**3. Fabrication processes**

310 Solar Cells – New Aspects and 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

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

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,

*q q* 

*B m* 

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

> *<sup>m</sup>*

heterojunctions, and those of PEDOT:PSS or AlN with III-nitrides including AlN, GaN and

(1)

values of these TCP/semiconductor

*<sup>m</sup>* is the workfunction of the metallic material, and

*<sup>B</sup>*, is given

is

produced and efforts to improve their conductivity have been made.

(PEDOT), and polyaniline (PANI) (Heeger, 2001).

by following equation (Schottky, 1939; Mott, 1939):

PEDOT:PSS/ZnO (Nakano et al., 2008). The ( )

where *q* is the unit electronic charge,

InN, are summarized in Table 1.

**2.4 Transparent conducting polymers as Schottky contacts** 

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

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 was as follows:


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 solutions.

Transparent Conducting Polymer/Nitride Semiconductor Heterojunction Solar Cells 313

photoelectric workfunction. Thus, if we modify Eq. (2) to the following form (Eq. (2)′), we

 

We employed a photoemission yield spectrometer (AC-3, Riken Keiki Co., Ltd.) to determine the workfunctions of the TCPs. The sample, which was prepared as described in section 3.1, was installed in the spectrometer and the photoemission yield was measured in

The diode (rectifying) and photovoltaic characteristics were evaluated using an electronic measurement system consisting of an electrometer and a light source. It is necessary for the diode characterization to cover a wide current range from ~10-11 to ~10-1 A to estimate the Schottky barrier height (SBH) based on the saturation current of the thermionic emission theory (Crowell, 1965). Thus, for the evaluation of the diode characteristics, we employed a high-precision electrometer with a built-in voltage source (Keithley 6487) and performed the measurement under dark conditions. The sample was put on a measurement stage and probe needles were connected to the indium and TCP parts. A xenon-arc light source (HX-504/Q, Wacom Electric Co., Ltd.) was utilized for the evaluation of the photovoltaic characteristics. The light passed though an AM1.5 filter (Bunko Keiki Co., Ltd) and guided onto the TCP side by an aluminum mirror. The values for the source voltage and measured current were acquired by a computer through a GPIB-USB device (National Instruments

The depletion layer width and built-in potential of the GaN layer in the TCP/GaN heterojunction solar cell were estimated using a capacitance measurement setup. A solartron 1255B frequency response analyzer was utilized for the measurement. The sample was set on a sample stage, which was in a vacuum chamber to avoid any influences from light and

**5.1 Conductivity, transparency, and workfunction of polyaniline and PEDOT:PSS**  The electrical conductivity was evaluated using a current-voltage (*I*-*V*) measurement setup under dark conditions. The conductivities estimated from the result of the *I*-*V* measurements were 3.4×102 S/cm and 5.7×10-1 S/cm for PANI and PEDOT:PSS,

The optical transmittance was evaluated using a UV-visible-near-infrared spectrophotometer (UV-3150, Shimadzu Co., Ltd.). Figure 3(a) shows the optical transmittance spectra of the PEDOT:PSS and PANI films. Both of the films exhibited transmittance greater than 80% within the wavelength region between 250 and 1500 nm. This is superior to conventional transparent contact materials such as transparent conductive oxides or semi-transparent metals (Kim et al., 2002; Satoh et al., 2007), which exhibit significant drops in transparency particularly near the

*<sup>t</sup>* . (2)'

can determine *Et* by extrapolating the linear portion of a Y1/2 vs. *h* plot:

1/2 ( ) *Y hE*

**4.2 Evaluation of current-voltage characteristics** 

air.

Co. ltd.).

humidity.

respectively.

UV region, as seen in Figure 3.

**4.3 Capacitance measurements** 

**5. Experimental results and discussion** 

In order to measure the conductivity, a coplanar electrode was fabricated by adding Ag paste to the TCP/synthetic silica plate sample. The electrode-gap width were both 3.3 mm and the lengths were 10.7 and 11.2 mm, respectively, for the PEDOT:PSS and PANI samples.
