Hours Of Light Soaking Research Articles

Optical properties and stability of Coumarin 7 luminescent down shifting layers in different host polymers

In this work, pristine Coumarin 7 (C7) layers and mixtures of C7 with Poly(methyl methacrylate) (PMMA), Polycarbonate (PC) and Polyurethane (PU) were produced by drop casting and characterized by UV-vis absorption and Photoluminescence spectroscopy, aiming at investigating the effect of the different host polymers on the optical properties of the luminescent material. The optical properties of the luminescent down shifting LDS layers are adjusted by using the different polymers, different solvents and by changing the relative concentration of the host, resulting in Photoluminescence Quantum Yield (PLQY) values of 98, 92 and 96% for C7:PMMA, C7:PC and C7:PU layers, respectively. The overall enhancement in emission is about 5 times when compared to pristine C7 layers (PLQY = 19% in DMSO solvent). The effect of the different LDS layers in a simulated Perovskite Solar Cell (PSC) was assessed using Rothemundś model. The External Quantum Efficiency (EQE) curve of the simulated device was obtained using SCAPS-1D software and calculations with Rothemund's model show enhancements of about 11, 15 and 17% in the short-circuit current density when C7:PC, C7:PU and C7:PMMA LDS layers are used, respectively. The stability of the layers under different stressing conditions was examined with Differential Scanning Calorimetry DSC measurements and optical characterizations. There is no dramatic effect of the temperature on the optical properties of the layers, however, upon interaction with light, PLQY values are significantly reduced during the first hours of light soaking. C7:PMMA layers were the more stable combination after the light-soaking stability tests, indicating that PMMA is the most suitable option as host matrix for organic LDS dyes, with potential applications in photovoltaic devices. Our work highlights the importance of choosing the host polymer and evaluate its stability for the successful application of organic LDS layers in solar cells.

Reversible Efficiency Variation of Tandem Amorphous/Microcrystalline Si Photovoltaic Modules in Outdoor Operation

The Staebler-Wronski effect in amorphous silicon based photovoltaic devices is responsible for degradation of their power conversion efficiency, within approximately the first one thousand hours of light soaking. Several experimental studies led to highlight the performance instability phenomena for the mentioned devices, underling that recovery and improvement of such performance are observable, by subjecting such devices (both of single-junction and tandem types) to DC reverse bias stresses under illumination, or to operation in the Maximum Power Point (MPP) under variable conditions of temperature and illumination. In this work, we present and discuss the results of novel recent outdoor tests on stabilized specimens (i.e., exposed to 1000 h extended light soaking, before our tests) of tandem amorphous/microcrystalline Si (a-Si/µc-Si) photovoltaic (PV) minimodules operating in their MPP, by analyzing the causes of the performance instability effects, systematically observed on a daily scale. During the mentioned tests, we have monitored the solar cell operating temperature and the incident solar spectrum at various times in different days to verify the effect of cell temperature and solar spectrum changes on the cell performances. The experimental results show a clear correlation between performance improvements of the photovoltaic modules and their thermal history during the outdoor tests, proving the interplay between defect build-up at a lower temperature and defect annealing at a higher temperature, taking place in the solar cells operated in MPP during conventional outdoor operation.

Atmospherically Processed and Stable Cs-Pb Based Perovskite Solar Cells

ABSTRACTIn this work, a planar heterojunction superstrate n-i-p device based on Zn(O,S) electron transport layer and CsPbI2Br absorber material at 1.93 eV bandgap is presented. The CsPbI2Br films are deposited using a 2-step atmospheric solution deposition process and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis spectroscopy and photoluminescence (PL). Best device with an efficiency of 12.34 % and 11.94% in reverse and forward scans respectively and stabilized power output of 12.14 mW/cm2 has been demonstrated via atmospheric solution processing with minimal hysteresis between forward and reverse scans. The devices show voltage dependent current collection as well as light-dark crossover in forward bias. Light soaking tests at 65 °C and 1-sun at Voc, resulted in open-circuit voltage and fill-factor degradation. Electroluminescence (EL) after 100 hours of light soaking shows a reduction in overall EL intensity as well a shift in emission to lower wavelength. The devices exhibit a positive temperature coefficient of about 0.14 %/°C. It is found that Zn(O,S) is a viable alternative electron transport layer to replace TiO2. By replacing methylammonium cation with cesium and addition of Br has improved the stability of the perovskite phase.

