Blending organic electron donors and acceptors yields intermolecular charge-transfer states with additional optical transitions below their optical gaps. In organic photovoltaic devices, such states ...play a crucial role and limit the operating voltage. Due to its extremely weak nature, direct intermolecular charge-transfer absorption often remains undetected and unused for photocurrent generation. Here, we use an optical microcavity to increase the typically negligible external quantum efficiency in the spectral region of charge-transfer absorption by more than 40 times, yielding values over 20%. We demonstrate narrowband detection with spectral widths down to 36 nm and resonance wavelengths between 810 and 1,550 nm, far below the optical gap of both donor and acceptor. The broad spectral tunability via a simple variation of the cavity thickness makes this innovative, flexible and potentially visibly transparent device principle highly suitable for integrated low-cost spectroscopic near-infrared photodetection.
High photon energy losses limit the open‐circuit voltage (VOC) and power conversion efficiency of organic solar cells (OSCs). In this work, an optimization route is presented which increases the VOC ...by reducing the interfacial area between donor (D) and acceptor (A). This optimization route concerns a cascade device architecture in which the introduction of discontinuous interlayers between alpha‐sexithiophene (α‐6T) (D) and chloroboron subnaphthalocyanine (SubNc) (A) increases the VOC of an α‐6T/SubNc/SubPc fullerene‐free cascade OSC from 0.98 V to 1.16 V. This increase of 0.18 V is attributed solely to the suppression of nonradiative recombination at the D–A interface. By accurately measuring the optical gap (Eopt) and the energy of the charge‐transfer state (ECT) of the studied OSC, a detailed analysis of the overall voltage losses is performed. Eopt – qVOC losses of 0.58 eV, which are among the lowest observed for OSCs, are obtained. Most importantly, for the VOC‐optimized devices, the low‐energy (700 nm) external quantum efficiency (EQE) peak remains high at 79%, despite a minimal driving force for charge separation of less than 10 meV. This work shows that low‐voltage losses can be combined with a high EQE in organic photovoltaic devices.
The insertion of a thin interlayer at the D/A interface of a cascade organic solar cell leads to a reduction of the voltage losses by 180 mV, while maintaining peak external quantum efficiencies approaching 80%. A detailed analysis of the organic multilayer device with exceptionally low losses reveals a minimal driving force for charge separation and suppressed nonradiative recombination.
Three furan fused boron dipyrromethenes (BODIPYs) with a CF3 group on the meso-carbon are synthesized as near-infrared absorbing materials for vacuum processable organic solar cells. The best single ...junction device reaches a short-circuit current (j sc) of 13.3 mA cm–2 and a power conversion efficiency (PCE) of 6.1%. These values are highly promising for an electron donor material with an absorption onset beyond 900 nm. In a tandem solar cell comprising a NIR BODIPY subcell and a matching “green” absorber subcell, complementary absorption is achieved, resulting in PCE of ∼10%.
An understanding of the factors limiting the open-circuit voltage (V oc) and related photon energy loss mechanisms is critical to increase the power conversion efficiency (PCE) of small-molecule ...organic solar cells (OSCs), especially those with near-infrared (NIR) absorbers. In this work, two NIR boron dipyrromethene (BODIPY) molecules are characterized for application in planar (PHJ) and bulk (BHJ) heterojunction OSCs. When two H atoms are substituted by F atoms on the peripheral phenyl rings of the molecules, the molecular aggregation type in the thin film changes from the H-type to J-type. For PHJ devices, the nonradiative voltage loss of 0.35 V in the J-aggregated BODIPY is lower than that of 0.49 V in the H-aggregated device. In BHJ devices with a nonradiative voltage loss of 0.35 V, a PCE of 5.5% is achieved with an external quantum efficiency (EQE) maximum of 68% at 700 nm.
Near infrared absorber materials for organic solar cells are needed to cover larger parts of the solar spectrum and, therefore, achieve higher short circuit current densities. We present a push-pull ...chromophore with a quinoid merocyanine structure (QM1) as donor material in vacuum deposited organic solar cells with a thin film absorption up to 1100 nm. A blue shift of the main absorption peaks of the thin film as compared to the solution indicates H-aggregation of the material. Morphology investigations using scanning electron microscopy and electron diffraction reveal a crystalline growth in nanowires, with lengths of hundreds of nanometers to a few micrometers, and diameters of a few tens of nanometers. Organic solar cells incorporating a blend of QM1 with C60 reach power conversion efficiencies up to 1.9% under 1 sun illumination with an external quantum efficiency of over 18% from 600 nm to 1000 nm.
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•Push-pull chromophore as near infrared absorber for organic solar cells.•Thin film absorption of up to 1100 nm by H-aggregation of the molecules.•Formation of micrometer long small molecular nanowires upon vacuum deposition.•External quantum efficiency of 18% at 1000 nm in organic solar cells with C60.
