Suppressing Thermally Induced Fullerene Aggregation in Organic

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Article Cite This: J. Phys. Chem. C 2018, 122, 9843−9851

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Suppressing Thermally Induced Fullerene Aggregation in Organic Solar Cells by Employing Plastic Network Xiaoyu Yang,† Mengsi Niu,† Pengqing Bi,† Zhihao Chen,† Jianqiang Liu,*,† and Xiaotao Hao*,†,‡ †

School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, China ARC Centre of Excellence in Exciton Science, School of Chemistry, The University of Melbourne, Parkville, Victoria 3010, Australia

‡

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S Supporting Information *

ABSTRACT: Thermally induced fullerene aggregation restricts the long-term stability in organic solar cells. Herein, we demonstrate an effective method of incorporating crosslinkable small-molecule ethoxylated (2) bisphenol-A dimethacrylate (BPA2EODMA) into poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C 61 butyric acid methyl ester (P3HT:PCBM) blends to form an insoluble framework and inhibit fullerene aggregation. The BPA2EODMA exhibits excellent heat and flexural endurance characteristics, which enhance the thermal and morphological stabilities of the active layer. By systematic research with time-resolved optical spectroscopy and microscopy, the morphological changes and charge-transfer dynamics in P3HT:PCBM blends are explored to unravel the underlying mechanisms for improved photovoltaic efficiency by incorporating the crosslinker. The power conversion efficiency (PCE) increased from 3.6 to 4.2% due to the BPA2EODMA incorporation into the active layer, which is ascribed to the enhancement of exciton dissociation and carrier transportation. After heating at 150 °C for 5 h, the crosslinker-modified devices retained 50% of the initial PCE, whereas the devices without any cross-linker showed 15% PCE retention.

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INTRODUCTION Organic electronics have gained significant research focus over the past 20 years due to the tremendous success of organic light-emitting diodes (OLEDs) for flexible displays and solidstate lighting applications and of organic photovoltaics (OPVs) for solar energy harvesting.1−4 The commercialization of OPVs requires balanced values of power conversion efficiency (PCE), operational device stability, and cost.5 Recently, the PCE of polymer solar cells has exceeded 13%,6 which meets the commercial requirements. Meanwhile, novel materials and device architectures are being explored to further improve the OPV performance. In addition, reducing the synthetic complexity of active materials can effectively control the overall cost.7 However, the long-term stability of OPVs is still a problem, and it is important to understand the underlying failure mechanisms to overcome the device degradation problems. It was generally considered that the instability of the OPV active layer depends on a number of key factors, such as thermal fluctuations, illumination, oxygen and moisture contents, and mechanical deformations. It is worth noting that the influence of oxygen and moisture can be reduced by effective device encapsulation.8 The incorporation of UV blocking materials into the packaging module provides additional protection to endure polymer photobleaching.9 Furthermore, the exploration of novel materials and flexible structures can improve the bending durability issues in flexible OPVs. Nevertheless, thermal induction leads to the morphology deterioration of the photosensitive layer with large © 2018 American Chemical Society

fullerene aggregation in the polymer:[6,6]-phenyl C61 butyric acid (PCBM), which cannot be ameliorated by encapsulation. The fullerene-based OPVs suffer from the microscale crystal growth of fullerenes with extended annealing time, which leads to the reduction in exciton-dissociation and carrier-collection efficiencies.10 Therefore, the thermal-induced fullerene aggregation is one of the major sources of PCE loss. The suppression of fullerene migration, nucleation, and growth can improve the thermal stability of fullerene-based OPVs.11 The utilization of cross-linkable additives has been reported as a facile strategy to suppress thermal-induced fullerene aggregation and reduce costs in the industrial manufacturing process.12−14 Most of the research attention has been focused on functionalizing the donors and acceptors with cross-linkable side chains, such as bromo,15−17 azide,18 and benzocyclobutene.19 However, most of these approaches rely on an elaborate and multistep synthesis with expensive reagents, which restricts their utilization at the commercial scale. Alternatively, introducing small-molecule cross-linkers into the blend systems is considered as an effective approach. Recently, a series of small-molecule cross-linkers, such as fluorinated phenyl azide (s-FPA),20 4,4′-bis(azidomethyl)-1,1′-biphenyl,21 and bis-azide 1,6-diazidohexane,22 have been introduced to stabilize the morphology in various OPVs. Therefore, according to the longterm goal of developing printable electronics, it is important to Received: March 13, 2018 Revised: April 17, 2018 Published: April 23, 2018 9843

