Assessment of ultra-thin Si wafer thickness in 3D wafer stacking

3D integration with multi-stacked wafers is a promising option to enhance device performance and density beyond traditional device scaling limits. However, to bring wafer stacking into reality, there are many technological challenges to be resolved, and one of those is the problem of uniform Si wafer thinning. For multi-stacked devices, Si wafers must be drastically thinned down to less than 50 μm. Problems associated with such ultra-thin Si wafers range from basic wafer handling to difficulty in accurately assessing the thickness of the thinned wafer across the wafer. In this study, bonded wafer pairs have been prepared with different bonding materials, and the stacks were ground down to about 30 μm. The thickness of the ultra-thin wafers was measured by Fourier transform infrared spectrometry (FTIR) technique, and its stability based on bonding status as well as measuring issues will be discussed.     

Influence of defect type on hydrogen passivation efficacy in multicrystalline silicon solar cells

We examine the effectiveness of hydrogen passivation as a function of defect type and microstructure at grain boundaries (GBs) in multicrystalline silicon. We analyze a solar cell with alternating mm‐wide bare and SiN_x‐coated stripes using laser‐beam‐induced current, electron backscatter diffraction, X‐ray fluorescence microscopy, and defect etching to correlate pre‐ and post‐hydrogenation recombination activity with GB character, density of iron‐silicide nanoprecipitates, and dislocations. A strong correlation was found between GB recombination activity and the nature/density of etch pits along the boundaries, while iron silicide precipitates above detection limits were found to play a less significant role. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of surface texture on the defect‐induced breakdown behavior of multicrystalline silicon solar cells

The defect‐induced diode breakdown behavior in multicrystalline silicon solar cells, which is located at recombination active crystal defects, is influenced by the surface texturization because the wet chemical treatment selectively etches grain boundaries and dislocations, resulting in etch pits. On textured surfaces, the defect‐induced breakdown voltage is decreased, and the slope of the local reverse I–V characteristics in breakdown is steeper. We find that the local defect‐induced breakdown voltage correlates with the depth of the etch pits. It is suggested that the enhanced electric field in the space charge region at the tip could be superimposed by an electric field around metallic precipitates because of the internal Schottky contact formation with the surrounding silicon. The combined electric field could be responsible for the dependence of the defect‐induced breakdown behavior on the surface texture. Copyright © 2012 John Wiley & Sons, Ltd.     

Effect of anodic-oxide layer thickness on the performance of GaAs MOS solar cells

Native oxide layers on n-type GaAs have been grown by aqueous anodic oxidation technique using AGW electrolyte. The effect of thickness of the As2O3rich interfacial oxide layer having a resistivity ~ 10¹⁴Ω . cm on the performance of GaAs MOS solar cells has been investigated. An attempt has been made to optimize the oxide layer thickness for achieving optimum efficiency.     

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.     

Influence of metal-covered area on performance of solar cells

Analytical expressions are proposed showing that the efficiency and fill factor of solar cells depend on the three normalised parameters rm, rL and g. These expressions are obtained with a new theoretical model, representing the metal-covered area of the solar cell.     

A study of mottling phenomenon on textured multicrystalline silicon wafers and its potential effects on solar cell performance

The mottling phenomenon refers to the appearance of irregular dark patterns on HF–HNO₃ acid etching textured multicrystalline silicon (mc-Si) wafers. The mottles have been identified as clusters of dislocation etch pits. In the acidic texturization, conditions favoring light reflectivity reduction usually lead to enhanced mottling. In a belief of adverse effect of the mottles to cell performance, light reflectivity reduction is more or less compromised in industry to avoid the mottling. The present study aims to identify whether appearance of the mottles alone really adversely affects the wafers minority carrier lifetime. Both serial examinations of an acid etched mc-Si wafer sample and parallel examinations of neighboring pairs of mc-Si wafer samples etched in acids of different HF/HNO₃ ratios were carried out. The results show that development of the mottling, i.e., growth of dislocation etch pits, does not deteriorate mc-Si wafers in their minority carriers lifetime; rather it even slightly increases the lifetimes. Light reflectivity measurement and modeling show that the mottles can contribute to reduction of light reflectivity, and ~3% relative reduction of reflectivity is expected for multicrystalline silicon wafers of ordinary level of dislocation density. Removal of the defected zone surrounding a dislocation by the etching is postulated as a reason for the observed mottling-enhancement of the lifetime. It is further postulated that, in texturization of mc-Si wafers for cell production, instead of compromising light reflectivity reduction to avoid the mottling, it may be better to pursue lower light reflectivity, allowing some extent of the mottling. Meanwhile, more attention should be paid to compatibility of the cone-shaped dislocation etch-pit with grid printing of solar cells.     

