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Light trapping efficiency of organic solar cells with large period photonic crystals

Léo Peres, Valérie Vigneras, and Sophie Fasquel

Open Access Open Access

Abstract

We study the optical properties of a 2D Photonic Crystal (PC) inserted in the upper ITO electrode of a classical P3HT:PCBM solar architecture with an ultra-thin active layer. First, we analyze the optical response of the system when only the active layer is supposed to absorb light. This allows us to observe clear photonic crystal resonances in the absorption spectrum, which increase the cell efficiency even if the period of the PC is higher than the wavelength. This is in apparent contradiction with the common belief that PC should work in subwavelength regime. Then, by turning to a real system (with optical losses in all the layers), an optimized PC design is proposed, where the maximum of efficiency is obtained for a PC period of 1200 nm, much larger than visible wavelength.

© 2014 Optical Society of America

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Figures (6)
Fig. 1
Fig. 1 Schematic view of global nanopatterned solar cell. The ITO electrode contains a 2D cubic air holes photonic crystal of period a and diameter d.

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Fig. 2
Fig. 2 (a) Absorption spectrum in the active layer for several values of PC period a. The air filling factor is fixed at 0.44. The integrated absorption enhancement G, normalized by the reference, is given for each value of period. (b) Spectral density of absorbed photons per unit of area and time, and integrated absorption enhancement G, weighted by the AM1.5 solar spectrum.

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Fig. 3
Fig. 3 Number of modes calculated at k = Г (of ГXMГ Brillouin zone, see the inset) over the wavelength range [400-700] nm, as a function as the PC period a. The solid line shows the best fit to a function Y = α X2.

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Fig. 4
Fig. 4 (a) P3HT:PCBM absorption in the reference cell (solid line) compared to the P3HT:PCBM absorption in the patterned solar cell (dashed line) ; the period a is 1000 nm and air filling factor ffair is 0.44. (b) Overview of Fig. 4(a) showing the P3HT:PCBM absorption (solid line) in the patterned solar cell on the range λ = [400-600nm], for the ideal system. The total absorption of the patterned multilayer system (dashed line) is added for comparison (all imaginary parts of index are taken into account). (c) Calculation of absorption in each layers of the patterned cell. (d) Calculation of P3HT:PCBM absorption enhancement in patterned solar cell by taking into account all absorbing layers contributions: 20% enhancement is obtained compared to reference planar cell.

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Fig. 5
Fig. 5 The black lines show the 2D band diagram of a cubic array of air holes (period a = 1000 nm) in a dielectric slab whose index is the effective index of the multilayer at λ = 575 nm (Neff575). A PC mode is found, in Γ, when a/λ = 1.74. The corresponding band is represented in thick black line. Due to the difference between Neff 575 and Neff 650, the diagram for λ = 650 nm is shifted (see the arrow), and only one band has been represented for λ = 650 nm (thick blue line). On this band, a mode can be excited in Γ, when a/λ = 1.54.

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Fig. 6
Fig. 6 Contour map of absorption enhancement as a function as period and air filling factor.

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Equations (3)

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A(ω)= 1 2 ω ε 0 Im( ε r (ω)) ∭ | E(r,ω) | 2 dV,
τ PC = 2 Q PC ω ,
G= ∫ λ 1 00 λ 2 λ hc A(λ)I(λ)d(λ) ∫ λ 1 λ 2 λ hc A ref f(λ)I(λ)d(λ) ,
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