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

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

Improved Light Harvesting and Improved Efficiency by Insertion of an Optical Spacer (ZnO) in Solution-Processed Small-Molecule Solar Cells

We demonstrate that the power conversion efficiency can be significantly improved in solution-processed small-molecule solar cells by tuning the thickness of the active layer and inserting an optical spacer (ZnO) between the active layer and the Al electrode. The enhancement in light absorption in the cell was measured with UV–vis absorption spectroscopy and by measurements of the photoinduced carriers generation rate. The ZnO layer used to improve the light-harvesting increases the charge collection efficiency, serves as a blocking layer for holes, and reduces the recombination rate. The combined optical and electrical improvements raise the power conversion efficiency of solution-processed small-molecule solar cells to 8.9%, that is, comparable to that of polymer counterparts

Mapping Orientational Order in a Bulk Heterojunction Solar Cell with Polarization-Dependent Photoconductive Atomic Force Microscopy

New methods connecting molecular structure, self-organization, and optoelectronic performance are important for understanding the current generation of organic photovoltaic (OPV) materials. In high power conversion efficiency (PCE) OPVs, light-harvesting small-molecules or polymers are typically blended with fullerene derivatives and deposited in thin films, forming a bulk heterojunction (BHJ), a self-assembled three-dimensional nanostructure of electron donors and acceptors that separates and transports charges. Recent data suggest micrometer-scale orientational order of donor domains exists within this complex nanomorphology, but the link to the optoelectronic properties is yet unexplored. Here we introduce polarization-dependent, photoconductive atomic force microscopy (pd-pcAFM) as a combined probe of orientational order and nanoscale optoelectronic properties (∼20 nm resolution). Using the donor 7,7′-(4,4-bis(2-ethylhexyl)-4<i>H</i>-silolo[3,2-<i>b</i>:4,5-<i>b</i>′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl[2,2′-bithiophen]-5-yl)benzo[<i>c</i>][1,2,5]thiadiazole), p-DTS(FBTTh₂)₂, we show significant spatial dependence of the nanoscale photocurrent with polarized light in both pristine and BHJ blends (up to 7.0% PCE) due to the local alignment of the molecular transition dipoles. By mapping the polarization dependence of the nanoscale photocurrent, we estimate the molecular orientation and orientational order parameter. Liquid crystalline disclinations are observed in all films, in agreement with complementary electron microscopy experiments, and the order parameter exceeds 0.3. The results demonstrate the utility of pd-pcAFM to investigate the optical/structural anisotropy that exists within a well-performing BHJ system and its relationship to optoelectronic properties on both the nanometer and micrometer length scales