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Insights on Charge Separation and Transport in Ternary Polymer Solar Cells Qicong Li, Yang Sun, Xiaodi Xue, Shizhong Yue, Kong Liu, Muhammad Azam, Cheng Yang, Zhijie Wang, Furui Tan, and Yong-Hai Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18240 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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ACS Applied Materials & Interfaces

Insights on Charge Separation and Transport in Ternary Polymer Solar Cells

Qicong Li#†, Yang Sun#†, Xiaodi Xue†, Shizhong Yue†, Kong Liu†, Muhammad Azam†, Cheng Yang†, Zhijie Wang†,*, Furui Tan‡,* and Yonghai Chen†,*

†Key

Laboratory of Semiconductor Materials Science, Beijing Key Laboratory of Low

Dimensional Semiconductor Materials and Devices, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, 100083, China. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing, 100049, China. ‡

Key Laboratory of Photovoltaic Materials, Department of Physics and Electronics,

Henan University, Kaifeng 475004, Henan, China *Corresponding authors: Zhijie Wang, E-mail: [email protected]. Furui Tan, E-mail: [email protected]. Yonghai Chen, E-mail: [email protected].

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ABSTRACT Though ternary polymer solar cell is more potential in realizing a high power conversion efficiency than the binary counterparts, the mechanism of exciton separation and charge transport in such complicated ternary system is far from being understood. Herein we focus on this issue and give a clear view on the detailed roles of the ternary components in contributing to the device performance, through utilizing the technique of pump-probe photoconductivity spectroscopy combining with transient photoluminescence (PL) spectroscopy, for the first time to ternary polymer solar cells. The ternary photovoltaic devices are based on PBDB-T:ITIC:PC71BM, and present a dramatic improvement in efficiency in comparison with the binary counterparts. Systematic investigation reveals that the excitons generated in ITIC could be separated at the interface of PBDB-T:ITIC rather than ITIC:PC71BM with holes injecting to PBDB-T. These holes together with those generated in PBDB-T contribute to the photocurrent of the devices. The aggregation of holes in PBDB-T would also weaken the exciton generation herein and the electron injection to PC71BM and ITIC would also be influenced. The key role of PC71BM in the ternary devices is accepting the electrons from PBDB-T and transporting them to the cathode with a higher rate than ITIC. Thus, this manuscript is of importance in constructing high-efficient ternary polymer solar cells.

Keywords: Ternary polymer solar cells, Pump-probe photoconductivity, Exciton separation, Charge transport, PL spectroscopy. 2 ACS Paragon Plus Environment

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INTRODUCTION Through the development for more than two decades, the power conversion efficiency (PCE) of polymer solar cells (PSCs) has reached to 17.3% for the according tandem devices.1 This category of solar cells is of advantages in flexibility, semitransparency and low-cost.2-6 In the configuration of PSCs, donor and acceptor are usually blended in a single film and form the so-called bulk heterojunction (BHJ).7-11 This specific design is to overcome the high binding energy and short diffusion length of photogenerated excitons in the polymers, leading to the efficient separation of excitons around the interface of donor and acceptor. The electrons transport through the acceptor and are collected by the cathode with the electron conducting layer, while holes transport via the donor and are extracted by the anode modified by the hole conducting layer. Conventionally, the active layer is constructed by one donor and one acceptor.12 The relevant PCE of the devices is determined by the solar energy harvesting capability,1318

interactive morphology,19-20 and charge transport efficiency of the two materials.21-25

