New Insights into Morphology of High Performance BHJ Photovoltaics

Sep 3, 2014 - Department of Polymer Science and Engineering, University of Massachusetts, ... Materials Sciences Division, Lawrence Berkeley National ...
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New Insights into Morphology of High Performance BHJ Photovoltaics Revealed by High Resolution AFM Dong Wang,† Feng Liu,*,‡,§ Noritoshi Yagihashi,⊥ Masafumi Nakaya,⊥ Sunzida Ferdous,‡ Xiaobin Liang,† Atsushi Muramatsu,⊥ Ken Nakajima,*,† and Thomas P. Russell*,†,‡,§ †

WPI−Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, 2-1-1 Katahira, Aoba, Sendai 980-8577, Japan Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States § Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan ‡

S Supporting Information *

ABSTRACT: Direct imaging of the bulk heterojunction (BHJ) thin film morphology in polymer-based solar cells is essential to understand device function and optimize efficiency. The morphology of the BHJ active layer consists of bicontinuous domains of the donor and acceptor materials, having characteristic length scales of several tens of nanometers, that reduces charge recombination, enhances charge separation, and enables electron and hole transport to their respective electrodes. Direct imaging of the morphology from the molecular to macroscopic level, though, is lacking. Though transmission electron tomography provides a 3D, real-space image of the morphology, quantifying the structure is not possible. Here we used high-resolution atomic force microscopy (AFM) in the tapping and nanomechanical modes to investigate the BHJ active layer morphology that, when combined with Ar+ etching, provided unique insights with unparalleled spatial resolution. PCBM was seen to form a network that interpenetrated into the fibrillar network of the hole-conducting polymer, both being imbedded in a mixture of the two components. The free surface was found to be enriched with polymer crystals having a “face-on” orientation and the morphology at the anode interface was markedly different. KEYWORDS: Organic Photovoltaic, Morphology, Atomic Force Spectroscopy, X-ray Scattering, Mechanical Properties

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description of the morphology from one electrode interface to the other with molecular resolution is lacking. Imaging of the BHJ active layer morphology at the electrode interfaces and in the interior of the active layer using methods that are complementary to electron microscopy and scattering techniques is needed to understand the interplay between the morphology and device function. This is particularly important because the morphologies of the active layers are generally kinetically trapped, far removed from any equilibrium structure. Using a well-studied, high-performance, low band gap polymer poly[[4,8-bis[(2-ethylhexyl)- oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) as the donor material and (6,6)-phenyl C71-butyric acid methyl ester (PCBM) as the acceptor material (chemical structures shown in Scheme 1),9,22,23 we performed high-resolution AFM studies, in the tapping and nanomechanical modes, visualizing the surface and internal morphologies with sub-10-nanomater resolution.

olymer-based bulk heterojunction (BHJ) organic photovoltaics (OPV) hold promise as a potential renewable and economically viable energy source.1−4 Tremendous efforts have been devoted to develop new materials and optimize device performance to achieve a higher power conversion efficiency (PCE).5−7 PCE > 9% can now be routinely achieved for singlelayered devices and there is potential for further increases without new syntheses.8−11 Co-continuous domains of the electron and hole conductor, with characteristic length scales of several tens of nanometers, are necessary to reduce charge recombination, enhance charge separation, and ensure continuous pathways for electron and hole transport to their respective electrodes.12,13 From transmission electron microscopy (TEM) and resonance soft X-ray scattering,14,15 a fibrillar network of the hole conducting polymer has been reported, with a mesh size on the tens of nanometer length scale. Wideangle X-ray diffraction, small-angle X-ray, and neutron scattering have provided information on the ordering of the hole conducting polymer and aggregation of the electron acceptor,16−18 respectively, and transmission electron tomography (TEMT) has afforded insights into the 3D network structure of the hole conducting polymer.19−21 However, a 3D © 2014 American Chemical Society

Received: July 5, 2014 Revised: August 27, 2014 Published: September 3, 2014 5727

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Scheme 1. Chemical Structures of PTB7 and PC71BM

