Theoretical Study of Core Excitations of Fullerene-Based Polymer

Article pubs.acs.org/JPCC

Theoretical Study of Core Excitations of Fullerene-Based Polymer Solar Cell Acceptors Xiuneng Song,† Weijie Hua,*,† Yong Ma,†,‡ Chuankui Wang,‡ and Yi Luo†,¶ †

Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, S-106 91, Stockholm, Sweden ‡ College of Physics and Electronics, Shandong Normal University, Jinan, Shandong 250014, P. R. China ¶ National Synchrotron Radiation Laboratory and Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ABSTRACT: First-principles simulations for the C K-edge Xray photoelectron (XPS) and near-edge X-ray fine-structure (NEXAFS) spectra of representative polymer solar cell acceptor (PSCA) molecules have been carried out with special focuses on the [6,6]-phenyl-Cn+1-butyric acid methyl ester (PCnBM, n = 60, 70, 84). In the XPS spectra, evident red shift of core binding energies are observed in PCnBM as compared to the corresponding fullerene Cn, which is due to weak electronic charge transfer from the side chain to the fullerene backbone and consequently increased electron screening. Special emphasis is paid to a spectral peak at ca. 284.7 eV, which is caused by resonances of phenyl-ring carbons in the side chain and is important in characterizing the electronic structures of different PSCA molecules. Additionally, we propose a modified version of the building block (MBB) approach for quick estimation of the NEXAFS spectra, which is useful when computational resources are limited or massive systems are under study. This method is based on our component analysis on the side chain and fullerene backbone parts and adds only a further empirical fine-tuning after the conventional BB approach. Our calculated results are in good agreement with available experiments.

1. INTRODUCTION The solar cell, a device to harvest renewable and clean energy, has attracted much attention of researchers owing to the urgent needs for solving the worldwide problems of the energy shortage and environment pollution. Over the past few decades, conjugated polymer solar cells (PSCs)1−7 have offered new possibilities to produce energy from light. In comparison with traditional materials, they possess several outstanding properties, such as being lightweight and easy to fabricate and having low pollution and low cost, just to name a few. Chemically, such a device usually consists of a polymer electron donor [e.g., poly(3-hexylthiophene), P3HT] and a fullerene derivative electron acceptor (e.g., [6,6]-phenyl-C61-butyric acid methyl ester, PC60BM). Various materials have been designed and synthesized in order to achieve high solar cell performance.7−10 A better understanding of the electronic structure of such materials, the most fundamental feature to various properties, is no doubt helpful for revealing the underlying mechanisms and the development of new devices with even higher efficiency. Here we focus our attention on the electronic structure of the representative acceptors (PSCAs) composed of a fullerene cage and a side chain (SC). For example, the most common one, PC60BM, is made up of a C60 backbone and an ester side chain (Figure 1). Replacing either or both components, or adding © 2012 American Chemical Society

one or more side chains (viz, bis- or mutiadducts) has led to a large series of derivatives (see ref 7 for a review and extensive examples) that are good candidates for real applications. Various X-ray spectroscopies are known to be extremely powerful for investigating the electronic structure of matter through core excitations/deexcitations. For example, the X-ray photoelectron spectroscopy (XPS) measures the ionization of core electrons and reflects the chemical states in a quantitative matter, the X-ray absorption spectroscopy (XAS) (or near-edge X-ray absorption fine structure (NEXAFS) spectroscopy) probes the information of the virtual orbitals, and the X-ray emission spectroscopy (XES) maps out the features of the occupied orbitals. Since these techniques are selective to different elements and sensitive to the local chemical environment, they have been routinely used for measurements in molecules, surfaces, and materials.11 Their applications particularly in PSCA molecules, however, are by now in an early stage. To the best of our knowledge, NEXAFS spectra were measured on a few PSCA molecules including PC60BM,12−14 and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM),13 Received: August 7, 2012 Revised: October 19, 2012 Published: October 24, 2012 23938

dx.doi.org/10.1021/jp307834x | J. Phys. Chem. C 2012, 116, 23938−23944

The Journal of Physical Chemistry C

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much from alkenes.11 Here we examine the performance of such a simplified treatment particularly for the PSCA molecules and suggest possible further refinement.

