The electronic conductivity of semiconductors consists of two contributions, one is the electrons at the bottom of the conduction band, which are a kind of charge that participates in conduction, usually called electrons; the other is at the top of the valence band, left by the electrons leaving the valence band. The empty quantum states of which are also a type of electric charge involved in conduction, these empty quantum states are often called holes. The charge of a hole is opposite to that of an electron. Therefore, semiconductors conduct electricity with two types of charges electrons and holes. Generally, the charge transport materials can be divided into three categories according to different categories, namely hole transport materials, electron transport materials and bipolar transport materials that can transport both holes and electrons. The hole transport material is a P-type transport material, which requires a higher energy level of the highest occupied molecular orbital (referred to as HOMO); correspondingly, the electron transport material is an N-type transport material, which requires the lowest unoccupied molecular orbital (referred to as LUMO) energy level is lower.
Based on the consideration of charge injection and transport properties, charge transport materials require relatively high electrical conductivity and carrier mobility. The material should also have good film-forming properties, easy to form a dense, pinhole-free film, and the film itself has good thermal and chemical stability.
Different classes of charge transport materials have different properties. Hole transport materials generally have the following characteristics: low ionization potential, high HOMO energy level, high polarizability, and the existence of electron-pushing groups in the molecule (we can understand that hole transport materials should be electron-donating materials. material), the size of the molecule is smaller. Nitrogen atoms on hierarchical amines are the most common groups present in hole transport materials. Common hole transport materials containing such groups are N,N′-(3-methyl-phenyl)-1,1′-biphenyl-4,4′-diamine (TPD) and N,N′-diphenyl-N ,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4″-diamine (NPB). The primary condition of hole injection material and hole transport material (HTL) is high mobility and high glass transition temperature (T g), and its HOMO energy level should be as large as possible, LUMO should be as small as possible to facilitate the transport of holes and at the same time block the transmission of electrons in the light-emitting layer to the anode It can be seen that HTL also has the function of electron blocking. Common hole transport materials include NPB, 1,1-bis((di-4-tolylamino)phenyl)cyclohexane (TAPC), 4,4′,4″-Tri(N-carbazolyl)triphenylamine(TCTA), TPD and 4,4′,4″-Tris(3-methylphenylphenylamino)triphenylamine(m-MTDATA) etc.
The electron transport material should have a large electron affinity and high electron mobility, and a high excited state energy level, which can effectively avoid the energy transfer of the excited state, so that the exciton recombination region is confined to the light-emitting layer instead of electron transport. in the layer. The requirements for electron transport materials are: high electron affinity, low LUMO energy level, existence of electron-pulling groups in the molecule, and small Coulomb force. Common electron withdrawing groups are hydroxyl, pyridine and halogens. such as Bphen, TPBI, Alq3, etc. Electron transport materials are also required to have high mobility and glass transition temperature.