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Light Trapping Textures Designed by Electromagnetic Optimization for Subwavelength Thick Solar Cells

Figure 1

Figure 1
The bottom surface texture of the absorbing material is computationally optimized. This diagram is a schematic of one unit cell; there are periodic boundary conditions along the y–z and x–z planes.

Figure 2

Figure 2
(a) Initial AE as a function of frequency, at normal incidence and (b) top-down view of the surface texture; the colors show the height of the absorbing material (from the AR coating to the bottom dielectric, as seen in Fig. 1).

Figure 3

Figure 3
(a) Final AE as a function of frequency, at normal incidence and (b) top-down view of the surface texture; the colors show the height of the absorbing material (from the AR coating to the bottom dielectric, as seen in Fig. 1).

Figure 4

Figure 4
Reciprocal Formula$(k$-) space representation for the final texture seen in Fig. 3. The blue pie slices represent the phase of the complex exponential Fourier coefficients.

Figure 5

Figure 5
Absorption enhancement (AE) for the texture in Fig. 3, plotted as a function of frequency at normal incidence, averaged over both polarizations, (blue) and angle averaged (green).

Figure 6

Figure 6
Surface textures and AE as a function of frequency for different initial conditions, revealing a broad optimum.

Figure 7

Figure 7
Absorption enhancement factor (AE) averaged over frequency and polarization, as a function of incident angle Formula${\theta}$ in the x–z plane (blue) and the y–z plane (red).

Figure 8

Figure 8
Photonic bandstructure for the texture in Fig. 6(b).

Figure 9

Figure 9
(a) Initial AE as a function of frequency, at normal incidence and (b) top-down view of the surface texture, for a symmetric texture with a slight perturbation along the diagonal.

Figure 10

Figure 10
(a) Final AE as a function of frequency, at normal incidence and (b) top-down view of the surface texture, showing broken mirror symmetry, from almost symmetric initial conditions seen in Fig. 9.

Figure 11

Figure 11
Reciprocal space representation for the texture with broken mirror symmetry in Fig. 10. The blue pie slices represent the phase of the complex exponential Fourier coefficients.

Figure 12

Figure 12
Optimizations were carried out at periodicities from 50 to 800 nm, in increments of 50 nm. For each periodicity, at least three optimizations were completed for randomly chosen initial starting noise. The AEFOM (absorption enhancement for the worst performing frequency and polarization at normal incidence) is plotted for each optimization.

Figure 13

Figure 13
AE FOM (minimum absorption enhancement at normal incidence) plotted for 100 randomly generated textures of 710-nm periodicity. For comparison, the AEFOM for the optimized texture in Fig. 6(b) is shown by the dotted red line.

Figure 14

Figure 14
(a) Reciprocal Formula$k$-space diagram and (b) real-space top-down view of the median randomly generated texture from Fig. 13.

Figure 15

Figure 15
Absorption enhancement (AE) as a function of frequency at normal incidence for the optimized texture from Fig. 6(b) (blue) compared with the median random texture from Fig. 14 (green). Lines are averaged over the two orthogonal polarizations.

Figure 16

Figure 16
Absorption enhancement (AE) as a function of frequency, angle averaged, for the optimized texture from Fig. 6(b) (blue) compared with the median random texture from Fig. 14 (green).

Figure 17

Figure 17
AE FOM (minimum absorption enhancement at normal incidence) plotted for 11 randomly generated textures of 2300-nm periodicity. For comparison, the AEFOM for the optimized texture in Fig. 6(b) is shown by the dotted red line.

Figure 18

Figure 18
A top-down view of the surface texture, for the randomly generated texture with periodicity of 2300 nm = 10 Formula${\lambda}_{n = 3.5}$, with median AEFOM (minimum absorption enhancement at normal incidence).

Figure 19

Figure 19
Absorption enhancement (AE) as a function of frequency at normal incidence for the optimized texture from Fig. 6(b) (blue) compared with the median random texture with 2300-nm periodicity from Fig. 18 (green). Lines are averaged over the two orthogonal polarizations.

Figure 20

Figure 20
Absorption enhancement (AE) as a function of frequency, angle averaged, for the optimized texture from Fig. 6(b) (blue) compared with the median random texture with 2300-nm periodicity from Fig. 18 (green).

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