A current-generating BSF silicon solar cell fabricated through masked ion implantation

A preferentially current-generating back-surface-field (BSF) silicon solar cell with a near-ideal value of the short-circuit current density JSCunder AM1 conditions has been developed through masked ion implantation of the n⁺-p junction. The BSF cells, fabricated in industrial conditions, possess a conversion efficiency η and a fill-factor FF up to 14.2 and 76.8 percent, respectively. The dependence of JSC, η, and FF on technological parameters are outlined in this letter.     

Study of injection effects on BSF silicon solar cells

The behavior of p⁺-n-n⁺ and n⁺-p-p⁺ silicon solar cells in terms of short-circuit current, open-circuit voltage, fill factor, and efficiency is studied as a function of base doping and illumination levels. A theoretical model that is valid for any injection level in the base region is used. Experimental results for cells of n-type base (in the range of 0.3 to 1000 Ω-cm) and a p-type base (0.4 to 300 Ω-cm) are presented. The theoretical model is able to explain phenomena such as the superlinearity of I_sc with concentration and the degradation of short-circuit current and efficiency at very high concentrations. These effects are seen as connected with the ohmic electric field in the base region. For the emitter saturation currents considered here, it can be concluded that, for p-type substrates, low base resistivities (≅1 Ω-cm) are necessary to achieve high efficiencies under concentrated light (≅100 suns), while for flat-array cells a particular resistivity is not required. For n-type substrates, it is found that any resistivity level can be used for both flat-array and concentrator cells     

Passivated millimeter-wave silicon IMPATT diodes fabricated by ion implantation

Ion implantation has been combined with planar-mesa processing techniques to realize a passivated silicon IMPATT diode for millimeter-wavelength operation. A continuous-wave output power of 100 mW was obtained at 62 GHz from a fully passivated single-drift-region p⁺-n-n⁺structure.     

背电场硅太阳能电池离子辐照效应

空间太阳能电池受电子、质子，以及重离子辐照后，要受到损伤。文章对背电场太阳能电池低能质子、高能质子、碳、氧离子辐照效应进行了研究，对辐照前后太阳能电池光电参数进行测量，并用Ｍａｔｔｅ－Ｃａｒｌｏ方法对辐照粒子的硅中的能量损失过程进行了模拟计算。结果表明，各种离子辐照技术电池的损伤是不同的，低能质子对开路电压的损伤比对睡电压的损伤大得多，而高能质子 地开路电压的损伤较低能质子的反而要小，对短路电流的     

Impurity gettering of polycrystalline solar cells fabricated from refined metallurgical-grade silicon

A damage-gettering technique is described which reduces the impurity content in grown crystals and enhances cell performance of diffused solar cells. Crystalline ingots were Czochralski-grown from an acid-leached metallurgical-grade source. Damage gettering was performed by preparing a mechanically damaged layer on the wafer back surface and subsequent annealing. Optimum annealing conditions were investigated as a function of ambient gas species, temperature, and time. In an O2ambient, the fill factor of the cells degraded to 0.25, while cell performance was greatly improved by annealing in N2. Conversion efficiency tends to increase with annealing time at higher temperatures. Maximum conversion efficiencies attained for mono- and polycrystalline solar cells fabricated from MG-Si are 9.8 and 7.7 percent, respectively. Light current-voltage characteristics and the leakage-current variations with depth were analyzed. It was found that impurity gettering begins at the wafer surfaces and proceeds gradually into the bulk regions.     

Investigation of Al back contacts and BSF formation by in situ TEM for silicon solar cells

Back contacts for Si solar cells made by Al evaporation and screen printing Al paste were studied by transmission electron microscopy. Si was found to diffuse into the Al during heating. Si diffusion formed vacancies in the Si wafer and Al could then penetrate the Si wafer in spiked formations. The Al spikes retracted during cooling, leaving a doped back surface field region. Copyright © 2012 John Wiley & Sons, Ltd.     

Boron ion implantation in silicon through selectively deposited tungsten films

The effect of implanting boron into silicon through thin selective tungsten films and annealing to form silicided p⁺-n junctions is investigated. A rate limited thickness of 0.011-µm tungsten is shown to have the equivalent stopping power of 0.08-µm oxide and be similarly ineffective in eliminating axial boron channeling. Nonetheless, junction diodes as shallow as 0.25µm with sheet resistances of 7 Ω, exhibiting nearly idealI-Vcharacteristics from -40 to 100°C, are fabricated. Analysis of the areal and perimeter leakage currents suggests that defects at the WSi2-SiO2interface are the contributing generation-recombination sites.     

