Effect of porous silicon stain etched on large area alkaline textured crystalline silicon solar cells

This work shows the effects of porous silicon stain etched on alkaline textured antireflection coatings of large area monocrystalline silicon solar cells. The texturization process has been produced by immersion of the silicon wafers in different carbonate-based solutions. The porous silicon layers were formed by stain etching in a HNO₃/HF aqueous solution before or after the texturization process. We study the effects of different alkaline and acidic solutions and the etching times on the solar cell parameters and the surface reflectance of the device. We have found that the average reflectance of the surface is lowered when the porous etching is combined with the texturization in the alkaline solution. However, the solar cell characteristics are not improved.     

Texturization of multicrystalline silicon by wet chemical etching for silicon solar cells

Two kinds of surface texturization of mc-Si obtained by wet chemical etching are investigated in view of implementation in the solar cell processing. The first one was the acid texturization of saw damage on the surface of multicrystalline silicon (mc-Si). The second one was macro-porous texturization prepared by double-step chemical etching after KOH saw damage layer was previously removed.Both methods of texturization are realized by chemical etching in HF-HNO₃-H₂O with different additives. Macro-porous texturization allows to obtain effective reflectivity (R_eff) in the range 9–20% from bare mc-Si. This R_eff value depends on the time of second step etching that causes porous structure modification. The internal quantum efficiency (IQE) of cells with this kind of texturization has possibility to reach better conversion efficiency than the standard mc-Si solar cells. However, low shunt resistance depends on morphology of porous layer and it is the main factor which can reduce open circuit voltage and conversion efficiency of cells.The effective reflectivity is about 17% for acid texturized mc-Si wafer. The investigation of surface morphology by scanning electron microscopy (SEM) revealed that the dislocations are appearing during chemical etching and they can reduce open circuit voltage. The density of the dislocations can be reduced by controlling depth of etching and optimisation of acid solution.     

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%.     

Correlation between spatially resolved solar cell efficiency and carrier lifetime of multicrystalline silicon

The correlation between the spatially resolved carrier lifetime of multicrystalline silicon and the spatially resolved monochromatic solar cell efficiency is investigated by means of microwave-detected photoconductance decay (MW-PCD) measurements and illuminated lock-in thermography (ILIT). Local monochromatic solar cell efficiencies are determined from ILIT measurements under short-circuit conditions and at the maximum power point of the cell. The resulting efficiency images are compared with efficiency images obtained from MW-PCD lifetime images of unprocessed neighbouring wafers using PC1D simulations. We observe a qualitative correlation between the measured and the simulated efficiency images. Areas with reduced efficiency are found in the same locations using both methods. However, the dynamic range in the monochromatic efficiency is larger for the images obtained from ILIT measurements. Possible explanations for this difference are a change in carrier lifetime during cell processing and varying lifetimes on microscopic scales, leading to averaging faults in the lifetime images.     

晶体硅太阳能电池少子寿命测试方法

少数载流子寿命(简称少子寿命)是半导体晶体硅材料的一项重要参数,它对半导体器件的性能、晶体硅太阳能电池的光电转换效率都有重要的影响。分别介绍了常用的测量晶体硅和晶体硅太阳电池少子寿命的各种方法,包括微波光电导衰减法(MW-PCD),准稳态光电导方法(QSSPC),表面光电压(SPV),IR浓度载流子浓度成像(CDI),调制自由载流子吸收(MFCA)和光束(电子束)诱导电流(LBIC,EBLC),并指出了各种方法的优点和不足。     

Spectroscopic ellipsometry characterization of SiNx antireflection films on textured multicrystalline and monocrystalline silicon solar cells

We present a spectroscopic ellipsometry study of silicon nitride based antireflection films deposited on chemically textured multi- and monocrystalline silicon wafers. The ellipsometric parameters were measured from the near infrared to the ultra violet spectral region. We report the effective thickness and complex index of refraction parameters of the antireflection films from all studied surfaces, regardless of their microscopic morphology. We report on a method to make ellipsometric measurements of the effective optical constants and thickness parameters of thin films deposited on alkaline etched (100)-oriented monocrystalline silicon. The effect of the texture on the complex index of refraction can be described within an effective medium approximation approach. The optical properties are consistent with those obtained from a series of reference films deposited on flat silicon surfaces.     

