Synthesis, properties, and photovoltaic characteristics of p-type donor copolymers having fluorine-substituted benzodioxocyclohexene-annelated thiophene

The incorporation of an acceptor unit into p-conjugated systems is an effective approach to tune both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of organic semiconducting materials. We report on the design of a series of fluorinesubstituted benzodioxocyclohexene-annelated thiophene acceptor units and the synthesis of donor– acceptor (D–A) type copolymers based on these acceptor units and dithienosilole as a donor unit. Physical measurements of the copolymers indicate that the D–A characteristics are increased by increasing the number of introduced fluorine atoms and the HOMO and LUMO energy levels of the copolymers are fine-tuned depending on the acceptor units. Organic photovoltaics based on blend films of these D–A copolymers and [6,6]-phenyl-C71-butyric acid methyl ester show photovoltaic responses with a power conversion efficiency of up to 7.30%. Investigation of the device physics shows that the performance is mainly limited by the hole transport, which provides insight in the direction of material design toward the improvement of OPV performance. These results demonstrate the potential of fluorine-substituted benzodioxocyclohexene-annelated thiophene as an acceptor unit in organic semiconducting materials.


Introduction
2][3][4][5][6] Both hole-transporting (p-type) and electrontransporting (n-type) organic semiconductors are required for the fabrication of these devices.It has been well-established that electrons or holes can be selectively injected by tuning the relative energies between the Frontier orbital of organic semiconductors and the Fermi level of metal electrodes.Thus, tuning of both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy level is important. 2[9][10][11][12][13][14][15][16] We previously reported that dihexyl-substituted naphtho[2,3c]thiophene-4,9-dione (C 6 ) is effective as an electron-accepting unit in p-conjugated materials applicable to n-channel OFETs. 17Furthermore, the C 6 unit can also function effectively as an acceptor unit of donor-acceptor (D-A) type copolymers for bulk-heterojunction OPVs.2][23] With DTS-C 6 as a p-type donor material in combination with [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) as an n-type acceptor, 7.85% power conversion efficiency (PCE) has been attained aer precise optimization of the lm-forming method. 22This result motivated us to modify the chemical structure of C 6 toward stronger electron-accepting nature, [24][25][26][27] which will lead to a decreased HOMO energy level of the copolymers.This is expected to result in an increased opencircuit voltage (V oc ) in OPVs because it has been suggested that the V oc is directly proportional to the energy-level difference between the HOMO of the p-type donor and the LUMO of the ntype acceptor. 28Furthermore, the increased D-A character will reduce the band gap extending the light absorption to the longwavelength region.To exemplify this hypothesis, we introduced strongly electron-withdrawing uorine atoms into the benzene ring of C 6 and designed C 0 (F 2 ), C 0 (F 4 ), and C 6 (F 2 ) as new acceptor units (Fig. 1).We also designed C 0 as a reference to these units. 29In this contribution, these acceptor units were integrated into conjugated copolymers with DTS as a donor (DTS-C 0 , DTS-C 0 (F 2 ), DTS-C 0 (F 4 ), and DTS-C 6 (F 2 )) to examine the inuence of uorine atoms on the polymer properties and OPV characteristics.Furthermore, the blend-lm property investigation gave an insight into a way for the improvement of OPV performance based on copolymers containing the C 0 (F 2 ) unit.

