In-situ Raman spectroscopy used to study and control the initial growth phase of microcrystalline absorber layers for thin-film silicon solar cells

The continuous deposition of microcrystalline silicon has been monitored with in-situ Raman spectroscopy. The process and measurement settings were chosen such that one spectrum was taken during approximately 9 nm of layer growth. This allows observing the crystallinity in the initial growth phase of microcrystalline silicon absorber layers. The influence of different p-doped seed layers has been studied. Under constant deposition conditions, an initial decrease in crystallinity was observed over the first tens of nanometers. By profiling the process gas flows during the initial phase it was possible to reduce the amount of amorphous material that was detected during the initial phase of deposition.     

Influence of crystalline volume fraction on the performance of high mobility microcrystalline silicon thin-film transistors

The influence of the crystalline volume fraction of hydrogenated microcrystalline silicon on the device performance of thin-film transistors fabricated at temperatures below 200 °C was investigated. Transistors employing microcrystalline silicon channel material prepared close to the transition to amorphous growth regime exhibit the highest charge carrier mobilities exceeding 50 cm²/V s. The device parameters like the charge carrier mobility, the threshold voltage and the subthreshold slope will be discussed with respect to the crystalline volume fraction of the intrinsic microcrystalline silicon material.     

A simple quality factor for characterization of thin silicon films

Four series of intrinsic thin Si films were prepared by plasma enhanced chemical vapor deposition at standard and high growth rate conditions. We suggest a simple ‘μc-Si:H layer quality factor’ based on the ratio of subgap optical absorption coefficient values: α(1.4 eV)/α(1 eV). This ratio minimizes the light scattering effects for rough films and can serve as a reliable detection of the amorphous/microcrystalline structure transition and also as a figure of merit for the microcrystalline layer. The quality factor is evaluated for series of our samples with well known structure and also compared with samples from other laboratories with different deposition and measurement techniques.     

Properties of n-type hydrogenated nanocrystalline cubic silicon carbide films deposited by VHF-PECVD at a low substrate temperature

n-Type hydrogenated nanocrystalline cubic silicon carbide (nc-3C–SiC:H) films have been deposited by very high-frequency plasma-enhanced chemical vapor deposition at a low substrate temperature of about 360 °C to apply this material to the window layer of heterojunction crystalline silicon (HJ-c-Si) solar cells. We investigated the effect of in situ doping on deposition rate, crystalline volume fraction and dark conductivity to optimize properties of the material. We also fabricated HJ-c-Si solar cells with a n-type nc-3C–SiC:H window layer. The solar cells shows high internal quantum efficiency of 0.90 at a wavelength of 400 nm, indicating that n-type nc-3C–SiC:H deposited by VHF-PECVD is a promising candidate of the window layer of HJ-c-Si solar cells.     

Determination of Raman emission cross-section ratio in hydrogenated microcrystalline silicon

The determination of the crystalline volume fraction from the Raman spectra of microcrystalline silicon involves the knowledge of a material parameter called the Raman emission cross-section ratio y. This value is still debated in the literature. In the present work, the determination of y has been carried out on the basis of quantitative analysis of medium-resolution transmission electron microscopy (TEM) micrographs performed on one layer deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) close to the amorphous/microcrystalline transition. Subsequent comparison of these data with the crystallinity as evaluated from measured Raman spectra yields a surprisingly high value of y = 1.7. This result is discussed in relation to previously published values (that range from 0.1 to 0.9).     

A narrow process window for the preparation of polytypes of microcrystalline silicon carbide thin films by hot-wire CVD method

Effects of deposition conditions on the structure of microcrystalline silicon carbide (μc-SiC) films prepared by hot-wire chemical vapor deposition (hot-wire CVD) method have been investigated. It is found from X-ray diffraction patterns of the film that a diffraction peak from crystallites from hexagonal polytypes of SiC is observed in addition to those of 3 C-SiC crystallites. This result is obtained in the film under a narrow deposition conditions of SiH₃CH₃ gas pressure of 8 Pa, the H₂ gas pressure of 80–300 Pa and the total gas pressure of 40–300 Pa under fixed substrate and filament temperatures employed in this study. Furthermore, the grain size of hexagonal crystallites (about 20 nm) on c-Si substrates becomes larger than that of 3 C-SiC crystallites (about 10 nm) for the films deposited under the total gas pressure of 36–88 Pa. The fact that microcrystalline hexagonal SiC can be deposited under limited deposition conditions could be interpreted in the context of a result for c-SiC polytypes prepared by thermal CVD method.     

