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.     

Preparation of microcrystalline silicon solar cells on microcrystalline silicon carbide window layers grown with HWCVD at low temperature

N-type microcrystalline silicon carbide layers prepared by hot-wire chemical vapor deposition were used as window layers for microcrystalline silicon n–i–p solar cells. The microcrystalline silicon intrinsic and p-layers of the solar cells were prepared with plasma-enhanced chemical vapor deposition at a very high frequency. Amorphous silicon incubation layers were observed at the initial stages of the growth of the microcrystalline silicon intrinsic layer under conditions close to the transition from microcrystalline to amorphous silicon growth. ‘Seed layers’ were developed to improve the nucleation and growth of microcrystalline silicon on the microcrystalline silicon carbide layers. Raman scattering measurement demonstrates that an incorporation of a ‘seed layer’ can drastically increase the crystalline volume fraction of the total absorber layer. Accordingly, the solar cell performance is improved. The correlation between the cell performance and the structural property of the absorber layer is discussed. By optimizing the deposition process, a high short-circuit current density of 26.7 mA/cm² was achieved with an absorber layer thickness of 1 μm, which led to a cell efficiency of 9.2%.     

Infrared semiconductor laser irradiation used for crystallization of silicon thin films

We report the rapid thermal crystallization of silicon films using infrared semiconductor laser. Carbon films were used on silicon films to absorb the laser light. Uniform crystalline regions were achieved by a line shape laser beam with a length of 20 μm. Polycrystalline silicon thin film transistors were fabricated in crystallized regions. The effective electron carrier mobility and threshold voltage were achieved to be 130 cm²/Vs and 0.4 V, respectively, when the crystalline volume ratio of the silicon films was 0.95.     

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).     

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.     

Characteristics of p-type nanocrystalline silicon thin films developed for window layer of solar cells

Different rf-power and chamber pressures have been used to deposit boron doped hydrogenated silicon films by the PECVD method. The optoelectronic and structural properties of the silicon films have been investigated. With the increase of power and pressure the crystallinity of the films increases while the absorption decreases. As a very thin p-layer is needed in p–i–n thin film solar cells the variation of properties with film thickness has been studied. The fraction of crystallinity and thus dark conductivity vary also with the thickness of the film. Conductivity as high as 2.46 S cm⁻¹ has been achieved for 400 Å thin film while for 3000 Å thick film it is 21 S cm⁻¹. Characterization of these films by XRD, Raman Spectroscopy, TEM and SEM indicate that the grain size, crystalline volume fraction as well as the surface morphology of p-layers depend on the deposition conditions as well as on the thickness of the film. Optical band gap varies from 2.19 eV to 2.63 eV. The thin p-type crystalline silicon film with high conductivity and wide band gap prepared under high power and pressure is suitable for application as window layer for Silicon thin film solar cells.     

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.     

Properties of thin-film silicon solar cells at very high irradiance

The focussed beam of a low-power helium–neon laser is used to study accelerated light-induced degradation (Staebler–Wronski effect) and high steady-state photocarrier generation rates in amorphous and microcrystalline silicon thin-film solar cells, at up to 13 MW m^? 2 irradiance. Even at these high power densities, COMSOL® simulations indicate that heat diffusion into the substrate, aided by spreading conduction via the Ag back-contact, restricts the temperature rise to less than 14 °C. Short-circuit current may be measured directly, and the I–V characteristic estimated by taking into account shunting by the inactive part of the cell. The improved resistance to degradation of microcrystalline silicon cells is shown to persist to high irradiance. Computer simulations of an amorphous silicon solar cell are presented that are consistent with measured un-degraded and degraded properties, and offer insight into prevailing defect creation processes and carrier recombination mechanisms.     

Metastable effects in silicon thin films: Atmospheric adsorption and light-induced degradation

The effects of exposure to atmosphere (ageing) and light-soaking on coplanar dark- and photo-conductivity of silicon films of varying crystallinity are examined. Dark conductivity generally increases on ageing in films with significant amorphous fraction and decreases in largely crystalline films, and may be reversed by annealing under vacuum at 130 °C consistent with adsorption and desorption of atmospheric components. Thinner films are more strongly affected by ageing. Boron doping appears to compensate charge introduced by ageing, though there are disagreements in detail. In comparison with ageing, moderate light-soaking affects dark conductivity in transitional microcrystalline silicon films only slightly. Both processes change the majority carrier mu–tau product in line with shifts in Fermi level position.     

Spectroscopic ellipsometry study of nickel induced crystallization of a-Si

The aim of this work is to present a spectroscopic ellipsometry study focused on the annealing time effect on nickel metal induced crystallization of amorphous silicon thin films. For this purpose silicon layers with 80 and 125 nm were used on the top of which a 0.5 nm Ni thick layer was deposited. The ellipsometry simulation using a Bruggemann Effective Medium Approximation shows that films with 80 nm reach a crystalline fraction of 72% after 1 h annealing, appearing to be full crystallized after 2 h. No significant structural improvement is detected for longer annealing times. On the 125 nm samples the crystalline volume fraction after 1 h is only around 7%, requiring 5 h to get a similar crystalline fraction than the one achieved with the thinner film. This means that the time required for full crystallization will be strongly determined by the Si layer thickness. Using a new fitting approach the Ni content within the films was also determined by SE and related to the silicon film thickness.     

Crystallinity of the mixed phase silicon thin films by Raman spectroscopy

Raman spectra of the mixed phase silicon films were studied for a sample with transition from amorphous to fully microcrystalline structure using four excitation wavelengths (325, 514.5, 632.8 and 785 nm). Factor analysis showed the presence of two and only two spectrally independent components in the spectra within the range from 250 to 750 cm^?1 for all four excitation wavelengths. The 785 nm excitation was found optimal for crystallinity evaluation and by comparison with surface crystallinity obtained by atomic force microscopy, we have estimated the ratio of integrated Raman cross-sections of microcrystalline and amorphous silicon at this wavelength as y = 0.88 ± 0.05.     

