Cross Self-n-Doping and Electron Transfer Model ... - ACS Publications

Subscriber access provided by UNIV OSNABRUECK

Article

Cross Self n-Doping and Electron Transfer Model in a Stable and Highly Conductive Fullerene Ammonium Iodide: A Promising Cathode Interlayer in Organic Solar Cells Weiwei Chen, Weixiang Jiao, Debing Li, Xuan Sun, Xia Guo, Ming Lei, Qi Wang, and Yongfang Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00214 • Publication Date (Web): 29 Jan 2016 Downloaded from http://pubs.acs.org on January 31, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Cross Self n-Doping and Electron Transfer Model in a Stable and Highly Conductive Fullerene Ammonium Iodide: A Promising Cathode Interlayer in Organic Solar Cells Weiwei Chen,†,‡ Weixiang Jiao,†,‡ Debing Li,† Xuan Sun,† Xia Guo,‖ Ming Lei,*,† Qi ‖ Wang,*,† and Yongfang Li ,§ †

Department of Chemistry, Zhejiang University, Hanzhou 310027, China

‖

Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China §

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China E-mail: [email protected], [email protected] ABSTRACT: Self n-doped stable and highly conductive fullerene ammoniums with

excellent thickness-tolerance could act as promising cathode interlayers to facilitate electron transfer and improve power conversion efficiencies (PCEs) of large-area organic solar cells (OSCs). Herein, systematic studies on electronic and spatial structure of fullerene ammonium iodide (PCBANI), have been performed to elucidate the cross self n-doping mechanism. In PCBANI, partial electron transfer from iodide to core fullerene could result in n-doping and high conductivity. This doping process forms strong anion-π interactions between iodides and fullerene cores accompanied with side-chain’s head-to-tail cation-π interactions that contribute to the stabilization of the n-doped fullerene. Moreover, two possible pathways of the cross self n-doping involving intermolecular exchange and transfer of iodide have been verified by experiment combined with computational modeling. Based on all of the solid evidence, we propose an electron transfer model for PCBANI in which the iodide 1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

sandwiched in the n-doped fullerene core acts as a shuttle to transfer electrons via redox processes. This finding provides a strategy for electrically doping and assembling of fullerenes to improve their performance in photovoltaic devices and endow them with new functionalities that could be applied to optoelectronics and organic electronics.

1. Introduction Much effort have been taken on efficient and stable n-type doping of organic semiconductors to facilitate charge carrier transport both in organic solar cells (OSCs) and organic light-emitting diodes (OLEDs).1-9 Usually, the HOMO level of the dopant must be higher than the LUMO level of the matrix for n-doping; this makes such materials vulnerable to oxidation under ambient conditions.1 The present n-doping processes such as thermal evaporation of small molecules and in situ creation of volatile dopants from precursors need inert environment and have been hindered by aggregation of solid-state dopants.1,4 Therefore, the development of solution processible and stable n-doped electron transport materials for photovoltaic and organic electronic device fabrication is of great challenge. Recently, Lonergan reported solution phase n-doping of fullerene and PCBM with tetrabutylammonium fluoride to form fullerene anions via an initial chemical reaction followed by electron transfer to a second fullerene molecule. They believe the formation of ionic and radical intermediate species has significant implications for the use of ionically functionalized materials as electron-selective interlayers.10 2 ACS Paragon Plus Environment

Page 2 of 28

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We have developed a series of self n-doped stable and highly conductive fullerene derivatives (1.05~1.98 S/m) linked with different number of ammonium groups and –

–

various anions (I and Br ), in which fullerene ammonium diiodide (PCBDANI) acts as a solvent-resistant and thickness-tolerant cathode interlayer to facilitate electron transfer and improve power conversion efficiency of OSCs with different active layers (Scheme 1a).11,12 The inverted polymer solar cells with the structure of ITO/PCBDANI/P3HT:PCBM/MoO3/Ag retained reasonably high PCE even at a thickness of 82 nm of the PCBDANI film as the cathode interlayer.12 Thus, it makes large-area organic photovoltaic device fabrication possible. These excellent properties of PCBDANI should be attributed to its self n-doping which results in intrinsically high conductivity.

3 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

Scheme 1. (a) Molecular structures of the highly conductive fullerene ammonium salts. (b) Molecular structures of FPI. (c) Molecular structures of PCBAN and tetramethylammonium iodide (TMAI).

