Microcrystalline Silicon Thin Films Research Articles

Thin-film silicon solar cell development on imprint-textured glass substrates

Abstract In this work, we report on the fabrication of microcrystalline thin-film silicon solar cells on textured glass substrates. The development of transparent and conductive front contacts for these solar cells is presented. State-of-the-art random textures for light-trapping were replicated into a glass-like resist on glass substrates with an imprint process. We applied an industrial relevant soft polymer mold that gives excellent replication accuracy. The necessity of applying thin front contacts for enhanced incoupling of the incident light is shown. An increased series resistance of these thin front contacts caused a decrease of the fill factor of the solar cells. One way to surpass this decrease in fill factor by reducing the solar cell width is demonstrated. In addition, the light-trapping and the light-incoupling for solar cells deposited on three different types of random textures were compared.

Impact of hydrogen radical-injection plasma on fabrication of microcrystalline silicon thin film for solar cells

A plasma-enhanced chemical deposition system with hydrogen radical-injection (RI) is proposed for the fabrication of hydrogenated microcrystalline silicon (μc-Si:H) thin films. The plasma parameters and resultant growth characteristics obtained with the RI-capacitively coupled plasma (RI-CCP) system excited with 60 MHz power were compared with those obtained using a conventional CCP (C-CCP) system. The absolute density of hydrogen (H) radicals was measured by vacuum ultraviolet laser absorption spectroscopy (VUVLAS) to evaluate the effect of RI for controlling the H radical density. A higher density of H radicals was achieved with RI-CCP than with C-CCP by H RI. The crystallinity factor, preferential orientation, defect density, microstructure, and post-deposition oxidation of Si thin films deposited using C-CCP and RI-CCP were investigated. Crystallinity factor of 0.6 was realized with high deposition rate of about 2 nm/s even under a low plasma density using RI-CCP. The defect density of μc-Si:H thin films prepared using RI-CCP was lower than that in thin films prepared using C-CCP. In addition, post-deposition oxidation of the films with RI-CCP was lower than that with C-CCP. The high performance of RI-CCP for the fabrication of μc-Si:H thin films for solar cell devices is also demonstrated.

The Study of Microcrystalline Silicon Thin Films Prepared by PECVD

Under different growth conditions, microcrystalline silicon thin films are deposited successfully on glass substrates by the double-frequency plasma enhanced chemical vapor deposition (PECVD). We report the systematic investigation of the effect of process parameters (hydrogen dilution, substrate temperature, forward power, reaction pressure, et al.) on the growth characteristics of microcrystalline silicon thin films. Raman scattering spectra are used to analyze the crystalline condition of the films and the experimental results. Optimizing the process parameters, the highest crystalline volume fraction of microcrystalline silicon films was achieved. It is found that the crystalline volume fraction of microcrystalline silicon films reaches 72.2% at the reaction pressure of 450 Pa, H2/SiH4 flow ratio of 800sccm/10sccm, power of 400 W and substrate temperature of 350 °C.

Light Management Using Periodic Textures for Enhancing Photocurrent and Conversion Efficiency in Thin-Film Silicon Solar Cells

ABSTRACTPeriodically textured back reflectors with hexagonal dimple arrays are applied to thin-film microcrystalline silicon (μc-Si:H) solar cells for enhancing light trapping. The period and aspect ratio of the honeycomb textures have a big impact on the photovoltaic performance. When the textures have a moderate aspect ratio, the optimum period for obtaining a high short circuit current density (JSC) is found to be equal to or slightly larger than the cell thickness. If the cell thickness exceeds the texture period, the cell surface tends to be flattened and texture-induced defects are generated, which constrain the improvement in JSC. Based on these findings, we have fabricated optimized μc-Si:H cells achieving a high active-area efficiency exceeding 11% and a JSC of 30 mA/cm2.

Comparison between periodic and stochastic parabolic light trapping structures for thin-film microcrystalline Silicon solar cells

Light trapping is of very high importance for silicon photovoltaics (PV) and especially for thin-film silicon solar cells. In this paper we investigate and compare theoretically the light trapping properties of periodic and stochastic structures having similar geometrical features. The theoretical investigations are based on the actual surface geometry of a scattering structure, characterized by an atomic force microscope. This structure is used for light trapping in thin-film microcrystalline silicon solar cells. Very good agreement is found in a first comparison between simulation and experimental results. The geometrical parameters of the stochastic structure are varied and it is found that the light trapping mainly depends on the aspect ratio (length/height). Furthermore, the maximum possible light trapping with this kind of stochastic structure geometry is investigated. In a second step, the stochastic structure is analysed and typical geometrical features are extracted, which are then arranged in a periodic structure. Investigating the light trapping properties of the periodic structure, we find that it performs very similar to the stochastic structure, in agreement with reports in literature. From the obtained results we conclude that a potential advantage of periodic structures for PV applications will very likely not be found in the absorption enhancement in the solar cell material. However, uniformity and higher definition in production of these structures can lead to potential improvements concerning electrical characteristics and parasitic absorption, e.g. in a back reflector.

