Effect of deposition temperature on polymorphous silicon thin films by PECVD: Role of hydrogen

Pm-Si:H which has improved optical and transport properties as well as stability compared to hydrogenated amorphous silicon is studied. In order to understand the effect of the growth temperature on pm-Si:H films, hydrogen bonding and stability were analyzed in this work. Samples grown at different temperatures were compared and a change on the films morphology and structure was observed. HRTEM images evidence nanocrystals with approximate size of 9 nm. A growth surface reorganization was observed at an almost constant deposition rate. Increasing the deposition temperature leads to a more ordered, compact and smooth structure of the pm-Si:H films. Hydrogen interaction with the growing surface is related to the deposition temperature, changing the growth of the amorphous matrix due to hydrogen surface diffusion into lower energy and more stable positions. The total hydrogen in the film is reduced as temperature increases and hydrogen becomes more tightly bonded, which changes in a non monotonous way how the nanocrystals are incorporated and their environment. The optoelectronic properties of the films are directly related to the incorporation of hydrogen and whether it is weakly or tightly bonded. A diminution of the optical gap of the pm-Si:H films in the range from 1.71 to 1.65 eV was observed with the increase of the deposition temperature in the range from 175 to 275 °C.     

A simple analytical model of thin films crystalline silicon solar cell with quasi-monocrystalline porous silicon at the backside

An analytical model that simulates the performance of an elementary thin silicon solar cell with a thin film quasi-monocrystalline porous silicon (QMPS) at the backside reflector is developed. A complete set of equations for the photocurrent generated under the effect of the reflected light is solved analytically in each region. The collection of the light absorbed by the QMPS layer has been discussed and the analytical solution of the light-generated current in this layer is derived. The maximum of the photocurrent density calculated in the present study is in accordance with the numerical values established by Bergmann et al. Furthermore, the influence that the layer's number of double porosities and high porosity have on the photovoltaic parameters is studied. It is demonstrated that the photovoltaic parameters increase with the number of double porosities that the layer might have in a given structure. When the QMPS layer is formed by three double-porosity layers 20%/80% and for a 5-μm-thick film c-Si, the backside reflector gives a total improvement of about 6 mA/cm² for the photocurrent density and 3.2% for the cell efficiency.     

XPS analysis and characterization of thin films Cu2ZnSnS4 grown using a novel solution based route

A new technique to grow single phase Cu₂ZnSnS₄ (CZTS) thin films for solar cells applications using a chemical route is presented; this consist in sequential deposition of Cu₂SnS₃ (CTS) and ZnS thin films followed by annealing at 550 °C in nitrogen atmosphere, where the CTS compound is prepared in one step process by simultaneous precipitation of Cu₂S and SnS₂ performed by diffusion membranes assisted CBD (chemical bath deposition) technique and ZnS by conventional CBD technique.Measurements of X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM) were used to identify the phases present in the CTS and CZTS films as well as to study their structural and morphological properties. Further, the oxidation states and the chemical composition homogeneity in the volume were studied by X-ray photoelectron spectroscopy (XPS) analysis. Oxidation states and results regarding structural and morphological characterization of CZTS films prepared using the novel technique are compared with those results obtained from single phase CZTS films prepared by sequential evaporation of metallic precursors in presence of elemental sulfur. XRD and Raman spectroscopy studies were used to verify that the CZTS films prepared by the novel method do not present secondary phases.     

Microstructure evolution of amorphous silicon thin films upon annealing studied by positron annihilation

Amorphous silicon (a-Si) thin films were prepared on glass substrates by plasma enhanced chemical vapor deposition (PECVD). Influence of annealing temperature on the microstructure, surface morphology, and defects evolution of the films were studied by X-ray diffraction (XRD), atomic force microscope (AFM) and positron annihilation Doppler broadening spectroscopy (DBS) based on a slow positron beam, respectively. The S parameter of the as-deposited a-Si thin film is high, indicative of amorphous state of Si film with many defects. The a-Si gradually grows into polycrystalline silicon with increasing temperature to 650 °C. For the films annealed below ~450 °C, positron diffusion lengths are rather small because most positrons are trapped in the defects of the a-Si films and annihilated there. With further rising the temperature to 600 °C, the diffusion length of positrons increases significantly due to the removal of vacancy-type defects upon annealing at a high temperature. The results indicate that the coalescence of small vacancy-type defects in a-Si thin film and the crystallization of a-Si occur around 450 °C and 650 °C, respectively.     

