**1. Introduction**

These two last decades have been marked by a dizzying rise in the field of solar energy harvesting thanks to photovoltaic technologies. Among the three wellknown fields of solar device technologies, thin-film solar cells are nowadays gaining more interest among researchers and industrial applications due to the reduction in the quantities of materials, their flexibility, their low environmental impact, reduction in time and new opportunities for higher yields goals. The achievement of a record yields of 22.6% [1] for CIGSe-based thin-film solar cells at the end of 2016 marked a decisive turning point towards the quest for higher yields. On the basis of recent studies, the efficiency of CIGSe-based ultra-thin solar cells can be further considerably improved by directing studies on obtaining the best structural and opto-electrical properties of the different materials layers that constitute the cell structure. For example, among the alternative materials for the buffer layer, scientific community recognizes the Zinc Sulfide ZnS material as the one which stands

out as the most promising, although the highest yield obtained with it to date does not exceed 22%. In the process of reducing the thickness of the layers, numerous works have been identified. In 2015 a yield of about 20.75% was achieved with CIGSe device using only 1 μm of absorber layer thickness and ZnS as buffer layer [2]. A couple of years later, a yield of 22.62% is recorded using nanoscale thicknesses for the different layers but using the controversy CdS material as buffer layer [3]. By going into continuation of the above-mentioned works, a correct modeling with the trusted numerical simulation software SCAPS allow to investigate the influence of the values of certain properties of layers and interfaces mainly, their electrical susceptibility, their crystallographic structure, their intrinsic doping level and the mobilities of charge carriers, on the performances of CIGSe-ZnS based solar cell structures. In this chapter, we carry out investigations and we highlight key properties values that allow to record very promising results. A good knowledge of these values is a basis to better control the material design and layers deposition processes, to be closed to these theoretical results [4, 5]. To achieve this goal a good understanding of the micro-activities which occur at the level of the layer interfaces which are the recombination mechanisms of the photo-generated electron-hole pairs, and the effects of the defect states which inevitably appear with thicknesses reduction, is required to limit their harmful effects on the cell performances [6, 7].

• **Cadmium Telluride (CdTe) thin films**: the record yield is 22.1%. It is the leading thin film technology in the photovoltaic industry. However, a

is a highly harmful material for environment purpose.

*Thin-Film Solar Cells Performances Optimization: Case of Cu (In, Ga) Se2-ZnS*

conversion yield of 22.6% in 2016 [1].

*DOI: http://dx.doi.org/10.5772/intechopen.93817*

can offer a yield of up to 11.5% [1].

**2.2 CIGSe-based cells: evolution**

• *Modification of the structure*

• *Introduction of Gallium*

• **Influence of sodium**

**87**

electrical performance of the solar cell.

at the Swiss Federal Institute of Technology in Lausanne.

work.

promising future is not guaranteed because of the use of cadmium (Cd) which

• **Thin films based on Indium Gallium Copper DiSelenide Cu (In, Ga) Se2**, known as CIGSe based solar cells: which represent the best yields with a

The third generation includes all the new approaches proposed and developed in recent years, this generation is to reduce manufacturing costs (Organic solar cells, Gräetzel<sup>1</sup> cells, etc.). It seeks to overcome the current limits of yields by resorting to original concepts such as multi-junction cells, intermediate gap cells or using hot carriers. The majority of third generation systems are currently under development and target more or less long-term industrial applications. Gräetzel cells for example

The scientific advances which have enabled the realization of very high efficiency thin film solar cells have taken place by successive technological leaps, the study of these different key stages is essential to understanding the complexity of the structure of a solar cell based on standard CIGSe and the problematic of this

Initially intended for the manufacture of photo-detectors, the first solar cells consisted of single crystals of CuInSe2 (CISe) evaporated on an alumina/molybdenum substrate. Interest in photovoltaic applications grew very quickly in regards to the good yields of around 9% obtained by BOEING in 1981. Since the 1980s, four

Modification of the buffer layer by the Cadmium Selenide (CdS) layer and the introduction of the ZnO: Al window layer enhanced absorption of the solar spectrum at short wavelengths. As of today, many materials have already been used as a buffer layer in particular (In2S3, ZnS, ZnSe, Zn (S, OH)) and as a window layer we

Beginning in 1987, Chen et al. [9] attempted to incorporate Gallium atoms into the CISe structure. The partial substitution of indium by gallium has improved the

In the 1990s, Hedson et al., working on the substitution of the initial substrate to soda glass in order to reduce costs, found that the performance of solar cells was

<sup>1</sup> It was in 1991 that Gräetzel's cell was discovered by Michael GRÄETZEL, a Swiss chemist and professor

main developments have made it possible to obtain current yields.

will see for example zinc oxide doped with Boron (ZnO: B).

This chapter will be structured as follow, in the first section we are presenting the state of the art on CIGSe-based second-generation thin film solar cells. The different properties of materials are highlighted in the second section, and we recall the mathematical relationships that support the microscopic phenomena within layers and interfaces, and which constitute the basis set of our solar cell modeling. In the last section, investigations are carried out to understand the influence of certain material properties on the overall performance of our device and depending on the results obtained, we highlight those which lead to better yields.
