**2.1 Background**

In 2002, an epochal report on the *E*g of InN was published; the *E*g, which had been believed to be 2.0 eV for many years, was found to be less than 1.0 eV by photoluminescence characterization (Matsuoka et al., 2002). Subsequent investigations verified that the correct *E*g is 0.7 eV (Wu et al., 2003). This fact immediately impelled III-nitride-researchers to consider applying III-nitrides to solar cells because InxGa1-xN, which is the III-nitride compound obtained from InN (*E*g = 0.7 eV) and GaN (*E*g = 3.4 eV), is a direct transition semiconductor that would widely cover the solar spectrum. Furthermore, the strong Piezoelectric-field that forms in III-nitride semiconductors, which is a critical problem for optical emission devices due to the suppression of carrier recombination (Takeuchi, 1998), will be more advantageous to photovoltaic devices in which carrier separation is necessary. There have been reports on the theoretical predictions of the conversion efficiency of InxGa1-x N solar cells that suggest that the maximum conversion efficiency of InxGa1-x N solar cells will reach 35–40% (Hamzaoui, 2005; Zhang, 2008). Experimental results of InxGa1-x N-based solar cells have been also reported (Chen, 2008; Zheng, 2008; Dahal, 2009; Kuwahara, 2010). Although the potential conversion efficiency of InxGa1-xN solar cells is promisingly high, the highest one so far obtained through an InGaN/InGaN superlattice structure remains as low as 2.5% (Kuwahara, 2011).

The challenges for the development of high efficiency InGaN solar cells are mainly attributed to the necessity for: (1) a conductive crystalline substrate to grow high quality nitride layers in order to reduce series resistance, (2) a high quality film growth technique to reduce carrier recombination, (3) high-efficiency p-type doping, and (4) a novel cell design that allows absorption in a wide range of the solar spectrum and efficient collection of the photo-generated carrier.

Our research has targeted issues (3) and (4) above by introducing a novel Schottky contact consisting of a transparent conducting polymer/nitride semiconductor heterojunction. In this section, the advantages of the polymer/nitride semiconductor heterojunction are described in comparison with those of a conventional nitride p-n homojunction. In addition, the optical and electrical properties of the transparent conducting polymers are shown.