Grain boundary effect on spectral response and effective diffusion length in polycrystalline silicon solar cells

The effect of grain boundary recombination velocity, diffusion length in the bulk of the grain and the grain size on the photocurrent is studied with the help of a comprehensive mathematical formulation. The behaviour of the spectral response at long and short wavelengths is discussed. The effective diffusion length for polycrystalline silicon is determined as a function of the bulk grain diffusion length, grain size and the grain boundary recombination velocity.     

Comparison of the characteristics of solar cells fabricated from multicrystalline silicon with those fabricated from silicon obtained by the monolike technology

In order to raise the efficiency of solar cells and reduce the cost of their production, a new process for obtaining silicon ingots based on the so-called moonlike technology is developed. New technologies, which use “solar-grade” silicon, make it possible to fabricate solar cells at a lower cost with a higher efficiency of solar-energy conversion. It is exactly for this reason that the “monolike” process was tested and optimized by us for Kazakhstan solar-grade silicon. The aim of this study is a comparison of the characteristics of solar cells fabricated from “monolike” silicon with those of solar cells obtained on the basis of multicrystalline silicon grown by oriented crystallization. For our study, ingots of multicrystalline silicon are grown on an industrial scale and through the use of Kazakhstan-sourced silicon; solar cells are fabricated and the characteristics of the obtained silicon ingots and solar cells are studied.     

Operating limits of Al-alloyed high-low junctions for BSF solar cells

Experimental estimations of the effective surface recombination velocity of the high-low junction (S_eff) and of the base diffusion length are carried out for Al-alloyed n⁺pp⁺ bifacial cells and the results are presented in form of histograms. These results agree with calculated values of S_eff when the characteristics of the recrystallized Si layer and heavy doping effects are taken into account. It is concluded that thick Al layers and high alloying temperatures (over 800°C) are necessary to obtain low values of S_eff. This conclusion agrees with experimental results of other authors. Recomendations to avoid diffusion length degradation are given and the operating limits of the Al alloying technology are discussed.     

Physics of BSF solar cells at high levels of illumination

The theory of BSF solar cells by Fossum et al. [1] is extended to high levels of injection. For such intensities, more generalJ_{SC}-V_{OC}relationships are derived which show deviation from the idealexp (qV/kT)relationship.     

PbS photodiodes fabricated by Sb+ ion implantation

N-p junction photodiodes in PbS have been fabricated using Sb⁺ ion implantation to create the n-type layer. At 300°K, 15-mil-square diodes have shown typical zero-bias resistances of $\text{200 Ω}$ corresponding to a resistance-area product of $\text{0.28 Ω-}\text{cm}{}^2$. At 195°K, the zero-bias resistance increased to $\text{50}\text{k}\text{Ω}$ for a resistance-area product of $\text{70 Ω-}\text{cm}{}^2$, and at 77°K, the zero-bias resistance was $\text{5 × 10}{}^9\text{Ω}$ for a resistance-area product of $\text{7 × 10}{}^6\text{Ω-}\text{cm}{}^2$. Peak detectivities occurred at 2.55, 2.95, and 3.4 μm at 300°K, 195°K and 77°K, respectively. The corresponding measured detectivities were 4.8 × 10⁹, 1.1 × 10¹¹ and 4.2 × 10¹² cm Hz^{$\text{1}\text{2}$}/W. The 77°K detectivity was measured in a reduced background and was amplifier noise limited. Peak quantum efficiencies were typically 50–60 per cent.     

ISFET's with ion-sensitive membranes fabricated by ion implantation

Ion sensitive FETs (ISFETs) for sodium ions (Na⁺) fabricated by ion-implantation are investigated. The sensing layers are produced by implanting Na⁺ ions into the surface of an oxidized Si₃N₄ layer through an Al buffer layer deposited beforehand, in order to reduce the damage to the gate insulator of the ISFET during ion implantation. The Na⁺ sensitivity, selectivity, repeatability, thermal characteristics, and long-term stability are evaluated. The ISFET responds to Na⁺ ions independent of pH within the range of pH 7-10, and the Na⁺ sensitivity is nearly Nernstian. The ISFETs repeatability and long-term stability (about 1300 h), suggests that the ion-implantation technique is a suitable method for fabricating a stable ion-sensing layer     

