Laser Doping Research Articles

Laser doping of boron-doped Si paste for high-efficiency silicon solar cells

Boron laser doping (LD) is a promising technology for high-efficiency solar cells such as p-type passivated locally diffused solar cells and n-type Si-wafer-based solar cells. We produced a printable phosphorus- or boron-doped Si paste (NanoGram® Si paste/ink) for use as a diffuser in the LD process. We used the boron LD process to fabricate high-efficiency passivated emitter and rear locally diffused (PERL) solar cells. PERL solar cells on Czochralski Si (Cz-Si) wafers yielded a maximum efficiency of 19.7%, whereas the efficiency of a reference cell was 18.5%. Fill factors above 79% and open circuit voltages above 655 mV were measured. We found that the boron-doped area effectively performs as a local boron back surface field (BSF). The characteristics of the solar cell formed using NanoGram® Si paste/ink were better than those of the reference cell.

Boron laser doping using spin-on dopant for textured crystalline silicon solar cell

A novel precursor layer, polyboron film (PBF), was applied in the laser doping (LD) process. Localized LD using PBF was applied to a high-efficiency solar cell structure as a selective emitter (SE). Interface adhesion was improved by using an organic-polymer-based doping precursor, PBF. A strongly adhered interface contributed to the preservation of textured formations after disruption by LD, and to the homogeneous introduction of impurities. Surface geometries were retained almost completely even after LD. Sheet resistances were decreased by additional boron doping by LD. The photovoltaic characteristics were improved by SE formation using PBF with an optimum average laser power. Consequently, the SE formed by LD showed an increase in fill factor (FF), which contributed to the improvement in solar cell efficiency.

Simultaneous Femtosecond Laser Doping and Surface Texturing for Implanting Applications

Surface texturing of titanium implants codoped with silver by femtosecond laser doping is proposed. Not only are hierarchical surfaces formed, but silver atoms are simultaneously implanted beneath the titanium surface. The silver doped titanium is highly effective in inhibiting P. gingivalis bacteria strain infections, while exhibiting no cytotoxicity and high adhesion to the osteoblast-like cell MG63. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Passivation-Induced Cavity Defects in Laser-Doped Selective Emitter Si Solar Cells—Formation Model and Recombination Analysis

Laser-induced selective Si doping and simultaneous ablation of a dielectric passivation layer is a promising technology for the creation of efficient and cost-effective solar cells. In this paper, the electrical quality of emitters produced with a 532-nm continuous-wave laser will be discussed using elaborate analysis of quasi-steady-state photoconductance (QSSPC) measurements. It will be shown that these emitters cause good charge carrier shielding, which leads to emitter saturation current densities as low as 240 fA/cm $^2$ for unpassivated surfaces. If an SiN $_{\it x}$ layer is present during laser doping, the emitter recombination increases by a factor of three. This detrimental effect is put down to the formation of microcavities within the recrystallized Si. A model of the ablation mechanism and cavity formation for long laser pulses is proposed, with the experimental data in this study serving as a limiting case for long irradiation lengths.

Laser-Doped Back-Contact Solar Cells

We present laser-doped interdigitated back-contacted solar cells with a record efficiency $\bm{\eta}$ = 22.0%. The high versatility and spatial resolution of our laser doping process enable local n-type and p-type doping with a precision below 30 $\mu \hbox{m}$ and avoid any masking for doping. Nevertheless, the presented solar cells use photolithography (masking) to define the contact area and metallization layout. Quasi-steady-state photoconductance measurements prove the low-saturation current density of the laser doping. We process solar cells with varied pitch and emitter fraction and compare their measured current density–voltage characteristics with a 3-D solar cell simulation. The good agreement of the simulation and experimental data allows a reliable efficiency forecast when optimizations are applied. Furthermore, the influence of the base-busbar region on solar cell performance is discussed.

Phosphorus out-diffusion in laser molten silicon

Laser doping via liquid phase diffusion enables the formation of defect free pn junctions and a tailoring of diffusion profiles by varying the laser pulse energy density and the overlap of laser pulses. We irradiate phosphorus diffused 100 oriented p-type float zone silicon wafers with a 5 μm wide line focused 6.5 ns pulsed frequency doubled Nd:YVO4 laser beam, using a pulse to pulse overlap of 40%. By varying the number of laser scans Ns = 1, 2, 5, 10, 20, 40 at constant pulse energy density H = 1.3 J/cm2 and H = 0.79 J/cm2 we examine the out-diffusion of phosphorus atoms performing secondary ion mass spectroscopy concentration measurements. Phosphorus doping profiles are calculated by using a numerical simulation tool. The tool models laser induced melting and re-solidification of silicon as well as the out-diffusion of phosphorus atoms in liquid silicon during laser irradiation. We investigate the observed out-diffusion process by comparing simulations with experimental concentration measurements. The result is a pulse energy density independent phosphorus out-diffusion velocity vout = 9 ± 1 cm/s in liquid silicon, a partition coefficient of phosphorus 1 < kp < 1.1 and a diffusion coefficient D = 1.4(±0.2)cm2/s × 10−3 × exp[−183 meV/(kBT)].

