Photoelectric phenomena in ZnO(ITO)/a-Si:H(n)/c-Si(p)/Al solar cells

Solar cells Al/ZnO/a-Si:H(n)/c-Si(p)/Al and Al/ITO/a-Si:H(n)/c-Si(p)/Al were fabricated on single-crystal Si substrates. The photoelectric properties of these solar cells were investigated under exposure to natural and linearly polarized radiation at T=300 K. The polarization photosensitivity of the solar cells, which emerges under the conditions of oblique incidence of linearly polarized radiation, was observed. The origin of the induced photopleochroism of solar cells with ZnO and ITO antireflection coatings is ascertained. The oscillations in the spectrum of the induced photopleochroism, which are associated with interference phenomena in oxide films, are observed. The results obtained point to the possibility of using the solar cells as selective photosensors. At the same time, polarization spectroscopy can ensure effective monitoring of the antireflection coatings of solar cells.     

Nature of impurity centers of rare-earth metals and self-organization processes in a-Si:H films

The existence of self-organization processes in a-Si:H:Er films and the appearance of an ordered structure of [Er-O] crystallites in the unordered structural network of a-Si:H:Er were ascertained. The principal parameter of the self-organization process was shown to be the control parameter, which is the [Er-O]-complex content in the a-Si:H:Er films. A decrease in the sizes of crystallites and an increase in the particle density leads to an increase in the photoluminescence intensity for λ=1.54 μm, i.e., to an increase in the optical activation of Er ions in the a-Si:H:Er films.     

Gamma-irradiation-induced metastable states of undoped amorphous hydrogenated silicon

The conductivity of intrinsic amorphous hydrogenated silicon (a-Si:H) becomes higher upon gamma irradiation. This effect is due to an increase in the number of metastable D ⁺ states in the mobility gap. At the same time, the conductivity of extrinsic (undoped) irradiated a-Si:H decreases. Most likely, gamma irradiation creates hydrogen-containing complexes in this material. The results obtained are discussed in comparison with the known data for B-or P-doped a-Si:H.     

a-SiOx:H passivation layers for Cz-Si wafer deposited by hot wire chemical vapor deposition

In order to get the high photoelectric conversion efficiency a-Si:H/c-Si solar cells, high quality intrinsic hydrogenated passivation layer between the a-Si:H emitter layer and the c-Si wafer is necessary. In this work, hot wire chemical vapor deposition (HWCVD) is used to deposite intrinsic oxygen-doped hydrogenated amorphous silicon (a-SiO_x:H) and hydrogenated amorphous silicon (a-Si:H) films as the intrinsic passivation layer for a-Si:H/c-Si solar cells. The passivation effect of the films on the c-Si surface is shown by the effective lifetime of the samples that bifacial covered by the films with same deposition parameters, tested by QSSPC method. The imaginary part of dielectric constant (ε₂) and bonds structure of the layers are analyzed by Spectroscopic Ellipsometry(SE) and Fourier Transfom Infrared Spectroscopy(FTIR). It is concluded that: (1) HWCVD method can be used to make a-SiO_x:H films as the passivation layer for a-Si:H/c-Si cells and the oxidation of the filament can be overcome by optimizing the deposition parameters. In our experiments, the lowest surface recombination velocity of the c-Si wafer is 3.0 cm/s after a-SiO_x:H films passivation. (2) Oxygen-doping in the amorphous silicon layers can increase H content and the band-gap of films, similar as the phenomenon of the films deposited by PECVD.     

Study of the correlation properties of the surface structure of <Emphasis Type="Italic">nc</Emphasis>-Si/<Emphasis Type="Italic">a</Emphasis>-Si:H films with different fractions of the crystalline phase

The correlation properties of the structure of nc-Si/a-Si:H films with different volume fractions of the crystalline phase are studied using 2D detrended fluctuation analysis. Study of the surface relief of experimental samples showed that with increasing in volume fraction of the crystalline phase in the nc-Si/a-Si:H films, the size and number of nanoclusters on their surface grow. The size of Si nanocrystals in the a-Si:H matrix (6–8 nm) indicates the formation of coarse nanoclusters due to the self-organization of Si nanocrystals in groups under laser radiation. According to 2D detrended fluctuation analysis data, the number of correlation vectors (harmonic components) in the nc-Si/a-Si:H film structure increased with an increase in the nanocrystal fraction in the films.     

