NANO LETTERS
Nanoscale Morphology of High-Performance Polymer Solar Cells
2005 Vol. 5, No. 4 579-583
Xiaoniu Yang,†,# Joachim Loos,*,‡,§,# Sjoerd C. Veenstra,⊥,# Wiljan J. H. Verhees,⊥,# Martijn M. Wienk,|,# Jan M. Kroon,⊥,# Matthias A. J. Michels,†,# and Rene´ A. J. Janssen|,# Group Polymer Physics, Laboratory of Materials and Interface Chemistry, Laboratory of Polymer Technology, and Molecular Materials and Nanosystems, EindhoVen UniVersity of Technology, P.O. Box 513, NL-5600 MB EindhoVen, The Netherlands, Energy Research Centre of The Netherlands (ECN), P.O. Box 1, NL-1755 ZG, Petten, The Netherlands, and Dutch Polymer Institute, P.O. Box 902, NL-5600 AX EindhoVen, The Netherlands Received November 12, 2004; Revised Manuscript Received February 7, 2005
ABSTRACT Transmission electron microscopy and electron diffraction are used to study the changes in morphology of composite films of poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM) in bulk heterojunction solar cells. Thermal annealing produces and stabilizes a nanoscale interpenetrating network with crystalline order for both components. P3HT forms long, thin conducting nanowires in a rather homogeneous, nanocrystalline PCBM film. Both the improved crystalline nature of films and increased but controlled demixing between the two constitutes therein after annealing explains the considerable increase of the power conversion efficiency observed in these devices.
The discovery of conjugated, semiconducting polymers has created a new class of materials that combines the processing properties of polymers with the functional properties of traditional semiconductors. These novel materials can be applied in optoelectronic devices such as polymer lightemitting diodes and solar cells.1,2 The prospect that lightweight and flexible polymer solar cells can be produced by roll-to-roll production, in combination with high energyconversion efficiency, has spurred a worldwide interest in these novel photovoltaic devices.3-5 For high performance, the polymer photovoltaic device should fulfill several requirements: efficient absorption of sunlight, excellent charge carrier generation, transport, and collection. However, unlike many conventional inorganic semiconductors, in which photon absorption directly produces free charge carriers,6 optical absorption in organic semiconductors mainly creates electron-hole pairs (excitons) that are bound at room temperature.7 These excitons must be dissociated into free electrons and holes to create a photovoltaic effect. Such * Corresponding author. Email:
[email protected]. Phone: +31-40-2473033. † Group Polymer Physics, Eindhoven University of Technology. ‡ Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology. § Laboratory of Polymer Technology, Eindhoven University of Technology. | Molecular Materials and Nanosystems, Eindhoven University of Technology. ⊥ Energy Research Centre of The Netherlands. # Dutch Polymer Institute. 10.1021/nl048120i CCC: $30.25 Published on Web 03/02/2005
© 2005 American Chemical Society
efficient dissociation can be achieved at the interface of two organic semiconductors with an appropriate difference in electron affinity. Hence, organic solar cells generally consist of two electronically matched materials; an electron donor and electron acceptor. Because the typical exciton diffusion length in conjugated polymers is limited to ∼10 nm, the donor and acceptor materials should form nanoscale interpenetrating networks within the whole photoactive layer to ensure an efficient dissociation of excitons. Compared to a simple double-layer configuration,8 the conceptual advantage of such a bulk heterojunction is that the interface area is enhanced enormously and that a thicker photoactive layer, optimized for light absorption, can be implemented in the device.9,10 The challenge is to organize the donor and acceptor materials such that their interface area is maximized,11-13 while typical dimensions of phase separation are within the exciton diffusion range and continuous, preferably short, pathways for transport of charge carriers to the electrodes are ensured. To enhance charge transport within the interpenetrating networks (and thus reduce charge recombination), high charge carrier mobility for both holes and electrons is required. The general approach to enhance charge carrier transport in organic and polymer materials is increasing the mesoscopic order and crystallinity. Hence, a nanoscale interpenetrating network with crystalline order of both constituents seems a desirable architecture for the active layer