Large area p-type PERL cells featuring local p+ BSF formed by laser processing of ALD Al2O3 layers

In this work we present large area p-type PERL solar cells featuring local p+ Back-Surface-Field (BSF) obtained by pico-second (ps) laser processing of thin ALD Al2O3 layers capped either by PECVD SiNx or PECVD SiOx. In this specific approach, the laser processing is performed through the rear passivation stack, whereby the laser simultaneously patterns the dielectrics for the subsequent Al–Si contact formation, whilst incorporating the Al atoms from the Al2O3 film into the underlying Si bulk. With this technique, we register substantial dopant incorporation for 10nm of ALD Al2O3, without using additional doping sources. When this process is coupled to a front metallization scheme based on Cu plating, p-PERL cells can be fabricated adopting only low temperature process steps, which avoids thermal stability issues related to the Al2O3 layer, such as crystallization and blistering, and improves the properties of the rear reflector by avoiding Al–Si alloying. Efficiencies up to 20.7% are reported on 6-in. Cz–Si solar cells featuring local p+ BSF formed by laser doping through a rear stack composed of 10nm ALD Al2O3 and 120nm PECVD SiNx.

Laser doping of Sb into ZnO nanowires in the Sb nanoparticle-dispersed liquid

We succeed in fabricating Sb-doped ZnO nanowires by laser doping using Sb nanoparticles (Sb NPs). Vertically aligned ZnO nanowires with a diameter of 100 nm were synthesized by the nanoparticles-assisted pulsed laser deposition. Sb NPs were prepared by laser ablation in liquid. The average size of 50 nm Sb NPs was successfully fabricated by laser ablation in ethanol. Subsequently, laser doping was performed by irradiating Nd:YAG laser beam (355 nm) in Sb-dispersed ethanol. After laser doping, the tip of ZnO nanowires was slightly deformed into spherical shape by laser heating. A rectifying property with a threshold voltage of 4.5 V was observed between n-type ZnO nanowires and Sb-doped ZnO nanowires.

Laser Fired Local Back Contact C-Si Solar Cells Using Phosphoric Acid for Back Surface Field

We report on a laser doping process for the formation of a local back surface field (BSF) using phosphoric acid (H3PO4) for n-type passivated emitter rear totally diffused silicon solar cells. The sheet resistance of the BSF layer was varied by changing the H3PO4 concentration. The BSF layer was passivated using SiNx. With the passivated BSF, the LBC solar cell shows an improved open circuit voltage. A laser power of 44 mW with 10 kHz resulted in a 45-Ω/sq BSF layer with effective lifetime of 290 μs and a higher Voc of 623 mV. With the optimized laser parameters, devices with the best electrical results yielded a short circuit current density of 36 mA/cm2 and an efficiency of 18.26%.

Calculated and Experimental Research of Sheet Resistances of Laser-Doped Silicon Solar Cells

The calculated and experimental research of sheet resistances of crystalline silicon solar cells by dry laser doping is investigated. The nonlinear numerical model on laser melting of crystalline silicon and liquid-phase diffusion of phosphorus atoms by dry laser doping is analyzed by the finite difference method implemented in MATLAB. The melting period and melting depth of crystalline silicon as a function of laser energy density is achieved. The effective liquid-phase diffusion of phosphorus atoms in melting silicon by dry laser doping is confirmed by the rapid decrease of sheet resistances in experimental measurement. The plateau of sheet resistances is reached at around 15Ω/□. The calculated sheet resistances as a function of laser energy density is obtained and the calculated results are in good agreement with the corresponding experimental measurement. Due to the successful verification by comparison between experimental measurement and calculated results, the simulation results could be used to optimize the virtual laser doping parameters.