Direct evidence of a preponderance of “face-on” PTB7 crystals at the cathode interface was found. In addition, a continuous network of PCBM was observed. Distinctly different morphologies at the two electrode interfaces were found. These results provide new insights into the structure and morphology of the BHJ active layer with real-space imaging on a sub-10-nanometer level. Results and Discussions. BHJ thin films comprised of PTB7:PCBM blends, having different concentrations of PCBM (20, 60, and 80 wt %) were spin coated from solutions in chlorobenzene (CB) and CB/1,8-diiodooctane (DIO, 3 vol %) solvent mixtures. Typical high resolution AFM phase images of the surfaces of the active layers are shown in Figure 1 (and Supporting Information Figure S1 and S2) for a 60 wt % PCBM cast from CB and CB/DIO. In the CB case, large phaseseparated PCBM-rich domains, hundreds of nanometers in size, were observed, as found in previous studies.9,24,25 However, a closer look of the phase image (insert in Figure 1a and phase images in Supporting Information Figure S1) shows a morphology similar to that observed for smooth films cast from CB:DIO (phase images in Figure 1b and Supporting Information Figure S2). These results suggest that the large PCBM-rich domains are covered by a thin PTB7-rich layer with a finer scale morphology.25 By employing an ultrasharp AFM tip (∼1 nm), the tapping mode could be used to probe the surface morphology. Grains, 10−20 nm in size, of ordered PTB7 were observed where the average separation distance between adjacent chains was ∼2.03 nm. From grazing incidence X-ray diffraction (Supporting Information Figure S3), the (100) reflection of PTB7 has an average spacing of ∼1.9 nm. Consequently, the high resolution AFM shows a segregation of the PTB7 to the surface, which is expected due to its lower surface energy, and an ordering of the PTB7 into a “face on” morphology. In the case of the CB/DIO-processed PTB7/ PCBM mixtures, ordered domains of PTB7 were also evident in the phase image, whereas the height image showed that the active layer was uniform in thickness and smooth, with no evidence of large scale phase separation. When the nanomechanical mode of AFM was employed, the mechanical response of the surface was measured that, by comparison to the mechanical properties of pure components, enables identification of the components in the mixtures. The lowest Young’s modulus features arise from the PTB7 polymer, the highest from the PCBM, and the intermediate from PTB7:PCBM mixtures. Although the surface was enriched with PTB7 crystals, the PCBM aggregates (dark area in Figure 1b) provide conducting pathways for electrons. The active layer of a CB/DIO-processed film was inverse transfer printed onto a Si wafer and by dissolving the PEDOT:PSS in water,26 the interface of the active layer with the PEDOT:PSS was examined. Height and phase images of

Figure 1. Top surface morphology of PTB7:PCBM (4:6) blends processed from chlorobenzne (a) and chlorobenzene/DIO mixture (b). Inserts are phase image of 3 μm × 3 μm size; and line cut analysis of PTB7 surface crystals.

this interface are shown in Figures 2a and 2b. As seen, this interface is markedly different from the free surface. The bottom surface morphology was drastically different from the top surface. No evidence of ordered PTB7 was observed but rather a network of nanoscopically thin domains of aggregated PCBM imbedded in a PTB7/PCBM matrix was observed. Using an Ar+ beam (10 mA, 2 × 10−6 Pa) the active layer was slowly etched at a rate of ∼0.25 nm/s to expose the morphology of the active layer. The etching rates of PTB7 and PCBM under identical etching conditions were similar (With etching calibration shown in Supporting Information Figure S4). Consequently, variations in the height due to variations in the concentrations of the components would be produced. After removing 15 nm of the active layer, the CBprocessed thin film showed large PCBM-rich aggregates (Supporting Information Figure S5). At high resolution (Supporting Information Figure S5), a network structure, comprised of ∼10 nm wide PCBM domains with an average center-to-center distance or mesh size of ∼30 nm, was evident (Figure 3a). Nanomechanical AFM studies on the PTB7 and PCBM (Figure 4) show that the dark regions in phase images in Figure 3 (bright regions in Figure 4) constituting the network are PCBM or PCBM-rich. For the 60% PCBM mixture cast from CB/DIO, a similar type of network structure 5728