Gap states density measurement in copper oxide thin films

The density of gap states near the Fermi level have been measured in copper oxide (CuO) thin films deposited by spray pyrolysis technique. The measurement method is based on the exploitation of the current–voltage characteristics of the space charge limited current (SCLC) measured in a sandwich Au/CuO/Au structure. The measured gap states density is equal to 1.5×10¹⁴ cm⁻³ and 2.0×10¹⁴ eV⁻¹ respectively in films prepared at 300 and 400 °C substrate temperature, while the defect position are located at 16 and 20 meV above Fermi level. The carriers mobility and concentration are also determined from SCLC, the obtained results are in good agreement with Hall effect measurement ones.     

Optical characterization of hydrogenated amorphous silicon thin films deposited at high rate

The aim of this paper is to provide a better understanding of hydrogenated amorphous silicon thin films (a-Si:H) in relation to their optical properties: refractive index, optical gap, absorption coefficient, thickness and surface roughness. The transmission spectrum of the films, deposited with various rf discharge power densities by an optimized plasma enhanced chemical vapor deposition (PECVD) method, at a high rate (>10 Å/sec), was measured over a range in wavelength from 500 to 1100 nm. An approximate model is utilized to describe the surface roughness. In this model, the surface roughness is modeled as a mixed layer of 50 percent of a-Si:H and 50 percent of air and the optical constant of the rough layer is derived using the Bruggemann effective medium approximation (EMA). The gradient iteration method of numerical analysis is used to solve the nonlinear equations in the study. Our results show that the potential underestimation of refractive index and resulting overestimation of film thickness can be overcome by considering the reflection of the rough surface. The method is carried out on the transmission data and the influence of rf discharge power density on the properties of the film is discussed in detail.     

Processing and characterisation of PECVD silicon nitride films

Amorphous Si-N films are synthesised from an NH₃/SiH₄ gas mixture by plasma-enhanced chemical vapour deposition (PECVD) at fixed radio frequency (13.56 MHz) and total gas pressure (34 ± 4 Torr). The variable process parameters and their ranges are: (i) substrate temperature, 200–400°C; (ii) RF power density, 0.08–0.35 W cm⁻²; (iii) NH₃/SiH₄ flow ratio, 40:400–40: 1200 ml min⁻¹. Fundamental properties of the Si-N films are characterised through elemental composition, chemical speciation, optical and electrical properties, all of which are dependent on the process parameters.     

Auger,ellipsometry, and environmental studies of thin films applied to schottky (MIS) solar cells

Schottky (MIS) solar cells typically consist of the structure 5Å Cr/50Å Cu/40Å Cr/20Å oxide/silicon. A study of metal diffusion and precise oxide thickness is important to predict the stability of such a device. Devices fabricated using the same process are shown to have almost identical electronic performance. Auger profiles show that the metal films do not penetrate through adjacent regions except when Ag is used in place of Cu. Ag does penetrate through the Cr and into the Si which alters the electronic properties. Ellipsometer studies show the insulator to range in thickness from 20Å to 28Å which may be controlled by variation of the heat-treatment cycle. Environmental studies of encapsulated solar cells show that polystyrene offers more protection than Sylgard l84. Failures due to dirt coatings, discoloration, thermal stresses, poor wire bonds, voltage degradation, and fill factor degradation have been observed. Properly fabricated and encapsulated cells have performed well for more than a year. A hermetic encapsulation may be necessary for future long term stability.     