Photophysical properties
The electronic absorption spectra of DTS-C 0 , DTS-C 0 (F 2 ), DTS-C 0 (F 4 ), and DTS-C 6 (F 2 ) as well as a reference copolymer DTS-C 6 in both dilute CHCl 3 solutions and lms are shown in Fig. 2. 22 The photophysical data including the absorption maximum (l max ) in solution, absorption onset (l onset ) in lms, and the HOMO-LUMO energy gap (DE opt ) estimated from l onset are summarized in Table 1.Compared with DTS-C 0 and DTS-C 6 , the l max s of the corresponding uorinated copolymers DTS-C 0 (F 2 ) and DTS-C 6 (F 2 ) are red-shied.Furthermore, by increasing the number of uorine atoms (DTS-C 0 , DTS-C 0 (F 2 ), and DTS-C 0 (F 4 )), a progressive clear red-shi of l max was observed, indicating that the uorine atoms in the acceptor units have a certain inuence on the absorption owing to the increased D-A character.The absorption spectra of the copolymer lms were broadened and obviously red-shied in comparison with those in the solutions, indicating the appearance of intermolecular interactions in the solid state, which is benecial for chargecarrier transport.Particularly, l onset in the lm of DTS-C 0 (F 2 ) showed a signicant red shi compared to the l onset in solution, indicating that DTS-C 0 (F 2 ) might have better intermolecular interaction in the solid state.In addition, DTS-C 0 and DTS-C 0 (F 4 ) also showed a large red-shi from solution to the solid state.These results indicate that the removal of substituents from the benzene part of the acceptor unit reduces steric bulkiness and thus induces intermolecular electronic interactions.Consequently, HOMO-LUMO energy gaps (DE opt s) can be controlled by modifying the acceptor unit with electronwithdrawing uorine substituents, that is, DE opt of the copolymers is reduced sequentially as the electron deciency of the acceptor unit is increased.

Frontier-orbital energy levels
The HOMO energy levels (E HOMO s) of all copolymers in the lm state were estimated by photo-electron spectroscopy in air (PESA) as shown in Fig. 3.The E HOMO s of the copolymers were decreased with increasing the number of electron-withdrawing uorine substituents or removal of electron-donating hexyl chains in the acceptor unit (Table 1).The LUMO energy level (E LUMO ) was estimated by the addition of DE opt g to E HOMO (Table 1).E LUMO was more signicantly decreased with the addition of uorine atoms compared with E HOMO , which means that uorine substituents on the benzene part of the acceptor units more strongly affect the LUMO as compared to the HOMO of the copolymers.The resulting energy level diagram of all copolymers is shown in Fig. 4.

Carrier mobility
The hole mobility of donor polymers plays an important role in the OPV performance.In order to evaluate the hole mobility (m h )   of the pristine copolymer lms, hole-only devices with a conguration of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/copolymer/Au were fabricated, and m h was estimated using the space charge limited current (SCLC) model.Note that these lms did not show any clear peaks in X-ray diffraction (XRD), indicating the formation of amorphous lms.As summarized in Table 1, the copolymer DTS-C 0 (F 2 ) exhibited one order of magnitude higher m h than those of other newly developed copolymers and similar to that of DTS-C 6 , being expected to show high OPV performance among these copolymers.

Photovoltaic characteristics
To investigate the characteristics of newly synthesized copolymers as OPV donor materials, we fabricated a conventional device structure of glass/ITO/PEDOT:PSS/active layer/Ca/Al.The active layers consisted of a blend of the copolymer as a donor and PC 71 BM as an acceptor.The fabrication conditions of the active layer were optimized to be the copolymer:PC 71 BM blend composition of a 1 : 2 weight ratio, a concentration of 30 mg mL À1 in o-DCB with the use of 3% 1,8-diiodooctane (DIO) for spin-coating (Table S1, Fig. S2 †), and without thermal annealing.All the measurements were performed under simulated AM 1.5G solar irradiation (100 mW cm À2 ) in a nitrogen atmosphere.The best-performance current density-voltage (J-V) characteristic of each of the copolymers is shown in Fig. 5(a) and their key photovoltaic parameters are summarized in Table 2.All the data to ascertain the reproducibility of OPV performance are summarized in Fig. S3 and Tables S2-S6.† As shown in these data, all the copolymer-based devices showed typical photovoltaic responses.However, the device based on DTS-C 0 (F 4 ):PC 71 BM showed the lowest short circuit current density (J sc ) among the investigated devices, leading to the lowest PCE of 0.99%.This low J sc is caused by the lower LUMO energy level of DTS-C 0 (F 4 ), relative to that of PC 71 BM, which prevents efficient charge separation from the photoexcited state of the copolymer.The removal of hexyl groups (DTS-C 0 ) or the introduction of uorine atoms (DTS-C 6 (F 2 )) from or to the C 6 unit did not contribute to the increase in J sc .On the other hand, the DTS-C 0 (F 2 ):PC 71 BM-based device showed an improved J sc of 11.58 mA cm À2 compared to that of DTS-C 6 :PC 71 BM under the same fabrication conditions (10.59 mA cm À2 ).Examination of the external quantum efficiency (EQE) spectra in Fig. 5 S7, † the difference in absorption onsets between the pristine and blend lms is prominent for DTS-C 0 (F 2 ) and DTS-C 6 (F 2 ), which may indicate the appearance of strong intermolecular interactions between copolymers and PC 71 BM in the blend lms.When focusing on the V oc , no clear relationship against the E HOMO s of copolymers was observed.We will discuss this point in a later section.Based on these OPV results, we further optimized the DTS-C 0 (F 2 ):PC 71 BM-based device and found that the MeOH treatment of the active layer 30,31 and the use of a Ba thin cathode buffer layer instead of Ca led to 7.30% PCE with a J sc of 16.71 mA cm À2 , V oc of 0.86 V, and FF of 0.51.For the comparison with simulation, J-V characteristics are shown in Fig. 11.

