Chinese Chemical Letters  2019, Vol. 30 Issue (5): 995-999   PDF    
Effects of various donor: acceptor blend ratios on photophysical properties in non-fullerene organic bulk heterojunctions
Zhenchuan Wena, Xuejian Maa, Xiaoyu Yanga, Pengqing Bia, Mengsi Niua, Kangning Zhanga, Lin Fenga, Xiaotao Haoa,b,*     
a School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Ji'nan 250100, China;
b ARC Centre of Excellence in Exciton Science, School of Chemistry, The University of Melbourne, Parkville, Victoria, 3010, Australia
Abstract: The composition ratio of donor and acceptor materials in organic bulk heterojunction (BHJ) is one of the key parameters to govern the performance in organic solar cells (OSCs). Therefore, high-performance non-fullerene organic bulk heterojunction consisting of poly[(2, 6-(4, 8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1, 2-b:4, 5-b']dithiophene))-co-(1, 3-di(5-thiophene-2-yl)-5, 7-bis(2-ethylhexyl) benzo[1, 2-c:4, 5-c']dithiophene-4, 8-dione)] (PBDB-T) and 3, 9-bis(2-methylene-(3-(1, 1-dicyanomethylene)-indanone))-5, 5, 11, 11-tetrakis(4-hexylphenyl)-dithieno[2, 3-d:2', 3'-d']-s-indaceno[1, 2-b:5, 6-b']-dithiophene (ITIC) are used to investigate the correlation among various donor: acceptor (D:A) ratios, photophysical properties and photovoltaic performance. Interestingly, the function of short-circuit current (Jsc) and D:A ratios demonstrates an axisymmetric trend. When the blending ratio of D:A deviates from the optimal ratio, the symmetrically decreased Jsc is derived from a reduction in the D:A interface or amorphous region. Research on the steady-state photoluminescence (PL), the time-resolved fluorescence spectroscopy measurements, atomic force microscopic (AFM) and grazing-incidence small angle X-rays scattering (GIWAXS) indicates no significant variation in energy loss in the process of changing D:A ratios in BHJs. With high donor or acceptor content, the domain size improves significantly, but the distance of π-π stacking corresponding to molecular packing has not changed significantly, and the bi-continuous percolation pathways were not obviously influenced.
Keywords: Photophysical properties     Non-fullerene     Bulk heterojunctions     Various blend ratios     Organic solar cells    

Bulk-heterojunction (BHJ) organic solar cells (OSCs), as one of the most promising next-generation green technologies, have attracted considerable attention in recent years due to unique advantages such as low-cost solution processing, light-weight and flexibility [1-4]. BHJ structure is an effective way to utilize sunlight, providing an efficient approach to split the exciton into free carriers [5, 6]. For a long time, fullerene derivatives were believed to be critical materials in OSCs stemming from their high electron affinity, high electron mobility and isotropic charge transport [7, 8]. However, the extensive application of fullerene derivatives in OSCs are restricted, due to limited tunability of energy levels [7], weak absorption in the visible region and poor stability. Strong absorption in the visible and even near-infrared (NIR) region [9], tunable energy levels, and high electron mobility of non-fullerene materials have caused extensive concern among scientific researchers [10]. In the past two decades, the synthetic methods [8], materials design strategies [11] and devices engineering protocols [12] of fullerene have accumulated a great deal of experience, which is beneficial for the quick development of nonfullerene OSCs in the past few years [13, 14]. Chen and Hou et al. achieved very high efficiency of OSCs over 14%, based on a new polymer donor (PBDB-T-SF) and a new A-D-A type small-molecule acceptor NCBDT-4Cl, due to well complementary absorption and the low highest occupied molecular orbital (HOMO) energy offset [15, 16]. By designing a highly efficient low-bandgap non-fullerene acceptor (COi8DFIC) with strong NIR absorption, Ding et al. achieved 14.62% power conversion efficiency (PCE) in ternary OCSs [17, 18]. Recently, Chen and Ding et al. reported a remarkable new record 17.36% PCE for a 2-terminal (2 T) monolithic solution processed tandem organic photovoltaic [19]. The morphology of non-fullerene blend is one of the most important factors in determining exciton dissociation and charge transfer, and thereby affecting device performance. The donor: acceptor (D:A) blend ratios have largely influence on the morphology [20]. According to reported works, the effect of D:A ratio variation on OSC performance are investigated, based on different polymer donor materials with the fullerene acceptor PCBM [21-23]. Nevertheless, a systematical study has been rarely reported on the effect of different D:A ratios on morphology and photophysical process in the non-fullerene organic BHJ devices.

