Solar cells prepared on multicrystalline silicon subjected to new gettering and passivation treatments

New combined gettering and passivating procedures for solar cells prepared from multicrystalline silicon (mc-Si) have been considered. Passivation has been performed by (i) diamond-like carbon films deposition onto front or rear side of the wafers with following annealing, or (ii) hydrogen plasma treatments. Gettering region has been formed by deposition of Al film on specially prepared Si with developed surface. The advantages of such a gettering process in comparison with traditional gettering with Al are demonstrated. The improving influence of the treatments on diffusion length in mc-Si and efficiency of prepared solar cells have been found out. Physical mechanisms responsible for the observed effects of gettering and passivation are discussed.     

Polycrystalline silicon thin films and solar cells prepared by rapid thermal CVD

Polycrystalline silicon (poly-Si) films ( 10 μm) were grown from dichlorosilane by a rapid thermal chemical vapor deposition (RTCVD) technique, with a growth rate up to 100 Å/s at the substrate temperature (T_s) of 1030°C. The average grain size and carrier mobility of the films were found to be dependent on the substrate temperature and material. By using the poly-Si films, the first model pn⁺ junction solar cell without anti-reflecting (AR) coating has been prepared on an unpolished heavily phosphorus-doped Si wafer, with an energy conversion efficiency of 4.54% (AM 1.5, 100 mW/cm², 1 cm²).     

Antireflective coating fabricated by chemical deposition of ZnO for spherical Si solar cells

ZnO thin films as an antireflective (AR) coating have been successfully fabricated on spherical Si solar cells by chemical deposition, which enables uniform film formation. ZnO films were prepared chemically by immersing the cell in an aqueous solution of zinc nitrate and dimethylamineborane maintained at 80 °C. The current–voltage measurements of the solar cells confirmed the increase in short circuit current induced by the AR effect. The open circuit voltage and fill factor were improved by surface passivation. As a result, the conversion efficiency of cells without an AR coating (9.45%) increased to 11.8%, which represents a 25% (relative) increase. The results indicate that the chemical deposition of ZnO is effective for the AR coating of spherical Si solar cells.     

Solar cells based on DLC film – Si structures for space application

Significant increase in solar cell (SC) efficiency after the deposition of thin antireflection diamond-like carbon (DLC) films has been observed. Thick (1300–1500 nm) DLC films deposited on a working side of the SC allow us to increase their stability relating to the effect of proton and ultraviolet (UV) irradiation. So, one-stage (E=50 keV) or multi-energy (50+100 keV) implantation of proton into protected SC (the SC+DLC films system) did not practically influence the SC parameters. Moreover, SC covered by DLC film after UV irradiation show improvement of efficiency both for implanted and unimplanted samples. At the same time the unprotected SC deteriorated after those treatments.The effects observed are connected with the decrease in reflection losses due to the deposition of DLC films and self-annealing of radiation damages in implanted DLC films due to light-induced hydrogen bonding. Re-emission by DLC film of the absorbed ultraviolet light in visible spectral range was observed and mechanism of the effect was proposed. Application of nitrogen-doped DLC film as a fluorescent layer allows us to improve Si SC efficiency in UV spectral range. The results obtained have opened up a wide variety of potential applications of DLC films, especially for space solar cells.     

Recycling of solar cell silicon scraps through filtration, Part I: Experimental investigation

This study investigated the removal of SiC and Si₃N₄ inclusions from top-cut solar cell silicon scraps by filtration with foam filters. Laboratory experiments tested various models for the removal mechanism of inclusions and the efficiency of ceramic foam filters. Inclusions in solar cell silicon top-cut scraps were mainly needle-like Si₃N₄ particles and lumpy SiC inclusions. SiC and Si₃N₄ inclusions sometimes agglomerated as clusters. Si₃N₄ inclusions were usually more than 500 μm long with diameters of ∼20 μm, and SiC inclusions were usually smaller than 500 μm. After filtration, no Si₃N₄ inclusions were found. The remaining inclusions were mainly SiC inclusions smaller than 10 μm. Filters with smaller pores improved the removal of inclusions. It was discovered that contamination of the silicon may occur from components of the filters themselves, especially from binders. The crucible was also a source of contamination. A filtration process that does not produce contamination of its own should be developed before being used in real industrial processes. Mechanisms for the removal of inclusions from silicon through filtration are as follows: (1) cake filtration, for the removal of large Si₃N₄ rods and large SiC inclusions; (2) deep-bed filtration of smaller Si₃N₄ inclusions and most SiC particles; (3) formation of large SiC clusters and bridges across pores, and (4) silicon dissolution into carbon filters, and a subsequent reaction to form layers of SiC.     

