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Device Performance of APFO-3/PCBM Solar Cells with Controlled Morphology By Cecilia M. Bjo¨rstro¨m Svanstro¨m, Jakub Rysz, Andrzej Bernasik, Andrzej Budkowski, Fengling Zhang, Olle Ingana¨s, Mats R. Andersson, Kjell O. Magnusson, Jessica J. Benson-Smith, Jenny Nelson, and Ellen Moons* Polymer-based solar cells offer a promising technology for photovoltaic energy conversion thanks to the simple and inexpensive solution-based fabrication process of the active layer, i.e., a thin film containing both donor and acceptor materials. However, the device performance is very sensitive to the internal film structure. The reason for this is that for the photogenerated exciton to be able to separate into free charges, it has to be generated in a thin region near the donor/acceptor interface, the width of which is determined by the exciton diffusion length. Excitons created further away from the interface than this length (10 nm for conjugated polymers[1]) will recombine. One way to address this problem has been to increase the roughness of the interface, i.e., interdiffusion or intermixing of the donor and the acceptor layers.[2,3] Another way is to blend donor and acceptor in solution and cast one single layer,[4,5] forming a so-called bulk-heterojunction. The latter approach, in some cases completed by post-casting thermal annealing or vapor annealing steps,[6] has proven to be successful for polymer-fullerene devices,[7,8] but the performance of the bulk-heterojunction solar cells is usually strongly affected by the film morphology is not easily controlled.[9–12] A third film structure of potential interest for the use in polymer solar cells could be self-stratified ¨rstro ¨m Svanstro ¨m, Prof. K. O. Magnusson [*] Dr. E. Moons, Dr. C. M. Bjo Department of Physics and Electrical Engineering Karlstad University 651 88 Karlstad (Sweden) E-mail: [email protected] Dr. J. Rysz, Prof. A. Budkowski Institute of Physics, Jagiellonian University ´w (Poland) Reymonta 4, 30-059 Krako Dr. A. Bernasik Faculty of Physics and Applied Computer Science AGH – University of Science and Technology ´w (Poland) Al. Mickiewicza 30, 30-059 Krako ¨s Dr. F. Zhang, Prof. O. Ingana Biomolecular and Organic Electronics IFM and Center of Organic Electronic (COE) ¨ping University Linko ¨ping (Sweden) 581 83 Linko Prof. M. R. Andersson Department of Polymer Technology Chalmers University of Technology ¨teborg (Sweden) 412 96 Go Dr. J. J. Benson-Smith, Prof. J. Nelson Department of Physics, Imperial College London London SW7 2BW (UK)
DOI: 10.1002/adma.200900754
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multilayers with vertical compositional variations scaling with the exciton diffusion length.[13] We have fabricated solar cells based on spin-coated thin films of the alternating polyfluorene copolymer poly[(9,9-dioctylfluorenyl2,7-diyl)-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole)] (APFO3) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and compared the photocurrent/voltage performance of three different device structures: diffuse bilayer, spontaneously formed multilayer, and vertically homogenous thin films. The compositional depth profiles of the films were determined by means of dynamic secondary ion mass spectrometry (SIMS). We found that the multilayer structure yields the highest energy conversion efficiency, despite the excess holetransporting material (APFO-3) near the low work function metal electrode where the electrons are collected. A significantly higher short-circuit current and fill factor are found in these multilayer structure devices compared to both the devices having the homogeneous blend structure and those with the diffuse bilayer structure. This finding indicates that the present models for device operation of bulk heterojunction solar cells are not fully adequate and calls for additional studies of the effects of morphology and charge transport on device performance. In earlier reports diffuse bilayer structures of poly hexylthiophene (P3HT) and PCBM and of poly(2-methoxy-5(30 ,70 -dimethyloctyloxy)-1,4-phenylene vinylene) (MDMO-PPV) and PCBM have been fabricated by spin-coating PCBM from a dichloromethane solution at high spin-speeds onto the polymer film.[14,15] We show here that this method can also be used for fabricating diffuse bilayers of APFO-3 and PCBM. SIMS depth profiles for the bilayer structure are shown in Figure 1A. The identification of PCBM was facilitated by deuterium-labeling of the phenyl-group (d5-PCBM).[16] As a label for d5-PCBM the CD ions (m/q ¼ 14) were collected. The CN ions with m/q ¼ 26 and the S ions with m/q ¼ 32 originate from APFO-3 and the intensity profiles of these therefore reflect primarily the polymer distribution. The m/q ¼ 26 signal could, however, also correspond to C2D ions that originate from d5-PCBM, which has to be accounted for in the interpretation of the profile. The signals at m/ q ¼ 24 for C2 are also monitored and displayed in the profiles. To investigate the outermost region of the film in detail, the sample was covered with a sacrificial polystyrene layer that provides an equilibrated sputtering rate through the film. Four regions, separated by dotted lines, can be identified in each profile (Fig. 1A): from left to right, the sacrificial polystyrene layer, the d5-PCBM top layer, the APFO-3 bottom layer, and the silicon substrate. The dotted lines correspond to the positions where the
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intensity of the signal in question has reached 50% of its maximum value. The bilayer structure is evidenced by the two inner regions. The top layer rich in d5-PCBM is reflected by the low sulfur intensity (m/q ¼ 32 profile) and the maximum in the CD signal (m/q ¼ 14 profile). The bottom layer, rich in APFO-3 polymer, is represented by the broad plateaus in the sulfur signal (m/q ¼ 32 profile) and the m/q ¼ 26 profile with high intensity. The presence of the other component in each of these layers is confirmed in the SIMS profiles (the polymer in d5-PCBM region: up to 10% of the max intensity of the m/q ¼ 26 signal; d5-PCBM in polymer region: non-zero constant value of m/q ¼ 14, which is
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Figure 1. SIMS profiles showing collected ion intensity versus depth (for ions with m/q ratios: 14 (CD), 24 (C2), 26 (CN, C2D), and 32 (S), obtained for A) bilayer structures with d5-PCBM spin-coated from dichloromethane at 2000 rpm on top of a film of APFO-3, spin-coated from chloroform. B) APFO-3:d5-PCBM blend thin films, with blend ratio 1:4, spin-coated from a chloroform solution and C) from a chlorobenzene solution. The depth values are relative to the free surface and calculated from the sputtering time using the sputter rate. The solid line is a smoothed profile for m/q ¼ 14 (CD) obtained by averaging over five adjacent data points.
