Texturization of mono-crystalline silicon solar cells in TMAH without the addition of surfactant

Texturization of mono-crystalline silicon solar cell by chemical anisotropic etching is still a key issue due to metal ions contamination and consumption of large amount of isopropyl alcohol (IPA) in a conventional mixture of potassium hydroxide (KOH) or sodium hydroxide (NaOH) and IPA. In this study, etching was performed on (100) silicon wafers using silicon-dissolved tetramethylammonium hydroxide (TMAH) solutions without addition of surfactant. Experiments were carried out in different TMAH concentration solutions at different temperatures for different etching time. The surface phenomena, etching rates, surface morphology and surface reflectance have been analyzed. Experimental results show that the resulted surface covered with uniform pyramids can be realized due to small changes of etching rates during the etching process. The etching mechanism has been explained basing on the experimental results and the theoretical considerations. It was suggested that all the components in the TMAH solutions play important roles in the etching process. Moreover, TMA+ ions may increase the wettability of the textured surface. A good textured surface can be obtained on conditions that the absorption of OH- /H2O is equilibrium with that of TMA+/SiO2(OH)22-.     

Photonic crystal microcrystalline silicon solar cells

Enhancing the absorption of thin‐film microcrystalline silicon solar cells over a broadband range in order to improve the energy conversion efficiency is a very important challenge in the development of low cost and stable solar energy harvesting. Here, we demonstrate that a broadband enhancement of the absorption can be achieved by creating a large number of resonant modes associated with two‐dimensional photonic crystal band edges. We utilize higher‐order optical modes perpendicular to the silicon layer, as well as the band‐folding effect by employing photonic crystal superlattice structures. We establish a method to incorporate photonic crystal structures into thin‐film (~500 nm) microcrystalline silicon photovoltaic layers while suppressing undesired defects formed in the microcrystalline silicon. The fabricated solar cells exhibit 1.3 times increase of a short circuit current density (from 15.0 mA/cm² to 19.6 mA/cm²) by introducing the photonic crystal structure, and consequently the conversion efficiency increases from 5.6% to 6.8%. Moreover, we theoretically analyze the absorption characteristics in the fabricated cell structure, and reveal that the energy conversion efficiency can be increased beyond 9.5% in a structure less than 1/400 as thick as conventional crystalline silicon solar cells with an efficiency of 24%. © 2015 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.     

Economic feasibility of bifacial silicon solar cells

Bifacial solar cells and modules are a promising approach to increase the energy output of photovoltaic systems, and therefore decrease levelized cost of electricity (LCOE). This work discusses the bifacial silicon solar cell concepts PERT (passivated emitter, rear totally diffused) and BOSCO (both sides collecting and contacted) in terms of expected module cost and LCOE based on in‐depth numerical device simulation and advanced cost modelling. As references, Al‐BSF (aluminium back‐surface field) and PERC (passivated emitter and rear) cells with local rear‐side contacts are considered. In order to exploit their bifacial potential, PERT structures (representing cells with single‐sided emitter) are shown to require bulk diffusion lengths of more than three times the cell thickness. For the BOSCO concept (representing cells with double‐sided emitter), diffusion lengths of half the cell thickness are sufficient to leverage its bifacial potential. In terms of nominal LCOE, BOSCO cells are shown to be cost‐competitive under monofacial operation compared with an 18% efficient (≙ p_MPP = 18 mW/cm²) multicrystalline silicon (mc‐Si) Al‐BSF cell and a 19% mc‐Si PERC cell for maximum output power densities of p_MPP ≥ 17.3 mW/cm² and p_MPP ≥ 18.1 mW/cm², respectively. These values assume the use of $10/kg silicon feedstock for the BOSCO and $20/kg for the Al‐BSF and PERC cells. For the PERT cell, corresponding values are p_MPP ≥ 21.7 mW/cm² and p_MPP ≥ 22.7 mW/cm², respectively, assuming the current price offset (≈50%, at the time of October 2014) of n‐type Czochralski‐grown silicon (Cz‐Si) compared with mc‐Si wafers. The material price offset of n‐type to p‐type Cz‐Si wafers (≈15%, October 2014) currently accounts for approximately 1 mW/cm², which correlates to a conversion efficiency difference of 1%_abs for monofacial illumination with 1 sun. From p‐type mc‐Si to p‐type Cz‐Si (≈30% wafer price offset, October 2014), this offset is approximately 2.5 mW/cm² for a PERT cell. When utilizing bifacial operation, these required maximum output power densities can be transformed into required minimum rear‐side illumination intensities for arbitrary front‐side efficiencies η_front by means of the performed numerical simulations. For a BOSCO cell with η_front = 18%, minimum rear‐side illumination intensities of ≤ 0.02 suns are required to match a 19% PERC cell in terms of nominal LCOE. For an n‐type Cz‐Si PERT cell with η_front = 21%, corresponding values are ≤ 0.11 suns with 0.05 suns being the n‐type to p‐type material price offset. This work strongly motivates the use of bifacial concepts to generate lowest LCOE. Copyright © 2016 John Wiley & Sons, Ltd.     

