Emitter Saturation Current Density Research Articles

Very low recombination phosphorus emitters for high efficiency crystalline silicon solar cells

This work studies low recombination phosphorus emitters on c-Si. The emitters are fabricated by diffusion from solid sources and then passivated by thermal oxide yielding sheet resistances between 15 and 280 Ω/sq. Emitter saturation current densities lie in the 2.5–110 fA cm−2 range, leading to implicit open-circuit voltages between 674 and 725 mV. Bulk lifetime is limited by intrinsic recombination mechanisms. Surface recombination velocities between 80 and 300 cm s−1 have been obtained, appearing among the lowest reported in this range of emitter sheet resistances.

N‐type silicon solar cells with surface‐passivated screen‐printed aluminium‐alloyed rear emitter

AbstractAluminium‐doped p‐type (Al‐p+) silicon emitters fabricated by means of a simple screen‐printing process are effectively passivated by plasma‐enhanced chemical‐vapour deposited amorphous silicon (a‐Si). We measure an emitter saturation current density of only 246 fA/cm2, which is the lowest value achieved so far for a simple screen‐printed Al‐p+ emitter on silicon. In order to demonstrate the applicability of this easy‐to‐fabricate p+ emitter to high‐efficiency silicon solar cells, we implement our passivated p+ emitter into an n+np+ solar cell structure. An independently confirmed conversion efficiency of 19.7% is achieved using n‐type phosphorus‐doped Czochralski‐grown silicon as bulk material, clearly demonstrating the high‐efficiency potential of the newly developed a‐Si passivated Al‐p+ emitter. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface passivation schemes for high‐efficiency n‐type Si solar cells

AbstractAn effective passivation on the front side boron emitter is essential to utilize the full potential of solar cells fabricated on n‐type silicon. However, recent investigations have shown that it is more difficult to achieve a low surface recombination velocity on highly doped p‐type silicon than on n‐type silicon. Thus, the approach presented in this paper is to overcompensate the surface of the deep boron emitter locally by a shallow phosphorus diffusion. This inversion from p‐type to n‐type surface allows the use of standard technologies which are used for passivation of highly doped n‐type surfaces. Emitter saturation current densities (J0e) of 49 fA/cm2 have been reached with this approach on SiO2 passivated lifetime samples. On solar cells a certified conversion efficiency of 21.7% with an open‐circuit voltage (Voc) of 676 mV was achieved. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High efficiency n-type Si solar cells on Al2O3-passivated boron emitters

In order to utilize the full potential of solar cells fabricated on n-type silicon, it is necessary to achieve an excellent passivation on B-doped emitters. Experimental studies on test structures and theoretical considerations have shown that a negatively charged dielectric layer would be ideally suited for this purpose. Thus, in this work the negative-charge dielectric Al2O3 was applied as surface passivation layer on high-efficiency n-type silicon solar cells. With this front surface passivation layer, a confirmed conversion efficiency of 23.2% was achieved. For the open-circuit voltage Voc of 703.6mV, the upper limit for the emitter saturation current density J0e, including the metalized area, has been evaluated to be 29fA∕cm2. This clearly shows that an excellent passivation of highly doped p-type c-Si can be obtained at the device level by applying Al2O3.

Formation of Boron‐doped region using spin‐on dopant: investigation on the impact of metallic impurities

AbstractInvestigation on the electrical properties of p+‐doped regions formed by spin‐on‐dopant (SOD) technique was achieved. Using this technique, boron‐diffused regions were formed on both p‐type and n‐type float zone wafers. Homogeneous sheet resistances were obtained for both types of wafers. Bulk properties were investigated by measuring effective carrier lifetime. An iron contamination was observed after the boron diffusion step and interstitial iron concentrations were deduced from lifetime measurements. More investigations proved that the iron was initially present within the SOD film. A phosphorus diffusion allows to remove this bulk contamination, leading to an improvement of effective lifetime values. Nevertheless, the corresponding emitter saturation current density was estimated on n‐type wafers and presented a high value. It is likely that this poor electrical quality is the consequence of a high iron concentration which remains in the diffused region. Some possibilities are suggested to avoid or to limit this contamination. Copyright © 2008 John Wiley & Sons, Ltd.

