2.1. Crystalline silicon technology

Crystalline Si (c-Si) technologies dominate the current market share of PV modules (more than 90%). The aluminum back surface field (Al-BSF) [30] is the current industry standard technology but the passivated emitter and rear cell (PERC) [31] is gaining importance in the world market and is expected to replace the Al-BSF technology in the future [3]. The heterojunction (HIT) cells are also expected to gain some space with predictions of 15% of the total market share by 2027 [7]. Besides that, Si-based tandem solar technologies are expected to appear in mass production after 2019 [7].

There are different cell structures for crystalline silicon-based PV cells [32]. The cells are electrically interconnected (with tabbing), creating a string of cells in series (60 or 72 cells are standard numbers used) and assembled into modules to generate electricity (Figure 1).

A typical crystalline silicon (c-Si) PV module contains approximately 75% of the total weight is from the module surface (glass), 10% polymer (encapsulant and backsheet foil), 8% aluminum (mostly the frame), 5% silicon (solar cells), 1% copper (interconnectors) and less than 0.1% silver (contact lines) and other metals (mostly tin and lead) [33]. The rest of the components have a small percentages of the module weight [29, 34].

The EU directive [8] established recycling targets in terms of module weight and also expresses the intention to increase the collection rates to allow the progressive recycling of more material and less to be landfilled. Even with targets aiming for 65% recycling product weight, some of the current studied recycling processes can recycle over 80% of the weight of a PV module (Figure 2). However there is still incentive to improve, considering that most of the weight is from glass and frame, which are relatively easy to remove, depending on the recycling process.

### 2.2. Thin-film technologies

keeping them adequately bonded to withstand several years of outdoor exposure. The modules are made to minimize the amount of moisture that can come in contact with the solar cells and their contacts while keeping manufacturing costs down. The current standard c-Si module is bonded using two layers of EVA to bond the layers together. Because of that, recycling solar modules is a relatively complex task, since these materials need to be separated. Once the materials/layers of a solar module can be separated, metals such as lead, copper, gallium,

Originally created by PV CYCLE in 2007 and commercially available in Europe, the process of recycling mono or multicrystalline silicon modules begins with the separation of the aluminum frame and the junction boxes and then a mechanical process is used for the extraction of the remaining materials of the module (a process similar to recycling of glass or electronic waste). The problems with this process are that the value of the material recovered is low (as it is a downcycling process) and that the maximum amount of recovered materials is about 80%, which is not sufficient for future requirements, and the value of recovered materials is smaller than the original [25]. Thin film processes are under development or near implementation in Italy, Japan and South Korea but costs are not yet competitive. Even up to 90% recovery of materials is not sufficient when compared to production costs [26]. Lastly for recycling processes aiming to generate new materials, the aim is to keep the materials intact for reuse or direct recycling, recovering the frame, glass, tabbing and solar cells without breakages and in good condition. The recovery rates can achieve up to 95% and the materials recovered have higher commercial value. However, these processes are complex and are currently just at

Even with the difficulty of recovering rare, toxic and valuable materials from solar modules, the recycling process has a remarkable environmental advantage [28]. Nevertheless, the need to recycle this type of waste is imminent. The better knowledge of these technologies and growth on the waste amounts that could generate profitable outcomes has supported the development of the first PV recycling plants. Hence, PV manufacturing companies (e.g. First Solar, Pilkington, Sharp Solar, and Siemens Solar) are investing in the research on solar

The challenges to design the ideal PV recycling process are many. The focus should be on the avoidance of damage to the PV cells and module materials, economic feasibility, and high recovery rate of materials that have some monetary value or are scare or are hazardous, that can be reused in the supply chain. Finally, the next step for the industry and researchers is to

Crystalline Si (c-Si) technologies dominate the current market share of PV modules (more than 90%). The aluminum back surface field (Al-BSF) [30] is the current industry standard technology

cadmium, aluminum and silicon can be recovered and reused in new products.

laboratory scale, being studied by a few research groups [27].

create module designs that are "recycling-friendly" [29].

