Analysis of hydrogen distribution and migration in fired passivating contacts (FPC)

High temperature passivating contacts for c-Si based solar cells are intensively studied because of their potential in boosting solar cell efficiency while being compatible with industrial processes at high temperatures. In this work, the hydrogenation mechanism of fired passivating contacts (FPC) based on c-Si/SiOx/nc-SiCx(p) stacks was investigated. More specifically, the correlation between passivation and local re-distribution of hydrogen resulting from the application of different types of interfacial oxides (SiOx) and post-hydrogenation processes was analyzed. To do so, the applied processing sequence was interrupted at different stages in order to characterize the samples. To assess the hydrogen content, deuterium was introduced (alongside/instead of hydrogen) and secondary ion mass spectroscopy (SIMS) was used for depth profiling. Combining these results with lifetime measurements, the key role played by hydrogen in the passivation of defects at the c-Si/SiOx interface is discussed. The SIMS profiles show that hydrogen almost completely effuses out of the SiCx(p) during firing, but can be re-introduced by hydrogenation via forming gas anneal (FGA) or by release from a hydrogen containing layer such as SiNx:H. A pile-up of H at the c-Si/SiOx interface was observed and identified as a key element in the FPC's passivation mechanism. Moreover, the samples hydrogenated with SiNx:H exhibited higher H content compared to those treated by FGA, resulting in higher iVoc values. Further investigations revealed that the doping of the SiCx layer does not affect the amount of interfacial defects passivated by the hydrogenation process presented in this work. Eventually, an effect of the oxide's nature on passivation quality is evidenced. iVoc values of up to 706 mV and 720 mV were reached with FPC test structures using chemical and UV-O3 tunneling oxides, respectively, and up to 739 mV using a reference passivation sample featuring a ~25 nm thick thermal oxide.

Bulk Defects and Hydrogenation Kinetics in Crystalline Silicon Solar Cells With Fired Passivating Contacts

Christophe Ballif, Franz-Josef Haug, Andrea Ingenito, Mario Joe Lehmann, Audrey Marie Isabelle Morisset, Père Roca i Cabarrocas, Philippe Wyss

In this article, the effect of the various processing steps during the fabrication of c-Si/SiOx/SiCx fired passivating contacts on the silicon bulk lifetime is studied, and the kinetics of defect deactivation by hydrogenation is investigated. It is found that the firing step at 800 degrees C induces shallow bulk defects in float-zone silicon wafers, which can subsequently be passivated with hydrogen provided by an a-SiNx:H/D reservoir layer upon annealing at 450 degrees C. Experimental results and numerical data treatment indicate a rapid passivation of the surface within less than 1 min, followed by a slower passivation of the shallow bulk defects. In situ lifetime measurements are consistent with a slow bulk lifetime improvement by showing similar lifetime evolutions for both p-type and n-type SiCx layers. The kinetics of the hydrogenation process seems to be limited by the available hydrogen supply at the c-Si/SiOx interface, rather than by its diffusion within the bulk of the wafer. Moreover, it is affected by the bulk doping as well as the SiCx layer thickness. Finally, it is shown that hydrogenation is also possible with an a-SiNx:H/D reservoir layer deposited on one side of the wafer only, although resulting in a lower passivation level (s similar to 700 mu s compared to similar to 1300 mu s for symmetrical samples), and slower kinetics (similar to 5 min compared to similar to 0.8 min).

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2022

Characterization and Development of Fired Passivating Contacts for Silicon Solar Cells