Effective passivation of highly aluminum-doped p-type silicon surfaces using amorphous silicon

Highly aluminum-doped p-type emitters prepared by screen printing on crystalline silicon wafers are effectively passivated by plasma-enhanced chemical-vapor deposited amorphous silicon layers. Using the photoconductance decay technique, the authors measure emitter saturation current densities of 800±200fA∕cm2 for nonpassivated emitters and of 490±120fA∕cm2 for Al-p+ emitters passivated with a 20nm thick amorphous silicon layer deposited at 225°C. An additional annealing step at 300°C for 10min reduces the emitter saturation current density down to only 246±60fA∕cm2. The measured saturation current densities are the lowest values achieved so far for Al-doped p+ emitter.

Excellent passivation of highly doped p-type Si surfaces by the negative-charge-dielectric Al2O3

From lifetime measurements, including a direct experimental comparison with thermal SiO2, a-Si:H, and as-deposited a-SiNx:H, it is demonstrated that Al2O3 provides an excellent level of surface passivation on highly B-doped c-Si with doping concentrations around 1019cm−3. The Al2O3 films, synthesized by plasma-assisted atomic layer deposition and with a high fixed negative charge density, limit the emitter saturation current density of B-diffused p+-emitters to ∼10 and ∼30fA∕cm2 on >100 and 54Ω∕sq sheet resistance p+-emitters, respectively. These results demonstrate that highly doped p-type Si surfaces can be passivated as effectively as highly doped n-type surfaces.

Characterization of the Si − SiO2 Interface Following Room Temperature Ammonia Plasma Exposure

The electrical properties of the interface following nitridation by ammonia plasma exposure at room temperature indicate that a high density of interface defects and charges is generated by the plasma exposure. Combined high-frequency and quasistatic capacitance-voltage measurements show a dramatic increase in the midgap defect density and interface charge, while quasi-steady-state photoconductivity decay measurements indicate a large increase in carrier recombination at the surfaces, as measured by an increase in the emitter saturation current density. The majority of the defects are not paramagnetic and are annealed out by a thermal treatment at 400°C. However, some defects are also generated by the ammonia plasma exposure which is not removed by subsequent thermal treatments. A combination of ammonia plasma treatment and a subsequent 400°C anneal results in good interface passivation, despite the generation of additional paramagnetic defects, making oxynitride layers produced in this way attractive for potential photovoltaic applications.

Laser ablation of SiO2 for locally contacted Si solar cells with ultra‐short pulses

AbstractWe apply ultra‐short pulse laser ablation to create local contact openings in thermally grown passivating SiO2 layers. This technique can be used for locally contacting oxide passivated Si solar cells. We use an industrially feasible laser with a pulse duration of τpulse ∼ 10 ps. The specific contact resistance that we reach with evaporated aluminium on a 100 Ω/sq and P‐diffused emitter is in the range of 0·3–1 mΩ cm2. Ultra‐short pulse laser ablation is sufficiently damage free to abandon wet chemical etching after ablation. We measure an emitter saturation current density of J0e = (6·2 ± 1·6) × 10−13 A/cm2 on the laser‐treated areas after a selective emitter diffusion with Rsheet ∼ 20 Ω/sq into the ablated area; a value that is as low as that of reference samples that have the SiO2 layer removed by HF‐etching. Thus, laser ablation of dielectrics with pulse durations of about 10 ps is well suited to fabricate high‐efficiency Si solar cells. Copyright © 2007 John Wiley & Sons, Ltd.

Accurate measurement of extremely low surface recombination velocities on charged, oxidized silicon surfaces using a simple metal-oxide-semiconductor structure

The authors report a simple technique to determine the surface recombination velocity of silicon and other semiconductor surfaces which have been passivated with a dielectric layer, as a function of charge density. A metal-oxide-semiconductor structure, employing large area, partially transparent metal contacts, is used to enable the charging of the surfaces. Simultaneous measurement of the emitter saturation current density Joe and the effective instantaneous lifetime τinst allows accurate extraction of the effective surface recombination velocity Seff at any given injection level. Extremely low Joe values of 1.8fAcm−2 are measured on the silicon-silicon oxide (Si–SiO2) interface of a thermally oxidized, charged wafer.

