Time-Dependent Morphology Evolution by Annealing Processes on Polymer:Fullerene Blend Solar Cells

Transcription

1 Time-Dependent Morphology Evolution by Annealing Processes on Polymer:Fullerene Blend Solar Cells By Jang Jo, Seok-Soon Kim, Seok-In Na, Byung-Kwan Yu, and Dong-Yu Kim* Changes in the nanoscale morphologies of the blend films of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM), for high-performance bulk-heterojunction (BHJ) solar cells, are compared and investigated for two annealing treatments with different morphology evolution time scales, having special consideration for the diffusion and aggregation of PCBM molecules. An annealing condition with relatively fast diffusion and aggregation of the PCBM molecules during P3HT crystallization results in poor BHJ morphology because of prevention of the formation of the more elongated P3HT crystals. However, an annealing condition, accelerating PCBM diffusion after the formation of a well-ordered morphology, results in a relatively stable morphology with less destruction of crystalline P3HT. Based on these results, an effective strategy for determining an optimized annealing treatment is suggested that considers the effect of relative kinetics on the crystallization of the components for a blend film with a new BHJ materials pair, upon which BHJ solar cells are based. 1. Introduction Conjugated-polymer-based solar cells have been actively studied as low-cost alternatives for renewable energy sources. Of the many candidate materials, bulk-heterojunction (BHJ) structures composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl- C 61 -butyric acid methyl ester (PCBM) have been the most extensively investigated. This pair of materials has many advantages over other materials, including light-harvesting and charge-transport properties. [1] Moreover, the most outstanding advantage of this materials pair is that an ordered BHJ morphology with interpenetrating nanoscale networks can be formed by simple external treatments, such as annealing processes. [2,3] More specifically, high-temperature thermal annealing increases the opportunity for the polymer solar cells [*] Prof. D.-Y. Kim, J. Jo, S.-I. Na, B.-K. Yu Heeger Center for Advanced Materials Department of Materials Science and Engineering Gwangju Institute of Science and Technology 1 Oryong-Dong, Buk-Gu, Gwangju (Republic of Korea) kimdy@gist.ac.kr Prof. S.-S. Kim School of Materials Science and Chemical Engineering Kunsan National University Kunsan, Chonbuk, (Republic of Korea) DOI: /adfm to exceed a power conversion efficiency (h eff ) of 5%. [4] Thermal annealing above the glasstransition temperature (T g ) of P3HT enables crystallization and interdiffusion of the two components, leading to nanoscale phase separation of P3HT:PCBM blends. [5] In addition to thermal annealing, the solventannealing treatment can help the evolution of the nanoscale BHJ morphology as well. [6] This treatment enables a blend film with residual solvent to self-organize and solidify into a complete film with a well-ordered morphology during slow evaporation of the solvent. Recently, it was reported that polymer solar cells fabricated by using a new method without annealing treatments showed h eff of up to 3.6%. [7] However, at the present time, either thermal or solvent annealing seems to be necessary to fabricate high-efficiency P3HT:PCBM solar cells through the formation of an optimized BHJ morphology. These treatments usually help in the formation of an ordered morphology via two evolution processes: i) crystallization or chain stacking of P3HT and ii) diffusion and aggregation of PCBM molecules into the blend. [8] In fact, the thermal annealing of P3HT:PCBM blend films has been used for improving device efficiencies via systematic studies to find the optimized temperatures and annealing times in work done by several research groups. [9] Notably, the highly efficient devices came from a nanoscale phase separation of the two components with an optimum periodic length of approximately 20 nm. [3] Moreover, an enormous increase in device efficiency because of the optimized morphology due to nanoscale phase separation could be obtained by high-temperature thermal annealing of a completed device after evaporation of a metal top electrode, that is, postproduction thermal annealing. [4,5] Since the relatively high temperature (150 8C) can easily induce a degradation of morphology because of overgrowth of the PCBM clusters, postproduction annealing can impose thermal stability on the morphology evolution by suppressing extended growth of PCBM beyond a proper size. Because the two evolution processes in the formation of the ordered blend morphology have different kinetics under the same external conditions, the annealing treatments should be carefully controlled under proper conditions to segregate each component in an optimum length scale. Here, we demonstrate that well-ordered morphology evolution for high-performance BHJ solar cells depends on a relative time scale in which the P3HT crystallizes and PCBM diffuses and aggregates into the blend films. The results were investigated by 866 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2009, 19,

