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We present small molecule solar cells with α,ω-bis-(dicyanovinylene)-sexithiophene:C60 mixed heterojunctions, reaching power conversion efficiencies of 4.9±0.2%. We use substrate heating during deposition of the mixed layer to achieve an optimized morphology and show that this significantly improves the internal quantum efficiencies (IQEs) to values approaching 70%. By optical modeling, we evaluate the amount of loss due to absorption in inactive layers and show that IQE of the active layer itself is about 80%.

In this work we apply a joint experimental and theoretical approach to investigate thin films of side chain substituted dicyanovinyl quaterthiophenes (DCV4T-Et2) and DCV4T-Et2:C60 blends, prototypic absorbers for small molecule organic solar cells. Structural characterization of the morphology of thin films thermally deposited at different substrate temperatures on a silica surface was performed by variable angle spectroscopic ellipsometry, grazing incidence X-ray diffraction, and atomic force microscopy measurements. These methods, combined with full-atomistic molecular dynamic (MD) simulation, provide detailed information about thin film morphology, namely about molecular orientation, absorption, phase separation, and crystallinity, i.e., factors that affect the efficiency of organic solar cells. Using molecular dynamics simulation, we can constitute why the DCV4T-Et2 molecules arrange strongly tilted in pristine (69°to 70°tilt angle to the substrate normal) and DCV4T-Et2:C60 blend films (tilt angle of 65°to 69°).

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A low band gap, solution processable oligothiophene with a dialkylated diketopyrrolopyrrole chromophore for use in bulk heterojunction solar cells Appl. Phys. Lett. 94, 103301 (2009); 10.1063/1.3086897Bandgap renormalization in titania modified nanostructured tungsten oxide thin films prepared by pulsed laser deposition technique for solar cell applications J. Appl. Phys.We investigate the end-capped oligothiophene derivative ␣ , -bis-͑dicyanovinylene͒-sexithiophene with ethyl side chains ͑DCV6T͒ as donor material in heterojunctions with C60. The effect of the substrate temperature on the morphology and related photophysical properties of single DCV6T and mixed DCV6T:C60 layers is investigated. Single layers of DCV6T show crystalline features in UV-visible absorption and x-ray diffraction when grown on a substrate heated to 90°C. Investigations of DCV6T:C60 mixed layers by atomic force microscopy, UV-visible absorption, and photoluminescence measurements reveal that the elevated substrate temperature induces an increased phase separation between the two materials with larger domain size and higher surface roughness. Based on these observations, we present mixed heterojunction solar cells where the power conversion efficiency ͑ PCE ͒ is increased from 1.6% to 3.8% by increasing the substrate temperature from 30 to 90°C, respectively.

Thin vacuum deposited films of aluminum tris(8-hydroxyquinolinate) (Alq3) embedded in SiO2 matrix at concentrations corresponding to a single-molecule distribution are investigated. The spectral properties of the films are studied by luminescence spectroscopy. It is established that the dilution of Alq3 in the matrix leads to blueshift of the luminescence maximum up to 0.28eV compared to pure Alq3 layers. In contrast to the recently reported facial isomer of Alq3 in the crystalline δ phase, we conclude from our spectral data the observation of facial isomer molecules.

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Ellagic acid (EA), an antioxidant from fruits or other plants, has recently evoked interest in the field of organic electronics because of its weak electron donor properties. In this work, the preparation of uniaxial π-stacked EA films by thermal evaporation on different surfaces is reported for the first time. The (102) lattice plane of the π-electron system was confirmed as the contact plane for one monolayer equivalent on Ag(111) by lowelectron energy diffraction. X-ray and atomic force microscopy measurements revealed nanocrystalline grains with an average inplane size of 50 nm and considerably smaller average out-of-plane crystallite sizes (16−25 nm) in films of 16−75 nm thickness. The influence of different substrates was minor compared to the effect of the film thickness. An increase in the in-plane density of grains at larger film thicknesses was deduced from the trend in their uniaxial optical properties. Weak and strong intermolecular H-bonding interactions were identified in the EA crystal lattice, while a surplus of weak H-bonding was observed for the nanocrystallites in thin films, as compared to bulk EA. Finally, EA was coevaporated with the semiconducting thiophene molecule DCV4T-Et 2 to demonstrate principle interactions with a guest molecule by H-bonding analysis. Our results illustrate the feasibility of applying EA films as alignment layers for templating other semiconducting organic films used in organic electronic devices.