Microcrystalline Silicon Thin Films Research Articles

Optimization of microcrystalline silicon thin film solar cell isolation processing parameters using ultraviolet laser

This study used ultraviolet laser to perform the microcrystalline silicon thin film solar cell isolation scribing process, and applied the Taguchi method and an L 18 orthogonal array to plan the experiment. The isolation scribing materials included ZnO:Al, AZO transparent conductive film with a thickness of 200 nm, microcrystalline silicon thin film at 38% crystallinity and of thickness of 500 nm, and the aluminum back contact layer with a thickness of 300 nm. The main objective was to ensure the success of isolation scribing. After laser scribing isolation, using the minimum scribing line width, the flattest trough bottom, and the minimum processing edge surface bumps as the quality characteristics, this study performed main effect analysis and applied the ANOVA (analysis of variance) theory of the Taguchi method to identify the single quality optimal parameter. It then employed the hierarchical structure of the AHP (analytic hierarchy process) theory to establish the positive contrast matrix. After consistency verification, global weight calculation, and priority sequencing, the optimal multi-attribute parameters were obtained. Finally, the experimental results were verified by a Taguchi confirmation experiment and confidence interval calculation. The minimum scribing line width of AZO (200 nm) was 45.6 μm, the minimum scribing line width of the microcrystalline silicon (at 38% crystallinity) was 50.63 μm and the minimum line width of the aluminum thin film (300 nm) was 30.96 μm. The confirmation experiment results were within the 95% confidence interval, verifying that using ultraviolet laser in the isolation scribing process for microcrystalline silicon thin film solar cell has high reproducibility.

Influence of hydrogen addition and gas pressure on nitride layer formation on microcrystalline silicon thin films by a hot-wire chemical vapor method using nitrogen gas

Abstract Silicon nitride is employed for insulating layers, surface passivation layers and antireflection coatings. In this paper, we report on the influence of the addition of H 2 gas to N 2 gas and the total gas pressure on the nitride layer formation on a hydrogenated microcrystalline silicon (μc-Si:H) film surface region. Following deposition of the μc-Si:H thin films, a post-deposition treatment was carried out using a hot-wire chemical vapor technique, in a mixture of N 2 /H 2 or in a N 2 atmosphere, at varying H 2 gas flow rates and total gas pressures, to form a nitride layer in the film surface region. As the H 2 gas flow rate, F (H 2 ), was increased from 2 to 10 sccm, at a N 2 gas flow rate, F (N 2 ), of 20 sccm, the surface nitridation was dramatically enhanced. Conversely, the degree of surface nitridation remained virtually unchanged with a F (H 2 ) of 10 - 15 sccm and the surface nitridation scarcely took place at F (H 2 ) of 20 sccm. The surface nitridation occurred to some extent at 0.1 Torr (with F (N 2 ) = 20 and F (H 2 ) = 10 sccm) but was dramatically enhanced as the gas pressure was increased from 0.1 to 1.0 Torr. There was a reduction in the densities of both the N and Si atoms as the gas pressure was increased from 1.0 to 6.0 Torr, however, the density of the N atoms on the subsurface region of the films (at a depth of 20 nm from the surface) increased with increasing gas pressure. These findings show the breakdown of the Si network structure by high density H radicals and the diffusion of more N atoms into the deeper region of the films.

Inductively coupled plasma-assisted RF magnetron sputtering deposition of boron-doped microcrystalline Si films

An innovative custom-designed inductively coupled plasma-assisted RF magnetron sputtering deposition system has been developed to synthesize B-doped microcrystalline silicon thin films using a pure boron sputtering target in a reactive silane and argon gas mixture. Films were deposited using different boron target powers ranging from 0 to 350 W at a substrate temperature of 250 °C. The effect of the boron target power on the structural and electrical properties of the synthesized films was extensively investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and Hall-effect system. It is shown that, with an initial increase of the boron target power from 0 to 300 W, the structural and electrical properties of the B-doped microcrystalline films are improved. However, when the target power is increased too much (e.g. to 350 W), these properties become slightly worse. The variation of the structural and electrical properties of the synthesized B-doped microcrystalline thin films is related to the incorporation of boron atoms during the crystallization and doping of silicon in the inductively coupled plasma-based process. This work is particularly relevant to the microcrystalline silicon-based p– i– n junction solar cells.