A study on the fabrication of polycrystalline Si wafer by direct casting for solar cell substrate

Si-wafers for solar cells were cast in a size of 50 × 46 × 0.5 mm³ by a direct casting method. A graphite mold coated by boron nitride (BN) powder was used in order to prevent the reaction between carbon and the molten silicon. Without any coating, the reaction of the Si melt to the graphite mold was very severe. In the case of BN coating, SiC was formed in the shape of tiny islands on the surface of the Si wafer by the reaction between the Si-melt and the carbon of the graphite mold at high temperature. The grain size was about 1 mm. The efficiency of the Si solar cell was about 0.5% under AM1.5 conditions. It was lower than that of a Si solar cell fabricated with a common single- (sc, 3.0%) and poly-crystalline (pc, 1.0%) Si wafer, which showed much lower efficiency than that of other commercial pc- or sc-Si solar cell (10–15%).     

微晶硅材料和电池特性的相关研究

主要采用甚高频等离子体增强化学气相沉积技术制备了系列微晶硅材料和电池。通过对材料电学特性、结构特性和电池间性能关系的研究,获得了高效率微晶硅薄膜太阳电池所对应材料的基本特性:暗电导在10~(-8)s/cm量级上,光敏性大于1000,晶化率约50%。进行了制备电池的开路电压和表观带隙之间关系的研究。     

Fabricating high performance a-Si solar cells by alternately repeating deposition and hydrogen plasma treatment method

The performance of a p-i buffer layer in pin amorphous silicon solar cell was improved by the “alternately repeating deposition and hydrogen plasma treatment method (ADHT)”. The optical bandgap of the a-Si film was increased by hydrogen plasma treatement. The wide optical bandgap and the high photoconductive a-Si:H films without carbon could be fabricated by the ADHT method. The conversion efficiency of the solar cell with a-Si:H buffer layer was almost the same as that using an a-SiC:H buffer layer. Second, the a-Si (ADHT) films were applied to the n-i buffer layer. The insertion of a-Si (ADHT) films between the i-layer and the n-layer was effective to improve the cell performance, especially the fill factor. With the use of high performance a-Si p-i and n-i buffer layer deposited by ADHT method, a cell conversion efficiency of 12.9% was obtained.     

低温沉积硅薄膜电池及其在塑料衬底上的应用

采用射频等离子体增强化学气相沉积(RF-PECVD)技术,保持衬底温度在125℃沉积硅薄膜材料及电池,研究了硅烷浓度、辉光功率等沉积参数对材料和电池性能的影响。在125℃的低温条件下,通过优化沉积工艺,在玻璃衬底和PET塑料衬底上分别制备出效率达到6.8%和3.9%的单结非晶硅电池。在PET衬底上,将低温沉积非晶硅电池的技术应用于的叠层电池的顶电池,制备出效率为4.6%的非晶/微晶硅叠层电池。     

The efficiency of solar cells immersed in liquid dielectrics

The effect of an increase in the efficiency of solar cells (SCs) (in particular, common silicon SCs) by their immersion in an isotropic liquid dielectric is described. The presence of a dielectric thin film results in an increase in the SCs efficiency by 40–60% from the reference value. The current–voltage characteristic, fill factor and other characteristics of SCs are analyzed. The mechanisms of the increase in the efficiency of SCs are discussed.     

