P-type Microcrystalline Silicon Research Articles

Investigation of Microcrystalline Silicon Thin Film Fabricated by Magnetron Sputtering and Copper-Induced Crystallization for Photovoltaic Applications

Microcrystalline silicon, which is widely used in the microelectronics industry, is usually fabricated by chemical vapor deposition techniques. In recent years, magnetron sputtering has been considered as an alternative because it is a simpler, cheaper and more eco-friendly technique. The big drawback of this technique, however, is the need to recrystallize the as-deposited amorphous silicon, which can be done by metal-induced crystallization. Among the different suitable metals, copper has not been extensively investigated for this purpose. Furthermore, the applicability of the microcrystalline film prepared by this method has not been evaluated for photovoltaic device fabrication. Therefore, this paper reports the fabrication of p-type microcrystalline silicon thin film by magnetron sputtering and copper-induced crystallization techniques, and evaluates its appropriateness for solar cell fabrication. In the first step, 60 nm of silicon followed by 10 nm of copper were deposited on n-type silicon wafer and glass substrates, both by the magnetron sputtering technique. Then, the as-deposited samples were annealed at temperatures from 450 °C to 950 °C. The crystal properties of the resulting films were characterized by Raman and X-ray diffraction spectroscopies and optical and secondary emission microscopies, while their electrical characteristics were determined by Hall-effect, J-V curve and external quantum efficiency measurements. These characterizations confirmed the formation of a layer of microcrystalline silicon mostly in the <111> direction with a crystallization ratio of 93% and a largest grain size of 20 nm. The hole concentration and mobility of the fabricated p-type microcrystalline silicon layer were about 1017~1019 cm−3 and 8 cm2/V.s, respectively. By using the fabricated film as the emitter layer of a p-n junction solar cell, a good rectification ratio of 4100 and reverse saturation current density of 85 nA.cm−2 were measured under dark conditions. The highest photovoltaic conversion efficiency, i.e., 2.6%, with an open-circuit voltage of 440 mV and short-circuit current density of 16.7 mA/cm2, were measured under AM1.5G irradiance. These results indicate that microcrystalline silicon created by magnetron sputtering and copper-induced crystallization has considerable potential for photovoltaic device fabrication.

Toward the design of monolithic 23.1% efficient hysteresis and moisture free perovskite/c-Si HJ tandem solar cell: a numerical simulation study

In recent years, research and development of high-efficiency perovskite solar cells (PSCs) have gained momentum. However, the inherent issues of high-temperature stability and hysteresis have constrained the device from commercial feasibility. Researchers have proposed different electron transport layer (ETL) based PSCs to minimize the aforesaid issues. Recently, reduced cerium oxide/[6,6]-phenyl-C61-butyric acid methyl ester (CeOx/PCBM ETL) based PSC device is developed with power conversion efficiency (PCE) of 16.85% and improved stability. In the present work, CeOx/PCBM ETL based PSC device is simulated and calibrated to provide the scope for further improvement in terms of the overall conversion efficiency of the device. The device is further optimized by parametric variation such as doping and thickness of CeOx/PCBM ETL layers. The optimized device with added carbon nanotubes CNTs (to enhance moisture stability) is employed in the monolithic tandem solar cell, and the efficiency potential of a monolithic, hysteresis and moisture free perovskite/crystalline silicon heterojunction (c-Si HJ) tandem solar cell is investigated. Silicon-based, i.e. hydrogenated p-type microcrystalline silicon oxide (µc-Si1−xOx:H) and hydrogenated n-type amorphous silicon tunnel junction (TJ) is used to model the TJ between two diodes. Comprehensive analysis and optimization of the tandem device are done in terms of optical and electrical performance with different thicknesses of perovskite and c-Si. The tandem device proposed in this work yielded a maximum PCE of 23.08%.

Computational design of high efficiency a-Si:H/CIGS monolithic tandem solar cell

In this work, we investigate numerical simulations of single junction hydrogenated amorphous silicon solar cells (a-Si:H), Cu (In, Ga) Se2 (CIGS) solar cells, and tandem solar cells with a-Si:H as the top cell and (CIGS) as the bottom cell using an advanced semiconductor analysis program. The optical bandgaps and thicknesses of the tandem solar cell were studied to optimize current matching. The tunnel-recombinaton (TJ) junctions were studied with different structures. The optimal structure was investigated using the optimal TJ junction, which consists of p-type microcrystalline silicon, n-type nanocrystalline silicon and AZO. The characteristics of the optimal a-Si:H/CIGS solar cell were a short circuit current density of 17.57 mA/cm2, and an open circuit voltage of 1.67 V. These results can be used as a basis for effectively developing low-cost and high-efficiency solar cells.

