Silicon solar cells are the dominant solar PV technology today and is expected to remains so for the foreseeable future. The improvement of the designs of silicon solar cells is still a vibrant area of research. Processing that reduces costs and thermal budgets (heating things for long periods of time) is also of keen interest.

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Room Temperature Contacting Technique

We are looking at exploiting a patented room temperature contacting method for metal to silicon (this can be a key loss mechanism for solar cells as semiconductors and metals often don't like electrons moving from one to the other). Called Point-contacting by Localised Dielectric Breakdown (PLDB) we have shown it can be used for single and multi-crystalline silicon and provides a way to circumvent light and elevated temperature induced degradation. It is now being adapted to use in III-V on silicon multi-junction devices (since there is no annealing needed).

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Shown right is the basis of PLDB, where doping is done locally to form a series of points,  a layer of dielectric that passivates the silicon is then deposited. Metal pads are deposited over the region with the local doping and then applying a voltage between the metal pads and ground sees a dielectric breakdown occurring at the local doping site. The process is irreversible and has proven to have a low resistance associated with it. There is some variation in the breakdown voltage, but we have shown it is well described by a Weibull distribution for different dielectric layers (shown to the right). Recently we have shown that this technique prevents the occurrence of Light and Elevated Temperature Induced Degradation (LeTID) in both crystalline and multi-crystalline silicon solar cells. The room temperature nature of the technique makes it an excellent approach to contacting the bottom cell in silicon based multi-junction devices.

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Carrier Selective Contacts

We are also looking at materials for use with silicon to create carrier selective contacts (CSCs). This idea is inspired by the Heterojunction with Intrinsic Thin layer (HIT) solar cell, where thin layers of intrinsic amorphous silicon are deposited on crystalline silicon for surface passivation, followed by doped amorphous silicon, with each side having a different polarity. This induces a junction and most importantly helps to be selective in which carrier is favoured to be extracted through each doped layer. Viewed this way, the HIT cell is a CSC cell. Research on CSCs is focused on improving on the selectivity of a layer, whilst maintaining the surface passivation. Materials such as transition metal oxides have been of great interest, particularly Molybdenum Oxide (MoOx), but many others are being explored.

 

We are investigating Nickel Oxide (NiOx) and Vanadium Oxide (VOx) in particular, using a variety of deposition techniques. These include Pulsed Laser Deposition (PLD), spin coating, and Atomic Layer Deposition. Approaches to maintain surface passivation include amorphous silicon, Aluminium Oxide and Silicon Oxide intermediate layers. You can check out some of what we have reported in the references below.

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Articles of Interest

Some of our published work on PLDB

N. J. Western and S. P. Bremner, "Hydrogenation and Gettering Compatible p-Type Contacts for Multicrystalline Silicon Cells, Free of Light, and Elevated Temperature Induced Degradation," in IEEE Journal of Photovoltaics, vol. 10, no. 5, pp. 1232-1238, Sept. 2020, doi: 10.1109/JPHOTOV.2020.2999869.

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Q. Ye, N. J. Western, U. Römer and S. P. Bremner, "Characterizing Point Contacting by Localized Dielectric Breakdown and Its Use in Silicon Solar Cell Applications," in IEEE Journal of Photovoltaics, vol. 10, no. 1, pp. 78-84, Jan. 2020, doi: 10.1109/JPHOTOV.2019.2953394.

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N. J. Western,  I. Perez-Wurfl, S. R. Wenham, and S. P. Bremner, "Point-contacting by localised dielectric breakdown: Characterisation of a metallisation technique for the rear surface of a solar cell," Journal of Applied Physics 118, 045711 (2015); https://doi.org/10.1063/1.4927282

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N. J. Western, I. Perez-Wurfl, S. R. Wenham, and S. P. Bremner, "Point-Contacting by Localized Dielectric Breakdown With Breakdown Fields Described by the Weibull Distribution," in IEEE Transactions on Electron Devices, vol. 62, no. 6, pp. 1826-1830, June 2015, doi: 10.1109/TED.2015.2423292.

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N. J. Western, A. Sung, S. R. Wenham, and S. P. Bremner, "Localized ohmic contact through a passivation dielectric for solar cell rear surface design,"

Appl. Phys. Lett. 102, 222105 (2013); https://doi.org/10.1063/1.4809820