The buffer layer, also called the transport layer, It refers to the material with proper energy level inserted between the active layer and electrode layer of solar cell, which is used to dissociate the charge carriers into electrons and holes, so as to improve the efficiency of hole and electron collection[1-2]. It can be divided into two types according to the type of carriers it transmits: electron transport layer and hole transport layer. The electron transport layer performs the function of transporting electrons from the active layer to the cathode, and the hole transport layer, on the other hand, blocks electrons during the transport of holes. The common electrode buffer layer materials can be divided into organic semiconductor and inorganic semiconductor according to the types of materials
TiO2 is a wide-band gap N-type semiconductor with high chemical stability and good light transmittance in visible region. It has three crystal types: rutile, anatase and plate titanite. Rutile TiO2 is the most thermodynamically stable and has excellent light scattering property. The anatase TiO2 bulk phase material has high electron mobility, so it has a wider application as electron transport materials in organic and inorganic hybrid solar cells.
In addition to TiO2, ZnO also is a broadband gap N-type semiconductor with almost the same energy level structure as TiO2. At the same time, ZnO also has better electron transport than TiO2. The only drawback is that ZnO is less chemically stable than TiO2 and easily soluble in both acidic and alkaline environments. Nevertheless, ZnO is still a promising electron transport material.
CuI is a wide-band gap semiconductor with good light transmittance in the visible region, showing the properties of P-type semiconductor. It is a hole transport material with electron blocking function and has excellent hole transport property. In addition, CuI has better solubility than other inorganic semiconductors, soluble in acetonitrile, N,N-dimethylformamide (DMF) and other organic solvents.Therefore, CuI is one of the most commonly used buffer materials.
CuSCN is another widely used wide-band gap P-type semiconductor after CuI, which can more effectively block electrons while conducting holes. CuSCN does not degrade similar to CuI under light, and has better chemical stability than CuI. However, CuSCN has relatively poor hole transport and low hole mobility in thin film. Therefore, CuSCN has important applications in solar cells.
Compared with inorganic materials, organic interface materials have better compatibility with organic active layer, so they can significantly improve the interface contact properties. For example, the anode buffer layer material PEDOT:PSS has the characteristics of high conductivity, good transparency and so on. It basically has no absorption in visible light and near infrared, which reduces the loss of photons. Although PEDOT:PSS has excellent performance, its poor stability, easy decomposition at high temperature and strong acidity inevitably lead to ITO corrosion. Its hygroscopicity leads to low stability of the device and greatly reduced battery life.