The influence of local surroundings of Er atoms on the kinetics of decay of Er photoluminescence in amorphous hydrogenated silicon

Kinetics of the decay of photoluminescence of Er impurity in the films of amorphous hydrogenated Si a-Si:H〈Er〉 was studied for the first time. The films were obtained either by cosputtering of Si and Er targets with the use of the technology of dc silane decomposition in a magnetic field (MASD) or by radio-frequency decomposition of silane. In the second case, an Er(TMHD)₃ polymer powder was used as the source of Er. It is shown that, at room temperature, the a-Si:H〈Er〉 films obtained by the MASD method feature the characteristic times of Er photoluminescence decay equal to 10–15 μs, which is 20 times smaller than in the case of Er-doped crystalline Si (c-Si〈Er, O〉) as measured at liquid-nitrogen temperature. For the a-Si:H〈Er〉 films obtained by radio-frequency decomposition of silane, the decay times of Er photoluminescence amount to 2 μs. The difference in the photoluminescence decay times is related to dissimilarities in the local surroundings of Er atoms in the a-Si:H〈Er〉 films obtained by different methods. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 1, 2000, pp. 90–92. Original Russian Text Copyright ? 2000 by Terukov, Kudoyarova, Kon’kov, Konstantinova, Kamenev, Timoshenko.     

Special features of photoelectric properties of nanostructured films of hydrogenated silicon

Nanostructured Si films differing in hydrogen content, in the forms of Si-H bonds, and in certain characteristics of Si inclusions in an amorphous matrix (volume fraction, size, and structure) were studied. The behavior common to all the studied films, i.e., an increase in the defect density and nonmonotonic enhancement of photoconductivity at the “red wing” of the spectral characteristic compared to a-Si:H, was assessed. At the same time, there are films with either enhanced or reduced photoconductivity compared to a-Si:H.     

Kinetics of light-induced degradation in a-Si:H films investigated by computer simulation

In this work we investigated the stability of a-Si:H films under illumination and following recovery in darkness at different temperatures. The a-Si:H films were fabricated with 55 kHz PECVD and with conventional rf 13.56 MHz PECVD. We measured the steady-state photocurrent and the dark-current after switching off the light source as a function of time. We observed photocurrent degradation and the following recovery of the dark current. The kinetics of the photocurrent degradation as well as the dark-current recovery demonstrated stretched-exponential behavior. The results of these straightforward measurements in combination with computer simulation were used to determine the effect of light-induced degradation and thermal recovery on the density of states distribution in the band gap of a-Si:H. We have found that the photocurrent degradation and the corresponding increase in the total defect concentration have different kinetics. The different kinetics were also determined for the dark-current recovery and the corresponding decrease in the total defect concentration. The results point out that slow and fast types of defects in a-Si:H films control the kinetics of light-induced changes of the defect distribution in the band gap. A model is proposed that relates the origin of the fast and slow metastable defects with the distribution of Si-Si bond lengths.     

Properties of periodic α-Si:H/a-SiNx:H structures obtained by nitridization of amorphoussilicon layers

The kinetics of nitridization of a-Si:H layers, the properties of the structures that are formed and a-Si:H in them have been investigated. The changes occurring in the resistance of the a-Si:H layers in the course of nitridization are described in terms of the competition between doping, transport, and change in the thickness of the remaining a-Si:H layer. The experimental data on the band spectrum of superlattices with a-Si:H and a-SiN_x:H layer thicknesses ～35 Å and ～5 Å, respectively, are in agreement with calculations in a model of interacting quantum wells with m*=(0.36±0.1)m ₀. Comparison of the properties of superlattices obtained by deposition of successive layers and nitridization of the a-Si:H layers showed that the latter can have a higher “structural perfection.”     

