Fulvio G. Brunetti

Strain and Hückel Aromaticity: Driving Forces for a Promising New Generation of Electron Acceptors in Organic Electronics

The tangible possibility of fabricating flexible, lightweight organic photovoltaic devices (OPVs) by using roll-to-roll coaters, similar to those used in the production of print magazines and newspapers, renders this technology a valid alternative to expensive crystalline silicon photovoltaic cells. The most widely used active layer for these OPVs, the so-called bulk heterojunction (BHJ), 5] is based on photoinduced charge transfer from an electron-donating material, such as a light-absorbing and hole-conducting polymer, to an electron-accepting component, typically fullerene[60] and its derivative 1-(3-methoxycarbonyl) propyl-1phenyl-[6,6]-C61 ([C60]PCBM). [6, 7] Several research groups have reported a wide range of new polymeric donor structures that absorb light over a broad wavelength range, and have a narrow energy gap and increased charge transport and collection at the electrode. However, there have been fewer reports on new structures of acceptor components that do not contain fullerene derivatives. C60 and C70 PCBMs are currently considered the most successful acceptor architectures, despite only slight improvements when modifying these functionalized fullerenes. For example, the insertion of electron-donating groups on the phenyl ring of the [C60]PCBM to tune the lowest unoccupied molecular orbital (LUMO) energy levels improved the open-circuit voltage (Voc), while maintaining a relatively unchanged efficiency. Furthermore, [C70]PCBM, which absorbs a wider range of wavelengths than [C60]PCBM, [16] was employed with low-band-gap polymers such as poly[2,6-(4,4-bis-(2-ethylhexyl-4H-cyclopenta[2,1b;3,4b’]-dithiophene)-alt-4,7-(2,1,3-benzothiadizole)] (PCPDTBT), to broaden the photocurrent spectral range. Although encouraging photocurrent and photovoltage values were obtained, a low overall efficiency, which arises from loss mechanisms, was observed. Despite the wide use of these fullerene derivatives, the synthesis of new acceptors with energy levels significantly different from those of current C60 derivatives, and wide versatility in terms of derivatization and functionalization is urgently required. Herein, we report the inherent potential of a new generation of acceptor compounds based on the 9,9’-bifluorenylidene (99’BF) backbone. 99’BF could be considered a tetrabenzofulvalene with an atom numbering that reflects fluorene linked by a double bond between the 9 and 9’ carbon atoms. In the ground state, 99’BF is forced to be coplanar because of the presence of the double bond, but the repulsive interaction between the H1– H1’ and H8–H8’ protons twists the structure of the dimer. The addition of one electron across the C9–C9’ bond is highly favorable for two main reasons: steric (“twist”) strain relief and gain in aromaticity to a 14-p-electron system (Scheme 1).

Bulk Heterojunction Solar Cells with Large Open-Circuit Voltage: Electron Transfer with Small Donor-Acceptor Energy Offset

e 99BF 99BF(.) Power conversion effi ciencies (PCEs) (in response to solar AM1.5 radiation) as high as 6–8% have been reported for bulk heterojunction (BHJ) polymer solar cells. [ 1 , 2 ] In order to attain PCEs over 10%, BHJ materials capable of generating larger open circuit voltage (V oc ) are required. [ 3 , 4 ] One approach to increase V oc is to develop low-bandgap semiconducting polymers with deeper highest occupied molecular orbital (HOMO) energies. An alternative approach is to develop new electron acceptors with higher lowest unoccupied molecular orbital (LUMO) energies. The pathway to low-bandgap semiconducting polymers with deeper HOMOs is well understood, and BHJ solar cells fabricated by semiconducting polymers with deeper HOMOs have successfully exhibited larger V oc . [ 5 ] However, the discovery of new (non-fullerene) electron acceptors with higher LUMOs remains undeveloped. [ 6 ]

Organic electronics from perylene to organic photovoltaics: painting a brief history with a broad brush

The past ten years have witnessed the development of bulk-heterojunction (BHJ) solar cells, which have emerged as an attractive renewable energy source in response to rising energy costs and environmental pollution. In such a solar cell, charge transfer at the donor–acceptor interface is a crucial aspect that significantly affects overall device efficiency. Therefore, the choice of these two components and their design are important factors for the optimization of plastic solar cells (PSCs). This feature article correlates the performance of the device to the active layer composites, analyzing the motivations behind specific BHJ designs. Several low-bandgap polymers are described based on their different donor–acceptor units and their influence on both the optical absorption and the electrochemical properties. As for the accepting materials, we examine the effect of chemical functionalization in a series of fullerene derivatives, carbon nanotubes and non-fullerene based compounds on their performances in PSCs. The understanding of film-morphology control is also briefly discussed.

