Phosphorus-doped Emitter Research Articles

Silicon solar cells with interfacial passivation of the highly phosphorus-doped emitter surface by oxygen ion implantation

We investigated oxygen passivation at the interfacial silicon surface on the highly phosphorus-doped region by oxygen ion implantation and showed resulting enhanced electrical properties of silicon solar cells. The oxygen ion implantation converts the dead layer formed due to crystallographic defects in the doping process into an oxygen-induced insulation layer by fragmentation with an optimal amount of energy. The surface dead layer of the highly doped Si emitter was broken by oxygen ions with a fixed-dose and different energies. The optimized implanted oxygen ion energy shows an improved short circuit current after implantation and improved efficiency with enhanced quantum efficiency in the blue region. On the other hand, high energy implanted oxygen ions formed surface defects, resulting in decreased cell efficiency.

Efficiency Roadmap for Evolutionary Upgrades of PERC Solar Cells by TOPCon: Impact of Parasitic Absorption

Passivating contacts created via a thin interfacial oxide and a highly doped polysilicon layer, e.g., the TOPCon technology, are on the verge of being implemented in solar cell mass production. Investment decisions rely on R&D to identify the most promising implementation option, meaning a trustworthy roadmap based on predicted performance gains. This article shows how to thoroughly quantify the performance potential via numerical simulation, focusing on an evolutionary upgrade of a busbarless p-type bifacial passivated emitter rear cell (PERC) technology. We specifically highlight the need to consider not only the electrical gains of passivating contacts, but also the associated optical losses due to parasitic absorption in the polysilicon layers for front and rear illumination. The influence of free-carrier absorption in polysilicon on the solar cell optics is characterized on experimental test structures in order to verify our optical simulation model. Introducing TOPCon fully at the rear and also locally aligned to the front fingers can boost the PERC efficiency by approximately 1%abs. The final device is strongly limited by losses in the p-type c-Si bulk and phosphorus-doped front emitter. Consequently, the presented evolutionary TOPCon upgrades may well be of increased relevance for future improved p-PERC cells, as an alternative to the current focus on n-type TOPCon cells with boron emitter.

Effective Contact Formation Method on High-Sheet-Resistance Boron-Doped Emitter With Current Injection

We investigate the effect of current injection during contact formation of an Ag-based screen-printed electrode to boron-doped emitters, which differ by their sheet resistances. The average contact resistivities between the metal electrode and silicon of all the boron-doped emitter samples are ∼3 mΩ cm2, regardless of the sheet resistance (75–145 Ω/sq), and the lowest values are below 1 mΩ cm2 using the injection of a current density of 5 A/cm2 during the metallization process. Additionally, the injection of current to a phosphorus-doped emitter in the opposite direction suppressed the formation of the Ag precipitates and crystallites and increased the contact resistivity of over 300 mΩ cm2, which is comparable to that obtained when Ag paste is applied to a boron-doped emitter with no current injection. This finding indicates that electrons are essential for the reduction of Ag ions during high-temperature metallization process using the screen-printing technique and that the injection of current can control the contact formation and enhance the efficiency of solar cells. Finally, we suggest a suitable process for reducing the contact resistivity in manufacturing n-type Si solar cells.

Cu gettering by phosphorus-doped emitters in p-type silicon: Effect on light-induced degradation

The presence of copper (Cu) contamination is known to cause relevant light-induced degradation (Cu-LID) effects in p-type silicon. Due to its high diffusivity, Cu is generally regarded as a relatively benign impurity, which can be readily relocated during device fabrication from the wafer bulk, i.e. the region affected by Cu-LID, to the surface phosphorus-doped emitter. This contribution examines in detail the impact of gettering by industrially relevant phosphorus layers on the strength of Cu-LID effects. We find that phosphorus gettering does not always prevent the occurrence of Cu-LID. Specifically, air-cooling after an isothermal anneal at 800°C results in only weak impurity segregation to the phosphorus-doped layer, which turns out to be insufficient for effectively mitigating Cu-LID effects. Furthermore, we show that the gettering efficiency can be enhanced through the addition of a slow cooling ramp (-4°C/min) between 800°C and 600°C, resulting in the nearly complete disappearance of Cu-LID effects.

Fundamental consideration of junction formation strategies for phosphorus-doped emitters with J0e < 10 fA/cm2

This work shows the potential of further optimization of phosphorus-doped emitters in p-type silicon solar cells. We investigate the impact of different combinations of phosphorus doping profiles and surface passivation qualities on the saturation current density J0e by considering boundary conditions based on published experimental data. Our simulation study shows that there are two possible ways to achieve J0e values below 10 fA/cm2. One is the reduction of the electrically active phosphorus concentration nsurf at the surface beneath 2×1019 cm−3 and simultaneously reducing the surface recombination velocity Sp to below 103 cm/s. The other contrarily increases nsurf to values of up to 1×1021 cm−3 while ensuring full activation of all phosphorus dopants. In the latter case, J0e values below 10 fA/cm2 seem possible, even for Sp = 107 cm/s which is equal to the thermal velocity.