Jen et al. reported an in situ n-doped highly conductive fulleropyrrolidinium iodide (FPI, Scheme 1b) (2.0 S/m)13 and doping of fullerenes via a solution-processed blend of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and tetrabutylammonium salts to achieve high conductivity in thin films.14 Based on device experiments, they speculate that iodide (in the close vicinity of fullerene core) plays an important role as dopant to “turn on” the fullerene conductivity. They explain the in situ n-doping mechanism by employing density functional theory (DFT) calculations and they believe the compact and zwitterionic structure between the fullerene radical anion and the cationic nitrogen on FPI would stabilize the doped fullerene.13,15 Although the intrinsically high conductivity of the above-mentioned fullerenes is likely related to n-doping, the fine structure of these fullerenes and chemical origin of their high conductivities are not currently understood. Notably, compared with FPI –

–

that iodide is in the close vicinity of fullerene core, the anions (I and Br ) in PCBANI, PCBDANI and PCBDANBr seem far from the fullerene cores since the ammonium salts linked with fullerene cores via long side-chains. It remains unclear how anion dopant accesses fullerene core to form doping. Therefore, we performed systematic studies on the electronic and spatial structure of the fullerene core, iodide dopant and the function of the side-chain of the model compound PCBANI (Scheme 4 ACS Paragon Plus Environment

Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1a) to elucidate the mechanism of the so-called “cross self n-doping”. In order to reveal the structure of this amorphous compound and cross self n-doping mechanism, comprehensive methods were used, such as X-ray photoelectron spectroscopy (XPS), electrochemical and spectroelectrochemical analysis, nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), electronic devices, computational modeling, and experimental verification. This work provides a comprehensive methodology for studying structure-property relationship of n-doped fullerene derivatives. 2. Results and Discussion 2.1. Electronic and Spatial Structure Studies 2.1.1. XPS Survey In n-doped PCBANI, the iodide and fullerene core should act as an electron donor and acceptor, respectively. Thus, XPS survey was used to preliminarily probe the electronic structure and chemical state of the related elements in PCBANI. The XPS spectra are shown in Figure 1. For the iodide in PCBANI, the binding energy increases by 1.1 eV for I 3d3/2, and I 3d5/2 peaks (Figure 1a) suggesting a highly electron-deficient environment around the iodide compared with that in its analogue tetramethylammonium iodide (TMAI, Scheme 1c). The binding energy decreases by 0.4 eV for C1s peaks in PCBANI versus that of its semiconducting precursor (Figure 1b), PCBAN16 (see Scheme 1c) — this manifests an electron-rich environment around fullerene’s carbon. The relevant change in electronic structure of iodide and fullerene core indicates that the electron transfer from the iodide to fullerene core occurred during n-doping. 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. (a) XPS spectra of I 3d3/2 and I 3d5/2 for PCBANI in comparison to TMAI. (b) XPS spectra of C 1s for PCBANI in comparison to PCBAN. The binding energy scale was calibrated using the C 1s line (284.6 eV) from the carbon contamination.

2.1.2. Electrochemical and Spectroelectrochemical Studies Inspired by XPS survey and Ikenoue’s research on the doping mechanism of the self-doped conducting polymer,17,18 detailed electrochemical studies were carefully performed to understand the electronic structure of PCBANI. Figure 2a shows the cyclic voltammogram (CV) of PCBANI measured in 1-butyl-3-methylimidazolium hexafluorophosphate (BMIH) DMSO solution. Three couples of redox peaks were found in the potential range from 0 to -1.60 V vs. a saturated calomel electrode (SCE); the electrochemical reduction potentials are -0.42, -0.84, and -1.43 V vs. SCE, respectively. Fortunately, under electrolysis in the absence of supporting electrolyte BMIH, PCBANI is also electroactive in DMSO aqueous solution. Thus, the AgI precipitation –

method could be used to test for generation of I . When a 1.04 × 10-4 M PCBANI DMSO aqueous solution was electrolyzed at -0.60 V (a potential below the first 6 ACS Paragon Plus Environment

Page 6 of 28

Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reduction peak) versus SCE, pH values of the electrolyte solutions decreased with time during the reduction (Figure 2b; black curve), which indicates the generation of +

H . In a control experiment, AgI precipitation was observed when adding several drops of AgNO3 aqueous solution to the electrolyte solution as shown in Figure S1. Similarly, upon reduction of the PCBANI film on the working electrode, pH values of the electrolyte solution decreased (Figure 2b; red curve) and AgI precipitation was also observed as shown in Figure S1. The result indicates that partial oxidation of I occurred due to the n-doping and that I

–

–

was dissolved in the solution during

reduction.

