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C: Energy Conversion and Storage; Energy and Charge Transport

Diketopyrrolopyrrole Based Organic Solar Cells Functionality: The Role of Orbital Energy and Crystallinity Patricie Heinrichova, Jan Pospíšil, Stanislav St#íteský, Martin Vala, Martin Weiter, Petr Toman, David Rais, Jiri Pfleger, Martin Vondrá#ek, Daniel Šimek, Ladislav Fekete, Petra Horáková, Lenka Dokládalová, Lubomir Kubac, and Irena Kratochvilova J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01328 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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The Journal of Physical Chemistry

Diketopyrrolopyrrole Based Organic Functionality: The Role of Orbital Crystallinity

Solar Cells Energy and

Patricie Heinrichováa, Jan Pospíšila, Stanislav Stříteskýa, Martin Valaa, Martin Weitera, Petr Tomanb, David Raisb, Jiří Pflegerb, Martin Vondráčekc, Daniel Šimekc , Ladislav Feketec, Petra Horákovád, Lenka Dokládalovád, Lubomír Kubáčd and Irena Kratochvílová* c

Materials Research Centre, Faculty of Chemistry, Brno University of Technology, Purkyňova 118, CZ-612 00 Brno, Czech Republic a

b Institute

of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovský Sq. 2, 162 06 Prague 6, Czech Republic Institute of Physics, Academy of Sciences of the Czech Republic, v.v.i, Na Slovance 2, CZ-182 21, Prague 8, Czech Republic c

d

Centrum Organické Chemie, Rybitví 296, CZ-533 54 Rybitví, Czech Republic

*Corresponding author: [email protected]

Abstract In this work, we investigated diketopyrrolopyrrole (DPP) derivatives as potential donor materials for fullerene:DPP solar cells. The derivatives 3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(TBFu)2) and 3,6-bis(5-(benzothiophene-2yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(TBTh)2) were modified by introducing a nitrogen atom into the terminal moiety of the molecule. Our quantum chemical calculations predicted that this modification would increase the rigidity of the molecular 1

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structure and increase the ionization potential relative to the original DPP derivatives. The higher ionization potential primarily supports an enhancement in the open circuit voltage, and a more rigid molecular structure will contribute to reduced nonradiative losses. We experimentally verified the fullerene:DPP solar cell concept based on the coincidence of a smaller driving force for charge separation at the donor:acceptor interface and the crystallinity of the studied DPP derivatives for preparing effective photovoltaic devices. The reduction of the driving force for charge separation could be overcome by more structured / packed donor DPP materials - the delocalization of electrons and holes in such structured materials improves charge separation in OPV devices. Using wide range of experimental methods we determined the parameters of the studied DPP materials with PC70BM in thin films. This work contributes to practical applications by verifying the concept of this organic solar cell design.

1.

Introduction

Organic photovoltaics (OPVs) refers to the latest technologies in solar power generation, which allow highly automated, low-environmental-impact production of highly customized products through printing techniques. The benefits include low density, mechanical flexibility, and the possibility of having different colors or semitransparency. This is a breakthrough development relative to the existing solutions in the photovoltaic domain. Despite the progress that has been made, organic solar cells are still relatively inefficient compared to their inorganic counterparts1-3. Primary photogeneration processes take place in an active layer, which consists of materials that act as electron donors upon illumination and transport the generated free holes and efficient electron acceptors that can transport photogenerated electrons. The photoexcitation of the active layer is accompanied by the creation of excitons, which diffuse towards the donor-acceptor interface, where they dissociate into free charges.4,5-6.

Many materials for efficient organic solar cells have been intensively studied in recent years.7 Namely, new strategies have been developed for increasing organic solar cells open circuit voltage, VOC.8 Poly(3‐hexyl thiophene) was matched8 with a novel non‐fullerene acceptor. The first benzotriazole‐containing non‐fullerene acceptor containing solar cells with high VOC of 1.02 V were realized. A fullerene-free organic solar cells based on poly(3‐hexyl thiophene):benzotriazole realizing a high VOC of 1.22 V were presented by Xiao et al,9 a benzotriazole-based p-type polymer and three benzotriazole-based non-fullerene small molecule acceptors were used in organic solar cells by Tang et al. 10 It was shown that LUMO energy levels of small molecule acceptors could be fine-tuned by modifying the end-capping units, leading to high VOC (1.15-1.30 V). This group also used the "Same2


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Acceptor-Strategy" to minimize the trade-off between the VOC and the short circuit current density jSC. They showed that fluorination of a donor−acceptor copolymer in combination with a benzotriazole-based acceptor increases the open-circuit voltage from 1.05 V to 1.18 V 11. Simultaneously, the power conversion efficiency (PCE) is increased from 7.82 % to 8.36 %. These results demonstrate that the introduction of different atoms in p-type polymers can improve both the VOC and PCE. Eﬃcient polymer solar cell devices are based on the bulk heterojunction architecture, i.e. on charge pathways formed upon blending donor and acceptor semiconductors. It was shown that morphology of these devices can be controlled by the aggregation properties of the donor polymer.12 The photoinduced charge transfer (CT) from the donor to the acceptor is controlled by the energetic difference between the molecular electronic levels of the donor and acceptor. For electron transfer,13-16 the lowest unoccupied molecular orbital (LUMO) level (ELUMO) of the electron acceptor must be lower than the ELUMO of the donor. For hole transfer, the HOMO (highest occupied molecular orbital) level (EHOMO) of the electron donor must be higher than the EHOMO of the electron acceptor (see Fig. 1). Efficient free charge carrier photogeneration is expected when the free energy / driving force for charge separation, GSC, related to the described energetical differences, is larger than 0.3 eV17,18. At the same time, to achieve a high open circuit voltage, VOC, the energy difference, ECT, between the LUMO level of the electron acceptor and the HOMO level of the electron donor should be high.4 To fulfil these two requirements, it is necessary to prepare donors with a wide HOMO-LUMO energy gap, Eg (Fig. 1), which makes light harvesting less efficient due to the limited spectral range of light absorption. Our proposal for resolving the described problem is based on previously demonstrated findings that excitons in electron-donating materials can be effectively separated, although the value of GSC is lower than the general limit. This phenomenon was observed in materials with an increased tendency to crystallization19,20. This is in agreement with studies showing that the ultrafast kinetics of exciton dissociation in polymer:fullerene systems are correlated with exciton delocalization21-22. It was also observed that an increase in the structural ordering of the donor and acceptor domains can dramatically reduce losses from electron-hole pair recombination. The reduction in the electron-hole pair losses might result from the fact that the local crystallinity can promote the delocalization of the electron-hole pair, facilitating charge dissociation at the donoracceptor interface.

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The development of organic solar cells has also been based on the use of soluble π-conjugated polymers as electron donating materials. In this context, an alternative approach based on the replacement of polydisperse polymers with soluble, conjugated single molecules as electron donor materials has been developed in recent years23. The best performing concept for designing new chromophores for organic solar cells utilizes alternating donor-acceptor (D-A) moieties to lower the band gap due to the Figure 1: Schematic diagram of the electron transfer from donor formation of CT states. One of the to acceptor and hole transfer from acceptor to donor, with building blocks commonly used in driving force for charge separation ΔGSC (Gibbs energy of charge separation). The energy gap between HOMO level of electron such designs is diketopyrrolopyrrole donor and LUMO level of electron acceptor ECT is related to (DPP)24-25. It is necessary to develop open circuit voltage VOC value of OPV device. strategies to move the DPP LUMO in the optimal (low) position and simultaneously allow HOMO modulation.25:26 Generally, lowering the LUMO energy requires the introduction of an electron-deficient group 25. A common strategy is to use substituents on pendant phenyl or thienyl rings 27-28.

DPP(TBFu)2 O

N

DPP(TBOx)2

O

O S

O S

N

N

N

N S O

S O

O

O N

4


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DPP(TBTh)2 S

N

DPP(TBTz)2

S

O S

O S

N

N

N

N S O

S O

S

S N

Figure 2: Chemical structure of studied DPP derivatives capped by benzofuran DPP(TBFu)2, benzooxazol DPP(TBOx)2, benzothiofen DPP(TBTh)2, benzothiazol DPP(TBTz)2.

Based on predictions following quantum chemical calculations, we modified the molecular structures of well-known DPP(TBFu)2 (3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione) and its analogue DPP(TBTh)2 (3,6-bis(5(benzothiophene-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione)29 by incorporating nitrogen atoms into the capping moieties (see Fig. 2). The new derivatives DPP(TBOx)2 (3,6-bis(5-(benzooxazol-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4c]pyrrole-1,4-dione) and DPP(TBTz)2 (3,6-bis(5-(benzothiazol-2-yl)thiophen-2-yl)-2,5-bis(2ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione) have lower HOMO and LUMO levels, but their energy gap, Eg, remained unchanged compared to the original derivatives. In this work, we prepared and studied new dyes, DPP(TBOx)2 and DPP(TBTz)2, with nitrogen atoms incorporated into the capping moieties, and compared their properties with the previously described properties of the original dyes, DPP(TBFu)2 and DPP(TBTh)2.29 We investigated the effect of lowering the energy of the donor LUMO and HOMO orbitals on the efficiency of the photovoltaic power conversion. In these new DPP materials, the lack of extension of the HOMO– LUMO gap should reduce the driving force for charge separation (probability of charge transfer through the donor-acceptor interface). The reduction of the driving force for charge separation could be overcome by more structured / packed donor DPP materials. The delocalization of electrons and holes in such structured materials improves charge separation in OPV devices. Using experimental methods such as X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), X-ray diffraction (XRD), Atomic Force Microscopy (AFM), photogeneration efficiency measurements, pump-probe transient absorption spectroscopy, charge carrier mobility measurements and quantum chemical modeling, we studied a wide range of material properties29. The results were verified by photoelectrical characterization of OPV devices containing the studied DPP-based materials. We focused on how changing the driving 5



forces for charge separation, molecular packing / crystallinity and molecular rigidity correlate with the functionality of the studied materials in organic solar cells. Our main goal was to verify the whole concept based on the coincidence of a smaller driving force for charge separation, GSC, which increases the value of the open circuit voltage, VOC, and the crystallinity of these new DPP materials in effective organic photovoltaic devices.