MOS field effect transistors formed by gate masked ion implantation

MOS enhancement mode field effect transistors with a circular geometry and with drains offset from the gate by distances from 0.1 mil to 0.9 mil were implanted with boron ions to fill in the offset region and thus achieve perfect alignment (i.e., no overlap) between gate and drain. The energies used were 50 to 100 keV and a 4000 Å-thick aluminum gate acted as a mask to prevent ions from penetrating into the channel region. The best junctions were obtained with 100-keV ions, with the sheet resistances being typically 4000 ω/□ for the implanted region. This additional drain resistance was quite small compared to the channel resistance of the devices and so was not objectionable. Ordinary diffused MOSFET's were included on the same wafers for comparison with the ion implanted MOSFET's. It was found that the differences in noise, leakage, and drain breakdown voltage were not serious. The chief advantage of the ion implanted MOSFET is the extremely low feedback capacitance due to the lack of gate-drain overlap, but this advantage is difficult to exploit in a conventional package because of the package capacitance. However, a significant difference was noted in switching characteristics between diffused and ion implanted MOSFET's mounted on TO-18 headers.     

Monitoring of low-dose ion implantation in silicon

Monitoring of low-dose arsenic or boron ion implantation (doses: 5×10¹⁰ to 1×10¹³ cm^-2) in silicon, which is required for threshold voltage control of MOS transistors, is studied. The thermal-wave (TW) signal intensity decreases monotonically with decreasing dose. The lowest detection limit for As⁺ and B⁺ implantations is 5×10¹⁰ and 1×10¹¹ cm^-2, respectively. Correlation of the TW signal intensity versus damage density, TW intensity versus dose, and laser Raman intensity versus dose is obtained. The TW intensity is also correlated with the sheet conductance, and the threshold voltage of the transistor. Therefore, this technique is useful as a nondestructive, highly sensitive dose monitor for low-dose implantation to achieve tight threshold voltage control in MOS transistors     

The electrical properties of phosphorus doped silicon layers obtained by ion implantation through a passivating oxide

The electrical properties of phosphorus doped silicon layers obtained by ion implantation through a passivating oxide are studied. The quality of the fabricated n⁺p diodes is investigated. The influence of the process parameters and of the crystal orientation on the sheet resistance of the n⁺ layers is studied. It is shown that sheet resistances higher than 1 kΩ/□ with a good linearity and a very low temperature coefficient can easily be obtained in a reproducible way.     

High-voltage buried-channel MOS fabricated by oxygen implantation into silicon

A high-voltage offset-gate buried-channel MOS made on a SOS-like substrate is described. An isolation layer is formed by an oxygen implantation process called SIMOX. A 410 V buried SiO2 breakdown voltage and a 180 V drain breakdown voltage are obtained.     

The collection probability and spectral response in isotype heterolayers of tandem solar cells

An analytical model for the position-dependent collection probability in uniformly doped one-dimensional layers with abrupt compositional and bandgap changes is presented. The collection probability is derived from coupled system of diffusion equations for low-level injection of photo-generated minority carriers in a stack of isotype heterolayers. Collection probability maintained continuity across isotype heterojunctions despite discontinuities of excess minority carrier concentration. An estimate for window and BSF passivation showed that the effective surface recombination velocity exponentially depended on the energygap difference, and linearly depended on increased doping, reduced mobility and increased thickness of sub-diffusion length passivation layers. Analytical expressions for the photo-generated current, and the internal quantum efficiency from each heterolayer were developed, and applied to the analysis of reported spectral response of a dual junction GaInP/GaAs tandem solar cell. Calculated internal quantum efficiencies closely matched reported experimental results, with the exception of sub-band absorption due to sub-bandgap deficiencies in the optical models and photon recycling. Calculated spectral response showed that upper AlInP₂ window, quasi-neutral emitter, and SCR layers dominated collection of photo-generated carriers in the top GaInP₂ cell, whereas, the base dominated collection of photo-generated carriers in the bottom GaAs cell. Results show that augmenting Hovel’s three layers (emitter, SCR, and base) analysis with the response from the top window layer should be sufficient to capture the spectral response of solar cells with thin passivation layers.     

Determination of minority carrier lifetime and effective back surface recombination velocity in BSF silicon solar cells from transient measurements

The method of determining the base lifetime ?B and the effective surface recombination velocity Seff in a BSF solar cell from the transient decay of open-circuit voltage and short-circuit current is extended to include emitter recombinations. If the emitter recombinations in modern Si solar cells are neglected in interpreting the experimental data, the experimental value of Seff is found to be in large error.