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 of around 20%, 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., IEEE J. Photovolt. 6(4), 1380 (2014)]
Our work focuses on the implementation of practical tandem devices, as well as the further development of the Perovskite and Silicon cells. We employ advanced optical modelling tools to optimise each layer of the solar device structure and implement and measure the resulting devices in collaboration with researchers from the University of New South Wales and Monash University, as well as Arizona State University and commercial partners SunTech and Trina Solar.
Our work has already resulted in high efficiency perovskite cells and perovskite / silicon tandem devices, with efficiencies exceeding 19 and 23%, respectively. These are among the most efficient perovskite devices reported to date in the literature. A clear route exists to exceed the silicon solar cell efficiency record of 25.6%.
Perovskite Cell Fabrication and Characterisation
Several key challenges need to be addressed for commercial Perovskite-based solar cells to be realised. Key among these is the need to demonstrate stable operation under conditions simulating the real operating environment, where modules are exposed to intense radiation, heat and cold, and high humidity.
Our work focuses on understanding the physics of Perovskite cell operation, and elucidating the requirements for stable cell operation. To this end we fabricate Perovskite cells and test structures, and employ a variety of techniques for detailed characterisation. These include Time Resolved Photoluminescence, Photoluminescence Imaging, Cathodoluminescence, Impedance Spectroscopy, Atomic Force Microscopy, X-ray Diffraction, Raman Spectroscopy and Kelvin Probe, as well as conventional solar cell measurement techniques. In addition, we develop sophisticated optical and electrical models to improve physical understanding and guide device design.
This work has led to several new insights into perovskite devices, such a clarifying the origin of hysteresis commonly observed in the cell characteristics.
Photoluminescence images of a Perovskite cell (Wu et al, On the Origin of Hysteresis in Perovskite Solar Cells, Advanced Functional Materials)
Student Research Opportunities
Perovskite solar cells