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