Light-induced changes in hydrogenated amorphous silicon solar cells deposited at the edge of crystallinity

A series of hydrogenated amorphous silicon (a-Si:H) films were deposited in the transition region from amorphous to nanocrystalline phases by changing hydrogen dilution ratio R, deposition gas pressure, and RF power. Single junction a-Si:H solar cells were made using these materials as the intrinsic layers in the structure of n–i–p type on ZnO/Ag/stainless steel substrates. Light-induced degradations in the photovoltaic parameters were characterized on these cells after 1 Sun solar illumination for 150 h. The stabilized efficiencies were compared in conjunction with the structures in the intrinsic layers, which were revealed by high resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectrometry (FTIR). It was found that the solar cells incorporated protocrystalline intrinsic layer as the i-layer give a better initial efficiency, while solar cells made from nanostructured i-layers have a better stability of ～7% degradation against light soaking, as a result, both have nearly the same final stabilized efficiency. The best device stabilized efficiency reaches ～10.2% (0.25 cm², AM1.5G) for the intrinsic layer deposited at a high pressure of 2 Torr.     

Nanocalorimetric investigation of light-induced metastable defects in hydrogenated amorphous silicon

The electrical properties of hydrogenated amorphous silicon, a-Si:H, are degraded by light-induced metastable defects after exposure to visible light for extended periods. Using nanocalorimetry, we have directly measured the heat released when these defects are annealed. Although these low level measurements were close to the instrument noise limit, and were affected by extraneous signals from adsorbed gas, a total heat release of only a few tens of nJs could be resolved. For a heating rate of 12 000 K s⁻¹, a single broad peak of heat release, centered at 180 °C, was observed. The integrated heat release indicates that ∼8 × 10¹⁶ defects cm⁻³ h⁻¹ were generated. Polycrystalline Si samples, in which no defects are created by light-soaking, showed no heat release.     

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 0.95 eV. These results suggest a strongly inhomogeneous spatial distribution of photo-created DBs.     

DLTS response due to localized states in hydrogenated amorphous silicon

The transient response of the localized states in a-Si : H involved in DLTS measurements is analyzed in the frame of a detailed model for the band bending. The model is based on an exponential distribution of the density of states, spatially uniform throughout a Schottky-barrier depletion layer. The thermally emitted charge is calculated and compared with the estimate of the charge from the simple crystalline model to assess the effect of nonparabolic band bending. The influence of spatially nonuniform space charge density and uncompensated shallow level concentration is analyzed by calculating the DLTS signal and retrieving the density of states by the modified crystalline model. Our results show that the simple modified crystalline model accurately determines the localized density of states in a-Si:H from DLTS data obtained with large reverse bias voltages. for lower reverse bias voltages the analysis of the DLTS spectra based on the crystalline model yields substantially lower values for the minimum density of states.     

Photoluminescence of hydrogenated amorphous silicon

Photoluminescence has beeb proven to be a powerful technique in the characterization of a-Si:H. In particular, it has contributed to the elucidation of some aspects of the electronic structure. However, there is a set of controversial topics still under discussion including the idenntity of the luminescent transition and the origin of broadening of the emission spectrum. In this paper we study these problems and show that the specified width has its origin in both disorder and electron-phonon interaction.Luminescent decay at low temperature has been studied and lifetimes from 10^?8 to 10^?2 s have been confirmed.Photoconductivity and photoluminescence are shown to behave in a complementary way and activation energies for both processes are obtained. Also, the photoluminescence quenching and photoconductivity enhanced under an applied electric field have been measured an interpreted.     

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.     

Recombination and transport through localized states in hydrogenated amorphous and microcrystalline silicon

Electrical transport and recombination mechanisms in hydrogenated amorphous silicon, a-Si:H, are determined by localized band-tail states and deep defects. At low temperatures (T < 100 K) the photoluminescence originates from tunneling recombination between localized band-tail states and the photoconductivity arises from hopping in the band tail. This review describes the present understanding of transport and recombination mechanisms in this low-temperature regime with a focus on two aspects: (i) the kinetics of carrier recombination and the competition between geminate and non-geminate recombination, and (ii) the microscopic identification of recombination paths by magnetic resonance techniques and the proof of excitonic recombination. Inspite of its complex nanocrystalline morphology, hydrogenated microcrystalline silicon, μc-Si:H, behaves in many respects similarly to a-Si:H in that the low-temperature properties are also determined by disorder-induced localized band-tail states.     

Luminescence gap in hydrogenated amorphous silicon

The “luminescence gap” is used instead of the thermalization gap and the hopping-gap because the gap is obtained from the luminescence measurement. The luminescence gaps in hydrogenated amorphous silicon (a-Si:H) are observed in the temperature range from 4.2 to 225 K for the films prepared at different substrate temperatures 170 to 300 °C by plasma CVD. It is shown from the temperature dependence of the luminescence gap that the luminescence edges are at the localized band tail states at which the waiting time for the hopping is equal to the life time of the luminescence. The excitation energy dependence of the luminescence peak energy similar to that of the porous Si has been observed.     

