A new type of high-efficiency bifacial silicon solar cell with external busbars and a current-collecting wire grid

Results regarding bifacial silicon solar cells with external busbars are presented. The cells consist of [n⁺p(n)p⁺] Cz-Si structures with a current-collecting system of new design: a laminated grid of wire external busbars (LGWEB). A LGWEB consists of a transparent conducting oxide film deposited onto a Si structure, busbars adjacent to the Si structure, and a contact wire grid attached simultaneously to the oxide and busbars using the low-temperature lamination method. Bifacial LGWEB solar cells demonstrate record high efficiency for similar devices: 17.7%(n-Si)/17.3%(p-Si) with 74–82% bifaciality for the smooth back surface and 16.3%(n-Si)/16.4%(p-Si) with 89% bifaciality for the textured back surface. It is shown that the LGWEB technology can provide an efficiency exceeding 21%.     

Evaluation of the conversion efficiency of thin-film single-junction (a-Si:H) and tandem (μc-Si:H + a-Si:H) solar cells by analysis of the experimental dark and load current-voltage (I–V) characteristics

The aim of the study is to apply a method commonly used to determine the efficiency of multi-junction nanoheterostructure III?CV solar cells by analysis of the dark current-voltage (I?CV) characteristics to such an unconventional semiconducting material as amorphous silicon. a-Si:H and a-Si:H/??c-Si:H p-i-n structures without a light-scattering sublayer or an antireflection coating are studied. The results of measurements of the dark I?CV characteristics demonstrate that the voltage dependence of the current has several exponential portions. The conversion efficiency of solar cells (SCs) is calculated for each portion of the dark I?CV characteristic. This yields a dependence of the potential SC efficiency on the generation current density or on the photon flux. The observed agreement between the data derived from the experimental characteristics and results of calculations can be considered satisfactory and acceptable, thus the method suggested for measurement and analysis of dark I?CV characteristics and tested earlier on SCs based on crystalline III?CV compounds acquires a universal nature. The analysis of the characteristics of p-i-n amorphous silicon structures and the calculation of potential efficiencies, based on this analysis, extend the authors?? understanding of this class of devices and make it possible to improve the technology and photoconversion efficiency of SCs of this kind.     

Development of Nuclear Radiation Detectors by Use of Thick Single-Crystal CdTe Layers Grown on (211) p +-Si Substrates by MOVPE

Development of an electron-collecting-type pixel array by use of an epitaxially grown thick single-crystal CdTe layer on p ⁺-Si substrate is discussed. To achieve such an array with an n-CdTe/p-like CdTe/p ⁺-Si heterojunction diode structure, charge transport at the p-like CdTe/p ⁺-Si heterointerface was studied. It was confirmed that ohmic conduction via holes occurs at this interface. A single-element detector was then fabricated by growth of 40 μm thick undoped p-like CdTe then 5 μm thick n-CdTe layers on the p ⁺-Si substrate. Rectification by the diode detector was good, and its energy-resolving capability was demonstrated by detection of gamma peaks from the ²⁴¹Am source, thus confirming the feasibility of using this structure for fabrication of an electron-collecting-type array.     

Erbium ion electroluminescence in p ++/n +/n-Si:Er/n ++ silicon diode structures

Results of experimental studies of erbium ion electroluminescence in p ⁺⁺/n ⁺/n-Si:Er/n ⁺⁺ silicon diode structures grown by sublimation molecular-beam epitaxy are discussed. The distinctive feature of these structures is that the regions of electron flux formation of (n ⁺-Si) and impact excitation of erbium ions (n-Si:Er) are spaced. The influence of the n ⁺-Si layer thickness on electrical and electroluminescent properties of diodes was studied. It was shown that n ⁺-Si layer thinning causes the transformation of the structure breakdown mechanism from tunneling to avalanche. The dependence of the Er³⁺ ion electroluminescence on the thickness of the heavily doped n ⁺-Si region is bell-shaped. At the n ⁺-Si-layer doping level n ≈ 2 × 10¹⁸ cm^?3, the maximum electroluminescence intensity is attained at an n ⁺-Si layer thickness of ～23 nm.     