of polymer photovoltaic devices.14 Eventually, the electronic band gaps of the materials in the photoactive layer should be tuned to harvest more light from the solar spectrum. One of the most promising polymer solar cells developed to date in terms of efficiency and stability is based on the combination of regioregular poly(3-hexylthiophene) (P3HT) as donor and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as acceptor.15,16 External quantum efficiencies above 75% and power conversion efficiencies of up to 3.85% have been reported recently for these P3HT/PCBM devices.17 The high efficiency of these devices can be related to the intrinsic properties of the two components. Regioregular P3HT self-organizes into a microcrystalline structure18 and, because of efficient interchain transport of charge carriers, the (hole) mobility in P3HT is high (up to ∼0.1 cm2 V-1 s-1).19-21 Moreover, in thin films interchain interactions cause a red shift of the optical absorption of P3HT, which provides an improved overlap with the solar emission. The second component, PCBM, is a C60 fullerene derivative with an electron mobility of 2 × 10-3 cm2 V-1 s-1.22 Compared to C60, the solubility of PCBM in organic solvents is greatly improved, which allows the utilization of film deposition techniques requiring highly concentrated solution. Also PCBM can crystallize and control nucleation and crystallization kinetics, allowing the adjustment of the crystal size.23 However, continuous crystallization may result in single crystals with micrometer sizes. Interestingly, the efficiency of solar cells based on P3HT and fullerenes was shown to depend strongly on the processing conditions and to improve particularly by a thermal annealing step.16,24,25 We hope to utilize the effect of annealing to identify critical morphology parameters that determine the performance of these cells by studying the changes in morphology of these composites before and after annealing. In our experiments, a mixture (1:1 in weight) of P3HT (Mw ) 100,000 g mol-1, Mw/Mn ) 2.14, and regioregularity greater than 98.5% as determined by NMR, from Rieke Metals Inc.) and PCBM was dissolved in 1,2-dichlorobenzene (ODCB) and deposited by spin coating on glass substrates covered with 100 nm thick layers of indium tin oxide (ITO) and poly(ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT/PSS) that form the electrode for hole collection. The composite P3HT/PCBM layer was ∼70 nm thick. The devices were completed by evaporation of a metal back electrode (LiF/Al). Thermal annealing was performed on complete devices, i.e., with the photoactive layer between electrodes, at 120 °C for 60 min. This temperature is below the apparent melting point (Tm′ ≈ 215 °C) of P3HT as determined by DSC.26 Figure 1a shows the external quantum efficiencies (or incident photon-to-current conversion efficiency, IPCE) of the device with a pristine, spin-coated photoactive layer (Figure 1a, blue curve) and of a device after controlled annealing (Figure 1a, red curve). The latter device shows the expected high IPCE for a broad spectral range.16 In particular, a considerable amount of current is produced in the red part of the spectrum. Current density-voltage (J580
Figure 1. Device performance of pristine (blue) and thermally annealed (red) photoactive layers of P3HT/PCBM devices. (a) IPCE measured using monochromatic light (∼1.0 mW cm-2) calibrated against a calibrated Si photodiode. (b) Current density-voltage characteristics under illumination by a tungsten-halogen lamp (filtered by a Schott KG1 and GG385 filter resulting in a spectral range of 400-900 nm with a maximum at ∼650 nm). The inset shows the stability test of a cell with initial efficiency of 2.5% under thermally accelerated aging at 70 °C with an intensity of 1 sun illumination. Device characterization was performed in an N2 atmosphere at room temperature.
V) measurements (Figure 1b) under white light illumination of a tungsten/halogen lamp at approximately equivalent 1.15 sun intensity (115 mW cm-2) reveal a slight increase of the open-circuit voltage accompanied by a considerable improvement for the fill factor to Voc ) 0.615 V and FF ) 0.61, respectively, after annealing. Integrating the IPCE data with the AM1.5 (100 mW cm-2) solar emission spectrum gives an estimate for Jsc under AM1.5 conditions of 7.2 mA cm-2 and a calculated power conversion efficiency of 2.7%. The specimens for transmission electron microscopy (TEM) measurements were prepared by etching the top metal contact with 1 M HCl solution, followed by floating the photoactive layer onto a water surface, and transfer to a TEM grid. Typical bright-field (BF) transmission electron microscopy (TEM) images, recorded in slight defocusing conditions, of a pristine P3HT/PCBM composite film are shown in Figures 2a and 2b. Fibrillar P3HT crystals, which are relatively bright in contrast compared to the background, overlap with each other over the whole film. From the Nano Lett., Vol. 5, No. 4, 2005
Figure 2. BF TEM images show the overview (a) and the zoom in (b), and the corresponding schematic representation (c) of the pristine photoactive layer of a P3HT/PCBM plastic solar cell. The inset in Figure 2a is the corresponding SAED pattern. The TEM images were recorded on a JEOL JEM-2000FX transmission electron microscope operated at 80 kV.