2 comparative studies between the high-temperature thermal annealing and the controlled growth rate of the blend films. Each of these has a different driving force for each process of the morphology evolution, although both treatments contribute to the formation of the well-ordered nanoscale morphology of P3HT:PCBM blends. Generally, it is reported that P3HT crystallizes more easily and more rapidly than PCBM, [2] implying that the rate of PCBM diffusion and aggregation is a critical factor for determining a kinetic driving force of the annealing treatments for the formation of the ordered BHJ morphology. 2. Results and Discussion To investigate the time-dependent morphology evolution, we comparatively analyzed two different annealing conditions: hightemperature thermal annealing and room-temperature solvent annealing. In general, thermal annealing in P3HT:PCBM blend solar cells is carried out at C, which was found by several research groups to yield a high device efficiency. [8] Also, timing of the annealing process, before or after the deposition of a metal electrode, influenced the results. Post-production thermal annealing can improve the evolution of well-ordered nanoscale morphology because of a limitation of PCBM overgrowth, that is, the confinement effect. [8] However, to focus this study on the effect of PCBM diffusuon and aggregation on the morphology evolution, we conducted the thermal annealing at a relatively high temperature (150 8C) before the deposition of the metal electrode. In addition, all P3HT:PCBM films were fabricated with blend solutions based on 1,2-dichlorobenzene (DCB), which has a relatively high boiling point (180 8C), because spin-coated films must have some remaining solvent for controlling the growth rate of the films. The solvent-annealing time was controlled by a weak flow of N 2 gas (not annealed) over the films and by keeping the films outside (5 min) or inside (30 min) a covered glass jar directly after spin coating. To retard the growth rate of the films (60 or 120 min), a small amount of DCB solvent was added inside the insulated glass jar with the deposited blend films. Details of the blend-film fabrication procedures and subsequent annealing processes are described in the Experimental Section. As previously mentioned, the annealing treatments of P3HT:PCBM blend films aid the evolution of a well-ordered morphology through two processes: first, the crystallization of P3HT, and second, the diffusion and aggregation of PCBM molecules. The crystallinity of P3HT in the blend films can be evaluated by X-ray diffraction (XRD) measurements. Figure 1a shows changes of the relative XRD intensities at the specific position (2u ¼ 5.48), corresponding to the a-axis orientation of the P3HT crystallite, [10] as a function of thermal- and solventannealing times. The relative peak intensities at 2u ¼ 5.48 were estimated by considering a peak intensity at a position (2u ¼ 308) not affected by P3HT and PCBM crystals in the XRD spectra of the P3HT:PCBM blend films as a base (zero), and by setting the highest recorded intensity (the intensity at 2u ¼ 5.48 in a sample after solvent annealing for 120 min) as a maximum (100). The XRD spectra after thermal- and solvent-annealing treatments with different annealing times are shown in the Supporting Information (Figs. S1 and S2). In comparing the XRD peaks of the 120 min films with those of blend films annealed for 5 min, the degrees of crystallinity were quite similar because the two Figure 1. a) Variation of XRD peak intensity at 2u ¼ 5.48, corresponding to the a-axis orientation of the P3HT crystallite, in P3HT:PCBM blend films after thermal (150 8C) and solvent-annealing treatments with different annealing times. b) Normalized UV-vis absorption spectra of P3HT:PCBM films after thermal and solvent annealing for 30 min, compared to a film before the annealing processes. The annealing times (30 min each) were considered as the nearly saturated conditions for P3HT crystallinity from XRD measurements. same-thickness films show comparable intensities at this position. However, in the case of solvent annealing, the peak intensity increased further after 5 min, whereas films annealed at the high temperature showed nearly saturated peaks after further annealing treatments. Considering that the two blend films included almost the same amount of P3HT chains, it is obvious that the films fabricated by thermal annealing have room for further P3HT crystallization, that is, they are far from the optimized BHJ morphology. UV-vis absorption studies confirmed that P3HT in the thermally annealed films was insufficiently crystallized compared to the films fabricated by solvent annealing. Generally, a higher degree of P3HT crystallinity can be represented by a relatively large red-shift of the spectrum in the wavelength range of P3HT absorption, because of an increase of effective conjugations, and by clearer vibronic shoulders at the longer-wavelength side of an absorption maximum due to an enhanced interchain interaction. [11] When each film was annealed by the respective annealing method for 30 min, the blend film formed by solvent annealing showed a larger red-shift and more apparent vibronic features near the wavelengths of 550 and 600 nm than the thermally annealed film did (Fig. 1b). Thermal annealing of pristine P3HT films at 150 8C was commonly used for improving crystallization of the chains, resulting in enhanced hole mobility for application in organic Adv. Funct. Mater. 2009, 19, ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 867