Voids in hydrogenated amorphous silicon materials

Effusion measurements of hydrogen and of implanted helium are used to characterize the presence of voids in hydrogenated amorphous silicon (a-Si:H) materials as a function of substrate temperature, hydrogen content, etc. For undoped plasma-grown a-Si:H, interconnected voids are found to prevail at hydrogen concentrations exceeding 15–20 at.%, while isolated voids which act as helium traps appear at hydrogen concentrations ≤ 15 at.%. The concentration of such isolated voids is estimated to some 10¹⁸/cm³ for device-grade undoped a-Si:H deposited at a substrate temperature near 200 °C. Higher values are found for, e.g., doped material, hot wire grown a-Si:H and hydrogen-implanted crystalline Si. The results do not support recent suggestions of predominant incorporation of hydrogen in a-Si:H in (crystalline silicon type) divacancies, since such models predict a concentration of voids (which act as helium traps) in the range of 10²¹/cm³ and a correlation between void and hydrogen concentrations which is not observed.     

Luminescence decay in hydrogenated amorphous silicon and silicon nanostructures

The stretched exponential luminescence decay observed at temperatures lower than 20 K transits to the power law decay due to the electron-hopping at localized band tail states near 60 K in the hydrogenated amorphous silicon (a-Si:H). The luminescence decay at 4.2 K in a-Si:H is quite similar to that of Si-nanoparticles in the porous Si (p-Si). It is explained from the comparison with p-Si that the slow luminescence of the life time of ~ 1 ms is due to the recombination of excitonic electron–hole pairs at the spin triplet state quantum-confined in the hydrogen-free Si nanostructure in a-Si:H. The fast luminescence of the life time of ~ 1 μs is due to the recombination of the pairs at the spin-singlet state and the life time is explained as due to the indirect optical transition.     

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 t^β 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.     

Photo-induced changes of a.c. conduction in amorphous chalcogenides and hydrogenated silicon

Photo-induced changes of a.c. conductance have been observed for both amorphous Se and hydrogenated Si (a-Si:H). After cessation of the illumination, no metastable change of a.c. conductance for Se and a decrease in the a.c. conductance for a-Si:H which could be related to the Staebler-Wronski effect have been found. These are discussed in terms of the negative-U defects.     

Er-doped hydrogenated amorphous silicon: structural and optical properties

The effect of erbium-doping on the structural and optical properties of hydrogenated amorphous silicon (a-Si:H) is investigated. Optical absorption and Raman spectra indicate that erbium doping introduces defect states, and that above a concentration of 0.27 at.%, induces strong structural disorder. The photoluminescence measurements show that erbium doping introduces non-radiative decay paths for carriers in a-Si:H, leading to decrease in both the Er³⁺ and intrinsic a-Si:H luminescence intensity when the Er concentration is increased to more than 0.04 at.%. The results are compared to that of Er-doped crystalline Si, and the possible excitation mechanisms of Er in a-Si:H are discussed.     

Network structure and dynamics of hydrogenated amorphous silicon

In this paper we discuss the application of current ab initio computer simulation techniques to hydrogenated amorphous silicon (a-Si:H). We begin by discussing thermal fluctuation in the number of coordination defects in the material, and its temperature dependence. We connect this to the ‘fluctuating bond-center detachment’ mechanism for liberating H bonded to Si atoms. Next, from extended thermal MD simulation, we illustrate various mechanisms of H motion. The dynamics of the lattice is then linked to the electrons, and we point out that the squared electron-lattice coupling (and the thermally-induced mean square variation in electron energy eigenvalues) is robustly proportional to the localization of the conjugate state, if localization is measured with inverse participation ratio. Finally we discuss the Staebler–Wronski effect using these methods, and argue that a sophisticated local heating picture (based upon reasonable calculations of the electron-lattice coupling and molecular dynamic simulation) explains significant aspects of the phenomenon.     

Modification of amorphous hydrogenated silicon by co-sputtered aluminum

The electronic and optical properties of a-Si_1?xH_x have been modified by the incorporation of aluminum. Samples were prepared by rf sputtering in a hydrogenated atmosphere from a composite silicon-aluminum target. This paper reports on several modified material parameters including the optical band gap, electrical conductivity, and thermal activation energy. Aluminum concentrations up to 10.6% in the target have been investigated. It is observed that the optical band gap remains constant at 1.83 eV for Al concentrations up to 2.7%. For higher concentrations there is a marked decrease in optical gap. The conductivity initially decreases with small Al concentration and the activation energy increases, characteristic of compensation of the inherently n-type material. For higher Al concentrations the conductivity increases by seven orders of magnitude and the activation energy decreases to a minimum of about 0.2 eV. The increase in conductivity can be explained by both the movement of the Fermi level and the shrinking band gap. Microprobe analyses have also been performed to determine the amount of Al actually incorporated into the films. Finally, implications of these results are discussed and compared to previously reported results on gas phase doping and ion implantation.     

Laser crystallization of compensated hydrogenated amorphous silicon thin films

Raman backscattering and hydrogen effusion measurements were performed on compensated, highly P- and B-doped laser crystallized polycrystalline silicon. From hydrogen effusion spectra the hydrogen chemical potential, μ_H, is determined as a function of hydrogen concentration, which can be related to the hydrogen density-of-states distribution. Interestingly, hydrogen bonding is affected by doping of the amorphous starting material. Below the hydrogen transport states, four peaks are observed in the hydrogen density-of-states at 2.0, 2.2, 2.5 and 2.8 eV. The latest peak is not observed in B-doped samples. The hydrogen effusion results will be correlated with the results obtained from Raman backscattering measurements.