**2.3 Conducting polymers as electrodes**

Recently, the electronic properties of conducting polymers have been significantly improved and they have been extensively applied in various electric devices (Heeger, 2001).

The study of polymers began with the accidental discovery of vinyl chloride by H. V. Regnault (1835). Thereafter, various kinds of polymers were found and industrialized including ebonite (1851), celluloid (1856), bakelite (1907), polyvinyl chloride (1926), polyethylene (1898; 1933), nylon (1935), etc. Polymers show good electrical insulating properties due to the lack of free electrons; therefore, they have been extensively applied as electrical insulators. However, in 1963, D. E. Weiss and his colleagues discovered that polypyrrole became electrically conductive by doping it with iodine (Bolto et al., 1963). In 1968, H. Shirakawa and his colleagues accidentally discovered a fabrication process for thinfilm polyacetylene. In 1975, A. G. MacDiamid noticed the metallic-colored thin-film polyacetylene when he visited Shirakawa's laboratory. Thereafter, collaborative works by A. Heeger, A. G. MacDiamid, and H. Shirakawa began and soon they found a remarkable effect that the electrical conductivity of the polyacetylene thin-film increased over seven

Transparent Conducting Polymer/Nitride Semiconductor Heterojunction Solar Cells 311

AlN 0.25e) 5.05 GaN 3.3f) 2.0 InN 5.7g) -0.4

SrTiO3:Nb 4.1h) 1.1 ZnO 4.3i) 0.7 AlN 0.25e) 4.95 GaN 3.3f) 1.9 InN 5.7g) -0.5

> *<sup>m</sup>*

transparent conducting polymer/nitride semiconductor heterojunction solar cells.

**3.1 Sample preparation for optical transmittance, workfunction, and conductivity** 

Synthetic silica plates (500 m thick) were utilized as the substrates to prepare samples for characterization to determine their optical transmittance, workfunction, and conductivity. A conductive polymer-dispersed solution of PEDOT:PSS (Clevios PH500, H. C. Starck; without dimethyl sulfoxide dopant) or PANI (ORMECON - Nissan Chemical Industries, Ltd.) was utilized to form the transparent conductive polymer films on the substrate. The same fabrication process was applied to both the PEDOT:PSS and PANI samples. The procedure

1. The substrate (2 × 2 cm2) was cleaned using ethanol and acetone for 5 min each in an

2. The cleaned substrate was set in a spin coater (MIKASA Ltd., 1H-D7), and the polymer-

4. The drop and spin procedures were repeated 4 times in total to obtain a sufficient

The resulting PEDOT:PSS and PANI film thicknesses were measured using a surface profilometer (Dektak 6M) and were found to be 420 and 170 nm, respectively. In the spincoat process, we applied the same conditions to both the PEDOT:PSS and PANI samples. Their thicknesses unintentionally differed due to differences in the viscosities of their source

dispersed solution were dropped onto the substrate using a dropper.

5. The coated sample was baked in air at 130 °C on an electric hotplate for 15 min.

Thus, it was expected that combinations of these TCPs and III-nitrides would exhibit highquality Schottky contact properties. When light is irradiated on the Schottky contact, the hole-electron pairs that are photo-generated in the depletion region of the semiconductor are separated due to the strong electric field. As a result, the carriers can be collected as a photocurrent. This suggests that the TCP Schottky contact can be a novel window layer for III-nitride solar cells as an alternative to a p-type layer. Based on this, we began to study

Table 1. Summary of workfunction barrier height properties of TCP/inorganic

 *<sup>m</sup>* 

(eV)

n-type Si 4.05 c), d) 1.25 a) Brown et al., 1999;

(eV) References

b) Jang st al., 2008 c) Wang et al, 2007 d) da Silva et al., 2009 e) Grabowski et al., 2001 f) Wu et al., 1999 g) Wu et al., 2004 h) Yamaura, 2003 i) Nakano et al., 2008

is considerably high for AlN and GaN.

TCP Semiconductor

*<sup>m</sup>* (eV) material

The theoretical Schottky barrier height ( )

ultrasonic cleaning bath at ambient temperature.

3. The substrate was spun at a 4000 rpm rotating speed for 30 s.

material

PANI 5.3a), b)

PEDOT:PSS 5.2 a), b)

semiconductor heterojunction.

