Reaction of PC61BM Film with Potassium - The Journal of Physical

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Article 61

Reaction of PC BM Film with Potassium Guang-Hua Chen, Xin-Yuan Bai, Wen-Jie Li, Ying-Ying Du, De-Qu Lin, HaiYang Li, Jiaou Wang, Haijie Qian, Rui Wu, Kurash Ibrahim, and Hongnian Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06346 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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The Journal of Physical Chemistry

Reaction of PC61BM Film with Potassium Guang-Hua Chen,† Xin-Yuan Bai,† Wen-Jie Li,† Ying-Ying Du,† De-Qu Lin,† Hai-Yang Li,† Jia-Ou Wang,‡ Hai-Jie Qian,‡ Rui Wu,‡ Kurash Ibrahim‡ and Hong-Nian Li*,† †

Department of Physics, Zhejiang University, Hangzhou 310027, People’s Republic

of China ‡

Laboratory of Synchrotron Radiation, Institute of High Energy Physics, Chinese

Academy of Sciences, Beijing 100049, People’s Republic of China

1

ACS Paragon Plus Environment


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ABSTRACT: We have studied the reaction of PC61BM ([6,6]-phenyl-C61-butyric -acid-methyl-ester)

film

with

K

atoms

using

photoemission

spectroscopy

measurements and density functional theory calculations. It is found that the molecular structure of PC61BM keeps intact until the intercalation stage of K3PC61BM. This is because that the C60 cage of the molecule attracts the three first intercalated K atoms (per molecule) through the electron transfer from the K 4s states to the LUMO and LUMO+1 orbitals. The fourthly intercalated K atom then detaches the methyl group and bonds with the two O atoms of the molecule. Additional K atoms can still bond with the molecule until the LUMO+2 orbital is filled, and the highest stoichiometry of K-intercalated PC61BM is K7~8PC61BM. The results indicate that the electrode interface of PC61BM-based devices is far from understood.

2


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INTRODUCTION

The fullerene derivative PC61BM1 is one of the most popular electron acceptors adopted in bulk-heterojunction organic solar cells2-6 and has exhibited application in perovskite solar cells.7,8 In these devices reactive metals such as Ca, Mg and Ba have been extensively applied as interfacial materials between PC61BM and electrode for optimizing the contact property.9-13 The frequently-used Al electrode is also reactive to some extent. So the reactivity of PC61BM with metals is a crucial topic for the applications. X-ray photoemission spectroscopy14,15 and near edge X-ray absorption fine structure spectroscopy studies16 have observed some evidences of Al- and Ca-induced degradation of PC61BM. Some authors14,15 suggested that the degradation is related to the detachment of the methyl group of the molecule (see the molecular structure in Figure 3). However, decisive evidence is still desired. If the methyl group can be indeed detached, the detail of the detachment needs in-depth study. Furthermore, the knowledge about the metal-induced electronic state change of the molecule is desirable for designing high-performance electrode interface. In this work we study the reactivity of PC61BM by intercalating a PC61BM film with K atoms and carrying out in-situ photoelectron spectroscopy (PES) measurements. Density functional theory (DFT) calculations are performed to help to understand the experimental data. The chemical activity of K is similar to that of Ca, Mg and Ba, but K may not act as an interfacial material in actual devices because K atoms can very easily diffuse into the bulk of organic film. However, the diffusion is 3



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just advantageous for mechanism study: the reaction could be studied at the level of stoichiometry-dependence. Compared with the samples prepared by spin-coating,14-16 very clean samples in the present work were prepared by evaporation-deposition method in ultra-high vacuum (UHV). Then the reaction between PC61BM and K has been analyzed on the basis of the high-quality core level and valence state PES data. It is revealed that K atom can indeed detach the methyl group as suggested for Ca and Al atoms by the authors of refs 14 and 15, but the detachment only occurs after the intercalation stage is higher than K3PC61BM. As for the electronic states, three lowest unoccupied molecular orbitals (LUMO, LUMO+1 and LUMO+2) are successively occupied by the electrons transferred from the K atoms. The DFT results provide more than needed supports to these conclusions and reveal some electronic information which cannot be readily deduced from the experimental data.