N -type emitter surface passivation in c-Si solar cells by means of antireflective amorphous silicon carbide layers

Emitter saturation current densities (JOe) of phosphorus-diffused planar c-Si solar cell emitters passivated by silicon carbide (SiCx) layers have been determined in a wide sheet resistance range (20–500Ω∕sq). Phosphorus diffusions were performed using solid planar diffusion sources without employing any drive-in step. Stacks of two SiCx layers were deposited by plasma enhanced chemical vapor deposition: first a thin silicon rich layer with excellent passivating properties and then an antireflective carbon rich layer. The thickness of the passivating layer was optimized, reaching a trade-off between the better passivation achieved for thicker layers and the increased light absorption within the layer, which reduced the photocurrent. The surface recombination velocity and the optical losses were determined for each configuration and used to calculate photovoltaic conversion efficiency limits for 50 and 90Ω∕sq emitters. In both cases, optimum configuration is for the stacks with passivating layers that are about 8nm thick.

Development of a Phosphorus Spray Diffusion System for Low-Cost Silicon Solar Cells

Phosphoric acid was used an n-type doping source to make an emitter for silicon solar cells. This paper reports on a cold spray method to coat phosphoric acid on the silicon wafer without any additional complicated and corrosive heating system. The key spray parameters such as belt speed, flow rate of the carrier gas, and concentration of phosphoric acid are optimized to get uniform and reproducible sheet resistance for the emitter. The diffusion process has been studied by firing silicon wafers that are spray-coated with phosphoric acid. screen printed solar cells have been fabricated using the emitter formed by the spray coating. The effects of the emitter on cell performance have been investigated and compared with those of the conventional -diffused emitter. screen printed/spray-diffused cells give impressive cell efficiencies of on float zone wafers, which is nearly equal to those of emitter cells . Compared with the emitter cell, the spray-diffused cells show slightly lower quantum efficiency at the short-wavelength response and slightly higher emitter saturation current density ( compared with ). Further optimization can eliminate this small difference in efficiency.

Phosphorus-diffused silicon solar cell emitters with plasma enhanced chemical vapor deposited silicon carbide

We propose the use of annealed phosphorus doped amorphous silicon carbide layers (a-SiC:H(n)) deposited by PECVD as emitters in solar cells and explore the effect of the annealing time on the emitter saturation current density (). We use the quasy-steady state photoconductance method to determine the dependence of effective lifetime on excess carrier density and from these measurements we obtain values in the range of 300 fA cm-2 for sheet resistances around 100 O?sq. Finally, we obtain effective surface recombination velocity values around 104 cm s-1 by fitting the measured values with PC1D simulated ones.

Measuring and interpreting the lifetime of silicon wafers

The carrier lifetime is the most important electronic property of semiconductor materials for solar cells. In this paper we discuss traditional and novel methods for its experimental determination. Among the latter, the Quasi-steady-state photoconductance is particularly powerful since it permits measuring the injection level dependence of the lifetime. The analysis and interpretation of this dependence yields a wealth of information on the physical mechanisms that limit the performance of silicon solar cells. The effect of the surfaces of the silicon wafers, the emitter saturation current density and the Shockley–Read–Hall and Auger recombination mechanisms are explained and their possible determination from lifetime measurements discussed.

Numerical modeling of highly doped Si:P emitters based on Fermi–Dirac statistics and self-consistent material parameters

We have established a simulation model for phosphorus-doped silicon emitters using Fermi–Dirac statistics. Our model is based on a set of independently measured material parameters and on quantum mechanical calculations. In contrast to commonly applied models, which use Boltzmann statistics and apparent band-gap narrowing data, we use Fermi–Dirac statistics and theoretically derived band shifts, and therefore we account for the degeneracy effects on a physically sounder basis. This leads to unprecedented consistency and precision even at very high dopant densities. We also derive the hole surface recombination velocity parameter Spo by applying our model to a broad range of measurements of the emitter saturation current density. Despite small differences in oxide quality among various laboratories, Spo generally increases for all of them in a very similar manner at high surface doping densities Nsurf. Pyramidal texturing generally increases Spo by a factor of five. The frequently used forming gas anneal lowers Spo mainly in low-doped emitters, while an aluminum anneal (Al deposit followed by a heat cycle) lowers Spo at all Nsurf.

