Emitter Doping Research Articles

Development of a Simple Analytical PSpice Model for SiC-Based BJT Power Modules

A simple analytical Spice-type model has been developed and verified for the first time for 4H-SiC-based bipolar junction transistor (BJT) power module with voltage and current rating of 1200 V and 800 A. The simulation model is based on a temperature-dependent silicon carbide (SiC) Gummel-Poon model for high-power applications. PSpice simulations are performed to extract technology-dependent modeling parameters coupled with static and dynamic characteristics of BJTs at different temperatures and validated against the measured data. Influence of various circuit elements, for instance, stray inductance and base resistance and internal device modeling parameters, carrier life time, and emitter doping, on switching losses has been studied. The performance of the SiC BJT model is fairly accurate and correlates well with the measured results over a wide temperature range.

Boron Oxygen Pair Effect in p+ Emitter and Nanosized Boron Rich Layer by Fold Coordination Analysis for Crystalline Silicon Solar Cell Applications.

N-type substrates possess better material characteristics than p-type substrates for high efficiency mass producible Si solar cells with HIT, IBC structures. The major drawbacks of these structures are a complicated fabrication process and an expensive unit cost. In this paper, the boron emitter doping profile of a nanosized boron rich layer (BRL), for which the boron and oxygen concentrations are correlated, is optimized to fabricate high efficiency solar cells on an n-type substrate. Boron doping was carried out using a BBr3 furnace with varying oxygen gas ratios and the surface was treated with acid etching. The effect of the oxygen on the nanosized BRL was analyzed using both FTIR spectroscopy and XPS, where by conductivity and the Si-B bond were observed for the three-fold and four-fold coordinated borons, respectively. The results showed that the oxygen quantities in the boron doped emitter and the nanosized BRL affected the characteristics of the solar cell. Regarding the solar cells that were fabricated using the boron emitter and shallow emitter (90 ohm/sq) processes, the open-circuit voltage increased by 54 mV and the short circuit current (J(sc)) increased by 3.7 mA/cm2. The J(sc) increase was due to an increased quantum efficiency in the short wavelength range. The shallow emitter etch back process minimized the boron-oxygen defects in the doping profile.

Impact of the phosphorus emitter doping profile on metal contact recombination of silicon wafer solar cells

Metallisation of phosphorus-doped silicon (Si) surfaces using screen-printed silver (Ag) pastes is a well-established process and also a key process in the production of Si wafer solar cells. The metal-Si interface in a solar cell is a highly recombination-active region that impacts the device voltage. In this work, a facile test metallisation pattern with regions of varying front metal contact fractions is screen printed to create test cells on different phosphorus emitter doping profiles, all on commercially available multicrystalline Si wafers. On these and in conjunction with H-pattern cells and cells without front metallisation, intensity-dependent photoluminescence imaging is performed to extract both the metal-induced recombination saturation current densities and parameters related to wafer edge and peripheral recombination. It is observed that as the phosphorus emitter profile becomes lightly doped and shallower, the metallisation-induced recombination losses increase. The extracted metal recombination saturation current densities (J01-metal and J02-metal) on a 50Ω/sq phosphorus emitter surface were 1800fA/cm2 and 10nA/cm2 respectively when compared to values of 2500fA/cm2 and 120nA/cm2 respectively on a 130Ω/sq phosphorus emitter surface. As a consequence of high metal recombination current density values for the lightly doped emitters of this study (130Ω/sq), the solar cells fabricated from this group have a significant drop of 12mV in open-circuit voltage values after front grid metallisation.

Towards understanding the characteristics of Ag–Al spiking on boron-doped silicon for solar cells

With this work, we introduce numeric three-dimensional simulation of metal spiking into highly boron-doped surfaces of n-type silicon solar cells, which is moreover performed with a simulation of the quasi-steady-state photoconductance technique. This setup serves as a virtual experiment to simulate the dark saturation current density j0,met of metallized boron-doped emitters with respect to metal spikes originating from the silver–aluminum (Ag–Al) contact. With the results obtained from this simulation model we approach quality and quantity of increased j0,met and give detailed insight to which degree a solar cell’s performance is possibly harmed by this effect. We show that metal spikes penetrating into boron-doped emitters are of harmless nature concerning j0,met until their tips reach depths where boron doping concentration is lower than approximately 1018cm−3. Deeper spikes then lead to an exponential increase in j0,met as more and more carriers from emitter and also the base are able to diffuse to its tip and recombine there. With the help of j0-results obtained experimentally in combination with the simulation results, we discuss the influence of spikes on emitter recombination, the benefits that can be achieved with deeper emitter doping profiles, and suggestions for the further development of pastes to contact boron-doped surfaces.