absent in a pure APFO-3 film). The intermixing of the two materials in the bilayer structure is indicated also by the width (16 nm), considerably larger than the depth resolution of dynamic SIMS (10 nm), of the interface between the d5-PCBM rich and the APFO-3 rich layers (see Supporting Information). This intermixing provides evidence for the diffuse bilayer structure. Further indication for the structure of the interface and the film is given by results of additional AFM experiments, given in the Supporting Information. The rms roughness of the neat APFO-3 layer increased from 0.4 nm to 6.3 nm after spin-coating a drop of pure dichloromethane, and 7.6 nm after spin-coating d5-PCBM from a dichloromethane solution, on top of the APFO-3 film. The similarity of the two latter roughness values indicates that the roughness of the bilayer interface is mainly caused by the exposure to dichloromethane. The exposure to dichloromethane partially dissolves the APFO-3 layer (film thickness reduced from 90 nm to 43 nm). The SIMS depth profiles of APFO-3:d5-PCBM blend thin films with blend ratio 1:4, spin-coated from chloroform solutions are shown in Figure 1B. These depth profiles can be divided into three regions: the sacrificial polystyrene layer, the APFO-3:d5-PCBM blend layer, and the silicon substrate. In the blend region a four-fold multilayer structure can be distinguished. Compositional waves (spinodal waves) are observed, as indicated by the two maxima in the m/q ¼ 26 (CN, C2D) and m/ q ¼ 32 (S) signals, accompanied by minima in the m/q ¼ 14 (CD) profile. This demonstrates polymer-enriched regions, about 10 nm wide; one near the free surface and one in the bulk, corresponding with the regions depleted of d5-PCBM. This confirms our earlier results for films of APFO-3 blended with nondeuterated PCBM.[13] In addition, the CD-depth profile now gives direct information about the distribution of d5-PCBM in the film. The four-fold multilayer structure, with an APFO-3-enriched free surface, and subsequent regions enriched in d5-PCBM, APFO-3, and d5-PCBM layer adjacent to the silicon substrate is a result of surface-directed phase separation, as explained in earlier work.[17] The driving force for this spontaneous lamellar ordering is the difference in surface energy of the components. APFO-3 has a lower surface energy than PCBM, or in this case d5-PCBM, and is thus attracted to the interface with air to lower the energy of the free surface. The higher surface energy of PCBM is more compatible with the high surface energy substrate and the blend film is enriched with PCBM at this interface. The SIMS depth profile for the 1:4 blend film of APFO-3:d5-PCBM spin-coated from chlorobenzene is seen in Figure 1C. In this profile both the signatures for APFO-3, i.e., the CN, C2D, and S signals, and for d5-PCBM, i.e., the CD signal, are flat over the blend region. This indicates a homogenous distribution of the components throughout the film. Since chlorobenzene is a less volatile solvent than chloroform, it will evaporate more slowly during spin-coating. The effect of the solvent evaporation kinetics on the morphology of polyfluorene/PCBM blends is discussed in detail in our earlier study.[17] PCBM (and d5-PCBM) has a limited solubility in most solvents. The kinetics for crystallization of PCBM is however slow and nucleation of PCBM starts to compete kinetically with phase-separation when the drying time of the blend film is extended.[17] The nanometer-sized aggregates or crystals of PCBM that are then formed will be dispersed in the polymer
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Figure 3. I–V characteristics obtained under white light illumination, AM 1.5 (100 mW cm2) for the devices A, B, and C.
Figure 2. Schematic illustration of the solar cell cross-sections for active layers based on APFO-3 and PCBM with: a diffuse bilayer morphology (A), a spontaneously formed multilayer structure (B), and a homogenous blend morphology (C).
solution matrix, resulting in homogeneous film morphology on the scale of the SIMS and AFM resolution. Such nano-crystals of PCBM have been observed directly for other polymer:PCBM blends.[18,19] Solar cell structures based on APFO-3 and PCBM (not deuterated) active layers with the three morphologies discussed above have been fabricated. Schematic illustrations of these are shown in Figure 2. They all have PEDOT:PSS-coated ITO as the substrate and LiF/Al as the top contact. The exchange of the silicon substrate by PEDOT:PSS/ITO has no significant influence on the film structure.[20] The devices with diffuse bilayer (A), spontaneously formed multilayer (B), and vertically homogeneous structure (C) were characterized under white light illumination, AM1.5 (100 mW cm2) and the I–V characteristics are displayed in Figure 3. The open-circuit voltage, VOC, values of the three devices are not significantly different (1.02 V VOC 1.05 V), despite the different internal structure. This is in agreement with findings for diffuse bilayer devices compared to blend devices of MDMO-PPV and PCBM, as well as other fullerene-based acceptors,[21] whereas for a polymer/polymer system of polyfluorene copolymers a pristine bilayer device demonstrated a significantly larger VOC than the corresponding 1:1 blend device.[22] In turn, the short-circuit current, JSC, and fill factor, FF, for the three devices are significantly affected by morphology, resulting in different power conversion efficiencies, h. The diffuse bilayer structure device (A) has the lowest JSC value (4.23 mA cm2) and FF (0.38) resulting in the lowest overall power conversion efficiency (h ¼ 1.66%). The low JSC is mainly attributed to limited charge generation due to the smaller interface area between the donor and acceptor in the bilayer as
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compared to other structures. This limitation is apparent despite the diffuse character of the interface region. The low short-circuit current can also be attributed to limited charge transport in the device with bilayer structure compared to other device structures, because the absence of PCBM in the bottom part of the film results in a lower hole (and electron) mobility in this part.[23] The present APFO-3 layer in the bilayer structure limits therefore the long distance charge transport. This would lead to a high series-resistance in the device and may also decrease the fill factor.[24] Series resistance values, extracted from the dark I/V characteristics, were indeed found to be significantly larger (300 V) than for the devices with other morphologies (B: 76 V and C: 38 V), which indicates that the charge carrier transport is obstructed in the bilayer structure (See Supporting Information). It should be noted that the bilayer device has not been optimized with respect to the thicknesses of the APFO-3 and PCBM layers, which can have a strong influence on device performance.