Potentials of mixed-phase doped layers in p-type Si heterojunction solar cells with ZnO:Al

We present and discuss advances in the development of p-type Si heterojunction solar cells with Al-doped ZnO (AZO) on both front and rear side, by means of mixed-phase Si- and SiOx-based doped layers. We demonstrate that while rear AZO can be deleterious when using p-type amorphous silicon (a-Si:H), p-type microcrystalline silicon (μc-Si:H) allows for: (1) suppression of the p-layer/AZO Schottky-like barrier, (2) relevant Voc increase, and (3) improved transport and extraction of carriers. In contrast, Voc reduction and increased low-injection recombination is observed with n-type μc-Si:H in conjunction with AZO, likely due to the low defect density of the material which entails severe depletion effects. These issues are avoided with the more defective mixed-phase n-SiOx, which additionally provides for reduced parasitic absorption losses and is thus established as the optimal emitter choice.

Correlation Between the Raman Crystallinity of p-Type Micro-Crystalline Silicon Layer and Open Circuit Voltage of n-i-p Solar Cells.

This article mainly discusses the difference between p-i-n and n-i-p type solar cells. Their structural difference has an effect on cell performance, such as open circuit voltage and fill factor. Although the deposition conditions are the same for both p-i-n and n-i-p cases, the substrate layers for depositing p-type microcrystalline silicon layers differ. In n-i-p cells, the substrate layer is p-type amorphous silicon oxide layer; whereas, in p-i-n cells, the substrate layer is ZnO:Al. The interfacial change leads to a 12% difference in the crystallinity of the p-type microcrystalline silicon layers. When the p-type microcrystalline silicon layer's crystallinity was not sufficient to activate an internal electric field, the open circuit voltage and fill factor decreased 0.075 V and 7.36%, respectively. We analyzed this problem by comparing the Raman spectra, electrical conductivity, activation energy and solar cell performance. By adjusting the thickness of the p-type microcrystalline silicon layer, we increased the open circuit voltage of the n-i-p cell from 0.835 to 0.91 V.

Aluminum doped zinc oxide wide band-gap n-type optical window for μc-Si superstrate solar cell

ZnO:Al thin films were deposited onto glass substrates by RF-magnetron sputtering system. The crystallographic orientation of the films, determined using an X-ray diffractometer (XRD), had a high c-axis orientated crystalline structure along (002) plane. The grains are densely packed as shown in the surface micrograph. The electrical parameters were carried out using Hall Effect measurements. The optical band-gap of the films was estimated based on the thickness and the optical transmittance data and is about 3.78 eV for 50 W RF-power. All parameters obtained were used to simulate a new solar cell structure based on p-type microcrystalline silicon as an absorber and n-ZnO:Al as an optical window. The excellent optical properties of this layer result in a high light trapping yielding to efficiencies about 19%. In order to improve efficiency, we have used a p+-μc-Si thin layer highly doped as a back surface field which minimizes significantly the impact of rear surface recombination velocity on voltage and current leading to a high efficiency of 22%. Optoelectronic parameters were determined using the current density-voltage (J-V) curve by means of an AMPS-1D device simulator.

Hot-wire chemical vapor deposition prepared aluminum doped p-type microcrystalline silicon carbide window layers for thin film silicon solar cells

Al-doped p-type microcrystalline silicon carbide (µc-SiC:H) thin films were deposited by hot-wire chemical vapor deposition at substrate temperatures below 400 °C. Monomethylsilane (MMS) highly diluted in hydrogen was used as the SiC source in favor of SiC deposition in a stoichiometric form. Aluminum (Al) introduced from trimethylaluminum (TMAl) was used as the p-type dopant. The material property of Al-doped p-type µc-SiC:H thin films deposited with different deposition pressure and filament temperature was investigated in this work. Such µc-SiC:H material is of mainly cubic (3C) SiC polytype. For certain conditions, like high deposition pressure and high filament temperature, additional hexagonal phase and/or stacking faults can be observed. P-type µc-SiC:H thin films with optical band gap E04 ranging from 2.0 to 2.8 eV and dark conductivity ranging from 10−5 to 0.1 S/cm can be prepared. Such transparent and conductive p-type µc-SiC:H thin films were applied in thin film silicon solar cells as the window layer, resulting in an improved quantum efficiency at wavelengths below 480 nm.