2.

Materials and Methods 2.1. DPP synthesis

The compounds were synthesized according to the procedure summarized in Scheme S1. The synthesis of compound 3 is described in 30. In order to ensure the solubility of the final compounds, alkylation of the two nitrogen atoms of DPP with 1-bromo-2-ethylhexane was performed (compound 4). Bromination of the resulting compound with N-bromosuccinimide in chloroform gave the corresponding dibromo-compound 5. This intermediate was used for Suzuki coupling reaction with 2-(1-benzofuran-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane 6 to obtain DPP(TBFu)2. Product DPP(TBTz)2 was obtained by Suzuki coupling reaction too with 2tributylstannyl-1,3-benzothiazole 11. Similarly, derivative 5 was used for Stille coupling reaction with 2-tributylstannylbenzo[b]thiophene 8 to give product DPP(TBTh)2. Intermediate 8 was prepared as reported in the literature 31-32. DPP(TBOx)2 was obtained by the same coupling reaction with 2-tributylstannylbenzoxazole. Details of the synthetic procedures are summarized in Supporting Information.

2.2. Quantum chemical calculations The ground state molecular geometries of the studied DPP derivatives were determined by means of the minimization of the total energy calculated using the hybrid Hartree–Fock/Density Functional Theory method B3LYP. This widely used method combines Becke’s three-parameter exchange functional33 with the Lee–Yang–Parr correlation functional34. It has been found to produce reliable conformations and electronic properties of organic molecules in both the neutral and ionic forms6, 14, 16, 35-39. Simultaneously, the computational requirements of the B3LYP method are comparable with the Hartree–Fock method, which allows its application to moderately sized systems. The 6-311G(d,p) basis set was used and the calculations were done using the Gaussian 09 program package40. To reduce energy oscillations and increase numerical accuracy during the molecular geometry optimization, two-electron integrals and their derivatives were calculated using the pruned (99,590) integration grid, having 99 radial shells and 590 angular points per shell, which was requested by means of the Gaussian 09 keyword “Int=UltraFine”. As a standard 6


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simplification used in many quantum chemical studies, the calculated molecules were considered to be isolated. All possible conformers were taken into account in order to perform the conformational analysis and the obtained lowest energy conformer was used for further studies. The molecular conformations were fully optimized with no molecular symmetry constraints taken a priori into account, but the lowest energy conformers were found to possess the C2 rotational symmetry, which was confirmed by the vibrational frequency calculation (no imaginary frequencies were found). Starting with the optimized geometries, the relaxed potential energy surface (PES) scans along the torsion angle between thiophenes and their substituents were performed. In doing so, both torsion angles of each molecule were rotated simultaneously, thus the C2 rotational symmetry was retained during the relaxed PES scan. Calculation of the PES scans made it possible to determine the dependence of the ionization potential on the scanned torsion angle. By analogy with the Koopmans theorem for the Hartree– Fock method, the ionization potentials were estimated as the negative values of the highest occupied molecular orbital (HOMO) energies. Although this method lacks a firm theoretical background for the Kohn–Sham orbitals obtained by the B3LYP calculations, it usually provides reliable differences in ionization potentials of various conformers or derivatives with similar chemical structure.

2.3. Ultraviolet Photoelectron Spectroscopy

Spectroscopy,

X-ray

Photoelectron

X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS)41-43 measurements were performed using an Omicron nanotechnology NanoESCA instrument with a monochromatized Al Kα and He I laboratory excitation radiation sources. The photoemission spectrometer is based on a PEEM column and an imaging double hemispherical energy filter. The photoelectron signal was collected from roughly 100 × 100 µm2 areas of the sample surface. Several places on the sample were probed so that possible inhomogeneities or deviances from average could be recorded. HOMO level values were taken as the low binding energy spectrum cut-off of UPS spectra. IP values were obtained by subtracting the measured UPS spectrum width from the excitation energy value 21.2 eV. Acquired spectra were calibrated such that EF was put equal to 0 eV. We suspect some samples to have suffered from electrical charging during measurements due to insufficient electrical conductivity. Consequently, any spectra suspected to have been affected by the charging were omitted. 44-45 The XPS data were acquired from materials under study (Figs. S1,S2). The XPS spectra were obtained from surfaces of the samples. The results of the XPS quantitative analysis performed on the samples surfaces confirmed the proper setting of materials according to their types. 7



All the samples for XPS and UPS measurements were prepared in glovebox (nitrogen atmosphere) as follows: the active layers of pure DPP materials were deposited by spin-coating at 2000 rpm from solutions containing 12 mg/ml DPP material dissolved in anhydrous chloroform. Half of the samples was immediately annealed from 80 °C to 130 °C with gradient 10 °C/10 min. As substrates, silicon oxide layers on silicon wafers were used (1 × 1 cm2). Before deposition, the substrates were cleaned in ultrasonic bath with IPA (5 min) and subsequently with anhydrous chloroform (5 min).

2.4. X-ray Diffraction Experiments, Atomic Force Microscopy The X-ray diffraction (XRD) experiment was carried out on a vertical diffractometer Panalytical X'Pert PRO with parallel beam geometry and cobalt sealed tube. The angle of incidence was kept at three degrees to pronounce the layer diffraction signal and scanning was performed by detector with parallel plate collimator. Particular structure of the organic layers was unknown thus only observed interplanar distances of diffracting planes were determined without indexing.

The AFM measurements were taken by Bruker Dimension Icon ambient microscope. Topographies were taken by standard sharp tips ScanAsyst-Air (spring constant 0.4 N/m, frequency 70 kHz, nominal tip radius 5 nm). Pictures have resolution of 512 × 512 pixels.

2.5. Optical characterization Optical measurements were performed on thin films of studied materials cast on quartz glass slides (Herasil® 102, Heraeus Quarzglas Co.), which were pre-treated by ultrasonic cleaning in following baths: 1) Detergent Neodisher (Miele, Inc., NJ, USA) 10 min, 2) deionized (Milli-Q) water 20 min, 3) isopropyl alcohol (a. p.) 10 min. Absorption spectra of samples were measured employing Varian Cary Probe 50 UV-VIS spectrometer (Agilent Technologies Inc.). The fluorescence emission spectra of the thin films were recorded with a Horiba Jobin Yvon Fluorolog. Emission was detected in front face geometry to eliminate inner-filter effects.

2.6. Pump-probe transient absorption spectroscopy A pump-probe transient absorption (TA) spectrometer HELIOS (Ultrafast Systems LLC) was used for the time resolved absorption spectroscopy measurements46. The details of the systems 8


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were described in our previous report.47 In brief, we used TOPAS-C (Light Conversion) as a source of pulses of linearly polarized light of 675 nm central wavelength, to excite the samples (pump) at a repetition rate of 500 Hz. The linearly-polarized probe pulses were hitting the sample at a repetition rate of 1 kHz, and contained continuum of photon wavelengths ranging either 400– 800 nm or 850–1200 nm – depending on the configuration of the white-light continuum generation. The mutual orientation of the electric field vectors of the pump and probe pulses in the sample was set at the magic angle (54.7°) in order to eliminate any contribution of the transition moment rotational diffusion to the decay kinetics.47 The mutual temporal spacing of the pump and probe pulses (the delay) at the sample was adjusted with 4-pass mechanical linear translation stage with corner retro-reflector in range from − 200 to 6300 ps. The thin film sample was placed in the convergent beam ca. 100 mm behind the lens of focal length 125 mm, where the pump pulses were of approximately Gaussian intensity circular profile; its FWHM was 250–266 μm as measured by fitting a Gaussian function to an optical microscope image of a slightly photobleached spot. The collection of the TA data did not cause any significant photobleaching of the sample, because the TA signals recorded on one sample were reproducible in repeated experiments. The total energy in pulse was set in range 44–60 nJ with a reflective neutral density filter, which could be varied continuously. The resulting maximal pulse photon fluencies at sample in range 2.1–2.91014 photons/cm2. The probe pulses were focused at the sample with FWHM of about ∼ 80 μm. The global analysis method48 was used to factorize the data into time-invariant spectral profiles, which evolve in time according to a sequential kinetic model defined by a system of linear differential equations. Fitting of the kinetic models was performed with Glotaran49 and TIMP50 software packages.

