Fabrication of large-grain polycrystalline Ge films using absorptive films

Amorphous germanium (a-Ge) films in samples with or without an absorptive film are crystallized by short-pulse XeF excimer laser crystallization (ELC). An in situ time-resolved optical reflection and transmission (TRORT) monitoring system combining a cw He-Ne probe laser, a digital oscilloscope and two photodetectors is developed to investigate the melting and resolidification dynamics of Ge films during ELC. TRORT measurements reveal that the longest melt duration is prolonged from 250 to 1000 ns by adding absorptive films in the samples. Absorptive films are shown to be effective in improving the melt duration of the molten state and the grain size of polycrystalline Ge films. The grain size with a diameter of approximately 12 μm can be fabricated in the superlateral growth regime for 90-nm-thick a-Ge films at room temperature in air by single-shot ELC.     

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.     

Compatibility of glass textures with E-beam evaporated polycrystalline silicon thin-film solar cells

For polycrystalline silicon thin films on glass, E-beam evaporation capable of high-rate deposition of amorphous silicon (a-Si) film precursor up to 1 μm/minute is a potentially low-cost solution to replace the main stream a-Si deposition method—plasma enhanced chemical vapour deposition (PECVD). Due to weak absorption of near infrared light and a target of 2 μm Si absorber thickness, glass substrate texturing as a general way of light trapping is vital to make E-beam evaporation commercially viable. As a result, the compatibility of e-beam evaporation with glass textures becomes essential. In this paper, glass textures with feature size ranging from ～200 nm to ～1.5 micron and root-mean-square roughness (Rms) ranging from ～10 nm to 200 nm are prepared and their compatibility with e-beam evaporation is investigated. This work indicates that e-beam evaporation is only compatible with small smooth submicron sized textures, which enhances J _sc by 21 % without degrading V _oc of the cells. Such textures improve absorption-based J _sc up to 45 % with only 90 nm SiN _x as the antireflection and barrier layer; however, the enhancement degrades to ～10 % with 100 nm SiO _x +90 nm SiN _x as the barrier layer. The absorption-based J _sc is abbreviated by J _sc(A), which is deduced by integrating the multiplication product of the measured absorption and the AM1.5G spectrum in the wavelength range 300–1050 nm assuming unity internal quantum efficiency at each wavelength. This investigation is also relevant to other thin-film solar cell technologies which require evaporating the absorber onto textured substrate/superstrate.     

Chemical vapour etching for double-sided buried metallic contacts in polycrystalline silicon solar cells processing

In this paper, we investigate the effect of double-sided buried metallicontacts (BMCs) on the photovoltaic performances of polycrystalline-silicon (pc-Si) solar cells. Prior to junction formation, groove patterns were achieved on both surface sides of the pc-Si wafers using a chemical vapour etching-based technique. The BMCs were realized onto the grooved areas using the screen printing technique. Once the BMCs were formed, we point out a significant increase of the spectral response in the 400-1100 nm wavelength range, particularly at short and long wavelengths. These results were attributed to an enlargement of the contact areas in both emitter and base regions of the cells, leading to a reduction of their effective thickness and to an enhancement of the minority carrier collection. The dark I-V characteristics of BMCs-based pc-Si cells show a significant reduction of the reverse current together with an improvement of the rectifying behaviour. The I-V characteristic under AM1.5 illumination shows that the introduction of BMCs in polycrystalline silicon solar cells processing improves the conversion efficiency by about 2.5-3.5%.     

太阳电池用本征微晶硅材料的制备及其结构研究

采用VHF-PECVD技术制备了系列不同硅烷浓度和反应气压的微晶硅薄膜.运用拉曼散射光谱和 x射线衍射对制备的材料进行了结构分析.在实验研究的范围内，制备材料的晶化程度随硅烷 浓度的增加而降低.XRD的测试结果表明：制备的微晶硅材料均体现了(220)方向择优.应用在 电池的有源层中，制备出了效率达7.1%的单结微晶硅太阳电池，电池的结构是glass/ZnO/p( μc-Si:H)/i(μc-Si:H)/n(a-Si:H/Al)，没有ZnO背反射电极，有源层的厚度仅为1.2μm. 关键词： 本征微晶硅薄膜 拉曼光谱 x射线衍射     

Nanocystalline silicon solar cells

Nanocrystalline silicon material has made rapid progress in the last several years and at present it can be defined as real device quality as a photoactive layer for solar cells. A number of innovative ideas, such as the deposition at the crystalline to amorphous transition, at high pressure depletion condition, by taming of the ion energy, by grading of the material growth, at reduced unwanted dopant incorporation, have helped to reach an efficiency of 10% for single junction nanocrystalline silicon cells. In situ plasma and gas phase diagnosis have contributed to the fast optimisation of deposition process parameters. Deposition rate, open circuit voltage and light confinement are some of most critical issues that are currently pursued. Materials with a defect density as low as 10¹⁵ cm⁻³ have been made, however, they are still not good enough for n–p junctions; the device structure is still of drift type in a p–i–n or n–i–p configuration.     

