Estimation of angle-dependent mode coupling and attenuation in step-index plastic optical fibers from impulse responses

We report on a method for estimation of angle-dependent mode coupling and attenuation in step-index plastic optical fibers (SI-POFs) from the shapes of impulse responses at two different fiber lengths. While alternating the fiber lengths, deviations between simulated and reference impulse responses are minimized by optimizing both mode coupling and attenuation parameters using pattern-search routines. Applying a matrix-based finite-difference approach to Gloge's time-dependent power flow equation fast computation of simulated impulse responses is enabled. We demonstrate that mode-dependent coupling and attenuation parameters converge to values that reconstruct fiber characteristics reported by other authors. We show that our results can be used for prediction of impulse responses, yielding determination of frequency responses, fiber bandwidths and coupling lengths. We conclude that our method enables characterization of SI-POFs from fiber impulse response measurements

Novel low-temperature process for perovskite solar cells with a mesoporous TiO2 scaffold

The most efficient organic–inorganic perovskite solar cells (PSCs) contain the conventional n-i-p mesoscopic device architecture using a semiconducting TiO2 scaffold combined with a compact TiO2 blocking layer for selective electron transport. These devices achieve high power conversion efficiencies (15–22%) but mainly require high-temperature sintering (>450 °C), which is not possible for temperature-sensitive substrates. Thus far, comparably little effort has been spent on alternative low-temperature (<150 °C) routes to realize high-efficiency TiO2-based PSCs; instead, other device architectures have been promoted for low-temperature processing. In this paper the compatibility of the conventional mesoscopic TiO2 device architecture with low-temperature processing is presented for the first time with the combination of electron beam evaporation for the compact TiO2 and UV treatment for the mesoporous TiO2 layer. Vacuum evaporation is introduced as an excellent deposition technique of uniform compact TiO2 layers, adapting smoothly to the rough fluorine-doped tin oxide substrate surface. Effective removal of organic binders by UV light is shown for the mesoporous scaffold. Entirely low-temperature-processed PSCs with TiO2 scaffold reach encouraging stabilized efficiencies of up to 18.2%. This process fulfills all requirements for monolithic tandem devices with high-efficiency silicon heterojunction solar cells as the bottom cell

Nondestructive probing of perovskite silicon tandem solar cells using multiwavelength photoluminescence mapping

In this paper, spatially resolved photoluminescence (PL) spectroscopy with various excitation wavelengths is presented as a nondestructive and versatile technique providing access to the individual subcells of multijunction solar cells. This method is demonstrated on a state-of-the-art monolithic tandem solar cell composed of a planar perovskite solar cell and a silicon heterojunction solar cell. It is shown that the lateral distribution of inhomogeneities can be attributed unambiguously to the individual cells and be related to the manufacturing process. The approach of depth-selective probing of the silicon bottom cell is verified by comparison to reflection maps and by comparison to measurements of the silicon cell after removing the perovskite top cell. Analyzing subcells integrated into a monolithic tandem solar cell is challenging though crucial in order to identify performance limiting loss mechanisms. This method can be used to improve the study of the mutual influence of adjacent subcells in the fully fabricated device, which has been an unfeasible task up to now

Transient I-V Measurement Set-up for Photovoltaic Laser Power Converters under Monochromatic Irradiance

A new I-V measurement set-up called LaserSim has been developed for photovoltaic laser power converters \u96 photovoltaic cells optimized for monochromatic artificial light - providing irradiances of up to 89 W/cm². The evaluation of the set-up and first I-V curve and efficiency measurements are presented. Similar to flash simulators used for I-V characterization of solar cells, the LaserSim is using transient measurement routines to avoid an influence of heating of the samples during measurement. An excellent light uniformity in the designated test area was realized. A beam sampler allows for in-situ irradiance determination while recording I-V data. This is mandatory for characterization of devices showing a non-linear behavior of short circuit current with irradiance. Efficiency values under monochromatic light at 809 nm have been measured for a single junction GaAs cell (55.6 %) and a dual-junction GaAs device (53.5 %)

Towards Perovskite Silicon Tandem Solar Cells with Optimized Optical Properties

Perovskite silicon tandem solar cells can overcome the efficiency limit of single junction solar cells. For an optimized optical performance, we propose a 2-terminal device featuring a front-side anti-reflection structure, optimized layer thicknesses, a meso-porous scaffold for the perovskite solar cell and a rear-side light-trapping structure for the silicon solar cell. To maintain the functionality of the underlying layers, we have developed a low-temperature process to realize a meso-porous TiO 2 scaffold via ultra-violet (UV) curing. With perovskite solar cell efficiencies >15%, we achieve results comparable to our conventional high-temperature (> 500°C) route. For the optical optimization of the complex tandem device with elements of different feature sizes, we apply a matrix-based formalism and show how layer thickness optimization and the rear-side light trapping can significantly improve the current of the tandem device

Low Temperature perovskite solar cells with an evaporated TiO2 compact layer for perovskite silicon tandem solar cells

Silicon-based tandem solar cells can overcome the efficiency limit of single junction silicon solar cells. Perovskite solar cells are particularly promising as a top cell in monolithic tandem devices due to their rapid development towards high efficiencies, a tunable band gap with a sharp optical absorption edge and a simple production process. In monolithic tandem devices, the perovskite solar cell is deposited directly on the silicon cell, requiring low-temperature processes ( 500°C) processes. With both manufacturing routes, we obtain short-circuit current densities (JSC) of about 20 mA/cm², open-circuit voltages (VOC) over 1 V, fill factors (FF) between 0.7 and 0.8 and efficiencies (η) of more than 15%. We further show that the evaporated TiO2 layer is suitable for the application in tandem devices. The series resistance of the layer itself and the contact resistance to an indium doped tin oxide (ITO) interconnection layer between the two sub-cells are low. In addition, the low parasitic absorption for wavelengths above the perovskite band gap allow a higher absorption in the silicon bottom solar cell, which is essential to achieve high tandem efficiencies