[22,25–27] The homogeneous blend (APFO-3:PCBM 1:4) device (C) has a higher JSC (4.76 mA cm2) and fill factor (0.49), and thus also a higher power conversion efficiency (h ¼ 2.45%), than the bilayer device (A). We attribute the higher current to the increased interface area between the active components which results in more photocarriers being generated and the more efficient transport of the photo-generated charges to the electrodes than was the case in the bilayer structure. The homogenous blend device has indeed a tenfold lower series resistance, which also contributes to the high value of the fill factor.[28,29] Yet this device has the lowest parallel resistance (in dark) (C: RP ¼ 9.6 kV vs. A: RP ¼ 16 kV and B: RP ¼ 13 kV). The fine morphology in this device seems to result in a rather leaky diode (see Supporting Information). The multilayer device (B) has the highest JSC value (5.76 mA cm2) and FF (0.58) resulting in the highest overall power conversion efficiency (h ¼ 3.46%). One probable explanation for the higher short circuit current is the high rate of free charge generation and efficient separation. While regions of lower PCBM content are more efficient in light harvesting (due to higher polymer content), regions with higher PCBM content exhibit a higher exciton dissociation efficiency, as has been demonstrated for several polymer:PCBM systems including
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We have produced diffuse bilayer thin films by spin-coating PCBM from a dichloromethane solution on top of a thin APFO-3 polymer film. The structure of the film is confirmed using dynamic SIMS and deuterium-labeled PCBM, to analyze the chemical composition perpendicular to the surface. In addition we have, using the same method, confirmed that a vertical multilayer structure is spontaneously formed in the APFO-3:PCBM (1:4) blend films spin-coated from chloroform solutions. In this structure the top surface of the blend film is enriched with APFO-3. A third morphology consisting of a vertically homogeneous blend film, was obtained by spin-coating the same blend from a chlorobenzene solution. APFO-3:PCBM solar cells based on active layers with these three morphologies have been fabricated and show a strong influence of morphology on device performance. Despite the enrichment of holetransporting APFO-3 close to the electron collecting contact, the spontaneously formed multilayer structure gave the highest short-circuit current and fill factor of the three devices, and a power conversion efficiency of about 3.5%. We have shown that the multilayer structure is favorable for promoting the generation and separation of free charge carriers as compared to a film with homogeneous composition, and that efficient transport of the photo-generated charges towards the electrodes is possible. Even though our results could be rationalized using simple device analysis tools, in-depth investigations of the device physics are needed to fully understand the complex role of vertically phase separated structures for device performance. These results give new insight in solar cell device optimization, calling for nano-scale engineering of the film composition, especially in the vertical direction. This nano-scale engineering is here demonstrated by the spontaneous self-assembly process of surfacedirected spinodal decomposition.
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APFO-3:PCBM.[30–32] A multilayer structure such as the one discussed here, makes it therefore possible to preferentially absorb light in regions of low PCBM content and preferentially dissociate excitons efficiently in regions with high PCBM content, provided the excitons can diffuse from the PCBM-poor to the PCBM-rich regions. To make full use of this architecture, the regions should therefore have a width comparable with the exciton diffusion length. Based on this model, in which efficient photon absorption and exciton dissociation preferentially take place in different areas of the structure, we propose that the multilayer structure is favorable for promoting the generation and separation of free charge carriers as compared to a film with homogeneous composition. This can then partially explain the increased overall efficiency of the multilayer solar cell in comparison to the solar cell with a homogeneous composition. In addition, it has been demonstrated for various polymer:PCBM blends that both electron and hole mobility improve with increasing PCBM content.[23,30,33] Hence PCBM-rich regions are also advantageous for the transport of the charges. If, in the multilayer structure, the free charges are preferentially generated in a PCBM-rich phase, the transport of the charges through the PCBM-rich phase should be facilitated as compared to a homogeneous blend. Hence, the transport of the photogenerated holes through the PCBM-rich interface region to the PEDOT:PSS/ITO electrode is expected to cause no problems. For the electrons to reach the Al electrode, the composition gradients could at first appear as problematic, since they need to travel through a PCBM-poor region. However, this PCBM-poor layer near the free surface is very thin (10 nm) and contains enough PCBM so that electrons can efficiently transverse it. The average mass fraction of PCBM in this region near the free surface was calculated from the SIMS profile and found to be approximately 60% by weight. Efficient electron transport through a polymer-rich ‘‘skin’’ has also been reported for other systems, such as MDMO-PPV:PCBM.[34,35] One could argue that the choice of solvent could also have other effects than morphological ones, i.e., that it could affect the polymer conformation, and hence the hole mobility in the APFO-3. It has been reported for other conjugated polymers that a higher mobility is found for films spin-coated from solvents with higher boiling points.[36,37] However, the differences in how the used solvents affect the APFO-3 conformation is minimal compared to the observed morphological effects. Indeed, the polymer-solvent interaction parameters for APFO-3 and chloroform and for APFO-3 and chlorobenzene were evaluated by Nilsson at al. and differ only about 5%.[17] In this study, it was indeed shown that the dominant interaction in this system is the polymer:PCBM interaction and that the influence of changing the solvent from chloroform to chlorobenzene on the phase diagram is minimal. Hence differences in morphology (on the mesoscale) that arise during the spin-coating process due to kinetic differences caused by the solvents’ differences in vapor pressure, are more significant. Besides, such hypothetical effects of the solvent boiling point on the mobility in the present work should lead to a higher hole mobility for the APFO-3 films spin-coated from chlorobenzene as compared to chloroform, which can not explain the superior device behavior of the APFO-3:PCBM blend layer spin-coated from chloroform.
