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

Laser processing has a long history in the manufacturing of solar cells since most thin-film photovoltaic modules have been manufactured using laser scribing for more than thirty years. Lasers have also been used by many solar cell manufacturers for a variety of applications such as edge isolation, identification marking, laser grooving for selective emitters and cutting of silicon wafers and ribbons. In addition, several laser-processing techniques are currently being investigated for the production of new types of high performance silicon solar cells. There have also been research efforts on utilizing laser melting, laser annealing and laser texturing in the fabrication of solar cells. Recently, a number of manufacturers have been developing new generations of solar cells where they use laser ablation of dielectric layers to form selective emitters or passivated rear point contacts. Others have been utilizing lasers to drill holes through the silicon wafers for emitter-wrap-through or metal-wrap-through back-contact solar cells. Scientists at Fraunhofer ISE have demonstrated high efficiency silicon solar cells (21.7%) by using laser firing to form passivated rear point contacts in p-type silicon wafers. Investigators art both the University of Stuttgart and the University of New South Wales have produced high efficiency silicon solar cells using laser doping to form selective emitters, and some companies are now developing commercial products based on both laser doping and laser firing of contacts. The use of lasers in solar cell processing appears destined to grow given the advances that are continually being made in laser technology.

The present highly competitive state of the PV industry is putting the pressure on both crystalline and thin film manufactures to deploy advanced architectures as a means to differentiate their products and protect market share. c-Si back side contact passivation, selective emitter opening and selective emitter doping are three efficiency improving processes that can utilize lasers that the industry is rapidly moving to adopt. Emitter Wrap Through (EWT) is another advanced architecture which will likely see adoption in the coming years for which lasers are a critical component of the process. Improved laser TCO patterning is of interest not only for CdTe solar cells but also for display and other micro electronics applications. Laser processing results for these various solar PV materials are presented both in terms of performance, materials science, and morphology as they relate to temporal characteristics of the laser pulse.

Laser fired contacts (LFCs) and laser doped selective emitters can be used to improve manufacturing throughput of photovoltaic devices without sacrificing device conversion efficiency. However, the laser parameters used to form these features can vary significantly. LFCs can be formed with short pulses (hundreds of nanoseconds) while selective emitters can be formed using either a pulsed or CW mode. Here, mathematical models for a pulsed laser and CW laser are used to evaluate how variations in processing parameters affects alloy formation, molten pool geometry and dopant concentration profiles. The models solve the conservation equations for mass, energy, and momentum to study the effects of heat and mass transfer and fluid flow on the formation of LFCs and emitters. Comparisons between experimental data and theoretical calculations for molten pool geometry and concentration profiles demonstrate good agreement. For LFCs, when assuming complete melting and mixing of the Al contact layer, the Al concentration varies significantly with power level, which drastically impacts the calculated pool shape. The dimensionless Peclet number is used to understand dominant heat and mass transfer mechanisms. Conduction is the dominant heat transfer mechanism at power levels around 20W for both LFCs and emitters. As the power level is increased to 50W, however, the dominant heat transfer mechanism changes to convection. Changes in laser parameters also impact fluid flow velocities and dopant concentration profile for emitters processed in CW mode, which suggests that convection-based models should be used to accurately predict concentration profiles within emitters.

The effects of both low power laser (He - Ne) of 7.5mW, wavelength 632.8 nm, and conventional thermal (50 - 150°C) treatments on reclamation of two types of silicon solar cells samples have been investigated. Dark and under light current - voltage characteristics have been studied before and after laser and thermal processings. The measurements on both types were done for successive times until a steady state has reached. It is found, that the low power laser has no effects on the dark J - V parameters such as saturation current and ideality factor. However, for solar cell parameters under light measurements at 500W/cm2, it has been found that the treatment causes an increase in the short circuit current density, maximum output power, fill factor and hence the solar cell efficiency.

