In the energy field, solar energy has renewable characteristics that natural gas, oil, coal and other resources do not have, and has gradually become a strategic resource for countries and research institutions to break through the bottleneck of economic development.
After about 60 years of development, since solar cells spread from the military field to civilian households and other fields, the cell efficiency has developed from less than 1% at the beginning to more than 20% today. Up to now, inorganic materials are still the most important raw materials for solar cells and play a role that cannot be ignored, but they are subject to the prices and sources of raw materials.
Based on the advantages of simple preparation, wide source of raw materials, and low cost, organic materials have shown a trend of replacing inorganic materials in the laboratory. However, due to the low charge mobility of organic materials and the low open circuit voltage caused by non-radiative recombination, the So that the performance of organic solar cells can not be comparable to inorganic solar cells.
Based on this phenomenon, Cambridge University physics experts Richard Freund, Alexander J. Gillett, Professor Thuc-Quyen Nguyen of the Department of Chemistry and Biochemistry at the University of California, Santa Barbara, and David Beljon of the University of Monsiello et al. propose a novel hybridization strategy that makes it possible to increase the efficiency of organic solar cells above 20% by identifying, confirming and suppressing nonradiative loss pathways.
An ideal solar cell model should convert energy only through radiative recombination, that is, to obtain 100% external quantum efficiency (EQE) for electroluminescence.
In practice, however, there are often multiple non-radiative recombination pathways that affect cell performance, resulting in additional voltage losses. The factors affecting EQE are photoluminescence efficiency and radiation attenuation composite ratio. By changing the two factors, the regulation of the conversion efficiency of the solar cell can be achieved.
Most of the current research is to improve the EQE by increasing the photoluminescence efficiency, and this paper provides a new idea to improve the efficiency of solar cells by changing the ratio of radiation attenuation.
In organic solar cells, the recombination process of free charges proceeds through the formation of charge-transfer excitons, requiring the molecular triplet state in the donor-acceptor to have a lower energy than the spin triplet state. It is therefore possible to have an inverse charge transfer from the spin triplet state to the molecular triplet state. As shown in Figure 1, spin molecular triplet states can be generated by two pathways: twin and non-twin charge carrier pairs. Conversely, the generation of molecular triplet states by reverse charge transfer can also be generated by two different mechanisms.
In this paper, four polymer donors and seven non-fullerene acceptors are used as examples, and two of them are the main research objects.
Through a series of means of transient absorption and electron paramagnetic resonance to analyze the relationship between the device performance and electron-molecular triplet state of different solar cell materials, in the transient absorption spectrum of the material PM6:Y6, it represents the light-induced absorption band The 1250 nm and 1550 nm gradually disappear with the increase of the time scale, and a new light-induced absorption band appears at 1450 nm, which is the characteristic absorption band of the molecular triplet state of Y6.
Transient absorption kinetics show flux dependence in the generation of molecular triplet states, suggesting that molecular triplet states are generated by a bimolecular mechanism. When high-flux excitation is used, the non-twin recombination process occurs rapidly. According to the electron paramagnetic resonance results in FIG. 2 , it can be known that there are unpaired molecular triplet states in PM6: Y6.
In another material that cannot detect charge-transfer excitons to generate molecular triplet states, the appearance of light-induced absorption bands was not observed, and the kinetic tests did not reveal a flux-dependent phenomenon. This suggests that the unpaired molecular triplet recombination pathway cannot be detected. And in its electron paramagnetic resonance spectrum, a molecular triplet state formed by intersystem crossing mediated by spin-orbit coupling is observed.
In this paper, the intermolecular interaction mechanism between donor and acceptor in the process of molecular triplet formation is deeply studied by quantitative calculation method. In a certain range, the spin singlet state is stable, while the spin triplet state is unstable.
Since S1 has a higher energy than the singlet state, and the molecular triplet state has a lower energy than the triplet state, the inversion is caused by charge transfer and hybridization between localized excitons. Due to the same vertical node plane sequence of the highest occupied molecular orbital along the molecular axis and a large number of molecular overlaps, the electronic coupling of the complex material is enhanced, which in turn induces hybridization and inhibits the reverse charge transfer.
By demonstrating the key role of the molecular triplet state in the additional voltage loss, this paper proposes a new idea to reduce the voltage loss by suppressing the formation process of the molecular triplet state to hybridize the spin triplet state and the molecular triplet state. The proposal of this new method is enough to make the cell conversion efficiency reach more than 20%.