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structure shown in Figure 4a consists of pure PCBM, with a modulus of ∼12 GPa (Figure 4d). The matrix in which the PCBM network is embedded has a modulus of 3−7 GPa (Figure 4b), indicating that the matrix consists of a mixture of PTB7, which has a modulus of ∼1.1 GPa (Figure 4c), and PCBM. It should also be noted that the modulus of the mixed matrix was not uniform but, rather, increased in the proximity of the PCBM domain, reflecting a gradient in the composition. The average modulus of the PTB7/PCBM matrix is ∼5.8 GPa (Supporting Information Figure S10), which corresponds to ∼43 wt % PCBM. No PTB7 fibrils or crystals were resolved in this mixed matrix due to the solubility of the PCBM in the PTB7. The observation of PCBM network for the CB and CB/ DIO-processed films was unexpected, but it is consistent with the formation of a continuous electron transporting domain that is required for device performance. It should be noted that for the 60 wt % PCBM mixtures, the CB-processed thin film shows a PCBM network at the surface layer that is similar to that in the CB/DIO-processed films in the interior of the film. In both cases there is a high concentration of PCBM at the PEDOT:PSS interface. Resonance soft X-ray scattering (RSoXS) profiles at carbon K-edge,27 for the different samples are shown in Figure 5. For the CB-processed thin film, an interference at 315 nm indicates a coarse domain structure, in agreement with the AFM results. The scattering from this phase separated structure dominates the scattering, while the structure at the surface of the film is evident only as a weak interference from 0.02 to 0.04 A−1. For CB/DIO-processed thin film, when 20% PCBM was used, a weak interference was observed in the high q region (centered at ∼0.035 A−1), corresponding to a spacing of 18 nm. With the 60% PCBM blend, a broad peak center at 0.018 A−1, corresponding to a spacing of 34 nm, was observed, in keeping with the AFM results. For the 80 wt % PCBM blend, two separate interferences were observed, one centered at 0.00435 A−1 (144 nm, not seen in the AFM studies) and second centered at 0.025 A−1 (25 nm, also seen in the AFM studies). In all the scattering profiles, the peak widths were quite large, indicative of a broad distribution of length scales of the phase separated domains. Bright field TEM (Figure 6) was also used to study the internal structure of CB/DIO-processed BHJ blends. The 20% PCBM mixture was feature-less, due to the weak phase separation. A fibrillar network was clearly evident for the 60% and 80% PCBM mixtures where the mesh-sizes were consistent with the RSoXS and AFM results. The network of PCBM was not evident in the TEM data due, more than likely, to the small size of the domains and to the poor contrast between PCBM and adjoining mixed phase. From these combined studies, a conceptually new description of the BHJ morphology emerges. Two different interpenetrating, continuous networks, one of PTB7, with a large mesh size, and second of PCBM, with a smaller mesh-size, both embedded in a mixture of PTB7 and PCBM. Although the composition of this mixed phase is, on average, uniform, a gradient in the concentration of the component at the interfaces with the networks were observed. BHJ solar cells using PTB7:PCBM blends as active layers were fabricated with LiF (1.5 nm)/Al (100 nm) as the cathode. Device performances are shown in Table 1 and Supporting Information Figure S11. For 60% PCBM, CB-processed devices processed a short circuit current (Jsc) of ∼7 mA/cm2 and an

Figure 2. Morphology of bottom surface of PTB7:PCBM (4:6) BHJ blends processed from CB/DIO (a, height image; b, phase image).

was observed (Figure 3b and Supporting Information Figure S6). When the PCBM loading was decreased to 20%, a different morphology was observed. The size scale of the features decreased significantly, the network of PCBM was lost, and the crystalline fibrillar network of the PTB7 was the dominant morphological feature (Figure 3c and Supporting Information Figure S7). The difference from the 20% mixture is a direct consequence of the solubility of the PCBM in the PTB7 which was found to be ∼30%.25 For the mixture with 80% PCBM, the mesh-size of the phase separated network of PCBM decreased markedly to ∼20 nm (Figure 3d and Supporting Information Figure S7). From the Fourier transform of the AFM images (Supporting Information Figure S8), a broad size distribution of the domains, ranging from 10 to 100 nm, was found. It should be noted that CB/DIO processed thin films were uniform normal to the surface, making the results after 15 nm etching representative of the entire film morphology. In the CB processed case, we have seen large scale phase separation of PCBM, which complicates the interpretation of the results. Extended etching of CB-processed sample was performed to fully remove the top layer (∼50 nm) to penetrate into PCBM rich domains. Large aggregates were observed with no finer textured morphology (Supporting Information Figure S9). These aggregates are PCBM domains and it is quite pure as has been reported Collins et al.24 The nanomechanical mapping of the 60% PCBM mixture cast from CB/DIO is shown in Figure 4a. The network 5729