Electro-optical characterization of ruthenium-based dye sensitized solar cells: A study of light soaking, ageing and temperature effects

In this work, we report on the electro-optical characterization of Dye Sensitized Solar Cells (DSSCs) which use ruthenium complexes as sensitizers. In particular, several kinds of measurements have been performed to study the electrical performances and the efficiency levels of the cells at different operating conditions. In detail, the measurements have been conducted at varying temperatures and hours of light soaking. Our results show that the short circuit current density JSC, the open circuit voltage VOC and the conversion efficiency η increase with the hours of light soaking, while they decrease with temperature. A thorough investigation on the light soaking process at the device level has been carried out, giving a wealth of experimental data which can be very useful to better single out and quantify the underlying physical phenomena and the effects on the cell parameters, which are still controversial among the scientific community. Finally, the ageing study demonstrates that the cell performance rapidly deteriorates in the first few months of exposure to light. All the obtained results are of great importance for a better understanding of the light soaking phenomenon and for improving the fabrication process of the DSSCs investigated during this study.

Thinner silicon‐based tandem solar cell with high efficiency made by hot wire CVD

AbstractWe developed thin film silicon tandem cells with hot‐wire deposited amorphous (protocrystalline) and nanocrystalline i‐layers, with a total thickness of only 1.43 µm. This is considerably thinner than the thickness of 2.5–3.0 µm normally needed, which is important to significantly reduce production costs for this type of cells. An initial efficiency of 8.84% was obtained. The top cell is ∼220 nm and the bottom cell only ∼1210 nm thick. No intermediate reflector layer is introduced. Compared to standard PECVD tandem cells, the Voc is remarkably high, consistent with the beneficial effect of thin i‐layers on Voc and with the high Voc of 0.57 V obtained for single junction HWCVD nc‐Si:H cells. After over 2000 hours of light soaking, a stabilized efficiency of 7.63% was obtained (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis of nanocrystalline silicon thin films using the increase of the deposition pressure in the hot-wire chemical vapour deposition technique

Nanostructured thin silicon-based films have been deposited using the hot-wire chemical vapour deposition (HWCVD) technique at the University of the Western Cape. A variety of techniques including optical and infrared spectroscopy, Raman scattering spectroscopy, X-rays diffraction (XRD) and transmission electron microscopy (TEM) have been used for characterisation of the films. The electrical measurements show that the films have good values of photoresponse, and the photocurrent remains stable after several hours of light soaking. This contribution will discuss the characteristics of the hydrogenated nanocrystalline silicon thin films deposited using increased process chamber pressure at a fixed hydrogen dilution ratio in monosilane gas.

Room Temperature Recovery of Light Induced Changes in Amorphous Silicon Solar Cells

AbstractWe have observed the recovery of the performance of amorphous silicon (a-Si:H) based solar cell (especially the fill factor) at temperatures between 25°C and 170°C after ∼600 hours of light soaking under 1 sun illumination at ∼40°C. We find that there is some recovery of the fill factor of the cells even at temperatures below the temperature of light soaking. The recovery is significantly greater in cells of poor quality. There is also some evidence of enhancement of the recovery rate due to an external electric field. Above the light soaking temperature, a sort of thermally activated recovery of the fill factor is observed.

Light Induced Effects in A-Si:H Films Alloyed with Sulfur

ABSTRACTLight-induced effects are studied in hydrogenated amorphous silicon-sulfur alloys (a-SiSx:H) and compared to those that exist in a-Si:H. The a-SiSx:H films were grown by decomposition of pre-mixtures of SiH4 and H2 S. The light-induced effects were monitored using electrical (dark conductivity and photoconductivity, including the constant photocurrent method [CPM]) and optical (photoluminescence) measurements and electron spin resonance. It is found that sulfur alloying results in a significant reduction in the degradation in the dark- and photo-conductivity. For ana-SiSx:H film grown with a gas mixture of H2 S/SiH4= 0.02, there is an increase of over an order of magnitude in the dark conductivity and a small decrease in the photoconductivity after 50 hours of light soaking. The subgap deep defect density as measured by CPM increases with illumination time, following a stretched exponential to saturation. The saturated defect density is an order of magnitude higher than that observed in the annealed state.