Black Silicon formation using dry etching for solar cells applications

A study on the formation of Black Silicon on crystalline silicon surface using SF₆/O₂ and SF₆/O₂/CH₄ based plasmas in a reactive ion etching (RIE) system is presented. The effect of the RF power, chamber pressure, process time, gas flow rates, and gas mixtures on the texture of silicon surface has been analyzed. Completely Black Silicon surfaces containing pyramid like structures have been obtained, using an optimized mask-free plasma process. Moreover, the Black Silicon surfaces have demonstrated average values of 1% and 4% for specular and diffuse reflectance respectively, feature that is suitable for the fabrication of low cost solar cells.     

Role of majority and minority carrier barriers silicon/organic hybrid heterojunction solar cells

A hybrid approach to solar cells is demonstrated in which a silicon p-n junction, used in conventional silicon-based photovoltaics, is replaced by a room-temperature fabricated silicon/organic heterojunction. The unique advantage of silicon/organic heterojunction is that it exploits the cost advantage of organic semiconductors and the performance advantages of silicon to enable potentially low-cost, efficient solar cells.     

Dependence of carrier lifetime on Cu-contacting temperature and ZnTe:Cu thickness in CdS/CdTe thin film solar cells

Cu diffusion from a ZnTe:Cu contact interface can increase the net acceptor concentration in the CdTe layer of a CdS/CdTe photovoltaic solar cell. This reduces the space-charge width (W_d) of the junction and enhances current collection and open-circuit voltage. Here we study the effect of Cu concentration in the CdTe layer on carrier lifetime (τ) using time-resolved photoluminescence measurements of ZnTe:Cu/Ti-contacted CdTe devices. Measurements show that if the ZnTe:Cu layer thickness remains constant and contact temperature is varied, τ increases significantly above its as-deposited value when the contacting temperature is in a range that has been shown to yield high-performance devices (~ 280° to ~ 320 °C). However, when the contacting temperature is maintained near an optimum value and the ZnTe:Cu thickness is varied, τ decreases with ZnTe:Cu thickness.     

Improvement of multicrystalline silicon wafer solar cells by post-fabrication wet-chemical etching in phosphoric acid

In this study, we have improved electrical characteristics such as the efficiency (η) and the fill factor (FF) of finished multicrystalline silicon (mc-Si) solar cells by using a new chemical treatment with a hot phosphoric (H₃PO₄) acidic solution. These mc-Si solar cells were made by a standard industrial process with screen-printed contacts and a silicon nitride (SiN) antireflection coating. We have deposited SiN thin layer (80 nm) on p-type mc-Si substrate by the mean of plasma enhanced chemical vapour deposition (PECVD) technique. The reactive gases used as precursors inside PECVD chamber are a mixture of silane (SiH₄) and ammonia (NH₃) at a temperature of 380°C. The developed H₃PO₄ chemical surface treatment has improved η from 5·4 to 7·7% and FF from 50·4 to 70·8%, this means a relative increase of up to 40% from the initial values of η and FF. In order to explain these improvements, physical (AFM, EDX), chemical (FTIR) and optical (spectrophotometer) analyses were done.     

多孔硅对单晶硅少子寿命影响状况的研究

以p型单晶硅片为研究对象,在单晶硅片表面采用化学腐蚀方法制备多孔硅层,通过实验选取制备多孔硅的最佳工艺条件,采用SEM观察多孔硅表面形貌,以及用微波光电导法测试少子寿命的变化情况。结果表明,在相同的腐蚀溶液配比条件下腐蚀11min得到的多孔硅层的表面形貌最好,孔隙率最大。在850℃下热处理150min时样品少子寿命的提高达到最大,不同腐蚀时间的样品少子寿命提高程度不同,腐蚀11min的样品少子寿命提高最大,约有10%左右。多孔层的形成伴随着弹性机械应力的出现,引起多孔层-硅基底界面处产生弹性变形,这有利于缺陷和金属杂质在界面处富集。另外,多孔硅仍具有晶体结构,但其表面方向上的晶格参数要比初始硅的晶格参数大,也有利于金属杂质向多孔层迁移。     

Applications of microcrystalline hydrogenated cubic silicon carbide for amorphous silicon thin film solar cells

We demonstrated the fabrication of n-i-p type amorphous silicon (a-Si:H) thin film solar cells using phosphorus doped microcrystalline cubic silicon carbide (μc-3C-SiC:H) films as a window layer. The Hot-wire CVD method and a covering technique of titanium dioxide TiO₂ on TCO was utilized for the cell fabrication. The cell configuration is TCO/TiO₂/n-type μc-3C-SiC:H/intrinsic a-Si:H/p-type μc- SiC_x (a-SiC_x:H including μc-Si:H phase)/Al. Approximately 4.5% efficiency with a V_oc of 0.953 V was obtained for AM-1.5 light irradiation. We also prepared a cell with the undoped a-Si_1−xC_x:H film as a buffer layer to improve the n/i interface. A maximum V_oc of 0.966 V was obtained.     