In this contribution we present different results of our investigations regarding the use of aluminum foil as rear side metallization for solar cells with dielectric passivation and laser fired contacts (LFC). We investigate the impact of different laser processes on the resistance of the contacts, the adhesion properties of the foil and the efficiency potential. By fabricating highly efficient, 20×20 mm² sized solar cells with a conversion efficiency of 21.0 %, we demonstrate the high potential of this approach, which is equal to that of LFC-cells with common screen-printed or PVD metallization on the rear side. We investigated the optical properties of such metallized rear sides which benefit from an embedded air gap between foil and passivation layer. Finally, we present first solar cell results on industrially sized wafers (A=238 cm²) demonstrating again the equal efficiency potential compared to PVD metallization.

This work investigates the influence of the laser wavelength on laser doping (LD) and laser-fired contact (LFC) formation in solar cell structures. We compare the results obtained using the three first harmonics (corresponding to wavelengths of 1064 nm, 532 nm and 355 nm) of fully commercial solid state laser sources with pulse width in the ns range. The discussion is based on the impact on the morphology and electrical characteristics of test structures. In the case of LFC the study includes the influence of different passivation layers and the assessment of the process quality through electrical resistance measurements of an aluminium single LFC point for the different wavelengths. Values for the normalized LFC resistance far below 1.0 mΩcm² have been obtained, with better results at shorter wavelengths. To assess the influence of the laser wavelength on LD we have created n+ regions into p-type c-Si wafers, using a dry LD approach to define punctual emitters. J-V characteristics show exponential trends at mid-injection for a broad parametric window in all wavelengths, with local ideality factors well below 1.5. In both processes the best results have been obtained using green (532 nm) and, specially, UV (355 nm). This indicates that to minimize the thermal damage in the material is a clear requisite to obtain the best electrical performance, thus indicating that UV laser shows better potential to be used in high efficiency solar cells.

Selective laser doping is a versatile tool for the local adaption of doping profiles in a silicon substrate. By adjusting the laser fluence as well as the pulse width the maximum melt depth in the silicon can be controlled. Longer pulses lead to lower temperatures in the material and can help to enlarge the process window as ablation sets in at higher fluencies. For the fabrication of highly efficient silicon solar cells, laser doping can be used for efficiency improvement and process simplification. In passivated emitter and rear cells (PERC), selective laser doping can be used for selective emitter formation. Employing such a process, an efficiency boost of Δ ƞ= 0.4%_abs was observed on commercial Cz-Si material. Laser doping was also used for process simplification for the fabrication of locally doped point contacts at the rear of a solar cell. A simple approach employing a doped passivation layer and a laser doping process allows for efficiencies beyond 22% on high quality n-type silicon.

Development of laser doping process for the formation of a selective emitter (SE) for p-type and n-type silicon solar cells is presented. The SE is formed by laser doping of spin-on dopant sources using an intermediate barrier layer (BL). The BL serves to form shallow emitter and also offers advantage to avoid etch back step. The shallow emitter is formed by applying a controlled thermal diffusion step, which in turn reduces the laser induced defects in the SE. This process has an advantage that the shallow and selective emitters can be formed from a single dopant source. In this investigation, PECVD deposited SiOx was used as the barrier. KrF excimer laser at 248 nm was used for the selective doping. The dopant concentration and depth, as measured by SIMS, were controlled by variation of the laser parameters and barrier thickness. It was found relatively lower thickness PECVD deposited SiOx barrier layer with high dopant content in the spin-on layer at comparably low laser fluences resulted in the best electrical results. The SiOx layers acted as perfect barrier for the boron diffusion. It was also observed that multiple laser annealing above a threshold laser fluence resulted in the redistribution of the dopant along with deepening of selective emitter because of the limitedness of the dopant source. Also, this is attributed to the increase of the total absorbed energy by the successive laser pulses. The results were discussed and presented in detail.

Laser processing applied to thin film silicon is an interesting approach for solar cell fabrication. In this work, we investigate the effects of a continuous wavelength (CW) laser irradiation in solid phase or liquid phase of silicon on the structural and electrical properties of thin film silicon layers. Thus, results on CW laser induced crystallisation (LIC) of ultrathin amorphous silicon, laser induced epitaxy (LIE) of a thick amorphous silicon on a seed silicon layer, and laser induced thermal annealing (LIA) of polycrystalline silicon films are presented and discussed.