EXPERIMENTAL AND COMPUTATIONAL DETAILS

Experiments were carried out at the Photoelectron Spectroscopy Endstation of the Beijing Synchrotron Radiation Facility. The base pressure of the UHV system was better than 1×10-10 mbar. Thoroughly degassed PC61BM (Solenne company, purity>99.5%) was sublimed from a Ta boat onto an Ag(111) substrate. The substrate was cleaned by standard bombarding and annealing procedure. We evaporated PC61BM with caution to avoid thermal degradation by keeping the temperature of the Ta boat below 280 oC. The relatively low temperature required total deposition time 4


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of 105 min to prepare a PC61BM film almost completely attenuating the Ag 3d core-level photoemission (measured with hυ=700.0 eV). The thickness of the film was estimated to be ~7 monolayer based on the inelastic-mean-free-path (IMFP) data, with the assumption of the IMFP of PC61BM being near the same as that of C60.17 Then we annealed the film at 120 oC for 30 min to improve the crystallinity of the film. K-intercalation was performed step by step with a SAES getter source located at 11 cm from the sample surface. We kept the sample at 100-120 oC during K deposition to facilitate the diffusion of the K atoms into the bulk of the PC61BM film. A Gammadata Scienta R4000 analyzer was used to collect the photoelectrons, and the sample normal coincided with the entrance of the energy analyzer. The transmission mode (wide angle lens, ±19° acceptance angle) of the analyzer was adopted, and the photoelectrons with different emission angles were integrated. Valence state photoemission (hυ=21.2 eV) and core-level photoemission (hυ=700.0 eV) were measured at room temperature for every stage of the sample preparation history. The valence state and 4f7/2 core-level photoemission of an Au film (deposited on the sample holder just before the measurements) was also recorded for the purpose of checking and correcting the photon energies. The total energy resolution of the spectra was better than 0.05 and 0.65 eV in the valence state and core-level measurements, respectively. DFT calculations were performed with the DMol3 package.18,19 The molecular structure of PC61BM inferred from the X-ray diffraction data20 was firstly optimized. Then we constructed and optimized some K-PC61BM complexes. The convergence 5



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criteria for the structural optimization are 10−6 Ha on the energy, 10−4 Ha/Å on the gradient and 10−3 Å on the displacement. We adopted the PBE generalized gradient approximation functional21 and the double numerical basis sets plus polarization function (DNP) in the calculations. The orbital cutoff was 6.0 Å. Self-consistent field procedures were carried out with a convergence criterion of 10−7 Ha on the energy and electron density. Energy levels and densities of states (DOSs) were acquired with single-point energy calculations on the optimized structures. The solid state effect of film sample is ignored because the calculations were performed for isolated molecule or complex.

RESULTS AND DISCUSSION

Figure 1 shows the core level photoemission with increasing K intercalation. The sample of line (13) corresponds to intercalation saturation because the shapes and positions for the top three lines are basically the same. The nominal stoichiometries indicated next to the spectral lines were estimated by calculating the intensity ratios between the K 2p and C 1s signals and taking the photoionization cross sections22 into consideration. The diffusion coefficient of K into the bulk of the PC61BM film surely decreases with increasing K intercalation, which may result in higher K concentration near the sample surface. However, we think that the samples at lower intercalation stages such as K0.2-1.5PC61BM and even K3.1PC61BM are near homogeneous by considering the fact that K can diffuse rapidly into the bulk of C60 film even at room temperature below the stoichiometry of K3C60.23 The substrate temperature of 6


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100-120 oC (during the K deposition) in our experiments further facilitated the diffusion.

Figure 1. (a) K 2p and C 1s photoemission. The values of x indicate the estimated K/PC61BM number ratios of the samples. The spectral lines are normalized to the intensity of C 1s photoemission (including the shoulder for the higher intercalation stages). (b) O 1s photoemission. The spectral lines are normalized to intensity.

