Amlan J. Pal

Functionalized carbon nanotubes in donor/acceptor-type photovoltaic devices

We have introduced functionalized multiwalled carbon nanotubes (CNTs) in donor/acceptor-type photovoltaic devices. We fabricated the devices based on heterostructure between polymer-CNT composite and buckminsterfullerene (C60) layers. Due to the functional groups of the CNTs, a homogeneous blend of CNT-polymer composite could be obtained. In the composite, the nanotubes acted as exciton dissociation sites and also hopping centers for hole transport. The CNTs in the polymer-CNT∕C60 device provided higher exciton dissociation volume and increased mobility for carrier transport. We have observed an increase in open-circuit voltage and short-circuit current in the polymer-CNT∕C60 devices as compared to the polymer∕C60 ones.

Large conductance switching and memory effects in organic molecules for data-storage applications

We have observed a large electrical conductance switching (ON:OFF ratio=105) in single-layer sandwich structures based on organic molecules at room temperature. The switching devices showed an associated memory effect for data-storage applications. We could write or erase a state and read it for many cycles. In switching devices, the active semiconductor retained its high conducting state until a reverse voltage erased it. A high conducting state arose due to restoration of conjugation in the molecule via electroreduction. Such a high ON–OFF ratio in a single layer sandwich structure, as compared to contemporary switching devices, is due to low off-state leakage current. The concept of conjugation restoration has been verified in supramolecular structures by adding donor groups to the molecule, which resulted in increased off-state current and hence lower ON–OFF ratio. Our work set a generalized example of selecting organic molecules to obtain higher ON–OFF ratio in molecular switching devices.

Multilevel conductivity and conductance switching in supramolecular structures of an organic molecule

We have demonstrated conductance switching between multilevel states in devices based on Rose Bengal molecules embedded in supramolecular matrices. Two mechanisms, namely electroreduction and conformational change of the molecules, resulting in conjugation modification have been proposed to be applicable in these devices. In a low voltage region, reverse-bias induced electroreduction of Rose Bengal facilitated conjugation restoration in the backbone of the molecule and, hence, switching to a high-conducting state. At high biases, the two perpendicular planes present in Rose Bengal, which have permanent dipole moments, allowed forward-bias induced conformation change to occur, and results in conductance switching. We have demonstrated how the devices can switch between two pair of conducting states for random-access memory and read-only memory applications for several hours.

Memory device applications of a conjugated polymer: role of space charges

Conjugated polymers have been used in data-storage devices. A “state” has been written by applying a voltage pulse. The state of the device has been “read” from the current under a small probe voltage (0.2 V). The polymer retained the state for more than 1 h which can be refreshed or erased at will. The stored space charges under a voltage pulse have been found to control the charge injection and hence the device current. Their slow relaxation process has resulted in the use of conjugated polymers in memory device applications. Hysteresis-type behavior has been observed in the current–voltage characteristics. The density of stored charges at the polymer layer near the metal/polymer interface has been found to depend on the voltage amplitude. The relaxation of the stored charges has been studied by applying two voltage pulses. By varying the delay between the two pulses, during which the space charges relax or redistribute, the time constant for charge relaxation has been calculated. The time constant was found to be independent of the density of the space charges or of the pace at which they were stored.Conjugated polymers have been used in data-storage devices. A “state” has been written by applying a voltage pulse. The state of the device has been “read” from the current under a small probe voltage (0.2 V). The polymer retained the state for more than 1 h which can be refreshed or erased at will. The stored space charges under a voltage pulse have been found to control the charge injection and hence the device current. Their slow relaxation process has resulted in the use of conjugated polymers in memory device applications. Hysteresis-type behavior has been observed in the current–voltage characteristics. The density of stored charges at the polymer layer near the metal/polymer interface has been found to depend on the voltage amplitude. The relaxation of the stored charges has been studied by applying two voltage pulses. By varying the delay between the two pulses, during which the space charges relax or redistribute, the time constant for charge relaxation has been calculated. The time constant was ...