3 Figure 3. a) An optical microscopy image (200) and b) a magnified transmission electron microscopy (TEM) image (720) of the P3HT:PCBM blend film after controlled solvent annealing for 120 min. Figure 2. Optical microscopy images (200) of P3HT:PCBM blend films after thermal-annealing treatment with different annealing times from 1 to 60 min. field-effect transistors. [12] Considering that P3HT readily self-organizes and crystallizes under external annealing processes, the insufficient crystallinity of P3HT after thermal annealing under our experimental conditions could be attributed to another component (PCBM) of the blend films that was affected by the treatments. Swinnen et al. demonstrated that large PCBM clusters in blend films can rapidly grow up to a few hundred micrometers in length under a condition that accelerates PCBM diffusion, such as high-temperature annealing. [13] They also mentioned, considering the use of the blend films in BHJ solar cells, high-temperature and long-term thermal annealing can result in a poor morphology and a subsequent decrease in device performances. Therefore, to investigate diffusion and aggregation of PCBM molecules as a function of annealing time, we examined changes in the optical microscopy images of the blend films. As shown in Figure 2, micrometer-sized PCBM clusters begin to appear after 5 min of thermal annealing at 150 8C. The number and size of the clusters gradually increased in the images as the annealing time increased. For quantitative analysis of the increased PCBM clusters in the films, the lateral density of the PCBM clusters (d PCBM ) in the optical microscopy images (size: 681 mm 510 mm) was estimated by the following equation: d PCBM [%] ¼ Np PCBM 100/Np total (where Np PCBM is the number of pixels occupied by PCBM clusters and Np total is the total number of pixels in the images). d PCBM gradually increased as the annealing time increased: 1.85% at 5 min, 4.03% at 10 min, 7.66% at 30 min, and 26.88% at 60 min. In contrast, solvent annealing showed no PCBM aggregations in an optical microscopy image (200), even up to 120 min (Fig. 3a); however, PCBM-rich domains at a sub-micrometer scale were found as relatively dark regions in the field-emission transmission electron microscopy (FE-TEM) images (720), as shown in Figure 3b. Comparing the time-dependent evolution of the PCBM clusters with the XRD results in Figure 1a, it was expected that P3HT crystallinity would be related to the formation of the micrometer-sized PCBM clusters because the increase in peak intensity at 2u ¼ 5.48 is decelerated after 5 min of thermal annealing. Comparing the time-dependent morphological changes in the optical microscopy images at each annealing condition, thermal annealing at 150 8C was probably a greater driving force for the growth of PCBM clusters than solvent annealing. According to this result, under our experimental conditions, solvent annealing seems to have a milder effect on morphology evolution, especially in terms of PCBM diffusion and aggregation, where extended growth of PCBM was suppressed while P3HT crystallized via self-organization. In general, nanoscale BHJ morphology based on polymer:fullerene blends is evaluated by high-resolution TEM measurements under slight defocusing conditions. [2,3,14] Figure 4 shows the nanoscale BHJ morphologies of the P3HT:PCBM blend films before and after the different annealing processes. In these images, the bright regions against the dark background reflected Figure 4. High-resolution TEM images of P3HT:PCBM blend films formed a) before and b) after thermal and c) after solvent-annealing processes. Thermal annealing was carried out at 150 8C for 30 min before deposition of a metal electrode inside a N 2 -filled glovebox. The optimized solventannealing time (120 min) was determined by the highest performance of solar cells using the active layers. The insets show the corresponding selected-area electron diffraction (SAED) patterns. 868 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2009, 19,

4 the P3HTcrystals as a result of the density difference (P3HT: 1.1 g cm 3, PCBM: 1.5 g cm 3 ). [2] When the blend films were thermally annealed at 150 8C for 30 min, slightly elongated P3HT crystals could be observed (Fig. 4b), which is different from the film s appearance before annealing (Fig. 4a). Furthermore, it is notable that films grown by solvent annealing for 120 min showed fibrillar P3HT crystals with more extended lengths and narrower widths (Fig. 4c) than the thermally annealed films. The long fibrillar P3HT crystals were broadly distributed in this image with an optimum morphology for high-performance BHJ solar cells. It was reported that the BHJ network of P3HT:PCBM blends shown by the TEM images consists of a hierarchical architecture based on three different length scales of P3HT crystals: long fibrillar P3HTcrystals, short P3HTcrystals, and low-ordered crystals. [2] In this study, the thermally annealed film showed only the short P3HT crystals in the TEM image, as shown in Figure 4b. In contrast, the film fabricated by solvent annealing (Fig. 4c) showed the long fibrillar P3HT