Asim Guchhait

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.

Organic/inorganic hybrid pn-junction between copper phthalocyanine and CdSe quantum dot layers as solar cells

We have introduced an organic/inorganic hybrid pn-junction for solar cell applications. Layers of II-VIquantum dots and a metal-phthalocyanine in sequence have been used as n- and p-type materials, respectively, to form a junction. The film of quantum dots has been formed through a layer-by-layer process by replacing the long-chain ligands of the nanoparticles in each ultrathin layer or a monolayer with short-chain ones so that interparticle distance becomes small leading to a decrease in resistance of the quantum dot layer. With indium tin oxide and Au as electrodes, we have formed an inverted sandwiched structure. These electrodes formed ohmic contacts with the neighboring materials. From the current-voltage characteristics of the hybrid heterostructure, we have inferred formation of a depletion region at the pn-junction that played a key role in charge separation and correspondingly a photocurrent in the external circuit. For comparison, we have also formed and characterized Schottkydevices based on components of the pn-junction keeping the electrode combination same. From capacitance-voltage characteristics, we have observed that the depletion region of the hybrid pn-junction was much wider as compared to that in Schottkydevices based on components of the junction.

Photoinduced electron transfer from the inorganic core to the organic shell of hybrid core-shell nanoparticles: impedance spectroscopy.

In hybrid core-shell nanoparticles with inorganic nanocrystals in the core and organic molecules in the shell, photoinduced electron transfer occurs from the core to the shell. This leads to exciton dissociation through an ultrafast electron-transfer process that results in charge separation and finally photocurrent in the external circuit in devices based on such core-shell nanoparticles. In this work, we have fabricated and characterized sandwiched devices based on a series of core-shell systems. From impedance spectroscopy, we have observed that photoinduced charge separation in core-shell systems is associated with a decrease in the device resistance and an increase in the dielectric constant of the active material. In the series of core-shell systems, we have observed a one-to-one correlation between the photoinduced electron-transfer process and the changes in resistive and dielectric parameters upon illumination.