Room temperature synthesis of nanocrystalline silicon by aluminium induced crystallization for solar cell applications

The aim of this study is to synthesis large area, plastic compatible of p-type nanocrystalline silicon through conventional sputter system. The growth of and p-type doping of nanocrystalline silicon onto plastic substrates using D.C. magnetron sputtering was investigated. The film properties were examined by Raman spectroscopy, X-ray Diffraction, scanning electron microscopy and energy dispersive spectrometry. Nanocrystalline silicon was achieved with careful control of ion bombardment energy. Through a narrow experimental, window room temperature, nanocrystalline silicon can be synthesised on aluminium. It is believed the aluminium reduces the required energy for crystallite nucleation. PN junction was formed through sputtering of Al/Al–Si/n-type Si/AZO structure. The I–V characteristic showed good rectifying behaviour and confirms p-type doping via aluminium induced crystallization.     

Preparation of ITO thin films applied in nanocrystalline silicon solar cells

ITO thin films were prepared by changing the experimental parameters including gas flow ratio, sputtering pressure and sputtering time in DC magnetron sputtering equipment. The stable experimental parameters of Ar flow at 70 sccm, O₂ flow at 2.5 sccm ∼ 3.0 sccm, sputtering pressure around 0.5 Pa, and sputtering time of 80 s were obtained. Under these parameters, we had achieved the ITO thin films with low resistivity (<4 × 10⁻⁴ Ω ? cm) and high average transmissivity (95.48%, 350 nm ∼ 1100 nm). These ITO thin films were applied in nanocrystalline silicon solar cells as top transparent conductive layer. The solar cell test result showed that the open circuit voltage (Voc) was up to 534.9 mV and the short circuit current density (Jsc) was 21.56 mA/cm².     

Low-temperature preparation of flexible a-Si:H solar cells with hydrogenated nanocrystalline silicon p layer

In this paper we report on flexible a-Si:H solar cells prepared on polyethylene naphthalate (PEN) substrates using p-type hydrogenated nanocrystalline silicon thin films (p-nc-Si:H) as the window layer. The p-nc-Si:H films were prepared at low temperature (150 °C) using trimethylboron (TMB) as a dopant gas. The influence of the silane concentration (SC) on the electrical and structural properties of ultra-thin p-nc-Si:H as well as the performance of solar cells on PEN was investigated. The results show that the crystalline fraction and conductivity of p-nc-Si:H thin films diminished, while the deposition rate and RMS roughness of films increased, when the SC increases from 0.53% to 0.8%. For the a-Si:H solar cells on PEN with the non-textured electrodes, the best efficiency of 6.3% was achieved with the p-nc-Si:H thin films deposited at SC = 0.67%.     

Characteristics of bottom-gate low temperature nanocrystalline silicon thin film transistor fabricated by hydrogen annealing of gate dielectric layer

Thin film transistors having nanocrystalline silicon as an active layer were fabricated by catalytic-CVD at a low process temperature (≤ 200 °C). The tri-layer of the bottom-gate TFT was deposited continuously inside the Cat-CVD reactor. In order to improve the quality of the gate dielectric layer an in-situ hydrogen annealing step was introduced in between the silicon nitride and the nanocrystalline silicon deposition steps. The in-situ hydrogen annealing was effective in reducing the hysteresis in the C-V characteristics and in enhancing the breakdown voltage by decreasing the defects inside the SiN_x film.     

Hot wire CVD deposition of nanocrystalline silicon solar cells on rough substrates

In silicon thin film solar cell technology, frequently rough or textured substrates are used to scatter the light and enhance its absorption. The important issue of the influence of substrate roughness on silicon nanocrystal growth has been investigated through a series of nc-Si:H single junction p-i-n solar cells containing i-layers deposited with Hot-wire CVD. It is shown that silicon grown on the surface of an unoptimized rough substrate contains structural defects, which deteriorate solar cell performance. By introducing parameter v, voids/substrate area ratio, we could define a criterion for the morphology of light trapping substrates for thin film silicon solar cells: a preferred substrate should have a v value of less than around 1 × 10^- 6, correlated to a substrate surface rms value of lower than around 50 nm. Our Ag/ZnO substrates with rms roughness less than this value typically do not contain microvalleys with opening angles smaller than ~ 110°, resulting in solar cells with improved output performance. We suggest a void-formation model based on selective etching of strained Si-Si atoms due to the collision of growing silicon film surface near the valleys of the substrate.     