Oxygen modified diamond-like carbon as window layer for amorphous silicon solar cells

In the present study, the effect of in situ layer-by-layer oxygen plasma treatment (OPT) on optical, nano-mechanical and electrical properties of layer-by-layer diamond-like carbon (DLC) thin films was explored. In situ layer-by-layer OPT on layer-by-layer DLC films led to drastic variation of optical band gap from 1.25 eV to 2.6 eV and hardness from 16.1 GPa to 25.3 GPa. Wide band gap and the band gap feasibility over wide range may lead to its realization as p-type window layer in p–i–n solar cells and variable band gap layers in tandem solar cells. Simulations of a-Si:H based p–i–n solar cells was also carried out by considering OPT–DLC films as p-type window layers that yielded maximum efficiency of 8.9%. In addition, due to high hardness and other excellent nano-mechanical properties, these OPT–DLC films can be treated as hard, protective and encapsulate layers on solar cells particularly in n–i–p configuration. It is important to mention that OPT–DLC film as p-layer can minimize the use of additional hard, protective and encapsulate layer.     

Solar to chemical conversion using metal nanoparticle modified microcrystalline silicon thin film photoelectrode

Microcrystalline silicon (μc-Si:H) thin films, which are prospective low-cost semiconductor materials, are used as photoelectrodes for the direct conversion of solar energy to chemical energy. An n-type microcrystalline cubic silicon carbide layer and an intrinsic μc-Si:H layer are deposited on glassy carbon substrates using the hot-wire cat-CVD method. The μc-Si:H electrodes are modified with platinum nanoparticles through electroless displacement deposition. The electrodes produce hydrogen gas and iodine via photoelectrochemical decomposition of hydrogen iodide with no external bias under solar illumination. Surface modification with platinum nanoparticles and surface termination with iodine improve the conversion efficiency.     

Thin film poly-Si formation for solar cells by Flux method and Cat-CVD method

Two methods were examined for the formation of poly-Si films. One is flux method and the other is Cat-CVD method. Flux method was used for forming poly-Si seed films on glass substrates covered with rear electrode. Poly-Si films of a few μm grain size and of mainly (1 1 1) crystalline orientation were obtained at less than 600°C. To make the seed films function as BSF layer for solar cell, boron doping was applied and carrier concentration of 2×10¹⁹/cm³ was obtained which is suitable for highly efficient solar cells. Cat (catalytic)-CVD method was examined for forming poly-Si photo-active layers on the seed films. The films showed deposition gas pressure-dependent crystalline orientations and there was no amorphous incubation layer in (1 1 1) oriented films by Cat-CVD method when deposited on (1 1 1) oriented seed films prepared by Flux method. The electrical properties of the film are insufficient at present, may be due to high defect density and the film structure which allows impurity contaminations of oxygen and carbon after film deposition. Although the film quality needs to be improved, poly-Si films whose crystal fraction is more than 85% were obtained at deposition rate of up to around 40 Å/s. This result indicates high potential of Cat-CVD method for high throughput photo-active formation process necessary for low production cost thin film silicon solar cells.     

Silicon thin films prepared in the transition region and their use in solar cells

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (σ_ph), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p–i–n junction solar cell was prepared with the initial efficiency of 8.51% and a stabilized efficiency of 8.01% (AM1.5, 100 mw/cm²) at room temperature.     

Chemical bath deposition for the fabrication of antireflective coating of spherical silicon solar cells

A CdS film as an antireflective (AR) coating has been successfully deposited on spherical silicon solar cells by chemical bath deposition, which is a novel deposition method of AR coatings for spherical silicon solar cells. The CBD method is a growth method in an aqueous solution and enables film formation for electronic devices with arbitrary shapes. The solar cell performance of the cell with the CdS film showed a 16% increase in short circuit current compared to that without an ARC. The result confirms that the CBD method is useful for the ARC fabrication of spherical silicon solar cells.     

Titanium-containing amorphous hydrogenated silicon carbon films (a-Si:C:H/Ti) for durable solar absorber coatings

By the incorporation of silicon into titanium-containing amorphous hydrogenated carbon films (a-C:H/Ti), the lifetime stability at 250°C in air can be strongly enhanced. A combined PVD/PECVD process for the vacuum deposition of these titanium-containing amorphous hydrogenated silicon carbon films (a-Si:C:H/Ti) is described. Elemental compositions of the deposited films have been determined by in situ core-level photoelectron spectroscopy (XPS). Optical constants for these films have been determined in the wavelength range from 400 to 2500 nm by means of spectrophotometry. Single layers of a-Si:C:H/Ti and a-C:H/Ti deposited on aluminum and copper substrates have been subjected to comparative aging tests. At 250°C in air, the stability of the a-Si:C:H/Ti films is significantly higher than that of the a-C:H/Ti films. If the silicon content is not too high, the aging properties under humid conditions do not suffer a lot from the incorporation of silicon. However, if the silicon content is clearly higher than the carbon content, the humidity resistance will decrease. For an absorber coating for flat plate solar collectors, the optimized silicon content is expected to be in the range where the high-temperature stability in air is already improved, and where the humidity resistance is still good. For vacuum collectors, a higher silicon content might be advantageous.     