Due to the limit on the selectivity of the donors and acceptors, the absorption capability is of high difficulty to cover the whole solar energy spectrum, particularly the infrared region.26-27 Additionally, the efficiencies for exciton separation and charge transport of the active layer have also reached the bottleneck and are hard to be improved significantly.28-29 To address these issues and further enhance the PCE of PSCs, the active layer with ternary materials has been proposed. The addition of another donor or acceptor widens the absorption region of solar energy and provides an additional choice 3 ACS Paragon Plus Environment


for exciton separation and charge transport.30-36 Accordingly, the PCE of the PSCs has been reported to 14.62%, higher than the binary counterparts.37 However, unlike routes of exciton separation and charge transport in binary PSCs that have been investigated fundamentally, these issues in the ternary devices are complicated and lack of deserved attention. Herein, we focus on the exciton separation and charge transport in ternary PSCs based on the structure of ITO/PEDOT:PSS/PBDB-T:ITIC:PC71BM/Ca/Al. In comparison with the binary device based on ITO/PEDOT:PSS/PBDB-T:ITIC/Ca/Al, the addition of the second acceptor, PC71BM, could obviously enhance the PCE from 9.03% to 10.43%. By using the technique of pump-probe photoconductivity (PC) spectroscopy in combination with transient PL spectroscopy, for the first time to the ternary PSCs, we have acquired a clear view on the exciton generation and separation as well as charge transport in the complicated ternary PSCs.

RESULTS AND DISSCUSSIONS The molecular structures of the materials in active layer are given in Figure 1a where PBDB-T, ITIC, PC71BM are used as donor, acceptor 1 and acceptor 2, respectively. When the three materials form the active layer, the alignment of the according energy levels is of significance to the exciton separation and charge transport. To get the detailed information, ultraviolet photoelectron spectroscopy (UPS) spectra of these three molecules are measured and shown in Figure S1.38 The highest occupied molecular orbital (HOMO) positions and Fermi levels of PBDB-T, ITIC, PC71BM are obtained as 4.9 and 4.3 eV, 5.7 and 4.5 eV, 6.0 and 4.7 eV, respectively. Figure 1b 4 ACS Paragon Plus Environment

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shows the UPS spectrum of the active layer where the three materials are in equilibrium. The relevant Fermi level is realized as 4.3 eV and such value is the same as that of PBDB-T. This indicates that the energy level of PBDB-T is flat at the interfaces with ITIC and PC71BM, while a slight downward bending of energy levels of ITIC and PC71BM at the interfaces with the donor is formed, respectively. Such bending might possibly be the trap center for the charges due to the formation of energy barrier and future design of molecular structures of donors and acceptors should avoid this issue. The energy level alignments of the PBDB-T/ITIC, PBDB-T/PC71BM and ITIC/PC71BM are given in Figure 1c. The band gap values of these materials are obtained from the relevant absorption spectra given in Figure S2. In comparison with PBDB-T and ITIC, the absorption contribution of PC71BM to the device could be almost ignorable (Figure S2c). Thus, in this ternary system, exciton generation occurs mainly in PBDB-T and ITIC. The lowest unoccupied molecular orbital (LUMO) of PBDB-T is higher than that of PC71BM and ITIC, indicative of that the excitons in PBDB-T could be separated at the interfaces with the two acceptors and the electrons could be injected to PC71BM and ITIC, respectively. Alternatively, the exciton in ITIC could be mainly separated at the interface with PBDB-T and the holes could be injected to the donor. Due to that the LUMO positions of ITIC and PC71BM are quite close, the HOMO of ITIC is higher than that of PC71BM and the trap-shaped energy level bending forms at the interface, the exciton separation and charge injection at the ITIC:PC71BM is not so influential. Additionally, the absorption spectrum of PBDB-T ranges mainly from 350 to 700 nm and that of ITIC is in the region of 500 to 850 nm. This indicates 5 ACS Paragon Plus Environment


