Controlling Electronic Transitions in Fullerene van ... - ACS Publications

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Controlling Electronic Transitions in Fullerene van der Waals Aggregates via Supra-Molecular Assembly Saunak Das, Felix Herrmann-Westendorf, Felix H Schacher, Eric Täuscher, Uwe Ritter, Benjamin Dietzek, and Martin Presselt ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06800 • Publication Date (Web): 02 Aug 2016 Downloaded from http://pubs.acs.org on August 5, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Controlling Electronic Transitions in Fullerene van der Waals Aggregates via SupraMolecular Assembly Saunak Das1,5, Felix Herrmann-Westendorf1,5, Felix H. Schacher2,3, Eric Täuscher4, Uwe Ritter4, Benjamin Dietzek1,5, Martin Presselt1,5,*

1

Institute of Physical Chemistry (IPC), Friedrich Schiller University Jena, Helmholtzweg 4,

07743 Jena, Germany 2

Institute of Organic Chemistry and Macromolecular Chemistry (IOMC), Friedrich Schiller

University Jena, Humboldtstraße 10, Jena, 07743, Germany 3

Jena Center for Soft Matter (JCSM), Friedrich Schiller University Jena, Philosophenweg 7,

Jena, 07743, Germany 4

Institute for Chemistry and Biotechnology, Ilmenau University of Technology, D-98684

Ilmenau, Germany 5

Leibniz Institute of Photonic Technology (IPHT), Albert-Einstein-Str. 9, 07745 Jena,

Germany

*Corresponding author: [email protected]

ABSTRACT Morphologies crucially determine the optoelectronic properties of organic semiconductors. Therefore, hierarchical and supra-molecular approaches have been developed for targeted design of supramolecular ensembles of organic semi-conducting molecules and performance improvement of e.g. organic solar cells (OSCs), organic light emitting diodes (OLEDs), and organic field-effect transistors (OFETs). We demonstrate how the photonic properties of fullerenes change with formation of van der Waals aggregates. We identified supra-molecular structures with broadly tunable absorption in the visible spectral range and demonstrated how to form aggregates with targeted visible(vis)-absorption. To control supra-molecular structure formation we functionalized the C60-backbone with polar (bis-polyethylene glycol malonate 1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 29

MPEG) tails, thus yielding an amphiphilic fullerene derivative that self-assembles at interfaces. Aggregates of systematically tuned size were obtained from concentrating MPEGC60 in stearic acid matrices, while different supra-molecular geometries were provoked via different thin film preparation methods, namely spin casting and Langmuir-Blodgett (LB) deposition from an airwater interface. We demonstrated that differences in molecular orientation in LB films (C2v type point group aggregates) and spin casting (stochastic aggregates) lead to huge changes in electronic absorption spectra due to symmetry and orientation reasons. These differences in the supra-molecular structures, causing the different photonic properties of spin-cast and LB films could be identified by means of quantum chemical calculations. Employing supramolecular assembly we propounded that molecular symmetry in fullerene aggregates is extremely important in controlling vis-absorption to harvest photons efficiently, when mixed with a donor molecule thus improving active layer design and performance of OSCs.

KEYWORDS: supramolecular assembly, Langmuir Blodgett isotherms, fullerene acceptor, van der Waals aggregates, TDDFT modelling, organic solar cells

INTRODUCTION Organic (opto)electronics is an emerging field since the last decade due to convenient processing methods, flexibility, and light weight of the devices.1-5 In organic (opto)electronics the morphologies of the active materials crucially determine the device performances.6 Among the active layers, bulk heterojunctions (BHJ) as used in organic solar cells (OSCs) are particularly important due to their influence on photon absorption, charge generation and transport.7-10 But the BHJ’s morphologies are also particularly complex since they are comprised of a stochastic mixture of electron donor and acceptor materials. The donor materials were engineered in the past decade for optimized harvesting of the red part of the solar light.11-14 In contrast, the most successfully applied acceptor materials, fullerenes15-17, were considered to be superior electron acceptors, but inherently weak absorbers of the visible and red part of the solar spectrum.18-22 However, Shubina et al. recently highlighted the impact of the fullerene’s supra-molecular structure on their electronic properties in thin solid films.23 Shubina and co-workers found that fullerene van der Waals aggregates can form deep electron traps. It was also shown for OSCs that 2 ACS Paragon Plus Environment

Page 3 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the fullerene’s local environment and aggregation cause barrier-less charge splitting24 and increase of the short circuit current and decrease of the open-circuit voltage25-26, respectively. In contrast, there is no detailed study on the influence of the morphology of fullerene thin films on its absorption properties though, there are reports on fullerene aggregation in solution27, which induces changes in UV-vis-absorption spectra28-29. Therefore, we might suppose that if the morphology of fullerene thin films can be controlled and tuned in a broad range, an increase of the absorption of the visible and red part of the solar spectrum might be enabled. Such visabsorption increase via morphology tuning would render fullerenes useful for photon harvesting beyond the common variation of the donor’s absorption spectrum, thus enabling escalation of the power conversion efficiency of OSCs.18, 30. For a targeted design and maximum control over the morphology hierarchical approaches were developed.31-32 One promising approach for fabrication of defined supra-molecular structures that are thermodynamically stable and possibly show distinct UV-vis absorption spectra is the self-organization of amphiphilic donor or acceptor materials at interfaces.32-35 To introduce amphiphilicity to fullerenes polar groups can be attached to the C60-backbone36-37 , which were chosen to be two bis-polyethylene glycol malonate -MPEG chains in the present work, thus yielding amphiphilic MPEGC60 (polyethylene glycol-C60) as shown in Figure 1. The controlled aggregate formation of MPEGC60 is investigated by means of a simple experimental approach of diluting/concentrating MPEGC60 in matrices of amphiphilic stearic acid as schematically depicted in Figure 1. The changes in the absorption spectra upon aggregate formation of the dilute thin films were detected by means of the highly sensitive photothermal deflection spectroscopy. We employed the amphiphilic property of MPEGC60 and stearic acid for the production of quasi-two dimensional model layers via the Langmuir-Blodgett (LB) technique in addition to spin-casting MPEGC60:stearic acid BHJ thin films.38-39 By means of the dilution series we are able to demonstrate for the first time the ‘Herzberg Teller Intensity Stealing’ phenomenon40 in fullerene aggregates within solution processed spin casted thin films experimentally and demonstrate how this phenomenon can be controlled via supramolecular chemistry. Therewith, one of the essential drawbacks of fullerene-based organic solar cells, namely the poor vis-absorption of fullerenes18, could be eliminated and possibly enables further boosted power conversion efficiencies.

3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 29

Figure 1: a) Molecular structure of MPEGC60 (bis-polyethylene glycol malonate derivative of C60), b) Orientation of MPEGC60 and stearic acid at the air-water interface. METHODS Thin Film Preparation a.