Blend-lm properties
In order to investigate the lm morphology, atomic force microscopy (AFM) measurements of the blend lms were   carried out.As shown in Fig. 6, the DTS-C 6 (F 2 ):PC 71 BM lm showed relatively large grains, while the other blended lms showed favorable smooth surfaces with the average roughness (R a ) between 1.65 and 2.09 nm.3][34] As summarized in Table S8, † the SFEs as well as the contributions of the London dispersion (g d ) and polar (g p ) components of the copolymers were in a narrow range, suggesting that similar surface natures of the copolymer lms rationalize the formation of similar blend-lm interfaces and thus morphologies in combination with PC 71 BM.In X-ray diffraction (XRD) measurements, no obvious diffraction peaks could be observed for all the blend lms (Fig. S5 †), indicating that these thin lms have an amorphous nature.These results indicate that variation in the chemical structure of the acceptor unit in copolymers with DTS has little inuence on the blend-lm morphological properties.The V oc of OPVs is expected to be directly proportional to the difference between E HOMO of the donor and E LUMO of the acceptor.However, the V oc of DTS-C 0 (F 2 ):PC 71 BM-based OPVs was lower than that obtained from DTS-C 6 :PC 71 BM-based devices (0.84 V vs. 0.90 V), irrespective of the low-lying E HOMO for DTS-C 0 (F 2 ).To investigate the origin of this unexpected phenomenon, we measured the PESA of blend lms with different copolymer/PC 71 BM ratios of 1 : 1, 1 : 2, and 1 : 3 (Fig. 3).As summarized in Fig. 7, the E HOMO s of 1 : 1 blend lms for DTS-C 0 (F 2 ):PC 71 BM and DTS-C 6 (F 2 ):PC 71 BM were increased compared to those of the corresponding copolymer pristine lms (1 : 0), while the blend ratio has little inuence on their E HOMO s.On the other hand, the E HOMO s of DTS-C 0 , DTS-C 6 , and DTS-C 0 (F 4 ) did not show signicant changes under these conditions.This result indicates that DTS-C 0 (F 2 ) and DTS-C 6 (F 2 ) might induce intermolecular interactions with PC 71 BM, which were also suggested from the difference in their absorption onsets between pristine and blend lms, and the resulting increase of E HOMO in the mixed state caused the decrease of the V oc in the OPVs.In addition, as shown in Fig. S6, † the dark J-V characteristics of DTS-C 0 (F 2 ):PC 71 BM and DTS-C 6 (F 2 ):PC 71 BM showed higher current density compared to those of other lms.This phenomenon might be due to the inuence of intermolecular charge-transfer interactions.
To get deeper insight into the device physics, we focused on the best-performance DTS-C 0 (F 2 ):PC 71 BM devices.First, we measured the photocurrent (J ph ) of OPVs as a function of effective applied voltage (V eff ) under the light intensity of 10 and 100 mW cm À2 .The J ph is given by the equation J ph ¼ J L À J D , where J L and J D are the current densities under illumination and dark conditions, respectively.The effective voltage V eff is calculated as V eff ¼ V 0 À V, where V 0 is the voltage at J ph ¼ 0 and V is the applied bias.In this way, the effect of an electric eld on the generated free carriers can be studied.As shown in Fig. 8, the photocurrent gradually increases with increasing effective voltage, which is characteristic of a recombination-limited photocurrent.It is observed that J ph does not saturate even at high effective voltages.This is explained by a eld-dependent dissociation rate of electron-hole pairs, or, in other words, eld-dependent geminate recombination, giving rise to moderate ll factors.
The dominant geminate recombination process is conrmed by light-intensity-dependent measurements of the short-circuit current, as displayed in Fig. 9(a).6][37] In Fig. 9(a), we plotted the J sc against light intensity, resulting in a ¼ 0.97.The small deviation from unity indicates that bimolecular recombination losses are minor under short-circuit conditions.Therefore, the increasing photocurrent beyond short-circuit conditions must be ascribed to a geminate-recombination process.