Laser processing of materials for renewable energy applications

The significant advances in high-power lasers with the attainment of tens of kilowatts of optical power, high repetition rates (>MHz), reduction in size, lower cost per photon ( 30%) are driving the use of lasers for material processing for renewable energy materials. The significant advances in high-power lasers with the attainment of tens of kilowatts of optical power, high repetition rates (>MHz), reduction in size, lower cost per photon ( 30%) are driving the use of lasers for material processing with very high throughput. The use of renewable energy is also increasing as an alternative power source. This review examines the various aspects of laser processing for renewable energy materials and provides an overview of fundamentals of laser material interactions, advances in high-power lasers, and specific examples of laser processing of materials for photovoltaics, solar thermal energy, thermophotovoltaics, thermoelectrics, and thin films. High-power lasers have been adapted for solar cell manufacturing applications, and new processes such as laser doping, laser transfer of metal contacts, laser annealing, etc. are being advanced further for industrial applications. The future of laser processing for renewable energy materials looks very bright with further advances expected in high-power lasers, beam delivery systems, and decreasing cost with very high reliability. Lasers can provide noncontact localized energy deposition with the potential for all low-temperature processing of materials and a very low thermal budget.

Laser doping for ohmic contacts in n-type Ge

We achieved ohmic contacts down to 5 K on standard n-doped Ge samples by creating a strongly doped thin Ge layer between the metallic contacts and the Ge substrate. Thanks to the laser doping technique used, gas immersion laser doping, we could attain extremely large doping levels above the solubility limit, and thus reduce the metal/doped Ge contact resistance. We tested independently the influence of the doping concentration and doped layer thickness and showed that the ohmic contact improves when increasing the doping level and is not affected when changing the doped thickness. Furthermore, we characterised the doped Ge/Ge contact, showing that at high doping its contact resistance is the dominant contribution to the total contact resistance.

Efficiency enhancement of i-PERC solar cells by implementation of a laser doped selective emitter

In this work, we present the incorporation of a laser doped selective emitter into the i-PERC platform at Imec using large area magnetically confined boron-doped Czochralski grown silicon wafers. Cells were fabricated with self-aligned plated n-type contacts with a comparison between the use of a homogenous emitter with contact openings formed by picosecond laser ablation and a selective emitter formed by laser doping with a mode-locked UV laser using various processing speeds. Without modification to other processes in the i-PERC platform, improvements in efficiency of approximately 0.4% absolute were obtained with the inclusion of the selective emitter structure through improvements in open circuit voltage, fill factor and reduced series resistance. This resulted in peak efficiencies of 20.5% using a processing speed for laser doping of 5m/s.

A simulation study of the micro-grooved electrode structure for back-contact back-junction silicon solar cell

A micro-grooved electrode structure is investigated to illustrate its advantages when applied to the back-contact back-junction (BC–BJ) silicon solar cell. The finite element analysis shows that the micro-grooved electrodes enhances the photo-carrier collection and weakens the dependence of collection ability on pitch distance. The geometries of micro-groove are found to have little impact on the cell performance. These advantages open possibilities for the implementation of low cost fabrication methods. As a demonstration, a process involving laser doping and screen printing techniques are proposed and analyzed. The simulation results show that the laser induced lattice damage causes negligible deterioration of device electrical properties and the presence of parasitic metal insulator semiconductor structure near the screen printed electrodes actually leads to a performance improvement rather than degradation. Our preliminary results indicate that the micro-grooved electrode structure is promising for fabricating low cost high efficiency BC–BJ silicon solar cells.

Emitter formation using laser doping technique on n- and p-type c-Si substrates

In this work laser doping technique is used to create highly-doped regions defined in a point-like structure to form n+/p and p+/n junctions applying a pulsed Nd-YAG 1064nm laser in the nanosecond regime. In particular, phosphorous-doped silicon carbide stacks (a-SiCx/a-Si:H (n-type)) deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) and aluminum oxide (Al2O3) layers deposited by atomic layer deposition (ALD) on 2±0.5Ωcm p- and n-type FZ c-Si substrates respectively are used as dopant sources.Laser power and number of pulses per spot are explored to obtain the optimal electrical behavior of the formed junctions. To assess the quality of the p+ and n+ regions, the junctions are electrically contacted and characterized by means of dark J–V measurements. Additionally, a diluted HF treatment previous to front metallization has been explored in order to know its impact on the junction quality. The results show that fine tuning of the energy pulse is critical while the number of pulses has minor effect. In general the different HF treatments have no impact in the diode electrical behavior except for an increase of the leakage current in n+/p junctions.The high electrical quality of the junctions makes laser doping, using dielectric layers as dopant source, suitable for solar cell applications. Particularly, a potential open circuit voltage of 0.64V (1 sun) is expected for a finished solar cell.

