2.3 Discussion
There are two hypotheses for why ΔPC varies with optical power: 1) The
electrical and optical properties of nonmagnetic organic semiconductors
respond strongly to an applied magnetic field, known as organic magnetic
field response (OMFE). The difference between linearly and circularly
polarized light could arise from the spin state change due to the
electric field component of circularly polarized light, thereby
generating an equivalent magnetic field (the inverse Faraday
effect).[26] The generated equivalent magnetic
field can modulate the electrical properties of the active layer of the
device, resulting in different signal responses under different
polarization states.[27] 2) The circularly
polarized light directly transfers the photon angular momentum to the
electrons in the active layer to polarize the excitons, resulting in a
spin-orbit coupling effect, thereby changing the conversion process of
singlet excitons and triplet excitons, inducing the change
ΔPC.[28,29] Relative to assumption 1), if there is
an equivalent magnetic field, ΔPC should have a quadratic relationship
with optical power,[30] which is inconsistent with
our linear relationship; therefore, assumption 2) is the most probable
under our system.
In perovskite materials, due to the presence of heavy metal elements
such as Pb or Sn, there is a strong spin-orbit coupling effect under
polarization conditions.[31,32] While organic
semiconductor materials are mainly composed of light elements, it is
believed that there should be no internal spins-orbit effect. However,
in organic conjugated polymers with larger molecular scales, atomic
orbitals can be converted into molecular orbitals. When high-energy
electron excitation or injection makes the electron occupy the excited
state, the electron has a considerable orbital angular momentum, and the
spin-orbit effect also occurs at this time.[33,34]
Due to the law of spin selection, only singlet excitons can be generated
by light absorption in the active layer, and singlet excitons can be
converted into triplet excitons through intersystem
jumping.[35] The binding energy of excitons at
room temperature is large and cannot be directly separated. Singlet
excitons and triplet excitons can dissociate into singlet charge
transfer (CT) and triplet CT states, respectively, as shown in Figure
2d. The singlet CT state has lower binding energy than the triplet state
and thus significantly contributes to the charge carrier
generation.[36] By increasing the optical power,
the number of excited states in the active layer can be effectively
improved. The spin-orbit coupling effect of circularly polarized light
produces more triplet excitons, which is not conducive to direct
dissociation to generate charges. Therefore, ΔPC increases with the
increase of light intensity.
Next, we studied the relationship between the polarized photocurrent and
the applied voltage of the organic solar cell. The excitation light
still chooses the polarized light of 532 nm, and the voltage range is -1
V to 1 V. As shown in Figure 3c, the PC signal increases with the
increase of the applied bias voltage. And the average PC and ΔPC are
extracted and plotted in Figure S5a. It can be seen that ΔPC also has a
slightly increasing trend with the increased applied bias voltage. Since
the applied bias voltage increases the dissociation rate of the singlet
CT state, the current would increase with the applied bias voltage.
Under the same bias voltage, the linearly polarized photocurrent is
still larger than the circularly polarized photocurrent, indicating that
there are more singlet excitons excited by linearly polarized light than
those excited by circularly polarized light, which is consistent with
the conclusion obtained from the variable power test.
In organic semiconductor materials, the magnetic field effect of
photocurrent is easy to observe and can be used as an effective means to
study the conversion process of singlet and triplet excited states in
organic solar cells.[37] The working process of
organic solar cells mainly includes four processes: light absorption,
generation of excited states, separation of excited states to generate
electrons and holes, charge transport in the active layer, and
collection by the positive and negative electrodes. Among them, the
magnetic field effect mainly occurs in the process of separation of
excited states to generate electrons and holes. There are two main ways
to generate a charge from the excited state. One is the direct
dissociation of the excited state to generate electrons and holes, and
the other is the collision reaction between the excited state and the
charge. Because the singlet has a much larger dissociation rate than the
triplet, the excited state dissociation, which is essentially driven by
the electric dipole-dipole interaction between the molecules, is mainly
participated by the singlet state. The reaction between the excited
state and the charge is essentially coulomb scattering. Due to the long
lifetime of the triplet state, the reaction mainly involves the triplet
state. The triplet state excitons can collide with the charges in the
trap and free charges to generate new electron-hole
pairs.[17]
The applied magnetic field can affect the above two processes by
changing the ratio of singlet/triplet excitons and the rate of excited
state-charge reaction through the magnetic field response. However, the
magnetic field has different effects on these two processes. On the one
hand, the external magnetic field can increase the number of singlet CT
states through the intersystem jumping of the magnetic field response so
that the current increases with the magnetic field increase.[38] On
the other hand, an external magnetic field can weaken the triplet
excitons-charge reaction by degenerating the triplet excitons. As a
result, the current decreases with the magnetic field
increase.[39]
Further, we excite the samples with different polarized lights under the
magnetic field condition and observe the experimental phenomenon. Figure
3d shows the variation of the linearly and circularly polarized
photocurrent signals with the magnetic field intensity. The magnetic
field positively affects the polarized photocurrent, and the result
indicates that the dissociation of the singlet CT state dominates the
system in our study. The magnetic field increases the ratio of the
singlet state to the triplet state by disturbing the intersystem
crossing between the singlet state and the triplet state of the excited
state, thereby increasing the current. The linearly polarized light is
more affected by the magnetic field, which proves that there are more
singlet excitons in the system under the excitation of linearly
polarized light. After that, we performed Lorentzian fitting and
normalization of the data to obtain Figure S5b. In the process of
changing from circularly polarized light to linearly polarized light,
the peak shape narrows, and the half-peak width decreases from 515 mT to
435 mT. Generally, the narrower the peak shape indicates the higher the
proportion of singlet CT states.[40] This proves
that circularly polarized light increases the ratio of triplet excitons,
which is not conducive to separation to generate charges.
Through testing in a magnetic field, circularly polarized light can
increase the proportion of triplet excitons because circularly polarized
photocurrent directly transfers angular momentum to organic
semiconductors through the spin-orbit coupling effect. Manipulating