In this work, the binary system is demonstrated based on poly [(2, 6-(4, 8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1, 2-b:4, 5-b'] dithiophene))-co-(1, 3-di(5-thiophene-2-yl)–5, 7- bis(2-ethylhexyl) benzo[1, 2-c:4, 5-c']dithiophene-4, 8-dione)] (PBDB-T) and 3, 9-bis (2-methylene-(3-(1, 1-dicyanomethylene)-indanone))–5, 5, 11, 11-tetrakis(4-hexylphenyl)-dithieno[2, 3-d:2', 3'-d']-s-indaceno[1, 2-b:5, 6-b']-dithiophene (ITIC). PBDB-T:ITIC system has drawn much attention in photovoltaics due to its prominent performance, such as broader absorption, appropriate molecular energy level alignment, and higher PCE [24]. The physical mechanism of the effect of various D:A blend ratios on the physical properties of bulk heterojunctions is not clear. A series of OSCs are fabricated with various D:A ratios. With the increase of PBDB-Tcontent, no obvious change is observed for fill factor (FF) and open-circuit voltage (Voc) of devices. The short-circuit current (Jsc) and power conversion efficiency (PCE) show the same tendency, which increase first and then decrease. Corresponding to device performance variation characteristics, the steady-state photoluminescence (PL), timeresolved PL (TRPL) measurement, atomic force microscopic (AFM) and grazing incident wide-angle X-ray scattering (GIWAXS) are carried out to study the physical mechanism, which causes the performance change by various D:A ratio.

PBDB-T and ITIC were purchased from Solarmer Materials, Inc. All chemicals were commercially available products and used as received without purification. Atomic force microscopic (AFM) images of PBDB-T: ITIC films were obtained using a Multimode Scanning Probe Microscope (NanoScope-IIIA, Veeco Metrology Group) in the peak force tapping mode. The conventional absorption spectra of film samples were obtained by ultravioletvisible (UV-vis) dual beam spectrophotometer (TU-1900, PG instruments, Ltd.). The steady-state PL spectra of film samples were collected by a spectrometer (DU420A-OE, ANDOR) using an excitation wavelength of 500 nm. Fluorescence lifetime imaging microscopy (FLIM) (Nanofinder FLEX2. Tokyo Instruments, Inc.) combined with TCSPC module (Becker & Hickl, SPC-150) was performed to investigate the two-dimensional (2D) fluorescence lifetime distribution. The excitation wavelength of 500 nm was generated by frequency doubling the output of a Ti-sapphire laser (Mai Tai HP, Spectra-physics) with 80 MHz repetition. The power of the excitation light was measured by a power meter (PM100D). The pump intensity is 1 mW, 2 mW, 3 mW and 4 mW. Grazingincidence wide-angle X-ray scattering (GIWAXS) was performed at BL16B1 beamline of Shanghai Synchrotron Radiation Facility with a photon wavelength of 0.124 nm. The incident angle was set to be 0.12° and the sample-detector distance was set at 220 mm. A CCD detector (MAR-165) was used to collect the scattering signal. The 2-D GIWAXS images were analyzed using the software Fit2d. The J-V curves were recorded by a programmable voltage-current sourcemeter (2400, Keithley Instruments Inc. Cleveland, OH, USA) under the 100 mW/cm2 AM 1.5 illumination.

