High‐Performance Ambipolar and n‐Type Emissive Semiconductors Based on Perfluorophenyl‐Substituted Perylene and Anthracene

Abstract Emissive organic semiconductors are highly demanding for organic light‐emitting transistors (OLETs) and electrically pumped organic lasers (EPOLs). However, it remains a great challenge to obtain organic semiconductors with high carrier mobility and high photoluminescence quantum yield simultaneously. Here, a new design strategy is reported for highly emissive ambipolar and even n‐type semiconductors by introducing perfluorophenyl groups into polycyclic aromatic hydrocarbons such as perylene and anthracene. The results reveal that 3,9‐diperfluorophenyl perylene (5FDPP) exhibits the ambipolar semiconducting property with hole and electron mobilities up to 0.12 and 1.89 cm2 V−1 s−1, and a photoluminescence quantum yield of 55%. One of the crystal forms of 5FDPA exhibits blue emission with an emission quantum yield of 52% and simultaneously shows the n‐type semiconducting property with an electron mobility up to 2.65 cm2 V−1 s−1, which is the highest value among the reported organic emissive n‐type semiconductors. Furthermore, crystals of 5FDPP are utilized to fabricate OLETs by using Ag as source–drain electrodes. The electroluminescence is detected in the transporting channels with an external quantum efficiency (EQE) of up to 2.2%, and the current density is up to 145 kA cm−2, which are among the highest values for single‐component OLETs with symmetric electrodes.


Materials
The reagents and starting materials were commercially available and used as received otherwise specified elsewhere. 2,6-Dibromoanthracene was purchased from TCI. 3,9-Dibromoperylene was purchased from Shanghai Macklin Biochemical Co., Ltd. Pentafluorobenzene was purchased from J&K Scientific Ltd.

Characterization techniques
1 H NMR and 13 C NMR and 19 F NMR spectra were measured with Bruker AVANCE III 500 MHz and Bruker AVANCE NEO 700 MHz spectrometers. Highresolution mass spectral (HRMS) data were collected on either 9.4T Solarix Mass instrument and the mass analyzers are MALDI-FTICR (Fourier transform ion cyclotron resonance). Elemental analyses (EA) were performed on a Carlo-Erba-1106 instrument.
Absorption spectra were recorded on the HITACHI UH4150 spectrophotometer. Emission spectra and lifetime were recorded on the Hitachi FP-6000 spectrometer and Edinburgh FLS980. Photoluminescence quantum yield of solution and solid were measured on the HAAMATSU C11347.
Atomic force microscopy (AFM) images were recorded using a Digital Instruments Nanoscope IIIa multimode atomic force microscope in tapping mode under ambient conditions. Two-dimensional grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were conducted on a Xenocs SAXS/WAXS system with X-ray wavelength of 1.54 Å; the film samples were irradiated at a fixed angle of 0.2 o . The thickness of the film for AFM and GIWAXS measurement was about 40 nm, which was deposited under the vacuum (10 -6 mbar) at the rate of 0.1~0.2 Å/s. The UPS measurement was performed on an Axis Ultra DLD (Kratos, UK) spectrometer with an unfiltered He I (21.22 eV) excitation source. The thickness of the film for UPS measurement was about 15 nm, which was deposited under the vacuum (10 -6 mbar) at the rate of 0.1Å/s. XRD measurements were conducted on a PANalytical Empyrean with X-ray wavelength of 1.54 Å.
The diffraction data for the single crystals were collected with a Rigaku Saturn diffractometer with CCD area detector with Cu at 170 K or 110 K. 5FDPP were obtained by physical vapor transport (PVT) method heating at 260 o C in the high-temperature zone. Crystals-B of 5FDPA were obtained by physical vapor transport (PVT) method heating at 220 o C in the high-temperature zone. Crystals-C was obtained by recrystallization of 5FDPA in chloroform solution. Crystals-G was obtained by cooling crystals-B with liquid nitrogen for 10 s, followed by warming the crystals to room temperature. Crystals-B was obtained by heating crystals-G at 180 o C for 5 s, followed by cooling to room temperature. Crystallographic data reported in this paper were deposited in the Cambridge Crystallographic Data Centre ( The substrates were firstly cleaned by sonication in acetone and water and immersed in Piranha solution (2:1 mixture of sulfuric acid and 30% hydrogen peroxide) and heated to 100 o C for 30 min, followed by rinsing with deionized water and isopropyl alcohol for several times, and they were blow-dried with nitrogen. Then, the substrate was processed by UV ozone for about 10 min. After that, the substrates were placed into a petri dish, and one drop of n-octadecyltrichlorosilane (OTS) was dropped into the middle of the petri dish. The system was stored under vacuum at 125 o C for 4 h to form an OTS self-assembled monolayer. After the substrate surfaces were modified with OTS, they were washed with n-hexane, CHCl 3 and isopropyl alcohol sequentially. These substrates were used directly for fabrication of devices with 5FDPA.