Langmuir-Blodgett (LB) Films

Solutions to be spread on the LB-trough (KSV NIMA Alternate L & LB Trough) were prepared by mixing MPEGC60/CHCl3 stock solution with a stearic acid/CHCl3 solution (3500 µM) to produce solutions of different molar percentage of MPEGC60 in stearic acid matrix (100%, 67%, 50% and 33%). These solutions were then stored at 280K for one day. A volume of 75 µl of each of the solutions was carefully dispersed on the air-water interface of the LB-trough prefilled with ultrapure water. A waiting time of 8 minutes was allowed for complete evaporation of CHCl3. Thereafter the barrier was compressed at a rate of 6 mm/min to record LB-isotherms. The variation of surface pressure (Π) was sensed using a Wilhelmy plate balance. To ensure film stability the barrier was kept locked for 30 mins and the variation of surface pressure was monitored. At more compressed barrier positions the films were sufficiently stable for subsequent 4 ACS Paragon Plus Environment

Page 5 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


deposition on polished quartz glass substrates (30x5x1 mm3). These substrates were pre-cleaned in a heated ultrasonic bath (40oC) with acetone, chloroform and isopropanol and were stored in isopropanol at 280 K overnight. These quartz glasses provided smooth hydrophilic surfaces for deposition of the Langmuir films. The fabrication was done at surface pressure of 10 mN/m with an initial vertical upstroke of 5 mm/min. b.

Spin Casted Films

Working solutions of MPEGC60:stearic acid using CHCl3 in high dilution were prepared (molar percentage 9%, 4%, 1.3%, 0.9% which is equivalent to 272, 72.01, 49.32 and 37.51 µM of MPEGC60 in stearic acid matrix respectively). Following the same protocol as for cleaning LB film substrates, smooth quartz glass substrates were used for casting. A volume of 200 µl of MPEGC60:stearic acid blend was spread onto the substrate surface and was spun at 12 rps for 60 s. A constant rotation frequency, rotation time, and casting volume were maintained for all of the films to ensure similar film thicknesses. Spectroscopic Methods For absorption spectroscopic measurements we employed both a UV-vis (Varian: Cary 5000) spectrophotometer in transmission mode and a custom-made setup for photothermal deflection spectroscopy (PDS).41-47 The principle and basic setup of PDS is described elsewhere.43 In our setup the monochromatic light source was obtained from LOT-Quantum Design and consisting of a 450W Xe lamp and a monochromator with a focal length of 260 mm. The light is modulated by a chopper operating at a frequency of 5 Hz. Deflection of a 0.7 mW HeNe-laser is measured by a lateral effect sensor obtained from Thorlabs. A SR850 Lock-In amplifier is used to amplify the signals. The samples are dispersed in perfluorohexane and graphite is used as standard to calibrate the data. A self-written Labview program automatically collects and calibrates the data. Theoretical Methods Quantum chemical structure optimizations, and calculations of absorption spectra on MPEGC60 and PCBM monomers were performed using density functional theory (DFT) and its time-dependent derivative (TD-DFT) implemented in Turbomole48-49 and applying the GGA (generalized gradient approximation) B-P86 exchange-correlation functional, the def2-TZVP triple-ζ basis set50 and the MARI-J approximation49. A similar combination has been shown to give reliable geometries, electron density distributions and spectroscopic properties in 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 29

many cases at very reasonable computational cost.51-56 Recently, the BP-86 functional was successfully employed in calculations of the C60 absorption spectrum in n-hexane.57 For geometry optimization of the dimers we also employed the D3-dispersion-correction.58 This yields an acceptable description of the dimers23, even though the van der Waals coefficients between fullerenes are abnormally large.59 Four test dimers of MPEGC60, were chosen for TD-DFT absorption spectra computation which turned out to be computationally costly (see section 5 and Figure S4, S5 in the supporting information (SI)). To facilitate the calculation and to enable efficient scanning of various dimer geometries, the MPEGC60 monomer was simplified stepwise: The MPEG-chains were replaced by linear CN-groups to reduce the number of atoms and the bridging spiro-carbon was placed between two six-membered rings of C60 instead between a five- and a six-membered ring to allow for C2- and σ-symmetry operations. These simplifications result in the MPEGC60 model C61(CN)2. As shown in the supplementary information C61(CN)2 is a reasonable model for MPEGC60 and shows an UV-vis absorption spectrum virtually identical to the one of MPEGC60. The C61(CN)2-monomer allows for construction of dimers with various symmetries that were treated within their point groups. Optimization of their geometries and calculations of their UV-vis absorption spectra were done using the TZV-basis set according the recent work of Shubina et al.23. The excited state responses were calculated using Turbomole's escf-module, for singlet excitations (rpas).49 The spectra were obtained after single-point calculations at the energetically most favorable geometry. To cover the UV-vis spectral range between 200 and 800 nm computations of about 2700 excitations in case of the dimers (234 atoms) were performed with equal distribution to the irreducible representations that lead to allowed electronic transitions. According previous studies60 TD-DFT-derived spectra were blue shifted by 0.35 eV and broadened with Gaussian profiles (standard deviation of 0.04), thus yielding excellent correspondence with experimental results.

RESULTS AND DISCUSSION To determine how much the absorption of solar radiation can be tuned and increased in the visible and red spectral part via morphology-tuning the supra-molecular size and structure of 6 ACS Paragon Plus Environment

Page 7 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


amphiphilic MPEGC60 was systematically varied and UV-vis absorption spectra were recorded. For modifying the supra-molecular structure the Langmuir-Blodgett (LB) and spin-casting (SC) technique were used for thin film preparation as discussed in the following. Then, the UV-vis absorption spectra of diluted MPEGC60 solution in comparison to PCBM UV-vis absorption spectra and to those determined by TD-DFT calculations will be considered. Subsequently, we will focus on spectral changes due to formation of aggregates observed in spin cast and LB films. Structure-absorption relations are discussed and are supported by TD-DFT-derived results.

Langmuir Monolayer Characteristics at Air-Water Interface For all following experiments a good miscibility between the amphiphilic MPEGC60 and the stearic matrix molecules is essential to enable a systematic aggregate growth with concentration of MPEGC60 within the stearic acid matrix. Furthermore, to identify the range of lateral compression at which the MPEGC60 Langmuir monolayers can be deposited the quantification of MPEGC60’s amphiphilicity is important. Both, miscibility and suitable deposition pressure can be derived from the LB-isotherm, which is analyzed in the following. The isotherm of pure MPEGC60 shows a slow rise of the surface pressure Π with decreasing area per molecule in the quasi-liquid phase till about 230 Å2 (Figure 2). For lower areas per molecule, i.e. upon higher compression, the surface pressure strongly increases, indicating the formation of a 2D quasi-solid phase till a collapse surface pressure of about 45 mN/m, similar to other amphiphilic fullerenes38. At ~28 mN/m a point of inflection indicates a phase transition within the MPEGC60 Langmuir film to a densely packed phase as the slope gets even higher towards the collapse surface pressure. For this densely packed phase (30 mN/m < Π < 45 mN/m) we determined an area per molecule (A0) of about 112.5 Å2 (via a line fit), which is significantly larger than the expected area of 87 Å2 for close-packing of equal C60-spheres as shown in the top panel of Figure 2. The expected area per molecule was calculated assuming a rhombic cell space with the C60-diameter taken from quantum chemical calculations (7 Å) and the intermolecular distance (3 Å) taken from the work of Shubina et al.23 Considering the LB-determined molecular area of 112.5 Å2, from Table 1, we end up at rhombic side-lengths of 11.4 Å and a resulting average intermolecular distance of 4.4 Å in the Langmuir layer. This larger intermolecular distance compared to the values theoretically estimated by Shubina et al. (3 - 4 Å) is attributed to