2.7. Photovoltaic devices and field effect transistor fabrication characterization The structure of the prepared photovoltaic devices can be described by the following scheme: glass substrate/ITO/MoOx/active layer/Al electrode. The glass/ITO substrates were supplied by Ossila. Substrates were pre-treated by the same procedure as substrates for optical characterization before deposition. Layers of MoOx with thickness 10 nm were deposed by evaporation in vacuum chamber (MB-ProVap-5 MBRAUN). The active layers of DPP:PC70BM in weight ratio 3:2 were prepared by spin-coating at 2000 rpm from a solution containing 25 mg/ml solids dissolved in anhydrous chloroform. PC70BM was supplied by Ossila. Layers were annealed at 130 °C for 10 min. The photovoltaic active layers were fabricated in a glovebox filled with a controlled nitrogen atmosphere. The top Al electrode was prepared by evaporation performed in

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the vacuum chamber (MB-ProVap-5 MBRAUN). Photovoltaic devices were encapsulated by a UV curable epoxide and a glass slide. The photogeneration efficiency of the fabricated photovoltaic devices was determined by an automated current-voltage measurement station by Keithley (2601B) under the illumination of 100 mW/cm2 delivered by a certified solar simulator (Lot Oriel LS0916 with the AM1.5 filter). External quantum efficiency (EQE) was obtained using a combined apparatus consisting of Keithley 6478 picoampermeter, LSH502 LOT Oriel xenon lamp, MSH101 LOT ORIEL monochromator, SiQE120 RaRe Solutions photometric head connected to Keithley 485 picoampermeter. The active area of the electrodes was 0.046 cm2. Intensity of electroluminescence was detected by the R3896 photomultiplier (Hamamatsu Photonics K.K., Iwata, Japan). Photomultiplier head was connected to high-voltage power supply Hamamatsu C9525. Electroluminescence spectra were captured by Shamrock SR-303i-B Andor (Andor Technology plc., Belfast, North Ireland) equipped by iCCD istars DH740-18U-03 Andor camera. Electroluminescence characteristics with dark current-voltage characteristics were recalculated to external quantum efficiency EQEEL. The organic field-effect transistors have been prepared with bottom gate / bottom contact architecture on heavily doped Si wafers, with 240 nm SiO2 dielectric layer. The interdigital electrodes had different channel lengths (L = 2.5, 5, 10 and 20 µm) with 1 cm channel width. The substrates were cleaned as follows: rinsed with acetone and propan-2-ol, dried with nitrogen blow. The octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs) were formed by immersing the substrate in 10−2 mol/dm3 toluene solution for 2 hour at 60 °C. The pentafluorobenzene thiol SAMs were formed by immersing substrate in 5·10−2 mol/dm3 solution in anhydrous toluene for 5 min. The residues were cleaned by immersing the substrates in propan-2-ol and dried with nitrogen blow. All solutions of DPP were prepared from anhydrous chloroform at concentration 10 mg/ml. Active layer were deposited on substrates by spin-coating at 1500 rpm for 30 s.

3.

Results and Discussions 3.1. Quantum chemical calculations

The obtained total energy profiles with respect to the global energy minima of the given derivatives are shown in Figure 3. All energy profiles show a global minimum corresponding to a zero torsion angle between the thiophenes and their respective substituents and another minimum or double-minimum at 180°. The energy difference between the minima at the 180° and 0° torsion angles is the smallest (approximately 1 kT at room temperature) for the reference derivative (DPP(TBFu)2), ca. 2 kT for DPP(TBOx)2, and the largest difference (approximately 6 kT) 10


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was observed for DPP(TBTz)2. Only the DPP(TBTh)2 derivative has a broad double-minimum potential well centered at 180°, with energies of 2 to 3 kT above the global energy minimum. The energy barriers between the minima at the 180° and 0° torsion angles are also different: the highest barrier was observed with DPP(TBOx)2 and the lowest one was with DPP(TBTh)2. Thus, the widths of the potential wells at the global energy minima of the reference derivative (DPP(TBFu)2) and DPP(TBTz)2 are comparable, the energy minimum of DPP(TBOx)2 is the narrowest, and the energy minimum of DPP(TBTh)2 is significantly broader. The small energy difference between the minima of the reference derivative is caused by the attraction between the positively charged sulfur of the thiophene and the negatively charged nearby oxygen being partially blocked by the stearic hindrance from the adjacent CH groups (see Figure 4). Substitution of the oxygen with another sulfur, as in DPP(TBTh)2, changed the attractive S–O interaction into repulsion of two positively charged sulfur atoms. Consequently, the energy minima in this derivative are very broad, and the energy barrier between them is smaller. If the intermolecular interactions can be neglected (e.g., in a solution), the different energy profiles of the derivatives lead to different distributions of the torsion angles in these materials at room temperature, which in turn leads to different amounts of structural disorder. On the basis of these results, we expect that the energy minima corresponding to both the 0° and 180° torsion angles are populated in the reference derivative DPP(TBFu)2. On the other hand, only torsion angles close to zero should be found in DPP(TBTz)2. In both DPP(TBOx)2 and DPP(TBTh)2, the minimum at a torsion angle of 0° should be strongly preferred. If all samples are prepared by the same procedure, the significantly broader energy minima on the potential energy surface of DPP(TBTh)2 may result in a different amount of structural disorder than is seen in the other derivatives. For weak intermolecular forces, broader energy minima result in a broader distribution of torsion angles due to thermal motion and thus also lead to increased structural disorder. On the other hand, if the intermolecular forces are relatively strong, which is probably the case for thiophene-substituted DPPs, broader energy minima will increase the likelihood that the molecules find an energetically more stable packing arrangement due to the easy torsion of the substituents, which could result in a twisted polythiophene backbone conformation in the gas phase or in a solution and a planar conformation in the crystal phase 51-52. For sake of completeness, we have studied in the same way also the relaxed PES scans along the torsion angle between the DPP core and thiophenes, but we have found no difference of the potential profiles in the vicinity of the global energy minimum (see Figure 5). The only difference between the profiles of the new DPP derivatives and the original ones is in the vicinity of the local energy minimum close to 180°, which is, however, about 7 to 11 kT above the global one for all 11



derivatives. For these reasons, we do not expect any significant differences in the distribution of values of this torsion angle among the studied derivatives. Figure 6 shows the dependence of the ionization potential on the torsion angle between the thiophene and its substituent. The ionization potentials of DPP(TBOx)2 and DPP(TBTz)2 were higher than those of DPP(TBTh)2 and the reference material, DPP(TBFu)2, for any torsion angle between the thiophene and its substituent. Simultaneously, the dependence of the ionization potential on the torsion angle is much smaller in DPP(TBOx)2 and DPP(TBTz)2 than in the other two studied derivatives, in which the HOMO level has much more antibonding character with respect to the bond between the thiophene and its substituent. Quantum chemical calculations predicted that the introduction of nitrogen atoms into DPP(TBFu)2 and DPP(TBTh)2 can make DPP(TBOx)2 and DPP(TBTz)2 more rigid with IP values up to 0.3 eV higher. These properties can reduce nonradiative losses of excitation energy and ultimately increase the open circuit voltage of OPV devices. However, the rigidity of the torsional angles of the substituent in the ground state of the neutral molecule does not imply small Stokes shifts by themselves because small changes of the molecular geometry in conjunction with the charge redistribution caused by excitation or ionization can still significantly alter the reorganization energy.

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0.8

Energy difference (eV)

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


0.7

DPP(TBFu)2

0.6

DPP(TBTh)2

DPP(TBOx)2 DPP(TBTz)2

0.5 0.4 0.3 0.2 0.1 0.0 0

30

60

90

120 150 180 210 240 270 300 330 360 Substituent torsion angle (°)

Figure 3: Relaxed potential energy surface scans along the torsion angle between thiophenes and their respective substituent. Substitution of the oxygen by sulfur in DPP(TBTh)2 changed the attractive S–O interaction into repulsion of two positively charged sulfurs. Consequently, the energy minima in this derivative are very broad and the energy barrier between them is reduced.

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Figure 4: Equilibrium geometry of the reference derivative DPP(TBFu)2. Attraction between the positively charged sulfur of the thiophene and the close negatively charged oxygen is partially cancelled by the stearic hindrance of the adjacent CH groups.

0.8 DPP(TBFu)2

0.7

DPP(TBOx)2

0.6 Energy difference (eV)

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

DPP(TBTh)2 DPP(TBTz)2

0.5 0.4 0.3 0.2 0.1 0.0 0

30

60

90

120 150 180 210 240 270 300 330 360 0 Thiophene torsion angle ( )

Figure 5: Relaxed potential energy surface scans along the torsion angle between the DPP core and thiophenes.

14


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5.6 DPP(TBFu)2 DPP(TBOx)2

5.5 Ionization potential (eV)

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


DPP(TBTh)2 DPP(TBTz)2

5.4 5.3 5.2 5.1 5.0

0

30

60

90

120 150 180 210 240 270 300 330 360 Substituent torsion angle (°)

Figure 6: Ionization potential calculated as a function of the torsion angle between the thiophene and its substituent. It was found that ionization potentials of DPP(TBOx)2 and DPP(TBTz)2 are higher than that of DPP(TBTh)2 and the reference material DPP(TBFu)2 for any torsion angle between the thiophene and its substituent.

