Light-induced Defect Creation Research Articles

Light-induced defect creation processes and light-induced defects in hydrogenated amorphous silicon

We have proposed a model of light-induced defect creation processes and light-induced defects. Recently, important results using pulsed electron-nuclear double resonance (ENDOR) by Fehr et al. [M. Fehr, A. Schnegg, C. Teutloff, R. Bittl, O. Astakhov, F. Finger, B. Rech, K. Lips, Phys. Status Solidi A 207, 552 (2010)] have been reported, so that these results are interpreted on the basis of our model. Fehr et al. have observed ENDOR signals due to hydrogen nuclei distributed around a dangling bond. The ENDOR spectra due to hydrogen nuclei being located with distance of r from the dangling bond have been calculated, taking into accounts the dipolar interaction, and also the Fermi-type contact hyperfine interaction for the H-related dangling bond (HDB) that is a dangling bond having hydrogen at a nearby site. The typical features of the observed ENDOR spectra are that the spectrum has a shoulder at the low frequency side from the natural NMR frequency of hydrogen and it has a dip in the central part. The calculated ENDOR spectrum of HDB exhibits such a shoulder. This is consistent with our model of light-induced defects such as HDB. The ENDOR spectra with various values of r are calculated. In this paper, we also deal with the distant ENDOR precisely, using the theory of distant ENDOR by Lambe et al. [J. Lambe, N. Laurance, K.C. McIrvine, R.W. Terhune, Phys. Rev. 122, 1161 (1961)]. The calculated distant ENDOR spectrum shows a dip in the central part. Concerning the dip, Fehr et al. attribute the dip to be due to the suppression of the matrix ENDOR line (this is called the artifact). Thus, it is not obvious whether the dip is due to such an artifact or the central part of the distant ENDOR spectrum.

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.

Model-simulation of light-induced defect creation in hydrogenated amorphous silicon

The processes of light-induced defect creation in hydrogenated amorphous silicon consist of self-trapping of holes in a weak Si-Si bond adjacent to a Si-H bond, nonradiativerecombination of self-trapped holes with electrons, the Si-H bond switching towards the weak Si-Si bond, and hydrogen-hopping or -tunneling along the weak Si-Si bond. The Si-H bond switching and hydrogen movements are treated by Monte-Carlo computer simulation in a simple cubic lattice. From the result of <r2> vs t in which r and t designate diffusion distance of hydrogen and diffusion time, respectively, the density of light-induced defects is estimated in a good agreement with the observed density.

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.

Kinetics of light induced defect creation in organic solar cells

The kinetics of light-induced recombination centers in bulk heterojunction organic solar cells are measured as a function of exposure time, intensity, and the illumination photon energy. The density of induced centers increases with exposure but stabilizes partially due to self-annealing. UV exposure is roughly 50 times more effective for defect creation than white light or yellow-filtered white light. Light-induced breaking of C-H bonds to create H-related localized states is proposed as the underlying mechanism.

Oxygenated Protocrystalline Silicon Thin Films for Wide Bandgap Solar Cells

AbstractProtocrystalline silicon, which is a material that has enhanced medium range order (MRO), can be prepared by using high hydrogen dilution in PECVD, or, alternatively, using high atomic H production from pure silane in HWCVD. We show that this material can accommodate percentage-level concentrations of oxygen without deleterious effects. The advantage of protocrystalline SiO:H for application in multijunction solar cells is not only that it has an increased band gap, providing a better match with the solar spectrum, but also that the solar cells incorporating this material have a reduced temperature coefficient. Further, protocrystalline materials have a reduced susceptibility to light-induced defect creation. We present the unique result in the PV field that these oxygenated protocrystalline silicon solar cells have an efficiency temperature coefficient (TCE) that is virtually zero (TCE is between -0.08%/°C and 0.0/°C). It is thus beneficial to make this cell the current limiting cell in multibandgap cells, which will lead to improved annual energy yield.