Organic solar cells were fabricated with an inverted structure of ITO/ZnO (~40 nm)/active layer/MoO3 (8.5 nm)/Ag (100 nm). Indium tin oxide (ITO)-coated glass substrates with the sheet resistance of ~15 V sq-1 were cleaned sequentially with detergent, deionized water, acetone, and isopropanol by ultrasonic treatment for 20 min each time, and then treated with ultraviolet-ozone for 20 min. Zinc oxide (ZnO) precursor solution was available by dissolving 0.28 g of zinc acetate dihydrate (Zn(CH3COO)2·2H2O) and 1 g of ethanolamine (NH2CH2CH2OH) in 10 mL of 2-methoxyethanol (CH3OCH2CH2OH) which has been reported elsewhere [25]. ZnO thin films spin-coated onto cleaned ITO substrates at 3000 revolutions per minute (rpm) and then annealed for 1 h in the air immediately. A mixture of PBDB-T and ITIC (1:1 weight ratio) in CB:DIO (0.5 v% DIO), was stirred at 50 ℃ for about 12 h. Blend solution of donor and acceptor with the weight ratio of 1:4, 1:2.3, 1:1.5, 1:1, 1.5:1, 2:1, 2.5:1, 3.3:1, 4:1 were spin coated onto the ZnO layer at 2500 rpm. After spin coating, the blend films were annealed on a hot plate for 10 min at 90 ℃ in the N2-atmosphere glovebox. Finally, 8.5 nm molybdenum (MoO3) and 100 nm silver (Ag) were deposited on the active layers in vacuum thermal evaporator with a pressure of 1.2 × 10-4 Pa.

The chemical structures and the molecular energy-level diagram of PBDB-T and ITIC are shown in Figs. S1a and b (Supporting information), respectively. It can be clearly seen that the energy levels of PBDB-T match well with that of ITIC, indicating effective exciton dissociations are able to take place at the PBDB-T/ITIC interface in principle. Fig. S1c (Supporting information) shows the UV-vis absorption spectra of the neat films of PBDB-T and ITIC. Two characteristic absorption peaks can be observed on the absorption spectra of the films of both PBDB-T and ITIC. PBDB-T exhibits strong absorption band in the wavelength region from 500 nm to 700 nm. The peak at 580 nm should be attributed to π-π* transition along the conjugated backbone of the polymer, while the shoulder peak at 640 nm should be related to interchain π-π* transition due to the π-π stacking of the backbones [26-28]. The UV-vis absorption spectrum of ITIC is primarily in the region of 500 nm to 800 nmwith two characteristic absorption peaks successively at 640 nm and 700 nm. Fig. 1a and Fig. S2a (Supporting information) show the absorption spectra and the normalized absorption spectra of blend films with the various D:A ratio. Two distinct peaks can be seen at 640 nm and 700 nm. For PBDB-T: ITIC blend thin films with different D:A ratio, it is difficult to distinguish that the absorption at 640 nm arise from which component because both ITIC and PBDB-T have a strong absorption at 640 nm.With increasing donor component, the shoulder peak of PBDB-T at 580 nm does not notably improve, but the peak of ITIC at 700 nm decreased significantly. The absorption intensity of PBDB-T is more stabilized than ITIC. This indicates PBDB-T and ITIC have different contributions to optical capture of the devices with various D:A ratio [23]. Therefore, it is more important to optimize the D:A blend ratios to obtain the balance of optical absorption and charge separation.

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Fig. 1. (a) UV-vis absorption spectra with the various D:A ratio; (b) J-V curves of OSCs based on PBDB-T and ITIC with different weight ratios; (c) The normalized PL spectra of neat PBDB-T and neat ITIC; (d) The steady-state PL spectra of blend thin films with the pump intensity of 3 mW.

The OSCs were fabricated to investigate the photovoltaic performance of various D:A ratios. The J-V characteristics of devices are shown in Fig. 1b. When the D:A blend ratio is 1:1, the highest PCE is achieved. It is consistent with previous work [29]. The optimized device has an Voc of 0.90 V, a Jsc of 14.83 mA/cm2 and FF of 69.84%, achieving a best PCE of 9.37%. In order to reveal the effect of D:A blend ratio on photovoltaic parameters, the Jsc, Voc, FF and PCE values versus D:A ratios are plotted in Figs. 2ad. It is evidently that under the circumstance of more donors (or acceptors), the FF of devices slightly change. We assume that the bi-continuous percolation pathways remain well with the D:A ratio variation among the range 1:4 to 4:1, causing effectively transportation of the photo-generated charge carriers [30]. When the D:A ratio decreases from 4:1 to 1:1, Jsc increases from 5.39 mA/cm2 to 14.82 mA/cm2. As the D:A ratio decreases further from 1:1 to 4:1, Jsc of the devices decreases from 14.82 mA/cm2 to 7.86 mA/cm2. As the donor content improves, Jsc gradually increases to maximum (corresponding to the optimal D:A ratio) and then exhibits dropping tendency. The decreased Jsc is derived from a reduction in the D:A interface or amorphous region. There is no obvious difference for Voc of the devices, which means no significant energy loss variation exists in the process of changing the D:A ratio.