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Figure 3. Internal morphology (phase image) of BHJ thin film. (a) PTB7:PCBM (4:6) processed from CB; (b) PTB7:PCBM (4:6) processed from CB/DIO; (c) PTB7:PCBM (2:8) processed from CB/DIO; (d) PTB7:PCBM (8:2) processed from CB/DIO. Scale bar, 40 nm.

Figure 4. (a) Young’s modulus map of PTB7:PCBM (4:6) blends processed from CB/DIO; scale bar, 40 nm. (b) Line-cut analysis of image a. (c) and (d) Young’s modulus of PTB7 and PCBM pure thin film.

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due to finer length scale of the phase separated morphology. For the lowest PCBM mixture (20 wt %), a drastically reduced Jsc, was observed and the current density remained low in the high voltage regions, indicating poor transport through the film, reflective of the poorly defined morphology. The current device results clearly show that only the mixed region cannot effectively transport electrons. At a high PCBM loading (80%), Jsc decreased to 13.41 mA/cm2 and PCE decreased to 6.59%. Though the relative concentration of PTB7 was reduced by 50% (in comparison to the optimized conditions), the PCE decreased by only 14%, indicating that PCBM also plays an important role in light harvesting (see Supporting Information Figure S13 for absorption comparison.). Conclusion. In summary, using a combination of high resolution AFM, TEM, and X-ray scattering, a new conceptual picture of the morphology in BHJ active layers is suggested. This morphology consists of a network of fibrils of PTB7 with a mesh size of several tens nanometers, a second interpenetrating network of PCBM with a mesh size of several tens of nanometers, both imbedded in a mixture of PTB7 and PCBM. Devices prepared using active layers comprised of different compositions of PTB7 and PCBM had device efficiencies that correlated with the length scales of the observed morphologies. These new results reveal continuous pathways by which electrons and holes can be transported to their respective electrodes.

Figure 5. Internal morphology of BHJ thin film characterized resonant soft X-ray scattering.

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ASSOCIATED CONTENT

S Supporting Information *

Information on the experimental procedures; series of AFM supporting images; GIXD measurements, distribution of Young’s modulus; device performance; morphology model and absorption profiles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (F.L.) [email protected]. *E-mail: (K.N.) [email protected]. *E-mail: (T.P.R.) [email protected]. Notes

The authors declare no competing financial interest.

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Figure 6. TEM characterization of PTB7:PCBM blends processed from CB/DIO.

ACKNOWLEDGMENTS This work was supported by World Premier International Research Center Initiative (WPI), MEXT, Japan. F.L., S.F., and T.P.R. were supported by Polymer-Based Materials for Harvesting Solar Energy (PHaSE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under award number DESC0001087. Portions of this research were carried out at beamline 7.3.3 and 11.0.1.2 at the Advanced Light Source, Lawrence Berkeley National Laboratory, which was supported by the DOE, Office of Science, and Office of Basic Energy Sciences.

Table 1. Summary of Device Performance devices

Voc (V)

Jsc (mA/cm2)

fill factor (%)

efficiency (%)

CB-60% PCBM CB/DIO-20% PCBM CB/DIO-60% PCBM CB/DIO-80% PCBM

0.72 0.76 0.70 0.70

6.63 0.38 16.31 13.41

46.15 25.70 67.20 70.34

2.20 0.07 7.64 6.59

overall PCE of 2.20%, due to the large size scale of the PCBM aggregates.9,25 However, the short circuit current in this case was still much larger than other low band gap polymer:PCBM blends that showed similar large sized PCBM aggregates.28,29 This is because there is a thin finer phase separated BHJ layer covering the top of large PCBM domains, providing efficient light extraction at the top skin ∼15 nm of the active layer. A sketch of this morphology is shown Supporting Information Figure S12. CB/DIO-processed BHJ active layers from the 60% PCBM mixture had a Jsc of ∼16 mA/cm2 and a PCE of 7.6%,

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