Ishwor Khatri

Improving photovoltaic properties by incorporating both single walled carbon nanotubes and functionalized multiwalled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) and functionalized multiwalled carbon nanotubes (f-MWCNTs) are introduced together for photovoltaic application in a poly(3-octylthiophene)/n-Si heterojunction solar cell. The performance of the device was improved by manyfold by the incorporation of both SWCNTs and f-MWCNTs. The open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and power conversion efficiency (η) were 0.44 V, 6.16 mA/cm2, 36%, and 0.98%, respectively. Here, we expect that SWCNTs help in exciton dissociation and provide percolation paths for electron transfer, whereas f-MWCNTs provide efficient hole transportation. CNT incorporation yields better carrier mobility, easy exciton splitting, and suppression of charge recombination, thereby improving photovoltaic action.

Optical anisotropy in solvent-modified poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) and its effect on the photovoltaic performance of crystalline silicon/organic heterojunction solar cells

An investigation was carried out into the effect of uniaxial optical anisotropy in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) on the photovoltaic performance of crystalline Si/PEDOT:PSS heterojunction solar cells fabricated by spin coating using either a methanol (MeOH) solvent alone or using MeOH and ethylene glycol (EG) as cosolvents. Spectroscopic ellipsometry revealed that the extraordinary index of refraction increased by the use of the cosolvents. In contrast, the ordinary index of refraction indicated metallic properties and was almost independent of the concentration of MeOH or EG. The highest conductivity was found for a (PEDOT:PSS):(MeOH):(EG) weight ratio of 1:1:0.1, and this sample exhibited a relatively high power conversion efficiency of 11.23%. These findings suggest that the increase in the extraordinary index of refraction leads to an enhancement of the hole mobility in PEDOT:PSS, resulting in improved photovoltaic performance.

Green-tea modified multiwalled carbon nanotubes for efficient poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate)/n-silicon hybrid solar cell

Poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) (PEDOT:PSS)/n-Si hybrid solar cells were studied with and without embedding green-tea modify multiwalled carbon nanotubes (MWCNTs) at interface. Devices fabricated with embedding green-tea modified MWCNTs show much better performance than that of a device without MWCNTs with short circuit current density (Jsc), open circuit voltage (Voc), fill factor, and power conversion efficiency (η) as 30.31 mA/cm2, 0.54 V, 0.66, and 10.93%, respectively. Here, we believe that green-tea disperse MWCNTs bundles to individual and its incorporation improved built-in potential (Vb) of the device for better hole transport, easy exciton splitting, and suppression of charge recombination, thereby improving photovoltaic response.

Effects of molybdenum oxide molecular doping on the chemical structure of poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) and on carrier collection efficiency of silicon/poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) heterojunction solar cells

The effects of MoO3 molecular doping in poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) (PEDOT:PSS) on the chemical structure and, in turn, on the carrier collection efficiency of c-Si/PEDOT:PSS heterojunction solar cells are demonstrated. Scanning electron microscopy revealed that the hydrophilic PSS polymer chain was intercalated into the interlayer van der Waals gap of MoO3 flake sheets, which modified the chemical structure of PEDOT:PSS. MoO3 exhibited intense photoluminescence in the 350–550 nm region, which enhanced the carrier collection efficiency of c-Si/PEDOT:PSS heterojunction solar cells with no significant changes. These findings suggest that the intense photoluminescence of MoO3 and its light wavelength conversion contribute to the increased carrier collection efficiency.

Improved photovoltaic performance of crystalline-Si/organic Schottky junction solar cells using ferroelectric polymers

The effect of inserting an ultrathin layer of ferroelectric (FE) poly(vinylidene fluoride-tetrafluoroethylene) P(VDF-TeFE) at the crystalline (c-)Si/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) interface of a c-Si/PEDOT:PSS Schottky junction solar cell is demonstrated. P(VDF-TeFE) is a highly resistive material that exhibits a large, permanent, internal polarization electric field by poling of molecular dipole among the polymer chains. Because of these properties, performance can be enhanced by adjusting the thickness of the FE layer and subsequent poling process. Inserting a 3-nm-thick FE layer increases the power conversion efficiency η from 10.2% to 11.4% with a short-circuit current density Jsc of 28.85 mA/cm2, an open-circuit voltage Voc of 0.57 V, and a fill factor FF of 0.692. Subsequent poling of the FE layer under a reverse DC bias stress increased η up to 12.3% with a Jsc of 29.7 mA/cm2, a Voc of 0.58 V, and an FF of 0.71. The obtained results confirm that the spontaneous...