EPR Analysis and DFT Computations of a Series of Polynitroxides Ottaviani, M. Francesca; Modelli, Alberto; Zeika, Olaf ...
The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory,
01/2012, Letnik:
116, Številka:
1
Journal Article
Recenzirano
Polynitroxides with varying numbers of nitroxide groups (one to four) derived from different aromatic core structures show intramolecular electron spin–spin coupling. The scope of this study is to ...establish an easy methodology for extracting structural, dynamical, and thermodynamical information from the EPR spectra of these polynitroxides which might find use as spin probes in complex systems, such as biological and host/guest systems, and as polarizing agents in dynamic nuclear polarization (DNP) applications. Density functional theory (DFT) calculations at the B3LYP/6-31G(d) level provided information on the structural details such as bond lengths and angles in the gas phase, which were compared with the single crystal X-ray diffraction data in the solid state. Polarizable continuum model (PCM) calculations were performed to account for solvent influences. The electron paramagnetic resonance (EPR) spectra of the polynitroxides in chloroform were analyzed in detail to extract information such as the percentages of different conformers, hyperfine coupling constants a, and rotational correlation times τc. The temperature dependence on the line shape of the EPR spectra gave thermodynamic parameters ΔH and ΔS for the conformational transitions. These parameters were found to depend on the number and relative positions of the nitroxide and other polar groups.
Two novel aza‐BODIPY derivatives are synthesized, with the fluorine atoms in the BF2 moiety replaced by cyano groups. The introduction of cyano groups changes the phenyl substituents on the 3,5 ...positions from parallel to antiparallel. The HOMO/LUMO energy levels are stabilized gradually upon increasing the number of cyano groups, and the photophysical properties show corresponding shifts. With high thermal stability, the derivatives can be purified by sublimation and used to prepare vacuum‐deposited thin films. This research extends the aza‐BODIPY family with cyano‐substituted derivatives.
Novel aza‐BODIPY derivatives: The fluorine atoms in the BF2 moiety in aza‐BODIPY dyes are for the first time replaced by one or two cyano groups (see figure), and two corresponding derivatives are achieved with the presence of different Lewis acids. The outstanding photophysical properties are retained in the derivatives. The frontier molecular orbital energy levels can be tuned with different numbers of cyano groups.
Charge generation and recombination processes at interfaces between electron donating (donor, D) and accepting molecules (acceptor, A) are mediated by intermolecular charge-transfer (CT) states. ...Since organic photovoltaic and photodetecting devices rely on D–A interfaces, an understanding of the molecular and morphological aspects governing CT state properties is crucial. In this paper, we synthesize a novel series of bi(thio)pyranylidene donor molecules and show how the interplay of molecular structure and energy levels in a D–C60 blend affect the line shape of the CT absorption cross section. By rationally designing the molecule 2,2′,6,6′-tetra-(2-methylthienyl)-4,4′-bithiopyranylidene, we achieve a 2 times stronger CT absorption peak than the literature-known molecule 2,2′,6,6′-tetraphenyl-4,4′-bipyranylidene when blended with C60. The low CT state energy combined with relatively strong CT absorption of this new material blend is exploited by fabricating near-infrared, cavity enhanced narrowband detectors. The photodetectors cover an impressive wavelength range from 810 to 1665 nm with line widths between 30 and 50 nm.
Persistent luminescence from triplet excitons in organic molecules is rare, as fast non‐radiative deactivation typically dominates over radiative transitions. This work demonstrates that the ...substitution of a hydrogen atom in a derivative of phenanthroimidazole with an N‐phenyl ring can substantially stabilize the excited state. This stabilization converts an organic material without phosphorescence emission into a molecular system exhibiting efficient and ultralong afterglow phosphorescence at room temperature. Results from systematic photophysical investigations, kinetic modeling, excited‐state dynamic modeling, and single‐crystal structure analysis identify that the long‐lived triplets originate from a reduction of intrinsic non‐radiative molecular relaxations. Further modification of the N‐phenyl ring with halogen atoms affects the afterglow lifetime and quantum yield. As a proof‐of‐concept, an anticounterfeiting device is demonstrated with a time‐dependent Morse code feature for data encryption based on these emitters. A fundamental design principle is outlined to achieve long‐lived and emissive triplet states by suppressing intrinsic non‐radiative relaxations in the form of molecular vibrations or rotations.
The substitution of a hydrogen atom with an N‐phenyl ring is demonstrated to convert an organic material without phosphorescence emission into a molecular system exhibiting efficient and ultralong afterglow phosphorescence at room temperature, originating from a reduction of intrinsic non‐radiative molecular relaxations, which can be used for data safety applications.
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