Cyano substituted benzotriazole based polymers for use in organic solar cells †

a A new synthetic route to the electron accepting di-cyano substituted benzo[ d ][1,2,3]triazole (BTz) monomer 2-(2-butyloctyl)-4,7-di(thiophen-2-yl)-2 H -benzotriazole-5,6-dicarbonitrile ( dTdCNBTz ) is reported. The cyano substituents can be easily introduced to the BTz unit in one step via the nucleophilic aromatic substitution of the ﬂ uorine substituents of the ﬂ uorinated precursor 2-(2-butyloctyl)-4,7-di(thiophen-2-yl)-2 H -benzotriazole-5,6-di ﬂ uoro ( dTdFBTz ). Co-polymers were prepared with distannylated benzo[1,2-b :4,5-b 0 ]dithiophene (BDT) monomers containing either 2-ethylhexylthienyl (T-EH) side chains or 2-butyloctylthienyl (T-BO) side chains via Stille coupling to yield the novel medium band gap polymers P1 and P2 respectively. Whilst the organic photovoltaic (OPV) performance of P1 was limited by a lack of solubility, the improved solubility of P2 resulted in promising device e ﬃ ciencies of up to 6.9% in blends with PC 61 BM, with high open circuit voltages of 0.95 V.

2][3] When co-polymerised, the orbitals of the donor and acceptor monomers hybridise resulting in a reduced band gap through the simultaneous raising of the highest occupied molecular orbital (HOMO) and lowering of the lowest unoccupied molecular orbital (LUMO).5][6] It is therefore generally preferable to reduce the polymer band gap through selective lowering of the LUMO level whilst maintaining a deep HOMO level.[9][10][11][12][13] We have previously shown that the strength of the common electron accepting monomer 4,7-di(thiophen-2-yl)-2,1,3-benzothiadiazole (dTBT) can be increased by substituting one or two cyano groups onto the central 2,1,3-benzothiadiazole (BT) ring. 14,15When these cyano substituted BT acceptor units (see Fig. 1A) were incorporated into donor-acceptor polymers with dithienogermole (DTG), the polymer LUMO level was systematically lowered to reduce the band gap without raising the HOMO level.We found that optimising the position of the polymer LUMO level led to large increases in OPV device efficiency through improved photocurrent without sacricing V OC . 14Cyano substituents have also been shown to induce strong dipole moments, 16 promote balanced ambipolar charge transport and thermal stability in semiconducting polymers, 17 as well as allow ne tuning of donor polymer energy levels. 18urther exploring the use of cyano substituents to increase the strength of common electron accepting monomers, we here report the synthesis of the di-cyano substituted benzo[d] [1,2,3]  triazole (BTz) monomer, 2-(2-butyloctyl)-4,7-di(thiophen-2-yl)-2H-benzotriazole-5,6-dicarbonitrile (dTdCNBTzsee Fig. 1B).
Fig. 1 (A) Di-and mono-cyano substituted BT derivatives can be synthesised from their fluorinated precursors 14,15 (B) di-cyano substituted BTz can be synthesised from its fluorinated precursor.
Unlike BT, the benzo[d] [1,2,3]triazole (BTz) ring can be alkylated in the N-2 position.Alkylating at the N-2 position instead of other sites (such as the 5 and 6 BTz positions or anking thiophene rings) avoids unfavourable steric interactions along the polymer backbone.This allows higher backbone planarity to be achieved, encouraging p-p stacking between chains which can help to improve charge carrier mobility.However, the BTz unit is signicantly less electron withdrawing than the BT unit, as the sulfur atom in the thiadiazole ring is replaced with a more electron donating nitrogen atom in the triazole ring.When unsubstituted or uorinated BTz-based monomers are co-polymerised with benzo[1,2-b:4,5-b 0 ]dithiophene (BDT), the resulting polymers (dTBTz-BDT and dTdFBTz-BDT, Fig. 2) have a relatively wide bandgap ($2 eV) due to the weak electron accepting ability of the BTz monomer. 10,19Whilst a wide band gap can be useful to obtain high V OC , the J SC can be limited due to reduced light absorption.Despite the large band gaps of these materials, the un-substituted and uorinated BTz-BDT polymers give promising power conversion efficiencies of over 4% and 7% respectively in OPV devices. 19By adding cyano substituents to the BTz unit and therefore increasing the electron accepting ability, we aimed to reduce the polymer band gap in order to increase the J SC whilst either maintaining or improving the V OC .
Previously we have shown that the mono and di-cyano substituted BT derivatives can be easily synthesised from their uorinated precursors in one step through nucleophilic aromatic substitution of the uorine substituents with cyanide (see Fig. 1A).Herein we report the analogous route to the di-cyanosubstituted BTz derivative.Whilst the di-cyano substituted BTz monomer has recently been published by You and co-workers, 10 we offer an alternative, shorter synthetic route from the common uorinated analogue.You and co-workers 10 co-polymerised the brominated dTdCNBTz monomer with distannylated BDT, dialkylated with 3-butyloctyl chains to form the polymer dTdCNBTz-BDT (see Fig. 2).In this work, we co-polymerise brominated dTdCNBTz with distannylated BDT dialkylated with 2-ethylhexylthienyl and 2-butyloctylthienyl groups to yield the novel twodimensional conjugated polymers P1 and P2 respectively (see Fig. 2).4][25] Unfortunately, P1 was highly insoluble and proved difficult to process.The alkyl chain length was therefore increased to 2-butyloctyl leading to the more soluble P2.
You and co-workers 10 found that the cyanated polymer dTdCNBTz-BDT had a reduced band gap in comparison to uorinated analogue dTdFBTz-BDT due to a stronger stabilisation of the LUMO in comparison to the HOMO level.This is in agreement with work we have carried out previously comparing uorinated and cyanated BT based donor-acceptor polymers. 15ere we report that the inclusion of thienyl units in the BDT side chains results in P1 and P2 having slightly reduced band gaps in comparison to dTdCNBTz-BDT (see Fig. 2).Promisingly P2 exhibited signicantly improved device performance over dTdCNBTz-BDT, with a device power conversion efficiencies up to 6.9% in blends with PC 61 BM using PEDOT:PSS as the hole transporting layer (HTL).