2. COMPUTATIONAL METHODS To investigate the soft X-ray spectra of polymer solar cell molecules, we choose six representative systems under study. As illustrated in Figure 1, these molecules include PCnBM (n = 60, 70, 84) (PC84BM = [6,6]-phenyl-C85-butyric acid methyl ester), two PC60BM-like C60 derivatives ([6,6]-thienyl-C61butyric acid methyl ester, ThC60BM; bis(4-methoxyphenyl)methano[60]fullerene, DPM), and one C60 bisadduct (bis[6,6]phenyl-C 61-butyric acid methyl ester, bisPC60BM). We performed direct calculations (DC) for all these molecules. Meanwhile, to examine the applicability of the building block approach in such systems, we also decomposed the PSCA molecule into the Cn backbone and side chain parts for individual study. Terminal groups were added for the latter part for valence saturation. Specifically, they were simply chosen to be two hydrogen atoms for all molecules, based on our tests for PC60BM. All geometries were optimized at the DFT level with the B3LYP30 functional and the 6-31G(d,p) basis set by using the Gaussian 09 package.31 After obtaining the optimized structures, C K-edge XPS and NEXAFS spectra were computed at the DFT level by the StoBe program.32 We followed the computational procedure and settings as described in our previous works (see, e.g., refs 29, 33−35). Briefly, we first calculate the atom-specific stick spectrum for each nonequivalent carbon center; next, each spectrum is broadened and a summation weighted by their relative abundance leads to the total spectrum. The gradient corrected Becke (BE88) exchange36 and the Perdew (PD86) correlation37 functionals were chosen for the calculations. Functional dependence is not tested in this work, since usually the choice of functionals is only sensitive to computed excitation energies but insensitive to term values and the spectral shape.38−40 Such a combination has been widely used in our group and literatures and has been convinced by good agreement with experiment (see, for example, refs 18, 29, 41−44). The core-excited atom was described by the triple-ζ quality individual gauge for localized orbital (IGLO-III) basis set45 and the triple-ζ plus valence polarization (TZVP) basis set for the rest. In addition, to facilitate the self-consistent field (SCF) convergence, a miscellaneous auxiliary basis set is also set for each atom, and an optional model core potential is used for each nonexcited carbon. In practice, the XPS is evaluated by a Gaussian broadening [full width at half-maximum (fwhm) = 0.4 eV] of the core ionic potential (IP), that is, the energy difference between the core-ionized [i.e., full core hole (FCH)] and the ground states (GS),

Figure 1. B3LYP-optimized structures of PC60BM, PC70BM, PC84BM, ThC60BM, DPM, and bisPC60BM. Symmetry of each molecule is provided below the structure and the number of nonequivalent carbons is included in parentheses. Yellow, sulfur; red, oxygen; black, carbon; gray, hydrogen.

but high quality theoretical NEXAFS calculations are even rare.14 A systematic theoretical study on X-ray spectroscopy of representative systems will be useful for understanding their electronic structure and the assignment of future experimental spectra. Herein, we conduct a theoretical investigation of PC60BM and several representative derivatives, and calculate the carbon K-edge XPS and NEXAFS spectra at the density functional theory (DFT) level. Within a single-electron picture in terms of the Kohn−Sham (KS) orbitals, extensive theoretical X-ray spectroscopies were computed for carbon-based systems, including fullerenes15−20 and derivatives,20−22 and graphene,23 as well as single-walled carbon nanotubes,24 and good agreement with experiment was achieved. In this work, we try to address the structure-spectroscopy relation by careful analysis of the influence of different carbon cages and side chains to the spectra. Another theme of this work is to examine the possibility of quick estimation of the absorption spectroscopy of this family, considering their structural similarity and specialty (relatively loose connection between the cage and the side chain parts). Historically, the so-called “building block (BB) principle”,11,25−28 is usually considered as an approximate method to estimate the spectra of medium- or large-sized systems. It states that the total spectrum of a target molecule could be approximately estimated as a linear combination of the spectra of all existing functional groups. In terms of a solar cell acceptor molecule, it can be considered as building blocks of a carbon cage and one (or several) side chains. The physical foundation lies in relatively weak interaction between the components as well as the locality of the X-ray spectroscopy technique, and thus the performance of this approach varies for different systems. For instance, the C1s NEXAFS spectrum of acetonitrile (CH3CN) looks quite like the summation of the spectra of ethane (CH3 −CH 3) and hydrogen cyanide (HCN),11 and the nitrogen K-shell X-ray absorption and emission spectra of a DNA fragment can be well reproduced by the constituting base pairs.29 By contrast, the BB approach does not work well when the conjugation effect is strong, for instance, the C1s absorption spectroscopy of benzene differs

