Influence of ion bombardment on microcrystalline silicon material quality and solar cell performances

Microcrystalline hydrogenated silicon growth with VHF-PECVD was examined in an industrial type parallel plate KAITM reactor. The influence of pressure on material quality was studied in single junction solar cells. Solar cells with their intrinsic layer prepared at higher pressures exhibit remarkable improvements, reaching 8.2% efficiency at 3.5 mbar. Further analyzes showed that μc- Si:H intrinsic layers grown at higher pressures have a significantly lower defect density. These results are attributed to lower ion bombardment energies due to higher working pressures, which improve the microcrystalline material quality. Layer amorphization with increasing power density is observed at low pressure. Calculations show that the average ion energy drops from roughly 20 eV to a few eV in the pressure range studied

Study of the microstructure transition width from amorphous to microcrystalline silicon as a function of the input silane concentration

Amorphous and microcrystalline silicon have been proven to be very interesting for low cost thin film photovoltaic devices. Usually these two materials are deposited using the same large area plasma-enhanced chemical vapor deposition reactors from silane and hydrogen gases. The transition from amorphous deposition regime to microcrystalline deposition regime is generally done by reducing the silane concentration in the input gas flow and the optimum deposition parameters to achieve high performance device stands just at the transition between the two microstructures. In the present work, a study of the transition width from amorphous to microcrystalline silicon is presented as a function of the input silane concentration. It is shown that the higher the input silane concentration, the wider is the microstructure transition. As a consequence, the process is less sensitive to fluctuations of the silane concentration when silane concentrations higher than 10 % are used and better uniformity and reproducibility can be then achieved

Input silane concentration effect on the a-Si:H to uc-Si:H transition width

In this work the microstructure transition width from amorphous to microcrystalline silicon is discussed. It is shown that the width of the transition depends on the input silane concentration level and indirectly on the silane depletion level. The higher the input silane concentration and depletion, the wider the transition. Experimental results are then compared to an analytical model and good agreement is obtained with a semi-empirical approach that takes into account the e®ect of the silane density in the plasma on the electron density

Thin-Film Barriers for Durable Thin-Film PV Modules

This contribution describes thin-film barrier coatings to enhance the stability of silicon thin-film solar modules. Thin-film barriers are applied to protect the modules from degradation in humid environment. Demonstrator modules were exposed to indoor and outdoor tests to evaluate failure modes by electrochemical corrosion during operation. Test results reveal some major failure modes, but we demonstrate long lifetimes under operational conditions for certain modules without conventional encapsulation. These modules without conventional encapsulation showed less than 1% relative degradation during 1000 hours of damp-heat tests with light exposure at open circuit voltage. These promising results will guide the way to new encapsulation concepts for longer lifetimes and/or new low-cost concepts. This approach might be applied to any thin-film technology and will be an important aspect for consumer electronics with less strict requirements for very long-term operation

Diagnostics of thin-film silicon solar cells and solar panels/modules with variable intensity measurements (VIM)

A simple and low-cost method for analyzing amorphous silicon solar cells and modules, which have low values of the fill factor (FF), is proposed. Low fill factors can occur mainly because of 3 reasons: (a) excessive recombination due to "bad" intrinsic layers; (b) shunts and (c) very high series resistance. The method described here allows one to discriminate between (a), (b) and (c). It consists of measuring the J-V curves at different light intensities, varying typically from 0.05 to 1 sun. It has been called the "variable intensity method (VIM)". Here, one plots R-sc=partial derivative V/partial derivative J (at V=0) and R-oc=partial derivative V/partial derivative J (at J=0) as a function of J(sc). From the slope of the R-sc-J(sc) curve, one derives the "collection voltage V-coll"; from the asymptotic value of R-sc for low values of J(sc) ( < 0.1 mA/cm(2)) one obtains the "true" shunt resistance R-shunt; from the asymptotic value of R-oc for high values disc J(sc) (around 10 mA/cm(2)) one obtains the "true" series resistance R-series. This paper shows quantitatively how too low values of V-coll and of R-shunt as well as how too high a value of R-series lead to a low value of FF for both cells and panels/modules. (c) 2010 Elsevier B.V. All rights reserved

Optimized short-circuit current mismatch in multi-junction solar cells

Multi-junction photovoltaic devices include two or more component sub-cells which are electrically interconnected in series. At any power point, the current output of the total device is limited by the sub-cell with the smallest current density. Therefore, the maximum efficiency is reached when the sub-cells have equal current densities at their respective maximum power points. In this case the sub-cells are so called "power matched". We report an experimental procedure in which the current voltage characteristics of tandem solar cells can be measured under various irradiance spectra, i.e. under various shortcircuit current matching conditions. This permits the probing of the optimized short circuit current mismatch, where the sub-cells are power matched, which is essential to define design rules for the tandem stack. The method applies well to devices where one of the sub-cells is metastable. We show that, in the case of thin-film silicon tandem cells, the optimum mismatch changes significantly after light induced degradation. Consequently, the degradation factor of such devices is shown to depend not only on material quality but also on the initial short circuit current matching. This experiment also provides relative quantification of the fill factors of each sub-cell. Our example suggests that a high bottom cell deposition rate can be detrimental to the fill factor of the top cell in the case of thin-film silicon tandem cells deposited in superstrate configuration. (C) 2013 Elsevier B.V. All rights reserved

Degradation in PV encapsulant strength of attachment: An interlaboratory study towards a climate-specific test

Reduced strength of attachment of the encapsulant resulting from the outdoor environment, including ultraviolet (UV) radiation, may decrease photovoltaic (PV) module lifetime by enabling widespread corrosion of internal components. To date, few studies exist showing how the adhesion of PV components varies with environmental stress. We have conducted an interlaboratory experiment to provide an understanding that will be used to develop climatic specific module tests. Factors examined in the study included the UV light source (lamp type), temperature, and humidity to be proposed for use in accelerated aging tests. A poly (ethylene-co-vinyl acetate) (EVA) formulation often used in veteran PV installations was studied using a compressive shear test - to quantify the strength of attachment at the EVA/glass interface. Replicate laminated glass/polymer/glass coupon specimens were weathered at 12 institutions using a variety of indoor chambers or field aging. Shear strength, s hear strain, and toughness were measured using a mechanical load-frame for the compressive shear test, with subsequent optical imaging and electron microscopy of the separated surfaces

Degradation in PV encapsulation transmittance: An interlaboratory study towards a climate-specific test

Reduced optical transmittance of encapsulants resulting from ultraviolet (UV) degradation has frequently been identified as a cause of decreased PV module performance through the life of service in the field. The present module safety and qualification standards, however, apply short UV doses only capable of examining design robustness or "infant mortality" failures. Essential information that might be used to screen encapsulation through product lifetime remains unknown. For example, the relative efficacy of xenon-arc and UVA-340 fluorescent sources or the typical range of activation energy for degradation is not quantified. We have conducted an interlaboratory experiment to provide the understanding that will be used towards developing a climate-and configuration-specific (UV) weathering test. Five representative, known formulations of EVA were studied in addition to one TPU material. Replicate laminated silica/polymer/silica specimens are being examined at 14 institutions using a variety of indoor chambers (including Xenon, UVA-340, and metal-halide light sources) or field aging. The solar-weighted transmittance, yellowness index, and the UV cut-off wavelength, determined from the measured hemispherical transmittance, are examined to provide understanding and guidance for the UV light source (lamp type) and temperature used in accelerated UV aging tests