This PDF file contains the front matter associated with SPIE Proceedings Volume 9180, including the Title Page, Copyright information, Table of Contents, Authors, and Conference Committee listing.

This paper will provide an overview of various laser processing techniques used in the fabrication of solar cells. There are numerous applications of lasers including laser doping, annealing, patterning, drilling and welding that vary based on material system (e.g. silicon wafer, polycrystalline thin-film) and the cell architecture. Laser annealing has been identified as a potential route to high quality thin-films for polycrystalline semiconductors such as CdTe and Cu(In,Ga)(Se,S)₂. Also for thin-film solar cell technologies, including amorphous-silicon, laser patterning has been widely adopted within the industry for creating monolithic interconnections and performing edge isolation. For silicon wafer based technologies there are a number of promising laser processing techniques, however most of these techniques are still in the development stages and are not yet incorporated into industrial production lines. These techniques that represent the next generation of high-efficiency crystalline silicon devices, include laser-fired contacts, laser doping of selective emitters, laser drilling for “wrap-through” device structures, and laser grooved buried contacts. This paper will present a review of each technique, the specific processing applications and the current state of development.

Laser processing has demonstrated great capabilities for processing the flat panel display glass, strengthened glass, and flexible glass for consumer electronics. In this paper, a variety of laser processing techniques and their applications are discussed. The techniques include glass cutting, drilling, and surface modification. To assess each technique, a matrix of criteria, such as speed, surface quality, strength, and process stability is proposed. Based on the matrix, future needs for laser processing of glass are outlined.

Laser microprocessing technologies offer an important tool to fulfill the needs of many industrial sectors. In particular, there is growing interest in applications of these processes in the manufacturing areas such as automotive parts fabrication, printable electronics and solar energy panels. The technology is primarily driven by our understanding of the fundamental laser-material interaction, process control strategies and the advancement of significant fabrication experience over the past few years. The wide-ranging operating parameters available with respect to power, pulse width variation, beam quality, higher repetition rates as well as precise control of the energy deposition through programmable pulse shaping technologies, enables pre-defined material removal, selective scribing of individual layer within a stacked multi-layer thin film structure, texturing of material surfaces as well as precise introduction of heat into the material to monitor its characteristic properties are a few examples. In this research, results in the area of laser surface texturing of metals for added hydrodynamic lubricity to reduce friction, processing of ink-jet printed graphene oxide for flexible printed electronic circuit fabrication and scribing of multi-layer thin films for the development of photovoltaic CuInGaSe₂ (CIGS) interconnects for solar panel devices will be discussed.

Light confinement strategies play a crucial role in the performance of thin-film (TF) silicon solar cells. One way to reduce the optical losses is the texturing of the transparent conductive oxide (TCO) that acts as the front contact. Other losses arise from the mismatch between the incident light spectrum and the spectral properties of the absorbent material that imply that low energy photons (below the bandgap value) are not absorbed, and therefore can not generate photocurrent. Up-conversion techniques, in which two sub-bandgap photons are combined to give one photon with a better matching with the bandgap, were proposed to overcome this problem. In particular, this work studies two strategies to improve light management in thin film silicon solar cells using laser technology. The first one addresses the problem of TCO surface texturing using fully commercial fast and ultrafast solid state laser sources. Aluminum doped Zinc Oxide (AZO) samples were laser processed and the results were optically evaluated by measuring the haze factor of the treated samples. As a second strategy, laser annealing experiments of TCOs doped with rare earth ions are presented as a potential process to produce layers with up-conversion properties, opening the possibility of its potential use in high efficiency solar cells.

Commercial polycrystalline silicon solar cells were textured by the Laser Beam Interference Ablation technique to produce periodical grating in the antireflective and passivation layer. Modeling of periodical hole arrays and refractive index gratings in the antireflective coating on silicon substrate was used to select the fabrication regime and explain alterations in optical properties of the laser treated samples. Visual and elemental analysis of the laser treated areas was performed as well as measurement of photo-electrical characteristics before and after the laser treatment. Two modification regimes were established: ablation and oxidation of the antireflective layer. Changes in surface structure and composition as well as optical and electrical properties of patterned solar cells are discussed.

