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.

This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional information. We compare the dark current-voltage ͑IV͒ characteristics of three different thin-film solar cell types: hydrogenated amorphous silicon ͑a-Si:H͒ p-i-n cells, organic bulk heterojunction ͑BHJ͒ cells, and Cu͑In, Ga͒Se 2 ͑CIGS͒ cells. All three device types exhibit a significant shunt leakage current at low forward bias ͑V Ͻ ϳ 0.4͒ 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 ͑I sh ͒, across all three solar cell types considered, is characterized by the following common phenomenological features: ͑a͒ voltage symmetry about V =0, ͑b͒ nonlinear ͑power law͒ voltage dependence, and ͑c͒ 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 ͑SCL͒ 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.

Infrared photodetectors (IRPDs) have become important devices in various applications such as night vision, military missile tracking, medical imaging, industry defect imaging, environmental sensing, and exoplanet exploration. Mature semiconductor technologies such as mercury cadmium telluride and III-V material-based photodetectors have been dominating the industry. However, in the last few decades, significant funding and research has been focused to improve the performance of IRPDs such as lowering the fabrication cost, simplifying the fabrication processes, increasing the production yield, and increasing the operating temperature by making use of advances in nanofabrication and nanotechnology. We will first review the nanomaterial with suitable electronic and mechanical properties, such as two-dimensional material, graphene, transition metal dichalcogenides, and metal oxides. We compare these with more traditional lowdimensional material such as quantum well, quantum dot, quantum dot in well, semiconductor superlattice, nanowires, nanotube, and colloid quantum dot. We will also review the nanostructures used for enhanced lightmatter interaction to boost the IRPD sensitivity. These include nanostructured antireflection coatings, optical antennas, plasmonic, and metamaterials.

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.

We report a normal-incident quantum well infrared photodetector ͑QWIP͒ strongly coupled with surface plasmon modes. A periodic hole array perforated in gold film was integrated with In 0.53 Ga 0.47 As/ 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 ϳ8 m was ϳ7 A/ W at the bias of 0.7 V at 78 K with the peak detectivity as high as ϳ7.4ϫ 10 10 cm Hz 1/2 / W. The full width at half maximum of the response spectrum was only ϳ0.84 m due to a narrow plasmonic resonance.Since Ebbesen et al. 1 found the extraordinary optical transmission of periodic metal nanohole arrays by surface plasmons, properties of surface plasmons have attracted a lot of research interests and been applied into many applications. 2 For example, surface plasmons have been used to improve the efficiency of optoelectronic devices such as solar cells, 3 light emitters, 4 semiconductor lasers, 5 and photodetectors. 6 Recently, periodic metal hole arrays have been applied to quantum dot infrared photodetectors to enhance the efficiency. 7,8 However, as far as we know, there is still no experimental study on applying surface plasmons to enhance the sensitivity of quantum well infrared photodetector ͑QWIP͒. Unlike quantum dot infrared photodetectors, QWIP is only sensitive to the electromagnetic ͑EM͒ waves which have electric field component normal to the quantum wells surface ͑TM mode͒. 9 In the mid and long-infrared wavelengths, where most QWIP devices are operational, the penetration depth of surface plasmons in metals is greatly reduced. Therefore, it yields a low optical loss, and surface plasmon waves have a long propagation length. 10 Surface plasmon supports TM mode and requires the electric field being normal to the surface because of the generation of surface charge. 11 In addition, a carefully designed plasmonic array forms standing waves and produces a cavity effect, which leads to an enhanced transverse plasmonic mode. Therefore, if properly coupled, surface plasmons can resonate with electron intersubband transitions, and efficiently excite carriers in the QWs to generate a strong photocurrent.To simulate the surface plasmon waves and electric field component distribution we used three-dimensional finitedifference time-domain methods. The simulated structure is shown in Fig. 1͑a͒, where the source is a normally incident plane wave. After optimizing the electric field intensity distribution at the wavelength of ϳ8 m, where our QWIP device works, we selected the diameter of the holes d as 1.4 m, the lattice constant of the array a as 2.9 m, and the thickness of Au layer t as 40 nm. The Au layer perforated with holes array is on top of In 0.53 Ga 0.47 As/ InP semiconductor layers. Figure 1͑b͒ shows the simulated spectrum of ͉E z ͉ 2 averaged across the whole quantum well active region between 140 and 584.8 nm below the Au/Semiconductor interface ͓the exact structure is shown in Fig. 2͑...