Bifacial concentrator Ag‐free crystalline n‐type Si solar cell

We report results obtained using an innovative approach for the fabrication of bifacial low‐concentrator thin Ag‐free n‐type Cz‐Si (Czochralski silicon) solar cells based on an indium tin oxide/(p⁺nn⁺)Cz‐Si/indium fluorine oxide structure. The (p⁺nn⁺)Cz‐Si structure was produced by boron and phosphorus diffusion from B‐ and P‐containing glasses deposited on the opposite sides of n‐type Cz‐Si wafers, followed by an etch‐back step. Transparent conducting oxide (TCO) films, acting as antireflection electrodes, were deposited by ultrasonic spray pyrolysis on both sides. A copper wire contact pattern was attached by low‐temperature (160°C) lamination simultaneously to the front and rear transparent conducting oxide layers as well as to the interconnecting ribbons located outside the structure. The shadowing from the contacts was ~4%. The resulting solar cells, 25 × 25 mm² in dimensions, showed front/rear efficiencies of 17.6–17.9%/16.7–17.0%, respectively, at one to three suns (bifaciality of ~95%). Even at one‐sun front illumination and 20–50% one‐sun rear illumination, such a cell will generate energy approaching that produced by a monofacial solar cell of 21–26% efficiency. Copyright © 2014 John Wiley & Sons, Ltd.     

The electronic and emissive properties of Au-doped porous silicon

The temperature dependences of photovoltage induced by intense pulses of red and white light, along with the time-resolved spectral dependences of photoluminescence, are studied for porous silicon structures por-Si/p-Si). These structures have been obtained by anode etching of p-Si with subsequent Au doping from an aqueous solution with Au ion concentrations of 10^?4 and 10^?3 M. The current-voltage characteristics and electroluminescence of the resulting por-Si/p-Si and por-Si:Au/p-Si structures are also studied after a deposition of semitransparent Au electrodes on por-Si. It is shown that the Au doping changes the sign of the boundary potential of p-Si from positive to negative, alters the magnitude and sign of the photovoltage in the por-Si films, and eliminates photomemory phenomena, which are associated with the capture of nonequilibrium electrons at grain-boundary traps and por-Si traps. The formation of Au nanocrystals in por-Si substantially affects the current-voltage and photoluminescence characteristics. Electroluminescence is observed for the Au/por-Si:(Au, 10^?3 M)/p-Si/Al structures and is attributed to emission from the nanocrystals.     

High injection phenomena in p+in+ silicon solar cells

A very simple analytic analysis of p⁺in⁺ solar cell structures is undertaken based in the quasi-neutrality condition (n = p), which validity is verified. The only differential equation to be solved in the model is the classical continuity one. Non linearities, appea ronly in the boundary conditions. This model can be applied to p⁺nn⁺ or n⁺pp⁺ cells under very high injection conditions. High carrier concentrations are required to support the current flow even in short-circuit conditions, so enhancing the recombination which, on the other hand, is reduced by high injection lifetime increase. Under-linearity between the short-circuit current and the photon flux is also deduced at very high irradiance. The short-circuit current under bifacial illumination is higher than the sum of those currents under front and back illumination alternatively, so leading to inherently better bifacial cells. In the V_oc vs J_sc curve values of m in the logarithmic slope mkT/q range from values slightly below one to two. The value m = 1 is not to be expected even if the base recombination is negligible due to Dember potential effects.     

An isolated Al-poly Si-(p)Si-(n+)Si switching device

A new MISS switching device structure was designed and fabricated, which consists of Al/poly Si/p/n⁺/p-Si layers and is isolated by diffusing n-well to the buried n⁺ layer.The switching voltage increases with increasing junction area of the poly-Si junction and the n⁺p junction, due to surface recombination current in the emitter-base junction, respectively. The holding voltage is kept nearly constant of 0.9 V for the 886 Å poly Si devices.     

p +-Si/nano-SiO2/n +-Si capacitor-based tunnel diode with negative differential resistance and a quartz resonator

To demonstrate the extending functionality of the simplest MOS (metal-oxide-semiconductor) capacitor, a structure with a p ⁺-Si/nano-SiO₂ heterojunction in which strongly degenerate n ⁺-Si is used instead of a metal electrode is considered. As a result, a tunnel diode with negative differential resistance and a quartz resonator is obtained. Its electrical characteristics are superior to those of the corresponding Esaki diode and are controlled not only by the Silicon doping level, but also by the SiO₂ thickness.     