To investigate whether trap-assisted recombination plays a role, we also investigated the light-intensity dependence of V oc .Since all photogenerated carriers recombine in the cell at V oc (J ¼ 0), the dependence of V oc on the light intensity (P) gives information on the recombination process.In the case of bimolecular recombination, it has been reported that the slope of V oc against the logarithm of P equals kTq À1 , where k is Boltzmann's constant, T is the temperature, and q is the elementary charge. 35,38This slope increases when trap-assisted recombination is present. 39The V oc of the DTS-C 0 (F 2 ):PC 71 BMbased device as a function of P between 1.0 mW cm À2 and 100 mW cm À2 is shown in Fig. 9(b).A slope of 1.01 kTq À1 was determined, indicating that trap-assisted recombination is effectively absent.
As can be observed from Fig. 8, the dissociation of electrons and holes is complicated in this system, which might be caused by poor charge transport.To reveal the charge-transport characteristics, we fabricated double-carrier, hole-only, and electron-only devices by using Au/PEDOT:PSS/DTS-C 0 (F 2 ):PC 71 BM/Ba/Al, Au/PEDOT:PSS/DTS-C 0 (F 2 ):PC 71 BM/ MoO 3 /Al, and Al/DTS-C 0 (F 2 ):PC 71 BM/Ba/Al congurations, respectively.The observed J-V characteristics between 213 K and 295 K are shown in Fig. S7, † and the plots of JL 3 against V À V bi , where L is the lm thickness and V bi is the built-in voltage are shown in Fig. 10.By using the SCLC equation, double-carrier (m eff ), hole (m h ), and electron (m n ) mobilities of this blend lm at 295 K were calculated to be 1.2 Â 10 À7 , 4.0 Â 10 À9 , and 3.0 Â 10 À8 m 2 V À1 s À1 , respectively.While the electron mobility is in the expected range for transport through PC 71 BM, the hole mobility through the polymer phase appears to be relatively low.
Based on the mobility values and the equation: the Langevin prefactor (g pre ) at 295 K is determined to be 5.1 Â 10 À2 . 40The Langevin prefactor determines the deviation from a Langevin recombination rate.The found value indicates that the bimolecular-recombination rate is reduced by a factor of 20 with respect to Langevin recombination.
With the mobilities and bimolecular-recombination rate known, we simulated the J-V characteristics of DTS-C 0 (F 2 ):PC 71 BM devices under illumination with a dri-diffusion model (Fig. 11). 41While the ll factor is well described, the voltage-dependent photocurrent at negative voltages is not reproduced.The reason for the poor description is the fact that the generation rate (G) of electrons and holes was considered eld independent, whereas Fig. 8 suggests a eld-dependent generation rate.To get an accurate t to the experimental data, a eld dependent generation rate has to be included according to the Onsager-Braun theory. 42By using a chargetransfer state decay rate k f of 1.0 Â 10 6 s À1 and an initial separation distance of electron-hole pairs a of 1.9 nm, the experimental data are well described.The experimental values for the mobilities and Langevin prefactor were used.The simulation once more conrms that, next to bimolecular recombination, eld-dependent electron-hole dissociation is responsible for the poor ll factors in the solar cell, giving rise to strong geminate recombination losses under operating conditions.We   postulate that the relatively poor hole transport inhibits efficient electron-hole pair separation.
As shown in Fig. S8, † there is a signicant difference in temperature dependence between hole and electron mobilities.Activation energies of 0.22 eV and 0.36 eV were determined for electron and hole transport, respectively.In combination with the one order of magnitude lower hole mobility than electron mobility, this suggests more energetic disorder for hole transport.In view of the difficult separation of electrons and holes at the donor-acceptor interface, we concluded that the most critical point to determine the photovoltaic performance in a DTS-C 0 (F 2 ):PC 71 BM system is the relative low hole mobility.In other words, the molecular design to increase the hole mobility using DTS and C 0 (F 2 ) as donor and acceptor units may lead to an increase in the PCEs in terms of improving J sc and FF.