Data-storage devices based on layer-by-layer self-assembled films of a phthalocyanine derivative

Abstract An organic dye, namely nickel phthalocyanine, has been used in data-storage devices. A “high state” has been written by applying a voltage pulse. The state of the device has been “read” by applying a small probe voltage. The dye embedded in an inert polymer matrix retained the high state for more than an hour, which can be refreshed or erased at will. Hysteresis-type behaviour has been observed in the current–voltage characteristics. The space charges at the metal/semiconductor interfaces, stored under the voltage pulse, have been found to control the charge injection and hence the current in these devices. The formation of space charges near the interfaces, and relaxation have been studied in the data-storage devices. The space charges’ slow relaxation process has been shown to result in the memory device applications of the semiconducting dyes.

Multilevel conductance and memory in ultrathin organic films

The present letter reports conductance switching in Langmuir–Blodgett films of an organic semiconductor. We have achieved multiple conducting levels in devices based on ultrathin films for increased density of memory bits in the same space. We have shown that multiple conducting levels in a device can be achieved by controlling the density of high-conducting molecules in the structures. We have observed one low- and three high-conducting states of the devices. All four states have associated memory for data-storage applications. Any of the four states, namely 00, 01, 10, and 11, can be “read” for several hours for read-only memory applications. We could “erase” a state, “write” another, and “read” the state for multibit random-access-memory applications.

Transient electroluminescence: Mobility and response time in quinquethiophene Langmuir–Blodgett films

Transient characteristics of organic light-emitting diodes with Langmuir–Blodgett films of quinquethiophene as the emitting material have been studied. Field and thickness dependence of the response time have been studied. From the time lag between the voltage pulse and the first appearance of electroluminescence emission, the mobility parallel to the chain of the molecules has been calculated. In thicker films, anisotropy in mobility has been observed when compared with previous results obtained from field-effect studies. The interfaces have been found to play an important role in thinner films.

Switching and memory devices based on a polythiophene derivative for data-storage applications

In this article, we report electrical characteristics of devices based on oriented and unoriented films of a polymer, namely poly[3-(6-methoxyhexyl)thiophene]. The current-voltage characteristics of sandwiched devices, based on unoriented polymer, showed hysteresis behavior, while oriented versions exhibited switching characteristics, i.e. presence of two conducting states depending on sweep direction of voltage scans. The ratio between the device current of two conducting states has been as high as 10 5 . This is comparable, if not better, than the results reported so far with complicated device architecture or doped polymeric materials. We have also demonstrated that the switching devices have an associated memory effect for data-storage applications.

Near-IR activity of hybrid solar cells: Enhancement of efficiency by dissociating excitons generated in PbS nanoparticles

Photovoltaic devices based on PbS nanoparticles remained inactive in the near-IR region due to a not-so-favorable energy band-diagram that does not allow dissociation of excitons generated in PbS. In this work, with the introduction of TiO2 nanostructures in the PbS-based hybrid system, we show an enhancement of photovoltaic performance in both visible and near-IR regions. The addition of TiO2 increases the power conversion efficiency from 0.006% to 0.12%. With the aid of energy band-diagram, we show that excitons generated in PbS even in the near-IR range can now become dissociated to yield photocurrent in the external circuit.

Copper-Diffused AgInS2 Ternary Nanocrystals in Hybrid Bulk-Heterojunction Solar Cells: Near-Infrared Active Nanophotovoltaics

We have grown copper-diffused AgInS2 ternary nanocrystals in order to introduce the nanoparticles in organic/inorganic bulk-heterojunction devices for photovoltaic applications. Here, copper diffuses to vacant sites and improves conductivity of the nanocrystals. Upon use of such copper-diffused nanoparticles that led to a decrease in internal resistance of sandwiched devices based on the bulk-heterojunction, there has been a marked improvement in short-circuit current under white light illumination. Due to a red-shift in the optical absorption spectrum of the nanoparticles upon copper diffusion, the devices moreover acted as near-infrared (IR) active photovoltaic solar cells. From current-voltage characteristics and impedance spectroscopy of the devices, we optimized performance of the photovoltaic devices. To do so, we have varied the content of diffused copper in AgInS2 nanoparticles and also the weight-ratio between the polymer and the nanoparticles of the hybrid bulk-heterojunction devices.