Optical Characterization of the Hole Polaron in a Series of Diketopyrrolopyrrole Polymers Used for Organic Photovoltaics

Abstract: A strategy that is often used for designing low band gap polymers involves the incorporation of electron-rich (donor) and electron-deficient (acceptor) conjugated segments within the polymer backbone. In this paper we investigate such a series of Diketopyrrolopyrrole (DPP)-based co-polymers. The co-polymers consisted of a DPP unit attached to a phenylene, naphthalene, or anthracene unit. Additionally, polymers utilizing either the thiophene-flanked DPP or the furan-flanked DPP units paired with the naphthalene comonomer were compared. As these polymers have been used as donor materials and subsequent hole transporting materials in organic solar cells, we are specifically interested in characterizing the optical absorption of the hole polaron of these DPP based copolymers. We employ chemical doping, electrochemical doping, and photoinduced absorption (PIA) studies to probe the hole polaron absorption spectra. While some donor-acceptor polymers have shown an appreciable capacity to generate free charge carriers upon photoexcitation, no polaron signal was observed in the PIA spectrum of the polymers in this study. The relations between molecular structure and optical properties are discussed. Keywords: organic solar cell; organic photovoltaic; diketopyrrolopyrrole; chemical doping; spectroelectrochemistry; photoinduced absorption; hole polaron

Time-independent charge carrier mobility in a model polymer:fullerene organic solar cell

The technique of photo-CELIV (charge extraction by linearly increasing voltage) is one of the more straightforward and popular approaches to measure the faster carrier mobility in measurement geometries that are relevant for operational solar cells and other optoelectronic devices. It has been used to demonstrate a time-dependent photocarrier mobility in pristine polymers, attributed to energetic relaxation within the density of states. Conversely, in solar cell blends, the presence or absence of such energetic relaxation on transport timescales remains under debate. We developed a complete numerical model and performed photo-CELIV experiments on the model high efficiency organic solar cell blend poly[3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene] (PDPP-TNT):[6,6]-phenyl-C71-butyric-acid-methyl-ester (PC70BM). In the studied solar cells a constant, time-independent mobility on the scale relevant to charge extraction was observed, where thermalisation of photocarriers occurs on time scales much shorter than the transit time. Therefore, photocarrier relaxation effects are insignificant for charge transport in these efficient photovoltaic devices.

Thiophene–tetrafluorophenyl–thiophene : a promising building block for ambipolar organic field effect transistors

A thiophene–tetrafluorophenyl–thiophene donor–acceptor–donor building block was used in combination with a furan-substituted diketopyrrolopyrrole for synthesizing the polymer semiconductor, PDPPF-TFPT. Due to the balance of tetrafluorophenylene/diketopyrrolopyrrole electron-withdrawing and furan/thiophene electron-donating moieties in the backbone, PDPPF-TFPT exhibits ambipolar behaviour in organic thin-film transistors, with hole and electron mobilities as high as 0.40 cm2 V−1 s−1 and 0.12 cm2 V−1 s−1.

Identification of the sources of primary organic aerosols at urban schools : a molecular marker approach

Children are particularly susceptible to air pollution and schools are examples of urban microenvironments that can account for a large portion of children’s exposure to airborne particles. Thus this paper aimed to determine the sources of primary airborne particles that children are exposed to at school by analyzing selected organic molecular markers at 11 urban schools in Brisbane, Australia. Positive matrix factorization analysis identified four sources at the schools: vehicle emissions, biomass burning, meat cooking and plant wax emissions accounting for 45%, 29%, 16% and 7%, of the organic carbon respectively. Biomass burning peaked in winter due to prescribed burning of bushland around Brisbane. Overall, the results indicated that both local (traffic) and regional (biomass burning) sources of primary organic aerosols influence the levels of ambient particles that children are exposed at the schools. These results have implications for potential control strategies for mitigating exposure at schools.

