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Three-Dimensional Nanoscale Organization of Highly Efficient Low Band-Gap Conjugated Polymer Bulk Heterojunction Solar Cells Mark Dante, Andres Garcia, and Thuc-Quyen Nguyen* Department of Chemistry & Biochemistry and Center for Polymers and Organic Solids, UniVersity of California, Santa Barbara, California 93106 ReceiVed: October 31, 2008
Scanning probe examination on cross sections of bulk heterojunction blends containing an amorphous conjugated polymer, poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiopene)-alt-4,7-(2,1,3bezothiadiazole)], and [6,6]-phenyl C71-butyric acid methyl ester reveals a structural change in the internal features of hole and electron transporting networks when the film is cast from a solution containing 2% by volume 1,8-octanedithiol. Phase separation of the nanoscalar domains becomes more defined, and the average sizes of hole and electron transporting networks double upon addition of the additive. Devices capable of electron- and hole-only transport show no significant improvement of the charge carrier mobilities. The increase in the size of the domains likely gives rise to less charge recombination. Introduction Bulk heterojunction (BHJ) solar cells comprised of conjugated polymer donor materials and fullerene acceptors provide an excellent example of how self-assembling principles need to converge for efficient solar to electrical energy conversion.1-5 The donor and acceptor phases form three-dimensional interpenetrating networks. Large phase separation is counterproductive due to the limited exciton diffusion length, which has been estimated to be in the order of tens of nanometers.6-9 The internal organization, for example crystallinity, within the domains determines the charge carrier mobilities10-14 and thereby influences the device efficiency.15-18 It was recently shown that the addition of a small percentage of 1,8-octanedithiol to the solution from which BHJ films are spin-coated leads to solar cell power conversion efficiencies of greater than 5%.19,20 This “additive” approach circumvents the need of postdeposition processing that improve efficiencies, such as thermal or solvent annealing, and thus has the potential to greatly simplify the fabrication methods, an important consideration when comparing polymer solar cell devices to their inorganic counterparts. Recent studies have provided some insight into physical features that make good additives and have independently confirmed the improvement of device function.21,22 For example, they need to have a higher boiling point than the host solvent and need to preferentially dissolve one of the components in the BHJ system. Structural characterization of the active layer comes primarily from techniques such as atomic force microscopy (AFM) and transmission electron microscopy (TEM), which enlarge yield detailed information of surface features. The internal structure is less well understood, and it is of interest to understand how the deposition history perturbs the size, sharpness, and vertical distribution of donor acceptor domains. Surface and internal features16,23 influence internal processes such as exciton diffusion, efficiency of charge generation, charge carrier mobilities, and charge collection efficiencies, all of which * To whom correspondence
[email protected].
should
be
addressed.
E-mail:
Figure 1. Chemical structures of PCPDTBT (a) and C71-PCBM (b).
contribute in determining the power conversion efficiency of the solar cell.24-33 In this contribution we provide direct imaging of the surface and internal features of the BHJ layers that were spun with and without 1,8-octanedithiol. Focused ion beam techniques were used to prepare cross sections of BHJ films. Subsequently, the cross section sample was transferred onto an indium tin oxide (ITO) substrate for AFM and conducting atomic force microscopy (C-AFM) studies. The study system was a blend of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b’]dithiopene)-alt-4,7-(2,1,3-bezothiadiazole)] and [6,6]-phenyl C71butyric acid methyl ester (PCPDTBT:C71-PCBM), the molecular structures of which are shown in Figure 1. This binary system was chosen on the basis of the excellent performance obtained within a solar cell device.34,35 We also show the impact of the internal features on the hole and electron transport across the active layer. Experimental Section Sample Preparation. Films were cast under nitrogen from a warm chlorobenzene solution containing 0.7% w/v PCPDTBT (Konarka) and 2.45% w/v C71-PCBM (Nano-C) that was stirred overnight at 60 °C. The effect of additive was observed by examining samples prepared from solutions containing 2% 1,8octanedithiol by volume. Cross-Section Preparation. Details of thin film cross-section fabrication are published elsewhere.16,36,37 Briefly, a 300 nm thick
10.1021/jp809650p CCC: $40.75 2009 American Chemical Society Published on Web 01/08/2009
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Figure 2. Topographic (a and d), phase (b and e), and current (c and f) AFM images of PCPDTBT:C71-PCBM films cast without (top) and with (bottom) 1,8-octanedithiol. Image sizes are 5 µm × 5 µm.
