Application of graphene-related materials in organic solar cells

Graphene-related materials (GRMs) such as graphene quantum dots (GQDs), graphene oxide (GO), reduced graphene oxide (rGO), graphene nanoribbons (GNRs), and so forth have recently emerged as photovoltaic (PV) materials due to their nanodimensional structure and outstanding properties such as high electrical and thermal conductivity, large specific surface, and unique combination of mechanical strength and flexibility. They can be a crucial part of transparent electrodes, hole/electron transport materials, and active layers in organic solar cells (OSCs). Besides their role in charge extraction and transport, GRMs act as device protectors against environmental degradation through their compact bidimensional structure and offer good durability. This review briefly presents the synthesis methods of GRMs and describes the current progress in GRM-based OSCs. PV parameters (short circuit current, open circuit voltage, power conversion efficiency, and fill factor) are summarized and comparatively discussed for the different structures. The efficiency recently surpassed 15% for an OSC incorporating polymer-modified graphene as a transparent electrode. The long-term stability of OSCs incorporating GRMs is also discussed. Finally, conclusions and the outlook for future investigation into GRM-based devices for PVs are presented

Counter electrode materials based on carbon nanotubes for dye-sensitized solar cells

Efficiency, stability, and cost-effectiveness are the prime challenges in research of materials for solar cells. Technologically as well as scientifically, attention gained by dye-sensitized solar cells (DSSCs) stems from their low material and fabrication costs as well as high efficiency projections. The aim of this study is to explore the carbon nanotubes (CNTs) based counter electrode (CE) materials for DSSCs and to reconnoiter the suitable alternative materials in place of noble metals such as Platinum (Pt), and Gold (Au).. Various classes of CE materials based on CNTs including pure single walled, double walled, and multiwalled CNTs, doped CNTs and their hybrid composites with various polymers, and transition metal compounds are discussed comprehensively in light of the research work started since the inspection of DSSCs and CNTs.The properties associated with such materials, including surface morphology, structural determination, thermal stability, and electrochemical activity, are also thoroughly analyzed and compared. This work provides a thorough insight into the possibility of exploiting CNTs as alternative CE materials. In addition to the above, this study also includes the working and brief overview of materials for other components of DSSCs such as photoanode, electrolyte, and sensitizer.

Single-Walled Carbon Nanotube electrodes for all-plastic, electronic device applications

In this thesis, new mechanically robust, high performance transparent conducting films of commercially sourced arc-made Single-Walled Carbon Nanotubes (SWCNTs) on both glass and flexible substrates were produced using spin-coating or spray deposition, interlayer or stencil patterning methods and used for fabricating efficient, flexible polymer-fullerene bulk hetero-junction solar cells. After carefully optimizing the dispersion process of SWCNTs with H2O:SDS (up to 0.03 wt.%) and developing and efficient surfactant removal/p-doping procedure with nitric acid, highly conductive and smooth SWCNT thin films (ca. 30 nm) were obtained with more than 6,500 Scm-1 at > 69 % transmittance and 7 nm (r.m.s.) roughness. In particular, SWCNT films spray coated from H2O:SDS exhibited electrical conductivities of up to 7694 ± 800 Scm-1. To our knowledge, these values are the highest so far reported for SWCNT electrodes. Peak values for the ratio of the dc conductivity to the optical conductivity (σdc/σop) were obtained as up to 24, which is quite similar to state of the art SWCNT films so far reported. In addition, two patterning methods were developed to define electrode patterns of SWCNT thin films for electronic device applications. Interlayer lithography provided a fast and high resolution patterning procedure for SWCNT thin films at micron and sub-micron length scales, which is important for the fabrication of high-speed transistors requiring short channel lengths, and offers an attractive route to fabricating high-density integrated circuits. In addition, stencil patterning provides a simple and fast method, which is well suited for low resolution electronic device applications such as organic solar cells. The patterned highly conductive SWCNT electrodes were incorporated into P3HT:PCBM bulk heterojunction solar cell applications, obtaining the best device performance of 3.6 %, which is the best result so far reported in the literature. Finally, to break through the limited performance (σdc/σop < 25) of SWCNT thin films, layered hybrid thin films of SWCNTs on reduced Graphene-Oxide were fabricated by a simple spray coating method and the optimised hybrid films were incorporated into relatively efficient organic solar cells (2 % efficiency)

From highly graphitic to amorphous carbon dots: a critical review

The emergence of carbogenic nanoparticles (C-dots) as a new class of photoluminescent (PL) nanoemitters is directly related to their economical preparation, non-toxic nature, versatility and tuneability. C-dots are typically prepared by pyrolytic or oxidative treatment of suitable precursors. While the surface functionalities critically affect the dispesrsibility and the emission intensity of C-dots in a given environment, it is the nature of the carbogenic core that actually imparts their intrinsic PL properties. Depending on the synthetic approach and the starting materials, the structure of the carbogenic core can vary from highly graphitic all the way to completely amorphous. This critical review focuses on correlating the functions of C-dots with the graphitic or amorphous nature of their carbogenic cores. The systematic classification on that basis can provide insights on the origins of their intriguing photophysical behaviour and can contribute in realising their full potential in challenging applications

Low-temperature titania-graphene quantum dots paste for flexible dye-sensitised solar cell applications

