Hooman Mohseni

Room-temperature ferroelectricity in supramolecular networks of charge-transfer complexes

Materials exhibiting a spontaneous electrical polarization that can be switched easily between antiparallel orientations are of potential value for sensors, photonics and energy-efficient memories. In this context, organic ferroelectrics are of particular interest because they promise to be lightweight, inexpensive and easily processed into devices. A recently identified family of organic ferroelectric structures is based on intermolecular charge transfer, where donor and acceptor molecules co-crystallize in an alternating fashion known as a mixed stack: in the crystalline lattice, a collective transfer of electrons from donor to acceptor molecules results in the formation of dipoles that can be realigned by an external field as molecules switch partners in the mixed stack. Although mixed stacks have been investigated extensively, only three systems are known to show ferroelectric switching, all below 71 kelvin. Here we describe supramolecular charge-transfer networks that undergo ferroelectric polarization switching with a ferroelectric Curie temperature above room temperature. These polar and switchable systems utilize a structural synergy between a hydrogen-bonded network and charge-transfer complexation of donor and acceptor molecules in a mixed stack. This supramolecular motif could help guide the development of other functional organic systems that can switch polarization under the influence of electric fields at ambient temperatures.

Universality of non-ohmic shunt leakage in thin-film solar cells

We compare the dark current-voltage (IV) characteristics of three different thin-film solar cell types: hydrogenated amorphous silicon (a-Si:H) p-i-n cells, organic bulk heterojunction (BHJ) cells, and Cu(In,Ga)Se2 (CIGS) cells. All three device types exhibit a significant shunt leakage current at low forward bias (V<∼0.4) and reverse bias, which cannot be explained by the classical solar cell diode model. This parasitic shunt current exhibits non-Ohmic behavior, as opposed to the traditional constant shunt resistance model for photovoltaics. We show here that this shunt leakage (Ish), across all three solar cell types considered, is characterized by the following common phenomenological features: (a) voltage symmetry about V=0, (b) nonlinear (power law) voltage dependence, and (c) extremely weak temperature dependence. Based on this analysis, we provide a simple method of subtracting this shunt current component from the measured data and discuss its implications on dark IV parameter extraction. We propose a space charge limited (SCL) current model for capturing all these features of the shunt leakage in a consistent framework and discuss possible physical origin of the parasitic paths responsible for this shunt current mechanism.We compare the dark current-voltage (IV) characteristics of three different thin-film solar cell types: hydrogenated amorphous silicon (a-Si:H) p-i-n cells, organic bulk heterojunction (BHJ) cells, and Cu(In,Ga)Se2 (CIGS) cells. All three device types exhibit a significant shunt leakage current at low forward bias (V<∼0.4) and reverse bias, which cannot be explained by the classical solar cell diode model. This parasitic shunt current exhibits non-Ohmic behavior, as opposed to the traditional constant shunt resistance model for photovoltaics. We show here that this shunt leakage (Ish), across all three solar cell types considered, is characterized by the following common phenomenological features: (a) voltage symmetry about V=0, (b) nonlinear (power law) voltage dependence, and (c) extremely weak temperature dependence. Based on this analysis, we provide a simple method of subtracting this shunt current component from the measured data and discuss its implications on dark IV parameter extraction. We propo...

Growth and characterization of InGaAs/InGaP quantum dots for midinfrared photoconductive detector

We report InGaAs quantum dot intersubband infrared photodetectors grown by low-pressure metalorganic chemical vapor deposition on semi-insulating GaAs substrates. The optimum growth conditions were studied to obtain uniform InGaAs quantum dots constructed in an InGaP matrix. Normal incidence photoconductivity was observed at a peak wavelength of 5.5 μm with a high responsivity of 130 mA/W and a detectivity of 4.74×107 cm H1/2/W at 77 K.

Plasmonic enhanced quantum well infrared photodetector with high detectivity

We report a normal-incident quantum well infrared photodetector (QWIP) strongly coupled with surface plasmon modes. A periodic hole array perforated in gold film was integrated with In0.53Ga0.47As/InP QWIP to convert normal-incident electromagnetic waves into surface plasmon waves, and to excite the intersubband transition of carriers in the quantum wells. The peak responsivity of the photodetector at ∼8 μm was ∼7 A/W at the bias of 0.7 V at 78 K with the peak detectivity as high as ∼7.4×1010 cm Hz1/2/W. The full width at half maximum of the response spectrum was only ∼0.84 μm due to a narrow plasmonic resonance.

