Due to the advantages and promise mentioned in the previous section, PEDOT:PSS has been studied intensively as TEs in organic devices [74-82] as well as in some other applications where the high
Formation of Closure of gap
Figure 5.12 Evolution of the energy band structure according to dopant concentration in (a) trans-polyacetylene and (b) poly(heterocycles) such as polypyrrole and polythiophene. Examples shown here are for oxidizing (electron accepting) dopants [65,66].
conductivity of ITO or similar TCO is not necessarily required. One example of the latter may be found in the case where PEDOT:PSS was adopted as TEs for electroluminescent (EL) light sources used in a keypad of cellular phones . The contoured shape of a keypad benefits from the conformal coating capability inherent to the solution processability of PEDOT:PSS, and the EL light source barely requires a large electrical current and thus high sheet resistance is tolerable.
In the case of OLEDs or OPVs, however, a certain level of Rsheet should be secured as discussed in previous sections. The standard grade PEDOT:PSS exhibits a conductivity of merely 1 S/cm, which is far smaller than that of conventional TCOs (~104 S/cm). In that respect, numerous studies have been devoted to improving its conductivity, as in some of the examples summarized in Table 5.3. While the conductivity of PEDOT:PSS can be influenced by several factors, such as the ratio between PEDOT and PSS, a small addition of solvent with high boiling point (b. p.) or a polar nature into an originally water-based PEDOT:PSS dispersion was consistently proven effective in improving the conductivity to a significant degree [76,84-88]. Upon addition of dimethyl sulfoxide (DMSO) or ethylene glycol (EG), for example, a certain grade of commercially available PEDOT:PSS can now exhibit conductivity as high as 1,000 S/cm [71,89]. Other types of treatment have also been reported recently, which involve the addition of fluoro-surfactant called Zonyl [9 0] or hexafluoroacetone (HFA) ; or immersion of solidified PEDOT:PSS films into methanol . The proposed mechanisms of the conductivity enhancement due to addition of these solvents include (i) reorientation of polymers for better interconnection incurred by a high b. p.-solvent functioning as a plasticizer ; screening effect due to polar solvents hindering Coulomb electrostatic interaction between carriers in PEDOT and negatively charged counter-ions (PSS) ; (iii) washing of PSS during a film-forming stage ; (iv) conformational change into a case where linear or extended coil conformation is dominant . Another notable method involves the so-called “vacuum vapor phase polymerization” (VPP), and the conductivity as high as 3400 S/cm has recently been reported for PEDOT films prepared by the VPP technique .
With the significantly enhanced conductivity, PEDOT:PSS may be considered suitable, although not perfect, as a stand-alone TE in organic solar cells. Vosgueritchian et al. improved the opto-electrical characteristics of PEDOT:PSS by adding a fluorosurfactant and demonstrated a sheet resistance as low as 46 Q/sq at T (A = 550 nm) of 82% . The power conversion efficiency (PCE) of the P3HT: PCBM device applying the conductive polymer TE showed ca. 2.16% (device area = 0.05 cm2), which is quite comparable to 2.22% of their control device. Xia et al. modified the PEDOT:PSS with HFA . By increasing the number of treatments, they obtained a similar range of Rsheet and T. With the modified PEDOT:PSS solution, they obtained PCE of 3.56% with FF as large as 0.64 for P3HT:PCBM-based OPVs. These results are clearly different from those observed in most of the early seminal experiments where the cell performance was severely limited by the high sheet resistance of PEDOT:PSS. [74,75]
Table 5.3 Summary of PEDOT:PSS treatment results.
To overcome the limitation of sheet resistance, whose effect becomes especially severe in large-area devices, auxiliary grids were often shown to be essential as briefly pointed out in Section 18.104.22.168. In particular, Galagan et al. demonstrated that PEDOT:PSS used in combination with Ag grids embedded within barrier layers can address problems of electrical shorts commonly found in such devices due to the thinness of PEDOT:PSS/organic active layers (see Fig. 5.14) .
Another approach that can ease the limitation of high Rsheet may be to use multiple spin-coating techniques. Once PEDOT:PSS gets solidified, its integrity is relatively well kept with additional PEDOT:PSS solution dispensed onto it, making multiple spin-coating processes possible to some degree. Shinar and coworkers demonstrated that efficient OLEDs can be realized based on a double-layer PEDOT:PSS , and Kim et al. demonstrated that the
T-Rsheet relationship of a multilayer PEDOT:PSS can be well described by Eq. (5.9) with 0’dc/0’opt of approximately 36  (see Fig. 5.15 for T and Rsheet as a function of the thickness of PEDOT:PSS layers prepared by multiple spin-coating technique). Because the benefits of the reduced Rsheet in a multi-layer PEDOT:PSS for the efficiency of OPVs can differ depending on various factors (e. g., device area, level of photocurrent/light intensity, etc.), it is not so trivial to predict how many layers are optimal for an OPV device; nevertheless, this multiple coating technique is expected to offer a significant degree of freedom in designing/engineering an OPV cell based on PEDOT:PSS electrodes.
■Ag (honeycombs) / HC-Pedot ■Ag (lines) /HC-Pedot
Figure 5.14 (a) Schematic diagram and photograph of an organic solar cell
with embedded Ag grids and PEDOT:PSS electrodes (b) J-V characteristics of a cell shown in (a) compared with that of ITO-based reference cells  (Reprinted with permission from Sol Energy Mater Sol Cells, Copyright 2012. Elsevier B. V.)
Figure 5.15 (a) T and Rsheet presented as a function of thickness prepared
by a multiple spin-coating method. The trend curve shown by solid line is Rsheet=p0/thickness with R0 of 8.2 x 10-3 Q-cm  (Reprinted with permission from Adv Funct Mater, Copyright 2011. Wiley).
The differentiating feature of PEDOT:PSS or similar conducting polymers may be found in the fact that it inherits the excellent mechanical properties of polymers. The recent report by Vosgueritchian et al. indeed shows that the PEDOT:PSS films prepared on poly(dimethylsiloxane) (PDMS) substrates can exhibit an excellent stretchability with no degradation in Rsheet even after 5000-time stretching cycles .