Potential of amorphous silicon for solar cells

This paper reviews recent developments in the field of amorphous-silicon-based thin-film solar cells and discusses potentials for further improvements. Creative efforts in materials research, device physics, and process engineering have led to highly efficient solar cells based on amorphous hydrogenated silicon. Sophisticated multijunction solar cell designs make use of its unique material properties and strongly suppress light induced degradation. Texture-etching of sputtered ZnO:Al films is presented as a novel technique to design optimized light trapping schemes for silicon thin-film solar cells in both p-i-n and n-i-p device structure. Necessary efforts will be discussed to close the efficiency gap between the highest stabilized efficiencies demonstrated on lab scale and efficiencies achieved in production. In case of a-Si:H/a-Si:H stacked cells prepared on glass substrates, significant reduction of process-related losses and the development of superior TCO substrates on large areas promise distinctly higher module efficiencies. A discussion of future perspectives comprises the potential of new deposition techniques and concepts combining the advantages of amorphous and crystalline silicon thin-film solar cells. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 14 June 1999     

High-temperature laser annealing for thin film polycrystalline silicon solar cell on glass substrate

Thin film polycrystalline silicon films grown on glass substrate were irradiated with an infrared continuous wave laser for defects annealing and/or dopants activation. The samples were uniformly scanned using an attachment with the laser system. Substrate temperature, scan speed and laser power were varied to find suitable laser annealing conditions. The Raman spectroscopy and Suns-V _oc analysis were carried out to qualify the films quality after laser annealing. A maximum enhancement of the open circuit voltage V _oc of about 100?mV is obtained after laser annealing of as-grown polysilicon structures. A strong correlation was found between the full width half maximum of the Si crystalline peak and V _oc. It is interpreted as due to defects annealing as well as to dopants activation in the absorbing silicon layer. The maximum V _oc reached is 485?mV after laser treatment and plasma hydrogenation, thanks to defects passivation.     

Fabrication and characterization of polycrystalline silicon nanowires with silver-assistance by electroless deposition

Single crystal silicon wafers are widely used as the precursors to prepare silicon nanowires by employing a silver-assisted chemical etching process. In this work, we prepared polycrystalline silicon nanowire arrays by using solar-grade multicrystalline silicon wafers. The chemical composition and bonding on the surface of silicon nanowire arrays were characterized by Fourier Transform Infrared spectroscope, and X-ray photoelectron spectroscope. The photoluminescence spectra of silicon nanowires show red light emissions centered around 700 nm. Due to the passivation effect of Si dangling bonds by concentrated HNO₃ aqueous solution, the photoluminescence intensities are improved by 2 times. The influences of surface chemical states on the wettability of silicon nanowire arrays were also studied. We obtained a superhydrophobic surface on the as-etched silicon nanowire arrays without surface modification with any organic low-surface-energy materials, and realized the evolution from superhydrophobicity to superhydrophilicity via surface modifications with HNO₃ solutions.     

A glass thinning and texturing method for light incoupling in thin‐film polycrystalline silicon solar cells application

For polycrystalline silicon (poly‐Si) thin‐film solar cells on ～3 mm borosilicate glass, glass thinning reduces the glass absorption and light leaking to neighbouring cells; the glass texturing of the sun‐facing side suppresses reflection. In this Letter, a labour‐free wet etching method is developed to texture and thin the glass at the same time in contrast to conventionally separated labour‐intensive glass thinning and texturing processes. For 2 cm² size poly‐Si thin‐film solar cells on glass superstrate, this wet etching successfully thins down the glass from 3 mm to 0.5 mm to check the ultimate benefit of the process and introduces a large micron texture on the sun‐facing glass surface. The process enhances J_sc by 6.3% on average, with the optimal J_sc enhancement of 8%, better than the value of 4.6% found in the literature. This process also reduces the loss in external quantum efficiency (EQE loss), which is due to light leaking to neighbouring cells, dramatically. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Boron-doped nanocrystalline silicon thin films for solar cells

This article reports on the structural, electronic, and optical properties of boron-doped hydrogenated nanocrystalline silicon (nc-Si:H) thin films. The films were deposited by plasma-enhanced chemical vapour deposition (PECVD) at a substrate temperature of 150 °C. Crystalline volume fraction and dark conductivity of the films were determined as a function of trimethylboron-to-silane flow ratio. Optical constants of doped and undoped nc-Si:H were obtained from transmission and reflection spectra. By employing p⁺ nc-Si:H as a window layer combined with a p′ a-SiC buffer layer, a-Si:H-based p-p′-i-n solar cells on ZnO:Al-coated glass substrates were fabricated. Device characteristics were obtained from current-voltage and spectral-response measurements.     

