**2. Solar cells technologies and design recent challenges**

In spite of better performance of traditional junction-based solar cells, during the past few decades, reports have appeared in literature that describe the construction of cells based metal-oxides (Bauer et al., 2001; Sayamar et al., 1994; He et al., 1999; Tennakone et al., 1999;

A New Guide to Thermally Optimized Doped Oxides Monolayer

**2.2 Low-cost deposition/incorporation methods** 

promising candidates for solar applications.

**2.4 Non toxicity and environmental concerns** 

thermal, mechanical and opto-electric performance.

**2.3 Abundance of the elements** 

**3. Materials optimisation 3.1 Primal selection protocols** 

these proprieties together.

**3.2 Opto-thermal analysis** 

section.

Where *D* is the thermal diffusivity and

Spray-Grown Solar Cells: The Amlouk-Boubaker Optothermal Expansivity

by U.S. Geological Survey 2008) with the last decades' exploitation.

enough tests to show reassuring safety levels (Amlouk, 2010).

The Amlouk-Boubaker optothermal expansivity is defined by:

For example, in silicon-based solar cells, recombination occurring at contact surfaces at which there are dangling silicon bonds (Wu, 2005) is generally caused by material/phase discontinuities. This phenomenon limits cell efficiency and decreases conversion quality.

Deposition techniques and incorporation methods have been developed drastically and several deposition improved methods have been investigated for fabrication of solar cells at high deposition rates (0.9 to 2.0 nm/s), such as hot wire CVD, high frequency and microwave PECVD, , and expanding thermal plasma CVD. Parallel to these improvements, vacuum conditions and chemical processes cost increased the manner that serial fabrication becomes sometimes limited. Nowadays, it is expected that low processing temperature allow using a wide range of low-cost substrates such as glass sheet, polymer foil or metal. These features has made the second-generation low- cost metal-oxides thin-film solar cells

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

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

Cost and toxicity concerns led to less and less use of Se and Cd-like materials. Additionally, increasing interest in conjoint heat-light conversion took some bad heat-conducting materials out from consideration. Selection protocols are becoming more concentrated on

Since thermal conductivity, specific heat and thermal diffusivity has always been considered as material intrinsic properties, while absorbance and reflexivity depend on both material and excitation, there was a need of establishing advanced physical parameters bringing

> AB ˆ *D*

(1)

ˆ is the effective absorptivity, defined in the next

AB 29

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

Recent challenges concerning newly designed solar cells are namely Band-gap concerns, cost, abundance and environmental concerns.

### **2.1 Band gaps matching the solar spectrum**

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 crystalline structures particularly in band gap sense.

Fig. 1. Solar spectrum

For example, in silicon-based solar cells, recombination occurring at contact surfaces at which there are dangling silicon bonds (Wu, 2005) is generally caused by material/phase discontinuities. This phenomenon limits cell efficiency and decreases conversion quality.
