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

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 TCP/epi.-GaN heterojunction solar cell.