等离子体刻蚀对CdTe太阳电池性能的影响

采用不同干法腐蚀条件下的CdTe薄膜制成器件, 通过I-V、C-V和光谱响应等测试 了电池性能参数。结果表明,溅 射时间太短和功率太小时不能完全去除氧化层,溅射时间过长和功率过高会对薄膜表面造成 损伤, 影响器件性能。 通 过选择器件性能较好的电池、 找出适合等离子束溅 射工艺的条件,所制成的电池转化效率 达 到10.99%;而湿法腐蚀所 制成器件的转化效率为 10.26%。由此可以认为,等离子束轰击溅射的 腐蚀方法较湿法腐蚀更适用于CdTe太阳电池的制备。     

Influence of GaInP ordering on the performance of GaInP solar cells

CuPt-type ordering with undesirable properties always occurs in GaInP at growth conditions that are very close to those leading to the highest quality material in metal organic chemical vapor deposition. In this work, highly disordered GaInP with high crystalline quality was obtained by optimizing growth conditions. Room-temperature and low-temperature photoluminescence (PL) spectra of AlGaInP/GaInP/AlGaInP double heterostructures (DHs) reveal that the band edge emission intensity is enhanced by optimizing growth temperature, V/III ratio, and reactor pressure at the expense of low energy peak originating from spatially indirect recombination due to the ordering-related defects. The DH sample with less ordering-related defects demonstrates a longer effective minority carrier lifetime, consequently, the GaInP solar cell shows a significant improvement in the performance.     

High‐efficiency solar cells on phosphorus gettered multicrystalline silicon substrates

Measurements of the dislocation density are compared with locally resolved measurements of carrier lifetime for p‐type multicrystalline silicon. A correlation between dislocation density and carrier recombination was found: high carrier lifetimes (>100 µs) were only measured in areas with low dislocation density (<10⁵ cm⁻²), in areas of high dislocation density (>10⁶ cm⁻²) relatively low lifetimes (<20 µs) were observed. In order to remove mobile impurities from the silicon, a phosphorus diffusion gettering process was applied. An increase of the carrier lifetime by about a factor of three was observed in lowly dislocated regions whereas in highly dislocated areas no gettering efficiency was observed. To test the effectiveness of the gettering in a solar cell manufacturing process, five different multicrystalline silicon materials from four manufacturers were phosphorus gettered. Base resistivity varied between 0·5 and 5 Ω cm for the boron‐ and gallium‐doped p‐type wafers which were used in this study. The high‐efficiency solar cell structure, which has led to the highest conversion efficiencies of multicrystalline silicon solar cells to date, was used to fabricate numerous solar cells with aperture areas of 1 and 4 cm². Efficiencies in the 20% range were achieved for all materials with an average value of 18%. Best efficiencies for 1 cm² (20·3%) and 4 cm² (19·8%) cells were achieved on 0·6 and 1·5 Ω cm, respectively. This proves that multicrystalline silicon of very different material specification can yield very high efficiencies if an appropriate cell process is applied. Copyright © 2006 John Wiley & Sons, Ltd.     

Influence of bandgap narrowing on the performance of silicon n-p solar cells

This paper deals with the incorporation of bandgap narrowing in the modelling of n⁺-p solar cells. First, the physcial model on which the computations are based, is explained. Second, the technology used to fabricate the solar cells, and the measurement technique for the minority carrier lifetime are commented. Then a detailed comparison between measured and computed results of I_sc, V_oc and P_max follows and the importance of inclusion I72 of the bandgap narrowing is illustrated. The theoretical case where Auger—recombination is the only recombination process is also treated. Finally computed results obtained for solar cells with different surface concentrations are shown.     

Influence of the layer parameters on the performance of the CdTe solar cells

Influence of the layer parameters on the performances of the CdTe solar cells is analyzed by SCAPS-1D. The ZnO:Al film shows a high efficiency than SnO2:F. Moreover, the thinner window layer and lower defect density of CdS films are the factor in the enhancement of the short-circuit current density. As well, to increase the open-circuit voltage, the responsible factors are low defect density of the absorbing layer CdTe and high metal work function. For the low cost of cell production, ultrathin film CdTe cells are used with a back surface field (BSF) between CdTe and back contact, such as PbTe. Further, the simulation results show that the conversion efficiency of 19.28% can be obtained for the cell with 1-μm-thick CdTe, 0.1-μm-thick PbTe and 30-nm-thick CdS.     