Graphene possesses excellent mechanical strength and chemical inertness with high intrinsic carrier mobility and superior flexibility making them exceptional candidates for optoelectronic applications. Graphene quantum dots (GQDs) derived from graphene domains have been widely explored to study their photoluminescence properties which can be tuned by size. GQDs are biocompatible, low cytotoxic, strongly luminescent and disperse well in polar and non-polar solvents showing bright promise for the integration into devices for bioimaging, light emitting and photovoltaic applications. In the present study, graphene quantum dots were synthesized by an electrochemical cyclic voltammetry technique using reduced graphene oxide (rGO). GQDs have been incorporated into binder free TiO2 paste and studied as a photoelectrode material fabricated on ITO/PEN substrates for flexible dye sensitized solar cells (DSSCs). DSSC based on GQDs-TiO2 exhibited open circuit output potential difference (Voc) of 0.73 V, and short circuit current density (Jsc) of 11.54 mA cm-2 with an increment in power conversion efficiency by 5.48 %, when compared with those with DSSC build with just a TiO2 photoanode (open-circuit output potential difference (Voc) of 0.68 V and short circuit density (Jsc) of 10.67 mA cm-2). The results have been understood in terms of increased charge extraction and reduced recombination losses upon GQDs incorporation

Carbon Nanotube–Polymer Composites: Device Properties and Photovoltaic Applications

This chapter provides an in-depth coverage of recent advances in the areas of the development and characterization of electro-optically active, device-grade carbon nanotube (CNT)–polymer blends. These new organic–inorganic multifunctional nanocomposites share many advanced characteristics which make them ideally suited for industrial scale, high-throughput manufacturing of lightweight, flexible electronic, light switching and emitting as well as energy harvesting devices of extremely low cost. The fundamental aspects and the physical mechanisms controlling light–matter interaction, photo-conversion, and photo-generated charge-carrier transport in these nanotube–polymer composites as well as the influence of the processing conditions on the electronic properties and device-related performances are further reviewed and discussed

DEVELOPMENT AND EVALUATION OF CARBON-BASED QUANTUM DOTS FOR CARBON DIOXIDE PHOTOCONVERSION

World energy consumption has increasingly grown over the past several decades.Because of its potential in photochemical energy conversion, photocatalysis has been the subject of much recent research. Recently, carbon or graphene-based quantum dots have attracted growing attention in solar energy conversion applications, because of its unique optoelectronic properties, broad-band optical absorption, bright fluorescence emissions, favorable photoinduced electron transfer properties, reliable chemical inertness and stability, cost-effectiveness, and non-toxicity. While nanosized wide band gap semiconductor-based systems were largely at the center of attention in such studies, carbon-based quantum dots have recently emerged as a new class of semiconductor like photoactive materials, due to some of its excellent optical figures of merit suited for light harvesting applications. In this dissertation, we have demonstrated the possibility of using quantum-sized carbon particles as chromophores for photosensitized energy conversion and visible-light photocatalysts for carbon dioxide conversion to organic acids as well as results supporting photoinduced redox properties in carbon nanodots. Metal- and semiconductor-doped carbon nanodots in various configurations have been developed for their utility in photocatalytic conversion of carbon dioxide. Our results demonstrate that nanoscale carbon dots represent a promising new alternative platform for light-driven energy conversion applications, competitive to conventional nanoscale semiconductor-based photocatalytic systems

Novel Nanostructured Titania and Titania Nanocomposites for Photovoltaics and Photocatalysis

With the consumption of energy continually increasing around the world and the main source of this energy, fossil fuels, slowly being depleted, the need for alternate sources of energy is becoming more and more pertinent. Demand for solar energy has experienced exponential increase over the last decade. Nanostructured TiO2 has attracted significant attention due to its nontoxicity, low cost and wide applications in photovoltaics and photocatalysis. This research is focused on novel synthesis and surface modification of TiO2 nanotube arrays for applications in advanced dye-sensitized solar cells (DSSCs) and efficient photocatalysis. The first part of this work entails fast synthesis of bamboo-type TiO2 nanotube arrays with large surface area via anodization of Ti substrates for applications as photo-anodes in high-efficiency DSSCs. In addition, titania nanotubes are modified with other nanomaterials for further increased efficiency of DSSCs. For example, uniformly-sized Ag nanoparticles are deposited onto TiO2 nanotube array via pulse electrodeposition for plasmon effect, leading to enhanced light absorption in DSSCs. Also, reduced graphene oxide nanosheets are deposited onto a TiO2 nanotube array using electrophoretic deposition, for increased electronic conductivity and improved electron transport in DSSCs. Additionally, ultra-thin two dimensional TiO2 nanosheets are synthesized via exfoliation of layered protonated titanate into separate layers using bulky organic ions, for application as photo-anodes with enhanced light scattering and dye loading in high-efficiency DSSCs. The second part of the work concentrates on synthesis of Ag-modified bamboo-type TiO2 nanotube arrays for efficient photocatalysis. Such novel titania-based nanocomposite structure provides large surface area for organic pollutant absorption and subsequent degradation; the ordered structure of nanotube array also offers direct pathway for fast electron transport. Moreover, Ag nanoparticles deposited onto TiO2 nanotubes function as reservoirs for photogenerated electrons to improve charge separation and facilitate catalytic reactions