DOI: 10.1021/acs.jpcc.8b02447 J. Phys. Chem. C 2018, 122, 9843−9851

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The Journal of Physical Chemistry C

Figure 1. (a) Device structure and (b) molecular structure of P3TH, PC60BM, and BPA2EODMA. (c) AFM image of three-dimensional network structure formed by the cross-linker in the blend films.

Device Fabrication and Characterization. The organic solar cells, with a conventional structure of indium tin oxide (ITO)/PEDOT:PSS/P3HT:PCBM/LiF/Al, were fabricated by following the given steps. First, the ITO patterned glass substrates were cleaned by the glass cleaner, deionized water, ethanol, and isopropanol in an ultrasound bath for 15 min. Then, the 40 nm PEDOT:PSS layer was spin-coated on the substrate at 1700 rpm for 50 s. The PEDOT:PSS layer was thermally annealed at 140 °C for 10 min. To activate the crosslinkers, 2 wt % of Irgacure-819 (BASF) was added as photoinitiator into the BPA2EODMA. The P3HT:PCBM (1:1 weight ratio) was dissolved in ODCB with a polymer concentration of 20 mg/mL. The solutions were continuously stirred for 12 h at 60 °C. Then, 10 wt % of BPA2EODMA, relative to the total weight of the materials, was introduced in the P3HT:PCBM solution and stirred for 2 h at room temperature. The pristine P3HT:PCBM or blends with the 10 wt % cross-linker were fabricated by spin-coating in a nitrogenfilled glovebox, at a speed of 900 rpm for 60 s; this resulted in an average thickness of 130 nm (Figure S2). The films were then cross-linked according to the conditions mentioned above. Thermal aging was carried out for 10 min, 1 h, and 5 h at 150 °C in the glovebox. The LiF (0.8 nm) and Al (100 nm) were prepared by thermal evaporation under vacuum at a pressures of about 1.0 × 10−4 Pa through a shadow mask to define effective device areas of 4 mm2. The OPV performances were characterized in an air atmosphere under simulated solar illumination (AM 1.5G, 100 mW/cm2) using a Keithley 236 source meter unit. The light source intensity was calibrated by using a silicon reference diode. The external quantum efficiency (EQE) was measured using the 7-SCSpecIII system (Sofn Instrument Co., Ltd). Optical Characterization. The fluorescence decay profiles were recorded by femtosecond time-resolved fluorescence upconversion spectroscopy (HALCYONE, Ultrafast Systems, LLC). To eliminate the effects of rotational motion, a pump light polarization was set at the magic angle (54.7°). The timeresolved fluorescence anisotropy measurement was carried out by using the Halcyone system, provided with a time-correlated

explore cost-effective cross-linked small molecules to improve the morphological stability in OPVs. In this study, we introduce a commercially available crosslinkable material, ethoxylated (2) bisphenol-A dimethacrylate (BPA2EODMA), which suppresses the thermal-induced fullerene aggregation in poly(3-hexylthiophene-2,5-diyl):[6,6]phenyl C61 butyric acid methyl ester (P3HT:PCBM) bulk heterojunction (BHJ). The cross-linker handles the issue of morphological instability in the polymer:fullerene devices for long-period operation under high temperature. The underlying mechanism of the morphological stability and enhanced PCE is elucidated by using time-resolved optical spectroscopy and microscopy. Under UV irradiation, BPA2EODMA forms a three-dimensional framework structure in the blend films, which results in excellent thermal and flexural endurance. We further demonstrate the effect of BPA2EODMA addition on exciton dissociation and carrier transportation. Compared to the aforementioned methods for stability improvement, the proposed method offers a flexible and cost-effective way to improve the PCE and stability, which shows the potential for commercial applications.