Synthesis of monomer and polymer
The synthetic route to the dTdCNBTz monomer reported by You and co-workers 10 is quite different to the synthesis utilised in this work.You and co-workers reported an elegant eight step synthesis in which a thiophene-anked triazole-fused 1,4-diketone intermediate undergoes a nucleophilic condensation reaction with succinonitrile to form dTdCNBTz, which is then brominated under the same conditions as reported here.A related system has also recently been reported by the direct arylation of 2-octyl-5,6-dicyano-2H-benzo[d][1,2,3]triazole with a mono-protected thiophene, 12 although the synthesis of the starting dicyanated benzotriazole is rather low yielding. 26In this work, we report the initial synthesis of the thiophene-anked uorinated precursor (2) via Negishi coupling of commercially available 1 with 2-thienylzinc bromide (Scheme 1).The uoride substituents of 2 were readily displaced when treated with excess potassium cyanide in the presence of 18-crown-6 to yield Fig. 2 Chemical structures of dTBTz-BDT, 19 dTdFBTz-BDT, 10,19 dTdCNBTz-BDT 10 and the novel polymers P1 and P2.Scheme 1 Synthesis of di-cyano substituted benzotriazole monomer bis-Br(dTdCNBTz).
the dicyano-substituted product dTdCNBTz (57%).We have previously found that excess molecular bromine with heating was required to brominate the dicyano-substituted BT derivative (see Fig. 1) due to the reduced reactivity of the a 1 and a 2 thiophene positions of the electron-poor monomer. 15Despite BTz being a weaker electron acceptor than BT, N-bromosuccinimide (NBS) was still found to be ineffective as a brominating agent, and reaction with excess molecular bromine over 30 hours was required to form the nal brominated monomer bis-Br(dTdCNBTz).
Monomer bis-Br(dTdCNBTz) was reacted with the distannylated BDT monomers BDT(T-EH) (2-ethylhexylthienyl chain) and BDT(T-BO) (2-butyloctylthienyl chain) via Stille coupling under microwave irradiation to yield polymers P1 and P2 respectively (see Scheme 2).Aer 40 min of microwave heating from 100 C to 200 C, the polymers were precipitated into methanol before Soxhlet extraction (methanol, acetone, hexane, chloroform, chlorobenzene).Polymer P1 was highly insoluble, with the hot chloroform and chlorobenzene fractions containing only $10 mg (3-4% yield) of polymer each.The remaining undissolved polymer (P1) was heated in 1,2,4-trichlorobenzene (TCB) and precipitated into methanol.However, much of P1 remained undissolved even in hot TCB.The longer branched chains of P2 increased the solubility of the resulting polymer.However, hot chlorobenzene was still required to fully dissolve the higher molecular weight material.Due to the insolubility of polymer P1, the molecular weight could not be determined by gel permeation chromatography (GPC).The molecular weights of the chloroform and chlorobenzene fractions of P2 (P2-CF and P2-CB respectively) were determined using GPC in hot chlorobenzene (80 C) and are summarised in Table 1.P2-CF (76 mg, 42% yield) had a number-average molecular weight (M n ) of 26.0 kDa with a dispersity index (Đ) of 3.37, whilst P2-CB (88 mg, 49% yield) had a much higher M n of 78.2 kDa and a lower Đ of 1.46.The more soluble P2-CF was used for subsequent analysis (UV/Vis, cyclic voltammetry, 1 H NMR). Neither polymer exhibited any obvious thermal transitions by differential scanning calorimetry up to 300 C (Fig. S1 †).