Efficiency-limiting defects in silicon solar cell material

The precipitation rate of intentionally introduced iron during low-temperature heating is studied among a variety of single-crystal and polycrystalline silicon solar cell materials. A correlation exists between the iron precipitation rate and the carrier recombination rate in dislocation-free as-grown material, suggesting that diffusion-length-limiting defects in as-grown material are structural defects which accelerate iron precipitation. Phosphorous diffusion gettering was found to be particularly ineffective at improving diffusion length after intentional iron contamination in materials with high iron precipitation rates. We propose that intragranular structural defects in solar cell silicon greatly enhance transition metal precipitation during cooling from the melt and become highly recombination-active when decorated with these impurities. The defects then greatly impair diffusion length improvement during phosphorus gettering and limit carrier lifetimes in as-grown material.     

Two‐dimensional modeling of front contact silicon solar cells

The modeling of a new type of silicon solar cell intended for operation at very high concentration, with all the contacts at its front face, is presented. The two‐dimensional model developed makes use of the theory of the complex variable, and is able to explain the main features of the operation of these cells. It is shown that if all the parameters reach good state‐of‐the‐art values, and with the appropriate layout, this structure can reach 25% efficiency for a range of concentrations wider than any other known silicon cell. Copyright © 2004 John Wiley & Sons, Ltd.     

Epitaxial solar cells on titanium-doped silicon substrates

Single crystal substrates (0.2 Ω cm, boron doped) purposely doped at 2 × 10¹⁴ cm⁻³ with titanium were used to assess the effect of titanium on solar cell performance. Comparisons were made of all-epitaxial, diffused junction epitaxial, and all diffused junction solar cells fabricated on these substrates. In all cases lower than normal short-circuit current densities were obtained due to diminished red response. However, the short-circuit currents and efficiencies for the epitaxial cells were higher than those for the cells made by direct diffusion into the bulk titanium-doped silicon. The highest efficiency obtained for an epitaxial cell on a titanium-doped substrate was 11.7%. The research reported herein was supported by Jet Propulsion Laboratory, California Institute of Technology under contract No. 954817 and RCA Laboratories, David Sarnoff Research Center, Princeton, New Jersey.     

Structural inhomogeneities in polycrystalline silicon on glass solar cells and their effects on device characteristics

The effects of electrostatic fluctuations due to charged extended defects and strain‐induced bandgap fluctuations are examined in polycrystalline silicon on glass solar cells. The analysis is based on models previously applied to Cu(In,Ga)Se₂ solar cells, but with a new interpretation of the local ideality factor associated with electrostatic fluctuations. It is shown that electrostatic fluctuations become influential to the cell voltage properties as the absorber dopant concentration falls below a certain threshold (a few 10¹⁵ cm⁻³), and the degradations to the open circuit voltage and fill factor are expected to increase with further lowering of dopant density. It is equally plausible that the electrostatic fluctuations originate from charged dislocations or grain boundaries. Bandgap fluctuations on the other hand can be detrimental to the open circuit voltage of cells of any absorber dopant density. However, this voltage degrading effect is seen only in the cells deposited by electron‐beam evaporation, and not amongst those made by plasma enhanced chemical vapour deposition. Copyright © 2011 John Wiley & Sons, Ltd.     