Experimental The synthesis of APFO-3 is described elsewhere [38]. The molecular weight (Mw) of APFO-3 was 11800. PCBM and the PCBM with a deuteriumsubstituted benzene-ring, d5-PCBM, were purchased from Solenne B.V., the Netherlands. The substrates used for the SIMS and AFM analysis were RCA-cleaned silicon (001) substrates [39,40]. The cleaning procedure for silicon leaves the surface hydrophilic with a native silicon oxide. Thickness measurements of the spin-coated films and devices were made using contact mode AFM (Nanoscope IIIa Multimode, Veeco/Digital Instruments, USA) across a scratch in the deposited film. Solutions of APFO-3 (12 mg ml1) in chloroform and of d5-PCBM (12 mg ml1) in dichloromethane were used. Bilayers were fabricated by first spin-coating a layer of APFO-3 on the substrate (silicon) at 1500 rpm, letting it dry in air for a few minutes and then spin-coating d5-PCBM from the dichlormethane solution (spin-speed 2000 rpm) on top of the polymer layer, resulting in a total film thickness of 120 nm. APFO-3:d5-PCBM blend solutions (blend ratio 1:4 by weight) were prepared in chloroform and in chlorobenzene with a total concentration of solids of 12 mg ml1 and 30 mg ml1, respectively. Thin films were spin-coated at 1500 rpm, resulting in film thicknesses in the range of 60 to 80 nm both for the blend in chloroform and in chlorobenzene. AFM images of the pure APFO-3 film and of the bilayers are given in the Supporting Information, and AFM images of the blend samples have been reported earlier [12,41]. SIMS: SIMS depth profiles, with depth resolution of 10 nm, were measured using a VSW apparatus equipped with a liquid metal ion gun (FEI Company,USA) [42]. The sample was sputtered with a Gaþ primary ion beam of 5 keV (2 nA) over a region of 100 mm 100 mm, and only the
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secondary ions from the central (50%) part of the sputtered area were collected for analysis. Selected secondary ions with mass to charge ratios (m/q) equal to 12, 14, 24, 26, and 32 were collected and analyzed in a quadrupole mass spectrometer (Balzers, Liechtenstein) and are interpreted as labels for the blend components and the substrate. The signal for m/q ¼ 14 is relatively weak and to reduce the noise in the SIMS profiles for this signal an averaging over n ¼ 5 adjacent data points was performed. The averaging returns the smoothed value at index i as the average of the data points in the inclusive interval fiðn 1Þ=2; i þ ðn 1Þ2g. To be able to investigate the outermost region accurately, all samples were covered with a 50–100 nm thick sacrificial layer of polystyrene, spin-coated from a toluene solution onto a glass substrate and then lifted off from the glass and floated onto the film using a water bath. This layer provides an additional region of polymer material necessary to reach equilibration of the sputtering process, before reaching the active film [43]. To convert the sputtering cycles to depths, the actual total film thickness was determined for all of the films using contact mode AFM. Sputter rates were also determined for pure films of APFO-3 and d5-PCBM. Devices: Solar cell devices were prepared by spin-coating first the hole injection polymer layer (PEDOT:PSS) and then the active layer (blend or bilayer) onto cleaned ITO-coated glass substrates, followed by vacuum depositing LiF and Al through a mask as the top electrode. The ITO substrates were first patterned and cleaned with acetone and boiled in a mixture of water, ammonia, and hydrogen peroxide (5:1:1 by volume) for 5 minutes at 85 8C to remove organic contaminants, followed by a thorough rinse with deionized water. The hole injection layers, the polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) (EL Grade, from Bayer AG, Germany) was spin-coated at 3000 rpm on top of the ITO, and annealed at 120 8C in air for 5 min to remove residual water. The different active layers were spin-coated on top of the PEDOT-PSS layers. We have previously observed that the morphology of the blend films spin-coated on PEDOT:PSS-coated ITO are similar to the morphology of the films spin-coated on silicon substrates [41]. Bilayer active layers were deposited by sequential spin-coating. First a layer of APFO-3 was spin-coated at 2000 rpm from a solution (12 mg ml1) in chloroform. Then PCBM was spin-coated on top, also at a spin-speed of 2000 rpm, from a solution (12 mg ml1) in dichloromethane. Blend layers of APFO-3:PCBM, with blend ratio 1:4 by weight, were spin-coated (spin-speed 1000 rpm) from a chloroform solution (12 mg ml1) and from a chlorobenzene solution (24 mg ml1). The mean film thickness of the organic layer, including the active layer and the PEDOT:PSS layer, was determined using contact mode AFM. Then the samples were transferred into a vacuum chamber (at a pressure of 106 Torr; 1 Torr ¼ 133.3 Pa) to deposit sequentially LiF (0.5–1 nm) and Al (60 nm) on top of the spin-coated polymer film by thermal evaporation. The actual area for each diode, defined by the mask (4–5 mm2), was measured using an optical microscope. The solar cells were characterized in air under white light illumination from a solar simulator AM1.5 (100 mW cm2) (Oriel, Germany).
Acknowledgements E. M. acknowledges the invaluable help of Dr. Andreas Opitz, Augsburg University, Germany, with the device physics discussions. C. B. S. acknowledges the support of the Swedish National Graduate School in Materials Science. The support of the Swedish Research Council and the Swedish Energy Agency is highly appreciated. E. M., J. R., and A. B. acknowledge the support by the European Community under the Marie Curie Host Fellowships for the Transfer of Knowledge. J. R. is grateful to the Reserve of the Rector of the Jagiellonian University, and A. B. acknowledges the financial support by the Polish State Committee for Scientific Research. Supporting Information is available online from Wiley InterScience or from the author. Received: March 3, 2009 Published online: July 13, 2009
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[40] W. Kern, D. A. Puotinen, RCA Rev. 1970, 31, 187. ¨rstro ¨m, S. Nilsson, K. O. Magnusson, E. Moons, A. Bernasik, [41] C. M. Bjo J. Rysz, A. Budkowski, F. Zhang, O. Ingana¨s, M. R. Andersson, in Proc. of the Soc. of Photo-Optical Instrumentation Engineers (Eds: P. L. Heremans, M. Muccini, E. A. Meulenkamp), SPIE, Bellingham, WA 2006, 6192, X1921. [42] A. Bernasik, J. Rysz, A. Budkowski, K. Kowalski, J. Camra, J. Jedlinski, Macromol. Rapid Commun. 2001, 22, 829. ´ski, in [43] A. Bernasik, J. Rysz, A. Budkowski, K. Kowalski, J. Camra, J. Jedlin Proc. of the 7th European Conf. on Applications of Surface and Interface Analysis (ECASIA 97) (Eds: I. Olefjord, L. Nyborg, D. Briggs), Wiley, Chichester, UK 1997, 775.
