and has yielded 10.3% effi cient solar cells with a V oc defi cit of 0.60 V [ 10 ] or recently, even up to 11.8% measured on active area. [ 11 ] Some commonly reported problems of the DMSO-processed kesterite layers are their high porosity, nonuniformity, and numerous grain boundaries that can lead to undesirable recombination. [ 12 ] Here, we employ a three-stage annealing process under controlled selenium atmosphere in an SiO x coated graphite box to drastically improve the grain size and morphology of the absorber layer. Importantly, the V oc defi cit can be reduced to 0.57 V, which appears to be one of the lowest values reported for kesterite devices. Systematic electrical characterization of absorbers and fi nished solar cells with time-resolved photoluminescence (TRPL), temperature-dependent currentvoltage measurements ( JV-T ), and admittance spectroscopy (AS) are used to identify the reasons of the improved voltage. Figure 1 shows the scanning electron microscopy (SEM) cross sections of four different Cu 2 ZnSn(S,Se) 4 (CZTSSe) absorbers A-D yielding effi ciencies from 6.6% to 10.1% (total cell area of 0.3 cm 2 including metal grid lines). The annealing conditions are varied from uncoated graphite box (sample A,B) to SiO x -coated graphite box (sample C,D) and two-stage temperature gradient (sample A,C) to three-stage temperature gradient (sample B,D); temperature gradients are presented in Figure S1 (Supporting Information). The selenization of sample A was conducted in an uncoated graphite box employing a two-stage temperature gradient, and the absorber layer exhibits a distinct bilayer structure with a thick small-grain bottom layer. [ 13 ] Sample B was selenized in an uncoated graphite box similar to A but employing a three-stage temperature gradient. The SEM cross section shows an improved crystallization and grain size in both upper crust and bottom layer. However, the distinct bilayer structure of the absorber layer remains. The selenization of sample C was conducted in an SiO x -coated graphite box using the two-stage process, and the morphology of the fi lm exhibits a comparably thin upper layer with small grains, but an improved crystallization in the bottom layer in contrast to sample A. Finally, sample D was selenized in the SiO x -coated graphite box with the three-stage temperature gradient and shows an overall improved crystallization with large grains and a signifi cant reduction of the small-grain bottom layer. X-ray diffraction (XRD) pattern in Figure 2 b shows a double kesterite refl ex at 53.4° for all samples, indicating two regions with different S/(S + Se) ratio in the absorber layer. Grazing incidence XRD with varying incident angles confi rms that the region with lower S/(S + Se) ratio coincides with the upper crust and the region with higher S/(S + Se) ratio belongs to the small-grain bottom layer. The refl exes corresponding to the higher S/(S + Se) ratio extenuate with the shift from uncoated On the way towards a marketable and industrially-relevant photo voltaic technology, ke...

Efficient light detection in the near-infrared (NIR) wavelength region is central to emerging applications such as medical imaging and machine vision. An organic upconverter (OUC) consists of a NIR-sensitive organic photodetector (OPD) and an visible organic light-emitting diode (OLED), connected in series. The device converts NIR light directly to visible light, allowing imaging of a NIR scene in the visible. Here, we present an OUC composed of a NIR-selective squaraine dye-based OPD and a fluorescent OLED. The OPD has a peak sensitivity at 980 nm and an internal photon-to-current conversion efficiency of ∼100%. The OUC conversion efficiency (0.27%) of NIR to visible light is close to the expected maximum. The materials of the OUC multilayer stack absorb very little light in the visible wavelength range. In combination with an optimized semitransparent metal top electrode, this enabled the fabrication of transparent OUCs with an average visible transmittance of 65% and a peak transmittance of 80% at 620 nm. Visibly transparent OUCs are interesting for window-integrated electronic circuits or imaging systems that allow for the simultaneous detection of directly transmitted visible and NIR upconverted light.

In light-emitting electrochemical cells (LECs), the position of the emission zone (EZ) is not predefined via a multilayer architecture design, but governed by a complex motion of electrical and ionic charges. As a result of the evolution of doped charge transport layers that enclose a dynamic intrinsic region until steady state is reached, the EZ is often dynamic during turn-on. For thick sandwich polymer LECs, a continuous change of the emission colour provides a direct visual indication of a moving EZ. Results from an optical and electrical analysis indicate that the intrinsic zone is narrow at early times, but starts to widen during operation, notably well before the electrical device optimum is reached. Results from numerical simulations demonstrate that the only precondition for this event to occur is that the mobilities of anions (μa) and cations (μc) are not equal, and the direction of the EZ shift dictates μc > μa. Quantitative ion profiles reveal that the displacement of ions stops when the intrinsic zone stabilizes, confirming the relation between ion movement and EZ shift. Finally, simulations indicate that the experimental current peak for constant-voltage operation is intrinsic and the subsequent decay does not result from degradation, as commonly stated.