Figure 2. Electrochemical and spectroelectrochemical study of PCBANI. (a) Cyclic voltammogram of PCBANI (1.33 × 10-4 M) in BMIH (1.23 × 10-2 M, supporting



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

electrolyte) DMSO solution under nitrogen atmosphere at a scan rate of 0.05 V/s. (b) The pH value changes of the solution as a function of time during reduction at -0.6 V vs. SCE. (The reduction of PCBANI films cast from DMSO solution (red curve) and the reduction of PCBANI DMSO aqueous solution (black curve). (c) In situ UV-vis-NIR absorption spectra of PCBANI at different times during the reduction of PCBANI (1.33 × 10-4 M PCBANI 1.84 × 10-2 M BMIH in deoxygenated super dry DMSO using a Pt wire working electrode) at -0.60 V vs. SCE. (d) Equation of electrolysis reduction of PCBANI in DMSO aqueous solution.

An in situ spectroelectrochemical investigation was then performed to study the change of electronic structure of the fullerene core under reduction. When a PCBANI DMSO (anhydrous without oxygen) solution was electrolyzed at -0.60 V vs. SCE, an absorption peak at 1026 nm appeared that is characteristic of the NIR absorption of fullerene anion radicals (Figure 2c).19 The intensity of the absorption increased as a function of time during the reduction. Thus, it revealed the formation of fullerene anion radicals during the reduction. Based on the experimental results, The mechanism for the reduction of PCBANI in its DMSO aqueous solution might be expressed as shown in Figure 2d. So far the results of XPS and electrochemistry studies indicate that a partial redox –

occurred between I and fullerene core and resulted in n-doping of fullerene. It is clearly unlikely that this doping process involves an integral electron transfer from I –

–

to fullerene core. Thus I will oxidize into I atom and subsequently form I2. Therefore, 8 ACS Paragon Plus Environment

Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


partial redox is reasonable and helpful for us to understand the structure and propose the later electron transfer model of PCBANI. 2.1.3. EPR Study To confirm the self n-doping, EPR measurements of PCBANI sample were conducted and the spectra were recorded as shown in Figure S2. The paramagnetic signals were observed in PCBANI solution and film respectively. Therefore, we concluded that the delocalization of valence electrons from iodide to fullerene core resulted in the n-doping based on XPS survy, electrochemical and EPR studies. We believe this doping effect is stronger than normal noncovalent anion-π interactions but weaker than covalent bond interactions,20-22 in which iodide shares valence electron in the molecular orbital with core fullerene propelled by the difference in electron –

affinity between the two counterparts. Under reduction, the generation of pure I and fullerene anion radical agrees well with the changes in their chemical states. 2.1.4. Electron-Only Device Study Electron-only devices23 for PCBANI, PCBAN, and PCBM with the structure ITO/TIPD/Film/Al (TIPD, titanium (diisopropoxide) bis(2,4-pentanedionate)) were tested to make a comparison. In Figure 3, under the same film thickness at 100nm, it shows that the conductivity of PCBANI is higher than that of semiconducting PCBAN and PCBM. This demonstrates that the enhanced charge transfer property is related with n-doping of PCBANI.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. J-V characteristics of the electron-only devices of PCBAN, PCBANI, and PCBM films.

2.1.5. Solution and Solid NMR Studies After we clarified the electronic structure and chemical state of the iodide and core fullerene in PCBANI, conformational structure of the side-chain was studied. Surprisingly, a 2-dimensional (2-D) 1H-1H NOESY (nuclear Overhauser effect spectroscopy) map of PCBANI (Figure 4b, for 1HNMR see Figure 4a) showed clear cross-peak for the protons (9 H1) in the ammonium methyl (-N(CH3)3) coupled through-space with the aromatic protons (H9 and H10). At same time, the H2 and H4 were coupled with the aromatic protons (H9 and H10), which undoubtedly suggests the side-chain’s head-to-tail cation-π interactions.20,24-27 Therefore, the aryl terminated side-chain plays an important role in iodide’s doping of fullerene because it not only links ammonium salt dopant group to the core fullerene within suitable distances, but also stabilizes the ammonium cation via an intramolecular noncovalent cation-π effect that recognizes the terminal aromatic ring.27


Page 10 of 28

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To further investigate the cation-π interactions in the solid state, we performed solid-state NMR (ssNMR) experiments on PCBANI and used