Effect of Bi addition on the electrical properties of a-Ge–Se–In thin films

This paper reports on the steady state and transient photoconductivity of Ge20Se70−xIn10Bix (x = 2, 4, 6, 8, 10) thermally evaporated thin films. The dark conductivity (σd) increases with the increase in the Bi content. The value of σd increases slowly from x = 2 to x = 6, but for x ⩾ 8 the increase is quite abrupt. Photoconductivity (σph) also increases with the increase in the Bi content. The dark activation energy (ΔEd) decreases with the increase in the Bi content. There is a drop in the photosensitivity (σph/σd) of films with the increase in Bi addition. The result of the intensity dependent steady state photoconductivity (σph) follows the power law with intensity (F), i.e. σph α Fγ where the value of γ is close to 0.5, suggesting the predominance of bimolecular recombination in these materials. The rise and decay of photocurrent show the increase in the differential lifetime (τd), with the increase in the Bi content, indicating a delay in the recombination rate of photo-generated charge carriers. This change is governed by the photo-generated carriers trapped in the intrinsic defects or light induced defect creation through structural changes. One can therefore also expect that the density of states increases with the increase in the Bi content in amorphous thin films. The dispersion parameter (α) of the localized state energy distribution, determined from the decay curves, is also reported.

The kinetics of the light-induced defect creation in hydrogenated amorphous silicon – Stretched exponential relaxation

Our model for light-induced defect creation in hydrogenated amorphous silicon is applied to its kinetics, i.e., the growing curve of light-induced dangling bond density as a function of illumination time, which is fitted to a stretched exponential function. Two parameters β and τ involved in the function are estimated as functions of saturated dangling bond density in terms of our model. These are compared with two experimental results, i.e., our results obtained from ESR measurements and Shimakawa et al.’s results obtained from photoconductivity measurements. The saturated dangling bond density is also measured as a function of the generation rate of free carriers. The experimental results are compared with calculated results and discussed.

Thermal stability of light-induced defects in hydrogenated amorphous silicon: Effect on defect creation kinetics and role of network microstructure

The kinetics of light-induced defect creation in a-Si:H is studied in early-time limit and as function of pre-existing defects of different thermal stability by electron spin resonance and optical spectroscopy techniques. Both for cw and for laser pulse exposures, the early-time kinetics follows sublinear tβ time dependences, similar to the long-time limit. In addition, the overall defect creation rate is not a single function of the total defect number. Instead, it depends on the thermal stability, or annealing energy distribution, of the defects present in the film. Furthermore, creation of the thermally less stable defects is unaffected by the presence of a large number of stable defects introduced by pre-exposure at a higher temperature. These findings question the existing defect creation models. Thermal stability of the light-induced defects depends on the network microstructure, the less stable defects being created in a-Si:H deposited near microcrystalline transition.

Modeling of light-induced degradation of amorphous silicon solar cells

Light-induced degradation of hydrogenated amorphous silicon (a-Si:H) solar cells has been modeled using computer simulations. In the computer model, the creation of light-induced defects as a function of position in the solar cell was calculated using the recombination profile. In this way, a new defect profile in the solar cell was obtained and the performance was calculated again. The results of computer simulations were compared to experimental results obtained on a-Si:H solar cell with different intrinsic layer thickness. These experimental solar cells were degraded under both open- and short-circuit conditions, because the recombination profile in the solar cells could then be altered significantly. A reasonable match was obtained between the experimental and simulation results if only the mid-gap defect density was increased. To our knowledge, it is the first time that light-induced degradation of the performance and the quantum efficiency of a thickness series of a-Si:H solar cells has been modeled at once using computer simulations.

Modeling of light-induced defect creation in hydrogenated amorphous silicon

Light-induced defect creation in hydrogenated amorphous silicon $(a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H})$ is discussed in terms of a model based on the breaking of weak Si--Si bonds by self-trapping of holes under illumination. In this model, two separate dangling bonds, i.e., a normal dangling bond and a hydrogen-related dangling bond that is a dangling bond having hydrogen at a nearby site, are created under illumination. Further, we take into account dissociation of hydrogen atoms from hydrogen-related dangling bonds, termination of two types of dangling bonds by dissociated hydrogen atoms (metastable hydrogen atoms), insertion of dissociated hydrogen atoms into nearby weak Si--Si bonds, and formation of hydrogen molecules by collision of dissociated hydrogen atoms. Taking into account these processes, rate equations governing the kinetics of two types of dangling bonds and metastable hydrogen atoms under illumination are numerically solved for the case of continuous illumination. The calculated results are compared with experimental results taken from literature. Furthermore, we discuss the case of low-temperature illumination, the kinetics of light-induced dangling bonds given by stretched exponential function, and saturated density of light-induced dangling bonds in $a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H}$.

