3 research outputs found
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(FBTTh<sub>2</sub>)<sub>2</sub>, 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