content (donor: acceptor ) 1:1) of electron-donating polymer. However, it was slightly better than the comparative binary blend solar cell with the same content of electron-donating polymer (donor:acceptor ) 1:2), indicative of further improvement in the performance of ternary blend solar cells through the control of blend composition and/or thickness. Introduction
Experimental Section
Recently, polymer solar cells have attracted significant interest due to their potential for cheaper electricity generation compared with conventional inorganic solar cells.1-6 This expectation is primarily based on simple device structure (single active layer) as well as inexpensive wet processes such as slit coating (slotdie coating), screen printing, spin coating, spray coating, etc, although improved efficiency has been reported for a tandem cell with enhanced open circuit voltage (VOC) at the expense of short circuit current (JSC).2,7 To date, most high-efficiency polymer solar cells with a single active layer (namely, singlestacked solar cells) employ a bulk heterojunction nanostructure made using an electron-donating polymer and an electronaccepting soluble fullerene [1-(3-methoxycarbonyl)propyl-1phenyl-(6,6)C61 (PCBM)].8-15 In order to achieve high power conversion efficiency (PCE), these polymer:fullerene solar cells are subject to annealing by either thermal or solvent treatment.10-14 In terms of thermal annealing, interestingly, the temperature range has been reported to be quite broad from 110 to 150 °C in the absence of obvious temperature exhibiting a maximum efficiency at the same annealing time.9-12,14 This can be attributed to the rigid nature of the constituents which has been proven from the extremely low signal of the glass transition temperature (Tg) for the most popular electron-donating polymer, regioregular poly(3-hexylthiophene) (P3HT).9 In this work we find a distinct annealing temperature exhibiting a maximum JSC and PCE for ternary blend solar cells with an electron-accepting polymer as a third component. The pronounced annealing temperature is ascribed to the influence of the added third component polymer, poly(9,9-dioctylfluoreneco-benzothiadiazole) (F8BT) by taking into account the thermal transition behavior of F8BT.
Materials and Solutions. A ternary blend solution was prepared by dissolving P3HT (regioregularity > 96%, Merck Chemical), F8BT (American Dye Source, Inc.), and PCBM (Nano-C) in chlorobenzene as a solvent. The weight ratio of these materials was P3HT:PCBM:F8BT ) 1.0:0.6:1.4 by weight. Films and Device Fabrication. To fabricate ternary blend solar cells, this solution was spin-coated on indium-tin oxide (ITO)-coated glass substrates on which a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS, Baytron P standard grade) was coated and then annealed at 230 °C for 15 min (thickness ) 70 nm). The thickness of ternary blend layers was ∼150 nm. Next, these samples were put into a vacuum chamber to deposit Al electrode (100 nm thick). Finally, the fabricated devices were thermally annealed by varying temperatures from room temperature to 200 °C for 10 min inside a nitrogen-filled glovebox (see Figure 1). In order to evaluate the current performance level of ternary blend solar cells made in this work, the binary blend (P3HT:PCBM ) 1:1 and 1:2 by weight) solar cells were fabricated using the same substrates as for the ternary blend solar cells. Then the comparative solar cells were subject to thermal annealing for 2 h at 140 °C.10,14 The thickness of binary blend films was adjusted to ∼150 nm for exact comparison. Measurements. The current density-voltage (J-V) characteristics of devices that are mounted in a sample holder (nitrogen atmosphere) were measured using a solar simulator system (Oriel) equipped with an electrometer (Keithley 2400). The incident light intensity of simulated solar light (air mass 1.5 G) was 50 mW/cm2. The external quantum efficiency (EQE) of devices was measured using an integrated system with a Xe lamp (150 W), a monochromator (CM110, CVI), and an electrometer (Keithley 2400/6517). The thermal transition behavior of F8BT was measured using a differential scanning calorimeter (DSC, TA Instrument). The surface morphology of ternary blend layers that are spin-coated on the PEDOT:PSS-
* To whom correspondence should be addressed. Telephone: +82-53950-5616. E-mail:
[email protected]. † Kyungpook National University. ‡ Imperial College London.
10.1021/jp809589n CCC: $40.75 2009 American Chemical Society Published on Web 01/05/2009
Polymer:Fullerene:Polymer Solar Cells
Figure 1. Structure of materials and device (active area ) 0.09 cm2) fabricated in this work: FE-SEM image (bottom right) shows a tilted cross section of P3HT:PCBM:F8BT ternary blend solar cell (note that the interface between active layer and PEDOT:PSS layer is less clear owing to tilting).
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Figure 3. AFM images (note the scale bar in each image for the size of 2D images) of P3HT:PCBM:F8BT ternary blend films coated on PEDOT:PSS/ITO glass substrates: as-fabricated (a-1 and a-3, height mode; a-2 and a-4, amplitude mode), annealed at 130 °C (b-1 and b-3, height mode; b-2 and b-4, amplitude mode), and annealed at 200 °C (c-1 and c-3, height mode; c-2 and c-4, amplitude mode).