crystals, which were connected by many P3HT crystals with relatively short lengths. The relatively high peak intensities of the XRD spectra at 2u ¼ 5.48 and larger redshift and clearer vibronic shoulders of the UV-vis absorption spectra (Fig. 1) in the blend films formed by solvent annealing compared to those from thermal annealing were probably attributed to the growth of the P3HT crystals to more extended lengths. In addition, the selected-area electron diffraction (SAED) patterns from each of the TEM images of Figure 4 show the change in the crystal nature of P3HT and PCBM with different annealing treatments (see insets of Fig. 4). In general, the SAED pattern of a well-ordered BHJ film based on the P3HT:PCBM blend exhibits two distinct diffraction rings: the outer ring corresponds to the p p stacking distance (0.39 nm) of P3HT chains, and the inner ring corresponds to the d spacing (0.46 nm) of the PCBM nanocrystals homogeneously dispersed in the film. [2,14] In Figure 4a, the SAED pattern of a blend film without annealing shows low and diffuse intensity of the P3HT ring as well as of the PCBM ring. After thermal annealing of the film, the intensities of both the inner and outer rings in the SAED patterns increased, as shown in Figure 4b. This suggests that the thermalannealing treatment enhanced the crystallization of the two materials. Also, analogous to the morphological changes observed in the TEM images, solvent annealing developed better crystallinity in P3HT and PCBM, as can be observed in the SAED pattern of Figure 4c. The intensity of the outer ring, corresponding to the fibrillar P3HT crystals, is clear enough to be distinguished from the inner ring, corresponding to the PCBM nanocrystals. When the morphological effects of the two annealing treatments were compared, it was evident that, under the conditions used, solvent annealing was a superior process to thermal annealing for the formation of nanoscaled BHJ morphology. In other words, the evolution of an optimized morphology could be achieved from a relatively mild annealing condition by suppressing PCBM diffusion and aggregation during the time taken for the development of P3HT crystals. Based on the comparative analysis of morphology changes after thermal (fast-growth condition of PCBM clusters) and solvent (slow-growth condition of PCBM clusters) annealing, it seems that the annealing condition with a high driving force for PCBM diffusion and aggregation induced a degradation of nanoscaled BHJ morphology. This was probably due to a prevention of the formation of the highly elongated fibrillar P3HT crystals by the fast diffusion and aggregation of PCBM molecules. Under the thermal-annealing conditions (at 150 8C before deposition of a metal electrode), PCBM molecules could freely diffuse into the blend films and readily form large PCBM clusters based on two possible causes: i) a sufficiently large driving force of the external treatment to quickly move PCBM molecules, and ii) a blend film with a large free volume due to randomly distributed polymer chains. First, high-temperature annealing treatments may force the fast diffusion of PCBM molecules into the blend films and subsequently interrupt evolution of the highly elongated P3HT crystals. In general, temperature is related to the diffusion coefficients (D) of the materials according to: [15] D ¼ D o exp Q d (1) RT where D o, Q d, R, and T are a temperature-independent preexponential, the activation energy for diffusion, the gas constant, and the absolute temperature, respectively. As shown in Equation 1, the magnitude of diffusion is strongly dependent on increasing temperature. Although the diffusion of relatively large PCBM molecules, compared to gaseous molecules, in the blend films is expected to be related to a conformational motion of the polymer chains, [16] the high-temperature annealing treatment may drive the PCBM molecules to self-diffuse into the blend by a thermally activated motion. The relatively fast diffusion of PCBM molecules during thermal annealing as compared to during solvent annealing will be further discussed below, along with the shape of the PCBM clusters. In addition to the large driving force for PCBM diffusion, the enlargedgrowthofpcbmcausedbythelargefreevolumeintheblend films after thermal annealing can be explained by the easy formation of PCBM clusters with micrometer size in the case of an amorphous polymer matrix, such as a poly[2-methoxy-5-(3,7-dimethyloctyloxy)- 1,4-phenylen]-alt-(vinylene) (MDMO-PPV):PCBM blend. However, in this study, high-temperature thermal annealing of the P3HT:PCBM blends also caused the large growth of PCBM clusters, although P3HTcould readily crystallize. Therefore, to investigate the effect of time balance on morphology evolution of the two components, we conducted additional thermal annealing, that is, thermal-annealing treatments on well-ordered blend films fabricated by an optimized solvent annealing time (120 min). Figure 5 shows the growth of the micrometer-sized PCBM clusters after additional thermal annealing at 150 8C as a function of time. Compared with the thermal annealing of the blend films with low-ordered P3HT (Fig. 2), the additional thermal annealing of the solvent-annealed blend films with highly ordered P3HT crystals resulted in a relatively small number and size of PCBM clusters, although the d PCBM value in the images gradually increased as the annealing time increased: 0.19% at 5 min, 0.26% at 10 min, 4.34% at 30 min, and 4.7% at 60 min. Figure 6a and b shows the annealing time-dependent variation of PCBM cluster size in the longer and shorter dimension in the optical microscopy images of Figure 2 and 5, respectively. The linear fit in the figures indicates that the growth rates of the clusters were constant with thermal-annealing time. However, Adv. Funct. Mater. 2009, 19, ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 869

