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2D materials boost carrier multiplication

- IBS researchers discover a carrier multiplication process in 2D semiconductors that could increase the efficiency of future solar cells -

Physicists at the Center for Integrated Nanostructure Physics (CINAP), within the Institute for Basic Science (IBS, South Korea), have discovered an intriguing phenomenon, known as carrier multiplication (CM), in a class of semiconductors with incredible thinness, outstanding properties, and possible applications in electronics and optics. Published in Nature Communications, these new findings have the potential to boost the photovoltaics and photodetector fields, and could improve the efficiency of solar cells produced with these ultrathin materials to up to 46%.

An interesting class of 2D materials, the van der Waals layered transition metal dichalcogenides (2D-TMDs), are expected to create the next-generation of optoelectronic devices, such as solar cells, transistors, light emitting diodes (LED), etc. They consist of individual thin layers separated by very weak chemical bonds (van der Waals bonds), and have unique optical properties, high light absorption, and high carrier (electron and hole) mobility. Beyond allowing the option to tune their band gap by changing composition and layer thickness, these materials also offer an ultrahigh internal radiative efficiency of >99%, promoted by the elimination of surface imperfections and large binding energy between carriers.

Absorption of sunlight in semiconducting 2D-TMD monolayers reaches typically 5-10%, which is an order of magnitude larger than that in most common photovoltaic materials, like silicon, cadmium telluride, and gallium arsenide. Despite these ideal characteristics, however, the maximum power conversion efficiency of 2D-TMDs solar cells has remained below 5% due to losses at the metal electrodes. The IBS team in collaboration with researchers at the University of Amsterdam aimed to overcome this drawback by exploring the CM process in these materials.

Figure 1. The schematic demonstrates the photogenerated carriers with carrier multiplication (CM). The discovery of the highest CM efficiency and the lowest CM threshold energy are displayed in the quantum yield data for 2H-MoTe2 and 2H-WSe2 thin films.
▲ Figure 1. The schematic demonstrates the photogenerated carriers with carrier multiplication (CM). The discovery of the highest CM efficiency and the lowest CM threshold energy are displayed in the quantum yield data for 2H-MoTe2 and 2H-WSe2 thin films.

CM is a very efficient way to convert light into electricity. A single photon usually excites a single electron, leaving behind an ‘empty space’ (hole). However, it is possible to generate two or more electron-hole pairs in particular semiconductors if the energy of the incident light is sufficiently large, more specifically, if the photon energy is twice the material’s bandgap energy. While the CM phenomenon is rather inefficient in bulk semiconductors, it was expected to be very efficient in 2D materials, but was not proved experimentally due to some technical limitations, like proper 2D-TMD synthesis and ultrafast optical measurement. In this study, the team observed CM in 2D-TMDs, namely 2H-MoTe2 and 2H-WSe2 films, for the first time; a finding that is expected to improve the current efficiency of 2D-TMD solar cells, even going beyond the Shockley-Queisser limit of 33.7%.

“Our new results contribute to the fundamental understanding of the CM phenomenon in 2D-TMD. If one overcomes the contact losses and succeeds in developing photovoltaics with CM, their maximum power conversion efficiency could be increased up to 46%,” says Young Hee Lee, CINAP director. “This new nanomaterial engineering offers the possibility for a new generation of efficient, durable, and flexible solar cells.”

Figure 2. Comparison of quantum yield for our samples (2H-MoTe2 and 2H-WSe2), various nanostructures and bulk materials.
▲ Figure 2. Comparison of quantum yield for our samples (2H-MoTe2 and 2H-WSe2), various nanostructures and bulk materials.

Notes for editors

- References
Ji-Hee Kim, Matthew R. Bergren, Jin Cheol Park, Subash Adhikari, Michael Lorke, Thomas Fraunheim, Duk-Hyun Choe, Beom Kim, Hyunyong Choi, Tom Gregorkiewicz & Young Hee Lee. Carrier multiplication in van der Waals layered transition metal dichalcogenides. Nature Communications (2019). DOI: 10.1038/s41467-019-13325-9

- Media Contact
For further information or to request media assistance, please contact Professor Ji-Hee Kim (kimj@skku.edu); Mr. Kyungyoon Min, Head of IBS Communications Team (+82-42-878-8156, kymin@ibs.re.kr); or Ms. Dahee Carol Kim, Public Information Officer of IBS & Science Communicator (+82-42-878-8133, clitie620@ibs.re.kr)

- About the Institute for Basic Science (IBS)
IBS was founded in 2011 by the government of the Republic of Korea with the sole purpose of driving forward the development of basic science in South Korea. IBS has 29 research centers as of October 2019. There are nine physics, two mathematics, six chemistry, six life science, one earth science, and five interdisciplinary research centers.

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Communications Team : Baek Seo Yun   042-878-8238
Last Update 2019-01-30 19:14