3.2. Ultraviolet Photoelectron Spectroscopy

Spectroscopy,

X-ray

Photoelectron

IP values were obtained by subtracting the measured UPS spectrum width from the excitation energy value 21.2 eV (Fig. S3). Ionisation potential IP of annealed reference material DPP(TBFu)2 was found 5.2 eV. In case of DPP(TBOx)2, value of IP increased (0.3 eV) in comparison with DPP(TBFu)2, see Tab. 1. For DPP(TBTh)2 and DPP(TBTz)2 IP values were found to be 5.1 eV and 5.3 eV, respectively. There is a clear evidence that incorporation of nitrogen in heterostructure of studied DPP materials caused increase of value of IP by 0.3 eV for DPP(TBOx)2 and 0.2 eV for DPP(TBTz)2 in comparison with DPP(TBFu)2 and DPP(TBTh)2, respectively. This observation is in good agreement with predicted trend between IP values determined by quantum chemical calculations for the most preferred torsion angle between thiophene and end-group, see Tab. 1. The absolute shift of all experimental values in comparison with theoretical values are expected53 and can be explained by intermolecular interaction, which were not included in calculations. The 15



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effect of annealing on IP value of measured samples was very low-practically the same spectra were obtained for as prepared and annealed samples. Table 1: Molecular energetic levels of studied DPP derivatives: ionization potential IP determined by quantum chemical calculations IPcalc. for the most preferred conformation and experimentally determined values IPUPS, and optical parameters of thin films: position of absorption local maximum corresponding to zero phonon transition λABS 0-0 of as-cast and annealed films, respectively, position of fluorescence maximum λPLmax and optical bandgap of donor Eg opt of only annealed films. As-cast IPUPS (eV) λABS 0-0 (nm) 5.2 662

Thermal annealing at 130 °C λABS 0-0 (nm) λPLmax (nm) Eg opt (eV) 666 818 1.77

Material DPP(TBFu)2

IPcalc. (eV) 5.02

DPP(TBOx)2

5.32

5.5

654

664

811

1.78

DPP(TBTh)2

5.08

5.1

679

683

689

1.82

DPP(TBTz)2

5.28

5.3

668

684

811

1.74

3.3. X-Ray Diffraction, Atomic Force Microscopy The measured diffraction patterns are shown in Fig. 6. The peak around 7° (corresponding to interplanar spacing d = 14.5 Å) in XRD spectra is characteristic for the crystalline form of studied DPP derivatives11. Data obtained by fitting showed only minor differences between the well-

pronounced peaks, only in case of DPP(TBTh)2 this peak is shifted to approximately 9° (d = 11.8 Å) and in XRD pattern is other distinguishable peak about 26° (d = 4.05 Å). This difference is pointing to another type of packing in layer than other derivatives. Intensity of observed peaks was significantly increased after layers annealing in comparison with as-cast layers, especially for DPP(TBTh)2. It means that crystallinity of all the annealed samples slightly increased compared to as-cast counterparts. DPP(TBTh)2 packing has already been described and the reasons on the molecular scale was thoroughly discussed by Liu et al29. Our quantum chemical calculations predicted that the introduction of nitrogen atoms make molecules DPP(TBOx)2 and DPP(TBTz)2 more rigid compared to DPP(TBFu)2 and DPP(TBTh)2. In general, the greater rigidity of the basic molecular building elements can reduce the variability of the entire structure and corresponding XRD spectra at given thermal annealing condition. Bigger difference in XRD spectra between DPP(TBTh)2 and DPP(TBTz)2 is affected by annealing conditions (both materials were annealed at 130°C). If layers of DPP(TBTz)2 were annealed at higher temperatures (up to 300°C) more similar crystallinity and absorption spectra (see part 3.4 in this paper) of DPP(TBTz)2 like DPP(TBTh)2 can be expected. However layers annealed at such high temperature conditions

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were not deeply studied, because such annealing temperatures practically can’t be applied for OPV fabrication. Bigger surface crystallinity annealed (compared to as prepared) blends DPP: PC70BM was confirmed by AFM for all investigated DPP materials. (for details see Supplementary materials,

Figs. S4, S5). Grow of DPP(TBFu)2 domain in heterojunction structured active layer as an effect of annealing was described by many autors 25, 54-57. In case of DPP(TBTh)2 annealing effect on heterojunction structure was observed by AFM in publication 29. In this work AFM confirmed that crystallization increased after annealing of blends DPP(TBOx)2 : PC70BM and DPP(TBTz)2 : PC70BM s.

Figure 7: X-Ray diffraction patterns of DPP(TBFu)2, DPP(TBTh)2 and their new derivate DPP(TBOx)2 and DPP(TBTz)2 before (solid line) and after thermal annealing abbreviated as T (dotted lines). Intensity of observed peaks was significantly increased after layers annealing in comparison with as-cast layers, especially for DPP(TBTh)2. It means that crystallinity of all the annealed samples increased compared to as-cast counterparts.

17



3.4. Steady state optical spectra Optical properties of DPPs thin films were studied by absorption and fluorescence spectroscopy and provided information about optical band gap Eg and exciton properties in relation to their morphology. The absorption spectra of pristine DPP thin films are plotted in Figure 8. We compare the spectra of the as-cast films and films after thermal annealing at 130 °C. The main absorption band is situated between 500 and 700 nm. This band was attributed to the internal charge transfer state between the diketopyrrolopyrrole core (electron acceptor) and the electron donating moieties29, 58. The band consisted of at least three peaks that correspond to vibrational modes. The first maximum can be attributed to the zero-phonon line, as confirmed by the fluorescence spectra (shown below). The positions of the zero-phonon lines are listed in Table 1 The effect of annealing on IP value of measured samples was very low-practically the same spectra were obtained for as prepared and annealed samples. According to the position of the zero-phonon line, the introduction of nitrogen into the structure of DPP derivatives DPP(TBOx)2 and DPP(TBTz)2 caused a small hypsochromic shift in the spectra before the annealing of the thin films relative to films of the original structures, DPP(TBFu)2 and DPP(TBTh)2. After annealing, the position of the absorption maxima of the layers from the DPPs with introduced nitrogen is similar to that of layers from the original materials. For DPP(TBFu)2 and DPP(TBOx)2, the maxima were at approximately 665 nm, and for DPP(TBTh)2 and DPP(TBTz)2 the maxima were at approximately 684 nm.

18


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Figure 8: Absorption spectra of as-cast and thermal annealed thin films prepared from studied DPPs. Annealing was realized at 130 °C. The main absorption band is situated between 500 and 700 nm. This band was attributed to internal charge transfer state between diketopyrrolopyrrole core (electron acceptor) and electron donating moieties. The band consisted at least from three peaks which origin from vibration modes.

The thin films of DPP(TBFu)2 show a characteristic significant decrease in the zero-phonon absorption band (662 nm) and a simultaneous increase in the intensity of the left shoulder absorption band due to annealing. This spectral behavior is typical for H-aggregation (face-toface molecular stacking). The opposite behavior is observed in the spectra of the DPP(TBTh)2 layers. This layer shows a spectrum characteristic of J-aggregates (head-to-tail molecular stacking) with a significant zero phonon absorption band. The annealing caused only a slight increase in absorption with a simultaneous degrease in the intensity of the left shoulder absorption band. Thus, the DPP(TBTh)2 layers seems to be highly ordered at the nanoscale after deposition. The differences in the molecular arrangement between DPP(TBFu)2 and DPP(TBTh)2 were demonstrated by their X-ray diffraction patterns. Notably, J-aggregation of DPP derivatives 19



was found to be more suitable for achieving a high charge transfer rate in an OPV than Haggregation59. However, Liu et al.11 reported that the smaller intermolecular interaction area and the longer - distance (caused by the strong repulsion between sulfur atoms) in DPP(TBTh)2 relative to that in DPP(TBFu)2 are the reasons for the reduced charge carrier mobility and thus the smaller photovoltaic conversion efficiency of the corresponding OPV devices. Thus, the molecular arrangement has two contradictory effects on the photovoltaic conversion efficiency. The spectra of the as-cast films of DPP(TBOx)2 and DPP(TBTz)2 have very similar shapes; however, the spectral changes after annealing show that have different aggregation tendencies. Qualitatively, the spectral changes of DPP(TBOx)2 showed a trend similar to that of DPP(TBFu)2 upon annealing of the thin film, and the trends in DPP(TBTz)2 showed behavior similar to those of DPP(TBTh)2. However, after thermal annealing at 130 °C, the absorption spectra of both films indicated the presence of both J- and H-aggregates. Hence, the films still seem to be disordered. Moreover, it was found that both derivatives, DPP(TBOx)2 and DPP(TBTz)2, can show absorption spectra with shapes similar to those of the original derivatives, but the annealing temperature has to be increased to at least 230 °C (see Fig. S4 in the Supplementary Information). Unfortunately, these thermal conditions are not suitable for the fabrication of flexible electronic devices. The absorption characteristics in conjunction with XRD characterization indicate that the layers of DPP(TBOx)2 and DPP(TBTz)2 are only partly recrystallized after thermal annealing at 130 °C. The layer of DPP(TBTz)2 seems to be even polymorphic because the XRD pattern of the layer annealed at 130 °C is similar to those of DPP(TBOx)2 and DPP(TBFu)2, but the absorption spectra of this layer after annealing at temperatures higher than 230 °C have characteristics similar to that of DPP(TBTh)2. Polymorphism was observed in other types of thiophene-DPP derivatives60-62. The fluorescence spectra of the pristine thin films were collected in the range of 600 to 850 nm (Figure 7). A relatively large portion of the spectra was at wavelengths longer than 850 nm, beyond the range of available detectors. The as-cast layers of all materials show very similar spectra with local maxima at 810 nm and very low intensities indicating very low fluorescence quantum efficiencies (see plot S7 in the Supplementary Information). After thermal annealing of the layers, the fluorescence intensity increased, and the shape of the spectra changed. The DPP(TBFu)2 layer clearly had the greatest fluorescence quantum yield. The annealing caused the fluorescence intensity to increase by order of magnitude. The annealing of the DPP(TBTh)2 layer mainly impacted the spectral shape, which became a mirror image of the absorption spectrum with spectral overlap of the first vibronic peaks, which represents the zero-phonon transition. Annealing increased the fluorescence intensity by a factor of four for DPP(TBTh)2 and by a factor of two for DPP(TBOx)2 and DPP(TBTz)2. The positive effect of annealing on fluorescence intensity is additional evidence of the film structural arrangement. Higher crystallinity in the layers 20


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contributed to the reduction of nonradiative recombination of the excitons as well as to longer exciton lifetimes and diffusion lengths63. This increases the probability that the excitons generated in the DPP domain after light absorption by the active layer of the OPV will be dissociated into a charge carrier. According to the observed effect of 130oC annealing on the fluorescence intensities of the films, DPP(TBOx)2 and DPP(TBTz)2 probably provide smaller amounts of “usable” excitons for photogeneration processes than are generated in DPP(TBFu)2 and DPP(TBTh)2. There is an increased probability of nonradiative losses of excitation energy. The fluorescence emission spectra in Figure 9 are presented together with the fluorescence excitation spectra and absorption spectra. The intersection point between the emission and excitation spectra is attributed to the optical band gap, Eg, of the materials. The values are presented in Table 1.