Impact of Photon Recycling on GaAs Solar Cell Designs

Radiatively dominated III–V semiconductor solar cells are strongly influenced by the effects of photon recycling. From a modeling standpoint, the semiconductor transport equations must account for this to predict accurate open-circuit voltages, even for cells on substrate. Using Shockley–Read–Hall (SRH) lifetimes based on internal quantum efficiency measurements, the current–voltage characteristics of cells on substrate and thin-film cells (where the substrate is removed) are predicted to good accuracy. Using this calibrated photon recycling model, the influences of base thickness and SRH lifetimes on device performance are studied in order to determine an optimal cell design using a conventional silver back reflector and a near-perfect back reflector. The result is efficiencies of >28% under AM1.5g standard testing conditions for thin-film cells with electron and hole SRH lifetimes of 1 μs and 100 ns, respectively. Finally, the doping concentration in the emitter is found to be unimportant in dictating the cell's open-circuit voltage as the cell approaches the radiative limit, whereas for nonradiatively dominated cells, the open-circuit voltage is found to depend on the cell's built-in voltage via emitter doping.

Pushing the limits of concentrated photovoltaic solar cell tunnel junctions in novel high‐efficiency GaAs phototransducers based on a vertical epitaxial heterostructure architecture

AbstractA monolithic compound semiconductor phototransducer optimized for narrow‐band light sources was designed for achieving conversion efficiencies exceeding 50%. The III‐V heterostructure was grown by metal‐organic chemical vapor deposition, based on the vertical stacking of 5 partially absorbing GaAs n/p junctions connected in series with tunnel junctions. The thicknesses of the p‐type base layers of the diodes were engineered for optimal absorption and current matching for an optical input with wavelengths centered near 830 nm. Devices with active areas of ~3.4 mm2 were fabricated and tested with different emitter gridline spacings. The open circuit voltage (Voc) of the electrical output is five times or more than that of a single GaAs n/p junction under similar illumination. The device architecture allows for improved Voc generation in the individual base segments because of efficient carrier extraction while simultaneously maintaining a complete absorption of the input photons with no needs for complicated fabrication processes or reflecting layers. With illumination powers in the range of a few 100 mW, the measured fill factor (FF) varied between 88 and 89%, and the Voc reached over 5.75 V. The data also demonstrated that a proper combination of highly doped emitter and window layers without gridlines is adequate for sustaining such FF values for optical input powers of several hundred milliwatts. As the optical input power is further increased and approaches 2 W (intensities ~58 W/cm2), the multiple tunnel junctions sequentially exceed their peak current densities in the case for which typical (n++)GaInP/ (p++)AlGaAs concentrated photovoltaic tunnel junctions are used. Lower bandgap tunnel junctions designed with improved peak current densities result in phototransducer devices having high FF and conversion efficiencies for up to 5 W optical input powers (intensities ~144 W/cm2). Measurements at different temperatures revealed a Voc reduction of −6 mV/°C at ~59 W/cm2. Copyright © 2015 John Wiley & Sons, Ltd.

Understanding the Optimization of the Emitter Coverage in BC-BJ Solar Cells

In this work, by exploiting two-dimensional (2-D) TCAD numerical simulations, we performed a study of optimum emitter coverage ratio (Ropt) to reach maximum performance on back contact-back junction (BC-BJ) solar cells. Ropt exhibits a strong dependence on pitch, emitter and back surface field (BSF) doping and bulk resistivity, ranging between 0.6 and 0.95. By fixing BSF doping, emitter doping and bulk resistivity, BSF and emitter width can be optimized independently one another. The optimum BSF width and the optimum emitter width are given by a trade-off between series resistance and electrical shading losses. From the design perspective, focusing on optimizing the BSF and emitter width is more effective than optimizing R at fixed pitch or BSF width.

Shallow B-implanted Emitters with Laser Overdoping from AlOx Passivating Layers

A simple process for the fabrication of selective emitter structures on n-PERT cells is investigated, using shallow Boron emitters obtained by ion implantation. By tuning the emitter doping process parameters, J0e values as low as 10 fA.cm-2 have been obtained with highly resistive profiles. Laser overdoping processes from AlOx passivating layers are tested on these profiles to locally increase the emitter conductivity and allow better contact properties. Through this process the emitter sheet resistance and doping profile may be locally controlled with a limited impact on the J0e values.

