An overview of some recent developments of the chemistry of molecular donor materials for organic photovoltaics (OPV) is presented. Although molecular materials have been used for the fabrication of ...OPV cells from the very beginning of the field, the design of molecular donors specifically designed for OPV is a relatively recent research area. In the past few years, molecular donors have been used in both vacuum‐deposited and solution‐processed OPV cells and both fields have witnessed impressive progress with power conversion efficiencies crossing the symbolic limit of 10 %. However, this progress has been achieved at the price of an increasing complexity of the chemistry of active materials and of the technology of device fabrication. This evolution probably inherent to the progress of research is difficult to reconcile with the necessity for OPV to demonstrate a decisive economic advantage over existing silicon technology. In this short review various classes of molecular donors are discussed with the aim of defining possible basic molecular structures that can combine structural simplicity, low molecular weight, synthetic accessibility, scalability and that can represent possible starting points for the development of simple and cost‐effective OPV materials.
Various classes of molecules used as donor materials in heterojunction organic solar cells are presented. Special emphasis is placed on molecular structures that combine low molecular weight, (in general inferior to 500) structural simplicity and synthetic accessibility with reasonable yield. Such systems are discussed as possible working structures for the development of simple and cost‐effective materials for organic photovoltaics.
The synthesis of acceptor–donor–acceptor (A–D–A) molecules based on a septithiophene chain with terminal electron acceptor groups is reported. Using a dicyanovinyl‐ (DCV) substituted molecule as ...reference, another symmetrical A–D–A donor containing thiobarbituric (TB) groups is synthesized and these two acceptor groups are combined to produce the unsymmetrical A–D–A′ compound. The electronic properties of the donors are analyzed by cyclic voltammetry and UV‐Vis absorption spectroscopy and their photovoltaic properties are characterized on bilayer planar heterojunction cells that include spun‐cast donor films and vacuum‐deposited C60 as acceptor. Optical and electrochemical data show that replacement of DCV by TB leads to a small increase of the HOMO level and to a larger decrease of the LUMO, which result in a reduced band‐gap. The desymmetrized compound presents the lowest oxidation potential in solution but the highest oxidation onset in the solid state, which leads to a significant increase of the open‐circuit voltage of the resulting solar cells.
Breaking the symmetry of acceptor–donor–acceptor conjugated systems used as molecular donors in heterojunction organic solar cells modifies the polarity of the system and produces an increase of the ionization potential in the solid state and thus a large increase in cell voltage.
The synthesis of donor‐acceptor molecules involving triarylamines and dicyanovinyl blocks is described. Optical and electrochemical results show that rigidification of the acceptor part of the ...molecule by a covalent bridge leads to a ca. 0.20 eV increase of the band gap due to a parallel increase of the lowest unoccupied molecular orbital level. A preliminary evaluation of these compounds as donor materials in organic solar cells shows that although this structural modification reduces the light‐harvesting properties of the donor molecule, it nevertheless induces an increase of the efficiency of the resulting solar cells due to a simultaneous improvement of the open‐circuit voltage and fill factor.
Covalent bridging of the dicyanovinyl group with the adjacent thiophene ring of small push‐pull molecular donors based on triarylamines leads to a significant improvement of the efficiency of the resulting organic solar cells due to the simultaneous increase of the fill factor and open‐circuit voltage.
New three-dimensional hole-transporting materials (HTMs) based on either 9,9′-spirobifluorene (SBF) or spiro-cyclopenta1,2-b:5,4-b′dithiophene-4,9′-fluorene (SDTF) core have been synthesized. All ...three HTMs, namely, SBFCz2, SDTFCz2, and SDTCz2F, are end-capped with two peripheral 3,6-dimethoxydiphenylaminyl-carbazole (CzDMPA) units. The HTMs behave as molecular glasses with glass transition temperature (T g) close to or higher than that of the reference HTM Spiro-OMeTAD. Thermal and optoelectronic properties strongly depend upon the nature of the bridging core unit between the two CzDMPA units. The two fluorene-bridged molecules SBFCz2 and SDTFCz2 exhibit similar properties. On the contrary, SDTCz2F, where the CzDMPA units are bridged to the cyclopentadithiophene ring, displays lower highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels and smaller band gaps. Upon doping of the HTM layer in perovskite solar cells, in spite of a much lower hole mobility, SDTCz2F leads to the highest power conversion efficiency (16.4% compared to 14.5 and 14.3% for SBFCz2 and SDTFCz2, respectively).