Photovoltaic effect in a-Si:H/n-InSe heterostructures

Heterostructures in an a-Si:H/InSe system were grown by the deposition of a-Si:H films onto the surface (001) of InSe single-crystal wafers and also by deposition of pure indium films with their subsequent selenization, in which case InSe films were synthesized at the a-Si:H surface. The photovoltaic effect was observed and studied for both types of heterostructures. It was concluded that the heterostructures obtained may be used as wide-band photoconverters of radiation.     

Nanostructured a-Si:H films obtained by silane decomposition in a magnetron chamber

Nanostructured a-Si:H films grown by the MASD method at various deposition temperatures (T _s =300–390°C) were studied. Among these films, those “on the verge of crystallinity” are of particular interest, because they tend to crystallize. In addition, although their electron-transport parameters are slightly inferior to those of conventional device-grade a-Si:H, they are preferable because of the higher stability of their photoconductivity under exposure to light.     

Effect of thermal treatment on structure and properties of a-Si:H films obtained by cyclic deposition

The effect of thermal treatment in a vacuum on the structure and properties of amorphous hydrogenated silicon (a-Si:H) films obtained by cyclic deposition with intermediate annealing in hydrogen plasma was studied. a-Si:H films deposited under optimal conditions are characterized by the nonuniform distribution of the nanocrystalline phase (<1 vol %) across the film thickness and have the optical gap E _g=1.85 eV, the activation energy of conductivity E _a =0.91 eV, and a high photosensitivity (σ_ph/σ_d≈10⁷ under illumination of 100 mW/cm² in the visible spectral range). Transmission electron microscopy studies demonstrated that thermal treatment in a vacuum leads to blurring of the initial layered structure of a-Si:H films and to a somewhat higher amount of nanocrystalline inclusions in the amorphous phase matrix. Thermal treatment above 350°C causes a dramatic increase in the dark conductivity, as well as the resulting decrease in the photosensitivity, of a-Si:H films.     

A quantum-well model and the optical absorption edge in structurally nonuniform a-Si:H-based alloys

This paper describes studies of the microstructure and optical absorption edge of films of a-Si: H and a-SiN_x: H alloys (x=0.0–0.72), obtained by decomposing gas mixtures in an rf glow discharge. For the a-Si: H films, the gas mixture was monosilane diluted by hydrogen, for the a-SiN_x: H alloys it was SiH₄+NH₃. Structurally nonuniform films with “device-quality” optoelectronic characteristics were obtained when the rates of growth were increased (to 8 Å/s). Atomic force microscopy and infrared absorption spectroscopy are used to identify a characteristic feature of the microstructures of these films: the presence of islets ～500 Å in diameter, whose boundaries are formed by clusters of hydrogen atoms (in the case of a-Si: H) or of hydrogen and nitrogen (in the case of a-SiN_x: H). In this case the optical band gap of a-Si: H (a-SiN_x: H) is determined by the concentration of SiH (SiN) bonds in the interior of the islets and is not sensitive to changes in the content of hydrogen (nitrogen) at their boundaries. This result is explained by a quantum-well model which takes into account the characteristic sizes of the microstructures formed by hydrogen or nitrogen atoms.     

The density of states in the mobility gap of amorphous hydrogenated silicon doped with erbium

The effect of doping films of amorphous hydrogenated silicon (a-Si:H) with erbium on the density of the states in the mobility gap is studied. The data obtained are compared with those for a-Si:H films doped with arsenic. The data on the density of the states in the lower and upper halves of the mobility gap are determined from measurements of the spectral dependences of the absorption coefficient and the temperature dependences of the constant and modulated components of the photoconductivity in films exposed to modulated light, respectively. It is shown that doping the a-Si:H films with erbium leads to an increase in the density of states both in the lower and upper halves of the mobility gap.     

