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The Australian National University

Tandem devices and Perovskite cells

Solar cells based on a single material, such as Silicon, are inherently limited in their efficiency because the single band gap of the material means that the energy of many photons cannot be utilised at all, or is utilised inefficiently. The best silicon cell efficiency is 25%. Further work may increase this to 26-27% which will be close to the theoretical limit. However, the conversion efficiency can be dramatically increased by creating so-called tandem structures, where two or more cells are stacked on top of each other. The top cell has a higher bandgap and efficiently converts high energy photons to electrical energy. Lower energy photons are not absorbed by the top cell but are instead passed to the lower bandgap cell, which can efficiently convert these photons to electrical energy.

A two-cell tandem configuration, where the cells are wired independently. Cell 1 (absorber thickness W), is made from a direct bandgap semiconductor material, characterized by its bandgap Eg , absorption coefficient α0 , carrier diffusion length Ld , and luminescence efficiency Φ. Cell 2 is a high-efficiency c-Si cell. Inset: distribution of incident energy absorbed in an idealized tandem cell. [White et al., IEEE J. Photovolt. 4 (1), 208 (2014)]

The tandem cell approach is used extensively to make high efficiency concentrator devices, using sophisticated and expensive materials and processes. For non-concentrating applications which constitute the vast majority of the Photovoltaic market, all the materials and fabrication processes used must be cheap. This has so far prevented tandem devices from becoming a reality.

The efficiency of such tandems depends critically on the efficiency and properties of the top cell. This is illustrated in the graph below, which shows the efficiency of the top cell required for various overall efficiencies, given a Silicon bottom cell efficiency of 25%. It can be seen that all currently available thin film cells do not meet the requirements for higher overall conversion efficiency.

The recent discovery that certain Perovskites are excellent materials for solar cells, as shown by already demonstrated efficiencies over 16%, now opens the door for high efficiency tandem devices.

Required top cell efficiencies to break-even (blue) and reach 30%tandem efficiencies (magenta) as a function of top cell bandgap and sub-bandgap transparencies (dashed lines). [Lal et al., to be published]

Our work focuses on the implementation of practical tandem devices, as well as the further development of the Perovskite and Silicon cells. Proof-of-concept development work is being carried out with GaAs top cells. This work has already yielded a tandem efficiency of 27.8%. This efficiency is expected to increase further as there is clear scope to exceed 30%.

Collection of solar photons by the GaAs (Red) and Silicon (blue) cells, in an experimentally realised 27.8% efficient 4 terminal tandem device.

In addition, tandem device structures based on Perovskite cells are being developed. Work on Perovskite cells is in the early stages but expected to ramp up during 2014 with the installation of dedicated fabrication equipment. This work will build on existing expertise within the group on high efficiency solar cells, advanced characterisation, modelling, nanostructures and device fabrication.

Schematic of independently wired Silicon and Perovskite cells in a tandem configuration.

Student Research Opportunities

Research Group Contacts

  • Klaus Weber: klaus.weber@anu.edu.au

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