Letter pubs.acs.org/JPCL
Effects of Processing Solvent on the Photophysics and Nanomorphology of Poly(3-butyl-thiophene) Nanowires:PCBM Blends Wei-Long Xu,† Peng Zeng,‡ Bo Wu,§ Fei Zheng,† Furong Zhu,§ Trevor A. Smith,‡ Kenneth P. Ghiggino,‡ and Xiao-Tao Hao*,†,‡ †
School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, China School of Chemistry, The University of Melbourne, Parkville, Victoria 3010, Australia § Department of Physics, Institute of Advanced Materials and Institute of Research and Continuing Education (Shenzhen), Hong Kong Baptist University, Hong Kong ‡
ABSTRACT: We report the effect of the processing solvent on the nanoscale morphology and photophysical dynamics of poly(3-butyl-thiophene) nanowires (P3BT-nw). P3BT-nw assembled in ortho-dichlorobenzene (ODCB) show higher crystallization and a longer conjugation length with increased exciton delocalization compared with those assembled in chlorobenzene (CB). It is proposed that this solvent effect is associated with the higher ordered structures formed from ODCB solution state. Charge-transfer dynamics and phase separation for P3BT-nw:PCBM blends were investigated by ultrafast fluorescence techniques. The more efficient fluorescence quenching observed in P3BT-nw:PCBM blend films processed from ODCB suggests that there is intimate contact between P3BTnw and PCBM that facilitates charge transfer. The superior performance of organic photovoltaic devices based on P3BT-nw:PCBM bulk heterojunctions processed using ODCB is attributed to the higher crystallization of P3BT-nw, optimized phase separation, and more efficient charge transfer from P3BT-nw to PCBM.
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room temperature. Zhang et al.17 successfully obtained highefficiency OSCs based on P3HT:PCBM by incorporating P3BT-nw to induce P3HT crystallization. The optimal solvent to achieve high efficiency OSCs varies for different polymers owing to the range of solubility parameters.18 Sirringhaus19 found that solvent has an effect on the π−π stacking of crystalline planes. In addition, it has been reported that during the film deposition process solvents with different boiling temperature resulted in very different electron mobilities in polymers.20 Liang et al.18 also pointed out that the phase separation of the blend film can be varied by using different solvents. Schwartz et al. have also concluded that the chain conformation, degree of interchain contact, and rate of energy transfer in polymers can be controlled by the choice of solvent.21 In this work, the effect of the processing solvent on the morphology and crystallinity of P3BT-nw was studied by atomic force microscopy (AFM) and grazing-incidence X-ray diffraction (GIXD), and the phase separation between the polymer and PCBM was quantitatively analyzed by the grazingincidence X-ray scattering (GISAXS) technique. The photophysical processes of P3BT-nw in solution and their corresponding blend films produced from different solvents
rganic solar cells (OSCs) have received widespread attention in recent years due to their light weight, flexibility, and potential for low-cost roll-to-roll production methods.1 To date, single junction OSCs of around 10% power conversion efficiency have been achieved by improvements in molecular design, device engineering, and theoretical understanding.2,3 One of the most studied conjugated polymers for OSCs is poly(3-hexyl-thiophene) (P3HT), which has a variety of attractive features such as high carrier mobility, large optical absorption coefficient, and good chemical stability relative to some other polymers.4−6 However, the highest efficiency OSC based on P3HT is ∼5% due to the limited absorption range and the high binding energy of the excitons, which diminishes the efficiency of exciton dissociation.7,8 The rapid development of ternary OSCs has led to renewed interest in conventional conjugated homopolymers.9−12 Goh et al.13 realized an efficiency enhancement from 7 to 8.2% by adding a moderate amount of P3HT into a PTB7:PCBM blend film. The addition of P3HT expanded the spectral absorption range and prolonged the exciton lifetime in the ternary system. In comparison with P3HT, however, few investigations have been focused on the related poly(3-butyl-thiophene) (P3BT), perhaps due to its reported shorter conjugation length and lower solubility.14,15 Xin et al.16 have taken advantage of these properties to fabricate P3BT nanowires (P3BT-nw) through polymer self-organization in ortho-dichlorobenzene (ODCB) solution that occurs when cooling from high temperature to © XXXX American Chemical Society
Received: April 15, 2016 Accepted: May 3, 2016
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DOI: 10.1021/acs.jpclett.6b00808 J. Phys. Chem. Lett. 2016, 7, 1872−1879
Letter
The Journal of Physical Chemistry Letters
the BL16B1 beamline. The incidence angle was 0.3° and the distance from sample to detector was 2050 mm. Picosecond transient absorption spectra were obtained on a high pulse repetition rate system as previously described.22 The pump beam (400 nm) was mechanically chopped at 4.6 kHz with a spot size of 0.132 mm2 on the sample. The excitation density was kept at 20 μJ/cm2 to reduce exciton−exciton and exciton− charge annihilation effects.23 Visible white-light probe pulses were generated by focusing a small amount of the fundamental pulses (800 nm) into a 3 mm thick sapphire substrate (Crystal Systems), and the probe transmission was recorded using a high-frequency CMOS-based spectrometer (Ultrafast Systems). The relative orientation of the pump and probe beam polarization was 54.7°. The steady-state PL spectra were collected using an emission spectrometer (PG2000 Pro, Idea Optics) with an excitation wavelength of 450 nm. Timeresolved PL decay profiles were measured by a fluorescence upconversion system. The excitation light with 400 nm was generated from frequency doubling a femtosecond Ti-sapphire laser (Maitai HP, Spectra-Physics) at 80 MHz. The gate beam with 800 nm wavelength was produced by the Ti-sapphire laser. The spontaneous emission from the example was mixed with the “gate” beam in a nonlinear crystal for sum frequency generation. The excitation intensity is 5.0 mW and the instrumental response function (IRF) is ∼400 fs, details of which are provided elsewhere.24 The global fitting method was used to analyze the decay curves. The OSCs were fabricated with a structure of ITO/ PEDOT:PSS/P3BT-nw:PCBM/ZnO/Ag. The electron and hole transport layers were solution-prepared as previously reported.25 Ag was deposited by thermal evaporation in vacuum with a base pressure of
