ORIGINAL RESEARCH Modifying electronic properties of ICBA through chemical substitutions for solar cell applications Eliezer Fernando Oliveira1 & Lucas Castorino Silva2 & Francisco Carlos Lavarda1,3 Received: 1 December 2016 /Accepted: 25 January 2017 /Published online: 10 February 2017 # Springer Science+Business Media New York 2017 Abstract Fullerene derivatives are the most widely used type of acceptor material in the organic solar cells (OSCs) active layers, but there are still some problems to be overcome, such as increased solubility and adjustment of the frontier electron- ic levels for a better combination with the donor materials in the active layer. Chemical modification of the materials al- ready employed in active layers is an interesting way to vary the electronic properties in order to find new materials, because it is possible, in principle, to tune the intrinsic prop- erties of the material aiming to improve the solar cell efficien- cy. Thus, we studied theoretically the effect caused by chem- ical substitutions on the electronic properties of the ICBA, one of the fullerene derivatives employed in OSCs. Geometry op- timizations and electronic structure data were obtained by DFT/PBE/6-311G(d,p) calculations for 13 ICBA derivatives. We show that by chemical substitutions of ICBA, it is possible to modify the energies of the frontier electronic levels, in- crease the solubility, and find new derivatives that show improvements in open circuit voltage and morphology of the active layer, potentially bringing better efficiency for OSCs. Keywords Computational modeling . Engineering of electronic properties . Chemical modifications . ICBA . Organic solar cells Introduction The most common electron acceptor for bulk heterojunctions organic solar cells is the phenyl-C61-butyric-acid-methyl-ester (PCBM) [1–4]. Looking for better electron acceptor materials, the indene-C60 bisadduct (ICBA) was proposed [5] (see Fig. 1) and has shown to be a promising substitute for the PCBM, once (i) it presents an easier synthesis [5] and (ii) higher open circuit voltages (VOC) and electric currents in the devices were achieved in active layers with common elec- tron donor materials, such as poly(3-hexylthiophene) (P3HT) [6, 7]. Despite the advantages of ICBA, lower values for fill factor (FF) are observed compared with those using PCBM, probably due to a lower solubility [5, 6, 8]. We showed previously for conjugated polymers that the use of chemical substitutions is an efficient tool to modify the intrinsic properties of the materials, since it was possible to obtain new derivatives with varying levels of solubility, better ionization potentials, and electronic levels close to what is considered ideal in the literature [9–12]. In the case of ful- lerene derivatives, it is still not clear if there is the possibility to significantly vary the electronic properties by chemical sub- stitutions [13]. Thus, as the ICBA is a promising material for use in active layers of OSC and still has some deficiencies regarding the most used PCBM, in this study, we evaluated the effects of chemical substitutions on the electronic proper- ties of the ICBA. Electronic supplementary material The online version of this article (doi:10.1007/s11224-017-0916-0) contains supplementary material, which is available to authorized users. * Eliezer Fernando Oliveira eliezer@fc.unesp.br 1 UNESP - Univ Estadual Paulista, POSMAT - Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, Av. Eng. Luiz Edmundo Carrijo Coube 14-01, Bauru, SP 17033-360, Brazil 2 UNESP - Univ Estadual Paulista, Graduação em Física, FC - Faculdade de Ciências, Av. Eng. Luiz Edmundo Carrijo Coube 14-01, Bauru, SP 17033-360, Brazil 3 DF-FC, UNESP - Univ Estadual Paulista, Av. Eng. Luiz Edmundo Carrijo Coube 14-01, Bauru, SP 17033-360, Brazil Struct Chem (2017) 28:1133–1140 DOI 10.1007/s11224-017-0916-0 http://dx.doi.org/10.1007/s11224-017-0916-0 http://crossmark.crossref.org/dialog/?doi=10.1007/s11224-017-0916-0&domain=pdf In this work, we studied theoretically 13 ICBA derivatives in which we performed chemical substitutions in one of its indene groups. In the first part of the work, we studied the effects caused in the electronic structure, in which was possi- ble to find new promising derivatives. The most interesting result was that the chemical substitutions were able to signif- icantly modify the dipole moment of the ICBA, which favors a good morphology and charge transport in the active layer. The largest dipole moments were obtained for the derivatives substituted with groups with strong electron-releasing or electron-withdrawing character. In the second part of the work, we compared the energies of the frontier electronic levels of P3HT and polythieno[3,4-b]- thiophene-co-benzodithiophene (PTB7) [10, 14] as electron donor materials with ICBA derivatives as electron acceptor materials. Overall, it is not predicted problems in the exciton dissociation