The samples at higher intercalation stages are inhomogeneous despite of the substrate temperature of 100-120 oC, as will be revealed by the analyses of the O 1s spectra, and thus the stoichiometries should be considered as the mean stoichiometries in the detection depths of the core level photoemission measurements. The detection depth (three times of IMFP) is ~5 nm for the K 2p and C 1s signals and ~2.5 nm for the O 1s signals if we adopt the IMFP data of C60.17 The mean stoichiometries cannot 7



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be very accurate yet, since X-ray photoemission is not an exact method for quantitative analysis. What is more, there should be excess K atoms on the sample surface for the samples around the intercalation saturation (say, the samples of lines (10)-(15)); the nominal stoichiometries for these samples are most possibly seriously over-estimated. Whatever, the nominal stoichiometries are helpful to the analyses of the spectra. They can at least act as the labels of the samples, let alone the nominal stoichiometries of K0.2-3.1PC61BM should be basically reliable. The C 1s photoemission of the pristine PC61BM film (the bottom line in Figure 1a) centers at 284.95 eV with a very weak satellite peak around 286.95 eV. The position of the main peak changes with increasing K intercalation. Strikingly, the main peak splits at higher intercalation stages. The higher the intercalation stage, the more obvious the splitting is. These observations indicate the strong interaction between PC61BM and K atoms. The interaction will be discussed in more detail on the basis of the O 1s photoemission, the valence state photoemission and the DFT results, and the more specific reasons for the C 1s splitting will be revealed thereafter. In Figure 1b the pristine PC61BM film exhibits two well-separated O 1s peaks. The separated peaks can be easily understood by considering the different chemical environment of the two O atoms of PC61BM. One O atom doubly bonds with a C atom, and the other singly bonds with two C atoms in the phenyl-butyric-acid-methyl -ester side tail (see the molecular structure in Figure 3). In the following we refer to the doubly- and singly-bonded O atoms as Od and Os, respectively. The assignments of the Od and Os PES peaks in Figure 1b are based on our DFT calculations. Lines 8


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(2)-(5) also exhibit the well-separated Od and Os peaks except for rigid shifts towards higher binding energy (BE). The rigid shifts should be mainly due to the Fermi level (Ef) shifts. So there is no indication of the detachment of the methyl group at the lower intercalation stages. From line (6) on, some spectral weight moves to lower BE, and a new feature around 530.80 eV can be clearly seen in line (7). Owing to the very low intensity of the new feature in line (6), we think that the molecular structure of PC61BM keeps intact until the intercalation stage of K3PC61BM. In line (8) another new feature emerges around 532.30 eV. With more and more intercalation of K the original Od and Os peaks eventually disappear, and the spectral weight

merges

to

form

one

asymmetric

peak

at

531.65

eV

for

the

intercalation-saturated sample. The corresponding chemical shifts are -3.35 eV and -1.80 eV for the Os and Od 1s photoemission respectively, in relative to the peak positions in line (6). The large chemical shifts support the detachment of the methyl group and the bonding between reactive metals and the O atoms suggested by the authors of refs 14 and 15. Strictly, however, the large chemical shift of the unresolved Od and Os 1s photoemission14,15 is a strong indication but not a decisive evidence of the detachment of the methyl group. The decisive evidence lies in the electronic state evolution from the resolved Od and Os peaks to one peak in Figure 1b. After the detachment of the methyl group (the detached methyl groups may form ethane gas and escape from the sample), the difference between C=O double bond and C-O single bond disappears. So we can observe the merging of the Os and Od peaks. The merged photoemission locates at much lower BE, indicating the formation of O-K 9