Low temperature deposition of boron-doped microcrystalline Si:H thin film and its application in silicon based thin film solar cells

Boron-doped hydrogenated microcrystalline silicon thin films (p-μc-Si:H) have been deposited by RF-PECVD method at different temperature, and the temperature dependence of growth kinetics and optoelectronic properties of p-μc-Si:H thin films have been studied. Both the deposition rate and the dark-conductivity of the p-μc-Si:H thin films drop down when the substrate temperature decreases. XRD and Raman measurements are used to characterize the micro-structure of p-μc-Si:H thin films prepared at different substrate temperature. Grain size of p-μc-Si:H thin films with different thickness as a function of substrate temperature has been investigated. Amorphous silicon thin film solar cells with p–i–n structures were fabricated on deposited boron doped μc-Si:H layers. The best cells performance is obtained for p-layers processed at 90 °C.

Surface Reactions and Growth Kinetics of Plasma-deposited Amorphous and Microcrystalline Silicon Thin Films

The growth kinetics of amorphous silicon (a-Si) and microcrystalline silicon (μc-Si) thin films fabricated by plasma-enhanced chemical vapor deposition (PECVD) is discussed for developing low-cost and high-efficiency solar cells. Surface reactions are reviewed associated with the a-Si growth and the nucleation of μc-Si. With respect to the film growth of μc-Si after the nucleation, roughness evolutions were evaluated using an atomic force microscope, and scaling analyses were carried out on fractal structures of growing surfaces, because scaling exponents give a great insight into the growth kinetics. On the basis of results of scaling analyses in conjunction with Monte Carlo simulations, the growth kinetics of microcrystalline silicon thin films, actually used for the high-efficiency solar cells, is discussed.

Performance optimization of p-i-n type microcrystalline silicon thin films solar cells deposited in single chamber

In-situ hydrogen plasma treating technique and burial method using microcrystalline silicon layer were used respectively to reduce the boron contamination in intrinsic layer for the p-i-n type microcrystalline silicon thin film solar cells deposited in single chamber. The measurement results of J-V relation and the quantum efficiency of solar cells proved that both of them improve the short circuit current density of solar cells to some extent. However, each method showed different effects on the other characteristic parameters of solar cells. By optimizing the hydrogen treating time and light trapping structure, single junction microcrystalline silicon thin film solar cell with 6.39% conversion efficiency has been fabricated in single chamber.

Influence of n-type layer structure on performance and light-induced degradation of n-i-p microcrystalline silicon solar cells

A series of n-i-p microcrystalline silicon thin film solar cells with different values of crystalline volume fraction Xc of n-type layers are prepared by radio frequency plasma enhanced chemical vapor deposition. It is found that the structure of intrinsic layer is strongly dependent on the structure of n-type layer, especially the incubation layer thickness at n/i interface and Xc of intrinsic layer. This series of solar cells were light-soaked under 100 mW/cm2 for 400 h. The experiment results demonstrate that the solar cell with the highest Xc of intrinsic layer (Xc(i)=65%) has the lowest light-induced degradation ratio. Then the solar cell with n-type layer deposited in an amorphous silicon/microcrystalline silicon transition region (Xc(i) =54%) is light-soaked under the irradiations of white light, red light and blue light with the same light intensities, separately. After 400 h light-soaking, the light degradation ratio is only 2% for the red light irradiation, while it is 8% for the blue light irradiation.

