A solar cell technology is to directly convert light, that is, solar energy into electric energy by utilizing a photovoltaic effect, and most commercialized solar cells are inorganic solar cells using inorganic materials such as silicon, etc. However, the inorganic solar cells have disadvantages in that manufacturing cost is increased due to a complicated manufacturing process, and high-priced materials are required, and accordingly, research into a dye-sensitized solar cell and an organic solar cell manufactured with low cost through a relatively simple manufacturing process and with low-priced materials has been actively conducted.
The organic solar cell has been expected to be a low-priced solar cell in the future since it is capable of being manufactured with simple manufacturing processes and low installation expenses as compared to conventional silicon or compound semiconductor solar cells. A number of materials, structures, etc., of the organic solar cell have currently researched to improve efficiency of solar cells. Among them, a bulk heterojunction structure using mixture of electron donors and electron acceptors is known to have the highest efficiency.
However, in the bulk heterojunction structure, a diffusion distance of excitons which are electron-hole pairs formed by electron donors such as conductive polymers, etc., by sun light is only about 10 nm in the polymer. Accordingly, when the excitons do not reach interfaces of the electron donors and the electron acceptors within this distance, they reunite again and become extinct. In addition, after the excitons are separated into electrons and holes at the interfaces of the electron donors and the electron acceptors, the respective electrons and holes move toward a metal electrode and a transparent electrode, respectively. To this end, it is required that all of the electron donors are linked to be in contact with an anode, and all of the electron acceptors are linked to be in contact with a cathode while having a co-continuous structure. However, the structures of the electron donors and the electron acceptors may not be artificially determined, but depend on phase separation properties after mixing materials, and accordingly, it is not possible to obtain the ideal structure.
To obtain facilitate electron injection in the modern optoelectronic devices such as the organic solar cell, the incorporation of solution processed interfacial layers on stable metal electrodes has been widely used. The appropriate choice of interface materials administer a platform to carefully regulate the electrode work function (WF), protect the semiconductor layer from the diffusion of electrodes, mitigate the charge accumulation and recombination at the electrodes.
In order to improve the interface properties, CsF, LiF, Cs2CO3, conjugated or non-conjugated polymer electrolytes, fullerene derivatives, etc., are used. However, this method has problems in that an effect for improving interface properties is not significant, reactivity with air and water is high, synthesis and purification methods are complicated, reproducibility in batches is low, and only a deposition via expensive vacuum deposition is applicable, and further, it is difficult to apply a solution process in manufacturing a large area device.
Documents related to solar cell technology include Korean Patent Laid-Open Publication No. 10-2014-0112494; Adv. Funct. Mater. 20, 4381-4387 (2010); Adv. Energy Mater. 2, 532-535 (2012); Adv. Mater. 21, 759-763 (2009); and Adv. Energy Mater. 5, 1401692 (2015).