SiO2 layer was first deposited on top of the organic layer using an electron beam evaporator (BOC Edwards Temescal). A focused ion beam microscope (FEI Strata DB 235) was then used to cut a thin slice (15 µm × 5 µm × 250 nm) of the sample, which was transferred onto ITO coated glass using a micromanipulator with a pulled glass pipet. Scanning Probe Measurements. All AFM measurements were performed in a glovebox under Ar using a commercial scanning probe microscope (MultiMode equipped with C-AFM module and the Nanoscope IIIa Controller, Veeco Inc.). AFM images were collected in tapping mode used silicon probes with a spring constant of 3 N/m and resonant frequency of 75 kHz (Budget Sensors). Platinum-coated silicon tips with a spring constant of 0.2 N/m and a tip radius of ca. 15 nm were used for current imaging. The bias was applied to the conducting substrate and the current was measured by a preamplifier. For each sample, images were collected on multiple locations to examine the film uniformity. Diode Fabrication and Characterization. Bulk devices were prepared and tested inside the nitrogen atmosphere of a glovebox. For hole-only diodes, the solutions were spin coated on an ITO-coated glass slide with a 40-nm layer of poly(3,4ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). A gold electrode was thermally evaporated onto the polymer: fullerene layer at an initial rate of less than 0.1 nm/s to a final thickness of ∼150 nm. For electron-only diodes, Al was thermally evaporated onto a glass slide. The polymer:fullerene solution was spin coated onto the Al substrate and Al electrodes were thermally evaporated through a shadow mask. A Keithley 2602 source-measure unit was used to measure the bulk current-voltage (I–V) characteristics. Results and Discussion Surface Characterization. Topographic, phase, and current images are included in Figure 2, which shows the surface
examination of BHJ films prepared with and without 1,8octanedithiol. These data were obtained directly from the surface of as-cast films. The film obtained from the 1,8-octanedithiol solution is rougher (rms roughness of 1.0 nm) than the film cast without additive (rms roughness of 0.5 nm). The phase images (parts b and d of Figure 2) reveal that 1,8-octanedithiol also leads to larger phase separation in the BHJ structure. That these features are due to the donor and acceptor phases and are supported by the current images (parts c and f of Figure 2), where the regions of high and low hole current are well matched with the phase contrast. This correspondence suggests that the bright and dark regions in the phase image correspond to separation at the surface of the film between polymer-rich and fullerene-rich phases, respectively. More detail of the surface features can be gained by examination of the images in Figure 3, which shows the topographic and phase images at higher magnification (250 nm × 250 nm). Little additional new information can be obtained by the topographic images (parts a and b of Figure 3). However, Figure 3d shows a higher level of organization than that observed in Figure 2e. Indeed, the originally observed two phases contain substructures corresponding to a mixture of smaller domains. Comparison of parts c and d of Figure 3 shows that the 1,8-octanedithiol treatment leads to an increase of the average domain diameter from ∼7 nm to ∼13 nm. Internal Morphology. Phase images of cross-sectional samples without and with additive are shown in Figure 4. These data provide information that was “hidden” from the surface examination in Figures 2 and 3. Indeed, they give insight into the BHJ organization throughout the bulk and more importantly along the charge transport direction toward the electrodes. That the donor and acceptor domains are larger and better separated in Figure 4b (∼8 nm), relative to Figure 4a (∼5 nm), gives unambiguous indication that the presence of 1,8-octanedithiol during the spin coating step controls the morphology throughout
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Figure 3. AFM topographic (top) and phase (bottom) images of PCPDTBT:C71-PCBM films without (a and c) and with (b and d) 1,8-octanedithiol additive. The image sizes are 250 nm × 250 nm.
