Electron-de ﬁ cient truxenone derivatives and their use in organic photovoltaics †

A series of electron-de ﬁ cient truxenone derivatives are investigated as fullerene alternatives in organic photovoltaic applications. These new electron-accepting molecules have easily tunable absorption pro ﬁ les, more than ten-fold higher absorptivities than PCBM, slightly higher electron a ﬃ nities than PCBM and clearly de ﬁ ned and highly reversible reductive characteristics. Fabrication of e ﬃ cient bilayer solar cells with a subphthalocyanine (SubPc) donor illustrates the promise of this class of materials as electron acceptors in organic solar cells.


Introduction
][6][7][8][9][10] Although fullerene acceptors such as phenyl-C 61 -butyric acid methyl ester (PCBM) have several highly benecial properties for organic photovoltaic (OPV) applications, non-fullerene acceptors are oen superior to fullerenes in terms of optical absorptivity, ease of synthesis and ease of frontier molecular orbital energy level ne tuning.Additionally, in contrast to fullerenes, nonfullerene acceptors hold the potential to be processed from non-halogenated solvents, which is highly advantageous in relation to scale-up and commercialisation of organic photovoltaics.
We have recently shown that solution processable truxenone derivatives with highly electron-withdrawing functionalities such as the dicyanovinylene moiety can be used as efficient acceptors in bilayer OPV devices with evaporated phthalocyanine donor materials. 11Here, we investigate further truxenone acceptors for OPV applications and the role of the electron-withdrawing moiety as well as the effects of introducing one, two or three of these electronwithdrawing functionalities on the C3-symmetric truxenone core.

Synthesis and thermal properties
As illustrated in Scheme 1, mono-, bis-and tris-adducts T1A, T2A and T3A were obtained from the Knoevenagel reaction between the 5-hexyl-2-thienyl-substituted truxenone (T) and ethyl cyanoacetate.Depending on the exact reaction conditions, favouring either partial (mono and bis) or complete (tris) addition, T1A was obtained in yields of 4-19%, T2A in 28-31% and T3A in 14-23% with combined yields ranging from 58 to 61%.The three adducts were isolated by column chromatography as red solids and NMR spectroscopy was used to conrm the identity of the three adducts.
Thermogravimetric analysis revealed good thermal stability for all three adducts as depicted in Fig. 1 with 5% weight loss in all cases observed above 365 C. When compared to the truxenone starting material (T), it is evident that each subsequent Knoevenagel adduct formation with ethyl cyanoacetate results in a thermogravimetric weight loss of approximately 4-5% around 400-410 C. It should be noted that, although less thermally stable, the higher adducts, with no more than 2% weight loss observed at 350 C, are still perfectly suitable for organic electronic applications.
Differential scanning calorimetry (DSC, Fig. 2) showed that while the unsubstituted truxenone (T) had a very distinct thermal transition around 230 C, all three truxenone adducts had DSC traces with no clear thermal transitions.This observation is most likely a result of the contortion of the planar truxenone core upon Knoevenagel adduct formation as will be discussed in the following section in greater detail.images of the energy-minimised conformations (Fig. 3) clearly illustrates how the truxenone core of T as expected adopts a completely coplanar conformation (thiophene rings are twisted 13 relative to the core), whereas all three adducts adopt a curved conformation owing to the steric repulsions between the four substituents on the newly formed carbon-carbon double bonds.This is in full agreement with the crystal structure reported by Zhang and co-workers on a closely related truxenone tris-adduct without the 5-hexyl-2-thienyl substituents. 124][15] This method was used to derive pyramidalization angles for each carbon atom in the central ring of the truxenone core for all three adducts.For T1A, the largest pyramidalization angle, which relates directly to the curvature, was found to be 1.46 , while the corresponding   angles were 2.01 for T2A and 2.68 for T3A (Fig. 3) illustrating the increased molecular curvature when going from T1A to T2A to T3A.
The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distributions are visualised for all three adducts in Fig. 4. T1A has its HOMO localised on one of the three electron-rich peripheral phenylene-thienylene "arms", while the LUMO is localised around the electron-decient cyanoacetate moiety on the truxenone core.In this context it is worth emphasising that it is the meta-linkage between the central benzene ring and the outer thiophene that disrupts the conjugation over the entire p-system.Compared to T1A, the bis-adduct T2A has a similar HOMO distribution reected in the identical calculated HOMO energy values of À5.67 eV.The LUMO of T2A, on the other hand, is delocalised more than that of T1A due to the additional cyanoacetate moiety, which explains the stabilisation of the LUMO energy from À3.07 eV for T1A to À3.13 eV for T2A.By formation of the tris-adduct T3A, the C3-symmetry of the molecule is restored and as a consequence both the HOMO and the LUMO is found to be doubly degenerate.The HOMO is thus equally distributed on all three peripheral "arms" and therefore signicantly stabilised (À5.85 eV) compared to T1A and T2A.The LUMO of T3A is likewise evenly distributed over the entire electron-decient core causing a slight stabilisation (À3.14 eV) relative to T2A.Due to the stabilisation of the LUMO in T2A, the calculated HOMO-LUMO gap is smallest for the bis-adduct, while the stabilisation of the HOMO in T3A explains why the tris-adduct has the highest HOMO-LUMO gap of the three.