Surface recombination velocity of phosphorus-diffused silicon solar cell emitters passivated with plasma enhanced chemical vapor deposited silicon nitride and thermal silicon oxide

The emitter saturation current density (JOe) and surface recombination velocity (Sp) of various high quality passivation schemes on phosphorus-diffused solar cell emitters have been determined and compared. The passivation schemes investigated were (i) stoichiometric plasma enhanced chemical vapor deposited (PECVD) silicon nitride (SiN), (ii) forming gas annealed thermally grown silicon oxide, and (iii) aluminum annealed (alnealed) thermal silicon oxide. Emitters with sheet resistances ranging from 30 to 430 and 50 to 380 Ω/□ were investigated for planar and random-pyramid textured silicon surfaces, which covers both industrial and laboratory emitters. The electronic surface passivation quality provided by PECVD SiN films was found to be good, with Sp values ranging from 1400 to 25 000 cm/s for planar emitters. Thin thermal silicon oxides were found to provide superior passivation to PECVD SiN, with the best passivation provided by an alnealed thin oxide (Sp values between 250 and 21 000 cm/s). The optimized PECVD SiN films are, nevertheless, sufficiently good for most silicon solar cell applications.

Comprehensive study of rapid, low-cost silicon surface passivation technologies

A comprehensive and systematic investigation of low-cost surface passivation technologies is presented for achieving high-performance silicon devices such as solar cells. Most commercial solar cells today lack adequate surface passivation, while laboratory cells use conventional furnace oxides (CFO) for high-quality surface passivation involving an expensive and lengthy high-temperature step. This investigation tries to bridge the gap between commercial and laboratory cells by providing fast, low-cost methods for effective surface passivation. This paper demonstrates for the first time, the efficacy of TiO/sub 2/, thin (<10 nm) rapid thermal oxide (RTO), and PECVD SiN individually and in combination for (phosphorus diffused) emitter and (undiffused) back surface passivation. The effects of emitter sheet resistance, surface texture, and three different SiN depositions (two direct PECVD systems and one remote plasma system) were investigated. The effects of post-growth/deposition treatments such as forming gas anneal (FGA) and firing of screen printed contacts were also examined. This study reveals that the optimum passivation scheme consisting of a thin RTO with a SiN cap followed by a very short 730/spl deg/C anneal can 1) reduce the emitter saturation current density, J/sub 0e/, by a factor of >15 for a 90 /spl Omega//sq. emitter, 2) reduce J/sub 0e/ by a factor of >3 for a 40 /spl Omega//sq, emitter, and 3) reduce S/sub back/ below 20 cm/s on 1.3 /spl Omega/cm p-Si. Furthermore, this double-layer RTO+SiN passivation is relatively independent of the deposition conditions (direct or remote) of the SiN film and is more stable under heat treatment than SiN or RTO alone. Model calculations are also performed to show that the RTO+SiN surface passivation scheme may lead to 17%-efficient thin screen-printed cells even with a low bulk lifetime of 20 /spl mu/s.