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 29

imperfection of the close-packing throughout the Langmuir layer and interactions between the MPEG-chains. For all the mixed MPEGC60:stearic acid films, there are two characteristic inflection points in the isotherms, one at ~30 mN/m and the other at about 44 mN/m which provides evidence for molecular reorientation in the solid phase with higher barrier pressure. The second inflection point is inherent to the mixed films, since stearic acid undergoes a γ (phase with high packing density) to δ (stabilization phase after film fracture) phase change.61 This phase change is characteristic for long chained fatty acids and gives typically rise to a sharp peak in the LBisotherm at high pressures.61-62 In conclusion, the surface pressure at deposition of the Langmuir layer should be lower than 20 mN/m to ensure similar phases for differently diluted MPEGC60Langmuir layers to avoid the collapse of Langmuir layers at high surface pressures. A good miscibility of MPEGC60 in the stearic acid phase can assert that stearic acid serves as a good matrix for MPEGC60 solutions. The miscibility can be characterized by calculating the surface excess AE. For a given pressure in a binary system the surface excess is defined as63 AE = A1,2 -Aid , where Aid = A1N1 + A2N2

,

where AE is the surface excess, A1,2 is the experimentally observed molecular area of the blend at a definite pressure, A1 and A2 are single component surface areas and N1 and N2 their molar fractions in the blend. Thus, Aid is simply the weighted sum of the molecular areas of the individual components, in our case stearic acid and MPEGC60, assuming no change in intermolecular interactions due to mixing as compared to the pristine components. This is referred to ideal mixing and would cause no surface excess, i.e. AE = 0. As shown in the inset of Figure 2 the surface excess shows a minimum at balanced blending ratios for the defined dipping pressure of 10 mN/m. This indicates good miscibility of stearic acid and MPEGC60 and ensures systematic aggregate growth with concentration of MPEGC60 which is the precondition for discussion of the vis-absorption dependence on the size of the supra-molecular structures in the following section.


Page 9 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2: (Top) Schematic close-packing of fullerenes as viewed from top of a twodimensional film. (Bottom) Surface pressure vs. area (Π(A)) isotherms for Langmuir films at different MPEGC60-stearic acid blending ratios. Inset shows a surface excess AE plot as a function of mole percentage of MPEGC60 in the MPEGC60:stearic acid blend. Molecular areas to calculate AE were determined at 10 mN/m. Table 1: Characteristics of the monolayers of LB isotherms for various blending ratios MPEGC60:stearic acid

Lift-off area (Å2)

A0 (Å2)

100:0

327

112

67:33

299

93

50:50

182

71

33:67

136

48



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0:100

56

Page 10 of 29

28

Solutions Absorption Spectra Figure 3 shows the UV-vis absorption spectra of MPEGC60 and the common [6,6]-phenylC61-butyric acid methyl ester (PCBM) as reference, both in chloroform. Additionally, it depicts quantum chemically computed spectra. Within the experimentally accessible spectral window in chloroform (λ>240 nm) the MPEGC60 spectrum is very similar, but slightly hypsochromic shifted if compared to the spectrum of PCBM64 and features two prominent peaks at 256 and 326 nm. According to TD-DFT calculations, the MPEGC60-peak at 256 nm is constituted by two pronounced electronic transitions, similar to the two prominent transitions to nT1u-excited states (n=6,7) in Ih-symmetric C6065-66. The peak at 326 nm is a superposition of several mediumintense and weak transitions which might be compared to the transition to the 3T1u state in Ih-C60 (335 nm). Additionally, an extremely sharp peak appears at 425 nm appears, which is typical for fullerene derivatives with [6,6]-bridged carbons.66 Absorption at longer wavelength than 400 nm is very weak. Thus the UV-vis absorption spectra of the dissolved fullerene derivatives (MPEGC60 and PCBM) match the common view on fullerenes showing only weak visabsorption.18-19 However, the weak vis-absorption originates from several transitions that are symmetryforbidden in Ih-C60, but gain intensity because of Herzberg-Teller intensity stealing. Furthermore, combination bands contribute to these transitions.67 These combination bands comprise totally symmetric modes and non-totally symmetric Jahn-Teller active modes, which partially favors these otherwise forbidden transitions.65 As shown later in this work, these transitions in the visible range can be drastically enhanced via supra-molecular assembly.


Page 11 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3: Experimental and TDDFT-derived absorption spectra of MPEGC60 and PCBM. TDDFT results are shown as stick spectra as well as dashed line spectra obtained by Gaussian broadening. The region below 240 nm is not discussed because of strong chloroform absorption in this spectral range.

Thin Films Absorption Spectra of Stochastic Aggregates in Spin-Cast Films After having characterized the absorption features of dissolved MPEGC60 in solution, we will focus on the spectroscopic properties of aggregates in thin solid films. Therefore, we will analyze the spectra of spin-cast films with systematically varied fractions of stearic acid matrix molecules and MPEGC60. This molar blending ratio is varied from pristine MPEGC60 (100%) to high dilutions of 0.9%. Recording the absorption spectra of diluted thin films is enabled by PDS spectroscopy, as described in the methods section. At low concentration of 0.9% (equals 36 µM with respect to stearic acid) the spin-cast films are supposed to be largely dominated by monomers as the solution spectra do not show any sign of aggregation between 15 and 120 µM. The ratio A256nm/A326nm between the prominent peak at 256 nm and the peak at 326 nm at 0.9% MPEGC60 content in the SC-films is with A256nm/A326nm≈3 about identical to the ratio found for the diluted chloroform solutions. However, upon slightly increasing the MPEGC60 content to 1.3% the sharp UV-absorption peak gets 11 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 29

weaker and extends to higher energies as shown in Figure 4. These spectral changes are assigned to MPEGC60-dimers. If we normalize the absorption spectra to the peak at 326 nm (3.6 eV) the prominent peak at 256 nm progressively diminishes upon concentration of MPEGC60 in the spincast films, while the broad absorption between 350 and 750 nm systematically increases. Finally, the pristine 100% MPEGC60 spin-cast film shows a broad and featureless absorption where the 326 nm absorption actually became the maximum of the absorption spectra as shown in Figure 4. Additionally, the peak at 256 nm gets significantly broader due to aggregation of the MPEGC60 molecules.

Absorption Spectra of Aggregates in Langmuir-Blodgett Films As discussed above, the spectral changes shown in Figure 4 are due to formation of various dimers, trimers and higher aggregates of MPEGC60. In Langmuir films however the hydrophilic MPEG chains adhere to the water surface and the monomeric units tend to orient themselves parallel to one another depending on the barrier pressure. The UV-vis absorption of those LB-thin films as detected by PDS shows only small changes with concentration of MPEGC60. Therefore, we assume that the spectral shapes presented in Figure 4 are representative for particular dimers and but also higher aggregates of similar symmetries as detailed in the subsequent theoretical section.


Page 13 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: (Top) PDS spectra for spin casted films of MPEGC60 from CHCl3 solvent for different MPEGC60:stearic acid concentrations. All spectra were normalized to the absorbance at 344 nm. (Bottom) PDS spectra on Langmuir-Blodgett films for different MPEGC60:stearic acid concentrations.

