Enhancement of optoelectronic properties in multicrystalline silicon via combination of grooving grain boundaries and porous silicon gettering

In multi-crystalline silicon (mc-Si), the detrimental effect of impurities and grain boundaries (GBs) on charge carrier transport has driven the research focus since many years. In view of curing these limitations, we present an innovative method to enhance the optoelectronic performance of mc-Si wafers via a combination between GBs grooving and porous silicon (PS) gettering. A preferential grooving of GBs was achieved using the HF/HNO₃ based solution, the PS layers were formed on both sides of the samples using stain-etching method and the gettering experiment was performed at temperatures ranging from 750 to 900 °C. As a result, it has been shown that the rapid thermal annealing process with chemical grooving gives a positive trend of improvement of the electronic quality and found to be more efficient when used in combination with PS. After removing the PS layer, the minority carrier lifetime increases by a factor of more than 27. In addition, a significant enhancement of majority carrier mobility was obtained, which led to an important decrease of the resistivity.     

低温退火磷吸杂工艺对低少子寿命铸造多晶硅电性能的影响

本文针对低少子寿命铸造多晶硅片进行试验, 通过一种将多温度梯度磷扩散吸杂工艺与低温退火工艺结合的新型低温退火吸杂工艺, 去除低少子寿命多晶硅片中影响其电性能的Fe杂质及部分晶体缺陷, 提高低少子寿命多晶硅所生产的太阳电池各项电性能. 通过低温退火磷扩散吸杂工艺与其他磷扩散吸杂工艺的比较, 证明了低温退火吸杂工艺具有更好的磷吸杂和修复晶体缺陷的作用. IV-measurement发现经过低温退火工艺处理后的低少子寿命多晶硅, 制备的太阳电池光电转换效率比其他实验组高0.2%, 表明该工艺能有效地提高低少子寿命多晶硅太阳电池各项电性能参数及电池质量. 本研究结果表明新型低温退火磷吸杂工艺可将低少子寿命硅片应用于大规模太阳电池生产中, 提高铸造多晶硅材料在太阳能领域的利用率, 节约铸造多晶硅的生产成本. 关键词： 低温退火 磷吸杂 低少子寿命多晶硅 太阳电池     

Gettering of metallic impurities in photovoltaic silicon

 This work addresses the issue of structural defect-metallic impurity interactions in photovoltaic silicon and their effect on minority carrier diffusion length values. Aluminium and phosphorus segregation gettering studies were performed on photovoltaic silicon in order to gain insight into these interactions and quantify the effect of gettering on solar cell performance. Integrated circuit grade silicon was also studied for comparative purposes. Additionally, a novel rapid thermal annealing technique, designed to dissolve metallic impurity precipitates, and Deep Level Transient Spectroscopy were utilized to determine the as-grown impurity concentration in both grades of materials. Significant differences in gettering responses between the two grades of silicon are observed. Gettering treatments greatly improve I.C. grade silicon with a specific gettering temperature providing the optimal response. Photovoltaic grade silicon does not respond as well to the gettering treatments and, in some cases, the material degrades at higher gettering temperatures. The degradation is primarily observed in dislocated regions of multicrystalline photovoltaic silicon. Additionally, these dislocated regions were found to possess the highest as-grown metallic impurity concentration of all the materials studied. The dislocation-free photovoltaic silicon has a higher diffusion length relative to dislocated silicon but could not be improved by the gettering methods employed in this study. A model is presented to describe these phenomena where the high concentration of metallic impurities at dislocations produce relatively low minority carrier diffusion lengths as well as the degrading response with higher gettering temperatures while microdefects create an upper limit to the photovoltaic grade material’s diffusion length. Received: 21 June 1996/Accepted: 2 September 1996     

Effects of phosphorus diffusion gettering on minority carrier lifetimes of single-crystalline,multi-crystalline and UMG silicon wafer

Phosphorus diffusion gettering, which can effectively reduce the transition-metal impurities in the bulk of Si wafer and enhance the minority carrier lifetime (MCLT), is a well-known process to improve the performances of solar cells. Especially, the appropriate gettering process is further required for manufacturing solar cells using an upgraded metallurgical-grade silicon (UMG Si) wafer. In this work, an improvement in the MCLT of the UMG Si wafer including the single-crystalline and multi-crystalline Si wafer after phosphorus diffusion gettering was confirmed by using the quasi-steady state photo-conductivity (QSSPC) measurement and the microwave photo-conductance decay (μW-PCD) method. The experimental results were compared with the MCLT variations calculated through the simulation of the Fe distributions in the Si wafers. It was also observed that the efficiency of the UMG Si solar cell increased by 0.53% due to the two-step gettering process.     