5 Figure 5. Optical microscopy images (200) after additional thermal annealing (150 8C) of well-ordered P3HT:PCBM blend films. The ordered blend films were fabricated by the optimized solvent-annealing process (120 min). In the blend films, PCBM clusters were barely observed before a 10 min thermal anneal. we note that the PCBM growth in Figure 5b depends linearly not on the annealing time (t) but on the square root of the annealing time (t 1/2 ). Yang et al. showed that the kinetics of crystal growth of PCBM in the blend films is controlled by both the long-range diffusion rate of PCBM molecules and the local incorporation rate of PCBM molecules. [16] Moreover, they demonstrated that crystal growth with a relatively low diffusion rate of molecules compared to the incorporation rate, that is, a diffusion-limited system, showed the size increment linearly proportional to the square root of growth time (L / t 1=2 ) rather than to growth time (L / t). Therefore, we could identify that the highly ordered P3HTcrystals suppressed or retarded the diffusion of PCBM molecules into the blend under the thermal-annealing condition and subsequently limited the enlarged growth of the PCBM clusters. In addition to the sizes, the shapes of the PCBM clusters in Figure 5 clearly differed from those in Figure 2. Interestingly, their shape was relatively isotropic; conversely, the PCBM clusters of Figure 2 had anisotropic shapes. As shown in Figure 6c, the PCBM clusters of Figure 2 resulted in various aspect ratios from 2.8 (mean value at 10 min) to 4.5 (mean value at 60 min). However, the additional thermal annealing of the blend films with highly ordered P3HT crystals showed relatively small aspect ratios (1.5 at all annealing times) of the PCBM clusters, with values close to those of a regular square or circle. This result is also consistent with the result of Yang et al., who demonstrated that less elongated (i.e., isotropic) PCBM clusters could be obtained under a diffusion-limited condition. [16] The limited growth of PCBM clusters under this condition could contribute to the improved morphological stability of the BHJ solar cells. The morphological stability could also be investigated by TEM studies after thermal annealing with already-ordered blend films. As shown in the TEM images in Figure 7, P3HT:PCBM blend films thermally annealed at 110 and 150 8C for 30 min showed fewer changes in their long fibrillar P3HT crystals than in the state before annealing (Fig. 4c), an observation also confirmed by considering the nearly unchanged SAED patterns of both images of Figure 7a and b. Because the conformational motion of P3HT Figure 6. Time-dependent variations of the PCBM cluster size, measured in the optical microscopy images of a) Figure 2 and b) Figure 5, in the longer and shorter dimensions, and c) their aspect ratios. Note that the cluster sizes were plotted against the annealing time (t) in (a) and against t 1/2 in (b). The solid lines are the linear fit to the mean values. can be limited in the highly ordered blend films because of the small free volume, the diffusion and aggregation of the PCBM molecules by thermal annealing seems to be minimized. In spite of this, the PCBM molecules that remained outside the P3HT crystalline domain may have aggregated with each other. The clusters with relatively small sizes in Figure 5 are regarded as thermally grown PCBM clusters unable to participate in the well- 870 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2009, 19,

6 Figure 7. High-resolution TEM images of well-ordered P3HT:PCBM blend films after additional thermal annealing at a) 110 and b) 150 8C for 30 min. The insets show the corresponding SAED patterns. ordered BHJ morphology. This speculation indicates the importance of the P3HT:PCBM blend ratio used in the active layer for the high-performance BHJ solar cells; this is because the higher content of PCBM, in excess of the proper amount for the formation of a nanoscale interpenetrating network with the P3HT crystalline domain, can readily induce the large growth or aggregation of PCBM molecules by external driving forces, such as thermal annealing. Although the PCBM aggregates hardly affect the P3HT crystalline domain and well-ordered BHJ morphology, they can cause defects in the electronic properties, such as charge transport, when the blend films are applied to solar-cell devices. [17] Figure 8 shows the device performances of BHJ solar cells with P3HT:PCBM active layers fabricated by several time-dependent annealing treatments. In Figure 8a, the current density voltage ( J V ) curve of the devices fabricated without annealing treatments shows relatively poor performance: the open-circuit voltage (V