Figure 9: Fluorescence emission and related excitation spectra (marked by lines with same color) of annealed layers of pristine DPPs (green lines) and blends with PC70BM (red lines). Excitation spectra are compared with absorption spectra of pure DPPs (black line). Intersection point between emission and excitation spectrum is attributed to optical band gap Eg of materials. 21



3.5. Fluorescence of DPP:PC70BM Blends Figure 8 shows the fluorescence emission and excitation spectra of the films cast from a solution of DPPs mixed with PC70BM. The fluorescence intensities of DPP(TBFu)2 and DPP(TBTh)2 are effectively quenched, while the fluorescence intensities of DPP(TBOx)2 and DPP(TBTz)2 increased slightly relative to those of pristine DPP layers. In the following section, we will discuss two possible processes: resonance energy transfer and charge transfer, which can explain the observed effects. Förster resonance energy transfer (FRET) requires a spectral overlap between the emission of the donor and the absorption of the acceptor5. Examples of spectral overlaps are shown in Figure 10 with the normalized absorption and emission spectra of pristine layers of DPP(TBFu)2, DPP(TBTh)2 and PC70BM. The absorption spectrum of PC70BM is the superposition of singlets and CT state absorption bands64; thus, the spectrum is normalized to the peak corresponding to the lowest energetic transition related to the fluorescence emission peak at approximately 720 nm. This band seems to be relevant for the spectral overlap calculations5, 64. The thin films of the studied DPP derivatives have optical band gaps similar to those of films of PC70BM; thus, their absorption and emission spectra mutually overlapped. This means that FRET from DPP to PC70BM as well as in the reverse direction is possible. The FRET from DPP to PC70BM has a negligible contribution to fluorescence quenching because for DPP(TBTh)2, DPP(TBOx)2, and DPP(TBTz)2, there have similar conditions (very small fluorescence quantum yields and similar spectral overlaps), but their fluorescence quenching efficiencies are quite different. On the other hand, the fluorescence intensity of DPP(TBTz)2:PC70BM is higher than that of pristine DPP(TBTz)2, and that of DPP(TBOx)2:PC70BM is higher than that of pristine DPP(TBOx)2, which can be explained by FRET from coabsorbing PC70BM to DPPs. This mechanism was confirmed by the excitation spectra, see S6 in the Supplementary Information. The normalized excitation spectra of the pristine materials and blends with fullerene showed that in the cases of DPP(TBOx)2, DPP(TBTz)2 and DPP(TBTh)2, the excitons in the blends contribute more to the fluorescence, and these excitons were generated after absorption of light in the range 400–550 nm. This absorption band is characteristic of PC70BM, but the characteristic PC70BM fluorescence emission band does not appear in the spectra. The reason for PC70BM fluorescence quenching may be hole transfer (see below). Another less probable explanation for the intensity increase is the contribution of the CT state emission. Table 2 summarizes the expected band gap energies of the CT states, ECT values. These values were determined from the difference between the IP of DPP determined by UPS (IP is approximately equal to − EHOMO) and the electron affinity, EA, of PC70BM (approximately equal to − ELUMO). The EA of PC70BM (3.9 eV) was taken from the literature7. The new derivatives, DPP(TBTz)2 and DPP(TBOx)2, blended with PC70BM can provide CT states with higher HOMO–LUMO gaps (up 22


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to 1.6 eV); however, the potential emission maxima are expected to be 0.2–0.3 eV lower due to the reorganization energy4–6; thus, in the best case scenario, the maximum would be at approximately 890 nm. The observed emission was at lower wavelength than 850 nm and therefore, this eventuality was eliminated. The second fluorescence quenching process was investigated for charge transfer. The efficiency of the charge transfer is closely related to the driving force for the charge separation, ΔGSC, as explained in the introduction. The values of GSC for DPP systems were estimated according to the following equations4, 17: Δ𝐺eCS = 𝐸g(D) + IP(D) ― EA(A)

(1)

𝛥𝐺hCS = EA(A) ― 𝐸g(A) ― IP(D) (2) The term Eg represents a singlet exciton energy, thus representing the optical band gap of DPP for the electron transfer process and the optical band gap of PC70BM for the hole transfer process, which were determined from optical spectra. IP(D) is the ionization potential of DPPs (determined by UPS), and EA(A) is the electron affinity of PC70BM (taken from the literature20). Note that the determined theoretical GCS values can vary by at least ± 0.1 eV. The GCS values for the new derivatives, DPP(TBOx)2 and DPP(TBTz)2, are 0.3 eV smaller than those of DPP(TBFu)2 and DPP(TBTh)2, which is consistent with quantum chemical calculations. The GCS values found for DPP(TBOx)2 are 0.1 eV lower than the generally accepted boundary value of 0.3 eV, which is considered the minimum energy for efficient charge separation. Thus, electron and hole transfer will be less efficient in this derivative. In the case of DPP(TBTz)2, the GCSe value is equal to 0.3 eV, and thus the efficiency of the electron transfer will likely be strongly dependent on system ordering. Fluorescence quenching experiments showed that the fluorescence of both new materials, DPP(TBOx)2 and DPP(TBTz)2, is not quenched by fullerene; thus, electron transfer is inefficient. This result is consistent with the determined GCSe values. Because the first approximation of the Gibbs energy for hole transfer, GCSh, is the same as that for electron transfer, GCSe, the efficiency of hole transfer can be expected to be very similar to the efficiency of electron transfer. The exception to this is the case of DPP(TBTz)2 for which the value of GCS for hole transfer is 0.1 eV higher than for electron transfer, and the absolute value (0.4 eV) is higher than the boundary value (0.3 eV) for efficient CT. Thus, hole transfer from PC70BM to DPP(TBTz)2 can be expected to be more efficient than electron transfer from DPP(TBTz)2 to PC70BM. In summary, the fluorescence quenching experiments showed that the fluorescence of new derivatives with nitrogen atoms introduced into the terminal moieties, DPP(TBOx)2 and 23



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DPP(TBTz)2, is not quenched, unlike what is seen in the original structures, DPP(TBFu)2 and DPP(TBTh)2. As indicated in the above discussion about the conditions for CT and FRET, FRET probably occurred in the studied systems but played a minor role, and the CT processes seemed to be hindered by low GCS in the cases of DPP(TBOx)2 and DPP(TBTz)2.

Figure 10: Example of overlapping absorption and fluorescence emission spectra of DPP with PC70BM films. Filled area is spectral overlaps, which represents the necessary condition for FRET from PC70BM to DPP (black area) or from DPP to PC70BM (green area). Plots are construed for two extreme cases of spectra shapes given by the way of molecular arrangement in film: left plot for DPP(TBFu)2 (spectra shape characteristic for H-aggregation) and right plot for DPP(TBTh)2 (J-aggregation). Table 2: Parameters describing charge transfer energetic states of DPP:PC70BM system: GCS – gibbs energy of charge separation predicted from experimentally determined IP of donor, ECT – energy of charge transfer state. material DPP(TBFu)2 DPP(TBOx)2 DPP(TBTh)2 DPP(TBTz)2

GCSe (eV) 0.5 0.2 0.6 0.3

GCSh (eV) 0.5 0.2 0.6 0.4

ECT (eV) 1.3 1.6 1.2 1.4

3.6. Ultrafast transient absorption The spectro-temporal evolution of the transient absorption (TA) signal in each of the composite thermal annealed samples is represented by the two Species-Associated Differential Spectra (SADS), Δε1(λ) and Δε2(λ), shown in Figure 11. The Δε1(λ) and Δε2(λ) obtained from the TA 24


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experiments in the vis- and NIR-probe spectral ranges with given composite sample were merged together in a single graph. The corresponding sets of two exponential decay time constants, τ1 and τ2, (the lifetimes of individual excited states) that are summarized in Table 3. The fit parameters Δε1(λ), Δε2(λ), τ1 and τ2 were optimized by global analysis method48 for each TA experiment individually, i.e. separately for vis-probe and for NIR-probe data for each composite sample. A detailed summary of the global analysis results obtained for each TA dataset is given in Figures S9–S16 in the Supporting information. From the individual fit summaries, one can see that the model captures the experimental evolutions of the TA signal quite well in all the cases, replicating the main spectral as well as the major temporal features. This is also reflected in the good fit of the full lines to the experimental points shown in Figure 12. All the composite samples showed similar spectral features of the initially excited state, Δε1(λ); see red curves in Figure 11. We can distinguish strong negative TA signal in the broad range of probe photon energies 2.0–2.3 eV, and around 1.8 eV. This can be attributed to the ground-state bleaching (GSB) phenomenon, because the correspondence with steady-state absorption spectrum of the given composite sample (shown with blue dashed curves in the Figure 11). The GSB reflects by definition the sum of populations of all excited states. On either side of the GSB region, we can distinguish two excited-state absorption (ESA) regions in all the composite samples: One weak ESA, extending to ca. 2.6–2.7 eV at the high-energy side of the negative GSB band, and one relatively strong ESA extending down to ca. 1.1 eV at the low-energy side of the negative GSB band. The shape of the low-energy ESA is specific for each composite sample. The lifetimes 1 obtained for the Δε1(λ) from both the vis- and the NIR-probe TA data for given composite sample are very close to each other (see the Table 3). This indicates, that the Δε1(λ) in both probe regions originate from the population of the same excited state.