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Fig. 2. The photovoltaic parameters of the OSCs based on PBDB-T and ITIC with the various D:A ratio under the illumination of AM 1.5 G, 100 mW/cm2.

PL studies were carried out to clarify exciton dynamic processes of blend films with different D:A ratio. As described in Fig. 1c, the neat PBDB-T and ITIC thin films exhibit clear PL emission with the emission peak at approximately 690 nm and 770 nm, respectively. The steady-state PL spectra of blend thin films with the pump intensity of 3 mW is shown in Fig. 1d. As the donor PBDB-T content increasing from 1:4 to 1.5:1, the emission peak of PBDB-Tat 690 nm is almost invisible, indicating that efficient charge transfer occurred between PBDB-T and ITIC, which has been reported in previous work [20, 31, 32]. With the PBDB-T content further increases, the PBDB-T shows an evident emission peak at 690 nm. The results verify that rational D:A ratio enhanced efficient charge transfer, while the enlarged difference in ratios inhibits this dynamic since the generated excitons mostly decayed through geminate recombination.

In order to investigate the effect of pump intensity to charge transfer, we measure the PL spectra with the various pump intensity, shown in Figs. S2b–d and S3 (Supporting information). The PL intensity increases gradually with the various pump intensity from 1 mW to 4 mW. Then the normalized PL spectra with optimal ratio (D:A = 1:1) of blended films can be seen in Fig. S3h. The overlap four PL spectra in different pump intensity suggest that the change of light intensity does not affect π-π stacking [33]. To further investigate the exciton dissociation and charge transfer dynamics between donor and acceptor, the FLIM images of blend thin films with different D:A ratios are recorded as shown in Fig. 3 (the pump intensity is 3 mW). The decay signals are well fitted by a bi-exponential function:

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Fig. 3. Time-resolved imaging (10×10 μm) of PBDB-T: ITIC with pump intensity of 3 mW. The blend films with various ratio of D:A: (a) 1:4; (b) 1:2.3; (c) 1:1.5; (d) 1:1; (e) 1.5:1; (f) 2.3:1; (g) 4:1.

where Ai is the exponential coefficient (%) for the τi decay times [34]. The fitted decay times and corresponding fluorescence decay profiles are shown in Table S1 (Supporting information) and Fig. S3a. When the ratio of D:A increases from 1:4 to 4:1, the average lifetimes of blend thin films are 214 ps, 151 ps, 68 ps, 89 ps, 76 ps, 50 ps, 64 ps, respectively. Lifetime shows a trend of initial decreasing and then remaining unchanged. Normally, regions of various degrees of aggregation of the polymer can cause film inhomogeneities, and these non-uniformities, especially the micro-scale and sub-micron inhomogeneity, can lead to the different fluorescence decay behavior [35-37]. In previous work [31, 38], an optimized D:A ratio (1:1) of blend thin film can obtain maximum photogenerated exciton dissociation into free charges. The more efficient charge transfer between PBDB-T and ITIC, the more increased Jsc of OSCs will be obtained. With high acceptor content, the fluorescence lifetime of the blended film reaches 214 ps, which is a longer lifetime. That means the amorphous domain and the exciton dissociation efficiency is decreased leading to a longer lifetime, which is consistent with previous work [20, 39, 40]. The photoinduced charges which can transfer electrons from donor to acceptor with high accepter content are relatively rare, resulting in a lower Jsc in devices. As the donor content increases, the blend ratio of donor and acceptor changing from 1:4 to 1:1, there is a significant decrease in the lifetime, and corresponding, the Jsc of the device is notably improved. We expect that amorphous domain increased significantly with more acceptor content in the D:A blend, allowing more excitons to dissociate. In the blend film, when the component of the donor is higher than that of the acceptor, the fluorescence lifetime is consistent with the lifetime of the blend film of the optimal D:A ratio. Due to the emission lifetime is monitored at 770 nm, most acceptors are used to perform efficient charge transfer with the donors at high donor component. The radiation recombination in acceptor is suppressed. As the donor component continuously increases, the amorphous domain also decreases, resulting in a decline in device short-circuit current. The time-resolved images of PBDB-T:ITIC blend films with different pump intensity are shown in Figs. S4-S6 (Supporting information), and corresponding fitting lifetimes are shown in Tables S2-S4 (Supporting information). The lifetime curves of D:A (1:1) with different excitation light intensity is shown in Fig. S3b. With different excitation light intensity, fluorescence lifetime does not change significantly, while the change tendency of lifetime is consistent.

