**2.2 Issues with solar cell window layer**

Figure 1 shows a schematic structure of the InxGa1-x N-based solar cell that exhibits 2.5% conversion efficiency (Kuwahara, 2011). Due to the low doping efficiency and activity of Mg in p-type III nitride semiconductors, the InxGa1-x N-based solar cell requires a highly conductive front layer on top of the p-type layer to collect the photo-generated carriers.

Aiming at developing multijunction solar cells based on III-nitrides, we have focused on the potential of a transparent conductive polymer (TCP) as a UV-transparent window layer for the cell instead of adopting the conventional all-inorganic p-i-n structure. In this chapter, we describe the concept and experimental results of the development of TCP/nitride semiconductor heterojunction solar cells. In addition, prospects for their further

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

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

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

Figure 1 shows a schematic structure of the InxGa1-x N-based solar cell that exhibits 2.5% conversion efficiency (Kuwahara, 2011). Due to the low doping efficiency and activity of Mg in p-type III nitride semiconductors, the InxGa1-x N-based solar cell requires a highly conductive front layer on top of the p-type layer to collect the photo-generated

development are discussed.

as 2.5% (Kuwahara, 2011).

photo-generated carrier.

**2.2 Issues with solar cell window layer** 

shown.

carriers.

**2. Basic concepts** 

**2.1 Background** 

Fig. 1. Schematic of InxGa1-x N-based solar cell exhibiting 2.5% conversion efficiency (Kuwahara, 2011).

In Figure 1, the electrode on the window side consists of a Ni/Au semitransparent thin film similar to that in the conventional III-nitride-based photoelectric devices. Despite the transparency of the Ni/Au thin-film being as low as 67%, this material is utilized because it forms good ohmic contact with the III-nitride semiconducting layer (Song et al., 2010). With the aim of increasing the transparency of the window-side electrode, indium tin oxide (ITO) was applied to a III-nitride light-emitting diode (LED) (Shim et al, 2001; Chang et al., 2003). In the same study, although the light emitting intensity in the ITO/GaN LED was enhanced compared with that of a Ni/Au/GaN LED under the same current density, the lifetime of the device was significantly shortened due to the heat generated by the high contact resistance between ITO and GaN. Thus, ITO is not a suitable alternative candidate for the metal semitransparent layer unless the contact resistance problem is solved. The low optical transparency and/or the high contact resistance of the front conductive layer are a critical disadvantage for solar cell applications; therefore, new materials that can overcome these issues are highly desirable.