Bisphenol A sorption by organo-montmorillonite: Implications for the removal of organic contaminants from water

Remediation of bisphenol A (BPA) from aqueous solutions by adsorption using organoclays synthesized from montmorillonite (MMT) with different types of organic surfactant molecules was demonstrated. High adsorption capacities of the organoclays for the uptake of BPA were observed and these demonstrated their potential application as strong adsorbents for noxious organic water contaminants. The adsorption of BPA was significantly influenced by pH, with increased adsorption of BPA in acidic pH range. However, the organoclays intercalated with highly loaded surfactants and/or large surfactant molecules were less influenced by the pH of the environment and this was thought to be due to the shielding the negative charge from surfactant molecules and the development of more positive charge on the clay surface, which leads to the attraction of anionic BPA even at alkaline pH. The hydrophobic phase created by loaded surfactant molecules contributed to a partitioning phase, interacting with BPA molecules strongly through hydrophobic interaction. Pseudo-second order kinetic model and Langmuir isotherm provided the best fit for the adsorption of BPA onto the organoclays. In addition, the adsorption process was spontaneous and exothermic with lower temperature facilitating the adsorption of BPA onto the organoclays. The described process provides a potential pathway for the removal of BPA from contaminated waters.

Visible light photocatalytic synthesis of fine organic chemicals with new photocatalysts

This project was a step forward in developing new recyclable photocatalysts for chemical reactions. These new photocatalysts can facilitate reactions by using visible light under moderate reaction conditions which is suitable for a sustainable, green and eco-friendly modern chemical industry. The outcome of the study greatly extended our understanding in metal nanoparticle photocatalysis, which reveals new photocatalytic mechanisms for the controlled transformation of chemical reactions. The prospect of sunlight irradiation driving chemical reactions may provide opportunity for the organic synthesis via a more controlled, simplified, and greener process in the future.

Assessment of volatile organic compounds in urban schools

There is an increased concern about airborne particles not only because of their environmental effects, but also due to their potential adverse health effects on humans, especially children. Despite the growing evidence of airborne particles having an impact on children’s health, there have been limited studies investigating the long term health effects as well as the chemical composition of ambient air which further helps in determining their toxicity. Therefore, a systematic study on the chemical composition of air in school environment has been carried out in Brisbane, which is known as “Ultrafine Particles from Traffic Emissions on Children’s Health” (UPTECH). This study is also a part of the larger project focusing on analysis of the chemical composition of ambient air, as well as source apportionment and the quantification of ambient concentrations of organic pollutants in the vicinity of schools. However, this particular paper presents some of the results on concentration of different Volatile Organic Compounds in both indoor and outdoor location from different schools. The database consisted of 750 samples (500 outdoor and 250 indoor) collected for VOCs at 25 different schools. The sampling and analysis were conducted following the standard methods. A total of 90 individual VOCs were identified from the schools studied. Compounds such as toluene, acetic acid, nonanal, benzaldehyde, 2- ethyl 1- hexanol, limonene were the most common in indoors whereas isopentane, toluene, hexane, heptane were dominant in outdoors. The indoor/ outdoor ratio of average sum of VOCs were found to be more than one in most of the schools indicating that there might be additional indoor sources along with the outdoor air in those schools. However, further expansion of the study in relation to source apportionment, correlating with traffic and meteorological data is in progress.

Volatile organic compounds: Characteristics, distribution and sources in urban schools

Long term exposure to organic pollutants, both inside and outside school buildings may affect children’s health and influence their learning performance. Since children spend significant amount of time in school, air quality, especially in classrooms plays a key role in determining the health risks associated with exposure at schools. Within this context, the present study investigated the ambient concentrations of Volatile Organic Compounds (VOCs) in 25 primary schools in Brisbane with the aim to quantify the indoor and outdoor VOCs concentrations, identify VOCs sources and their contribution, and based on these; propose mitigation measures to reduce VOCs exposure in schools. One of the most important findings is the occurrence of indoor sources, indicated by the I/O ratio >1 in 19 schools. Principal Component Analysis with Varimax rotation was used to identify common sources of VOCs and source contribution was calculated using an Absolute Principal Component Scores technique. The result showed that outdoor 47% of VOCs were contributed by petrol vehicle exhaust but the overall cleaning products had the highest contribution of 41% indoors followed by air fresheners and art and craft activities. These findings point to the need for a range of basic precautions during the selection, use and storage of cleaning products and materials to reduce the risk from these sources.

Does Australia need laws for 'Biotrespass' to protect organic farmers?

