Emitter Saturation Current Density Research Articles

Investigating the Application and Impact of Laser Etching Technology in the Fabrication of Solar Cells

The most crucial initial step in the metallization process of electroplating is the local contact openings in dielectric layers. In this article, an ultraviolet picosecond laser (UV‐ps), is used to open the front and back dielectric layer of the precursor of n‐TOPCon solar cells. By changing the laser parameters and spot overlap rate, the surface morphology of laser etching under different conditions is obtained and characterized by optical microscope and scanning electron microscope. The results indicate that there are three separate laser shock zones in the dielectric layer under the action of Gaussian light spot, and the corresponding ablation mechanisms are proposed. The longitudinal distribution of B element is characterized by secondary ion mass spectrometry to explore the effect of laser on its pn junction. In addition, the electrical characterization of lifetime photoluminescence is measured and calibrated using WCT120, and this indicates that the majority of the amorphous silicon is recrystallized when applying a fast firing oven process, and this hence improve the minority carrier lifetime and implied open‐circuit voltage. The laser damage to the emitter at the laser‐ablated regions is investigated using the emitter saturation current density, J0laser extracted by WCT120.

Titanium oxide nanomaterials as an electron-selective contact in silicon solar cells for photovoltaic devices

To obtain high conversion efficiency, various carrier-selective contact structures are being applied to the silicon solar cell, and many related studies are being conducted. We conducted research on TiO2 to create an electron-selective contact structure that does not require a high-temperature process. Titanium metal was deposited using a thermal evaporator, and an additional oxidation process was conducted to form titanium oxide. The chemical compositions and phases of the titanium dioxide layers were analyzed by X-ray diffraction. The passivation effects of each titanium oxide layer were measured using the quasi-steady-state photoconductance. In this study, the layer properties were analyzed when TiO2 had a passivation effect on the silicon surface. The charge and interface defect densities of the layer were analyzed through CV measurements, and the passivation characteristics according to the TiO2 phase change were investigated. As a result, by applying optimized TiO2 layer thickness and annealing temperature conditions through the experiment for passivation to the cell-like structure, which is the structure before metal and electrode formation, an implied open-circuit voltage (iVoc) of 630 mV and an emitter saturation current density (J0) value of 60.4 fA/cm2 were confirmed.

Tube‐type plasma‐enhanced atomic layer deposition of aluminum oxide: Enabling record lab performance for the industry with demonstrated cell efficiencies >24%

AbstractIn this work, single‐side aluminum oxide (Al2O3) deposition enabled by a new tube‐type industrial plasma‐assisted atomic layer deposition (PEALD) technique is presented to meet the increasingly stringent requirements for high‐efficiency solar cell mass production. Extremely low emitter saturation current densities, J0e, down to 15 fA/cm2 are achieved on an industrial textured boron emitter with a sheet resistance of 104 Ω/sq, passivated by PEALD Al2O3/PECVD SiNx stack after firing. An implied open‐circuit voltage of up to 721 mV is obtained on symmetrical lifetime samples. The underlying passivation mechanisms of this new tube‐type PEALD Al2O3 are investigated by contactless corona‐voltage measurements. The results indicate that the superior passivation is mainly attributed to a low interface defect density down to 1.1 × 1011 cm−2 eV−1 and a high negative fixed charge density up to 4.5 × 1012 cm−2. Simulations show that the obtained J0e is close to its intrinsic limit. Lastly, the developed tube‐type PEALD Al2O3 is applied to industrial TOPCon solar cells achieving an average cell efficiency above 24% and a maximum Voc of 707 mV. This work shows that the record level of surface passivation available from lab‐scale PEALD reactors is now available in a flexible high‐throughput industrial PEALD platform, which opens a new route for mass production of high‐efficiency industrial TOPCon solar cells with a lean process at low costs.