Mobility of charge carriers in porous silicon layers

The (conduction) mobility of majority charge carriers in porous silicon layers of the n and p types is estimated by joint measurements of electrical conductivity and free charge carrier concentration, which is determined from IR absorption spectra. Adsorption of donor and acceptor molecules leading to a change in local electric fields in the structure is used to identify the processes controlling the mobility in porous silicon. It is found that adsorption of acceptor and donor molecules at porous silicon of the p and n types, respectively, leads to a strong increase in electrical conductivity, which is associated with an increase in the concentration of free carrier as well as in their mobility. The increase in the mobility of charge carriers as a result of adsorption indicates the key role of potential barriers at the boundaries of silicon nanocrystals and may be due to a decrease in the barrier height as a result of adsorption.     

Efficiency characteristics of a silicon oxide passivation layer on p-type crystalline silicon solar cell at low illumination

Silicon dioxide (SiO₂) is widely used to improve the surface passivation properties of silicon solar cells. To minimize solar cell potential-induced degradation when the PV module is installed outdoors, a silicon oxide film is widely used as an insulator. However, experiments have confirmed that solar cells with a silicon oxide (SiO₂) film have a lower efficiency than solar cells without a silicon oxide (SiO₂) film at low illumination (<0.4 sun). Actually, the efficiency in the low illumination condition affects the average power output per day because the PV module mostly operates when the solar irradiation dose is less than 1 sun. To maximize the performance of the PV module, the output at a low light intensity level should also be considered. Shunt resistance (R_shunt) is known to cause a decrease in solar cell efficiency under low illumination conditions. PC1D simulation was used to analyze parameters, such as the series resistance, parallel resistance, and surface recombination, that affect the characteristics of the solar cell at low light intensity. In this study, we confirmed how the SiO₂ layer affected the low illumination properties of solar cells, even though these cells were more efficient at 1 sun. Silicon solar cells with a SiN_x/SiO₂ bilayer or a SiN_x single film were fabricated, and their characteristics were evaluated. Passivation characteristics were measured using the quasi-steady-state photoconductance (QSSPC) technique to evaluate the minority carrier lifetime and the implied open-circuit voltage (V_OC), and capacitance-voltage measurements were used to analyze the fixed charges. The values of the shunt resistance and series resistance in solar cells with different passivation layers were compared, and the cause of the decrease in the efficiency under low illumination was also analyzed via fill factor calculation.     

Effect of rapid thermal oxidation on structure and photoelectronic properties of silicon oxide in monocrystalline silicon solar cells

This paper concerns the topic of surface passivation properties of rapid thermal oxidation on p-type monocrystalline silicon wafer for use in screen-printed silicon solar cells. It shows that inline thermal oxidation is a very promising alternative to the use of conventional batch type quartz tube furnaces for the surface passivation of industrial phosphorus-diffused emitters. Five minutes was the most favorable holding time for the rapid thermal oxidation growth of the solar cell sample, in which the average carrier lifetime was increased 19.4 μs. The Fourier transform infrared spectrum of the rapid thermal oxidation sample, whose structure was Al/Al-BSF/p-type Si/n-type SiP/SiO₂/SiN_x/Ag solar cell with an active area of 15.6 cm², contained an absorption peak at 1085 cm⁻¹, which was associated with the Si–O bonds in silicon oxide. The lowest average reflectance of this sample is 0.87%. Furthermore, for this sample, its average of internal quantum efficiency and conversion efficiency are respectively increased by 8% and 0.23%, compared with the sample without rapid thermal oxidation processing.     