This work discusses the impact of laser annealing on a picosecond laser ablation process of anti-reflection layers on damage etched and random pyramid textured silicon wafers. The laser ablation is realized using picosecond pulsed laser radiation which facilitates a continuously ablated passivation layer but induces a significant reduction in charge carrier lifetime. It is demonstrated that the application of a nanosecond pulsed laser annealing step can improve the electrical properties of the picosecond laser treated area.

The ability to grow large-area, large-grained polycrystalline silicon on inexpensive substrates is becoming increasingly important for photovoltaic (PV) devices. With large-grained (grain size <10 μm) 10 μm thick films it is possible with light trapping to achieve PV efficiencies exceeding 15%. If crystallites could be nucleated and grown for longer times before native nucleation occurs, then potentially these much larger grain, thin film silicon material could be produced.The interaction of sub-crystallization threshold laser fluence with hydrogenated amorphous silicon (a-Si:H) has been demonstrated on a macroscopic scale to shorten the incubation time in subsequently thermally annealed films. Further examination of crystallite laser nucleation, found that nucleation was suppressed around PECVD a-Si:H thin film(50-100nm) sample edges, and scratches, in addition to laser-ablated areas, extending as much as 100-200 μm laterally from these features. Optical microscopy and stepwise high temperature thermal annealing were used to investigate this behavior for the a-Si:H films deposited on glass substrates. The nucleation rates were measured in the treated and untreated regions. The data suggests that these features (edges, scratches, and laser ablated areas) provide stress relief by interrupting the surface connectivity. We confirm the existence of stress and stress relief by μ-Raman measurements of the crystallite transverse optical peak position relative to that of c-Si. PECVD films were annealed at temperatures between 540-600C, to enable a determination of r_n at each anneal temperature. The temperature dependent measurements enabled the determination of the nucleation rate activation energies (E_A), and how they are affected by film stress.

Cu(In,Ga)Se₂ (CIGS) thin film photovoltaic absorber layers are primarily synthesized by vacuum based techniques at industrial scale. In this work, we investigate non-vacuum film synthesis by electrochemical deposition coupled with pulsed laser annealing (PLA) and or continuous wave laser annealing (CWLA) using 1064 nm laser. PLA results indicate that at high fluence (≥100 mJ/cm²) CuInSe₂ films melt and dewet on both Mo and Cu substrates. In the submelt PLA regime (≤70 mJ/cm²) no change in XRD results is recorded. However CWLA at 50 W/cm² for up to 45 s does not result in melting or dewetting of the film. XRD and Raman data indicate more than 80% reduction in full width at half maximum (FWHM) in their respective main peaks for annealing time of 15 s or more. No other secondary phases are observed in XRD or Raman spectrum. These results might help us in setting up the foundation for processing CIGS through an entirely non-vacuum process.

The best conversion efficiency of champion, small-area CIGSeS cells and mass-produced modules are 20.3% and ~12.0% respectively. Molybdenum back-contact layer is scribed with a laser (P1). Development of laser scribing for P2 and P3 scribes will reduce the dead area and improve the reliability. Development of a laser annealing technique can minimize and passivate micro-non-uniformities and grain boundaries thus reducing carrier recombination. In-situ characterization of the sample through a nonintrusive method such as reflectivity measurement during the laser recrystallization process can enhance insight into laser-material interaction and the effect of material structures on the photoelectric properties of solar cells. The improvement in efficiency achieved with laser processing can help to bring down the cost.

Lasers have proven to be efficient tools in a variety of micro/nanoscale material processing and diagnostics. In this talk, recent research activities will be presented, focused on the laser-assisted manufacturing and diagnostics technologies for solar applications. Examples include laser scribing and spectroscopic in-situ monitoring for thin film solar cells, surface structuring to achieve improved light trapping at superior conversion efficiency, laser-assisted local doping based on wide range of laser parameters, via hole fabrications for back contact formation, and direct synthesis of multi-bandgap nanomaterials. In addition, the advanced in-situ diagnostics of laser interaction with photovoltaic materials system will be discussed.