1

H-13C 2-D

heteronuclear correlation (HetCor) spectroscopy to identify the proximity between the benzene ring and the trimethylammonium cation. As expected, the correlation between the -CH2- group next to trimethylammonium (δC13 ~ 64 ppm, δH1 ~ 4 ppm) and the benzene ring (δC13 ~ 130 ppm, δH1 ~ 7.3 ppm) were observed as shown in Figure 4c. These HetCor experiments were performed with a short cross polarization time of 0.1 ms and therefore probe the correlations of short distances (within 2~3 chemical bonds). The ssNMR evidence supports the finding that PCBANI experiences cation-π interactions even in the solid state.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. (a) 1HNMR spectrum of PCBANI in DMSO-d6.

Page 12 of 28

13

CNMR, HMQC and

HMBC spectra are shown in Figure S3-S5. (b) Selected region of 2-D 1H-1H NOESY spectrum of PCBANI with a mixing time of 200 ms. Even with a mixing time of 100 ms, a similar NOESY spectrum was acquired along with some minor noise as shown in

Figure S6. (c) Selected region of solid-state 2-D 1H-13C heteronuclear correlation

(HetCor) NMR spectrum of PCBANI (0.1 ms).


Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Thus, we believe that this doping effect is much stronger than the normal noncovalent anion-π interactions that would cause separation of loose cation-anion pair in the ammonium salt. As a result, ammonium ion accesses the aromatic ring face to stabilize itself through cation-π interactions. This suggests that the noncovalent cation-π interaction contributes to the stabilization of the n-doped fullerene and the coiled head-to-tail side-chain over the fullerene core. Indeed, as an overwhelming electron acceptor, the fullerene core may initiate a doping process by withdrawing the iodide. However, it is not easy for the iodide in PCBANI to get close to the fullerene core in the same molecule to initiate the doping process. Thus, it remains unclear how the fullerene core accesses the iodide. 2.2. Cross Self n-Doping Mechanism 2.2.1. Computational Modeling For further insights of the self n-doping mechanism of PCBANI at the atomic and electronic levels, quantum mechanics calculations were carried out with Gaussian 03. The initial structures before and after self n-doping were proposed and fully optimized at LanL2DZ pseudo-potential for iodide as well as at the 6-31G* level of theory for other atoms. Figure 5a shows the optimized structure with natural bond orbital (NBO) atomic charge distribution on each atom before and after n-doping as well as the charge transfer from iodide to the fullerene core. The major species before and after self n-doping are denoted as A and B, respectively. Figure 5b shows the electron density distribution of the frontier molecular orbitals for major species A and B. The distance from the iodide to the fullerene core is approximately 9.6 Å. The point 13 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

charges on iodide is -0.86 e in species A, but the distance between the iodide and fullerene core is reduced to about 3.0 Å, and the point charges on iodide reduces to -0.61 e in species B. This implies that anion-π interactions and charge transfer (i.e., self n-doping) occurred. This result agrees with the XPS and electrochemical data. Moreover, ammonium ion accesses the benzene ring face of PCBANI to form a cation-π interaction, which agrees with the results of 2-D 1H-1H NOESY and ssNMR. The calculated cation-π interaction energy is about 8.6 kJ/mol, which is approximately 2/3 of the hydrogen bond strength if two water molecules are H-bonded.28 The properties of species B are consistent with the data above that suggest that species B is produced by self n-doping. Because the iodide is far from the fullerene core (9.6 Å) before self n-doping, it is reasonable that the n-doping occurs between two molecules (intermolecular doping) rather than in one molecule (intramolecular doping). Consequently, the mechanism of so-called “cross self n-doping” is logical.


Page 14 of 28

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. NBO atomic charge distribution on each atom before (A) and after (B) cross self n-doping. a) The point charges of iodide in the species A and B are -0.86 e and -0.61 e, respectively. b) Electron density distribution of the frontier molecular orbitals for major species before (A) and after (B) self n-doping.