Low temperature synthesis of mesoporous silicon carbide via magnesiothermic reduction

Mesoporous silicon carbides (SiC) with high surface areas (above 300 m²/g) have been prepared successfully at a relative low temperature of 650 °C via magnesiothermic reduction of mesoporous silica/carbon (SiO₂/C) composites. The physicochemical properties and the structure of the products were characterized by various techniques such as XRD, FT-IR, SEM, TEM and N₂ adsorption-desorption isotherm. The experimental results indicate that the obtained SiC materials by this new method have similar structure to corresponding silica matrix templates. It was found that the magnesium (Mg) plays an important role in determining the structure and properties of the final products, which is used as a dual role agent both reducer and catalyst. The formation mechanism of mesoporous SiC has been also discussed.     

Poly-silicon thin films by aluminium induced crystallisation of microcrystalline silicon

Al-induced crystallisation of microcrystalline Si thin films prepared by electron cyclotron resonance plasma-enhanced chemical vapour deposition (ECR-PECVD) on glass and SiO₂ coated Si wafers has been studied. The starting structure was substrate/μc-Si/Al. Annealing this structure in the temperature range 370-520 °C, immediately following deposition of the Al layer, resulted in successful layer exchange and the formation of a substrate/Al+Si layer/poly-Si geometry. The top poly-Si layer exhibited grain sizes generally in the range ∼2-6 μm, although larger grains were also sparsely present. The films did not exhibit any appreciable degree of preferred orientation. The surface roughness was relatively high with a R_a value of ∼20 nm.     

Effect of nanoporous silicon coating on silicon solar cell performance

The surface modification of silicon solar cells was used for improvement of photovoltaic characteristics of silicon solar cells. A screen-printed solar cell technology is used to fabricate n⁺-p silicon solar cell. Nanoporous silicon (PS) layer on n⁺-type Si wafers or on the frontal surface of (n⁺-p)Si solar cell was formed by electrochemical etching in HF-containing solution. The surface morphology, porosity, spectra of photoluminescence and reflectance of PS layers were analyzed. The photovoltaic characteristics of two silicon solar cell type with and without PS layer (PS/(n⁺-p)Si and (n⁺-p)Si cell) were measured and compared. The spectra of photosensitivity of cells were measured in the wavelength range of 300-1100 nm. An average reflection of the porous silicon layer, fabricated on a polished silicon surface, is decreased to 4%. A remarkable increment of the conversion efficiency by 20% have been achieved for PS/(n⁺-p)Si solar cell comparing to (n⁺-p)Si solar cell without PS layer. The results, related with improving of the performance of PS/(n⁺-p)Si solar cell, have been attributed to the effective antireflection and the wide-gap window role of nanoporous silicon on the silicon solar cell.     

Nanostructure formation and passivation of large-area black silicon for solar cell applications

Nanoscale textured silicon and its passivation are explored by simple low-cost metal-assisted chemical etching and thermal oxidation, and large-area black silicon was fabricated both on single-crystalline Si and multicrystalline Si for solar cell applications. When the Si surface was etched by HF/AgNO(3) solution for 4 or 5 min, nanopores formed in the Si surface, 50-100 nm in diameter and 200-300 nm deep. The nanoscale textured silicon surface turns into an effective medium with a gradually varying refractive index, which leads to the low reflectivity and black appearance of the samples. Mean reflectance was reduced to as low as 2% for crystalline Si and 4% for multicrystalline Si from 300 to 1000 nm, with no antireflective (AR) coating. A black-etched multicrystalline-Si of 156 mm × 156 mm was used to fabricate a primary solar cell with no surface passivation or AR coating. Its conversion efficiency (η) was 11.5%. The cell conversion efficiency was increased greatly by using surface passivation process, which proved very useful in suppressing excess carrier recombination on the nanostructured surface. Finally, a black m-Si cell with efficiency of 15.8% was achieved by using SiO(2) and SiN(X) bilayer passivation structure, indicating that passivation plays a key role in large-scale manufacture of black silicon solar cells.     

