Branched Electron-Donor Core Effect in D-π -A Star-Shaped Small Molecules on Their Properties and Performance in Single-Component and Bulk-Heterojunction Organic Solar Cells

: Star-shaped donor-acceptor molecules are full of promise for organic photovoltaics and electronics. However, the effect of the branching core on physicochemical properties, charge transport and photovoltaic performance of such donor-acceptor materials in single-component (SC) and bulk heterojunction (BHJ) organic solar cells has not been thoroughly addressed. This work shows the comprehensive investigation of six star-shaped donor-acceptor molecules with terminal hexyldicyanovinyl blocks linked through 2,2 (cid:48) -bithiophene π -conjugated bridge to different electron-donating cores such as the pristine and fused triphenylamine, tris (2-methoxyphenyl)amine, carbazole-and benzotriindole-based units. Variation of the branching core strongly impacts on such important properties as the solubility, highest occupied molecular orbital energy, optical absorption, phase behavior, molecular packing and also on the charge-carrier mobility. The performance of SC or BHJ organic solar cells are comprehensively studied and compared. The results obtained provide insight on how to predict and ﬁne-tune photovoltaic performance as well as properties of donor-acceptor star-shaped molecules for organic solar cells.


Introduction
Organic solar cells are attracting considerable attention due to their ability to produce flexible, lightweight devices with solvent and printing technologies [1][2][3][4][5][6].Power conversion efficiency (PCE) has exceeded 16% over the last few years [7][8][9].In the design of small molecules, the donor-acceptor (D-A) concept is the most successful one for organic photovoltaics and other optoelectronic applications.In general, the concept includes variation of types and numbers of electron-donor and electron-withdrawing groups or their sequence within the molecular structure, which results in a lower bandgap by reason of orbital mixing of the donor and the acceptor blocks and shifts absorption into the long-wave region due to the intramolecular charge transfer (ICT) effects [10][11][12][13].Investigations of D-A small molecules demonstrate that even minor adjustments in the molecule structure can lead to significant changes in material characteristics, which may be used for fine tuning the material properties [14][15][16].Donor-acceptor molecules of star-shaped architecture have several advantages over linear ones, for example, enhanced solubility, lower anisotropy of electrical and optical properties, higher value of molar extinction coefficients and increased number of pathways for light conversion [17][18][19][20][21][22][23][24][25].Triphenylamine (TPA) is a branching core widely used to design hole-transporting star-shaped and branched molecules for various optoelectronic applications [22,23,[26][27][28][29][30][31][32].The propeller-shaped form of TPA inhibits strong π-π interactions between neighboring groups and because of that the great number of TPA derivatives are amorphous and soluble.Benefits of amorphous materials include absence of the necessity to carry out a posttreatment of functional layers in optoelectronic devices and formation of homogeneous contacts at interfaces of organic-metal or organicorganic layers.However, it is well-known that amorphous materials, when compared to crystalline organic semiconductors, demonstrate much less efficient charge transport.One of the possible approaches to solve the problem is a chemical modification of the TPA core or usage of other more ridged and planar cores.It was shown that the addition of the methoxy groups into the TPA can increase crystallinity of the target materials [33].Phenyl-substituted carbazoles can be considered as the nearest but partly fused and more planar analogs of the TPA core [34][35][36][37].The complete covalent fusing of the TPA core is another tool to convert propeller-like TPA to the planar core [38][39][40].Benzotriindole (BTI) core [41] being a truxene analogue, but with three nitrogen atoms instead of carbon ones, is one of the promising planar cores to design star-shaped structures of the molecules for organic photonics and photovoltaics [42][43][44][45].
Single-component (SC) OSCs, being a separate field of organic photovoltaics, have attracted much attention due to a simple device structure and the lack of time-dependent active layer phase segregation [46][47][48].SC OSCs are showing rapid advances and the most efficient devices based on so-called "double-cable" polymers are now approaching over 8% PCE [49].In fact, the "double-cable" polymers or similar small molecule dyads consist of donor and acceptor units linked to each other through an aliphatic spacer within one molecule.The drawbacks of these concepts are a relatively complex synthesis and the necessity to carry out a posttreatment of functional layers in the devices to tune the miscibility between conjugated backbones and side units.In contrast to them, simple conjugated small molecules and polymers do not suffer from that and demonstrate high open circuit voltage (V OC ) in SC OSCs [50,51].The PCE of SC OSCs based on such materials have recently reached a remarkable 2.9% [52].However, the library of appropriate small molecules for this purpose is scarce and many aspects, such as the correlation between the structure, properties and performance in SC devices, remain poorly addressed, which limits the further boost of the PCE for SC OSCs.Therefore, the development and comprehensive investigation of novel conjugated small molecules for SC OSCs are urgently needed.
In our recent reports we have demonstrated that TPA-based donor-acceptor starshaped molecules have a field-assisted mechanism of charge photogeneration and demonstrate small energy losses resulting in high open-circuit voltage up to 1.19 V and the power conversion efficiency over 1% [53][54][55].However, a detailed investigation of the impact of core type on the performance of donor-acceptor star-shaped molecules in SC OSCs has never been conducted.

