Broadband light absorption enhancement in dye-sensitized solar cells with Au-Ag alloy popcorn nanoparticles

Abstract

In this paper, we present an investigation on the use of Au-Ag alloy popcorn-shaped nanoparticles (NPs) to realise the broadband optical absorption enhancement of dye-sensitized solar cells (DSCs). Both simulation and experimental results indicate that compared with regular plasmonic NPs, such as nano-spheres, irregular popcorn-shaped alloy NPs exhibit absorption enhancement over a broad wavelength range due to the excitation of localized surface plasmons (LSPs) at different wavelengths. The power conversion efficiency (PCE) of DSCs is enhanced by 16% from 5.26% to 6.09% by incorporating 2.38 wt% Au-Ag alloy popcorn NPs. Moreover, by adding a scattering layer on the exterior of the counter electrode, the popcorn NPs demonstrate an even stronger ability to increase the PCE by 32% from 5.94% to 7.85%, which results from the more efficient excitation of the LSP mode on the popcorn NPs.

Figure 1. Schematic structure of plasmonic enhanced dye-sensitized solar cells (DSCs) with Au-Ag alloy popcorn NPs.

Popcorn NPs have several fine structures of different shape and size upon which various LSP modes of different wavelengths, polarisations and field patterns could be excited simultaneously.

Figure 2. Theoretical investigation of the localized surface plasmon (LSP) effect of the popcorn NPs.

For simplicity, we use a 2D model instead of a 3D model and assume the popcorn NP model to be a large ellipsoidal structure (approximately 180 nm) with several small ellipsoidal structures (approximately 20–50 nm) on it. (a) to (c): The LSP field distribution of the popcorn NP illuminated by incident light in one direction at wavelengths of (a) 490 nm, (b) 650 nm and (c) 790 nm, respectively. (d) to (f): The LSP field distribution of the popcorn NP illuminated by incident light in two opposite directions at wavelengths of (d) 490 nm, (e) 650 nm and (f) 790 nm, respectively.

Figure 3

(a) Scanning electron microscopy (SEM) image of popcorn NPs. (b) Energy dispersive spectroscopy (EDS) of popcorn NPs refers to point 1# and point 2# in Fig. 3 (a). (c) Appearance of popcorn NPs with (sample 2)/without (sample 1) electrolyte. (d) Optical absorption of popcorn NPs before/after being mixed with electrolyte. (e) Optical absorption spectra of popcorn NPs (black curve), N719 dye molecules (blue curve), and a mixture of popcorn NPs and dye in ethanol solution (red curve).

Figure 4

(a) Scanning electron microscopy (SEM) image of dye-sensitized TiO₂ film incorporated with popcorn NPs. (b) Energy dispersive spectroscopy (EDS) of TiO₂ film incorporated with popcorn NPs. (c) Optical absorption spectra of TiO₂ film with (red curve) or without popcorn NPs (black curve) and dye (N719) sensitized TiO₂ film with (green curve) or without popcorn NPs (blue curve). (d) Relative light absorption change (ΔOA/OA) of dye-sensitized TiO₂ film with popcorn NPs. The thickness of the film is 3 μm. Here, ΔOA(λ)/OA(λ) = (OA_{TiO2+dye+popcorn}(λ) − OA_TiO2+dye(λ))/OA_TiO2+dye(λ), where OA_{TiO2+dye+popcorn}(λ) and OA_TiO2+dye(λ) are the optical absorptions at wavelength λ of dye-sensitized TiO₂ films with and without popcorn NPs, respectively.

Figure 5

(a) The photocurrent density-voltage characteristics (J–V curves) of plasmonic DSCs incorporated with popcorn NPs at the optimised concentration of 2.38 wt% (device 3) and TiO₂-only DSCs (device 1), (b) The J–V curves of plasmonic DSCs incorporated with popcorn NPs (device 6) and TiO₂-only DSCs (device 5). Insets show the schematic structure of the DSCs with or without a reflection film.

Figure 6

(a) Incident photon-to-electron conversion efficiency (IPCE) of DSCs incorporated with popcorn NPs at optimised concentration of 2.38 wt% (device 3) and TiO₂-only DSCs (device 1). (b) The IPCE enhancement ratio of the plasmonic DSCs (device 3). IPCE enhancement ratio = (IPCE_popcorn(λ)/IPCE_TiO2-only(λ), where IPCE_popcorn(λ) and IPCE_TiO2-only(λ) are the IPCEs of the popcorn NP-enhanced DSCs and TiO₂-only DSCs at wavelength λ, respectively.

Figure 7

(a) Cyclic voltammogram of N719 sensitized TiO₂ anode incorporated with/without popcorn NPs measured at a sweep rate of 200 mV/s. (b) Electrochemical impedance spectra of DSCs incorporated with popcorn NPs of different concentrations. Inset is The equivalent circuit.