Improving the material quality of silicon ingots by aluminum gettering during crystal growth

We present a method for the purification of silicon ingots during the crystallization process that reduces significantly the width of the low charge carrier lifetime region at the ingot top. The back‐diffusion of impurities from the ingot top is suppressed by adding a small amount of pure aluminum into the silicon melt right at the end of the solidification. We study the aluminum gettering effect by instrumental neutron activation analysis (INAA) and Fe_i imaging. Furthermore, we present a model for aluminum gettering of Fe in the silicon ingot that is in agreement with literature data for aluminum gettering at lower temperature. The distribution of iron in the ingots with and without aluminum is fairly well predicted by a combination of this model with a model for Fe contamination from the crucible system. A simulation with varying Al content exhibits further potential for an increased yield of silicon wafers with high charge carrier lifetime. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Impurity photovoltaic effect in silicon solar cell doped with sulphur: A numerical simulation

The impurity photovoltaic effect (IPV) has mostly been studied in various semiconductors such as silicon, silicon carbide and GaAs in order to increase infrared absorption and hence cell efficiency. In this work, sulphur is used as the IPV effect impurity incorporated in silicon solar cells. For our simulation we use the numerical device simulator (SCAPS). We calculate the solar cell performances (short circuit current density J_sc, open circuit voltage V_oc, conversion efficiency η and quantum efficiency QE). We study the influence of light trapping and certain impurity parameters like impurity concentration and position in the gap on the solar cell performances. Simulation results for IPV effect on silicon doped with sulphur show an improvement of the short circuit current and the efficiency for sulphur energy levels located far from the middle of the band gap especially at E_c-E_t=0.18 eV.     

Characteristics of silicon solar cell emitter with a reduced diffused phosphorus inactive layer

The diffusion of phosphorus using a phosphorous oxychloride (POCl₃) source in silicon has been used widely in crystalline silicon solar cells. The thermal diffusion process in the furnace consists of two steps: pre-deposition and drive-in. The phosphorous doping profile via thermal diffusion often exhibits high concentrations in the surface-near emitter, which result in a recombination increase. This layer, called the dead layer, should be inhibited in order to fabricate high efficiency silicon solar cells. In this paper, the amount of the POCl₃ flow rate was varied during the pre-deposition process in order to minimize the dead layer, and the characteristics of the phosphosilicate glass (PSG) and emitter were analyzed. From the secondary ion mass spectroscopy (SIMS) and electrochemical capacitance–voltage profiler (ECV) measurements, the emitter formed using a POCl₃ flow rate of 1000 sccm contained the least amount of inactive dopant and resulted in reasonable performance in the silicon solar cell. As the POCl₃ flow rate increased, the doped silicon wafer included electrically inactive P near the surface, which functions as a defect degrading the electrical performance of the emitter. As a result of this, the removal of the dead layer containing the inactive P was attempted through dipping the doped wafer in a HF solution. After this process, the emitter saturation current density and implied V_oc were improved. The completed solar cells and their external quantum efficiencies at a short wavelength also demonstrated improved performance. A quantitative analysis of the emitter can provide a deeper understanding of methods to improve the electrical characteristics of the silicon solar cell.     

The effect of anti-reflection coating of porous silicon on solar cells efficiency

Anti-reflection coatings of solar cells have been fabricated using different techniques. The techniques used include SiO₂ thermal oxidation, ZnO/TiO₂ sputtering deposition and porous silicon prepared by electrochemical etching. Surface morphology and structural properties of solar cells were investigated by using scanning electron microscopy and atomic forces microscopy. Optical reflectance was obtained by using optical reflectometer. I-V characterizations were studied under 80 mW/cm² illumination conditions. Porous silicon was found to be an excellent anti-reflection coating against incident light when it is compared with another anti-reflection coating and exhibited good light-trapping of a wide wavelength spectrum which produced high efficiency solar cells.     

Doped SiO x emitter layer in amorphous/crystalline silicon heterojunction solar cell

In this work we propose to replace the emitter layer of the n-type doped a-Si:H/p-type doped crystalline silicon heterojunction solar cell, with an n-type doped SiO _x amorphous oxide layer. The n-type doped SiO _x :H shows a lower activation energy and higher carrier mobility value with respect to the n-type doped a-Si:H. Moreover, higher transmission, below 500 nm of wavelength, and higher conductivity are measured. The relevance of transparency of the (n) a-SiO _x :H has been studied using that film in solar cells. The electrical parameters revealed a solar cell efficiency of 15.8 %. Moreover, the effect of TCO as a front side cell electrode is considered and discussed on the base of its workfunction when applied on top of the n-type doped SiO _x emitter layer using also numerical simulations.     