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ionic bonds with the K 4s electrons transferred to the O atoms. Having demonstrated the detachment of the methyl group at higher intercalation stages, we discuss Figure 1(b) in more detail. The most dramatic changes of the spectra occur at the intercalation stages from K3.9PC61BM to K5.7PC61BM (lines (7)-(9)) with the appearance of two new spectral features. There are two possible explanations for the new features. First, the features around 532.30 and 530.80 correspond to the Os and Od photoemission of intact PC61BM, respectively. Second, the feature around 530.80 eV originates from K4PC61B with one K atom bonding with the O atoms, and the feature at 532.30 eV originates from K5PC61B with two K atoms bonding with the O atoms; the two O atoms have identical (or near identical) BE either in K4PC61B or in K5PC61B. Hereafter, we omit the character "M" in the chemical formula if the methyl group has been detached. Our DFT results (shown later) support the second explanation, and we continue the discussion with this explanation. The peak position of K5PC61B (532.30 eV) differs evidently from that of lines (11)-(13), indicating that more than five K atoms can bond with PC61BM. The additional K atoms should again bond with the C60 cage. The bond orders of the former C=O and C-O bonds (2 and 1 respectively) become the same (1.5) after the methyl group is detached, and thus each O atom can at most accept 0.5 electrons to stabilize the 2p shell. One K atom can already provide the necessary electron to fill the 2p orbitals of the two O atoms. Of course, the 4s electron of a K atom (bonding with the O atoms) may not completely transfer to the O atoms because the nearby C60 10


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cage, a strong electron acceptor, most possibly accepts a portion of the 4s electron. Another K atom can still donate some electrons to the O atoms. It is reasonable that two K atoms mainly bond with the O atoms (K5PC61B), but we cannot expect a third K atom bonding with the O atoms. Figure 1(b) also provides some additional information about the sample stoichiometries. The sample of nominal K3.9PC61BM (line (7)) is the mixture of K3PC61BM, K4PC61B and a small portion of K5PC61B. The small portion of K5PC61B is indicated by the notable spectral weight between the peak at 533.45 eV and the feature around 530.80 eV. There are substantial signals of K3PC61BM in line (8) (nominal K5PC61B), and the actual mean stoichiometry of the sample seems no more than K4PC61B. Then the actual mean stoichiometries for the samples of lines (9) and (10) are probably around K4.5PC61B and slightly greater than K5PC61B, respectively. The K concentration in the samples is inhomogeneous at the higher intercalation stages. The actual stoichiometry of the intercalation-saturated sample will be deduced on the basis of the valence state photoemission. Figure 2a shows the valence state photoemission, which provides the direct observation of the electron transfer between the K atoms and the PC61BM molecules. The energy level diagram of PC61BM obtained by our DFT calculations is shown in Figure 2b for helping to understand the experimental data. The spectrum of the pristine PC61BM film exhibits two features labeled with A and B. Feature A corresponds to a bundle of five energy levels (HOMO-HOMO-4 in Figure 2b) split from the 5-fold degenerated HOMO (the highest occupied molecular orbital) of C60. 11



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Feature B contains 13 energy levels, nine from the C60 cage and four from the phenyl-butyric-acid-methyl-ester side tail with some extent of hybridization between them.24 Upon K intercalation new features (labeled with C) emerge in the former gap region (between the Fermi level and ~2.0 eV BE), indicating the electron transfer from K to PC61BM.

Figure 2. (a) Valence state photoemission of the same samples as of Figure 1. The spectra are normalized to the height of peak A. (b) Energy level diagram of PC61BM molecule.

The valence state evolution with increasing K intercalation resembles that of K-intercalated C60 film25 because the electronic states near the HOMO and LUMO levels of PC61BM are derived from the C60 cage.24 Line (6) is somewhat like the literature spectrum of K3C60 measured with hυ=21.2 eV,25.26 and thus the nominal stoichiometry of K3.1PC61BM estimated with the core level PES data is basically 12


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reliable. One difference between K-intercalated C60 and PC61BM systems is that metallic phase (photoemission signal at the Fermi level) is not observed in Figure 2a. Thorough understanding of the valence state PES data is beyond the scope of the present work, and we concentrate our attention on the electron transfer. The position of feature A shifts evidently towards higher BE for line (2) as compared with line (1). This is due to the partial filling of the LUMO orbital by the electrons transferred from the K atoms. The Fermi level is then pinned at the LUMO level. From line (2) to line (6) the positions of features A and B shift back towards lower BE (lines (2)-(6) in Figure 1(a) are also the case), which can be understood by the screening effect of increasing number of electrons occupying the LUMO and LUMO+1 orbitals. The increasing electron transfer is clearly indicated by the rapid increments of the intensity ratio between feature C and A (C/A ratio) from line (2) to line (6). Considering the fact that K 4s electrons almost completely transfer to C60 in KxC60 fullerides,25-27 the LUMO+1 orbital is half filled for the sample of line (6) (K3.1PC61BM). The electron transfer at the low intercalation stages explains the intact molecular structure until K3PC61BM. The C60 cage of the molecule, as a strong electron acceptor, attracts the three first arrived K atoms through the electron transfer. Different from the dramatic changes of the O 1s photoemission at the intercalation stages of lines (7)-(9), the changes of the valence state photoemission are the least at these intercalation stages. Especially, the peak positions keep almost invariant and differ very slightly from that of lines (6) and (10), see the vertical lines in Figure 2a. Actually, the core level and valence state photoemission is consistent: the intercalated 13