High deposition rate processes for the fabrication of microcrystalline silicon thin films

Abstract The increase of deposition rate of microcrystalline silicon absorber layers is an essential point for cost reduction in the mass production of thin-film silicon solar cells. In this work we explored a broad range of plasma enhanced chemical vapor deposition (PECVD) parameters in order to increase the deposition rate of intrinsic microcrystalline silicon layers keeping the industrial relevant material quality standards. We combined plasma excitation frequencies in the VHF band with the high pressure high power depletion regime using new deposition facilities and achieved deposition rates as high as 2.8 nm/s. The material quality evaluated from photosensitivity and electron spin resonance measurements is similar to standard microcrystalline silicon deposited at low growth rates. The influence of the deposition power and the deposition pressure on the electrical and structural film properties was investigated.

Enhanced photocurrent and conversion efficiency in thin-film microcrystalline silicon solar cells using periodically textured back reflectors with hexagonal dimple arrays

Periodically textured back reflectors with hexagonal dimple arrays are applied to thin-film microcrystalline silicon (μc-Si:H) solar cells for enhancing their photon absorption and photovoltaic performance. In a systematic survey of 1 -μm-thick μc-Si:H cells, the best performance is obtained with a period of 1.5 μm and an aspect ratio of 0.20–0.25 with a high current density exceeding 26 mA/cm2 and a marked efficiency of 10.1%. These results demonstrate the high potential of periodic textures or surface gratings for improving the conversion efficiency of thin-film silicon solar cells.

Effects of ion bombardment on microcrystalline silicon growth by inductively coupled plasma assistant magnetron sputtering

Hydrogenated microcrystalline silicon (μc-Si:H) thin films were deposited by inductively coupled plasma assistant magnetron sputtering (ICP-MS) in an Ar-H2 gas mixture. The role of ion bombardment in the growth of μc-Si:H films was studied with increasing negative bias voltages on the substrate holder from 0 to −100 V. Raman scattering, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM) were performed to investigate the microstructure changes of deposited Si films. Raman scattering showed that the high energy ion bombardment resulted in crystalline degradation of Si films. The XRD results showed the decrease and even elimination of preferential growth orientation of crystalline Si films with ion bombardment energy increase. The SiH bonding configuration changes and the increase of bonded hydrogen concentration were determined with the analysis of FTIR spectra. Furthermore, the dramatic evolution of cross-sectional morphology of Si thin films was detected by TEM observation.

Structural analyses of seeded thin film microcrystalline silicon solar cell

ABSTRACTThis contribution investigates the effect of seeding the growth of thin film microcrystalline silicon (µc‐Si : H) deposited by radio frequency plasma‐enhanced chemical vapor deposition on the material properties of µc‐Si : H film and the device performance of p‐i‐n and n‐i‐p µc‐Si : H solar cells. By means of Raman measurement, x‐ray diffraction (XRD) and transmission electron microscopy (TEM), we investigate the structure of seeded µc‐Si : H. In particular, the effect of seed layers on the crystallinity development is investigated. Measurements of the depth profile of the crystalline mass fraction using Raman spectroscopy show that seed layers lead to a more rapid and uniform crystallinity development in growth direction. The amorphous incubation layer is suppressed and crystallization begins directly from onset of film growth without evolving through the intermediate growth phases. From TEM analyses, we observe that crystal sizes are not affected by seed layers. Horizontal cracks are however observed to dominate the early growth of µc‐Si : H in p‐i‐n solar cell and this is reduced upon seeding. For the n‐i‐p cells, these cracks are not affected by seeding. XRD results also indicate that the use of seed layers does not affect the crystal sizes but affects the direction of preferential orientation. Solar cell external parameters show that seeding of p‐i‐n solar cells leads mainly to increase in short‐circuit current density, Jsc with a slight drop in open‐circuit voltage, Voc. For the n‐i‐p cells, a reverse effect is observed. In this case, the Voc increases and the Jsc decreases. Copyright © 2012 John Wiley & Sons, Ltd.

