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J. Phys. Chem. C 2010, 114, 20672–20677
Imaging Local Trap Formation in Conjugated Polymer Solar Cells: A Comparison of Time-Resolved Electrostatic Force Microscopy and Scanning Kelvin Probe Imaging† Obadiah G. Reid,‡ Glennis E. Rayermann, David C. Coffey,‡ and David S. Ginger* Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195, United States ReceiVed: June 19, 2010; ReVised Manuscript ReceiVed: September 27, 2010
We study local photooxidation and trap formation in all-polymer bulk-heterojunction organic photovoltaics (OPVs) using both time-resolved electrostatic force microscopy (trEFM) and conventional scanning Kelvin probe microscopy (SKPM). We create electron-trapping defects at known locations by locally photooxidizing blends of poly[(9,9′-dioctylfluorene-alt-(bis(N,N′-(4-butylphenyl))-bis(N,N′-phenyl-1,4-phenylenediamine)] and poly[9,9′-dioctylfluorene-alt-1,4-benzothiadiazole]. We then compare the local surface photovoltage shifts measured via SKPM and the changes in local photoinduced charging rates measured via trEFM with changes in the performance of macroscopic photodiodes that have been exposed to similar photooxidation. We find that the trEFM charging rate images can identify local photooxidation and trap formation with much better sensitivity than conventional SKPM images. In addition, the changes in the trEFM charging rates correlate well with the external quantum efficiencies of the macroscopic photodiodes. In contrast, the SKPM images not only are less sensitive to trap formation but also show a more complicated response. We conclude that trEFM is well suited to studying local trap formation in organic solar cells and caution that SKPM data by itself can be difficult to interpret on OPV films, especially when materials have been exposed to photooxidation. Introduction Organic solar cells are the subject of intense academic and industrial research because they may one day enable cheap, ubiquitous, solar energy conversion.1-5 The most efficient organic solar cells to date are bulk heterojunction photodiodes, consisting of a blend of electron-donating and electron-accepting materials.6-8 Bulk heterojunction solar cells are intrinsically nanostructured devices, and it is well accepted that materials and processing can dramatically alter the nanoscale morphology, and thus performance, of bulk heterojunction organic photovoltaics.9-11 Charge trapping in these complicated blends can reduce carrier mobility and increase recombination losses, leading to decreased performance.12-16 To date, however, the nature of charge traps in organic semiconductors remains one of the most poorly understood aspects of these complex blended materials. There are reasons to suspect that trap formation may be associated with specific morphological features in blends; indeed, recent studies on pentacene and anthradithiophene have produced evidence that trapping can be as sensitive as mobility is to film morphology in transistors.17 If true, this would provide a route by which the lifetime of materials could be altered or improved via better processing. However, studies correlating local trap formation with device performance on organic solar cells are practically nonexistent, in part, because there are few methods that can image local trap formation. To address this experimental challenge, we explored the suitability of two scanning-probe techniques, time-resolved electrostatic force microscopy (trEFM) and scanning Kelvin probe microscopy (SKPM), to study the trap formation in model blends of the conjugated polymers poly[(9,9′-dioctylfluorene†
Part of the “Mark A. Ratner Festschrift”. * To whom correspondence should be addressed. Phone: 206-685-2331. Fax: 206-685-8665. E-mail:
[email protected]. ‡ Current address: National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401-3305.
alt-(bis(N,N′-(4-butylphenyl))-bis(N,N′-phenyl-1,4-phenylenediamine)] (PFB) and poly[9,9′-dioctylfluorene-alt-1,4-benzothiadiazole] (F8BT) (see Figure 1a for chemical structures). These blends were chosen not because they offer any particular performance advantages but because they represent a wellstudied model system, both morphologically18-26 and photochemically.27-30 In particular, it has been demonstrated that photooxidation of polyfluorenes leads to the formation of electron-trapping fluorenone defects.28,29,31-33 Methods for imaging charge trap formation in organic photovoltaic blends using scanning probe microscopy are not well-established. Thus, as a first step, we take the approach of intentionally creating local trap sites via photooxidation of the polyfluorene blends and comparing the resulting trEFM and SKPM images. We find that, although average values from local trEFM measurements on photooxidized samples correlate well with macroscopic device efficiency, the surface photovoltage measured via SKPM does not. The SKPM data only indirectly track local performance changes due to photooxidation, most likely reflecting the density of charge traps via the steady-state surface charge density. These results have important implications not only for future studies of nanoscale charge trapping in OPVs but also, more generally, for the growing field of scanning probe microscopy on organic electronic devices.34 Experimental Methods The devices used in this study were bulk heterojunction blends of PFB and F8BT. The polymers were obtained from American Dye Source and used as received, with molecular weights and PDI values of 23 kg mol-1 and 2.7 for the PFB and either 28 kg mol-1 and 2.2 or 30 kg mol-1 and 3.7 for the F8BT. Solutions of each polymer were prepared in a glovebox using anhydrous chlorobenzene at a concentration of 20 mg/mL. The solutions were heated for ∼4 h at 50 °C before being mixed together and filtered sequentially though 1, 0.45, and 0.2 µm PTFE filter
10.1021/jp1056607 2010 American Chemical Society Published on Web 10/14/2010
Imaging Local Trap Formation Using trEFM and SKPM
Figure 1. (a) Chemical structures of the electron donor PFB and acceptor F8BT. (b) External quantum efficiency (EQE) spectra of photochemically degraded PFB/F8BT blend bulk heterojunction solar cells spin-coated from chlorobenzene. As expected, with increasing photooxidation (red to black curves), the quantum efficiency decreases. (c) Normalized average device characteristics plotted as a function of a 405 nm photon energy dose in air, including the fill factor (FF, purple diamonds), open-circuit voltage (VOC, inverted green triangles), and EQE (black squares). Error bars represent the standard deviation of the mean.
membranes using all-polypropylene syringes. The blend solution was mixed and allowed to stand at 50 °C for ∼1 h before spincoating at 2000 rpm for 2 min. The substrates comprised 15 × 15 mm squares of ITO-coated glass (TFD Inc.) coated in ∼40 nm of cured poly(ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS; Baytron P). After spin-coating, the samples were allowed to dry in a nitrogen glovebox for at least 1 h before being subjected to controlled photooxidation. The devices were completed by depositing 75 nm thick aluminum top-contacts at 0.2 nm/s via thermal evaporation. The completed devices had an active area of ∼1.5 mm2, defined by the overlap of the ITO and Al electrodes. All device fabrication steps except for spin-coating and annealing of the PEDOT:PSS and controlled photooxidation were carried out in a dry nitrogen glovebox (
