Currently, c-Si modules are mainly treated in bulk recycling plants designed for treating laminated glass, electronic waste and metals. In these facilities, a mechanical approach of crushing and sorting is employed to recover only bulk materials such as glass, Al frames and Cu [11,25]. Meanwhile, the remaining silicon cells and other materials including plastics are incinerated or disposed. According to IRENA [4], the cumulative recovery yields of these bulk materials can achieve more than 85% by panel mass just through mechanical separation. However, the treatment capacity of these facilities is low and the glass recovered is recycled as low-grade product [57]. Thus, in order to accomplish high-value recycling with greater recovery yields, further treatment of thermal, chemical or metallurgical methods are needed to recover the semiconductor materials [4].

The first step of recycling c-Si PV modules is to separate the major components, which are the Al frames, junction boxes, wiring and laminated glass [22,35]. This can be done through manual separation, thermal treatment or automatic separation [25,35,58]. Next, the EVA encapsulant layer sealing the silicon cells has to be removed through thermal, chemical or mechanical treatment. These studies have shown that thermal treatment of the EVA layer is more preferable compared to chemical treatment which requires usage of expensive and toxic reagents [35,59]. Besides, the effectiveness chemically treating the EVA layer is quite low, as the time duration needed to delaminate the glass is too long. Additionally, thermal treatment is usually able to recover glass and the silicon cells without breakage compared to most of the mechanical processes [19,57]. The heating condition or requirement of a pretreatment during thermal treatment is fairly important so as to avoid damaging the silicon cells, which can fetch a higher price in an unbroken state [50,60].

After removing the EVA layer, the next step is to recover the silicon wafer from the silicon cells. Conventionally, this process is carried out via chemical etching, which utilises acidic or basic solutions to sequentially remove the metal electrodes, anti-reflective coating (ARC) and n-p junction on the surface of the silicon cells [35,61]. However, there also exists laser treatment for removing the coating on the silicon cells, as demonstrated by this study [59]; however, the cost of employing this method is high and its effectiveness is low compared to chemical etching. In addition, a study [62] proposed a combination of chemical and mechanical treatment to remove the silicon cell coatings in an effort to reduce the use of hazardous chemicals such as hydrogen fluoride (HF), nitric acid (HNO₃) and phosphoric acid (H₃PO₄).

The processes discussed above are general treatment methods for a complete recycling of c-Si PV modules. As summarised in Table 1, there are variations in the methods used depending on the studies and corporations.

The recycling processes provided by Granata et al. [56], the laminated glass recycling facility (LGRF) [25,53], YingLi Solar [64] and NPC group [66] are considered to be bulk recycling as semiconductor materials and precious metals are not recovered. On the other hand, the studies by Jung et al. [50], Park et al. [62] and the industrial pilot-scale project (Full-Recovery End of Life Photovoltaics (FRELP)) [58] are examples of high-value recycling where both bulk materials and semiconductor materials are reclaimed. The other treatment processes [35,59,65] are considered as semi-high-value recycling, as some of them recover only silicon wafer but no other precious metals. Meanwhile, the Deutsche Solar AG recycling process is closed-loop recycling which integrates reclaimed cells back into a standard PV module production line [34,63].

With the environmental impacts from the production of virgin silicon cells in mind, it is highly recommended to employ high-value and closed-loop recycling processes to recover silicon wafer, especially in an unbroken state [57,61]. This is because even if the cells have damage such as microcracks or edge chipping, they cannot be recycled into a whole wafer and have to be crushed into powder form to be used for silicon ingot production again [57,62]. Besides, high-value recycling processes have lower environmental impacts due to the displacement of primary production from the increased yields and greater profits from the higher quality of the materials recovered [53,70].

Although most of the recycling processes are carried out on intact c-Si PV modules, some processes, such as those from Klugmann-Radziemska and Ostrowski [35] as well as Deutsche Solar, are applicable for broken or damaged PV modules. After the removal of external major components and initial thermal delamination treatment, the modules are usually crushed for further treatment; thus, broken modules can still be recycled [67] but the broken wafers cannot be remade into new wafers.

In a study by IEA [57], most patents on thin-film modules recycling indicated a focus on high-value recycling and the patent assignees for thin-film modules recycling were mostly corporations (95% of patents for thin-film modules recycling), suggesting that thin-film modules recycling patents are more likely to be commercialised [57]. Furthermore, in thin-film modules recycling, the recovery of semiconductor materials, despite being in very small quantities, is more important than the recovery of glass due to the scarcity of semiconductor materials [30]. Hence, CdTe thin-film modules are more commonly recycled in dedicated recycling plants instead of being bulk recycled in existing recycling plants [25].

