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Local Electronic Structure Changes in Polycrystalline CdTe with CdCl2 Treatment and Air Exposure

ACS Applied Materials and Interfaces

Berg, Morgann B.; Kephart, Jason M.; Munshi, Amit; Sampath, Walajabad S.; Ohta, Taisuke O.; Chan, Calvin C.

Postdeposition CdCl2 treatment of polycrystalline CdTe is known to increase the photovoltaic device efficiency. However, the precise chemical, structural, and electronic changes that underpin this improvement are still debated. In this study, spectroscopic photoemission electron microscopy was used to spatially map the vacuum level and ionization energy of CdTe films, enabling the identification of electronic structure variations between grains and grain boundaries (GBs). In vacuo preparation and inert transfer of oxide-free CdTe surfaces isolated the separate effects of CdCl2 treatment and ambient oxygen exposure. Qualitatively, grain boundaries displayed lower work function and downward band bending relative to grain interiors, but only after air exposure of CdCl2-treated CdTe. Analysis of numerous space charge regions at grain boundaries showed an average depletion width of 290 nm and an average band bending magnitude of 70 meV, corresponding to a GB trap density of 1011 cm-2 and a net carrier density of 1015 cm-3. These results suggest that both CdCl2 treatment and oxygen exposure may be independently tuned to enhance the CdTe photovoltaic performance by engineering the interface and bulk electronic structure.

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TYPE Journal Article YEAR 2018
DOIOSTIScopus

Experimental Determination of the Ionization Energies of MoSe2, WS2, and MoS2 on SiO2 Using Photoemission Electron Microscopy

ACS Nano

Keyshar, Kunttal; Kunttal; Berg, Morgann B.; Zhang, Xiang; Xiang; Vajtai, Robert; Robert; Gupta, Gautam; Gautam; Chan, Calvin C.; Laros, James H.; Ajayan, Pulickel M.; Pulickel; Mohite, Aditya D.; D, Aditya; Ohta, Taisuke O.

Here, the values of the ionization energies of transition metal dichalcogenides (TMDs) are needed to assess their potential usefulness in semiconductor heterojunctions for high-performance optoelectronics. Here, we report on the systematic determination of ionization energies for three prototypical TMD monolayers (MoSe2, WS2, and MoS2) on SiO2 using photoemission electron microscopy with deep ultraviolet illumination. The ionization energy displays a progressive decrease from MoS2, to WS2, to MoSe2, in agreement with predictions of density functional theory calculations. Combined with the measured energy positions of the valence band edge at the Brillouin zone center, we deduce that, in the absence of interlayer coupling, a vertical heterojunction comprising any of the three TMD monolayers would form a staggered (type-II) band alignment. This band alignment could give rise to long-lived interlayer excitons that are potentially useful for valleytronics or efficient electron–hole separation in photovoltaics.

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TYPE Journal Article YEAR 2017
DOIDOIOSTIOSTI
Results 51–75 of 159
Results 51–75 of 159