Fabrication of FETs with Thin-films of 5FDPA
Bottom gate bottom contact (BGTC) field-effect transistors were fabricated to explore the semiconducting performance. 5FDPA was slowly deposited on an OTS-treated Si/SiO 2 substrate under a high vacuum (10 -6 mbar) as the active layer with the thickness of 40 nm at the rate of 0.1~0.2 Å/s. Then, an interdigital mask with a channel length of 7140 um and a channel width of 270 um was used. Ag electrode was deposited to the surface with a thickness of 50 nm. After peeling the mask, the BGTC devices were fabricated and used for measurement directly. All these processes were carried out in the glove box.

The device of single crystals
The substrates were firstly cleaned by sonication in acetone and water and immersed in Piranha solution (2:1 mixture of sulfuric acid and 30% hydrogen peroxide) and heated to 100 o C for 30 min, followed by rinsing with deionized water and isopropyl alcohol for several times, and they were blow-dried with nitrogen. Then, the substrate was heated at 80 °C for 1 h under vacuum. After that, the substrate was washed with n-hexane, CHCl 3 and isopropyl alcohol sequentially. A thin layer of PMMA was spin-coated onto the substrates with a speed of 5000 r/min for 50 s. The concentration of PMMA was 1% in toluene. The thickness of PMMA was about 20 nm according to the AFM height image. Then, the substrates were heated to 120 o C for 10 min. After cooling to room temperature, these substrates were used directly for fabrication of devices with crystals of 5FDPP and 5FDPA.

Fabrication of OFETs with single crystals of 5FDPP and 5FDPA
Figure S1 Schematic diagram of the fabrication processes of OFETs with single crystals of 5FDPP and 5FDPA.

Single-crystal OFETs with 5FDPP (step1-step4)
The micro-co-crystals were grown by micro-spacing in-air sublimation of co-crystals when heating to 280 o C for 10 min. S1 After cooling to temperature, the microcrystals were transferred onto the substrates. The copper mesh was used as a mask to the microcrystals. Then Ag was deposited to the surface with a thickness of 100 nm as source-drain electrodes. After peeling the mask, the BGTC single-crystal field-effect transistors based on co-crystals were fabricated and used for measurement directly in the glove box.

Single-crystal OFETs with crystals-B (step1-step4)
The microcrystals were grown by micro-spacing in-air sublimation of 5FDPA when heating to 235 o C for 10 min. S1 After cooling to temperature, the microcrystals were transferred onto the substrates. The copper mesh was used as a mask to the microcrystals. Then, Ag was deposited to the surface with a thickness of 100 nm as source-drain electrodes. After peeling the mask, the BGTC single-crystal field-effect transistors with crystals-B were fabricated and used for measurement directly in the glove box.