多晶Si太阳电池新型制绒工艺研究

提出一种采用二次酸腐蚀的多晶Si制绒新方法,首先在HF/HNO3的富HNO3体系中对Si片进行一次腐蚀,之后在富HF体系中进行二次腐蚀,以优化表面织构,减少光在Si表面的反射损失。制绒后,用扫描电子显微镜(SEM)对Si片进行了表面形貌分析,用Carry 5000紫外-可见-近红外分光光度计测量反射谱线,得到未镀减反射膜(ARC)的二次腐蚀制绒的最低反射率为20.34%,比一次腐蚀制绒(22.70%)低2.36%。将二次腐蚀新工艺应用于太阳电池工业制备中,对电池输出参量进行检测分析。结果表明,经过二次腐蚀工艺处理的太阳电池开路电压(VOC)、短路电流(JSC)和效率η均比采用一次腐蚀工艺的太阳电池有不同程度的提高,制成的太阳电池最高效率为14.93%。     

An efficient method for monitoring the shunts in silicon solar cells during fabrication processes with infrared imaging

, and energy dispersive X-ray spectroscopy. Approaches for diminishing shunts are presented. The methods are beneficial for the optimization of the cell fabrication processes and the improvement of the cell performances.     

一种用红外热像监测硅太阳能光伏电池生产工艺的新方法

为了监测硅太阳电池生产工艺,设计了一套红外热像系统对单晶硅太阳电池的漏电情况进行研究。外加直流偏压时,太阳电池的漏电区域会明显发热。红外热像仪可以发现这些发热区域,从而确定漏电的位置。结合金相显微镜、扫描电子显微镜和能量色散谱等分析手段,总结了刻边、镀膜、丝网印刷和烧结工艺所造成的七种常见漏电形式。本文还提出了解决各类型漏电的可能方案,为进一步优化太阳电池生产工艺提供指导。     

超短脉冲激光对单晶硅太阳能电池的损伤效应

太阳能是未来的主要能源之一,关于太阳能电池的研究也逐渐成为热点。长期以来,人们对太阳能电池的高能粒子辐射特性进行了广泛的研究,对其激光辐照损伤特性的研究却很少。随着光电对抗技术的发展,对这方面的研究需求也越来越迫切。研究了532 nm、20 ns和300 ps脉冲激光对单晶硅太阳能电池的辐照效应,分析了超短脉冲激光对单晶硅太阳能电池的损伤机理。对比了超短脉冲激光和长脉冲激光、连续激光的损伤机理的异同。阐述了在激光单脉冲能量一定的情况下,损伤效果与脉宽和重频的关系。通过分析,指出了太阳能电池损伤的主因,激光对太阳能电池的破坏主要是依靠热效应。     

种子层对微晶硅锗太阳电池的影响

Using plasma enhanced chemical vapor deposition（PECVD） at 13.56 MHz,a seed layer is fabricated at the initial growth stage of the hydrogenated microcrystalline silicon germanium(μc-Si_1-xGe_x:H) i-layer.The effects of seeding processes on the growth ofμc-Si_1-xGe_x:H i-layers and the performance ofμc-Si_1-xGe_x:H p-i-n single junction solar cells are investigated.By applying this seeding method,theμc-Si_1-xGe_x:H solar cell shows a significant improvement in short circuit current density(J_sc) and fill factor（FF） with an acceptable performance of blue response as aμc-Si:H solar cell even when the Ge content x increases up to 0.3.Finally,an improved efficiency of 7.05%is achieved for theμc-Si_0.7Ge_0.3:H solar cell.     

