Staebler-Wronski Effect Research Articles

Structural and electrical properties of metastable defects in hydrogenated amorphous silicon

The structural and electrical properties of metastable defects in various types of hydrogenated amorphous silicon have been studied using a powerful combination of continuous wave electron-paramagnetic resonance spectroscopy, electron spin echo (ESE) decay measurements, and Doppler broadening positron annihilation spectroscopy. The observed dependence of the paramagnetic defect density on the Doppler S parameter indicates that porous, nanosized void-rich materials exhibit higher spin densities, while dense, divacancy-dominated materials show smaller spin densities. However, after light soaking more similar spin densities are observed, indicating a long-term defect creation process in the Staebler-Wronski effect that does not depend on the a-Si:H nanostructure. From ESE decays it appears that there are fast and slowly relaxing defect types, which are linked to various defect configurations in small and large open volume deficiencies. A nanoscopic model for the creation of light-induced defects in the a-Si:H nanostructure is proposed.

DEGRADATION OF THREE–JUNCTION AMORPHOUS SI : H BASED SOLAR CELLS

The operating experience of hydrogenated amorphous silicon (a−Si : H) based solar cells has shown that besides their low efficiency this type of photovoltaics degrade much faster compared to single crystal based solar cells. As far as the processes deter- mining the degradation of amorphous materials based solar cells are not well studied, and the degradation of similar cells without light exposure has also been reported, we conducted an experiment to compare the temporal change characteristics of main solar cell parameters in darkness and under natural light. The demonstration of short circuit current reduction in darkness aged solar cells should be considered as one of the most interesting results of the work. Moreover we have shown that the change of this parameter is on average the same for the illuminated cells, while for some cells short circuit current reduction is substantially higher. This is indicative of the fact that the observed effect is not related to the Staebler—Wronski effect.

Power change in amorphous silicon technology by low temperature annealing

Amorphous silicon (a-Si) is one of the best established thin-film solar-cell technologies. Despite its long history of research, it still has many critical issues because of its defect rich material and its susceptibility to degrade under light also called as Staebler-Wronski effect (SWE). This leads to an increase in the defect density of a-Si, but as a metastable effect it can be completely healed at temperatures above 170 °C. Our study is focused on investigating the behavior of annealing of different a-Si modules under low temperature conditions below 80 °C indicated by successive change of module power. These conditions reflect the environmental temperature impact of the modules in the field, or integrated in buildings as well. The power changes were followed by STC power rating and investigation of module-power evolution under low irradiance conditions at 50 W/m2. Our samples were recovered close to their initial state of power, reaching as high as 99% from its degraded value. This shows the influence of low temperature annealing and light on metastable module behavior in a-Si thin-film modules.

Outdoor degradation of thin film amorphous silicon based PV modules

One of the main problems in thin film silicon based modules is the deterioration of their performance upon exposure to light. The presented work focuses on a methodology for evaluation of thin-film photovoltaic module degradation behavior under real operating conditions. The outdoor degradation of double junction a-Si:H/a-Si:H modules was investigated using automated measurement setup for a period of two years. A deterioration of the module's maximum power was observed due to the well known Staebler-Wronski effect, which main causes are the decrease of open circuit voltage and the fill factor of the module. The obtained results can be correlated to the technology and construction of the thin film silicon based modules.

The Nature and the Kinetics of Light-Induced Defect Creation in Hydrogenated Amorphous Silicon Films and Solar Cells

The nature and the kinetics of light-induced defect creation in hydrogenated amorphous silicon (a-Si:H) films and solar cells are investigated by means of Doppler broadening positron annihilation spectroscopy, Fourier transform photocurrent spectroscopy, and J-V characterization. There is a strong correlation between the open volume deficiencies in a-Si:H and the Staebler-Wronski effect (SWE). The carrier generation and recombination profiles in the absorber layer are spatially correlated, and the recombination due to defects in the top and bottom parts of the absorber layer is different. Furthermore, the various defect distributions in the bandgap have different defect creation kinetics. It is demonstrated that the SWE defect formation kinetics in a solar cell are very complex and can impossibly be described by one time scaling ~ t <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">β</sup> as is often claimed.