Behavior of the parameters of microcrystalline silicon TFTs under mechanical strain

N-type and P-type microcrystalline silicon top-gate TFTs, processed directly on PEN plastic substrate at maximum temperature of 180°C, were mechanically stressed.These TFTs were bent by different curvature radii varying between infinite (flat) and 0.5cm. The tensile stress increases the electron mobility and the compressive stress decreases it. The tensile stress decreases the threshold voltage of N-type TFTs while the compressive stress increases it. These trends are inversed if the type of stress changes OR the type of TFTs changes.The total behavior under mechanical stress is exactly similar to that of single crystalline silicon MOSFETs in nano-scale technologies (90, 65, 45, 32nm), where nano-scale stress is introduced in the goal to engineer the electrical parameters. The similarity originates from the microcrystalline silicon active layer that behaves like single crystalline silicon even if the stress effects are softened by the grain boundaries and the multiple crystalline orientations of the grains.

Remarkable progress in thin-film silicon solar cells using high-efficiency triple-junction technology

Despite the many advantages of thin-film silicon (Si) solar cells, their low efficiencies remain a challenge that must be overcome. Efficient light utilization across the solar spectrum is required to achieve efficiencies over 15%, allowing them to be competitive with other solar cell technologies. To produce high-efficiency thin-film Si solar cells, we have developed triple-junction solar cell structures to enhance solar spectrum utilization. To maximize the light management, in-house ZnO:Al layers with high haze ratios and high transmittances were developed. In addition, novel doping layers, such as n-type microcrystalline silicon oxide (µc-SiOx:H), which has a very low refractive index, and p-type microcrystalline silicon oxide (µc-SiOx:H), which has a wide bandgap, were successfully applied to the optical reflector and the window layer, respectively. Thin-film quality control techniques for the deposition of hydrogenated amorphous silicon (a-Si:H) in the top cell, hydrogenated amorphous silicon-germanium (a-SiGe:H) or hydrogenated microcrystalline silicon (μc-Si:H) in the middle cell, and hydrogenated microcrystalline silicon (μc-Si:H) in the bottom cell were also important factors leading to the production of high-efficiency triple-junction solar cells. As a result of this work, an initial efficiency of 16.1% (in-house measurement) in the a-Si:H/a-SiGe:H/μc-Si:H stack and a stabilized efficiency of 13.4% (confirmed by NREL) in the a-Si:H/μc-Si:H/μc-Si:H stack were successfully achieved in a small-area triple-junction solar cell with dimensions of 1cm×1cm.

Microcrystalline silicon oxide (μc-SiO:H) alloys as a contact layer for highly efficient Si thin film solar cell

We observe >6% efficiency enhancement in silicon thin film solar cell using a p-type microcrystalline silicon oxide (μc-SiO:H) contact layer between transparent conducting oxide (TCO) electrode and the hydrogenated amorphous silicon (a-Si:H) layer. The role of the above contact layer is to reduce the Schottky barrier effect as well as the hetero-junction barrier formation at the interface. Despite its nanometer scale thickness, the properties of the contact layer significantly affect the solar parameters. Based on our results, p-type doped μc-SiO:H can be an ideal material as a contact layer due to its good optical response without noticeable degradation in its electrical property.

Study of the contact property between BZO and p-a-SiC in amorphous silicon solar cell

Highly conductive p-type microcrystalline silicon thin layer is inserted between the front layer (ZnO:B) and the window layer(p-a-SiC) in a p-i-n amorphous silicon solar cell, and the inserted layer is found to be able to eliminate the non-ohmic contact, which is caused by the difference in the work function between the ZnO:B and p-a-SiC. The properties of the p-type microcrystalline silicon are studied by varying layer thickness, hydrogen dilution ratio and B2H6/SiH4 ratio. The optimized p-type microcrystalline silicon film can have a dark conductivity as large as 4.2 S/cm at a thickeness of 20 nm. The p-i-n type amorphous silicon solar cell with the p-type microcrystalline silicon is shown to have a good open circuit voltage and fill factor compared with without the p-type microcrystalline silicon layer.

