Perovskite solar cells: the rising trend of new photovoltaic technologies

Perovskite is a mineral with the chemical formula CaTiO3. Its name comes from the Russian scientist Lev Perovski (Лев Александрович Перовский), who first discovered this mineral in 1839. Perovskite is widely found in nature and has important applications in the field of materials science, particularly in the area of solar cells. Perovskite solar cells are considered one of the most promising new types of solar cells currently available.

In 1978, Weber introduced methylammonium ions into crystals, forming organic-inorganic hybrid perovskite materials with a three-dimensional structure.

Japanese scientists Kojima and Miyasaka applied the perovskite material to dye-sensitized solar cells, achieving a conversion efficiency of 3.8%.

In 2012, the Park group replaced the traditional liquid electrolyte with a solid-state spiro-OMeTAD as a hole-transporting layer to prepare a fully solid-state perovskite solar cell, achieving a conversion efficiency of 9.7%. In the same year, the conversion efficiency broke through 10% for the first time.

Michael Grätzel, the father of dye-sensitized solar cells, and his collaborators proposed a two-step deposition method for forming perovskite dyes in porous metal oxide films. Solid-state mesoporous perovskite solar cells prepared using this method achieved approximately 15% photoelectric conversion efficiency. In the same year, Henry Snaith et al. used alumina instead of titanium dioxide and achieved a conversion efficiency of 15.4%.

California University's Yang Yang and colleagues improved the light absorption layer of perovskite, selected more suitable electron transport materials, and further increased the efficiency of perovskite solar cells to 19.3%.

Sang II Seok and colleagues improved the efficiency of the solar cells to 20.2% by using a cation-exchange method.

Grätzel's team incorporated the inorganic cesium ion (Cs+) into a mixed methylammonium and formamidinium perovskite, resulting in a three-cation perovskite with higher thermal stability, less phase impurity, and lower sensitivity to processing conditions. The resulting device achieved a record-high power conversion efficiency of 21.1%.

Sang II Seok and colleagues improved the efficiency of perovskite solar cells by introducing triiodide ions to repair defects in the perovskite, combined with a two-step spin-coating method. The perovskite solar cells achieved a record efficiency of 22.1%.

The research group led by You Jingbi at the Institute of Semiconductors, Chinese Academy of Sciences, used organic halide salt phenethylammonium iodide (PEAI) for surface defect passivation on FA-MA mixed perovskite thin films, and improved the certified efficiency of perovskite solar cells to 23.3%. Subsequently, they also produced devices with an efficiency of 23.7%.

MIT and Korea Advanced Institute of Science and Technology (KAIST) jointly created a record efficiency of 25.2% by enhancing charge carrier management to improve the performance of perovskite solar cells.

Seok's group at the Ulsan National Institute of Science and Technology in Korea used Cl-SnO2 and chlorine-containing perovskite precursors to form an intermediate layer between the SnO2 electron transport layer and the halide perovskite absorption layer, enhancing charge extraction and transport in the perovskite layer while reducing interface defects. This method resulted in a single-junction perovskite solar cell with an efficiency record of 25.7%.

In June, the research team of Professor Tan Hairong from Nanjing University achieved a steady-state photovoltaic conversion efficiency of 28.0% with their all-perovskite tandem solar cell, which was certified by the international authoritative organization JET. 

In December, the research team of Renergy Co. Ltd. achieved a steady-state photovoltaic conversion efficiency of 29.0% with their all-perovskite tandem solar cell, which was certified by the Japanese JET third-party certification agency.

a) The raw materials for the perovskite layer are basic chemical materials with abundant reserves and low prices;

b) The amount of raw materials used is small, with a thickness of only about 500nm for the perovskite layer, while the average thickness of silicon wafers for silicon-based solar cells is 150 microns;

c) The purification requirements for perovskite materials are not high. Compared with silicon-based solar cells that must use 99.9999% high-purity silicon, the purity requirement for perovskite materials is only above 95% for solar-grade materials.

