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End-Group Engineering of Low-Bandgap Compounds for HighDetectivity Solution-Processed Small-Molecule Photodetectors Ji Qi, Jinfeng Han, Xiaokang Zhou, Chang Guo, Dezhi Yang, Wenqiang Qiao, Yuning Li, Dongge Ma, and Zhi Yuan Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08471 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 26, 2015

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The Journal of Physical Chemistry

End-Group Engineering of Low-Bandgap Compounds for High-Detectivity Solution-Processed Small-Molecule Photodetectors Ji Qi, †,‡ Jinfeng Han, †,‡ Xiaokang Zhou, †,‡ Chang Guo, ǁ Dezhi Yang, † Wenqiang Qiao,*,† Yuning Li, ǁ Dongge Ma, † and Zhi Yuan Wang*,†,§ †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China ‡

University of Chinese Academy of Sciences, Beijing, 100049, P. R. China

§

Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario,

Canada K1S 5B6 ǁ

Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L

3G1 KEYWORDS small-molecule photodetector, high detectivity, end group, mobility, film morphology

ACS Paragon Plus Environment

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ABSTRACT

Several π-conjugated compounds based on diketopyrrolopyrrole and trithiophene substituted with different end groups (alkyl, alkyloxy, and alkylthio) were designed and synthesized for investigation of the material properties and photodetector performance brought by subtle changes in the end groups. Among all, compound 4 with hexylthio groups exhibits the most red-shifted absorption, strongest molecular stacking, highest mobility, and ideal film morphology. These unique properties make it a promising material for use in small-molecule photodetectors. Photodetector SMPD-4 based on compound 4 exhibits broad response from 300 to 900 nm and a high specific detectivity (D*) of 1.3 x 1013 Jones at 650 nm under -0.1 V. This result is among the best values reported for solution-processed small-molecule photodetectors and even in the same order of conventional silicon photodetector. The molecular structure-material property-device performance relationships are established with these compounds. This work suggests that endgroup engineering is a useful method in tuning the material properties and device performance of organic semiconductors.

INTRODUCTION Among most of organic optoelectronic devices, such as organic field-effect transistors (OFETs), light-emitting diodes (OLEDs), and solar cells (OSCs),1-4 organic photodetectors (OPDs) are relatively underdeveloped.5-7 In particular, the development of small-molecule photodetectors (SMPDs) falls far behind the polymer photodetectors,8-10 as the detectivity of solution-processed SMPDs is still in the order of 1012 Jones.11-13 In order to increase the detectivity of SMPDs, it is


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necessary to systemically investigate the relationships between molecular structure and material property and the factors related to device performance.14,15 Modification of molecular structures is a valid method to increase the performance of optoelectronic devices.16-18 Recent study indicates that side groups play a significant role in tuning the material properties and device performance of conjugated polymers and small molecules.19-23 Pei et al. found that the branching position of branched alkyl side chains greatly affected the FET performance of isoindigo-based conjugated polymers.21 In the work reported by Leclerc et al., the alkyl chain lengths of thienopyrrolodione-based polymers showed a significant impact on the solubility, molecular stacking, and active layer morphology, which remarkably influenced the corresponding solar cell performance.22 Chen et al. reported that by changing alkyloxy to alkylthio groups in small molecules, the power conversion efficiency (PCE) of the small-molecule photovoltaic cells increased greatly.23 The presence of sulfur atom seems unique, as the compounds containing alkylthio group exhibit more ordered intermolecular stacking and consequently show unique optoelectronic properties.24,25 End-group engineering of small molecules may represent a useful strategy in tuning the material properties that may contribute to the device performance.26,27 In comparison with the πconjugated polymers, small molecules possess several intrinsic advantages for device applications, such as well-defined chemical structure, good reproducibility, and high purity.28,29 These properties give rise to reproducible and trustworthy results, which are desirable for practical applications. Therefore, it is absolutely necessary and conceivably useful to further understand the side-group influence on small-molecule materials for high-detectivity photodetectors.