Improvement in the optical absorption of PECVD microcrystalline Si thin film through modification of the crystalline fraction through an annealing process

Intrinsic microcrystalline silicon (μc-Si) thin films with varying crystalline fractions ( X C) were deposited using plasma-enhanced chemical vapor deposition. Effort was made to modify the X C and optical properties by annealing at different temperatures, pressures, and ambient gases. After annealing, the amount of Si–H in the μc-Si thin film was decreased for an initial X C of 7%, while that of Si–H 2 mainly decreased to an initial X C of 60–70%. The X C of μc-Si with an initial X C of 7% increased by annealing. However, initial X C of 60% and 70% resulted in a decrease of X C upon annealing. It is suggested that an increase in crystal size of μc-Si films by annealing decreases X C, while a decrease in crystal size increases X C. Depending on the annealing conditions, the optical absorption property of the μc-Si thin film changed. When the initial X C was 7%, the absorption coefficient increased as X C increased to 16%, but slightly decreased with X C values from 51% to 70%. Therefore, we suggest that there is an optimum X C that exhibits the highest optical absorption.

Light trapping in thin-film silicon solar cells with submicron surface texture

The influence of nano textured front contacts on the optical wave propagation within microcrystalline thin-film silicon solar cell was investigated. Periodic triangular gratings were integrated in solar cells and the influence of the profile dimensions on the quantum efficiency and the short circuit current was studied. A Finite Difference Time Domain approach was used to rigorously solve the Maxwell's equations in two dimensions. By studying the influence of the period and height of the triangular profile, the design of the structures were optimized to achieve higher short circuit currents and quantum efficiencies. Enhancement of the short circuit current in the blue part of the spectrum is achieved for small triangular periods (P<200 nm), whereas the short circuit current in the red and infrared part of the spectrum is increased for triangular periods (P = 900nm) comparable to the optical wavelength. The influence of the surface texture on the solar cell performance will be discussed.

From amorphous to microcrystalline silicon: Moving from one to the other by halogenated silicon plasma chemistry

Amorphous and microcrystalline silicon thin-films have been deposited on a large variety of substrates including crystalline silicon, Corning glass and polyimide, exploiting SiF4–H2–He radiofrequency (13.56 MHz) plasma-enhanced chemical vapor deposition. Spectroscopic ellipsometry has been mainly applied to investigate the optical and structural characteristics of the deposited films and to investigate the microstructure depending on experimental parameters, such as H2 and He dilution of plasmas, deposition temperature and substrate plasma treatment prior to deposition. Film growth dynamics have also been investigated by analyzing films with different thicknesses. The peculiarities of the SiF4 chemistry involved in heterogeneous processes during film deposition, and responsible for the amorphous to microcrystalline transition are presented. It is demonstrated that the SiF4-H2 chemistry leads to tailoring of film microstructure from amorphous to fully and dense microcrystalline silicon films without any amorphous incubation layer at temperatures as low as 100°C on any substrate, including Corning glass 7059 and polyimide, independently of film thickness. These results and peculiarities of SiF4-based plasmas are discussed within the framework of the literature debate on SiH4 versus SiF4 as silicon precursor.

Formation of Silicon Nitride Layer on Microcrystalline Silicon Thin Films by Hot-wire Chemical Vapor Method using Nitrogen and Hydrogen Gases

Right after the deposition of hydrogenated microcrystalline silicon (μc-Si:H) thin films, a post-deposition treatment was carried out using the hot-wire chemical vapor technique in N2/H2 atmosphere, to form a nitride layer in the film surface region. The influence of the gas pressure during the post-deposition treatment on the nitride layer formation was investigated. Nitriding at the surface was negligible at 0.1 Torr but was enhanced dramatically with an increase in the gas pressure from 0.1 to 1.0 Torr. Consequently, the surface was almost completely covered with the nitride layer. However, the density of N atoms at the surface reduced with increasing gas pressure from 1.0 to 6.0 Torr. On the other hand, the nitriding on the inside of the films (a depth of 20 nm from the surface) was enhanced by the increase in the gas pressure.

