Ivan Gordon

Photonic assisted light trapping integrated in ultrathin crystalline silicon solar cells by nanoimprint lithography

We report on the fabrication of two-dimensional periodic photonic nanostructures by nanoimprint lithography and dry etching and their integration into a 1-μm-thin mono-crystalline silicon solar cell. Thanks to the periodic nanopatterning, a better in-coupling and trapping of light is achieved, resulting in an absorption enhancement. The proposed light trapping mechanism can be explained as the superposition of a graded index effect and of the diffraction of light inside the photoactive layer. The absorption enhancement is translated into a 23% increase in short-circuit current, as compared to the benchmark cell, resulting in an increase in energy-conversion efficiency.

Design, fabrication and optical characterization of photonic crystal assisted thin film monocrystalline-silicon solar cells.

In this paper, we present the integration of an absorbing photonic crystal within a monocrystalline silicon thin film photovoltaic stack fabricated without epitaxy. Finite difference time domain optical simulations are performed in order to design one- and two-dimensional photonic crystals to assist crystalline silicon solar cells. The simulations show that the 1D and 2D patterned solar cell stacks would have an increased integrated absorption in the crystalline silicon layer would increase of respectively 38% and 50%, when compared to a similar but unpatterned stack, in the whole wavelength range between 300 nm and 1100 nm. In order to fabricate such patterned stacks, we developed an effective set of processes based on laser holographic lithography, reactive ion etching and inductively coupled plasma etching. Optical measurements performed on the patterned stacks highlight the significant absorption increase achieved in the whole wavelength range of interest, as expected by simulation. Moreover, we show that with this design, the angle of incidence has almost no influence on the absorption for angles as high as around 60°.

Electrical activity of intragrain defects in polycrystalline silicon layers obtained by aluminum-induced crystallization and epitaxy

Defect etching revealed a very large density (∼109cm−2) of intragrain defects in polycrystalline silicon (pc-Si) layers obtained through aluminum-induced crystallization of amorphous Si and epitaxy. Electron-beam-induced current measurements showed a strong recombination activity at these defects. Cathodoluminescence measurements showed the presence of two deep-level radiative transitions (0.85 and 0.93eV) with a relative intensity varying from grain to grain. These results indicate that the unexpected quasi-independence on the grain size of the open-circuit voltage of these pc-Si solar cells is due to the presence of numerous electrically active intragrain defects.

High open-circuit voltage values on fine-grained thin-film polysilicon solar cells

Grain boundaries are known to be the main limiting factor for a high performance of polysilicon solar cells. Defects at these grain boundaries serve as recombination centers for minority and majority carriers. Grain boundaries are also known to be paths for enhanced hydrogen diffusion, which results in passivation of part of the defects. In this paper, we show that grain boundaries are also paths for an enhanced phosphorus diffusion that limits the effect of hydrogen passivation. Phosphorus spikes along the grain boundaries enhance the junction area and determine the collection and the recombination volumes. Avoiding this preferential diffusion of phosphorus atoms during emitter formation, we obtained open-circuit voltages (Voc) up to 536mV on polysilicon material with a grain size of only 0.2μm. These high Voc values can only be accounted for by theory if a much smaller grain boundary recombination velocity is assumed than what was previously accepted for p‐n junctions on fine-grained polysilicon solar c...

Intragrain defects in polycrystalline silicon layers grown by aluminum-induced crystallization and epitaxy for thin-film solar cells

Polycrystalline silicon (pc-Si) thin-films with a grain size in the range of 0.1–100 μm grown on top of inexpensive substrates are economical materials for semiconductor devices such as transistors and solar cells and attract much attention nowadays. For pc-Si, grain size enlargement is thought to be an important parameter to improve material quality and therefore device performance. Aluminum-induced crystallization (AIC) of amorphous Si in combination with epitaxial growth allows achieving large-grained pc-Si layers on nonsilicon substrates. In this work, we made pc-Si layers with variable grain sizes by changing the crystallization temperature of the AIC process in order to see if larger grains indeed result in better solar cells. Solar cells based on these layers show a performance independent of the grain size. Defect etching and electron beam induced current (EBIC) measurements showed the presence of a high density of electrically active intragrain defects. We therefore consider them as the reason fo...

Large-area monocrystalline silicon thin films by annealing of macroporous arrays: Understanding and tackling defects in the material

A concept that could provide a thin monocrystalline-silicon absorber layer without resorting to the expensive step of epitaxy would be very appealing for reducing the cost of solar cells. The empty-space-in-silicon technique by which thin films of silicon can be formed by reorganization of regular arrays of cylindrical voids at high temperature may be such a concept if the high quality of the thin film could be ensured on centimeter-large areas. While previous works mainly investigated the influence of the porous array on the final structure, this work focuses on the practical aspects of the high-temperature step and its application to large areas. An insight into the defects that may form is given and the origin of these defects is discussed, providing recommendations on how to avoid them. Surface roughening, pitting, formation of holes, and silicon pillars could be attributed to the nonuniform reactions between Si, SiO2, and SiO. Hydrogen atmospheres are therefore preferred for reorganization of macropo...

