Energy levels and corresponding wavelengths of the radiative transition of trivalent erbium. The population of higher energy levels by the absorption of several photons is illustrated by the curved arrows. A photon with more energy than the absorbed ones can be emitted from a higher energy level by transition to the ground state. The data for this graph was taken from Dieke (Ref. 7).
Spectral optical UC efficiency of the in a logarithmic scale calculated from calibrated photoluminescence measurements. While keeping the irradiance constant at the excitation wavelength was varied from 1430 to 1630 nm in 2 nm steps and the photoluminescence spectrum was measured at each excitation wavelength . Emission peaks occur at 660 nm, 810 nm, and a dominating one at 980 nm.
The integrated optical UC efficiency is calculated by integration over the luminescence wavelength in Fig. 2. Photons with a wavelength of are most efficiently upconverted. In the spectral region from 1492 to 1547 nm, photons are upconverted with quantum efficiencies exceeding 1.5%. The shape of the excitation spectrum is formed by the subenergy levels of the .
Spectral optical UC efficiency of the calculated from calibrated PL measurements in a logarithmic scale. While keeping the excitation wavelength constant at 1523 nm, the irradiance was varied from 17 to and the PL spectrum was measured at each irradiance. The emission peak at 980 nm is visible for all applied irradiances, while the other emissions are only detectable over certain excitation thresholds.
Due to the nonlinearity of the UC, the integrated optical UC efficiency increases with the irradiance . At high irradiance values the increase slowly saturates. Therefore, the characteristic exponent decreases from 1.86 to 1.35. An extrapolation of the fitted curve measured at Fraunhofer ISE agrees well with the measurement from the UOL.
Schematic graph of the experiment for EQE measurements of a solar cell with upconverter in the IR spectral region. On the planar grid free side of a bifacial silicon solar cell the solidified upconverter is attached with a refractive index matching liquid.
Spectra of the absorption coefficients of the binding agents silicone gel Sylgard 184 and zapon varnish. For comparison, the absorption coefficient of the powder is shown as well. The data was taken from Refs. 21 and 26). The absorption coefficient is higher for the zapon varnish over a broad spectral range but in the absorption range of the upconverter, the silicone shows the higher absorption. Furthermore, less material of the zapon varnish is used in the later devices, making the zapon varnish the more favorable binder.
EQE measurements of a silicon solar cell with two different upconverter samples optically coupled to its back: one sample with the upconverter immersed in silicone with a weight concentration of the upconverter in the silicone of 25%, and one sample with the upconverter glued together with zapon varnish with a weight concentration of the upconverter of 96%. The incident irradiance on the silicon solar cell is for both samples. The sample with zapon varnish shows a much higher . This is attributed to the higher upconverter concentration and also to unwanted absorption and scattering in the silicone sample. The strong oscillations in the are caused by the oscillating transmission of the silicon solar cell (see Fig. 10).
The of the solar cell upconverter device increases with higher irradiances. The , defined as the ratio of and the irradiance , decreases. The characteristic exponent was determined by a least-square fit with Eq. (3). The characteristic exponent of 1.89 compared to the optical measurement indicates that the irradiance impinging on the upconverter is in the range of the low irradiances of the optical measurement.
Transmission of used the silicon solar cell in the IR spectral region. Interference effects within the cell cause strong oscillations.
The integrated optical efficiency , scaled according to the dependence on the irradiance (Fig. 5) to the level of the effective irradiance on the upconverter of , in comparison to the and the . The is much lower than the because absorption in the UC layer is not complete. The takes this effect into account. Hence, and are in fairly good agreement. Remaining differences are attributed to uncertainties of the used absorption coefficient data, effects such as inhomogeneous illumination of the solar cell and uncertainties of the optical properties, such as transmission and reflection losses.
Summary of the UC efficiencies and their description. ( wavelength of the incident photons. wavelength of the upconverted photons. irradiance of the photons. reduced irradiance impinging on the upconverter due to the transmission of the solar cell.)
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