Growth of n-Si layers by molecular-beam epitaxy on the substrates heavily doped with boron

Epitaxial n-Si layers doped with phosphorus or erbium have been grown by sublimation molecularbeam epitaxy at 500°C on heavily boron-doped p ⁺-type substrates with resistivity ρ = 0.005 Ω cm. Distribution profiles of the B, Er, and O impurity concentrations in the samples were determined by secondary-ion mass spectrometry. A thermal annealing of the substrate in vacuum at 1300°C for 10 min and growth at a very low substrate temperature made it possible to obtain an extremely abrupt profile for doping impurities at the layer-substrate interface. This method for growth of n-p ⁺ junctions considerably improves their electrical and luminescent characteristics.     

Electrical analysis of Al/ZnO/p-Si,Al/PMMA/p-Si and Al/PMMA/ZnO/p-Si structures: Comparison study

Al/ZnO/p-Si, Al/PMMA/p-Si and Al/PMMA/ZnO/p-Si structures were fabricated. Based on the measured current–voltage (C–V) and capacitance–voltage curves, the electrical characteristics of these heterostructures such as ideality factor, barrier height and series resistance of each structure were analyzed and then compared with those of Al/PMMA/ZnO/p-Si. According to C–V measurement, it was found that the Al/PMMA/ZnO/p-Si structure indicates the better electronic performance rather than other structures. The obtained results represent low series resistance (19.3 Ω) after coating with polymethyl methacrylate (PMMA) over ZnO/p-Si heterojunction structure for Al/PMMA/ZnO/p-Si heterostructure.     

Study of Deep Levels in a HIT Solar Cell

The results of studying a HIT (heterojunction with an intrinsic thin layer) Ag/ITO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n⁺)/ITO/Ag solar cell by the capacitance–voltage characteristic and current deep-level relaxation transient spectroscopy methods are presented. The temperature dependence of the capacitance–voltage characteristics of the HIT structure and deep-energy-level parameters are studied. The results of comprehensive studies by the above methods are used to determine the features of the energy-band diagram of actual HIT structures.     

Electrical analysis of organic dye-based MIS Schottky contacts

In this work, we prepared metal/interlayer/semiconductor (MIS) diodes by coating of an organic film on p-Si substrate. Metal(Al)/interlayer(Orange GOG)/semiconductor(p-Si) MIS structure had a good rectifying behavior. By using the forward-bias I-V characteristics, the values of ideality factor (n) and barrier height (BH) for the Al/OG/p-Si MIS diode were obtained as 1.73 and 0.77 eV, respectively. It was seen that the BH value of 0.77 eV calculated for the Al/OG/p-Si MIS diode was significantly larger than the value of 0.50 eV of conventional Al/p-Si Schottky diodes. Modification of the potential barrier of Al/p-Si diode was achieved by using thin interlayer of the OG organic material. This was attributed to the fact that the OG organic interlayer increased the effective barrier height by influencing the space charge region of Si. The interface-state density of the MIS diode was found to vary from 2.79 × 10¹³ to 5.80 × 10¹² eV⁻¹ cm⁻².     

Temperature dependence of resistivity and hole conductivity mobility in p-type silicon

A new method is employed to determine the temperature dependence of the resistivity and hole conductivity mobility of p-type silicon. This method involves the use of an aluminum-on-p-Si ohmic diode and a magnesium on p-Si Schottky barrier diode on the same silicon chip. The resistivity is determined from the Al/p-Si ohmic diode. The hole concentration is evaluated from the C-V data of the Mg/p-Si Schottky barrier diode. The conductivity mobility is then computed from the resistivity and hole concentration data. The following ranges are covered: 77–300 K, 0.4–100 ohm-cm and 5 × 10¹⁶?2 × 10¹⁴ holes/cm³ at room temperature.     

Postgrowth Annealing of CdTe Layers Grown on Si Substrates by Metalorganic Vapor-Phase Epitaxy

Annealing conditions of CdTe layers grown on Si substrates by metalorganic vapor-phase epitaxy were studied. Typically, 3-μm-thick n-type (211) CdTe layers were annealed for 60 s in flowing hydrogen at atmospheric pressure by covering their surfaces with bulk CdTe wafers. At annealing temperatures above 700°C, improvement of crystal quality was confirmed from full-width at half-maximum values of double-crystal rocking-curve measurements and x-ray diffraction measurements. Photoluminescence measurements revealed no deterioration of electrical properties in the annealed n-CdTe layers. Furthermore, annealing at 900°C improved the performance of radiation detectors with structure of p-like CdTe/n-CdTe/n ⁺-Si substrate.     

