Scribing Process Research Articles

Polycrystalline CdTe thin film mini-modules monolithically integrated by fiber laser

The CdTe thin film technology for photovoltaics (PV) is attractive because of its potential low cost and good performance. In thin film technology the efficiency of large area cells can be maintained if small segments are interconnected in series to reduce the photocurrent and resistance losses. In respect to this, the scribing process is critical for the performance of the device. Today, fiber lasers represent the most advanced and cheap technology that can be used in PV industry to carry out the cuts, needed for the monolithic integration, at different deposition stages. We will present our results on the scribing of CdTe thin film solar cells by means of fiber lasers, with pulse duration of a few nanoseconds and solid state lasers in the picosecond regime. The quality of the scribing was evaluated by optical and scanning electron microscopy. Finally, mini-modules with a total area of 10×10cm2 were fabricated, in which the cells were interconnected in series by means of a scribing system, equipped with a fiber laser with the same characteristics of the system mounted on production lines. The mini-modules were characterized by photovoltaic and electrical measurements.

Domain refinement of grain oriented electrical steel with high power laser beam sources

Laser magnetic domain refinement (LMDR) is applied to reduce the power losses of grain oriented electrical steel, which is used as core material in power or distribution transformers, by generating thermal stress due to the absorbed laser energy. The most widely used laser beam source for the industrial utilization is the CO 2 laser beam source with typical core loss improvements of about 10%. In this paper the dependency of treatment parameters with respect to the magnetic properties of the steel sheets are investigated for the laser scribing process using a high power fiber laser beam source. Especially the treatment properties, which are influenced by the laser beam source, such as focusability, wavelength and absorption behavior, are addressed in this paper. Therefore, single sheets 300 mm × 30 mm × 0.27 mm (high-permeability, comparable to M103-27P) are measured before and after the laser scribing process to evaluate the magnetic properties, as well as the loss components. The analysis has shown that the core loss can be improved by approximately 13% using the fiber laser by scribing invisible lines on the sheet surface.

Precise selective scribing of thin-film solar cells by a picosecond laser

In this paper, precise scribing of thin-film solar cells (CIGS/Mo/Glass) via a picosecond laser is investigated. A parametric study is carried out for P1 and P2 scribing to study the effects of laser fluence and overlap ratio on scribing quality and ablation depth. Three ablation regimes are observed for P1 scribing in different laser fluence ranges, due to the involvement of different ablation mechanisms. The optimum scribing conditions are determined for both P1 and P2 scribing, and the potential processing speed is significantly increased. The heat accumulation effect at different repetition rates is studied to extrapolate the results from low to high repetition rates. A two-temperature model-based model is developed to simulate the scribing process for multiple thin films, providing decent prediction of the slot depth for both P1 and P2 scribing.

Development of Low AC Loss TFA-MOD Coated Conductors

TFA-MOD process is expected to be promising for future applications since it can produce high performance YBCO coated conductors (CCs) with low cost. Applying YBCO CCs to the power electric devices such as transformers and power cables, the reduction of alternating current (AC) loss for long wire is necessary. Multi-filamentation process, which is one of the most effective approaches for AC loss reduction, has been developed by the scribing process. We have developed the filamentation process using chemical etching. MOD derived CCs are, however, easily damaged in the chemical etching process due to existence of pores in a YBCO layer, resulting in critical current (Ic)-degradation and weak delamination strength. Consequently, it is difficult to scribe MOD derived CCs into 1mm-wide filaments for long length using the chemical etching process. Accordingly, we have studied a scribing process using an excimer laser without chemical etching. We defined P’ value in this study as a function of irradiated laser power [J] divided by processing speed[m/s]. We studied relationship between the P’ value and the results of scribing. It was found that we could scribe the C.C. with a sufficient depth in the condition of large P’ value. Furthermore, we found that the Ic was degraded with further increase of the P’ value. A 5mm wide short sample was divided into 10 filaments by the excimer laser scribing process at the P’ of 9[J/(m/s)]. The sample revealed reduction of the hysteresis loss down to 1/10 which could be expected from a theoretical prediction using the numbers of the filaments. Ic-degradation was suppressed as 28%, which was smaller than that of the scribed sample using chemical etching (Ic degradation was 38%). Subsequently, we applied the technique to a 100m long YBCO CC. The hysteresis loss of the 100m long MOD derived CC was reduced down to 1/10 (1/the number of filaments) after the multi-filamentation.