Application of the chemical vapor-etching in polycrystalline silicon solar cells

This paper reports a study of the application of chemical vapor-etching (CVE) for the rear surface and in the emitter of polycrystalline silicon (pc-Si) solar cells. The CVE technique consists of exposing pc-Si wafers to a mixture of HF/HNO₃. This technique is used to groove the rear surface of the pc-Si wafers for acid vapors rich in HNO₃ (HNO₃/HF > 1/4), in order to realize rear-buried metallic contacts (RBMC) and the formation of a porous silicon (PS) layer on the frontal surface of the cell for volume ratio of HNO₃/HF = 1/7. A significant increase of the spectral response in the long wavelength range was observed when a RBMC is formed. This increase was attributed to the reduction of the effective thickness of the base of the cells and grain boundary Al gettering. The achievement of a PS layer on the emitter of the pc-Si cells passivates the surface and reduces the reflectivity. The dark I-V characteristics of pc-Si cells with emitter-based PS show an important reduction of the reverse current together with an improvement of the rectifying behaviour. The I-V characteristic under AM1.5 illumination shows an enhancement of both short circuit current density and fill factor. The internal quantum efficiency is improved, particularly in the short wavelengths region.     

多晶硅中的氧碳行为及其对太阳电池转换效率的影响

认识及控制多晶硅中杂质行为对于实现低成本、高效率多晶硅太阳电池有着重要的意义.利用红外光谱技术研究了定向凝固多晶硅锭中不同部位材料热处理前后的氧浓度、碳浓度变化,结合少子寿命、光电转换效率、内量子效率等电池性能,探索不同含量的氧、碳杂质对电池性能影响的物理机制.提出一种考虑碳影响的氧沉淀生长模型,并模拟了热处理后氧沉淀的尺寸分布和数量.研究发现,碳除了使利用硅锭顶部材料制备得到的电池转换效率降低外,还是决定氧沉淀作用的重要因素.由于碳含量多造成中部材料氧沉淀的尺寸大、数量多,引起缺陷,增加复合,而碳在底部 关键词： 氧 碳 太阳电池 转换效率     

Effect on the reduction of the barrier height in rear-emitter silicon heterojunction solar cells using Ar plasma-treated ITO film

In this study, we investigated the effect of plasma treatment on an indium tin oxide (ITO) film under an ambient Ar atmosphere. The sheet resistance of the plasma-treated ITO film at 250 W (37.6 Ω/sq) was higher than that of the as-deposited ITO film (34 Ω/sq). Plasma treatment was found to decrease the ITO grain size to 21.81 nm, in comparison with the as-deposited ITO (25.49 nm), which resulted in a decrease in the Hall mobility. The work function of the Ar-plasma-treated ITO (WF_ITO=4.17 eV) was lower than that of the as-deposited ITO film (WF_ITO = 5.13 eV). This lower work function was attributed to vacancies that formed in the indium and oxygen vacancies in the bonding structure. Rear-emitter silicon heterojunction (SHJ) solar cells fabricated using the plasma-treated ITO film exhibited an open circuit voltage (V_OC) of 734 mV, compared to SHJ cells fabricated using the as-deposited ITO film, which showed a V_OC of 704 mV. The increase in V_OC could be explained by the decrease in the work function, which is related to the reduction in the barrier height between the ITO and a-Si:H (n) of the rear-emitter SHJ solar cells. Furthermore, the performance of the plasma-treated ITO film was verified, with the front surface field layers, using an AFORS-HET simulation. The current density (J_SC) and V_OC increased to 39.44 mA/cm² and 736.8 mV, respectively, while maintaining a WF_ITO of 3.8 eV. Meanwhile, the efficiency was 22.9% at V_OC = 721.5 mV and J_SC = 38.55 mA/cm² for WF_ITO = 4.4 eV. However, an overall enhancement of 23.75% in the cell efficiency was achieved owing to the low work function value of the ITO film. Ar plasma treatment can be used in transparent conducting oxide applications to improve cell efficiency by controlling the barrier height.     

