Firing stability of tube furnace‐annealed n‐type poly‐Si on oxide junctions

Abstract

Stability of the passivation quality of poly-Si on oxide junctions against the conventional mainstream high-temperature screen-print firing processes is highly desirable and also expected since the poly-Si on oxide preparation occurs at higher temperatures and for longer durations than firing. We measure recombination current densities (J 0 ) and interface state densities (D it ) of symmetrical samples with n-type poly-Si contacts before and after firing. Samples without a capping dielectric layer show a significant deterioration of the passivation quality during firing. The D it values are (3 ± 0.2) Â 10 11 and (8 ± 2) Â 10 11 eV/cm 2 when fired at 620 C and 900 C, respectively. The activation energy in an Arrhenius fit of D it versus the firing temperature is 0.30 ± 0.03 eV. This indicates that thermally induced desorption of hydrogen from Si H bonds at the poly-Si/SiO x interface is not the root cause of depassivation. Postfiring annealing at 425 C can improve the passivation again. Samples with SiN x capping layers show an increase in J 0 up to about 100 fA/cm 2 by firing, which can be attributed to blistering and is not reversed by annealing at 425 C. On the other hand, blistering does not occur in poly-Si samples capped with AlO x layers or AlO x /SiN y stacks, and J 0 values of 2-5 fA/cm 2 can be achieved after firing. Those findings suggest that a combination of two effects might be the root cause of the increase in J 0 and D it : thermal stress at the SiO z interface during firing and blistering. Blistering is presumed to occur when the hydrogen concentration in the capping layers exceeds a certain level.

Monolithic Perovskite/Silicon Tandem Solar Cells Fabricated Using Industrial p‐Type Polycrystalline Silicon on Oxide/Passivated Emitter and Rear Cell Silicon Bottom Cell Technology

“…However, firing of POLO junctions without any degradation is even possible at low firing belt velocities, which can make additional H supply unnecessary. [ 18 ]…”

“…However, firing of POLO junctions without any degradation is even possible at low firing belt velocities, which can make additional H supply unnecessary. [18] After local ablation of the passivation stack on the rear side using a laser with 532 nm wavelength and pulses in the picosecond range, we screen print Al fingers on top of the laser contact openings. The wafers are fired in a conveyor belt furnace at 810 C set point temperature with a belt speed of 7.2 m min À1 .…”

In Situ Reflectometry and Diffraction Investigation of the Multiscale Structure of p-Type Polysilicon Passivating Contacts for c-Si Solar Cells

“…The detrimental impact of increasing the effective thermal budget of firing on the interface quality is in good agreement with observations made by Hollemann et al in a study published recently. 21 However, we note that in this study, they performed the firing step after previous activation of the poly-Si contact through a long-annealing step, whereas in the present study, the firing step was performed directly after deposition of the a-SiC x layer.…”

“…Peak temperature is only one component of the thermal profile. Several recent studies reported the degradation of the surface passivation of the poly-Si contact after firing (i.e., high-temperature thermal process featuring fast heating and cooling rates), suggesting that the thermal profile to which the poly-Si/SiO x contact is submitted strongly impacts its final structural and functional properties. , In the following, we discuss our current understanding of the effects of the heating ramp rate, dwell time, and overall thermal budget by comparing samples annealed in different setups as described in detail in the experimental section (Table ).…”

“…A prominent example of a key process step whose effect on the poly-Si final properties is not completely understood yet is the high-temperature annealing performed to “activate” the formation of the poly-Si contact. Other than crystallization and doping activation, such high-temperature annealing has been shown to cause a shallow diffusion of dopants into the underlying c-Si substrate (and thus through the thin SiO x layer) as well as chemical/structural changes of the thin SiO x . − Moreover, lately, several studies have emphasized the detrimental impact of firing on the surface passivation properties provided by poly-Si contacts. − It is worth noting that in the currently foreseen integration of poly-Si contacts in c-Si solar cells, it is most likely that poly-Si contacts will be submitted to a firing process applied at the very end of the solar cell fabrication to contact the metal paste to the c-Si and/or poly-Si layer. , Thus, it is of utmost importance to better understand the impact of such high-temperature processes on the final properties of poly-Si contacts.…”