As a new type of green energy technology, organic solar cells (OPVs), the core of which is to use photoelectric conversion organic materials to convert light energy into electrical energy, have the advantages of all-solid-state, low cost, low temperature fabrication and large-area flexible devices. The photocurrent flow of OPV depends not only on photogenerated carriers but also on the interface properties between the photoactive layer and the electrode. In order to improve the interface properties between the photoactive layer and the electrode, buffer layer materials are usually used to improve the performance of OPV devices. For different electrodes and photosensitive layers, the buffer layers used are also different. The buffer layer with excellent performance has the following common functions: 1) modifying the surface of the electrode to make the surface smooth and orderly; 2) separating the active layer and the electrode, keeping the photo-generated carriers away from the electrode, preventing the quenching of photo-generated carriers at the electrode; 3) block the transport of carriers to the opposite direction of the photocurrent collection direction; 4) make ohmic contact between the electrode and the active layer; 5) improve the stability of the device. According to different application objects in OPV devices, buffer layer materials can be divided into anode buffer layer and cathode buffer layer. The application of buffer layer materials can effectively improve the stability and photoelectric conversion efficiency of OPV devices.
Figure 1. Buffer layers in OPV devices
- OPV anode buffer layer: OPV anode generally uses indium tin oxide transparent conductive film with excellent conductivity, good light transmission performance and high hardness. However, this conductive film has poor energy level matching with the organic photoelectric conversion layer, and an anode buffer layer needs to be used to improve the performance of OPV. The use of the anode buffer layer can effectively reduce the floor laying, double the open circuit voltage, and improve the stability of the OPV device.
- OPV cathode buffer layer: The cathode generally uses metals with excellent electrical conductivity and stable properties. However, the work function of this cathode material is too high and does not match the LUMO (Lowest Unoccupied Molecular Orbital) energy level of the acceptor, resulting in low electron transport efficiency. Therefore, inserting a buffer layer between the cathode and the acceptor can improve the performance of OPV. The function of the cathode buffer layer to improve OPV is as follows: prevent the diffusion of hot metal atoms to the acceptor layer; reduce the quenching of photogenerated excitons at the interface between the organic layer and the cathode, so that the photogenerated excitons can be more fully dissociated into electrons and holes ; prevent the penetration of oxygen molecules and water molecules into the active layer to improve the stability of the device; as an optical spacer layer to adjust the light field, so that sunlight can be more fully absorbed.
- Lo M F, Ng T W, Lai S L, et al. Operation stability enhancement in organic photovoltaic device by a metal doped organic exciton blocking layer[J]. Appl. Phys. Lett., 2010, 97, 143304.