New Hot Paper in Physics: Improving the Development of Perovskite Solar Cells

The article “Band filling with free charge carriers in organonietal halide perovskites” (Nat. Photonics 8[9]: 737-43 September 2014), was recently named a New Hot Paper for Physics in Essential Science Indicators from Clarivate Analytics. Currently in the Web of Science, this paper has 205 citations.

Below, the paper’s corresponding author, Professor Prashant Kamat, discusses the paper and its influence in the field of Physics. Professor Kamat is the John A. Zahm Professor of Science at the University of Notre Dame and is the Editor-in-Chief of ACS Energy Letters.

Why do you think your paper is highly cited?

Perovskite solar cells boasting efficiencies greater than 20% have emerged as a new class of devices with the potential to revolutionize the photovoltaic industry. At the time of our publication, reported efficiencies were only beginning to reach double digits, and little was known about the underlying properties of these hybrid organic-inorganic semiconductors. Our study was one of the first to examine the fundamental processes induced by light absorption in the now prototypical perovskite absorber methylammonium lead iodide. Using femtosecond time-resolved laser spectroscopy, we provided early insight into the photophysical properties of this compound, uncovering behavior more analogous to traditional high-quality semiconductors fabricated at high temperature than typical solution-processed materials. Gaining a more comprehensive understanding of how the material interacts with light provides insight into the operating mechanisms of perovskite solar cells and light-emitting diodes. Such information can then be used to inform design of devices that capture and convert energy more efficiently. Our work thus has broad implications for both fundamental and applied research on perovskite semiconductors. These areas of study have seen enormous growth in recent years, as have the number of published articles.

Does it describe a new discovery, methodology, or synthesis of knowledge?

The study provides new understanding of the excited-state processes in lead halide perovskites and the mechanisms by which charge carriers recombine in these materials. We observed, for the first time, bimolecular charge recombination in a metal halide perovskite under laser pulse excitation. The study also provides evidence for a transient band filling phenomenon, an interesting property that originates in part from the long lifetime of photogenerated carriers in methylammonium lead iodide.

Would you summarize the significance of your paper in layman's terms?

Semiconductors are the workhorse of the solar cell. When semiconductor materials capture sunlight, an internal charge-separation process occurs that generates electrons and holes. These charge carriers are ultimately responsible for generating electricity in a solar cell. These carriers have opposite charge, and if they are not separated from one another fast enough, they will recombine and will not be available to generate electric current. This reduces the efficiency of the device. It is therefore necessary to capture the electrons and holes at two different electrodes before they can recombine within the film. The lifetime of the photogenerated charge carriers becomes an important property to evaluate the performance of a semiconductor material used in photovoltaic applications. The present study offers new insights into the excited-state processes of methylammonium lead halide perovskite films and shows the dependence of charge carrier lifetime on the intensity of excitation.

How did you become involved in this research, and how would you describe the particular challenges, setbacks, and successes that you've encountered along the way?

We have been actively engaged in utilizing semiconductor nanocrystals for light energy harvesting applications supported by the U.S. Department of Energy. When lead halide hybrid perovskites emerged as new contenders for photovoltaic materials, we were eager to track the photoinduced processes. The interpretation of absorption changes observed under laser irradiation at early times and the light intensity dependence of charge carrier recombination was different than the familiar dynamics of other semiconductors. It took nearly a year for us to map out these excited-state dynamics and establish the kinetic processes in the sub-picosecond to nanosecond time scale. 

Where do you see your research leading in the future?

The metal halide perovskite field continues to grow exponentially. More researchers are working on more projects than ever in this exciting area. Materials scientists, physicists, chemists, engineers, and computational scientists are all heavily involved. Our work provided a piece of the initial foundation in our expanding understanding of these unique and promising compounds. We are now focusing our efforts to understand defect-driven excited-state processes and explain their role in governing the photoconversion efficiency in solar cells. In addition, we are investigating halide ion segregation in mixed halide perovskite films that impacts the stability of metal halide perovskite alloys.

Do you foresee any social or political implications for your research?

Basic understanding of the excited-state processes in lead halide hybrid perovskite materials is crucial in developing new photovoltaic materials that can compete with other high-efficiency technologies. The thin-film technology offers simplicity in manufacturing and new opportunities for development of high-efficiency tandem solar cells that absorb a broad range of frequencies across the solar spectrum. Ultimately, we hope the culmination of our work and the work of others leads to the realization of perovskite devices that make an impact in the energy sector, whether through lower-cost, energy-efficient lighting or photovoltaic panels.

 

Prashant V. Kamat
John A. Zahm Professor of Science
University of Notre Dame
Notre Dame, IN, USA