pubs.acs.org/JPCL
Molecular Miscibility of Polymer-Fullerene Blends Brian A. Collins,† Eliot Gann,† Lewis Guignard,† Xiaoxi He,‡ Christopher R. McNeill,‡ and Harald Ade*,† †
Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, United States, and Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge, CB3 0HE, U.K.
‡
ABSTRACT The device function of polymer bulk heterojunction (BHJ) solar cells has been commonly interpreted to arise from charge separation at discrete interfaces between phase-separated materials and subsequent charge transport through these phases without consideration of phase purity. To probe composition, the miscibility of poly(3-hexylthiophene) (P3HT)and poly(2-methoxy-5-(30 ,70 -dimethyloctyloxy)1,4-phenylenevinylene) (MDMO-PPV) with phenyl-C61-butyric acid methyl ester (PCBM) has been determined, while the effects of polymer crystallinity on miscibility are probed using P3HT grades of varying regioregularity. It is found that, while no intercalation occurs in P3HT crystals, amorphous portions of P3HT and MDMO-PPV contain significant concentrations of PCBM, calling into question models based on pure phases and discrete interfaces. Furthermore, depth profiles of P3HT/PCBM bilayers reveal that even short annealing causes significant interdiffusion of both materials, showing that under no conditions do pure amorphous phases exist in BHJ or annealed bilayer devices. These results suggest that current models of charge separation and transport must be refined. SECTION Macromolecules, Soft Matter
performance of a device.9,10 In fact, initial studies of molecular morphology have revealed that a fullerene intercalated into polymer crystals can substantially improve device performance over nonintercalated crystals.11 Other studies have suggested that amorphous blends do not contain pure phases12,13 even after annealing.14-16 Here we probe the thermodynamic forces at play by allowing two key model polymers of poly(3-hexylthiophene) (P3HT) and poly(2-methoxy-5-(30 ,70 -dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) blended with phenylC61-butyric acid methyl ester (PCBM) to achieve equilibrium concentrations upon annealing and directly measure their composition via near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, thus creating a miscibility phase diagram for amorphous portions of the systems investigated. To examine the influence of P3HTcrystallinity, we have studied several grades of the polymer exhibiting different degrees of regioregularity (RR) and characterized their crystallinity via grazing-incidence wide-angle X-ray scattering (GIWAXS). In addition, we have measured depth profiles of annealed P3HT/ PCBM bilayers of the two materials using dynamic secondary ion mass spectrometry (SIMS) to study their interdiffusion. The results of this study show that within amorphous regions, pure phases are not thermodynamically favored. This has a direct impact on how charge separation and transport is thought to occur in OPV devices, and in light of this finding, a modified paradigm of device operation may be required.
P
hotovoltaic devices based on solution-processable organic molecules and polymers hold immense potential in providing a cheap, scalable, and renewable source of energy using environmentally friendly materials and have thus been under intense investigation over the past several years. The payoff has been an orders of magnitude improvement of their power conversion efficiency since their first demonstration,1,2 yet they are still in need of further improvement before fully replacing current technology.3-6 Because these organic photovoltaic (OPV) devices operate via separation of a short-lived exciton at a donor/acceptor interface, high interfacial area devices from blends of donor and acceptor materials in so-called bulk heterojunctions (BHJs) have resulted in much higher efficiencies than bilayer constructions. Under the current paradigm of device operation, blends create a BHJ or bicontinuous network of phase separated materials, which provide a maximized area of discrete interfaces to separate the excitons into free charges and percolation pathways in which to transport those charges to the appropriate electrodes.7 The phases are often implicitly or explicitly assumed to be pure.4 Mixed phases are thought to be counterproductive to device performance, since isolated molecules could act as traps for separated charges and eventually centers for charge recombination within the percolation pathways.8 Short exciton diffusion lengths require phase separation to be on the order of 10 nm, making interface morphology and phase purity difficult to measure, yet this may be critical as recent transient absorption spectroscopy measurements have suggested that the intimate molecular morphology and ordering of the two materials may determine the
r 2010 American Chemical Society
Received Date: September 9, 2010 Accepted Date: October 7, 2010 Published on Web Date: October 19, 2010
3160
DOI: 10.1021/jz101276h |J. Phys. Chem. Lett. 2010, 1, 3160–3166
pubs.acs.org/JPCL
Figure 1. (a) STXM image of a high-RR P3HT:PCBM blended and annealed film acquired at 284.4 eV, an absorption peak of PCBM. Dark regions are PCBM crystals. The arrow designates a typical area used to generate a spectrum. Each scan includes I0 data taken simultaneously (right 30% of the line) to avoid milli-electron volt-scale drift of the energy from temperature drift in the beamline. (b) P3HT, (c) PCBM, and (d) MDMO-PPV molecular structures. (e) Reference spectra used in fits and (f) ratio of P3HT reference spectra acquired at various incident angles, showing the effect of crystallite orientation on absorption peaks of a linearly polarized photon beam. Angles posted are those the photon electric field makes with the surface. Ratios displayed represent relative edge-on P3HT crystal orientation (see Figure 4f).
For the P3HT-based system, three grades ; noncrystallizing regiorandom [Rieke 4007], 93-95% RR [Rieke 4002], and a >98% RR [Plextronics OS 1200] ; were used. PCBM was obtained from Nano-C. MDMO-PPV was supplied by American Dye Source [ADS104RE]. With record power conversion efficiencies of 3.3% for the latter17 and 5% for the former,18,19 these systems represent some of the highest performing materials that are commercially available whose morphologies are thought to be largely optimized. Blended films were spincast onto poly(styrenesulfonate) (PSS)-coated glass substrates from chlorobenzene solutions in a N2 glovebox containing
