48,642 research outputs found
Investigation of the charge transport through disordered organic molecular heterojunctions
We develop a new three-dimensional multiparticle Monte Carlo ({\it 3DmpMC})
approach in order to study the hopping charge transport in disordered organic
molecular media. The approach is applied here to study the charge transport
across an energetically disordered organic molecular heterojunction, known to
strongly influence the characteristics of the multilayer devices based on thin
organic films. The role of energetic disorder and its spatial correlations,
known to govern the transport in the bulk, are examined here for the bilayer
homopolar system where the heterojunction represents the bottleneck for the
transport. We study the effects of disorder on both sides of the
heterojunction, the effects of the spatial correlation within each material and
among the layers. Most importantly, the {\it 3DmpMC} approach permits us to
treat correctly the effects of the Coulomb interaction among carriers in the
region where the charge accumulation in the device is particularly important
and the Coulomb interaction most pronounced. The Coulomb interaction enhances
the current by increasing the electric field at the heterojunction as well as
by affecting the thermalization of the carriers in front of the barrier. Our MC
simulations are supplemented by the master equation (ME) calculations in order
to build a rather comprehensive picture of the hopping transport over the
homopolar heterojunction.Comment: 26 pages, 11 figures, LaTe
Self-Powered, Highly Sensitive, High Speed Photodetection Using ITO/WSe2/SnSe2 Vertical Heterojunction
Two dimensional transition metal di-chalcogenides (TMDCs) are promising
candidates for ultra-low intensity photodetection. However, the performance of
these photodetectors is usually limited by ambience induced rapid performance
degradation and long lived charge trapping induced slow response with a large
persistent photocurrent when the light source is switched off. Here we
demonstrate an indium tin oxide (ITO)/WSe/SnSe based vertical double
heterojunction photoconductive device where the photo-excited hole is confined
in the double barrier quantum well, whereas the photo-excited electron can be
transferred to either the ITO or the SnSe layer in a controlled manner. The
intrinsically short transit time of the photoelectrons in the vertical double
heterojunction helps us to achieve high responsivity in excess of A/W
and fast transient response time on the order of s. A large built-in
field in the WSe sandwich layer results in photodetection at zero external
bias allowing a self-powered operation mode. The encapsulation from top and
bottom protects the photo-active WSe layer from ambience induced
detrimental effects and substrate induced trapping effects helping us to
achieve repeatable characteristics over many cycles
Light Generation and Harvesting in a Van der Waals Heterostructure
Two-dimensional (2D) materials are a new type of materials under intense
study because of their interesting physical properties and wide range of
potential applications from nanoelectronics to sensing and photonics.
Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have
been proposed as promising channel materials for field-effect transistors
(FETs). Their high mechanical flexibility, stability and quality coupled with
potentially inexpensive production methods offer potential advantages compared
to organic and crystalline bulk semiconductors. Due to quantum mechanical
confinement, the band gap in monolayer MoS2 is direct in nature, leading to a
strong interaction with light that can be exploited for building
phototransistors and ultrasensitive photodetectors. Here, we report on the
realization of light-emitting diodes based on vertical heterojunctions composed
of n-type monolayer MoS2 and p-type silicon. Careful interface engineering
allows us to realize diodes showing rectification and light emission from the
entire surface of the heterojunction. Electroluminescence spectra show clear
signs of direct excitons related to the optical transitions between the
conduction and valence bands. Our pn diodes can also operate as solar cells,
with typical external quantum efficiency exceeding 4%. Our work opens up the
way to more sophisticated optoelectronic devices such as lasers and
heterostructure solar cells based on hybrids of two-dimensional (2D)
semiconductors and silicon.Comment: Submitted versio
Identification of Ultrafast Photophysical Pathways in Photoexcited Organic Heterojunctions
The exciton dissociation and charge separation occurring on subpicosecond
time scales following the photoexcitation are studied in a model donor/acceptor
heterojunction using a fully quantum approach. Higher-than-LUMO acceptor
orbitals which are energetically aligned with the donor LUMO orbital
participate in the ultrafast interfacial dynamics by creating photon-absorbing
charge-bridging states in which charges are spatially separated and which can
be directly photoexcited. Along with the states brought about by
single-particle resonances, the two-particle (exciton) mixing gives rise to
bridge states in which charges are delocalized. Bridge states open up a number
of photophysical pathways that indirectly connect the initial donor states with
states of spatially separated charges and compete with the efficient
progressive deexcitation within the manifold of donor states. The diversity and
efficiency of these photophysical pathways depend on a number of factors, such
as the precise energy alignment of exciton states, the central frequency of the
excitation, and the strength of carrier-phonon interaction.Comment: Final, published versio
Method for fabricating solar cells having integrated collector grids
A heterojunction or Schottky barrier photovoltaic device comprising a conductive base metal layer compatible with and coating predominately the exposed surface of the p-type substrate of the device such that a back surface field region is formed at the interface between the device and the base metal layer, a transparent, conductive mixed metal oxide layer in integral contact with the n-type layer of the heterojunction or Schottky barrier device having a metal alloy grid network of the same metal elements of the oxide constituents of the mixed metal oxide layer embedded in the mixed metal oxide layer, an insulating layer which prevents electrical contact between the conductive metal base layer and the transparent, conductive metal oxide layer, and a metal contact means covering the insulating layer and in intimate contact with the metal grid network embedded in the transparent, conductive oxide layer for conducting electrons generated by the photovoltaic process from the device
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