Optimization of Effective Doping Concentration of Emitter for Ideal c-Si Solar Cell Device with PC1D Simulation

Increasing silicon solar cell efficiency plays a vital role in improving the dominant market share of photo-voltaic systems in the renewable energy sector. The performance of the solar cells can be evaluated by making a profound analysis on various effective parameters, such as the sheet resistance, doping concentration, thickness of the solar cell, arbitrary dopant profile, etc., using software simulation tools, such as PC1D. In this paper, we present the observations obtained from the evaluation carried out on the impact of sheet resistance on the solar cell’s parameters using PC1D software. After which, the EDNA2 simulation tool was used to analyse the emitter saturation current density for the chosen arbitrary dopant profile. Results indicated that the diffusion profile with low surface concentration and shallow junction depth can improve the blue response at the frontal side of the solar cell. The emitter saturation current density decreases from 66.52 to 36.82 fA/cm2 for the subsequent increase in sheet resistance. The blue response also increased from 89.6% to 97.5% with rise in sheet resistance. In addition, the short circuit density and open circuit voltage was also observed to be improved by 0.6 mA/cm2 and 3 mV for the sheet resistance value of 130 Ω/sq, which resulted in achieving the highest efficiency of 20.6%.

Enhanced Surface Passivation by Atomic Layer-Deposited Al2O3 for Ultraviolet-Sensitive Silicon Photomultipliers

This article describes a superior passivation of p-type (p^&#x002B;) Si surface by an Al₂O₃ thin film that is synthesized by plasma-assisted atomic layer deposition (ALD) for ultraviolet-sensitive silicon photomultipliers (SiPMs), compared to conventional SiO₂ and SiN_<i>x</i> passivation schemes. The superiority of Al₂O₃ passivation is due not only to a sufficiently low interface defect density, but also to a high density of built-in negative charges. A 7-nm thin Al₂O₃ film can yield an emitter saturation current density of &#x007E;8 fA/cm² on a high sheet resistance p^&#x002B; layer, compared to &#x007E;60 and &#x007E;1480 fA/cm² for thermal SiO₂ and SiN_<i>x</i> passivation. This superior surface passivation allows the photon-generated carriers to have higher probabilities to reach the high-field region to trigger an avalanche event. In addition, the Al₂O₃ thin film provides very low values of effective surface recombination velocity on low resistivity n- and p-type Si surfaces, which can lead to well-passivated surface features on the guard ring and trench isolation regions of SiPM. These demonstrate the potential of the Al₂O₃ thin film passivation to improve the quantum efficiency and thus photo-detection efficiency (PDE) of ultraviolet-sensitive SiPM with p^&#x002B;/n^&#x2212;/n/n^&#x002B; structure.

Formation of emitter by boron spin-on doping from SiO2 nanosphere and properties of the related n-PERT solar cells

In this paper, spin-on doping (SOD) process using commercial boron dopant source was applied to form front p-type emitter on n-type mono-crystalline CZ substrate. Results showed that the thickness of boron rich layer (BRL) decreased from 180 nm to 20 nm with SiO2 nanosphere assisted SOD and the BRL can be removed entirely by further in-situ oxidation. Furthermore, the hole concentration of the emitter reduced from 5.5 × 1019 cm3 to 2.8 × 1019 cm3 by integrating these two processes. In a symmetric sample without BRL, a very low emitter saturation current density of 32 fA/cm2 with implied-Voc over 700 mV was obtained. The minority carrier lifetime got improved more than 30% after diffusion. Finally, n-PERT solar cell with a conversion efficiency of 20.26% was achieved using optimized boron emitter, showing an enhanced conversion efficiency of 0.54%.

Comparison of passivation properties of plasma-assisted ALD and APCVD deposited Al2O3 with SiNx capping

In this paper, we report on the direct comparison of Al2O3 films deposited by plasma-assisted atomic layer deposition (ALD) and atmospheric pressure chemical vapor deposition (APCVD) techniques on the passivation properties of both bare and boron diffused Si surfaces. It is found that ALD and APCVD deposited Al2O3 layers with SiNx capping have very similar chemical composition, hydrogen concentration at the Al2O3/Si interface, and interface defect density (Dit), but the ALD film has slightly higher negative fixed charge (Qf) after a high temperature (~750 °C) contact firing cycle typically used for solar cell fabrication. Both films showed excellent surface recombination velocities of <5 cm/s on planar and textured un-diffused Si wafers, but the ALD films gave 1 to 8 fA/cm2 lower emitter saturation current densities (J0e) on implanted boron emitters with sheet resistance in the range of 90–180 Ω/□. This is attributed to the observed slightly higher negative charge-induced field-effect passivation in the ALD films. However, SiNx/Al2O3/p+/n/n+ passivated emitter rear totally diffused (PERT) cell structures fabricated with 110 and 150 Ω/□ emitter in this study showed no appreciable difference in cell efficiency. Device analysis and simulations showed that even for >23% high-performance solar cells design, with very low bulk and rear contact recombination, the observed small difference in J0e has negligible effect on cell efficiency. Thus, APCVD Al2O3 passivation of B emitter can produce comparable efficiencies to their counterpart ALD Al2O3 passivated n-type Si solar cells.