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[33] L. M. Andersson, F. Zhang, O. Ingana¨s, Appl. Phys. Lett. 2007, 91, 071108. [34] H. Hoppe, T. Glatzel, M. Niggemann, A. Hinsch, M. C. Lux-Steiner, N. S. Sariciftci, Nano Lett. 2005, 5, 269. [35] C. R. McNeill, B. Watts, L. Thomsen, W. J. Belcher, A. K. D. Kilcoyne, N. C. Greenham, P. C. Dastoor, Small 2006, 2, 1432. ¨lling, M. Giles, I. [36] J.-F. Chang, B. Sun, D. W. Breiby, M. M. Nielsen, T. I. So McCulloch, H. Sirringhaus, Chem. Mater. 2004, 16, 4772. [37] W. Geens, S. E. Shaheen, B. Wessling, C. Brabec, J. Poortmans, N. S. Sariciftci, Org. Electron. 2002, 3, 105. [38] O. Ingana¨s, M. Svensson, F. Zhang, A. Gadisa, N. K. Persson, X. Wang, M. R. Andersson, Appl. Phys. A 2004, 79, 31. [39] W. Kern, J. Electrochem. Soc. 1990, 137, 1887.
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Device Performance of APFO-3/PCBM Solar Cells with Controlled Morphology By Cecilia M. Bjo¨rstro¨m Svanstro¨m, Jakub Rysz, Andrzej Bernasik, Andrzej Budkowski, Fengling Zhang, Olle Ingana¨s, Mats R. Andersson, Kjell O. Magnusson, Jessica J. Benson-Smith, Jenny Nelson, and Ellen Moons* Polymer-based solar cells offer a promising technology for photovoltaic energy conversion thanks to the simple and inexpensive solution-based fabrication process of the active layer, i.e., a thin film containing both donor and acceptor materials. However, the device performance is very sensitive to the internal film structure. The reason for this is that for the photogenerated exciton to be able to separate into free charges, it has to be generated in a thin region near the donor/acceptor interface, the width of which is determined by the exciton diffusion length. Excitons created further away from the interface than this length (10 nm for conjugated polymers[1]) will recombine. One way to address this problem has been to increase the roughness of the interface, i.e., interdiffusion or intermixing of the donor and the acceptor layers.[2,3] Another way is to blend donor and acceptor in solution and cast one single layer,[4,5] forming a so-called bulk-heterojunction. The latter approach, in some cases completed by post-casting thermal annealing or vapor annealing steps,[6] has proven to be successful for polymer-fullerene devices,[7,8] but the performance of the bulk-heterojunction solar cells is usually strongly affected by the film morphology is not easily controlled.[9–12] A third film structure of potential interest for the use in polymer solar cells could be self-stratified ¨rstro ¨m Svanstro ¨m, Prof. K. O. Magnusson [*] Dr. E. Moons, Dr. C. M. Bjo Department of Physics and Electrical Engineering Karlstad University 651 88 Karlstad (Sweden) E-mail: [email protected] Dr. J. Rysz, Prof. A. Budkowski Institute of Physics, Jagiellonian University ´w (Poland) Reymonta 4, 30-059 Krako Dr. A. Bernasik Faculty of Physics and Applied Computer Science AGH – University of Science and Technology ´w (Poland) Al. Mickiewicza 30, 30-059 Krako ¨s Dr. F. Zhang, Prof. O. Ingana Biomolecular and Organic Electronics IFM and Center of Organic Electronic (COE) ¨ping University Linko ¨ping (Sweden) 581 83 Linko Prof. M. R. Andersson Department of Polymer Technology Chalmers University of Technology ¨teborg (Sweden) 412 96 Go Dr. J. J. Benson-Smith, Prof. J. Nelson Department of Physics, Imperial College London London SW7 2BW (UK)
DOI: 10.1002/adma.200900754
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multilayers with vertical compositional variations scaling with the exciton diffusion length.[13] We have fabricated solar cells based on spin-coated thin films of the alternating polyfluorene copolymer poly[(9,9-dioctylfluorenyl2,7-diyl)-alt-5,5-(40 ,70 -di-2-thienyl-20 ,10 ,30 -benzothiadiazole)] (APFO3) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and compared the photocurrent/voltage performance of three different device structures: diffuse bilayer, spontaneously formed multilayer, and vertically homogenous thin films. The compositional depth profiles of the films were determined by means of dynamic secondary ion mass spectrometry (SIMS). We found that the multilayer structure yields the highest energy conversion efficiency, despite the excess holetransporting material (APFO-3) near the low work function metal electrode where the electrons are collected. A significantly higher short-circuit current and fill factor are found in these multilayer structure devices compared to both the devices having the homogeneous blend structure and those with the diffuse bilayer structure. This finding indicates that the present models for device operation of bulk heterojunction solar cells are not fully adequate and calls for additional studies of the effects of morphology and charge transport on device performance. In earlier reports diffuse bilayer structures of poly hexylthiophene (P3HT) and PCBM and of poly(2-methoxy-5(30 ,70 -dimethyloctyloxy)-1,4-phenylene vinylene) (MDMO-PPV) and PCBM have been fabricated by spin-coating PCBM from a dichloromethane solution at high spin-speeds onto the polymer film.[14,15] We show here that this method can also be used for fabricating diffuse bilayers of APFO-3 and PCBM. SIMS depth profiles for the bilayer structure are shown in Figure 1A. The identification of PCBM was facilitated by deuterium-labeling of the phenyl-group (d5-PCBM).[16] As a label for d5-PCBM the CD ions (m/q ¼ 14) were collected. The CN ions with m/q ¼ 26 and the S ions with m/q ¼ 32 originate from APFO-3 and the intensity profiles of these therefore reflect primarily the polymer distribution. The m/q ¼ 26 signal could, however, also correspond to C2D ions that originate from d5-PCBM, which has to be accounted for in the interpretation of the profile. The signals at m/ q ¼ 24 for C2 are also monitored and displayed in the profiles. To investigate the outermost region of the film in detail, the sample was covered with a sacrificial polystyrene layer that provides an equilibrated sputtering rate through the film. Four regions, separated by dotted lines, can be identified in each profile (Fig. 1A): from left to right, the sacrificial polystyrene layer, the d5-PCBM top layer, the APFO-3 bottom layer, and the silicon substrate. The dotted lines correspond to the positions where the
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intensity of the signal in question has reached 50% of its maximum value. The bilayer structure is evidenced by the two inner regions. The top layer rich in d5-PCBM is reflected by the low sulfur intensity (m/q ¼ 32 profile) and the maximum in the CD signal (m/q ¼ 14 profile). The bottom layer, rich in APFO-3 polymer, is represented by the broad plateaus in the sulfur signal (m/q ¼ 32 profile) and the m/q ¼ 26 profile with high intensity. The presence of the other component in each of these layers is confirmed in the SIMS profiles (the polymer in d5-PCBM region: up to 10% of the max intensity of the m/q ¼ 26 signal; d5-PCBM in polymer region: non-zero constant value of m/q ¼ 14, which is
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Figure 1. SIMS profiles showing collected ion intensity versus depth (for ions with m/q ratios: 14 (CD), 24 (C2), 26 (CN, C2D), and 32 (S), obtained for A) bilayer structures with d5-PCBM spin-coated from dichloromethane at 2000 rpm on top of a film of APFO-3, spin-coated from chloroform. B) APFO-3:d5-PCBM blend thin films, with blend ratio 1:4, spin-coated from a chloroform solution and C) from a chlorobenzene solution. The depth values are relative to the free surface and calculated from the sputtering time using the sputter rate. The solid line is a smoothed profile for m/q ¼ 14 (CD) obtained by averaging over five adjacent data points.