Synthesis
The synthesis of m-TPA and f-TPA molecules was carried out according to the previously developed scheme for the preparation of similar star-shaped compounds (Figure 2) [58,59].

Solubility, Thermal and Structural Properties
Thermal and thermo-oxidative stability were investigated by thermogravimetric analysis (TGA) and the data are summarized in Figure 3 and Table 1.The materials showed a high thermal-oxidative stability with the decomposition temperature (T d ) exceeding 360 • C (Figure 3a, Table 1) and no clear dependence of the T d from the branching core type.Considering the thermal stability in the inert atmosphere (Figure 3b), one can see that the m-TPA and BTI compounds that have the thermally less stable aliphatic substituents at the core show the lowest T d within the series of molecules.In contrast, f-TPA shows the highest T d since it has phenyl substituents at the core.The phase behavior of the materials was studied by differential scanning calorimetry (DSC).The results are summarized in Figure 4 and Table 1.According to the DSC scans of the first heating (see Figure 4a) and X-Ray diffraction data (see below) most of the molecules as received are amorphous.Molecules based on rather similar triphenylamine and carbazole cores with no substituents (TPA, s-CBZ and t-CBZ) showed alike glass transition temperatures (T g ) at the 64-66 • C region.However, it should be noted that t-CBZ with T-like molecular geometry is able to crystalize above T g .Modification of the TPA core by introduction of the methoxy substituents leads to the slight increase in T g up to 71 • C due to enhanced intermolecular interactions [59,60].Comparing TPA and f-TPA one can see that the latter's T g is more than twice that of the former's (64 • C vs. 137 • C), which can be explained by a higher molecular weight and an increased molecular rigidity of the fused molecule.An important difference in the phase behavior of f-TPA is the ability to crystalize above T g followed by melting at 300 • C with an enthalpy (∆H m ) of 34 Jg −1 .The phase behavior of the planar BTI is like that of f-TPA but does not show any glass transition and melts at 231 • C with ∆H m of 42 Jg −1 .It is interesting to note that among this series of molecules the t-CBZ and BTI molecules retain the ability to crystallize from the melt as well, whereas crystallization of the other materials is hindered, and they show only a glass transition on DSC scans at the subsequent heating (Figure 4b).