that the utilization of these two organic materials could have a wide absorption spectrum of the active layer. These results suggest that the presence of the second acceptor is of benefit to improve the device performance. Figure 2a shows the schematic structure of the devices, where ITO/PEDOT:PSS is for extracting the holes and Ca/Al is to collect the electrons. The according crosssectional scanning electron microscopy (SEM) is given in Figure 2b and the thickness of the active layer is gauged as 98 nm. Figure 2c shows the current density-voltage (JV) curves of the devices based on PBDB-T:PC71BM, PBDB-T:ITIC and PBDBT:ITIC:PC71BM, respectively. In comparison with the binary counterparts, the ternary device presents a dramatic improvement in short-circuit current density (Jsc), fill factor (FF) and a slight enhancement in open-circuit voltage (Voc). These performance parameters are given in Table S1. The incident photon-to-electron conversion efficiency (IPCE) spectra of these devices are given in Figure 2d and the results demonstrate that the presence of ITIC widens the IPCE spectrum over 800 nm and the solar energy absorbed by ITIC indeed contributes to the photocurrent. In comparison with the PBDB-T:ITIC binary device, the PBDB-T:ITIC:PC71BM ternary device delivers a wider IPCE spectrum. Due to the ignorable absorption of PC71BM around 800 nm, this implies that the contribution of PC71BM to the device performance is of high possibility from the role of the efficient electron acceptor and transport route. The integrated currents from the IPCE spectra are also given in Figure 2d, and the values are consistent with those obtained from J-V curves. Herein, the ratio of PBDB-


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T:ITIC:PC71BM in the ternary blend has been optimized as 1:1:0.2 and other ratios of the three components in the blend deliver a low efficiency (Figure S3 and Table S2). Dark J-V curves of the corresponding devices are given in Figure S4, where the ternary devices deliver the smallest leakage current. This indicates the ternary devices are of advantages in suppressing the current leakage, thus beneficial for realizing a high Voc.39 (Detailed discussion is given in the Section 1 of supporting information.) The dependence of photocurrent density (Jph) on effective voltage (Veff) is shown in Figure S5, where Jph increases as Veff increasing and saturates at high Veff value. (Detailed discussion is given in the Section 2 of supporting information.) The saturated photocurrent density is positively related to the maximum exciton generation rate, since the efficiency of exciton generation is unit at high voltage.40 In comparison with the other devices, the ternary devices exhibit largest saturated photocurrent density, indicating the largest exciton generation rate. The morphology of the three components in the active layer is of importance to the exciton separation and charge transport, thus measurements on grazing incidence wideangle X-ray scattering (GIWAXS) of the active layers with different components were performed. Figure 3a-d display the two-dimensional (2D) GIWAXS patterns of neat PBDB-T, blend films of PBDB-T:PC71BM, PBDB-T:ITIC and PBDB-T:ITIC:PC71BM on the Si substrates. The diagram of PBDB-T does not possess the (010) π-π stacking peak, indicating that the polymer shows a poor crystallinity in the neat PBDB-T film. For the blend film, however, the arc-like scatting appears in the diagram and indicates that the binary and ternary systems both show the preferential face-on orientation which 7 ACS Paragon Plus Environment


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is helpful for the charge transport with the respect to the substrate.41 Figure 3e shows the corresponding out-of-plane (OOP) and in-plane (IP) line cuts, thus the π-π stacking corresponding length (Lc) shown in Table S3 could be calculated by Scherrer’s equation using the full width at half maxima (FWHM) of the peak in the diagram.42-44 Lc reflects the crystalline domain size in the active layer.45-46 Accordingly, the Lc of the ternary system is obtained as 20.57 Å, longer than that of the binary systems (19.79 Å; 16.54 Å). The fitted curves for FWHM are given in supporting information (Figure S6). This result indicates that the ternary system is of advantage in crystallinity and charge transport. To get more details on the exciton separation and charge transport, transient PL spectroscopy was measured.47-48 Figure 4a shows the relevant spectra of the films based on

PBDB-T,

PBDB-T:ITIC,

PBDB-T:PC71BM

and

PBDB-T:ITIC:PC71BM,

respectively. The spectra were obtained by exciting the film with the laser of 450 nm and the emission was probed at the wavelength of 528 nm (the emission peak of PBDBT). This experimental design is to monitor the separation of exciton generated in PBDBT. In comparison with the bare PBDB-T film, the blend binary and ternary films show a remarkably shorter PL lifetime. The fitted lifetime of PBDB-T, PBDB-T:ITIC, PBDB-T:PC71BM and PBDB-T:ITIC:PC71BM was obtained as 4.0 ns, 2.5 ns, 2.2 ns, and 2.4 ns, respectively (shown in Table S4).49 For the two binary blends, the presences of ITIC and PC71BM in PBDB-T could obviously shorten the PL lifetime of PBDB-T, thus indicating that both ITIC and PC71BM have the capability to separate the PBDBT exciton at the respective interface with PBDB-T. Due to the weight ratios of ITIC 8 ACS Paragon Plus Environment