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EXPERIMENTAL SECTION Materials. P3HT (Sigma Aldrich, >98% head-to-tail region regular, average Mn 54 000−75 000) and PC60BM (1-Materials, >99%) were used as received. BPA2EODMA (>99%) was purchased from DECOMER Co, Ltd, Guangzhou. ODichlorobenzene (ODCB) was bought from Sigma Aldrich. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (CLEVIOS PVP AI4083) solution was prepared by adding isopropanol into PEDOT:PSS (5:1 v/v) to improve the wettability. Photocross-Linking. To obtain the cross-linked network, the films were blended with cross-linker in a nitrogen atmosphere box, with O2 and H2O contents 35 J/cm2, under UV light (λ = 365 nm) irradiation. The curve of the dose with respect to time is presented in Figure S1. 9844

DOI: 10.1021/acs.jpcc.8b02447 J. Phys. Chem. C 2018, 122, 9843−9851

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Figure 2. Optical microscope images of the P3HT:PCBM active layer after annealing at 150 °C for 10 min, 1 h, and 5 h: (a) P3HT:PCBM and (b) P3HT:PCBM:BPA2EODMA. Note the formation of a large number of micron-scale fullerene aggregates in the pristine films after prolonged annealing, whereas the addition of cross-linker films resulted in a more stable morphology by hindering the fullerene diffusion.

between 200 and 300 nm. To further confirm the phase separation and elemental distribution of network inside the active layer, the EDS mapping of P3HT:BPA2EODMA was employed (Figure S3). The EDS maps confirm the presence of oxygen (O), sulfur (S), and silicon (Si), arising from the crosslinker, P3HT, and the silicon substrate, respectively. The O was uniformly distributed along the network, which is consistent with the AFM observations. Therefore, the combination of AFM and EDS demonstrates that the cross-linked BPA2EODMA forms a contiguous interpenetrating framework into the active layer. To further elucidate the effect of cross-linker on the aging test, along with morphological changes, optical microscopy was employed to monitor the nucleation and growth of PCBM aggregations during thermal annealing (Figure 2). The initial morphology of the active layer, with fine nanoscale phase separation, is in a metastable state, which transforms to a thermodynamical equilibrium state with time under the influence of sunlight irradiation or thermal stress.23 As shown in Figure 2a, in the case of the pristine P3HT:PCBM film, a large amount of tiny PCBM black dots becomes visible after 1 h of thermal treatment at a high temperature of 150 °C. After 5 h, the number and size of these dots gradually grew more prominent along the surface of the film, producing needleshaped patterns. These aggregates can be ascribed to the diffusion, nucleation, and growth of PCBM in the active layer.24 The large-scale PCBM phase segregation domains exceed far more than its optimal length scale, which can cause quick performance degradation due to inefficient exciton dissociation and carrier transport.25 In the case of P3HT:PCBM:BPA2EODMA (Figure 2b), few aggregates were observed after 5 h of annealing. Optical microscopy might be insufficient to visualize the suppression of PCBM crystallization. Figure S4 shows the AFM images of the crosslinked blend films before and after thermal annealing at 150 °C for 5 h. The root-mean-square roughness of the annealed film is about 1.07 nm, which is very close to that of the pristine crosslinked film (0.86 nm). Like the pristine morphology, Figure S4b shows no detectable morphology changes after the heat treatment. All of these results demonstrate that cross-linked

single photon counting (TCSPC) module, and the necessary polarizers. The two-dimensional fluorescence lifetime imaging (Nanofinder FLEX2, Tokyo Instruments, Inc.) was collected with high-resolution confocal microscopy, combined with the TCSPC (SPC-150, Becker & Hickl) technique. Femtosecond transient absorption (TA) spectroscopy was performed by an optical instrument, which combined an oscillator and an optical parametric amplifier system (Coherent Opera Solo). The fundamental 800 nm light, with a pulse width of 195 fs, is split into two parts, one for generating 400 nm pump beam and the other for generating 450−760 nm visible probe light. The pump beam at a dose of 20 μJ/cm2 was chopped at 6 kHz and focused on the sample with 2 mm spot size. BPA2EODMA Network Measurement. As the crosslinked BPA2EODMA is not soluble in ODCB, atomic force microscopy (AFM, NanoScope IIIA) measurement was used to assess the formation of the insoluble network in the P3HT:PCBM films by dropping the ODCB diluted with ethanol on the films to remove the P3HT and PCBM. Scanning electron microscopy (SEM, Zeiss-Sigma) and energy-dispersive X-ray spectroscopy (EDS, Oxford) were carried out to observe the morphology and elemental composition of the network in P3TH films, spin-coated on the HF-etched silicon wafer.