Figure 4. EQE spectra of P3HT:PCBM:F8BT ternary blend solar cells annealed at various temperatures. NAN denotes unannealed (asfabricated) device.
Figure 2. Light (top) and dark (bottom) J-V characteristics of P3HT: PCBM:F8BT ternary blend solar cells annealed at various temperatures. NAN denotes unannealed (as-fabricated) device.
coated ITO glass substrates were measured using an atomic force microscope (AFM, Nanoscope IIIa). The optical absorption spectra of P3HT:PCBM:F8BT ternary blend films were measured using an UV-visible spectrophotometer (JASCO 560). In order to make the same condition as for the devices, these films were coated on the PEDOT:PSS-coated ITO glass substrates and then thermally annealed at the same condition as for the devices. Results and Discussion As shown in the light J-V curve (Figure 2, top panel), JSC was increased by annealing at 130 °C but decreased again by annealing at higher temperatures. This trend is supported by the dark J-V curves in which the device annealed at 130 °C exhibited the highest current at forward bias, indicating the lowest series resistance. In contrast to the JSC trend, VOC was decreased by thermal annealing. This infers that the F8BT and/ or PCBM components in the ternary blend layer moved out of the PEDOT:PSS interface by thermal annealing so that the P3HT component became enriched at the PEDOT:PSS interface, considering the highest occupied molecular orbital (HOMO) energy of three components (HOMO of F8BT ) 5.9 eV; HOMO
Figure 5. Optical density as a function of wavelength for the P3HT: PCBM:F8BT (1.0:0.6:1.4) ternary blend films annealed at three different temperatures (see the figure legend).
of PCBM ) 6.1 eV; HOMO of P3HT ) 4.9 eV).3-6,16,17 Here the reason for the poorer device performance by annealing at 200 °C can be attributed to the disruptive morphology change in the ternary blend layer, leading to a large scale of phase segregation (see Figure 3), as similarly observed in P3HT:PCBM binary blend.5 Hence, as shown from the AFM images, the best device performance in the present device structure is attributed to the formation of fine nanomorphology of the ternary blend film by annealing at 130 °C which could secure both charge separation and charge percolation paths.6,8 The changed JSC by thermal annealing is also evidenced from the EQE spectra in Figure 4. Thermal annealing at 130 °C resulted in the increased EQE over the entire range of wavelengths, whereas the EQE of the device annealed at 200 °C was decreased from that of the as-fabricated device at the wavelength range of 400∼570 nm. Based on the optical absorption spectra of ternary blend films (see Figure 5), the lightharvesting effect by thermal annealing is not so large and partially applicable in the wavelength range of 500∼650 nm. In more detail, the optical density was slightly increased over the entire wavelength by thermal annealing. However, this
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Kim et al.
Figure 7. Differentiated DSC thermogram of F8BT polymer as a function of temperature (scan rate, 10 °C/min); a and b denote glass transition and melting temperature ranges, respectively. The inset shows the energy-minimized structure of eight-unit F8BT segment.
Figure 6. Characteristics of P3HT:PCBM:F8BT ternary blend solar cells as a function of device annealing temperatures. Series (RS) and shunt (RSH) resistances were calculated by taking light J-V data at open circuit and short circuit conditions (∆V ) 0.1 V), respectively.
degree of increment in optical density cannot account for the huge increase in the short circuit current density as well as the external quantum efficiency. Therefore, we consider that charge transport in the ternary blend films is responsible for the huge change in JSC and EQE by thermal annealing. Overall characteristics of ternary blend solar cells with annealing temperature are summarized in Figure 6. As discussed in Figure 2, the highest JSCwas achieved by thermal annealing at 130 °C. In particular, the JSC trend as a function of annealing temperature shows quite a sharp (narrow-distributed) temperature range leading to JSC increase, which is different from wellestablished P3HT:PCBM binary blend solar cells.3,4,6,8-10 As the annealing temperature increased, the VOC of ternary blend solar cells was in general decreased, showing a minimum at 130 °C in the presence of marginal increase at higher temperatures. The trend of fill factor (FF) was similar to that of JSC. These trends are well supported with the marked reduction in series resistance (RS) at 130 °C and with higher shunt resistance (RSH) compared to the as-fabricated device (see bottom panel in Figure 6). In particular, we note that annealing at 200 °C led to greatly increased RS, though RSH was not much changed, which can be attributed to the poor charge transport in the ternary blend layer due to the coarse morphology (see Figure 3). As a result, the maximum PCE was achieved by annealing at 130 °C that is distinctive compared to other annealing temperatures. Here we need to discuss the narrow shape of annealing temperature observed in the present ternary blend solar cells. Since rigid and/or highly crystalline materials, such as regioregular P3HT and PCBM, have a limited portion of amorphous phase, their glass transition signal is in principle very weak as reported previously.10 This gives rise to an indefinite and broad range of annealing temperature.9-12 In contrast, F8BT, which is a class of semicrystalline polymer, has bulky dioctyl groups that prevent its chain backbone from packing closely (see the energy-minimized structure in Figure 7). Hence F8BT has a relatively sufficient amount of amorphous phase to exhibit a clear glass transition. This nature of F8BT resulted in the sharp shape of annealing temperature with a distinct peak at 130 °C for the present ternary blend solar cells. We note that the peak annealing temperature (130 °C) is in excellent agreement with the inflection point of glass transition of F8BT (see the upward peak point of a in Figure 7).
