Felipe A. Larrain

Solution-based electrical doping of semiconducting polymer films over a limited depth

Solution-based electrical doping protocols may allow more versatility in the design of organic electronic devices; yet, controlling the diffusion of dopants in organic semiconductors and their stability has proven challenging. Here we present a solution-based approach for electrical p-doping of films of donor conjugated organic semiconductors and their blends with acceptors over a limited depth with a decay constant of 10-20 nm by post-process immersion into a polyoxometalate solution (phosphomolybdic acid, PMA) in nitromethane. PMA-doped films show increased electrical conductivity and work function, reduced solubility in the processing solvent, and improved photo-oxidative stability in air. This approach is applicable to a variety of organic semiconductors used in photovoltaics and field-effect transistors. PMA doping over a limited depth of bulk heterojunction polymeric films, in which amine-containing polymers were mixed in the solution used for film formation, enables single-layer organic photovoltaic devices, processed at room temperature, with power conversion efficiencies up to 5.9 ± 0.2% and stable performance on shelf-lifetime studies at 60 °C for at least 280 h.

A Study on Reducing Contact Resistance in Solution-Processed Organic Field-Effect Transistors

We report on the reduction of contact resistance in solution-processed TIPS-pentacene (6,13-bis(triisopropylsilylethynyl)pentacene) and PTAA (poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]) top-gate bottom-contact organic field-effect transistors (OFETs) by using different contact-modification strategies. The study compares the contact resistance values in devices that comprise Au source/drain electrodes either treated with 2,3,4,5,6-pentafluorothiophenol (PFBT), or modified with an evaporated thin layer of the metal-organic molecular dopant molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3), or modified with a thin layer of the oxide MoO3. An improved performance is observed in devices modified with Mo(tfd)3 or MoO3 as compared to devices in which Au electrodes are modified with PFBT. We discuss the origin of the decrease in contact resistance in terms of increase of the work function of the modified Au electrodes, Fermi-level pinning effects, and decrease of bulk resistance by electrically doping the organic semiconductor films in the vicinity of the source/drain electrodes.

A simplified methodology to estimate the impact of wind power over operating reserves in isolated power systems

This paper presents a simplified methodology to estimate the impact of wind power over operating reserves in isolated power systems. The methodology is based on a convolution approach that merges the fluctuations of wind speed with the natural variations of demand, and formulates the probability distribution function of variables as a non-Gaussian behavior. The estimation renders a histogram containing the probability distribution of operating reserves required in different time spans. The methodology also estimates the LOLP indicator to assess the operative reserve of the system. An application on the expansion plan of a Chilean isolated power system is presented and results indicate that the methodology is able to estimate operating reserve needs in a time span ranging from 1 to 30 minutes, which adequately satisfy the future needs of this isolated power system.

Wind power curtailment and energy storage in transmission congestion management considering power plants ramp rates

The integration of intermittent generation in power grids, such as wind energy, imposes new challenges for transmission congestion management. In order to solve this problem, energy storage systems (ESS) have been proposed in the literature, as they provide an efficient mechanism for balancing variability while reducing operational costs. This paper presents a comprehensive analysis of the dynamic interactions between wind energy curtailment and an energy storage system (ESS) when the ramping rates of power plants are considered. An analytical framework is developed to study different mitigation measures in terms of total energy curtailed, total congestion costs, line load factor and congestion probability. This framework is tested in a real case study and a sensitivity analysis is performed to identify the influence of the main ESS design parameters in congestion mitigation performance.

Self-forming electrode modification in organic field-effect transistors

We report on self-forming electrode modification by mixing 2,3,4,5,6-pentafluorothiophenol (PFBT) directly into the solution of the organic semiconductor prior to film formation on top of existing metal electrodes. During the formation of the semiconductor layer from the mixed solution, PFBT chemisorbs on the underlying source/drain electrodes and modifies their electronic properties. The modification of evaporated silver, gold, or printed silver electrodes with PFBT is analyzed by X-ray photoelectron spectroscopy. The use of this self-forming electrode modification is applied to solution-processed p-channel top-gate 6,13-bis(triisopropylsilylethynyl)pentacene/poly[bis(4-phenyl) (2,4,6-trimethylphenyl) amine] organic field-effect transistors (OFETs) that comprise bare silver or gold source/drain electrodes. The proposed new method simplifies device fabrication while yielding OFETs with a performance level that is comparable to that of reference devices in which the metal electrodes are modified with PFBT prior to the fabrication of the semiconductor layer.