Solution flow rate influence on properties of copper oxide thin films deposited by ultrasonic spray pyrolysis

The present work is an investigation of the solution flow rate influence on copper oxide (CuO) thin film properties deposited by ultrasonic spray pyrolysis. A set of CuO thin films were deposited, with various solution flow rates, on glass substrate at 300 °C. The precursor solution is formed with copper salt dissolution in distilled water with 0.05 molarity. The solution flow rate was ranged from 10 to 30 ml/h. Films composition and structure were characterized by means of XRD (X Rays diffraction) and Raman scattering. The optical properties were studied using UV–visible spectroscopy. The electrical conductivity, carrier mobility and concentration were determined by Hall Effect measurements. The obtained results indicate that flow rate is a key parameter controlling CuO films growth mechanism and their physical properties. The prepared films are mainly composed with a CuO monophase, the crystallite size is reduced with increasing the flow rate. A ZnO/CuO heterojunction structure has been realized and its rectifying behavior is tested.     

Spray deposition and characterization of zirconium-oxide thin films

Zirconium oxide films were prepared by the pyrosol process using zirconium acetylacetonate as source material onto clear fused quartz and (100) silicon at substrate temperatures ranging from 300°C to 575°C. X-ray diffraction (XRD) measurements show that samples prepared at substrate temperatures lower than 425°C are amorphous. Films deposited at higher temperatures and short deposition times show a cubic crystalline structure. However, for long deposition times, the samples show monoclinic crystalline structure. A similar phase transformation is observed on samples deposited at short time if they are annealed at high temperature. The cubic and monoclinic phases of the corresponding samples were confirmed by infrared (IR) and Raman spectroscopy, respectively. The ZrO₂ films with cubic phase show an almost stoichiometric chemical composition and refractive index values of the order of 2.1 with an energy band gap of 5.47 eV. The current-density electric-field characteristics of metal-oxide semiconductor (MOS) structures show a small ledge from 2 MV/cm to 4.5 MV/cm, indicating current injection and charge trapping. For higher electric fields, the current is associated with oxygen ion diffusion through the zirconium oxide film. The dielectric breakdown is observed at 6 MV/cm, which is a value higher than those observed in the monoclinic and tetragonal phases.     

Electrospray deposition and characterization of cobalt oxide thin films

Cobalt oxide thin films were fabricated by means of electrospray deposition. The obtained films were characterized by Raman spectroscopy, X-ray diffraction and Scanning electron microscopy. The solution that was used gave the Co₃O₄ phase at different growth temperatures. The best granular surfaces were obtained at 250 °C as verified by all characterization techniques, while flaky surfaces were obtained at higher temperatures. The surface morphology is mostly granular except for high temperatures where the cobalt oxide is formed as flakes instead of grains.     

The path to 25% silicon solar cell efficiency: History of silicon cell evolution

The first silicon solar cell was reported in 1941 and had less than 1% energy conversion efficiency compared to the 25% efficiency milestone reported in this paper. Standardisation of past measurements shows there has been a 57% improvement between confirmed results in 1983 and the present result. The features of the cell structure responsible for the most recent performance increase are described and the history of crystalline and multicrystalline silicon cell efficiency evolution is documented. Copyright © 2009 John Wiley & Sons, Ltd.     

Optimization of physical properties of vacuum evaporated CdTe thin films with the application of thermal treatment for solar cells

This paper reports the optimization of physical properties of cadmium telluride (CdTe) thin films with the application of thermal treatment. The films of thickness 650 nm were deposited on glass and indium tin oxide (ITO) coated glass substrates employing vacuum evaporation followed by thermal annealing in the temperature range 250–450 °C. The films were characterized using X-ray diffraction (XRD), source meter and atomic force microscopy (AFM) for structural, electrical and surface topographical properties respectively. The X-ray diffraction patterns reveal that films are polycrystalline with predominant zinc-blende structure having preferred reflection (111). The structural parameters are calculated and discussed in detail. The current–voltage characteristics show Ohmic behavior and the electrical conductivity is found to increase with annealing treatment. The AFM studies show that the surface roughness of films is observed to increase with annealing. The experimental results reveal that the thermal annealing plays an important role to enhance the physical properties of CdTe thin films and annealed films may be used as absorber layer in CdTe/CdS solar cells.     

