Effects of grain boundaries on the current-voltage characteristics of polycrystalline silicon solar cells

The current components associated with the grain boundaries of diffused p/n junction polysilicon solar cells made on n- and p-type Wacker substrates are analyzed and experimentally identified. New electrical methods for determining the presence or absence of preferential diffusion along the grain boundaries and for determining the average doping density of preferentially diffused regions along the grain boundaries are described. For p-type substrates, these methods revealed preferential phosphorus diffusion along grain boundaries; no preferential boron diffusion along grain boundaries was observed. The recombination current components were analyzed for the cells in which preferential diffusion occurred. The analysis shows that the dominant current component at small bias levels (0-300 mV) is the recombination current at the grain boundaries within the p/n junction space-charge region. At higher bias levels (V simeq V_{OC} simeq 500-600mV), both this current component and the current component due to recombination at that part of the grain boundary below the preferentially diffused region are important. The grain-boundary shunt resistance does not contribute a significant current component. It is shown that the preferential diffusion makes negligible the recombination current injected into the sidewall of the preferentially diffused region. This is consistent with a model in which the phosphorus diffusion significantly lowers the surface recombination velocity at the grain boundaries and in which the retarding built-in electric field further decreases the recombination current.     

Electronic processes at grain boundaries in polycrystalline semiconductors under optical illumination

The dependence of minority carrier lifetime (τ) on the doping concentration Nd, grain sizedand interface state density Nisat the grain boundaries in (n-type) polycrystalline semiconductors has been calculated analytically. The recombination velocity at grain boundaries is enhanced by the diffusion potential Vdadjacent to the boundaries, and ranges fromsimeq 10^{2}to 10⁶cm . s^-1depending on Nisand Nd. Under illumination, the population of the interface states is altered considerably from its dark level and as a result, Vddecreases to that value which maximizes recombination (equal concentrations of electrons and holes at the boundary). This causes τ to decrease with increasing Nd. Sample calculations for polycrystalline silicon show that for low angle boundaries with interface state densities ofsimeq 10^{11}cm^-2eV^-1, τ decreases from 10^-6to 10^-10s as the grain size is reduced from 1000 to 0.1 µm (forN_{d} = 10^{16}cm^-3). For a constant grain size, τ decreases with increasing Nd. The open-circuit voltage of p-n junction solar cells decreases fortau leq 10^{-7}s, whereas that for Schottky barrier cells remains at its maximum value untiltau lsim 10^{-8}s.     

Recombination velocity at grain boundaries in polycrystalline Si under optical illumination

Calculations have been performed of recombination currents at grain boundaries in polycrystalline silicon for three different energy distributions of recombination centers. These results show that the relationship between the recombination current density and the minority carrier concentration at the grain boundary, under conditions of optically induced separation of the electron and hole quasi-Fermi levels, is highly nonlinear. It is further shown that the recombination velocity is an increasing function of the minority carrier concentration at the grain boundary. In this calculation, it is necessary to relax the earlier assumptions [1] of equal capture cross sections for electrons and holes of the grain boundary recombination centers and [2] of a flat minority-carrier quasi-Fermi level in the space-charge region, since these assumptions prove to be unjustified in the general case.     

Effects on the open-circuit voltage of grain boundaries within the junction space-charge region of polycrystalline solar cells

A physical explanation is given for the observed dependence of open-circuit voltage on grain size in polycrystalline solar cells when no such dependence is seen for short-circuit current. This explanation identifies carrier recombination through grain-boundary surface states within the junction space-charge region as a primary mechanism underlying these dependencies. Experimental data that support this explanation are discussed, and possible ways of improving the conversion efficiency of polycrystalline solar cells are indicated.     

Examination of CCDs using voltage contrast with a scanning electron microscope

The voltage contrast mode of a scanning electron microscope (SEM) is utilized to observe charge-coupled devices (CCDs) which have been cross sectioned. A new cross sectioning technique which uses anisotropic etching to accurately define the axis along which fracture occurs is presented. Lapping is not required in this technique, as smooth surfaces result from the controlled fracturing. SEM imaging of the region just beneath a CCD double-level, polycrystalline silicon electrode structure revealed a region of contrast which appeared and disappeared under the presence and absence of an applied pulsed bias. Application of a pulsed bias to the CCD output gate is seen to result in the transient charging of the output floating diffusion, while application of a dc bias is seen to not result is such charging.     

