Laser Doping Research Articles

Crystalline Solar Cells with Selective Emitters

AbstractSelectively higher doped areas below the metal contacts increase the efficiency of silicon solar cells. Laser doping is one of the technologies deployed for the production of such cells. Industrial applications of laser doping processes require high throughputs as in automated systems with integrated process controls.

Laser-Doped SiC as Wireless Remote Gas Sensor Based on Semiconductor Optics

An uncooled SiC-based electro-optic device is developed for gas sensing applications. P-type dopants Ga, Sc, P and Al are incorporated into an n-type crystalline 6H-SiC substrate by a laser doping technique for sensing CO2, CO, NO2 and NO gases, respectively. Each dopant creates an acceptor energy level within the bandgap of the substrate so that the energy gap between this acceptor level and the valence band matches the quantum of energy emitted by the gas of interest. The photons of the gas excite electrons from the valence band to the acceptor level, which alters the electron density in these two states. Consequently, the refractive index of the substrate changes, which, in turn, modifies the reflectivity of the substrate. This change in reflectivity represents the optical signal of the sensor, which is probed remotely with a laser such as a helium-neon laser. Although the midwave infrared (3-5 mm) band is studied in this paper, the approach is applicable to other spectral bands.

High performance of ultralow temperature polycrystalline silicon thin film transistor on plastic substrate

A high performance ultralow temperature polycrystalline silicon (poly-Si) thin film transistor (TFT) was obtained on a plastic substrate using the optimization of a triple layered buffer process for a suppression the damage on plastic substrate during laser dopant activation, the high quality SiO2 interface layer formation between the gate dielectric film and the poly-Si film using plasma oxidation, and a successful crystallization of large grain poly-Si films with a sequential lateral solidification (SLS) method. High performances with field effect mobilities of 180 and 62cm2V−1s−1, threshold voltages of 1.4 and −1.6V and sub-threshold swings of 0.78 and 0.92V/decade were obtained for n-channel metal–oxide-semiconductor (nMOS) and p-channel metal–oxide-semiconductor (pMOS) TFT on plastic substrate, respectively.

High Quality Passivation Scheme Combined with Laser Doping from SiN:P and SiN:B Layer for Silicon Solar Cell

In this paper strategy combining both high passivation quality on n+ surface depleted emitter and P and B dopant source availability for further laser doping process from SiN:P and SiN:B layers is investigated. Passivation improvement is demonstrating by adding SiO2 based interfacial layer between n+ emitter and doped SiNx layer. Moreover efficient Excimer laser P and B doping is highlighted even through additional passivation layers. Surface concentration, profile shape and depth of both p+ and n+ doped regions could be tuned by using SiN:P or SiN:B layers, SiO2 interfacial layer type and laser fluence conditions. It is highlighted fluence ranges where the initial emitter can be fully compensated or over-compensated. Such processes could be helpful to simplify the complex fabrication of photovoltaic structures like IBC solar cells.

Laser Doping Strategies Using SiN:P and SiN:B Dielectric Layers for Profile Engineering in High Efficiency Solar Cell

In this paper an Excimer laser doping process is investigated from SiN:P and SiN:B PECVD layers used as dopant sources. It is demonstrated efficient doping effect with P and B with large doping range on both p+ and n+ pre-diffused regions. Doped regions are shown to be modified in term of surface concentration and depth by both laser-driven re-distribution and additional over-doping from doped dielectric source. When using laser doping from SiN:B layer, partial or complete compensation of the initial n+ emitter is highlighted. Moreover similar study with SiN:P demonstrated the potential for over-compensate the initial p+ emitter. These laser processes could be used for realization of adjacent p+ and n+ regions with controlled profiles. Moreover fluence ranges where material could be fully compensated are pointed.