IP = N − 1 E FCH − NEGS

(1)

With respect to NEXAFS calculations, we used the full core hole approximation in combination with a double-basis technique, where normal basis set described above is used for SCF iterations, and an augmented diffuse basis set (19s,19p,19d) is added for spectral calculations.46 The absorption oscillator strength for transition i → f is given by fif = 23939

2mεif 3ℏ2

(|⟨ψf |x|ψi ⟩|2 + |⟨ψf |y|ψi ⟩|2 + |⟨ψf |z|ψi ⟩|2 )

(2)



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wher ψi,f is the two molecular orbitals (MOs) of the FCH state, and εif = εf − εi is the corresponding orbital energy difference. The summation over x, y, and z components is to account for the random orientation of molecules. Raw spectra were then calibrated by aligning the 1s to lowest unoccupied molecular orbital (LUMO) transition to that obtained from a ΔSCF approach.47,48 Stick spectra below the IP were convoluted by a Gaussian line shape with fwhm of 0.4 eV. In the continuum region, the photonionization cross section is obtained by the Stieltjes imaging (SI) method49−51 which is a moment-theorybased technique to reconstruct the spectral distribution from the discrete excitation energies and moments. Finally, a uniform shift of +0.2 eV was finally employed to the XPS or NEXAFS spectra to account for the relativistic effect due to the introduced core hole.47 It is noted that for fullerene Cn, only a few (1, 5, and 11 for C60, C70, and C84, respectively) atomic-specific calculations need to be performed due to their possessed symmetries (Ih, D5h, and D2d, respectively). These symmetries are lowered for all PSCA molecules. Since direct calculations require separate treatment for each nonequivalent carbon, more atom-specific calculations are performed (see Figure 1). For an extreme example, PC70BM has no symmetry and totally 82 individual spectral calculations were performed, while a building block calculation, which fully takes advantage of the preobtained result of the corresponding fullerene backbone, only requires a few underlying calculations for carbons in the side chain and therefore greatly reduces the computational cost. Meanwhile, the gain of efficiency meets with the loss of the accuracy since the conjugation effect between the components are completely neglected. Here we use the term “conjugation” as proposed by Stöhr11 to describe the electron “delocalization”. In an effort to improve the accuracy, a modified version of the BB approach (denoted as the MBB approach for simplicity) is suggested. This new method simply introduces an empirical correction to the BB spectra to approximately account for the influence of the conjugation effect, which will be presented in section 3.2.3.

Figure 2. Calculated XPS of PCnBM (n = 60, 70, 84) in comparison with those of Cn. The energy shifts of main peaks, as indicated by δ, are −0.3, −0.2, and −0.3 eV for PC60BM, PC70BM and PC84BM, respectively. Schematic molecular structure of polymer solar cell molecules is given, and new features (as compared to naked fullerene cages) are assigned.

value for the sum of Mulliken charges in the Cn cage part, which are −0.1e, −0.2e, and −0.2e for PC60BM, PC70BM, and PC84BM, respectively. Interestingly, phenyl carbons in the side chain exhibit very close IP values with those in the Cn cage and make contributions to the main peak, which can be attributed to their similar bonding types. Two carbons in the butyl middle chain (labeled by 2, 5), as well as the two carbons in Cn which has a direct bonding with the side chain (labeled by 6, 7) show IPs of some 0.8 eV above the main peak (at ca. 290.4, 290.4, and 290.3 eV, respectively). It is noted that carbons 6 and 7 are changed from sp2 (in Cn cages) to sp3 (in solar cell molecules) hybridizations which causes their blue shift as compared to the other fullerene carbons. All PSCA molecules exhibit a peak at 292.0 eV, which comes from the carbon in the -OCH3 group (labeled by 1′) Here the same value is simply because they are spatially far from the cage backbone and thus less influenced. Similarly, carbon atoms in the carboxyl groups (labeled by 1) show a peak at 293.0 eV. It is the largest IP value among all carbons, which results from the strong pulling of electrons from the two oxygens. Our calculated results demonstrate that different chemical environment have distinct influence to the IPs of carbons. 3.2. Near Edge X-ray Absorption Fine Structure. 3.2.1. Direct Calculations. In Figure 3, we plot the calculated C1s NEXAFS spectra of PCnBM with n = 60, 70, 84 (panel b) as compared to those of fullerene Cn (panel a). Available experiments13,15,16 are also recaptured for comparison in both panels, and major spectral features are summarized in Table 1. Generally, each theoretical profile matches well with the measured spectrum except a uniform underestimation of the excitation energies by ca. 0.2−0.3 eV. To have a better comparison among different molecules, in this work, we always keep the originally calculated data (i.e., without doing any further alignment with experiment). Panel a simply illustrates the fact that different fullerene molecules have distinctive X-ray