Electrical and Mechanical Properties of Plated Ni/Cu Contacts for Si Solar Cells

Plated Ni/Cu/Ag contacts are an industrially feasible metallization approach for high efficiency c-Si solar cells with low surface doping concentrations (1018 cm-3 <ND < 1020 cm-3). The 2d-simulations of this work define the minimum requirements on the contact resistivity of metal contacts in a high efficiency solar cell design. The following experimental study of the contact resistivity of plated Ni/Cu/Ag contacts on lowly doped phosphorus emitter demonstrates low contact resistivities in the mΩcm2 regime, which enable solar cells with high fill factors. Furthermore, the paper analyzes the influence of the thermal silicidation process on pseudo-fill factor losses and on the mechanical contact adhesion. The contact adhesion is also studied with respect to the laser contact opening process. The results of this work demonstrate that the right choice of back-end processes enable plated Ni/Cu/Ag contacts with low contact resistivities in combination with high contact adhesions above 1 N/mm.

Doping Level Effect on Sample Temperatures in Infrared Belt Furnace Firing

A firing study has been conducted for a variety of samples with different surface morphologies as well as with different bulk and emitter doping concentrations. In this study the firing settings were fixed and the temperature profile on the sample was measured with thermocouples. We have observed that the measured peak temperature on solar-cell R&D samples in an infrared (IR) conveyer belt firing furnace is strongly dependent on the surface morphology of the sample and its doping level. The surface morphology determines the coupling of IR radiation into the substrate. Samples with a random pyramids (RP) surface texture have reached temperatures that were 150oC higher than that of their polished counterparts. In addition to that, the doping level determines the free carrier absorption (FCA) and thus the heating up due to absorption of IR radiation. We have derived an analytical expression for a Planck-spectrum weighed FCA coefficient that is inversely proportional with the absolute black-body temperature (T) squared. Since the total energy radiated by a black body scales with T4, the radiation absorption is proportional with T2. The total doping level of a sample was quantified by Gtot, the summation of the sheet conductance values [Ω-1/sq] of the base and emitter region(s). For a sample with a base resistivity of 3Ωcm and two emitters of 100 Ohm/sq on either side, yielding Gtot=0.028Ω-1/sq, the measured peak temperature was found to be 150oC higher than that of a ‘bare’ (lifetime) sample of 10Ωcm with Gtot=0.0016Ω-1/sq.

New Method for Determination of Electrically Inactive Phosphorus in n-type Emitters

The precise knowledge of the amount and the location in depth of inactive phosphorus in an n-type emitter is still a challenge. As a new approach, we determine the total amount of phosphorus (P dose) in the emitter stepwise in dependence of etching depth with the characterization tool ICP-OES. A comparison of the data with the electrically active P concentration profile measured by ECV allows to determine in which depths electrically inactive phosphorus is present. For a highly doped emitter, we show that most of the inactive P dose is located next to the sample surface. Furthermore, we compare the determined P dose in dependence of depth with the P dose extracted from a SIMS profile. In a second experiment, we investigate the amount of inactive phosphorus in the whole emitter for various n-type emitters, depending on the POCl3-N2 gas flow as a significant diffusion parameter. It is shown that an increase of the POCl3-N2 gas flow results in a saturation effect of the active phosphorus, while the amount of inactive phosphorus is strongly increasing.

Analysis of n-type IBC solar cells with diffused boron emitter locally blocked by implanted phosphorus

Interdigitated back-contact (IBC) solar cells were fabricated with a process sequence combining local ion implantation of phosphorus and full area BBr3 furnace diffusion resulting in conversion efficiencies of up to 22.4%. The highly doped emitter and BSF are in direct contact to each other (p+n+ junction) leading to a controlled junction breakdown at low reverse-bias voltages of around 5V. The breakdown was located at the p+n+ junction and found to be homogeneously distributed over the whole cell area. This is not critical for module integration as the absolute temperature rise of a reverse-biased cell was determined to be less than 35K. After reverse breakdown, the conversion efficiency degraded by 1–2% absolute due to additional recombination at the p+n+ junction. The cell performance could be fully recovered by a short annealing at 300°C indicating that the Al2O3 passivation was altered by the reverse breakdown. This might be a fundamental issue for Al2O3 passivated IBC solar cells without gap between emitter and BSF, independent from the doping method.