Assignment of Dimer-Geometries to Experimental Absorption Features We assume that in the highest diluted SC-film (0.9%) MPEGC60-aggregates were not formed, but the 1.3%-MPEGC60 containing SC-film comprises monomers and dimers of different symmetries, cf. dynamic light scattering studies of solutions with the same MPEGC60 concentration in the SI. Hence, we might identify spectral features of dimers and by taking the 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 29

spectrum of the most diluted spin-cast film (0.9% MPEGC60) as a representative for monomer spectrum and subtracting this monomer reference from the spectrum of the next higher concentrated spin-cast film (1.3% MPEGC60). As already seen in Figure 4, the 1.3% MPEGC60 containing film shows absorption broadening of the intense 256 nm band, particularly towards the deeper UV-spectra range. If the monomer reference spectrum is subtracted with increasing weight, the UV-absorption at shorter wavelength than 256 nm becomes more pronounced but the difference (∆SC) spectrum features an unphysical minimum when approaching balanced weights of the 1.3% and the 0.9% monomer spectra. In Figure 6 the ∆SC spectrum is shown for a relative weight of the monomer reference of 0.5 to highlight the UV-absorption at 232 nm (5.3 eV), while the >256 nm-UV-vis absorption basically resembles the typical LB-film absorption spectrum. Since the 232 nm peak is not present in LB-films it might be used as a spectroscopic fingerprint allowing identification of different dimer geometries that are present in LB-films and those which are present in spin-casted films only. Thereby we can unravel those supramolecular structures which give rise to exclusive UV-absorption (LB-type) and ones causing intense vis-absorption (spin-casted-type) at large supra-molecular structures as shown in Figure 4.

Figure 5: Relative Energies of all optimized dimer geometries with respect to two times the energy of reference monomer C61(CN)2, belonging to different molecular point groups as


Page 15 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


indicated, with their individual intermolecular distances. The color scheme is in accordance to the simulated spectrum in the lower panel of Figure 6.

Figure 6: (Top) Comparison between LB-film and spin-cast films (SC) with 0.9% and 1.3% MPEGC60 content assigned to be representative for monomers and a mixture of monomers and dimers, respectively. The ∆SC difference spectrum is calculated by subtracting SC-monomer spectrum

from

the

1.3%-MPEGC60

containing

spin-cast

film

with

weights

A(λ,c=1.3%):A(λ,c=0.9%)=2:1. (Bottom) Simulated electronic absorption spectra for van der Waals C61(CN)2-dimers with different symmetries. Molecular orbitals are sketched for discussion of the individual absorption features. 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 29

To assign the essential spectral features shown in Figure 6 the absorption spectra of C61(CN)2dimer models for MPEGC60-dimers are computed by means of TDDFT. The C61(CN)2-model allows to create dimers of various symmetries which is of particular importance for the discussion of the absorption spectra of C60-derivatives. Figure 6 illustrates the calculated absorption spectra of eight different geometries of van der Waals-dimers belonging to 5 different symmetry groups, namely Cs, D2d, D2h, C2h, C2v and the reference monomer which is actually a C2v-dimer at a huge intermolecular distance (93 Å). The molecular structures and relative energies versus intermolecular distance of all the computed dimers are represented in Figure 5. The dimers belonging to the D2h and the C2h point group are expected to show the weakest visabsorption of all considered dimers since they are centrosymmetric dimers. In general, molecules of those symmetries involving a center of inversion have forbidden transitions between states of same parity (gerade-gerade in C2h and D2h) according to Laporte’s selection rule. This effect has been used to explain the forbiddance of HOMO-LUMO transitions for non-substituted fullerene65, 68 and further centrosymmetric molecules.69 The school of C2v-dimers can be viewed as representatives of LB aggregates since their dimer-geometries show a non-vanishing dipole moment and a net-amphiphilicity. At the air-water interface the hydrophobic -C60 moiety is directed away from the water surface, for which dimers belonging to point groups D1h, C2h, D2d have least chance of domination in LB films. Dimer 8 Cs has minimal chance of formation in a well-assembled monolayer, since the plane cutting the broad bases of the individual fullerenes horizontally is not parallel to the water surface. Furthermore, in accordance to Figure 5 the school of C2v-dimers have higher energetic stability than dimers 8 Cs, 4 C2h and 1 D2h. Based on these arguments, one can expect the school of C2v dimers to dominate the studied LB films. Although in the solid films various dimers without perfect point-group-symmetry are supposed to be present, the absorption spectra of the symmetric dimers sketched in Figure 6 are assumed to resemble the essential absorption features observed experimentally. The absorptions calculated at photon energies of 3.9 and 4.9 eV are attributed to core-fullerene transitions65 and depend negligibly on the point-group of the dimer. The remaining spectral signatures depend on the dimer symmetry, particularly those between 1.8 and 2.8 eV, between 2.8 and 3.6 eV, between 4.4 and 5.2 eV, and between 5.2 to 5.6 eV. The first weak absorption feature between 1.8 and 2.8 eV is attributed to the HOMO-LUMO transition which inherently depends on the dimer symmetry. It is most pronounced for C2v-type dimers but Laporte-forbidden for centrosymmetric dimers 1 D2h and 4 C2h due to transition 16 ACS Paragon Plus Environment

Page 17 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


between MOs of same parity, namely 46 b2g to 62 ag and between 107 ag to 108 ag respectively (MO-numbering according ascending energies). However, these absorptions are generally weak and very broad in the wavelength scale what might explain why just negligible absorption differences are present between LB-films and highly diluted SC-films below 2.8 eV as shown in Figure 6. The second absorption features between 2.8 and 3.6 eV are attributed to singlet excited states where the adjacent p-orbitals at the inter-fullerene junction interact to varying extends depending on molecular geometry and symmetry. As shown in Figure 6, maximum overlap for noncentrosymmetric clusters is observable in the UMO 90b2 of 3 C2v with minimal dipole-dipole repulsion followed by 108a1 for 6 C2v and 114 a1 for 5 C2v. Accordingly, in the same order the UMO energies are lowered, thus yielding maximal red shift for dimer 3 C2v followed by 6 C2v and 5 C2v. Thus, at the optimized geometries with intermolecular distances between 3.4 and 2.6 Å (see section 9 and Table S2 of S1 for details) the C2v-type dimers contribute most to the visabsorption. However, with increasing aggregate size LB-type structures do not change their absorption features in the vis-spectral range significantly. Therefore, we deduce that basically the remaining symmetric dimers shown in Figure 6 and those with similar, though not perfectly symmetric, geometries, as well as a large distribution of different intermolecular distances cause the large vis-absorption of the SC-films shown in Figure 4. The third absorption range between 4.4 and 5.2 eV involves a core fullerene based transition at 4.9 eV, whose energy remains constant for all other dimers. However, for 1 D2h and 2 D2d, excited states like 49 b3u and 62 b2 respectively, with adequate fullerene-fullerene interaction a shoulder at 4.6 eV for these two dimers shows ample oscillator strength in comparison to the monomer. It seems plausible that the growth of this shoulder can be a result of vibronic coupling between the transitions 44 b3u to 48 b2g (at 4.28 eV) and 45 b2g to 49 b3u (at 4.59 eV) for 1 D2h. This shoulder, which shows an absorption similarly strong as the 4 eV-core-fullerene absorption just for the D2h and D2d dimers, might be attributed to the slightly stronger absorption of the ∆SCspectrum as compared to the C2v-dominated LB-spectra at ~4.6 eV shown in Figure 6. The fourth feature between 5.2 and 5.6 eV is also due to excitations to overlapped states (like 112 a1 for 6 C2v). This absorption has the highest energy for 4 C2h, 1 D2h, and 2 D2d dimers. Therefore, it least overlaps with the core-fullerene absorption at 4.9 eV for these non-C2v dimers and is likely to be the origin of the experimentally observed peak at 5.3 eV in the ∆SC-spectrum,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

which is not present in the absorption spectra of the LB-films as shown in Figure 6. In the pristine SC-films, this “peak” is not resolved but visible as a shoulder.