Emitter Profile Tailoring to Contact Homogeneous High Sheet Resistance Emitter

In this work we report on successful direct contacting of high sheet resistance (RSH) emitter at 100Ω/sq by emitter profile manipulation. The formation of lightly doped emitter via POCl3 diffusion was investigated and optimized by the variation of temperature, time and gas fluxes. Sheet resistance mapping and emitter profile analysis have been done by four-point-probe and Electrochemical Capacitance Voltage (ECV) measurements. By increasing the depth of the n++ layer and at the same time reducing the peak concentration of inactive phosphorous doping, an efficiency gain of up to 0.7% absolute was achieved for multicrystalline silicon (mc-Si) solar cells. Suns-VOC measurements show an even higher gain of up to 1% absolute. In this work five different silver pastes are analysed accompanied by three different simulated grid designs. Electroluminescence imaging technique was used to characterize the spatially-resolved electrical properties of the solar cells. Based on these investigations we evaluated a 160Ω/sq emitter and could demonstrate by laser doping that the minimum n++ layer depth for high fill factors is approx. 25nm leading to 0.4%abs efficiency gain.

Evaluating The Quality of Selective Emitter Structures by Imaging the Emitter Saturation Current Density

A method to derive the emitter saturation current density J0e with lateral resolution is applied to investigate selective emitter structures. The method uses PL lifetime imaging at several injection densities to laterally evaluate J0e by applying the method of Kane and Swanson [1] pixel by pixel. Samples with two-sided diffused emitters on lowly-doped Cz wafers were used to produce selective emitter structures by laser doping of the phosphorus-rich glass (LD-SE). By comparison of experimental and numerical simulation results of J0e linescans, a limited resolution of a feature size of an inhomogeneous emitter is determined to be theoretically between 0.5-1.0mm and experimentally about 2mm. The method was successfully applied to investigate the dependence of J0e on the laser power of a selective emitter structure. The expected behaviour of a maximum J0e for medium laser intensities is observed. The method is suitable to evaluate the selective emitter process and its optimization.

Simplified Interdigitated Back Contact Solar Cells

In this work, the fabrication of an interdigitated back contact solar cell is investigated on p-type Czochralski silicon wafers using a novel laser doping approach to form both polarities of rear contacts. Using only one conventional thermal diffusion which forms the n+ active emitter on the rear and n+ floating emitter on the front of the device, implied open circuit voltages exceeding 690mV have been achieved on partly processed devices prior to metallisation, with virtually full-area emitter coverage and both polarities of contacts formed, indicating the potential of this structure for achieving high efficiencies with a simple process. Severe shunting post-metallisation due to the poor electrical isolation properties of the rear surface passivation layer currently limits the final device voltage to 625mV. Further investigations must be undertaken in order to minimise the parasitic shunting effect and maintain a high open circuit voltage post-metallisation.

Parameterization of local laser doping and laser-fired contacts for high efficiency c-Si solar cells

Abstract Local laser doping (LLD) and laser fired-contacts (LFC) are among the most promising techniques based on laser tools to be industrially applied in high efficiency crystalline silicon (c-Si) solar cells production. Here we present a comprehensive parameterization of LLD and LFC processes using commercial ns laser sources. The best process conditions are assessed both analysing the morphology and the electrical characteristics of the irradiated test samples. Values for the normalized LFC resistance far below 1.0 mΩcm 2 have been obtained. For LLD experiments we found irradiation conditions giving excellent diode behaviour, with ideality factors below 1.5.

Optical and electrical properties of laser doped Si:B in the alloy range

We have probed the dopant activity of silicon B-doped by Gas Immersion Laser Doping (GILD). Here, we report on the comparison of optical, electrical and structural properties of Si:B, over a wide concentration range, up to 1.5×1021cm−3 by steps of 1.5×1019cm−3. Data obtained by reflectance FTIR spectroscopy are used within a Drude model to extract concentration, thickness and mobility. Resulting carrier concentration and conductivity are checked with 4-point probe electrical and X-ray diffraction measurements. FTIR proved to be very sensitive to the dopant distribution inside the layer, despite its thinness. It clearly reveals a moderate dopant accumulation at the interfaces.

Defect Formation in Silicon During Laser Doping

Laser doping in industrial crystalline solar cells creates a selective emitter and thereby enhances the efficiency. Nevertheless, if not done carefully, the irradiation of the semiconductor introduces defects. We have suppressed defect formation by using a laser beam that is focused to a line with a width of only several micrometers. The maximal line width for a defect free recrystallization, however, depends on the surface orientation of the silicon. Using a transmission electron microscope, we find a dislocation-free recrystallized layer on (100)-oriented silicon wafers that are irradiated with a line focus smaller than or equal to 15 μm. For (111)-oriented surfaces, this holds for the use of a 5.2-μm-wide line focus. Wafers that are irradiated with a circular laser focus of diameter D = 36 μm show the formation of microcracks, but no hints of dislocations are found using transmission electron microscopy (TEM).

