Figure 4. The molecular structures of D18 (a) and Y6 (b). (c) the I-V curve of the best device. (d) Photocurrent measured by switching phase angles with different laser power.
spins in organic solar cells and exploiting triplet excitons can help us gain insight into the excited state and charge transport processes in organic photonic devices.
In order to further prove whether this phenomenon can be observed in other systems, the device of the current highest efficiency system with the active layer D18:Y6 was prepared. The schematic molecular structure of the donor and acceptor is shown in Figure 4 a and b. The exploration of the annealing process shows that the efficiency of the D18:Y6 system decreases after annealing, which is different from the previous system. To make the acceptor distribution in the active layer more uniform, we perform solvent annealing without heating. The effect of solvent annealing time on device performance was studied, and a device with an energy conversion efficiency of 17.35% was finally prepared (Figure 4c). We carried out the polarization photocurrent test on the prepared high-efficiency device and obtained similar results as PBDB-TF:IT-4F, which also has periodic changes, and the linearly polarized photocurrent effect is greater than the circularly polarized photocurrent effect, as shown in Figure 4d. This indicates that the polymer with a large conjugated structure has a spin-orbit coupling effect under polarized light conditions. By adjusting the polarization conditions of the excitation light, the ratio of singlet and triplet excited states in the active layer can be changed, thereby generating different photocurrent signals..
3. Conclusion
In summary, we investigated the effect of the exciton state on photocurrent in the organic solar cell by manipulating the spin state of the excited exciton with different polarization light and varied external magnetic fields. In the PBDB-TF:IT-4F system, the photocurrent signal under linearly polarized light is larger than that of circularly polarized light. Because the circularly light would increase the ratio of triplet exciton and the singlet charge transfer state has lower binding energy than the triplet state, the singlet exciton plays the primary role in photocurrent. The varied power and bias voltage test certify that the ratio of singlet exciton would increase under the linearly polarized light, thus improving the photocurrent. Combined with the magnetic field photocurrent response, the narrower peak of the normalized profile reflects that the contribution of free charge is mainly from the direct dissociation of the singlet CT state. The identical trend was also observed in the D18:Y6 system, which possesses the highest PCE in organic solar cells. This result is entirely different from the case of a low-efficiency system, where the main contribution to the photocurrent comes from the triplet excitons. Our findings provide a new explanation for the high PCE in OSCs devices and pave the way for further improving the performance by manipulating the spin state of the exciton.
4. Experimental Section