Metal fingers on grain boundaries in multicrystalline silicon solar cells

We have developed a method of applying a net‐like finger grid to the front of multicrystalline (mc) silicon solar cells, which lies mainly on the grain boundaries (grain‐boundary‐oriented‐finger GBOF grid). This net has no busbars. It is drawn by a plotter, using screen printing paste dispensed through a fine tube. The power output of cells contacted in this manner has been tested in a statistical study of pairs and triplets of cells of size 100 × 100 mm² (Bayer) and 103 × 103 mm² (Eurosolare). In the pairs study, pairs of neighbouring wafers of the original ingot were processed into solar cells. One wafer received a GBOF grid, the other got the same grid rotated by 90°, and so had little coverage of grain boundaries. In the study of triplets the third wafer of each triplet was equipped with a standard H‐pattern of the same shading as the GBOF grid. In the pairs study, we find that under approximately standard conditions there is an 89% chance that the GBOF grid increases power output over cells with an identical, but 90° rotated, grid, the most probable increase being 2.6%. The triplets study shows that there is an 87% chance that the GBOF grid increases power output over cells with the standard H‐pattern, the most probable increase being 2.5%. Copyright © 2002 John Wiley & Sons, Ltd.     

Differential amplitude imaging in scanning optical microscope

A differential amplitude scanning optical microscope (DASOM) is described. The system, which uses a simple optical and electronic arrangement, is capable of detecting variations in optical reflectivity of 9 × 10?6 in a 10kHz bandwidth. Results of surface studies of polished stainless steel and of natural diamond are presented.     

The effects of grain boundaries in the electrical characteristics of large grain polycrystalline thin-film transistors

High-performance low-voltage thin-film transistors (TFTs) can be fabricated by grain-enhancement methods such as nickel-seeded metal-induced lateral crystallization (MILC). Electrical characteristics of the TFTs may vary due to the existence of the grain boundaries in the device active region. To obtain the best device characteristics, the effect of grain boundaries on the device must be investigated. In this paper, the cumulative distributions of the device properties such as leakage current, threshold voltage, subthreshold slope, and field-effect mobility as a function of different channel lengths and widths were studied. In general, the grain boundary effects decrease with device size. Devices with short channel lengths and wide channel widths may suffer from degradation due to large leakage current. Moreover, the effects due to the location of the nickel-seeding region on device characteristics were investigated. These include the effect of the longitudinal and lateral grain boundaries and the distance between the nickel seeding region and the device. Finally, a design guideline to reduce the grain boundary effect is presented.     

The SEM-EBIV signals near the grain boundary in a polycrystalline solar cell

A new technique of EBIV measurements has been proposed. The open-circuit voltage signal modified by a grain boundary (GB) assuming a simple equivalent circuit of a polycrystalline solar cell has been analysed. The numerical calculations of the voltage contrast as a function of the distance between the electron-hole generation sphere and the GB were performed. The EBIC and EBIV characteristics have been compared on account of their usefulness for evaluating the carrier diffusion length, the recombination velocity at the grain boundary, and the shunt resistance of the cell. It was found that EBIV measurements provided more detailed information about the electrical properties of the GB and the junction parameters of a polycrystalline solar cell.     

The contrast characteristics of the grain boundary near the boundary layer in polycrystalline solar cells

The expression for the short-circuit current near the grain boundary of a polycrystalline solar cell is obtained. The grain boundary is characterised by five parameters: left- and right-side diffusion lengths and appropriate effective recombination velocities and a boundary layer width. The radial generation function is described by the Gaussian distribution, and the axial one by a trigonometric function.     

High efficiency polycrystalline silicon solar cells using lowtemperature PECVD process

Conventionally directionally solidified (DS) and silicon film (SF) polycrystalline silicon solar cells are fabricated using gettering and low temperature plasma enhanced chemical vapor deposition (PECVD) passivation. Thin layer (~10 nm) of PECVD SiO₂ is used to passivate the emitter of the solar cell, while direct hydrogen rf plasma and PECVD silicon nitride (Si₃N₄) are implemented to provide emitter and bulk passivation. It is found in this work that hydrogen rf plasma can significantly improve the solar cell blue and long wavelength responses when it is performed through a thin layer of PECVD Si₃N₄. High efficiency DS and SF polycrystalline silicon solar cells have been achieved using a simple solar cell process with uniform emitter, Al/POCl₃ gettering, hydrogen rf plasma/PECVD Si₃N₄ and PECVD SiO₂ passivation. On the other hand, a comprehensive experimental study of the characteristics of the PECVD Si₃N₄ layer and its role in improving the efficiency of polycrystalline silicon solar cells is carried out in this paper. For the polycrystalline silicon used in this investigation, it is found that the PECVD Si₃N₄ layer doesn't provide a sufficient cap for the out diffusion of hydrogen at temperatures higher than 500°C. Low temperature (⩽400°C) annealing of the PECVD Si₃N ₄ provides efficient hydrogen bulk passivation, while higher temperature annealing relaxes the deposition induced stress and improves mainly the short wavelength (blue) response of the solar cells     

Enhanced carrier collection at grain-boundary barriers in solar cells made from large grain polycrystalline material

The case of the solar cell in polycrystalline material is investigated specifically in view of the electronic properties of grain-boundaries. Material in thin film form, ribbons or cast ingots display a preponderance of vertically oriented grains. This and the fact that grain-boundaries are excellent photo-electric converters, when correctly contacted, is the basis for a new diffusion- and contacting scheme. It makes use of the grain-boundaries as vertical junctions, enhancing the photo-current. The argument is substantiated by a discussion of the electronic features of grain-boundaries under these conditions and the enhancement of the photovoltage due to the frequency extension of the photoresponse.     