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Herein, we report high-detectivity SMPDs by end-group engineering of diketopyrrolopyrrole (DPP) and trithiophene based π-conjugated molecules. We aim to study the influence of subtle changes in the end groups of organic compounds on the material properties and electronic properties, and find some design guidelines for high-performance electronic devices. The photodetector performance was enhanced, as a result of improvement of molecular stacking, mobility, and film morphology. The molecular structure-material property-device performance relationships were established based on these small molecules. The results indicate that subtle changes in the end groups can significantly affect the material property and photodetector performance.

EXPERIMENTAL SECTION Materials: All the chemicals and reagents were purchased from commercial sources and used without purification. Solvents for chemical synthesis were purified by distillation. Detailed syntheses and characterizations are given in the Supporting Information. Characterizations: 1H (400 MHz) and 13C (100 MHz) NMR spectra were collected on a Bruker Avance 400 NMR spectrometer with tetramethylsilane (TMS) as the internal reference. MALDITOF MS spectra were obtained on a Bruker Daltonics Autoflex III TOF/TOF. Elemental analyses (EA) were carried out on a FlashEA1112 Elementar Analysis Instrument. The UV-visNIR absorption spectra were conducted using a Shimadzu UV-3600 spectrophotometer. Cyclic voltammetry (CV) measurements were recorded on a CHI660b electrochemical workstation in the solution of tetrabutylammonium hexafluorophosphate (0.1 M) in anhydrous dichloromethane at a scan rate of 50 mV/s. The solution was purged with argon for more than 20 min before the


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measurement. The working electrode was a Pt disk (2-mm diameter), the counter electrode was a Pt wire, and the reference electrode was a Ag/AgCl electrode. Ferrocene was used as an internal standard to calibrate the redox potentials. Thermogravimetric analysis (TGA) measurements were run using a PerkinElmer Pyris Diamond TG under nitrogen (10 ºC/min). Differential scanning calorimetry (DSC) measurements were conducted under N2 on a TA-DSC Q100 at a heating/cooling rate of 10 ºC/min. The two-dimensional grazing incidence X-ray diffraction (2DGIXD) profiles were recorded at beamline BL14B1 of the Shanghai Synchrotron Radiation Facility (SSRF, λ = 1.2398 Å). The out-of-plane and in-plane grazing incidence X-ray diffraction (GIXRD) profiles were obtained by using a Rigaku SmartLab X-ray diffractometer equipped with Cu target, CBO mirror, thin film sample stage, and a scintillation detector. The scan rate was in 0.05°step size (2θ) and 5 s per step. Atomic force microscopy (AFM) studies were performed with a SPI 3800N Probe Station (SPA 300HV, Seiko Instruments Inc., Japan) in tapping mode in ambient condition. Bottom-gate, bottom-contact FETs were used for evaluating compounds 1-4. The fabrication process and device structure were similar with our previous report.35 The chlorobenzene solution of the given compound (10 mg/mL) was spin-coated on the substrate to form a film of about 50-nm thickness. The evaluation of the device was carried out in the absent of light in a N2-filled glovebox by using an Agilent B2900A Semiconductor Analyzer. Fabrication and characterization of photodetectors: Most of the fabrication and characterization were carried out as we previously reported.15,43 For photodetector fabrication, indium tin oxide (ITO) coated glass substrates were cleaned by the standard procedure with detergent, de-ionized water, acetone, and isopropyl alcohol. The clean ITO-coated slide was treated by UV-plasma for 15 min, and spin-coated a 35-nm film of PEDOT:PSS (Baytron P VP


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A1 4083), then annealed at 120 oC for 30 min. The ITO/PEDOT:PSS substrate was moved to a nitrogen-filled glovebox and spin-coated the mixed solution of a given compound and PCBM (ADS71BFA) in a weight ratio of 1:1.5 in chlorobenzene (total concentration 30 mg/mL) at a speed of 1000 rpm for 60 s to form an active layer of about 170-nm thickness. After that, the coated substrate was transferred to a thermal evaporator, and BCP (10 nm) was deposited on top of the active layer. Finally, Al electrode (100 nm) was deposited through a shadow mask. The active area of the device is 0.16 cm2. For photodetector characterization, the J-V characteristics were recorded using a Keithley 236 Source Measure Unit. The EQE measurements were recorded by using a calibrated crystalline silicon diode (S1337-1010BQ, Hamamatsu) as the reference, and a halogen lamp (250 W) as the light source.