0.4% absolute efficiency increase for inline-diffused screen-printed multicrystalline silicon wafer solar cells by non-acidic deep emitter etch-back

Emitter formation is one of the most critical and crucial process steps in the fabrication of standard silicon wafer solar cells. Typically the photovoltaic industry uses tube based phosphorus diffusion, using phosphorus oxychloride as the dopant source. Alternately, a low-cost inline diffusion using phosphoric acid as a dopant source can be used. However, proper process conditions must be used to meet solar cell energy conversion efficiencies obtained by tube diffusion. In this work, we present the application of a non-acidic homogeneous emitter etch-back process – the ‘SERIS etch’ – for inline-diffused emitters in order to raise the efficiency of multicrystalline silicon (multi-Si) wafer solar cells. We apply both light and heavy emitter etch-backs on inline-diffused emitters with sheet resistance (Rsq) values in the 40–60Ω/sq range to achieve emitters with a target Rsq of ~70Ω/sq. The emitter surface reflectance and doping uniformity are maintained even after an etch-back that results in a Rsq change of ~30Ω/sq. An average cell efficiency gain of 0.4% (absolute) is reported for cells with heavy etch-back when compared to the as-diffused non-etch-back screen-printed full-area aluminum back surface field solar cells and efficiencies up to 17.9% are achieved. Besides, best lot of the etch-back inline-diffused cells shows a 0.2% (absolute) efficiency gain over the standard tube-diffused cells. These results show that the ‘SERIS etch’ etch-back process can enable higher-efficiency industrial inline-diffused multi-Si wafer solar cells.

The investigation on the stratification phenomenon of aluminum rear alloyed layer in silicon solar cells

A stratification phenomenon of aluminum rear alloyed layer was found in the study of aluminum rear emitter N-type solar cells. It is related to the composition of the paste. The outer aluminum alloyed layer can be called as aluminum doped emitter, and it gives the contribution to the junction formation. The inner layer is only the Al/Si mixed layer. The aluminum atoms in this layer are not bonded with silicon atoms. This inner layer will ruin the quality of the rear junction. The shunt resistance, reverse current density and the junction electric leakage value are getting worse when the thickness of the inner layer increases. The thickness of the inner Al/Si mixed layer increases with the increasing of firing temperature, while the depth of the aluminum doped emitter almost does not change. From the analyses, the inner Al/Si mixed layer is redundant and deleterious. Only a single deep aluminum doped rear emitter is needed for N-type solar cells. The highest power conversion efficiency of 19.93% for aluminum rear emitter N-type cells without the stratification phenomenon has been obtained.

Reverse Bias Behavior of Diffused and Screen-Printed n-Type Cz-Si Solar Cells

In this study, we investigate current flow in reverse bias mode and its impact on conversion efficiency for large-area n-type Cz-Si H-pattern and n-type Cz-Si metal wrap through (MWT) solar cells. Shunting is studied as a function of the boron emitter doping profile and by comparing MWT cells with two different phosphorus-doped back surface field (BSF) structures. Less shunting is observed for cells with deeper boron-doped emitters (depth d ≈ 700 nm) compared with cells with shallower emitters (d ≈ 500 nm). Cells with a deeper doping profile have initial shunt resistances of R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</sub> > 30 kΩ · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (without prior reverse load), while cells with shallower emitters exhibit initial values of R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</sub> ≈ 9 kΩ · cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , irrespective of the cell type. Furthermore, cells with deeper boron doping profiles show significantly lower current flows under reverse bias. We observe a halving of the RP-values after reverse biasing the H-pattern and the MWT cells with structured BSF where, on the other hand, the conversion efficiencies are hardly affected. MWT cells featuring a BSF below the external p-type contacts show a drop in conversion efficiency of 0.3%abs. This is due to degradation of the electrical insulation between via paste and BSF after reverse bias stress.