Spatial Atomic Layer Deposition of Aluminum Oxide as a Passivating Hole Contact for Silicon Solar Cells

Herein, tunneling aluminum oxide (Al2O3) passivation layers are demonstrated to be a candidate for hole collecting, passivating contacts when coupled with a boron‐doped surface. These very thin Al2O3 films (1.5–3 nm) are deposited using spatial atomic layer deposition (ALD) on boron diffused (110–115 Ω □−1) hydrophilic surfaces operating as metal–insulator–semiconductor (MIS) contacts. The emitter saturation current density values of ≈57 fA cm−2 are achieved for the Al2O3 film thicknesses of 2 nm before metallization. At this same thickness, the contact resistivity values of 33 mΩ cm2 are obtained after metallization. Most importantly, the boron diffusion, wet chemical surface preparation, and spatial ALD of Al2O3 used to create these MIS structures are all performed with industrially relevant equipment on Cz wafers.

A simulation approach to improve photocurrent through a double-layer of the emitter in a-Si1-xCx/c-Si heterojunction solar cell

Abstract In this work, a simulation approach to improve the contribution of light generated current by the emitter layer is presented. Single emitter layer made of nanocrystalline carbide of silicon is replaced with a double layer of the same compound material but different elemental composition and optical properties in a microcrystalline silicon-based solar cell. Each layer of the emitter is made of carbides of silicon with different carbon to silicon fractions. The simulated study results showcase short circuit current and efficiency up to 37.86 mAcm−2 and 20.56% respectively for the device with a double emitter layer. The quantum efficiency also justifies an increase in photocurrent in the double layer device. This increase is mainly observed in the wavelength range of photons absorbed by the emitter layer. The calculated emitter saturation current density for the same device is as small as 1.7854 × 10−17 Acm−2. Furthermore, a strong dependence of device parameters on the physical properties of the intrinsic layer is observed. The overall results introduce double emitter as a new approach to improve the photocurrent of heterojunction solar cells.

Laser Crystallization and Dopant Activation of a-Si:H Carrier-Selective Layer in TOPCon Si Solar Cells

Herein, we present a pulsed-laser processing method for crystallization and dopant activation of a highly n-doped amorphous silicon (a-Si:H) carrier-selective layer in a tunnel oxide passivated contact (TOPCon) Si solar cell structure. The laser method provides enhanced conductivity and implied open circuit voltage while reducing emitter saturation current density and surface heating, as opposed to conventional high-temperature furnace annealing of the bulk Si wafer with a TOPCon structure. We identify an appropriate laser wavelength, fluence, and layer thickness using modeling and simulations. Raman and Hall effect measurements demonstrate increased crystallinity and dopant activation, whereas photoconductive decay shows enhanced surface and interface passivation quality. Additionally, we examine the role of subsequent SiNx deposition on further improving the passivation of laser-processed TOPCon layers to achieve a 5.9 kΩ/sq film sheet resistance, 2 ms effective carrier lifetime, 718 mV implied open-circuit voltage (iVOC), and 8.6 fA/cm2 one-side recombination current density (J0) with the potential for further passivation improvement via laser process and a-Si layer thickness optimization.