Performance enhancement of multicrystalline silicon solar cells and modules using double‐layered SiNx:H antireflection coatings

Silicon nitride coating deposited by the plasma‐enhanced chemical vapor deposition method is the most widely used antireflection coating for crystalline silicon solar cells. In this work, we employed double‐layered silicon nitride coating consisting of a top layer with a lower refractive index and a bottom layer (contacting the silicon wafer) with a higher refractive index for multicrystalline silicon solar cells. An optimization procedure was presented for maximizing the photovoltaic performance of the encapsulated solar cells or modules. The dependence of their photovoltaic properties on the thickness of silicon nitride coatings was carefully analyzed. Desirable thicknesses of the individual silicon nitride layers for the double‐layered coatings were calculated. In order to get statistical conclusions, we fabricated a large number of multicrystalline silicon solar cells using the standard production line for both the double‐layered and single‐layered antireflection coating types. On the cell level, the double‐layered silicon nitride antireflection coating resulted in an increase of 0.21%, absolute for the average conversion efficiency, and 1.8 mV and 0.11 mA/cm² for the average open‐circuit voltage and short‐circuit current density, respectively. On the module level, the cell to module power transfer factor was analyzed, and it was demonstrated that the double‐layered silicon nitride antireflection coating provided a consistent enhancement in the photovoltaic performance for multicrystalline silicon solar cell modules than the single‐layered silicon nitride coating. Copyright © 2015 John Wiley & Sons, Ltd.     

Boron‐doped hydrogenated silicon carbide alloys containing silicon nanocrystallites for highly efficient nanocrystalline silicon thin‐film solar cells

Boron‐doped hydrogenated silicon carbide alloys containing silicon nanocrystallites (p‐nc‐SiC:H) were prepared using a plasma‐enhanced chemical vapor deposition system with a mixture of CH₄, SiH₄, B₂H₆ and H₂ gases. The influence of hydrogen dilution on the material properties of the p‐nc‐SiC:H films was investigated, and their roles as window layers in hydrogenated nanocrystalline silicon (nc‐Si:H) solar cells were examined. By increasing the R_H (H₂/SiH₄) ratio from 90 to 220, the Si―C bond density in the p‐nc‐SiC:H films increased from 5.20 × 10¹⁹ to 7.07 × 10¹⁹/cm³, resulting in a significant increase of the bandgap from 2.09 to 2.23 eV in comparison with the bandgap of 1.95 eV for p‐nc‐Si:H films. For the films deposited at a high R_H ratio, the Si nanocrystallites with a size of 3–15 nm were formed in the amorphous SiC:H matrix. The Si nanocrystallites played an important role in the enhancement of vertical charge transport in the p‐nc‐SiC:H films, which was verified by conductive atomic force microscopy measurements. When the p‐nc‐SiC:H films deposited at R_H = 220 were applied in the nc‐Si:H solar cells, a high conversion efficiency of 8.26% (V_oc = 0.53 V, J_sc = 23.98 mA/cm² and FF = 0.65) was obtained compared to 6.36% (V_oc = 0.44 V, J_sc = 21.90 mA/cm² and FF = 0.66) of the solar cells with reference p‐nc‐Si:H films. Further enhancement in the cell performance was achieved using p‐nc‐SiC:H bilayers consisting of highly doped upper layers and low‐level doped bottom layers, which led to the increased conversion efficiency of 9.03%. Copyright © 2015 John Wiley & Sons, Ltd.     

非晶硅太阳电池的衰减与退火

讨论了影响非晶硅太阳电池稳定性的因素,介绍了改善非晶硅材料稳定性的方法,进行了非晶硅太阳电池光致衰减测试.描述了电流注入退火和热退火对非晶硅太阳电池性能的改善.     

