Aggregate formation in crystalline blends of α-sexithiophene and para-sexiphenyl

Abstract

Earlier reports on rod-like conjugated molecules of similar shape and size such as α-sexithiophene (6T) and para-sexiphenyl (6P) indicated mixed crystal growth in equimolar blends. The spectral overlap between the 6P fluorescence and 6T absorption might there give rise to resonant energy transfer between the two species. In marked contrast to H-type aggregation found for 6T bulk crystals, isolated monolayers of 6T as well as 6T monolayers sandwiched between 6P multilayers have been reported to show pronounced green (instead of red) fluorescence, which has been attributed to J-type aggregation. Here, we investigate whether these altered optical properties of 6T translate from the monolayer to a bulk equimolar blend with 6P. Insight into the mixed crystal structure for vacuum co-deposited films of 6T + 6P is provided by using synchrotron grazing-incidence x-ray diffraction on different substrates. By correlating the optical properties of the pure and the mixed systems using absorption and photoluminescence spectroscopy we identify the green emission known from 6T monolayers to prevail in the blend. Our analysis indicates the formation of aggregates which are promoted by the molecular arrangement in the mixed crystal structure highlighting that the remarkable optical properties of 6T/6P heterostacks translate into mixed crystalline films. This study underlines that tuning the opto-electronic properties of organic semiconductors by blending species of similar shape but distinct opto-electronic properties is a promising pathway to achieve altered material properties.

Energetic disorder impacts energy-level alignment of alpha-sexithiophene on hydrogen-terminated silicon and silicon oxide

“…Strikingly, taking advantage of the orientation-dependent IE of 6 T thin films [16] even OPVs without a donor-acceptor interface could be facilitated [25]. Furthermore, evaporated 6 T thin films are well-known for pronounced polymorphism [20] and for their aggregation-dependent optical properties [26]. The transport gap of 6 T multilayer thin films (as determined by the combination of UPS and inverse photoemission) is 2.8 eV [27] and the IEs range from 4.7 eV for standing 6 T to 5.4 eV for lying 6 T as determined by UPS [16,27,28] and by photoelectron yield spectroscopy [25].…”

Thiophene–furan oligomers: beyond-DFT study of electronic and optical properties

“…Thiophene (T ) polymers or oligomers are a common choice for the active layer of hybrid organic-inorganic solar cells, given the special qualities regarding optical absorption energies, and convenient alignment of ionization potentials (IPs) with band gaps of consolidated semiconducting oxides [1][2][3][4][5][6]. Oligothiophenes films can be built from clean oligomers [7], or most often from alkyl-functionalized units which, depending on the specific preparation method, can either bring disorder [8] or be used to neatly arrange [9] the film. In any case, the final result is very stable.…”

Theoretical and experimental insight into nano-optoelectronics

“…Another seminal technique, which also allows insight in the unoccupied density of states, was covered in another focus issue of Electronic Structure: 'X-ray Absorption Spectroscopy of Energy Materials' [20]. Half of the contributions to our focus issue are from America, which seems to be a bit unbalanced, but two of them are from Brazil [15,16] (and the two others from the USA [17] and from Canada [11]). Europe is represented by two contributions from Germany [10,14] and Asia by China [13] and Japan [12].…”

“…While the focus issue is regionally quite balanced, only one of the corresponding authors' names is identified as female [17], which is as low as the proportion for all Electronic Structure articles published in 2021. This shows that the whole community has still a long way ahead in order to reach gender equality and a first step is made as the first authors of our focus issue include at least two female names [11,17].…”

“…In the article by Bhagat et al it is demonstrated that precise knowledge about the structure of mixed organic thin films, which was acquired by state-of-the-art gracing-incidence x-ray diffraction, is the key to understand the photoluminescence properties of these organic heterostructures [11]. The article by Nakayama et al deals with related organic thin films, but focuses on the energy-level alignment and the valence band dispersion as measured by ultraviolet photoelectron spectroscopy (UPS) [12].…”