Microcrystalline silicon for solar cells deposited at high rates by hot-wire CVD

Using two tungsten (W) filaments and a filament–substrate spacing of 3.2 cm, we have explored the deposition of microcrystalline silicon (μc-Si) solar cells, with the i-layer deposited at high deposition rates (R_d), by the hot-wire CVD (HWCVD) technique. These cells were deposited in the n–i–p configuration on textured stainless steel (SS) substrates, and all layers were deposited by HWCVD. Thin, highly crystalline seed layers were used to facilitate crystallite formation at the n–i interface. Companion devices were also fabricated on flat SS substrates, enabling structural measurements (by XRD) to be performed on i-layers used in actual device structures. Using a filament temperature of 1750 °C, device performance was explored as a function of i-layer deposition conditions, including variations in i-layer substrate temperature (T_sub) using constant H₂ dilution, and also variations in H₂ dilution during i-layer deposition. The intent of the latter is to affect crystallinity at the top surface of the i-layer (i–p interface). We report device performance resulting from these studies, with all i-layers deposited at R_d>5 Å/s, and correlate them with i-layer structural studies. The highest device efficiency reported is 6.57%, which is a record efficiency for an all-hot-wire solar cell.     

Status report: solar cell related research and development using amorphous and microcrystalline silicon deposited by HW(Cat)CVD

This article reviews the research and development of a-Si:H and μc-Si:H based solar cells by using hot wire chemical vapor deposition (HWCVD). The groups involved and the present status of conversion efficiencies attained are listed and will be discussed for different cell structures realized entirely or partly using this method. There are three main advantages of HWCVD: a quite simple set up, higher useable deposition rates and higher stability of HW-a-Si:H. It will be discussed how these advantages can be exploited to make HWCVD an alternative to plasma enhanced chemical vapor deposition.     

Intrinsic microcrystalline silicon prepared by hot-wire chemical vapour deposition for thin film solar cells

Microcrystalline silicon (μc-Si:H) prepared by hot-wire chemical vapour deposition (HWCVD) at low substrate temperature T_S and low deposition pressure exhibits excellent material quality and performance in solar cells. Prepared at T_S below 250 °C, μc-Si:H has very low spin densities, low optical absorption below the band gap, high photosensitivities, high hydrogen content and a compact structure, as evidenced by the low oxygen content and the weak 2100 cm⁻¹ IR absorption mode. Similar to PECVD material, solar cells prepared with HWCVD i-layers show increasing open circuit voltages V_oc with increasing silane concentration. The best performance is achieved near the transition to amorphous growth, and such solar cells exhibit very high V_oc up to 600 mV. The structural analysis by Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows considerable amorphous volume fractions in the cells with high V_oc. Raman spectra show a continuously increasing amorphous peak with increasing V_oc. Crystalline fractions X_C ranging from 50% for the highest V_oc to 95% for the lowest V_oc were obtained by XRD. XRD-measurements with different incident beam angles, TEM images and electron diffraction patterns indicate a homogeneous distribution of the amorphous material across the i-layer. Nearly no light induced degradation was observed in the cell with the highest X_C, but solar cells with high amorphous volume fractions exhibit up to 10% degradation of the cell efficiency.     