Multi-resonant silver nano-disk patterned thin film hydrogenated amorphous silicon solar cells for Staebler-Wronski effect compensation

We study polarization independent improved light trapping in commercial thin film hydrogenated amorphous silicon (a-Si:H) solar photovoltaic cells using a three-dimensional silver array of multi-resonant nano-disk structures embedded in a silicon nitride anti-reflection coating to enhance optical absorption in the intrinsic layer (i-a-Si:H) for the visible spectrum for any polarization angle. Predicted total optical enhancement (OE) in absorption in the i-a-Si:H for AM-1.5 solar spectrum is 18.51% as compared to the reference, and producing a 19.65% improvement in short-circuit current density (JSC) over 11.7 mA/cm2 for a reference cell. The JSC in the nano-disk patterned solar cell (NDPSC) was found to be higher than the commercial reference structure for any incident angle. The NDPSC has a multi-resonant optical response for the visible spectrum and the associated mechanism for OE in i-a-Si:H layer is excitation of Fabry-Perot resonance facilitated by surface plasmon resonances. The detrimental Staebler-Wronski effect in a-Si:H solar cell can be minimized by the additional OE in the NDPSC and self-annealing of defect states by additional heat generation, thus likely improving the overall stabilized characteristics of a-Si:H solar cells.

Preillumination – Induced change of electronic transport properties of TlGaSe2 semiconductor

The effect of the preillumination on the dark and the photo-conductivity of the TlGaSe2 layered semiconductor is investigated within the temperature range of 80–300 K. After the illumination predominantly at high temperatures, a substantial decrease in both dark and photo-conductivities was observed. Observed phenomena resemble the Staebler–Wronski effect, which is typical for the amorphous semiconductors. The main contribution of this work is to show that the TlGaSe2 single crystals with well apparent crystalline structure can demonstrate certain characteristics peculiar to amorphous semiconductors.

In situ manipulation of the sub gap states in hydrogenated amorphous silicon monitored by advanced application of Fourier transform photocurrent spectroscopy

A profound comprehension of the nanostructure of hydrogenated amorphous silicon (a-Si:H) and the defect states in this material is currently still lacking despite several decades of research on this topic. Investigating the nature of defects in a-Si:H devices is especially important in view of the poorly understood Staebler–Wronski effect (SWE). Therefore we present the latest insights into the gap states in the a-Si:H bandgap by manipulating these sub gap states in three different ways: via the application of a voltage bias, via in situ light soaking, and via in situ annealing. To monitor the changes in the sub gap absorption we employ an advanced application of Fourier Transform Photocurrent Spectroscopy (FTPS). A recently published optical model that removes interference fringes from the photocurrent spectrum is presented to obtain the absorption coefficient. For FTPS measurements on solar cells the photocurrent spectrum is scaled to the external quantum efficiency (EQE) of the solar cell. Further, a mathematical data fitting routine is employed to accurately quantify the sub gap absorption and the changes therein during light soaking and annealing. The high sensitivity of FTPS is particularly helpful in identifying the sub gap states that play a role in the SWE in a-Si:H devices.

The effects of dispatch strategy on electrical performance of amorphous silicon-based solar photovoltaic-thermal systems

Previous work has shown that high-temperature short-term spike thermal annealing of hydrogenated amorphous silicon (a-Si:H) photovoltaic thermal (PVT) systems results in higher electrical energy output. The relationship between temperature and performance of a-Si:H PVT is not simple as high temperatures during thermal annealing improves the immediate electrical performance following an anneal, but during the anneal it creates a marked drop in electrical performance. In addition, the power generation of a-Si:H PVT depends on both the environmental conditions and the Staebler–Wronski Effect kinetics. In order to improve the performance of a-Si:H PVT systems further, this paper reports on the effect of various dispatch strategies on system electrical performance. Utilizing experimental results from thermal annealing, an annealing model simulation for a-Si:H-based PVT was developed and applied to different cities in the U.S. to investigate potential geographic effects on the dispatch optimization of the overall electrical PVT systems performance and annual electrical yield. The results showed that spike thermal annealing once per day maximized the improved electrical energy generation.