and recombination in these possible active layers. Also, the calculations indicate that some ICBA derivatives could have a higher VOC in the device than the active layers using ICBA or PCBM; this suggests that the solar cell effi- ciency tends to increase using these ICBA derivatives together with P3HT or PTB7. Materials and methods All geometry optimizations were performed using density functional theory (DFT) due to its broad and successful em- ployment in organic materials studies [15–19] and even coupled with quantitative structure-properties relationship studies [20, 21]. However, for good reliability of the results obtained by DFT, we must first choose the correlation and exchange functional that best suits to the system studied [16, 22]. To determine which functional would be used in our calculations, we conducted a preliminary study for the ICBA, performing geometry optimizations with some of the most commonly used functional in the literature, among them hybrids with different amounts of Hartree–Fock exact ex- change as well as those that include corrections for long- range interactions; the choice of the functional will be guided by that one that presents the best predictions for the electronic properties of ICBA compared to the experimental data. We tested the functionals B3LYP [23, 24], CAM-B3LYP [25], BHLYP [24], M06HF [26], X3LYP [27], and PBE [28], employing the basis set functions 6-311G(d,p) and the soft- ware GAUSSIAN09 [29]. Geometry optimizations were per- formed for various initial structures, to assess all possible con- formations and to ensure that only those of lowest energy would be chosen for the study. The equilibrium geometries were confirmed by vibrational spectrum calculations since no imaginary frequencies were found. Table 1 presents the results obtained for each functional tested and the deviation obtained in relation to the experimental data. We analyzed the energies of the highest occupied and the lowest unoccupied molecular orbitals (EHOMO and ELUMO, respectively). As can be seen, the smallest deviations obtained for both EHOMO and ELUMOwere with the PBE functional (below 10%). This result is in agreement with previous studies, where the PBE func- tional has shown good results for fullerene derivatives in com- parison with experimental data [30–33]. Thus, we decided to conduct our studies using the DFT/PBE/6-311G(d,p) methodology. After determining the calculation methodology that would be adopted, we performed chemical substitutions in the ICBA in one of the indene groups that are inserted into the fullerene in order to remove the system symmetry. We modified the most extreme position, which is highlighted in Fig. 1. The following substituents were chosen: chlorine (Cl), bromine (Br), fluorine (F), hydroxy (OH), cyano (CN), amino (NH2), methyltio (SCH3), trifluoromethyl (CF3), methyl (CH3), dimethylamino (N(CH3)2), metoxy (OCH3), carboxy (COOH), and ethenyl (CH=CH2). It is interesting to note that the selected substituents have different effects and intensities of charge releasing/withdrawing, which is important tomake a comprehensive analysis of the influences of the substituents on the electronic properties of the ICBA. We also tested more than one chemical substitution in the indene group. However, the results were very similar to that obtained with only one substitution; based on this fact, we continued the study with only one chemical modification. Results and discussion Electronic properties of ICBA derivatives We present in Table S1 in the Electronic Supplementary Material the theoretical results for EHOMO and ELUMO of ICBA and its derivatives. As we know that there is a deviation in the theoretical results for the ICBA, the derivatives would have a double deviation, i.e., a deviation in relation to the theoretical data of ICBA which already has a deviation from the experimental data. To let our analysis be more realistic, we decided to insert scale factors (SF) on the theoretical data of ICBA derivatives. So, based in the experimental data, we Fig. 1 Structure of ICBA. The red point indicates the position where the substitutions were performed 1134 Struct Chem (2017) 28:1133–1140 multiply the theoretical values of EHOMO and ELUMO of the ICBA derivatives by the following SF: SFHOMO = (5.6/5.2) and SFLUMO = (3.7/3.8). The data of the electronic structure recalculated for ICBA and its derivatives are shown in Fig. 2; these recalculated data are also reproduced in Table S2 of the Electronic Supplementary Material. In order to make some comparisons in our analysis, we also inserted in Fig. 2 the energies EHOMO and ELUMO of PCBM, which are, respective- ly, −6.0 and −4.2 eV [35]. As we can see, the chemical sub- stitutions were able to change the electronic properties of ICBA, with percentage variations between −4.0 and 2.5% for ELUMO and between −1.3 and 12.0% for EHOMO. Although it was not observed large changes due to chemical substitutions, it