Effects of crystallization on trap state densities at grain boundaries in polycrystalline silicon

The relationship between crystallization processes in the formation of polycrystalline-silicon (poly-Si) films and trap state densities at grain boundaries is described. Three different crystallization techniques were used to obtain poly-Si films: 1) LPCVD, 2) solid-phase crystallization, and 3) laser recrystallization. Trap state densities in laser-recrystallized poly-Si are 9.2-9.6 × 10¹¹cm^-2, regardless of grain size. These values are half of those in LPCVD and solid-phase crystallized poly-Si. It is indicated quantitatively that laser-induced melting and the subsequent solidification process exert a significant influence on the electrical activity of silicon grain boundaries.     

Picosecond electrical sampling using a scanning force microscope

A scanning force microscope probe for measuring ultrafast voltage signals is demonstrated. The new technique is based on mixing due to the square-law force interaction present between the microscope tip and sample. Correlation of 100 ps pulses and mixing up to 20 GHz have been achieved.<>     

用扫描近场光学显微镜观察微畴

本文利用反射式扫描近场光学显微镜观察了硫酸三甘氨酸 [(NH3CH2 COOH) 3·H2 SO4 ) ](简称TGS)晶体的电畴结构和铝酸镧晶体的孪晶畴。横向分辨率约为 5 0nm。对原有的TopometrixAuroraNSOM系统作了较大的改进。采用音叉 (tuningfork)检测光纤探针与样品间的剪切力取代了原有的光学法振动检测。对TGS的观察说明 ,反射式扫描近场光学显微镜适合研究垂直B轴切割的TGS( 0 1 0 )面的自发极化。对这种 1 80°极化的多畴 ,可获得光学衬度较好的电畴分布图像。与形貌图像相比电畴与形貌无关。无论是新鲜解理的原子级光滑表面和表面水解的较为粗糙表面均可观察到分布较为均匀的电畴分布。采用探针发光的透射式和反射式观察铝酸镧晶体 ,发现只有反射式探测能够给出晶体表面的孪晶畴     

Damping vibration of scanning near-field optical microscope probe using the Timoshenko beam model

The effect of interactive damping on the flexural vibration frequency for the scanning near-field optical microscope (SNOM) fiber probe based on the Timoshenko beam theory, including the effects of shear deformation and rotary inertia, has been analyzed. In the analysis, the effects of the transverse contact stiffness, damping factor and the ratio of different probe dimensions on the damping vibration frequency were studied. The results show that increasing the ratio of probe length to radius increases the damping vibration frequency of mode 1. Besides, the damping vibration frequencies, based on the Bernoulli-Euler beam theory and the Timoshenko beam theory, are compared. When the contact stiffness is very large for the higher modes, the effects of shear deformation and rotary inertia on the frequency becomes significant. Furthermore, increasing the damping factor increases the vibration frequency. The trend is more obvious, especially dimensionless damping factor η_f>0.4.     

The preparation and properties of thin polycrystalline GaAs solar cells with grain boundary edge passivation

This paper describes the preparation and properties of thin-film (9-µm) polycrystalline GaAs solar cells, fabricated on molybdenum substrates by the metalorganic process. These cells, of a Schottky structure, have an efficiency of 5.7 percent under simulated AM1 conditions, with no antireflective (AR) coating. This is achieved by passivating the grain boundary edges, resulting in a reduction of shunt leakage by five to six decades. The effect of treatments on the resulting electrical properties is described; both treatments during fabrication as well as on complete devices are considered here. These include a critic acid etch treatment prior to anodization, and hydrochloric acid treatments on completed devices. The effect of annealing is also described briefly together with techniques by which full recovery of cell performance can be achieved. Finally, transport properties are outlined briefly in order to highlight the primary current transport mechanisms in these cells.     

偏振光调制的扫描近场光学显微镜应用

本文利用电光调制技术结合反射式扫描近场光学显微镜，获得了可以对光的偏振方向进行调制的针尖发光远场接收反射光的近场显微镜。对原有的Topometrix Aurora NSOM系统作了较大的改进。采用音叉(tuning fork)检测光纤探针与样品间的剪切力取代了原有的光学法振动检测，避免了杂散光干扰。观察了(Ba0.5Sr0.5)TiO3薄膜和LiTaO3晶体表面的电畴结构。横向分辨率约为50nm。观察说明，反射式扫描近场光学显微镜适合研究固体表面的电畴结构，可获得光学衬度较好的电畴分布图像。与形貌图像相比电畴与形貌无关。