A deep sub-wavelength process for the formation of highly uniform arrays of nanoholes and nanopillars

We report a low-cost and high-throughput process for the realization of two-dimensional arrays of deep sub-wavelength features using silica and polystyrene spheres. The pattern size in this method is a weak function of sphere size, and hence excellent size uniformity is achievable. Also, the period and diameter of the holes and pillars formed with this technique can be controlled precisely and independently. Moreover, the patterns can be formed in conventional negative and positive photoresists, and hence this approach is compatible with a wide range of existing processing methods. Although we achieved hole sizes of ~250 nm with a broadband UV source centered at 400 nm, our simulation results show that patterns as small as 180 nm should be achievable at a wavelength of 365 nm.

High-performance InAs/GaSb superlattice photodiodes for the very long wavelength infrared range

We report on the demonstration of high-performance p-i-n photodiodes based on type-II InAs/GaSb superlattices with 50% cut-off wavelength λc=16 μm operating at 80 K. Material is grown by molecular beam epitaxy on GaSb substrates with excellent crystal quality as evidenced by x-ray diffraction and atomic force microscopy. The processed devices show a current responsivity of 3.5 A/W at 80 K leading to a detectivity of ∼1.51×1010 cmHz1/2/W. The quantum efficiency of these devices is about 35% which is comparable to HgCdTe detectors with a similar active layer thickness.

Growth and characterization of InAs/GaSb photoconductors for long wavelength infrared range

In this letter we report the molecular beam epitaxial growth and characterization of InAs/GaSb superlattices grown on semi-insulating GaAs substrates for long wavelength infrared detectors. Photoconductive detectors fabricated from the superlattices showed photoresponse up to 12 μm and peak responsivity of 5.5 V/W with Johnson noise limited detectivity of 1.33×109 cm Hz1/2/W at 10.3 μm at 78 K.

Fabrication of Large Area Periodic Nanostructures Using Nanosphere Photolithography

Large area periodic nanostructures exhibit unique optical and electronic properties and have found many applications, such as photonic band-gap materials, high dense data storage, and photonic devices. We have developed a maskless photolithography method—Nanosphere Photolithography (NSP)—to produce a large area of uniform nanopatterns in the photoresist utilizing the silica micro-spheres to focus UV light. Here, we will extend the idea to fabricate metallic nanostructures using the NSP method. We produced large areas of periodic uniform nanohole array perforated in different metallic films, such as gold and aluminum. The diameters of these nanoholes are much smaller than the wavelength of UV light used and they are very uniformly distributed. The method introduced here inherently has both the advantages of photolithography and self-assembled methods. Besides, it also generates very uniform repetitive nanopatterns because the focused beam waist is almost unchanged with different sphere sizes.

Very long wavelength infrared type-II detectors operating at 80 K

We report a demonstration of very long wavelength infrared detectors based on InAs/GaSb superlattices operating at T=80 K. Detector structures with excellent material quality were grown on an optimized GaSb buffer layer on GaAs semi-insulating substrates. Photoconductive devices with 50% cutoff wavelength of λc=17 μm showed a peak responsivity of about 100 mA/W at T=80 K. Devices with 50% cutoff wavelengths up to λc=22 μm were demonstrated at this temperature. Good uniformity was obtained over large areas even for the devices with very long cutoff wavelengths.

A Novel Self-aligned and Maskless Process for Formation of Highly Uniform Arrays of Nanoholes and Nanopillars

Fabrication of a large area of periodic structures with deep sub-wavelength features is required in many applications such as solar cells, photonic crystals, and artificial kidneys. We present a low-cost and high-throughput process for realization of 2D arrays of deep sub-wavelength features using a self-assembled monolayer of hexagonally close packed (HCP) silica and polystyrene microspheres. This method utilizes the microspheres as super-lenses to fabricate nanohole and pillar arrays over large areas on conventional positive and negative photoresist, and with a high aspect ratio. The period and diameter of the holes and pillars formed with this technique can be controlled precisely and independently. We demonstrate that the method can produce HCP arrays of hole of sub-250 nm size using a conventional photolithography system with a broadband UV source centered at 400 nm. We also present our 3D FDTD modeling, which shows a good agreement with the experimental results.