Liquid-Phase Diffusion of Phosphorus Atoms in Laser-Doped Crystalline Silicon Solar Cells

In this paper, a nonlinear numerical model of laser melting of crystalline silicon solar cells and phosphorus diffusion has been established by finite difference method implemented in MATLAB. Based on the features of liquid-phase diffusion of phosphorus atoms in melting silicon, theoretical simulation for phosphorus concentration profiles and explanation for the mechanism of laser doping are achieved. The theoretical phosphorus concentration profile is in good agreement with the experimental SIMS data.

Improved LDSE processing for the avoidance of overplating yielding 19.2% efficiency on commercial grade crystalline Si solar cell

A record in laser doped selective emitter (LDSE) solar cells with an efficiency η=19.2% is reported. In this study, we investigate the effect of SiN x films for laser doped selective emitter solar cells with plated front contacts. It is observed that the condition of processes such as silicon nitride and laser doping (LD) is of critical importance prior to light induced plating. If these processes are not performed optimally, localized shunts may form during the light induced plating (LIP) process that then inhibit plating in the surrounding areas. In the previous work an efficiency of 18.3% has been achieved, even though the fill factor was only 74.2% and the cell suffered from additional shunting and shading losses due to overplating. However, in this work, we demonstrate that with the optimization of the PECVD SiN x and metallization processes, cells have reached efficiencies of more than 19% on commercial grade p-type CZ Si substrates.

Improving the efficiency of crystalline silicon solar cells by an intersected selective laser doping

The selective emitter technology is the most effective method to improve the silicon solar cell performance. However, the strict alignment requirements between the Ag gridlines and the selective emitters should be addressed. In the present study, a novel front metal contact patterning scheme for crystalline silicon (c-Si) solar cells is developed. An intersected selective laser doping process is applied. The control experiment indicates that the best efficiency increases for ∼23.8% from ∼14.39% to ∼17.74% after receiving the laser doping process. Moreover, our laser doping process does not affect the open-circuit voltage and short-circuit current significantly and the emitter properties have not been degraded in terms of the emitter saturation current. The improved front metal contact results in the improved energy conversion efficiency. The developed process offers an elegant and reliable approach for the low-cost, high throughput industrial-scale production process.

Co‐optimization of emitter profile and metal grid of selective emitter silicon solar cells

ABSTRACTCo‐optimization of the metallization and emitter dopant profile is fully investigated for selective emitter crystalline silicon solar cells. The simulation parameters for the laser doping selective emitter, metallization by plating, silicon nitride passivation, and aluminum back surface field are identified and reached. Internal light flux reflection is also considered in the model. In particular, the influence of the increased light trapping ability of a textured surface on the optimization results is clarified by comparing a cell with a non‐textured surface. In this paper, the optimization results, including the electrical performances of a solar cell are discussed in detail. On the basis of these simulation results, an optimized metallization and emitter dopant profile is proposed. Copyright © 2011 John Wiley & Sons, Ltd.

Improved Ghost Plating of Light-Induced Plating on Crystalline Silicon Solar Cells by SiO<sub>2</sub>/SiN Selective Etching

In this paper, the ghost plating problems of crystalline silicon solar cells is studied in theory and experiment. After laser doping process, a pretreatment process is needed to remove SiO2 in the heavy doping area and keep SiN mask simultaneously by using chemical solution containing HF. Otherwise, the unexpected non-heavy-doping area would be plated with silver, resulting in the ghost plating problems. The mechanism of HF etching SiO2 and SiN is analyzed and the feasibility of selective etching is discussed. By changing the main aspects of affecting the etching rate, the ghost plating of crystalline silicon solar cells is improved.