Improving the Quality of Epitaxial Foils Produced Using a Porous Silicon-based Layer Transfer Process for High-Efficiency Thin-Film Crystalline Silicon Solar Cells

A porous silicon-based layer transfer process to produce thin (30-50 μm) kerfless epitaxial foils (epifoils) is a promising approach toward high-efficiency solar cells. For high efficiencies, the epifoil must have high minority carrier lifetimes. The epifoil quality depends on the properties of the porous layer since it is the template for epitaxy. It is shown that by reducing the thickness of this layer and/or its porosity in the near-surface region, the near-surface void size is reduced to <;65 nm and in certain cases achieve a 100 nm-thick void-free zone below the surface. Together with better void alignment, this allows for a smoother growth surface with a roughness of <;35 Å and reduced stress in the porous silicon. These improvements translate into significantly diminished epifoil crystal defect densities as low as ~420 defects/cm 2. Although epifoils on very thin porous silicon were not detachable, a significant improvement in the lifetime (diffusion length) of safely detachable n-type epifoils from ~85 (~300 μm) to ~195 μs (~470 μm) at the injection level of 10 15/cm 3 is achieved by tuning the porous silicon template. Lifetimes exceeding ~350 μs have been achieved in the reference lithography-based epifoils, showing the potential for improvement in porous silicon-based epifoils.

Micrometer-Thin Crystalline-Silicon Solar Cells Integrating Numerically Optimized 2-D Photonic Crystals

A 2-D photonic crystal was integrated experimentally into a thin-film crystalline-silicon solar cell of 1-μm thickness, after numerical optimization maximizing light absorption in the active material. The photonic crystal boosted the short-circuit current of the cell, but it also damaged its open-circuit voltage and fill factor, which led to an overall decrease in performances. Comparisons between modeled and actual optical behaviors of the cell, and between ideal and actual morphologies, show the global robustness of the nanostructure to experimental deviations, but its particular sensitivity to the conformality of the top coatings and the spread in pattern dimensions, which should not be neglected in the optical model. As for the electrical behavior, the measured internal quantum efficiency shows the strong parasitic absorptions from the transparent conductive oxide and from the back-reflector, as well as the negative impact of the nanopattern on surface passivation. Our exemplifying case, thus, illustrates and experimentally confirms two recommendations for future integration of surface nanostructures for light trapping purposes: 1) the necessity to optimize absorption not for the total stack but for the single active material, and 2) the necessity to avoid damage to the active material by pattern etching.

Study of the hydrogenation mechanism by rapid thermal anneal of SiN:H in thin-film polycrystalline-silicon solar cells

A considerable cost reduction in photovoltaics could be achieved if efficient solar cells could be made from thin polycrystalline-silicon (pc-Si) films. Although hydrogen passivation of pc-Si films is crucial to obtain good solar cells, the exact mechanism of hydrogen diffusion through pc-Si layers is not yet understood. In this letter, the influence of the junction and the grain size are investigated. We find that the presence of a p-n junction acts as a barrier for hydrogen diffusion in thin-film polysilicon solar cells. Therefore, pc-Si solar cells should preferably be passivated before junction formation. Furthermore, pc-Si layers with large grains retain less hydrogen after passivation than layers with small grains. This indicates that hydrogen atoms get mainly trapped at the grain boundaries.

Optical bandgap of ultra-thin amorphous silicon films deposited on crystalline silicon by PECVD

An optical study based on spectroscopic ellipsometry, performed on ultrathin hydrogenated amorphous silicon (a-Si:H) layers, is presented in this work. Ultrathin layers of intrinsic amorphous silicon have been deposited on n-type mono-crystalline silicon (c-Si) wafers by plasma enhanced chemical vapor deposition (PECVD). The layer thicknesses along with their optical properties –including their refractive index and optical loss- were characterized by spectroscopic ellipsometry (SE) in a wavelength range from 250 nm to 850 nm. The data was fitted to a Tauc-Lorentz optical model and the fitting parameters were extracted and used to compute the refractive index, extinction coefficient and optical bandgap. Furthermore, the a-Si:H film grown on silicon was etched at a controlled rate using a TMAH solution prepared at room temperature. The optical properties along with the Tauc-Lorentz fitting parameters were extracted from the model as the film thickness was reduced. The etch rate for ultrathin a-Si:H layers i...