High Efficiency Solar Cell Structures Research Articles

Hydrogen concentration at a-Si:H/c-Si heterointerfaces—The impact of deposition temperature on passivation performance

We studied the effect of deposition temperature on the hydrogen distribution and the passivation performance of hydrogenated amorphous silicon (a-Si:H) coated crystalline silicon (c-Si) heterojunctions as a model of high efficiency solar cell structures. Nuclear reaction analysis (NRA) was employed to obtain hydrogen depth profiles of the heterojunctions prepared at temperatures from 80 to 180 °C. The implied open circuit voltage (i-VOC) and carrier lifetime monotonically increased with increasing deposition temperature in the as-deposited samples. NRA clarified that the hydrogen concentration (CH) at the a-Si:H/c-Si interface and in the a-Si:H layer decreased with deposition temperature. The hydrogen concentration around the interface was roughly 3 × 1021 cm-3 for the sample deposited at 180 °C. The NRA results are supplemented by optical constants obtained with spectroscopic ellipsometry (SE). At higher growth temperature, larger refractive indices and extinction coefficients were confirmed by SE analysis, suggesting that fewer hydrogen atoms are incorporated into the a-Si:H layers prepared at higher growth temperature. Furthermore, the passivation performance was enhanced by post deposition annealing (PDA) at 200 °C for 30 min. No significant change of the hydrogen distribution and optical constants was observed after PDA, suggesting that improved passivation is due to a local rearrangement of hydrogen at the molecular level that results in enhanced hydrogenation of dangling bonds.

Simple fabrication of back contact heterojunction solar cells by plasma ion implantation

A back-contact amorphous-silicon (a-Si)/crystalline silicon (c-Si) heterojunction is one of the most promising structures for high-efficiency solar cells. However, the patterning of back-contact electrodes causes the increase in fabrication cost. Thus, to simplify the fabrication of back-contact cells, we attempted to form p-a-Si/i-a-Si/c-Si and n-a-Si/i-a-Si/c-Si regions by the conversion of a patterned area of p-a-Si/i-a-Si/c-Si to n-a-Si/i-a-Si/c-Si by plasma ion implantation. It is revealed that the conversion of the conduction type can be realized by the plasma ion implantation of phosphorus (P) atoms into p-a-Si/i-a-Si/c-Si regions, and also that the quality of passivation can be kept sufficiently high, the same as that before ion implantation, when the samples are annealed at around 250 °C and also when the energy and dose of ion implantation are appropriately chosen for fitting to a-Si layer thickness and bulk c-Si carrier density.

Boron laser doping using spin-on dopant for textured crystalline silicon solar cell

A novel precursor layer, polyboron film (PBF), was applied in the laser doping (LD) process. Localized LD using PBF was applied to a high-efficiency solar cell structure as a selective emitter (SE). Interface adhesion was improved by using an organic-polymer-based doping precursor, PBF. A strongly adhered interface contributed to the preservation of textured formations after disruption by LD, and to the homogeneous introduction of impurities. Surface geometries were retained almost completely even after LD. Sheet resistances were decreased by additional boron doping by LD. The photovoltaic characteristics were improved by SE formation using PBF with an optimum average laser power. Consequently, the SE formed by LD showed an increase in fill factor (FF), which contributed to the improvement in solar cell efficiency.

Excellent Passivation of p+Silicon Surfaces by Inline Plasma Enhanced Chemical Vapor Deposited SiOx/AlOxStacks

Excellent surface passivation of boron emitters is demonstrated for industrial plasma-enhanced chemical vapor deposited (PECVD) SiOx/AlOx stacks. Emitter saturation current densities of 39 and 34 fA/cm2, respectively, were achieved at 300 K on 80 Ω/sq boron emitters after activation by (i) a standard industrial firing process and (ii) a forming gas anneal followed by industrial firing. We find that the surface passivation by SiOx/AlOx stack can be effectively controlled by varying the SiOx layer thickness. This stack is directly applicable to certain high-efficiency solar cell structures, by optimising the SiOx thickness accordingly.

Excellent Passivation of p+ Silicon Surfaces by Inline Plasma Enhanced Chemical Vapor Deposited SiOx/AlOx Stacks

Excellent surface passivation of boron emitters is demonstrated for industrial plasma-enhanced chemical vapor deposited (PECVD) SiOx/AlOx stacks. Emitter saturation current densities of 39 and 34 fA/cm2, respectively, were achieved at 300 K on 80 Ω/sq boron emitters after activation by (i) a standard industrial firing process and (ii) a forming gas anneal followed by industrial firing. We find that the surface passivation by SiOx/AlOx stack can be effectively controlled by varying the SiOx layer thickness. This stack is directly applicable to certain high-efficiency solar cell structures, by optimising the SiOx thickness accordingly.