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RESULTS AND DISCUSSION The function of the insulating molecule, BPA2EODMA, in the active layer was systematically investigated. Figure 1a,b illustrates the device and the molecular structure of the P3HT, PCBM, and the cross-linker, BPA2EODMA. Figure 1c shows an AFM image of a three-dimensional network structure formed by mixing the cross-linker with blend films. The crosslinker was mixed with P3HT:PCBM in ODCB solution and spin-coated to form a uniform film, which was then cross-linked with UV light irradiation under nitrogen. The AFM was used to identify how BPA2EODMA integrates into the film, as shown in Figure 1c (details in the Experimental Section). After removing organic materials with the required solvents, we can see that the cross-linked BPA2EODMA provides a desired insoluble matrix, serving as a scaffold for improving the film stability. The diameter of the network frame structure varies 9845

DOI: 10.1021/acs.jpcc.8b02447 J. Phys. Chem. C 2018, 122, 9843−9851

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Figure 3. (a) Optical microscope image of the P3HT:PCBM film after heating at 150 °C for 1 h. (b) Distribution histogram deduced from the fluorescence lifetime images of regions ① and ②, scanning over an area of 30 μm × 30 μm. The lifetime images are shown in the insets.

Figure 4. (a) Change in PCE with respect to annealing treatment time. (b) Best performance of P3HT:PCBM with and without cross-linker, where (c) shows the corresponding EQE spectra of the devices. (d) J−V characteristics of the hole-only device and electron-only device (with and without BPA2EODMA), fitted by the space-charge-limited current (SCLC) model.

marked as regions ① and ②. Region ① represents the fine phase-segregated area, consisting of well-mixed P3HT and PCBM. Region ② represents a depletion of PCBM in the crystal vicinity, and the spreading of this zone is faster than the crystal growth (Figure 2). The depletion zone formed due to the significant movement of the PCBM molecules into the PCBM nucleation location, which is a direct result of softening of the P3HT chains during annealing.24,26 Moreover, by using transient absorption microscopy with simultaneously high spatial and temporal resolution, Huang et al. 25 have

blend films have good thermal stability. From the aforementioned AFM and EDS, the lack of PCBM aggregation in the BPA2EODMA modified film can support the hypothesis that the framework structure formed by the cross-linker can effectively lead to a reduced free movement of PCBM in the active layer. Interestingly, the enlarged optical micrograph of the PCBM cluster in the pristine film (Figure 3a) shows three distinct regions: the PCBM crystal, the orange area surrounding the crystal, and a fine homogeneous zone. The last two regions are 9846

DOI: 10.1021/acs.jpcc.8b02447 J. Phys. Chem. C 2018, 122, 9843−9851

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The Journal of Physical Chemistry C Table 1. Average Device Characteristics of P3HT:PCBM and P3HT:PCBM with BPA2EODMA Organic Solar Cells devices P3HT:PC60BM

P3HT:PC60BM:BPA2EODMA

a

Voc (V)

annealing time (h) 0.17 1 5 0.17 1 5

0.60 0.59 0.45 0.61 0.60 0.60

± ± ± ± ± ±

0.003 0.003 0.003 0.003 0.003 0.003

Jsc (mA/cm2) 9.50 8.50 2.48 9.72 10.16 6.53

± ± ± ± ± ±

0.05 0.16 0.25 0.09 0.16 0.21

FF (%) 62.6 56.6 48.7 66.9 68.3 54.1

± ± ± ± ± ±

0.4 0.8 1.5 0.3 0.5 1.3

PCE (%)a 3.57 2.85 0.54 3.96 4.16 2.13

± ± ± ± ± ±

0.04 0.10 0.31 0.08 0.18 0.26

Average PCE was calculated from the 12 devices tested.