Origins of hole traps in hydrogenated nanocrystalline and amorphous silicon revealed through machine learning

Genetic programming is used to identify the structural features most strongly associated with hole traps in hydrogenated nanocrystalline silicon with very low crystalline volume fraction. The genetic programming algorithm reveals that hole traps are most strongly associated with local structures within the amorphous region in which a single hydrogen atom is bound to two silicon atoms (bridge bonds), near fivefold coordinated silicon (floating bonds), or where there is a particularly dense cluster of many silicon atoms. Based on these results, we propose a mechanism by which deep hole traps associated with bridge bonds may contribute to the Staebler-Wronski effect.

Metastable Defect Formation at Microvoids Identified as a Source of Light-Induced Degradation ina−Si:H

Light-induced degradation of hydrogenated amorphous silicon (a-Si:H), known as the Staebler-Wronski effect, has been studied by time-domain pulsed electron-paramagnetic resonance. Electron-spin echo relaxation measurements in the annealed and light-soaked state revealed two types of defects (termed type I and II), which can be discerned by their electron-spin echo relaxation. Type I exhibits a monoexponential decay related to indirect flip-flop processes between dipolar coupled electron spins in defect clusters, while the phase relaxation of type II is dominated by 1H nuclear spin dynamics and is indicative for isolated spins. We propose that defects are either located at internal surfaces of microvoids (type I) or are isolated and uniformly distributed in the bulk (type II). The concentration of both defect type I and II is significantly higher in the light-soaked state compared to the annealed state. Our results indicate that in addition to isolated defects, defects on internal surfaces of microvoids play a role in light-induced degradation of device-quality a-Si:H.

Structural Order and Staebler–Wronski Effect in Hydrogenated Amorphous Silicon Films and Solar Cells

The structure of hydrogenated amorphous silicon films is investigated by Raman spectroscopy and X-ray diffraction. Raman spectroscopy probes the phonon density of states, whereas X-ray diffraction measures the distribution of the electron density. Yet, both methods can yield information on the microstructure of the material represented by certain parameters like, e.g., the position or the width of the transverse optical phonon or the width of the first scattering peak. Interdependences between these parameters are investigated and evaluated. A correlation was found between the structural disorder and the relative efficiency loss caused by the Staebler-Wronski effect for intrinsic films applied as absorbing layers in solar cells. This correlation could be used to estimate the solar cell degradation without time-consuming light-soaking experiments.

Role of heterogeneities in Staebler–Wronski effect

The effect of light soaking (LS) on the properties of hydrogenated amorphous silicon (a-Si:H) presents many challenging puzzles. We look at some of them, along with their present understanding. In particular, the role of the heterogeneities in LS is examined. We find that for most of the solved as well as unsolved puzzles, the long-range potential fluctuations arising from the heterogeneities in the films, afford an alternative view which looks quite plausible. The implications of such considerations are used to speculate on the possibility of making stable a-Si:H solar cells.

High Efficiency Thin Film Silicon Solar Cells

High-efficiency solar cells based on amorphous silicon technology are designed. Multi-junction amorphous silicon solar cells are discussed, how these are made and how their performance can be understood and optimized. Although significant amount of work has been carried out in the last twenty-five years, the Staebler-Wronski effect has limited the development of a-Si:H solar cells. As an alternative material, nc-Si:H has attracted remarkable attention. Taking advantage of a lower degradation in nc-Si:H than a-Si:H and a-SiGe:H alloys, the light induced degradation in triple junction structures has been minimized by designing a bottom-cell-limited current mismatching, and obtained a stable active-area cell efficiency. All this has been investigated in this paper.

