Using spacer layers to control metal and semiconductor absorption in ultrathin solar cells with plasmonic substrates

We systematically explore the performance of ultrathin amorphous silicon solar cells integrated on plasmonic substrates of several different morphologies. Angle-resolved reflectance, external quantum efficiency measurements, and finite-difference time-domain simulations highlight the importance of the spacer layer in determining the mode profiles to which light can couple. Coupling mechanisms are found to strongly differ between periodic silver nanovoid arrays and randomly textured silver substrates. Tailoring the spacer thickness leads to 50$\%$ higher quantum efficiencies and short-circuit current densities by tuning the coupling between the near-field and trapped modes with enhanced optical path lengths. The balance of absorption for the plasmonic near field at the metal/semiconductor interface is analytically derived for a broad range of leading photovoltaic materials. This yields key design principles for plasmonic thin-film solar cells, predicting strong near-field enhancement only for CdTe, CuInGaSe${}_2$, and organic polymer devices.

Figure 1

(a) Schematic representation of substrate and layer structure. (b, c) Scanning electron micrographs of silver nanovoids (radius 250 nm) and silver-coated Asahi glass. Nanovoids and Asahi glass coated with (d, e) 30  and (f, g) 200 nm of Al:ZnO and 50 nm a-Si.

Figure 2

Angle reflectance scans of silver nanovoids (250-nm radius) (a) uncoated, (c, d) coated with Al:ZnO of 30  and 200 nm, (e, f) and complete solar cells as indicated. Color scale is log(reflectance) with blue indicating high reflectance and red-white indicating low reflectance. Modes labeled according to Ref. 30. (b) Short-circuit current density for nanovoids cells versus Al:ZnO thickness, decomposed for $\lambda <$ 600 nm and $\lambda >$ 600 nm.

Figure 3

Angularly resolved reflectance for (a) silver-coated randomly textured glass, (b, c) coated with Al:ZnO and (d, e) complete cell layers. (f) Photocurrent measurements with varying Al:ZnO thickness. Labelling discussed in text.

Figure 4

(a) Short-circuit current density and (f) EQE measurements for flat cells with changing Al:ZnO thickness. Angle-resolved reflectance of Al:ZnO (b, c) coated silver (d, e) and with additional solar cell layers.

Figure 5

FDTD simulations of ${\left|E\right|}^2$ intensity at 600 nm with increasing Al:ZnO spacer thicknesses of 30 100 and 200 nm (left to right) for (a-c) nanovoids, (d-f) Asahi, and (g–i) flat solar cells.

Figure 6

Extinction = (1 reflectance) spectra for flat and Asahi substrates coated with (a) Al:ZnO and (b) additional cell layers. Transfer matrix calculations (grey lines) show the position of Fabry-Perot peaks within planar substrates.

Figure 7

External quantum efficiency of a-Si solar cells fabricated on Asahi VU type glass, Nanovoids, and Flat silver substrates. LSPR = localized surface plasmon resonance, FP = Fabry-Perot resonance, GM = Guided mode.

Figure 8

Short-circuit current density versus Al:ZnO thickness for each substrate.

Figure 9

$J_{\mathrm{sc}}$ vs curvature for cells with 30 nm Al:ZnO on flat silver, nanovoids with radius 100, 150, 200, and 250 nm, and silver-coated Asahi glass.

Figure 10

Ratio of power absorbed in semiconductor and metal for various semiconductors. A silver substrate with a surface plasmon polariton is excited at the interface.

Figure 11

Angle reflectance scans in TE of silver nanovoids (a) uncoated, (c, d) coated with Al:ZnO of 30  and 200 nm, and (e, f) complete solar cells as indicated. Color scale is log(reflectance) with blue indicating high reflectance and red-white indicating low reflectance. (b) EQE for nanovoids cells with varying Al:ZnO thickness.

Figure 12

Angle reflectance scans in TE of (a) silver-coated Asahi glass, (c, d) coated with Al:ZnO of 30 and 200 nm, (e, f) and complete solar cells as indicated. Color scale is log(reflectance) with blue indicating high reflectance and red-white indicating low reflectance. (b) EQE for Asahi cells with varying Al:ZnO thickness.

Figure 13

Angle reflectance scans in TE of flat silver (a) uncoated, (c, d) coated with Al:ZnO of 30  and 200 nm, (e, f) and complete solar cells as indicated. Color scale is log(reflectance) with blue indicating high reflectance and red-white indicating low reflectance. (b) EQE for flat cells with varying Al:ZnO thickness.

Figure 14

External quantum efficiency versus wavelength for solar cells with 30 nm coating of Al:ZnO and varying substrate geometry.

Figure 15

Angle reflectance (TM) scans of nanovoids with varying radius coated with 30 nm of Al:ZnO (top) and complete solar cell layers (bottom). Left to right 250, 200, 150, and 100 nm.