Effect of the femtosecond laser treatment of hydrogenated amorphous silicon films on their structural, optical, and photoelectric properties

The effect of the femtosecond laser treatment of hydrogenated amorphous silicon (a-Si:H) films on their structural, optical, and photoelectric properties is studied. Under the experimental conditions applied in the study, laser treatment of the film with different radiation intensities induces structural changes that are nonuniform over the film surface. An increase in the radiation intensity yields an increase in the contribution of the nanocrystalline phase to the structure, averaged over the sample surface, as well as an increase in the conductance and photoconductance of the samples. At the same time, for all of the samples, the absorption spectrum obtained by the constant-photocurrent method has a shape typical for those of amorphous silicon. Obtained results indicate the possibility of a-Si:H films photoconductance increase by femtosecond pulse laser treatment.     

Study of Deep Levels in a HIT Solar Cell

The results of studying a HIT (heterojunction with an intrinsic thin layer) Ag/ITO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n⁺)/ITO/Ag solar cell by the capacitance–voltage characteristic and current deep-level relaxation transient spectroscopy methods are presented. The temperature dependence of the capacitance–voltage characteristics of the HIT structure and deep-energy-level parameters are studied. The results of comprehensive studies by the above methods are used to determine the features of the energy-band diagram of actual HIT structures.     

Characterization of an <Emphasis Type="Italic">a</Emphasis>-Si:H/<Emphasis Type="Italic">c</Emphasis>-Si interface by admittance spectroscopy

The capabilities of admittance spectroscopy for the investigation of a-Si:H/c-Si heterojunctions are presented. The simulation and experimental results, which compare very well, show that the admittance technique is sensitive to the parameters of both the a-Si:H layer and the a-Si:H/c-Si interface quality. In particular, the curves showing capacitance versus temperature have two steps, accompanied by two bumps in the temperature dependence of the conductance. The first step, occurring in the low temperature range (100–200 K), is related to the transport and response of gap states in the a-Si:H layer. The second step, occurring at higher temperatures (>200 K), is caused by a carrier exchange with interface states and appears when the interface defect density exceeds 5 × 10¹² cm^?2. Then, the interface defects affect band bending, and, thus, the activation energy of de-trapping, which favors exchange with electrons from a-Si:H and holes from c-Si, respectively, for an increasing defect density.     

Numerical calculation of the temperature dependences of photoconductivity in the p-type a-Si:H

Temperature dependences of photoconductivity in p-type a-Si:H were calculated numerically. The calculations were performed in terms of a simple recombination model in which the tunneling recombination of electrons captured by the states in the conduction-band tail and holes trapped at the states of the valence-band tail is accounted for at low temperatures. The results of calculations are consistent with experimental data, according to which the photoconductivity of p-type a-Si:H depends only slightly on the Fermi level position and on the total concentration of dangling bonds. The computer-assisted numerical simulation suggests that the possible cause of this weak dependence may consist in the differing extent of the valence-and conduction-band tails. It is shown that the statistics of occupancy of the states in the mobility gap in a-Si:H under illumination differs vastly from that in the state of equilibrium.     

Transformation of an unordered structural network in amorphous hydrogenated silicon films as a result of doping with boron

The influence of the technological parameters of deposition, the purity and relative concentration of diborane, and the substrate temperature on electrical parameters of (a-Si:H):B films obtained by high-frequency decomposition of a gaseous mixture in a multielectrode system was studied. Simultaneous existence of the mechanisms for doping and modification in the case of introduction of boron from diborane in the course of deposition of (a-Si:H):B films is proposed.     

Photoinduced conductivity change in erbium-doped amorphous hydrogenated silicon films

Changes in the dark conductivity of erbium-doped amorphous hydrogenated silicon (a-Si:H(Er)) films after their preliminary illumination at room temperature have been studied. The effect of a compensating boron impurity on the photoinduced change in the conductivity of a-Si:H(Er) films is analyzed. It is established that the magnitude and the sign of the change in conductivity depend on the duration of illumination and position of the Fermi level in the mobility gap. Possible mechanisms leading to a photoinduced change in the conductivity of a-Si:H(Er) films are discussed.