Microcrystalline Silicon Carbide p-Layer with Wide-Bandgap and Its Application to Single- and Triple-Junction Silicon Thin-Film Solar Cells

Wide-bandgap, high-quality p-type microcrystalline silicon carbide (p-µc-SiC) films have been prepared by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD) technique for use as window layers for single- and triple-junction thin-film silicon solar cells. We have found that the p-µc-SiC films have wider optical bandgaps and lower absorption spectra in the short-wavelength region than the conventional p-µc-Si films. The new p-type doping layer was applied as a window layer of a microcrystalline silicon (µc-Si:H) single-junction cell, and the thin-film solar cell with the new window layer showed higher open circuit voltage (Voc) and conversion efficiency. In addition, the insertion of an optimized p/i buffer layer was essential for reducing atomic damage at the p/i interface and obtaining a higher conversion efficiency. The optimized p-µc-SiC layer and p/i buffer layer were adopted successfully as a new window layer for the bottom cell within the triple-junction cell structure.

Analysis on the interfacial properties of transparent conducting oxide and hydrogenated p-type amorphous silicon carbide layers in p–i–n amorphous silicon thin film solar cell structure

Quantitative estimation of the specific contact resistivity and energy barrier at the interface between transparent conducting oxide (TCO) and hydrogenated p-type amorphous silicon carbide (a-Si1−xCx:H(p)) was carried out by inserting an interfacial buffer layer of hydrogenated p-type microcrystalline silicon (μc-Si:H(p)) or hydrogenated p-type amorphous silicon (a-Si:H(p)). In addition, superstrate configuration p–i–n hydrogenated amorphous silicon (a-Si:H) solar cells were fabricated by plasma enhanced chemical vapor deposition to investigate the effect of the inserted buffer layer on the solar cell device. Ultraviolet photoelectron spectroscopy was employed to measure the work functions of the TCO and a-Si1−xCx:H(p) layers and to allow direct calculations of the energy barriers at the interfaces. Especially interface structures were compared with/without a buffer which is either highly doped μc-Si:H(p) layer or low doped a-Si:H(p) layer, to improve the contact properties of aluminum-doped zinc oxide and a-Si1−xCx:H(p). Out of the two buffers, the superior contact properties of μc-Si:H(p) buffer could be expected due to its higher conductivity and slightly lower specific contact resistivity. However, the overall solar cell conversion efficiencies were almost the same for both of the buffered structures and the resultant similar efficiencies were attributed to the difference between the fill factors of the solar cells. The effects of the energy barrier heights of the two buffered structures and their influence on solar cell device performances were intensively investigated and discussed with comparisons.

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.

Effect of thermal annealing and hydrogen-plasma treatment in boron-doped microcrystalline silicon

We have investigated the effects of temperature (during film growth and post-deposition thermal annealing) and H2-plasma treatment on the electronic and structural properties of p-type microcrystalline silicon films (p-μc-Si:H) for solar cell applications. The highest dark conductivity is obtained in the thermally annealed p-μc-Si:H prepared at low substrate temperature of 50°C. This dark conductivity is decreased by two orders of magnitude when the film is exposed to H2-plasma, being completely restored after thermal annealing. Namely, reversible dual-conductivity cycle is observed between thermally annealed state and H2-plasma-treated state in p-μc-Si:H. The dual-conductivity cycle is accompanied with the reversible change in the infrared-absorption spectrum at around 1845cm−1 assigned as SiHB complex in p-μc-Si:H network structure. Taking into account of the reversible structural change by H2-plasma-exposure and thermal-annealing cycles, necessary process-procedure condition has been proposed for obtaining high photovoltaic performance in thin-film-Si solar cells with high quality p-μc-Si:H.