2.2.2. Cross Self n-Doping Mechanism Based on the experimental facts and computational results, a mechanism for cross self n-doping of fullerene ammonium iodide was proposed and schematically illustrated in Figure 6a. PCBAN first transforms to PCBANI (species A) via a fast methylation step. Once the iodide formed and it was close to the fullerene core of another PCBANI (species A) molecule from the sterically favourable hemisphere, also the atomic net charge distribution favourable area (In general, the upper hemisphere is slightly negative and the lower half is slightly positive as shown in Figure S7), there was an intermolecular iodide doping of the fullerene core accompanied with intramolecular cation-π interaction formation. Therefore, two possible pathways of cross self n-doping were proposed. One of them is that, two molecules of structure A collision results in an exchange of iodine in the molecule. This iodine would form an anion-π interaction with fullerene core. At same time, ammonium ion forms a cation-π interaction with benzene ring face to reduce the potential energy of a single molecule and forms structure B (concerted step). Another pathway is a series of molecules of with A-structure colliding alternately. It results in transfer of iodine between molecules to form structure B (relay step). 2.2.3. Verification of Cross Self n-Doping Process



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

To further experimentally verify the exchange and transfer of iodide in the cross self n-doping process, a quaternization reaction of PCBAN in presence of its analogue PCBPiP with bulky amino group was performed by adding iodomethane dropwise as shown in Figure 6b (for synthesis and characterization of PCBPiP see Figure S8 in Supporting Information). Since PCBAN reacts faster than PCBPiP, in order to observe cross doping easily, excess amount of PCBPiP was used to enclose PCBAN and ensure it could access to iodide formed from quaternization reaction of PCBAN. –

The formation of the complex PCBPiPI •PCBAMN

+

was corroborated in

MALDI-TOF-MS with an isotopically enriched ion of [M+1] 2131.7 m/z as shown in Figure S9 (Supporting Information). This information undoubtedly demonstrated an intermolecular cross doping reaction process. Notably, this indicates an in situ homogeneous reaction strategy for efficient n-doping of the fullerene semiconductors with suitable redox potential.


Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a O

O

N

O

N

I

O

Cross n-doping

MeI

O

N

Fast step

I

O

PCBAN

Cross n-doping

O

I O

O

O

O

N

O

...... N

N

I

I Concerted step

Relay step

Cation-π interaction N O O

PCBANI δ¯

δ¯ I

Anion-π interaction

b N

O O

N

I PCBAN O H3C

+

I

N

O

O O

I

drop wise O O O

4

O

N

N

PCBPiPI PCBAMN+ m/z [M] calcd for C155H51IN2O4 2130.3

Cross n-doping PCBPiP

O N

O

Figure 6. (a) Schematic of the cross self n-doping mechanism. (b) Cross n-doping reaction between PCBPiP and PCBANI.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2.3. Electron Transfer Model for PCBANI Thus far, based on all of the above studies, we propose the electron transfer model for PCBANI. As shown in Figure 7, when applying electric field on PCBANI films, the iodide sandwiched in the n-doped fullerene core oscillates and acts as a shuttle to transfer electrons via redox processes. Herein, the function of iodide in electron transfer process was clarified. In addition, our previous work had demonstrated that the conductivity of PCBDANI (1.98 S/m) is higher than that of PCBANI (1.50 S/m).11 Therefore, it indicates that increasing iodide dopant number and tightly stacking of the doped fullerenes would benefit electron transfer so as to enhance their conductivity.

Figure 7. Electron transfer model for fullerene ammonium iodide.

It is clear that the supramolecular arrangement of conjugated molecules may significantly tune their optoelectronic properties.29,30 Introducing noncovalent interactions combined with molecular shape tuning is an effective alternative strategy for the controlled assembly of fullerenes than solvent-assisted self-assembly31,32 and


Page 18 of 28

Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


crystalline co-assemblies approaches.33 The proposed model also suggests that further improvement of charge transfer with fullerenes through tight and ordered arrangement governed by intermolecular noncovalent interaction, such as cation-π interaction, could be achieved. 3. Conclusions In conclusion, the electronic and spatial structure of highly conductive fullerene ammonium iodide were disclosed by using various methods and proposed cross self n-doping mechanism involving intermolecular exchange and transfer of iodide. Notably, the n-doping process forms a unique molecular structure with a strong anion-π interaction between iodide and core fullerene in concert with the side-chain’s head-to-tail cation-π interaction. The cation-π interaction contributes to the stabilization of the n-doped fullerene. We believe that delocalization of the valence electron from iodide to core fullerene resulted in the cross n-doping and high conductivity. Based on all of the above studies, the function of iodide in highly conductive PCBANI was clarified and the electron transfer model for PCBANI was proposed. This study would shed light onto an in situ homogeneous procedure for efficient n-doping of fullerene. Also, it indicates that increasing iodide dopant number and tightly stacking of the doped fullerenes would benefit electron transfer so as to enhance their conductivity and further improve electron transport both in optoelectronics and organic electronics. More importantly, our observations suggest a strategy for the assembling of fullerene derivatives via the non-covalent interactions. So, the studies on tuning properties of fullerenes through cross n-doping and assembly 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

are in progress. Interestingly, the fullerene ammonium bromide (PCBDANBr) also exhibits same properties and behavior as PCBANI.12 Therefore, studies are underway –

–

on the motivation of other halide ion (Br , Cl ) doping of fullerene.