Similar to c-Si PV modules, the first step of recycling CdTe thin-film modules is to remove the junction box. Next, the encapsulant sealing the semiconductors between the cover glass and glass substrate has to be removed, mainly through mechanical or thermal means. However, there is a study by Palitzsch (as cited by [57]) that used an optical approach to separate the glass layers in thin-film modules. Then, the semiconductor materials on the glass substrate are stripped off, usually by chemical etching; however, the mechanical method of stripping semiconductors off the glass is also being investigated [45,67]. The various recycling processes for CdTe thin-film modules are presented in Table 1.

These studies [67,68,69,71] presented the recycling processes of CdTe modules by First Solar, a commercial CdTe thin-film module manufacturer. These studies have displayed the evolution of First Solar recycling processes, although a high-value recycling principle has been applied since the first study. Through First Solar’s recycling processes, approximately 90% of the weight of a CdTe module can be recovered, consisting mainly of glass which can be reused in new glass product [68].

ANTEC Solar has developed a patented process for recycling CdTe and CdS thin-film PV modules by treating the modules in a gaseous environment [44]; this proves is distinct from the other chemical etching processes where wet chemicals are used. Meanwhile, Berger et al. [45] have examined the feasibility of the wet-mechanical process in recovering metals from thin-film modules. The advantage of the wet-mechanical process is its lack of chemical usage; however, there are considerable losses of valuables, resulting in low efficiency compared to conventional chemical etching [45].

For thin-film modules, as there are no materials requiring to be intact during recycling (as in Si cell), the mechanical approach to delaminating thin-films modules is not discouraged. Furthermore, crushing the modules into smaller particles enhances the performance of the subsequent chemical etching process [57], hence even broken modules can be treated in the same processes. Aside from that, most processes are not just applicable to CdTe and can be suitable for other thin-film modules as well. For instance, First Solar recycling process is applicable to CIS thin-film modules, albeit with the recovery chemistry for indium requiring further investigation [71].

Recycling EoL PV modules provides numerous benefits, especially to the environment. The environmental performance of recycling can be analysed via the approach of LCA, which has been carried out by many studies, as summarised in [22]. The main benefit of recycling PV modules is the reduction of the energy use and emissions associated with raw material production and the usage of secondary materials. In contrast to its zero emissions operational stage, the EoL recycling stage of PV modules consumes energy and release emissions due to the usage of fossil fuel in the recycling processes. However, a few studies have shown that the environmental impacts from recycling are very little compared to those of the production of the PV modules [25,36,69,70,72]. In addition, the recovery of materials from recycling to produce secondary resources offsets the energy use and emissions related to virgin material production, as shown by several LCA studies studies [16,25,41,53,63,69,73,74,75]. A recent LCA study has demonstrated that the environmental impacts of producing c-Si cells from recycled materials were 58% lower than production of cells from virgin materials. The results were mainly due to decreased energy consumption from processing raw silicon [61]. Furthermore, a study has shown that the supply of semiconductor materials, including In, Ga, Se and tellurium (Te), will have their reserves depleted in 5 to 50 years at the current rate of extraction [76]. Therefore, there are greater motivations to recycle PV modules, including the recovery of the valuable materials contained within them to prevent the deficit of raw materials [20,29] and the reduction of the environmental impacts caused by the processing of raw materials.

Compared to a landfill disposal scenario, the recycling of PV modules is able to reduce the amount of waste and waste-related emissions [4,77]. According to Vellini et al. [37], the recycling of poly-Si modules was able to reduce the terrestrial eco-toxicity potential (TETP) by 73.58%, fresh water aquatic eco-toxicity potential (FAETP) by 67.4% and acidification potential (AP) by 37.48% compared to EoL without recycling scenario. Furthermore, environmental credits are given when energy recovery is established during thermal treatment or incineration of PV modules, leading to significant reduction in terms of ozone layer depletion potential (OLDP) (27%), ionising radiation human health (25%) and freshwater eutrophication (18%), as evidence by this LCA study [58].

In addition to environmental benefits, recycling PV modules also brings about economical profitability. IRENA has estimated that the material value of material that can be recovered from recycling PV modules by 2030 and 2050 amounts to USD 450 million and 15 billion, respectively [4]. These impressive economic values are obtained from the secondary materials embodied within the EoL PV modules, which are locked away when PV modules are manufactured and cannot be accessed until the lifetime of a PV module is over [78]. Recycling PV modules recovers these materials, and reselling them into the global market helps stimulate a market for secondary raw materials [4]. Besides this, these secondary materials can help keep the costs of PV materials low due to their lower pricing [77]. Furthermore, the establishment of new PV EoL industries can yield new employment opportunities such as waste collectors, pre-treatment companies, producers and installation companies [4,23], hence providing economic growth to the country.

Additionally, setting up proper recycling infrastructure for PV modules further reinforces the ‘clean’ image of the solar PV system. Proper recycling can answer public concerns about the toxicity of hazardous materials present in PV modules by demonstrating that those materials can be recovered instead of being disposed in an unregulated way [71,79]. This can assist the large-scale penetration of PV in the market [72] due to the assurance offered by the proper management of EoL PV modules.