Electronic Characteristics of Ultra‐Thin Passivation Layers for Silicon Photovoltaics

Abstract

on PERC technologies and get even closer to the theoretical single-junction efficiency limit, electrical losses in the contacted regions must be reduced. [3][4][5] Passivating contacts can help alleviate such losses by simultaneously suppressing the current of non-collected carriers to the contact, and by reducing recombination sites at the interface. Introducing a passivating interlayer between the metal/silicon interface provides a route to reducing the recombination current density, J 0 , [6,7] thereby increasing device voltage. [3] Passivating contacts have achieved some success to date, with the strongest candidates being polysilicon on top of thin silicon oxide layers (e.g., tunnel oxide passivating contacts (TOPCon) or poly silicon on oxide (POLO)) and amorphous silicon (a-Si) heterojunctions. [3,7,8] TOPCon is an efficient electron-selective contact but has a high thermal budget with temperatures around 900 °C needed to reduce the contact resistivity to acceptable levels. [9] An efficient hole-selective layer that can match or exceed the performance of the current electron-selective materials would be of considerable interest. The use of SiO 2 -based hole-selective contacts has so far failed to reach equivalent levels. [10,11] The most promising hole-selective contacting materials are p-type a-Si and siliconrich SiC, but conventional high-temperature Ag screen printing methods are not necessarily compatible with such contacts. [10] Surface passivating thin films are crucial for limiting the electrical losses during charge carrier collection in silicon photovoltaic devices. Certain dielectric coatings of more than 10 nm provide excellent surface passivation, and ultra-thin (<2 nm) dielectric layers can serve as interlayers in passivating contacts. Here, ultra-thin passivating films of SiO 2 , Al 2 O 3 , and HfO 2 are created via plasma-enhanced atomic layer deposition and annealing. It is found that thin negatively charged HfO 2 layers exhibit excellent passivation properties-exceeding those of SiO 2 and Al 2 O 3 -with 0.9 nm HfO 2 annealed at 450 °C providing a surface recombination velocity of 18.6 cm s −1 . The passivation quality is dependent on annealing temperature and layer thickness, and optimum passivation is achieved with HfO 2 layers annealed at 450 °C measured to be 2.2-3.3 nm thick which give surface recombination velocities ≤2.5 cm s −1 and J 0 values of ≈14 fA cm −2 . The superior passivation quality of HfO 2 nanolayers makes them a promising candidate for future passivating contacts in high-efficiency silicon solar cells.

Mechanisms of Silicon Surface Passivation by Negatively Charged Hafnium Oxide Thin Films

“…HfO 2 has the potential to become a useful passivation layer for photovoltaics as it can be grown by established ALD processes and can have either positive or negative charge polarity [7], [8], [9], [10], [11], [12], [13]. The benefits of HfO 2 as a passivation layer are especially apparent for ultrathin films below 5 nm, where it has been shown to outperform Al 2 O 3 [14]. This advantage suggests a potential application for HfO 2 films in passivating contact structures.…”

Stable chemical enhancement of passivating nanolayer structures grown by atomic layer deposition on silicon

“…To determine the viability of Al 2 O 3 capping layers for nanolayer HfO 2 , three film structures were deposited at 200 °C onto 5 Ω cm 125–150 μm thick n -type Cz silicon wafers: (1) 1 nm HfO 2 (10 ALD cycles, denoted Si/HfO 2 . The growth rate of HfO 2 under the conditions used in this work was characterised in our previous work 13 ); (2) 1 nm HfO 2 capped with 30 nm Al 2 O 3 (250 ALD cycles, denoted Si/HfO 2 /Al 2 O 3 ); and (3) 30 nm Al 2 O 3 (denoted Si/Al 2 O 3 ). Samples were annealed (“activated”) in air ambient in a tube furnace at temperatures between 350–600 °C for 30 min to improve the passivation.…”

“…7 Al 2 O 3 passivates extremely well when relatively thick (>10 nm) but passivates poorly when thin (<1 nm). 13 There is a strong motivation to develop dielectrics with thicknesses at the single nanometre scale which also passivate well for new passivating contact structures.…”

“…We have recently studied passivation by ultra-thin HfO 2 films and determined SRVs < 2.5 cm s −1 for 2.2–3.3 nm thick films, with a negative fixed charge density on the order ∼10 12 q cm −2 . 13 Although this is promising and better than Al 2 O 3 of similar thickness, when the HfO 2 thickness is reduced to ∼1 nm – as required for passivating contacts – the SRV is ∼19 cm s −1 . Work in this current paper aims to enhance this.…”

SiNx and AlOx Nanolayers in Hole Selective Passivating Contacts for High Efficiency Silicon Solar Cells

“…While Kaur achieved a significantly lower J0 of 12.8 fA/cm 2 for ALD AlOx hole contact using a n-type wafer with a boron diffusion from a p + poly-Si [34] and 40 fA/cm 2 for an LPCVD SiNx on an n-type wafer before poly-Si deposition [36]. DFPCs incorporating AlOx into a dielectric stack have so far achieved an SRV of 20 cm/s [76] and J0 of 580 fA/cm 2 [75], while single AlOx nanolayers have reached 100 fA/cm 2 with just 10 ALD cycles [77]. From the small collection of work on AlOx and SiNx nanolayers, it can be seen that both can provide reasonable passivation, while maintaining low resistivity, however, a J0 as low as 2 fA/cm 2 is required to match that achieved for SiOx p-type poly-Si contacts.…”