Optimization of 1D photonic crystals to minimize the reflectance of silicon solar cells

This paper presents a process to easily fabricate photonic crystals (PCs) on silicon to increase the efficiency of solar cells by reducing the sunlight reflection in the front surface of the cell. The process, based on laser interference lithography (LIL) and reactive ion etching (RIE), allows creating nanostructures over large areas with different shapes and dimensions. The reflectance of the resulting surface depends on the height, pitch, width and shape of the created PC. In this work, these parameters have been optimized by computer simulation and the best PC so far found has been fabricated on silicon. We obtain a normal reflectance under 10% in the spectral region between 500 and 900 nm without any other material employed as antireflecting coating.     

Investigation on antireflection coatings for Al:ZnO in silicon thin-film solar cells

We numerically investigate the role of antireflection (AR) coatings, composed of SiO₂ and/or ZnO, in suppression of interfacial reflections in the presence of the transparent conducting oxide, Al-doped ZnO (AZO). Three structures are simulated: (a) AR coatings in organic light-emitting diodes (OLEDs) and flat panel displays, (b) AR coating located between the glass and the AZO, and (c) same as the case b except the involvement of another AR coating between the AZO and the amorphous silicon layers. The weighted average transmittance according to the AM1.5 solar spectrum, the photovoltaic transmittance (T_pv), suggests that there is no evident difference between the structures a and b, especially when the layer number of AR coatings is less than three. An effective way to improve T_pv is presented in the structure c, where T_pv is increased from 73.54% to 76.32% with a three-layered AR coating located between AZO and a-Si. It implies that the suppression of interfacial reflections, resulting from the considerable mismatch of refractive indices at the interface of AZO and a-Si, would benefit the efficiency improvement of silicon thin-film solar cells.     

Theoretical analysis of trapping and recombination of photogenerated carriers in amorphous silicon solar cells

We have made theoretical studies on the trapping and recombination of photogenerated carriers in hydrogenated amorphous silicon (a-SiH) p-i-n solar cells. We discuss in detail the following points: 1) The limitations of the assumptions in the previous analysis. It has been clarified that the single-level Shockley-Read-Hall model for carrier recombination and the treatment of trap occupation in terms of quasi-Fermi levels are inadequate for exact analysis. 2) The superlinear dependence of carrier recombination rate on the free-carrier density which can explain the enhancement of photo-induced changes ina-SiH under high intensity light. 3) The estimation of capture cross section of the tail states ina-SiH. We show that the charged and neutral tail states have rather small capture cross sections of less than 10^–16 cm² and of less than 10^–19 cm², respectively. 4) The effect of the recombination of photogenerated (PG) carriers at the p/i and the n/i interfaces. We estimate the recombination velocityS of PG carriers at these interfaces to be about 10³ cm/s. It has been also clarified that the decrease inS is effective to improve the cell performance, especially the open circuit voltage.     

基于地面实测光谱的湿地植物全氮含量估算研究

随着再生水越来越多的应用于城市湿地,湿地植物生长状态的大面积监测对于利用再生水的湿地恢复与重建具有重要意义。目前遥感技术已成为植物生长状态大面积监测的重要手段。本研究以北京市典型再生水城市湿地奥林匹克公园南园湿地为研究区,以反映植物生长状态的重要指标全氮(TN)为研究对象,在测定研究区湿地植物芦苇(Phragmites australis)和香蒲(Typha angustifolia)的叶片光谱及TN含量的基础上,对数据进行预处理并建立二者的关系模型,包括单变量模型(比值光谱指数(SR)模型和归一化差值光谱指数(ND)模型),与多变量模型(逐步多元线性回归(SMLR)模型和偏最小二乘回归(PLSR)模型),并利用交叉验证决定系数(R2CV)和均方根误差(RMSECV)对模型精度进行检验。结果表明,不同湿地植物类型相比,利用芦苇反射光谱建立的各种预测模型的精度都高于香蒲;不同回归模型相比,多变量回归模型的精度较高;多变量回归模型中,PLSR模型的精度高于SMLR模型,其R2CV可达0.80,RMSECV仅为0.24,是建立湿地植物光谱与TN含量关系的最优模型。研究成果不仅为湿地植物生长状态遥感探测提供参考借鉴,而且可以为利用再生水的城市湿地监测与管理提供有力的科学依据。     

Influence of oxidation on optical diffuse reflectance spectra in nanoscale silicon powder

Received: 5 June 1997/Accepted: 30 July 1997     

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.     

Realization of conformal doping on multicrystalline silicon solar cells and black silicon solar cells by plasma immersion ion implantation

Emitted multi-crystalline silicon and black silicon solar cells are conformal doped by ion implantation using the plasma immersion ion implantation （PⅢ） technique. The non-uniformity of emitter doping is lower than 5 %. The secondary ion mass spectrometer profile indicates that the PⅢ technique obtained 100-rim shallow emitter and the emitter depth could be impelled by furnace annealing to 220 nm and 330 nm at 850 ℃ with one and two hours, respectively. Furnace annealing at 850 ℃ could effectively electrically activate the dopants in the silicon. The efficiency of the black silicon solar cell is 14.84% higher than that of the mc-silicon solar cell due to more incident light being absorbed.     

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.     

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.     