Ultrathin Plasma Oxide for Passivation of Phosphorus-Diffused Silicon Solar Cell Emitters

Different passivation layers on phosphorous diffused emitters have been studied. A comparison between silicon nitride (SiNx) single layer at different deposition temperatures, stacks of “outgassing” silicon oxide (that was grown during the moving in/out of the furnace in ambient air during the outgassing process) and an ultrathin SiOx layer formed by plasma oxidation capped with SiNx layers have been investigated. Optimization of plasma enhanced chemical vapor deposition (PECVD) SiNx at different deposition temperatures showed good passivation quality depending on the hydrogen content. Plasma oxidation process by a PECVD tool without utilizing a silicon source in the process gas flow is an obvious choice to form a thin SiOx as it allows the formation of a SiOx/SiNx stack in a sequence of deposition processes in the same inline PECVD tool. Such an ultrathin SiOx layer was found to not only improve the surface passivation after a fast firing process but also to maintain the excellent anti-reflection coating (ARC) property of the SiNx. Low emitter saturation current density J 0 e values of 15 fA/cm2 for planar and 24 fA/cm2 for textured surfaces for an emitter with a sheet resistivity of 161 Ω/ sq has been reached after firing at 850 °C, which is comparable with the outgassing oxide. These stacks gave an improvement of the J 0 e values with a factor of 6–9 for emitters with low surface concentration comparing to single SiNx layer.

A low cost additive-free acid texturing process for large area commercial diamond-wire-sawn multicrystalline silicon solar cells

Diamond-wire-sawing (DWS) technique allows slicing of silicon ingots to produce wafers at cheaper price due to its reduced kerf-loss and increased cutting rate. However, there is no cost-effective and industrially viable method available for texturing DWS wafers, especially for multicrystalline silicon (mc-Si) wafers. Currently additive-based acid texturing process is availed by PV manufacturing units for texturing DWS mc-Si wafers. An additive-free, low cost and energy-efficient acid texturing process is demonstrated for DWS mc-Si wafers in industrial production line. Nearly 10% absolute reduction in weighted average reflectance values (WAR) are noticed for the newly textured wafers from that of as-cut DWS mc-Si wafers and the WAR values are comparable to that of existing additive-based acid textured mc-Si wafers. An absolute reduction in emitter saturation current density by ~17 fA-cm-2 and improved implied open circuit voltage of ~5 mV (absolute) are reported for lifetime sample fabricated using newly textured wafers when compared to the additive-based acid textured lifetime samples. An impressive batch average efficiencies of 18.20% and 18.24% are achieved for the additive-free and existing additive-based acid textured mc-Si cells, respectively. Detailed analysis conclude that ~1.5% (relative) enhancement in short circuit current density can be achieved by further process optimization. Cost analysis indicates that 60% of the chemical cost involved the texturing process can be cut down by replacing the existing additive-based acid process with the new method using the same process equipment. Hence, the texturing process presented has great potential for producing large area high efficiency mc-Si cells favoring cost considerations without compromising performance.

EFFECT OF PHOSPHOROUS/BORON DOPING PROFILE DIFFERENCES ON THE PERFORMANCE OF SILICON SOLAR CELLS

This research work was done under title “Effect of phosphorous/boron doping profile differences on the performance of silicon solar cells”. Emitter diffusion either phosphorous or boron is quite challenging in photovoltaic industry. It directly affects the emitter saturation current density and the emitter quantum efficiency of silicon solar cells. Our main objective was to make the comparison of both phosphorous and boron diffused emitters for different peak dopant concentrations in silicon solar cells. It was done by using EDNA 2 simulations. We used different parameters in EDNA 2 and simulated the high efficiency solar cells with boron as back ground and phosphorous as emitter. Then we simulated the solar cells with phosphorous as back ground and boron as emitter. We varied the peak dopant concentration of phosphorous as well boron from 1.6E+17 to 3.9E+20. The best internal quantum efficiency of emitter for phosphorous diffused emitters was 95.1 %, obtained at 1.6E19 (cm-3 ) with an effective emitter depth of 0.675 (µm). However, the best internal quantum efficiency of emitter for boron diffused emitters was 80.6 %, obtained at 3.9E19 (cm-3 ). It has an effective emitter depth of 0.732 (µm) that is greater than obtained from phosphorous diffused emitters. We concluded that the phosphorous diffused emitters have much better performance than boron diffused emitter in silicon solar cells. They have better internal quantum efficiency of emitters at lower peak dopant concentration. They have lower emitter sheet resistance with lower effective emitter depth, as also required during silicon solar cell fabrication.