Figure 5. (a) Time-resolved fluorescence anisotropy of P3HT:PCBM with and without BPA2EODMA. (b) Time-resolved fluorescence upconversion spectra of P3HT:PCBM with and without BPA2EODMA.

demonstrated the faster decay signals of the P3HT+ polarons in the PCBM-depleted regions than that in the finely phasesegregated regions, which indicates that the PCBM-depleted regions do not undergo charge separation. To further investigate the consequences of the PCBMdepleted regions on exciton-dissociation dynamics, we have measured the time-resolved fluorescence imaging using confocal optical microscopy.27 Figure 3b presents the timeresolved images (inset panel b) and fluorescence lifetime distribution histograms from region ① and region ②. The samples were scanned over an area of 30 μm × 30 μm selected from the two representative regions. The lifetime distribution was extracted from two-dimensional fluorescence lifetime images. The shorter average lifetime corresponds to the fine phase-segregated region ①, which is located around 50−56 ps with a narrow peak at 53 ps. However, the average lifetime of the PCBM-depleted region ② exhibits a much longer lifetime, in the range of 70−80 ps, with a broad peak around 75 ps. This can be ascribed to the high-efficiency donor−acceptor interfacial contact for exciton dissociation in the uniformly mixed region and less interfacial area in the PCBM-depleted region. To confirm the potential benefit of the BPA2EODMA network, the devices were fabricated with and without the addition of a cross-linker (see the Supporting Information for details). The device performance parameters are presented in Figure 4 and Table 1. The fabricated devices have the configuration of ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al on incorporating 10 wt % BPA2EODMA into the BHJ of P3HT:PCBM. Recently, Blom et al.28 have reported that only 10 wt % of the UV-assisted cross-linkable material (ethoxylated (4) bisphenol-A dimethacrylate, SR540) in OLED is required to obtain an insoluble layer, with minimal damage to the charge transport properties. Thus, incorporating 10 wt % BPA2EODMA (Figure S5) into P3HT:PCBM blends is reasonable to

freeze the morphology and stabilize the PCE upon thermal annealing. Moreover, Fourier transform infrared spectroscopy (FTIR, Figure S6) was used to confirm that P3HT:PCBM does not get damaged by UV illumination in a nitrogen environment. The FTIR also confirmed that P3HT:PCBM did not react with the cross-linker. Figure 4a shows the change in PCE with respect to the thermal annealing time. In the absence of BPA2EODMA, the initial PCE dramatically reduced from 3.6 to 0.54% within 5 h, which can be attributed to the decreased short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF). This trend is in good agreement with the morphology of PCBM crystals and PCBM-depleted region, on the time scale, as discussed in Figures 2 and 3. However, the presence of BPA2EODMA exhibits a positive effect on device performance within the first 1 h, yielding an increased PCE from 4 to 4.2%. In addition, the devices with the cross-linked additive get stabilized at >50% of the initial PCE after 5 h. On the contrary, the devices without cross-linked additives retain only 15% of the initial PCE. The improved performance in the presence of BPA2EODMA can be attributed to the unchanged Voc and increased FF, which result from the desired morphology. As shown in Figure 4b, the highest PCE of 4.2% was achieved after heating for 1 h, which can be ascribed to the soft curing process after the UV exposure.22 It is worth noting that both the modified and reference devices have a similar film thickness, which implies that the PCE improvement cannot be assigned to film thickness. Furthermore, we have chosen the best performing modified and reference devices to explore the underlying mechanism of performance improvement. The suppressed charge-carrier recombination was measured by the dark current measurement, and the results are presented in Figure S7. The dark J−V curve of the cross-linker incorporated devices displays a lower leakage current in the reverse direction and higher current in the forward direction. 9847

DOI: 10.1021/acs.jpcc.8b02447 J. Phys. Chem. C 2018, 122, 9843−9851

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Figure 6. Transient absorption measurements with a 400 nm pump light at different time delays of (a) P3HT:PCBM film and (b) P3HT:PCBM with BPA2EODMA. The kinetic profiles are shown in (c) of the GSB signals probed at 605 nm and (d) of PIA signals at 720 nm.