RESULTS AND DISCUSSION Synthesis and Characterization The target compounds 1-4 were readily synthesized via Stille cross-coupling reaction of the monotributyltin bithiophene derivatives and the dibromo-DPP compound (Scheme 1). The dibromo-DPP was prepared according to the previously reported procedures.30-32 The end-group substitution on the electron-donating thiophene unit includes the hexyl, hexyloxy, and hexylthio groups. The detailed syntheses and structure characterizations of all the compounds are described in the Supporting Information (SI). The final products were first purified by chromatography and then by recrystallization from hexane several times. The products were thoroughly characterized by 1H NMR, 13C NMR, mass spectrum, and elemental analysis. These compounds are quite soluble in common organic solvents such as tetrahydrofuran, chloroform, and chlorobenzene.


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Thermal stability and phase transition of compounds 1-4 were analyzed by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). All the compounds show good thermal stability with 5% weight loss (Td) of above 300 oC from the TGA traces (Figure S1). The DSC scans reveal similar melting-crystallization transition characteristics for all the compounds (Figure S2 and Table 1), although the melting and crystallization occur at quite different temperatures due to the end-group effect on crystal packing and intermolecular interaction.33 Scheme 1. Synthesis of DPP-Based Compounds 1-4 Having Different End-Groups.

N

Br

S

O S

O

R

+ Bu3Sn

S S

R

Pd(PPh3)2Cl 2 THF

N

N

S S

Br

S

O S

O

N

S S

R

1: R = H 2: R = C6H13 3: R = O-C6H13 4: R = S-C6H13

Optical and Electrochemical Properties The normalized UV−vis-NIR absorption spectra of compounds 1-4 in chlorobenzene solutions and in thin films are depicted in Figure 1. All the compounds show similar absorption profile or typically an intense and broad absorption peak from 500 to 700 nm. Going from hydrogen atom to hexyl and to hexyloxy, the maximum absorption peaks in solution gradually shift to the red about 5 nm, due to the increased electron-donating property of the end groups. In comparison


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with compound 3, the absorption maximum of 4 is slightly blue shifted (5 nm), since the electron-donating property of sulfur is a little weaker than oxygen. Interestingly, the absorption spectra of the films are quite different from those in solution, showing large red shifts (53, 75, 73, and 86 nm for compound 1 to 4, respectively) and broadness due to molecular aggregation in the solid state.34,35 As presented in Figure 1b, the normalized absorption peaks are located at about 650 nm and the intensity of the low-energy shoulder peaks increases in the order of 1 < 3 < 2 < 4, which seems to correlate with the red-shift values (Table 1) and may be attribute to different molecular stacking properties. As shown in Figure 1, the molecular solutions exhibit similar color, while the colors of their films are a bit different. The electrochemical properties of the four compounds were investigated by cyclic voltammetry (CV) with 0.1 M of tetrabutylammonium hexafluorophosphate in anhydrous dichloromethane. As shown in Figure 2, all the compounds show similar electrochemical property and undergo reversible oxidation and reduction reactions. The highest occupied molecular orbital (HOMO) energy level, the lowest unoccupied molecular orbital (LUMO) energy level, and the electrochemical bandgap (EgEC) are calculated according to the equations: EHOMO = −e(Eonox + 4.38) (eV) and ELUMO = −e(Eonred + 4.38) (eV), where Eonox is the onset oxidation potential and Eonred is the onset reduction potential. As a result, the HOMO energy levels are calculated to be −5.36, −5.33, −5.28, and −5.36 eV for compounds 1-4, respectively. The LUMO energy levels are all −3.71 eV, which provide adequate driving force for exciton dissociation in the blends with PCBM. It is well-known that the HOMO and LUMO energy levels are affected by the electron-donating and -withdrawing units in D-A type compounds, respectively.36 Accordingly, the end groups with different electron-donating property in the donor parts are able to finely tune the HOMO energy levels or bandgaps of compounds 1-4.