absent in a pure APFO-3 film). The intermixing of the two materials in the bilayer structure is indicated also by the width (16 nm), considerably larger than the depth resolution of dynamic SIMS (10 nm), of the interface between the d5-PCBM rich and the APFO-3 rich layers (see Supporting Information). This intermixing provides evidence for the diffuse bilayer structure. Further indication for the structure of the interface and the film is given by results of additional AFM experiments, given in the Supporting Information. The rms roughness of the neat APFO-3 layer increased from 0.4 nm to 6.3 nm after spin-coating a drop of pure dichloromethane, and 7.6 nm after spin-coating d5-PCBM from a dichloromethane solution, on top of the APFO-3 film. The similarity of the two latter roughness values indicates that the roughness of the bilayer interface is mainly caused by the exposure to dichloromethane. The exposure to dichloromethane partially dissolves the APFO-3 layer (film thickness reduced from 90 nm to 43 nm). The SIMS depth profiles of APFO-3:d5-PCBM blend thin films with blend ratio 1:4, spin-coated from chloroform solutions are shown in Figure 1B. These depth profiles can be divided into three regions: the sacrificial polystyrene layer, the APFO-3:d5-PCBM blend layer, and the silicon substrate. In the blend region a four-fold multilayer structure can be distinguished. Compositional waves (spinodal waves) are observed, as indicated by the two maxima in the m/q ¼ 26 (CN, C2D) and m/ q ¼ 32 (S) signals, accompanied by minima in the m/q ¼ 14 (CD) profile. This demonstrates polymer-enriched regions, about 10 nm wide; one near the free surface and one in the bulk, corresponding with the regions depleted of d5-PCBM. This confirms our earlier results for films of APFO-3 blended with nondeuterated PCBM.[13] In addition, the CD-depth profile now gives direct information about the distribution of d5-PCBM in the film. The four-fold multilayer structure, with an APFO-3-enriched free surface, and subsequent regions enriched in d5-PCBM, APFO-3, and d5-PCBM layer adjacent to the silicon substrate is a result of surface-directed phase separation, as explained in earlier work.[17] The driving force for this spontaneous lamellar ordering is the difference in surface energy of the components. APFO-3 has a lower surface energy than PCBM, or in this case d5-PCBM, and is thus attracted to the interface with air to lower the energy of the free surface. The higher surface energy of PCBM is more compatible with the high surface energy substrate and the blend film is enriched with PCBM at this interface. The SIMS depth profile for the 1:4 blend film of APFO-3:d5-PCBM spin-coated from chlorobenzene is seen in Figure 1C. In this profile both the signatures for APFO-3, i.e., the CN, C2D, and S signals, and for d5-PCBM, i.e., the CD signal, are flat over the blend region. This indicates a homogenous distribution of the components throughout the film. Since chlorobenzene is a less volatile solvent than chloroform, it will evaporate more slowly during spin-coating. The effect of the solvent evaporation kinetics on the morphology of polyfluorene/PCBM blends is discussed in detail in our earlier study.[17] PCBM (and d5-PCBM) has a limited solubility in most solvents. The kinetics for crystallization of PCBM is however slow and nucleation of PCBM starts to compete kinetically with phase-separation when the drying time of the blend film is extended.[17] The nanometer-sized aggregates or crystals of PCBM that are then formed will be dispersed in the polymer
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Figure 3. I–V characteristics obtained under white light illumination, AM 1.5 (100 mW cm2) for the devices A, B, and C.
Figure 2. Schematic illustration of the solar cell cross-sections for active layers based on APFO-3 and PCBM with: a diffuse bilayer morphology (A), a spontaneously formed multilayer structure (B), and a homogenous blend morphology (C).