The effect of annealing on the properties of diamond-like carbon protective antireflection coatings

In addition to its similarity to genuine diamond film, diamond-like carbon (DLC) film has many advantages, including its wide band gap and variable refractive index. Therefore, as one of the diverse applications, DLC film can be utilized as a protective coating for IR windows and an anti-reflective coating for solar cells. For this study, DLC films were prepared by the radio frequency-plasma enhanced chemical vapor deposition (RF-PECVD) method on silicon substrates using methane (CH₄) and hydrogen (H₂) gas. We examined the effects of the post-annealing temperature and the annealing ambient on structural, electrical and optical properties of DLC films. The films were annealed at temperatures ranging from 300 to 900 °C in steps of 200 °C using rapid thermal annealing equipment in nitrogen ambients. The thickness of the film was observed by scanning electron microscopy (SEM) and surface profile analysis. The variation of structure according to the annealing treatment was examined using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The reflectance of DLC thin film was investigated by UV–vis spectrometry and its electrical properties were investigated using a four point probe and I–V meter. The carrier lifetime of the film was also checked.     

PECVD grown silicon nitride AR coatings on polycrystalline silicon solar cells

Silicon nitride films produced by plasma enhanced chemical vapor deposition (PECVD) have been studied as antireflection (AR) coating on polycrystalline silicon solar cells. A substantial enhancement (28%) in the short circuit current (I_sc) has been obtained. The open circuit voltage (V_oc) of these cells has also been found to improve after silicon nitride deposition. The deposition conditions to optimise the improvement in the cell performance have been discussed.     

Boron-doped diamond-like amorphous carbon as photovoltaic films in solar cell

In this paper, the photovoltaic feature of metal-boron carbide-silicon (MCS) solar cell was reported. The boron-doped diamond-like carbon thin film on n-silicon substrate has been prepared using arc-discharge plasma chemical vapor deposition (PCVD) technique. The conductivity and the resistivity of the film were measured by Bio-Rad Hall5500PC system to be p-type semiconductor and 3–12 Ω cm/□, respectively. The boron content in the films was about 0.8–1.2%, obtained from Auger electron spectroscopy (AES), and some microcrystalline diamond grains (0.5–1.0 μm) embedded in the mainly amorphous network were revealed through scanning electron microscope (SEM) and Raman spectrum. The performance of Au/C(B)/n-Si heterojunction solar cells has been given under dark I–V rectifying curve and I–V working curve (with 100 mW cm⁻² illumination). A measurement of open-circuit voltage V_oc=580 mV and short-circuit current density J_sc=32.5 mA cm⁻² was obtained. Accordingly, the energy conversion efficiency of the device was tentatively determined to be about 7.9% in AM 1.5, 100 mW/cm² illuminated.     

Boron doped amorphous diamond window layer deposited by filtered arc for amorphous silicon alloy p-i-n solar cells

In order to improve the conversion efficiency of amorphous silicon (a-Si:H) alloy p-i-n solar cells, the original p-a-Si:H window layer is substituted by the boron-doped amorphous diamond (a-D:B) films deposited using filtered cathodic vacuum arc technology. The microstructural, optical and electrical properties as functions of the boron concentrations in the films were, respectively, evaluated by an X-ray photoemission spectroscopy, an ultraviolet-visible spectrometer and a semiconductor parameter analyzer. The photovoltaic parameters of the solar cell modules were also detected as functions of boron concentration. It has been shown that the conductive a-D:B films could be obtained and still remained a wide optical gap. The p-i-n structural amorphous silicon solar cell using the a-D:B window layer increased the conversion efficiency by a roughly 10% relative improvement compared to the conventional amorphous silicon solar cell because of the enhancement of short wavelength response.