However, we were able to determine the trend of P3HT fluorescence anisotropy decay kinetics, which shows distinct differences in the two films. The overall lower fluorescence anisotropy of the incorporated BPA2EODMA film compared to that of the pristine film suggests that a more efficient and fast depolarizing process occurred in the former.31 Interestingly, the long nonzero anisotropy of over 1 ns was observed in both solid films, which is due to the overall polymer torsional relaxation.31 Particularly, if the overall polymer molecule is not completely free to rotational diffusion, a nonzero limiting anisotropy value occurs in the decay curve. In Figure 5a, the lower nonzero value suggests a favorable torsional relaxation of P3HT chains in the BPA2EODMA-incorporated films, implying that exciton hopping can be improved along the polymer backbone. Thus, the exciton formed in P3HT would be easier to reach the nearby acceptor for splitting into charges, leading to the enhanced PCE. Further evidence for exciton dissociation at the donor/ acceptor interface is collected by the femtosecond timeresolved fluorescence upconversion spectroscopy time-resolved photoluminescence (TRPL), and the results are shown in Figure 5b. The profiles were normalized to compare the changes in the decay time. With the addition of BPA2EODMA, the PL evolution becomes faster. The fluorescence lifetime is usually a sum of multiple exponential behavior decays. The measured curves can be nicely fitted with biexponential function I(t) = A1 exp(−t/τ1) + A2 exp(−t/τ2) with short instrument response function (IRF, 270 fs). The inset shows the fitted results with two components (τ1 and τ2) and the corresponding relative amplitude (A1 and A2). The incorporation of BPA2EODMA into the active layer resulted in a

These results indicate the excellent diode quality with high rectification ratio by adding the BPA2EODMA into the active layer, which enhances the FF to 68.3%. The slight improvement in the Jsc is also in good agreement with the EQE measurement, given in Figure 4c. Furthermore, the charge-carrier mobility was also investigated, and the results are shown in Figure 4d. The charge-carrier mobility curves can be fitted by the space-chargelimited current (SCLC) model. The calculated electron and hole mobilities in the reference devices were 3 × 10−4 and 0.9 × 10−4 cm2/(V s), respectively. On the contrary, the modified devices have shown enhanced electron and hole mobilities of 6.3 × 10−4 and 2.2 × 10−4 cm2/(V s), respectively. These results indicate that the addition of BPA2EODMA improved the carrier transport capability, which might contribute to the enhanced PCE. The time-resolved fluorescence anisotropy measurements were used to investigate the depolarization dynamics of the P3HT:PCBM films, with and without the BPA2EODMA addition, at the molecular level.29 It offers a direct insight into the depolarization dynamics, which are correlated with the exciton hopping or diffusion and torsional relaxation processes.30 The measured temporal anisotropy is shown in Figure 5a. The detection wavelength was set to be 650 nm, where the signals obtained are considered to be solely from a photophysical process that occurred in the P3HT donor materials, with 400 nm wavelength excitation. The anisotropy (r) provides the degree of randomization, and r(t) defines the kinetics of the process. The depolarization dynamics cannot be accurately measured in this system, owing to the limited resolution of the instrument (instrument response time, IRF:150 ps). 9848