Mario Joe Lehmann

One of the key elements to improve mainstream crystalline silicon (c-Si) solar cell performance is surface passivation, which is at the center of the ongoing transition from cells with direct silicon-metal contacts to full area passivating contacts. In theory, these contacts combine an optimal passivation and current extraction. Among those, the fired passivating contact (FPC) studied in this thesis is a promising candidate. Based on a 1.3 nm thin tunneling oxide capped with a doped SiCx layer, it is compatible with the high temperature firing process typically used in industry for metallization. Moreover, its fabrication does not require a long annealing step, reducing its thermal budget.In this thesis, effective minority carrier lifetime measurements were combined with advanced characterization techniques, such as Secondary Ion Mass Spectrometry (SIMS), to investigate the surface passivation mechanism. Chemical passivation was found to be predominant, achieved through hydrogen diffusion from a SiNx:H reservoir layer to the c-Si/SiOx interface where it accumulates. The composition of the interfacial oxide was found to influence the surface passivation quality too, reaching implied open circuit voltage (iVoc) values around 710, 720 and 740 mV with chemical, UV-O3 and thick thermal oxides, respectively. After firing, the boron-doped SiCx(p) layer was observed to consist of 5-10 nm large c-Si grains embedded in a carbon rich amorphous matrix.Surprisingly, it was found that the firing step can induce shallow bulk defects within the used Float-Zone (FZ) wafers, which can be passivated by hydrogen. Kinetics experiments indicate a rapid passivation of the c-Si/SiOx interface, happening in less than 1 min, followed by a slower passivation of the bulk. The kinetics of the hydrogenation process seem to be limited by the available hydrogen supply, rather than by its diffusivity within the bulk. In addition, the possibility to hydrogenate both surfaces with a SiNx:H layer deposited on only one side of the wafer was demonstrated, providing further flexibility to the fabrication process of FPC based solar cells.Pursuing the same goal, diffusion of fluorine from SiCx(p):F layers upon firing was tested. The results indicate that this approach can also passivate the c-Si/SiOx interface even though the iVoc after firing seems to be limited by the creation of shallow bulk defects. Nevertheless, values over 730 mV could be reached with UV-O3 SiOx layers upon subsequent hydrogenation.Regarding the stability of this passivation, no degradation was observed for FPCs upon 80 days of light soaking at 80°C. Moreover, it remained stable upon thermal treatments of 30 min up to 170°C.Finally, metallization of these FPCs was studied. A layer stack with a total thickness of ~90 nm was developed to be contacted by a screen printed Al paste fired through the SiNx:H layer. Unfortunately, so far good contact resistivities could only be achieved at the expense of passivation. Closer microstructural investigations revealed diffusion of Al from the metallic paste into the SiNx:H layer, forming an Al-N compound, and in some cases accumulating in the SiOx layer. Nevertheless, using a low temperature metallization approach, solar cells featuring co-fired SiCx(n) and SiCx(p) FPCs at the front and rear were processed, reaching up to 20.5% in efficiency. This corroborates the potential of FPCs for high efficiency solar cells with a simple and low-cost fabrication process.

EPFL2022

Transparent Passivating Contacts for Front Side Application in Crystalline Silicon Solar Cells

Josua Andreas Stückelberger

In this thesis, we present the development and characterization of novel approaches to carrier-selective passivating contacts for crystalline silicon (c-Si) solar cells. In our first approach, we benefit from a wide-bandgap material, namely mixed-phase silicon oxide (SiOx). High hydrogen dilution during the plasma-enhanced chemical vapor deposition of a SiOx layer leads to the growth of vertically oriented silicon filaments embedded in a silicon oxide matrix. The resulting mixed-phase material combines transparency and vertical conduction through the layer. We propose a layered contact structure starting with an ultra-thin chemically grown SiOx (~1.2 nm) at the c-Si wafer surface, followed by a phosphorus-doped bilayer of the mixed-phase SiOx, finishing with a nanocrystalline silicon layer that ensures contact with the metallization. With such a stack, an electron-selective passivating contact with a saturation current density J0 of 5.3 fA/cm2 is achieved on n-type wafers with hydrogenation, whereas on p-type wafers the contact reached 5.8 fA/cm2 even without hydrogenation. The contact is analyzed in detail with respect to its structure and its change upon thermal treatment. Two different growth regions are detected: one with the vertically oriented silicon inclusions and a second one in which the silicon phases have more lateral growth. Crystallization and atomic redistribution are analyzed during in-situ annealing in an electron microscope, leading to a better understanding of the change in distribution of phosphorus and oxygen as well as the nucleation sites. The influence of the annealing temperatures and dwell times as well as the initial doping concentration on the resulting in-diffused region and the passivation quality is reported. We use simulations to relate the influence of these parameters on passivation to the different recombination mechanisms. From the analyzed phosphorus doping profiles, an interesting observation is reported. The chemical oxide varies in thickness and density depending on the wafer polarity as well as the base resistivity. Proof-of-concept cells including this novel structure demonstrate the suitability for electron-selective passivating contacts at the device level with conversion efficiencies of 19.0% on planar wafers and 20.1% on textured wafers. Promisingly high short-circuit current densities (Jsc) of 35 mA/cm2 (40 mA/cm2) on planar (textured) with low parasitic absorption indicate its potential as a front contact in next-generation solar cells. Additionally, the beneficial feature of the smooth refractive index change within the layers that leads to a low reflectance over a broad wavelength range, similar to a built-in anti-reflection coating, is presented. A second approach to reach highly transparent electron-selective passivating contacts is demonstrated by the use of a plasma process based on the precursor gases phosphine (PH3), silicon tetrafluoride (SiF4), hydrogen (H2), and argon (Ar). The beneficial effect of fluorine in terms of passivation without the need for additional hydrogenation processes is shown, reaching implied open-circuit voltages of 730 mV on planar surfaces and 707 mV on textured surfaces. These layers are implemented in first proof-of-concept working devices with efficiencies of 19.5% on single-side-textured wafers and Jsc values of up to 40 mA/cm2.