In this paper we present a simple technique for approximating laser process parameters needed for laser processing of crystalline silicon solar cells. The calculation computes the changes of silicon material properties during the time of laser-material interaction. As the laser pulse energy modifies optical and thermal properties of silicon, the chronological segmentation illustrates the temperature rise within the irradiated volume and indicates the time needed for melting or evaporation. Depending on the desired material modification, commercially available laser sources are analyzed regarding their process suitability. Simulating the laser system performance reveals its theoretical output and determines its expected efficiency. Simulations in this paper correlate well to experimental data and are done for different fields of interest: a) ablation rate during laser drilling for EWT cells, using IR wavelengths in the order of 1 μs b) depth and width of laser grooves as used for Laser Grooved Buried Contact cells (LGBC) or edge isolation, using wavelengths in the IR and VIS c) process windows during selective laser doping with 532 nm using PSG as sole phosphorous source d) laser parameters needed for Laser-Fired Contacts (LFC).

Laser processing of photovoltaic cells enables the manufacturing of high-efficiency cells and offers opportunities to cut down costs. Structuring of selective emitters or laser fired contacts requires precise laser beam shaping. In this paper we identify key design parameters for industrial applications of Gaussian-to-top hat converters. We derive the physical limits for top hats close to the diffraction limited and compare the performance of actual beam shapers to these limits. Additionally we present details and first measurements of our improved solution to scan a 50μm top hat, allowing to structure a 156mm wafer in a few seconds.

The flattop (uniform) and other intensity distributions of a laser beam are frequently considered as techniques to improve performance of laser technologies in manufacturing of solar cells. This task of creating these beam profiles can be easily solved with using beam shaping optics, for example, the field mapping refractive beam shapers like piShaper. The operation principle of these devices presumes transformation of laser beam intensity from Gaussian to flattop one with saving beam consistency, providing collimated output beam of low divergence, high transmittance, extended depth of field, capability to work with galvo-mirror scanning optics. The flattop, inverse Gauss, super Gauss, donut and other intensity distributions can be provided for the laser spots which size spans from microns to millimetres and centimetres; this makes these devices a powerful tool to improve the laser technologies like patterning, scribing, drilling, edge isolation, firing contacts, etc. This paper will describe some design basics of refractive beam shapers πShaper and optical layouts of their applying in various technologies for solar cell manufacturing. Examples of real implementations and experimental results will be presented as well.

Laser ablation of passivation layers is one of the most promising processes for high efficiency cell concepts in high throughput solar cell production. Especially on the front side a depth- or material-selective ablation process is required to avoid damage to the sensitive emitter. To develop a fast and reliable laser ablation process with a minimum amount of damage to the emitter it is vital to use the most suitable laser source and to optimize the processing parameters. For identification of the influence of pulse duration on cell performance after ablation a new experimental approach is chosen, where full crystalline solar cells are used as samples. In an iterative experimental sequence ablation of lines between the fingers is alternated with Suns-Voc measurements. The measurements reveal the impact of the laser ablation process on the electrical properties of the solar cell, like pseudo fill factor and open circuit voltage. The method has two decisive advantages compared to other approaches presented in earlier works: a) the preparation of special samples (e.g. full cells without front metallization) is not required and reliable commercially available standard cells can be used instead; b) the iterative nature of the approach allows an extrapolation to larger ablated areas.

This work reports on the elaboration of a new industrial process based on laser selective ablation of dielectric layers for Interdigitated Back Contact Silicon Heterojunction (IBC Si-HJ) solar cells fabrication. Choice of the process is discussed and cells are processed to validate its performance. A pulsed green laser (515nm) with 10-20ns pulse duration is used for hydrogenated amorphous silicon (a-Si:H) layers patterning steps, whereas metallization is made by screen printed. High Open-Circuit Voltage (Voc=699mV) and Fill Factor (FF=78.5%) values are obtained simultaneously on IBC Si-HJ cells, indicating a high surface passivation level and reduced resistive losses. An efficiency of 19% on non textured 26 cm² solar cells has been reached with this new industrial process.