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K atoms now mainly bond with the O atoms. The consistency is reflected on the same samples of lines (7)-(9) in Figures 2(a) and 1(b), indicating the detection depths differ not much for the valence state photoemission and the O 1s photoemission. Indeed, it is a consensus that the IMFP of fullerene samples is about one molecular layer (slightly smaller than 1 nm in thickness) for the valence state photoemission measured with the photon energy of 21.2 eV. That is, the detection depth (three times of IMFP) is near that (~2.5 nm) of the O 1s photoemission. The invariant peak positions for lines (7)-(9) indicate that the additional K atoms do not provide sufficient electrons to fully occupy the LUMO+1. Otherwise the spectral peaks would move towards higher BE due to the Fermi level shift. Consistently, the increments of the C/A ratio are small from line (6) to line (9). Features A and C move obviously towards higher BE, and the C/A ratio increases evidently from line (10) to line (13). These observations indicate the occupation of the LUMO+2 orbital. Line (13) is much like the literature spectrum of K6C60 measured with hυ=21.2 eV,25 which is another indication of the occupation of the LUMO+2 orbital. Electronically, the intercalation saturation corresponds to the full occupation of the LUMO+2. The LUMO+3 cannot be occupied because its energy position is too high (see Figure 2b). On the basis of the orbital occupation, the actual stoichiometry of the intercalation-saturated sample can be estimated as follows. The LUMO+1 is half filled for K3PC61BM. Two K atoms bond with the O atoms in K5PC61B, but the O atoms can at most accept one electron as discussed previously. Another electron of the two K atoms thus transfer to the C60 cage. So the LUMO+1 is fully occupied for 14


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K5PC61B. The filling of the LUMO+2 requires two additional K atoms bonding with the C60 cage. The stoichiometry of the intercalation-saturated sample is thus estimated to be K7PC61B. Of course, the electron transfer from the K atoms to either the O atoms or the C60 cage may not be absolutely complete (90-94% according to the Mulliken occupation analyses of our DFT results). So the actual stoichiometry of the intercalation-saturated sample may be slightly higher than K7PC61B, say, K8PC61B. The nominal stoichiometry of K11.2PC61B is due to excess K atoms on the sample surface. Figure 3 shows the DFT results of the isolated PC61BM, K3PC61BM, K4PC61B and K5PC61B molecules or complexes. The results of K1PC61BM and K2PC61BM are not shown here for clarity. We did not perform DFT calculations for Kx>5PC61B because those calculations are too time-consuming; the results of Kx≤5PC61B(M) have been sufficient for understanding the PES data. The solid state effects were neglected in the calculations. PC61BM solid is combined with van de Waals interaction and weak hydrogen bonds.28 So the calculations for isolated molecule can satisfactorily explain the photoemission spectrum.29 Intercalation of K enhances the solid state effects, but the solid state effects on the core levels should not be significant especially for the lower intercalation samples (e.g. K3PC61BM). In K4PC61B or K5PC61B the K atoms bonding with the O atoms are localized to individual molecule and do not result in much more solid state effects as compared with the case of K3PC61BM. The most stable structures of these molecules or complexes are shown in Figure 3a. It is reasonable that the K atom bonding with the O atoms (abbreviated to KO in the 15



