Full-Area Passivating Hole Contact in Silicon Solar Cells Enabled by a TiO_x/Metal Bilayer

Abstract

Passivating contacts, featuring dual functions of defect passivation at the semiconductor surface and extracting one type of charge carrier, are recognized as the key enabler in achieving high-efficiency Si solar cells. In particular, a dopant-free and full-area passivating hole contact is critical to replace the conventional rear structure that features a partial Si-metal contact design with insulator interlayers. Herein, titanium oxide (TiO x ) nanolayers (∼5 nm) grown by atomic layer deposition over the full area of the Si surface followed by metal capping such as Ag are shown to provide efficient passivation and hole extraction with high optical reflectivity at the rear of Si solar cells. The proof-of-concept solar cells with either a p- or an n-Si absorber demonstrate ∼20% efficiency, exhibiting a higher infrared response compared with the conventional rear structure. Photoluminescence and electrical measurements on different TiO x /metal bilayers revealed that the field-effect passivation mechanism plays a major role in device performance, exploiting the high-concentration negative charge (>1012 q cm–2) at the Si/TiO x interface and the high work function (≥4.6 eV) of the capping metal. The developed contact offers great potential for boosting the efficiency and simplifying manufacturing of commercial Si solar cells.

Recombination in Passivating Contacts: Investigation Into the Impact of the Contact Work Function on the Obtained Passivation

“…The surface passivation is degraded with a 4.2 eV work function contact (Al) before improving again with a further decrease in the contact work function. Matsui et al [20] also reported a significant degradation of the surface passivation when using Al contacts for devices with p-type Si wafers, which is in good agreement with our results.…”

“…Significant degradation of the surface passivation was noticed when an Al layer was used for p ‐type devices. [ 20 ] These results indicate that the contact work function influences the surface recombination statistics despite the interlayer between the contact and the Si. However, no direct evidence, such as a variation of the surface saturation current density ( J 0s ) before and after metallization, has been presented, mainly since common lifetime measurement techniques have severe limitations when applied to metallized samples.…”

“…[ 12 ] The third method is based on the deposition of a material with a different work function compared to the c‐Si substrate, resulting in a higher concentration of one type of carrier near the interface. [ 20 ] Therefore, it is expected that the work function of the metal applied as a contact will impact the carrier concentrations near the interface and hence the surface recombination rate.…”

“…Van de Loo et al [ 23 ] reported variations in the injection‐dependent minority carrier lifetime ( τ eff ) of c‐Si wafers passivated by stacks of silicon dioxide, zinc oxide, and aluminum oxide (AlO x ) films as a function of the ZnO doping level. More relevant, using photoluminescence images, Matsui et al [ 20 ] showed that the work function of metal contacts deposited on a 5‐nm titanium oxide (TiO x ) layer (used as a passivating contact) significantly impacts the surface passivation quality regardless of the wafer polarity. Significant degradation of the surface passivation was noticed when an Al layer was used for p ‐type devices.…”

Addressing the stability challenges of TiOx-based passivating contacts for high-efficiency c-Si solar cells

“…Finally, while in this work we have focused on applications as an electron selective contact, it is worth noting that TiO x has also been demonstrated as an efficient hole-selective passivating contact in c-Si solar cells. 23,24,47 This manipulation of TiO x to operate as an efficient hole-selective contact would potentially allow the fabrication of novel architectures of c-Si solar cells, using TiO x /TCO stacks on both sides of Si to form heterojunction devices. However, in order to induce the TiO x /TCO stack to serve as efficient hole-selective contacts without signicant electrical losses (ll factor issues), the TCO material must have a high work function with high mobility and low sheet resistance.…”