**1. Introduction**

306 Solar Cells – New Aspects and Solutions

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Energy supplies that depend on fossil fuels evoke significant concern about the future depletion of those resources and the emission of carbon dioxide and sulfidizing gas, which are believed to cause environmental problems including climate change and acid precipitation (Solomon et al., 2007). Solar cells, which convert sunlight directly to electric power, are one of the most promising devices for a clean and enduring energy source. The standard energy-weighted power density of sunlight, which is defined as air mass 1.5, is 1kW/m2 under clear and sunny weather conditions (Myers et al., 2000). The maximum available amount of sunlight is usually lower than the value described above due to the weather and the total hours of sunlight in the region.

Thus, the first important aim for developing a solar cell is to derive the highest possible photovoltaic conversion efficiency from the utilized materials and structure. When a solar cell with a single bandgap, *E*g, is exposed to the solar spectrum, a photon with less energy than *E*g does not contribute to the cell output. Therefore, a multilayer structure comprising a variety of bandgaps is effective for the collection of photons in a wide range of the solar spectrum.

The current (2010) best research-cell efficiencies of typical solar cells are as follows (Green, 2010): crystalline Si (25.0%), multicrystalline Si (20.4%), crystalline GaAs (26.4%), CuInGaSe (19.4%), CdTe (16.7%), amorphous Si (10.1%), dye-sensitized polymers (10.4%), and organic polymers (5.15%). In addition to these, there have been a number of studies focused on developing "third-generation photovoltaics" with ultra-high conversion efficiencies at a low cost (Green, 2001). More recently, after the discovery of the wide band gap range of 0.65–3.4 eV in InxGa1-x N, this material is considered to be one of the most promising candidates for third-generation photovoltaic cells.

Transparent Conducting Polymer/Nitride Semiconductor Heterojunction Solar Cells 309

**10 pairs**

**50 pairs**

**3 nm/3 nm**

**3 nm/0.6 nm**

**Ga0.09In0.10N:Si/GaN:Si**

**Ga0.83In0.17N/Ga0.93In0.07N**

**Ti/Au Ni (5 nm)/Au(5 nm)**

**Ti/Al/Ti/Au**

(Kuwahara, 2011).

issues are highly desirable.

**2.3 Conducting polymers as electrodes** 

Fig. 1. Schematic of InxGa1-x N-based solar cell exhibiting 2.5% conversion efficiency

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

Recently, the electronic properties of conducting polymers have been significantly improved

The study of polymers began with the accidental discovery of vinyl chloride by H. V. Regnault (1835). Thereafter, various kinds of polymers were found and industrialized including ebonite (1851), celluloid (1856), bakelite (1907), polyvinyl chloride (1926), polyethylene (1898; 1933), nylon (1935), etc. Polymers show good electrical insulating properties due to the lack of free electrons; therefore, they have been extensively applied as electrical insulators. However, in 1963, D. E. Weiss and his colleagues discovered that polypyrrole became electrically conductive by doping it with iodine (Bolto et al., 1963). In 1968, H. Shirakawa and his colleagues accidentally discovered a fabrication process for thinfilm polyacetylene. In 1975, A. G. MacDiamid noticed the metallic-colored thin-film polyacetylene when he visited Shirakawa's laboratory. Thereafter, collaborative works by A. Heeger, A. G. MacDiamid, and H. Shirakawa began and soon they found a remarkable effect that the electrical conductivity of the polyacetylene thin-film increased over seven

and they have been extensively applied in various electric devices (Heeger, 2001).

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 development are discussed.