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following) locates at a symmetric position with respect to the O atoms in the optimized structures of K4PC61B. The nearest distances between the KO atom and the C atoms of the C60 cage is 3.132 Å, while the nearest distances for the three K atoms bonding with the C60 cage (abbreviated to KC60 in the following) are 2.961, 2.941 and 2.829 Å. The difference in nearest K-C distance between the KO and KC60 atoms is obvious but not too large. So there is weak interaction between the KO atom and the C60 cage. In the optimized structure of K5PC61B one KO atom also locates at a near symmetric position. We tested to place the two KO atoms on the outsides of the O atoms, but the optimization process automatically moved one KO atom to the near symmetric position. The nearest KO-C distances are 2.980 and 3.016 Å, apparently smaller than the case of K4PC61B (3.132 Å) and only slightly greater than the nearest KC60-C distances. So the two KO atoms in K5PC61B strongly interact with the C60 cage, and there must be electron transfer from the KO atoms to the C60 cage, supporting the previous analyses of the PES data. Figures 3b and 3c show the DOS curves of the O 1s and C 1s states. There are large energy differences of the DOS peaks between PC61BM and K3PC61BM, which are mainly due to the shift of the calculated Fermi level. For PC61BM the calculated Fermi level is taken as the HOMO level according to the custom of DFT calculations. For K3PC61BM the calculated Fermi level is at the LUMO+1 level. In fact, the Fermi level of the experimental PC61BM film is very near the LUMO level (rather than at the HOMO level) due to the fact that PC61BM behaves as n-type semiconductor. So the energy differences of the experimental peaks between PC61BM and K3PC61BM 16


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(lines (1) and (6) in Figure 1) are much smaller than that observed in Figure 3. For K4PC61B and K5PC61B the Fermi level also locates at the LUMO+1 level according to the discussion of Figure 2a (and the calculated DOS curves of the valence states which are not shown here). So the energy differences of the peaks between the top three DOS curves in Figures 3b and 3c are chemical shifts and can be compared with the experimental data.

Figure 3. (a) Optimized structures of PC61BM, K3PC61BM, K4PC61B and K5PC61B. The grey, red, white and purple balls represent C, O, H and K atoms, respectively. The blue dotted lines indicate the K-O distances (in Å). (b) DOS curves of the O 1s states. The blue lines are the superimposed 17



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DOSs of the two O atoms. (c) DOS curves of the C 1s states. The red lines are the LDOSs of the side tails, and the dashed lines are the LDOSs of the C60 cage.

In Figure 3b the BEs of the O 1s states of the two O atoms are almost identical for K4PC61B, which have been already implied by the optimized structure: the K atom locates almost symmetrically about the two O atoms. The calculated BE variations of -4.36 and -2.69 eV from K3PC61BM to K4PC61B excellently agree with the experimental data of -4.20 (=530.80-535.00) and -2.65 (=530.80-533.45) eV. For K5PC61B the BEs of the O 1s states of the two O atoms differ slightly (smaller than the resolution of the experimental measurements). The overlapping of the DOS peaks results in one peak with the BE greater than that of K4PC61B by 1.40 eV. We have cited the DFT result of near identical BE for the two O atoms in the analyses of the PES data. The calculated BE change of 1.40 eV coincides excellently with the experimental observation of 1.50 (=532.30-530.80) eV. So it is theoretically verified that the methyl group is detached at the intercalation stage of K4PC61B, and our analyses of the experimental data are reasonable. In Figure 3c the local DOS (LDOS) of the side tail exhibits triplet structure for PC61BM, see the short vertical lines. The strongest peak of the side tail LDOS locates at the lower-BE (higher energy) side of the C60 LDOS by 0.9 eV, which is not resolved in Figure 1a possibly due to the energy resolution (0.65 eV) of our experimental setup being not high enough. The highest-BE peak of the side tail LDOS separates from the C60 LDOS by ~2 eV and was experimentally observed as a very weak feature at ~286.95 eV in Figure 1a. The side tail LDOS steadily moves towards 18


