Passivating Contacts

Impact of metal work-function on surface recombination

Understanding the impact of metal contacts on recombination within a passivated crystalline silicon wafer is crucial for the optimisation of various photovoltaic devices, such as passivating-contact-based solar cells. In this type of device, the metal contacts are offset from the silicon wafer surface by additional layers. The latter (1) minimise recombination losses—either by chemically reducing the density of defects at the wafer surface or by increasing the imbalance between the majority and minority carrier density near the surface—and (2) are selective for one type of carrier. An asymmetric population of electrons and holes near the c-Si surface can be obtained by applying a contact layer with a different work function than that of silicon. Therefore, we expect that the presence of a metal contact forms an extremely thin accumulation or inversion layer close to the wafer surface. This imbalance in concentration of the two carrier types will change the recombination statistics at the passivated surface. To date, it has been challenging to assess the impact of the metal work function on the obtained passivation as photoconductance-based lifetime measurements cannot be used for metallised devices. Using our novel photolumnscence lifetime tester, we investigate for the first time the impact of work function. In the figure to the right we compare the obtained surface passivation for aluminium oxide deposited samples with and without metalisation.

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(a) Metal work functions and structures used to extract J0s,total, J0s, and J0m; (b) J0m values of Structures A and B determined from J0s,total and J0s values. The rear side J0s values of these structures before metallisation are shown as reference (filled symbols).

PECVD-based passivating contacts

This project aims to raise cell efficiencies by capitalising on the recent developments in contact formation while using more conventional industrial techniques that can be easily integrated into existing manufacturing capacity. We aim to do this via plasma enhanced chemical vapour deposition (PECVD) of transition metal oxides (TMOs) titanium oxide (TiOx) at the electron contact, and tungsten oxide (WOx) at the hole contact. The project brings together a uniquely positioned consortium of industrial partners ‒ the PV industry’s largest PEVCD equipment manufacturer and a world-leading cell manufacturer ‒ whose individual goals coalesce around a shared vision to introduce advanced contact technologies in an industrially feasible way. Doing so will increase cell efficiencies in production, lowering the cost of PV generated electricity and ultimately enabling silicon PV to become a technology that can address global electricity needs and contain the effects of climate change.

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FTIR spectra of TiOx films deposited by our modified PECVD tool.