The Importance of Exploring and Developing a Wide Variety of Photovoltaic Technologies

1. Introduction

According to the latest Copernicus report [1], 2024 is expected to be the warmest year on record and the first year with temperatures more than 1.5 °C above pre-industrial levels; however, this growth is concerningly in line with the temperatures recorded last year and in previous years [2]. Furthermore, in recent decades, the effects of climate change have become increasingly evident. The impact of global warming on ecosystems also affects the lives and livelihoods of millions of people worldwide, making natural disasters more frequent and powerful while also exacerbating existing social and economic inequalities. It is well known that the natural climate has oscillated between warm periods and ice ages for the past million years. This fluctuation is closely related to Milankovitch cycles. However, the scientific community agrees, based on numerous studies, data, and simulations, that the changes in global temperature during the twentieth century can only be explained if both natural and anthropogenic processes are considered [3]. In particular, global warming is associated with the increasing concentration of greenhouse gases in the atmosphere. The Earth’s temperature is determined by the energy balance between absorbed solar radiation and emitted infrared radiation, and greenhouse gases absorb infrared radiation from the Earth’s surface. Without them, the Earth’s temperature would be close to minus 20 °C, making life as we know it impossible. On the other hand, an increase in greenhouse gases in the atmosphere reduces the emission of infrared radiation, creating an imbalance that results in an increase in temperature [4]. Approximately three-quarters of emissions of these gases come from energy use and consumption [5]; this is why replacing conventional energy sources with clean, renewable energy technologies is paramount. Furthermore, energy demand is expected to rise exponentially in the coming years, making it increasingly necessary to resort to energy sources that, unlike fossil fuels, are not at risk of depletion [6].

Renewable energy sources, such as solar, wind, hydropower, ocean and geothermal energy, biomass, and biofuels, are inexhaustible and cleaner alternatives to fossil fuels. They can lower greenhouse gas emissions, diversify our energy options, and reduce our dependence on volatile fossil fuel prices. Exploiting all these sources and utilizing every possible technology to diversify energy production is essential. Many of these sources are not present everywhere, nor are they constant over time. On the other hand, each source can be efficient for energy production if utilized in the right location and for the most suitable application. Therefore, it is essential not to focus on just one alternative energy source; the combined development and use of multiple sources can solve this pressing issue.

One of the technologies that harness renewable energy sources, specifically solar energy, is photovoltaic (PV) PV technologies have been classified, depending on their development over time, into first-, second-, third-, and fourth-generation devices:

• The first generation is based on monocrystalline and polycrystalline silicon and gallium arsenide.
• The second generation focuses on developing thin-film PV technologies, such as CdTe and CIGS.
• The third generation includes “emerging technologies”, characterized by low manufacturing costs, non-toxicity, and elemental abundance of their constituent, such as perovskites and organic cells, as well as multi-junction devices.
• The fourth generation refers to a new hybrid technology under development that uses nanoparticles or organic nanomaterials such as graphene, carbon nanotubes, and graphene derivatives.

Subsequent generations have been developed, in some cases, to reduce production costs and, in others, to address new technological needs or applications, without replacing or interrupting research on earlier technologies. To date, the market is still dominated by panels from the first two generations: approximately 97% consist of monocrystalline silicon, with the remainder consisting of CdTe thin-film devices [7]. This does not mean that research on new materials is useless; rather, it is essential to differentiate technologies and develop them all to ensure the proper technology is available for each purpose. Research is now focused not only on the development of traditional photovoltaic panels but also on, for example, devices for powering the Internet of Things (IoT) [8], and cells integrated into construction elements, building materials (building-integrated photovoltaics, BIPV), or space applications. Therefore, it is essential to develop materials that enable the development of devices with diverse characteristics, such as semi-transparent or colored cells, materials that can be deposited into light and flexible substrates, devices in superstrate or substrate configurations, etc.

In light of these observations, the Special Issue “Advances on Solar Energy Materials and Solar Cells” highlights a wide range of photovoltaic research, covering a wide range of technologies and applications.

2. The Wide Variety of Photovoltaic Technologies: A Short Review

The International Renewable Energy Agency (IRENA) reported that, in 2023, the levelized cost of electricity produced by solar PV panels was 56% lower than the weighted average fossil fuel-fired alternatives compared to 2010, when it cost 414% more [9]. In 2023, the global spot price of mono-facial monocrystalline Si modules dropped below 0.15 USD/Wdc; this technology accounted for 98% of PV panel shipments, 80% of which came from Chinese production. The drop in the price of these modules is also due to the exponential growth of the Chinese market, which, in 2009, represented only 1% of exports [10]. Moreover, about 97% of the world’s silicon wafer production occurs in China. Even the United States has no active production of c-Si ingots, wafers, or cells; these wafers are shipped from China and turned into solar cells [11]. China’s rise began in 2010, when the Chinese government, in its market development programs, initiated targeted support for the sector, making it a world leader in both the production and diffusion of photovoltaics [12]. The Chinese market is still favored due to the lower costs of materials, electricity, and labor. This push by the Chinese government has focused mainly on crystalline silicon technology, which is the traditional application as it was the first to be developed: researchers at Bell Laboratories presented the first practical silicon solar cell in 1954. It is worth noting that this technology is also very close to that used for producing electronic chips. Today, the efficiency of monocrystalline silicon cells on a laboratory scale has reached 27.3% while that of modules is 24.9% [13].

However, the success of these panels should not halt research into alternative devices. This technology is suitable for traditional photovoltaics with classic modules but can hardly be used in BIPV as it cannot be deposited on flexible or alternative substrates. This is intrinsic to its manufacturing process, which includes growing a silicon ingot using the Czochralski (CZ) process, cutting the ingot into wafers, processing the wafers into solar cells, interconnecting the cells into circuits, and finally completing the assembly of a module [14].

