History of organic photovoltaic cells
Photovoltaic effect refers to the phenomenon in which a potential difference is generated between different regions of a semiconductor or a combination of semiconductors and metals that are illuminated by light. The term "photo-voltaic" appeared in English in 1849, which means that electromotive force is generated by light, that is, light generates volts. With the gradual understanding of the physical properties of semiconductors and the advancement of processing technology, in 1954, researchers from Bell Laboratories in the United States discovered that when silicon was doped with a certain amount of impurities, it became more sensitive to light. The first solar cell with practical application value was born in Bell Laboratories in 1954, marking the arrival of the era of solar cells.
Basic principles of organic photovoltaic cells
The basic principle of organic photovoltaic cells is similar to inorganic solar cells:
(1) After a certain amount of light is irradiated to an organic photovoltaic device, a photon having energy is absorbed by the organic semiconductor layer, and an electron is excited to transition from the valence band to the conduction band, leaving a vacancy at the valence band. This vacancy called holes and the holes have a positive charge.
(2) In conventional semiconductors, the excited electrons and formed holes move freely toward the opposite electrode. In conductive polymers, the electrons and holes formed by the excitation of incident photons will exist in a bound form, that is, they become excitons.
(3) Usually these electron holes are formed when excited by photons. If they are in the electric field or at the interface, these electron hole pairs will be separated into electrons and holes, which are called charged carriers. Photocurrents are formed during the migration of charged carriers.
Classification of organic photovoltaic cell materials
According to mechanical properties and processability, organic photovoltaic cell materials can be divided into insoluble materials, soluble materials, and liquid crystal materials. These materials usually include small molecules or oligomers, polymers, and liquid crystal molecules. Among them, oligomers or monomers capable of absorbing visible light are called chromophores, those with solubility are called dyes, and those without solubility are called pigments. At present, the polymer materials used in photovoltaic device research mainly include polythiophene (PTH) derivatives, polyphenylacetylene (PPV) derivatives, polyparaphenylene (PPP) fluorene derivatives, polyaniline (PANI) and other polymer materials.
Advantages of organic photovoltaic cells
Compared with inorganic solar cells, organic solar cells have the following advantages:
(1) Compared with the materials used for inorganic solar cells, the raw materials of organic semiconductor materials are widely available, cheap, light weight, high environmental stability, and have a high absorption coefficient (usually> 105cm-1) and good photovoltaic effect.
(2) The preparation process of organic solar cells is more flexible and simple. It can be prepared by vacuum evaporation, coating, printing or spraying. The energy consumption of organic solar cells in production is lower than that of inorganic materials, and the production process has no environmental pollution. Organic solar cells can be processed on flexible or non-flexible substrates, and have the characteristics of large manufacturing area, ultra-thin, cheap, good flexibility and so on.
(3) Organic solar cell products are translucent, easy to decorate and use.
Application prospects of organic photovoltaic cells
As fossil energy is limited and non-renewable, finding an alternative energy source has become an important task to maintain the sustainable development of the global economy. Photovoltaic power generation directly converts solar energy into electricity, which is a green renewable energy source with high commercial development value. And it is expected to become one of the main energy sources at the end of this century. With the continuous improvement of photovoltaic cell manufacturing technology and the continuous rise in the price of fossil energy, the photovoltaic power generation industry has entered a stage of rapid development. The international community is also increasingly optimistic about the development prospects of the photovoltaic power generation industry. The International Energy Agency (IEA) predicted in the solar photovoltaic roadmap published in May 2010 that the proportion of photovoltaic power generation in total global electricity will reach 1.3% in 2020 and rise to 4.6% in 2030.
The current research direction of photovoltaic cells is to develop efficient and low-cost battery materials and manufacturing technologies. Organic polymer photovoltaic cells use conjugated polymer as the photovoltaic material, and the manufacturing process is simple, the cost is low, and they can be manufactured on a large area, which makes the research of organic polymer photovoltaic cells more and more important. Although the research of polymer photovoltaic cells has made remarkable progress in recent years, its photoelectric conversion efficiency is still very low. Only by obtaining highly efficient and stable photovoltaic cells can the commercialization of polymer photovoltaic cells be achieved. The improvement of the efficiency of organic polymer photovoltaic cells can be achieved through the selection of materials and optimization of the structure of photonic and optical devices.