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Figure 1. Absorption spectra of compounds 1-4 (a) in chlorobenzene solutions and (b) as thin films spin-coated on quartz. The photographs of the molecular solutions and films are shown below.

Figure 2. Cyclic voltammograms (CV) of the compounds (black line: 1; red line: 2; blue line: 3; green line: 4).


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Table 1. Characterizations of Compounds 1-4. λabssoln.

λabsfilm

Δλmax

HOMO

LUMO

EgCV

Egopt.

Td

Tm

Tc

(nm) a

(nm) b

(nm)c

(eV) d

(eV) d

(eV) e

(eV) f

(oC)

(oC) g

(oC) g

1

639

629, 692

53

-5.36

-3.71

1.55

1.78

401

198

170

2

646

648, 721

75

-5.33

-3.71

1.52

1.76

389

154

129

3

651

641, 724

73

-5.28

-3.71

1.47

1.74

315

161

126

4

645

657, 731

86

-5.36

-3.71

1.55

1.76

361

132

85

Compound

a

Maximum absorption in chlorobenzene (0.03 g/L). b Maximum absorption of the thin films

spin-coated on quartz from chlorobenzene solutions. c Red-shift of the low-energy absorption peaks from solutions to films. d Energy levels estimated from the onset oxidation and reduction potentials of CV curves. e Electrochemical band gaps calculated from EHOMO subtracting ELUMO. f Optical bandgaps estimated from the onset wavelength of the absorption spectra. g The melting and crystallization temperatures determined from the second heating and cooling scan cycles by DSC.

Molecular Stacking and Film Nanostructure In order to gain in-depth insight into the film property of these compounds, the microstructure of the thin films were investigated. The two dimensional grazing incidence X-ray diffraction (2DGIXD) profiles of the pristine molecular films are presented in Figure 3. The 2D-GIXD patterns clearly indicate that these compounds exhibit different molecular stacking behaviors with subtle changes in end groups. The films of compounds 2-4 reveal apparent diffraction peaks at about 0.5 Å-1 in the out-of-plane direction and no peaks are observed in other position, suggesting that


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these molecules favor the edge-on orientation.37,38 In contrast, there are nearly no diffraction peaks observed for compound 1 with hydrogen in the molecular chain end for its low crystallinity. The diffraction intensity in the out-of-plane direction increases in the order of 1 < 3 < 2 < 4, and thus the film crystallinity is the lowest for 1 and the highest for 4. Compounds 2-4 exhibit similar diffraction peaks, and compound 4 with the hexylthio group possesses the best crystalline property. Therefore, the end groups have great impact on the molecular stacking nature, which may give rise to distinct electronic property.35,37 Apart from the crystalline nature, the film with high quality is also important for good device performance but rather difficult for small molecules to form.27,39 In order to examine the nanoscale structure of pristine molecular films, tapping-mode atomic force microscopy (AFM) measurements were carried out to study the surface topography. As shown in Figure 4, the rootmean-square (RMS) roughnesses of the films formed by compounds 1-4 are 3.04, 6.15, 4.87, and 3.03 nm, respectively. The difference in surface morphology suggests quite different crystal and aggregation behaviors. There are a few isolated fiber-like aggregates in the film of compound 1 and the rest parts are really smooth without any features. The film surface of compound 2 is rather rough, as some large crystal structures are observed. In contrast, the film of compound 3 is smooth with small intercalating grains, which is consistent with the relatively low XRD diffraction intensity. Interestingly, the film of compound 4 is quite uniform with continuous crystal-like features and poses homogeneous network, which is responsible for the high crystallinity, and is most promising morphology for thin-film devices. The differences in film morphology and molecular stacking may result in diverse charge carrier mobility. Therefore, the bottom-contact field-effect transistors (FETs) were made to characterize the mobility of the four compounds (SI). The hole mobility of compounds 1-4 are


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1.4 x 10-3, 4.5 x 10-3, 2.6 x 10-3, and 2.7 x 10-2 cm2/V s, respectively. Compound 4 has the highest carrier mobility, which is about one order of magnitude higher than the other analogs. While, compound 1 possesses the lowest mobility among all, which is partially due to its low crystallinity. The results are in accordance with the conclusion that better crystalline usually associates with higher mobility, which could improve the carrier transport and collection efficiency, and therefore give better photoresponse.14,17

Figure 3. 2D-GIXD patterns of the pristine molecular films. (a) to (d) correspond to compounds 1-4, respectively.