Plasma‐immersion ion implantation: A path to lower the annealing temperature of implanted boron emitters and simplify PERT solar cell processing

AbstractIon implantation is a suitable and promising solution for the massive industrialization of boron doping, which is a crucial process step for most next‐generation solar cells based on crystalline silicon (c‐Si). However, the use of ion implantation for boron doping is limited by the high temperature (in the 1050°C range) of the subsequent activation anneal, which is essential to dissolve the boron clusters and reach a high‐emitter quality. In this work, we propose the use of plasma‐immersion ion implantation (PIII) from B2H6 gas precursor instead of the standard beamline ion implantation (BLII) technique to decrease this temperature down to 950°C. PIII and BLII boron emitters were compared with annealing temperatures ranging from 950°C to 1050°C. Contrary to BLII, no degradation of the emitter quality was observed with PIII implants annealed at 950°C along with a full activation of the dopants in the emitter. At 1000°C, emitter saturation current densities (J0e) below 21 fA/cm2 were obtained using the PIII technique regardless of the tested implantation doses for sheet resistances between 110 and 160 Ω/sq. After metallization steps, the metal/emitter contact resistances were assessed, indicating that these emitters were compatible with a conventional metallization by screen‐printing/firing. The PIII boron emitters' performances were further tested with their integration in n‐type passivated emitter rear totally diffused (PERT) solar cells fully doped by PIII. Promising results already show a conversion efficiency of 20.8% using a lower annealing temperature than with BLII and a reduced production cost.

Recombination Analysis of Tunnel Oxide Passivated Contact Solar Cells

In this paper, an analytical model has been developed to observe the recombination losses involved in a tunnel oxide passivated contact solar cell with a carrier-selective layer constituting polycrystalline silicon (poly-Si). The main focus of this paper is to observe the contribution of each component of recombination current density and its effect on open-circuit voltage, Voc. The effect of emitter saturation current density (Joe), surface recombination velocity, substrate thickness, and doping density of carrier-selective layer on Voc has been presented. The analysis indicates that Voc = 766 mV can be achieved for Joe = 2 fA/cm2, and a doping density of poly-Si > 1021cm−3 in a c-Si/SiO2/poly-Si contact for a 50- $\mu \text{m}$ -thick substrate in case of ideal surface passivation. For identical conditions, Voc attains a lower value of 750mV for a 180- $\mu \text{m}$ -thick substrate. The dependence of Voc on bandgap of tunnel layers has also been observed by considering other materials like Si3N4 and SiC. These results from the analyticalmodel have been compared with results froma simulation using automat FOR simulation of HETerostructures v2.5 and have been found to be in good agreement with each other.

Application of SiO2 passivation technique in mass production of silicon solar cells

Effective surface passivation is one of the primary prerequisites for high-efficiency silicon solar cells. In this paper, high-quality silicon dioxide (SiO2) films with excellent surface passivation abilities have been realized by thermal oxidation and plasma-enhanced chemical vapor deposition, respectively. By employing SiO2 capped with hydrogenated silicon nitride (SiNx:H) (SiO2/SiNx:H stacks) as the front passivation layers, the emitter saturation current density has been reduced from 90.5 fA/cm2 to 62.3 fA/cm2. We have successfully mass-produced thermal-oxidized p-type Czochralski-Si (CZ-Si) solar cells with high conversion efficiency (η) of 20.1%, which is 0.2% absolutely higher than that of the conventional Al back surface field (Al-BSF) solar cells. With a rational design of process integration, we have further presented a cost-effective way to fabricate high-efficiency SiO2 passivated emitter and rear cells (PERCs) at an existing production line. The introduction of SiO2/SiNx:H stacks on the rear surface can effectively increase the long wavelength response and the rear surface recombination is also suppressed with a rather low surface recombination velocity of 26 cm/s achieved after a post-annealing process. The industrial SiO2 passivated p-type CZ-Si PERCs possess outstanding performances with the average η of 21.3% and the highest η of over 21.9%, absolute 1.3% increment in average η compared with the conventional Al-BSF solar cells. Moreover, we have demonstrated that the relative low illumination response (below 50 W/m2) loss of our SiO2 PERCs over a day is extremely limited to be less than 0.2%.