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%.     

Studies on the polycrystalline silicon/SiO_2 stack as front surface field for IBC solar cells by two-dimensional simulations

Interdigitated back contact(IBC) solar cells can achieve a very high efficiency due to its less optical losses. But IBC solar cells demand for high quality passivation of the front surface. In this paper, a polycrystalline silicon/SiO_2 stack structure as front surface field to passivate the front surface of IBC solar cells is proposed. The passivation quality of this structure is investigated by two dimensional simulations. Polycrystalline silicon layer and SiO_2 layer are optimized to get the best passivation quality of the IBC solar cell. Simulation results indicate that the doping level of polycrystalline silicon should be high enough to allow a very thin polycrystalline silicon layer to ensure an effective passivation and small optical losses at the same time. The thickness of SiO_2 should be neither too thin nor too thick, and the optimal thickness is 1.2 nm.Furthermore, the lateral transport properties of electrons are investigated, and the simulation results indicate that a high doping level and conductivity of polycrystalline silicon can improve the lateral transportation of electrons and then the cell performance.     

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.     

Radiation effects in silicon solar cells

Silicon solar cells have been utilized as the principal source of electrical energy for space satellites during the past decade. Despite the reliability of these photovoltaic devices, degradation of their power output by charged particle radiation in the earth's geomagnetic field has continued to be the primary problem for their use on flights of long-duration. A study of radiation damage induced by 1 MeV electrons in a variety of current silicon solar cell types has been conducted as a function of dopant impurity and resistivity of the base region. A companion study of radiation damage induced by nominal 0.2 MeV protons was performed in solar cells with coverslips having small cell areas exposed alongside the coverslip. The photovoltaic current-voltage characteristics were measured under a solar simulator emitting 140mW/cm² at air mass zero. Irradiations were performed at room temperature to fluences of 1 × 10¹⁵ e/cm² and 1 × 10¹⁵ p/cm². The efficiency of 10 ohm-cm cells after large fluences was superior to cells of 2 ohm-cm base resistivity. No significant differences were observed between boron- and aluminum-doped cells. Solar cell exposure to low energy Protons resulted in an 8 per cent loss in power at a fluence of 1 × 10¹⁴ p/cm² when as little as 2 per cent of the solar cell surface was left unshielded.     

钙钛矿/晶硅叠层太阳能电池的研究进展

有机—无机杂化钙钛矿电池因其禁带宽度可调、光吸收系数高、光电转化效率高、制备成本低等优点而被用于硅基叠层太阳能电池中,使得太阳能电池的转换效率提高,生产成本降低,应用范围也更为广泛。文章介绍了钙钛矿吸收材料和钙钛矿/晶硅叠层电池的工作原理,对钙钛矿/晶硅叠层电池的类别、影响其性能的主要因素进行了归纳综述,对钙钛矿/晶硅叠层电池未来发展进行了展望。     

Thin macroporous silicon heterojunction solar cells

We demonstrate the processing of a heterojunction solar cell from a purely macroporous silicon (MacPSi) absorber that is generated and separated from a monocrystalline n‐type Cz silicon wafer by means of electrochemical etching. The etching procedure results in straight pores with a diameter of (4.7 ± 0.2) µm and a distance of 8.3 µm. An intrinsic amorphous Si (a‐Si)/p⁺‐type a‐Si/indium tin oxide (ITO) layer stack is on the front side and an intrinsic a‐Si/n⁺‐type a‐Si/ITO layer stack is on the rear side. The pores are open when depositing the layers onto the 3.92 cm²‐sized cell. The conductive layers do not cause shunting through the pores. A silicon oxide layer passivates the pore walls. The energy‐conversion efficiency of the (33 ± 2) µm thick cell is 7.2%. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High efficiency nanotextured silicon solar cells

We report the computational modeling of forward scattering phenomena arising in Au nanoparticles array near their localized surface plasmon resonance, which by producing a strong field enhancement effect on the substrate leads to higher optical absorption and, therefore, higher efficiencies of operation. Computational calculations indicate that the ultimate efficiency of an optimized silicon nanoholes (SiNH) array texture surface in combination with the surface and bottom-of-a-trench Au nanoparticles array described herein, is 39.67%, which compares favorably with the ultimate efficiency of 31.11% for an optimized silicon nanoholes array texture surface. Furthermore, the utilization of an optimized silicon nitride antireflection coating increases the ultimate efficiency to a promising value of 41.88%, while the utilization of a single-crystal silicon layer of thickness 2.8 μm will be instrumental in drastically reducing solar cell manufacturing cost.