Review: Advances in the CIGS Thin Films for Photovoltaic Applications

The copper indium gallium selenium (CIGS) thin film is widely acknowledged as the most promising material for photovoltaic applications. Mainly due to appealing chemical and physical structures properties, low fabrication cost, high efficiency, and uncomplicated integration especially with the advancement in the use of the flexible substrate. Promising results have been achieved in CIGS-based solar cells in the last few years and these devices could be key in unlocking the potential of green energy. Therefore, it is necessary to un-derstand the parameters that are critical to improving the efficiency of these devices. Parameters such as doping concentration, thickness, substrates, and energy bandgap. In this review, we comprehensively report on these parameters with an aim of showing the recent progress on the various methods used to optimize them, all geared towards efficient and low cost solar cells for PV applications.

. (a) Shows the world's share of renewable energy technologies in the last 35 years. Insert shows, the breakdown of these renewable technologies in Japan. (b) Shows the global solar energy consumption and the leading consumer countries [8].
competitive to the fossil sources of energy. For example, although hydropower plants are fundamental to the sustainable growth of renewable energy [4], the increased demand for water and other sources like irrigation may be in direct conflict with the objectives of river conservation. Wind power has been gaining worldwide attention as a large-scale energy source, however, its reliability is a serious challenge due to the intermittent nature of wind power [6]. To this end, the solar energy option offers great potential especially considering that the solar power received on earth is ~10 4 times larger than humanity's mean consumption [7]. In the last 12 years, the demand for solar energy has soared [8], which calls for intense research in this field to meet this demand. Solar energy harvesting forms what is referred to as photovoltaic energy harvesting and deals with the direct conversion of photons to electrons being key to meeting the world's demands for clean, sustainable, and abundant energy. In the recent past exponential development of photovoltaic (PV) technologies (polycrystalline thin-film like copper-indium-gallium-diselenide, perovskite solar c ls, silicon solar cells, dye-sensitized solar cells or organic solar cells) has led to significant reduction in the price of solar electricity, making it a potential competitor to the commonly used power sources [9].
The application of PVs however can further be enhanced by having more efficient solar cells. According to the Solar cell efficiency tables (Version 55) [10] the efficiency of solar cells (measured by a recognized test centers) is 26% and 38.8% for single and multiple junctions cells respectively. However, various research groups have reported efficiencies as high as 43% [11]. The key challenge is to not only improve the efficiency of solar cells but also make them affordable.
Currently, the bulk semiconductor dominates the market due to their easier processing and manipulation [12]. Among them is the important copper-indium-gallium-diselenide (CIGS) thin film solar cells which are characterized by high absorption coefficient and adjustability of graded band gap for solar spec-  [19].
Expanding applications of CIGS in PV devices demands for the optimization of parameters that are keys to the enhancement of the efficiency of these devices.
To fully realize the potential of CIGS for PVs applications parameters such doping concentration, thickness, substrates and energy band gap must be well understood. In addition to effect of forming CIGS based heterojuctions for enhanced performance. Therefore, this review mainly focuses on recent (mostly within past 10 years) progress on the improving the performance of CIGS based devices by optimizing the parameters.

1) Thickness
The standard thickness of the CIGS layer is ≈2 µm. Reducing this thickness can be a key in reducing the overall production cost and the materials usage.
However reducing the thickness can reduce the amount of light absorbed by the layer and back contact recombination because of electrons being generated near the back coat. Soumaila et al. [20] investigated the influence of absorber back surface region grading [20] in CIGS solar cells. To achieve the optimal performance, thickness of the back surface grading layer and the absorber bulk thickness were varied. The results showed that back surface grading greatly improved the performances of CIGS [20] [21] [22]. Figure 2(a) shows that the efficiency of the device increased with increasing the absorber bulk thickness d. Primarily due to an increased short-circuit current density (J sc ), which varies by 3.2 mA/cm 2 from 29 mA/cm 2 when d changed from 0.5 μm -2 μm (see Figure 2  [30].

3) Effects of temperature
Varying temperatures during CIGS thin film deposition can be used to determine the effect of thin film growth in relation to solar cell efficiency. Stuckelberger et al. [31] investigated the complex temperature dependence of defects and voltage in CuIn x Ga 1-x Se2 thin solar cells. The growth temperature ranged from room temperature to 100˚C and the thin films were deposited by Mia-Sole on flexible stainless steel substrates. They concluded that a crucial understanding of light-induced and heat-induced metastabilities at the microscale is vital in relation to the overall module performance especially the efficiency values. In addition, the deposition of polycrystalline CIGS thin films onto Mo-coated sodalime glass substrates using the three-stage co-evaporated process was done [32]. tor-based absorber layers [33]. Another study used cadmium sulphide (CdS) buffer layer, and showed that the solar cell performance is affected by the operating temperature [34].

