**4.2 Evaluation of current-voltage characteristics**

312 Solar Cells – New Aspects and Solutions

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

**3.2 Transparent conducting polymer/nitride semiconductor heterojunction solar cells**  We fabricated a TCP/III-nitride heterojunction solar cell structure by employing PEDOT:PSS or PANI for the TCP layer and epitaxial GaN (epi.-GaN) for the III-nitride layer (Matsuki et al, 2009, 2010, 2011). Silicon-doped gallium nitride (GaN) was grown on a sapphire (0001) substrate (sapp (0001)) surface by typical metal-organic vapor-phase epitaxy (MOVPE). Ammonia and trimethylgallium were used as the N and Ga sources, respectively. Nitrogen was used as the carrier gas. An undoped buffer GaN layer with a thickness of 1 m was deposited, followed by the growth of a 2 m thick Silicon-doped layer. The carrier concentration and electron mobility of the GaN film was determined to be 6.3 × 1017 cm-3

The PEDOT:PSS or PANI thin film was formed on the epi.-GaN surface using the same process described in section 3.1. Then, in order to fabricate isolated cells, the TCP film was divided into several ~3–9 mm2 square-shaped sections using a scratching tool. Finally, an ohmic contact for the GaN layer was made by soldering indium metal onto the area from which the TCP layer was removed. Figure 2 shows the schematic structure of the fabricated

The workfunctions of the TCPs were determined using photoemission electron spectroscopy. The photoemission electron yield *Y* is expressed as follows Kane (1962):

> ( )*<sup>n</sup> Y hE*

threshold energy, and the value of *n* ranges from 1 to 5/2 depending on the system. For

metallic materials, an *n* value of 2 is recommended, and the *Et* is consistent with the

*<sup>t</sup>* (2)

is the frequency, *Et* is the

**4.1 Photoemission electron spectroscopy for workfunction determination** 

is a proportional constant, *h* is Planck's constant,

lengths were 10.7 and 11.2 mm, respectively, for the PEDOT:PSS and PANI samples.

and 360 cm2/V·s, respectively, by Hall measurement.

TCP/epi.-GaN heterojunction solar cell.

Fig. 2. Schematic of TCP/epi.-GaN sample.

**4. Characterization methods** 

where

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 Co. ltd.).
