Organic bulk heterojunction photovoltaic devices predominantly use the fullerene derivatives [C60]PCBM and [C70]PCBM as the electron accepting component. This report presents a new organic electron accepting small molecule 2‐[{7‐(9,9‐di‐n‐propyl‐9H‐fluoren‐2‐yl)benzo[c][1,2,5]thiadiazol‐4‐yl}methylene]malononitrile (K12) for organic solar cell applications. It can be processed by evaporation under vacuum or by solution processing to give amorphous thin films and can be annealed at a modest temperature to give films with much greater order and enhanced charge transport properties. The molecule can efficiently quench the photoluminescence of the donor polymer poly(3‐n‐hexylthiophene‐2,5‐diyl) (P3HT) and time resolved microwave conductivity measurements show that mobile charges are generated indicating that a truly charge separated state is formed. The power conversion efficiencies of the photovoltaic devices are found to depend strongly on the acceptor packing. Optimized K12:P3HT bulk heterojunction devices have efficiencies of 0.73±0.01% under AM1.5G simulated sunlight. The efficiencies of the devices are limited by the level of crystallinity and nanoscale morphology that was achievable in the blend with P3HT.

n-Type organic semiconductors are important for a range of optoelectronic applications including organic photovoltaic devices, light-emitting diodes, and field effect transistors (FETs). In spite of this clear motivation there has been significantly less development of n-type compounds relative to p-type systems. We have developed a simple, small molecule n-type material, 2-[(7-{9,9-di-n-propyl-9Hfluoren-2-yl}benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (K12), that can be processed either by spin-coating from solution or evaporation in vacuum. The thermal properties of K12 enable the film morphology to be controlled at easily accessible temperatures allowing the charge mobility to be tuned over two orders of magnitude. The electron mobility in the films was found to be independent of the initial processing conditions (solution or evaporation).

We report the electronic properties of a new class of non-fullerene electron acceptor molecules with electron affinities tunable over an approximately 1 eV range. This tunability allows us to vary the thermodynamic driving force for electron transfer (∆G°) such that it is equal-and-opposite-to the reorganization energy for the ionized states (λ). We utilize this design principle, derived from Marcus-Hush theory, to optimize the rate of charge transfer in blends of these acceptors with poly(3-n-hexylthiophene-2,5-diyl) (P3HT)-a standard organic solar cell donor material. We show that computationally inexpensive calculations can be used to parameterize Marcus-Hush theory so as to correctly predict whether quenching will occur. Arguments based solely on energetics are common in the literature and we show that such theories do not predict the trends observed in our photoluminescence quenching experiments. This is the case whether the energies determined from experiments [cyclic voltammetry (CV) and the optical gap] or calculated from density functional theory for the solid state. We predict essentially barrier-less photoelectron transfer (PET) from P3HT to the acceptor 2-[{7-(9,9-di-npropyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile (or K12), consistent with the experimental photoluminescence quenching efficiencies found for P3HT:K12 blends. Our results clearly show that energetics alone is not sufficient to predict PET between the acceptor-donor pair, and that kinetics are an important determining factor.

We report an n-type organic semiconductor [2-({7-(9,9-di-n-propyl-9H-fluoren-2-yl}benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (herein referred to as K12) for use in organic field-effect transistors (OFETs). K12 can be processed by spin-coating from solution or by vacuum deposition, organizing into highly orientated microcrystalline structures at modest (75 °C) annealing temperatures. OFETs with n-octyltrichlorosilane or hexamethyldisilazane monolayers, or poly(propylene-co-1-butene) (PPCB) modified dielectric surfaces were prepared. The mobility, ON/OFF ratio, threshold voltage, and current hysteresis were found to be dependent on the thermal history of the film and surface onto which it was deposited. The highest OFET mobility achieved was 2.4×10−3 cm2/V s, for spin-coated films with a PPCB modified silicon dioxide dielectric.

We report on the charge transport properties of a simple solution processable small-molecule electron acceptor, 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile (K12), designed for use in bulk-heterojunction organic photovoltaic cells. It was found that the molecular order in as-cast films can be dramatically improved by annealing at moderate temperatures (60 °C), which leads to a greatly enhanced electron mobility. Using the photoinduced charge extraction in linearly increasing voltage technique we measured bulk electron mobilities to be as high as 10−4 cm2 V−1 s−1, comparable with some of the best nonfullerene acceptor materials reported to date.