How the type of bombarding ion affects the formation of radiation defects in silicon

The method of nonstationary capacitance was used to study how the chemical nature of implanted ions affects the creation of electrically active defects in silicon. Oxygen O⁺ and nitrogen N⁺ ions were implanted into Si at a target temperature of 300 K, in doses of 2×10¹¹cm⁻² with energies of 75 keV, and argon Ar⁺ ions were implanted in a dose of 7×10¹⁰cm⁻² with energies of 150 keV, in such a way that all the samples of n-and p-Si received approximately the same number and spatial distribution of primary radiation defects. It was observed that the spectrum of stable radiation defects depends on the nature of the bombarding ion. Thus, the DLTS spectrum of n-Si irradiated by O⁺ ions has three peaks, whereas the spectrum of n-Si implanted by N⁺ ions exhibited only one of these peaks. The DLTS spectra of samples of n-and p-Si implanted by O⁺ and N⁺ ions revealed peaks of reverse (anomalous) polarity, whose energy positions matched the most clearly defined DLTS peaks of silicon samples with the opposite type of conductivity. Fiz. Tekh. Poluprovodn. 32, 523–526 (May 1998)     

Dark Current Characteristics of a Radiation Detector Array Developed Using MOVPE-Grown Thick CdTe Layers on Si Substrate

We present reverse bias current (dark current) characteristics of a two-dimensional monolithic pixel-type nuclear radiation detector array fabricated using metalorganic vapor-phase epitaxy (MOVPE)-grown thick CdTe epitaxial layers on Si substrate. The (14?×?8) pixel array was formed by cutting deep vertical trenches using a dicing saw, where each pixel possesses a p-CdTe/ n-CdTe/n ⁺-Si heterojunction diode structure. The dark currents showed pixel-to-pixel variations when measured at higher applied biases exceeding 100?V. The dark current had a dependence on the pixel thickness, where pixels with lower CdTe thickness exhibited higher currents. Moreover, the temperature dependence of the dark current revealed that a deep level with activation energy of around 0.6?eV is responsible for the observed dark currents and their pixel-to-pixel variation. We discuss that the effective ratio of Te to Cd at the growth surface is a major factor that controls the thickness variation, and is also responsible for the formation of 0.6?eV deep levels.     

Electron surface trapping effects on the switching voltage of metal-insulator (tunnel)-Si(n)-Si(p+) devices

Using double-delay pulse method for investigation of the dynamic characteristics of MIS switching devices, we have observed an abnormal dependence of the switching voltage on the pulse separation time in AuSiO₂ (tunnel)-Si(n)-Si(p⁺) diodes. This abnormal dependence may be attributed to the electron trapping and detrapping in the surface states at the SiSiO₂ interface. A simple model of the surface discrete traps is given and the calculation is in good agreement with experiment.     

Low-temperature processing of shallow junctions using epitaxial and polycrystalline CoSi2

Cobalt disilicide is grown epitaxially on (100) Si from a 15 nm Co/2 nm Ti bilayer by rapid thermal annealing (RTA) at 900°C. Polycrystalline CoSi₂ is grown on (100) Si using a 15 nm Co layer and the same annealing condition. Silicide/p⁺-Si/n-Si diodes are made using the silicide as dopant source:¹¹B⁺ ions are implanted at 3.5–7.5 kV and activated by RTA at 600–900°C. Shallow junctions with total junction depth (silicide plus p⁺ region) measured by high-resolution secondaryion mass spectroscopy of 100 nm are fabricated. Areal leakage current densities of 13 nA/cm² and 2 nA/cm² at a reverse bias of -5V are obtained for the epitaxial silicide and polycrystalline silicide junctions, respectively, after 700°C post-implant annealing.     

Electromigration of Cu and Ti atoms and dopant junction profiles in the p+-Si implanted channel under high-density current

Ultrafast diffusion of Cu and Ti atoms in p⁺-Si channel was achieved in samples stressed with high current density. The junction was changed under high current stress and copper silicide was formed in the junction. The current effects of high current on the silicide line formation and symmetrical end-of-range defects elimination near the center of p⁺-Si channel are discussed.