A method of doping germanium using 1064 nm pulsed fiber laser was demonstrated. The secondary ion mass spectrometry showed a p-n junction of 800 nm deep with a peak phosphorus concentration of 2×10¹⁹ cm^-3. Germanium photodiodes were fabricated on the laser-doped p-n junctions. Low bulk and surface leakage current values were obtained which were comparable to diodes fabricated by rapid thermal diffusion. Laser doping allows low thermal budget, minimization of surface desorption and selective doping without requiring photolithography. Laser doping was shown to be an effective method for fabrication of electronic and optoelectronic devices.

Silicon remains as the main material used in solar cell production, because of its low cost, abundance in nature and well-established technologies. However, its surface reflects considerable part of light due to its high refraction index. Light harvesting pays an important role for further progress to high-efficient solar cells. Texturing of the substrate surface is an efficient method to enhance the light absorption leading to the higher solar-to-electricity conversion efficiency in crystalline silicon solar cells. We present the novel method for silicon surface texturing using the direct laser beam interference ablation in addition with selective chemical etching. This technique enables production of high aspect ratio structures on a large surface area with just a single laser exposure. Characterization of the laser textured surfaces was performed using SEM. Theoretical simulation of light interaction with such structures was conducted in parallel and was used to adjust the laser process for more efficient light harvesting.

Near field optics concepts have introduced a paradigm shift in a wide variety of engineering fields in the recent past and the most significant applications of this fundamental physics concepts have been in the applied engineering problems such as improved broad band light absorption thereby enhancing the conversion efficiency of thin silicon solar cells. Also, for writing patterned structures or features using non contact optical methodologies have enabled near field optics assisted fabrication and related applications. The technology involving optics concepts and methodologies targeting energy sector have seen the impact of the same with a challenging trend to achieve smaller features or devices with micro- or nano-scale features. This demands automatically the need for achieving much smaller features beyond the forecasted sub- 30nm feature patterning methodologies. To meet such demands, a new branch of near- field optical concepts for improving patterning resolution has started developing which have been receiving considerable attention for its ability to produce high density sub-wavelength features that can find tremendous energy harvesting applications. This paper in this context mainly focuses on the review of different near field optical concepts and approaches developed for patterning by the author’s group at NTU. Different concepts were explored incorporating surface Plasmon waves ( LSPs, SPPs, LRSPs), gap modes as well as their interference in order to high resolution features and pattern dimensions at nano-scales. The absorbance of near band gap light is small and hence structuring of thin film solar cell is very important for increasing the absorbance by light trapping. The manuscript conclude by correlating the above said aspects and the challenges in achieving improved light conversion in thin film solar cells.

Recent work on laser-induced crystallization of thin films and nanostructures is presented. Characterization of the morphology of the crystallized area reveals the optimum conditions for sequential lateral growth in a-Si thin films under high-pulsed laser irradiation. Silicon crystal grains of several micrometers in lateral dimensions can be obtained reproducibly. Laser-induced grain morphology change is observed in silicon nanopillars under a transmission electron microscopy (TEM) environment. The TEM is coupled with a near-field scanning optical microscopy (NSOM) pulsed laser processing system. This combination enables immediate scrutiny on the grain morphologies that the pulsed laser irradiation produces. The tip of the amorphous or polycrystalline silicon pillar is transformed into a single crystalline domain via melt-mediated crystallization. The microscopic observation provides a fundamental basis for laser-induced conversion of amorphous nanostructures into coarse-grained crystals. A laser beam shaping strategy is introduced to control the stochastic dewetting of ultrathin silicon film on a foreign substrate under thermal stimulation. Upon a single pulse irradiation of the shaped laser beam, the thermodynamically unstable ultrathin silicon film is dewetted from the glass substrate and transformed to a nanodome. The results suggest that the laser beam shaping strategy for the thermocapillary-induced de-wetting combined with the isotropic etching is a simple alternative for scalable manufacturing of array of nanostructures.