Dispersive processes of light-induced defect creation in hydrogenated amorphous silicon

The growing curve of light-induced dangling bonds under illumination has been observed for various intensities of illumination in a-Si:H. It is fitted to a stretched exponential function and then two parameters β and τ involved in the function are estimated as a function of saturated dangling bond density N ss . The experimental values of β , τ , and N ss are compared with those calculated based on our model of light-induced defect creation in a-Si:H.

Transient Current Behavior of Vertically Integrated Amorphous Silicon Diodes

AbstractMonolithic image sensors based on Thin Film on CMOS (TFC) Technology are becoming more and more attractive as an alternative solution to conventional active pixel sensors (APS). Imager with high sensitivity, high dynamic coupled with low dark current values (10-100 pA/cm2 @ 104 V/cm) have been developed. However, issues such as light-induced degradation and image lag hinder the commercial development of a-Si:H based image sensors. The problem of image lag is caused by residual current due to the release of trapped charges after the switch off of the illumination.In this paper, we present a comprehensive study of the transient behavior of the photocurrent in a-Si:H photodiodes deposited on glass, as well as in corresponding diodes implemented in a TFC image sensor when illumination is switched off or periodically varied. The influence of the pixel architecture for two different cases is also analyzed: One setup reproduces the typical 3 transisor APS pixel architecture behavior, in which the bias voltage of the diode varies with the photogenerated charge while the second setup keeps a constant bias voltage applied to the diode by using a charge integrator.The influence of the light-induced defect creation on the performance of the sensors is also presented and discussed.

Light-induced creation of defects related to low energy photoluminescence in hydrogenated amorphous silicon

We have studied intensity and lifetime distribution of low energy photoluminescence (PL) in a-Si:H films containing native defects of various densities, prepared at various substrate temperatures, and those containing photo-created defects after illumination of pulsed light from a YAG–OPO laser system. A relation between the density of dangling bonds (DBs) and intensity of low energy PL is obtained for the films before illumination. The low energy PL in the films after illumination is stronger than that expected from the relation. The illumination does not cause sizable change of the lifetime distribution of the low energy PL of 0.95eV. These results suggest a strongly inhomogeneous spatial distribution of photo-created DBs.

Effect of Light Illumination on Threshold Voltage and Sub-threshold Slope of Amorphous Silicon Thin Film Transistors

AbstractIn this study the light induced defect state creation under 1 sun illumination is investigated through changes in sub-threshold slope, Δ, and threshold voltage, VT, in a-Si:H TFT structures. Threshold voltage changes can be due to either charge trapping or defect state creation near the interface of gate insulator and a-Si:H active layer. By incorporating high quality thermal silicon dioxide in the a-Si:H TFT, the charge trapping effects have been minimized and the changes in threshold voltage can be attributed to induced defect states in the a-Si:H. Since the sub-threshold current is closely related to the band gap defect states, both VT and Δ can be used as a new probe to study the light induced defect creation in the a-Si:H. The information obtained about the nature of gap defect states from the analysis of changes in VT and Δ under light illumination is presented. The results obtained show that under 1 sun illumination there are very fast increases (within 10 seconds) in VT with subsequent slow increases and no observable changes in Δ, similar to those observed in the light induced photoconductivity changes in a-Si:H thin films. The increase in VT under the initial light illumination are associated with the Fermi level in a-Si:H active layers moving downward and causing band bending near the interface region of the gate insulator and a-Si:H active layer. Results are also presented on changes in VT under gate-bias stressing. Comparisons are then made between the results obtained under 1 sun light illumination and those under gate-bias stressing. The information obtained in this study about the nature of the gap defect states derived from characterization of the sub-threshold regimes is discussed.