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and PC71BM in the respective binary blend are the same, the relatively shorter PL lifetime of PBDB-T:PC71BM than that of PBDB-T:ITIC suggests that the exciton separation efficiency at PBDB-T:PC71BM is slightly higher than that at PBDB-T:ITIC. After exciton separation, the electrons would be injected to the two acceptors and the injection rate to the ITIC and PC71BM could be calculated to 1.5×108 s-1 and 2.0×108 s-1, respectively, using the equation,50 1

1

(1)

𝑘𝑒𝑡 = 𝜏1 ― 𝜏2

Figure S7 presents the transient PL spectra excited at 660 nm and probed at 713 nm. In this case we could focus on the separation of exciton in ITIC. In comparison with the bare ITIC film that presents a PL life time of 2.1 ns, the PBDB-T:ITIC film shows a shorter PL lifetime (1.7 ns) as shown in Table S5 and this suggests that the exciton in ITIC could be separated at PBDB-T:ITIC interface and the holes could be injected to PBDB-T. According to the equation, the hole injection rate is obtained as 1.1×108 s-1. However, for the ITIC:PC71BM film, the PL life time shows the same value with that of bare ITIC film, indicating that the exciton separation at ITIC/PC71BM interface could be ignorable in consistency with the analysis of band energy alignment. To

investigate

exciton

separation

and

charge

transport

route

deeply,

photoconductivity spectra on the active layer were measured.51-52 A laser source is used to provide excitation source with a 5 ps pulse, a repetition rate of 40 MHz and an average power of 4 W. The PC signals were recorded by a lock-in amplifier in phase with a mechanical chopper (the reference frequency is 220 Hz). As shown in Figure S8, the active layers with PBDB-T or ITIC show the photocurrents in the range of 50-150 9 ACS Paragon Plus Environment


pA mW-1, while those with the PBDB-T:ITIC, PBDB-T:PC71BM and PBDBT:ITIC:PC71BM present much larger photocurrents in the range of 300-650 nA mW-1 in Figure 4b. This indicates that exciton separation and charge transport in the blends of PBDB-T:ITIC, PBDB-T:PC71BM and PBDB-T:ITIC:PC71BM are much more efficient than those in the bare PBDB-T and ITIC films. In addition, the PC peaks of the PBDB-T, ITIC are located at 650 nm, 730 nm, respectively, in agreement with the corresponding absorption peak positions of PBDB-T, ITIC. For the PC71BM involved films, due to relevant ignorable absorption contribution, the PC71BM could not affect the PC peaks but the according positive role is providing the interface with the donor to separate the excitons, accepting the electrons and transporting them to the cathode. The dynamics of charge transfer were explored using the pump-probe PC system that was set up by adding a pump light on the base of the PC spectroscopy (the details of this technique are given in the supporting information section 4). A Ti: sapphire laser with spectral width of 10 nm and a repetition rate of 80 MHz is employed as the pump source. In this case, the probed PC signal is solely from the probe light. Figure 4c presents the pump-probe PC spectra of the samples based on PBDB-T:ITIC. At the pump light of 690 nm (light intensity: 7 mW), the PC signal exhibits a dramatic drop. The signal could be further reduced to 41 nA mW-1, as we use the pump light of 730 nm (light intensity: 7 mW). This result indicates that the pump light could indeed impact the PC signal and the relevant influence is determined by the optical feature of the pump light. Though the pump light of 690 nm is not at the PC peaks positions of PBDB-T or ITIC, it can still generate excitons in both PBDB-T and ITIC. The generation of such 10 ACS Paragon Plus Environment