4. Experimental Section XPS Measurements: XPS examinations of the samples were carried out in a Thermo Scientific ESCALAB 250Xi spectrometer. All spectra were taken using a monochromatic Al Kα (1486.8 eV) X-ray radiation at 180W. The typical operating pressure was 2×10–7 Pa. The binding energy scale was calibrated using the C 1s line (284.6 eV) from the carbon contamination. Cyclic Voltammogram (CV) Measurement of PCBANI: Cyclic voltammogram of PCBANI (1.33 × 10-4 M) in BMIH (1.23 × 10-2 M, supporting electrolyte) DMSO solution under nitrogen atmosphere at a scan rate of 0.05 V/s. Three couples of redox peaks were found in the potential range from 0 to -1.60 V vs. a saturated calomel electrode (SCE); the electrochemical reduction potentials are -0.42, -0.84, and -1.43 V vs. SCE, respectively. Computational Modeling: Quantum mechanics calculations were performed with the hybrid B3LYP functionals. The LanL2DZ pseudo-potential was chosen for iodide and the 6-31G* basis was employed for all other elements. Based on various experimental facts, the initial structures before and after self n-doping were first guessed and then fully optimized. Frequency analyses were performed to confirm the resulting optimized stationary points. To show the quantity of the charge transfer,


Page 20 of 28

Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


atomic point charges were calculated with natural bond orbital (NBO) theory. All calculations were performed using the Gaussian 03 program package.

Supporting Information SI1:Electrochemical reduction of PCBANI (Figure S1) SI2:EPR measurement of PCBANI solution and film (Figure S2) SI3:NMR characterization of PCBANI (Figure S3-S6) SI4:NBO atomic charge distribution on each atom before cross self n-doping (Figure S7) SI5:Synthesis and characterization of PCBPiP(Figure S8) SI6:MALDI-TOF-MS for complex PCBPiPI¯•PCBAMN+ (Figure S9).

Acknowledgements We acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 21442005). We would like to thank Dr. X. Kong and Dr. Y. Z. Jin for their help in analysis of data and manuscript revision, Mrs. L. He for her help in solution state NMR measurements and 1H-1H NOESY NMR control experiments. All solid state NMR experiments were performed at the Beijing NMR Center and the NMR facility of National Center for Protein Sciences at Peking University. We would like to thank Dr. S. L. Wang from Peking University for his help in solid state NMR experiments. We thank Dr. X. W. Chen from Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia for helpful discussion.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

AUTHOR INFORMATION Corresponding Author [email protected], [email protected] Author Contributions ‡

W. C. and W. J.contributed equally to these work.

Notes The authors declare no competing financial interest. References [1] Walzer, K.; Maennig, B.; Pfeiffer, M.; Leo, K. Highly efficient organic devices based on electrically doped transport layers. Chem. Rev. 2007, 107, 1233-1271. [2] Wei, P.; Menke, T.; Naab, B. D.; Leo, K.; Riede, M.; Bao, Z. N. 2-(2-Methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide as a new air-stable n-type dopant for vacuum-processed organic semiconductor thin films. J. Am. Chem. Soc. 2012, 134, 3999-4002. [3] Wei, P.; Oh, J. H.; Dong, G. F.; Bao, Z. N. Use of a 1H-benzoimidazole derivative as an n-type dopant and to enable air-stable solution-processed n-channel organic thin-film transistors. J. Am. Chem. Soc. 2010, 132, 8852-8853. [4] Naab, B. D.; Guo, S.; Olthof, S.; Evans, E. G. B.; Wei, P.; Millhauser, G. L.; Kahn, A.; Barlow, S.; Marder, S. R.; Bao, Z. N. Mechanistic study on the solution-phase n-doping of 1,3-dimethyl-2-aryl-2,3-dihydro-1H-benzoimidazole derivatives. J. Am. Chem. Soc. 2013, 135, 15018-15025.