Passivating TiO2 coatings for silicon solar cells by pulsed laser deposition

Pulsed laser deposition was used to deposit TiO₂ anti-reflection coatings for silicon solar cells. We deposited smooth coatings with an optimal refractive index of 2.3 for use as anti-reflection coating. The introduction of passivating qualities was achieved by deposition in different gasses. The best result was obtained with deposition in a water vapour ambient. The plasma shape and the position of the substrate in the plasma appeared important for properties such as the smoothness, the thickness distribution and the passivating quality. An increase in the measured effective lifetime of up to 137% during modulated free carrier absorption measurements was observed.     

Effect of dielectronic recombination excitation on solar line intensities

The intensities of lines emitted from the solar atmosphere and observed on earth are calculated by considering electron-impact excitation followed by radiative emission. In order to determine the contribution of dielectronic-recombination -excitation (DRE), the calculations have been repeated with the inclusion of DRE as a process for excitation, along with electron-impact excitation; the excited electron is assumed to deexcite through radiative emission. The DRE is found to contribute significantly and the contribution generally increases with a decrease in wavelength and an increase of ionic charge.     

Microcrystalline silicon and micromorph tandem solar cells

“Micromorph” tandem solar cells consisting of a microcrystalline silicon bottom cell and an amorphous silicon top cell are considered as one of the most promising new thin-film silicon solar-cell concepts. Their promise lies in the hope of simultaneously achieving high conversion efficiencies at relatively low manufacturing costs. The concept was introduced by IMT Neuchatel, based on the VHF-GD (very high frequency glow discharge) deposition method. The key element of the micromorph cell is the hydrogenated microcrystalline silicon bottom cell that opens new perspectives for low-temperature thin-film crystalline silicon technology. According to our present physical understanding microcrystalline silicon can be considered to be much more complex and very different from an ideal isotropic semiconductor. So far, stabilized efficiencies of about 12% (10.7% independently confirmed) could be obtained with micromorph solar cells. The scope of this paper is to emphasize two aspects: the first one is the complexity and the variety of microcrystalline silicon. The second aspect is to point out that the deposition parameter space is very large and mainly unexploited. Nevertheless, the results obtained are very encouraging and confirm that the micromorph concept has the potential to come close to the required performance criteria concerning price and efficiency. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 1 July 1999     

Effect of synthesis conditions protective and decorative titanium nitride coatings on their optical and color parameters

Optical and color parameters of titanium nitride coatings deposited from an unseparated vacuum-arc plasma flux and a flux which is separated from macroparticles are culculated in varying sputtering regimes. Physics and Engineering Institute, National Academy of Sciences of Belarus, 4, Zhodinskaya St., Minsk, 220141. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 87–94, January–February, 1998.     

Polycrystalline silicon on glass for thin-film solar cells

Although most solar cell modules to date have been based on crystalline or polycrystalline wafers, these may be too material intensive and hence always too expensive to reach the very low costs required for large-scale impact of photovoltaics on the energy scene. Polycrystalline silicon on glass (CSG) solar cell technology was developed to address this difficulty as well as perceived fundamental difficulties with other thin-film technologies. The aim was to combine the advantages of standard silicon wafer-based technology, namely ruggedness, durability, good electronic properties and environmental soundness with the advantages of thin-films, specifically low material use, large monolithic construction and a desirable glass superstrate configuration. The challenge has been to match the different preferred processing temperatures of silicon and glass and to obtain strong solar absorption in notoriously weakly-absorbing silicon of only 1–2 micron thickness. A rugged, durable silicon thin-film technology has been developed with amongst the lowest manufacturing cost of these contenders and confirmed efficiency for small pilot line modules already in the 10–11% energy conversion efficiency range, on the path to 12–13%.     

Effect of incidence angle on the electrical parameters of vertical parallel junction silicon solar cell under frequency domain

The effect of incidence angle on the electrical parameters of vertical parallel silicon solar cell under frequency domain was theoretically analyzed. Based on the diffusion-recombination equation, the expression of excess minority carrier density in the base was established according to the modulation frequency and the illumination incidence angle. The excess minority carrier density, the photocurrent density, the photo voltage, series resistance, shunt resistance, electric power and the space charge region capacitance were calculated and plotted. The objective of this work was to show the effects of solar cell modulation frequency and the illumination incidence angle on these electrical parameters, electric power and space charge region capacitance. Plots of solar cell’s electric power with the junction recombination velocity gave the maximum solar cell’s electric power; Pm. Influence of various parameters of incidence angles on the solar cell’s electric power was also studied.     

硅片及其太阳电池的光衰规律研究

采用氙灯模拟太阳光源,将光强调至1000 W/m2,研究常规太阳能级单晶硅片、多晶硅片和物理提纯硅片的原片、去损减薄片、热氧化钝化片、双面镀氮化硅(SiN x:H)膜钝化片、碘酒钝化片以及太阳电池的光衰规律.利用WT-2000少子寿命测试仪以及太阳电池I-V特性测试仪分别对硅片的少子寿命和太阳电池的I-V特性参数随光照时间的变化进行了测试.结果表明:所有硅片以及太阳电池在光照的最初60 min内衰减很快随后衰减变慢,180 min之后光衰速率变得很小,几乎趋于零.     

the specular reflectance of ir-opaque coatings

A reflecting-layer model of the specular reflectance of a rough, thick, absorbing, dielectric film has been developed and fitted to the spectra of seven optical-black coatings between 12 and 300 m. The coatings were quite different. At 100 m their reflectance varied by a factor of nearly 10³. The model determines optical constants and surface parameters, and indicates which parameters control reflectance in the far IR.