is worth to point here that a significant impact on solar cell efficiency parameters can be predicted, as discussed in the following section. Half of the ICBA derivatives showed an EHOMO higher than that observed for the pure ICBA, being the most pro- nounced difference for ICBA-N(CH3)2, 12% higher; the low- est EHOMO was found for P3HT-CN, 3% lower. In relation to ELUMO, smaller variations were obtained compared with those obtained for the EHOMO. Six ICBA derivatives presented an increase in ELUMO, being the highest observed for ICBA- N(CH3)2, 2.4% higher; ICBA-CN had the lowest ELUMO, ap- proximately 4% lower. Considering a bulk heterojunction, the higher the ELUMO of the acceptor material, the better the VOC of the device [35–37]; that is why the use of ICBA has brought significant improvements in the solar cell performance [6, 7], once the ICBA’s ELUMO is higher than that of PCBM (see Fig. 2). For ICBA derivatives, six of them showed an increase in ELUMO and would tend to have a higher VOC in the device than the ICBA or PCBM.However, we have to be careful with the EHOMO of the electron acceptor material, because depend- ing on its value relative to EHOMO of the electron donor ma- terial, problems with dissociation and/or recombination of ex- citons could arise [9, 37]. A more detailed analysis regarding these issues will be made further on. We can also evaluate the oxidation stability of the studied materials when exposed to the environment. As the acceptor material in the solar cell’s active layer receives the electrons of the donor material in the LUMO orbital, it is necessary to have a ELUMO approximately equal to or less than −4.0 eV to ensure the stability to oxidation [38]. According to this criterion, the ICBA is not as stable to oxidation as one would like, because it has a ELUMO of −3.7 eV. PCBM is more stable than ICBA with a ELUMO of −4.2 eV. Our results showed that the ICBA derivatives still present values for ELUMO above the limit of −4.0 eV, but our calculations suggest that seven of them are more stable to oxidation than ICBA (substituted with Cl, Br, F, CN, CF3, COOH, and CH=CH2). ICBA-Cl, ICBA-Br, ICBA- CN, and ICBA-CF3 are the best derivatives, with a ELUMO around −3.8 eV. As for the energy gap Eg between the LUMO and HOMO orbitals (ELUMO-EHOMO), the majority of derivatives have Table 1 Electronic structure data of ICBA from some different functionals ICBA EHOMO (eV) EHOMO Deviation (%) ELUMO (eV) ELUMO Deviation (%) Experimental [5, 34] −5.60 – −3.70 – B3LYP −4.49 19.82 −3.82 3.34 CAM-B3LYP −6.16 −10.07 −2.27 38.63 BHLYP −5.76 −2.88 −2.39 35.22 M06HF −7.82 −39.71 −1.88 49.31 X3LYP −4.98 10.97 −3.18 13.95 PBE −5.22 6.81 −3.83 −1.04 Fig. 2 Results obtained for EHOMO and ELUMO of ICBA and its derivatives. The data for PCBM was also inserted. The dashed lines indicate the position of the ICBA values Struct Chem (2017) 28:1133–1140 1135 values around 1.9 eV, equal to that for the ICBA, except the derivatives ICBA-N(CH3)2, ICBA-SH3, and ICBA-NH2, with an Eg of 1.16, 1.76 and 1.58 eV, respectively; PCBM has an Eg of 1.8 eV, narrower than that for the ICBA and most of its derivatives. The Eg of the acceptor material is also important for the solar cell, because once they are exposed to the solar spectrum, their excited electrons can also contribute to the photocurrent [37, 39]. For this fact, we note that the deriva- tives ICBA-N(CH3)2, ICBA-SH3, and ICBA-NH2 will be bet- ter than the ICBA and PCBM, since the photons of lower energy may be absorbed so that, together with the donor ma- terial, a wider range of the solar spectrum can be used. It is known from literature that the variation of EHOMO and ELUMO, and consequently the Eg, is closely related to the electron-releasing or electron-withdrawing ability of the sub- stituent [9, 10, 37]: electron-withdrawing substituents tend to stabilize the energies of the frontier orbitals, whereas electron- releasing substituents bring destabilization. For the ICBA dervatives, we note that this trend is confirmed, but the vari- ations obtained were not as significant as expected. This hap- pens because the substitutions in fullerene derivatives are usu- ally made at locations far from the electron density of the frontier orbitals, which are located completely in the fullerene, as can be seen in Fig. 3a, where the HOMO and LUMO orbitals of the ICBA are presented. In the case of polymers, for example, it was verified in previous studies that the varia- tions in the electronic properties due to chemical substitutions are in fact much more significant [9, 10, 12], once the substit- uents can interact directly with the electron density of the frontier orbitals, causing major changes in EHOMO and ELUMO energies. We also noted in the ICBA derivatives, that substituents with strong electron-releasing character (e.g., NH2 and N(CH3)2) are able to displace the HOMO orbital region to the indene group (see the frontier orbitals in