Synthesis of highly conductive boron-doped p-type hydrogenated microcrystalline silicon (μc-Si:H) by a hot-wire chemical vapor deposition (HWCVD) technique

Boron-doped hydrogenated microcrystalline silicon (μc-Si:H) films were prepared using hot-wire chemical vapor deposition (HWCVD) technique. Structural, electrical and optical properties of these thin films were systematically studied as a function of B₂H₆ gas (diborane) phase ratio (Variation in B₂H₆ gas phase ratio, dopant gas being diluted in hydrogen, affected the film properties through variation in doping level and hydrogen dilution). Characterization of these films from low angle X-ray diffraction and Raman spectroscopy revealed that the high conductive film consists of mixed phase of microcrystalline silicon embedded in an amorphous network. Even a small increase in hydrogen dilution showed marked effect on film microstructure. At the optimized deposition conditions, films with high dark conductivity (0.08 (Ω cm)⁻¹) with low charge carrier activation energy (0.025 eV) and low optical absorption coefficient with high optical band gap (2.0 eV) were obtained. At these deposition conditions, however, the growth rate was small (6 Å/s) and hydrogen content was large (9 at%).     

Rough ZnO layers by LP-CVD process and their effect in improving performances of amorphous and microcrystalline silicon solar cells

Doped ZnO layers deposited by low-pressure chemical vapour deposition technique have been studied for their use as transparent contact layers for thin-film silicon solar cells.Surface roughness of these ZnO layers is related to their light-scattering capability; this is shown to be of prime importance to enhance the current generation in thin-film silicon solar cells. Surface roughness has been tuned over a large range of values, by varying thickness and/or doping concentration of the ZnO layers.A method is proposed to optimize the light-scattering capacity of ZnO layers, and the incorporation of these layers as front transparent conductive oxides for p–i–n thin-film microcrystalline silicon solar cells is studied.     

Amorphous silicon/p-type crystalline silicon heterojunction solar cells with a microcrystalline silicon buffer layer

Undoped hydrogenated amorphous silicon (a-Si:H)/p-type crystalline silicon (c-Si) structures with and without a microcrystalline silicon (μc-Si) buffer layer have been investigated as a potential low-cost heterojunction (HJ) solar cell. Unlike the conventional HJ silicon solar cell with a highly doped window layer, the undoped a-Si:H emitter was photovoltaically active, and a thicker emitter layer was proven to be advantageous for more light absorption, as long as the carriers generated in the layer are effectively collected at the junction. In addition, without using heavy doping and transparent front contacts, the solar cell exhibited a fill factor comparable to the conventional HJ silicon solar cell. The optimized configuration consisted of an undoped a-Si:H emitter layer (700 Å), providing an excellent light absorption and defect passivation, and a thin μc-Si buffer layer (200 Å), providing an improved carrier collection by lowering barrier height at the interface, resulting in a maximum conversion efficiency of 10% without an anti-reflective coating.     

整形激光掺杂制备PERC电池选择性发射极研究

研究整形激光掺杂制备选择性发射极(SE)对p型单晶硅钝化发射极局域背接触(PERC)太阳电池的表面金字塔形貌、掺杂分布及电池性能的影响。整形激光具有能量分布均匀、对硅片损伤小等优点。通过改变激光的功率以及激光划线速度在p型单晶硅PERC太阳电池制备了不同掺杂分布的发射极。结合3D激光显微镜、扫描电子显微镜、EDS能谱分析、四探针方阻测试仪、电化学电容法等测试分析方法表征了样品的表面形貌、方阻变化、掺杂浓度曲线和电学性能。本文结合光斑重叠率将不同激光功率和激光划线速度采用公式统一转化为激光能量密度,从而得出制备选择性发射极的最佳激光能量密度。研究结果表明,当激光能量密度为0.97 J/cm2时,电池效率可以稳定提升0.25%以上,体现了整形激光SE技术应用于PERC电池的应用潜力。     

Hall effect analysis of liquid phase epitaxy silicon for thin film solar cells

We use variable temperature Hall effect measurements to determine the doping concentration, impurity compensation, and mobility of n- and p-type liquid phase epitaxy (LPE) silicon layers that are grown from indium solutions onto silicon substrates. Our theoretical analysis of carrier concentration versus temperature data considers temperature-dependent effective masses, Fermi-Dirac statistics, multiple majority impurity levels, excited impurity states, and the temperature dependence of the Hall scattering factor. The measured Hall mobilities and computed compensation ratios in these LPE silicon thin films are within the range of values that have been measured in bulk silicon crystals. Such LPE layers are therefore suitable for the fabrication of high efficiency silicon thin film solar cells.     

