Abstract
In organic solar cells, the singlet and triplet excitons dissociate into free charge carriers with different mechanisms due to their opposite spin state. Therefore, the ratio of the singlet and triplet excitons directly affects the photocurrent. Many methods were used to optimize the performance of the low-efficiency solar cell by improving the ratio of triplet excitons, which shows a long diffusion length. Here we observed that in high-efficiency systems, the proportion of singlet excitons under linearly polarized light excitation is higher than that of circularly polarized light. Since the singlet charge transfer state has lower binding energy than the triplet state, it makes a significant contribution to the charge carrier generation and enhancement of the photocurrent. Further, the positive magnetic field effect reflects that singlet excitons dissociation plays a major role in the photocurrent, which is opposite to the case of low-efficiency devices where triplet excitons dominate the photocurrent.
1. Introduction
In recent years, spintronics has become an emerging discipline in materials physics. Through realizing the measurement and regulation of the spin state, many new phenomena, which could provide a new scheme to optimize the related devices, were observed.[1-5]Manipulating spin statistics is usually achieved by applying external field regulations such as electric, magnetic, and optical fields in many organic devices.[1, 6-8] In the photovoltaic sector, the organic solar cells (OSCs) performance can be improved by singlet fission or control of the spin-orbital effect.[9, 10] The external magnetic field mainly changes the ratio of singlet and triplet in the excited state in two ways: the magnetic field effect in the formation of the excited state and the intersystem crossing caused by the magnetic field.[11] However, the role of singlet and triplet excitons in contributing to photocurrent in OSCs seems unclear. Hence, figuring out the effect of singlet and triplet excitons on photocurrent is a meaningful direction for profoundly understanding the mechanism of OSCs.
Since the exciton binding energy in organic semiconductors is large at room temperature,[12] its excited state forms an intramolecular excited (Frenkel exciton)[13] or intermolecular excited state (Wannier excitons).[14] The spins of electrons have multiplicity, and both electrons and holes in the excited state have spin-antiparallel (singlet) and spin-parallel (triplet) states. With the help of hyperfine interaction and spin-orbit coupling effects, singlet and triplet excitons could be converted into each other.[15] In the domain of OSCs, the short diffusion length (LD) of the exciton is the main limiting factor to the effective charge carrier separation.[16] Because the LD of triplet exciton is longer than that of singlet exciton, increasing the ratio of triplet exciton helps enhance the PCE of OSCs. Therefore, many methods have been reported to increase the proportion of triplet excitons through orbital coupling effects, such as introducing heavy metal molecules or atoms.[1, 9, 17] The above conclusions are all summarized in the devices with a low-efficiency system. In 2020, Hu et al. proved that the dipoles in the non-fullerene acceptor molecule could be polarized by light excitation, which makes the excited state easier to separate into free charge, thus inducing the high efficiency in the non-fullerene system.[18] However, in this high-efficiency system of OSCs, rare attention focuses on investigating the effect of the exciton spin state on the photocurrent by polarized light. It is worth reconsidering the function of singlet and triplet excitons in contribution to photocurrent for the following reasons. First, if the uniform dispersion of the bulk heterojunction can guarantee the efficient diffusion of singlet excitons to the interface, then increasing the proportion of triplet states may not be highly important. In addition, the exciton binding energy of the charge transfer (CT) triplet state is higher than that of the singlet CT state. Increasing its proportion in excitons may not necessarily contribute to improving photocurrent.
Based on the above analysis, we investigated the photocurrent responses of high-efficiency OSCs under different polarizations of light and analyzed the contributions of triplet excitons and singlet excitons to the current. Here, we found that the separation of singlet excitons plays a major role in the photocurrent, which is different from the case of a low-efficiency system. In the organic solar cells with PBDB-TF and IT-4F as active layers, an anisotropic axis was confirmed in as-prepared samples by changing the incident azimuth angle and the azimuth angle of the sample. The variable power and the external bias test further prove that the singlet exciton ratio generated under linearly polarized light excitation is improved, inducing a higher photocurrent. On the other hand, the circularly polarized light increases the proportion of triplet excitons, which is not conducive to the direct dissociation of charges. A similar trend is observed on the D18:Y6 OSCs with a power conversion efficiency (PCE) of 17.35%. The positive correlation between photocurrent and magnetic field indicates that the free charge in the high-efficiency device mainly comes from the direct dissociation of the singlet excitons.
2. Results and Discussion