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Improved understanding of fundamental structure–property relationships, particularly the effects of molecular shape and intermolecular packing on film morphology and active layer charge transport characteristics, enables more rational synthesis of new p-type small molecules. Here we investigate a series of small molecules consisting of an acene-based electron-rich core flanked by one or two electron-deficient diketopyrrolopyrrole (DPP) moieties. Through minor changes in the molecule structures, measurable variations in the crystal structure and sizable differences in macroscopic properties are achieved. The molecular symmetry as well as the conformation of the side chains affects the unit cell packing density and strength of the intermolecular electronic coupling in single crystals of all molecules in this series. The addition of a second DPP unit to the benzodithiophene (BDT) core increases molecular planarity leading to decreased reorganization energy, strong cofacial coupling, and moderate hole mobility (2.7 × 10−4 cm2 V−1 s−1). Increasing the length of the acene core from benzodithiophene to naphthodithiophene (NDT) results in a further reduction in reorganization energy and formation of smaller crystalline domains (∼11 nm) when mixed with PCBM. Decreasing the aspect ratio of the core using a “zig-zag” naphthodithiophene (zNDT) isomer results in the highest hole mobility of 1.3 × 10−3 cm2 V−1 s−1 due in part to tight lamellar (d = 13.5 A) and π–π stacking (d = 3.9 A). The hole mobility is directly correlated with the short-circuit current (11.7 mA cm−2) and solar cell efficiency (4.4%) of the highest performing zNDT:PCBM device. For each of these small molecules the calculated π-coupling constant is correlated with the hole mobility as a function of crystal structure and orientation indicating the importance of designing molecules that create extended crystalline networks with maximal π-orbital overlap.

The recent emergence of a wide variety of two-dimensional (2D) materials has created new opportunities for device concepts and applications. In particular, the availability of semiconducting transition metal dichalcogenides, in addition to semi-metallic graphene and insulating boron nitride, has enabled the fabrication of all 2D van der Waals heterostructure devices. Furthermore, the concept of van der Waals heterostructures has the potential to be significantly broadened beyond layered solids. For example, molecular and polymeric organic solids, whose surface atoms possess saturated bonds, are also known to interact via van der Waals forces and thus offer an alternative for scalable integration with 2D materials. Here, we demonstrate the integration of an organic small molecule p-type semiconductor, pentacene, with a 2D n-type semiconductor, MoS2. The resulting p-n heterojunction is gate-tunable and shows asymmetric control over the anti-ambipolar transfer characteristic. In addition, the pentacene-MoS2 heterojunction exhibits a photovoltaic effect attributable to type II band alignment, which suggests that MoS2 can function as an acceptor in hybrid solar cells., Comment: 5 figures and supporting information. To appear in Nano Letters

Perylenediimides (PDIs) are among the best performing organic luminescent materials, both in terms of emission efficiency and chemical and photochemical stability because of their rigid, symmetric and planar structure; however, they exhibit very small Stokes shifts. The sizeable reabsorption of the emitted light limits the performances of perylenediimides in imaging applications and luminescent solar concentrators. Perylenemonoimides (PMIs) having an electron donating substituent in one of the free peri positions feature larger Stokes shift values while retaining high chemical stability. The selection of the most appropriate donor, both in terms of electron donating capability and steric demand, boosts emission efficiency and limits reabsorption losses. The synthesis, optical spectroscopy, molecular orbital computations, UPS, electrochemical, spectroelectrochemical, and multinuclear NMR investigation of a series of PMI derivatives functionalized with donors having different electronic characteristics and steric demands are discussed. Results are relevant for the fabrication of single layer plastic luminescent solar concentrators (LSC).

We compare and contrast the properties of hybrid organic-inorganic self-assembled nanodielectrics (SANDs) based on alternating layers of solution-processed ZrOxand either of two phosphonic acid-functionalized azastilbazolium Ï -units having opposite dipolar orientations. Conventional Zr-SAND and new inverted IZr-SAND are characterized by Kevin probe, optical spectroscopy, capacitance-voltage measurements, AFM, X-ray reflectivity, and electronic structure computation. The molecular dipolar orientation affects thin-film transistor (TFT) threshold and turn-on voltages for devices based on either p-channel pentacene or n-channel copper perfluorophthalocyanine. Specifically, Zr-SAND shifts the threshold and turn-on voltages to more positive values, whereas IZr-SAND shifts them in the opposite direction. Capping these SANDs with -SiMe3groups enhances the effect, affording a 1.3 V difference in turn-on voltage for IZr-SAND vs Zr-SAND-gated organic TFTs. Such tunability should facilitate the engineering of more complex circuits

Soluble molecular semiconductors are a promising alternative to semiconducting polymers in the field of organic photovoltaics. Here, three custom-made symmetric 1,3-bis(N,N-alkylated-2,6-dihydroxy-anilino)squaraines containing systematic variations in their molecular structures are compared regarding their applicability as donor materials in bulk-heterojunction solar cells. The terminal substitution pattern of the squaraines is varied from cyclic over linear to branched including a stereogenic center. Single crystal structures are determined, and, in the case of chiral squaraine, unusual formation of stereoisomer co-crystals is revealed. The thin film absorbance spectra show characteristic signatures of H- and J-bands or hint at the formation of tautomers. The general feasibility of these model compounds for photovoltaic applications is studied by light-induced electron spin resonance spectroscopy. The impact of the different molecular substitution patterns on aggregation behavior and, consequently, their optoelectronic solid state properties including charge carrier mobility and finally the solar cell performance are investigated.

The design, synthesis and characterization of five diphenylhydrazone end capped molecules as donors for Bulk Heterojunction (BHJ) solar cells are described. The use of the hydrazone donor instead of the more common amine one is advantageous in terms of synthetic access and overall stability. We show that the variation of both the nature and the substitution pattern of the electron deficient conjugated bridge enables the tuning of the optical energy gap as well as of the HOMO and LUMO. In particular, our data show that the low energy shift of the HOMO level along the series of hydrazone compounds, one of the critical parameters affecting the performance of BHJ cells, is as high as 0.42 eV. We tested all derivatives as donors in a BHJ configuration, using the fullerene derivative [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) as the acceptor. We found that the trend we observed in the progressive stabilization of the chromophore HOMO level upon increasing the electron accepting capabilities of the conjugated bridge is reflected in the progressive increase of the cell open circuit voltage (V-oc). Our preliminary results show that it is possible to obtain a V-oc as high as 0.74 V, along with a power conversion efficiency of 1.2%.

Highly efficient plastic based single layer Luminescent Solar Concentrators (LSCs) require the design of luminophores having complete spectral separation between absorption and emission spectra (large Stokes shift). We describe the design, synthesis and characterization of a new perylene dye possessing Stokes shift as high as 300 meV, fluorescent quantum yield in the LSC slab of 70% and high chemical and photochemical stability. © 2013 The Royal Society of Chemistry.