Compounds
Solubility a , gL −1  The solubility of the molecules was measured in o-dichlorobenzene (ODCB) solutions (Table 1).The solubility of the s-CBZ and t-CBZ molecules that have carbazole cores is slightly higher as compared to that of TPA molecule.The presence of the both methoxy and bulky p-tolylmethene substituents at the TPA core leads to a more than twice increase in solubility of m-TPA and f-TPA molecules as compared to the pristine TPA.The BTI compound, having the completely planar core, showed the lowest and quite modest solubility.
X-ray structural data (Figure 5a) are in a good agreement with the DSC data.X-ray diffraction patterns of all but t-CBZ compounds reveal only a broad halo corresponding to the amorphous state of the matter.Crystalline structure of t-CBZ was described in detail in our earlier work [35].However, after annealing, the f-TPA and BTI samples become highly crystalline, each manifesting tens of narrow reflections, their halfwidths corresponding to the crystallite sizes higher than 30 nm.Such separation can be easily understood, as in all crystalline compounds the core is flat and end-to-end conjugated providing good possibilities for tight packing of the molecules in the crystal lattice.On the other hand, cores of molecules of the amorphous compounds are substantially warped, having a fan-type shape.
1 Indexing of the provided X-ray data for three crystalline compounds was carried out using Accelrys Material Studio program (Table 2 and Figure 5).As expected, in all three cases, the crystallization is induced by the wide planar contact of the conjugated parts of the molecules.Such packing is the necessary condition for intermolecular electron transfer.

Optical and Electrochemical Properties
The optical properties of the molecules in diluted THF solutions and thin films were studied by UV-vis absorption spectroscopy and the data are summarized in Figure 6 and Table 3.In the solutions, all the compounds showed quite similar absorption spectra shape: the bands at the high-energy region (300-420) nm are usually ascribed to π-π* transitions, whereas the intensive bands at 490-550 nm are ascribed to the ICT [17,61] or to a mixed character [62].Among this series of molecules, s-CBZ and t-CBZ have the most blue-shifted absorption spectra.In contrast, m-TPA and f-TPA with the modified TPA core have the most red-shifted absorption spectra.The absorption maxima (λ max ) of BTI and TPA were found to be red-shifted as compared to s-CBZ and t-CBZ and blue-shifted as compared to m-TPA and f-TPA (Figure 6, Table 3).These facts can be explained by the narrowing of the bandgap as confirmed by cyclic voltammetry (CV) data (see below).The absorption spectra of films are significantly shifted to the long-wavelength region and broadened for all materials in comparison with the spectra of solutions (Figure 6b).The most pronounced shift was found to be for the BTI molecule, which has the planar core and thus an increased ability to aggregate.
as compared to m-TPA and f-TPA (Figure 6, Table 3).These facts can be explained by the narrowing of the bandgap as confirmed by cyclic voltammetry (CV) data (see below).The absorption spectra of films are significantly shifted to the long-wavelength region and broadened for all materials in comparison with the spectra of solutions (Figure 6b).The most pronounced shift was found to be for the BTI molecule, which has the planar core and thus an increased ability to aggregate.The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies were calculated from oxidation and reduction potentials (see SI).All the molecules have close LUMO energies, since the reduction proceeds mainly at the hexyldicyanovinyl electron-withdrawing groups which are the same for all the compounds.In contrast, the type of branching core has a pronounced impact on the HOMO energies.The HOMO energies were found to range from −5.45 eV to −5.20 eV, indicating an increase in the core electron-donating ability in the following sequence: s-CBZ-t-CBZ-BTI-TPA-f-TPA-m-TPA.As a result, the electrochemical bandgap of the molecules can be tuned from 1.93 to 2.04 eV, which agrees well with the optical absorption data.

Single-Component Solar Cells
The active layer of SC OSC should have comparable hole and electron mobilities to ensure efficient charge transport to electrodes after exciton dissociation.The hole and electron mobilities in films measured by space charge limited current (SCLC) technique (for details see Supplementary Materials) are presented in Table 4 (J-V characteristics of the hole and electron only devices are presented in Figures S3.1-3.12).The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies were calculated from oxidation and reduction potentials (see SI).All the molecules have close LUMO energies (Table 3), since the reduction proceeds mainly at the hexyldicyanovinyl electron-withdrawing groups which are the same for all the compounds.In contrast, the type of branching core has a pronounced impact on the HOMO energies.The HOMO energies were found to range from −5.45 eV to −5.20 eV, indicating an increase in the core electron-donating ability in the following sequence: s-CBZ-t-CBZ-BTI-TPA-f-TPA-m-TPA.As a result, the electrochemical bandgap of the molecules can be tuned from 1.93 to 2.04 eV, which agrees well with the optical absorption data.

