This PDF file contains the front matter associated with SPIE Proceedings Volume 8826, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Laser processing has been the tool of choice last years to develop improved concepts in contact formation for high efficiency crystalline silicon (c-Si) solar cells. New concepts based on standard laser fired contacts (LFC) or advanced laser doping (LD) techniques are optimal solutions for both the front and back contacts of a number of structures with growing interest in the c-Si PV industry. Nowadays, substantial efforts are underway to optimize these processes in order to be applied industrially in high efficiency concepts. However a critical issue in these devices is that, most of them, demand a very low thermal input during the fabrication sequence and a minimal damage of the structure during the laser irradiation process. Keeping these two objectives in mind, in this work we discuss the possibility of using laser-based processes to contact the rear side of silicon heterojunction (SHJ) solar cells in an approach fully compatible with the low temperature processing associated to these devices. First we discuss the possibility of using standard LFC techniques in the fabrication of SHJ cells on p-type substrates, studying in detail the effect of the laser wavelength on the contact quality. Secondly, we present an alternative strategy bearing in mind that a real challenge in the rear contact formation is to reduce the damage induced by the laser irradiation. This new approach is based on local laser doping techniques previously developed by our groups, to contact the rear side of p-type c-Si solar cells by means of laser processing before rear metallization of dielectric stacks containing Al2O3. In this work we demonstrate the possibility of using this new approach in SHJ cells with a distinct advantage over other standard LFC techniques.

The laser processing of thick aluminum foil (8 μm) has the potential of providing low resistance metal contacts and reducing the fabrication cost for silicon solar cells. A high-power nanosecond pulsed laser with wavelength of 1064 nm was used in combination with a soda-lime glass substrate to make electrical contacts between aluminum foil and silicon where the glass substrates allowed flattening of the foil for laser processing. The initial demonstration was performed by passing a laser beam through glass and irradiating an aluminum foil to contact p-type silicon through a passivation layer of SiOx. Cross-section morphologies of resultant line contacts were investigated. A specific contact resistivity as low as 1.8 mΩ-cm2 was achieved based on measurements by the Transmission Line Method (TLM). A non-vacuum laser assisted rear metallization process based on cost-effective aluminum foil is feasible for silicon solar cell fabrication.

Laser processing is a single step, attractive alternative to current multi-step formation of ohmic contacts between an aluminum metallization layer and a silicon substrate in solar cell devices. However, small changes in laser parameters such as pulse duration, power density and laser wavelength can result in significant differences in the contact geometry and electrical properties. Here, the effects of power density and pulse duration on the morphology, resistance and surface concentration of laser fired contacts (LFCs) are examined experimentally. The minimum fluence threshold for forming a contact with measureable resistance through the 100nm SiO₂ layer is determined to be 8 J/cm². In addition, when forming the contact, an outer rim region accumulates on the surface that is comprised of aluminum and silicon. As a result, the entire contact is actually governed by the size of an inner crater region plus this outer rim material, which is in contrast to results reported in the literature for nanosecond pulse durations. These results are in good agreement with independent results reported in the literature for LFCs processed on wafers with substantially different base resistivity and using significantly different processing parameters.

Presented here are the results of a three dimensional, finite element simulation that models pulsed, ultraviolet (UV) laser annealing of polycrystalline CdTe. The model considers heat generated by the absorption of a 25 ns, 248 nm laser pulse normally incident to a 5 μm thick CdTe thin film deposited on a polycrystalline alumina substrate. In particular, focus is on the spatial and temporal distribution of temperature from laser fluences that achieve a sub-melting condition. The model shows that there are very large temperature gradients both in depth and in-plane directions. These predictions, as well as the onset of melting, are confirmed with cross sectional scanning electron microscopy. Additionally, the model predicts that the heat generated dissipates rapidly after the pulse has ended. This has implications if pulse trains are to be used experimentally.