Comparison of POCl3 diffusion and phosphorus ion-implantation induced gettering in crystalline Si solar cells

Most p-type Si solar cells involves phosphorus-doped emitter by POCl3 diffusion or phosphorus ion-implantation. Although the formation of the phosphorus emitter is known to getter impurities like Fe, the difference in the impact of these two gettering techniques on cell performance is not well quantified. Therefore, this paper compares the gettering efficiency of POCl3 diffusion and phosphorus ion-implantation on Czochralski(Cz) and cast quasi-mono Si wafers. Cz-Si wafers were used to measure bulk lifetime and iron concentration before and after POCl3 diffusion and phosphorus implantation with different doses. Increase in phosphorus implantation dose improved the gettering efficiency by increasing bulk lifetime and decreasing iron concentration but remained inferior to POCl3 diffusion partly due to single side gettering as opposed to double side gettering with higher phosphorus surface concentration during POCl3 diffusion. Moreover, large-area solar cells were fabricated on cast quasi-mono and Cz-Si wafers to quantify the impact of these emitters on cell parameters. POCl3 diffused cast quasi-mono cells showed ~0.4% higher efficiency due to higher bulk lifetime compared to phosphorus implanted emitter. However, phosphorus implanted Cz cells gave ~0.3% higher efficiency due to lower emitter saturation current, resulting from the benefit of in-situ oxide surface passivation during the implant anneal.

Influence of Particle Velocity of Copper on Emitter Contact by Cold-Spray Method

In this study, we investigated cold-sprayed copper as a front contact for crystalline silicon solar cells. Copper powder was deposited on a monocrystalline silicon wafer with variation of the particle velocity during deposition. The particle velocity was varied by varying the heating temperature from 250 to 400 °C using a gas pressure of 0.45 MPa. The particle velocities were calculated using empirical equations, and were found to increase with an increase in the carrier gas temperature. Grid patterns were formed on a phosphorus-doped n-type emitter of a p-type silicon substrate. The electrode thickness increased with increasing particle velocity. The electrical properties of the grids were evaluated using the transfer length method. The specific contact resistance of the n-type emitter was in the range of 2.6-26.4 mΩ-cm2. Damage to the p-n junction was investigated via minority carrier lifetime measurement of the substrate. The copper-silicon interface was evaluated using transmission electron microscopy. The contact properties were affected by the interface conditions.

Phosphorus emitter engineering by plasma-immersion ion implantation for c-Si solar cells

Ion Beam Services (IBS) has developed processes dedicated to silicon-based solar cell manufacturing using a plasma-immersion ion implantation equipment. It enables the realization of various doping profiles for phosphorus-doped emitters which fit the requirements of high-efficiency solar cells. PH3 plasma-implanted emitters are chemically, physically and electrically characterized to demonstrate their excellent quality. Those emitters are then integrated into a low cost p-type monocrystalline silicon solar cell manufacturing line from the National Solar Energy Institute (INES) in order to be compared with usual POCl3 diffusion. Starting from a basic process flow with blanket emitter and conventional full-area aluminum back-surface field, plasma-immersion implanted emitters enable to raise conversion efficiencies above 19.1%. Thanks to an optimized double layer anti-reflective coating, a 19.4% champion cell has been achieved. Depending on different plasma process parameters, lightly doped emitters are then engineered aiming to study doping modulation using a dedicated laser.

Heavily doped Si:P emitters of crystalline Si solar cells: recombination due to phosphorus precipitation

The measured saturation current density J0e of heavily phosphorus-doped emitters of crystalline Si solar cells is analysed by means of sophisticated numerical device modelling. It is concluded that Shockley–Read–Hall (SRH) recombination exceeds Auger recombination significantly; it is caused by inactive phosphorus. This explains the large discrepancies between measured and simulated J0e values, observed persist-ently over the last two decades in industrially fabricated Si solar cells. As a consequence, the heavily phosphorus-diffused emitters still bear a significant potential to contribute to higher Si solar cell efficiency levels, if the amount of inactive phosphorus can be reduced. (© 2014 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

A numerical simulation study of gallium-phosphide/silicon heterojunction passivated emitter and rear solar cells

