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Preface

In recent years, global energy demand has increased dramatically. This continuously increasing demand brought on by rapid population growth and industrialization represents one of the biggest challenges facing mankind. Currently, this demand is mainly being fulfilled by fossil fuels, which are limited and will be depleted in the near future due to the very high consumption rate. Moreover, the burning of fossil fuels produces a large amount of harmful gases, such as CO₂, SO_x, and NO_x, which are being released into the atmosphere and polluting the environment. The higher concentration of harmful gases in the environment increases the risk of global warming and acid rain. In addition to this, direct dumping of different organic and inorganic waste into water systems also pollutes water reservoirs and causes waterborne diseases, leading to environmental problems. Therefore, since the ultimate global challenge is to achieve environmental sustainability for our society, it is very important to develop alternative long-lasting, clean and renewable energy sources to replace fossil fuels.

Among all the alternative energy sources, solar energy is considered one of the most promising alternatives to fossil fuels due to its easy availability, cleanness, and accessibility. Moreover, the sun is the fundamental and ultimate source of renewable energy for our planet, which can provide endless carbon emission-free energy for mankind. Around 3.85 yottajoules of solar energy reaches the Earth every year, which is approximately 10⁴ times higher than the world energy consumption annually. However, the availability of solar light depends on the time of day, weather, and seasonal conditions. Therefore, it is critical to convert sunlight to usable or storable energy that can be used as required. In this context, different approaches, such as solar heating, solar thermal electricity, photocatalysis, and photovoltaics, have been involved in harvesting solar energy. Among all these approaches, photovoltaics (PV) is regarded as the most efficient and promising approach, which can directly convert solar radiation into electric power using the photovoltaic effect. Therefore, PV is a successful case of effective utilization of solar energy to generate electricity using semiconducting materials, which can fulfill global energy demand and ease environmental problems. Moreover, PV is the crucial machinery that can provide a carbon-free (decarbonized economy) power supply to achieve a sustainable society. A PV system is made up of solar arrays (solar panels), a solar inverter, and other electrical accessories, and solar cells are building blocks of the PV system. Therefore, solar cells are the electrical device in the PV system which can directly convert solar radiation to electric power. The first PV solar cell was built by Edmond Becquerel in 1839 at age 19. The first solid-state PV solar cell with 1% efficiency was created by Charles Fritts in 1883 with the help of a selenium semiconductor with a thin layer of gold. However, the first practical PV solar cell was constructed by Daryl M. Chapin, Calvin S. Fuller, and Gerald L. Pearson in 1954 based on silicon solar cells with 4.5 to 6% efficiency.

Different types of PV solar cells have been developed, including first-generation (1G) solar cells based on single-crystalline silicon; second-generation (2G) solar cells based on cadmium telluride (CdTe), copper indium gallium selenide (CIGS) and amorphous silicon; and third-generation (3G) solar cells based on organic solar cells (OSCs), dye-sensitized solar cells (DSSCs), multijunction cells, quantum-dot-sensitized solar cells (QDSCs) and perovskite solar cells (PSCs). The commercial 1G solar cells based on crystalline silicon that are currently dominating the PV market exhibit remarkable stability with a maximum power conversion efficiency (PCE) of about 25% in the lab, but the theoretical limit of the PCE is about 31%. The PCE of 2G solar cells is lower than 1G but they are significantly less expensive than 1G solar cells due to their lower processing cost and materials. Since 1G solar cells made up of nontoxic crystalline silicon have about a 25-year performance warranty and show fairly high PCE with negligible degradation, they are leading the PV solar cell market. Although 1G solar cells exhibit high PCE, they are not free from some fundamental drawbacks such as high manufacturing and installation costs. Although there is a relatively large global demand for silicon solar cells, the device formation at high temperature is very expensive due to the high melting point of silicon, limiting their future development. Therefore, it is crucial to explore new PV materials with high PCE and low production costs to replace or complete the state-of-the-art PV technology for the future development of solar cells. In order to fulfill the gap, scientists have been on a quest to develop 3G solar cells with the aim of high PCE with lower cost. Among all 3G solar cells, perovskite solar cells (PSCs) have recently been attracting much attention and have also emerged as a hot research area of competing materials for silicon PV due to their comparable PCE, easy fabrication, long charge-carrier lifetime, low binding energy, low defect density, and low cost.

Perovskite is a mineral consisting largely of calcium titanate (CaTiO₃), which was discovered by the German mineralogist Gustav Rose in the Ural Mountains of Russia in 1839 and named after the Russian mineralogist Lev Aleksevich Perovski. Since then, the term perovskite is generally applied to the class of materials having the same type of crystal structure and stoichiometry as CaTiO₃. A perovskite is an organic/inorganic hybrid and possesses a general formula of ABO₃, in which A and B stand for cations of rare-earth or alkaline-earth metals and transition metals of 3d, 4d, and 5d configurations, and O stands for oxygen, which bonds to both cations. The ABO₃ structure of perovskite is ideally cubic, whereas A atom is bigger than B and forms the corner of cubic cells, coordinated by a 12-fold cuboctahedral, B atoms are in the center and surrounded by an octahedral of anions with 6-fold coordination and the oxygen atom at the centers of the faces of the unit cells. The oxide perovskites show good activity in various fields such as dielectrics, piezoelectrics, photocatalysis, ferroelectrics and pyroelectrics, etc., but are not good for PV applications. Some compounds, like LiNbO₃, BiFeO₃, and PbTiO₃, exhibit some PV activity due to the ferroelectric polarization effect, which makes them suitable for PV applications. However, halide perovskite, a new class of perovskite, differs from oxide perovskite in that it has a halide anion instead of oxide anions with general formula ABX₃, presenting the excellent semiconducting properties that are desired for PV solar cells. Moreover, the A is a monovalent metallic cation from group I of the periodic table instead of divalent in the case of oxide perovskite, B is divalent cations that form transition metals instead of tetravalent, and X is the non-metallic anion (halide).

Halide perovskites were discovered in the 1890s, but they were first comprehensively studied by Wells et al. in 1893. Mitzi’s group investigated the physical properties of two-dimensional (2D) halide perovskite materials having organic group in the 1990s. In 2006, the Miyasaka group used perovskites as the first photovoltaic absorber, which provided only 2.2% PCE. After that, the development of PSCs continued to boom and exhibited fast improvement in efficiency to 3.8% in 2009, 6.5% in 2011, 20.1% in 2015, 22.1% in 2017, 24.2% in 2019, and 25.5% in 2020. This fast improvement in the performance of PVCs might have been due to their unique properties such as small bandgap, high dielectric constant, lower excitation binding energies, high absorption coefficient, high charge-carrier mobility and length, and so on.

This book provides a state-of-the-art summary and discussion about the recent progress in the development and engineering of PSCs materials along with the future directions it might take. Moreover, the recent advances in perovskite materials for energy conversion and environmental applications have been compiled herein.

Khursheed Ahmad

Waseem Raza

April 2022