Fig. 3b, c), but without major changes in the LUMO. In the case of substituents with a strong electron-withdrawing char- acter, as the CN substituent, the frontier orbitals remain con- centrated in the fullerene, as we can see in Fig. 3d. While chemical modifications in the ICBA bring only slight changes in EHOMO and ELUMO energies, we realize that for the dipole moment (DM) they produce a considerable ef- fect. It is known that the DMof the materials used in the active layer of the solar cell influences the morphology and the sol- ubility of the material (and consequently the FF), as well as in the charge transfer between the donor and the acceptor mate- rials [40, 41]. Then, the larger the DM, the better these prop- erties will be. Because of the high symmetry of the ICBA, it is expected that it has a low DM, a fact that influences in its low solubility verified experimentally [42, 43]. Figure 4 shows the theoretical DM obtained for ICBA and its derivatives.We also calculated the theoretical DM of the PCBM with the same methodology employed for the ICBA, as described in section 2; the value was also inserted in Fig. 4. As we can see, indeed the ICBA has a low DM, close to 0 Debye; in the case of ICBA derivatives, as expected, all modify the symmetry and the charge distribution in the system inducing a larger DM. The DM of the derivative ICBA-CN was the largest of all, suggesting to be a good candidate to be used in solar cells to improve the morphology and charge transfer. We note that PCBM also has a DM larger than that for the ICBA and that is why it is observed in the literature that the best morphol- ogies for the active layers are achieved using PCBM [44]. Overall, the structures that have been inserted substituents with strong electron-releasing or electron-withdrawing char- acter showed the largest DM of all the substituents employed. Analysis of the solar cell performance parameters for active layer that could employ P3HTor PTB7 combined with ICBA derivatives To evaluate the real usefulness of the ICBA derivatives, it is interesting to make an analysis of how would be its perfor- mance in active layers of solar cells. In this section we will assess whether it could be attractive to use some ICBA deriv- atives in combination with P3HT or polythieno[3,4-b]-thio- phene-co-benzodithiophene (PTB7), the electron donor mate- rials most employed in solar cells [45]; this step is important, because it is known that the energies of the frontier electronic levels of the acceptor (A) and the donor (D) materials have a direct correlation with the parameters of efficiency of organic solar cells [37]. Figure 5 shows an energy scheme of the frontier electronic levels of the donor and acceptor materials in the active layer of the solar cell. Photons absorbed by the donor material generate Fig. 3 Kohn–Sham orbital representation (isovalue surface of 0.015 au) of LUMO (up) and HOMO (down) for a ICBA, b ICBA-NH2, c ICBA-N(CH3 )2, and d ICBA-CN 1136 Struct Chem (2017) 28:1133–1140 electron-hole pairs (excitons) that must be dissociated to cre- ate free charge carriers [35, 37]. The exciton dissociation will only occur if the difference ΔELL between the energy of the LUMO of the donor (ELUMO,D) and the energy of the LUMO of the acceptor (ELUMO,A) is at least equal to the exciton bind- ing energy (Eb) of the electron donor material [9, 37]. When the exciton is dissociated, the hole remains in the donor ma- terial and the electron goes to the acceptor material. In similar way, to prevent the hole that remained in the donor material recombine with an electron in the acceptor material, we need to ensure that the differenceΔEHH between the energy of the HOMO of the donor (EHOMO,D) and the energy of the HOMO of the acceptor (EHOMO,A) is also at least equal to Eb [35, 37]. The VOC in the device is proportional to the energy difference ΔEHL between EHOMO,D and ELUMO,A [36, 37]. The values obtained for the VOC in the device also depend on other factors related to the morphology of the active layer [36, 37, 46], but it is still experimentally shown that the higherΔEHL , the higher VOC can be achieved [35, 37]. Thus, the adjustment of the frontier electronic levels concurrently with the parameters ΔELL,ΔEHL, andΔEHH are very important to achieve a more efficient OSC. Fig. 6 shows the results forΔELL,ΔEHL, andΔEHH in the combinations performed with ICBA derivatives as electron acceptor and P3HT and PTB7 as electron donors (these data are also shown in Tables S3 and S4 in the Electronic Supplementary Material); combinations with PCBM are also presented. The values for EHOMO and ELUMO of P3HT and PTB7 employed in these analyzes were, respectively, −5.1 and −2.9 eV [47] and −5.2 and −3.3 eV [48]. The parameters ΔELL and ΔEHH will be compared with the Eb of P3HT and PTB7, which are approximately 0.3 and 0.2 eV [49, 50], respectively. As can be seen in Fig. 6a, b, with exception of the deriva- tives ICBA-NH2 and