Dangling-bond defect in a-Si:H: Characterization of network and strain effects by first-principles calculation of the EPR parameters

The performance of hydrogenated amorphous silicon (a-Si:H) solar cells is severely affected by the light-induced formation of metastable defects in the material (Staebler-Wronski effect). The common notion is that the dangling-bond (db) defect, a threefold coordinated silicon atom, plays a key role in the underlying mechanisms. To support the characterization of this defect by electron paramagnetic resonance (EPR), we present in this work a first-principles study of the EPR parameters for a structural ensemble of the db defect. We show that the a-Si:H dangling bond is a network defect for which charge and spin localization substantially depend on the actual coordination of the db atom and the local geometric and electronic structure of the immediate surrounding. It consequently differs by its very nature from its crystalline counterpart, which is typically related to the presence of a vacancy. The application of hydrostatic strain to our models yields further insights into the dependence of the hyperfine interaction on the structural characteristics of the defect. The observed trends are shown to result from the interplay between delocalization and sp hybridization.

Spectroscopic Detection of Medium Range Order in Hydrogenated Amorphous Silicon, a-Si(H): Applications in Photovolatics, Thin Film Transistors and Si-based Microelectronics

Abstract Hydrogenated amorphous silicon, a-Si1-xHx, with ~ at.10% or x ~0.1±0.02, is used in photovoltaics (PV), and thin film transistors (TFT's). Amorphous Si (a-Si) thin films, thought to be free of H, are used as precursors for polycrystalline gate electrodes in microelectronics. PV and TFT alloys has been deposited by the glow discharge method (GD), remote plasma-enhanced chemical vapor deposition (RPECVD, and reactive magnetron sputtering (RMS) with bonded-H determined by deposition precursors and substrate temperatures. Two conditions are required for low Si dangling bond densities ~0.5 to 1x1016 cm−3: (i) a bonded monohydride, Si-H, concentration of ~10 at. % H, and (ii) a deposition, and/or a post-deposition anneal at ~240 °C to 300 °C. These reduce strain by introducing medium range order (MRO) as nano-meter scale aperiodically-assembled clusters. Si L2,3 X-ray absorption spectroscopy (XAS) has been used to confirm MRO extending beyond short-range order (SRO) of continuous random networks (CRN) through observation of ligand-field split d-state features. These are associated with symmetry-adapted linear combinations (SALCs) of atomic states forming molecular orbital valence bands. XAS and photoelectron spectroscopy (PES) studies have confirmed that a-Si used for microelectronic applications also has MRO as well. This is associated with a H-transfer reaction from an Si-H bond of the cluster into a Si-H-Si bonding arrangement on near-neighbor sites. MRO has also identified by other spectroscopic techniques: spectroscopic ellipsomentry (SE), and ultra-violet or X-ray photoemission spectroscopy (UPS or XPS). The same H-atom motion induced by absorption of sun-light is responsible for the Staebler-Wronski Effect (SWE); the generation of Si-atom dangling bonds that degrade PV performance.

Seasonal power fluctuations of amorphous silicon thin-film solar modules: distinguishing between different contributions

Several works report on seasonal fluctuations of power production of amorphous silicon (a-Si). These oscillations are due to two overlapping phenomena (i) spectral and (ii) the Staebler–Wronski effects. It is hence difficult to assess—for a given location and climatic conditions—which one has the largest impact. By means of a straightforward approach based on two sets of single-junction a-Si photovoltaic modules (stored indoors/exposed outdoors) and on two different I–V measurement set-ups (indoor and outdoor), we were able to separate the different contributions to this phenomenon. For the test-site of Lugano, seasonal oscillations account for performance variations of a-Si of ~10% (±5% around an annual average value with a minimum around the mid of January and a maximum around mid-July). The time-phase of the overall effect lies in between that of the two distinguished phenomena. (i) Spectral variations seem to have the highest impact on the outdoor performance of a-Si with an amplitude corresponding to 10.5% (± ~5.2%). Moreover, the influence of spectral variations on the outdoor performance of a-Si (and for comparison of c-Si) was modeled, and the experimental data were found to be in excellent agreement with the theoretical simulation; (ii) the Staebler–Wronski effect has a slightly lower influence with an amplitude of ~8% (±4% with a minimum at the middle of February and a maximum around mid-August). Because of the position (46°N) and average climatic conditions (southern Alpine climate) of Lugano, these observations are possibly representative of a large part of continental Europe. Copyright © 2012 John Wiley & Sons, Ltd.

