Amir Dastgheib-Shirazi

Minimizing the electrical losses on the front side: Development of a selective emitter process from a single diffusion

In this paper we present latest results in the development of a process for the fabrication of a selective emitter structure on mono- and multicrystalline silicon solar cells. The process is based on an approach that was first introduced by Zerga et al. [1]. We have chosen a wet chemical route for an emitter etch back where the areas of the wafer that are intended for emitter metallization are shielded from etching by a screen printable etch barrier. The etch barrier is later removed by wet chemical etching. The process has yielded a gain in open circuit voltage of more than 1% and a gain in short circuit current of more than 2%. The overall efficiency gain was more than 0.3%abs due to slightly lower fill factor of the cells.

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 c...

Optimizing phosphorus diffusion for photovoltaic applications: Peak doping, inactive phosphorus, gettering, and contact formation

The phosphosilicate glass (PSG), fabricated by tube furnace diffusion using a POCl3 source, is widely used as a dopant source in the manufacturing of crystalline silicon solar cells. Although it has been a widely addressed research topic for a long time, there is still lack of a comprehensive understanding of aspects such as the growth, the chemical composition, possible phosphorus depletion, the resulting in-diffused phosphorus profiles, the gettering behavior in silicon, and finally the metal-contact formation. This paper addresses these different aspects simultaneously to further optimize process conditions for photovoltaic applications. To do so, a wide range of experimental data is used and combined with device and process simulations, leading to a more comprehensive interpretation. The results show that slight changes in the PSG process conditions can produce high-quality emitters. It is predicted that PSG processes at 860 °C for 60 min in combination with an etch-back and laser doping from PSG layer results in high-quality emitters with a peak dopant density Npeak = 8.0 × 1018 cm−3 and a junction depth dj = 0.4 μm, resulting in a sheet resistivityρsh = 380 Ω/sq and a saturation current-density J0 below 10 fA/cm2. With these properties, the POCl3 process can compete with ion implantation or doped oxide approaches.

A model for phosphosilicate glass deposition via POCl3 for control of phosphorus dose in Si

Effective control of the dose of diffused phosphorus emitter profiles is crucial for optimization of crystalline silicon solar cells, but it requires detailed understanding of the POCl3 doping process. We measure concentration profiles within the deposited phosphosilicate glass (PSG) layer for a range of POCl3 doping conditions and find that (i) its composition is nearly independent of process conditions and (ii) it is separated from Si by a thin SiO2 layer. We also find strong accumulation of P at the SiO2-Si interface. As common linear-parabolic models cannot fully explain the observed kinetics of PSG thickness and phosphorus dose in Si, we present an improved model including oxygen depletion and dose saturation, giving a better explanation of the experimental data. In contrast to previous models that adjust the peak phosphorus concentration at the Si surface to match the measured profiles, our models accurately predict the time-dependent dose behavior under different experimental conditions. We further...

Improving the predictive power of modeling the emitter diffusion by fully including the phosphsilicate glass (PSG) layer

Presently, the PV industry is switching to the selective emitter design, where the phosphorus density is significantly reduced between the front metal fingers. Current diffusion models simply adjust the peak phosphorus density at the Si surface to match the measured profiles, and therefore they are unable to predict the necessary gas flows and the temperatures during predeposition and drive-in to realize an optimum emitter profile. In order to achieve better prediction capabilities, we implement a model for the phosphosilicate glass (PSG) layer and for its coupling to silicon. We combine this model with coupled dopant/defect diffusion models in Si to calculate the resulting dopant profiles. With our improvements, we reproduce the profile measurements for a range of POCl3 flows at temperatures typically chosen in industrial fabrication.

The etchback selective emitter technology and its application to multicrystalline silicon

We have developed a simple and industrially applicable selective emitter cell process using only one diffusion step and an emitter etchback to create the high sheet resistance emitter [1, 2]. The process generates a deeper doping profile with a lower surface phosphorous concentration than a directly diffused emitter with the same sheet resistance. This results in an extremely low emitter saturation current j0E even at a moderate sheet resistance of 60–80 Ω/□. The highest independently confirmed cell efficiency on Cz-Si (146 cm2 was 18.7%. In this work the etching behavior of the acidic solution at the grain boundaries is studied by SEM imaging and high resolution LBIC measurements at 405 nm wavelength. The etchback also leads to a change in reflectivity, which is quantified by reflectance measurements. We furthermore investigate the influence of the base material quality on the gain that can be achieved by this process. Large area solar cells have been processed from solar grade and UMG mc silicon.

