7922
J. Phys. Chem. C 2008, 112, 7922–7927
The Changing Face of PEDOT:PSS Films: Substrate, Bias, and Processing Effects on Vertical Charge Transport† Liam S. C. Pingree, Bradley A. MacLeod, and David S. Ginger* Department of Chemistry, UniVersity of Washington, Box 351700, Seattle, Washington 98195-1700 ReceiVed: December 17, 2007; ReVised Manuscript ReceiVed: January 18, 2008
Poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) is widely used as a semitransparent anode layer in organic light-emitting diodes and polymer photovoltaics. We use conductive atomic force microscopy (c-AFM) to map the electronic properties of PEDOT:PSS films during a variety of processing steps to better explain how the observed changes in macroscopic electronic properties arise from local changes in charge transport. We observe only small conductive regions, ∼20 nm in size, surrounded by more insulating regions in all of the films studied. We confirm that these features dominate the c-AFM measurements, independent of the substrate. We observe a marked increase in the density of the conductive regions with increasing annealing times, increasing applied bias (independent of polarity), and decreasing PSS concentration (achieved by altering PEDOT:PSS grades). We also find an increase in current flow following a chlorobenzene wash, suggesting the solvents used in processing the active semiconductor layers on top of PEDOT:PSS anodes may affect the quality of the interface and subsequently alter device performance. Introduction Poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT: PSS) is one of the most widely used materials in organic elecronics.1 Indeed PEDOT:PSS can be found in the vast majority of small-molecule organic light-emitting diodes (OLEDs), polymer light-emitting diodes (PLEDs),2–4 and organic photovoltaic (OPV) devices under study today.5–7 PEDOT:PSS is often included as an interface layer between the tin-doped indium oxide (ITO) anode and the active organic semiconductor layers to improve both morphological and electronic figures of merit.2 Although research on the electronic properties of PEDOT:PSS has a rich history,8–10 the role this layer plays in subsequent device structures,5 the evolution of its electronic properties and morphology with processing,11–13 and the optimal processing parameters for specific device architectures3,4,12 are all still areas of active research. It is believed that spin-coated PEDOT:PSS films consist of a lamellar-type structure of conductive ellipsoidal-shaped PEDOT particles separated by insulating PSS layers.14–16 In the surfacenormal direction, the insulating PSS layers can reach a thickness of 3-4 nm,15–17 whereas the in-plane PSS thickness is often less than 1 nm, resulting in anisotropic conductivity14,17,18 with an in-plane conductivity up to 3 orders of magnitude higher than the vertical conductivity relevant in most organic electronic device geometries. Not surprisingly, the ratio of conducting PEDOT to insulating PSS (typically 1:6 or 1:2.5)19 can also dramatically alter the conductivity of the films in both directions.18,20 However, given the ubiquitous role PEDOT:PSS plays as a supporting anode in organic electronics, it is surprising that more attention has not been paid to the basic transport properties of PEDOT:PSS films17,18 and to the inter-related effects of film morphology,12–14,17,21–24 solution composition,20,25–28 and processing12,15,28,29 on the properties of these films. Recently, Ionescu† Part of the “Larry Dalton Festschrift”. * To whom correspondence should be addressed. E-mail: ginger@ chem.washington.edu.
Zanetti et al.21 employed conductive atomic force microscopy (c-AFM) to study the local charge transport variations in relatively thick (∼1 µm) solution-cast PEDOT:PSS films on ITO and concluded that the lamellar structure had a strong impact on transport.21 Furthermore, Kemerink and co-workers14,17,18,30 have published several papers using scanning tunneling microscopy (STM) to examine the impact of the lamellar structure on the local charge transport of thinner (100-200 nm), spincoated PEDOT:PSS films (1:2 wt ratio, supplied by AGFA,30 1:6 wt ratio supplied by Stark,17 and a form of low Na+ PEDOT: PSS17). They found that PEDOT:PSS-blended films contain elongated PEDOT particles on the order of 10-50 nm in length with insulating PSS regions between.18,30 Furthermore, they noted the conductivity was highly anisotropic, and could be nonohmic in the surface-normal direction.14 Kemerink and coworkers subsequently fit their findings to one of two transport models (dependent upon the direction of conduction).18 As noted above, the macroscopic conductivity of PEDOT: PSS films can vary widely with formulation (PEDOT to PSS ratio) and processing (annealing times, solvent washing, etc.). In this paper, we use c-AFM to study the changes in local charge transport that underpin the known changes in bulk film conductivity for different PEDOT:PSS formulations and processing conditions. We start by demonstrating that the observed local transport variations are substrate independent. Subsequently we study the role of the applied electric field and postdeposition thermal annealing on the PEDOT:PSS surface. We examine the effect of PSS concentration on the vertical charge transport by varying the PEDOT:PSS grade deposited, and finally we study how subsequent processing may alter the PEDOT:PSS interface. We find the surface to be highly nonuniform, as observed previously,17,18 a result independent of the substrate, and we find that the density of the more conductive regions of the surface increases with the applied electric field, annealing time, PEDOT concentration, and exposure to organic solvent.
10.1021/jp711838h CCC: $40.75 2008 American Chemical Society Published on Web 04/08/2008
The Changing Face of PEDOT:PSS Films
J. Phys. Chem. C, Vol. 112, No. 21, 2008 7923
Experimental Methods Samples. ITO coated glass substrates (Thin Film Devices Inc., Anaheim, CA) were cleaned by sonication in acetone and then isopropanol and were then dried under a nitrogen stream. The ITO substrates were then plasma cleaned (Harrick Plasma, model PDC-32G, 18 W applied, 4 min cleaning time), and in all cases, the PEDOT:PSS films were spin-coated onto the ITO within 60 s of plasma cleaning. 100 µL of various grades of Baytron PEDOT:PSS (“VP Al 4083”, PEDOT:PSS ratio of 1:6; “P” or “4071”, PEDOT:PSS ratio 1:2.5; and “F-HC”, PEDOT: PSS ratio 1:2.5 with unspecified additives) were deposited onto the substrates via spin-coating. Spin-coating was performed in ambient conditions as follows: 500 rpm for 5 s, 4000 rpm for 2 min, followed by 5000 rpm for 2 min to accelerate the final drying times. All PEDOT solutions were filtered with a 0.45 µm PVDF syringe filter prior to use. Unless otherwise noted, samples were transferred to a custom heating stage and annealed for 30 min at 140 °C under a nitrogen stream on a hot plate and thereafter stored in a dry-nitrogen glovebox (