Realization of conformal doping on multicrystalline silicon solar cells and black silicon solar cells by plasma immersion ion implantation

Emitted multi-crystalline silicon and black silicon solar cells are conformal doped by ion implantation using the plasma immersion ion implantation (PIII) technique. The non-uniformity of emitter doping is lower than 5%. The secondary ion mass spectrometer profile indicates that the PIII technique obtained 100-nm shallow emitter and the emitter depth could be impelled by furnace annealing to 220 nm and 330 nm at 850 °C with one and two hours, respectively. Furnace annealing at 850 °C could effectively electrically activate the dopants in the silicon. The efficiency of the black silicon solar cell is 14.84% higher than that of the mc-silicon solar cell due to more incident light being absorbed.

High-efficiency nanostructured silicon solar cells on a large scale realized through the suppression of recombination channels.

Nanostructured silicon solar cells show great potential for new-generation photovoltaics due to their ability to approach ideal light-trapping. However, the nanofeatured morphology that brings about the optical benefits also introduces new recombination channels, and severe deterioration in the electrical performance even outweighs the gain in optics in most attempts. This Research News article aims to review the recent progress in the suppression of carrier recombination in silicon nanostructures, with the emphasis on the optimization of surface morphology and controllable nanostructure height and emitter doping concentration, as well as application of dielectric passivation coatings, providing design rules to realize high-efficiency nanostructured silicon solar cells on a large scale.

Back-junction back-contact n-type silicon solar cell with diffused boron emitter locally blocked by implanted phosphorus

The highest energy conversion efficiencies in the field of silicon-based photovoltaics have been achieved with back-junction back-contact (BJBC) silicon solar cells by several companies and research groups. One of the most complex parts of this cell structure is the fabrication of the locally doped p- and n-type regions, both on the back side of the solar cell. In this work, we introduce a process sequence based on a synergistic use of ion implantation and furnace diffusion. This sequence enables the formation of all doped regions for a BJBC silicon solar cell in only three processing steps. We observed that implanted phosphorus can block the diffusion of boron atoms into the silicon substrate by nearly three orders of magnitude. Thus, locally implanted phosphorus can be used as an in-situ mask for a subsequent boron diffusion which simultaneously anneals the implanted phosphorus and forms the boron emitter. BJBC silicon solar cells produced with such an easy-to-fabricate process achieved conversion efficiencies of up to 21.7%. An open-circuit voltage of 674 mV and a fill factor of 80.6% prove that there is no significant recombination at the sharp transition between the highly doped emitter and the highly doped back surface field at the device level.

Synthesis of Metal Oxide-Coated Conductive Metal Powders and Their Application to Front Electrodes for Solar Cells

Recently, improvement in the conversion efficiency of silicon-based solar cells has been achieved by decreasing emitter doping concentration, because the lightly doped emitter can effectively prevent the recombination of electrons and holes generated by solar light irradiation. This type of emitter is very thin due to the low doping concentration, thus conductive materials (i.e., silver) used for front electrodes can easily penetrate the emitter during a firing process because of their large diffusivity in silicon. This results in junction leakage currents which might reduce cell efficiencies. In this study, -coated Ag powders were synthesized by an ultrasonic spray pyrolysis method and applied to the conductive materials of the front electrode to control the junction leakage current. The shell obstructs the Ag diffusion into the emitter during the firing process. The powder is spherical with a core-shell structure and the thickness of the shell is tens of nanometers. Solar cells were fabricated using pure Ag powders or the -coated Ag powder as front electrode materials, and the conversion efficiency and junction leakage current were compared to investigate the role of the shell during the firing processes.

Contacting Moderately Doped Phosphorous Emitters of Silicon Solar Cells With Dopant Segregation During Nickel Silicidation

Emitter performance is crucial for the efficiency of solar cells. A further increase of efficiency requires reducing recombination losses while keeping parasitic resistances and shadowing at a minimum. In silicon solar cells, recombination can be lowered by decreasing the emitter doping concentration; however, this comes at the cost of increasing contact and sheet resistances. Dopant segregation during nickel silicidation resolves this tradeoff since it allows ohmic contact formation to emitters with almost arbitrary doping profile and concentration. Here, we elaborate on using dopant segregation during silicidation and demonstrate contact formation, as well as a substantial performance improvement of silicon solar cells exhibiting a lowly doped emitter with a peak doping concentration of 6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . Furthermore, we show that the small achievable contact resistances in principle allow a strong reduction of the contact area. In turn, this enables lowering the shadowing while, at the same time, decreasing the sheet resistance due to a larger number of front contacts that can be placed close to each other. Using PC1D simulations, we study the solar cell performance as a function of the emitter dopant concentration estimating the achievable efficiency increase when employing dopant segregation.