Hole transport material (HTM) is a kind of selective high-rate hole transport material that transports the holes generated by the perovskite layer to the anode. The introduction of HTM materials with excellent optoelectronic and physical properties into perovskite solar cells can improve device stability, lifetime, and performance, while enhancing charge transfer rates and reducing non-radiative recombination inside the device. An HTM with superior performance needs to meet the following conditions: 1) it has an energy level matching the perovskite layer, and the highest occupied molecular orbital of the molecule is higher than the valence band of the perovskite, so as to ensure the transport of charges; 2) high hole mobility and electrical conductivity; 3) excellent light, thermal and moisture stability, improving the stability of the device; 4) good solubility and film-forming properties, which are beneficial to reduce interface defects and reduce the difficulty of device processing ; 5) lower material preparation cost, improving the possibility of device industrialization.
Figure 1. Some organic hole transport material(HTM) structures
- Perovskite solar cells (PSC): HTM insertion between perovskite and metal electrodes can promote the separation of electrons and holes at the functional layer interface, reducing charge recombination, while facilitating hole transport, improving cell efficiency and device stability. At present, HTM can be roughly divided into three categories: organic small molecule hole transport materials, polymer hole transport materials, and inorganic hole transport materials. Small molecule hole transport materials of organic materials have the advantages of adjustable energy level and fixed molecular weight. The most common ones are fluorenes, thiophenes and aromatic amines. Polymer hole transport materials have the advantages of low temperature solution processing, strong mechanical flexibility, large conjugated structure, etc. At the same time, they have better film-forming and covering ability, which can reduce the defects between interfaces. Inorganic hole transport materials have the advantages of high stability, high hole mobility and low cost, and commonly used Cu2O, CuI, CuSCN, NiO, etc. Each class of materials has its own advantages and can meet the needs of multifunctional PSC devices. Organic small molecule hole transport materials can prepare PSCs with good monodispersity; polymer hole transport materials can prepare lightweight PSCs; inorganic hole transport materials can prepare high-efficiency PSCs.
- Laura Calio, Dr. Samrana Kazim, Hole-Transport Materials for Perovskite Solar Cells [J]. Angew. Chem. Int. Ed., 2016, 55, 14522-14545.