Page 22 of 28

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[5] Guo, S.; Kim, S. B.; Mohapatra, S. K.; Qi, Y. B.; Sajoto, T.; Kahn, A.; Marder, S. R.; Barlow, S. n-Doping of organic electronic materials using air-stable organometallics. Adv. Mater. 2012, 24, 699-703. [6] Cho, N.; Yip, H. -L.; Hau, S. K.; Chen, K. -S.; Kim, T. -W.; Davies, J. A.; Zeigler. D. F.; Jen, A. K. -Y. n-Doping of thermally polymerizable fullerenes as an electron transporting layer for inverted polymer solar cells. J. Mater. Chem. 2011, 21, 6956-6961. [7] Cho, N.; Yip, H. -L.; Davies, J. A.; Kazarinoff , P. D.; Zeigler. D. F.; Durban, M. M.; Segawa, Y.; O’Malley, K. M.; Luscombe, C. K.; Jen, A. K. -Y. In-situ crosslinking and n-doping of semiconducting polymers and their application as efficient electron-transporting materials in inverted polymer solar cells. Adv. Energy. Mater. 2011, 1, 1148-1153. [8] Russ, B.; Robb, M. J.; Brunetti, F. G. P.; Miller, L.; Perry, E. E.; Patel, S. N.; Ho,V.; Chang, W. B.; Urban, J. J.; Chabinyc, M. L.; Hawker, C. J.; Segalman, R. A. Power factor enhancement in solution-processed organic n-type thermoelectrics through molecular design. Adv. Mater. 2014, 26, 3473-3477. [9] Li, F. H.; Pfeiffer, M.; Werner, A.; Harada, K.; Leo, K.; Hayashi, N.; Seki, K., Liu, X. J.; Dang, X. D. Acridine orange base as a dopant for n doping of C60 thin films. J. Appl. Phys. 2006, 100, 023716. [10] Weber, C. D.; Bradley, C.; Lonergan, M. C. Solution phase n-doping of C60 and PCBM using tetrabutylammonium fluoride. J. Mater. Chem. A 2014, 2, 303-307.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[11]Li, S. S.; Lei, M.; Lv, M. L.; Watkins, S. E.; Tan, Z. A.; Zhu, J.; Hou, J. H.; Chen, X. W.; Li, Y. F. [6,6]-Phenyl-C61-butyric acid dimethylamino ester as a cathode buffer layer for high-performance polymer solar cells. Adv. Energy Mater. 2013, 23, 1569-1574. [12] Jiao, W. X.; Ma, D.; Lv, M. L.; Chen, W. W.; Wang, H. Q.; Zhu, J.; Lei, M.; Chen, X. W. Self n-doped [6,6]-phenyl-C61-butyric acid 2-((2-(trimethylammonium)ethyl)-(dimethyl)ammonium)ethyl ester diiodides as a cathode interlayer for inverted polymer solar cells. J. Mater. Chem. A 2014, 2, 14720-14728. [13] Li, C. Z.; Chueh, C. C.; Yip, H. -L.; Ding, F. Z.; Li, X. S.; Jen, A. K.-Y. Solution-processible highly conducting fullerenes. Adv. Mater. 2013, 25, 2457-2461. [14] Li, C. Z.; Chueh, C. C.; Ding, F. Z.; Yip, H. -L.; Liang, P. W.; Li, X. S.; Jen, A. K. -Y. Doping of fullerenes via anion-induced electron transfer and its implication for surfactant facilitated high performance polymer solar cells. Adv. Mater. 2013, 25, 4425-4430. [15] Guldi, D. M.; Luo, C.; Kotov, N. A.; Ros, T. D.; Bosi, S.; Prato, M. Zwitterionic acceptor moieties: small reorganization energy and unique stabilization of charge transfer products. J. Phys. Chem. B. 2003, 107, 7293-7298. [16] Hong, D.; Lv, M. L.; Lei, M.; Chen, Y.; Lu, P.; Wang, Y. G.; Zhu, J.; Wang, H. Q.; Gao, M.; Watkins, S. E.; Chen, X. W. N-acyldithieno[3,2-b:2′,3′-d]pyrrole based low-band-gap conjugated polymer solar cells with amine-modified


