Fig. 1 The investigated solar cells consist of a glass superstrate, a transparent conductive oxide (TCO) layer, a pin-absorber of hydrogenated amorphous silicon (a-Si:H), and a back contact of TCO and silver (Ag). The Bragg filter is applied on top of the front glass after a first characterization of the cell without filter. The filter reflects light with incidence angles θ > θ_th in the respective wavelength range. (Thicknesses are not to scale.)

Fig. 2 (a) The total reflectance (r ₀/r _fi, without/with filter, black/orange line) of a 322 nm thick a-Si:H solar cell prepared on ZnO:Al on Corning glass increases for wavelengths λ > 600 nm because non-absorbed light is re-emitted from the cell. The filter increases the reflectance at wavelengths λ < 350 nm and λ > 770 nm but suppresses the re-emission of photons for 650 nm < λ < 770 nm. (b) The difference Δr = r _fi-r ₀ is reduced due to enhanced light trapping to a minimum of −40% at λ < 764 nm. (c) The external quantum efficiency EQE of the same solar cell measured with and without filter and (d) the difference ΔEQE=EQE _fi-EQE ₀ show that the filter acts as antireflective coating for 350 nm < λ < 600 nm and increases the light trapping for 650 nm < λ < 770 nm.

Fig. 3 (a) The optical path length improvement κ_r calculated from the data in Fig. 2(a) demonstrates the enhanced light trapping in the device for wavelengths 650 nm < λ < 770 nm. In the range 350 nm < λ < 650 nm the improvement is not due to the spectral selectivity, but rather to the anti reflective properties of the filter. The effective path length improvement κ_EQE shows the same wavelength dependence but a weaker increase than κ_r. (b) The maximum values of the improvement factors $\overline{\kappa }$ _r and $\overline{\kappa }$ _EQE depend on absorber layer thickness and the used superstrate (dots: AsahiU, squares: Corning glass). The error bars represent the standard deviation of the 5-12 investigated samples for each data point.

Fig. 4 (a) The dependence of the external quantum efficiency EQE _fi of a 414 nm thick a-Si:H solar cell (prepared on Corning glass) on the angle of incidence θ is used to characterize the Bragg filter. (b) The effect of the filter is visualized by dividing EQE _fi by EQE ₀ obtained from a sample without filter prepared in the same run. The dashed line is calculated from Eq. (3) using λ₀ = 767 nm and λ₁ = 600 nm.