Explanation of Strong vis-Absorption of Spin-Cast Films As shown in Figure 6, none of the considered dimers at their energetically most favorable structure shows the strong vis-absorption observed for spin-cast films with large MPEGC60 weight fractions that were shown in Figure 4. Instead of the computationally expensive treatment of larger and larger fullerene aggregates one might vary the dimer intermolecular distance to identify trends that occur with closer packing upon concentration of MPEGC60 in spin cast thin films. This distance variation seems reasonable since the potential minima of fullerene van der Waals-dimers are supposed to be broad according to the recent work of Shubina et al.23. As discussed in the previous section, C2v-dimers are assumed to dominate LB-films, which do not develop a strong vis-absorption upon higher aggregation as shown in Figure 4. Therefore, one might consider the remaining 8 Cs, 2 D2d, 1 D2h and 4 C2h dimers to explain increasing visabsorption of the spin-cast films with progressive aggregation as shown in Figure 4. However, the perfectly centrosymmetric 1 D2h-dimer is not allowed to show strong vis-absorption with distance variation (see section 6 and Figure S6 in SI) and the 4 C2h-dimer is energetically less favorable than e.g. the 2 D2d-dimer as can be seen in Figure 5. Additionally, all dimers interacting via parallel bonds or rings tend to form covalent bonds by [2+2]-cycloaddition leading to polymerization which is not synthetically straight-forward23,

70

and beyond the focus of the

present work on van der Waals-aggregates (see section 8 and Figure S8 in SI). Hence, we focus on the 2 D2d-type dimer with perpendicular nearest neighbor bonds in the following section. For the D2d-dimer, we observed a large diagonal π-orbital overlap for the OMO (occupied molecular orbital) 61 a1 as shown in Figure 7. On decreasing the van der Waals distance, the transitions around 480 to 590 nm (2.58 to 2.10 eV) escalated due to appearance of several new electronic transitions. The electronic transitions between 480 to 590 nm are listed in Table 2 for the dimers as a function of the van der Waals distance. As the distance between monomeric C61(CN)2 units in the D2d dimer is decreased from 3.2 to 2.0 Å the diagonal overlap for the OMO 61 a1 increases effectively and causes a MO stabilization of 0.3 eV. However, the corresponding UMO (unoccupied molecular orbital) 62 b2, which is attributed to core fullerene π-system, remains energetically degenerate at -4.62 eV in both cases. Hence, the electronic transition at 18 ACS Paragon Plus Environment

Page 19 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


560 nm (2.21 eV) (61 a1→ 62 b2) in D2d shifts hypsochromically to 496 nm (2.5 eV) upon shortening the intermolecular distance from 3.2 to 2.0 Å as shown in Table 2. On the contrary, OMOs 94 e, 61 b2 and the UMOs 97 e, 63 a1 have strong and extended π-orbital overlap between the C61(CN)2 units, thus stabilizing the molecular orbitals and effectively causing bathochromic shifts for decreased intermolecular distance (3.2 to 2.0 Å). Hence, the absorption in the range 480-590 nm heavily depends on the listed transitions, which acquire appreciable oscillator strength for a short intermolecular distance of 2 Å compared to other dimer geometries. Thus, on decreasing the distance from 3.2 to 2Å within the van der Waals distance regime, one observes progressively higher and higher visible absorption. For a dimer with a van der Waals distance of 3.2 Å the lowest energy transition is nearly at 520 nm whereas for the one with a van der Waals distance of 2 Å there is a shift above 750 nm. This means, that the extent of delocalization in the collective states of the dimer increases upon closer packing which in turn provides evidence that a 100% spin cast film, which has innumerable fullerenes embedded in a cluster with higher packing, can have even pronounced low energy transitions.

Table 2: Computed electronic transitions for 2 D2d at different van der Waals-distances in the range 480-590 nm Range

van der Waals distance (d) (Å)

Electronic transitions

3.2

61 a1 – 62 b2 (560 nm) 61 a1 – 62 b2 (531 nm)

2.6

94 e – 97 e (476 nm) 94 e – 97 e (482 nm)

480-590 nm

2.3

61 b2 – 63 a1 (496 nm) 61 a1 – 62 b2 (512 nm) 39 a2- 40 b2 (483 nm)

2

94 e – 97 e (495 nm) 61 a1 – 62 b2 (496 nm) 61 b2 -63 a1 (596 nm)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

Figure 7: Electronic absorption spectra of 2 D2d as a function of van der Waals-distance.

CONCLUSION An amphiphilic fullerene derivative, MPEGC60, was utilized to enable control of the supramolecular structure of thin solid films made from these fullerenes. The focus of this fundamental research is on the change of absorption properties with modification of the supra-molecular structure. Therefore, we systematically enlarged the supramolecular structures by concentrating MPEGC60 within a stearic acid matrix and additionally changed the molecular orientation between random and parallel orientation in spin cast films and Langmuir-Blodgett films, respectively. The resulting absorption spectra differ dramatically: While the increase in size of LB-type aggregates causes just marginal variations in the UV-dominated absorption spectra the growth of stochastic aggregates in spin cast-films causes a strong increase of the vis-absorption till the absorption spectrum covers the whole vis-spectral range homogeneously. To get deeper insights into the supra-molecular structures the smallest aggregates, i.e. dimers which are present at the highest diluted spin-cast MPEGC60-films, were quantum chemically investigated for various geometries and symmetries. The van der Waals-dimers assigned to point groups D2d, D2h, C2h, Cs, and C2v represent the experimental absorption features of smaller spin casted-aggregates observable at higher dilution (concentration≤1.3%), while small LB-aggregates can be mainly described by C2v-type dimers. 20 ACS Paragon Plus Environment

Page 21 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Thus, we have demonstrated that the fullerene absorption in the visible spectral range can be strongly increased by controlling their supra-molecular size and structure.

Our research

illustrates that the common notion of fullerenes having weak visible absorption holds true only for single molecules but with progressive van der Waals aggregation the vis-absorption escalates. We have shown how and why this spectral variation comes in both experimentally and theoretically from monomers to dimers with various symmetries for which materials engineers can now craft supramolecular aggregates of specific symmetries to boost the weak electronic absorption of fullerenes. In retrospect, the LB-technique allows for preparation of defined films with strong UV-absorption. Contrarily, spin casting the amphiphilic MPEGC60 leads to tunable strong vis-absorption. In a subsequent work, we will focus on further tuning the supramolecular structure of amphiphilic fullerenes to enable strong vis-absorption even in smaller fullerene clusters matching the exciton diffusion length of about 10 nm. Additionally, we demonstrated can expect that the supra-molecular structures are inherently stable if amphiphilicity is utilized to form the supramolecular structures via self-assembly. These findings are vital to guide the development of acceptor materials in organic electronics with potentially improved performance and supramolecular design.