The search for photovoltaic devices that could be produced with faster and cheaper manufacturing methods gave rise to the second generation of PV. Attention was paid to direct band gap semiconductors with the goal of saving material, as they require only a few microns of material to absorb all the solar radiation, unlike crystalline Si, which requires hundreds of microns. The first thin-film cells were based on amorphous silicon, later using alternative semiconductors, such as CdTe and CIGS. These absorbers enable high conversion efficiencies even when used in the polycrystalline form due to the possibility of passivating the grain boundaries [15]. This allows the use of a wide range of physical or chemical deposition methods and direct growth onto large substrates [16]. These panels can therefore be manufactured through a fast and cost-effective in-line production process, from glass to the finished product. Furthermore, it is possible to use light and flexible substrates [17].

As mentioned, about 3% of the global market is occupied by CdTe thin-film devices; however, according to data from the U.S. Manufacturing of Advanced Cadmium Telluride Photovoltaics (US-MAC) Consortium, this technology supplies 40% of the U.S. utility-scale photovoltaic (PV) market. The United States has made significant investments in developing CdTe devices, which have seen a major increase in efficiency over the last 10 years, [18] both on a laboratory scale, now at 23.1%, and in modules, now at almost 20% [13].

Among other thin-film technologies, CIGS has achieved the highest efficiency of 23.35% [19], and modules show an efficiency of 19.2% [13,19]. However, this is lower than CdTe due to the greater effort required to scale up the deposition of a quaternary compound on large areas.

The third generation of PV builds from the advantages of thin films while focusing on the pursuit of low production costs, non-toxicity, and elemental abundance of their constituents. From the crystal structure of CIGS, by replacing two group III elements with one group II (Zn) cation and one group IV (Sn) cation, kesterite (CZTS) is derived, which is formed only by non/low-toxic and abundant constituents on earth [20]. The record efficiency of these devices is 14.9% [21], which is modest compared to the previous ones. Still, they can be successfully fabricated by non-vacuum solution-based growth techniques, which is unique for their low cost [22].

Over the past 12 years, a new family of materials has emerged that seems promising as absorbers for photovoltaic applications: antimony chalcogenides [23], in particular Sb₂S₃, Sb₂Se_3, and Sb₂(S,Se)₃, which have demonstrated a promising trajectory of rapid performance improvement, achieving power conversion efficiencies of 8.0%, 10.57%, and 10.75%, respectively [24,25,26]. Their main feature is a one-dimensional nature, being composed of repeating (Sb₄(S,Se)₆)n chains oriented along the c-axis, which are weakly bound together by van der Waals interactions. This peculiar quasi-1D structure makes them strongly anisotropic materials in which charge transport occurs only along the ribbons, leading to intrinsically benign grain boundaries. Furthermore, they have a very high absorption coefficient, about 10⁵ cm⁻¹, which allows the use of a thin layer down to 300-400 nm [27].

Recently, a new class of inorganic–organic hybrid compounds CH₃NH₃-PbX₃ (X = I, Br, Cl) with perovskite crystal structure has emerged. The most surprising thing is that their efficiency has increased from 3.8% to 26.7% in just 15 years [13]. They can be produced at a low cost, and their tunable band gap enables them to be used for engineering tandem solar cells with excellent results: currently, the perovskite/Si tandem has achieved an efficiency of 34.2%, while the perovskite/perovskite tandem has achieved an efficiency of 30.1% [13]. Tandem devices are another emerging technology, which consists of multi-junction cells, introduced to overcome the Shockley–Queisser limit for single solar cells. However, perovskites have stability issues that arise from the organic components that easily decompose or leak when exposed to moisture or external thermal forces; this is the main challenge for their future application [28]. Another key goal is replacing lead in the compound to reduce its toxicity, but this replacement currently lowers conversion efficiency [29].

The fourth generation of PV was recently developed to create devices that combine the low cost and flexibility of polymer thin films with the stability of organic nanostructured functional materials, such as metal nanoparticles, metal oxides, carbon nanotubes, graphene, and their derivatives; these devices are also referred as “nano-photovoltaics” [30,31]. Currently, the study of nanotechnology is gaining increasing attention, representing a revolutionary approach to technological development focused on managing matter at the nanoscale. Indeed, nanotechnology can be defined as creating functional materials, devices, and systems through controlling matter at the atomic and molecular scale [32]. A new challenge today in the field of photovoltaics is the implementation of these innovative nanotechnologies in cell design. These materials, which can be bulk nanostructured materials [3D], quantum wells [2D], nanowires [1D], and quantum dots/nanoparticles [0D], can therefore be used to exploit a deep theoretical understanding of nanoscale physical phenomena during the photovoltaic process to enhance light absorption in terms of light extinction, broad-spectrum absorption, exciton dissociation, charge separation and collection, and recombination reduction [33].

3. Conclusions

Currently, photovoltaics is a mature technology capable of producing electricity at more competitive prices than traditional fossil fuels. The market and the global cumulative installed PV capacity are growing exponentially, with 1.6 TW in 2023, compared to 1.2 TW in 2022 [34]. These are very positive figures, as this technology presents a significant opportunity to mitigate the effects of climate change. However, we should not stop at the current level of technology, i.e., first- and second-generation cells. On the contrary, we should be motivated to continue improving and exploring all innovative alternative technologies and applications. The articles in the Special Issue “Advances on Solar Energy Materials and Solar Cells” prove that photovoltaic research remains highly relevant and multifaceted, demonstrating that there is still much to learn, develop, and improve.