The thickness of the i-layer was chosen such that an interference maximum

occurs at 950 nm, increasing the BMS-907351 cell line transmission at this wavelength. As a result, more light can be absorbed by the upconverter layer in the case of the flat solar cell configuration. Concentration levels of up to 25 times were reached using near-infrared light from a solar simulator. The absorption and emission spectra of the upconverter are shown in Figure 4. The absorption is highest around 950 nm. The upconverter was excited with filtered light of a xenon lamp at 950 ± 10 and 980 ± 10 nm. The 4F7/2 state at 2.52 eV is reached after two energy transfer events from Yb to Er. The upconverter was already shown to be very efficient at low light intensities. Saturation was measured under light intensities of less than 1 W/cm2. Although the

absorption at 950 nm (1.31 eV) is higher, excitation at 980 nm (1.26 eV) leads to two times higher upconverted emission intensity. This may be attributed to the perfectly resonant energy transfer step of 980 nm (1.26 eV) since the 4F7/2 state is at 2.52 eV. Figure 4 Upconverted emission and absorption spectra of the upconverter in PMMA layer. The emission spectrum is obtained when Proteases inhibitor the upconverter shows no saturation and only emission peaks from the 4S3/2, 2H11/2 (510 to 560 nm), and 4F9/2 (650 to 680 nm) states are SB431542 concentration observed. For further experiments, the upconverter was excited at 980 nm with a pulsed Opotek Opolette laser. Because upconversion is a two-photon process,

the efficiency should be quadratically dependent on the excitation power density. Cediranib (AZD2171) The intensity of the laser light was varied with neutral density filters. Upconversion spectra were recorded in the range of 400 to 850 nm under identical conditions with varying excitation power. Varying the intensity shows that for low light intensities, the red part is less than 6% of the total emission (see Figures 4 and 5). Only when the emission from the green-emitting states becomes saturated does the red emission become more significant and even blue emission from the 2H9/2 state is measured (see Figure 5). By comparing the emission intensities, it becomes clear that the emission intensity is not increasing quadratically with excitation power density. Instead, emissions from higher and lower energy states are visible. The inset in Figure 5 shows the integrated emission peaks for the green and total emissions, showing that at very high laser intensities, the total emission is saturated. Figure 5 Upconverted emission spectra under low and high excitation density. For the low excitation power, the green state was not yet saturated. The intensities may be compared. New peaks (italic) are assigned: 2H9/2 → 4I15/2 transition at 410 nm, 4I9/2 → 4I15/2 transition at 815 nm, and the intermediate transition 2H9/2 → 4I13/2 at 560 nm.