Perovskites-Based Solar Cells: Materials, Methods, and Future Perspectives

• With increasing global energy consumption and environmental pollution, traditional fossil energy sources cannot meet the sustainable development of human society. The utilisation of clean, renewable energy sources has become a prerequisite for the development of human society.  Among a variety of new energy technologies, solar power is undoubtedly one of the most promising technologies. A solar cell is a device that converts light energy directly into electrical energy via photovoltaic effects or photochemical reactions. In 1839, the French physicist Becquerel discovered the photovoltaic effect for the first time. In 1876, British scientists Adams et al. found that a selenium semiconductor could produce electricity when it was radiated under sunlight [1]. In 1883, Fritts successfully prepared the first semiconductor/metal junction solar cell with a piece of germanium coated with a thin layer of gold although the efficiency was only ~1%. In 1954, Pearson et al. from US Bell Labs developed the first piece of crystalline silicon solar cell and achieved a conversion efficiency of 4.5%, thus beginning a new era for the utilisation of solar power [2]. The monocrystalline silicon/polycrystalline silicon solar cells currently employed in industrial applications have achieved a photovoltaic conversion efficiency of more than 20% [3, 4]. However, such silicon-based solar cells are characterised by a high cost, harsh preparation conditions, and serious environmental pollution. Cadmium telluride and copper indium gallium selenium thin-film solar cells have achieved a high efficiency of photovoltaic conversion in the laboratory, but the industrial applications are restricted by the high production cost, environmental pollution, and other problems [5]. In recent years, dye-sensitized solar cells, as the representative of the third-generation solar cells, have achieved a photoelectric conversion efficiency of more than 13% in the laboratory and have developed rapidly due to their significant advantages, including low cost, simple process, and high efficiency [4]. However, dye-sensitized cells still have two disadvantages. Firstly, in order to ensure the full absorption of sunlight’s energy, the absorbing layer is thick (>10 μm) because it is difficult to achieve complete light absorption using a thinner absorbing layer in the solid-state cells [6, 7]. Secondly, organic dyes cannot avoid the phenomenon of light bleaching. These two problems have prompted researchers to develop excellent all-solid dye materials.
  

  
  

  In 2009, Japanese scientists Kojima et al. found that the organic metal halide perovskite was similar to dyes and can absorb sunlight. The perovskite can be applied in the dye-sensitized solar cells with a liquid electrolyte as a sensitizer to achieve power conversion efficiency (PCE) of 3.8% [8]. In 2012, Kim et al. reported all-solid-state perovskite solar cells with a PCE of 9.7% for the first time [9]. Because of the high efficiency and low cost, perovskite solar cells have attracted extensive attention from researchers worldwide and have developed rapidly in recent years. So far, the highest conversion efficiency has been 22.1% in 2016, which was certified by the National Renewable Energy Laboratory (NREL) [10, 11]. Further improvements in the performance of perovskite solar cells are expected to break the bottleneck of conversion efficiency and production cost. As one of the most promising novel photovoltaic cells, perovskite solar cells are of great scientific value and practical significance.

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• Perovskite Materials for Solar Cells

  The perovskite material is derived from the calcium titanate (CaTiO3) compound, which has the molecular structure of the type ABX3. Perovskite materials have attracted wide attention because of the cubic lattice-nested octahedral layered structures and the unique optical, thermal, and electromagnetic properties. Perovskite materials used in solar cells are a kind of organic-inorganic metal halide compound with the perovskite structure, in which Group A (methylammonium, CH3, MA+, or formamidinium, , FA+) is located in the vertex of the face-centred cubic lattice, and the metal cation B (Pb2+, Sn2+, etc.) and halogen anion X (Cl-, Br-, or I-, or a coexistence of several halogens) occupy the core and apex of the octahedra, respectively. The metal-halogen octahedra are joined together to form a stable three-dimensional network structure. 
  

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• Preparation Methods of the Perovskite Light-Absorbing Layer

  
  

  The synthesis methods of the light-absorbing layer of perovskite solar cells can be roughly divided into three types: the solution method, the vapour-deposition method, and the vapour-assisted solution method. The solution method is simple and economical, but more internal defects will be produced in synthetic crystals and the hole transport layer is in direct contact with the electron transport layer, thus reducing the device’s filling factor and the open-circuit voltage. The perovskite films prepared by the vapour-deposition method show a high surface density and fewer defects, which improve the filling factor and the open-circuit voltage. However, this method requires a high-vacuum environment and involves high energy consumption. The vapour-assisted solution method integrates the advantages of the solution method and the evaporation method. At a lower vacuum, the perovskite materials with fewer internal defects can be synthesized. Figure 5 illustrates the different deposition methods for perovskite layer.