Conclusions
In summary, we described here the synthesis and electronic properties of a series of new D-A type copolymers having uorine-substituted naphtho[2,3-c]thiophene-4,9-diones as acceptor units.The uorine substitution in naphtho[2,3-c] thiophene-4,9-dione chromophores contributes to a reduced band gap and low-lying HOMO energy level of the D-A copolymers, and we revealed that the photophysical properties as well as the energy levels are precisely controlled by the number of substituted uorine atoms.All the copolymers exhibited photovoltaic characteristics in combination with PC 71 BM, and high efficiencies of up to 7.30% were obtained for DTS-C 0 (F 2 ).The XRD and AFM measurements of the blend lms indicated that all the lms showed an amorphous character and similar morphologies.The detailed investigation of DTS-C 0 (F 2 ):PC 71 BM device characteristics based on light-intensity dependent measurements and carrier mobility measurements as well as simulation of the J-V characteristics suggested that the increase of hole mobility becomes the key issue to improve the PCE.These results demonstrate the potential of uorine-substituted naphtho[2,3-c]thiophene-4,9-dione units, and studies on the development of second-generation copolymers containing these units with improved hole mobility are underway in our laboratories.

Fig. 1 Scheme 1
Fig. 1 Chemical structures of the acceptor units and copolymers used in this study.
(b) revealed the generation of a broad photocurrent in a longer wavelength region for DTS-C 0 (F 2 ):PC 71 BM and DTS-C 0 :PC 71 BM with maximum EQEs of approximately 50%, whereas both DTS-C 0 (F 4 ):PC 71 BM and DTS-C 6 (F 2 ):PC 71 BM devices gave much lower EQE responses.Although the overall shapes of these EQE spectra well reect the UV-vis absorption spectra of the corresponding blend lms (Fig. S4 †), photocurrent generation in the region exceeding 800 nm in the case of DTS-C 0 (F 2 ):PC 71 BM is particularly signicant, since this region exceeds the absorption onset of the pristine DTS-C 0 (F 2 ) lm (779 nm).As summarized in Table

Fig. 4
Fig. 4 Energy diagram of the copolymers.

Fig. 8 J
Fig. 8 J ph -V eff characteristics of DTS-C 0 (F 2 ):PC 71 BM devices under illumination of a simulated solar spectrum.The arrows indicate the points of V oc and J sc conditions under 100 mW cm À2 irradiation conditions.

Fig. 10
Fig. 10 Plots of JL 3 against V À V bi for DTS-C 0 (F 2 ):PC 71 BM at 295 K. Corresponding calculated results are shown in the straight line.Film thicknesses of double-carrier, holy-only, and electron-only devices are 85, 88, and 55 nm, respectively.

Fig. 11 J
Fig. 11 J-V characteristics of the optimized DTS-C 0 (F 2 ):PC 71 BMbased OPV device under illumination (filled red circle) and in the dark (red circle).Simulated J-V curves with a field-dependent and fieldindependent generation rate are shown in solid and dashed black lines, respectively.

Table 1
Properties of compounds a In CHCl 3 .b In lms.c DE opt ¼ 1240/l onset .d Determined by PESA measurements.e Determined by E HOMO and DE opt .f Ref. 22.

Table 2
Photovoltaic characteristics of the copolymer:PC 71 BM films a The average of 4 devices is provided in parentheses, see the ESI for details.b Aer optimization of the device structure and fabrication conditions.Active layers were fabricated using a 33 mg mL À1 solution in o-DCB with 3% DIO.c Ref.22.