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excitons could limit the exciton generation by the probe light, due to the fact electron density in the HOMO of the two molecules is reduced by the pump light. In addition, the aggregation of the pump-generated excitons and the according separated charges at PBDB-T:ITIC could also inhibit injection of charges generated by the probe-involved excitons. These two factors are responsible for the reduction of PC signals generated by the probe light. Such PC signals could be even decreased further, as we use the pump light at 730 nm (light intensity: 7 mW). The wavelength of this pump light is at the PC peak position of ITIC and this makes the pump light could only excite ITIC. In this case, the generation of the probe-involved exciton is severely inhibited and the aggregation of the pump-involved excitons at PBDB-T:ITIC could prohibit the electron injection from PBDB-T. The inset shows the normalized PC curves and the three curves exhibit the same profile, indicating that the pump light could only impact the intensity of the PC signals in the binary system. This phenomenon could also be observed in the binary system based on PBDB-T:PC71BM (Figure S9). Figure 4d shows the pump-probe PC spectra of the samples based on PBDBT:ITIC:PC71BM and the ternary device presents a complicated and interesting phenomenon. As we tune the wavelength of the pump light from 690 nm to 730 nm, the probed PC signal shows a decrease feature, similar to the situation in the binary system based on PBDB-T:ITIC. When the pump light wavelength is further modulated to a longer value, however, the PC signals increase. The overall evolution of PC signals upon the wavelengths of pump light is plotted in the inset. The highest attenuation of PC signal at the pump light of 730 nm demonstrates that the generation of exciton by 11 ACS Paragon Plus Environment


the probe light in ITIC is inhibited to the largest extent. Once the electrons of the excitons occupy the states around LUMO of ITIC, the electron injection from PBDBT could also be limited. These two issues are responsible for that attenuation of PC signals herein. However, the lowest PC signal for the ternary system is still much higher than that in the binary system based on PBDB-T:ITIC. Though exciton generation and electron acceptance in ITIC have been inhibited to the largest extent, the probegenerated excitons in PBDB-T can still separate at the interface with PC71BM with electrons injection to that material. To get more information on the charge injection, the pump-probe PC spectra for the ternary system were normalized and given in Figure 4e. Interestingly, unlike those in the binary system whose profiles are the same, FWHM of the curves shows an attenuation tendency as we tune the pump wavelength from 690 nm to 730 nm. Further lengthening the wavelength of pump light makes the FWHM wide by approaching to the original value (shown as the inset). By comparing the ternary samples with the PBDB-T:ITIC counterparts, the only difference is the addition of the PC71BM and this is of high possibility to cause the change of the FWHM as we tune the wavelength of pump light. In PBDB-T:ITIC system, the pump light inhibits the exciton generation by the probe light, and the occupation of the pump-generated excitons and charges both in PBDB-T and ITIC would also prohibit the probe-generated electron injection from PBDB-T to ITIC and hole injection from ITIC to PBDB-T. These two issues cause the attenuation of PC signals. Due to the only one charge injection route in this binary system, as the wavelength of the pump light close to 730 nm, both of two factors would 12 ACS Paragon Plus Environment