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higher BE in relative to the C60 LDOS for K1PC61BM, K2PC61BM (not shown here) and K3PC61BM, but the triplet structure remains. The strongest peak of the side tail LDOS has been moved to the higher-BE side of the C60 LDOS for K3PC61BM, interpreting the broadening of the C 1s photoemission at the intercalation stage of K3.1PC61BM (line (6) in Figure 1a). This observation can be understood in-depth by the electron transfer from the K atoms to the C60 cage which decreases the BEs of the C 1s states of the C60 cage but has negligible effect on the C 1s states of the side tail. The magnitude of the BE change for the triplet structure from PC61BM to K3PC61BM in Figure 3c is near the same as that for the O 1s states in Figure 3b; these BE changes of the side tail core levels are due to the Fermi level shift. The BE changes of the C 1s states of the C60 cage are decided by the chemical shifts induced by the electron transfer, in addition to the Fermi level shift. So the electron transfer to the C60 cage is one of the reasons for the C 1s splitting observed in Figure 1a. The detachment of the methyl group and the K-O bonding drastically alter the side tail LDOS as can be seen in the DOS curves of K4PC61B and K5PC61B in Figure 3c, but the strongest peak of the side tail LDOS remains at the higher-BE side of the C60 LDOS. So the detachment of the methyl group and the K-O bonding play a minor role in the C 1s splitting; the electron transfer to the C60 cage is the main reason. The splitting is certainly more distinct at higher intercalation stages because more electrons are transferred to the C60 cage.

CONCLUSIONS 19



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Each molecule in PC61BM film can combine with 7~8 K atoms, and the electronic structure of PC61BM plays conspicuous role in the K-PC61BM reaction. The C60 cage of the molecule is a strong electron acceptor, which offers a threshold stoichiometry (K4PC61B) below which the K atoms are attracted by the C60 cage and do not react with the O atoms. The fourthly arrived K atom detaches the methyl group and bond with the two O atoms of the molecule. The O atoms can still attract the fifthly arrived K atom but cannot accept all the 4s electrons of the two K atoms. So the two KO atoms meanwhile bond with the C60 cage. More intercalated K atoms again bond with the C60 cage. The upper limit of the number of K atoms bonding with the molecule is restricted by the full occupation of the LUMO+2 orbital by the electrons transferred from the K atoms. These results have many implications to actual device interfaces. A threshold stoichiometry for detaching the methyl group may also exist for other reactive metals when reacting with PC61BM. So the molecular layers nearest to the Al electrode (or Ca, Ba and Mg interfacial materials) are the mixture of PC61BM and PC61B. The electron transfer up to the occupation of the LUMO+2 orbital implies that the physical properties differ for different molecular layers in the depth range of the diffusion of the metal atoms, even if the molecules are intact PC61BM. These complexities need further studies for high-performance electrode interfaces.

AUTHOR INFORMATION

Corresponding Author 20


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*Tel: 86-571-87952880. Fax: 86-571-87952880. E-mail: [email protected]. ORCID iD: 0000-0002-6148-9669.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China under Nos. 11374258 and 11079028.

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TOC Graphic

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Figure 1. (a) K 2p and C 1s photoemission. The values of x indicate the estimated K/PC61BM number ratios of the samples. The spectral lines are normalized to the intensity of C 1s photoemission (including the shoulder for the higher intercalation stages). (b) O 1s photoemission. The spectral lines are normalized to intensity. 125x94mm (300 x 300 DPI)



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Figure 2. (a) Valence state photoemission of the same samples as of Figure 1. The spectra are normalized to the height of peak A. (b) Energy level diagram of PC61BM molecule. 123x105mm (300 x 300 DPI)


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Figure 3. (a) Optimized structures of PC61BM, K3PC61BM, K4PC61B and K5PC61B. The grey, red, white and purple balls represent C, O, H and K atoms, respectively. The blue dotted lines indicate the K-O distances (in Å). (b) DOS curves of the O 1s states. The blue lines are the superimposed DOSs of the two O atoms. (c) DOS curves of the C 1s states. The red lines are the LDOSs of the side tails, and the dashed lines are the LDOSs of the C60 cage. 167x174mm (300 x 300 DPI)



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34x14mm (300 x 300 DPI)


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Reaction of PC61BM Film with Potassium - The Journal of Physical

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