Development of high efficiency p-i-n amorphous silicon solar cells with the p-μc-Si:H/p-a-SiC:H double window layer

A structure is developed to help improve the TCO/p contact and efficiency of the solar cell. A p-i-n amorphous silicon (a-Si:H) solar cell with high-conversion efficiency is presented via use of a double p-type window layer composed of microcrystalline silicon and amorphous silicon carbide. The best efficiency is obtained for a glass/textured TCO/p-μc-Si:H/p-a-SiC:H/buffer/i-a-Si:H/n-μc-Si:H/GZO/Ag structure. Using a SnO₂/GZO bi-layer and a p-type hydrogenated microcrystalline silicon (p-μc-Si:H) layer between the TCO/p-a-SiC:H interface improves the photovoltaic performance due to reduction of the surface potential barrier. Layer thickness, B₂H₆/SiH₄ ratio and hydrogen dilution ratio of the p-μc-Si:H layer are studied experimentally. It is clearly shown that the double window layer can improve solar cell efficiency. An initial conversion efficiency of 10.63% is achieved for the a-Si:H solar cell.     

Preparation of highly conductive p-type μc-Si:H window layer using lower concentration of hydrogen in the rf glow discharge plasma

Boron doped p-type hydrogenated microcrystalline silicon (μc-Si:H) films have been prepared by radio-frequency glow discharge method. Highly conductive p-type μc-Si:H films can be obtained even with lower concentration of hydrogen in the rf glow discharge plasma if chamber pressure is low. Effects of increase in hydrogen (H₂) flow rate and chamber pressure have been studied. The structural properties of the films have been studied by X-ray diffractometry. The electrical and optical characterization have been done by dark conductivity, Hall measurements and optical absorption measurements respectively. Film with conductivity 0.1(Ω-cm)⁻¹ with band gap 2.1 eV has been obtained.     

Preparation of highly conductive p-type μc-Si:H window layer using lower concentration of hydrogen in the rf glow discharge plasma

Boron doped p-type hydrogenated microcrystalline silicon (μc-Si:H) films have been prepared by radio-frequency glow discharge method. Highly conductive p-type μc-Si:H films can be obtained even with lower concentration of hydrogen in the rf glow discharge plasma if chamber pressure is low. Effects of increase in hydrogen (H₂) flow rate and chamber pressure have been studied. The structural properties of the films have been studied by X-ray diffractometry. The electrical and optical characterization have been done by dark conductivity, Hall measurements and optical absorption measurements respectively. Film with conductivity 0.1(Ω-cm)⁻¹ with band gap 2.1 eV has been obtained.     

A metallurgical route to produce upgraded silicon and monosilane

We studied alumothermic reduction of silica from silicate slag to obtain silicon-containing Alloy-I and Alloy-II. Phosphorous industry waste and synthetic slag are used as a silicate slag that consists of more than 90% silicon and calcium oxides and less than l0% other elemental oxides. Silicon-containing Alloy-I was upgraded by acid leaching to silicon of a fine powder structure. Using this powder, we grew poly- and mono-crystalline p-type silicon, with resistivity of 0.24 Ω cm, by the Czochralski method. Silicon-containing Alloy-II was used for obtaining monosilane by aqueous treatment with hydrochloric acid under atmospheric conditions and without any catalyst. There was no trace of diborane, which is a common source for boron contamination in crude silane.     

Control of μc-Si/c-Si interface layer structure for surface passivation of Si solar cells

Outstanding passivation properties for p-type crystalline silicon surfaces were obtained by using very thin n-type microcrystalline silicon (μc-Si) layers with a controlled interface structure. The n-type μc-Si layers were deposited by the RF PE-CVD method with an insertion of an ultra-thin oxide (UTO) layer or an n-type amorphous silicon (a-Si : H) interface layer. The effective surface recombination velocity (SRV) obtained was very small and comparable to that obtained using thermal oxides prepared at 1000°C. The structural studies by HRTEM and Raman measurements suggest that the presence of UTO produces a very thin a-Si : H layer under the μc-Si. A crystal lattice discontinuity caused by these interface layers is the key to a small SRV.     

