Sean Erik Foss

Temperature dependent ablation threshold in silicon using ultrashort laser pulses

We have experimentally investigated the ablation threshold in silicon as a function of temperature when applying ultrashort laser pulses at three wavelengths. By varying the temperature of a silicon substrate from room temperature to 320 °C, we observe that the ablation threshold for a 3 ps pulse using a wavelength of 1030 nm drops from 0.43 J/cm2 to 0.24 J/cm2, a reduction of 43%. For a wavelength of 515 nm, the ablation threshold drops from 0.22 J/cm2 to 0.15 J/cm2, a reduction of 35%. The observed ablation threshold for pulses at 343 nm remains constant with temperature, at 0.10 J/cm2. These results indicate that substrate heating is a useful technique for lowering the ablation threshold in industrial silicon processing using ultrashort laser pulses in the IR or visible wavelength range. In order to investigate and explain the observed trends, we apply the two-temperature model, a thermodynamic model for investigation of the interaction between silicon and ultrashort laser pulses. Applying the two-temp...

Optimization of multilayer porous silicon antireflection coatings for silicon solar cells

Efficient antireflection coatings (ARC) improve the light collection and thereby increase the current output of solar cells. In this work, multilayered refractive index stacks optimized for antireflection, in bare air and within modules, are modeled. The relation between porous silicon (PS) etching parameters and PS structure is carefully investigated using spectroscopic ellipsometry, gravimetry, x-ray photoelectron spectroscopy, and scanning electron microscopy. The close relation between porosity and refractive index, modeled using the Bruggeman effective medium approximation, allows PS multilayers to be tailored to fabricate the optimized ARCs. Limits imposed by efficient application in photovoltaics, such as thickness restrictions and the angular distribution of incident light, are examined and accounted for. Low reflectance multilayer ARCs are fabricated with integrated reflectances of ∼3% in air and 1.4% under glass in the wavelength range 400–1100 nm.

Damage free laser ablation of SiO2 for local contact opening on silicon solar cells using an a-Si:H buffer layer

We have used a Q-switched Nd:YVO4, diode pumped 532 nm laser with nanosecond pulses, and a spot diameter of 40 μm to ablate a layer of plasma enhanced chemical vapor deposited (PECVD) SiO2 on n-type Cz silicon, with the aim of making local contact openings on back-junction silicon solar cells. Laser pulses within the ns range are usually believed to be incompatible with processing of high efficiency solar cells because such long pulses induce too much damage into the underlying silicon lattice. This is due to thermal dissipation. In this work, a PECVD layer of a-Si:H between the n-type silicon and the dielectric layer is shown to absorb much of the laser radiation and allows for ablation at laser fluences lower than the ablation threshold of crystalline silicon. In addition, the a-Si:H layer serves as an excellent surface passivation layer for the silicon substrate. We show that it is possible to ablate PECVD SiO2 in a damage free way with fluences five times lower than those needed to ablate crystalline ...

Light-Trapping Properties of a Diffractive Honeycomb Structure in Silicon

Thinner solar cells will reduce material costs, but require light trapping for efficient optical absorption. We have already reported development of a method for fabrication of diffractive structures on solar cells. In this paper, we create these structures on wafers with a thickness between 21 and 115 μm, and present measurements on the light-trapping properties of these structures. These properties are compared with those of random pyramid textures, isotropic textures, and a polished sample. We divide optical loss contributions into front-surface reflectance, escape light, and parasitic absorption in the rear reflector. We find that the light-trapping performance of our diffractive structure lies between that of the planar and the random pyramid-textured reference samples. Our processing method, however, causes virtually no thinning of the wafer, is independent of crystal orientation, and does not require seeding from, e.g., saw damage, making it well suited for application to thin silicon wafers.

Influence of pre-annealing of printed silver electrodes on ultrafast laser ablation of short thin-film transistor channels on flexible substrates

The cutoff frequency and current from an organic thin-film transistor (OTFT) are strongly dependent on the length and to some extent on the uniformity of the transistor channel. Reducing the channel length can improve the OTFT performance with the increase in the current and frequency. Picosecond laser ablation of the printed Ag electrodes, compatible with roll-to-roll fabrication, has been investigated. The ablation threshold was found to be similar for the laser wavelengths tested: 515 nm and 1030 nm. Short transistor channels could be opened both after light annealing at 70 °C and after annealing at 140 °C. The channels in the lightly cured films had a significantly less scale formation, which is critical for avoiding shunts in the device. By moving from bottom electrodes fully defined by printing to the bottom electrodes where the transistor channel is opened by the laser, the channel length could be reduced from 40 μm to less than 5 μm.

A Thermodynamic Model for the Laser Fluence Ablation Threshold of PECVD SiO2 on Thin a-Si:H Films Deposited on Crystalline Silicon

We have developed a thermodynamic model that predicts the heat distribution in a stack of PECVD SiO2 and a-Si:H on crystalline Si after laser irradiation. The model is based on solving the total enthalpy heat equation with a finite difference scheme. The laser used in the model is a frequency doubled Nd:YVO 4 green laser with pulse duration in the nanosecond range. The modeling was done with the aim of getting a better understanding of our newly developed laser ablation process for making local contacts on back-junction silicon solar cells. Lasers with pulse duration within the nanosecond range are usually believed to induce too much thermal damage into the underlying silicon to make them suitable for high efficiency solar cells. In our case, insertion of a thin layer of a-Si:H between the SiO 2 and the Si absorbs much of the laser irradiation both optically and thermally. This makes it possible to form local contacts to Si in a damage-free way. In addition, the residual a-Si:H serves as an excellent surface passivation layer for the Si substrate. We have also developed a simple static model to determine the onset of SiO 2 ablation on a-Si:H layers of varying thickness. The models, both the static and the dynamic, are in good agreement with experimental data.