In-process Evaluation of Electrical Properties of CIGS Solar Cells Scribed with Laser Pulses of Different Pulse Lengths

Abstract The optimization of laser scribing for the interconnection of CIGS solar cells is a current focus of laser process development. In addition to the geometry of the laser scribes the impact of the laser patterning to the electrical properties of the solar cells has to be optimized with regards to the scribing process and the laser sources. In-process measurements provide an approach for reliable evaluation of the electrical characteristics. In particular, the parallel resistanceR p that was calculated from the measured I–V curves was measured in dependence on the scribing parameters of a short-pulsed ns laser in comparison to a standard ps laser at a wavelength of 1.06 μm. With low pulse overlap of ∼20% a reduction of R p to 2/3 of the initial value has been achieved for ns laser pulses. In comparison to ps laser slightly more defects were observed at the investigated parameter range.

Loss Analysis for Laser Separated Solar Cells

Half-cell modules are promising candidates for new innovative module designs as they offer major advantages. Modified connection schemes reduce the serial resistance losses yielding a higher overall module performance. The reduced size of the cells allows a more flexible module design that is needed for special applications such as implementations on curved surface. Furthermore, a better performance under partial shading can be achieved. However, these advantages lead to a benefit only if the losses induced by the cell separation process are negligible. In this work, we study the different sources of power reduction, i.e. increased shunting and recombination, for mono-crystalline and multi-crystalline silicon solar cells separated using different laser process parameters. It is shown that recombination plays the major role for an optimized laser separation process. Additionally we identify the laser scribing process as the major source of losses in comparison to the mechanical breaking.

The influence of the laser parameter on the electrical shunt resistance of scribed Cu(InGa)Se2 solar cells by nested circular laser scribing technique

Laser scribing of thin film solar cells attracts increasing attention for performing integrated interconnection. For high efficiency solar modules, low damage laser scribing techniques are needed. The optimization of the laser scribing process concerns all functional properties of the device but in particular the efficiency. The recently demonstrated nested circular scribing technique allows the in-process measurement of the shunt formation due to laser scribing (P3) of the thin film solar cell. By using this technique, the influence of laser fluence and pulse overlap on the electrical shunt resistance formation at laser scribing of Cu(InGa)Se2 solar cell with ultrashort laser pulses (tp=10ps, λ=1.06μm, v=2000mm/s) was investigated. The TCO layer is removed when the laser fluence is about 0.3J/cm2. The Cu(InGa)Se2 is removed completely to expose molybdenum when the laser fluence is about 3.5J/cm2. For low defect scribing, a clear tendency of using a low pulse overlap and low laser fluences were found.

In-process measuring of the electrical shunt resistance of laser-scribed thin-film stacks by nested circular scribes

Laser scribing of functional thin-film stacks attracts increasing attention especially for applications of flexible electronics or photovoltaics. Laser can perform selective removal of the thin-film stacks that is essential for the isolation and interconnection of the solar cells. The optimization of the laser scribing process concerning the functional properties of the device requires customized characterization techniques minimizing side effects. The proposed and demonstrated nested circular laser scribing technique allows the in-process measurement of the electrical characteristics, e.g., the shunt formation due to laser scribing of the thin-film stack, minimizing secondary effects originating from aging, contacting, changing of sample characteristics, or alterations of the measurement conditions. This technique enables the identification of reliable and quick information on the changes of the solar cell characteristics by laser scribing as this is demonstrated in this work.

Laser irradiation of ZnO:Al/Ag/ZnO:Al multilayers for electrical isolation in thin film photovoltaics

Laser irradiation of ZnO:Al/Ag/ZnO:Al transparent contacts is investigated for segmentation purposes. The quality of the irradiated areas has been experimentally evaluated by separation resistance measurements, and the results are complemented with a thermal model used for numerical simulations of the laser process. The presence of the Ag interlayer plays two key effects on the laser scribing process by increasing the maximum temperature reached in the structure and accelerating the cool down process. These evidences can promote the use of ultra-thin ZnO:Al/Ag/ZnO:Al electrode in large-area products, such as for solar modules.