Low concentrator hetero-junction microcrystalline silicon solar cells

Multi-junction silicon-based thin-film solar cells are attractive materials for further cost-reduction and high efficiency. Meanwhile, it is also well considered that a concentrator solar cell is another alternative approach to enhance the conversion efficiency. In concentrator solar cells, the photocurrent linearly increases with the concentration ratio of incident light. At the same time, the open-circuit voltage (V_oc) of solar cells increases logarithmically with the photocurrent. This leads to an increase in efficiency with increasing sunlight intensity.We proposed a novel hetero-junction structure microcrystalline silicon (μc-Si:H) solar cell structure using wide-gap microcrystalline silicon oxide (μc-Si_1 ? xO_x:H) as p-layer and it has some advantages over conventional homo-junction μc-Si:H solar cells under low concentrations. It was observed that wide-gap doped layers can reduce carrier recombination rate especially in p-layer and at the p/i interface and V_oc enhancement with increasing light intensity improves as the band gap of p-layer is increased. Our best solar cell has efficiencies of 9.2% at 1 sun and 10.4% at 11.8 suns. We also investigated the degradation behavior of hetero-junction μc-Si:H solar cells. The degradation in efficiency for this type of solar cell was less than 6%. Therefore, hetero-junction μc-Si:H solar cell is the promising alternative for low-light concentration.     

Enhanced spectral response by silicon nitride index matching layer in amorphous silicon thin-film solar cells

We employed the low temperature hydrogenated amorphous silicon nitride (a-SiN_x:H) prepared by plasma-enhanced chemical vapor deposition as a refractive index (n) matching layers in a silicon-based thin-film solar cell between glass (n = 1.5) and the transparent conducting oxide (n = 2). By varying the stoichiometry, refractive index and thickness of the a-SiN_x:H layers, we enhanced the spectral response and efficiency of the hydrogenated amorphous silicon thin-film solar cells. The refractive index of a-SiN_x:H was reduced from 2.32 to 1.78. Optimizing the a-SiN_x:H thickness to 80 nm increased the J_SC from 8.3 to 9.8 mA/cm² and the corresponding cell efficiency increased from 4.5 to 5.3%, as compared to the cell without the a-SiN_x:H index-matching layer on planar substrate. The a-SiN_x:H layers with graded refractive indices were effective for enhancing the cell performance.     

Microstructures of high-growth-rate (up to 8.3 nm/s) microcrystalline silicon photovoltaic layers and their influence on the photovoltaic performance of thin-film solar cells

Microstructures of microcrystalline silicon (μc-Si) deposited at a high-growth-rate have been investigated in order to apply to the photovoltaic i-layer. μc-Si films were prepared by very-high-frequency (100 MHz) plasma-enhanced chemical vapor deposition at 180 °C. High growth rates of 3.3–8.3 nm/s have been achieved utilizing high deposition pressures up to 24 Torr and large input powers. Applying μc-Si to n–i–p junction solar cells, as the optimum result in this experimental series, a conversion efficiency of 6.30% (J_SC: 22.1 mA/cm², V_OC: 0.470 V, and FF: 60.7%) has been achieved employing the i-layer deposited at 8.1 nm/s. Raman scattering and X-ray diffraction measurements revealed the crystalline volume fraction of around 50% with the (2 2 0) crystallographic preferential orientation, respectively. The cross-sectional transmission electron microscope image shows densely columnar structure grown directly on the underlying n-layer. These structural features are basically in good agreement those of low-growth-rate μc-Si used for a high efficiency solar cell as previously reported, implying advantages of the use of high pressures with regard to providing the photovoltaic i-layers. Finally, the implication is discussed from the photovoltaic performance as a function of the crystalline volume fraction of i-layer, and current problems in improving the photovoltaic performance are extracted.     

Argon-plasma-induced growth of crystalline grains in microcrystalline silicon: Formation mechanism of grains

A thick (∼300 nm) microcrystalline silicon (μc-Si:H) film with a low crystalline volume fraction (∼24%) and a columnar grain size of about 100 nm was exposed to an argon plasma at a substrate temperature of 220 °C after deposition. It is shown that argon plasma treatment significantly enhances film-crystallinity throughout the μc-Si:H layer: over a factor of 2 in crystalline fraction and by a factor of 3 in columnar grain size after a 90-min argon treatment. Based on these experimental results, it is proposed that crystallization of μc-Si:H is likely mediated by the energy transferred from energetic argon atoms.     