Similar Device Architectures for Inverted Organic Solar Cell and Laminated Solid-State Dye-Sensitized Solar Cells

Here, we examine the device architecture of two different types of solar cells mainly inverted organic solar cells and solid state dye-sensitized solar cells (DSSCs) that use organic materials as hole transportation. The inverted organic solar cells structure is dominated by work on titanium dioxide (TiO2) and zinc oxide (ZnO). These layers are sensitized with dye in solid state DSSCs. Because of the similar device architecture, it becomes possible to fabricate laminated solid-state DSSCs. The performance of the device was improved by varying the top metal electrode. In laminated solid-state DSSC, we expect that excited dye molecules inject electron into the conduction band of nanocrystalline TiO2 layer, whereas P3HT provides efficient hole transportation. These solar cells are promising for future energy source as they are cheaper, light weight, flexible and made into large areas, which are showing growing importance.

Surface sulfurization on MBE-grown Cu(In1−x,Gax)Se2 thin films and devices

Molecular beam epitaxy (MBE) grown Cu(In1−x,Gax)Se2 (CIGS) thin films were sulfurized at temperatures of 450–550 °C for 30 min in a 10% H2S–N2 mixture gas. The micro-roughness together with the S diffusion in the CIGS surfaces increased with increasing sulfurization temperature. Both near-band-edge PL intensity and decay time of the CIGS absorber layer enhanced after sulfurization. PL sub-peak around 80 meV below the main peak almost disappeared after sulfurization above 500 °C, which is expected due to the occupation of Se vacancies (Vse) with S. The open-circuit voltage (Voc), hence conversion efficiency, improved after sulfurization. The photovoltaic performance of the solar cells was consistent with PL intensity. Moreover, it is found for the first time from the SIMS analysis that the Cu atoms were depleted at the surface of CIGS layer after sulfurization, which could result in the improved Voc.

Improved performance of poly(3,4-ethylenedioxythiophene):poly(stylene sulfonate)/n-Si hybrid solar cell by incorporating silver nanoparticles

We report an enhancement in the efficiency of poly(3,4-ethylenedioxythiophene):poly(stylene sulfonate) (PEDOT:PSS)/n-Si hybrid solar cell by incorporating silver nanoparticles (AgNPs) with PEDOT:PSS. AgNPs were prepared by reducing silver nitrate in green-tea solution, which showed characteristic absorption peak due to the surface plasmonic resonance effect. AgNPs incorporated PEDOT:PSS/n-Si hybrid device shows power conversion efficiency (η) of 10.21%, which is comparatively higher to the performance of pristine device without AgNPs. Here, we noticed that incorporation of AgNPs decreases sheet resistance and enlarged surface roughness of PEDOT:PSS film for the efficient collection of charges, rather than plasmonic effect.

Optical and carrier transport properties of graphene oxide based crystalline-Si/organic Schottky junction solar cells

We investigated the graphene oxide (GO) based n-type crystalline silicon (c-Si)/conductive poly(ethylene dioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) Schottky junction devices with optical characterization and carrier transport measurement techniques. The optical transmittance in the UV region decreased markedly for the films with increasing the concentration of GO whereas it increased markedly in the visible-infrared regions. Spectroscopic ellipsometry revealed that the ordinary and extraordinary index of refraction increased with increasing the concentration of GO. The hole mobility also increased from 1.14 for pristine film to 1.85 cm2/V s for the 12–15 wt. % GO modified film with no significant increases of carrier concentration. The highest conductivity was found for a 15 wt. % GO modified PEDOT:PSS film: the c-Si/PEDOT:PSS:GO device using this sample exhibited a relatively high power conversion efficiency of 11.04%. In addition, the insertion of a 2–3 nm-thick GO thin layer at the c-Si/PEDOT:PS...

Carbon Nanotubes Towards Polymer Solar Cell

The semiconducting polymer thin film has gained substantial interest in the research community because of the possibility to produce polymer based photovoltaic devices by roll to roll type manufacture, which is impossible by conventional technologies. Hole transferring semiconducting polymer and electron accepting fullerene (C60) derivative are of special interest because of their stability and high power conversion efficiencies. With the discovery of new carbon “carbon nanotubes” (CNTs), researchers have started blending them with polymer for improving the solar cell efficiency. CNT-incorporated solar cell shows better power conversion efficiency than pristine solar cell without CNTs. This is because of the wonderful properties of CNTs. CNTs have outstanding properties like ballistic conductive, high aspect ratio, high surface area, flexible, strong, rigid, environmental stable, capability of charge dissociation, transportation and so on, which are believed to be an ideal material for fabricating high performance solar cell. In this chapter, some of the works done on polymer–CNTs based solar cells are summarized. A variety of device architecture and band diagram proposed by different authors have been included and described.