3. RESULTS AND DISCUSSION For simplicity, the following discussions on XPS (section 3.1) and NEXAFS spectra (section 3.2) will focus only on the PCnBM molecules (n = 60, 70, 84). With them, the influence of the cage size is also partially considered. Then in section 3.3, the ThC60BM, DPM and bisPC60BM, serve as test systems to assess the modified building block approach for NEXAFS computation. Since these three systems are all C60-based derivatives, the influence of different side chains can thus be highlighted. 3.1. X-ray Photoelectron Spectra. The XPS spectra of the PCnBM molecules are shown in Figure 2, as compared with corresponding Cn molecules. One can see that C60 only exhibits a single peak at 290.0 eV. With the increase in size and decrease in symmetry, the main peak is slightly red-shifted to 289.8 eV (289.6 eV) in C70 (C84), and a shoulder structure (a weaker peak) is observed at 290.1 eV. Generally, the introduction of the side chain leads to a small red shift (ca. 0.3, 0.2, and 0.3 eV for n = 60, 70, and 84, respectively) of the IPs of the cage carbons, as indicated by δ in the figure. Relatively smaller core binding energies in PCnBM can be ascribed to the increased screening of the K-shell electrons by the outer-shell electrons,52 since weak electronic charge transfer from the side chain to the fullerene backbone happens. This is verified by the negative 23940



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absorption spectra although similar C−C and CC alternating structures. Especially, C84 has a comparatively lower first absorption region (peak a, 283.9 eV) than the other smaller ones (284.2 eV). By comparing corresponding data in the two panels, we find that the attached side chain introduces a new feature b at some 284.7 eV in the spectra of PC60BM and PC84BM, which are contributed by transitions from the phenylring carbons. The same happens to PC70BM, but since C70 already has a 1s to π* resonance in this region, visually this peak b is relatively strengthened. Results show that the introduced side chain adds noticeable perturbation to the unoccupied-level electronic structure as compared with the fullerene backbone. To further understand the spectral features, it is worthwhile to analyze the contributions from different functional groups. For example, the individual contributions of the side chain and fullerene backbone of PC60BM are drawn in Figure 4a. One can clearly see that the peak b in the total spectrum comes solely from the side chain contribution at 284.7 eV (denoted by star), which is assigned as transition from phenyl-ring C1s to LUMO in each atom-specific calculation. In a recent work, Bazylewski et al.14 conducted a combined theoretical and experimental NEXAFS study for this molecule. Their computed spectra at a similar theoretical level showed a contribution from the C60 backbone which coincides with the transitions from the phenylring carbons. This is suspected due to the inconsistency of their reported total and functional-group-specific spectra. It turned out that this experimental shoulder feature b at 284.8 eV (appears at ca. 285.1 eV and labeled as A in their work) was incorrectly associated to transitions from the carbon atoms around the side chain attachment, while the phenyl ring contribution was assigned to the experimental peak c at 285.8 eV. Although an optimized geometry with C1 symmetry was obtained which is different from our structure with Cs symmetry, this should not be the reason for such a large discrepancy in the transition energy. In this figure, our calculations also show that peaks a, c, and d are purely features from the fullerene cage, while peaks e are contributed from

Figure 3. Comparison of calculated (red) and experimental (black) C1s NEXAFS spectra of (a) Cn and (b) PCnBM (n = 60, 70, 84). The experimental data of C60 and C70 is taken from refs 15 and 16, respectively, and those of PC60BM and PC70BM are from ref 13. Major features are labeled.