Low temperature plasma deposition of silicon thin films: From amorphous to crystalline

We report on the epitaxial growth of crystalline silicon films on (100) oriented crystalline silicon substrates by standard plasma enhanced chemical vapor deposition at 175 °C. Such unexpected epitaxial growth is discussed in the context of deposition processes of silicon thin films, based on silicon radicals and nanocrystals. Our results are supported by previous studies on plasma synthesis of silicon nanocrystals and point toward silicon nanocrystals being the most plausible building blocks for such epitaxial growth. The results lay the basis of a new approach for the obtaining of crystalline silicon thin films and open the path for transferring those epitaxial layers from c-Si wafers to low cost foreign substrates.     

Influence of deposition conditions for bottom cell on micromorph tandem device performance

We have realized micromorph tandem solar cells on Asahi U-type TCO-covered glass substrates. The intrinsic layers of both amorphous top cell and microcrystalline bottom cell are grown by very high frequency plasma enhanced chemical vapour deposition (VHF-PECVD) at 100 MHz at low substrate temperature (150 °C). For the bottom cell different growth regimes have been explored by changing both chamber pressure and plasma power. The effect of the structural composition of the microcrystalline absorber layer on the electrical parameters of the device has been investigated. High short circuit current density and constant FF in a wide silane concentration range are obtained when using large power to pressure ratio (0.5 W/Pa). However, low open circuit voltage is generally found in this regime. The largest V_OC values are found at 67 Pa and power to pressure ratio of 0.3 W/Pa, where the highest efficiency (11.1%) is reached. An evaluation of device stability has been done by exposing the tandem solar cells to white light (AM 1.5-like spectrum) for 200 h.     

Heat treatment of amorphous silicon p-i-n solar cells with high-pressure H2O vapor

We report improvement in characteristics of hydrogenated amorphous silicon (a-Si:H ) p-i-n structured solar cells by high-pressure H₂O vapor heat treatment. a-Si:H p-i-n solar cells were formed on glass substrates coated with textured SnO₂ layer. P-, i-, and n-type a-Si:H layers were subsequently formed by plasma enhanced chemical vapor deposition. Finally an indium-tin-oxide layer was coated on the n-type a-Si:H surface. Heat treatment at 210 °C with 2 × 10⁵ Pa H₂O vapor for 1 h was applied to the a-Si:H p-i-n solar cells. Electrical characteristics were measured when samples were kept in dark and illuminated with light of AM 1.5 at 100 mW/cm². The heat treatment with H₂O vapor increased fill factor (FF) and the conversion efficiency from 0.54 and 7.7% (initial) to 0.57 and 8.4%, respectively. Marked improvement in solar cell characteristics was also observed in the case of a poor a-Si:H p-i-n solar cell. FF and the conversion efficiency were increased from 0.29 and 3.2% (initial) to 0.56 and 7.7%, respectively.     

Improving the performance of amorphous and crystalline silicon heterojunction solar cells by monitoring surface passivation

The influence of thermal annealing on the crystalline silicon surface passivating properties of selected amorphous silicon containing layer stacks (including intrinsic and doped films), as well as the correlation with silicon heterojunction solar cell performance has been investigated. All samples have been isochronally annealed for 1 h in an N₂ ambient at temperatures between 150 °C and 300 °C in incremental steps of 15 °C. For intrinsic films and intrinsic/n-type stacks, an improvement in passivation quality is observed up to 255 °C and 270 °C, respectively, and a deterioration at higher temperatures. For intrinsic/n-type a-Si:H layer stacks, a maximum minority carrier lifetime of 13.3 ms at an injection level of 10¹⁵ cm^? 3 has been measured. In contrast, for intrinsic/p-type a-Si:H layer stacks, a deterioration in passivation is observed upon annealing over the whole temperature range. Comparing the lifetime values and trends for the different layer stacks to the performance of the corresponding cells, it is inferred that the intrinsic/p-layer stack is limiting device performance. Furthermore, thermal annealing of p-type layers should be avoided entirely. We therefore propose an adapted processing sequence, leading to a substantial improvement in efficiency to 16.7%, well above the efficiency of 15.8% obtained with the ‘standard’ processing sequence.     

