**3. Spray-coating: a low-cost technique for chalcogenide solar cells**

In general, the success of the deposition of a thin film by spray-pyrolysis depends mainly on three factors: the composition of the precursor solution, the atomization and droplet transport process and the substrate temperature [30]. The control or modification of these parameters directly impacts the quality of the obtained film. Considering the production of the precursor solution, it can be composed of inorganic salts or organometallic compounds dissolved in aqueous or organic solvents, respectively.

The choice of the precursor reagent and solvent is an important step, since it will define the maximum concentration of salt that can be used and will directly affect the process of formation and transport of the aerosol in the atomization step [30]. The use of aqueous solvents and inorganic salts are the most used reagents, since they have lower degree of toxicity and environmental pollution than organic solvents and organometallic compounds. In addition, it has been observed that the use of alcoholic organic solvents can lead to carbonaceous impurities that are harmful to the photovoltaic device. They affect the growth of crystals in the film and acting as barriers for extracting loads, leading to low values of open-circuit voltage (Voc) and fill factor (FF) of the solar cell [31, 32]. A common approach in the production of precursor solutions for deposition of copper-based chalcogenides thin films, such as CIS and CIGS, is to prepare aqueous solutions stocking the chloride or nitrate salts of the metals of interest, and the sulfur source, which is normally the thiourea. The concentration of the sulfur precursor is usually 5 to 10 times more concentrated than metallic salts. The excess of the sulfur has a compensatory role due to losses during deposition, and when using molybdenum substrate, it prevents the formation of oxides of this metal [33, 34]. These solutions are diluted and mixed to obtain the final precursor solution, the concentration of each component in the final solution must be varied in order to assess the influence of each metal on the chemical and physical properties of the obtained films. Solar cells with a CIGS absorbing layer obtained by spray-pyrolysis using this approach in the preparation of the precursor solution have achieved 5 to 10% efficiency [31, 35].

The typical atomizers used for depositing chalcogenides films are the pneumatic and ultrasonic. The aerosol formation mechanism is quite different between the two models, while in the pneumatic the aerosol is formed by the action of a pressurized gas, in the ultrasonic, the aerosol is produced by ultrasonic waves generated by a piezoelectric component that is in contact with the solution, the formed aerosol is transported to the substrate surface by a carrier gas [36, 37]. Although ultrasonic atomizers are more expensive and complex than pneumatic ones, these atomizers have better control of average droplet size and the rate of aerosol formation is independent of the flow rate of the carrier gas. The efficient control of the droplet size distribution is important due to its influence on uniformity of the deposited film [38]. The two types of atomizers mentioned are also differentiated by the atomization rate and the initial speed of the drops, because as the pneumatic works with a pressurized gas carrier, both the atomization rate and the velocity of the droplets are much higher [30, 39]. In pneumatic spraying, the main factor to be tested is the pressure of the carrier gas, since the distribution of the average size of the droplets and the initial velocity of the droplets, as well as the rate of aerosol formation are directly linked to this factor. On the other hand, when using an ultrasonic atomizer the main factor to be monitored is the atomization frequency, since the distribution of the average droplet size is defined by this parameter [40]. When choosing carrier gas, one must consider whether it is reactive or not. Oxidizing gases, such as O2, should be avoided, as they can react with the precursor metals in the pyrolysis phase,

**Figure 2.**

*Schematic illustration of thin film deposition by spray-pyrolysis and the effect of increasing substrate temperature on the spray-pyrolysis deposition.*

leading to the formation of unwanted oxides. In this sense, inert gases such as N2 or Ar are the most suitable for depositing chalcogenides by spray-pyrolysis [41].