Optical properties
The UV-vis absorption spectra of the three truxenone adducts were recorded in dichloromethane (DCM) solution and as thin lms spin-coated from chloroform solution; the obtained spectra are depicted in Fig. 5 and summarized in Table 1.A gradual red-shi of the absorption maximum of 20-30 nm was observed, both in solution and the solid state, when going from T1A to T2A to T3A (Table 1).For the transition from solution to solid state, we observe small red-shis for the mono-and bisadduct of 2-4 nm, while a small blue-shi of 3 nm is seen for the tris-adduct.In all three cases, a signicant broadening of the absorption prole, especially on the low-energy side, is seen when going from solution to solid state.Optical bandgaps, as estimated from the onset of absorption in the solid state, are very similar for the three compounds, varying from 2.05 eV for T2A to 2.10 eV for T3A and 2.12 eV for T1A.
Time-Dependent Density Function Theory (TDDFT, Gaussian B3LYP/6-31G* with DCM solvation) was used to compute the excitation energies and the oscillator strengths associated with the rst twenty vertical excitations for each truxenone adduct (raw data in Tables S1-S3 in ESI †).With this data, the theoretical UV-vis spectra were calculated using the SWizard program as depicted in Fig. 6. 16,17 The theoretical data nicely corroborates the experimental data (also obtained in  DCM solution) with respect to the gradual red-shi when going from mono-adduct to bis-and subsequently tris-adduct.The calculations furthermore predict a gradual increase in molar absorptivity when going from T1A to T2A to T3A and this trend is also found experimentally with a maximum molar extinction coefficient of roughly 9 Â 10 4 M À1 cm À1 measured for T3A (Table 1).In this context it should be noted that the C3symmetry of T3A would imply that the S 0 -S 1 transition is orbitally forbidden, but mixing with strong transitions nearby are likely responsible for the low-energy absorption. 18e note that the absorption prole of these truxenone acceptors effectively can be blue-or red-shied depending on the degree of functionalization; a highly advantageous feature when seeking complementarity of absorption proles between the donor and the acceptor materials in OPV applications.Additionally, the molar extinction coefficients of 67 000 to 89 000 M À1 cm À1 exceed the value reported for PCBM by more than one order of magnitude. 19

