**2.3 Abundance of the elements**

28 Solar Cells – New Aspects and Solutions

Bandara & Tennakone, 2001) and composite nanocrystalline materials (Palomares et al., 2003; Kay & Gratzel, 2002). Since that time, several other semiconductors have been tested

Recent challenges concerning newly designed solar cells are namely Band-gap concerns,

The recently adopted layered structure of PVC raised the problem of solar spectrum matching (Fig.1) as well as lattice mismatch at early stages. In fact, the heterogeneous structure: Contact/window layer/buffer layer/Contact causes at least three differently structured surfaces to adhere under permanent constraints. It is known that the electronic band gap is the common and initial choice-relevant parameter in solar cells sensitive parts design. It is commonly defined as the energy range where no electron states exist. It is also defined as the energy difference between the top of the valence band and the bottom of the conduction band in semiconductors. It is generally evaluated by the amount of energy required to free an outer shell electron the manner it becomes a mobile charge carrier. Since the band gap of a given material determines what portion of the solar spectrum it absorbs, it is important to choose the appropriate compound matching the incident energy range. The choice of appropriated materials on the single basis of the electronic band gap is becoming controversial due the narrow efficient solar spectrum width, along with new thermal and mechanical requirements. It is rare to have a complete concordance between adjacent

with less success.

Fig. 1. Solar spectrum

cost, abundance and environmental concerns.

**2.1 Band gaps matching the solar spectrum** 

crystalline structures particularly in band gap sense.

W/m2 nm

The first challenge for PV cells designer is undoubtedly the abundance of materials for buffer and window layers. The ratio of abundance i. e. of Tungsten-to-Indium is around 104, that of of Zinc-to-Tin is around 40. Although efficiency of Indium and Gallium as active doping agents has been demonstrated and exploited (Abe & Ishiyama, 2006; Lim et al., 2005), their abundance had decreased drastically (510 and 80 tons, respectively as reported by U.S. Geological Survey 2008) with the last decades' exploitation.

## **2.4 Non toxicity and environmental concerns**

Among materials being used, cadmium junctions (Cd) and selenium (Se) are presumed to cause serious health and environmental problems. Risks vary considerably with concentration and exposure duration. Other candidate materials haven't gone though enough tests to show reassuring safety levels (Amlouk, 2010).