High‐quality surface passivation of silicon solar cells in an industrial‐type inline plasma silicon nitride deposition system

We have studied the surface passivation of silicon by deposition of silicon nitride (SiN) in an industrial‐type inline plasma‐enhanced chemical vapor deposition (PECVD) reactor designed for the continuous coating of silicon solar cells with high throughput. An optimization study for the passivation of low‐resistivity p‐type silicon has been performed exploring the dependence of the film quality on key deposition parameters of the system. With the optimized films, excellent passivation properties have been obtained, both on undiffused p‐type silicon and on phosphorus‐diffused n⁺ emitters. Using a simple design, solar cells with conversion efficiencies above 20% have been fabricated to prove the efficacy of the inline PECVD SiN. The passivation properties of the films are on a par with those of high‐quality films prepared in small‐area laboratory PECVD reactors. Copyright © 2004 John Wiley & Sons, Ltd.     

Studies in CuInS2 based solar cells,including ZnS and In2S3 buffer layers

The influence of the growth conditions on the surface chemistry and on the homogeneity of the chemical composition of CuInS₂ (CIS) thin films, prepared by sequential evaporation of metallic precursors in presence of elemental sulfur in a two-stage process, was studied by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It was found that the growth temperature affects the phase in which this compound grows. The samples deposited at temperatures around 500 °C (2nd stage) contain mainly the CuInS₂ phase; however, secondary phases like In₂S₃, Cu₂S were additionally identified at the surface and in the bulk of CuInS₂ samples deposited at temperatures greater than 550 °C. Also, the elemental composition of the layers constituting the Glass/Mo/CuInS₂/buffer/ZnO structure was studied through Auger electron spectroscopy (AES) depth profile measurements. AES measurements carried out across the Glass/Mo/CuInS₂/buffer/ZnO heterojunction gave evidence of Cu diffusion from the CuInS₂ layer towards the rest of the layers constituting the device, and of the formation of a MoS₂ layer in the Mo/CuInS₂ interface. The performance of CuInS₂-based solar cells fabricated using CBD (chemical bath deposition) deposited ZnS as buffer layer was compared to that of cells fabricated using CBD deposited In₂S₃ as buffer.     

Deposition of intrinsic hydrogenated amorphous silicon for thin‐film solar cells – a comparative study for layers grown statically by RF‐PECVD and dynamically by VHF‐PECVD

Hydrogenated amorphous silicon (a‐Si:H) is conventionally deposited using static plasma‐enhanced chemical vapor deposition (PECVD) processes. In this work, a very high frequency (VHF) dynamic deposition technique is presented, on the basis of linear plasma sources. This configuration deploys a simple reactor design and enables continuous deposition processes, leading to a high throughput. Hence, this technique may facilitate the use of flexible substrates. As a result, the production costs of thin‐film silicon solar cells could be reduced significantly. We found a suitable regime for the homogeneous deposition of a‐Si:H layers for growth rates from 0.35–1.1 nm/s. The single layer properties as well as the performance of corresponding a‐Si:H solar cells are investigated and compared with a state‐of‐the‐art radio frequency (RF) PECVD regime. By analyzing the Fourier transform infrared spectroscopy spectra of single layers, we found an increasing hydrogen concentration with deposition rate for both techniques, which is in agreement with earlier findings. At a given growth rate, the hydrogen concentration was at the same level for intrinsic layers deposited by RF‐PECVD and VHF‐PECVD. The initial efficiency of the corresponding p–i–n solar cells ranged from 9.6% at a deposition rate of 0.2 nm/s (RF regime) to 8.9% at 1.1 nm/s (VHF regime). After degradation, the solar cell efficiency stabilized between 7.8% and 5.9%, respectively. The solar cells incorporating intrinsic layers grown dynamically using the linear plasma sources and very high frequencies showed a higher stabilized efficiency and lower degradation loss than solar cells with intrinsic layers grown statically by RF‐PECVD at the same deposition rate. Copyright © 2012 John Wiley & Sons, Ltd.     