Charge Transport and Photovoltaic Properties 2.4.1. Single-Component Solar Cells
The active layer of SC OSC should have comparable hole and electron mobilities to ensure efficient charge transport to electrodes after exciton dissociation.The hole and electron mobilities in films measured by space charge limited current (SCLC) technique (for details see Supplementary Materials) are presented in Table 4 (J-V characteristics of the hole and electron only devices are presented in Figures S3.1-3.12).The balanced transport of holes and electrons should be beneficial for SC OSC to avoid space charge effects.At the same time charge carrier mobilities that are too low can increase recombination losses, which reduce the device performance.All investigated compounds showed ambipolar charge transport with almost the same ratio µ h /µ e ~10 (Table 4), with the exception of BTI for which this ratio is three times greater.TPA, m-TPA and BTI showed the highest values of hole and electron mobilities, so we can expect the best performance of SC OSCs based on them.
The main photovoltaic properties of SC OSCs are presented in Figure 7 and summarized in Table 5 (averaged parameters are presented in SI, Table S1).For the OSCs based on f-TPA, t-CBZ and BTI, which are able to crystallize according to the DSC data (Figure 4), thermal annealing for ordering of the active layer was applied.In the case of f-TPA and t-CBZ, thermal annealing did not result in an increased PCE due to decreased V OC .However, SC OSC based on BTI showed 24% higher efficiency due to increased J SC and FF.The balanced transport of holes and electrons should be beneficial for SC OSC to avoid space charge effects.At the same time charge carrier mobilities that are too low can increase recombination losses, which reduce the device performance.All investigated compounds showed ambipolar charge transport with almost the same ratio µh/µe~10 (Table 4), with the exception of BTI for which this ratio is three times greater.TPA, m-TPA and BTI showed the highest values of hole and electron mobilities, so we can expect the best performance of SC OSCs based on them.
The main photovoltaic properties of SC OSCs are presented in Figure 7 and summarized in Table 5 (averaged photovoltaic parameters are presented in SI, Table S1).For the OSCs based on f-TPA, t-CBZ and BTI, which are able to crystallize according to the DSC data (Figure 4), thermal annealing for ordering of the active layer was applied.In the case of f-TPA and t-CBZ, thermal annealing did not result in an increased PCE due to decreased VOC.However, SC OSC based on BTI showed 24% higher efficiency due to increased JSC and FF.Note: a J SC EQE -short-circuit current integrated from the EQE spectrum.b Data for the annealed (180 • C) active layer.
The exceptional position of BTI in terms of transport and photovoltaic properties is in agreement with the data of atomic force microscopy (AFM) (SI, Figure S3.19).While films based on all other oligomers have smooth surfaces with indistinguishable morphologies and RMS roughness in the range of 0.30-0.37nm, the BTI film has a much higher roughness of 4.60 nm, which confirms its high crystallinity.
As expected from transport properties, TPA, m-TPA and BTI demonstrated higher PCE values in SC OSCs, and the most efficient was BTI.SC OSCs differ mainly in J SC , while the values of V OC ~1.1-1.2V (except for f-TPA and BTI) and FF ~25% (except for t-CBZ and BTI) are about the same.Such V OC values are among the highest for SC OSCs [62].The relatively low V OC value for BTI is possibly related to high crystallinity according to AFM data (Figure S3.19f), since in this case the formation of shunts and increased leakage currents are possible.The low V OC value for f-TPA can be associated with the presence of bulky PhMe groups surrounding the donor core, which can hinder the effective charge transport and, as a consequence, enhance recombination losses.The low FF could be explained by a strong dependence of the charge photogeneration efficiency on the electric field as the current density does not saturate at the negative voltages (Figure 7a).The J SC values for all compounds are in full accordance with the maximum EQE values.The J SC values calculated from the EQE spectra are in good agreement with the J SC value obtained from the J-V curves.The highest J SC values for TPA, m-TPA and BTI also correspond to the most red-shifted EQE spectra (Figure 7c).