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Figure 4. AFM topography images (5 μm x 5 μm) of the pristine films spin-coated on quartz from clorobenene solutions (a-d correspond to compounds 1-4, respectively).

Photodetector Performance In order to probe the effect of end groups on the optoelectronic properties of the materials, solution-processed SMPDs with the optimized device structure of ITO/PEDOT:PSS/active layer/BCP/Al were fabricated. The active layer of the photodetector was prepared by spincoating a mixed solution of a given compound and phenyl C71 butyric acid methyl ester (PCBM) in an optimized weight ratio of 1:1.5 in chlorobenzene. A thin layer (10 nm) of 2,9-dimethyl-4,7diphenyl-1,10-phenanthroline (BCP) was used as the hole blocking layer, in order to suppress the charge injection from the cathode.15,40 The current density-voltage (J-V) characteristics in the dark under reverse and forward biases exhibit the rectification behaviors but with different diode property and dark current values. As shown in Figure 5c, device SMPD-4 has the lowest dark current, and SMPD-3 shows the highest value. According to the standard diode equation, the dark current is strongly influenced by the energy difference, EPF, between the HOMO energy level of electron donor material and the LUMO energy level of PCBM.8,41 Compound 3 has the highest HOMO energy level, which is one possible reason for the relatively high dark current. Compounds 1 and 4 display similar HOMO levels but different dark current, which may be ascribed to their different film morphology. SMPD-4 shows the best diode characteristic of about 104 at ±1 V (Figure 5d). As shown in Figure 5e and SI, all the devices show broad response in the spectral region of 300-900 nm and with high responsivity from 300 to 900 nm, where these compounds absorb intensely. It


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is clear that the external quantum efficiency (EQE) profiles match well with the absorption characteristics of the active films (Figure S6). However, the EQE of SMPD-3 is different from other devices, as the absorption and response in the low-energy region are relatively low, and SMPD-3 also exhibits different color compared to other devices. One possible reason may be due to the poor miscibility of compound 3 with PCBM and some unfavorable morphology. As expected, the EQE profiles and spectral responsivity are dependent on the applied biases (0, −0.1, −0.5, −1.0, and −2.0 V) or the field-induced exciton dissociation.42,43 The specific detectivity (D*) is a key figure of merit to characterize the performance of a photodetector. Assuming that the shot noise from the dark current is the major contribution to the noise,8,9,44,45 the specific detectivity can be expressed as: D* = R/(2qJd)1/2 = (Jph/Plight)/(2qJd)1/2 (Jones) Where q is the absolute value of electron charge (1.6 × 10−19 Coulombs), R is the responsivity, a ratio of photocurrent (Jph) to incident-light intensity (Plight), and Jd represents the dark current density. Accordingly, the spectral detectivity (D*) of the devices is calculated and shown in Figure 5g, h and Table 2. SMPD-4 based on compound 4 gives the highest specific detectivity among all and its detectivity (D*) remains fairly constant about 1013 Jones in a broad spectral region of 350-750 nm. The specific detectivity of 1.3 x 1013 Jones at 650 nm at a biased voltage of -0.1 V for SMPD-4 represents one of the best values reported to date for solution-processed SMPDs.11-13 The high specific detectivity of SMPD-4 is clearly attributed to low dark current and the high responsivity. These two key parameters relate to the morphological characteristic and crystalline behavior in molecular film, which is crucial for exciton separation and charge transport and evidently brought by minor modification at the molecular chain ends.