The supreme court of Western Australia handed down a landmark decision yesterday, on genetically modified crop liability. The ruling in Marsh v Baxter is an enormous win for the agricultural biotechnology industry, and has disappointed organic farmers and their advocates.

Stabilization of stormwater biofilters: Impacts of wetting and drying phases and the addition of organic matter to filter media

Ripening period refers to a phase of stabilization in sand filters in water treatment systems that follows a new installation or cleaning of the filter. Intermittent wetting and drying, a unique property of stormwater biofilters, would similarly be subjected to a phase of stabilization. Suspended solids, is an important parameter that is often used to monitor the stabilization of sand filters in water treatment systems. Stormwater biofilters however, contain organic material that is added to the filter layer to enhance nitrate removal, the dynamics of which is seldom analysed in stabilization of stormwater biofilters. Therefore, in this study of stormwater biofiltration in addition to suspended solids (Turbidity), organic matter (TOC, DOC, TN and TKN) was also monitored as a parameter for stabilization of the stormwater biofilter. One Perspex bioretention column (94 mm internal diameter) was fabricated with filter layer that contained 8% organic material and fed with tapwater with different antecedent dry days (0 – 40 day) at 100 mL/min. Samples were collected from the outflow at different time intervals between 2 – 150 minutes and were tested for Total Organic Carbon, Dissolved Organic Carbon, Total Nitrogen, Total Kjeldhal Nitrogen and Turbidity. The column was observed to experience two phases of stabilization, one at the beginning of each event that lasted for 30 minutes while the other phase was observed across subsequent events that related to the age of filter.

Electrochemical restructuring of copper surfaces using organic additives and its effect on the electrocatalytic reduction of nitrate ions

This work describes the fabrication of nanostructured copper electrodes using a simple potential cycling protocol that involves oxidation and reduction of the surface in an alkaline solution. It was found that the inclusion of additives, such as benzyl alcohol and phenylacetic acid, has a profound effect on the surface oxidation process and the subsequent reduction of these oxides. This results in not only a morphology change, but also affects the electrocatalytic performance of the electrode for the reduction of nitrate ions. In all cases, the electrocatalytic performance of the restructured electrodes was significantly enhanced compared with the unmodified electrode. The most promising material was formed when phenylacetic acid was used as the additive. In addition, the reduction of residual oxides on the surface after the modification procedure to expose freshly active reaction sites on the surface before nitrate reduction was found to be a significant factor in dictating the overall electrocatalytic activity. It is envisaged that this approach offers an interesting way to fabricate other nanostructured electrode surfaces.

Design and fabrication of linear pixel arrays for organic photovoltaic modules to achieve scalable power output

We describe a design and fabrication method to enable simpler manufacturing of more efficient organic solar cell modules using a modified flat panel deposition technique. Many mini-cell pixels are individually connected to each other in parallel forming a macro-scale solar cell array. The pixel size of each array is optimized through experimentation to maximize the efficiency of the whole array. We demonstrate that integrated organic solar cell modules with a scalable current output can be fabricated in this fashion and can also be connected in series to generate a scalable voltage output.

Designs and architectures for the next generation of organic solar cells

Organic solar cells show great promise as an economically and environmentally friendly technology to utilize solar energy because of their simple fabrication processes and minimal material usage. However, new innovations and breakthroughs are needed for organic solar cell technology to become competitive in the future. This article reviews research efforts and accomplishments focusing on three issues: power conversion efficiency, device stability and processability for mass production, followed by an outlook for optimizing OSC performance through device engineering and new architecture designs to realize next generation organic solar cells.

Enhancing current density using vertically oriented organic photovoltaics

We report a new organic photovoltaics (OPV) design, a wrapped OPV, which can circumvent both challenges of short exciton diffusion length [1], and low charge carrier mobility [2] of organic semiconductors by orienting the OPV vertically, to capture; manage; guide and use all incident photons and therefore, generate higher current. Resonant light, on being transmitted into a wrapped OPV, makes multiple passes through the photoactive layer and is absorbed completely, thus achieving benefits of thick photoactive layer while maintaining its ultra-thin thickness requirement. The current density generated from a wrapped OPV is twice than that generated by a similar OPV with flat orientation.