Evaluation and Control of the Orientation of Small Molecules for Strongly Absorbing Organic Thin Films

Abstract

In the photoactive film of organic solar cells, the orientation of the absorber molecules is one of the key parameters to achieve high absorption and high photocurrents as well as efficient exciton and charge transport. However, most organic absorber small molecules, such as zinc-phthalocyanine (ZnPc) or diindenoperylene (DIP) grow more or less upright standing in crystalline thin films. Considering absorption, this molecular alignment is unfavorable. In this work we control the orientation of ZnPc and DIP in crystalline absorber films by varying the substrate or organic underlayer appropriately. For this purpose, a precise evaluation of the molecular orientation and packing is important. We find that a combination of the methods variable angle spectroscopic ellipsometry (VASE) and grazing incidence X-ray diffraction (GIXRD) can fulfill this requirement. The combination of these complementary methods shows that the growth of DIP and ZnPc is nearly upright standing on weakly interacting substrates, like glass or amorphous charge transport films. In contrast, on strongly interacting metal sublayers and PTCDA templating layers, both molecules arrange in a strongly tilted orientation (mean tilt angle 54°-71°with respect to the substrate normal), inducing a significant enhancement of absorption (maximum extinction coefficient from 0.72 to 1.3 for ZnPc and 0.14 to 0.4 for DIP). However, even when deposited on metal or PTCDA sublayers, not all ZnPc and DIP molecules in the film are oriented in the desired flat-lying fashion. This highlights that classifying organic films into either solely flat lying structures or solely upright standing structures, as often made in literature, is a too simplified picture.

Critical CuI buffer layer surface density for organic molecular crystal orientation change

“…As is well known, the presence of the CuI buffer changes the surface energy of the weakly interacting substrate, which means that the molecular-surface van der Waals interaction is stronger than the intermolecular interaction, and results in flat-lying rather than upright CuPc molecules. 1,4 According to this growth model, the molecules grown on a substrate surface partially covered with CuI islands should have either edge-on or face-on orientations, depending on whether they are in contact with the bare substrate or CuI. This prediction seems to be correct for the CuPc film on the 0.4 nm thick CuI buffer layer, as shown in Figs.…”

“…[1][2][3][4][5][6][7] In organic field-effect transistors (OFETs), the charges are transported along the semiconductor-insulator interface, so the edge-on orientation with upright molecules produces higher mobility because of the shorter intermolecular pi-pi distance along the interface. 3 In contrast, the face-on orientation with flat-lying molecules is preferable in organic solar cells because the charges within the molecules can effectively absorb photon energies by moving along the electric field of the solar light, and thus oscillating within the sample surface perpendicular to the direction of photon travel.…”

“…3 In contrast, the face-on orientation with flat-lying molecules is preferable in organic solar cells because the charges within the molecules can effectively absorb photon energies by moving along the electric field of the solar light, and thus oscillating within the sample surface perpendicular to the direction of photon travel. 1,2,4,5,7 Molecular orientation is also important for organic light-emitting diodes (OLEDs). 6 Since the overlayer packing is affected by the mutual actions between intermolecular interaction and molecule-substrate interaction, growth conditions, 6,8 substrate types, 6,9 and the insertion of a thin buffer layer [1][2][3][4][5][6][7]10,11 can be applied to control the preferred orientation of a molecular crystal.…”

Crystallization in organic semiconductor thin films: A diffuse-interface approach

“…Furthermore, the extension of our model to account for additional physical phenomena may help explain certain experimentally observed morphologies and relative crystallization rates of differently oriented grains that cannot yet be reproduced by the current model. For example, here it has been assumed that films tend toward either face-on or edge-on crystallization, but films with intermediate orientations have also been experimentally observed [8,13].…”

“…A highly complex relationship exists between the processing methods used to fabricate the film and its final morphology since intermolecular and intramolecular interactions with the substrate and solvent play a role in determining the structural organization of the film [9,10]. Molecular orientation strongly influences the electrical mobility of organic semiconductor films [11,12] and can also affect optical absorption [13], while grain boundaries and film morphology can have a strong impact on charge transport processes [14][15][16]. A better understanding of the physical mechanisms that lead organic semiconductor thin films to crystallize in different molecular orientations and microstructures as well as greater knowledge of the relationship between these two characteristics are thus crucial for improved control of material and device properties.…”