Figure 8. Comparison of P3HT:PCBM:F8BT ternary blend solar cell annealed at 130 °C (a) with the P3HT:PCBM binary blend solar cells: (b) P3HT:PCBM ) 1:2 and (c) P3HT:PCBM ) 1:1 by weight.
TABLE 1: Performance Summary of Polymer Solar Cells Given in Figure 8a solar cells
JSC (mA/cm2)
VOC (V)
FF (%)
PCE (%)
a b c
3.22 2.73 5.16
0.67 0.61 0.57
45.2 41.7 53.8
1.94 1.38 3.16
a
Incident light intensity (AM 1.5) ) 50 mW/cm2.
Finally we compared the best performance P3HT:PCBM: F8BT ternary blend solar cells made in this work with the wellestablished P3HT:PCBM binary blend solar cells. As shown in Figure 8, the present best ternary blend solar cell exhibited slightly better J-V curve shape than the binary blend solar cell with the same donor-acceptor composition (see b device in Figure 8). This can be attributed to the better light-harvesting effect for the ternary blend film (P3HT:PCBM:F8BT ) 1.0: 0.6:1.4) than the binary blend film (P3HT:PCBM ) 1:2) due to the presence of F8BT.16 However, the performance of the ternary blend solar cell was poorer than the well-optimized binary solar cell with the P3HT:PCBM ) 1:1 composition (see c in Figure 8 and Table 1). Conclusion In summary, the performance of ternary blend solar cells with an electron-accepting polymer as a third component was sharply dependent on the annealing temperature, exhibiting a maximum at 130 °C. This distinct annealing temperature trend was mainly attributed to the critical role of F8BT in the ternary blend layer, since the peak annealing temperature is exactly the same as the Tg of F8BT. Although the present best ternary blend solar cell annealed at 130 °C exhibited slightly higher performance than the comparative binary solar cell with the same P3HT composition, its performance was lower than the well-optimized binary solar cell with the P3HT:PCBM ) 1:1 composition. However, we expect further improvement in the performance of ternary blend solar cells by adjusting the blend composition (increasing the electron donor content) and/or the film thickness. Acknowledgment. The authors thank Prof. D. D. C. Bradley, Prof. J. Nelson, and Prof. J. R. Durrant of Imperial College London for their helpful comments on this work at an early stage. Y.K. thanks Merck Chemicals Ltd. for supplying P3HT
Polymer:Fullerene:Polymer Solar Cells and Prof. I. McCulloch of Imperial College London for his help with organizing the supply of these materials. This work was supported by the Korea Science and Engineering Foundation (KOSEF-R01-2007-000-10836-0) and the Korea Research Foundation (KRF-2007-331-D00121). References and Notes (1) Service, R. F. Science 2008, 319, 718. (2) Gaudiana, R.; Brabec, C. Nat. Photonics 2008, 2, 287. (3) Jørgensen, M.; Norrman, K.; Krebs, F. C. Sol. Energ. Mater. Sol. Cell. 2008, 92, 686. (4) Thompson, B. C.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2008, 47, 58. (5) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91, 954. (6) Gu¨nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. ReV. 2007, 107, 1324. (7) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222.
J. Phys. Chem. C, Vol. 113, No. 4, 2009 1623 (8) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841. (9) Kim, Y.; Cook, S.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C. J. Mater. Sci. 2005, 40, 1371. (10) Kim, Y.; Choulis, S. A.; Nelson, J.; Bradley, D. D. C.; Cook, S.; Durrant, J. R. Appl. Phys. Lett. 2005, 86, 063502. (11) Li, G.; Shrotriya, V.; Yao, Y.; Yang, Y. J. Appl. Phys. 2005, 98, 043704. (12) Reyes-Reyes, M.; Kim, K.; Carroll, D. L. Appl. Phys. Lett. 2005, 87, 083506. (13) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864. (14) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C. S.; Ree, M. Nat. Mater. 2006, 5, 197. (15) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J. Nat. Mater. 2007, 6, 497. (16) Kim, Y.; Cook, S.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C. Chem. Mater. 2004, 16, 4812. (17) Kim, Y.; Cook, S.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C. Synth. Met. 2005, 152, 105.
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