Novel aspects in thin film silicon solar cells-amorphous, microcrystalline and nanocrystalline silicon

The improvement of photodegradation of a-Si:H has been studied on the basis of controlling the subsurface reaction and gaseous phase reaction. We found that higher deposition temperature, hydrogen dilution and triode method are effective to reduce the SiH₂ density in the film and to suppress the photodegradation of solar cells. These results are explained in terms of the hydrogen elimination reaction in the subsurface region and the contribution of the higher silane radicals to the film growth. The high-rate deposition of μc-Si:H was obtained by means of a high-pressure method and further improvement in deposition rate and the film quality was achieved in combination with the locally high-density plasma, which enables effective dissociation of source gases without thermal damage. It was also found that the deposition pressure is crucial to improve the film quality for device. This technique was successfully applied to the solar cells and an efficiency of 7.9% was obtained at a deposition rate of 3.1 nm/s. The potential application of nanocrystalline silicon is also discussed.     

Tungsten filament effect on electronic properties of hot-wire CVD silicon films for heterojunction solar cell application

In this study, we describe the correlation between cell efficiency and wire aging during hot-wire chemical vapor deposition in detail. The new and aged tungsten (W) filaments were used to deposit the n-type microcrystalline silicon (μc-Si) films for heterojunction (HJ) Si solar cell applications. Tungsten silicide (WSi_x) was coated on the W catalyzer surface (center and end regions) after each deposition, and which was investigated and determined by scanning electron microscopy and electron probe microanalysis. The wire age has an effect on the resulting electronic properties of the grown film, thought to be related to differences in dark conductivity with aged versus new wires. It was found that the aging process is related to the formation of a silicide at the surface. A limited amount of silicon was observed in the bulk of catalyzer, suggesting that silicon diffusion into the wire has occurred. The original single-side HJ solar cell with efficiency of 15.3% has been fabricated using the new wires. The quality of n-type μc-Si films and efficiency of HJ solar cells were reduced when the aged W filament was employed. The quality of silicon films and the efficiency of HJ solar cell could be improved after regeneration process.     

Recent advances in very high frequency plasma enhanced CVD process for the fabrication of thin film silicon solar cells

We have deposited amorphous silicon (a-Si) and nanocrystalline silicon (nc-Si) materials and the total p-i-n configurations for solar cells in a high vacuum multichamber system ASTER using very high frequency plasma enhanced chemical vapour deposition (VHF PECVD) process. The deposition process is monitored and controlled by in-situ diagnostic tools to maintain reproducibility of the material quality. In this paper we show our recent results on single junction (amorphous silicon) and tandem (a-Si/nc-Si) cells on plastic foil using the Helianthos concept. The tandem cell efficiency on Asahi U-type SnO₂:F coated glass is ~ 12% and this is achieved by employing nc-Si deposited at high pressure (p) conditions of 5 mbar and a small inter-electrode distance (d) of 5 mm. The deposition scheme of this cell on glass was adapted for the SnO₂:F coated Al foil substrates from Helianthos b.v., especially taking into account the expansion of the foil during deposition. The inter-electrode distance d was one of the variables for this optimisation process. Depositions at four inter-electrode distances of 6 mm, 8 mm, 10 mm and 12 mm (keeping the pressure-distance product constant) revealed that the deposition rate increases at higher distances, reaching 0.6 nm/s at a d of 10 mm and pressure p of 3.0 mbar. The Raman crystalline ratio showed a monotonic increase with the combination of higher d and lower p. Tandem cells with an area of 2.5 cm² on plastic foil fabricated by the Helianthos concept and employing the above mentioned nc-Si made at 0.6 nm/s in the bottom cell and a-Si in the top cell, showed an efficiency of 8.12%, with a short circuit current density of 10 mA/cm². The combined deposition time of the photoactive silicon layers of the top and bottom cells amounted to only 85 min.     

A direct correlation between film structure and solar cell efficiency for HWCVD amorphous silicon germanium alloys

The film structure and H bonding of high deposition rate a-SiGe:H i-layers, deposited by HWCVD and containing ~ 40 at.% Ge, have been investigated using deposition conditions which replicate those used in n-i-p solar cell devices. Increasing the germane source gas depletion in HWCVD causes not only a decrease in solar cell efficiency from 8.64% to less than 7.0%, but also an increase in both the i-layer H preferential attachment ratio (PA) and the film microstructure fraction (R?). Measurements of the XRD medium range order over a wide range of germane depletion indicate that this order is already optimum for the HWCVD i-layers, suggesting that energetic bombardment of a-SiGe:H films may not always be necessary to achieve well ordered films. Preliminary structural comparisons are also made between HWCVD and PECVD device layers.     