First principles investigation of optical properties of zinc-blende Al xGa 1− xAs 1− yN y materials

We have performed a first-principle Full Potential Linearized Augmented Plane Waves calculation within the local density approximation (LDA) to the zinc-blende Al x Ga 1− x As 1− y N y to predict its optical properties as a function of N and Al mole fractions. The accurate calculations of electronic properties such as band structures and optical properties like refractive index, reflectivity and absorption coefficient of Al x Ga 1− x As and Al x Ga 1− x As 1− y N y with x≤0.375 and y up to 4% are presented. Al x Ga 1− x As on GaAs have a lattice mismatch less than 0.16% and the lattice constant of Al x Ga 1− x As has a derivation parameter of 0.0113±0.0024. The band gap energies are calculated by LDA and the band anticrossing model using a matrix element of C MN=2.32 and a N level of E N=(1.625+0.069 x) eV. The results show that Al x Ga 1− x As can be very useful as a barrier layer in separate confinement heterostructure lasers and indicate that the best choice of x and y Al x Ga 1− x As 1− y N y could be an alternative to Al x Ga 1− x As when utilized as active layers in quantum well lasers and high-efficiency solar cell structures.

Analysis of Tunnel Junction Suitable for 4000 Suns in Tandem High Efficiency Solar Cell Structures

AbstractThe road to achieve ultra high efficiency is through multi-junction solar cells operating at high solar concentrations, larger than 1000 suns. Critical to the success of this approach is the development of tunnel junctions (TJ) that serve as electrically low loss interconnections, yet are optically transparent, using high band gap semiconductor material systems. We have previously reported the fabrication of a TJ made of n+-InGaP/ p+-AlGaAs with a band gap about 1.9 eV using Se and C doping, respectively. This TJ structure has a peak current density of 88 A/cm2 allowing it to be implemented in a three junction cell structure at solar concentrations as high as 4000 suns (x4000). Almost all reported conversion efficiencies higher than 40% have used this tunnel junction. This very high peak current density is unexpected in a high band gap material system, which is good news for the multi junction solar community. This seems to be due to the fact that the InGaP/AlGaAs interface has a staggered band line up. We will present the effect of this band line up at the heterointerface and its effect on the width of the depletion region and the peak current density. We also compare the current result from this heterostructure junction with an artificial homojunction made of n+-AlGaAs/ p+-AlGaAs doped to the same levels as that of the heterojunction. Results from the homojunction showed that peak current density is about one half of that obtained from the heterojunction at the same doping levels. A reasonable match between experimental result and the model was obtained when a value of 150 meV was used for ΔEc, the conduction band discontinuity at the interface. Both experiment and theory predicted that at a current density of about 80 A/cm2 with only about a few tens of meV drop across the TJ. This will have a minimal effect on the overall efficiency of the tandem solar cell structure when used at high solar concentrations.

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 ( 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.

Enhancing the surface passivation of TiO2 coated silicon wafers

In this letter, we demonstrate good surface passivation of lightly diffused n-type solar cell emitters using titanium dioxide (TiO2) thin films treated with a furnace oxidation process. Transient-photoconductance decay, x-ray photoelectron spectroscopy, and scanning electron microscopy measurements indicate that the silicon dioxide layer formed at the TiO2:Si interface provides excellent surface passivation. Emitter dark saturation current densities of 4.7×10−14 A/cm2 are achieved by this method, demonstrating that TiO2 films are compatible with high-efficiency solar cell structures.

Crystalline Silicon Technology for Photovoltaics

This paper reviews the CZ and FZ processes of growth of single crystal silicon ingots, the important directional solidification, including casting, techniques of growth of multicrystalline silicon ingots and crystalline silicon ribbon or sheet processes for obtaining wafers for fabrication of large area terrestrial solar cells. It also discusses the advancements which have brought about improvements in solar cell design and processing and have yielded small area laboratory cells of AM 1.5 efficiency, ~ 24% under normal sunlight and ~28% under concentrated sunlight. Of these, some advanced structures have been adopted for silicon space cells because of improved power to weight ratio and radiation resistance of these structures. However, the PV industry has so far refrained from adopting the more sophisticated high efficiency solar cell structures, e.g., laser grooved buried contact structure, for commercial terrestrial silicon solar cells as they lower the throughput of solar cell production and do not seem to offer, at this stage, any cost per watt benefit despite giving higher efficiency. Thus, there still remains a need and the challenge to develop a solar cell structure for silicon commercial cells which would yield cells of 16-18% or higher production level efficiencies using a low cost and high throughput process that would help achieve the cost goal of US $2 per watt on module level.