The effects of blend ratio on exciton dissociation, charge transfer are closely related to the morphology of the film [41, 42]. A bi-continuous interpenetration network structure can form the channel that facilitates charge transport [23, 43]. Atomic force microscopy (AFM) and grazing incidence wide-angle X-rays scattering (GIWAXS) were carried out to investigate the microstructures of PBDB-T: ITIC blend films. The surface morphology of different composition can be investigated by AFM (Fig. S7 in Supporting information). At the D:A ratio of 1:4, the blend film exhibits a typical wrinkled morphology with the lager root-meansquare (RMS) roughness of 8.43 nm, which should be attributed to the aggregation of the ITIC. Under optimal ratio (1:1) of D:A, the RMS reduce to 1.56 nm, the larger aggregations formed by small molecule ITIC will be less apparent with higher PBDB-T content. The RMS is similar for the films with blend ratio form 1:1 to 4:1. That means the surface of the film is dominantly occupied by PBDB-T with high PBDB-T content [23]. GIWAXS was adopted to investigate the detail information about components crystallinity under various D:A ratios. In our previous work [44], neat PBDB-T film shows an obvious (100) peak at 3.3 nm-1 in the out-of-plane direction and 2.9 nm-1 in the in-plane direction, that implies the formation of lamellar structures. For the neat film of ITIC, the (100) peak is located at 4.48 nm-1. The (010) π-π stacking peaks of PBDBT and ITIC are located at 17.97 nm-1 and 15.38 nm-1, respectively. The out-of-plane and in-plane profiles are shown in Fig. 4, and the 2D GIWAXS patterns of different D:A ratio are shown in Fig. S8 (Supporting information). The blend film shows an obvious (100) peak at 3 nm-1 at the in-plane direction, which means the diffraction peaks are dominated by the donor. When the ratio of D:A is 1:4, a weak (100) peak of ITIC at 4.5 nm-1 can be seen. The conclusion is consistent with previous reports [20, 45]. Upon the loading of more donor polymer PBDB-T molecules, the (100) peak of PBDB-T at 3 nm-1 further increase, and the (100) peak of ITIC at 4.5 nm-1 gradually weakens. That means ITIC domain size reduced and donor domain size significantly enhanced [46]. The π-π stacking distance (d-spacing), obtained through d = 2π/q, are described in Table S5 (Supporting information). Where q is the location of the peak. The d-spacing of PBDB-T and ITIC do not show significant changes with different D:A ratio, the distance of π-π stacking corresponding to molecular packing has not changed significantly [47, 48]. The results verify that the blend PBDB-T:ITIC system can maintain high charge transfer capacity, which is beneficial to obtain high fill factor.

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Fig. 4. 1D cuts of 2D GIWAXS patterns to PBDB-T: ITIC blend thin films with the various D:A ratio.

In conclusion, the D:A ratio effected photophysical properties of non-fullerene organic solar cells are comparatively investigated. When the D:A ratio increased from 1:4 to 4:1, the device is capable of maintaining a high bi-continuous interpenetrating network structure, resulting in a relative high fill factor. Upon the loading of more donor (or acceptor), the domain size increases significantly, resulting in the reduction of Jsc. Interestingly, the trend of Jsc is axisymmetric, that means there is no significant variation in energy loss variation in the process of changing the ratio of D:A. The GIWAXS results show that the molecular arrangement of PBDB-T or ITIC does not change with the various D:A blend ratio. We infer that even with large D:A blending changes, the PBDB-T: ITIC system can still maintain structural stability. With rational D:A ratio, efficient charge transfer, fine morphology of films, stable channel for charge transport can be gained, leading to high device performance.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 61631166001, 11574181, 11774204) and Key R & D Programs of Shandong Province, China (No. 2018GGX103004), X.T. Hao also acknowledges support from the ARC Centre of Excellence in Exciton Science (No. CE170100026). The authors would like to thank the Shanghai Synchrotron Radiation Facility (beamline BL16B1) for providing the beam time for GIWAXS measurements.

Appendix A. Supplementary data

Supplementary materialrelatedtothisarticle canbefound, inthe online version, at doi:https://doi.org/10.1016/j.cclet.2019.01.028.

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