Stable solvent for solution-based electrical doping of semiconducting polymer films and its application to organic solar cells

The immersion of polymeric semiconducting films into a polyoxometalate (PMA) solution was found to lead to electrical doping over a limited depth, enabling the fabrication of organic photovoltaic devices with simplified geometry; yet, the technique was highly solvent selective and the use of nitromethane was found to be limiting. Here, we report on the use of acetonitrile as an alternative solvent to nitromethane. Morphology studies on pristine and PMA doped P3HT films suggest that dopants reside in between the lamella of the polymer, but cause no distortion to the P3HT π–π stacking. With this information, we propose an explanation for the observed solvent-selectivity of the doping method. Degradation studies reveal a superior stability of films doped with PMA in acetonitrile. Based on these findings, we believe that the post-process immersion technique, when dissolving PMA in acetonitrile, is a more suitable candidate to conduct solution-based electrical p-doping of organic semiconductors on an industrial scale.

Organic photovoltaic devices with a single layer geometry(Conference Presentation)

Organic photovoltaics (OPV) can lead to a low cost and short energy payback time alternative to existing photovoltaic technologies. However, to fulfill this promise, power conversion efficiencies must be improved and simultaneously the architecture of the devices and their processing steps need to be further simplified. In the most efficient devices to date, the functions of photocurrent generation, and hole/electron collection are achieved in different layers adding complexity to the device fabrication. In this talk, we present a novel approach that yields devices in which all these functions are combined in a single layer. Specifically, we report on bulk heterojunction devices in which amine-containing polymers are first mixed in the solution together with the donor and acceptor materials that form the active layer. A single-layer coating yields a self-forming bottom electron-collection layer comprised of the amine-containing polymer (e.g. PEIE). Hole-collection is achieved by subsequent immersion of this single layer in a solution of a polyoxometalate (e.g. phosphomolybdic acid (PMA)) leading to an electrically p-doped region formed by the diffusion of the dopant molecules into the bulk. The depth of this doped region can be controlled with values up to tens of nm by varying the immersion time. Devices with a single 500 nm-thick active layer of P3HT:ICBA processed using this method yield power conversion efficiency (PCE) values of 4.8 ± 0.3% at 1 sun and demonstrate a performance level superior to that of benchmark three-layer devices with separate layers of PEIE/P3HT:ICBA/MoOx (4.1 ± 0.4%). Devices remain stable after shelf lifetime experiments carried-out at 60 °C over 280 h.

Recent advances in organic photodiodes (Conference Presentation)

Although the detection of photons is ubiquitous, man-made photon detectors still limits the effectiveness of applications such as light/laser detection, photography, astronomy, quantum information science, medical imaging, microscopy, communications, and others. The performance of the technologically most advanced detectors based on CMOS semiconductor technology has improved during the last decades but at the detriment of increased complexity, higher cost, limited portability and compactness, and limited area. On the other hand, nature has produced a relatively simple detector with remarkable properties: the human eye. The exploration of new paradigms in photon detection using new material platforms might therefore provide a path to further challenge the frontiers of applications enabled by light. In this talk, we will report on the realization of solution-processed organic semiconductor visible spectrum photodetectors with a high specific detectivity above 1014 Jones, at least an order of magnitude larger than values found in photodiodes based on silicon. These detectors demonstrate a sub-pA current under reverse bias in the dark, making them suitable for detecting very low levels of light. The small dark current under reverse bias allows the characterization of these devices over 9 orders of magnitude of increasing light irradiance. The detectors are based on the device structure: tin-doped indium oxide / ethoxylated polyethylenimine / poly(3-hexylthiophene) : indene C60 bisadduct / molybdenum oxide / silver and present a path toward fabrication on flexible substrates. We will show that these detectors can operate over a large dynamic range in the self-powered photovoltaic mode where the light produces a photovoltage that can be measured directly without any external bias source. We believe that large-area flexible photodetectors with detectivity values comparable to or better than those displayed by silicon-based photodiodes will enable a wide variety of applications from the detection of radiation to non-planar imaging arrays.

Power supply of a rural off-grid health center — A case study

Presently, more than 7 million Haitians have no access to power nor basic energy related services. Available generation capacity of Haiti reaches 212 MW, which is insufficient to meet the estimated peak demand of more than 500 MW in the whole country. This deficit severely impacts basic essential facilities such as health care centers. The IEEE Student Branch PES Chapter at Georgia Tech established a project to design and implement a microgrid to supply power to a recently established health center in the mountains of Thoman, Haiti. Several combinations of power generating units were evaluated on an economic basis, including: a standalone diesel generator (DG), photovoltaic (PV) panels with batteries, and PV panels with batteries and a DG. Key parameters including power rating, daily energy production, maximum annual capacity shortage, etc., were also incorporated into the economic evaluation. This paper outlines the preliminary microgrid design steps, assessment of topology alternatives, site visit, detailed design and the fundraising process. Only commercial off-the-shelf parts were considered for device selection. To verify the preliminary design, a site visit was conducted in February 2015. Installation and commissioning is expected to take place later this year.