Optoelectronic properties of hot-wire silicon layers deposited at 100 °C

Hot-wire chemical vapor deposition is employed for the deposition of amorphous and microcrystalline silicon layers at substrate temperature kept below 100 °C with the aid of active cooling of the substrate holder. The hydrogen dilution is varied in order to investigate films at the amorphous-to-microcrystalline transition. While the amorphous layers can be produced with a reasonably low defect density as deduced from subgap optical absorption spectra and a good photosensitivity, the microcrystalline layers are of a lesser quality, most probably due to a decrease of crystallinity during the film growth. In the amorphous growth regime, the Urbach energy values decrease with increasing hydrogen dilution, reaching a minimum of 67 meV just before the microcrystalline threshold. By varying the total gas pressure, the growth rate of the films is changed. The lowest deposition rate of this study (0.16 nm/s) produced the amorphous sample with the highest photoresponse (1 × 10⁶).     

Electron and hole transport in microcrystalline silicon solar cells studied by time-of-flight photocurrent spectroscopy

A photocurrent time-of-flight study of carrier transport in microcrystalline silicon pin diodes prepared over a range of crystallinities is presented. Electron and hole drift mobilities at a crystalline volume fraction >0.35 are typically 3.8 and 1.3 cm²/(V s) respectively at 300 K and a thickness to electric field ratio of 1.8 × 10⁻⁷ cm²/V. A factor of five enhancement in hole mobility over amorphous silicon persists at a crystalline volume fraction as low as 0.1. Current decays are dispersive and mobilities are thermally activated, although detailed field-dependence is still under investigation. Evidence for a sharp fall in the density of states at 0.13 eV above the valence band edge is presented. Similarities in behaviour with certain amorphous and polymorphous silicon samples are identified.     

Variation of the defect density in a-Si:H and μc-Si:H based solar cells with 2 MeV electron bombardment

Improvement of the performance of solar cells based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon requires understanding of the role of the deep defects – dangling bonds – in the bulk of the intrinsic a-Si:H or μc-Si:H absorber layers. A straightforward way to understand how these defects may affect the performance of the cells is to investigate changes in the device performance upon variation in the defect density.In the present work solar cells with a-Si:H and μc-Si:H absorber layers were exposed to 2 MeV electron bombardment. The performance of the cells after various bombardment doses and annealing steps was evaluated in view of the changes in the defect density of intrinsic layers, measured with ESR on nominally identical absorber layers irradiated in parallel with the cells.The defect density was varied over a range of 2 orders of magnitude. In the solar cells a strong degradation of performance is observed upon irradiation with the biggest effect on the short circuit current density J_SC for both types of absorber layers. In most cases both V_OC and J_SC recover after the final annealing step (at 160 °C) for both types of cells.     

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.     

Toward the fast deposition of highly crystallized microcrystalline silicon films with low defect density for Si thin-film solar cells

In this study, employing a high-density, low-temperature SiH₄–H₂ mixture microwave plasma, we investigate the influence of source gas supply configuration on deposition rate and structural properties of microcrystalline silicon (μc-Si) films, and demonstrate the plasma parameters for fast deposition of highly crystallized μc-Si films with low defect density. A fast deposition rate of 65 Å/s has been achieved for a SiH₄ concentration of 67% diluted in H₂ with a high Raman crystallinity of X_c > 65% and a low defect density of (1–2) × 10¹⁶ cm⁻³ by adjusting source gas supply configuration and plasma conditions. A sufficient supply of deposition precursors, such as SiH₃, as well as atomic hydrogen H on film growing surface is effective for the high-rate synthesis of highly crystallized μc-Si films, for the reduction in defect density, and for the improvement in film homogeneity and compactability. A preliminary result of p–i–n structure μc-Si thin-film solar cells using the resulting μc-Si films as an intrinsic absorption layer is presented.     

Undoped and arsenic-doped low temperature (∼165 °C) microcrystalline silicon for electronic devices process

Microcrystalline silicon films are deposited at 165 °C by plasma enhanced chemical vapor deposition (PECVD) from silane, highly diluted in hydrogen–argon mixtures. Ar addition during the deposition allows to increase the crystallinity from 24% to 58% for 20 nm thick films. The final crystallinity for 350 nm thick films reaches 72% with an increase in the grain size. A further increase, still 80%, is provided by substrate pre-treatment using hydrogen plasma before the deposition process. Arsenic doped μc-Si films, deposited on previous optimized (5 W power and 1.33 mbar pressure) undoped films without stopping the plasma between the deposition of both layers, show high electrical conductivity up to 20 S cm⁻¹.