Highly doped p-type microcrystalline silicon thin films fabricated by a low-frequency inductively coupled plasma at a low temperature

Highly doped p-type microcrystalline silicon thin films are fabricated by a low-frequency inductively coupled plasma at a low temperature of 150 °C. The effects of radio frequency power on the microstructures of thin films such as x-ray diffraction patterns, crystalline volume fraction, grain size and surface morphology are investigated. The deposition rate of the thin films increases from 22 to 35 nm min−1 when the power increases from 1200 to 2000 W. The optical properties of the thin films are studied using UV–VIS–near IR spectroscopy. The Hall effect measurements show that the fabricated thin films present a high doping concentration from 1.2 × 1019 to 1.3 × 1020 cm−3 with increasing power from 1200 to 2000 W. The effect of power on these properties of thin films, especially for high doping, is understood from the impact of power on the plasma properties of the species in the chamber such as the electron energy distribution function (EEDF), electron density and Debye length. The EEDF presents an evolution with increasing power accompanying an increase in electron density and a decrease in Debye length. It is found that the higher the electron density, the higher is the doping concentration. This shows that high electron density is favourable for heavy doping. In fact, the high electron density is determined by the electromagnetic (H) mode of the low-frequency inductively coupled plasma.

Influence of p/i Interface Treatment on the Flexible Thin Film Solar Cells for Application in Building Integrated Photovoltaics

Nip type flexible a-Si solar cells for application in building integrated photovoltaics (BIPV) were deposited by plasma enhanced chemical vapor deposition (PECVD) method. In order to improve the efficiency and stability of the device, p-type microcrystalline silicon (μc-Si:H) film was used as the window layers. H plasma treatment was applied on the p/i interface and nucleation layer was introduced to enhance the deposition of p-type μc-Si:H film on the surface of a-Si:H. Results suggest that with the application of H plasma treatment and the nucleation layer introduction, high quality p-type μc-Si:H film and high efficiency flexible solar cells were obtained.

Preparation of P-Type Microcrystal Si:H Films by ECR-PECVD

P-type microcrystalline silicon films prepared by electron cyclotron resonance（ECR）PECVD are studied here. Silane diluted with Ar (SiH4/Ar=1/19) is used as a source gas and diborane (B2H6) diluted with H2 (100ppm) as doping gases. The effect of flow rate of doping gas on the microstructures and electrical properties of the films were investigated. Raman spectroscopy and X-ray diffraction were used to determine the film structure; AFM was employed to characterize the film surface topography; Hall measurements were carried out on the doped films to determine the carrier type, carrier concentration, and Hall mobility. The optical quality was measure by transmission spectrum.

DEVELOPMENT OF HIGHLY CONDUCTIVE P-TYPE MICROCRYSTALLINE SILICON FILMS FOR N–I–P FLEXIBLE SOLAR CELLS APPLICATION

In this paper, we reported highly conductive p-type microcrystalline silicon (μc- Si:H ) films deposited on amorphous silicon (a- Si:H ) surface by very high frequency plasma enhanced chemical vapor deposition (VHF PECVD) technique. Hydrogen plasma treatment of amorphous silicon surface and nucleation layers were introduced prior to μc- Si:H films deposition. The film properties were investigated by using Raman spectra, scanning electron microscope (SEM), optical transmission and reflection, as well as dark conductivity measurements. The influence of plasma treatment and nucleation layer on the growth and properties of the thin p-type μc- Si:H films was studied. It is demonstrated that the hydrogen plasma treatment of a- Si:H films gives rise to the deposition of μc- Si:H on the a- Si:H surface. Also, the growth and properties of the μc- Si:H films are strongly dependent on the nucleation layer. The dark conductivity (σd) and crystalline fraction increase with the plasma treatment time and attain high values at about 600 s. A p-type μc- Si:H film with conductivity of 0.0875 Scm-1 at a thickness of 30 nm was obtained. The film was introduced as window layers for flexible solar cells. An efficiency of about 7.15% was obtained.

Highly Conductive p-Type Silicon Carbon Alloys Deposited by Hot-Wire Chemical Vapor Deposition

P-type microcrystalline silicon carbide (µc-SiC:H) alloys for application as a window layer in silicon based thin film solar cells were grown by hot-wire chemical vapor deposition using hydrogen diluted monomethylsilane and trimethylaluminum. Conductivities up to 0.1 S/cm were obtained for p-type material. The optical properties were studied by photothermal deflection spectroscopy. At photon energies below 1.25 eV, both free carrier and defect absorption lead to a high absorption coefficient. For photon energies >2.0 eV, the absorption coefficient is affected by the crystallinity and the structural composition. The structure of Al-doped µc-SiC:H thin films were investigated by infrared and Raman spectroscopy. It was found that increase of the deposition pressure can compensate for the loss of crystallinity caused by Al-doping. At high deposition pressure (>100 Pa), increasing contributions of hexagonal SiC alloys, and separated carbon phases are observed.