Applications for laser patterning in Si photovoltaics include (i) patterning SiO₂ or SiN layers with openings for local contacts and (ii) laser-doped selective emitter (LDSE) processes, in which the contact open is accompanied by the diffusion of dopants into a locally melted Si area. While contact open processes are best performed with UV wavelengths that can be strongly absorbed by the SiN or SiO₂ (allowing layer ablation with a minimum of Si heating), the Si melt depth required by LDSE requires irradiation at longer laser wavelengths where these antireflection coatings (ARCs) no longer absorb well. An optimized LDSE process must thus produce Si melting as well as the least amount of Si vaporization sufficient to lift off the overlying ARC. In this work, we investigate the mechanisms for lifetime degradation in Si(p-type, 100-oriented)/ARC samples resulting from 20 ns pulsed laser irradiation at 532 nm at fluences near the threshold for ARC removal. To differentiate between lifetime degradation induced by changes in the passivation layer vs. changes in the Si itself, samples were lifetime mapped after patterned laser irradiation and then again after a wet ARC strip and repassivation. Samples with ARCs of thermal SiO₂ and PECVD SiN typically showed some residual Si damage after irradiation at fluences sufficient for contact open. Interestingly, irradiation of the SiO₂ samples at lower fluences, between the threshold for Si melting and ARC removal, showed damage to the SiO₂ passivation, but no residual Si damage. Explanations for these observations and related results will be discussed.

Different sidewall characters of the silicon microholes drilled with fiber laser pulses at 1065.2 nm were examined by a scanning electron microscope. It shows two different recast phases on the hole-wall: mild melt phase with smooth interface and vaporization phase with rough interface. These two recast phases show different etch rate when alkaline texturing, which sometimes leave some undesirable damages on the sidewall. After alkaline texturing, the taper holes transform into octagonal holes. And the eight sidewalls of the octagonal-hole alternating appear two different micro-nano structures, one is pyramid structure, the other is lamellar structure.

The use of selective emitters in p-n junction solar cells is a well-known way to increase cell efficiency by 0.4 - 0.5% (absolute) with the addition of a few processing steps. In a selective emitter, the region directly below the metal-contact fingers is more heavily doped than the shallow p-n junction. This allows for enhanced carrier collection by shielding minority carriers from the contacts, thereby lowering recombination at the metal-semiconductor interface. In contrast to earlier expensive techniques involving fine-line lithography, laser processing provides an ideal way to create these selective emitters because of its ability to locally heat and dope the surface of the cell without any external patterning steps. In this study, Q-switched lasers of wavelengths 1064, 532, and 355 nm are used at a range of pulse energies to create selective emitters on a p-type FZ silicon wafer with a thin n+ dopant film deposited on the top surface of the wafer. In addition to the Q-switched lasers, a 1070 nm continuous wave laser is also used and both the pulse energy and pulse duration are varied. To determine the effect of the n+ dopant film, the thickness of the film is also varied and processed with all of the lasers. The results from these lasers and the different dopant layers are characterized electrically through current-voltage measurements and compared to determine the optimal processing wavelength and energy for the selective emitters which maximize diode performance while minimizing crystal lattice damage and series resistance.

The laser as an industrial tool is an essential part of today’s solar cell production. Due to the on-going efforts in the solar industry, to increase the cell efficiency, more and more laser-based processes, which have been discussed and tested at lab-scale for many years, are now being implemented in mass production lines. In order to cope with throughput requirements, standard laser concepts have to be improved continuously with respect to available average power levels, repetition rates or beam profile. Some of the laser concepts, that showed high potential in the past couple of years, will be substituted by other, more economic laser types. Furthermore, requirements for processing with less-heat affected zones fuel the development of industry-ready ultra short pulsed lasers with pulse widths even below the picosecond range. In 2011, the German Ministry of Education and Research (BMBF) had launched the program “PV-Innovation Alliance”, with the aim to support the rapid transfer of high-efficiency processes out of development departments and research institutes into solar cell production lines. Here, lasers play an important role as production tools, allowing the fast implementation of high-performance solar cell concepts. We will report on the results achieved within the joint project FUTUREFAB, where efficiency optimization, throughput enhancement and cost reduction are the main goals. Here, the presentation will focus on laser processes like selective emitter doping and ablation of dielectric layers. An indispensable part of the efforts towards cost reduction in solar cell production is the improvement of wafer handling and throughput capabilities of the laser processing system. Therefore, the presentation will also elaborate on new developments in the design of complete production machines.