oc ) was 0.66 V, the short-circuit current density (J sc ) was 4.39 ma cm 2, the fill factor (FF) was 0.38, and h eff was 1.09%, as calculated from the equation h eff ¼ FF V oc J sc /P in (where P in is the incident power of solar radiation on the device). After thermal annealing of the active layer at 150 8C for 30 min, the device performance was slightly improved (V oc ¼ 0.63 V, J sc ¼ 9.95 ma cm 2, FF ¼ 0.49, and subsequently, h eff ¼ 3.10%) mainly because of the slightly improved blend morphology due to the increased domains of relatively short P3HT cystals, as previously demonstrated in Figure 4b. Also, devices fabricated by the slow-growth (solvent-annealing) method for 120 min showed a relatively high device efficiency: V oc ¼ 0.60 V, J sc ¼ ma cm 2, FF ¼ 0.63, and h eff ¼ 4.14%. Although the device showed the highest efficiency among all devices based on the different annealing treatments because of superior nanoscale BHJ morphology, as shown in Figure 4c, higher efficiencies than this may be obtained via further optimization of other variables affecting device performance, such as the thickness of the active layer and the blend ratio of components. The incident photon-to-current conversion efficiency (IPCE) spectra of the devices also confirmed the increases in the efficiencies after the annealing processes (Fig. 8b). In particular, red-shifts of the IPCE spectra, which are consistent with the UV-vis absorption spectra of the blend films (Fig. 1b), indicated that the increased J sc in the J V curves was mainly due to an enhanced P3HT crystallinity and subsequent increased charge-carrier mobility. Figure 8. a) Current density voltage ( J V ) curves and b) incident photonto-current conversion efficiency (IPCE) spectra of P3HT:PCBM BHJ solar cells based on various annealing processes. All thermal-annealing treatments were carried out on a digital hotplate for 30 min before deposition of a metal electrode, and the solvent-annealing time was 120 min. All annealing processes were carried out in a N 2 -filled glovebox. Devices based on the active layers fabricated by additional thermal annealing showed slightly reduced efficiencies, as shown in the J V curves and IPCE spectra. The additional 30 min thermal anneal resulted in the following device performances: V oc ¼ 0.61 and 0.60 V, J sc ¼ and ma cm 2, FF ¼ 0.59 and 0.54, and subsequent h eff ¼ 3.98 and 3.27% at 110 and 150 8C, respectively. Considering the formation of the small PCBM clusters after additional thermal annealing, as shown in optical microscopy images (Fig. 5), the slight reduction in the efficiencies, especially J sc and FF, can be attributed to increased series resistance and an imbalance of charge transport caused by electrical defect sites on the charge transport routes as a result of the enlarged growth of PCBM. The morphological stability of the thermal annealing may be further improved by control of the P3HT:PCM blend ratio. In addition, before and after additional thermal annealing, the devices showed relatively identical V oc values ( V), although h eff decreased as the annealing condition became stronger. Typically, V oc is determined by the energy difference between the highest occupied molecular orbital (HOMO) level of the electron donor (P3HT) and the lowest unoccupied molecular orbital (LUMO) level of the electron Adv. Funct. Mater. 2009, 19, ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 871

7 acceptor (PCBM). [18] Also, it can be expected that the HOMO level of P3HT shifts upward and V oc decreases as P3HT crystallinity increases, because a P3HT film with higher crystallinity has a reduced bandgap compared to a film with lower crystallinity. This correlation of change of V oc with polymer crystallinity is supported by the previous work of Vandewal et al. [19] They demonstrated that an improved crystallinity of the polymer caused a reduction of the effective bandgap determined from the energetic onset of the charge-transfer band and resulted in a proportional reduction of V oc. As shown by XRD, UV-vis absorption, and SAED patterns by high-resolution TEM studies, solvent annealing resulted in higher crystallinity of P3HTthan thermal annealing in these experimental conditions. On the basis of the origin of V oc in the BHJ solar cells, the relative changes of the values with different annealing processes (pristine: 0.66 V, thermal annealing: 0.63 V, solvent annealing: 0.60 V) were consistent with the degree of P3HT crystallinity previously mentioned in Figure 1. These results were confirmed by the individual devices over 10 groups, which showed the coherent result that V oc gradually decreased as P3HT crystallinity increased. Therefore, the relatively stable V oc values of devices based on additional thermal annealing indicates that the diffusion and aggregation of the PCBM molecules in the wellordered morphology after thermal annealing hardly affected P3HT crystallinity but maintained the ordered morphology, as previously demonstrated in Figure 7. As a result of unaffected P3HT crystal domains, IPCE spectra after additional thermal annealing at 110 and 150 8C showed consistent spectrum shapes without any wavelength shifts or changes of vibronic shoulders, although the IPCE spectra values slightly decreased in overall wavelength ranges. Clearly, annealing