Table 1. Calculated Excitation Energies (eV) for Significant Features in the C1s NEXAFS Spectra of Cn and PCnBM Compared with Corresponding Experiments Where Available a C60 PC60BM C70 PC70BM C84 PC84BM

284.2 284.5 284.3 284.5 284.2 284.4 284.2 284.4 283.9 284.0

b

c

d

e

method

284.7 284.8 284.9 285.2 285.0 285.2

285.5 285.8 285.5 285.8 285.9 286.3 285.9 286.4 285.2 285.3

286.0 286.3 286.0 286.2 287.2 287.6 287.6 287.6 285.8 285.8

287.8 288.2 287.8 288.4 288.2 288.6 288.3 288.6

calcd expt15 calcd expt13 calcd expt16 calcd expt13 calcd calcd

284.7

Figure 4. C K-edge NEXAFS spectra of PC60BM: (a) Theoretical total spectrum by direct calculation (blue solid) and contributions from the C60 (blue dash) and side chain (blue dot) components in comparison with experiment13 (black solid). (b) Calculated total spectrum by the building block approach (red solid) as well as spectra of C60 (red dash) and isolated side chains (red dot) with three possible terminal groups at carbon 5 (chemical structure illustrated near each curve). Major features are labeled and stars denote transitions from phenyl ring carbons. 23941



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cage. It has a value of 0.3 eV for the C60, while it increases slightly for bigger cages due to a stronger conjugation effect (i.e., the electron becomes more delocalized), for instance, 0.4 and 0.5 eV for C70 and C84, respectively. This is quite empirical and is only to provide a quick estimate for various fullerenebased solar cell acceptor systems. The computed spectra for PCnBM by the three approaches are shown in Figure 5. We find

both the cage and carbons in the butyl middle chain. The situation is similar for PC70BM and PC84BM, except that in the former molecule the carbon backbone also contributes to the peak b, as we mentioned above. 3.2.2. Building Block Approach. Figure 4b displays the theoretical C1s NEXAFS spectra of PC60BM from the building block approach, that is, a simple addition of the spectra of respectively optimized isolated side chain and C60, respectively. It is worth noticing that the side chain needs to be terminated at the C5 position for valence saturation. In practice, we have tested three kinds of terminals: two hydrogens, two methyl groups, and one cyclopropyl group. In this figure, one can see that each spectrum all exhibits a strong phenyl-ring resonance at 285.0 eV. Meanwhile, a very small difference is observed in the high-energy region at ca. 287−289 eV, which only has tiny influence to the peak e in the predicted total spectra. Calculations demonstrate that the spectra are not sensitive to different choices of terminal groups, therefore, all BB calculations hereafter employs the simplest hydrogen terminals. By comparing the spectra to those in Figure 4a, one can see that the BB approach acceptably reproduces the direct calculated result. One evident disagreement is peak b, which is blue-shifted by 0.3 eV (from 284.7 to 285.0 eV). Visually, it moves from the high-energy shoulder of peak a to the lowenergy shoulder of peak c. Such a difference lies in the influence of fullerene cage to the phenyl ring resonance. Moreover, this shift of phenyl ring resonance in the spectra of the isolated and embedded side chain reaches 0.4 and 0.5 eV for PC70BM and PC84BM, respectively. The increasing shift shows that a largersized fullerene also has stronger influence to the electronic structure of the side chain. Except such a shift, the spectra of the isolated SC (panel b) and that contributed from the SC component in PCnBM (panel a) look quite similar. This is ascribed to small deformation of the isolated and embedded SC. This is verified by the deformation energy of the side chain which is only 3.4, 1.0, and 3.5 kcal/mol for PC60BM, PC70BM and PC84BM, respectively. Here the deformation energy refers to the energy cost of the side chain from the gas-phaseoptimized geometry to the geometry extracted from PCnBM.53,54 By contrast, the Cn cage suffers larger geometrical changes, and the deformation energy reaches 28.5, 31.9, and 25.2 kcal/mol respectively for the three systems, and structurally, the bond lengths of fullerene carbons which locate near the attachment are stretched by ca. 0.1−0.2 Å. In the case of PC60BM, the deformation leads to a breaking of the original Ih symmetry. Consequently, each individual nonequivalent cage carbon presents distinctive profile in the regions of peaks c and d. As a result, the two peaks present a different relative intensity from both approaches. Interestingly, when comparing the calculated spectra of PC60BM with experiment, the BB approach happens to predict this feature much better than that from direct calculation. It suggests that our direct calculation in the gas phase overestimates the geometrical deformation of the C 60 backbone. The intermolecular interactions in the condensed phase (thin film as in the experimental condition) might be the reason that keeps more geometrical similarity than in the gas phase. 3.2.3. Building Block Approach, Modified Version. Guided from the above discussions, we propose a modified building block approach which can better predict the C1s NEXAFS spectra of such systems. In practice, this is simply realized by manually red-shifting the excitation energies of the isolated side chain by a value δ which depends on the size of the fullerence

