Center for Integrated Nanostructure Physics

Carrier multiplication for solar power conversion breakthrough and hot carrier dynamics in 2D materials

In the currently available solar cells, hot electron-hole pairs generated upon absorption of high-energy photons thermalize, thus converting their excess energy as heat and lowering the photovoltaic conversion efficiency. Several methods have been offered to increase the power conversion efficiency of solar cells including the development of tendem cells, impurity band and intermediate band devices, hot electron extraction, and carrier multiplication (CM). Harvesting this energy via efficient carrier multiplication would have a large impact by improving solar-cell power-conversion efficiency to up to ~45%, above the Shockley-Queisser limit (~34%).

The CM is a photo-physical process to generate multiple electron-hole pairs by exploiting excess energy of free carriers. In other words, the absorption of a single photon can lead to the excitation of multiple electrons: two for one. The CM conversion efficiency heavily depends on competition between the electron-electron scattering rate and the electron-phonon cooling rate of hot carriers, which determines the number of electron-hole pairs at the final stage with absorption of a photon.

Two-dimensional van der Waals layered materials emerging recently have revealed strong Coulomb interactions, giving rise to large exciton binding energies, and weak exciton-phonon coupling, which could lead to lower carrier cooling rates. In addition, the 2D materials has bandgap range of 0.7-1 eV which could maximize the power conversion efficiency. From the CM point of view, those properties identify 2D materials as most promising new materials.

The research of this team focuses on (1) carrier multiplication and carrier-cooling mechanism via femtosecond laser spectroscopy, (2) new quantum phenomena in individual or hybrid 2D materials, (3) Realization of carrier multiplication based 2D solar cells.

Key publications

○ Ji-Hee Kim*+, Mattew R. Bergren+, Jin Cheol Park+, Subash Adhikari, Michael Lorke, Thomas Fraunheim, Duk-Hyun Choe, Beom Kim, Hyunyong Choi, Tom Gregorkiewicz* and Young Hee Lee*, "Carrier multiplication in van der Waals layered transition metal dichalcogenides," Nature Communications 10, 5488 (2019).
Top 50 Nature Communications physics articles published in 2019

○ Sungtae Kim, Ji-Hee Kim*, and Young Hee Lee*, "Carrier multiplication in PbS quantum dots anchored on an Au Tip using conductive atomic force microscopy," Advanced Materials 32, 1908461 (2020)
Selected for the Inside Front Cover of Advanced Materials