As global energy consumption continues to grow and environmental pollution becomes increasingly severe, replacing traditional energy sources with clean, renewable energy sources is imminent. Solar energy is widely used because it is widely distributed. Solar cell is the most important way to use solar energy, and a new type of solar cell with perovskites as its light-absorbing material has developed rapidly. The conversion efficiency of organic solar cells with perovskite structure can be as high as 22.1%, and the cost of solar cells can be significantly reduced. Perovskite crystals exhibit excellent performance and strong competitiveness in the field of solar cells. They have the advantages of low cost of organic materials, solution preparation, and easy film formation. They also have the advantages of high mobility of inorganic materials and high absorption coefficient. The titanium ore crystal itself can absorb light, generate carriers and transmit carriers, and the battery performance quickly exceeds DSSC and BSC (third generation solar cells).
Figure 1. Perovskite solar cells
Work mechanism of perovskite solar cells
Perovskite refers to an organic-inorganic hybrid material with a crystal structure of a perovskite crystal whose molecular formula is ABX3 (A = organic cation, B = Pb, Cd, or Sn, X = I, Cl, or Br). For example, the CH3NH3PbI3 cell consists of one Pb2+ ion, one CH3NH3+ ion, and three I- ions. The metal cation and halogen anion form a positive octahedral structure, and the organic cation balances the charge. Under light irradiation, the electrons of I can be excited on Pb , and electron migration occurs.
Figure 2. Crystal structure of ABX3
Perovskite solar cells originate from the third generation solar cell DSSC. The initial structure Meso-super-structured solar cells (MSSCs) is the evolution of DSSC. The simple structure is described as follows: Firstly, a dense TiO2 layer is prepared as an electron transport layer (ETL) on a fluorine-doped tin oxide/indium-doped tin oxide (FTO/ITO) conductive glass, and a TiO2 (or Al2O3) mesoporous is prepared as well. Material (or nanoparticle layer), then the perovskite light absorbing layer is prepared, if necessary, a hole transport layer (HTL) may be spin-coated on the light absorbing layer, and finally the metal electrode is evaporated to obtain a battery.
Figure 3. Structure for typical MSSCs
The earliest perovskite solar cells used a perovskite layer as a light-absorbing layer and a transport layer to prepare MSSC, and later a perovskite cell containing a porous metal oxide structure was developed. Due to the preparation of TiO2/Al2O3 mesoporous materials after high-temperature sintering, many research groups began to develop and process normal-temperature Planar Heterojunction (PHJ) perovskite solar cells. For example, professor Chen design the structure of ITO/PEDOT (PEDOT = polyethylene Dioxythiophene) / CH3NH3PbI3/C60/BCP (BCP = Bath Copper) / Al battery.
Figure 4. Planar heterojunction perovskite solar cells
The main factors affecting the performance of perovskite solar cells
Perovskite crystal ABX3 contains organic cation (A+), metal cation (B2+), and halogen anion (X-) parts. Changing the chemical composition will change the crystal's energy level, mobility, absorption spectrum, and cell performance.
The crystallinity and morphology of the perovskite light-absorbing layer largely determine the performance of perovskite solar cells. Scientists have invented a variety of new methods to obtain uniform and perovskite light absorption layers with high crystallinity and complete coverage, thereby increasing the mobility of the light-absorbing layer, reducing the series resistance, greatly increasing the fill factor, photocurrent and cell efficiency. In order to prevent a short circuit in batteries, the perovskite crystal film must be uniform and free of holes.
References
- Mitzi D B, Feild C A, Harrison W T A. Conducting tin halides with a layered organic-based perovskite structure[J]. Nature, 1994, 369(6480): 467.
- Wang Y, Gould T, Dobson J F. Density functional theory analysis of structural and electronic properties of orthorhombic perovskite CH3NH3PbI3[J]. Physical Chemistry Chemical Physics, 2013, 16(4): 1424-1429.
- Gao P, Grätzel M, Nazeeruddin M K. Organohalide lead perovskites for photovoltaic applications[J]. Energy & Environmental Science, 2014, 7(8): 2448-2463.