Single-crystal OFETs with crystals-G (step1-step5')
The microcrystals were grown by micro-spacing in-air sublimation of 5FDPA when heating to 235 o C for 10 min. S1 After cooling to temperature, the microcrystals were cooled to 78 K in liquid nitrogen for 10 s. After warming to room temperature, the microcrystals were transferred onto the substrates. The green-emission microcrystals were picked up under a microscope and the copper mesh was used as a mask to the microcrystals. Then Ag was deposited to the surface with a thickness of 100 nm as source-drain electrodes. After peeling the mask, the BGTC single-crystal S5 field-effect transistors based on crystals-G were fabricated and used for measurement directly in the glove box.

OFETs measurements
Characteristics of the devices were measured in the glove box using a Keithley 4200 SCS semiconductor parameter analyzer. The mobility of the OFETs in the saturation region was extracted from the following equation: Where I DS is the current collected by the drain electrode; L and W are the channel length and width, respectively; μ is the mobility of the device; C i is the capacitance per unit area of the gate dielectric layer (11.5 nF/cm 2 for film devices and 10.0 nF/cm 2 for single-crystal devices); V G is the gate voltage, and V Th is the threshold voltage.

Fabrication of OLETs with single crystals of 5FDPP
The Single-crystals were prepared by physical vapor transfer method under argon environment. A thin layer of PMMA was spin-coated onto Si/SiO 2 substrate from a PMMA solution (6 mg ml −1 ; chlorobenzene as a solvent) with a speed of 5000 r/min for 50 s in the glove box and then annealed at 90°C for 2 hours. Then large sized single crystals were transferred to the PMMA treated Si/SiO2 substrate by shearing friction in the glove box. The symmetric electrodes Ag (50 nm) or CuPc (20 nm)/Ag (50 nm) were fabricated by thermal evaporation with a mask for hole injection and electron injection, respectively. The PMMA buffer layer (20 nm, see Figure S14) has a negligible effect on the capacitance of the SiO 2 (300 nm), and thus, the capacitance used in the calculation of the mobility of OLET is still 10 nF cm −2 .

OLETs measurements
The electrical characteristics measurement was conducted by PDA FS-Pro 380 under inert atmosphere and the optical image was taken by a Nikon digital camera. The electroluminescence spectra were obtained by a spectrometer made by Ocean Optics. the EQE of EL device is acquired from the number of the collection emissive photons divided by the number of injected carriers according to the following equation:

=
The photocurrent of standard light source collected by PMT can be calculated by the following equation: I PMT =∫ ( ) ( ) = ∫ ( ) · ·̃( ) Where ( ) and ̃( ) are spectral response curve and normalized curve of PMT, respectively. The spectrum of a standard light source P C (λ) is a known value. Then, the value of B can be calculated directly according to the calculation formula of I PMT . The P S (λ) of the sample light source can also be normalized to A·̃S(λ), where the normalized spectrum ̃S (λ) can be measured directly by the spectrometer.
For the sample source, photocurrent of sample source collected by PMT can be calculated by the following equation: I PMT =∫ ·̃S(λ) ·̃S( ) , only A is an unknown quantity, then the optical power spectrum of the sample source P S (λ) can be directly calculated by the formula. Hence, the number of total photons from detected emissive spectra ranging from starting and ending wavelength can be calculated through mathematical integral.
= ∫ ( ) ℎ Where λ is wavelength of the emissive light, in nm. h is Planck constant (6.626 × 10 -34 J s). c is velocity of light in vacuum (3.0 × 10 8 m s -1 ). The number of injected carriers during the test is calculated from the recombination current I at saturation region of OLET devices divided by the elementary charge e = 1.602×10 -19 C. Therefore, the EQE is calculated by following equation: The device structure of the single-crystal OLETs; two electrodes in grey color are on the top of the crystal (in green color); W and L are the channel width and length, respectively.

Crystals-B to Crystals-G transformation
By putting blue-emissive crystals-B into liquid nitrogen for 10 s, the green-emission crystals-G were obtained. However, the conversion was not complete according to XRD patterns in Figure S4.