Electrochemical properties
The electron-accepting behaviour of the truxenone adducts were investigated by reductive cyclic voltammetry (CV) in dichloromethane solution and the resulting voltammograms are depicted in Fig. 7 and results are summarised in Table 2. Congruent with the increasing number of cyanoestervinylene moieties, we observe two, three and four semi-reversible reduction waves for T1A, T2A and T3A, respectively.For all three truxenone adducts, we observe the rst and second half-wave potentials around À0.55 V and À0.80 V, respectively.In addition, T2A shows a third reduction event with a half-wave potential at À1.13 V, while T3A has a third and a fourth reduction event at À1.01 V and À1.39 V, respectively.From the onset of reduction, we estimate the LUMO energy levels to be À3.87 to À3.88 eV for all three adducts, while the corresponding value for PC 61 BM measured under identical conditions was found at À3.75 eV.We thus nd all three truxenone-adducts to be slightly stronger electron acceptors than PC 61 BM.It is also worth emphasising that the CV results clearly indicate that the LUMO level is not signicantly altered when going from T1A to T2A and T3A.This is in good agreement with the semi-empirical calculations presented above (Fig. 4), where the three LUMO levels are predicted to differ no more than 0.07 eV.From a molecular point of view, the 1,3,5-positioning of the electronwithdrawing moieties around the central benzene ring and thus the limited electronic communication between them is likely responsible for the nearly identical LUMO levels.The distributions of the calculated LUMO+n orbitals (Fig. S5-S7 in ESI †) support the assumption that each cyanoestervinylene moiety will accept one electron upon reduction.Subsequently, another electron can be accepted by either the remaining carbonyl group (for T1A and T2A) or the aromatic core of the truxenone system (for T3A), which explains why two reduction events are observed for the mono-adduct, three reduction events for the bis-adduct and four reduction events for the tris-adduct.We also note that the area of the second reduction wave of T1A is roughly twice that of the area of the rst reduction wave, in agreement with one cyanoestervinylene moiety being reduced Fig. 5 UV-vis absorption spectra for the three truxenone adducts in dichloromethane solution (dashed lines) and as thin films spin-cast from chloroform (solid lines).  in the rst reduction process and two carbonyl groups being reduced in the second event.

Photovoltaic properties
To investigate the fundamental photovoltaic properties of these electron-accepting truxenone adducts, a bilayer device conguration was chosen over a bulk heterojunction conguration in order to eliminate any potential interference from blend morphology.Thus, with an inverted bilayer device conguration, these new solution-processable truxenone derivatives were spin-coated onto TiO 2 -coated ITO substrates aer which either subphthalocyanine (SubPc) or zinc phthalocyanine (ZnPc) was applied as the donor material by evaporation. 20,21Both phthalocyanine compounds have complementary absorption to the truxenone derivatives as SubPc absorbs between 450 and 600 nm, while ZnPc between 600 and 800 nm.Finally a thin MoO 3 and Ag top contact was deposited.As illustrated in Fig. 8, both phthalocyanine donor materials used in conjunction with the truxenone-based acceptors herein have sufficiently high-lying LUMO levels (À3.5 eV for ZnPc and À3.4 eV for SubPc) to allow for energetically favorable charge transfer to the acceptor material.
As illustrated in Fig. 9 and summarised in Table 3, the solar cells with SubPc (30 nm) show much higher open circuit voltages (V oc ) than the cells with the ZnPc donor (50 nm) due to the much lower HOMO value of SubPc relative to ZnPc (Fig. 8).We also note that there is basically no variation of the V oc with the three different truxenone acceptors, which is in good agreement with the nearly identical LUMO levels determined for the three compounds.Fill factors (FF) for these bilayer devices are in all cases rather low with values ranging from 19 to 25% with slightly higher values observed for T3A than for T1A and T2A.With both SubPc and ZnPc donors, a clear trend of increasing short circuit current (J sc ) when going from T1A to T2A to T3A is discernible.The highest J sc -values are obtained with SubPc as the donor ranging from 0.53 mA cm À2 with T1A to 0.85 mA cm À2 with T3A (for JV and EQE curves, see ESI).Consequently, the best bilayer OPV devices are obtained with the subphthalocyanine donor and the tris-adduct giving a power conversion efficiency (PCE) of 0.28% while the best ZnPc-device with T3A has a PCE of 0.08%.In comparison, reference OPV devices with PCBM as the electron acceptor material gave power conversion efficiencies of 0.73% with SubPc and 1.30% with ZnPc in the investigated device conguration (Table 3).
In light of the above results, we set out to investigate further the underlying reasons for the inferior OPV device performance of these truxenone derivatives despite favourable frontier energy levels and high optical extinction coefficients.Attempts to incorporate the truxenone adducts in bottom-gate bottom-contact organic eld-effect transistors (OFETs) were in all cases unsuccessful; no transistor behaviour could be observed neither for n-type nor for p-type charge transport.This is in good agreement with the very low ll factors observed for the OPV devices and we thus conclude that very poor charge transport properties is most likely the limiting factor for this class of materials.Further to this observation, we turn our attention once again to Fig. 4 and the highly localised LUMO distributions around the truxenone core for all three truxenone adducts.While this strong spatial separation of HOMO (localised around periphery of molecule) and LUMO (localised around the core of the molecule) has proven benecial for independently altering the HOMO and LUMO energy levels, we believe that it can also explain the very poor electron transport properties of these truxenone acceptors.Numerous reports in the recent literature have highlighted a strong spatial overlap of HOMO and LUMO distributions for highly efficient electron-transporting materials. 22,23Furthermore, while the contortion of the planar truxenone core is benecial for reducing pi-pi stacking and hence crystallinity (for improved bulk-heterojunction blend morphology with Table 2 Reductive electrochemical properties for truxenone adducts  donor materials), it is likely to strongly limit intermolecular charge transport as well.