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be influential synchronously to the probe-involved exciton generation and charge injection. In this case, the pump light would attenuate the PC signals proportionally according to the wavelength of the probe light. Thus, the normalized PC curves in the binary system at the different pump lights have the same profile. In the ternary system, however, the situation becomes complicated. Though the above two factors are still effective, another charge injection route to PC71BM has to the considered. This route could be approved by the remarkable wider FWHM of the ternary system than the PBDB-T:ITIC system (129 nm vs 101 nm) for the PC curves without pump light. In case that the ternary samples are pumped and electron injection from PBDB-T to ITIC is inhibited, the electrons in PBDB-T could be still accepted by PC71BM. This additional electron injection route might not be influenced directly by the pump light close 730 nm, since this pump light could mainly inhibit the probeinvolved exciton generation in ITIC and the charge transfer between ITIC and PC71BM is ignorable. Thus, as the exciton generation in ITIC and charge injection between PBDB-T and ITIC are prohibited synchronously, the acceptance of electrons of PC71BM from PBDB-T is still influential. This causes the wider FWHM of the ternary sample than the PBDB-T:ITIC under the pump light of 730 nm (104 nm vs 101 nm). However, the electron injection from PBDB-T to PC71BM could be still affected indirectly by the pump light close 730 nm. As the pump generates a large amount of excitons in ITIC, these pump-related exciton would separate and the holes would be injected to PBDB-T. As the pump-related holes occupy the HOMO of PBDB-T, the generation of probe-involved exciton would be inhibited in some sense and the amount 13 ACS Paragon Plus Environment


of the electrons accepted by PC71BM would decrease accordingly. This response to the pump light at 730 nm might be not as straightforward as those of the exciton generation and charge transfer in ITIC, leading to the disproportional attenuation effect to the pump light. More evidences were obtained by focusing the pump light at 730 nm and tuning the intensities. As shown in Figure 4f, with the increase of pump light intensity from 0.07 to 30 mW, the PC signals keep attenuating and the strengthened inhibiting effect for exciton generation in PBDB-T and ITIC as well as the charge transfer between these two materials are responsible for this phenomenon. In addition, in Figure S10, FWHM of the normalized PC curves also shows a shrinking tendency and the values reach to the lowest value. The PC curves under the pump light with the intensity of 7 mW, 12mW and 30 mW share nearly the same profile. This indicates that the generation of exciton in PBDB-T has been inhibited to the largest extent, though the pump light of 730 nm is not relevant to the absorption of PBDB-T. These results demonstrate that exciton generation in PBDB-T and ITIC influences the exciton separation between them, the presence of PC71BM provides another interface of separation of exciton in PBDB-T. Accordingly, the exciton dissociation possibility (P(E,T)) of the ternary devices is the highest by comparing with the binary counterparts, as demonstrated in Figure 5a. (Detailed discussion is given in the Section 2 of supporting information.) After exciton separation and charge injection, the transport efficiencies of electrons in the acceptors and holes in the donor are also of significance to the device performance. Figure 5b shows the J1/2-V curves of the series of hole only devices. Using 14 ACS Paragon Plus Environment

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the well-known Mott-Gurney law to fit the curves, the hole conductivities of the devices based on PBDB-T:ITIC, PBDB-T:PC71BM and PBDB-T:ITIC:PC71BM are obtained as 1.01×10-4cm2V-1s-1, 2.39×10-4cm2V-1s-1 and 1.94×10-4cm2V-1s-1, respectively.53 In addition, the hole trapping states of these devices could also be acquired as 4.15×1016cm-3, 2.34×1016cm-3 and 3.25×1016cm-3respectively.54 Detailed discussion on this issue is given in the supporting information (Section 3, Figure S11). Apparently, the PBDB-T:PC71BM device presents a larger hole mobility and a lower hole trapping state than the PBDB-T:ITIC device, demonstrating that PC71BM has a superior capability in capturing the electrons and reducing the recombination possibility of electron and holes than ITIC. In this case, the hole transport in PBDB-T and to the cathode is improved in the PC71BM based binary devices. The values of hole conductivity and trap state in the ternary devices are between the two binary devices. The J1/2-V curves of the electron only devices with structure of PBDB-T:ITIC, PBDB-T:PC71BM and PBDB-T:ITIC:PC71BM, are given in Figure 5c. Interestingly, the PBDB-T:PC71BM device presents the highest electron mobility and lowest electron trap states in comparison with the PBDB-T:ITIC and PBDB-T:ITIC:PC71BM devices. To get a straightforward evidence, impedance spectra were measured and shown in Figure 5d. The PBDB-T:PC71BM device delivers the lowest resistance by comparing with the other devices, considering that the semicircle in a Nyquist plot at high frequencies is characteristic of the charge transfer process and a small semicircle usually indicates a low charge transfer resistance. The detailed parameters obtained by fitting the plots using the equivalent circuit shown as the inset are given in Table S8. 15 ACS Paragon Plus Environment