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Figure 5. a) Schematic device structure. b) Schematic energy-level diagrams of the materials in the photodetector. c) The dark current density of SMPD-1 to SMPD-4 at different voltages (black square at -0.1 V, red circle at -2 V, and blue up-triangle at +2 V). d) The dark current density-voltage (J–V) characteristic of SMPD-4. e) EQE of the photodetectors based on each compound under a biased voltage of -2 V. f) Spectral responsivity of SMPD-4 at different biased voltages. g) Specific detectivity of the devices based on each compound under a biased voltage of -0.1 V. h) Responsivity (color strip) at 650 nm at -2 V and the specific detectivity (solid square) at 650 nm at -0.1 V of the photodetectors for each compound.

Table 2. Photodetector Performance

Device

a

Jd

EQE

R

D*

(A/cm2) a

(%) b

(mA/W) b

(Jones) b

Compound

SMPD-1

1

8.2 × 10-9

34.5

180.6

3.5 × 1012

SMPD-2

2

3.5 × 10-9

42.0

220.2

6.5 × 1012

SMPD-3

3

2.6 × 10-8

31.8

166.7

1.8 × 1012

SMPD-4

4

1.3 × 10-9

48.0

251.4

1.3 × 1013

The dark current density at -0.1 V. b Data at 650 nm at -0.1 V.

Film Morphology It is widely acknowledged that the appropriate morphology and phase separation of the active layer is critically important for high-performance organic solar cells and photodetectors.46-49 As the end groups are expected to influence the miscibility of the donor materials with PCBM, they can also affect the corresponding film nanoscale morphology and phase property. Therefore, the


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surface topography and phase texture of the photoactive films were investigated with tapping mode AFM and the images are shown in Figure 6. The active layers of each of compounds 1-4 and PCBM show the RMS roughness of 0.34, 0.65, 2.47, and 0.97 nm, respectively. The film of 1/PCBM is quite even and featureless with poorly defined domains, which are not ideal for exciton dissociation due to charge carrier recombination.43 The film of 2/PCBM shows good domain sizes and phase separation, and there are some small crystal-like structure, which leads to the second best device performance among all. The film of 3/PCBM consists of some severely aggregated domains and phase separation due to poor miscibility of compound 3 with PCBM, leading to inefficient exciton dissociation and low EQE or relatively low photovoltaic response in the low-energy region relative to other devices (Figure 5c).50,51 The film of 4/PCBM exhibits a lot of long crystal-like nanowire structure, which can effectively facilitate exciton separation and charge transport. It has been widely reported that the optimal domain size of the active film for organic photovoltaic devices is 10-20nm, which is in accord with the diameters of the nanowire structures in the 4/PCBM film.46,52 These results suggest that the active layer of compound 4 and PCBM has the most favorable nanostructured morphology and phase separation, ideal and critically important for the devices to have good photoresponse and low dark current. Additionally, from the out-of-plane GIXRD diagrams of the active films (Figure S11), the mixed film of compound 4 and PCBM displays the highest diffraction intensity among all, which is responsible for the high responsivity.53,54 The results indicate that the type of end group is critical in adjusting the miscibility of donor and acceptor materials and the film nanostructure of active layer, which in turn affects the resulting photodetector performance, in particular, with regards to the responsivity and dark current.


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Figure 6. AFM topography and phase images (2 μm x 2 μm) of the active films. a-d) Topography images correspond to compounds 1-4, respectively. e-h) Phase images correspond to compounds 1-4, respectively.

We made several important observations from end-group engineering of DPP-based small molecules. Firstly, the electronic property of the end groups would influence the energy levels of D-A type compounds. End groups with more electron-donating property in the donor unit would increase the HOMO energy level, and thus reduce the bandgap, suggesting that side groups are nonnegligible in tuning the energy levels or absorption spectra of organic semiconducting materials. Secondly, the end groups can have a vital impact on the crystalline nature of the molecular films. The better molecular stacking is beneficial for charge carrier transport in electronic devices. In this work, compound 4 with the hexylthio groups shows better molecular stacking in both pristine and mixed films, which results in a higher charge mobility in FET devices and increased photoresponsivity in photodetectors. Thirdly, side groups greatly affect the miscibility of the components in active film, which is considerably important for photovoltaic