Conclusions
In conclusion, we have introduced a truxenone moiety with peripheral thiophene groups for extended pi-conjugation and presented a synthetic route that allows us to access three different Knoevenagel adducts of this molecule in order to form highly electron-decient materials with electron affinities slightly higher than PCBM.The three adducts are thermally stable and show a lack of crystallinity due to the contortion of the otherwise planar truxenone motif.In addition, these novel truxenone derivatives have promising optical properties with high molar extinction coefficients and easily tunable absorption maxima, while they also show good electrochemical stability during reduction with clearly dened electrochemical events.Satisfactorily, initial assessment in photovoltaic devices showed that all three adducts behaved as electron materials with phthalocyanine based donor materials.Modest device performances were obtained in an inverted bilayer device architecture with the tris-adduct T3A showing the best performance (PCE of 0.28%) with an evaporated subphthalocyanine donor.

Fig. 1
Fig. 1 TGA traces for the truxenone precursor (T) and the three truxenone adducts recorded in a nitrogen atmosphere at a heating rate of 10 C min À1 .

Fig. 2
Fig. 2 DSC traces (exotherm down) for the truxenone precursor (T) and the three truxenone adducts recorded with a heating and cooling rate of 10 C min À1 .

Fig. 6
Fig. 6 Experimental (solid lines, left y-axis) and theoretical (dashed lines, right y-axis) UV-vis absorption spectra for the three truxenone adducts in dichloromethane solution.

Fig. 7
Fig. 7 Reductive cyclic voltammograms for the three truxenone adducts in dichloromethane solution with tetrabutylammonium hexafluorophosphate as the supporting electrolyte (curves are shifted vertically for improved clarity).

Fig. 8
Fig. 8 Frontier energy levels (all values in eV) of electron donor and acceptor materials used herein.

Fig. 9
Fig. 9 Photovoltaic device parameters of bilayer OPV devices with the three truxenone adducts (T1A, T2A and T3A) tested with both ZnPc (blue circles) and SubPc (red squares) donor materials.Values are averages and standard deviations measured on 4-8 cells under 100 mW cm À2 and AM1.5G simulated solar illumination.

Table 1
Experimental and theoretical optical properties for truxenone adducts

Table 3
Photovoltaic properties for truxenone adducts