Bulk-Heterojunction Solar Cells
Figure 8 compares J-V curves and EQE spectra of bulk-heterojunction solar cells based on blends with PC 71 BM, and their photovoltaic properties are summarized in Table 6 (averaged photovoltaic parameters are presented in SI, Table S2).For a correct comparison we fabricated solar cells based on all blends with donor:acceptor mass ratio of 1:2 (as the optimal mass ratio for TPA:PC 71 BM and BTI:PC 71 BM was found to be 1:2 [56]).By using thermal annealing for the blends based on f-TPA, t-CBZ and BTI, which are able to crystallize according to the DSC data, we managed to slightly increase PCE within 10%.Note that solvent vapor annealing (o-dichlorobenzene) did not result in an increased PCE.
AFM study for BHJ solar cells was also carried out (SI, Figure S3.20).Similar to pristine films, a blended film based on BTI had a significantly different morphology and the highest RMS roughness of 1.28 nm, while the other blended films had similar smooth surfaces with a roughness in the range of 0.30-0.36nm.Thus, it can be assumed that BTI:PC 71 BM film is partially crystalline and differences in morphology of other films are present in the bulk.
As in the case of SC OSCs, BHJ solar cells based on BTI showed the highest efficiency, which may be due to the partial crystallinity of the active layer in accordance with AFM data (Figure S3.20f).TPA-based BHJ solar cells also had sufficiently high PCE.f-TPA-based BHJ solar cells had practically the same high efficiency, in contrast to the f-TPA-based SC OSC with the lowest PCE.CBZ-based BHJ solar cells had relatively low PCE values, as in the case of SC OSCs.AFM study for BHJ solar cells was also carried out (SI, Figure S3.20).Similar to pristine films, a blended film based on BTI had a significantly different morphology and the highest RMS roughness of 1.28 nm, while the other blended films had similar smooth surfaces with a roughness in the range of 0.30-0.36nm.Thus, it can be assumed that BTI:PC71BM film is partially crystalline and differences in morphology of other films are present in the bulk.
As in the case of SC OSCs, BHJ solar cells based on BTI showed the highest efficiency, which may be due to the partial crystallinity of the active layer in accordance with AFM data (Figure S3.20f).TPA-based BHJ solar cells also had sufficiently high PCE.f-TPAbased BHJ solar cells had practically the same high efficiency, in contrast to the f-TPAbased SC OSC with the lowest PCE.CBZ-based BHJ solar cells had relatively low PCE values, as in the case of SC OSCs.
BHJ solar cells differ mainly in JSC, EQE and FF.The values of VOC ~0.95-1.05V are about the same (except for m-TPA).These small differences in VOC correspond to identical differences in HOMO energies within 0.1 eV.The lowest VOC of m-TPA-based solar cells in agreement with the highest HOMO energy for m-TPA.
To understand the reasons for the differences in JSC, EQE and FF of BHJ solar cells, we measured the hole mobilities in the blended films (Figures S3.13-3.18,Table 7).We suggest that the electron mobilities are similar in all blends, as they are determined mainly by PC71BM.The blended films based on TPA, m-TPA and BTI had the highest hole mobilities, which is in agreement with hole transport in the pristine films (Table 4).This explains the highest PCE for TPA and BTI.CBZ-based blends had relatively low hole mobilities and so the corresponding BHJ solar cells had the lowest efficiencies.BHJ solar cells differ mainly in J SC , EQE and FF.The values of V OC ~0.95-1.05V are about the same (except for m-TPA).These small differences in V OC correspond to identical differences in HOMO energies within 0.1 eV.The lowest V OC of m-TPA-based solar cells in agreement with the highest HOMO energy for m-TPA.
To understand the reasons for the differences in J SC , EQE and FF of BHJ solar cells, we measured the hole mobilities in the blended films (Figures S3.13-3.18,Table 7).We suggest that the electron mobilities are similar in all blends, as they are determined mainly by PC 71 BM.The blended films based on TPA, m-TPA and BTI had the highest hole mobilities, which is in agreement with hole transport in the pristine films (Table 4).This explains the highest PCE for TPA and BTI.CBZ-based blends had relatively low hole mobilities and so the corresponding BHJ solar cells had the lowest efficiencies.