Influence of laser power on the properties of laser doped solar cells

In recent years, increased attention has been focused on the use of lasers in different fabrication steps of solar cells, in particular laser doping to form emitter and/or selective emitter. In this method the laser energy is used to melt silicon, allowing the diffusion of dopant atoms to occur in the liquid phase. The main advantage of this method is the localised nature of the laser beam, which melts and diffuses a limited area without heating the bulk, therefore reducing the possible degradation associated with high temperature processes. At the University of New South Wales a novel laser doping method was developed, which combines the formation of the selective emitter with a self-aligned metallisation pattern. Despite achieving high efficiencies, concerns arose regarding the adhesion of the metal to the shallow laser doped areas. This issue may be alleviated by increasing the roughness of the surface or even more so by creating holes/grooves in the laser doped areas. One simple way of achieving this is by carrying out the laser doping at higher laser energies to deliberately create some ablation. This paper examines the influence of the laser power on the solar cell electrical parameters to ascertain the relationship and the tradeoff between surface roughness and electrical performance. Efficiencies above 18% on a large area commercial grade p-type CZ substrate were achieved despite some ablation, confirming the potential for using this method to improve adhesion. Efficiency of 18.7% on the same substrate, using lower laser power, demonstrates the capability of the laser doping method.     

Performance of p-type silicon-oxide windows in amorphous silicon solar cell

a-SiO_x films have been prepared using silane and pure oxygen as reactive gases in plasma CVD system. Diborane was introduced as a doping gas to obtain p-type conduction silicon oxide. Infrared absorption spectra show the incorporation of Si–O stretch mode around 1000 cm⁻¹. The optical bandgap increases with the oxygen to silane gas ratio, while the electrical conductivity decreases. Hydrogenated amorphous silicon solar cells have been fabricated using p-type a-SiO_x with around 1.85 eV optical bandgap and conductivity greater than 10⁻⁷ S/cm. The measured current–voltage characteristics of the solar cells under 100 mW/cm² artificial light are V_oc=0.84 V, J_sc=14.7 mA/cm², FF=0.635 with a conversion efficiency of 7.84%.     

Electrical/electronic effects of titanium and iron impurities in EFG and FZ solar cell silicon: SPV/EBIC analysis

In this paper results on surface photovoltage (SPV) and electron beam induced conductivity (EBIC) studies of edge-defined film-fed growth (EFG) and floating zone (FZ) silicon solar cell materials (both p-type) are presented. A systematic comparison based on minority carrier diffusion length and carrier recombination is made between: (i) samples contaminated with Ti and/or Fe, (ii) samples gettered by phosphorous diffusion, and (iii) as-received samples. Deep level transient spectroscopy (DLTS) measurements, together with the iron-boron (FeB) pairing kinetics [1] have successfully been used to detect the presence of Fe in the samples. Even though this process is effective in revealing Fe impurities in p-type FZ silicon it is evidently not suitable for Fe identification in p-type EFG silicon. Ti, like Fe, is found to be a prominent lifetime-limiting metallic impurity in both EFG and FZ silicon. Phosphorous diffusion is proven to be an effective external gettering technique for fast-diffusing impurities such as Fe, but not for Ti.     

Crystalline silicon surface passivation by amorphous silicon carbide films

This article reviews the surface passivation of n- and p-type crystalline silicon by hydrogenated amorphous silicon carbide films, which provide surface recombination velocities in the range of 10 cm s⁻¹. Films are deposited by plasma-enhanced chemical vapor deposition from a silane/methane plasma. We determine the passivation quality measuring the injection level (Δn)-dependent lifetime (τ_eff(Δn)) by the quasi-steady-state photoconductance technique. We analyze the experimental τ_eff(Δn)-curves using a physical model based on an insulator/semiconductor structure and an automatic fitting routine to calculate physical parameters like the fundamental recombination velocities of electrons and holes and the fixed charge created in the film. In this way, we get a deeper insight into the effect of the deposition temperature, the gas flow ratio, the doping density of the substrate and the film thickness on surface passivation quality.     