Analysis of controlled mixed-phase (amorphous+microcrystalline) silicon thin films by real time spectroscopic ellipsometry

Engineered thin films consisting of periodic arrays of silicon microcrystallites in a hydrogenated amorphous silicon host matrix have been prepared by plasma-enhanced chemical vapor deposition where the hydrogen dilution of silane is modulated in multiple cycles. These types of films have been guided by a phase evolution diagram, depicting the deposition conditions and film thickness at which the material exhibits amorphous, microcrystalline, or mixed-phase (amorphous+microcrystalline) characteristics, developed for intrinsic Si:H prepared with varying H2 dilution on unhydrogenated a-Si:H. Real time spectroscopic ellipsometry (RTSE) has been used in situ to noninvasively determine the phase evolution of the resulting hydrogenated mixed-phase (amorphous+microcrystalline) silicon thin films and corroborated with dark-field transmission electron microscopy. Such tailored microstructures are of growing interest as components of thin film photovoltaic devices, and RTSE is shown to be a key technique for structure verification.

Influence of the substrate geometrical parameters on microcrystalline silicon growth for thin-film solar cells

The effect of substrate morphology on the growth and electrical properties of single-junction microcrystalline silicon cells is investigated. A large variety of V-shaped and U-shaped substrates are characterized by scanning electron microscopy (SEM) and the growth of thin-film microcrystalline silicon (μc-Si:H) devices is observed by cross-sectional transmission electron microscopy (TEM). It is shown that enhanced electrical properties of solar cells are obtained when U-shaped substrates are used and the effect is universal, i.e. independent of the substrate or feature size. U-shaped substrates prevent the formation of two dimensional “cracks”, which are identified as zones of porous material, from propagating throughout the active part of the solar cell. A numerical growth simulation program reproduces satisfactorily these experimental observations. According to these simulations, shadowing effect due to surface morphology and low adatom surface diffusion length are responsible for the formation of cracks in μc-Si:H material.

Effect of self-orderly textured back reflectors on light trapping in thin-film microcrystalline silicon solar cells

Light trapping is a key issue to boost the efficiency of thin-film Si solar cells including μc-Si:H. In this work, effect of textured back reflectors on light trapping in μc-Si:H cells has been investigated with self-orderly patterned Al substrates obtained by anodic oxidation. By increasing the period of the patterned substrates from 0 to 1.1 μm, the short circuit current densities of 1-μm-thick μc-Si:H cells on the patterned substrates significantly increase from 18 to over 24 mA/cm2, which is attributed to the improved light trapping in the near infrared region. The optimum period for light trapping is determined to be around 1 μm for 1-μm-thick μc-Si:H cells. In addition, it has been demonstrated that the optimized patterned substrate enhances light trapping effect in a wide range of cell thicknesses of 0.5–4 μm. It is also confirmed that natural surface texturing during μc-Si:H film growth improves light scattering for flat substrates but deteriorates the superior light scattering property of the well-designed substrates.

Modulated photonic-crystal structures as broadband back reflectors in thin-film solar cells

A concept of a modulated one-dimensional photonic-crystal (PC) structure is introduced as a back reflector for thin-film solar cells. The structure comprises two PC parts, each consisting of layers of different thicknesses. Using layers of amorphous silicon and amorphous silicon nitride a reflectance close to 100% is achieved over a broad wavelength region (700–1300 nm). Based on this concept, a back reflector was designed for thin-film microcrystalline silicon solar cells, using n-doped amorphous silicon and ZnO:Al. Simulations show that the short-circuit current of the cell with a modulated PC back reflector closely resembles that of a cell with an ideal reflector.

Transport and structural properties of silicon films in the amorphous-to-microcrystalline transition region

Undoped hydrogenated microcrystalline silicon (μc-Si:H) thin films have been deposited by plasma enhanced chemical vapor deposition (PECVD) at low temperature with different hydrogen dilutions and rf powers. Large complexity of microstructure in hydrogenated microcrystalline silicon and the existence of different sizes of crystallites are demonstrated by different characterizations. The authors correlate the transport properties with the structural properties of the rf PECVD grown μc-Si:H in the amorphous-to-crystalline transition region. For chamber pressure of 2.0Torr and rf power density of 310mW∕cm2, the onset of crystallinity is observed for the film deposited at a hydrogen dilution of 94%. At a hydrogen dilution of 95%, amorphous-to-microcrystalline transition have been observed. This film exhibits a dark conductivity of 2.7×10−7Scm−1 and a crystalline volume fraction of 21%. The mobility-lifetime product for these films are 3.8×10−6cm2∕V and hole diffusion length is 70nm. Fourier transform infrared study shows mainly monohydride bonding in this film. This film becomes stable after 30h of light soaking.