SUPPORTING INFORMATION MPEGC60 synthesis, Figures S1-S8, Tables S1-S2, compressibility test parameters, relative energies of modelled structures and intermolecular distances, XYZ coordinates of optimized geometries.

ACKNOWLEDGEMENTS We gratefully acknowledge financial support from the “Bundesministerium für Bildung und Forschung” (FKZ: 03EK3507). Furthermore, we thank Torsten Sachse for computational support and Karin Kobow for support in the lab.

REFERENCES 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.

Page 22 of 29

Matsumoto, F.; Iwai, T.; Moriwaki, K.; Takao, Y.; Ito, T.; Mizuno, T.; Ohno, T. Controlling the Polarity of

Fullerene Derivatives to Optimize Nanomorphology in Blend Films. ACS Appl. Mater. Interfaces 2016, 8, 48034810. 2.

Li, N.; Baran, D.; Forberich, K.; Machui, F.; Ameri, T.; Turbiez, M.; Carrasco-Orozco, M.; Drees, M.;

Facchetti, A.; Krebs, F. C.; Brabec, C. J. Towards 15% Energy Conversion Efficiency: A Systematic Study of the Solution-Processed Organic Tandem Solar Cells Based on Commercially Available Materials. Energy Environ. Sci. 2013, 6, 3407-3413. 3.

Reinspach, J. A.; Diao, Y.; Giri, G.; Sachse, T.; England, K.; Zhou, Y.; Tassone, C.; Worfolk, B. J.;

Presselt, M.; Toney, M. F.; Mannsfeld, S.; Bao, Z. Tuning the Morphology of Solution-Sheared P3HT:PCBM Films. ACS Appl. Mater. Interfaces 2016, 8, 1742-1751. 4.

Khatiwada, D.; Venkatesan, S.; Chen, Q.; Chen, J.; Adhikari, N.; Dubey, A.; Mitul, A. F.; Mohammed, L.;

Qiao, Q. Improved Performance by Morphology Control Via Fullerenes in PBDT-TBT-Alkobt Based Organic Solar Cells. J. Mater. Chem. A 2015, 3, 15307-15313. 5.

Youn, H.; Park, H. J.; Guo, L. J. Organic Photovoltaic Cells: From Performance Improvement to

Manufacturing Processes. Small 2015, 11, 2228-2246. 6.

Sweetnam, S.; Prasanna, R.; Burke, T. M.; Bartelt, J. A.; McGehee, M. D. How the Energetic Landscape in

the Mixed Phase of Organic Bulk Heterojunction Solar Cells Evolves with Fullerene Content. J. Phys. Chem. C 2016, 120, 6427-6434. 7.

Minh Trung, D.; Hirsch, L.; Wantz, G.; Wuest, J. D. Controlling the Morphology and Performance of Bulk

Heterojunctions in Solar Cells. Lessons Learned from the Benchmark Poly(3-Hexylthiophene): 6,6 -Phenyl-C-61Butyric Acid Methyl Ester System. Chem. Rev. 2013, 113, 3734-3765. 8.

Hoppe, H.; Sariciftci, N. S. Morphology of Polymer/Fullerene Bulk Heterojunction Solar Cells. J. Mater.

Chem. 2006, 16, 45-61. 9.

Renz, J. A.; Keller, T.; Schneider, M.; Shokhovets, S.; Jandt, K. D.; Gobsch, G.; Hoppe, H. Multiparametric

Optimization of Polymer Solar Cells: A Route to Reproducible High Efficiency. Sol. Energy Mater. Sol. Cells 2009, 93, 508-513. 10.

Gagorik, A. G.; Mohin, J. W.; Kowalewski, T.; Hutchison, G. R. Effects of Delocalized Charge Carriers in

Organic Solar Cells: Predicting Nanoscale Device Performance from Morphology. Adv. Funct. Mater. 2015, 25, 1996-2003.


Page 23 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11.


Burkhart, B.; Khlyabich, P. P.; Cakir Canak, T.; LaJoie, T. W.; Thompson, B. C. “Semi-Random”

Multichromophoric Rr-P3ht Analogues for Solar Photon Harvesting. Macromolecules 2011, 44, 1242-1246. 12.

Huang, J.-S.; Goh, T.; Li, X.; Sfeir, M. Y.; Bielinski, E. A.; Tomasulo, S.; Lee, M. L.; Hazari, N.; Taylor,

A. D. Polymer Bulk Heterojunction Solar Cells Employing Forster Resonance Energy Transfer. Nat. Photonics 2013, 7, 479-485. 13.

Park, S. H.; Roy, A.; Beaupré, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.;

Heeger, A. J. Bulk Heterojunction Solar Cells with Internal Quantum Efficiency Approaching 100%. Nat. Photonics 2009, 3, 297-302. 14.

You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.-C.; Gao, J.; Li, G.;

Yang, Y. A Polymer Tandem Solar Cell with 10.6% Power Conversion Efficiency. Nat. Commun. 2013, 4, 1446. 15.

Alam, M. A.; Ray, B.; Khan, M. R.; Dongaonkar, S. The Essence and Efficiency Limits of Bulk-

Heterostructure Organic Solar Cells: A Polymer-to-Panel Perspective. J. Mater. Res. 2013, 28, 541-557. 16.

Minh Trung, D.; Hirsch, L.; Wantz, G. P3HT:PCBM, Best Seller in Polymer Photovoltaic Research. Adv.

Mater. 2011, 23, 3597-3602. 17.

Hoppe, H.; Sariciftci, N. S. Organic Solar Cells: An Overview. J. Mater. Res. 2004, 19, 1924-1945.

18.

Sauvé, G.; Fernando, R. Beyond Fullerenes: Designing Alternative Molecular Electron Acceptors for

Solution-Processable Bulk Heterojunction Organic Photovoltaics. J. Phys. Chem. Lett. 2015, 6, 3770-3780. 19.

Bin, H.; Zhang, Z.-G.; Gao, L.; Chen, S.; Zhong, L.; Xue, L.; Yang, C.; Li, Y. Non-Fullerene Polymer Solar

Cells Based on Alkylthio and Fluorine Substituted 2d-Conjugated Polymers Reach 9.5% Efficiency. J. Am. Chem. Soc. 2016, 138, 4657-4664. 20.

Lin, Y.; Zhan, X. Non-Fullerene Acceptors for Organic Photovoltaics: An Emerging Horizon. Mater. Horiz.

2014, 1, 470-488. 21.

Xia, D.; Gehrig, D.; Guo, X.; Baumgarten, M.; Laquai, F.; Mullen, K. A Spiro-Bifluorene Based 3d

Electron Acceptor with Dicyanovinylene Substitution for Solution-Processed Non-Fullerene Organic Solar Cells. J. Mater. Chem. A 2015, 3, 11086-11092. 22.