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.     

Optical properties of microcrystalline 3C–SiC:H films measured by resonant photothermal bending spectroscopy

Optical absorption coefficient spectra of hydrogenated microcrystalline cubic silicon carbide (μc-3C–SiC:H) films prepared by Hot-Wire CVD method have been estimated for the first time by resonant photothermal bending spectroscopy (resonant-PBS). The optical bandgap energy and its temperature coefficient of μc-3C–SiC:H film is found to be about 2.2 eV and 2.3 × 10⁻⁴ eV K⁻¹, respectively. The absorption coefficient spectra of localized states, which are related to grain boundaries, do not change by exposure of air and thermal annealing. The localized state of μc-3C–SiC:H has different properties for impurity incorporation compared with that of hydrogenated microcrystalline silicon (μc-Si:H) film.     

Excimer laser crystallization of microcrystalline silicon for TFTs on flexible substrate

Microcrystalline silicon (μc-Si) films have been deposited on PDMS as well as on PEN substrate. Excimer laser annealing was used to improve the crystalline structure and so to obtain high mobility TFTs. The effect of the laser annealing on the crystalline structure of silicon films is studied using different characterization techniques and discussed. Mobility values of 60 cm²/V s with PDMS and 46 cm²/V s with PEN are obtained.     

Characterization of mixed phase silicon by Raman spectroscopy

We have examined the common methods for determination of the crystallinity of mixed phase silicon thin films from the TO–LO phonon band in Raman spectra. Spectra are decomposed into contributions of amorphous and crystalline phase and empirical formulas are used to obtain crystallinity either from the integral intensities (peak areas) or from magnitudes (peak maxima). Crystallinity values obtained from Raman spectra excited by Ar⁺ laser green line (514.5 nm) for a special sample with a profile of structure from amorphous to fully microcrystalline were compared with surface crystallinity obtained independently from atomic force microscopy (AFM). Analysis of the Raman collection depth in material composed of grains with absorption depth 1000 nm in an amorphous matrix (absorption depth 100 nm), was used to explain reasons for systematic difference between surface and Raman crystallinities. Recommendations are given for obtaining consistent results.     

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.     

Boron-doped hydrogenated microcrystalline silicon oxide (μc-SiOx:H) for application in thin-film silicon solar cells

We report on the development of p-type μc-SiO_x:H material, in particular the relationship between the deposition parameters and the material properties like band gap, electrical conductivity, and crystalline volume fraction. The material was deposited from gas mixtures of silane, carbon dioxide and hydrogen by RF-PECVD. The gas flows were varied systematically to evaluate their influence on the material properties. An increase of the oxygen content in the material disturbs the crystalline growth. This can be counteracted by appropriate hydrogen dilutions. Materials with a combination of reasonably high conductivity of 4 × 10^? 6 S/cm at a high optical band gap E₀₄ of 2.56 eV and a refractive index of 1.95 are obtained. Applied in single junction μc-Si:H pin solar cells the improved properties of the μc-SiO_x:H p-layers are reflected in higher quantum efficiency in the short wavelength range by 10% compare to cells without adding CO₂ during p-layer deposition.     

The role of ion-bulk interactions during high rate deposition of microcrystalline silicon by means of the multi-hole-cathode VHF plasma

The high rate deposition of microcrystalline silicon (μc-Si:H) by means of the novel multi-hole-cathode very high frequency (MHC-VHF) plasma technique has been studied in the high-pressure depletion region (9.3 Torr). A distinct relationship between vacancy incorporation, the crystalline volume fraction and a qualitative measurement of the energy of the ions bombarding the substrate has been found. The observed relation is explained with the help of an ion-phase-diagram: we claim that the most energetic ions, containing at least one silicon atom, are responsible for the local amorphization of the μc-Si:H films via the ion induced Si bulk displacement mechanism.     

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.     

Nanocrystalline silicon TFT process using silane diluted in argon–hydrogen mixtures

We have investigated the effect of Ar dilution on the deposition process of intrinsic nc-Si:H (hydrogenated nanocrystalline silicon) thin films used as active layers of top-gate TFTs, in order to improve the TFTs performances. The nc-Si:H films were deposited by plasma enhanced chemical vapor deposition (PECVD) at low temperature (165 °C) and the related TFTs were fabricated with a maximum process temperature of 200 °C. During the nc-Si:H films deposition, the SiH₄ fraction and the total flow of the diluting gases Ar + H₂ mixture was kept constant, H₂ being substituted by Ar. We have pointed out the active role played by the metastable states of excited Ar atoms in both the dissociation of SiH₄ and H₂ by quenching reactions in the plasma. The role of the atomic hydrogen during the film deposition seems to be promoted by the addition of argon into the discharge, leading to an increase of the deposition rate by a factor of about three and an enhancement of the crystalline quality of the nc-Si:H films. This effect is maximized when the Ar fraction in the Ar + H₂ gases mixture reaches 50%. The evolution with Ar addition of the carriers mobility of the related TFTs is closely connected to the evolution of the crystalline fraction of the intrinsic nc-Si:H film. Mobilities values as high as 8 cm² V^?1 s^?1 are obtained at the Ar fraction of 50%. For higher Ar fractions, the fall of the mobility comes with a degradation of the I_D–V_G transfer characteristics of the processed TFTs due to a degradation of the nc-Si:H films quality. OES measurements show that the evolution of the H_α intensity is closely connected to the evolution of the deposition rate, intrinsic films crystalline fraction and TFTs mobility, providing an interesting tool to monitor the TFTs performances.