Perovskite solar cells can be prepared by solution processing, and the production process temperature does not exceed 150℃, while the casting and pulling of silicon materials for single-crystal silicon solar cells require temperatures above 1500℃, resulting in a large difference in production energy consumption. 

The energy consumption for manufacturing a single-crystal module is about 1.52 KWh per watt, while the energy consumption for a perovskite module is only about 0.12 KWh per watt, which is only 1/10 of that of silicon.

The entire production process of calcium-titanate solar cells, from raw materials to the final module assembly, only takes about 45 minutes, which significantly improves production efficiency. In terms of the cost composition of calcium-titanate modules, the proportion of calcium-titanate is about 5%, while glass, targets, and other materials account for the remaining two-thirds of the cost. The total cost is about 0.5-0.6 yuan per watt, which is 50% of the limit cost of silicon modules.

The wide downstream application scenarios of perovskite solar cells are mainly due to the high light absorption coefficient of perovskite materials, which can effectively utilize sunlight with a thinner layer compared to traditional silicon solar cells (the thickness of silicon wafer is about 150 microns, while the thickness of perovskite layer is about 500 nanometers). The characteristics of perovskite materials allow for the use of lightweight and flexible substrates in the fabrication of perovskite solar cells. The lightweight and flexible characteristics make perovskite solar cells suitable for a wider range of applications, such as building-integrated photovoltaics (BIPV) and automotive photovoltaics.

Factors that affect the stability of perovskite solar cells mainly include:

Inherent instability of perovskite materials (a decisive factor). Perovskite materials can decompose faster under the effects of water, oxygen, heat, and light.

Various functional layers in the device (hole transport layer, electron transport layer, and electrode) can have a mutual influence on the perovskite layer. TiO2 and ZnO metal oxides are commonly used as electron transport layers in the formal structure, and these two materials can produce photogenerated holes and catalyze the decomposition of perovskite materials under light. Spiro-OMeTAD is a commonly used material for hole transport layer, and it is sensitive to iodine ions. 

When iodine ions diffuse into Spiro-OMeTAD from perovskite materials, it can reduce its charge transfer performance. A metal top electrode is currently a more mainstream choice, but metal atoms can diffuse into the perovskite layer through diffusion, causing the decomposition of perovskite materials. 

The built-in electric field formed by the photovoltaic effect can exacerbate the diffusion of atoms and accelerate decomposition. In addition, halogen ions in perovskite materials can diffuse to the metal electrode and cause corrosion, which affects the performance.

Failure in any of these areas can lead to a decrease in product performance, thereby affecting the stability of the solar cell.

The lifespan of perovskite solar cells is still much shorter than traditional silicon solar cells. Although some companies have started to introduce commercial products of perovskite solar cells, further technological breakthroughs and market validation are still needed due to the stability issues of perovskite solar cells.

Currently, the high-efficiency perovskite solar cells are mostly small laboratory-scale devices (less than 1 square centimeter), with the highest recorded efficiency of 25.7% achieved on a 0.1 square centimeter device. The average conversion efficiency of commercial-scale perovskite solar cells is currently around 16%. As the cell area increases, the conversion efficiency inevitably decreases. 

For example, for silicon, cadmium telluride thin-film, dye-sensitized, and organic solar cells, each order of magnitude increase in device area results in a decrease of approximately 0.8% in conversion efficiency. 

However, the decline in efficiency for perovskite solar cells is even greater, mainly due to the following reasons:

 1) when preparing large-area perovskite films, the uniformity of the films decreases, and the number of holes and defects increases due to the limitations of the preparation process. 

The solution-spinning method is mainly used in laboratory-scale perovskite film preparation. The amount of anti-solvent used is a key factor affecting the quality of the perovskite layer and has an edge effect, which causes the uneven thickness of the perovskite film; 

2) when the size increases, the non-light-active dead zone (grid line area, etching area) of the cell increases, resulting in a reduction in the effective light-receiving area and a decrease in the short-circuit current density of the component; and 

3) related to the series-parallel structure design and component process, the series resistance of the component increases, resulting in a decrease in conversion efficiency.