Flexible Semiconductor Technologies with Nanoholes-Provided High Areal Coverages and Their Application in Plasmonic-Enhanced Thin Film Photovoltaics

Abstract

Mechanical flexibility and advanced light management have gained great attentions in designing high performance, flexible thin film photovoltaics for the realization of building-integrated optoelectronic devices and portable energy sources. This study develops a soft thermal nanoimprint process for fabricating nanostructure decorated substrates integrated with amorphous silicon solar cells. Amorphous silicon (a-Si:H) solar cells have been constructed on nanoholes array textured polyimide (PI) substrates. It has been demonstrated that the nanostructures not only are beneficial to the mechanical flexibility improvement but also contribute to sunlight harvesting enhancement. The a-Si:H solar cells constructed on such nanopatterned substrates possess broadband-enhanced light absorption, high quantum efficiency and desirable power conversion efficiency (PCE) and still experience minimal PCE loss even bending around 180°. The PCE performance without antireflection coatings increases to 7.70% and it improves 40% compared with the planar devices. Although the advantages and feasibility of the schemes are demonstrated only in the application of a-Si:H solar cells, the ideas are able to extend to applications of other thin film photovoltaics and semiconductor devices.

Figure 1

Schematic illustration of fabrication of PDMS stamps and a-Si:H solar cells on nanopatterned plastic substrates: (a) Si master stamp, (b) An anti-sticking coating on Si master stamp by a molecular vapor deposition process, (c) PDMS precursor casted onto the anti-sticking surface of the Si master stamp, (d) PDMS soft mold separated from the Si master stamp, (e) Pre-cleaned PI film, (f) TiO₂ sol film by spin coating on the PI substrate, (g) Textured TiO₂ sol film on the PI substrate via soft thermal nanoimprint process, (h) Ag back electrodes deposited on TiO₂ nanostructures by DC magnetron sputtering, (i) AZO spacer layer deposited on the Ag bottom, (j) n-i-p a-Si:H stacks constructed on top of the AZO layer, (k) the ITO top electrode deposited on amorphous silicon layer.

Figure 2

(a) Scanning electron microscopy (SEM) images of a slab of PI film with patterned surface in the geometry of hexagonally ordered nanohole arrays; (b) Finite element modeling (FEM) of this structure in a biaxially stretched state, FEM results of the distribution of maximum principal strain and the two components of the strain tensor at the top surface of the nanoholes decorated substrates; (c) Top and (d) Cross sectional scanning electron microscopy (SEM) images of a-Si:H solar cells constructed on the patterned substrate.

Figure 3

(a) Plots of calculated absorption in each layer of the patterned devices (Top coated ITO configuration); (b) Plots of calculated absorption in each layer of the planar devices (Planar configuration); (c) Plots of calculated absorption in each layer of the patterned devices (Filled ITO top configuration); (d) Schematic structures of fabricated solar cells, Top coated ITO configuration, Filled ITO top configuration and Planar configuration, respectively; (e) Experimental absorption spectra of the a-Si:H solar cells on the planar and patterned PI films; (f) Simulated absorption spectra of the a-Si:H solar cells on the planar and patterned PI films.

Figure 4

(a) Dispersion plots of samples at $\overline{t}=0.25$ and $\phi =0$ with nanohole diameter of 800 nm at different photo energies and incident angles. Lines show theory for Bragg plasmons; (b) Mie plasmon absorption for samples with nanohole diameter of 800 nm, showing plasmon mode energy vs t at normal incidence; (c) Angle reflectance scans of complete solar cells as indicated. Color scale is log(reflectance) with red representing high reflectance and blue-white representing low reflectance; (d–i) The cross-sectional views of electric field intensity (E) distributions in patterned devices and planar devices at wavelengths of 733 nm, 860 nm and 916 nm.

Figure 5

(a) Current-voltage characteristics. The short circuit current density (J_sc), open circuit voltage (V_oc), fill factor (FF), and power conversion efficiency (PCE); (b) External quantum efficiency (EQE) and reflectance of a-Si:H devices based on PI film with TiO₂-gel patterns and planar substrates; (c) Relative PCE efficiency of the patterned devices as a function of bending angles, the inset illustrates the definition of the bending angle; (d) PCEs and PCE enhancement of patterned devices with respect to flat devices as a function of incident angles; (e) Incident Photon-Electron Conversion Efficiency spectrum (IPCE) and reflectance of a-Si:H devices based on PI film with TiO2-gel patterns and planar substrates.