We compare two types of laser ablation for ARC removal on polished and textured surfaces. Selective ablation with limited impact on the underlying substrate is performed with short wavelength picosecond sources working at relatively low repetition rate (<200 kHz). By adapting wavelength and fluence, the SiNx could be removed efficiently with slight change initial topography. Crystal damage is detected whatever the laser parameters but could be reduced using low fluence in UV regime. The second ablation process uses ultra-high repetition rate picosecond laser (80 MHz) and targets both SiNx ablation and over-doping of the initial n⁺ emitter. The thermal effect induced by the short duration between pulses performs simultaneously SiNx removal and selective emitter structure with with deep dopant profiles and low surface concentration. We investigate the correlation between the post-ablation properties and a nickel silicidation process using Excimer Laser Annealing of a thin layer of Ni. A reference process is first described on pyramid topography without pre-ablation of SiN_x. It is demonstrated efficient formation of SixNiy compounds on the silicon substrate depending and laser fluence. The similar silicidation process is transferred on sample after the SiN_x ablation step. A continuous nickel silicide layer is observed but its thickness distribution reveals non-uniformity over the pyramids due to post ablation roughness.

Industrial solar cell fabrication generally adopts printing process to deposit the front electrodes, which needs additional heat treatment after printing to enhance electrical conductivity. As a heating method, laser irradiation draws attention not only because of its special selectivity, but also because of its intense heating to achieve high electric conductivity which is essential to reduce ohmic loss of solar cells. In this study, variation of electric conductivity was examined with laser irradiation having various beam intensity. 532 nm continuous wave (CW) laser was irradiated on inkjet-printed silver lines on glass substrate and electrical resistance was measured in situ during the irradiation. The results demonstrate that electric conductivity varies nonlinearly with laser intensity, having minimum specific resistance of 4.1 x 10^-8 Ωm at 529 W/cm² irradiation. The results is interesting because the specific resistance achieved by the present laser irradiation was about 1.8 times lower than the best value obtainable by oven heating, even though it was still higher by 2.5 times than that of bulk silver. It is also demonstrated that the irradiation time, needed to finish sintering process, decreases with laser intensity. The numerical simulation of laser heating showed that the optimal heating temperature could be as high as 300 oC for laser sintering, while it was limited to 250 oC for oven sintering. The nonlinear response of sintering with heating intensity was discussed, based on the results of FESEM images and XRD analysis.

Monolithic cell interconnection is a technique used in solar devices to allow for interconnection of adjacent cells through patterning of the thin films during fabrication. In the case of CuIn_1-xGa_xSe_2-yS_y (CIGS) solar cells, Molybdenum is commonly used as the back contact. Patterning of this layer is required in the interconnection scheme to electrically isolate adjacent cells. Laser scribing has been adopted for patterning of this layer. This paper reports on the effect of the molybdenum thin film deposition technique, and the resulting film properties, on the characteristic of the laser scribe. Films were deposited using DC magnetron sputtering over a range of working gas pressures and powers as well as in single and multilayer configurations. It was found that the residual stress within the film lead to significantly different laser ablation processes. This required independent tuning of the laser processing parameters to create a clean, defect free scribe for different samples. Experimentation was carried out using both film-side and glass-side processing. It was shown that glass-side processing leads to a reduction in cracks and delamination originating from the scribe. The processing conditions that produced successful scribe lines for the various films are presented and discussed.

Laser-assisted localized growth of semiconducting nanostructures is reported. As is the case of conventional crystal growth, localized laser enables three kinds of crystal growth: (1) melt growth (recrystallization) of amorphous silicon nanopillars by pulsed laser; (2) vapor growth (chemical vapor deposition) of germanium nanowires; (3) solution growth (hydrothermal growth) of zinc oxide nanowires. The results not only demonstrate programmable and digital fabrication of laser-assisted crystal growth, but also reveal unusual growth chacracteristics (grain morphologies, growth kinetics). Related to solar applications, it is suggested that these structures can act as epitaxial seeds for growth of coarse grains and as multi-spectral centers for enhanced and engineered light absorption.