Conclusions
In summary, novel D-A star-shaped molecules m-TPA and f-TPA were synthesized.Investigation of their properties in comparison with the previously published analogs (s-CBZ, t-CBZ, BTI, TPA) made it possible to reveal the impact of the core type on the structure-properties relationship for this series of star-shaped molecules.Varying the type of electron-donor core allows us to control the solubility, thermal, optical, electrochemical properties and photoelectric characteristics of SC and BHJ OSCs.The molecules that have a more rigid and planar core such as t-CBZ, f-TPA and BTI are able to crystallize as compared to the amorphous s-CBZ, TPA, m-TPA.The introduction of either methoxy or p-tolylmethene substituents into the TPA core leads to an increase in solubility compared to the pristine TPA.There is no unambiguous influence of the star-shaped molecule core type on the photovoltaic properties.In BHJ OSCs, the new molecule f-TPA is among the most efficient, but at the same time in SC OSCs f-TPA is the least efficient.For both types of OSCs, BTI and TPA have shown the best efficiencies.Thus, the results obtained demonstrate that the BTI and TPA cores are attractive building blocks to design promising small molecules for SC and BHJ OSCs.S1: Photovoltaic parameters of SCOSCs under AM1.5G illumination (100 mW/cm 2 ) averaged over eight devices, Table S2: Photovoltaic parameters of BHJ solar cells under AM1.5G illumination (100 mW/cm 2 ) averaged over eight devices.

Figure 5 .
Figure 5. (a) XRD patterns of as-received (solid lines) and annealed (dotted lines) samples of the studied compounds.For the sake of clarity experimental curves are shifted along the intensity axis.Molecular modelling of crystal cell packing solutions for BTI (b) and f-TPA (c).

Figure 6 .
Figure 6.UV-vis absorption spectra of oligomers in THF solutions (a) and films cast from THF (b).
32  2.04 Notes: a Measured in THF solution.b Films cast from THF solution.c Determined as an intersection between the tangent to the film's absorption edge and the abscissa axis.d Calculated from CV experiments.

Figure 6 .
Figure 6.UV-vis absorption spectra of oligomers in THF solutions (a) and films cast from THF (b).
JSC EQE-short-circuit current integrated from the EQE spectrum.b Data for the annealed (70 °C for f-TPA, 100 °C for t-CBZ, 80 °C for BTI) active layer.

Table 1 .
Solubility, thermal data and phase behavior of oligomers.

Table 1 .
Solubility, thermal data and phase behavior of oligomers.

Table 2 .
Crystal cell parameters of t-CBZ, f-TPA and BTI.

Table 3 .
Optical and electrochemical data of oligomers.

Table 3 .
Optical and electrochemical data of oligomers.
a Measured in THF solution.b Films cast from THF solution.c Determined as an intersection between the tangent to the film's absorption edge and the abscissa axis.d Calculated from CV experiments.

Layer Material J SC , mA/cm 2 J SC EQE a , mA/cm 2 V OC , V FF, % PCE, % EQE, % TPA:PC 71 BM
Note: a J SC EQE -short-circuit current integrated from the EQE spectrum.b Data for the annealed (70 • C for f-TPA, 100 • C for t-CBZ, 80 • C for BTI) active layer.

Table 7 .
Hole mobilities in the blended films.