Effects of thickness and thermal annealing of the PEDOT:PSS layer on the performance of polymer solar cells

We report the influence of thickness and thermal annealing of the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer on the performance of bulk heterojunction solar cells made from blends of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C₆₁. Results show that the power conversion efficiency was significantly improved by inserting the PEDOT:PSS layer but was not strongly sensitive to either the layer thickness or the annealing temperature. Although the short circuit current density was enhanced slightly by annealing the PEDOT:PSS layer at high temperatures, the fill factor was slightly decreased. The trend in device performance could not be explained by the observed changes in the work function of the PEDOT:PSS layer.     

Influence of wafer thickness and carrier recombination on the cutoff frequency of alloy-junction transistors

The influence of wafer thickness and carrier recombination on the -cutoff frequency of the “intrinsic” transistor is studied. First, M' solution of the diffusion of minority carriers in a symmetrical junction transistor is used to find the d.c. transport factor. This is then converted to the a.c. case and finally an implicit equation is found for the cutoff frequency as a function of wafer thickness θ and surface recombination velocity S. It is shown that, for S < 3000 cm/sec, the influence of surface recombination is negligible.

At very narrow basewidths the current density is very high at the emitter edge and this decreases the diffusion length. More important seems to be the fact that the carriers' path (from slanted edge of emitter to slanted edge of collector) is longer than the geometrical basewidth. A reduction in wafer thickness decreases this effect and thus increases the cutoff frequency, in spite of increasing the current density at the emitter edge. The most important equations have been represented graphically for the particular case of the STC TK20 transistor.     

Influence of the dislocation density on the performance ofheteroepitaxial indium phosphide solar cells

The authors point out that heteroepitaxial indium phosphide solar cells developed to date have low efficiency due to misfit dislocations. Dislocations act as recombination centers and strongly influence the solar cell performance. Calculations have been made to study the dependence of heteroepitaxial InP solar cell efficiency on dislocation density. The effects of surface recombination velocity and cell emitter thickness are also considered. Calculated results are compared with the available experimental results on representative InP solar cells. It is shown that heteroepitaxial InP cells with over 20% AM0 efficiency could be fabricated if dislocation density can be reduced to <10⁵ cm^-2 and the surface recombination velocity reduced to <10⁵ cm/s     

An experimental study of drift-field silicon solar cells

Silicon solar cells were constructed with drift fields of various widths and magnitudes. Both initial performance and performance after irradiation with up to 10¹⁶1 MeV electrons/cm²are compared with theory. Behavior is much as expected if the radiation damage is assumed to vary with doping level. This latter assumption leads to the conclusion that little change in cell performance occurs because of the field. Such a result was not anticipated at the start of the investigation, since the neglect of the capture cross-section variation gave the prediction of appreciably improved radiation tolerance.     

Effect of low segregation coefficient on Ga‐doped multicrystalline silicon solar cell performance

High‐quality Ga‐doped ingots are grown in different casting furnaces at optimized growth parameters; 3·5 kg ingots exhibit normal distribution of diffusion lengths along their height with very high diffusion lengths at the center of the ingot. Effective lifetimes as high as 1·1 ms are realized in 10 Ω cm Ga‐doped wafers after proper P‐diffusion and hydrogen passivation. Average effective lifetimes above 400 µs are also realized after P‐diffusion and hydrogen passivation for Ga‐doped wafers cut from 75 kg ingot where the response to P‐diffusion and hydrogen passivation is pronounced. High effective lifetimes are realized over the whole ingot with minimum values of 20 µs at the top of the ingot, indicating the possible use of about 85% of the ingot for solar cell production. Conversion efficiencies above 15·5% were realized in utilizing more than 80% of the ingot. High efficiencies of about 16% were realized in wafers with resistivities higher than 5 Ω cm p ‐type multicrystalline silicon wafers. Copyright © 2005 John Wiley & Sons, Ltd.     

Film thickness effect on the performance of small molecular solar cell

An efficient organic photovoltaic （OPV） cell with an indium-tin-oxide/CuPc/C60/Ag structure has been investigated by changing the film thickness of organic layers. A high olin-circuit voltage （Yoc） of 0.5 V, a short-circuit current density （Jsc） of 5.81 mA/cm^2, and a high power conversion efficiency （ηp） of 1.2% were achieved at an optimum film thickness. The results demonstrate that material thickness is an important factor to cell optimization, especially for maximizing the absorption rate as will as reducing the cell resistance. Experimental results also indicate that the power conversion efficiency increases from 1.2% to 1.54% as a BCP exciton blocking layer of 10 nm is introduced.