absence of other clearly detectable crystalline features, we infer that PCBM is rather homogeneously distributed in the matrix. The bright appearance of the P3HT crystals relative to a dark background is caused by the lower density of P3HT (1.10 g cm-3)18 compared to PCBM (1.50 g cm-3).27 The width of these fibrillar crystals is approximately 15 nm and their length is mostly less than 500 nm. The P3HT crystals have a tendency to form a network, although not entirely connecting each other. The inset of Figure 2a shows the selected area electron diffraction (SAED) pattern of this film. Two diffraction rings can be observed. The outer ring corresponds to a distance of 0.39 nm. This ring is attributed to the (020) reflection of P3HT crystals, which is associated with the typical π-π stacking distance of P3HT chains. A similar reflection with the same d-spacing has previously been observed for the SAED pattern of P3HT whiskers.28 The crystallinity and the perfection of present P3HT crystals seems not very pronounced, as revealed by the low intensity of the reflection ring. The inner ring in the SAED pattern, corresponding to a d-spacing of 0.46 nm, appears even more diffuse and has been observed in pure PCBM layers.23 We have previously shown that in pure PCBM layers the small size of PCBM nanocrystals and their dense stacking within the film in both lateral and vertical directions inhibit the formation of a pronounced bright-field contrast, while the SAED pattern reveals the crystalline order inside the film.23 The inner ring is therefore attributed to PCBM nanocrystals that are homogeneously dispersed throughout the film. Because P3HT apparently crystallizes faster than PCBM, Nano Lett., Vol. 5, No. 4, 2005
Figure 3. BF TEM images show the overview (a) and the zoom in (b), and the corresponding schematic representation (c) of the thermal annealed photoactive layer. The inset in Figure 3a is the corresponding SAED pattern. The arrow is to indicate the increased intensity of (020) Debye-Scherrer ring from P3HT crystals compared to the SAED pattern shown in the inset of Figure 2a. Note: for Figure 3c, the dash line bordered regions represent the extension of existing P3HT crystals in the pristine film (Figure 2) or newly developed PCBM-rich domains during the annealing step. The TEM images were recorded on a JEOL JEM-2000FX transmission electron microscope operated at 80 kV.
small fibrillar P3HT crystals are formed while the crystallization of PCBM is almost suppressed and only nanometersized crystals are formed. The main morphological features of the pristine composite film are schematically visualized in Figure 2c. The advantage of the morphology created in this first preparation step is that a homogeneous distribution of the two components on the nanoscale is achieved and that P3HT forms elongated crystals. Both features are essential for the success of the annealing step; the reasons will be discussed below. Figure 3 shows the BF TEM images of the composite film after controlled annealing (120 °C for 60 min). The annealing was performed on a completed device, i.e., with bottom and top contacts present that were later removed to obtain the images. The most pronounced feature in the BF TEM image of the annealed sample is the increased contrast and the appearance of bright fibrillar P3HT crystals throughout the entire film. The width of these crystals remains almost constant compared to the pristine composite film, but on average their length has increased over 50% as inferred from a statistical measurement on the images. We note that a conventional BF TEM image is a two-dimensional projection of the three-dimensional morphology in a thin film, which causes missing morphology information in a direction perpendicular with respect to the film plane. Because there 581
is no strong interaction between P3HT and the substrates and bending of P3HT fibrils is clearly visible in Figure 3a and 3b, we presume that the orientation of P3HT crystals in the film should be rather homogeneous, including the perpendicular direction. The increased crystallinity of P3HT after thermal treatment is evidenced by the increased intensity of the (020) reflection ring in the SAED pattern (inset of Figure 3a). In addition, various larger dark (PCBM rich) areas can be observed that evidence an increased demixing between P3HT and PCBM. In addition, the intensity of the PCBM reflection rings in the SAED increases slightly. Despite the increased demixing, the image does not show any evidence of the large PCBM crystals (up to several microns) that were previously observed upon annealing of mixed MDMO-PPV/ PCBM films where demixing is much more pronounced.29 These results point to an important characteristic of the P3HT/PCBM blend. The crystallinity of P3HT is improved upon annealing and the demixing between the two components is increased, but large-scale phase separation does not occur. The resulting interpenetrating networks composed of P3HT crystals with high aspect ratio and aggregated nanocrystalline PCBM domains provide continuous pathways in the entire photoactive layer for efficient hole and electron transport. To rationalize the changes in morphology, we infer that during