Supporting Information

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1 Supporting Information 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 Peng Wei, Torben Menke, Benjamin D. Naab, Karl Leo, Moritz Riede, Zhenan Bao*, Department of Chemical Engineering, Stanford University, Stanford, California 94305, U.S. Institut für Angewandte Photophysik, Technische Universität Dresden, George-Bähr-Str. 1, Dresden, Germany zbao@stanford.edu 1. Chemicals S2 2. Instruments S2 3. Synthesis of 2-(2-methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide (o-meo-dmbi-i) S2 4. Full reference citation for ref. 25 in the communication S3 Figure S1. The 1 H-NMR (DMSO, 400 MHz) spectrum of o-meo-dmbi-i S5 Figure S2. 13 C-NMR (DMSO, 400 MHz) spectrum of o-meo-dmbi-i S5 Figure S3. Output characteristics of undoped and o-meo-dmbi-i doped C 60 OTFTs in a glovebox S6 Figure S4. Output characteristics of undoped and o-meo-dmbi-i doped C 60 OTFTs in a glovebox S7 Figure S5. Root-Mean-Square (rms) of the morphology of undoped and o-meo-dmbi-i doped C 60 films (2 μm х 2μm) S7 Figure S6. AFM images of undoped and o-meo-dmbi-i doped C 60 films S8 Figure S7. Mass spectrum (250 ~ 260 amu) measured in situ in the vacuum chamber while evaporating o-meo-dmbi-i at the deposition temperature (180ºC~200ºC).---S9 S1

2 1. Chemicals Divinyltetramethyldisiloxane bis(benzocyclobutene) (BCB) was purchased from Dow Chemicals. Fullerene C 60 was purchased from CreaPhys GmbH. The remaining chemicals and solvents were purchased from Sigma-Aldrich. All of the chemicals were used as received without further purification. 2. Instruments NMR ( 1 H and 13 C) spectra were recorded on a Varian Mercury 400 NMR spectrometer at room temperature. UV-Vis-NIR spectra were recorded with a Varian Cary 6000i UV-Vis spectrophotometer under ambient conditions. X-Ray photoelectron spectroscopy (XPS) spectra were measured with a PHI 5000 VersaProbe system with an Al Kα radiation (1486 ev) source. A Digital Instruments (DI) MMAFM-2 scanning probe microscope was used to perform tapping mode AFM on the samples with a silicon tip of 300 khz frequency. DI Nanoscope software was used to process the raw AFM images. All thin-film transistor measurements were measured with a Keithley 4200 semiconductor parameter analyzer in a N 2 -filled glovebox without encapsulation. In situ film conductivity measurements were performed at a base pressure of 1х10-7 mbar, using a Keithley SMU 236 source measure unit. The sample layout consisted of a glass substrate with two parallel gold contacts of 5 mm distance, 40 nm thickness and a length of 20 mm each, and a voltage of 10 V was used to measure the conductivity. The substrate temperature was regulated to and kept at 25 C during preparation and conductivity measurement. The doped organic layers were produced by thermal co-deposition of matrix and host material, controlling the rates, and thus the doping concentration, by two independent quartz crystal monitors. 30 nm thick layers were produced at a nominal rate of 0.15 Å/s. Mass spectra were measured using a Pfeiffer QMG220 mass spectrometer, attached to the vacuum chamber used to prepare the samples. Base pressure prior to S2

3 processing was 1х10-7 mbar, reaching o-meo-dmbi-i's evaporating temperature (180 C~200 C) the pressure increased to 1~10х10-6 mbar. The scan speed was 20 s/amu. 3. Synthesis of 2-(2-methoxyphenyl)-1,3-dimethyl-1H-benzoimidazol-3-ium iodide (o-meo-dmbi-i) 5.0 g (23.85 mmol) 2-(2-hydroxyphenyl)-1H-benzoimidazole was dissolved in 40 ml 5N KOH and 10 ml methanol solution. Then 20 ml (45.60 g, mmol) methyl iodide (MeI) was added dropwise. Reaction temperature was kept at 45 C with oil bath overnight. Then the precipitated white solid was filtered, and recrystallized from ethanol twice to obtain the pure product, o-meo-dmbi-i (70% yield). 1 H NMR (400 MHz, DMSO) δ/ppm: 8.12 (dd, J = 6.23, 3.14 Hz, 2H), 7.84 (t, J = 7.75 Hz, 1H), 7.45 (d, J = 8.48 Hz, 1H), 7.32 (t, J = 7.54 Hz, 1H), 3.88 (s, 3H), 3.85 (s, 6H). 13 C NMR (400 MHz, DMSO) δ/ppm: , , , , , , , , , , 65.83, Full reference citation for ref. 25 in the communication (24) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,M. A.; Cheeseman, J. R.; Montgomery, J. A. Jr; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cvossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, J. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dappriach, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Nalick,D. K.; Rabuck, A. D.; Raghavachari, J.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; S3

4 Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. A.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalex, C.; Pople, J. A. Gaussian 03, Gaussian, Inc.: Pittsburgh, PA, S4

5 Figure S1. The 1 H-NMR (DMSO, 400 MHz) spectrum of o-meo-dmbi-i. Figure S2. 13 C-NMR (DMSO, 400 MHz) spectrum of o-meo-dmbi-i. S5

6 Figure S3. Output characteristics of undoped and o-meo-dmbi-i doped C 60 OTFTs in a glovebox. S6

7 Figure S4. Output characteristics of undoped and o-meo-dmbi-i doped C 60 OTFTs (V gs = 0 V) in a glovebox. Figure S5. Root-Mean-Square (rms) of the morphology of undoped and o-meo-dmbi-i doped C 60 films (2 μm х 2μm). S7

8 Figure S6. AFM images of undoped and o-meo-dmbi-i doped C 60 films; Left: 10 μm х 10μm), right: 2 μm х 2μm. S8

9 Figure S7. Mass spectrum (250 ~ 260 amu) measured in situ in the vacuum chamber while evaporating o-meo-dmbi-i at the deposition temperature (180ºC~200ºC). S9