Rapid thermal processing of next generation silicon solar cells

Rapid and potentially low-cost process techniques are analysed and successfully applied towards the fabrication of high-efficiency mono-and multicrystalline Si solar cells. First, a novel dielectric passivation scheme (formed by stacking a plasma silicon nitride film on top of a rapid thermal oxide layer) is developed that serves as antireflection coating and reduces the surface recombination velocity (S{sub eff}) of the 1.3 {omega}-cm p-Si surface to approximately 10 cm/s. The essential feature of the stack passivation scheme is its ability to withstand short 700 - 850degC anneal treatments used to fire screen printed (SP) contacts, without degradation in S{sub oeff}. The stack also lowers the emitter saturation current density (J{sub oe}) of 40 and 90 {omega}/{open_square} emitters by a factor of three and 10, respectively, compared to no passivation. Next, rapid emitter formation is accomplished by diffusion under tungsten halogen lamps in both belt line and rapid thermal processing (RTP) systems (instead of in a conventional infrared furnace). Third, a combination of SP aluminium and RTP is used to form an excellent back surface field (BSF) in 2 min to achieve an effective back surface recombination velocity (S{sub eff}) of 200 cm/s on 2.3 {omega}-cm Si. Finally, the above individual processes are integrated to achieve: (1) >19% efficient solar cells with emitter and Al-BSF formed by RTP and contacts formed by vacuum evaporation and photolithography, (2) 17% efficient manufacturable cells with emitter and Al-BSF formed in a belt line furnace and contacts formed by SP. (Author)

Understanding and implementation of rapid thermal technologies for high-efficiency silicon solar cells

Rapid and potentially low-cost process techniques are analyzed and successfully applied toward the fabrication of high-efficiency monocrystalline Si solar cells. First, a methodology for achieving high-quality screen-printed (SP) contacts is developed to achieve fill factors (FF's) of 0.785-0.795 on monocrystalline Si. Second, rapid emitter formation is accomplished by diffusion under tungsten halogen lamps in both beltline and rapid thermal processing (RTP) systems (instead of in a conventional infrared furnace). Third, a combination of SP aluminum and RTP is used to form an excellent back surface field (BSF) in 2 min to achieve an effective back surface recombination velocity (S/sub eff/) of 200 cm/s on 2.3 /spl Omega/-cm Si. Next, a novel dielectric passivation scheme (formed by stacking a plasma silicon nitride film on top of a rapid thermal oxide layer) is developed that reduces the surface recombination velocity (S) to approximately 10 cm/s on the 1.3 /spl Omega/-cm p-Si surface. The essential feature of the stack passivation scheme is its ability to withstand short 700-850/spl deg/C anneal treatments (like the ones used to fire SP contacts) without degradation in S. The stack also lowers the emitter saturation current density (J/sub oe/) of 40 and 90 /spl Omega//sq emitters by a factor of three and ten, respectively, compared to no passivation. Finally, the above individual processes are integrated to achieve (1) >19% efficient solar cells with emitter and Al-BSF formed by RTP and contacts formed by vacuum evaporation and lift-off, (2) 17% efficient manufacturable cells with emitter and Al-BSF formed in a beltline furnace and contacts formed by SP, and (3) 17% efficient gridded-back contact (bifacial) cells with surface passivation accomplished by the stack and gridded front and back contacts formed by SP and cofiring.

A simultaneously diffused, textured, in situ oxide AR-coated solar cell process (STAR process) for high-efficiency silicon solar cells

A novel device fabrication process called the STAR process is presented, which incorporates a Simultaneously diffused emitter and Back Surface Field (BSF), on a textured silicon wafer, with an in situ thermal oxide for surface passivation and Anti-Reflection (AR) coating. In a single high-temperature step, the STAR process provides four important quality-enhancement features: (1) emitter oxide passivation, (2) back surface passivation via a boron back surface field, (3) a low reflectance (SiO/sub 2/) single layer AR coating, and (4) a back surface reflector (BSR) for light trapping. The STAR process is implemented using a novel diffusion technique which can simultaneously form boron and phosphorus diffusions and grow an in situ thermal oxide in a conventional diffusion furnace, without the deleterious effects of cross doping. Conversion efficiencies as high as 20.1% have been obtained for this structure on 2.0 /spl Omega//spl middot/cm float zone silicon. This paper presents a detailed characterization of the impact of each of the above quality enhancement features, using a combination of an extended IQE analysis, minority carrier lifetime measurements, and measurements of the emitter saturation current density J/sub oe/. Computer simulations are used to improve the understanding of STAR cells and show that the STAR process is capable of producing device efficiencies over 19% on thin, relatively modest quality, solar grade silicon materials.