Figure 4. Phase images of cross-sectional samples cut from the PCPDTBT:C71-PCBM films with (a) and without (b) 1,8-octanedithiol. The image sizes are 250 nm × 250 nm.
the film. Larger separation of the electron and hole transport networks would be anticipated to reduce charge recombination. At the same time, the size of the domains is sufficiently small that the fraction of excitons that migrate to the interface is not drastically reduced. The additive is capable of reaching the fine balance needed for optimization of optical conversion and charge collection. We also imaged the regions adjacent to the top and bottom surfaces to investigate the vertical phase segregation. Vertical phase segregation has been reported for the poly(3hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester blend (P3HT: C61–PCBM) system in which the top and bottom surfaces are enriched with P3HT and PCBM, respectively,38,39 and vice versa.40 From the AFM studies, there is no evidence of vertical phase segregation; however, if the enriched P3HT layer is only a few nanometers, it may not be possible to detect in our phase images. Deposition of P3HT:C61-PCBM films with 1,8-octanedithiol leads to a higher level of crystallinity in the conjugated polymer
domains. The better interchain registry within the ordered domains improves the hole mobilities. The hole mobility increases from 2.6 × 10-5 cm2/V · s to 1.8 × 10-4 cm2/V · s upon the addition of the 1,8-octanedithiol additive (Supporting Information). The electron mobility changes very little from 1.7 × 10-3 cm2/V · s for the control film to 1.2 × 10-3 cm2/V · s with the additive. These values are in agreement with results from a previous study.22 However there is little change in the size of the donor and acceptor domains (see Supporting Information). To examine whether similar perturbations in the charge carrier transport take place in the PCPDTBT:C71-PCBM blends as a function of additive processing, hole- and electrononly diodes were fabricated. Charge Transport. Figure 5 shows the current density (J)-voltage (V) of the single carrier devices as a function of processing history. Mobilities were calculated using the space charge limited current (SCLC) model.41-47 Most significantly, the hole mobility of the blend decreases from 1.7 × 10-5 cm2/
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Figure 5. J–V characteristics of hole-only and electron-only diodes (a and c, respectively) and corresponding fit to the SCLC model (b and d) of the PCPDTBT:C71-PCBM films with and without 1,8-octanedithiol.
V · s to 7.8 × 10-6 cm2/V · s when deposited in the presence of 1,8-octanedithiol. The electron mobility increases slightly from 1.4 × 10-3 cm2/V · s to 3.2 × 10-3 cm2/V · s for the film cast from the additive containing solution. Therefore, in contrast to the situation observed with P3HT, there is no significant improvement in mobility associated with 1,8-octanedithiol processing. Conclusions In conclusion, BHJ solar cells made from a PCPDTBT: C71-PCBM active layers have shown increased efficiency when spin coated with a small amount of 1,8-octanedithiol present in the solution. Such a processing option is important for this system, as thermal annealing leads to device deterioration. We have shown by a combination of cross-sectional and surface analysis that the additive effect leads to the formation of larger donor and acceptor domains throughout the active layer and therefore to a restructuring of the networks responsible for charge separation, charge recombination, and charge transport. The average domain sizes increase from ∼5 nm to ∼8 nm. Furthermore, the single carrier devices show no large differences in carrier mobilities. Overall, these structural and electronic features suggest that the improved performance is due to larger average separation between charge carriers and thereby lower probability of recombination. This effect stems from the spatial distribution of donor and acceptor domains rather than intrinsic differences in carrier mobilities, as in the case with the crystalline P3HT system. Also fortuitous is that the BHJ organization remains within the appropriate nanoscalar regime where excitons are sufficiently close to interfaces where charge generation takes place. These studies highlight the importance of being able to examine the three-dimensional internal structure to fully understand the effect of different processing options on the self-assembly of multicomponent polymer solar cell devices. Understanding the device improvement mechanism of 1,8-octanedithiol additive opens the possibility to apply this processing method to other materials used in organic optoelectronic devices. Acknowledgment. We thank Konarka for providing the polymer and the Office of Naval Research Young Investigator Program for financial support and Jeffrey Peet for helpful discussion. Supporting Information Available: Chemical structures of P3HT and PCBM, topographic, phase, and current images of
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