processes can help the formation of a wellordered morphology via crystallization and nanoscale phase separation of the components in the BHJ solar cells. Recently, it was reported that blend films based on new polymers for application in BHJ solar cells revealed unsuccessful device performances after conventional annealing processes were used for the P3HT:PCBM blend solar cells, in spite of these films having better optical and electrical properties than P3HT. [20 22] These results seem to be mainly attributed to the undesirable evolution of BHJ morphologies. Interestingly, some research groups also reported that blend films using conjugated polymers with different intrinsic properties, such as molecular weight and regioregularity, demand different annealing conditions for the formation of the well-ordered BHJ morphology. [23,24] These results may be attributed to ineffective combinations between the crystallization kinetics of the polymer and PCBM, as previously mentioned. For the evolution of a well-ordered blend morphology with new BHJ materials, we suggest a practicable strategy that considers the effect of relative kinetics on the crystallization of the components. First of all, a fullerene derivative as an electron acceptor, constituting the blend morphology with a polymer as an electron donor, should be selected by a detailed investigation of crystallization abilities. In general, fullerene derivatives such as PCBM have a relatively slow rate of crystallization because of an introduction of solubilizing substituents. [25] Therefore, various modifications of the solubilizing moiety in fullerene derivatives for the application of BHJ solar cells can provide a feasible solution for controlling the crystallization rates of fullerene molecules. For a morphology evolution with new polymers, it may be critical to develop new fullerene derivatives with relatively slow rates of crystallization compared to the polymers under external treatment. Furthermore, the treatment condition should be carefully controlled by detailed studies of relative time scales in crystallization of the two components. The polymers should form a crystalline network frame or boundary under the condition that least affects the diffusion and aggregation of the fullerene derivatives. The optimum condition can be determined by individual examination of the time-dependent morphological changes of each component in the blend film after several annealing processes. If micrometer-scale fullerene clusters grow before the long-range ordered fibrillar crystals and the nanoscale networks of the polymer are completely formed, an evolution of a well-ordered BHJ morphology may be suppressed. In addition, imperfect crystallization of the polymer under a non-optimized condition may further degrade a blend morphology during the ongoing external treatment because it can accelerate diffusion and enlarged growth of the fullerene molecules because of a large free volume of the blend films. However, controlled annealing, considering the crystallization kinetics of the two components, can reduce the degradation of the BHJ morphology by forming the stable network frame of the polymer crystals before the overgrowth of the fullerene clusters. Finally, for a morphology evolution toward high-performance solar cells, a polymer:fullerene ratio in the three-dimensional blend films with confined volume should also be optimized to prevent large aggregations of the remaining fullerene molecules outside the well-ordered BHJ morphology. The optimized blend ratio will further improve the morphological stability of well-ordered blend films by excluding the undesirable growth of fullerene molecules caused by additional annealing treatments. 3. Conclusions Morphological changes in P3HT:PCBM blend films for the application of BHJ solar cells were compared and evaluated under different annealing conditions over a time scale relevant for PCBM diffusion and aggregation; high-temperature thermal annealing (150 8C) was a strong condition, and room-temperature solvent annealing was a relatively mild condition. Comparing with the morphological changes of the blend films after the two annealing treatments, solvent annealing resulted in a more favorable BHJ morphology than thermal annealing. The poor BHJ morphology after thermal annealing under these experimental conditions was attributed to the relatively fast diffusion and aggregation of the PCBM molecules during P3HT crystallization, which interfered with the growth of the elongated fibrillar P3HT crystals and subsequent evolution of the wellordered BHJ morphology. However, additional thermal annealing of the blend films after the formation of the well-ordered morphology by an optimized solvent annealing showed a relatively stable morphology, an observation confirmed by TEM images, SAED patterns, and device performances. The enhanced stability was attributed to the already-formed network frame of P3HT, which suppressed the diffusion and aggregation of the PCBM molecules. The results suggest an effective strategy that determines an optimized annealing treatment for the application 872 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2009, 19,