**3. Fabrication processes** 

**characterizations** 

was as follows:

thickness.

solutions.

orders of magnitude, from 3.2×10-6 to 3.8×102 -1 cm-1, with iodine doping (Shirakawa et al., 1977). Since these early studies, various sorts of -conjugated polymer thin films have been produced and efforts to improve their conductivity have been made.

We briefly describe the origin of conductivity in degenerate -conjugated polymers below (Heeger, 2001). In degenerate -conjugated polymers, stable charge-neutral-unpairedelectrons called solitons exist due to defects at the counterturned connection of the molecular chain. When the materials are doped with acceptor ions like I2, the acceptor ion abstracts an electron from the soliton; then the neutral soliton turns into a positivelycharged soliton while I2 becomes I3-. If the density of the positively-charged solitons is low, the positively-charged soliton tends to pair with a neutral soliton to form a polaron. The polaron is mobile along the polymer chain, thus it behaves as a positive charge. However, the mobility of the polaron is quite low due to the effect of Coulomb attraction induced by the counterion (I3-). The Coulomb attraction is reduced by increasing the density of the counterions, which block the electric field. Thus, a high doping concentration of up to ~20% is required to gain high conductivity of over 102 -1 cm-1. Typical conducting polymers that have high conductivity are fabricated based on polyacetylene (PA), polythiophene (PT), polypyrrole (PPy), polyethylenedioxythiophene (PEDOT), and polyaniline (PANI) (Heeger, 2001).

### **2.4 Transparent conducting polymers as Schottky contacts**

Among the various kinds of conducting polymers, we have focused primarily on polyaniline (PANI) and poly(ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) because of their high conductivity (~1000 -1 cm-1) and high optical transparency (>80%) (Lee et al., 2006; Ha, 2004). Conducting polymers with high optical transparency are known as transparent conducting polymers (TCPs). PANI and PEDOT:PSS also have the advantage in a high workfunction of 5.2–5.3 eV (Brown, 1999; Jang, 2008). This workfunction value is comparable to that of Ni (5.1 eV) and Au (5.2 eV). The high workfunction properties of PANI and PEDOT:PSS make them feasible candidates as hole injection layers in polymer light emitting devices (Jang, 2008). If we assume that a heterojunction consists of a metallic layer and an n-type semiconductor, it is expected that electric barrier, or Schottky barrier, will form at the metal-semiconductor interface. The ideal Schottky barrier height, *<sup>B</sup>*, is given by following equation (Schottky, 1939; Mott, 1939):

$$
\eta \phi\_{\mathcal{B}} = q \left( \phi\_m - \mathcal{X} \right) \tag{1}
$$

where *q* is the unit electronic charge, *<sup>m</sup>* is the workfunction of the metallic material, and is the electron affinity of the semiconductor. In general, the experimentally observed Schottky barrier is modified due to the influence of image-force surface states of the semiconductor and/or the dipole effect (Tung, 2001; Kampen, 2006). Nevertheless, the ideal Schottky barrier height estimated from Eq. (1) is still useful to evaluate the potential barrier formation. There have been precedential reports on heterojunctions consisting of TCPs and inorganic monocrystalline semiconductors including: sulfonated-PANI/n-type Si (Wang et al., 2007; da Silva et al., 2009), PEDOT:PSS/SrTiO3:Nb (Yamaura et al., 2003), and PEDOT:PSS/ZnO (Nakano et al., 2008). The ( ) *<sup>m</sup>* values of these TCP/semiconductor heterojunctions, and those of PEDOT:PSS or AlN with III-nitrides including AlN, GaN and InN, are summarized in Table 1.


Table 1. Summary of workfunction barrier height properties of TCP/inorganic semiconductor heterojunction.

The theoretical Schottky barrier height ( ) *<sup>m</sup>* is considerably high for AlN and GaN. Thus, it was expected that combinations of these TCPs and III-nitrides would exhibit highquality Schottky contact properties. When light is irradiated on the Schottky contact, the hole-electron pairs that are photo-generated in the depletion region of the semiconductor are separated due to the strong electric field. As a result, the carriers can be collected as a photocurrent. This suggests that the TCP Schottky contact can be a novel window layer for III-nitride solar cells as an alternative to a p-type layer. Based on this, we began to study transparent conducting polymer/nitride semiconductor heterojunction solar cells.