Reliable fabrication process for long-length multi-filamentary coated conductors by a laser scribing method for reduction of AC loss

We have developed a reliable fabrication process based on a laser scribing method for multi-filamentary coated conductors with a low alternating current loss. This process consists of shallow laser irradiation and two-step chemical etching. The shallow laser irradiation, which penetrates into only a masking polymer tape and a silver stabilizing layer, suppresses generation of dross that is formed due to melting and hardening of a metal substrate. The two-step chemical etching individually for the stabilizing and the superconducting layers prevents over-etching of the superconducting layer and thus suppresses degradation of the critical current (Ic). By using the developed laser scribing process, we realized an improvement of processing speed and a reduction of Ic degradation at the same time. Subsequently, we developed reel-to-reel equipment to apply this process to coated conductors several hundred meters long. We succeeded in fabricating 280 m and 70 m long coated conductors with 5 mm width divided into three and five filaments, respectively. It was confirmed that they had one-third and one-fifth hysteresis loss, respectively, in comparison with that before processing, and high inter-filament resistance, over 0.1 MΩ cm. The typical degradation rates of Ic in the developed laser scribing process were less than 20%.

Parameter optimization of Nd:Yag laser scribing process on core loss of grain-oriented magnetic silicon steels

The core loss of the grain-oriented silicon steel influences the energy loss of transformer. Pulse energy, point spacing, and scribing spacing are the three key parameters of laser scribing process, which determine the core loss of silicon steel. In this study, the relationship between the three key parameters and core loss reduction was investigated and analyzed. Experiments of laser scribing on 30Q130 silicon steel with different parameter combinations were implemented. The results showed that there was no obvious silicon coating burns in the surface of the steel after laser scribing and that the core loss reduction rate (reduction rate) significantly increased with the increasing laser energy density. However, when the energy density exceeded a certain range, the reduction rate decreased and gradually tended to a stable level. Hence, the parameters had significant influence on the reduction rate of core loss, and the two-factor interaction effects of reduction rate of core loss were prominent when parameters of high pulse energy, small point spacing, and small scribing spacing were selected. The optimization combination values of the three key parameters of laser scribing process on 30Q130 silicon steel was acquired by genetic algorithm–enhanced artificial neural network algorithm. For achieving high reduction of core loss, the optimized combination of pulse energy of 2.52 mJ, small point spacing of 0.22 mm, and small scribing spacing of 3.03 mm were selected, and the reduction rate of 13.12 % was verified through experiment.

Influence of Laser Scribing System in the Electrical Properties for a-Si Thin Film Solar Cell Preparation

Laser scribing of hydrogenated amorphous silicon (a-Si) is a crucial step in the fabrication of thin film photovoltaic modules. The required line width of the laser scribing process for a-Si thin film solar cell preparation is 30 m~50 m, the dead zone is less than 300 m in size, and the line depth should be compliant with the process requirements. Thus, the high imaging quality and focal depth of the optical system is required in the laser scribing system. Three crucial laser patterning steps (known as P1, P2 and P3 in the photovoltaic literature) are integrated in the thin film silicon module manufacturing sequence. Therefore, efforts to optimize these laser processes are demanded by the photovoltaic industry. In particular, the state of the remaining material after laser treatment is known to have a critical influence on the electrical properties of the final devices. This paper focuses on the P3 laser scribing process with the peculiarity that it has been done in single solar cells. By evaluating it in single solar cells rather than in finished module, it is possible to isolate its effect on the device characteristics since the P1 and P2 scribings are omitted. To study the effect of the P3 scribing length, several scribings can be done in the same cell. As it will be shown, the high speed motion systems needed for precision laser scribing plays an important role in this experiment. They can be responsible for the electrical losses after the scribing of the solar cells. If this is dealt with properly, it can be seen that the P3 scribings have very little effect on the electrical characteristics of the processed solar cells.