Among the several factors that must be taken when depositing thin films by spray-pyrolysis, the substrate temperature is pointed out as the most important parameter [38]. Many stages of the thin film deposition process depend directly on the substrate temperature, among which are: droplet spreading, solvent evaporation and salt decomposition and compound formation. All these steps are important for obtaining the compound with the desired chemical and physical characteristics in the form of a homogeneous thin film. **Figure 2** summarizes the effects of increasing the temperature on the deposition process [30, 38, 39]. At low temperatures, the solvent still does not evaporate and the droplets of the precursor solution collide with the substrate and undergo decomposition (process A). At intermediate temperatures the solvent evaporates completely during transport, reaching the substrate in the form of a dry precipitate which then decomposes (process B). From intermediate to high temperatures, the solvent evaporates, and the solid precipitate is formed, but before reaching the substrate the precipitate melts and vaporizes, leading to chemical vapor deposition (CVD) (process C). Finally, at high temperatures (process D) the vapor phase formed after melting the precipitate decomposes to form the compound powder, which is deposited on the substrate. In this context, the processes A and D are not indicated for chalcogenides deposition, because the films obtained are often rough or the form of poorly adherent powder [38]. Thus, temperatures where processes B or C can occur (300° to 450°C), are the most suitable for the deposition of thin films of chalcogenides such as CIGS and CZTS.

**Table 2** shows the parameters used in the spray-pyrolysis deposition of absorber layers of copper-chalcogenide thin film from aqueous precursor solutions and the parameters obtained in the solar cells. As can be seen, most of the chalcogenide films are deposited using pneumatic atomizer, this reflects the simplicity and cost– benefit of this type of system. Other points to note are the prevalence of the use of N2 as a carrier gas, and the flow rate that varies between 0.25 and 3 ml min−1.

Mo-coated glass is among the most used substrates for depositing chalcogenides for PV applications. However, the preheating temperatures of the substrates required in the spray-pyrolysis process are high enough (T ≥ 200°C) for the formation of Mo oxides. The oxide layer can lead to loss of charge carrier collection efficiency and worsen the adhesion of the film that will be deposited later. The solution found by Ho and colleagues [31] was to deposit the CIS layer with a gradual increase in substrate temperature from 120°C to 300–350°C. The XRD patterns shown in **Figure 3a** indicate that in the initial phase of deposition (red line) with temperatures between 120 and 150°C, in the intermediate phase with temperatures

#### *Solution-Processed Chalcogenide Photovoltaic Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94071*


**Table 2.**

*Atomizer parameters used in the spray-pyrolysis deposition of some copper-chalcogenides and the parameters obtained in solar cells.*

#### **Figure 3.**

*(a) XRD patterns of the CIS films deposited by spray-pyrolysis at different substrate heat stage. (b) Crosssection SEM image of CIS film de posited on the preheated substrate. (c) Cross-section SEM image of CIS film deposited with gradual substrate heating. (d) CISSe film after heat treatment and selenization processes. Adapted with permission from [31]. Copyright (2014) American Chemical Society.*

between 150 and 250°C (blue line) and the final phase with temperatures between 250 and 350° (green line) there were no characteristic peaks of MoO2. On the other hand, in the films in which the Mo substrate was preheated between 300 and 350°C (violet line), peaks related to MoO2 were found. **Figure 3b** and **c** show the scanning electron microscopy (SEM) images of films deposited with preheating and gradual heating, respectively. As can be seen, the CIS film deposited with preheating was less uniform and with pores at the CIS/Mo interface. The formation of Mo oxides is identified as the main cause of film malformation. Finally, most of the chalcogenides thin films deposited by spray-pyrolysis require a thermal treatment after their deposition, this is essential to increase the crystallinity, as shown in **Figure 3d**, and by means of sulfurization or selenization serves also for the stoichiometric improvement of S or Se deficient films.

#### **4. Conclusions**

Solution-processed chalcogenide thin film solar cells have already reached similar efficiencies to the ones prepared by using vacuum techniques. Spin-coating and spray-coating are inexpensive alternatives to fabricate highly efficient devices. The progress in the field since the hydrazine-based solution to deposit chalcogenide led to the development of environmentally friendly and low-cost molecular inks, nanocrystal inks and hybrid inks. Although solution-processed CIS and CZTS solar cells are equally to or more efficient than the vacuum-based devices, many improvements need to be done to put solution-based CIGS solar cells on top of the

*Solution-Processed Chalcogenide Photovoltaic Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94071*

efficiency charts. Spin- and spray-coating are undoubtedly more advantageous for the process of solar cells fabrication, and more efforts are still needed to develop inks using environmentally friendly solvents, metals and chalcogen precursors. It is still needed to work on decreasing the residues in the films to eliminate any possible site of recombination. Optimization of these techniques will be essential to the scalability of the fabrication of the highly stable and highly efficient solar cells.
