1. Field of the Invention
The present invention is generally related to a solar cell and a method for preparing the same, and more particularly to a solar cell having a tree-like nanostructure and a method for preparing the same.
2. Description of the Prior Art
At present, polymer solar cells attract great research interests because the various advantages compared to the traditional silicon-based solar cells. For example, the fabrication cost is low and it is easy to fabricate a cell with large area, transparent cell, and flexible cell, etc. However, the cell efficiency of a polymer solar cell is still low. Thus, developing a polymer solar cell with high efficiency becomes an important research target.
The operating principle of polymer solar cells is as follows. When a polymer cell is irradiated by sun, the conjugate polymer in the polymer solar cell absorbs sunlight to have its electron in the highest occupied molecular orbital (HOMO) excited to the lowest unoccupied molecular orbital (LUMO) and then a hole is generated in HOMO to form an electron-hole pair, called exciton. This exciton can generate effective electron-hole separation at the interface between the electron acceptor and the electron donor and then the electron and the hole are transported to the external circuit to generate electric current, via electron transport substance and hole transport substance, respectively. Thus, solar energy is transformed into electric energy. In the process of this energy conversion, solar cell efficiency is highly related to the mechanism of separating the electron-hole pair, the life time of the exciton, and the pathway of electric current transport.
In order to increase solar cell efficiency, the current polymer solar cell uses the mixture of semiconductor nanoparticles and conjugate polymers by using solvent to blend semiconductor nanoparticles and conjugate polymers so as to thereby increase the area of the interface between the electron acceptor and the electron donor. Generally, it requires forming bi-continuous phase between nanoparticles and conjugate polymers to prevent from lowering the efficiency of charge transport. In such case, higher nanoparticle concentration is required. However, as the nanoparticle concentration is high, coagulation between nanoparticles occurs and the area of the interface between the electron acceptor and the electron donor is thereby decreased to further lower the efficiency of electron-hole separation and the quantum efficiency of energy transfer. On the other hand, as the properties of nanoparticles are significantly different from those of conjugate polymers, directly physically blending these two substances will result in serious phase separation so that the exciton generated by light excitation can not have good charge transfer to thereby lower the efficiency of energy conversion of the device. Even though the nanoparticles and the polymers are well blended, the charge transport speed is still limited due to the non-linear transport pathway.
To solve the above-mentioned problems associated with the current method, the invention discloses a novel solar cell having a tree-like nanostructure and a method for preparing the same.