solution matrix, resulting in homogeneous film morphology on the scale of the SIMS and AFM resolution. Such nano-crystals of PCBM have been observed directly for other polymer:PCBM blends.[18,19] Solar cell structures based on APFO-3 and PCBM (not deuterated) active layers with the three morphologies discussed above have been fabricated. Schematic illustrations of these are shown in Figure 2. They all have PEDOT:PSS-coated ITO as the substrate and LiF/Al as the top contact. The exchange of the silicon substrate by PEDOT:PSS/ITO has no significant influence on the film structure.[20] The devices with diffuse bilayer (A), spontaneously formed multilayer (B), and vertically homogeneous structure (C) were characterized under white light illumination, AM1.5 (100 mW cm2) and the I–V characteristics are displayed in Figure 3. The open-circuit voltage, VOC, values of the three devices are not significantly different (1.02 V VOC 1.05 V), despite the different internal structure. This is in agreement with findings for diffuse bilayer devices compared to blend devices of MDMO-PPV and PCBM, as well as other fullerene-based acceptors,[21] whereas for a polymer/polymer system of polyfluorene copolymers a pristine bilayer device demonstrated a significantly larger VOC than the corresponding 1:1 blend device.[22] In turn, the short-circuit current, JSC, and fill factor, FF, for the three devices are significantly affected by morphology, resulting in different power conversion efficiencies, h. The diffuse bilayer structure device (A) has the lowest JSC value (4.23 mA cm2) and FF (0.38) resulting in the lowest overall power conversion efficiency (h ¼ 1.66%). The low JSC is mainly attributed to limited charge generation due to the smaller interface area between the donor and acceptor in the bilayer as
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compared to other structures. This limitation is apparent despite the diffuse character of the interface region. The low short-circuit current can also be attributed to limited charge transport in the device with bilayer structure compared to other device structures, because the absence of PCBM in the bottom part of the film results in a lower hole (and electron) mobility in this part.[23] The present APFO-3 layer in the bilayer structure limits therefore the long distance charge transport. This would lead to a high series-resistance in the device and may also decrease the fill factor.[24] Series resistance values, extracted from the dark I/V characteristics, were indeed found to be significantly larger (300 V) than for the devices with other morphologies (B: 76 V and C: 38 V), which indicates that the charge carrier transport is obstructed in the bilayer structure (See Supporting Information). It should be noted that the bilayer device has not been optimized with respect to the thicknesses of the APFO-3 and PCBM layers, which can have a strong influence on device performance.[22,25–27] The homogeneous blend (APFO-3:PCBM 1:4) device (C) has a higher JSC (4.76 mA cm2) and fill factor (0.49), and thus also a higher power conversion efficiency (h ¼ 2.45%), than the bilayer device (A). We attribute the higher current to the increased interface area between the active components which results in more photocarriers being generated and the more efficient transport of the photo-generated charges to the electrodes than was the case in the bilayer structure. The homogenous blend device has indeed a tenfold lower series resistance, which also contributes to the high value of the fill factor.[28,29] Yet this device has the lowest parallel resistance (in dark) (C: RP ¼ 9.6 kV vs. A: RP ¼ 16 kV and B: RP ¼ 13 kV). The fine morphology in this device seems to result in a rather leaky diode (see Supporting Information). The multilayer device (B) has the highest JSC value (5.76 mA cm2) and FF (0.58) resulting in the highest overall power conversion efficiency (h ¼ 3.46%). One probable explanation for the higher short circuit current is the high rate of free charge generation and efficient separation. While regions of lower PCBM content are more efficient in light harvesting (due to higher polymer content), regions with higher PCBM content exhibit a higher exciton dissociation efficiency, as has been demonstrated for several polymer:PCBM systems including
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We have produced diffuse bilayer thin films by spin-coating PCBM from a dichloromethane solution on top of a thin APFO-3 polymer film. The structure of the film is confirmed using dynamic SIMS and deuterium-labeled PCBM, to analyze the chemical composition perpendicular to the surface. In addition we have, using the same method, confirmed that a vertical multilayer structure is spontaneously formed in the APFO-3:PCBM (1:4) blend films spin-coated from chloroform solutions. In this structure the top surface of the blend film is enriched with APFO-3. A third morphology consisting of a vertically homogeneous blend film, was obtained by spin-coating the same blend from a chlorobenzene solution. APFO-3:PCBM solar cells based on active layers with these three morphologies have been fabricated and show a strong influence of morphology on device performance. Despite the enrichment of holetransporting APFO-3 close to the electron collecting contact, the spontaneously formed multilayer structure gave the highest short-circuit current and fill factor of the three devices, and a power conversion efficiency of about 3.5%. We have shown that the multilayer structure is favorable for promoting the generation and separation of free charge carriers as compared to a film with homogeneous composition, and that efficient transport of the photo-generated charges towards the electrodes is possible. Even though our results could be rationalized using simple device analysis tools, in-depth investigations of the device physics are needed to fully understand the complex role of vertically phase separated structures for device performance. These results give new insight in solar cell device optimization, calling for nano-scale engineering of the film composition, especially in the vertical direction. This nano-scale engineering is here demonstrated by the spontaneous self-assembly process of surfacedirected spinodal decomposition.
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APFO-3:PCBM.[30–32] A multilayer structure such as the one discussed here, makes it therefore possible to preferentially absorb light in regions of low PCBM content and preferentially dissociate excitons efficiently in regions with high PCBM content, provided the excitons can diffuse from the PCBM-poor to the PCBM-rich regions. To make full use of this architecture, the regions should therefore have a width comparable with the exciton diffusion length. Based on this model, in which efficient photon absorption and exciton dissociation preferentially take place in different areas of the structure, we propose that the multilayer structure is favorable for promoting the generation and separation of free charge carriers as compared to a film with homogeneous composition. This can then partially explain the increased overall efficiency of the multilayer solar cell in comparison to the solar cell with a homogeneous composition. In addition, it has been demonstrated for various polymer:PCBM blends that both electron and hole mobility improve with increasing PCBM content.[23,30,33] Hence PCBM-rich regions are also advantageous for the transport of the charges. If, in the multilayer structure, the free charges are preferentially generated in a PCBM-rich phase, the transport of the charges through the PCBM-rich phase should be facilitated as compared to a homogeneous blend. Hence, the transport of the photogenerated holes through the PCBM-rich interface region to the PEDOT:PSS/ITO electrode is expected to cause no problems. For the electrons to reach the Al electrode, the composition gradients could at first appear as problematic, since they need to travel through a PCBM-poor region. However, this PCBM-poor layer near the free surface is very thin (10 nm) and contains enough PCBM so that electrons can efficiently transverse it. The average mass fraction of PCBM in this region near the free surface was calculated from the SIMS profile and found to be approximately 60% by weight. Efficient electron transport through a polymer-rich ‘‘skin’’ has also been reported for other systems, such as MDMO-PPV:PCBM.[34,35] One could argue that the choice of solvent could also have other effects than morphological ones, i.e., that it could affect the polymer conformation, and hence the hole mobility in the APFO-3. It has been reported for other conjugated polymers that a higher mobility is found for films spin-coated from solvents with higher boiling points.[36,37] However, the differences in how the used solvents affect the APFO-3 conformation is minimal compared to the observed morphological effects. Indeed, the polymer-solvent interaction parameters for APFO-3 and chloroform and for APFO-3 and chlorobenzene were evaluated by Nilsson at al. and differ only about 5%.[17] In this study, it was indeed shown that the dominant interaction in this system is the polymer:PCBM interaction and that the influence of changing the solvent from chloroform to chlorobenzene on the phase diagram is minimal. Hence differences in morphology (on the mesoscale) that arise during the spin-coating process due to kinetic differences caused by the solvents’ differences in vapor pressure, are more significant. Besides, such hypothetical effects of the solvent boiling point on the mobility in the present work should lead to a higher hole mobility for the APFO-3 films spin-coated from chlorobenzene as compared to chloroform, which can not explain the superior device behavior of the APFO-3:PCBM blend layer spin-coated from chloroform.