DOI: 10.1021/acs.jpcc.8b02447 J. Phys. Chem. C 2018, 122, 9843−9851

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The Journal of Physical Chemistry C similar lifetime of the fast decay component, τ1 = 3.6 ps, but a higher amplitude (56 vs 51%) than those in the pristine film. The slow decay component decreased from 23 to 20.5 ps with BPA2EODMA incorporation. The average lifetime is calculated according to the equation ⟨τ⟩ = (A1τ1 + A2τ2)/(A1 + A2).32 The average lifetime of P3HT:PCBM was 13.13 ps and was obviously decreased to 11 ps for P3HT:PCBM:BPA2EODMA blend films with the addition of 10 wt % BPA2EODMA. These trends can be attributed to the fast exciton dissociation from the polymer donor to the PCBM acceptor after photoexcitation.32,33 Femtosecond transient absorption (TA) spectroscopy was performed to probe the effect of the BPA2EODMA network on charge-transfer dynamics in the BHJ blend films. The TA spectra and kinetics are presented in Figure 6. As shown in Figure 6a,b, the negative ground-state bleach (GSB) signals, at the 460−630 nm region, and the positive photoinduced absorption (PIA) signals, from 630 to 750 nm, show mirror symmetrical dynamics, suggesting P3HT exciton decay is the primary photophysical process in the films.34 For both films, the GSB signals clearly show three peaks at 510, 550, and 605 nm, which coincide with the (0−2), (0−1), and (0−0) transitions of the steady-state absorption spectra. The bleaching signals decay probed at 605 nm is able to describe the exciton decay process,31 as shown in Figure 6c. The GSB signals in the P3HT:PCBM:BPA2EODMA film occur on a faster time scale than that in the P3HT:PCBM film. The faster dynamics may be attributed to the exciton population decay from exciton to charges, which is consistent with upconversion spectroscopy measurement.35 However, TA has a slightly faster decreasing rate than that of TRPL. This might be due to the stimulated emission effects.36 The signals centered around 720 nm represent positive charge carriers (P3HT+ polarons) absorption, which is the product of charge transfer.35,37 Figure 6d compares the charge-transfer kinetics, at 720 nm, for P3HT:PCBM and P3HT:PCBM:BPA2EODMA blend films. It is worth noting that the photoinduced negative polarons, generated in PCBM, have no contribution in this spectral range (680−750 nm) because the absorption signals of PCBM− anions are centered in the 1030−1400 nm region.38 The PIA signals in the film with BPA2EODMA decayed much slower than that in the pristine film in the whole time range. Although the short lifetime of the initial rising PIA signals is partially obscured, the longer PIA component, beyond 100 ps, became more obvious due to the BPA2EODMA addition. These results suggest effective charge transfer, with suppressed charge recombination, in P3HT:PCBM:BPA2EODMA blend films compared to that in P3HT:PCBM films. In addition to the morphological stability, mechanical degradation also hinders the commercialization of organic solar cells. We demonstrate that mechanical degradation can be mitigated by the incorporation of BPA2EODMA into the active layer. One should note that due to the inevitable film fracture, there were many cracks after the bending test, which is undesirable in flexible OPVs. The P3HT:PCBM films coated on the flexible ITO/poly(ethylene terephthalate) substrate were bent 20 times with a bending radius of 8 mm, as shown in Figure S8. Relatively sharp creases were observed in the pristine film after bending, whereas the films with the BPA2EODMA additive maintained a uniform and undamaged morphology. The fracture resistance can be attributed to the excellent fracture toughness and mechanical strength of the crosslinkable BPA2EODMA scaffold, which is a kind of plastic. It

can effectively release mechanical stresses associated with the bending test, and the films demonstrated a remarkable flexural endurance, making it possible to apply in flexible organic solar cells.39,40

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CONCLUSIONS In this work, the UV-cross-linkable small molecule, BPA2EODMA, is introduced into PCBM-based bulk heterojunction to inhibit PCBM aggregation and improve device performance. The BPA2EODMA additive can form a cross-linked network to overcome the thermal instability and improve the mechanical performance simultaneously. Moreover, the enhanced PCE of 4.2% was achieved due to the improved exciton dissociation and charge transport. Furthermore, the excellent stability of the devices has been demonstrated. The efficiency of the devices with the cross-linker decayed to 50% of the initial PCE, whereas the efficiency of the devices without the cross-linkers decayed to 15% after 5 h of thermal annealing. Future work shall focus on exploring the versatility of this type of cross-linker molecules to other organic photovoltaic systems.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b02447. Curve showing the relationship between dose and time under UV light; cross-sectional view of the SEM image; EDS maps and the FTIR spectrum of different films with or without UV light at a dose of 42 J/cm2; dark J−V curves of the devices with and without the BPA2EODMA cross-linker; optical microscope images of P3HT/ PCBM films with and without BPA2EODMA under the bending test (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.L.). *E-mail: [email protected] (X.H.). ORCID

Xiaotao Hao: 0000-0002-0197-6545 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (Nos. 11574181, 61631166001, and 51372141), Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (2015014), and the “National Young 1000 Talents” Program of China. We also acknowledge support by ARC Centre of Excellence in Exciton Science (CE170100026).

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REFERENCES

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DOI: 10.1021/acs.jpcc.8b02447 J. Phys. Chem. C 2018, 122, 9843−9851

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DOI: 10.1021/acs.jpcc.8b02447 J. Phys. Chem. C 2018, 122, 9843−9851

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DOI: 10.1021/acs.jpcc.8b02447 J. Phys. Chem. C 2018, 122, 9843−9851