4) Effects of post-selenization and use of a precursor
The quality of CIGS thin films can be improved by deposition of the precursors followed by post-selenization which in the process improves the cell efficiency. Post-selenization of copper, gallium and indium precursors to fabricate CuIn x Ga 1_x Se 2 (CIGS) thin films can be achieved by the use of Se vapour, diethylselenide or H 2 Se gas. Using Cu-In-Ga precursors and H 2 Se gas Cu(In1_x-Gax)Se2 (CIGS) thin films fabricated [35]. To improve the optoelectronic properties, a high temperature selenization and in situ annealing process was conducted. Morphological and crystal characterization showed that the films had large grain size and with improved crystallinity. Conversely, sputtering of CuIn-Ga precursors followed by chalcogenization was done to fabricate CIGS thin films. Two stage selenization processes were employed and then the microstructural characteristics of CIGS films studied [36]. The selenization temperature for the two processes was varied between 450˚C and 580˚C to establish the relationship between the microstructural characteristics and compositions of the CIGS films. From the results, the CIGS thin films formed using isothermal selenization were found to have dense grain structure whose grains increased in size after an increase in the selenized temperature. However, the Se/(Cu + In + Ga) ratios of the films indicated that Se was distributed non-uniformly in the films.
Further investigation employed a rapid thermal process of stacked elemental layers. Here, the properties of the Cu,Ga and In layers deposited by DC-sputtering were studied [37]. By varying the thickness ratio of the In/CuGa layer, the chemical compositions of the metallic precursor were optimized. The optimized precursor was then selenized under various temperatures after which the performance of the fabricated CIGS solar cells could be investigated and analyzed.
The experimental results showed that the performance of the CIGS solar cells enhanced at higher selenization temperatures. The use off non-vacuum coating techniques for CIGS thin films is an interesting thing and many efforts have been made to develop for solar cell applications. The approach may either use solution type precursors or particle-based precursors [38]. For instant, Gas flow sputtering of CIGS with slightly Cu-poor stoichiometry was performed with two opposing CIGS targets i.e selenium only provided by target and additional selenium from an elemental source inside the sputtering system [39]. From the results, the thin films deposited without extra selenium produced cells of efficiency  In addition, CIGS thin films were prepared onto different substrates by thermal evaporation technique in a high vacuum system of (10 −5 ) torr [43]. Sodium is another interesting candidate for doping. Ideally, CIGS absorbers have the following shortcomings; including poor crystallinity, large porosity, and rough surfaces, which result in lower power conversion efficiency as compared to vacuum-based CIGS solar cells. Therefore, promoting absorber grain growth is fundamental to enhancing the performance of these devices especially the solution-based solar cell. The use of Sodium which is alkali based has been shown to improve the grain growth and enhance the absorbing ability of the CIGS layer [48]. Specifically it leads to morphological changes leading to improved carrier collection and minority carrier lifetimes. Another way of doping is the use of Cs-PDT although the mechanism of doping remains controversial. The entry of only locate at the grain boundary but also enter the grains. Implying that they could passivate the defects both at the grain boundary and grain interior, improving the hole carrier concentration and minority carrier lifetime [49].

Conclusion
From the above discussion, it shows that CIGS quaternary compound is good candidate for use as an absorber layer in high efficiency thin films solar cells.
However, the efficiency of the CIGS thin films solar cells is dependent on the following factors: substrate and growth temperature, deposition techniques and the stoichiometry composition. Unlike the other compounds, in thin films of the alloy CIGS copper, indium, and gallium typically redistribute during growth to create composition profiles in the final layers completely different from their initial distribution. Therefore, it is necessary to discuss the essential materials such as gallium and its impact on the structural properties of CIGS solar cells. This provides a better understanding of the relationship between the emitter and absorber bulk in relation to electronic fields, carrier transport, and recombination processes that determine device performance. In this review, we have discussed these parameters with an aim of showing the recent progress on the various methods used to optimize them, all geared towards efficient and low cost solar cells for PV applications.