Organic solar cells are solar cells with organic materials as the core part. They mainly use photosensitive organics as semiconductor materials to generate voltage and form current through the photovoltaic effect to achieve the effect of solar power generation. The first organic photoelectric conversion device was prepared by Kearns and Calvin in 1958. Its main structure is to use two electrodes with different work functions to sandwich the active layer material magnesium phthalate tube. This structure is very simple, but it is observed With an open circuit voltage of 200mV, organic solar energy utilization technology has developed rapidly since then.
For more than two decades, there has not been much innovation in the field of organic solar cells. All reported device structures are similar to the 1958 version, except that various organic semiconductor materials are used between two electrodes with different work functions. The principle of this type of device: the electrons in the organic semiconductor are excited from the HOMO energy level to the LUMO energy level under light, and a pair of electrons and holes are generated. Electrons are extracted by electrodes with low work function, and holes are filled with electrons from electrodes with high work function, thereby forming a photocurrent under light. Theoretically, when an organic semiconductor film is in contact with two electrodes with different work functions, different Schottky barriers will be formed. This is the basis for the directional transfer of photoelectric charge. Therefore, batteries with this structure are usually called “Schottky-type organic solar cells.”
In 1986, a milestone breakthrough occurred in the industry. The device is a double-layer membrane structure composed of a derivative of tetrasophenanthrene and copper phthalate (CuPc). This double-layer film structure is actually an early heterojunction. Using this idea, organic solar cells are prepared by imitating inorganic solar cells. The photoelectric conversion efficiency of this kind of solar cell reaches about 1%. Although it is far from silicon cell, it is a big improvement compared to the previous Schottky cell. This is a successful idea, which opens up a new direction for organic solar cell research. To this day, this double-layer heterojunction structure is still one of the focuses of organic solar cell research.
In 1992, the Turk Saliqivqi discovered that excited electrons can be injected from organic semiconductor molecules into C60 molecules very quickly, but the reverse process is very slow. Therefore, it can be said that excitons can easily achieve charge separation and transport at the interface between the semiconductor material and C60, but the recombination process is difficult to occur. It is based on this that C60 is considered to be an excellent electron acceptor material.
In 1993, a new type of P-type organic semiconductor material “polyp-styrene” appeared. On the basis of this discovery, Saliqivqi made a PPV/C60 double-layer film heterojunction solar cell. Subsequently, more and more solar cells using C60 as the electron acceptor appeared.
In 1995, researchers from UCSB and UNIAX at the University of California, Santa Barbara provided a milestone key step for the advancement of solar cells. They proposed the important concept of “bulk heterojunction”. Compared with the previous double-layer heterojunction battery, this bulk heterojunction structure greatly improves the efficiency of the battery, because the micro-topography of IPNs is formed inside the battery. Since then, the efficiency of bulk heterojunction solar cells has been developing rapidly, creating the highest efficiency of organic photovoltaics so far.
In 2001, Shaheen et al. increased the efficiency of bulk heterojunction solar cells to 2.5% by controlling the micro-morphology of PPV and fullerene derivatives.
In 2003, Padinger et al. increased the efficiency to 3.5% and the internal quantum efficiency to 70% by post-processing the P3HT:PC60BM system.
In 2005, Al-lbrahim et al. increased the efficiency of bulk heterojunction solar cells of aggregation and thiophene and fullerene derivatives to 5% through solvent heat treatment.
In 2006, AlanJ.Heeger et al. prepared a TiO2 cathode buffer layer by using a low-temperature solution method to adjust the light field distribution and enhance the absorption of light by the active layer, thereby increasing the efficiency to 5.0%.
In 2009, Alan J. Heeger and others used a new type of narrow band gap conductive polymer material and a TiO2 modified layer, which further increased the efficiency to 6.1%. In the same year, Yang Yang et al. used a narrow band gap PBDTTT4 to increase the efficiency to 7.73%.
In 2011, Cao Pu and others increased the battery efficiency to 8.37%.
In 2012, Professor Wu Hongbin of the School of Materials Science and Engineering of South China University of Technology, etc., used PTB7 and PC71BM, and adopted the inverted structure of PFN, which increased the efficiency of single-layer cells to 9.3%. In the same year, Sumitomo Chemical of Japan used P3HT; ICBA and PBDTT-DPP: PC71BM made laminated cells with an efficiency of 10.6%.