Thermal stability of Al2O3/TiO2 stacks for boron emitter passivation on n-type silicon solar cells

Boron-diffused samples passivated by Al2O3/TiO2 stacks with different types of TiO2 were investigated. An emitter saturation current density of 11 × 10−15 A/cm2 was obtained for Al2O3/TiO2 passivated samples with symmetrical 100 Ω/sq. boron diffusion. The surface passivation of Al2O3/TiO2 stacks was strongly impacted by the temperature, gas, and sequence during two-step annealing. In addition, the saturation current density of the Al2O3/TiO2 stacks is influenced by the TiO2 layers with an O3 treatment. Enhanced passivation performance of the Al2O3/TiO2 stacks was achieved by a combination of appropriate TiO2 capping layer and post-annealing. The results of this study provide reliable criteria for the design of thermally-stable Al2O3/TiO2 stacks on high-efficiency silicon solar cells.

The Effect of Nonuniform Emitter Sheet Resistance on PERC Solar Cell Performance

The aim of this research work is to study the effects of non-uniform emitter sheet resistance on the performance of PERC solar cells. For this purpose, we used different simulation techniques including EDNA 2, MATLAB and Griddler 2.5 Pro. We calibrated the phosphorous doping profiles with Nmax of 4E20, 3.5E20, 3E20, 2.5E20, 2E20, 1.5E20 and 1E20 by using EDNA 2. The results show that the emitter saturation current densities are decreased for corresponding increase in sheet resistance during phosphorous diffusion. In this way we find a relationship between emitter sheet resistances and recombination current densities and developed a MATLAB function from it. By assuming maximum emitter sheet resistance and the corresponding minimum recombination current density at the centre of the cell, we determined their average values from the same MATLAB function. These average values were then used as input parameters for computer simulations Griddler 2.5 Pro, to assess the effect of non-uniformities in emitter sheet resistance and recombination currents on the performance of p- PERC solar cells. The open-circuit voltage, short-circuit current density, fill factor and efficiency are calculated as a function of the difference between the maximum and minimum value of emitter sheet resistance. The results show that the open-circuit voltage and the efficiency of p-PERC solar cell are decreased if the difference between maximum and minimum value of emitter sheet resistance is increased from 0.45 ohm/sq to 19.3 ohm/sq. Whereas the fill factor is increased for the corresponding increase in the difference between maximum and minimum value of emitter sheet resistance.

Impact of boron rich layer on performance degradation in boric acid diffused emitters for n-type crystalline Si solar cells

Boron rich layer (BRL) formed beneath the borosilicate glass layer during p-type emitter formation is an undesirable phenomenon. It influences different cell parameters and can degrade the device performance. In this work, the device degradation study is done for different BRL thicknesses produced with different concentrations of the boric acid dopant source. The bulk carrier lifetime reduces to more than 75% and emitter saturation current density becomes more than 10−12 mA cm−2 for 60 nm of BRL thickness. The observed Jsc and Voc values become zero for BRL thicknesses higher than 40 nm as seen in this work and the device properties could not be enhanced. So, higher thicknesses of BRL should be avoided.

Implementation of Selective Emitter for Industrial-Sized PERCs Using Wet Chemical Etch-Back Process

This paper introduces a selective phosphorus emitter formed by screen-printed resist masking combined with a wet chemical etch-back process for an industrial-sized passivated emitter and rear cell (PERC). Applying the selective emitter (SE) concept is expected to decrease the recombination losses at the front surface of the PERC cell. With the SE structure, we observed an increase in the open-circuit voltage $(V_{{\rm{oc}}})$ and short-circuit current density $(J_{{\rm{sc}}})$ , but a decrease in the fill factor (FF), compared with homogeneous emitter cells. The sheet resistance $(R_{{\rm{sheet}}})$ of the lightly doped emitter (n+-emitter) by the etch-back process had a considerable impact on the $V_{{\rm{oc}}}$ and $J_{{\rm{sc}}}$ , which we attributed to the reduced emitter saturation current density $(J_{0e})$ caused by a reduction in the Auger recombination in the n+ -emitter. The diminished FF was owing to the higher $R_{{\rm{sheet}}}$ , resulting in an increased series resistance of the cell. These results suggest that an SE structure made by the wet etch-back process is a very promising technology to improve the conversion efficiency of industrial-sized PERC solar cells.