Optical properties
The optical properties of P1 and P2 were investigated using UV-Vis absorption spectroscopy.The absorption spectra of both polymers in room temperature and heated (80 C) 1,2,4-trichlorobenzene (TCB) solutions are shown in Fig. 3A, whilst the absorption spectra of the polymers as thin lms are shown in Fig. 3B.The optical properties of the polymers are summarised in Table 2.The room temperature TCB solution and thin lm absorption spectra of P1 are almost identical, both having a l max at 641 nm.This suggests that P1 was aggregated in room temperature TCB solution.Indeed upon heating to 80 C, the solution l max blue shis to 622 nm as the aggregated polymer becomes more solvated (see Fig. 3A).The room temperature solution spectrum of P2 is signicantly blue shied in comparison to that of P1, having a l max at 627 nm and a high energy shoulder at $596 nm.Upon heating (80 C) the high energy shoulder of the solution absorption spectrum of P1 blue shis and increases in intensity giving a l max at 584 nm with    a low energy shoulder at $618 nm (see Fig. 3A), again indicative of reduced aggregation upon heating.This suggests that both polymers are at least partly aggregated in room temperature TCB.The absorption spectrum of P2 undergoes a small red shi of 9 nm from solution to thin lm, and appears almost identical to the solid state spectra of P1.
The optical band gaps of P1 and P2 are slightly smaller (1.73-1.74eV) than the reported optical band gap of dTdCNBTz-BDT (1.77 eV).The inclusion of thienyl groups onto the BDT core therefore has the effect of slightly broadening the absorption spectra of P1 and P2.

Electronic properties
The solid state electronic properties of polymers P1 and P2 were investigated using cyclic voltammetry (CV) of lms spun onto uorine-doped tin oxide (FTO).Measurements were performed in anhydrous, degassed solutions of acetonitrile with tetrabutylammonium hexauorophosphate (0.1 M) electrolyte using an Ag/Ag + reference electrode.Both polymers gave almost identical oxidation and reduction traces (Fig. S2 †), with rather poorly dened oxidation onsets around 1.2 V and reduction onsets around À0.67 V. Assuming that the ferrocene/ferrocenium reference redox couple is 4.8 eV below vacuum level, this corresponds to relatively deep HOMO levels of À5.6 eV and a LUMO level around À3.73 eV.As the polymers only differ by the length of the branched alkyl chain it is not surprising that they show very similar electronic properties.The HOMO levels of P1 and P2 are slightly shallower than those of the reported 10 polymer dTdCNBTz-BDT (see Fig. 2), which according to literature has a HOMO level of À5.73 eV.This may be due to the inclusion of the thienyl unit in the side chain, which has previously been shown to inuence the polymer HOMO and LUMO levels, 21 although we note that comparison with reported oxidation potential is difficult due to the inherent error in the measurement and the different experimental conditions used.
To further investigate the role of the thienyl groups, density functional theory calculations (DFT) were performed with a B3LYP level of theory and a basis set of 6-31G(d).The lowest energy conformation of the monomer dTdCNBTz was initially investigated by running a potential energy scan in which the thiophene-benzotriazole angle was systematically changed and the rest of the molecule was allowed to relax to its energy minimum (Fig. S3 †).The lowest energy conformer was then used to model trimers of this unit with carbazole (Fig. S4 †).These gas phase calculations show that the HOMO was delocalised over the polymer backbone, whereas the LUMO was more localised on the triazole unit.The alkylated thienyl sidegroups were twisted approximately 60 with respect to the BDT unit, limiting their involvement in the frontier molecular orbitals.