Experimental The synthesis of APFO-3 is described elsewhere [38]. The molecular weight (Mw) of APFO-3 was 11800. PCBM and the PCBM with a deuteriumsubstituted benzene-ring, d5-PCBM, were purchased from Solenne B.V., the Netherlands. The substrates used for the SIMS and AFM analysis were RCA-cleaned silicon (001) substrates [39,40]. The cleaning procedure for silicon leaves the surface hydrophilic with a native silicon oxide. Thickness measurements of the spin-coated films and devices were made using contact mode AFM (Nanoscope IIIa Multimode, Veeco/Digital Instruments, USA) across a scratch in the deposited film. Solutions of APFO-3 (12 mg ml1) in chloroform and of d5-PCBM (12 mg ml1) in dichloromethane were used. Bilayers were fabricated by first spin-coating a layer of APFO-3 on the substrate (silicon) at 1500 rpm, letting it dry in air for a few minutes and then spin-coating d5-PCBM from the dichlormethane solution (spin-speed 2000 rpm) on top of the polymer layer, resulting in a total film thickness of 120 nm. APFO-3:d5-PCBM blend solutions (blend ratio 1:4 by weight) were prepared in chloroform and in chlorobenzene with a total concentration of solids of 12 mg ml1 and 30 mg ml1, respectively. Thin films were spin-coated at 1500 rpm, resulting in film thicknesses in the range of 60 to 80 nm both for the blend in chloroform and in chlorobenzene. AFM images of the pure APFO-3 film and of the bilayers are given in the Supporting Information, and AFM images of the blend samples have been reported earlier [12,41]. SIMS: SIMS depth profiles, with depth resolution of 10 nm, were measured using a VSW apparatus equipped with a liquid metal ion gun (FEI Company,USA) [42]. The sample was sputtered with a Gaþ primary ion beam of 5 keV (2 nA) over a region of 100 mm 100 mm, and only the
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secondary ions from the central (50%) part of the sputtered area were collected for analysis. Selected secondary ions with mass to charge ratios (m/q) equal to 12, 14, 24, 26, and 32 were collected and analyzed in a quadrupole mass spectrometer (Balzers, Liechtenstein) and are interpreted as labels for the blend components and the substrate. The signal for m/q ¼ 14 is relatively weak and to reduce the noise in the SIMS profiles for this signal an averaging over n ¼ 5 adjacent data points was performed. The averaging returns the smoothed value at index i as the average of the data points in the inclusive interval fiðn 1Þ=2; i þ ðn 1Þ2g. To be able to investigate the outermost region accurately, all samples were covered with a 50–100 nm thick sacrificial layer of polystyrene, spin-coated from a toluene solution onto a glass substrate and then lifted off from the glass and floated onto the film using a water bath. This layer provides an additional region of polymer material necessary to reach equilibration of the sputtering process, before reaching the active film [43]. To convert the sputtering cycles to depths, the actual total film thickness was determined for all of the films using contact mode AFM. Sputter rates were also determined for pure films of APFO-3 and d5-PCBM. Devices: Solar cell devices were prepared by spin-coating first the hole injection polymer layer (PEDOT:PSS) and then the active layer (blend or bilayer) onto cleaned ITO-coated glass substrates, followed by vacuum depositing LiF and Al through a mask as the top electrode. The ITO substrates were first patterned and cleaned with acetone and boiled in a mixture of water, ammonia, and hydrogen peroxide (5:1:1 by volume) for 5 minutes at 85 8C to remove organic contaminants, followed by a thorough rinse with deionized water. The hole injection layers, the polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) (EL Grade, from Bayer AG, Germany) was spin-coated at 3000 rpm on top of the ITO, and annealed at 120 8C in air for 5 min to remove residual water. The different active layers were spin-coated on top of the PEDOT-PSS layers. We have previously observed that the morphology of the blend films spin-coated on PEDOT:PSS-coated ITO are similar to the morphology of the films spin-coated on silicon substrates [41]. Bilayer active layers were deposited by sequential spin-coating. First a layer of APFO-3 was spin-coated at 2000 rpm from a solution (12 mg ml1) in chloroform. Then PCBM was spin-coated on top, also at a spin-speed of 2000 rpm, from a solution (12 mg ml1) in dichloromethane. Blend layers of APFO-3:PCBM, with blend ratio 1:4 by weight, were spin-coated (spin-speed 1000 rpm) from a chloroform solution (12 mg ml1) and from a chlorobenzene solution (24 mg ml1). The mean film thickness of the organic layer, including the active layer and the PEDOT:PSS layer, was determined using contact mode AFM. Then the samples were transferred into a vacuum chamber (at a pressure of 106 Torr; 1 Torr ¼ 133.3 Pa) to deposit sequentially LiF (0.5–1 nm) and Al (60 nm) on top of the spin-coated polymer film by thermal evaporation. The actual area for each diode, defined by the mask (4–5 mm2), was measured using an optical microscope. The solar cells were characterized in air under white light illumination from a solar simulator AM1.5 (100 mW cm2) (Oriel, Germany).
Acknowledgements E. M. acknowledges the invaluable help of Dr. Andreas Opitz, Augsburg University, Germany, with the device physics discussions. C. B. S. acknowledges the support of the Swedish National Graduate School in Materials Science. The support of the Swedish Research Council and the Swedish Energy Agency is highly appreciated. E. M., J. R., and A. B. acknowledge the support by the European Community under the Marie Curie Host Fellowships for the Transfer of Knowledge. J. R. is grateful to the Reserve of the Rector of the Jagiellonian University, and A. B. acknowledges the financial support by the Polish State Committee for Scientific Research. Supporting Information is available online from Wiley InterScience or from the author. Received: March 3, 2009 Published online: July 13, 2009
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