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