Organic solar cells (OSCs) have drawn great attention as a promising renewable energy generator due to their potential for cheap processing on large areas, mechanical flexibility, and light weight [1, 2]. In spite of much research effort, the power conversion efficiency (PCE) of commercial OSCs is still lacking, compared to that of silicon-based inorganic solar cells . The main problem is that most organic materials have a serious limitation for the probability that an absorbed photon contributes to photocurrent, known as the internal quantum efficiency (IQE). The active layer must be kept thick enough to absorb most incident photons, which results in increasing the probability for charge recombination, which decreases the IQE. The trade-off between optical absorption and charge carrier transport should be carefully considered in the determination of device thickness in OSCs [4, 5].
Recently, various light-trapping schemes such as metal gratings , buried nanoelectrodes , scattering elements , and multiple-reflection structures  have been proposed to enhance light absorption in OSCs without thickening the active layer. Among them, V-shaped organic solar cells (VOSCs) have shown a double enhancement of PCE compared to planar cells . Part of the incident light that is not absorbed by one arm is reflected onto the other arm, which gives a greater chance for it to be absorbed in the active layer and increases the PCE due to the light-trapping effect . Moreover, VOSCs can be applicable to tandem or multiple-bandgap OSCs, where different bandgap materials are used on each arm of a V-shaped substrate to broaden the absorption spectrum [11, 12].
Optical modeling based on the finite element method (FEM) has been performed to investigate the optical behavior of VOSCs with respect to layer thickness and folding angle [11, 12]. Polarization of sunlight should be considered in the optical modeling of VOSCs because
In this paper, we present comprehensive optical modeling results based on the FEM, focusing on the dependence of light polarization on the absorption behavior of VOSCs. The spectral distribution of absorbance and the spatial distribution of power dissipation are each calculated as a function of the folding angle for
The direction of the electric field oscillations is composed of
For planar OSCs, the transfer matrix method (TMM) described by 2×2 matrices has been widely used to calculate the absorption behavior for both
The time-average power dissipation per volume, for one wavelength, is defined as where λ is the wavelength and the unit of
[FIG. 1.] Schematic diagram of the device structure along with material’s composition and thickness of each layer. The plane wave is assumed to enter the OSC structure from the air at the uppermost boundary. The direction of the electric field oscillations is perpendicular (s-polarization) or parallel (ppolarization) to the incident plane. Two arms of the VOSC are tilted to the normal incident light with a folding angle of α.
Because the number of the multiple reflections between two arms of the VOSC increases at smaller folding angles, the light-trapping effect is more prominent for smaller folding angles . Thus, the overall absorbance in Fig. 2 increases as the folding angle decreases and becomes saturated at an angle of 10°. The absorbance enhancement caused by the folded arms is more visible in the wavelength range of 450-600 nm, due to the significant absorption coefficient of the P3HT:PCBM active layer around 500 nm, as shown in Fig. 3. The small ripples in the absorbance spectra between 500 and 600 nm are ascribed to the ripples in the extinction coefficient of the P3HT:PCBM active layer, shown in Fig. 3.
Figures 4 and 5 show the spatial distributions of the power dissipation in the VOSC at folding angles of 20°, 45°, and 70° for
[FIG. 4.] The spatial distribution of the power dissipation in the VOSC at the folding angle of α = (a) 20°, (b) 45°, and (c) 70° for s-polarized light. The spatial distribution of the power dissipation is obtained by taking the summation over the whole wavelength range.
[FIG. 5.] The spatial distribution of the power dissipation in the VOSC at the folding angle of α = (a) 20°, (b) 45°, and (c) 70° for p-polarized light. The spatial distribution of the power dissipation is obtained by taking the summation over the whole wavelength range.
The reflectance at the planar OSC structure shown in the inset of Fig. 1 is calculated based on the TMM. For the folding angles α = 20°, 45°, and 70°, the corresponding incidence angles at the interface of the VOSC α = 70°, 45°, and 20°, in terms of the oblique incidence for planar solar cells. At the small folding angle α = 20° (α = 70°), almost all of the wavelength range than that for
[FIG. 6.] Calculated reflectance spectra at the interface of the VOCS at the folding angle of α = 20°, 45°, and 70° for s- and p-polarized light. For the folding angles of α = 20°, 45°, and 70°, the incidence angle at the interface of the VOSC corresponds to α = 70°, 45°, and 20° in terms of the oblique incidence for planar solar cells.
[FIG. 7.] Calculation results of polarization-dependent absorptance spectra in the active layer of the VOSC at the folding angles of α = 20°, 45°, and 70°. Both polarizations have relatively high absorptance at the small folding angle due to the enhanced light trapping effect.
In view of light polarization,
When the folding angle is smaller than 20°,
In the realistic performance evaluation of solar cells, the optical spectrum of input light should follow the AM 1.5 sunlight model in Fig. 9(a) instead of the normalized power across the whole wavelength range. Figure 9(b) shows the calculated solar absorbance of the VOSC as a function of folding angle for
We numerically investigated the polarization-dependent absorption behavior of the VOSC with respect to the folding angle. The absorption enhancement caused by the light-trapping effect was more prominent for