Laser Patterned Junctionless In-Plane-Gate Oxide Thin-Film Transistors Arrays

A novel laser scribing process was successfully developed in fabricating junctionless low-voltage oxide-based thin-film transistors (TFTs) arrays without any mask and photolithography steps at room temperature. Such junctionless TFTs feature that the channel and the source/drain electrodes are of the same thin indium-tin-oxide films without any intentional source/drain junction deposition process. Effective field-effect modulation of the drain current has been realized. Such junctionless in-plane-gate TFTs exhibit a good electrical performance with a small threshold voltage (-0.7V), a small subthreshold swing (0.25V/decade), a high mobility (~12.6cm2/Vs), and a large on/off ratio (>106), respectively. The developed laser scribing technology is highly desirable in terms of the low-cost fabrication process.

Characterization of direct- and back-scribing laser patterning of SnO2:F for a-Si:H PV module fabrication

In thin film photovoltaic modules, the different solar cells are interconnected monolithically during the production process, which gives a greater control over the size and output characteristics of the finished module. The interconnection is typically achieved through different laser scribing processes made at different production steps. In thin film modules built in the superstrate configuration, the first laser process is the patterning of the transparent front electrode. This paper presents results on the investigation of this first laser scribing process on fluorine-doped tin oxide deposited onto a glass substrate using nanosecond diode-pumped solid-state laser sources. Processes made with two different wavelengths (1064nm and 355nm) and executed from the film-side and from the substrate side are compared and evaluated. The quality of the scribes is assessed with confocal and scanning electron microscopy images. In addition, Raman microscopy is used to study the extension of the heat affected zones. While good quality scribes were obtained using both wavelengths and either film-side or substrate-side irradiation, only using 355nm and substrate-side scribing yielded grooves with no observable heat affected zones. It also needed the lowest values of energy per ablated volume and allowed for the highest processing speeds. As such, substrate side ablation with 355nm is proposed as the best ablation strategy.

Laser directly written junctionless in-plane-gate neuron thin film transistors with AND logic function

Junctionless oxide-based neuron thin-film transistors with in-plane-gate structure are fabricated at room temperature by a laser scribing process. The neuron transistors are composed of a bottom indium-tin-oxide floating gate and multiples of in-plane control gates. The control gates, coupling with the floating gate, control the “on” and “off” of the transistor. Effective field-effect modulation of the drain current has been realized. AND logic is demonstrated on a dual in-plane gate neuron transistor. The developed laser scribing technology is highly desirable in terms of the fabrication of high performance neuron transistors with low-cost.

Developing Monolithically Integrated CdTe Devices Deposited by AP-MOCVD

ABSTRACTThin film deposition process and integrated scribing technologies are key to forming large area Cadmium Telluride (CdTe) modules. In this paper, baseline Cd1-xZnxS/CdTe solar cells were deposited by atmospheric-pressure metal organic chemical vapor deposition (AP-MOCVD) onto commercially available ITO coated boro-aluminosilicate glass substrates. Thermally evaporated gold contacts were compared with a screen printed stack of carbon/silver back contacts in order to move towards large area modules. P2 laser scribing parameters have been reported along with a comparison of mechanical and laser scribing process for the scribe lines, using a UV Nd:YAG laser at 355 nm and 532 nm fiber laser.

Scanning electron microscopical examination of the impact of laser patterning on microscopic inhomogeneities of Cu(In,Ga)(Se,S)2 absorbers produced by rapid thermal processing

Laser scribing of the Mo back electrode is commonly applied to define the cell structure of Cu(In,Ga)(Se,S)2 (CIGSSe) thin film solar cells. The patterning process was performed on laboratory samples using ns and ps pulse length laser processes. After structuring, CIGSSe absorbers were processed by rapid thermal processing (RTP) of stacked elemental layer precursors. Microscopic inhomogeneities were investigated on different sample positions. For samples structured with ns pulse, the absorber morphology in the laser line vicinity is different as compared to the morphology in the unstructured cell area. Scanning electron microscopy and energy-dispersive X-ray spectroscopy show significant changes in the absorber grain size and chemical composition. Close to the laser line, the typically observed Ga accumulation on the back contact is less pronounced and more Ga is incorporated closer to the surface leading to a smaller grain size. The observed changes are attributed to partial damaging of a diffusion barrier between glass and Mo induced by the ns laser process, which allows diffusion of sodium from the glass substrate into the absorber during RTP. The enhanced Ga incorporation closer to the surface is an indication for the influence of sodium on the local phase development during RTP. The damages of the diffusion barrier can be effectively prevented by the application of a ps laser scribing process. CIGSSe absorbers processed on samples structured with ps pulse length do not show the described microscopic inhomogeneities around the laser line.