2. Photovoltaic technologies

2.1. Crystalline silicon technology

modules at EoL [29].

12 Solar Panels and Photovoltaic Materials

Thin-films represent less than 10% of the total PV industry [3]. The currently dominant technologies are cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous silicon (a-Si) with, approximately, 65%, 25% and 10% of the total thin-film market share, respectively [35].

Figure 1. Silicon solar module basic structure [32].

3. Photovoltaic recycling technologies

on the recycling of PV modules for all the technologies.

The summary of this process is shown in Figure 4.

Figure 4. Summary of PV cycle recycling process for c-Si modules [25].

shown in Figure 5.

PV modules are largely recyclable. Materials such as glass, aluminum and semiconductors can, theoretically, be recovered and reused. Hence it is vital that consumers, industry and PV producers take responsibility for the EoL of these modules. So far, the most common methods for recycling c-Si PV modules are based on mechanical, thermal and chemical processes.

A Review of Recycling Processes for Photovoltaic Modules

http://dx.doi.org/10.5772/intechopen.74390

15

Although thin-film solar cells use far less material than c-Si cells, there are concerns about the availability and toxicity of materials such as tellurium (Te), indium (In), and cadmium (Cd), for example. Furthermore, the production processes also generates greenhouse gases emissions during some reactor-cleaning operations. Because of these issues, it is very important to focus

PV Cycle is a not-for-profit organization which goal is to manage PV waste through their waste management programme for solar PV technologies [42]. PV Cycle was the first to establish a PV recycling process and PV waste logistics throughout the EU. In 2016 their process of recycling PV achieved a record recycling rate of 96% for c-Si PV modules (fraction of solid recycled) [25], which is a percentage that surpasses the current European WEEE standards. The process begins with the removal of the cables, junction box and frame from the PV module. Then, the module is shredded, sorted and separated. The separation of the materials allows them to be sent to specific recycling processes associated with each material.

FirstSolar [21] developed a recycling process for CdTe modules. The company manages the collection and transportation of EoL modules to the recycling centre; however, the recycling process itself must be financed. This is made by setting aside funds by the company itself at the time of the module sale, which also happens with WEEE. The summary of this process is

The recycling process starts with the shredding of the modules into large pieces and subsequently in to small fragments (5 mm or less) by a hammer mill. During the next 4–6 h the semiconductor films are removed in a slow leaching drum. The remaining glass is exposed to a mixture of sulfuric acid and hydrogen peroxide aiming, to reach an optimal solid–liquid ratio. After that process, the glass is separated again. The next step is to separate the glass from the larger ethylene vinyl acetate (EVA) pieces, via a vibrating screen. The glass is cleaned and sent to recycling. Sodium hydroxide is used to precipitate the metal compounds, after which they are sent to another company where they can be processed to semiconductor grade raw materials for use in new solar modules. This process recovers 90% of the glass for use in new

products and 95% of the semiconductor materials for use in new solar modules [21].

Figure 2. Total collection rate for WEEE in 2014 as a percentage of the average weight of EEE put on the market in the three preceding years (2011–2013) [8].

Thin-film solar cells were developed with the aim of providing low cost and flexible geometries, using relatively small material quantities. CdTe, CIGS and a-Si are the main technologies for thin-film PV modules [36]. CdTe is the most widely used thin-film technology. It contains significant amounts of cadmium (Cd), an element with relative toxicity, which presents an environmental problem that has been studied worldwide [37, 38]. CIGS has a very high optical absorption coefficient because it is a direct band gap material (can be tuned between 1.0 and 2.4 eV by varying the In/Ga and Se/S ratios [39]) and efficiency of approximately 15.7 0.5% for high bandgap [40]. A-Si has low toxicity and cost but also low durability and it is less efficient compared with the other thin-film technologies [41]. Current projections expect the a-Si module market to disappear in the near future, since they cannot compete on costs or efficiency [3].

Basically, thin-film modules consist of thin layers of semiconducting material (CdTe, CIGS or a-Si) deposited on a substrate (glass, polymer or metal) (Figure 3).

Figure 3. Thin-film solar module basic structures [36].