Hesse, H. C.; Weickert, J.; Hundschell, C.; Feng, X.; Müllen, K.; Nickel, B.; Mozer, A. J.; Schmidt‐Mende,

L. Perylene Sensitization of Fullerenes for Improved Performance in Organic Photovoltaics. Adv. Energy Mater. 2011, 1, 861-869.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

23.

Page 24 of 29

Shubina, T. E.; Sharapa, D. I.; Schubert, C.; Zahn, D.; Halik, M.; Keller, P. A.; Pyne, S. G.; Jennepalli, S.;

Guldi, D. M.; Clark, T. Fullerene Van Der Waals Oligomers as Electron Traps. J. Am. Chem. Soc. 2014, 136, 1089010893. 24.

Kesava, S. V.; Fei, Z. P.; Rimshaw, A. D.; Wang, C.; Hexemer, A.; Asbury, J. B.; Heeney, M.; Gomez, E.

D. Domain Compositions and Fullerene Aggregation Govern Charge Photogeneration in Polymer/Fullerene Solar Cells. Adv. Energy Mater. 2014, 4. 25.

Jamieson, F. C.; Domingo, E. B.; McCarthy-Ward, T.; Heeney, M.; Stingelin, N.; Durrant, J. R. Fullerene

Crystallisation as a Key Driver of Charge Separation in Polymer/Fullerene Bulk Heterojunction Solar Cells. Chem. Sci. 2012, 3, 485-492. 26.

Tada, K. Interplay between Annealing Temperature and Optimum Composition and Fullerene Aggregation

Effects in Bulk Heterojunction Photocells Based on Poly(3-Hexylthiophene) and Unmodified C60. Sol. Energy Mater. Sol. Cells 2014, 120, Part A, 136-142. 27.

Ying, Q.; Marecek, J.; Chu, B. Slow Aggregation of Buckminsterfullerene (C60) in Benzene Solution.

Chem. Phys. Lett. 1994, 219, 214-218. 28.

Nath, S.; Pal, H.; Palit, D. K.; Sapre, A. V.; Mittal, J. P. Aggregation of Fullerene, C60, in Benzonitrile. J.

Phys. Chem. B 1998, 102, 10158-10164. 29.

Nath, S.; Pal, H.; Sapre, A. V. Effect of Solvent Polarity on the Aggregation of C60. Chem. Phys. Lett.

2000, 327, 143-148. 30.

Nielsen, C. B.; Holliday, S.; Chen, H.-Y.; Cryer, S. J.; McCulloch, I. Non-Fullerene Electron Acceptors for

Use in Organic Solar Cells. Acc. Chem. Res. 2015, 48, 2803-2812. 31.

Burckstummer, H.; Tulyakova, E. V.; Deppisch, M.; Lenze, M. R.; Kronenberg, N. M.; Gsanger, M.; Stolte,

M.; Meerholz, K.; Würthner, F. Efficient Solution-Processed Bulk Heterojunction Solar Cells by Antiparallel Supramolecular Arrangement of Dipolar Donor-Acceptor Dyes. Angew. Chem. Int. Ed. 2011, 50, 11628-11632. 32.

Wurthner, F.; Chen, Z.; Hoeben, F. J.; Osswald, P.; You, C. C.; Jonkheijm, P.; Herrikhuyzen, J. V.;

Schenning, A. P.; van der Schoot, P. P.; Meijer, E. W.; Beckers, E. H.; Meskers, S. C.; Janssen, R. A. Supramolecular P-N-Heterojunctions by Co-Self-Organization of Oligo(P-Phenylene Vinylene) and Perylene Bisimide Dyes. J. Am. Chem. Soc. 2004, 126, 10611-10618. 33.

Zhang, X.; Gorl, D.; Stepanenko, V.; Würthner, F. Hierarchical Growth of Fluorescent Dye Aggregates in

Water by Fusion of Segmented Nanostructures. Angew. Chem. Int. Ed. 2014, 53, 1294-1298.


Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

34.


Habenicht, S. H.; Schramm, S.; Fischer, S.; Sachse, T.; Herrmann-Westendorf, F.; Bellmann, A.; Dietzek,

B.; Presselt, M.; Weiß, D.; Beckert, R.; Görls, H. Tuning the Polarity and Surface Activity of Hydroxythiazoles Extending the Applicability of Highly Fluorescent Self-Assembling Chromophores to Supra-Molecular Photonic Structures. J. Mater. Chem. C 2016, 4, 958-971. 35.

Brewer, N. J.; Beake, B. D.; Leggett, G. J. Friction Force Microscopy of Self-Assembled Monolayers:

Influence of Adsorbate Alkyl Chain Length, Terminal Group Chemistry, and Scan Velocity. Langmuir 2001, 17, 1970-1974. 36.

Halik, M.; Hirsch, A. The Potential of Molecular Self-Assembled Monolayers in Organic Electronic

Devices. Adv. Mater. 2011, 23, 2689-2695. 37.

Babu, S. S.; Mohwald, H.; Nakanishi, T. Recent Progress in Morphology Control of Supramolecular

Fullerene Assemblies and Its Applications. Chem. Soc. Rev. 2010, 39, 4021-4035. 38.

Jin, J.; Li, L. S.; Li, Y.; Zhang, Y. J.; Chen, X.; Wang, D.; Jiang, S.; Li, T. J.; Gan, L. B.; Huang, C. H.

Structural Characterizations of C60-Derivative Langmuir−Blodgett Films and Their Photovoltaic Behaviors. Langmuir 1999, 15, 4565-4569. 39.

Dynarowicz-Łątka, P.; Dhanabalan, A.; Oliveira Jr, O. N. Modern Physicochemical Research on Langmuir

Monolayers. Adv. Colloid Interface Sci. 2001, 91, 221-293. 40.

Orlandi, G.; Siebrand, W. Theory of Vibronic Intensity Borrowing. Comparison of Herzberg‐Teller and

Born‐Oppenheimer Coupling. J. Chem. Phys. 1973, 58, 4513-4523. 41.

Herrmann, F.; Engmann, S.; Presselt, M.; Hoppe, H.; Shokhovets, S.; Gobsch, G. Correlation between near

Infrared-Visible Absorption, Intrinsic Local and Global Sheet Resistance of Poly(3,4-Ethylenedioxy-Thiophene) Poly(Styrene Sulfonate) Thin Films. Appl. Phys. Lett. 2012, 100, 153301. 42.

Goris, L.; Haenen, K.; Nesládek, M.; Wagner, P.; Vanderzande, D.; De Schepper, L.; D’haen, J.; Lutsen, L.;

Manca, J. V. Absorption Phenomena in Organic Thin Films for Solar Cell Applications Investigated by Photothermal Deflection Spectroscopy. J. Mater. Sci 2005, 40, 1413-1418. 43.

Jackson, W. B.; Amer, N. M.; Boccara, A. C.; Fournier, D. Photothermal Deflection Spectroscopy and

Detection. Appl. Opt. 1981, 20, 1333-1344. 44.

Ambacher, O.; Rieger, W.; Ansmann, P.; Angerer, H.; Moustakas, T. D.; Stutzmann, M. Sub-Bandgap

Absorption of Gallium Nitride Determined by Photothermal Deflection Spectroscopy. Solid State Commun. 1996, 97, 365-370.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

45.