Solubility-limited extrinsic n-type doping of a high electron mobility polymer for thermoelectric applications.

The thermoelectric properties of a highperformance electron-conducting polymer, (P(NDIOD-T2), extrinsically doped with dihydro-1H-benzoimidazol-2-yl (NDBI) derivatives, are reported. The highest thermoelectric power factor that has been reported for a solution-processed n-type polymer is achieved; and it is concluded that engineering polymerdopant miscibility is essential for the development of organic thermoelectrics.

Power Factor Enhancement in Solution‐Processed Organic n‐Type Thermoelectrics Through Molecular Design

A new class of high-performance n-type organic thermoelectric materials, self-doping perylene diimide derivatives with modified side chains, is reported. These materials achieve the highest n-type thermoelectric performance of solution-processed organic materials reported to date, with power factors as high as 1.4 μW/mK(2). These results demonstrate that molecular design is a promising strategy for enhancing organic thermoelectric performance.

1,4-Fullerene Derivatives: Tuning the Properties of the Electron Transporting Layer in Bulk-Heterojunction Solar Cells**

Light and flexible photovoltaic devices based on organic materials are extensively studied as an alternative to expensive and fragile silicon-based solar cells. The efficiency of these devices is rapidly increasing with the most recent power conversion efficiency (PCE) of greater than 8% bringing them closer to commercial viability. Further improvements are needed and can be achieved by optimizing the ratio between donor and acceptor, modifying the electronic properties of the materials, and optimizing the morphology of the resulting bulk heterojunction (BHJ). A direct way to increase the efficiency is to lower the band gap of the donor material in order to absorb a greater fraction of the solar spectrum. This concept has been explored through the synthesis of a series of new low band gap polymers, which exhibit a decreased band gap as a result of lowering the lowest unoccupied molecular orbital (LUMO). An emerging challenge is the need for electronically compatible acceptors with sufficiently low LUMO levels so that charge separation is efficiently promoted. Optimum miscibility of a specific polymer and fullerene combination to create the optimum degree of phase separation is also a feature to take into account. Typically, the approach used to test new donor materials is to fabricate devices using PC61BM ([6,6]-phenyl-C61-butyric acid methyl ester), a well-studied benchmark acceptor. Only a limited number of other fullerene derivatives have been successfully employed. Although these fullerene derivatives bear different functional groups, they are related by the positioning of the substituents on carbons 1 and 2 of a six-membered ring. From literature studies it is apparent that even subtle modification of the nature and especially position of substituents can drastically alter the electronic properties of fullerene derivatives. For example, PC71BM has reduced symmetry that increases visible light absorption and has a positive effect on current generation in polymer BHJ cells. Here we report the synthesis of a series of novel fullerene derivatives functionalized through the “1,4” position and their use in organic photovoltaics (OPVs). Features that distinguish this class of fullerene derivatives are: 1) straightforward synthesis which includes versatility of functionalization with different substrates starting from the same material (fullerenol, Scheme 1); 2) tunable LUMO energy by appending electron-donating or electron-withdrawing groups; 3) lower symmetry which decreases the optical gap and produces an increased absorption in the visible (the extinction coefficient at 480 nm of a 1,4-adduct is approximately 8 times larger than that of PCBM) ; 4) tunable solubility which influences the morphology of the BHJ. Like PC71BM, 1,4addends have increased light absorption at ca. 500 nm (Figure 1) with the advantage that all the C60 derivatives

Twisted but conjugated: building blocks for low bandgap polymers.

Here we report a novel twisted monomer based on a distorted CC double bond for low bandgap conjugated copolymers. This new building block provides several unique characteristics when compared to classical planar systems such as high solubility, electron accepting ability, and isomeric tunability. The resulting copolymers exhibit broad absorption spanning both visible and near-infrared regions leading to promising solar cell performance.

Fullerene and Its Derivatives for Organic Solar Cells

We describe in detail the features and characteristics of fullerene and its derivatives, specifically as acceptors in the field of bulk heterojunction solar cells. Examples of alterations on the PCBM structure are schematically presented, examining the effects of these modifications on device performance. Additionally, we discuss alternatively functionalized fullerenes, including the emerging bis-adduct derivatives.