Irreversible light-induced degradation and stabilization of hydrogenated polymorphous silicon solar cells

In this work, hydrogenated amorphous and polymorphous silicon (a-Si:H and pm-Si:H) solar cells were tested for the reversibility of their light-induced degradation by subjecting them to light-soaking and thermal annealing cycles. The tests were done under one sun at 40°C. The pm-Si:H solar cells show an irreversible degradation at the initial stage of light-soaking (five hours), while having higher stabilized efficiency with respect to a-Si:H cells. To the best of our knowledge, such irreversible degradation of pm-Si:H solar cells has never been reported and cannot be explained by the conventional paradigm based on the Staebler–Wronski Effect. The results lead us to view the kinetics from a different perspective in which stimulated hydrogen motion—such as that which occurs in a-Si:H under extreme conditions—takes place in pm-Si:H under conditions close to typical device operation, and introduces structural changes that affect the solar cell parameters in an irreversible way.

The effect of hybrid photovoltaic thermal device operating conditions on intrinsic layer thickness optimization of hydrogenated amorphous silicon solar cells

Historically, the design of hybrid solar photovoltaic thermal (PVT) systems has focused on cooling crystalline silicon (c-Si)-based photovoltaic (PV) devices to avoid temperature-related losses. This approach neglects the associated performance losses in the thermal system and leads to a decrease in the overall exergy of the system. Consequently, this paper explores the use of hydrogenated amorphous silicon (a-Si:H) as an absorber material for PVT in an effort to maintain higher and more favorable operating temperatures for the thermal system. Amorphous silicon not only has a smaller temperature coefficient than c-Si, but also can display improved PV performance over extended periods of higher temperatures by annealing out defect states from the Staebler–Wronski effect. In order to determine the potential improvements in a-Si:H PV performance associated with increased thicknesses of the i-layers made possible by higher operating temperatures, a-Si:H PV cells were tested under 1 sun illumination (AM1.5) at temperatures of 25°C (STC), 50°C (representative PV operating conditions), and 90°C (representative PVT operating conditions). PV cells with an i-layer thicknesses of 420, 630 and 840nm were evaluated at each temperature. Results show that operating a-Si:H-based PV at 90°C, with thicker i-layers than the cells currently used in commercial production, provided a greater power output compared to the thinner cells operating at either PV or PVT operating temperatures. These results indicate that incorporating a-Si:H as the absorber material in a PVT system can improve the thermal performance, while simultaneously improving the electrical performance of a-Si:H-based PV.

Study on the fabrication of silicon nanoparticles in an amorphous silicon light absorbing layer for solar cell applications

Hydrogenated amorphous-silicon (a-Si:H) thin-film solar cells have advantages of relatively simple technology, less material consumption, higher absorption ratio compared to crystalline silicon, and low cost due to the use of cheaper substrates rather than silicon wafers. However, together with those advantages, amorphous-silicon thin-film solar cells face several issues such as a relatively lower efficiency, a relatively wider bandgap, and the Staebler-Wronski effect (SWE) compared to other competing materials (i.e., crystalline silicon, CdTe, Cu(InxGa(1−x))Se2 (CIGS), etc.). As a remedy for those drawbacks and a way to enhance the cell conversion efficiency at the same time, the employment of crystalline silicon nanoparticles (Si-NPs) in the a-Si matrix is proposed to organize the quantum-dot (QD) structure as the light-absorbing layer. This structure of the light absorbing layer consists of single-crystal Si-NPs in an a-Si:H thin-film matrix. The single-crystal Si-NPs are synthesized by using SiH4 gas decomposition with CO2 laser pyrolysis, and the sizes of Si-NPs are calibrated to control their bandgaps. The synthesized size-controlled Si-NPs are directly transferred to another chamber to form a QD structure by using co-deposition of the Si-NPs and the a-Si:H matrix. Transmission electron microscopy (TEM) analyses are employed to verify the sizes and the crystalline properties of the Si-NPs alone and of the Si-NPs in the a-Si:H matrix. The TEM results show successful co-deposition of size-controlled Si-NPs in the a-Si:H matrix, which is meaningful because it suggests the possibility of further enhancement of the a-Si:H solar-cell structure and of tandem structure applications by using a single element.