OPV performance
The OPV device performances of P1 and P2 were tested in a device conguration of glass/ITO/(PEDOT:PSS or CuSCN)/ polymer : PC 61 BM/Ca/Al.A blend weight ratio of 1 : 2 polymer : PC 61 BM was used with a concentration of $20 mg mL À1 in TCB for blends using P1 and $24 mg mL À1 in TCB for blends using P2.TCB was utilised since the polymers were not sufficiently soluble in CB or CF at the required concentrations.Both P2-CF and P2-CB were tested, whilst the re-precipitated TCB fraction of P1 was tested.Previously You and co-workers reported the device performance of dTdCNBTz-BDT was signicantly affected by the choice of hole transport layer (HTL), with overall efficiencies of $5.5-6% using CuSCN as the HTL and $4% using PEDOT:PSS as HTL. 10 The lower efficiency of the devices made using PEDOT:PSS was attributed to a mismatch in energy levels between the deep HOMO of dTdCNBTz-BDT (À5.73 eV) and the work function of PEDOT:PSS (À5.0 eV) compared to CuSCN (À5.5 eV).As such devices with P1 or P2 were made using either PEDOT:PSS or CuSCN as the hole transporting layer (HTL) and then compared.The OPV device characteristics are summarised in Table 3, whilst the J-V curves of average type devices are shown in Fig. 4.
Solutions containing P1 were difficult to dissolve fully, even when heated for 6 h at 135 C in TCB, resulting in poor device yields (ca.50%).Nevertheless devices made from P1 with PEDOT:PSS as the HTL did work, despite the low apparent energy offset between the LUMO of P1 and PCBM.The overall performance was poor, with low J SC ($5-6 mA cm À2 ) and FF ($0.3) resulting in low power conversion efficiencies of $1.5% (see Fig. S5 †).We ascribe the poor performance to the bad solubility and high aggregation tendency of P1 which likely results in poor mixing between the polymer and PC 61 BM, limiting the photocurrent.Due to the difficult processing of P1, no further optimisation was attempted.
Both P2-CF and P2-CB were tested in devices using either PEDOT:PSS or CuSCN as the HTL.Despite having higher molecular weight and narrower dispersity (Đ), P2-CB exhibited lower efficiencies in OPV devices (using both CuSCN and PEDOT:PSS) in comparison to the CF fraction.Higher molecular weight polymers generally tend to give improved performance due to improved active layer morphologies and charge transport properties. 27,28Whilst the J SC was improved ($13 mA cm À2 ) for P2-CB in comparison to devices made from the lower weight CF fraction ($11-12 mA cm À2 ), the ll factor was reduced in both the devices made using PEDOT:PSS (FF ¼ 0.42) and CuSCN (FF ¼ 0.38).Similarly to devices made from P1, this is likely to be due to the reduced solubility of P2-CB causing non-optimal active layer morphology.Films made from P2-CF appeared more visibly homogeneous than those made from P2-CB.Surprisingly, the use of CuSCN as the HTL reduced the efficiency of devices made from both P2-CF and P2-CB in our hands, mainly due to reduction in V OC and FF in both cases.The use of CuSCN did result in an increase in J SC , which we relate to the wide band gap of CuSCN (>3.5 eV) resulting in improved transparency between 400-1100 nm compared to PEDOT:PSS.][31] Atomic force microscopy (AFM) was used to investigate the effect of the CuSCN and PEDOT:PSS HTL's on the active layer morphologies of P2-CB and P2-CF.The thin lm morphology of just the CuSCN and PEDOT:PSS HTL layers on ITO coated glass are shown in Fig. S6.† In agreement with literature, the lm of CuSCN appeared to be nanocrystalline and signicantly rougher (root mean square (RMS) roughness of 5.13 nm) than the PEDOT:PSS layer (RMS 0.77 nm). 29AFM topography and phase images of P2-CF and P2-CB coated onto either CuSCN and PEDOT:PSS are shown in Fig. S7 and S8 † respectively.Both P2-CF lms (RMS 0.71-0.75nm) are slightly smoother than the P2-CB (RMS 0.89-1.36nm) on each HTL, in agreement with the improved solubility and processability of P2-CF.The P2-CF and P2-CB lms coated onto the CuSCN layer were marginally less smooth than those coated onto the PEDOT:PSS layers but, in general, their surface morphologies appear to be similar.The reduced ll factor and V OC of the P2-CB and P2-CF devices utilising CuSCN may therefore be related to poor contact of the polymer with the rough CuSCN surface.Previous studies using ZnO interlayers in an inverted solar cell device have noted a reduction in both FF and voltage as surface roughness increased. 32evices made from P2-CF with PEDOT:PSS as the HTL displayed the best performance with overall efficiencies up to 6.93% in combination with a high V oc of 0.95 V. Examination of the external quantum efficiency (EQE) curves (Fig. 4B) show that the blends generates current over the spectral range of the blend, with substantial photocurrent generated by the polymer.The performance is encouraging for a mid-gap and further demonstrates that dicyanobenzotriazole is a promising acceptor for the further development of high ionisation potential polymers.