25



Figure 11: The Species-Associated Differential Spectra (SADS) of the ‘early’, Δε1, and ‘late’, Δε2, excited state (full lines) found by the global analysis of the transient absorption experiments in composite samples indicated in the legend. The confidence intervals at the 95 % level of probability are shown by the semitransparent bands of the respective color. The actual values of the lifetimes are given in Table 3. The ‘late’ component was added also in scaled form (dashed green line) to allow comparison of its spectral shape with the ‘early’ component. The steady-state absorption spectrum (dashed blue line, multiplied by factor 0.002) was added to clarify the identification of the negative TA spectral features as the ground-state bleaching (GSB). 26


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Table 3: Summary of lifetimes extracted from the global analysis of the TA data. The confidence interval estimates of τ1 and τ2 are calculated at 95 % probability level.

material DPP(TBFu)2 DPP(TBOx)2 DPP(TBTh)2 DPP(TBTz)2

vis-probe τ1 (ps) τ2 (ns) 59 ± 1 13.5 ± 0.7 184 ± 2 16 ± 3 180 ± 2 23 ± 2 69.2 ± 0.7 44 ± 15

NIR-probe τ1 (ps) τ2 (ns) 64 ± 2 13.5* 186 ± 3 21 ± 12 175 ± 4 7.0 ± 0.3 81 ± 1 5.2 ± 0.4

*value was taken from fit of vis-probe data and fixed for fit of the NIR-probe data. As the delay between pump and probe pulses increases, the TA spectra drop in intensity and transform into the shape Δε2(λ), depicted by full green curves in Figure 11. The shape of the Δε2(λ) in each composite sample reflects the nature of excited state that was created from the initial excitation, Δε1(λ). The NIR-probe data yielded lower lifetime 2 values than those obtained from the vis-probe experiments in each composite, with only one exception, in DPP(TBOx)2:PC70BM composite – cf. the Table 3. In that material, the Δε2(λ) from the NIR-probe data was the weakest of the four samples, and therefore the error in its estimation was the highest. In the other composites, the differences in the lifetimes are quite high, thus we cannot assume the identity of the long-living excited states observed in the vis- and NIR-probe data. In Figure 12, we show the normalized evolutions of the TA signal for each of the studied DPP(Aryl)2:PC70BM composite samples, that were integrated over two ranges of photon energies 2.05–2.25 eV (GSB signal region measured with the vis-probe TA setup) and 1.05–1.30 eV (measured with the NIR-probe setup). As the GSB signal occurs generally due to the pumpinduced depletion of the population of the ground state, it is determined by summed contribution of all photoexcited states in the composite. In the case of charge separation event in the bulk heterojunction composite, one can expect the creation of a CT state or a pair of long-living polarons from the initial photoexcitation. From the two opposite charges, one will be located in the DPP domain and the other in the PC70BM domain. Therefore, in the simplest case, when the ionization process has no concurrence with energy loss process, the GSB signal in the DPP domain should remain constant during the event. However, we cannot exclude the possibility of spectral overlap between negative TA signal of the GSB and a positive TA signal of an excited state absorption (ESA) of the ionized molecules, which would influence the observed TA signal strength in the GSB region. This means, that in our case, we can interpret the normalized GSB signal evolutions in Figure 12a by partitioning it into 4 sequential temporal regions: (i) photo-induced creation of population of initial excited states (singlet excitons) with spectral profile Δε1(λ), manifested by sharp signal rise around time t=0; (ii) minor decaying region, almost a plateau, which extends up to tens of ps and reflects probably a diffusion 27



of the singlet excitons with occasional annihilations due to their mutual collisions; (iii) major decay region, which is probably caused by transformation of electronic configuration, manifested by change in TA spectral profile of the excited state (Δε1(λ) → Δε2(λ)) and also by simultaneous energy losses; (iv) “final” plateau region, that extends beyond the experimental time window of our TA apparatus, and represents the population of the long-living state represented by spectral profile Δε2(λ) shown in the respective panel of Figure 11.

Figure 12: Normalized evolutions of the TA signal averaged over the region of probe photon energies (a) 2.05–2.25 eV and (b) 1.05–1.30 eV. The colored circles are the experimental data transient absorption measured in composite samples indicated in the legend. The full lines of the respective color show the result of global analysis with best-fit two-exponential sequential decay model, averaged over the respective probe photon energies. 28


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We can use the strength of the long-living TA signal in region (iv) in Fig. 12a as a figure-of-merit for charge separation prerequisite, because without it, there is no evidence for an excited state in DPP to yield any charge separation. Therefore, we predict that the composites based on DPP(TBFu)2 and DPP(TBTh)2 have higher chances for production of ionized states, than the other two. The existence of the photoinduced charge-separated state in the composites DPP(TBFu)2, DPP(TBTh)2 can be deduced from the NIR-probe region, where we expect strong contribution of excitons,65 or polarons66 at the fullerene domains. In the Figure 12b, we can discern the same temporal partitioning (i)–(iv), as we described above for the vis-probe TA data. In temporal region (iv), we have observed relatively strong long-living signal in NIR-probe region in composites based on DPP(TBFu)2 and DPP(TBTh)2, whereas in the other two composites, the TA signal almost completely vanished. As the strength of the long-living TA signal in NIR-probe region is indicative for a charge-separated state formation, we can therefore expect higher current photogeneration from the composites based on DPP(TBFu)2 and DPP(TBTh)2, than form those based on the other two DPP derivatives. Thanks to the spectral resolution of the TA experiment, we can judge whether the major signal decay in the GSB range (Figure 12a) was caused by rise of an overlapping positive ESA from an emerging population of new excited state, or rather it reflects direct deexcitation to the groundstate – by comparing the shape of Δε1(λ) (full red lines) with scaled Δε2(λ) (dashed green lines) in the vis-probe spectral region in Figure 11: If the two are identical the decay of GSB signal shown in Figure 12a reflects exclusively the loss of the excitation at the DPP domains. It indicates an absence of a distinguishable TA spectrum of charge-separated state, and therefore a direct loss of the excitation into a ground state. The Δε1(λ) and Δε2(λ) showed almost identical positions of peaks as well as of isosbestic points (where Δε1=0) in composites with DPP(TBOx)2 and DPP(TBTz)2 – consequently, a poor photocurrent yield from solar cells based on those composites can be expected. Similar expectation arises also from the overall low intensity of the NIR probe Δε2(λ), found in those two composites. In the contrary cases found in composites with DPP(TBFu)2 and DPP(TBTh)2, the short- and longliving SADS in the vis-probe range differ substantially from each other (see the Figure 11a,c). Here, we can see an increase in the positive signal in the probe range 1.7–1.9 eV, which partially overlaps with the GSB region. Therefore in those cases, the observed drop in the TA signal strength in the GSB region shown in Figure 11a reflects the rise in positive contribution from longliving ESA to the overall TA signal. If the ESA originates from the charge-separated state, then there is chance, that the composite may perform relatively well in the photovoltaic application.

29



By looking at the spectral profiles of the Δε2(λ) in the composites with DPP(TBFu)2 and DPP(TBTh)2, we can try to identify the nature of the excitation, from which the observed ESA originates. In the case of the composite with DPP(TBFu)2, the TA signal shows spectrally flat base, which extends over region 1.0–1.3 eV. This bears strong similarity with the TA spectrum reported in pristine PCBM (a very closely related material) thin film65 and in pristine PC70BM thin film67. Over the flat base, the TA signal shows a peak at about 1.1 eV, which is relatively close to the reported position of peak in the PCBM radical anion extinction spectrum at 1.2 eV66. In the case of broad morphological variety in the samples, and therefore the spectral shapes of the individual excited states may be affected, most probably significantly broadened. Additionally, comparing the spectral shapes of Δε1(λ) (full red lines) with scaled Δε2(λ) (dashed green lines) in the vis-probe spectral region in Figure 11, we can see the pronounced negative TA signal in the high-energy probe above 2.6 eV in the composites with DPP(TBFu)2 and DPP(TBTh)2. This is additional evidence, that some excitation is shared also by the PC70BM domain, because of the pronounced steady-state absorption in the pristine PC70BM film (cf. dark red full curve in Figure 14). In summary, the results of the TA experiments show that photoexcitation in the bulkheterojunction composite samples based on DPP(TBFu)2 and DPP(TBTh)2 leads to the production of a photoexcited charge-separated state with opposite charges located in the DPP and PC70BM domains, and this process has the potential to produce a photocurrent in a photovoltaic cell. In contrast, the composite samples based on DPP(TBOx)2 and DPP(TBTz)2 predominantly show a direct transition of the photoexcited state to the ground state, which prevents efficient charge photogeneration in a PV cell.