Effects of process conditions for the n + -emitter formation in crystalline silicon

Nowadays new solar cell concepts are continually attracting the attention of the PV industry. Thereby emitter structures and the application of high performance emitters like the homogeneous and etched-back emitter on crystalline p- and n-type silicon solar cells continue to be very popular [1]-[4]. In this work we study the influence of process parameters on the phosphosilicate glass layer characteristics during the predeposition of a POCl3 diffusion process. The quantitative analysis of the highly doped layer gives a deeper understanding of the phosphorus diffusion process for industrial emitter structures.

Impact of Extended Contact Cofiring on Multicrystalline Silicon Solar Cell Parameters

During the temperature spike of the contact cofiring step in a solar cell process, it has been shown that the concentration of lifetime-killer dissolved metallic impurities increases, while adding an annealing after the spike getters most of the dissolved impurities toward the phosphorus emitter, where they are less detrimental. The contact cofiring temperature profile, including the after-spike annealing, has been called extended contact cofiring, and it has also been proposed as a means to decrease the emitter saturation current density of highly doped emitters, thus benefiting a wide range of materials in terms of detrimental impurity content. The aim of the present work is to determine the effect of performing this additional annealing on contact quality and solar cell performance, looking for an optimal temperature profile for reduction of bulk and emitter recombination without affecting contact quality. It presents the effect of the extended cofiring step on fill factor, series resistance, and contact resistance of solar cells manufactured with different extended cofiring temperature profiles. Fill factor decreases when extended cofiring is performed. Series resistance and contact resistance increase during annealing, and this happens more dramatically when the temperature peak is decreased. Scanning electron microscopic images show silver crystallites in contact with silver bulk before the annealing that allow a direct current path, and silver crystallites totally surrounded by glass layer (>100 nm thick) after annealing. Glass layer redistribution and thickening at low temperatures at the semiconductor-metal interface can be related to the series resistance increase. Degradation of series resistance during the temperature spike, when it is below the optimum one, can also be attributed to an incomplete silicon nitride etching and silver crystallite formation. To make full use of the beneficial effects of annealing, screen-printing metallic paste development supporting lower temperatures without a thick glass layer growth is needed.

Analyzing emitter dopant inhomogeneities at textured Si surfaces by using 3D process and device simulations in combination with SEM imaging

The lowering of the phosphorus dopant density in the emitter of Si solar cells is a current topic in the photovoltaic industry. In lowly-doped emitters, diffusion inhomogeneities between the tops of the pyramids and the valleys affect the saturation current density J0. We quantify diffusion inhomogeneities by means of 3D process simulations, and we evaluate J0 by means of 3D device simulations. Finally, we compare the simulated diffusion results with a 2D dopant-contrast analysis obtained with a scanning electron microscope (SEM). Both methods show a deeper in-diffusion at the top of the pyramid and a shallower in-diffusion in the valley regions. Within the pyramidal faces, the diffusion is between both of these extreme points and comparable with in-diffusion at planar structures. The results of the device simulations indicate that the increase of J0 from planar to textured surfaces depends on the dopant profile and the surface passivation, but that a factor of about 5 is observed in our example, as is observed experimentally.

Understanding coupled oxide growth and phosphorus diffusion in POCl 3 deposition for control of phosphorus emitter diffusion

Effective control of diffused phosphorus profiles in crystalline silicon requires detailed understanding of the doping process. We analyze concentration profiles within the deposited phosphosilicate glass (PSG) for a range of POCl3 conditions and develop a model to account for the experimentally observed time dependence of PSG thickness and dose of phosphorus in Si. A simple linear-parabolic model cannot fully explain the kinetics of thickness and dose; while an improved growth model including oxygen dependence and dose saturation gives better fits to the experiments. We further couple the growth model with phosphorus diffusion and deactivation models in silicon and provide full modeling of the POCl3 doping process.