Record Large-Area p-Type CZ Production Cell Efficiency of 19.3% Based on LDSE Technology

A record independently confirmed production cell efficiency of 19.3% is presented for a large-area p-type Czochralski (CZ) silicon solar cell, based on the University of New South Wales (UNSW) laser-doped selective emitter technology. In this paper, the innovative and patented laser-doping technology is simply added to a standard Centrotherm turnkey line, operating with a modified process and the addition of the laser-doping and light-induced plating steps. Impressively, this record efficiency is achieved by using standard commercial grade p-type CZ-grown silicon wafers on standard production equipment and exceeds the previous independently confirmed record for any technology of 19.2% using a standard aluminum back-surface field with full rear coverage. The avoidance of laser-induced defects is discussed in this paper to overcome previous limitations of the laser-doping technology using conventional Q-switched lasers or the laser chemical processing method. It is demonstrated that the use of appropriate lasers can avoid defect formation through thermal cycling while still allowing for the sufficient mixing of dopants and allow laser doping to be performed through a standard SiN layer with contacts formed through a self-aligning metallization scheme.

Optical response of laser-doped silicon carbide for an uncooled midwave infrared detector

An uncooled mid-wave infrared (MWIR) detector is developed by doping an n-type 4H-SiC with Ga using a laser doping technique. 4H-SiC is one of the polytypes of crystalline silicon carbide and a wide bandgap semiconductor. The dopant creates an energy level of 0.30 eV, which was confirmed by optical spectroscopy of the doped sample. This energy level corresponds to the MWIR wavelength of 4.21 μm. The detection mechanism is based on the photoexcitation of electrons by the photons of this wavelength absorbed in the semiconductor. This process modifies the electron density, which changes the refractive index, and, therefore, the reflectance of the semiconductor is also changed. The change in the reflectance, which is the optical response of the detector, can be measured remotely with a laser beam, such as a He-Ne laser. This capability of measuring the detector response remotely makes it a wireless detector. The variation of refractive index was calculated as a function of absorbed irradiance based on the reflectance data for the as-received and doped samples. A distinct change was observed for the refractive index of the doped sample, indicating that the detector is suitable for applications at the 4.21 μm wavelength.

Experimental and analytical study of saturation current density of laser-doped phosphorus emitters for silicon solar cells

Heavily doped emitters with low saturation current density are of particular interest for selective emitter solar cells. These emitters can be obtained by laser doping through the phosphosilicate glass layer formed after thermal diffusion from POCl 3 gas. The experimental results show that in contrast to purely POCl 3 furnace-diffused emitters, the saturation current density of laser-doped emitters does not increase linearly as sheet resistance decreases, but rather features two distinct regimes. In one of these regimes, the saturation current density is found to decrease as the sheet resistance decreases, reaching values lower than those of furnace emitters. This peculiar behaviour was explained by both qualitative analysis and numerical simulations.

Effects of laser scans on the diffusion depth and diffusivity of gallium in n-type 4H–SiC during laser doping

Abstract An n-type 4H–SiC substrate has been doped with gallium using a continuous wave Nd:YAG laser to heat the sample to high temperatures but below the peritectic temperature of SiC. Mathematical models have been presented for the temperature and Ga concentration distributions in the sample. The Ga atoms, which are produced due to the thermal decomposition of a metallorganic precursor, diffuse into the sample by the solid-phase diffusion process at high temperatures. This process is modeled by considering the temperature-dependent diffusion coefficient and the Ga concentration profile was measured by the secondary ion mass spectrometry (SIMS). The concentration of Ga (6.25 × 1020 cm−3) at the substrate surface was found to exceed the solid solubility limit (1.8 × 1019 cm−3) of Ga in SiC. Comparing the SIMS data to the results of the diffusion model, the activation energy, pre-exponential factor and diffusion coefficient of Ga were determined for different doping conditions. Four doped samples were produced by scanning the samples with a laser beam for different number of passes. The sample prepared with four passes showed the highest diffusion coefficient of 5.53 × 10−7 cm2/s with activation energy 1.84 eV and pre-exponential factor 1.05 × 10−2 cm2/s. The diffusion coefficient is five orders of magnitude higher than the typical diffusion coefficient of Ga in SiC. This indicates that the laser doping process enhances the diffusion coefficient of dopant significantly.