The performance of passivated emitter and rear (PERC) solar cells made of p-type Si wafers is often limited by recombination in the phosphorus-doped emitter. To overcome this limitation, a realistic PERC solar cell is simulated, whereby the conventional phosphorus-doped emitter is replaced by a thin, crystalline gallium phosphide (GaP) layer. The resulting GaP/Si PERC cell is compared to Si PERC cells, which have (i) a standard POCl3 diffused emitter, (ii) a solid-state diffused emitter, or (iii) a high efficiency ion-implanted emitter. The maximum efficiencies for these realistic PERC cells are between 20.5% and 21.2% for the phosphorus-doped emitters (i)–(iii), and up to 21.6% for the GaP emitter. The major advantage of this GaP hetero-emitter is a significantly reduced recombination loss, resulting in a higher Voc. This is so because the high valence band offset between GaP and Si acts as a nearly ideal minority carrier blocker. This effect is comparable to amorphous Si. However, the GaP layer can be contacted with metal fingers like crystalline Si, so no conductive oxide is necessary. Compared to the conventional PERC structure, the GaP/Si PERC cell requires a lower Si base doping density, which reduces the impact of the boron-oxygen complexes. Despite the lower base doping, fewer rear local contacts are necessary. This is so because the GaP emitter shows reduced recombination, leading to a higher minority electron density in the base and, in turn, to a higher base conductivity.

Use of QSSPC and QSSPL to Monitor Recombination Processes in P-type Silicon Solar Cells

This paper reports the use of injection-dependent local ideality factors, obtained from quasi-steady state photoconductance and photoluminescence measurements, to investigate the effects of various cell processing steps on recombination in solar cells, fabricated using boron-doped Czochralski silicon wafers, with an ∼ 100Ω/□ phosphorus-doped emitter and silicon nitride passivation of both surfaces. It is shown that activation of boron-oxygen complexes in the cells by light soaking can impact the pseudo fill factor and is manifest in increased local ideality factors in the injection range between maximum power point and open circuit voltage. The introduced recombination was modeled as a single Shockley Reed Hall (SRH) recombination centre at Ec - Et = 0.41eV and an electron:hole capture cross section ratio of ∼14. The effects of boron and phosphorus laser doping on the injection-dependent local ideality factor were also investigated. Boron laser doping was shown to introduce additional recombination in cells, as indicated by the increased local ideality factor in the injection range between maximum power point and open circuit voltage. However, in this case, the additional recombination was not well-modelled by a single SRH recombination centre, especially in the mid-injection range. Finally, it is shown that high temperature belt furnace anneals can place cells into a specific recombination state, and cells can be returned to this state by subsequent anneals even after additional recombination is introduced into cell by processes like laser doping.

Limitation of Industrial Phosphorus-diffused Emitters by SRH Recombination

It is commonly assumed in the PV community that Auger recombination is the dominant loss mechanism in heavily phosphorus-doped emitters of industrial Si solar cells. Contrary to this assumption, we show in this work that most of the losses are caused by Shockley-Read-Hall (SRH) recombination via defect states introduced by inactive phosphorus. Using numerical device simulations, we successfully reproduce all the measured J0e values of a series of profiles (with various concentrations of inactive phosphorus) with an effective SRH capture crosses-section for holes, σp = 7.5×10-18 cm2. The composition of inactive phosphorus (e.g. in form of clusters of interstitial phosphorus, fine and rod-shaped precipitates) may vary under different fabrication conditions, hence we do not claim that this σp value is globally valid. However, because the series of profiles is representative for many emitters in mass production, our result implies generally higher SRH than Auger recombination rates and has decisive implications on the improvement of such industrial emitters: many emitters have not yet reached the Auger-limit and they may still be significantly improved by reducing their density of inactive phosphorus.

Large-Area Diode Laser Defect Annealing of Polycrystalline Silicon Solar Cells

A new process was presented to anneal crystallographic defects in solid-phase crystallized silicon that produces higher Suns-V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> voltages than the conventional belt-furnace annealing (BFA) process. A few-millisecond continuous-wave diode laser treatment anneals defects present in the polycrystalline silicon and activates dopants. It is shown that the silicon/glass-interface system reaches an effective steady state at every point during the laser treatment, and the relative temperature profile is determined by the laser beam profile rather than its power or scanning speed. The peak temperature that is reached during a few-millisecond laser exposure is shown to increase and then level off with laser dose, indicating partial recrystallization of the film. A reduction in boron-doped p-type sheet resistance and phosphorus-doped emitter resistance is reported for a wide range of laser dose and scanning speed. The highest substrate temperature required during the process sequence is reduced from 960°C to 620°C. A peak 1-sun voltage of 492 mV is achieved using a 4-ms exposure at 500-J·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> laser dose, which is an improvement of 32 mV over the voltage after optimized BFA.

