Alternate material could cut solar cell costs by more than 75%
Anything that can drive costs down and efficiency up is a welcome development in solar energy, as the world turns inexorably toward more sustainable energy. Part of the solution may be a non-silicon, light-absorbing material that could reduce solar cell costs by more than 75%.
Perovskite — discovered in the Ural Mountains, and named after the Russian mineralogist Lev Perovski — is a light-absorbing semiconducting material that can be used to make solar cells in a simple process at very low cost. The resultant cells have a power conversion efficiency comparable to that for traditional silicon cells (~26% for the best perovskite solar cell, versus ~27% for the top silicon cell).
Another plus: The fabrication process can be as simple as printing newspapers. In our lab, a sophomore student can make 25% efficiency devices in two days. If translated into large-scale production, the solar energy price can drop to less than one-quarter of the price of an existing silicon solar module.
Still, challenges remain. Perovskite solar cells are less stable than silicon cells, as the perovskite material can react with moisture and oxygen and decompose. The long-term stability at elevated temperatures also is inadequate. Further, the interface of the perovskite light absorber and charge-transporting (charge-collecting) layers can degrade — meaning current perovskite solar cells typically last only a few months while silicon cells can operate for more than 25 years.
Our lab is propelling efforts to overcome these limitations.
To address the stability issue, we invented a molecularly tailored “glue” that can neutralize the defects on the perovskite surface and improve the interface between perovskite and other charge-collecting layers. These molecules, called conjugated ligands, are a type of molecular semiconductor with multiple-ring structures that help conduct the charge. Conjugated ligands also interact with perovskite to remove surface imperfections.
The benefits are substantial. We improve the overall power conversion efficiency by reducing defect density on perovskite materials. We facilitate charge extraction by enhancing the contact with other charge-transporting materials. And we increase device stability by blocking moisture penetration and ion migration, as well as lessening defects and undesired charge accumulation.
We’re using advanced spectroscopy to investigate the electronic structures of materials, which includes understanding the energy of electrons and how the electrons move at the interfaces. We also use advanced microscopes to explore material morphologies, to understand how crystalline materials grow, and to improve the physical contact between different materials.
In addition, we’re changing the role of organic molecules in the perovskite structure. Conventionally, the organic molecules are considered structural framework, without contributing to the electronic structure. We regard this organic molecule as both a framework and a semiconducting bridge that can manipulate the electronic structure of 2D perovskite.
Another breakthrough is enabling what are called polymeric hole-transporting materials to be used in high-efficiency devices. This type of material helps generate stable perovskite solar cells, but it sacrifices efficiency due to its poor contact with perovskite. We used the molecular engineering technique to solve the interface contact issue, a long-standing problem in this field. This strategy will pave the way for building more efficient and stable devices.
Our funding comes primarily from the Department of Energy; we also are discussing collaborations with the private sector to commercialize our technology. Through the Purdue Office of Technology Commercialization (OTC), we have filed several patents, now available for licensing.
Perovskite solar cells are developing extremely quickly these days. Within 15 years of development, their power conversion efficiency already is comparable to that of the Si-based solar cell. Although the stability of perovskite devices still lags, the research field is turning the “steering wheel” from investigating high efficiency to exploring high stability.
And based on how device efficiency of perovskite has been improved, we believe the stability of this material also will advance tremendously in the next few years. We like to compare it to organic light-emitting diodes (OLEDs) — the first OLED research papers demonstrated stability of only a few seconds, but after 30 years of effort OLED technology was commercialized with great success.
There already are many perovskite startup companies all over the world; some major solar companies additionally are investigating the possibility of perovskites. What also looks promising is combining perovskite cells with silicon solar cells to make “tandem” solar cells that can help boost the efficiency from ~26% to ~35% without a major increase in production cost.
Overall, the future appears promising. The next five to 10 years will be an exciting time for perovskite solar technology to make the next giant leap — from lab to fab.
Letian Dou, PhD
Charles Davidson Associate Professor of Chemical Engineering
Davidson School of Chemical Engineering
College of Engineering
Associate Professor of Chemistry (by courtesy)
James Tarpo Jr. and Margaret Tarpo Department of Chemistry
College of Science
Purdue University
Ke Ma, PhD
Lillian Gilbreth Postdoctoral Fellow
Davidson School of Chemical Engineering
College of Engineering
Purdue University
Jiaonan Sun, PhD
Postdoctoral Research Assistant
Davidson School of Chemical Engineering
College of Engineering
Purdue University