3.7. Photovoltaic devices characterizations

The probability of charge transfer has a direct impact on the efficiency of charge carrier photogeneration in heterojunction photovoltaic devices. The photogeneration efficiency was evaluated from current-voltage characteristics measured under light illumination. These characteristics of the as-cast devices and after annealing at 130 °C are summarized in the plot in Figure 13, and the determined parameters are given in Table 4. The most efficient devices were those prepared from DPP(TBFu)2. The highest photogeneration efficiency, , among the as-cast devices was 0.9 %, and after annealing at 130 °C, it increased to 3.5 %. The second most efficient device was that prepared from DPP(TBTh)2. Its efficiency was 1.5 % before thermal annealing, 30


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and annealing increased its efficiency to 2.9 %. The relatively high photogeneration efficiency of the as-cast device is consistent with quantum chemical calculations and optical experiments. The simultaneous increase in photogeneration efficiency is in agreement with fluorescence experiments demonstrating an increase in DPP(TBTh)2 domains in the layer as indicated by the increasing the fluorescence intensity. The as-cast devices with DPPs with nitrogen atoms introduced into the structure, DPP(TBOx)2 and DPP(TBTz)2, had efficiencies of only 0.4 and 0.2 %, respectively. While the photogeneration efficiency of the DPP(TBOx)2 device was only slightly improved to 0.5 % by annealing, and the efficiency of the DPP(TBTz)2 device increased by a factor of 4 to 0.8 %. The low photogeneration efficiency of both nitrogen atoms containing derivatives in OPV devices is in good agreement with our expectations based on previously presented transient absorption experiments. The increase of efficiency due to annealing is attributed mainly to the increase in the short circuit current density, jSC, of the devices and the fill factor, FF (see Table 4). Direct information about the utilization of absorbed light by the active layer to generate the photocurrent is provided by the external quantum efficiency (EQE) spectra. In Fig. 14, these spectra are compared with the normalized absorbance of the DPP:PC70BM blends. The absorbance spectrum of the pure PC70BM thin film is also included in each graph for comparison with the device EQE spectra. The shapes of the EQE and absorption spectra are significantly different in all cases. For all DPPs, the characteristic peak in the EQE spectrum is at approximately 1.8 eV. In all cases, this peak is more evident after annealing. This can be explained by the growth of DPP domains. Next, an important band lies at approximately 2.2–2.3 eV. This band is more or less the superimposition of DPP (peak maximum 2.0 eV) and PC70BM (2.22–2.75 eV). Especially in the case of DPP(TBTz)2, the EQE spectrum of the annealed device in this range follows the absorption spectrum of PC70BM. This suggests that the growth of the fullerene domain after thermal annealing is the main reason for the improvement in the photogeneration efficiency; thus, in this case, hole transfer is more efficient than electron transfer. This observation was predicted from values of GSC. Hole transfer from PC70BM to DPP seems to be significant in the cases of DPP(TBOx)2 and DPP(TBTh)2 before thermal annealing. In contrast, the device prepared from DPP(TBFu)2 shows an EQE spectrum in this range (and generally over the whole range) that is more proportional to the absorption spectrum of DPP; therefore, electron transfer appears to be more effective than hole transfer. The high energy range of the spectrum (from 2.5 to 3.5 eV) follows the absorption spectrum of PC70BM rather than that of DPP. This part of the spectrum is relatively flat, especially in the case of DPP(TBTh)2 devices. This can be explained by the optical thickness of the layers, which absorb 31



most of the incident light. Another effect influencing the shape of the spectrum might include light interference in the sandwich structure of the photovoltaic device. 68 Among material parameters that significantly influence the effectiveness of OPV devices, charge carrier mobility is an important issue. The charge carrier mobility affects the efficiency of the transport of photogenerated charge carriers to the electrodes and thus their nongeminate recombination, which has a direct impact on the jSC values. The charge carrier mobility was determined by characterization of an OFET device fabricated with a pristine DPP derivative used as the hole-transporting semiconductor material (see Table 5). While DPP(TBFu)2 and DPP(TBTh)2 have hole mobilities on the order of 103 cm2/Vs, even after annealing DPP(TBOx)2 and DPP(TBTz)2 show values that are smaller by more than an order magnitude. The hole mobility of DPP(TBTz)2 increased by more than a factor of 3 due to annealing. This effect of annealing was the largest of any of the derivatives and is likely the result of increasing DPP domains. However, the change in the hole mobility was smallest in the case of DPP(TBOx)2, indicating that the thin film structure of this material was not recrystallized efficiently under the tested thermal conditions. A similar trend was observed in the serial resistance, RS, of the OPV devices, which are summarized in Table S1 of the Supplementary Information. This parameter, along with shunt resistance, RSH (also in Table S1), explains the fill factor values. The devices prepared from DPP(TBOx)2 and DPP(TBTz)2 have lower fill factors than the devices prepared from DPP(TBFu)2 and DPP(TBTh)2 after annealing due to higher serial resistance (5–6 times). The shunt resistance is slightly higher and thus better for devices prepared with derivatives having incorporated nitrogen atoms. Thus, the efficiencies of OPV devices from DPP(TBOx)2 and DPP(TBTz)2 are hindered not only by the poor efficiency of the photogeneration processes but also by worse charge carrier transport phenomena. Incorporation of nitrogen atoms can result in OPV devices with higher VOC values than are seen devices with the original materials according to the ECT values (see Tabs 2). This phenomen was really observed in current voltage characteristics of as-cast devices. In the case of DPP(TBOx)2, these values change by 0.19 V relative to that of DPP(TBFu)2, and for DPP(TBTz)2, it changes by 0.13 V compared to that of DPP(TBTh)2. However, annealed OPV devices from DPP(TBOx)2 had VOC values only 0.04 V higher than those of devices made with DPP(TBFu)2, and DPP(TBTz)2 devices had a VOC value 0.03 V higher than that of devices from DPP(TBTh)2. VOC usually fluctuates with device quality by 0.05 V. Thus, the VOC values of devices made with the new derivatives with incorporated nitrogen atoms are quite similar to those of devices prepared with the original materials and are thus more limited by recombination losses. 32


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Deeper analyses of VOC losses can provide information about the efficacy of electronic processes at the donor-acceptor interface and help to identify reasons for losses.40 First, it is necessary to eliminate the possibility that the values of VOC are limited by the nonohmic contact between the active layer with electrodes.38, 69 The transfer of electrons from the fullerene to the aluminum electrode is usually barrierless, assuming good deposition of the aluminum. On the other hand, the collection of holes by ITO can be blocked by the energy barrier between the hole-transfer layer and electron-donating material. Commonly used PEDOT:PSS have a work function of approximately 5.2 eV 69, but the IP values of nitrogen atom containing DPPs are higher; therefore, the VOC of devices prepared with these DPP derivatives can be reduced. This hypothesis was confirmed; the devices with these materials and with PEDOT:PSS had VOC values up to 0.3 eV smaller than the other two materials (see Table S2 in the Supplementary Material). (This is indirect confirmation of the determined IP values.) Thus, molybdenum trioxide was used as the hole transport layer to eliminate the energy barrier for hole collection by ITO. It can be expected that the VOC values of devices with this structure are the result only of the materials used in the active layer and their order. Next, analyses of radiative and nonradiative loses were carried out to identify the reason why the devices with the nitrogen atom containing DPP derivatives have smaller VOC values than were theoretically predicted.38–40 Nonradiative losses, VOCnr, of VOC can be determined from the electroluminescence external quantum efficiency, EQEEL, of devices70. (For detailed data see the described spectra in Figs. S17 and S18 in the Supplementary Information, and the spectra do not show emissions from the CT states in any case.) The determined values of EQEEL are summarized in Table 5. The as-cast devices with DPP(TBFu)2 and DPP(TBTh)2 show EQEEL values that are an order of magnitude smaller than those of DPP(TBOx)2 and DPP(TBTz)2. It is generally known that a smaller band gap leads to a higher probability of nonradiative relaxation. Nonradiative losses, VOCnr, in the case of devices prepared from the original materials are thus 0.06 eV higher than those of devices prepared from the nitrogen atom containing DPP materials with incorporated nitrogen atoms (see Table 4). This observation is consistent with the relationship between the energy of the CT states and VOCnr.71 After annealing, the EQEEL values of devices with DPP(TBFu)2 and DPP(TBTh)2 increased by at least one order of magnitude, and thus, nonradiative losses were reduced by 0.11 eV in the case of DPP(TBFu)2 and 0.07 eV in the case of DPP(TBTh)2. The annealed device containing DPP(TBTz)2 had an EQEEL value quite similar to that of the as-cast device; thus, VOCnr remained unchanged. The EQEEL value of the device with DPP(TBOx)2 even decreased after annealing, so the nonradiative losses increased by 0.03 eV. This was caused by the shift of the emission maximum to smaller energy (see Fig. S6 in the Supplementary Information). The VOC values of the as-cast devices with the nitrogen atom containing DPP materials, DPP(TBOx)2 and 33