annealing, the already existing shorter fibrillar-like P3HT crystals in their pristine film, in combination with their continuative growth up into longer fibrils upon annealing, build borders that hamper the extensive diffusion of PCBM molecules and their large-scale crystallization. This limitation does not occur in MDMO-PPV/PCBM films because the MDMO-PPV does not crystallize.29 Consequently, in mixtures with P3HT only small PCBM crystals are formed during the annealing treatment (dark domains in Figures 3a and b, and area between the P3HT fibrillar-like crystals), while large crystals are formed in MDMO-PPV. Furthermore, since the growth of P3HT is preferably in one direction and finally leads to highly elongated fibrillar-like crystals, the increased crystallinity does not significantly reduce the interface area with the electron acceptor PCBM. More favorably, the increased length of the crystals enhances the formation of a P3HT network within the composite film. The percolation for charge carriers between the PCBM domains is established via PCBM nanocrystals that fill the space between the P3HT network and form a continuous pathway for electron transport, respectively. The nanowires in Figure 3 bear strong resemblance to P3HT whiskers.26,28 The molecular arrangement of P3HT (single-whisker) in such elongated crystals has been studied in detail by Smith et al. by using X-ray and electron diffraction.28 Remarkably, the P3HT chains in these fibrillar crystals are normal to the long axis of the crystal and typically folded with a fold period of 15 nm. In other words, the π-π stacking direction (b axis) coincides with the long axis of the crystal. The thickness of the whiskers was found to scale linearly with the side-chain length and was 2 or 3 times the a/2 lattice dimension of the unit cell (for P3HT a ) 3.36 nm).28 Recent AFM studies on fibrillar P3HT crystals 582
confirmed these dimensions, revealing lengths of 0.2-5 µm, heights of 3-7 nm, and widths of approximately 15 nm.30,31 Interestingly, conductance measurements along single nanofibers, yielded hole mobilities as high as 0.06 cm2 V-1 s-1, consistent with charge transport along the π-π stacking direction.30 Clearly the extensive formation of such elongated, high-mobility fibrillar crystals can be expected to enhance the performance of P3HT/PCBM solar cells considerably. The TEM results allow a moment’s speculation on how the morphology creates the conditions for the high monochromatic external quantum efficiencies of the annealed devices (Figure 1a). As evidenced by the TEM images as shown in Figure 2, the P3HT/PCBM network created during the two preparation steps consists of a hierarchical architecture at different length scales. First, pronounced, long fibrillar crystals create the frame and “main roads” of the network. Second, relatively short P3HT crystals connect to this frame. Third, low order crystalline or even amorphous P3HT forms the fundamental heterojunction with PCBM nanocrystals all over the entire composite film. This hierarchical network offers a maximum interface area between the constituents for efficient exciton dissociation and efficient and continuous pathways for electron and hole transport. An additional advantage of the thermal annealing treatment at temperatures of 120 °C is that the resulting morphology is stabilized and provides conditions for long-term performance under ambient conditions. Indeed, preliminary studies (as shown in the inset of Figure 1b) demonstrate that the lifetime of the devices based on this composite exceeds 1000 h at 70 °C under 1 sun illumination. In contrast, mixed MDMO-PPV/PCBM films show extensive phase separation at such temperatures, which seriously limits the device performance.29,32 In summary, we were able to demonstrate that crystallization and demixing induced by thermal annealing controls the nanoscale organization of P3HT and PCBM in the photoactive layer toward a morphology in which both components have a large interfacial area for efficient charge generation and have attained crystalline order that improves charge transport. We infer that the long, thin fibrillar crystals of P3HT in a homogeneous nanocrystalline PCBM layer are the key to the high device performance, because they are beneficial for charge transport and control the degree of demixing. We note that the crystallization-demixing process observed is not limited to P3HT and PCBM, but will likely apply to other material combinations that favor crystallization. Hence, for future optimization of the performance of polymer solar cells, a similar nanoscale organization via crystallization-demixing can be one of the design parameters in addition to an appropriate optical absorption, efficient electron transfer, and high charge carrier transport. Acknowledgment. The work forms part of the research program of the Dutch Polymer Institute (DPI), projects DPI #324, 325, 326. This work is also partly supported from the Dutch Ministries of EZ, O&W, and VROM through the EET program (EETK97115). We thank Prof. J. C. Hummelen (University of Groningen, The Netherlands) for a generous gift of PCBM. Nano Lett., Vol. 5, No. 4, 2005
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