硅基纳米结构的减反特性及其在太阳能电池中的应用

本文利用湿法化学腐蚀方法在硅基抛光衬底以及金字塔制绒的衬底上成功制备了纳米线阵列结构。在300~1000纳米波段,硅纳米线结构以及纳米线-金字塔混合结构都表现出了很好的减反特性,其平均反射率分别为2.53%、8%。利用传统工艺,我们在125mm125mm²的硅衬底上成功制备了短路电流密度为34.82mA/cm,开路电压为 594mv,效率为12.45%的纳米线太阳能电池。我们发现钝化对纳米结构的太阳能电池很重要,沉积钝化层之后可以将开路电压由420mv提高到560mv。我们通过分析所制备的太阳能电池的基本参数以及外量子效率,系统研究了硅基纳米结构太阳能电池的效率损失机制。实验证实钝化层以及电极的接触特性对提高纳米线太阳能电池的效率具有重要作用,并发现在已含PN结的硅衬底上制备纳米结构有助于提高太阳能电池的性能。     

Antireflection properties and solar cell application of silicon nanoscructures

Silicon nanowire arrays（SiNWAs） are fabricated on polished pyramids of textured Si using an aqueous chemical etching method.The silicon nanowires themselves or hybrid structures of nanowires and pyramids both show strong anti-reflectance abilities in the wavelength region of 300-1000 nm,and reflectances of 2.52%and less than 8%are achieved,respectively.A 12.45%SiNWAs-textured solar cell（SC） with a short circuit current of 34.82 mA/cm² and open circuit voltage(K_oc) of 594 mV was fabricated on 125×125 mm² Si using a conventional process including metal grid printing.It is revealed that passivation is essential for hybrid structure textured SCs,and K_oc can be enlarged by 28.6%from 420 V to 560 mV after the passivation layer is deposited.The loss mechanism of SiNWA SC was investigated in detail by systematic comparison of the basic parameters and external quantum efficiency（EQE） of samples with different fabrication processes.It is proved that surface passivation and fabrication of a metal grid are critical for high efficiency SiNWA SC,and the performance of SiNWA SC could be improved when fabricated on a substrate with an initial PN junction.     

Sunrise to sunset optimization of thin film antireflective coatings for encapsulated,planar silicon solar cells

We present an approach for the optimization of thin film antireflective coatings for encapsulated planar silicon solar cells in which the variations in the incident spectra and angle of incidence (AOI) over a typical day are fully considered. Both the angular and wavelength dependences of the reflectance from the surface, absorptance within the coating, and transmittance into the device are calculated for both single‐ and double‐layer antireflection coatings with and without thin silicon oxide passivation layers. These data are then combined with spectral data as a function of time of day and internal quantum efficiency to estimate the average short‐circuit current produced by a fixed solar cell during a day. This is then used as a figure of merit for the optimization of antireflective layer thicknesses for modules placed horizontally at the equator and on a roof in the UK. Our results indicate that only modest gains in average short‐circuit current could be obtained by optimizing structures for sunrise to sunset irradiance rather than AM1·5 at normal incidence, and fabrication tolerances and uniformities are likely to be more significant. However, we believe that this overall approach to optimization will be of increasing significance for new, potentially asymmetric, antireflection schemes such as those based on subwavelength texturing or other photonic or plasmonic technologies currently under development especially when considered in combination with modules fixed at locations and directions that result in asymmetric spectral conditions. Copyright © 2009 John Wiley & Sons, Ltd.     

Some prospective applications of silicon electrodeposition from molten fluorides to solar cell fabrication

Electrodeposition of both epitaxial and polycrystalline continuous films of dense, coherent, and well-adherent silicon coatings was achieved from molten fluorides. A dissolving Si anode and an operating temperature of about 750 C were utilized. Silicon electrocrystallization epitaxy (ECE) produced films with the (111) orientation on Si substrates of the same orientation. The unintentionally doped films were of p-type character with a resistivity in the range 0.05 - 0.10 Ω-cm. Polycrystalline Si films were similarly electroplated onto various polycrystalline metal substrates. Uniform coherent, and well-adherent coatings with grain diameters as large as 40 – 50 μm were obtained. The useful rate of electrodeposition of Si could be significantly increased by the application of an alternating square wave pulse (ASWP)₂technique. Cathodic current pulses as high as 300 mA/cm² (growth rate of about 5 μm/min) were demonstrated. The cathodic current efficiencies, for all modes of growth, were about 70 – 100%. The effects of the various operating parameters, and some prospective applications to the fabrication of solar cells, are discussed. This was paper A-6 in the 18th Annual Electronic Materials Conference, Salt Lake City, June 23–25, 1976.