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Donor Materials
Introduction
In organic solar cells, electron donor materials refer to a substance that supplies electrons in electron transport and a substance that receives oxidation, they generally have strong electron giving ability and contain more rich electric units (D) such as thiophene, pyrrole, carbazole, etc[1]. The electron donor material has two main functions in the active layer: absorption of sunlight and as a void hole transport material and the process of exciton diffusion, separation and charge transfer will take place simultaneously.
Materials
According to the arrangement of electron-rich unit (D) and electron-poor unit (A), the electron donor materials can be divided into two categories, one is pure D-type donor materials, the other is D-A-type donor materials.
- Pure D-type donor materials
Early studies on electron donor materials mainly focused on pure D-type polymers, among which P3HT, MEH-PPV and MDMO-PPV were the three most thoroughly studied polymers. Traditional poly (p-styrene) and poly (thiophene) are refractory and have poor processing performance. In the modification of its molecular side chain, a variety of base chains can greatly increase its solubility in conventional solvents such as chloroform, chlorobenzene, tetrahydrofuran and so on, thus benefiting its application in organic solar cells. Meh-PPV and MDMO-PPV are orange light-emitting polymers with strong absorption capacity to visible light. They are the first kind of polymers to be combined with fullerdilute electron acceptors to prepare bulk heterojunction cells.
In addition to poly (p-phenylene ethylenes) and poly (thiophene) two types of pure D-type polymer, people also try to use a variety of other structures of pure D-type polymer as the main material of organic solar cells, which has representative significance of polythiophene ethylene derivatives, polythiophene derivatives with conjugated branched chain, poly (3-phenylthiophene) derivatives.
- D-A type donor materials
- Benzene-type D-A polymer, in which the first and last two benzene units in the rich electric unit are connected with a five-membered ring or six-membered ring, such as fluorene connected to pentadiene, pyrrole connected to carbazole and so on.
- Polycyclic D-A polymer, which is rich in the first and last two benzene or thiophene using two or more thick ring system connected, such as the common trapezoidal monomer dininhydrin and dithiophene, naphthalene connected to two thiophene unit naphthalene and dithiophene;
- Other D-A polymers, such as backbone D side chain A type polymer, backbone containing metal atoms of polymer, single conjugated unit (such as thiophene, furan, selenium phenoe) do electrorich unit of polymer, polymonomer copolymerization of random polymer.
In addition to D-type electron donor materials, D-A-type donor materials are more widely used at present, that is, the alternating copolymerization of electric-rich unit (D) and electric-absorbing unit (A). And the most efficient polymer solar cells currently use D-A polymers. Considering that the majority of the electric-rich units in D-A type polymers are the linking system or co-ring system of two or more minimum conjugated groups (benzene or thiophene), according to the different combinations of their structures, type polymers are mainly divided into the following categories:
Reference
- B. Liu, Y. P. Zou, B. Peng, B. Zhao, K. L. Huang, et al. Low bandgap isoindigo-based copolymers: Design, synthesis and photovoltaic applications. Polymer Chemistry, 2011, 2: 1156-1162.