Hydrogen Passivation of Laser-Induced Defects for Laser-Doped Silicon Solar Cells

Hydrogen passivation of laser-induced defects (LasID) is shown to be essential for the fabrication of laser-doped solar cells. On first-generation laser-doped selective emitter solar cells where open-circuit voltages were predominately limited by the full-area back surface field, a 10-mV increase and 0.4% increase in the pseudo-fill factor were observed through hydrogen passivation of defects generated during the laser doping process, resulting in an efficiency gain of 0.35% absolute. The passivation of such defects becomes of increasing importance when developing higher voltage devices and can result in improvements in implied open-circuit voltage on test structures up to 50 mV. On n-type PERT solar cells, an efficiency gain of 0.7% absolute was demonstrated with increases in open-circuit voltage and pseudo-fill factor by applying a short low-temperature hydrogenation process using only hydrogen within the device. This process was also shown to improve the rear surface passivation, increasing the short-circuit current of approximately 0.2 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of wavelengths from 950 to 1200 nm compared with that achieved using an Alneal process. Subsequently, an average efficiency of 20.54% was achieved.

Influence of laser damage on the performance of selective emitter solar cell fabricated using laser doping process

This article investigates the impact of laser damage on the performance of a selective emitter solar cell fabricated using a laser doping process (LD). After the optimization of the laser doping process, an amorphous silicon layer with a thickness of approximately 100Å was observed on the laser-doped emitter. The amorphous laser damage decreased the power conversion efficiency due to poor contact between the Ag front electrode and the selective emitter. To effectively improve the quality of the front contacts, a solution etch-back process was applied after the laser doping step (LEB), and the series resistance of the selective emitter solar cell was restored to 0.55Ω cm2 from around 1.0Ω cm2. The contact formation during the LEB process clearly improved, and it was verified by comparing the densities of the Ag crystallites after Ag electrode removal from the LEB cell and the LD cell. The improvement in the formation of the front contact resulted in the LEB selective emitter solar cell recording a maximum 19.16% of power conversion efficiency, with a standard deviation of 0.055.

Laser etch back process to fabricate highly efficient selective emitter c-Si solar cells

We developed a novel cost effective process scheme for the fabrication of highly efficient selective emitter solar cells, which uses a laser doping method combined with an etch back process. The laser doping process using a 150ns pulse width green (532nm) laser effectively controls the doping profiles to form a selective emitter. However, laser damage was created on the laser-doped surface and eventually the performances and stabilities of laser-doped cells were degraded due to this damage. Using a transmission electron microscope (TEM), the damage was examined and found to have a thickness of 40nm of amorphous silicon. This thin damage layer was effectively removed in an acid mixture solution. The combined process of laser doping and etch back is called the laser etch back process. After removal of this thin damage layer, the cell efficiencies were significantly improved up to 19.17%.

Incorporation of deep laser doping to form the rear localized back surface field in high efficiency solar cells

Solar cells with rear localized contacts formed with and without the use of deep boron laser doping are compared as an approach to achieve a more repeatable process for high efficiency solar cells. In particular the paper investigates the impact of the belt firing conditions and screen printable Al paste on the quality of localized contact formation. By adjusting the firing profile, cells incorporating deep boron laser doping on the rear surface are shown to better avoid Kirkendall void formation at contact regions, while the cells without need to balance the percentage of Kirkendall void formation against the localized back surface field (LBSF) thickness. The boron laser doping can be integrated into the laser ablation process used to open the rear dielectric, which minimizes the impact on the process steps. Standard Al paste used for commercial screen printed solar cells is shown to be applicable on the boron laser doped solar cells with a peak firing temperature of 700°C. On the contrary, a special designed Al paste for LBSF applications has to be used on the cells without boron doping. An average cell efficiency of 19.8% is achieved on boron laser doped solar cells using both types of Al pastes.

Germanium p-n Junctions by Laser Doping for Photonics/Microelectronic Devices

A method of forming germanium p-n junction by laser doping is demonstrated. Low bulk and surface leakage current density of 5.4 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 2.0 μA/cm, respectively, were obtained. The leakage current density was comparable with the diodes formed by rapid thermal diffusion, but approximately two orders of magnitude lower than the diodes formed by ion implantation. The performance of the laser doped junction in photonic devices was demonstrated through the fabrication of 130-μm diameter photodetector, which showed a responsivity of 0.46 A/W at 1.55-μm wavelength at 0 V bias and a -3-dB bandwidth of 190 MHz.