The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. While a tremendous amount of research activity has occurred in assembling disparate 2D materials into “all-2D” van der Waals heterostructures,1, 2 this concept is not limited to 2D materials alone. Given that any passivated, dangling bond-free surface will interact with another via vdW forces, the vdW heterostructure concept can be extended to include the integration of 2D materials with non-2D materials that adhere primarily through noncovalent interactions.3 In the first part of this talk I will present our work on emerging mixed-dimensional (2D + nD, where n is 0, 1 or 3) heterostructure devices performed at Northwestern University. I will present two distinct examples of gate-tunable p-n heterojunctions.4-6 I will show that when a single layer n-type molybdenum disulfide(MoS2)(2D) is combined with p-type semiconducting single walled carbon nanotubes (1D), the resulting p-n junction is gate-tunable and shows a tunable diode behavior with rectification as a function of gate voltage and a unique anti-ambipolar transfer behavior.4The same concept when extended to p-type organic small molecule semiconductor(pentacene) (0D) and n-type 2D MoS2 leads to a tunable p-n junction with a photovoltaic effect and an asymmetric anti-ambipolar transfer response.6 I will present the underlying charge transport and photocurrent responses in both the above systems using a variety of scanning probe microscopy techniques as well as computational methods. Finally, I will show that the anti-ambipolar field effect observed in the above systems can be generalized to other semiconducting heterojunction systems and extended over large areas with practical applications in wireless communication circuits.5
The second part of talk will discuss my more recent work performed at Caltech on photovoltaic devices from 2D semiconductors such as transition metal dichalcogenides (TMDCs).High efficiency inorganic photovoltaic materials (e.g., Si, GaAs and GaInP) can achieve maximum above-bandgap absorption as well as carrier-selective charge collection at the cell operating point. But thin film photovoltaic absorbers have lacked the ability to maximize absorption and efficient carrier collection, concurrently often due to 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. I will show experimental demonstration of light confinement in ultrathin (< 15 nm) Van der Waals semiconductors (MoS2, WS2 and WSe2) leading to nearly perfect absorption.7 I will further present the fabrication and performance of our, broadband absorbing, heterostructure photovoltaic devices using sub-15 nm TMDCs as the active layers, with record high quantum efficiencies.7, 8 I will then present ongoing work on addressing the key remaining challenge for application of 2D materials and their heterostructures in high efficiency photovoltaics which entails engineering of interfaces and open-circuit voltage.9 I will conclude by giving a broad perspective of future work on 2D materials for optoelectronic applications.
1. Grigorieva, I. V.; Geim, A. K. Nature 2013, 499, 419-425.
2. Jariwala, D.; Sangwan, V. K. et al.ACS Nano 2014, 8, 1102–1120.
3. Jariwala, D.; Marks, T. J.; Hersam, M. C. Nat. Mater. 2017, 16, 170-181.
4. Jariwala, D.; Sangwan, V. K. et al.Proc. Nat. Acad. Sci. USA 2013, 110, 18076–18080.
5. Jariwala, D.; Sangwan, V. K. et al.Nano Lett. 2015, 15, 416-421.
6. Jariwala, D.; Howell, S. L. et al.Nano Lett. 2016, 16, 497–503.
7. Jariwala, D.; Davoyan, A. R. et al.Nano Lett. 2016, 16, 5482-5487.
8. Wong, J.; Jariwala, D. et al.ACS Nano 2017, 11, 7230–7240.
9. Jariwala, D.; Wong, J.; Davoyan, A. R.; Atwater, H. A. submitted 2017.