Reflectivity of porous-pyramids structured silicon surface

The antireflection of porous-pyramids structured silicon surface has been studied. The porous surface is formed by stain etching in HF/Fe(NO₃)₃ aqueous solution after textured in KOH/IPA solution. Reflectivity measurements show an overall reflectance of 4.2% for porous-pyramids textured silicon surface in the range from 400 to 900 nm. An optimal etching time of 30 min is obtained when both reflectivity and photo-generated carriers lifetime are considered. This technique may be probably used in the texturization process for high-efficiency silicon solar cells.     

Effect of temperature on crystalline silicon solar cells processed from chemical and metallurgical route

Effect of temperature on monocrystalline and multicrystalline silicon solar cells processed from chemical (EG-Si) and metallurgical (SoG_M-Si) routes was investigated in the range of 280–350 K. The temperature coefficients of important parameters related with the cell property were discussed. Experimental results indicate that the T-coefficient of conversion efficiency (η) of multicrystalline EG-Si cell processed from chemical is only 68% that of the monocrystalline EG-Si cell. Furthermore, the η of both types of SoG_M-Si cells decrease much less than that of the EG-Si cells with the increase in temperature. Additionally, the recombination fraction, the minority carrier lifetime, the carrier mobility decrease and the band-gap shrinkage were also investigated to reveal the intrinsic temperature dependence mechanism. In order to confirm the results, we used numerical simulation software AMPS-1D (analysis of microelectronic and photonic structure in one dimension program) to simulate the temperature dependence of solar cell performances. The results of numerical simulation were basically consistent with the experimental results.     

单晶硅表面均匀小尺寸金字塔制备及其特性研究

表面织构是一种通过有效的光俘获增加短路电流从而提高太阳电池效率的主要途径之一.在加入间隙式超声和NaClO添加剂的碱性四甲基氢氧化铵(TMAH)溶液中对单晶硅表面进行织构化处理,研究超声与NaClO在织构过程中对金字塔成核和生长的影响,以及金字塔大小对高温工艺之后的单晶硅少子寿命的影响.研究表明,通过在织构溶液中加入间隙式超声控制气泡停留在硅片表面的时间和脱离硅片表面速度,增强了小尺寸金字塔的均匀分布.织构之后硅片在AM1.5G光谱下的加权平均反射率能够达到12.4%,在高温扩散和氧化之后少子寿命的大小与金字塔大小之间存在近似于指数衰减函数的关系. 关键词： 表面织构化 反射率 少子寿命 单晶硅太阳电池     

Low-porosity porous silicon nanostructures on monocrystalline silicon solar cells

In this work we present a study of low-porosity porous silicon (PS) nanostructures stain etched on monocrystalline silicon solar cells. The PS layers reduce the reflectance, improve the diffusion of dopants by rapid thermal processes, and increase the homogeneity of the sheet resistance. Some samples were subjected to chemical oxidation in HNO₃ to reduce the porosity of the surface layer. After the diffusion process, deposition of a SiN_x antireflection layer, and screen printing of the samples, an efficiency of 15.5% is obtained for low-porosity PS solar cells, compared with an efficiency of 10.0% for standard PS cells and 14.9% for the reference Cz cells.     

Properties of erbium silicate doped with chromium in porous silicon

The possible formation of chromium-doped erbium silicate Er₂SiO₅: Cr in thin layers of porous silicon is demonstrated. This paper reports on studies of the photoluminescence, electron paramagnetic resonance, and transverse current transport in porous silicon layers (with different chromium and erbium contents) grown on n-and p-silicon single crystals heavily doped with shallow impurities. The Er₂SiO₅: Cr phase with the photoluminescence maxima at approximately 1.3 and 1.5 μm manifests itself after high-temperature annealing at 1000°C. The introduction of erbium and annealing at 700°C increase the intensity of the red photoluminescence of porous silicon by several factors. The decrease in the electrical conductivity of porous silicon suggests the onset of the formation of erbium silicate. The current-voltage characteristics exhibit a nonlinear behavior with an exponential dependence of the current on the voltage due to the discrete electron tunneling. An electron paramagnetic resonance spectrum of P _b centers in p-type heavily doped silicon is observed for the first time.