ICBA-N(CH3)2, the parameters ΔELL and ΔEHH of the majority of the cases studied had the Eb higher than that of P3HT and PTB7, indicating that in general there will be no problems related to exciton dissociation and recombination in the active layer. The derivatives substituted with NH2 and N(CH3)2, which are those with strong electron- releasing character, have ΔEHH < Eb due to EHOMO values very close to or above those of P3HT and PTB7. The values obtained for the parameters ΔEHL indicate that all ICBA de- rivatives studied would tend to have a higher VOC in the de- vice than the PCBM in combination with P3HT or PTB7; however, only the derivatives substituted with OH, NH2, SCH3, CH3, N(CH3)2, and OCH3 would present a higher VOC than that obtained with the ICBA. As the derivatives ICBA-NH2 and ICBA-N(CH3)2 presented problems in the parameter ΔEHH, to enhance the VOC without causing prob- lems in the exciton dissociation and recombination, the best derivatives to be employed would be the ICBA-OH, ICBA- SCH3, ICBA-CH3, and ICBA-OCH3. If we look again in the results shown in Fig. 4, we note that the cases that had the best compromise with the parameters ΔELL,ΔEHL, andΔEHH also had a dipole moment larger than that for ICBA; thus, these materials would tend to have better morphologies (facilitating the charge transport) than ICBA, favoring an increasing of the solar cell efficiency when com- bined with P3HT or PTB7. However, there are others ICBA derivatives with larger dipole moments that also would tend to Fig. 4 Dipole moment (in Debye) of ICBA and its derivatives and PCBM. The dashed lines indicate the ICBA value level Fig. 5 Energy levels sketch for acceptor and donor materials and the parameters that influence the efficiency of the organic solar cell Struct Chem (2017) 28:1133–1140 1137 have a decreased VOC than that for the ICBA; this is the case of the ICBA-CN, that presented the largest dipole moment compared to other derivatives, but with a smaller ΔEHL than ICBA. These derivatives could be used in solar cells due to the opportunity to get better morphologies, but already knowing in advance that a lower efficiency would be achieved in rela- tion to ICBAwhen combined with P3HT or PTB7. It is worth emphasizing that in this section we made estimates for the parameters that have a direct influence in solar cell efficiency when we are using ICBA derivatives in combi- nation with P3HT or PTB7 as electron donor material in the active layer. If the same analyzes were performed in combina- tion with other electron donor materials, possibly other ICBA derivatives could draw attention to the use of the active layer. Conclusions We studied theoretically the electronic structure of 13 new derivatives of ICBA for applications in active layers of solar cells in which chemical substitutions were performed in one of the indene group. The dipole moment of the ICBA was sig- nificantly modified, which favors a good morphology and charge transport in the active layer, without losses in the elec- tronic properties. The largest dipole moments were obtained for the derivatives substituted with groups with strong electron-releasing or electron-withdrawing character, for ex- ample, CN, NH2, and N(CH3)2. In active layers that combine P3HT or PTB7 as donor ma- terial with ICBA derivatives, it was verified that overall there would be no problems in the exciton dissociation and recom- bination; this happened because the parameters ΔEHH and ΔELL were higher or near to the Eb of P3HT and PTB7, except for the derivatives ICBA-NH2 and ICBA-N(CH3)2. Furthermore, the derivatives ICBA-OH, ICBA-SCH3, ICBA-CH3, and ICBA-OCH3 would tend to have a higher VOC in the device than the active layers using ICBA and PCBM, since they had the largest ΔEHL; these derivatives could be indicated in order to increase the solar cell efficiency. If we perform the same analysis using other donor material, possibly others ICBA derivatives could be feasible for use. In our opinion, these ICBA derivatives can have a great impact in OSC performance since our calculations suggest that they can enhance the morphology and the open circuit voltage, both crucial parameters for solar to electric energy conversion efficiency. Fig. 6 Obtained values for ΔEHH, ΔEHL, and ELL of ICBA derivatives and PCBM in combination with a P3HT and b PTB7. The dashed lines indicate the P3HT/ICBA and PTB7/ICBA values level.ΔEHH and ΔELL should be greater than Eb 1138 Struct Chem (2017) 28:1133–1140 Acknowledgments We would like to thank the Brazilian agency FAPESP (proc. 2012/21983-0 and 2014/20410-1) for financial support. This research was also supported by resources supplied by the Center for Scientific Computing (NCC/GridUNESP) of the São Paulo State University (UNESP). References 1. Chi D, Qu S,Wang Z,Wang J (2014) High efficiency P3HT:PCBM solar cells with an inserted PCBM layer. J Mater Chem C 2:4383 2. 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