Design of Ag nanograting for broadband absorption enhancement in amorphous silicon thin film solar cells

Light trapping is one of the key issues to improve the light absorption and increase the efficiency of thin film solar cells. The effects of the triangular Ag nanograting on the absorption of amorphous silicon solar cells were investigated by a numerical simulation based on the finite element method. The light absorption under different angle and area of the grating has been calculated. Furthermore, the light absorption with different incident angle has been calculated. The optimization results show that the absorption of the solar cell with triangular Ag nanograting structure and anti-reflection film is enhanced up to 96% under AM1.5 illumination in the 300–800 nm wavelength range compared with the reference cell. The physical mechanisms of absorption enhancement in different wavelength range have been discussed. Furthermore, the solar cell with the Ag nanograting is much less sensitive to the angle of incident light. These results are promising for the design of amorphous silicon thin film solar cells with enhanced performance.     

17.6% efficient tricrystalline silicon solar cells with spatially uniform texture

Up to now solar cells fabricated on tricrystalline Czochralski‐grown silicon (tri‐Si) have shown relatively low short‐circuit current densities of about 31–33 mA/cm² because the three {110}‐oriented grains cannot effectively be textured by commonly used anisotropic etching solutions. In this work, we have optimised a novel chemical texturing step for tri‐Si and integrated it successfully into our solar cell process. Metal/insulator/semiconductor‐contacted phosphorus‐diffused n⁺p junction silicon solar cells with a silicon‐dioxide‐passivated rear surface and evaporated aluminium contacts were manufactured, featuring a spatially uniform surface texture over all three grains on both cell sides. Despite the simple processing sequence and cell structure, an independently confirmed record efficiency of 17.6% has been achieved. This excellent efficiency is mainly due to an increased short‐circuit current density of 37 mA/cm² obtained by substantially reduced reflection and enhanced light trapping. Copyright © 2003 John Wiley & Sons, Ltd.     

Bifacial potential of single‐ and double‐sided collecting silicon solar cells

Bifacial applications are a promising way to increase the performance of photovoltaic systems. Two silicon solar cell concepts suitable for bifacial operation are the passivated emitter, rear totally diffused (PERT) and the both sides collecting and contacted (BOSCO) cell concepts. This work investigates the bifacial potential of these concepts by means of in‐depth numerical device simulation and experiment with a focus on the impact of varying material quality. It is shown that the PERT cell concept (representing a structure with front‐side emitter only) requires high‐minority‐carrier‐diffusion‐length substrates with L_bulk > 3 × W (with cell thickness W) to exploit its bifacial potential, while the BOSCO cell (representing a structure with double‐sided emitter) can already utilise its bifacial potential on substrates with significantly lower diffusion lengths down to L_bulk ≈ 0.5 × W. Experimentally, BOSCO cells with and without activated rear‐side emitter are compared. For rear‐side illumination, the activated rear‐side emitter is measured to increase internal quantum efficiency at wavelengths λ < 850 nm by up to 45%_abs (factor of 9) and 30%_abs (factor of 2) for cells processed on p‐type multicrystalline silicon substrates with L_bulk ≈ 0.3 × W and L_bulk ≈ 2.6 × W, respectively. For PERT cells processed on n‐type Czochralski‐grown silicon substrates, an according increase in internal quantum efficiency for rear‐side illumination of more than 20%_abs (factor of 1.3) is measured when changing from a substrate with L_bulk ≈ 3.0 to 10.0 × W. The performed simulations and experiments demonstrate that the BOSCO cell concept is a promising candidate to successfully exploit bifacial gain also on low‐ to medium‐diffusion‐length substrates such as p‐type multicrystalline silicon, while PERT cells require a high‐diffusion‐length substrate to utilise their bifacial potential. Furthermore, the BOSCO cell concept is shown to be a promising option to achieve highest output power densities, even when using lower quality and therefore possibly more cost‐effective silicon substrates. Copyright © 2016 John Wiley & Sons, Ltd.