Near-Unity Absorption in van der Waals Semiconductors for Ultrathin Optoelectronics

TitleNear-Unity Absorption in van der Waals Semiconductors for Ultrathin Optoelectronics
Publication TypeJournal Article
Year of Publication2016
AuthorsJariwala, D., A. R. Davoyan, G. Tagliabue, M. C. Sherrott, J. Wong, and H. A. Atwater
JournalNano Letters
Date Published09/2016
Keywordsbroadband, electrical-characterization, heterostructures, light trapping, near-unity absorption, photovoltaic, photovoltaics, Transition metal dichalcogenides, Transition metal dichalcogenidesbroadband

Key criteria for high efficiency photovoltaics include achieving high radiative efficiency, maximizing above-bandgap semiconductor absorption, and enabling carrier- selective charge collection at the cell operating point that exploits the full quasi-Fermi level separation for the carriers. High efficiency inorganic photovoltaic materials (e.g., Si, GaAs and GaInP) can achieve these criteria, but thin film photovoltaic absorbers have lacked the ability to fulfill one or more of these criteria, often due to surface and interface recombination effects. In contrast, Van der Waals semiconductors have naturally passivated surfaces with electronically active edges that allows retention of high electronic quality down-to the atomically thin limit and recent reports suggest that Van der Waals semiconductors can achieve the first criterion of high radiative efficiency. Here, we report that the second criteria for high efficiency of near-unity light absorption is possible in extremely thin ({\textless} 15 nm) Van der Waals semiconductor structures by coupling to strongly damped optical modes of semiconductor/metal heterostructures. We demonstrate near unity, broadband absorbing photovoltaic devices using sub-15 nm thick transition metal  dichalcogenides (TMDCs) as van der Waals semiconductor active layers. Our TMDC devices show a short circuit current density {\textgreater} 10 mA/cm2 at {\~{}}20 Suns and exhibits spectral response that parallels the spectral absorption over the above bandgap region. Our work addresses one of the key criteria to enable TMDCs to achieve high photovoltaic efficiency.