非晶孵化层对高速生长微晶硅电池性能的影响

采用高压高功率的甚高频等离子体增强化学气相沉积(VHF-PECVD)技术,以不同的反应气体总流量制备出沉积速率大于1nm/s、次带吸收系数(α0.8eV)小于2.5cm-1且具有相同晶化率的本征微晶硅薄膜,然而将其应用在微晶硅电池中时,电池性能却有明显差异.通过对微晶硅电池的光、暗态J-V,量子效率(QE)和微区拉曼(Raman)测试发现,微晶硅薄膜中非晶孵化层厚度的不同是引起电池性能差异的主要原因.反应气体总流量较低时沉积的微晶硅薄膜具有较厚的非晶孵化层,阻碍了载流子的输运,使电池的长波光谱响应下降,从而降低了电池的短路电流密度与填充因子;而增加总气体流量,有效减小了微晶硅薄膜中的非晶孵化层的厚度,从而使电池性能得到改善.最后在总气体流量为500sccm时,制备得到沉积速率为1nm/s,效率为7.3%的单结微晶硅太阳电池.     

Doping of a-SiCX:H films including μc-Si:H by hot-wire CVD and their application as a wide gap window for heterojunction solar cells

Impurity doping using B₂H₆ gas using hot-wire CVD has been tried for hydrogenated amorphous silicon-carbon (a-SiC_X:H) alloy films including hydrogenated microcrystalline silicon (μc-Si:H) with carbon content, C/(Si+C), of about 28%. The dark- and photoconductivities of B-doped samples are larger than those of undoped samples. The activation energy for dark conductivity of the B-doped sample with the doping gas ratio, B₂H₆/(SiH₄+CH₄), of about 0.054% is 0.17 eV. This value is smaller than that of the undoped sample. The P-doped samples also show larger dark- and photoconductivities and smaller activation energy than the undoped samples.     

Sprayed boric acid as a dopant source for silicon ribbons

This paper presents a new method for bulk boron doping of silicon ribbons for solar cells. The method is based on the spraying of the ribbon with a solution of boric acid followed by zone melting recrystallization in an argon atmosphere. Dopant incorporation is evaluated by measuring the spreading resistance of the silicon ribbon.A numerical model for the incorporation of boron into silicon was developed, considering two competing mechanisms: boron diffusion during the heating step and evaporated boron transport in the gas phase. The dependence of the incorporation of boron on experimental parameters such as the direction of recrystallization and the flux of argon in and out of the furnace was measured, yielding results that are compatible with the numerical model. It is also shown that the mean incorporation of boron into the samples depends linearly on the initial concentration of boric acid.     

Lifetime analysis of silicon solar cells by microwave reflection

Microwave photoconductive decay (μPCD) has become a standard technique for measuring the carrier lifetime of silicon used in solar cells. Here, we have used μPCD to examine the carrier lifetimes at common doping levels used in the base region of silicon photovoltaic devices. For the conductivity range used in the p-type base of n⁺–p structures, the microwave penetration depth is less than the wafer thickness. In this case, the reflectance–conductivity relationship is very nonlinear. We will show that quasi-steady-state photoconductivity (QSSPC) and resonance-coupled photoconductive decay (RCPCD) lifetime measurements track over a wide range of injection level, and generally agree at higher injection levels. Our μPCD data will be compared with the transient RCPCD data over the same range. The data from the latter agree at low-injection levels, but show serious disagreement at higher injection levels. The conclusion is that μPCD must be limited to low-injection levels in the doping range used for solar cells.     

Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry

Real time spectroscopic ellipsometry has been applied to develop deposition phase diagrams that can guide the fabrication of hydrogenated silicon (Si:H) thin films at low temperatures (<300°C) for highest performance electronic devices such as solar cells. The simplest phase diagrams incorporate a single transition from the amorphous growth regime to the mixed-phase (amorphous+microcrystalline) growth regime versus accumulated film thickness [the a→(a+μc) transition]. These phase diagrams have shown that optimization of amorphous silicon (a-Si:H) intrinsic layers by RF plasma-enhanced chemical vapor deposition (PECVD) at low rates is achieved using the maximum possible flow ratio of H₂ to SiH₄ that can be sustained while avoiding the a→(a+μc) transition. More recent studies have suggested that a similar strategy is appropriate for optimization of p-type Si:H thin films. The simple phase diagrams can be extended to include in addition the thickness at which a roughening transition is detected in the amorphous film growth regime. It is proposed that optimization of a-Si:H in higher rate RF PECVD processes further requires the maximum possible thickness onset for this roughening transition.