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- Solution-Processable Donor-Acceptor-Donor Oligomers with Cross-Linkable FunctionalityPublication . Brotas, Graça; Costa, Cristiana; Dias, Sandra I. G.; Costa, Pedro M. M.; Paolo, Roberto E. Di; Martins, João; Farinhas, Joana; Alcácer, Luís; Jorge Morgado; Matos, Manuel; Charas, AnaElectron-acceptor units, combined with bithiophene substituted with flexible chains end-functionalized with cross-linkable moieties, provide soluble donor-acceptor-donor (DAD) it-conjugated oligomer-type molecules with cross-linking ability and broad absorption in the visible spectrum. A study on the cross-linking conditions of the new oligomers to yield insoluble polymer networks is presented, including conditions for obtaining polymer films over poly(3,4-ethylenedioxythiophene):polystyrene sulfonate-covered substrates. The combination of the DAD molecular design and cross-linking functionality opens prospects for applications in solution-processed small-molecule solar cells with morphologically-stable organic layers.
- Band-gap tunable thiadiazolo[3,4-g]quinoxaline derivatives as non-fullerene acceptors in organic photovoltaic cells processed from low toxic ethanol/anisole mixturesPublication . Costa, Cristiana; Farinhas, Joana; Velho, Mariana F. G.; Avo, Joao; Matos, Manuel; Galvão, Adelino; Charas, AnaThe synthesis, characterisation and application as electron-acceptors in organic photovoltaic cells of new [1,2,5]thiadiazolo[3,4-g]quinoxaline-based small molecules with oligo(ethylene oxide) solubilising groups and tunable optical bandgaps are reported. A new p-type polymer consisting of a polythiophene derivative substituted with analogous solubilising groups was also synthesised to perform as a donor, in combination with the small molecules, for bulk-heterojunction solar cells using ethanol/anisole mixtures as low toxic solvents to process the active blends. Comparison of the structural, electrochemical and photophysical properties of the small molecules elucidated on the structural dependence of their optical bandgaps, ranging from 1.31 to 2.25 eV. The active layers composed of the polythiophene donor and the small molecules showed well-intermixed morphologies, and the best performing blends showed a fibrillar-type morphology, indicative of more ordered molecular packings. Blends processed from the 50 : 50 (v/v) mixture of ethanol and a low toxic solvent, anisole, yielded low power conversion efficiencies of 0.39% in non-optimized organic solar cells. Although the efficiencies are low, these are the first reported results of OSCs with active layers composed of non-fullerene acceptors processed from ethanol-based solvent formulations. This study provides guidelines for the design of non-fullerene acceptors for the fabrication of OSCs processed from environmentally friendly solvents.
- Combined organic photovoltaic cells and ultra low power CMOS circuit for indoor light energy harvestingPublication . Batista, Duarte; Oliveira, Luis; Paulino, Nuno; Carvalho, Carlos; Oliveira, João P.; Farinhas, Joana; Charas, Ana; Mendonça Dos Santos, PedroThis paper describes an energy harvesting system composed of an organic photovoltaic cell (OPV) connected to a DC-DC converter, designed in a 130 nm Complementary Metal-Oxide-Semiconductor (CMOS) technology, with a quasi- maximum power point tracking (MPPT) algorithm to maximize the system efficiency, for indoor applications. OPVs are an emerging technology with potential for low cost indoor light energy harvesting. The OPV current-voltage curves (I-V) under an irradiance of solar simulator Oriel Sol 3A, at room temperature, are obtained and an accurate electrical model is derived. The energy harvesting system is subjected to four different indoor light sources: 35 W halogen, 3.5 W LED, 5 W LED, and 7 W LED, positioned at three different heights (0.45 m, 0.26 m, and 0.11 m), to evaluate the potential of the system for indoor applications. The measurements showed maximum efficiencies of 60% for 35 W halogen and 45% for 7 W LED at the highest distance (0.45 m) and between 60% (5 W LED) and 70% (35 W halogen), at the shorter distance (0.11 m). Under irradiation, the integrated CMOS circuit presented a maximum efficiency of 75.76%, which is, to the best of the authors' knowledge, the best reported power management unit (PMU) energy system using organic photovoltaic cells.