Collectively, we could get a clear understanding on the exciton separation and charge transport in ternary PSCs based on PBDB-T:ITIC:PC71BM. First, ITIC could extend the absorption spectra to the infrared region and behaves as the second source for exciton generation. The exciton in ITIC could separate at the interface with PBDB-T with holes injecting to that polymer, but the exciton separation at ITIC:PC71BM could be ignorable. Furthermore, ITIC could also be the acceptor to capture the electrons generated by the separation of exciton in PBDB-T. With the two kinds of exciton generation and separation, visually, the efficiency of the device like PBDB-T:ITIC should have been high. However, the situation is complicated and the efficient transport of the exited charge carriers should be considered. The device based on PBDB-T:ITIC has been confirmed to present the lowest electron and hole conductivities as well as the highest charge transfer resistance in comparison with the devices based on PBDBT:PC71BM and PBDB-T:ITIC:PC71BM. This demonstrates that charge transport in the donor and acceptors also plays a key role in device performance. Second, though both ITIC and PC71BM could capture the electrons in PBDB-T and transport them to the cathode, the capabilities of electron capture and transport in PC71BM is of advantageous over those in ITIC. In order to further improve the device performance, future design for the new acceptor should focus on this issue and at least makes the two aspects of the new acceptor comparable to those in PC71BM, not only on improving the absorption ability to a wide spectrum. Thus, the role of PC71BM in the ternary device is providing an efficacious interface to separate the exciton in PBDB-T, capturing the electrons and transporting them to the cathode efficiently. Though the ratio of PC71BM in the active 16 ACS Paragon Plus Environment

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layer is low, it can still form effective pathway to transport electrons. Even in the case of binary device, such low ratio could still improve the PCE greatly, in comparison with the device based on only PBDB-T (Figure S12). This indicates that the hopping mechanism of electron transport in PC71BM is still effective, though the hopping distance is relatively longer than that in the system with a high ratio of PC71BM. Third, the charge injection between PBDB-T and ITIC (holes from ITIC to PBDB-T, electrons from PBDB-T to ITIC) would weaken the exciton generation in PBDB-T and ITIC in some sense, due to the occupation of the charges in the exited states. Though PC71BM does not contribute to exciton generation, the electron capture of PC71BM could also be influenced by the excited light.

CONCLUSION Summarily, we have performed a comprehensive investigation on PSCs based on PBDB-T:ITIC:PC71BM. Such ternary devices present a higher PCE than the binary counterparts. Through utilizing of the technique of pump-probe PC spectra in combination of transient PL spectra, for the first time to the ternary PSCs, we have obtained a clear understanding on the exciton separation and charge transport. In particular, the excitons generated in ITIC could be separated at the interface of PBDBT:ITIC rather than ITIC:PC71BM with holes injecting to PBDB-T. These holes in combination with those generated in PBDB-T contribute to the photocurrent of the devices. The aggregation of holes in PBDB-T would also weaken the exciton generation herein and the electron injection to PC71BM would also be influenced. The key role of PC71BM in the ternary devices is accepting the electrons from PBDB-T and 17 ACS Paragon Plus Environment


transporting them to the cathode with a higher rate than ITIC. Therefore, with the systematic view on exciton generation and separation as well as charge transport in ternary PSCs, this work is of significant to design new molecules as efficient donor or acceptor and to further improve the PCE of the ternary PSCs.

