Sudip K. Saha

Improvement in PbS-based Hybrid Bulk-Heterojunction Solar Cells through Band Alignment via Bismuth Doping in the Nanocrystals

We introduce dopants in lead sulfide (PbS) quantum dots (QDs) in forming hybrid bulk-heterojunction (BHJ) solar cells. Because an increase in the content of bismuth as dopants in PbS QDs transforms the intrinsic p-type semiconductor into an n-type one, the band alignment between a conjugated polymer and the doped QDs changes upon doping affecting performance of BHJ solar cells. From scanning tunneling spectroscopy (STS) of the doped QDs, we observe a shift in their Fermi energy leading to formation of a type II band alignment in the polymer:doped-QD interface. We also show that the dopants improve electron-conduction process through the QDs. With the dopants controlling both band alignments at the interface and the conduction process, we show that the dopant concentration in QDs influences open-circuit voltage unfavorably and short-circuit current in a beneficial manner. The device performance of hybrid BHJ solar cells is hence maximized at an optimum concentration of bismuth in PbS QDs.

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.

Schottky diodes between Bi2S3 nanorods and metal nanoparticles in a polymer matrix as hybrid bulk-heterojunction solar cells

We report the use of metal-semiconductor Schottky junctions in a conjugated polymer matrix as solar cells. The Schottky diodes, which were formed between Bi2S3 nanorods and gold nanoparticles, efficiently dissociated photogenerated excitons. The bulk-heterojunction (BHJ) devices based on such metal-semiconductor Schottky diodes in a polymer matrix therefore acted as an efficient solar cell as compared to the devices based on only the semiconductor nanorods in the polymer matrix or when gold nanoparticles were added separately to the BHJs. In the latter device, gold nanoparticles offered plasmonic enhancement due to an increased cross-section of optical absorption. We report growth and characteristics of the Schottky junctions formed through an intimate contact between Bi2S3 nanorods and gold nanoparticles. We also report fabrication and characterization of BHJ solar cells based on such heterojunctions. We highlight the benefit of using metal-semiconductor Schottky diodes over only inorganic semiconductor nanorods or quantum dots in a polymer matrix in forming hybrid BHJ solar cells.

Investigation of cross-hatch in In0.3Ga0.7As pseudo-substrates

Metamorphic buffer layers offer a wide variety of lattice constants for substrate on which devices can be grown. However, almost in all cases, the surface of the pseudo-substrate contains striations which are known as “cross-hatch.” Although, it is accepted that this surface undulation is related with the underlying gridlike misfit dislocations (MDs), the exact correlation is still to be determined. In this article, degree of polarization of photoluminescence maps and atomic force microscopy were used to analyze the correlation between surface undulation and the underlying strain field of the pseudo-substrate. From the correlation, it can be said that the surface undulation is not formed after MD nucleation, but MDs form in some of the troughs of the undulation.

Solution-processed reduced graphene oxide in light-emitting diodes and photovoltaic devices with the same pair of active materials

We have introduced organic-dispersed reduced graphene oxides (rGO) in light-emitting diodes and photovoltaic devices. Both the devices were based on a conjugated polymer and inorganic nanoparticles. While we used a derivative of poly(para-phenylenevinylene) (MEH-PPV) as the organic semiconductor, zinc diffused copper indium disulphide (CIZS) quantum dots have been used as the inorganic counterpart that were based on nontoxic elements. We have shown that the device architecture in relation to rGO determines the functionality of a device based on the hybrid materials. A layer of rGO acted as a hole transport layer in MEH-PPV/CIZS heterojunction LEDs. On the other hand, the carbon allotrope added to MEH-PPV:CIZS bulk-heterojunction quenched photoluminescence of the components through a photoinduced electron-transfer process thereby dissociating photogenerated excitons yielding PV properties. The power conversion efficiency maximized at an optimum rGO concentration. The results show the role of reduced graphene oxides with the same pair of active materials in light-emitting diodes and photovoltaic devices.

Enhancing the Photoelectrochemical Response of DNA Biosensors Using Wrinkled Interfaces

Photoelectrochemical (PEC) biosensors, with optical biasing and electrochemical readout, are expected to enhance the limit-of-detection of electrochemical biosensors by lowering their background signals. However, when PEC transducers are functionalized with biorecognition layers, their current significantly decreases, which reduces their signal-to-noise ratio and dynamic range. Here, we develop and investigate a wrinkled conductive scaffold for loading photoactive quantum dots into an electrode. The wrinkled photoelectrodes demonstrate an order of magnitude enhancement in the magnitude of the transduced PEC current compared to their planar counterparts. We engineer PEC biosensors by functionalizing the wrinkled photoelectrodes with nucleic acid capture probes. We challenge the sensitivity of the wrinkled and planar biosensors with various concentrations of DNA target and observe a 200 times enhancement in the limit-of-detection for wrinkled versus planar electrodes. In addition to enhanced sensitivity, the wrinkled PEC biosensors are capable of distinguishing between fully complementary and targets with a single base-pair mismatch, demonstrating the suitability of these biosensors for use in clinical diagnostics.