8 of a blend film with a new BHJ materials pair for BHJ solar cells. It is expected that the annealing condition can be optimized by comparative investigation of time-dependent morphological changes of each component under several annealing conditions with different kinetic driving forces for the components. 4. Experimental Device Fabrication: For the fabrication of BHJ solar cells, all materials were used without further purification. The indium tin oxide (ITO)-coated glass substrate was first cleaned by ultrasonic treatment with detergent (Mucasol, Merz), and subsequently with deionized water, acetone, and isopropyl alcohol. Each step was accompanied by N 2 -gas blowing to dry the residual solvent. After complete drying of the ITO glass in a vacuum oven for several hours, UV-O 3 treatment was applied for 30 min before deposition of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Baytron P VPAI4083). The PEDOT:PSS was spin-coated (5000 rpm, 60 s) on the hydrophilic ITO glass and subsequently dried at 140 8C for 10 min in air before transporting the films into a N 2 -filled glovebox for further device-fabrication processes. The thickness, as measured with a surface profiler (Kosaka ET-3000i), was nearly 25 nm. Blend films of P3HT (4002-E, purchased from Rieke Metal Inc.) and PCBM ([60]PCBM, purchased from Nano-C) were used for the active layer of the BHJ solar cells. The P3HT:PCBM weight ratio was fixed at 1:1. P3HT and PCBM (20 mg each) were dissolved in 1 ml of DCB and stirred at 60 8C for 12 h in a N 2 -filled glovebox. The active layers of each device were formed by spin coating (700 rpm, 60 s), with the blend solutions passing through a 0.2 mm PTFE filter on the PEDOT:PSS layer. The thickness of the active layer was measured as 180 nm by the surface profiler. For the devices fabricated by the solvent-annealing process, the growth rate of the blend films was controlled by keeping the spin-coated films in a small, capped glass jar to protect against fast solvent evaporation and penetration of outside N 2 gas into the glovebox. Small amounts of solvent (DCB) were added into the jar for retardation of the evaporation rate. In addition, the thermal-annealing process was performed on a digital hotplate in the glovebox, and, after annealing, the films were directly cooled by placement on a metal plate. Metal top electrodes (Ca/Al) were deposited on the active layers by a thermal evaporation method under high vacuum (<10 6 torr; 1 torr ¼ Pa) with two kinds of shadow masks, and these areas ( and cm 2 ) were defined as the photoactive areas of the devices. Characterization of Device Performances: The J V curves were measured with a Keithley 4200 source measurement unit. The solar cell devices were illuminated at an intensity of 100 mw cm 2 from a 1 kw Oriel solar simulator with an AM 1.5G filter in a N 2 -filled glovebox. For accurate measurement, light intensity was calibrated by a radiant power meter and a reference silicon solar cell certified by the National Renewable Energy Laboratory (NREL). IPCE spectra were measured with a tungsten quartz halogen light source, a monochromator, filters, reflective optics (to provide monochromatic light), a mechanical chopper (to modulate the light), and a transimpedance amplifier (to provide the test device signal to the digital signal processing equipment). Reference silicon photodiodes, calibrated for spectral response and traceable to NIST (National Institute for Standards and Technology), were used for monochromatic power-density calibrations. Optical and Morphological Characterization: P3HT:PCBM blend films for UV-vis absorption measurements were prepared on a glass substrate using the same method as that for device fabrication. The absorption spectra were obtained using a Perkin Elmer Lambda 12 UV-vis spectrophotometer. Optical microscopy images were measured by the Zeiss Axiovert 200M microscope (Germany) with the same films as those used for the UV-vis absorption measurements. For XRD measurement, the blend films were spin-coated on SiO 2 /Si wafers (2 cm 2 cm) with the same solution as that used for device fabrication. The thickness and area of the films obtained through different annealing conditions were all identical. For a more accurate comparison of the relative peak intensities at the specific position (2u ¼ 5.48), XRD spectra of the different samples were obtained by out-ofplane XRD spectrometry (Rigaku, RINT 2000) under the same condition. For the preparation of samples for use in TEM imaging measurements, the freestanding P3HT:PCBM active layers of completed devices were floated on a water surface and transferred to a Cu grid. The samples were subsequently dried at a low temperature and kept under vacuum conditions overnight to exclude any morphological effects of unwanted thermal annealing. The images and SAED patterns were obtained from a field-emission TEM (JEOL JEM-2100F) operated at 200 kv under slight defocusing conditions. Acknowledgements This work was financially supported by the Ministry of Education of Korea through the Brain Korea 21 (BK21) program, the National Research Laboratory (NRL) Program of the Korea Science and Engineering Foundation (KOSEF) funded by the Korean government (MOST) (M J ), and the Heeger Center for Advanced Materials (HCAM). TEM measurements were performed at the Korea Basic Science Institute (KBSI) in Daejeon. 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