Page 26 of 29

Herrmann, F.; Muhsin, B.; Singh, C. R.; Shokhovets, S.; Gobsch, G.; Hoppe, H.; Presselt, M. Influence of

Interface Doping on Charge-Carrier Mobilities and Sub-Bandgap Absorption in Organic Solar Cells. J. Phys. Chem. C 2015, 119, 9036-9040. 46.

Presselt, M.; Herrmann, F.; Hoppe, H.; Shokhovets, S.; Runge, E.; Gobsch, G. Influence of Phonon

Scattering on Exciton and Charge Diffusion in Polymer-Fullerene Solar Cells. Adv. Energy Mater. 2012, 2, 9991003. 47.

Presselt, M.; Herrmann, F.; Shokhovets, S.; Hoppe, H.; Runge, E.; Gobsch, G. Sub-Bandgap Absorption in

Polymer-Fullerene Solar Cells Studied by Temperature-Dependent External Quantum Efficiency and Absorption Spectroscopy. Chem. Phys. Lett. 2012, 542, 70-73. 48.

Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kölmel, C. Electronic Structure Calculations on Workstation

Computers: The Program System Turbomole. Chem. Phys. Lett. 1989, 162, 165-169. 49.

Furche, F.; Ahlrichs, R.; Hattig, C.; Klopper, W.; Sierka, M.; Weigend, F. Turbomole. Wiley Interdiscip Rev

Comput Mol Sci 2014, 4, 91-100. 50.

Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta

Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297-3305. 51.

Presselt, M.; Schnedermann, C.; Schmitt, M.; Popp, J. Prediction of Electron Densities, the Respective

Laplacians and Ellipticities in Bond-Critical Points of Phenyl-Ch-Bonds Via Linear Relations to Parameters of Inherently Localized Cd Stretching Vibrations and 1h-Nmr-Shifts. J. Phys. Chem. A 2009, 113, 3210-3222. 52.

Beenken, W. J. D.; Herrmann, F.; Presselt, M.; Hoppe, H.; Shokhovets, S.; Gobsch, G.; Runge, E. Sub-

Bandgap Absorption in Organic Solar Cells: Experiment and Theory. Phys. Chem. Chem. Phys. 2013, 15, 1649416502. 53.

Presselt, M.; Dietzek, B.; Schmitt, M.; Winter, A.; Chiper, M.; Friebe, C.; Schubert, U. S.; Popp, J. The

Influence of Zinc(Ii) in a Bis-Terpyridine-Phenylene Complex on the Pi-Conjugation between the Terpyridine and the Phenylene Moiety. J. Phys. Chem. C 2008, 112, 18651-18660. 54.

Presselt, M.; Dietzek, B.; Schmitt, M.; Rau, S.; Winter, A.; Jäger, M.; Schubert, U. S.; Popp, J. A Concept

to Tailor Electron Delocalization: Applying QTAIM Analysis to Phenyl-Terpyridine Compounds. J. Phys. Chem. A 2010, 114, 13163-13174. 55.

Beenken, W.; Presselt, M.; Ngo, T. H.; Dehaen, W.; Maes, W.; Kruk, M. Molecular Structures and

Absorption Spectra Assignment of Corrole Nh Tautomers. J. Phys. Chem. A 2014, 118, 862-871.


Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

56.


Preiß, J.; Jäger, M.; Rau, S.; Dietzek, B.; Popp, J.; Martínez, T.; Presselt, M. How Does Peripheral

Functionalization of Ruthenium(Ii)–Terpyridine Complexes Affect Spatial Charge Redistribution after Photoexcitation at the Franck–Condon Point? ChemPhysChem 2015, 16, 1395-1404. 57.

Menéndez-Proupin, E.; Delgado, A.; Montero-Alejo, A. L.; García de la Vega, J. M. The Absorption

Spectrum of C60 in N-Hexane Solution Revisited: Fitted Experiment and TDDFT/PCM Calculations. Chem. Phys. Lett. 2014, 593, 72-76. 58.

Grimme, S.; Hujo, W.; Kirchner, B. Performance of Dispersion-Corrected Density Functional Theory for

the Interactions in Ionic Liquids. Phys. Chem. Chem. Phys. 2012, 14, 4875-4883. 59.

Ruzsinszky, A.; Perdew, J. P.; Tao, J.; Csonka, G. I.; Pitarke, J. M. Van Der Waals Coefficients for

Nanostructures: Fullerenes Defy Conventional Wisdom. Phys. Rev. Lett. 2012, 109, 233203. 60.

Bauernschmitt, R.; Ahlrichs, R.; Hennrich, F. H.; Kappes, M. M. Experiment Versus Time Dependent

Density Functional Theory Prediction of Fullerene Electronic Absorption. J. Am. Chem. Soc. 1998, 120, 5052-5059. 61.

Overbeck, G. A.; Moebius, D. A New Phase in the Generalized Phase Diagram of Monolayer Films of

Long-Chain Fatty Acids. J. Phys. Chem. 1993, 97, 7999-8004. 62.

Brzozowska, A. M.; Duits, M. H. G.; Mugele, F. Stability of Stearic Acid Monolayers on Artificial Sea

Water. Colloids Surf., A 2012, 407, 38-48. 63.

Hussain, S. A.; Dey, D.; Chakraborty, S.; Bhattacharjee, D. J-Aggregates of Thiacyanine Dye Organized in

Lb Films: Effect of Irradiation of Light. J. Lumin. 2011, 131, 1655-1660. 64.

Wang, H.; He, Y.; Li, Y.; Su, H. Photophysical and Electronic Properties of Five Pcbm-Like C60

Derivatives: Spectral and Quantum Chemical View. J. Phys. Chem. A 2012, 116, 255-262. 65.

Orlandi, G.; Negri, F. Electronic States and Transitions in C-60 and C-70 Fullerenes. Photochem. Photobiol.

Sci. 2002, 1, 289-308. 66.

Hare, J. P.; Kroto, H. W.; Taylor, R. Preparation and Uv / Visible Spectra of Fullerenes C60 and C70.

Chem. Phys. Lett. 1991, 177, 394-398. 67.

Orlandi, G. The Evaluation of Vibronic Coupling Matrix Elements. Chem. Phys. Lett. 1976, 44, 277-280.

68.

Sassara, A.; Zerza, G.; Chergui, M.; Negri, F.; Orlandi, G. The Visible Emission and Absorption Spectrum

of C-60. J. Chem. Phys. 1997, 107, 8731-8741. 69.

Gouterman, M. Spectra of Porphyrins. J. Mol. Spectrosc. 1961, 6, 138-163.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

70.

Page 28 of 29

Montero-Alejo, A. L.; Menendez-Proupin, E.; Fuentes, M. E.; Delgado, A.; Montforts, F. P.; Montero-

Cabrera, L. A.; Garcia de la Vega, J. M., Electronic Excitations of C60 Aggregates. Phys. Chem. Chem. Phys. 2012, 14 (37), 13058-13066. 71.

Komatsu, K.; Wang, G.-W.; Murata, Y.; Tanaka, T.; Fujiwara, K.; Yamamoto, K.; Saunders, M.

Mechanochemical Synthesis and Characterization of the Fullerene Dimer C120. J. Org. Chem. 1998, 63, 9358-9366.


Page 29 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic


Controlling Electronic Transitions in Fullerene van ... - ACS Publications

Recommend Documents