Tailored Voltage Waveform Deposition of Microcrystalline Silicon Thin Films from Hydrogen-Diluted Silane and Silicon Tetrafluoride: Optoelectronic Properties of Films

Tailored Voltage Waveform Deposition of Microcrystalline Silicon Thin Films from Hydrogen-Diluted Silane and Silicon Tetrafluoride: Optoelectronic Properties of Films

Segmented Electrode Plasma Source with Anti-Phase Discharge

A new plasma source concept, consisting of segmented electrodes and an economical 13.56 MHz power generator, has been developed for the plasma-enhanced chemical vapor deposition (PECVD) of microcrystalline silicon thin films on large-area substrates. Using a plurality of rectangular column-type electrodes fed with RF (13.56 MHz) at various relative phases, a two-stage plasma, which consists of an upper and lower plasma, was generated. In the case of supplying the anti-phase RF, we could control the ion incident energy on a substrate to be lower, so as to keep the high crystallinity and high deposition rate during the film growth. The agreement between the observed light emission profiles from Ar or Ar/H2 plasma and the numerically calculated ones was good, confirming the accuracy of the simulation model, and we performed the thin-film growth of microcrystal silicon with this new plasma source and H2/SiH4 gas. Consequently, we could achieve a fast deposition rate of more than 1.5 nm/s with good crystallinity over 60%.

Tailored Voltage Waveform Deposition of Microcrystalline Silicon Thin Films from Hydrogen-Diluted Silane and Silicon Tetrafluoride: Optoelectronic Properties of Films

The use of tailored voltage waveforms (TVW's) to excite a plasma for the deposition of thin films of hydrogenated microcrystalline silicon (µc-Si:H) has been shown to be an effective technique to decouple mean ion bombardment energy (IBE) from injected power. In this work, we examine the changes in material properties controlled by this technique through Raman scattering and spectroscopic ellipsometry for films deposited from H2-diluted SiH4, and we examine the electrical properties of such films using temperature dependent conductivity. As the laboratory-scale deposition system used had neither a load lock nor an oxygen filter in the H2 line, accidental O-doping was observed for the µc-Si:H films. We investigated suppression of this doping by adding varying amounts of SiF4, and using an SiF4/Ar pre-etch step to clean the reactor. This technique is shown to be effective in decreasing the accidental doping of the films, and intrinsic µc-Si:H films are produced with an activation energy of up to 0.55 eV. As well, an important difference in the amorphous-to-microcrystalline transition is observed once SiF4 is included in the gas mixture.

Laser Marking on Microcrystalline Silicon Film

We present a compact dot marker using a CW laser on a microcrystalline silicon (Si) thin film. A laser annealing shows a continuous crystallization transformation from nano to a large domain (> 200 nm) of Si nanocrystals. This microscale patterning is quite useful since we can manipulate a two-dimentional (2-D) process of Si structural forms for better and efficient thin-film transistor (TFT) devices as well as for photovoltaic application with uniform electron mobility. A Raman scattering microscope is adopted to draw a 2-D mapping of crystal Si film with the intensity of optical-phonon mode at 520 cm(-1). At a 300-nm spatial resolution, the position resolved the Raman scattering spectra measurements carried out to observe distribution of various Si species (e.g., large crystalline, polycrystalline and amorphous phase). The population of polycrystalline (poly-Si) species in the thin film can be analyzed with the frequency shift (delta omega) from the optical-phonon line since poly-Si distribution varies widely with conditions, such as an irradiated-laser power. Solid-phase crystallization with CW laser irradiation improves conductivity of poly-Si with micropatterning to develop the potential of the device application.

Rapid Reversible Degradation of Silicon Thin Films by a Treatment in Water

Metastability effects in amorphous and microcrystalline silicon thin films induced by exposure to atmospheric gases and water are investigated. A simple procedure is described which allows studying such effects in a reproducible and reliable manner on a short time scale. The method is applied to thin film silicon materials with different structure composition ranging from amorphous to highly crystalline. It is shown that the materials can be brought back into a well defined state even after pro-longed and repeated degradation cycles.

Effect of the very high frequency plasma with a balanced power feeding on silicon film deposition

Characteristics of very high frequency plasma produced by a balanced power feeding method were experimentally and numerically investigated by comparison with those of a conventional power feeding method. The measurement results of the plasma parameters in H2 or SiH4/H2 plasma showed that the electron density was a few times higher and the gas pressure at which the plasma was stably sustained was much higher than that in the conventional power feeding. In addition, the electron temperature and the sheath potential near a substrate were lower by about 30%. The numerical simulation in accordance with the experimental condition suggested that the completely floating electrodes cause the decrease of the sheath potential. These characteristics of the plasma parameters seem to be effective for the decrease of ion bombardment damage. As a result of the microcrystalline silicon thin film fabrication, it was showed that the defect density in the balanced power feeding was lower.