Fluorine doped indium oxide films for silicon solar cells

The effect of conditions of preparation of the In₂O₃:F(IFO)/(pp⁺)Si solar cell (SC) by pyrosol method was systematically studied with the goal to maximize its photovoltage. Heterojunction IFO/(pp⁺)Si SC was obtained with the efficiency of 16.6% and photovoltage of 617 mV as well as the IFO/(n⁺pp⁺)Si SC with the efficiency of 19.2% using the following obtained optimal conditions: film-forming solution: 0.2 M InCl₃ + 0.05 M NH₄F + 0.1 M H₂O in methanol; carrier gas — Ar + 5% O₂; deposition temperature — 480 °C; duration of deposition — 2 min; two-minute annealing in argon with sprayed methanol at a temperature of 380 °C.     

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.     

Optimization of KOH etching process to obtain textured substrates suitable for heterojunction solar cells fabricated by HWCVD

In this work, we have studied the texturization process of (100) c-Si wafers using a low concentration potassium hydroxide solution in order to obtain good quality textured wafers. The optimization of the etching conditions have led to random but uniform pyramidal structures with good optical properties. Then, symmetric heterojunctions were deposited by Hot-Wire CVD onto these substrates and the Quasi-Steady-State PhotoConductance technique was used to measure passivation quality. Little degradation in the effective lifetime and implicit open circuit voltage of these devices (< 20 mV) was observed in all cases. It is especially remarkable that for big uniform pyramids, the open-circuit voltage is comparable to the values obtained on flat substrates.     

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.     

半导体材料少子寿命测试仪的研制开发

少数载流子寿命(简称少子寿命)是半导体材料的一项重要参数，它对半导体器件的性能、太阳能电池的效率都有重要的影响。我们采用微波反射光电导衰减法研制了一台半导体材料少子寿命测试仪，本文将对测试仪的实验装置、测试原理及程序计算进行了较详细的介绍，并与国外同类产品的测试进行比较，结果表明本测试仪测试结果准确、重复性高，适合少子寿命的实验室研究和工业在线测试。     

Nanocrystalline silicon films with high conductivity and the application for PIN solar cells

Intrinsic nanocrystalline silicon films (nc-Si:H) were prepared by plasma enhanced chemical vapor deposition (PECVD) method. Films’ microstructures and characteristics were studied with Raman spectroscopy and Atom Force Microscope (AFM). The electronic conductivity of nc-Si:H films was found to be 4.9×10⁰Ω⁻¹ cm⁻¹, which was one order of magnitude higher thanthe reported 10⁻³-10⁻¹ Ω⁻¹ cm⁻¹. And PIN solar cells with nc-Si:H film as intrinsic thin-layer (ITO/n⁺-nc-Si:H/i-nc-Si:H/p-c-Si/Ag) were researched. The cell's performances were measured, the open-circuit voltage V_oc was 534.7 mV, short-circuit current I_sc was 49.24 mA (3 cm²) and fill factor FF was 0.4228.     

Humid environment stability of low pressure chemical vapor deposited boron doped zinc oxide used as transparent electrodes in thin film silicon solar cells

The stability in humid environment of low pressure chemical vapor deposited boron doped zinc oxide (LPCVD ZnO:B) used as transparent conductive oxide in thin film silicon solar cells is investigated. Damp heat treatment (exposure to humid and hot atmosphere) induces a degradation of the electrical properties of unprotected LPCVD ZnO:B layers. By combining analyses of the electrical and optical properties of the films, we are able to attribute this behavior to an increase of electron grain boundary scattering. This is in contrast to the intragrain scattering mechanisms, which are not affected by damp heat exposure. The ZnO stability is enhanced for heavily doped films due to easier tunneling through potential barrier at grain boundaries.     

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.