Conclusions
We report a new route for the synthesis of 5,6-dicyano benzo[d] [1,2,3]triazole based monomers via with nucleophilic substitution of a uorinated precursor with cyanide.We highlight the utility of this route by synthesising the previously reported monomer 2-(2-butyloctyl)-4,7-di(thiophen-2-yl)-2H-benzotriazole-5,6-dicarbonitrile (dTdCNBTz) in one step from the uorinated precursor dTdFBTz.The resulting monomer was co-polymerised with two distannylated BDT monomers containing either 2-ethylhexylthienyl or 2-butyloctylthienyl side chains via Stille coupling to yield the novel medium band gap polymers P1 and P2 respectively.Polymer P1 was found to be poorly soluble even in hot TCB, whereas the 2-butyloctyl side chains of P2 improved solubility and processability.Both polymers had similar solid state optical and electronic properties, with notably high oxidation potentials around 5.6 eV.
Solar cell devices were prepared from both polymers as blends with PC 61 BM.The performance of P1 was rather poor, mainly due to processability problems but P2 afforded promising device performance with an overall efficiency up to 6.9% and a high open voltage circuit (0.95 V).The performance of P2 was found to be molecular weight dependent, with the lower molecular weight fraction (M n ¼ 26.0, Đ ¼ 3.37) performing signicantly better than the higher molecular weight (M n ¼ 78.2, Đ ¼ 1.46), which we ascribe to the better solubility of the lower weight fraction.The fact that the lower molecular weight fraction of P2 gave the highest performance shows that this polymer system is highly sensitive to solubility.If the solubility can be improved further, for example by using longer branched alkyl chains, higher molecular weight polymer with a narrower dispersity (Đ) may lead to further improvements in device performance.These results further demonstrate the utility of

Fig. 3 (
Fig. 3 (A) Absorption spectra of P1 and P2 in TCB solution and (B) P1 and P2 in thin film.

Fig. 4 (
Fig.4(A) J-V curves for either chlorobenzene (CB) or chloroform (CF) fractions of P2: PC 61 BM using either PEDOT:PSS or CuSCN as the hole transporting layers, (B) EQE of device made using P2 (chloroform fraction) with PEDOT:PSS as the hole transporting layer.

Table 1 Molecular
weights of chloroform (CF) and chlorobenzene (CB) fractions of P2 as measured by GPC versus polystyrene standards Polymer M n (kDa) M w (kDa) Đ

Table 2
Summary of optical and electronic properties of polymers P1 and P2 Polymer Soln l max(RT) (l max(80 C) ) nm l max (lm) nm E g (eV) HOMO (CV) b eV a In TCB solution.b Estimated from onset of rst oxidation.Error of AE0.1 eV associated with CV measurements.

Table 3
OPV device characteristics for polymers P1 and P2 a a CB ¼ chlorobenzene fraction, CF ¼ chloroform fraction.