Interface Engineering for Liquid‐Phase Crystallized‐Silicon Solar Cells on Glass

Abstract

Liquid‐phase crystallization (LPC) of silicon is a suitable method to grow large grained poly‐crystalline silicon with wafer equivalent electronic quality on cheap glass substrates. Dielectric layers between glass and silicon (called interlayer) are not only crucial for the solar cell performance, but, they also provide wetting of the silicon during crystallization. So far, LPC‐Si samples based on interlayers grown with plasma‐enhanced chemical vapor deposition (PECVD) were annealed prior to crystallization to remove hydrogen in order to prevent delamination during crystallization. However, this step increases manufacturing time and disables integrated processing without extended vacuum breaks. In this work, PECVD‐grown stacks with silicon oxide and silicon nitride, that is, SiOx/SiNx/SiOx (O/N/O), were developed aiming a high cell performance and a successful crystallization without prior annealing step. We found that the SiNx layer significantly influences adhesion and that an O/N/O stack with a nitrogen‐rich SiNx layer in which hydrogen is only bonded to nitrogen, that is, no Si–H bonds, provides wetting without annealing step. The total amount of bonded hydrogen in the SiNx film was not crucial. Finally, based on the presented O/N/O stack, LPC‐Si solar cells with almost 630 mV open circuit voltage and a conversion efficiency of up to 13.2% were obtained.

Passivation of Liquid‐Phase Crystallized Silicon With PECVD‐SiNx and PECVD‐SiNx/SiOx

“…Silicon‐rich SiN x films behave similarly to amorphous silicon layers which provide a high level of chemical passivation but a rather low field‐effect passivation . The SiN x film that we use is nitrogen‐rich . Thus, it contains a high number of K + ‐centers (NSi•) that are responsible for the high number of Q IL,eff that we observed (4.5 × 10 12 cm −2 ).…”

“…The SiN x layer appears compact, well defined and no voids are detected. We attribute the smooth interface achieved with the O/N IL stack to the nitrogen‐rich character of the developed SiN x layer in which hydrogen is bonded to nitrogen only (no SiH bonds present) . This makes the layer more stable upon annealing as compared to silicon‐rich SiN x layers that exhibit SiH bonds …”

“…More details on the deposition parameters and the resulting layer characteristics are supplied in ref. . After the IL deposition, the samples were shortly (around 10 min) exposed to air in order to change the sample holder.…”

“…Apart from passivating the buried LPC‐Si surface toward the glass substrate, the ILs need to prevent dewetting of the absorber during crystallization, they have to impede impurity diffusion from the glass into the silicon, and they should enhance light coupling into the absorber when illuminating the cell through the glass side. To meet all these requirements, multi‐layer stacks based on, for example, amorphous silicon oxide (SiO x ), amorphous silicon nitride (SiN x ), amorphous silicon oxynitride (SiO x N y ), amorphous silicon carbide (SiC x ), and aluminum oxide (AlO x ) deposited by using plasma‐enhanced chemical vapor deposition (PECVD), reactive RF‐magnetron sputtering (PVD), or atomic layer deposition (ALD) were successfully integrated into LPC‐Si solar cells.…”

“…Previously, we demonstrated a SiO x /SiN x /SiO x (O/N/O) IL stack deposited with PECVD that allowed for adhesion during crystallization without prior annealing treatment. We attributed adhesion properties to the nitrogen‐rich character of the developed SiN x layer in which hydrogen is only bonded to nitrogen and not to silicon atoms . Based on the developed PECVD O/N/O IL stack we manufactured LPC‐Si solar cells with 13 μm thin, n‐doped absorbers and interdigitated back contacts that exhibited a conversion efficiency of 13.2% as well as a test cell with a full emitter contact having an efficiency of 15.9% .…”