Figure 2. (a) The relationship of photocurrent with different
incident angles. (b) The relationship of photocurrent with different
azimuth angles. (c) Photocurrent measured by switching phase angle under
1/2 wave plate and 1/4 wave plate. (d) The schematic of the CT states.
In addition, we performed a polarization Raman test on the active layer
film to further verify their anisotropy. Figure S3a shows the
relationship between the angle (between the polarization direction of
the excitation light and the optical axis direction) and the intensity
of the polarization Raman scattering spectrum of the sample, where the
rotation angle changes from 0° to 360°, and the measurement range
locates in 1250~1650 cm-1 wave number.
At the wave number of 1430 cm-1, the peak intensity
varies periodically with the angle, as shown in Figure 1e. According to
previous studies,[22, 23] this wavenumber is the
Raman scattering peak of thiophene, which is consistent with the
conjugated structure in our conjugated polymer.
Since the polarization photocurrent test cannot be performed by the
reported method,[24] the anisotropy axis of the
film must be verified before the polarized light passes through the 1/4
wave plate. First, a 1/2 wave plate was placed between the polarizer and
the sample to explore the anisotropy axis of the film, as shown in
Figure 2c. Adjusting the 1/2 wave plate to the direction of the
anisotropy axis of the sample, a periodic function of 90° can be seen.
Then the 1/4 wave plate is placed between the 1/2 wave plate and the
sample, and the 1/4 wave plate is in the state of circularly polarized
light when the phase angle is 45°. By measuring the difference between
the phase angle of 45° and 0°, the difference of the 1/2 wave plate is
two times that of the 1/4 wave plate, which is consistent with our
assumption. To exclude the influence of sample anisotropy and accurately
measure the difference between the circularly polarized photocurrent
signal and the linearly polarized photocurrent signal, we rotated the
fixed 1/2 wave plate by 22.5°. The difference between the photocurrent
signals of circularly polarized light and linearly polarized light can
be obtained.
To demonstrate the reliability of our testing method, we apply this
method to standard Si solar cell testing, as shown in Figure S4a. First,
only the 1/2 wave plate was added for testing, and the polarization
direction of the linearly polarized light was changed by rotating the
1/2 wave plate. It is found that the photocurrent signal of the Si cell
does not change with the polarization angle. In Figure S4b, the 1/4 wave
plate was added between the 1/2 wave plate and the sample, and the 1/2
wave plate was fixed for testing. The photocurrent signal also did not
change with the polarization angle of the 1/4 wave plate. It is well
known that standard Si cells are optically isotropic. Therefore, the
photocurrent should also not vary with the polarization angle of the 1/2
wave plate, which agrees with our experiments. Moreover, the polarized
light would not affect the photocurrent because the photogenerated
exciton with lower binding energy dissociates into free charge
efficiently enough in silicon solar cell.[25]