Conjugated systems built by connecting two electron-donor side-chains to a diketopyrrolopyrrole (DPP) core have been synthesized and evaluated as donor material in heterojunction organic solar cells. ...The effects of composition of the side-chain on the electronic properties of the conjugated system have been analyzed on first series of compounds containing various combinations of benzofuran, benzothiophene, thiophene and furan units. In a second series of compounds, the keto groups of DPP have been replaced by one or two thioketo groups. Results of UV–vis absorption spectroscopy, fluorescence emission spectroscopy and cyclic voltammetry show that the composition of the side-chain has little effect on the HOMO and LUMO levels of the system, but strongly affects the sensitivity of the material toward thermal treatment and thus indirectly the performances of the resulting solar cells. On the other hand, replacement of the keto groups of DPP by thioketo ones leads at the same time to significant reduction of the band gap due to a decrease of the LUMO level, to a quenching of fluorescence and to dramatic decrease of the photovoltaic activity of the molecule.
► Small molecule heterojunction solar cells based on dithioketopyrrolopyrrole. ► Diketo and dithioketopyrrolopyrroles with furan and thiophene substituents. ► Structure–properties relationships.
Three bis-EDOT derivatives end capped with
n
-hexyl or
n
-hexylsulfanyl groups have been synthesized. The oxidation processes of these compounds have been studied using cyclic voltammetry and ...UV-Vis-NIR spectroscopy. The strong influence of the sulfur atoms of the hexylsulfanyl chains for stabilizing the oxidation states is evidenced. Thus, it is shown that the two compounds grafted with one or two hexylsulfanyl groups are reversibly oxidized into cation radical and dication states. By contrast, compounds end capped with hexyl groups present a fast σ-dimerization of the radical cation. The reduction of the σ-dimer to give the neutral bis-EDOT derivative is slow.
Synthesis, isolation, and characterization of isomerically pure syn- and anti-anthradiindole (ADI) derivatives are described. The anti- and syn-ADI structures are demonstrated by 13C NMR spectroscopy ...and by single-crystal X-ray diffraction. The spectroscopic and electrochemical properties as well as the stability of these newly synthesized π-conjugated systems are evaluated and supported by quantum-chemical calculations.
The hole transporting medium in solid-state dye-sensitized solar cells can be utilized to harvest sunlight. Herein we demonstrate that a triphenylamine-based dye, used as hole-transporting medium, ...contributes to the photocurrent in a squaraine-sensitized solid-state dye-sensitized solar cell. Steady-state photoluminescence measurements have been used to distinguish between electron transfer and energy transfer processes leading to energy conversion upon light absorption in the hole-transporting dye.
The synthesis of heterocyclic spirobifluorene (SBF) analogs generally requires long and complicated synthetic pathways. Despite this synthetic effort, such structural modification allows the ...(opto)electronic properties of this remarkable three-dimensional node to be tuned especially for molecular electronic applications. For this reason, the development of a simple, robust, and efficient synthetic methodology to introduce various heterocycles in place of classical phenyl rings in the spirofluorene motif is highly and timely desirable. In this context, we describe herein our efforts to develop a straightforward and efficient synthesis leading to replacement of 2 phenyl rings by various heterocycles in spiro compounds from 2,2′-dibromobenzophenone. As the same procedure to form fully heterocyclic compounds failed, an original theoretical approach based mainly on the uncommon Fukui dual function was developed in order to determine clearly the limitation of this strategy and provide an efficient predictive tool. Indeed, such calculation allows prediction of the thermodynamic and kinetic aspects of the synthesis of spiro derivatives using a double aromatic electrophilic substitution. If this procedure reproduces well our experimental results focused on (heterocyclic) SBF compounds, it can be certainly adapted and generalized to other intramolecular substitutions.
The dimer and trimer of 3,4‐phenylenedioxythiophene (PheDOT) have been synthesized. Unlike the parent systems based on 3,4‐ethylenedioxythiophene (EDOT), these compounds are quite stable under ...atmospheric conditions. The electronic absorption spectra of di‐ and tri‐PheDOT exhibit a well‐resolved vibronic fine structure indicative of self‐rigidification of the conjugated structure by noncovalent intramolecular sulfur–oxygen interactions. Comparison of UV‐visible data for the PheDOT oligomers with those of the corresponding EDOT oligomers reveals a faster decrease of the HOMO–LUMO gap with chain length for the former. Cyclic voltammetric data show that whereas PheDOT oxidizes at a lower potential than EDOT, the PheDOT dimer and trimer exhibit much higher oxidation potentials than their EDOT‐based analogues. A comparative analysis of the electropolymerization of the three PheDOT‐based systems shows that although PheDOT is very difficult to polymerize, its dimer and trimer can be readily electropolymerized. This unexpected increase of reactivity with chain extension is discussed with the aid of theoretical calculations.
Bigger is better: Contrary to other classes of conjugated oligomers, the electropolymerization efficiency of 3,4‐phenylenedioxythiophene (PheDOT, see scheme, left) oligomers increases with chain length. This unexpected increase of reactivity with chain extension is discussed with the aid of theoretical calculations.
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