We investigate how post-deposition laser annealing can be used to improve structural and electronic quality of room-temperature deposited CdTe. We use continuous-wave, 1064 nm laser light to anneal CdTe solar cell stacks prior to back contact deposition. Sub-bandgap optical absorption measurements by photothermal deflection spectroscopy show a reduction of sub-bandgap defects due to the annealing process. Since the 1064 nm light is only partially absorbed, in situ monitoring of the transmitted light during laser annealing gives real-time information about changes in the material. These results reveal an evolution of electronic defect annealing and surface roughness modification with laser exposure time. This hypothesis is supported by electron microscopy. Two distinct annealing regimes emerge: one at low laser power where electronic defect annealing saturates after about one minute exposure and another at high power where structural defects are annealed after several minutes exposure. Temperatures reached during laser annealing are estimated by finite element modeling of the thermal transport due to heat generation from optical absorption.

It is well known that lasers have helped to increase efficiency and to reduce production costs in the photovoltaic (PV) sector in the last two decades, appearing in most cases as the ideal tool to solve some of the critical bottlenecks of production both in thin film (TF) and crystalline silicon (c-Si) technologies. The accumulated experience in these fields has brought as a consequence the possibility of using laser technology to produce new Building Integrated Photovoltaics (BIPV) products with a high degree of customization. However, to produce efficiently these personalized products it is necessary the development of optimized laser processes able to transform standard products in customized items oriented to the BIPV market. In particular, the production of semitransparencies and/or freeform geometries in TF a-Si modules and standard c-Si modules is an application of great interest in this market. In this work we present results of customization of both TF a-Si modules and standard monocrystalline (m-Si) and policrystalline silicon (pc-Si) modules using laser ablation and laser cutting processes. A discussion about the laser processes parameterization to guarantee the functionality of the device is included. Finally some examples of final devices are presented with a full discussion of the process approach used in their fabrication.

Selective laser patterning for integrative serious connection has been industrially established in inorganic thin film solar cells based on glass substrates since a few years. In organic solar cells (OSC) the used materials significantly differ in terms of their patterning behavior. Due to their processability by wet chemical methods inverted architectures are often preferred in organic solar cells which allow the patterning by ultrashort laser pulses in substrate and superstrate configuration. Starting with an introduction of the ablation mechanisms taking place in OSC thin films, an overview of the current state-of-the-art in laser patterning of organic solar cells is presented. Besides progress in research also current achievements in industrial applications are illustrated.

Molybdenum is commonly used as the electrical back contact for Cu(In,Ga)(Se,S)₂ thin-film solar cells. In order to create a monolithically interconnected device, scribing of the molybdenum layer is required. This scribe, known as the P1 scribe, is commonly carried out through laser processing. Optimization of this laser scribing has been carried out using a 532nm pulsed Nd:YAG laser. It was found that two specific regimes of processing resulted in defect free scribes. These regimes are low fluency and high pulse overlap, and high fluency and low pulse overlap. Film properties, including the microstructure, surface oxidation, and internal stress, were studied to understand their effect on the laser ablation process. It was observed that a thin layer of oxidation resulted in significant heat affected zone during the laser ablation process. A discussion of the optimal film properties and laser processing parameters is presented.

A novel production process combining slot-die coating, transparent flexible IMI (ITO-Metal-ITO) electrodes and ultra-fast laser ablation can be used for the realization of P3HT:PCBM based thin film flexible OPV modules. The fast and precise laser ablation allows an overall efficiency over 3 % and a device geometric fill factor (GFF) over 95 %. Three functional layers can be ablated using the same wavelength only with varying the laser fluence and overlap. Different OPV device architectures with multilayers utilizing various materials are challenging for ablation but can be structured by using a systematical approach.