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