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DPP(TBTz)2, changed by smaller amounts due to less nonradiative recombination than was seen in devices with the original DPPs; however, annealing did not contribute to the reduction of these losses, unlike what was seen with devices with the original derivatives DPP(TBFu)2 and DPP(TBTh)2. This difference is the result of the differences in the ordering abilities of the nitrogen atom containing DPPs compared to the original derivatives. Radiative losses can be determined by subtracting the VOC and nonradiative losses from the energy, ECT.70 However, optical and opto-electrical experiments did not provide direct information about the ECT. The emission of CT states can be reduced by increasing the electrondonor and electron-acceptor domains in the active layer, as was reported for fullerene domains by Piersimoni et al.72 Therefore, only theoretical values derived from the theoretical value of ECT can be estimated. In this context, radiative losses for the as-cast devices with DPPs with incorporated nitrogen atoms are roughly the same as those of devices with the original DPP derivatives. After thermal annealing, radiative losses increased by 0.1 eV on average. This is evidence of phase separation and growth of the electron-donor and electron-acceptor domains. In summary, analyses of VOC energy losses confirmed that the introduction of nitrogen atoms into the structure of the nitrogen atom containing DPPs contributed to the reduction of nonradiative losses in the as-cast devices; however, the suppressed recrystallization did not allow other reductions of nonradiative losses, which are necessary to increase VOC. In addition, the estimated radiative losses are dependent on morphological changes in the active layer due to thermal annealing of the devices prepared with all the studied DPP materials. These results suggest that the intimate mixture of donor and acceptor materials in the as-cast active layer was separated into domains by thermal annealing in all the devices (increase in radiative loses), but the domains from DPP(TBFu)2 and DPP(TBTh)2 are more crystalline than those from DPP(TBOx)2 and DPP(TBTz)2 (decrease in nonradiative loses). This observation is consistent with results from EQE and optical experiments.

Table 4: The characteristics of the best-performing photovoltaic devices prepared from studied DPP materials blended with PC70BM.

DPP DPP(TBFu)2 DPP(TBOx)2 DPP(TBTh)2 DPP(TBTz)2

JSC

(mA/cm2) 3.0 1.3 4.5 0.8

As-cast VOC (mV) 1000 1100 960 1070

FF (%) 31 28 34 26

 (%) 0.9 0.4 1.5 0.2

JSC

(mA/cm2) 8.1 1.4 6.3 2.3

34


Thermal annealed VOC (mV) FF (%) 940 46 980 32 940 48 970 37

 (%) 3.5 0.5 2.9 0.8

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Figure 13: Current density-voltage characteristic of as-cast OPV devices (dashed lines) and after thermal annealing at 130 °C (solid lines).

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Figure 14: External quantum efficiency spectra of as-cast and annealed DPP:PC70BM devices (symbols connected with line) compared with absorption spectra of DPP:PC70BM blends (green lines) and PC70BM (brown lines) annealed films. Table 5: Values of electroluminescence external quantum efficiency EQEEL of OPV devices, quantified nonradiative losses VOCnr of open circuit voltage and holes mobility’s h of pristine DPP materials. As-cast Material DPP(TBFu)2 DPP(TBOx)2 DPP(TBTh)2 DPP(TBTz)2

EQEEL ) 1·104 1·103 2·104 2·103

VOCnr (V) 0.36 0.30 0.34 0.28

h (cm2/Vs) (4 ± 1) ·10−3 (0.7 ± 0.2) ·10−4 (1.2 ± 0.4) ·10−3 (0.5 ± 0.2) ·10−4

Annealed Material DPP(TBFu)2 DPP(TBOx)2 DPP(TBTh)2 DPP(TBTz)2

EQEEL ) 6·103 3·104 3·103 3·103

VOCnr (V) 0.25 0.33 0.27 0.27

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h (cm2/Vs) (7 ± 1) ·10−3 (1.1 ± 0.2) ·10−4 (3.5 ± 0.9) ·10−3 (1.7 ± 0.6) ·10−4

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4.

Conclusions

In this work, we investigated the properties of four DPP derivatives, DPP(TBFu)2, DPP(TBTh)2, DPP(TBOx)2 and DPP(TBTz)2, as donor materials for organic bulk heterojunction solar cells. The introduction of nitrogen atoms in the terminal moieties of the DPP molecule (DPP(TBOx)2 and DPP(TBTz)2) contributed to the reduction in the driving force behind charge separation, GSC, in the heterojunction system and contributed to the larger energy difference, ECT, between the HOMO level of DPP and the LUMO level of PC70BM, which relates to the maximum theoretical value of VOC of the OPV device. We focused on verifying this concept based on the coincidence of a smaller driving force for charge separation at the donor:acceptor interface and the crystallinity enhancing the charge delocalization of the studied DPP derivatives for preparing effective photovoltaic devices. Using quantum chemical calculations, we confirmed that DPP(TBOx)2 and DPP(TBTz)2 have higher ionization potentials (by 0.3 eV) than the original DPP derivatives. Fluorescence quenching and ultrafast transient absorption measurements were used to verify the occurrence of electron transfer processes in DPP(Aryl)2:PC70BM thin films. Efficient electron transfer was detected in the cases of DPP(TBFu)2 and DPP(TBTh)2. For DPP(TBOx)2 and DPP(TBTz)2 we expected that the reduction of the driving force for charge separation could be overcome by more structured materials. The delocalization of electrons and holes in such structured materials can improve charge separation in OPV devices. The utility of DPP(TBOx)2 and DPP(TBTz)2 to prepare OPV devices with a standard structure, ITO/MoO3/DPP:PC70BM/Al fabricated under gentle thermal conditions (up to 150 °C), was verified by optoelectrical characterization of these OPV devices. The devices with these materials had photogeneration efficiencies up to seven times smaller than those of devices without the nitrogen atom. This reduction in the power conversion efficiency was attributed mainly to the smaller short current density and fill factor values. The incorporation of nitrogen in the DPP structure contributed to higher VOC values, as was expected based on the IP values, only in the case of the as-cast devices. The VOC values of the annealed OPV devices were all similar. Photogeneration losses were studied using external quantum efficiency spectra and electroluminescence measurements. External quantum efficiency spectra confirmed the growth of DPP domains as a result of thermal annealing in all cases and show that as predicted, hole transfer is more efficient than electron transfer in DPP(TBTz)2:fullerene devices. In the cases of DPP(TBOx)2 and DPP(TBTh)2, hole transfer also significantly contributed to the photogeneration efficiency. Electroluminescence experiments confirmed that nonradiative losses can be reduced by incorporating nitrogen into the structure of the DPPs; however, annealing did not contribute 37



to further reductions in nonradiative losses, probably due to low temperature that was used for annealing of the whole solar cells. Our results indicate that charge transfer and separation processes were hindered in DPP(TBOx)2 and DPP(TBTz)2 by insufficient crystallinity and in turn affected photovoltaic efficiency more than the limited charge carrier transport in these materials. Annealing at temperatures up to 130 °C did not led to sufficient crystallinity and thus charge delocalization in the electron-donating domain in these DPP derivatives to compensate for the small driving force for charge separation, GSC, at the donor:acceptor interface. To obtain sufficient solar cell effectivity, the structural organization of DPP(TBOx)2 and DPP(TBTz)2 must be improved for proper charge delocalization. This work proved that the incorporation of nitrogen atoms into the terminal moieties of DPP(TBFu)2 and DPP(TBTh)2 derivatives resulted in some advantageous properties for OPVs on the molecular level; however, the practical application of these materials in OPV is limited by the lower ability of these materials to crystalize under standard thermal conditions for OPV fabrication. This seems to be a side effect of the higher molecular rigidity of these new materials. However, it was found that the new DPPs can generate a more organized structure with annealing at higher temperatures. We hypothesize that if the structural ordering of DPP(TBOx)2 and DPP(TBTz)2 are increased, the OPV devices made with DPP(TBOx)2 and DPP(TBTz)2 will be efficient.

5.

Acknowledgements

This work was supported by the Czech Science Foundation (with project No. GA17-21105S the synthesis and the steady-state optical and opto-electrical characterization, with project No. 1505095S the transient absorption spectroscopy, and with project No. 17-02578S the quantumchemical calculation was funded). The access to the CERIT-SC computing and storage facilities provided under the program Projects of Large Research, Development, and Innovations Infrastructures (CERIT Scientific Cloud LM2015085) is gratefully acknowledged. This work was also supported by operational program Research, Development and Education financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. SOLID21 - CZ.02.1.01/0.0/0.0/16_019/0000760). Research infrastructure used at Materials Research Centre, Brno University of Technology was supported by project MSMT No. LO1211. Project No. SAFMAT - CZ.02.1.01/0.0/0.0/16_013/0001406). Research infrastructure SAFMAT at Institute of Physics was support by MEYS through LM2015088 and LO1409. 38


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Supporting Information for Publication Details of investigated DPP materials synthesis, Atomic Force Microscopy, qantitative analysis of XPS core level spectra and UPS data and way of calculation of DPP materials ionization energy, DPP absorption spectra, studied DPP optoelectrical characteristics and electroluminescence spectra are presented.

6.

References

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Fllerene:DPP solar cell concept based on the coincidence of a smaller driving force for charge separation at the donor:acceptor interface and the crystallinity of the studied DPP derivatives for preparing effective photovoltaic devices.


Diketopyrrolopyrrole Based Organic Solar Cells ... - ACS Publications

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