Laser beam shaping systems converting Gaussian to flattop or other irradiance profiles are used in various solar cell manufacturing laser technologies to enhance their performance. Scanning over whole working field with using popular 2- and 3-axis galvo mirror scanners is very often important part of microprocessing systems. Therefore, combining of beam shaping optics with scanning heads is an important technical task in field of solar cells manufacturing. As the beam shaping optics it is suggested to apply field mapping refractive beam shapers πShaper having some important features: low output divergence, high transmittance, extended depth of field, capability to work with TEM00 and multimode lasers, as result providing a freedom in building various optical systems. De-magnifying of flattop laser beam can be realized with using imaging technique; the imaging optical system to be composed from F-ʘ lens of scanning head and additional collimating system to be used right after a πShaper. One of the problems in this approach is implementation of compact design of the collimating part. As a solution it is suggested to apply a specially designed Beam Shaping Unit being based on π Shaper and locating between a laser and a scanning head; the functions of that combined system are: conversion from Gaussian to flattop laser beam irradiance profile, compact collimator design, alignment features, easy adaptation to a laser and a scanning head used in particular equipment. There will be considered design features of refractive beam shapers π Shaper and Beam Shaping Unit, examples of optical layouts to generate flattop laser spots, which sizes span from several tens of microns to millimetres. Examples of real implementations and results of material processing will be presented as well.

One approach to realize a back contact solar cell design is to ‘wrap’ the front contacts to the backside of the cell [1]. This results in significantly reduced shadowing losses, possibility of simplified module assembly process and reduced resistance losses in the module; a combination of measures, which are ultimately expected to lower the cost per watt of PV modules. A large number of micro-vias must be drilled in a silicon wafer to connect the front and rear contacts. Laser drilling was investigated using a pulsed disk laser which provided independent adjustment of pulse width, repetition rate and laser power. To achieve very high drilling rates, synchronization of the laser pulses with the two-axis galvanometer scanner was established using a FPGA controller. A design of experiments (DOE) was developed and executed to understand the key process drivers that impact the average hole size, hole taper angle, drilling rate and hole quality. Laser drilling tests were performed on wafers with different thicknesses between 120 μm and 190 μm. The primary process parameters included the average laser power, pulse length and pulse repetition rate. The impact of different laser spot sizes (34 μm and 80 μm) on the drilling results was compared. The results show that average hole sizes between 30 – 100 μm can be varied by changing processing parameters such as laser power, pulse length, repetition rate and spot size. In addition, this study shows the effect of such parameters on the hole taper angle, hole quality and drilling rate. Using optimized settings, 15,000 holes per second are achieved for a 120 μm thick wafer with an average hole diameter of 40μm.

Ultra-short pulsed laser sources, with pulse durations in the ps and fs regime, are commonly exploited for cold ablation. However, operating ultra-short pulsed laser sources at fluence levels well below the ablation threshold allows for fast and selective thermal processing. The latter is especially advantageous for the processing of thin films. A precise control of the heat affected zone, as small as tens of nanometers, depending on the material and laser conditions, can be achieved. It enables the treatment of the upper section of thin films with negligible effects on the bulk of the film and no thermal damage of sensitive substrates below. By applying picosecond laser pulses, the optical and electrical properties of 900 nm thick SnO2 films, grown by an industrial CVD process on borofloat®-glass, were modified. The treated films showed a higher transmittance of light in the visible and near infra-red range, as well as a slightly increased electrical sheet resistance. Changes in optical properties are attributed to thermal annealing, as well as to the occurrence of Laser- Induced Periodic Surface Structures (LIPSSs) superimposed on the surface of the SnO₂ film. The small increase of electrical resistance is attributed to the generation of laser induced defects introduced during the fast heating-quenching cycle of the film. These results can be used to further improve the performance of SnO₂-based electrodes for solar cells and/or electronic devices.