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devices. Compound 3 with the hexyloxy group is poorly miscible with PCBM, and exhibits large domains and phase separation, thus giving rise to a low EQE, especially, in low-energy spectral region. Lastly but not the least important, the photodetector performance, particularly with respect to the dark current and responsivity, is apparently different in this system. For the dark current, the energy levels and active film morphology are vital. In the BHJ blend, dark current is influenced by the energy difference between the HOMO energy level of the donor material and the LUMO energy level of PCBM. Additionally, the active layer morphology also plays a crucial role in determining the dark current. The appropriate phase separation with bicontinuous network would reduce the probability of recombination at the interface of the active layer, resulting in low dark current. Compound 4 has a low HOMO energy level and the most favorable nanoscale morphology, therefore leading to the lowest dark current among all. For the responsivity, the film morphology and molecular stacking contribute simultaneously. The active layer films with appropriate domain size and phase separation of 10-20 nm are essential for high photoresponsivity. Compound 4 presents the most favorable domains and crystal-like nanowire structures, and the corresponding device has the highest responsivity.

CONCLUSION

Hgh-detectivity small-molecule photodetectors were demonstrated through molecular engineering of end groups in low-bandgap compounds. The excellent device performance is related to the ideal material and thin-film properties such as molecular stacking, mobility, and film morphology. The molecular structure-material property-device performance relationships were established. The best photodetector (SMPD-4) based on compound 4 with hexylthio end groups shows broad


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photovoltaic response in the spectral region of 300-900 nm, and remains fairly constant detectivity (D*) of about 1013 Jones in a broad spectral region from 350 to 750 nm, which is one of the best results reported to date for solution-processed small-molecule photodetectors and even comparable to conventional silicon photodetector, rendering promising potential for solution-processed smallmolecule photodetectors. The high specific detectivity stems from both low dark current and high responsivity, which correlate well with the thin-film properties. End-group engineering of small molecules offers an effective tool in tuning the material property and electronic device performance.

ASSOCIATED CONTENT Supporting Information TGA, DSC traces of the materials, UV-vis absorption spectra, GIXRD diagrams of the active films, and photodetector properties, as well as the syntheses and structure characterizations of the compounds. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail [email protected] (Z. Y. W.) *E-mail [email protected] (W. Q.) Notes The authors declare no competing ﬁnancial interest.


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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21134005, 21474102, 21474105, and 51403203), International Cooperation Foundation of China (2015DFR10700), the Basic Research Project of Jilin Province (20150520018JH), and the Natural Science and Engineering Research Council of Canada. REFERENCES (1) Kergoat, L.; Herlogsson, L.; Piro, B.; Pham, M. C.; Horowitz, G.; Crispin, X.; Berggren, M. Tuning the Threshold Voltage in Electrolyte-Gated Organic Field-Effect Transistors. Proc. Natl. Acad. Sci. USA 2012, 109, 8394-8399. (2) Wang, Q.; Ma, D. Management of Charges and Excitons for High-Performance White Organic Light-Emitting Diodes. Chem. Soc. Rev. 2010, 39, 2387-2398. (3) Hains, A. W.; Liang, Z.; Woodhouse, M. A.; Gregg, B. A. Molecular Semiconductors in Organic Photovoltaic Cells. Chem. Rev. 2010, 110, 6689-6735. (4) Dong, H.; Zhu, H.; Meng, Q.; Gong, X.; Hu, W. Organic Photoresponse Materials and Devices. Chem. Soc. Rev. 2012, 41, 1754-1808. (5) Clifford, J. P.; Konstantatos, G.; Johnston, K. W.; Hoogland, S.; Levina, L.; Sargent, E. H. Fast, Sensitive, and Spectrally Tuneable Colloidal-Quantum-Dot Photodetectors. Nat. Nanotech. 2009, 4, 40-44. (6) Hu, X.; Dong, Y.; Huang, F.; Gong, X.; Cao. Y. Solution-Processed High-Detectivity NearInfrared Polymer Photodetectors Fabricated by a Novel Low-Bandgap Semiconducting Polymer. J. Phys. Chem. C 2013, 117, 6537-6543.


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