Figure 5. Theoretical C1s NEXAFS spectra of PCnBM (n = 60, 70, 84) by direct calculation (red), the conventional (blue) and modified (green) building block approaches. Major features are labeled.

that the spectral profiles of PC60BM and PC84BM are effectively improved by the modified rather than by the conventional BB approach (using the direct calculations as a standard). With respect to PC70BM, two approaches exhibit similar spectral profiles around peak b with only a few changes in relative peak intensity due to a coincidence of fullerene cage resonance in this region. Results demonstrate the applicability of the MBB approach in extending to a more fullerene-based PSCA systems. 3.3. NEXAFS of ThC60BM, DPM, and bisPC60BM. The above-mentioned three approaches are applied to more systems. Since we have already examined the influence of fullerene cage, three C60-based molecules, ThC60BM, DPM, and bisPC60BM are now studied in order to see the influence of different side chains. As shown in Figure 6, their calculated C1s NEXAFS spectra are compared. From the directly calculated results, one can see that the spectra are sensitive to different side chains at peak b (at ca. 284.7−284.9 eV). ThC60BM has only a thienyl ring, which corresponds to a weak shoulder structure. While there are two phenyl rings in DPM and bisPC60BM, a stronger peak is observed. Relatively small differences are found for other features since they are solely (peaks a, c, and d) or mostly (peak e) contributed from the fullerene cage. Meanwhile, we find that the MBB approach well reproduces the spectra features a, b, and e, which verifies the generality of the method in such systems. The features c and d, however, are not well reproduced by both the BB and MBB approaches. Nevertheless, this can somehow be considered as a systematic error and should not cause trouble when studying 23942



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requires special attention, which is resulted from the resonances of phenyl-ring carbons in the side chain. Since the choice of side chain has more flexibility in the design of PSCA materials, this feature is important in characterizing the electronic structures of different PSCA molecules. Additionally, we have also proposed a modified version of the building block (MBB) approach for a quick estimation of the NEXAFS spectra of the fullerene-based PSCA materials, which is useful when computational resources are limited or when massive systems are under study. This method simply adds an empirical “fine tuning” to the conventional BB approach (to account for the conjugation effect between the cage and the side chain parts) but can effectively improve the feature at ca. 284.7 eV and consequently a better visualization of the whole spectra.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the Göran Gustafsson Foundation for Research in Natural Sciences and Medicine. The Swedish National Infrastructure for Computing (SNIC) is acknowledged for computer time.

Figure 6. Theoretical C1s NEXAFS spectra of ThC60BM, DPM and bisPC60BM by direct calculation (red), the conventional (blue) and modified (green) building block approaches. Major features are labeled.

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the effect of side chain, since within the same approach, the spectra of different molecules in this region look almost the same. Table 2. Comparison of Resolved Excitation Energies (eV) for Phenyl-Ring C1s → LUMO Transitions (See an Example for PC60BM in Figure 4, Denoted as Stars) in the NEXAFS Spectra of a Polymer Solar Cell Acceptor Molecule and in the Corresponding Isolated Side Chain (Terminated with Hydrogens)a molecule

εPSCA phenylC1s→LUMO

εSC phenylC1s→LUMO

Δ

PC60BM PC70BM PC84BM

284.7 284.6 284.5

285.0 285.0 285.0

−0.3 −0.4 −0.5

REFERENCES

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a SC The energies are denoted as εPSCA phenylC1s→LUMO and εphenylC1s→LUMO, respectively, and Δ is their difference.

4. SUMMARY AND CONCLUSION In summary, we have carried out density functional theory calculations for the C K-edge XPS and NEXAFS spectra of six representative polymer solar cell acceptor (PSCA) molecules. The comparison with available experiments shows good agreement. In the XPS spectra, evident red shift of core binding energies are observed in PCnBM (n = 60, 70, 84) as compared with those of fullerene Cn, which is ascribed as weak electronic charge transfer from the side chain to the fullerene backbone and consequently increased screening effect. With respect to the NEXAFS spectra, our direct calculations on representative systems match well with experimental results, which provide a reliable benchmark study for the large family of fullerene-based PSCA molecules. By detailed component analysis, we have assigned each spectral feature to the backbone and side chain parts. It is found that the peak b at ca. 284.7 eV 23943



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Theoretical Study of Core Excitations of Fullerene-Based Polymer

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