Multi-objective optimization of laser-scribed micro grooves on AZO conductive thin film using Data Envelopment Analysis

Laser scribing process has been considered as an effective approach for surface texturization on thin film solar cell. In this study, a systematic method for optimizing multi-objective process parameters of fiber laser system was proposed to achieve excellent quality characteristics, such as the minimum scribing line width, the flattest trough bottom, and the least processing edge surface bumps for increasing incident light absorption of thin film solar cell. First, the Taguchi method (TM) obtained useful statistical information through the orthogonal array with relatively fewer experiments. However, TM is only appropriate to optimize single-objective problems and has to rely on engineering judgment for solving multi-objective problems that can cause uncertainty to some degree. The back-propagation neural network (BPNN) and data envelopment analysis (DEA) were utilized to estimate the incomplete data and derive the optimal process parameters of laser scribing system. In addition, analysis of variance (ANOVA) method was also applied to identify the significant factors which have the greatest effects on the quality of scribing process; in other words, by putting more emphasis on these controllable and profound factors, the quality characteristics of the scribed thin film could be effectively enhanced. The experiments were carried out on ZnO:Al (AZO) transparent conductive thin film with a thickness of 500nm and the results proved that the proposed approach yields better anticipated improvements than that of the TM which is only superior in improving one quality while sacrificing the other qualities. The results of confirmation experiments have showed the reliability of the proposed method.

Light Extraction Efficiency Improvement by Curved GaN Sidewalls in InGaN-Based Light-Emitting Diodes

Polygonal GaN sidewalls are fabricated using a laser scriber and a wet-etching process. After a two-step laser scribing process and a hot acid etching treatment, curved GaN sidewalls are formed consisting of the chemically stable (10-1-2) crystallographic plane of GaN epilayer and the laser-cutting facets. Compared to light-emitting diodes (LEDs) with vertical and inclined GaN sidewalls, light output power of LEDs with these curved GaN sidewalls exhibits an improvement of 10.2% and 6.8%, respectively. I-V curve shows that there is no deterioration to the curved-sidewall LEDs' electronic properties. This enhancement of light output power is also confirmed by numerical simulations using the ray-tracing method.

AC loss reduction of TFA-MOD coated conductors in long length by laser scribing technique

Abstract TFA-MOD process is expected to be promising for future applications since it can produce high performance YBCO coated conductors with low cost. Applying YBCO coated conductors to the power electric devices such as transformer, cable, motors, reduction of AC loss for long wire is necessary. Multifilamentation, which is one of the effective approaches for AC loss reduction, has been developed by the scribing process. YBCO coated conductors produced by our standard TFA-MOD process delaminated into two parts by the laser scribing. The delamination was clarified to occur within the superconducting layer caused by the defects such as pores in the superconducting layer. In order to reduce the defects in the superconducting layer, we modify the heat treatment profile performed on the decomposed precursor films by applying the interim annealing(550-600°C) before crystallization heat treatment(740-770°C). The interim annealed samples had much less and smaller pores than the standard processed ones. The peel strength measured by transverse tensile test was as high as the PLD derived coated conductors which was successfully scribed into five filaments resulting in 1/5 AC loss. A 50m long YBCO coated conductor with the characteristics of 398A/cmwidth was obtained and cut into 5 mm width, followed by the laser scribing process into five filaments. The multifilamentation process was successfully performed without delamination throughout the wire. The hysteresis loss was down to 1/N (N: number of filaments), as we aimed. The IC properties of the filaments were 29±4A, indicating the wire was uniformly fabricated.