Light-induced defect creation in hydrogenated polymorphous silicon during repeated cycles of illumination and annealing

We have examined how the intensity of the ESR signal associated with dangling bonds in hydrogenated polymorphous silicon is affected by repeating light-soaking (LS) and 100°C annealing cycles. It was found that the light-induced degradation efficiency decreases with repeated LS–100°C annealing cycles. This result is accounted for in terms of the termination of dangling bonds by mobile hydrogen, i.e. the termination is enhanced by more mobile hydrogen as a result of modification of the amorphous network by the LS–annealing cycles, and consequently the net light-induced defect creation rate is reduced. This is in contrast with previous models in which the decrease in the light-induced degradation efficiency on repeated LS–annealing cycles is attributed to an increase in the amount of hydrogen being ineffective for light-induced defect creation.

Light-induced defect creation in hydrogenated polymorphous silicon

Light-induced defect creation in hydrogenated polymorphous silicon (pm-Si:H) is investigated from electron spin resonance measurements and is compared with that in hydrogenated amorphous silicon (a-Si:H). Light-induced defect creation occurs at room temperature similarly for both types of films prepared at 250 °C. Thermal annealing of light-induced defects is also investigated as a function of temperature. Different behaviours of annealing characteristics for pm-Si:H from those for a-Si:H are observed and discussed. In particular, we observed a decrease of the light-induced defect creation efficiency with repeated light-soaking–annealing cycles and discuss it with respect to the hydrogen bonding in pm-Si:H films.

A theoretical study of light induced defect creation, annealing and photoconductivitydegradation in a-Si:H

This work is a theoretical study of the defect creation and annealing, and of thephotoconductivity degradation of an intrinsic a-Si:H film under continuous illumination. Anew model is developed for the Staebler–Wronski effect (SWE). In this model, we considerthat a defect of the SiHD type is locally created by a non-radiative recombinationat a weak bond close to a SiHHSi configuration. Also, it is considered that thelight induced defect (LID) annealing requires a hydrogen diffusion motion. Usingthe fact that the areas of defect Gaussians increase with illumination time whiletheir positions and widths remain unchanged, we present the evolution of thegap state defect density, calculated by the defect pool model, with illuminationtime. This density of states, which varies during illumination, is introduced in thesimulation of the photoconductivity to obtain its degradation. In addition, thevalidity of the proportionality of the photoconductivity to the illumination intensityG and to the inverse dangling bond density is assessed. Our investigations show amonomolecular behaviour of the photoconductivity for moderate intensity. They also show an illumination intensity dependency onGα,with α varying between 0.5 and 1, in agreement with a large number of similar investigations.

Stability of deuterated amorphous silicon solar cells

In order to elucidate the microscopic mechanism for the earlier observed enhanced stability of deuterated amorphous silicon solar cells we conducted a side-by-side study of fully deuterated intrinsic layers on crystalline silicon substrates using the free-electron laser facility at Nieuwegein (FELIX) to resonantly excite the Si–D stretching vibration and measure the various relaxation channels available to these modes, and of p-i-n solar cells with identical intrinsic absorber layers on glass/TCO substrates to record the degradation and stabilization of solar cell parameters under prolonged light soaking treatments. From our comparative study it is shown that a-Si:D has a superior resistance against light-induced defect creation as compared to a-Si:H and that this can now be explained in the light of the `H collision model' since the initial step in the process, the release of H, is more likely than that of D. Thus, a natural explanation for the stability as observed in a-Si:D solar cells is provided.

Quantum efficiency of light-induced defect creation in hydrogenated amorphous silicon and amorphous As2Se3

The quantum efficiency (QE) of light-induced metastable defect creation in hydrogenated amorphous silicon (a-Si : H) and amorphous As2Se3 (a-As2Se3) by bandgap and subgap illumination has been deduced from photocurrent measurements. The QE decreases with increasing number of absorbed photons. A higher QE for a-As2Se3 than for a-Si : H has been observed and this is interpreted in terms of the higher structural flexibility of a-As2Se3. We have also found that, for both materials, subgap illumination yields a higher QE than does bandgap illumination.