Page 24 of 28

Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[6,6]-phenyl-C61-butyric acid ester cathode interlayers. ACS Appl. Mater. Interfaces 2013, 5, 10995-11003. [17] Ikenoue, Y.; Chiang, J.; Patil, A. O.; Wudl, F.; Heeger A. J. Verification of the "cation-popping" doping mechanism of self-doped polymers. J. Am. Chem. Soc. 1988, 110, 2983-2985. [18] Ikenoue, Y.; Uotani, N.; Patil, A. O.; Wudl, F.; Heeger A. J. Electrochemical studies of self-doped conducting polymers: verification of the ‘cation-popping’ doping mechanism. Synth. Met. 1989, 30, 305-319. [19] Ros, T. D.; Prato, M.; Carano, M.; Ceroni, P.; Paolucci, F.; Roffia, S. Enhanced acceptor character in fullerene derivatives. Synthesis and electrochemical properties of fulleropyrrolidinium salts. J. Am. Chem. Soc. 1998, 120, 11645-11648. [20] Chifotides, H. T.; Dunbar, K. R. Anion-π interactions in supramolecular architectures. Acc. Chem. Res. 2013, 46, 894-906. [21] Salonen, L. M.; Ellermann, M.; Diederich, F. Aromatic rings in chemical and biological recognition: energetics and structures. Angew. Chem. Int. Ed. 2011, 50, 4808-4842. [22] Hay, B. P.; Bryantsev,V. S. Anion-arene adducts: C-H hydrogen bonding, anion-π interaction, and carbon bonding motifs.Chem. Commun. 2008, 2417-2428. [23] Blom, P. W. M.; Vissenberg, M. C. J. M. Charge transport in poly(p-phenylene vinylene)light-emitting diodes. Mater. Sci. Eng. 2000, 27, 53-94. [24] Dougherty, D. A. Cation-π interactions in chemistry and biology: a new view of benzene, Phe, Tyr and Trp. Science 1996, 271, 163-168. 25 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[25] Ma, J. C.; Dougherty, D. A. The cation-π interaction. Chem. Rev. 1997, 97, 1303-1324. [26] Mahadevi, A. S.; Sastry, G. N. Cation-π interaction: its role and relevance in chemistry, biology, and material science. Chem. Rev. 2013, 113, 2100-2138. [27] Pletneva, E. V., Laederach, A. T., Fulton, D. B. & Kostić, N. M. The role of cation-π interactions in biomolecular association. Design of peptides favoring interactions between cationic and aromatic amino acid side chains. J. Am. Chem. Soc. 2001, 123, 6232-6245. [28] Kumar, R.; Schmidt, J. R.; Skinner, J. L. Hydrogen bonding definitions and dynamics in liquid water. J. Chem. Phys. 2007, 126, 204107. [29] Diao,Y.; Shaw, L.; Bao, Z. N., Mannsfeld, S. C. B. Morphology control strategies for solutionprocessed organic semiconductor thin films. Energy Environ.Sci. 2014, 7, 2145-2159. [30] Henson, Z. B. Müllen, K.; Bazan, G. C. Design strategies for organic semiconductors beyond the molecular formula. Nat. Chem. 2012, 4, 699-704. [31] Zhang, J. Y.; Li, C. Z.; Williams, S. T.; Liu, S. Q.; Zhao, T.; Jen, A. K. -Y. Crystalline co-assemblies of functional fullerenes in methanol with enhanced charge transport. J. Am. Chem. Soc. 2015, 137, 2167-2170. [32] Wei, L.; Yao, J. N.; Fu, H. B. Solvent-assisted self-assembly of fullerene into single-crystal ultrathin microribbons as highly sensitive UV-Visible photodetectors ACS Nano 2013, 7, 7573-7582. [33] Li, H. Y.; Tee, B. C-K.; Cha, J. J.; Cui, Y.; Chung, J. W.; Lee, S. Y.; Bao, Z. N. 26 ACS Paragon Plus Environment

Page 26 of 28

Page 27 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


High-mobility field-effect transistors from large-area solution-grown aligned C60 single crystals J. Am. Chem. Soc. 2012, 134, 2760-2765.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of contents Cross self n-doping mechanism and electron transfer model in a stable and highly conductive fullerene ammonium iodide (PCBANI) are proposed. The doping process forms strong anion-π interactions between iodides and fullerene cores accompanied with side-chain’s head-to-tail cation-π interactions. The iodide sandwiched in the n-doped fullerene core acts as a shuttle to transfer electrons via redox processes. W. W. Chen, W. X. Jiao, D. B. Li, X. Sun, X. Guo, M. Lei,* Q. Wang,* and Y. F. Li Cross Self n-Doping and Electron Transfer Model in a Stable and Highly Conductive Fullerene Ammonium Iodide: A Promising Cathode Interlayer in Organic Solar Cells


Page 28 of 28

Cross Self-n-Doping and Electron Transfer Model ... - ACS Publications

Recommend Documents