Optimization of Rear Point Contact Geometry by Means of 3-D Numerical Simulation

In this work three-dimensional (3-D) numerical simulations, validated by the experimental measurements of a reference cell, have been performed to optimize the rear contact geometry of a PERC-type solar cell, featuring a high sheet resistance (140Ω/sq) phosphorus-doped emitter and a front-side metallization with narrow and highly-conductive electro-plated copper lines (40μm wide) on lowly resistive Ti contacts. The simulation results show that an optimization of the rear point contact design potentially leads to an efficiency improvement of 0.68%abs compared to the reference cell.

Modelling carrier recombination in highly phosphorus-doped industrial emitters

Abstract One important parameter for modelling emitter recombination is the surface recombination velocity (SRV), which strongly depends on the surface doping concentration and the applied surface passivation. However, for highly phosphorus-doped surfaces with concentrations in excess of 2×1020 cm-3, where not all of the dopant is electrically activated, data is hardly available in the literature. Moreover, the bulk carrier lifetime in such supersaturated near surface regions is unknown. We use an analytical model to describe silicon-nitride-passivated phosphorus-diffused emitters. The model shows excellent agreement with a recently presented numerical solver, deviating less than 1%. In both cases we apply a Fermi-Dirac statistics correction and account for band gap narrowing to calculate the intrinsic carrier density. Our results from measurements of the emitter dark saturation current density indicate that either the SRV or the local carrier lifetime in the supersaturated region are strongly affected by the doping concentration, even if it exceeds the dopant activation limit by far. Assuming only Auger recombination in the supersaturated region, we derive an upper limit for the SRV that depends on the chemical phosphorus surface concentration.

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.

Record fast thermal processing of 17.5% efficient silicon solar cells

This paper presents the development and analysis of 17.5% efficient silicon solar cells on 0.9 Ω cm PV-grade Czochralski silicon (Cz–Si), featuring a record small thermal process time of 1 min for diffusion and oxidation. The cells have been processed by rapid thermal processing (RTP) using incoherent light as an energy source. The involved processes consist of a 5 s RTP step at 930 °C for the simultaneous formation of the phosphorus-doped emitter and of the aluminium back surface field (Al–BSF) and a 30 s rapid thermal oxidation (RTO) at 950 °C for the emitter surface passivation. Both processes feature high heating and cooling rates of 100 K s−1. The results show that no lengthy gettering or post diffusion annealing steps are required for the fabrication of high efficiency Cz–Si solar cells and that high heating and cooling rates are applicable. Detailed analysis reveals that the emitter saturation current J0e is reduced by a factor of 3–4 after the RTO passivation in spite of the high carrier surface concentrations of 3 to 4 × 1020 cm−3. Furthermore, one-dimensional device simulations are presented which show that the achieved efficiencies are limited by the effective back surface recombination velocity Sback resulting from the thin Al–BSF applied.

Characterization of phosphorus-doped and boron-doped diamond-like carbon emitter arrays

We synthesized phosphorus-doped and boron-doped emitters by using trimethylphosphite P(OCH3)3 and trimethylborate B(OCH3)3 as doping sources in a microwave plasma chemical vapor deposition system. Based on our experimental results from scanning electron microscopy and Raman spectra, there is much difference among undoped, phosphorus-doped, and boron-doped diamondlike material. In addition, doping both phosphorus and boron can enhance electric properties by reducing the turn-on voltage and can increase the emission current density. The turn-on voltages of undoped, boron-doped, and phosphorus-doped emitters in triode-type field emitter arrays are 15, 8, and 5 V, respectively. The emission currents of boron-doped and phosphorus-doped emitters are about 20 and 80 times larger than the undoped.

Application of an improved band-gap narrowing model to the numerical simulation of recombination properties of phosphorus-doped silicon emitters

The commonly used band-gap narrowing (BGN) models for crystalline silicon do not describe heavily doped emitters with desirable precision. One of the reasons for this is that the applied BGN models were empirically derived from measurements assuming Boltzmann statistics. We apply a new BGN model derived by Schenk from quantum mechanical principles and demonstrate that carrier degeneracy and the new BGN model both substantially affect the electron–hole product within the emitter region. Simulated saturation current densities of heavily phosphorus-doped emitters, calculated with the new BGN model, are lower than results obtained with the widely used empirical BGN model of del Alamo.

A low-complexity 62-GHz fT SiGe heterojunction bipolar transistor process using differential epitaxy and in situ phosphorus-doped poly-Si emitter at very low thermal budget

A low-complexity SiGe heterojunction bipolar transistor process based on differential epitaxy and in situ phosphorus doped polysilicon emitter technology is described. Silane-based chemical vapor deposition at reduced pressure was used for low-temperature SiGe epitaxy. Following SiGe epitaxy, the process temperature budget was kept very low with 900°C for 10 s as the highest temperature step. A very high current gain of almost 2000 and cut-off frequency of 62 GHz were achieved for a uniform 12% Ge profile. The breakdown voltage BV CEO and forward Early voltage were equal to 2.9 and 6.5 V, respectively.