Properties of plasma enhanced chemical vapor deposited silicon nitride for the application in multicrystalline silicon solar cells

Hydrogenated films of silicon nitride (SiNx:H) were investigated by varying the deposition condition in plasma enhanced chemical vapor deposition (PECVD) reactor and annealing condition in infrared (IR) heated belt furnace to find the optimized condition for the application in multicrystalline silicon solar cells. By varying the gas ratio (ammonia to silane), the silicon nitride films of refractive indices 1.85-2.45 were obtained. Despite the poor deposition rate, silicon wafer with the film deposited at 450 °C showed the best minority carrier lifetime. The film deposited with the gases ratio of 0.57 showed the best peak of carrier lifetime at the annealing temperature of 800 °C. The performance parameters of cells fabricated by varying co-firing peak temperature also showed the best values at 800 °C. The multicrystalline silicon (mc-Si) solar cells fabricated in conventional industrial production line applying the optimized film deposition and annealing conditions on large area substrate (125 mm × 125 mm) was found to have the conversion efficiency of 15%.     

Room temperature crystallization by RF plasma

The crystallization of amorphous thin films was achieved by radiofrequency (RF) plasma treatment. Although various amorphous films are crystallized after 2 min or so, the sample temperature is lower than 150 °C without compulsory cooling even when the films are treated for 1 h. This treatment works on amorphous films of various materials, independently of the film preparation method and substrate materials. Sol-gel-derived TiO₂ films were densified and simultaneously crystallized to anatase structure by the plasma treatment and the obtained films indicate almost the same photocatalytic activities as that of thermally crystallized TiO₂ films. Plasma-crystallized sputtered indium tin oxide (ITO) films have a bixbite structure and the resistivity reached to 1.6 × 10^− 4 Ω cm while the crystallization condition was not optimized. Amorphous silicon films with a small mount of crystallites were deposited by sputtering method and were crystallized by the plasma treatment.     

Arc-instability generated during the Joule heating induced crystallization of amorphous silicon films

Joule heating induced crystallization (JIC) was accomplished by applying an electric field to a conductive layer located beneath the amorphous silicon film. This study found that an intense arc is generated at the interface between the silicon and the electrode. The artificial modification of a JIC-sample structure led us to the finding that arc generation is caused by the dielectric breakdown of a SiO₂ layer that is sandwiched between the transformed polycrystalline silicon and a conductive layer at high temperatures during Joule heating.     

Nanostructure, electrical and optical properties of p-type hydrogenated nanocrystalline silicon films

In this paper, p-type hydrogenated nanocrystalline (nc-Si:H) films were prepared on corning 7059 glass by plasma-enhanced chemical vapor deposition (PECVD) system. The films were deposited with radio frequency (RF) (13.56 MHz) power and direct current (DC) biases stimulation conditions. Borane (B₂H₆) was a doping agent, and the flow ratio η of B₂H₆ component to silane (SiH₄) was varied in the experimental. Films’ surface morphology was investigated with atomic force microscopy (AFM); Raman spectroscopy, X-ray diffraction (XRD) was performed to study the crystalline volume fraction X_c and crystalline size d in films. The electrical and optical properties were gained by Keithly 617 programmable electrometer and ultraviolet-visible (UV-vis) transmission spectra, respectively. It was found that: there are on the film surface many faulty grains, which formed spike-like clusters; increasing the flow ratio η, crystalline volume fraction X_c decreased from 40.4% to 32.0% and crystalline size d decreased from 4.7 to 2.7 nm; the optical band gap E_g^opt increased from 2.16 to 2.4 eV. The electrical properties of p-type nc-Si:H films are affected by annealing treatment and the reaction pressure.