Ultrafast carrier dynamics in microcrystalline silicon probed by time-resolved terahertz spectroscopy

We present the results of optical-pump--terahertz probe experiments applied to a set of thin-film microcrystalline silicon samples, with structures varying from amorphous to fully microcrystalline. The samples were excited at wavelengths 800 and 400 nm and studied at temperatures down to 20 K. The character of nanoscopic electrical transport properties markedly change on a subpicosecond time scale. The initial transient photoconductivity of the samples is dominated by hot free carriers with a mobility of $\ensuremath{\sim}70\text{ }{\text{cm}}^{2}/\text{Vs}$. These carriers are rapidly (within 0.6 ps) trapped into weakly localized hopping states. The hopping process dominates the terahertz spectra on the picosecond and subnanosecond time scales. The saturated high-frequency value of the hopping mobility is limited by the sample disorder in the amorphous sample and by electron-phonon interaction for microcrystalline samples.

Back surface reflectors with periodic textures fabricated by self-ordering process for light trapping in thin-film microcrystalline silicon solar cells

Back surface reflectors (BSRs) with grating structures have been developed to enhance light trapping in thin-film hydrogenated microcrystalline Si (μc-Si:H) solar cells. As a grating structure, a periodic honeycomb-like dimple pattern with a period of ∼450 nm has been fabricated on Al substrates by a self-ordering process using anodic oxidation of Al. The clear diffraction effect has been confirmed on the patterned Al from the angle-dependent reflection. From quantum efficiency measurements, we found that the periodically patterned BSR can confine the incident light more effectively than the random textured BSR, especially at longer wavelengths.

Continuous hot-wire chemical vapor deposition on moving glass substrates

Hot Wire Chemical Vapor Deposition (HWCVD) is a very suitable technique for homogeneous deposition of thin films on continuously moving substrates in an in-line manufacturing system. This process is further aided by the fact that transport of insulating substrates (such as glass) during deposition can be easily arranged as the substrate is not part of the decomposition mechanism as in plasma enhanced CVD. Rigorous grounding of the moving substrates is not required, and no special care needs to be taken to make shields or liners equipotential planes. Moreover, as the creation of dust particles in the gas phase can be avoided, deposition could be undertaken with the substrates facing upward, thus further simplifying the mounting of the substrates. Amorphous as well as microcrystalline silicon thin films with device-quality properties have been achieved on moving substrates. The first p-i-n solar cells made with a 300-nm thick i-layer that was deposited on a linearly moving substrate already showed efficiencies of 6.4%, despite two air breaks that were needed in these tests.

N 2 decomposition by hot wire and N 2 post-deposition treatment on hydrogenated microcrystalline silicon thin films

Post-deposition treatment of hydrogenated microcrystalline silicon (µc-Si:H) was carried out using a hot wire in atmospheres of N 2, N 2/H 2 or H 2 and the states of the bonds in the µc-Si:H films were investigated using X-ray photoelectron spectroscopy. For the µc-Si:H film treated in N 2 at the filament temperature ( T f) of 1600 °C, a weak N1s peak was observed. It increased slightly with increasing T f from 1600 to 1900 °C and increased dramatically with increasing T f from 1900 to 2000 °C. The N1s peak of the µc-Si:H film treated in N 2/H 2 at T f = 2000 °C was one order of magnitude lower than that in N 2 at T f = 2000 °C . These findings indicate that N 2 molecules decompose on the heated filament and that the addition of H 2 prevents N 2 decomposition.