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]