Effects of showerhead hole structure on the deposition of hydrogenated microcrystalline silicon thin films by vhf PECVD

The authors present the characteristics of hydrogenated microcrystalline silicon (μc-Si:H) thin films deposited from an SiH4/H2 in 40 MHz plasma enhanced chemical vapor deposition system equipped with a multihole-array showerhead. The effects of a hole-array structure are analyzed in terms of deposition rate, crystallinity, and electrical conductivity of the μc-Si:H film. The effects of process parameters, such as SiH4 concentration, radio frequency power, and total gas flow rate are also investigated. The deposition rate of a multihole-array electrode is generally lower compared with that of the flat electrode. However, a higher deposition rate is found in the deep and dense hole region when the hole shape and density are varied on the electrode. The crystallinity and conductivity are almost not affected by hole dimension and density. It is demonstrated that the multihole-array electrode can be a useful tool for uniformity improvement in a large-sized very high frequency capacitively couple plasma μc-Si:H deposition system.

Deposition and Characterization of High-Efficiency Silicon Thin-Film Solar Cells by HF-PECVD and OES Technology

The optical emission spectrometer (OES) is an effective experimental tool for monitoring plasma states and the composition of gases during the growth of silicon thin films by plasma-enhanced chemical vapor deposition. In this paper, hydrogenated amorphous silicon (a-Si) (a-Si:H) and microcrystalline silicon (μc-Si) thin films have been deposited in a parallel-plate radio frequency (RF) plasma reactor using silane and hydrogen gas mixtures. The plasma emission atmosphere was recorded using an OES system during the growth of the Si thin films. The plasma was simultaneously analyzed during the process using an OES method to study the correlation between growth rate and microstructure of the films. In the deposition, the emitted species (SiH*, Si*, and H*) were analyzed. The OES analysis supported a chemisorption-based deposition model of the growth mechanism. The effects of RF power, electron-to-substrate distance, and H2 dilution of the emission intensities of excited SiH, Si, and H on the growth rate and microstructures of the film were studied. Finally, single-junction a-Si:H and μc-Si solar cells were obtained with initial aperture area efficiencies of 9.71% and 6.36%, respectively. A tandem a-Si/μc-Si cell was also realized with an efficiency of 12.3%.

Effects of growth temperature on μc-Si:H films prepared by plasma assistant magnetron sputtering

Hydrogenated microcrystalline silicon thin films (μc-Si:H) were deposited by plasma assistant magnetron sputtering in Ar-H2 gas mixture. The effects of growth temperature from 150 to 450 °C on properties of deposited Si films were investigated at two different hydrogen diluted gases with [H2]/([Ar]+[H2]) of 10% and 50% at 3 Pa. The crystallinity of Si films examined by Raman scattering exhibited higher degradation by lowering growth temperature from 250 to 150 °C in low hydrogen diluted gas of 10% than that in high hydrogen diluted gas of 50%. The IR absorption band around 845 and 890 cm−1 as well as calculated concentration of bonded hydrogen showed more obvious decrease of samples deposited in low hydrogen diluted gas of 10% than that in high hydrogen diluted gas of 50%. The optical band gasps of both groups of samples measured by ultraviolet-visible optical absorption were decreased with increasing temperature in both gas conditions.

The microstructure and optical properties of p-type microcrystalline silicon thin films characterized by ex-situ spectroscopic ellipsometry

Ex-situ spectroscopy ellipsometry (SE) was applied on the ~25nm thick p-type hydrogenated microcrystalline silicon (p-μc-Si:H) thin films deposited by very high frequency plasma enhanced chemical vapor deposition. Several optical models were built and compared with each other for ex-situ SE data analysis considering both the oxide surface and the heterogeneous bulk condition to reveal the material microstructure and obtain optical function. The SE results imply that the p-μc-Si:H has both an oxide rich surface and the bulk with a flexible amorphous phase, the bandgap of which depends on the gas doping ratio (DR) of B2H6/SiH4. At the same time, we were able to distinguish p-μc-Si:H materials in crystalline volume fraction by ex-situ SE as confirmed by Raman scattering. In this way, we showed the effect of DR on μc-Si:H thin films in microstructural and optical properties.

Al2O3 antireflection layer between glass and transparent conducting oxide for enhanced light trapping in microcrystalline silicon thin film solar cells

An Al2O3 antireflection layer was placed between a glass substrate and a transparent conducting oxide layer in order to decrease optical reflection in microcrystalline silicon (μc-Si:H) p–i–n solar cells. Optical simulations showed that reflections were decreased by Al2O3 thin films, these reflections were found to be at a minimum when a 40nm thick Al2O3 layer was used. Experimental results demonstrated that the measured reflectance of μc-Si:H solar cells was decreased by employing the proposed 40nm Al2O3 in all wavelength regions and the quantum efficiency was also increased. The short-circuit current was increased from 22.7 to 23.5mA/cm2 without sacrificing open circuit voltage or fill factor. The average efficiency of devices was improved from 6.02% to 6.32% by introducing 40nm Al2O3 antireflection layer.