Crystals-C to crystals-B and crystals-G transformation
Crystals-C cannot be transformed into green-emissive crystals-G directly by cooling at 78 K, but they can be converted to crystals-B firstly by heating at 180 o C for 5 min and then cooling them at 78 K for 10 s to give green-emissive crystals-G. Obviously, the conversion of crystals-C to crystals-B was nearly complete, but crystals-B cannot be fully transformed to crystals-G according to XRD patterns in Figure S5.

Crystals-G to Crystals-B transformation
When heating crystals-G to 180 o C for 5 s, it can be converted to crystals-B completely based on the XRD patterns in Figures S4 and S5. Besides, when dissolving crystals-B or crystals-G in chloroform solution, the plate-like crystals-C were yielded by slow evaporation of chloroform. (g) The crystallographic parameters of crystals-B, crystals-C, crystals-G and crystals-B. Crystals-B obtained by sublimation; crystals-C obtained by recrystallization in chloroform solution; crystals-G obtained by cooling crystals-B to 78 K for 10 s; crystals-B obtained by heating crystals-G to 180 o C for 5 s. Displacement ellipsoids are drawn at 50% probability level.

Figure S3
The XRD patterns of the crystalline samples under different conditions. State 1: the pristine solid (belonging to crystals-B phase) obtained by sublimation (black); state 2: cooling pristine solid at 78 K for 10 s, followed by warming to temperature (red); state 3: heating the crystalline sample that had been cooled at 180 o C for 5 s, followed by cooling to temperature (pink). The simulated XRD patterns of crystals-B (blue) and crystals-G (green).

Figure S4
The XRD patterns of the crystalline samples under different conditions. State 1: the pristine solid (belonging to crystals-C phase) obtained by recrystallization (black); state 2: heating the pristine solid to 180 o C for 5 min and then cooling to room temperature (purple); state 3: cooling the crystalline sample that had been heated at 78 K for 10 s and then warming to room temperature (red); state4: heating the crystalline sample that had been cooled at 180 o C for 5 s and then cooling to room temperature (pink). The simulated XRD patterns of crystals-C (orange), crystals-B (blue) and crystals-G (green).

Figure S7
The absorption and emission spectra of vacuum-deposited thin film of S10 5FDPA.

Table S1
The photophysical data of 5FDPA at different states.

Figure S10
The XRD patterns of 5FDPA at different states: before grinding (black line), after grinding (red line), and after grinding, followed by fuming with CH 2 Cl 2 vapor for 3 min (green line) and heating at 180 o C for 5 s, respectively.

Figure S12
The XRD pattern of the deposited thin film of 5FDPA (black) and simulated XRD pattern of crystals-B (red).

Figure S13
Electron mobility distribution of 27 devices with the vacuum-deposited films of 5FDPA.

Figure S14
The device structure of organic single-crystal field-effect transistors based on the crystals of 5FDPP and 5FDPA.

Figure S23
The GIWAXS (grazing-incidence wide-angle X-ray scattering) image (left) and corresponding out-of-plane cut (right) of vacuum-deposited film of 5FDPA. Figure S24 (a) The atomic force microscopy image of the deposited thin film of 5FDPP. (b) The step height of layer-by-layer growth of 5FDPP on the SiO 2 /Si substrate. The inset shows the molecular length of 5FDPP in crystal.

Figure S25
The calculated transfer integrals for the nearest neighboring molecules within crystals-B (a) and crystals-G (b).

Figure S26
The calculated transfer integrals for the nearest neighboring molecules within the single crystal of 5FDPP.

Figure S27
The photographs of the single-crystal devices with crystals-B (a) and crystals-G (c). The electroluminescence spectra of crystals-B (b) and crystals-G (d).
The inset photographs show the corresponding electroluminescence photos.  Where W and L OML are the width of the conducting channel and the length of one molecular layer respectively (W = 473μm; L OML = 18.6 Å).

Figure S30
Transfer curves of the OLETs with 5FDPP crystal after successive measurements for 12 times.

Table S3
The performances of 5FDPP-based OLETs devices.