EXPERIMENTAL SECTION Materials and solution Preparation: PBDB-T, ITIC and PC71BM were purchased from 1-Material. The PEDOT:PSS aqueous solution (Clevios P AI 4083) was purchased from H.C. StarckClevios. Other materials and solvents were commercially available and used without further purification. PBDB-T, ITIC and PC71BM were dissolved (1:1:0.2, wt%) in a mixed solution of chlorobenzene and 1,8-diiodooctane (99:1, vol%) with the PBDB-T concentration of 7.5mg mL-1. Device Fabrication: First, prepatterned ITO substrates were sequential ultrasonic cleaned in deionized water, acetone, and isopropanol and then dried under nitrogen flow before used. Prior to film deposition, the substrates were treated with UV-ozone for 15 min. PEDOT:PSS (filtrated with a 0.45 µm PTFE filter) was spin-coated on the ITO surface at 5000 rpm for 40 s and then annealed at 140 ℃ for 15 min in ambient atmosphere. Then the treated samples were transferred into a glove box which was filled with nitrogen. The polymer solution was spin-coated at 1500 rpm for 60 s to obtain a film thickness about 100 nm. The film was dried at 100 ℃ for 10 min. Finally, about 10 nm Ca as the buffer metal and 100 nm Al were deposited by thermal evaporation as cathode under vacuum at a pressure of 2×10-4 Pa.


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Characterization: The UPS data were obtained with AXIS ULTRA DLD. Thinfilm (on ITO substrates) absorption spectra were measured on ultraviolet-visible spectrophotometer (UV-vis, Persee TU-1950) in transmission mode. The crosssectional SEM images were measured by JSM-7401F to get the structure of samples. The current density-voltage (J-V) characteristics of samples were measured by a Keithley 2450 source under AM 1.5G illumination with an intensity of 100 mW cm-2. The IPCE measurements were performed by a QEPVSI-b Measurement System (Newport Corporation). GIWAXS images were acquired using a Xeuss WAXS system and all the samples were deposited on the Si substrate. Impedance spectroscopy measurements were performed with a CHI660E electrochemical workstation under dark condition with initial electrical level of 0.9 V, oscillating voltage of 100 mV and frequency scanning range of 100 Hz to 1 MHz. The transient PL measurements of BHJ and neat films were performed with time correlated single photon counting (TCSPC) system (Edinburgh F900) under excitation light at 450nm and 660 nm. To get the photoconductivity signals, a supercontinuum laser source combined with a monochromator was used, providing excitation source with wavelength ranging from 500-900 nm and the supercontinuum laser was about 5 ps pulse with a repetition rate of 40 MHz. After the monochromatic light with a line width of 1.5 nm went through a chopper, a Gaussian profile light spot with a diameter about 2 mm was distributed in the center of two electrodes with the power of 0.3 mW. The PC signals were obtained by applying a dc-bias voltage and detecting the PC signals by a preamplifier, recording by a lock-in amplifier in phase with a mechanically chopper. For further investigating 19 ACS Paragon Plus Environment


about the dynamics of charge generation and separation, a system of pump-probe PC spectroscopy was designed. The pump-probe PC spectroscopy was measured by adding a pump light on the base of the PC spectroscopy. The pump light was carried out by using a Ti: sapphire laser with spectral width of 10 nm and the repetition rate of 80 MHz. The spot area of the pump light was approximately twice the area of the probe light and the light spot was also distributed in the same position between the two electrodes.

ASSOCIATED CONTENT Supporting Information Experimental details, UPS spectra, absorbance spectra, photovoltaic data, PL spectra, photoconductivity spectra, schematic diagram of pump-probe PC spectroscopy and supporting tables. The Supporting Information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (Zhijie Wang) * E-mail: [email protected] (Furui Tan) * E-mail: [email protected] (Yonghai Chen)

Notes The authors declare no competing financial interest. 20 ACS Paragon Plus Environment

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ACKNOWLEDGEMENTS This work was mostly supported by the National Key Research and Development Program of China (Grant No. 2017YFA0206600), the Key Research Program of Frontier Science, Chinese Academy of Sciences (Grant No. QYZDB-SSW-SLH006), the National Natural Science Foundation of China (Contract No. 61674141, 61504134， 21503209，11574302 and 61474114), Z. Wang appreciates the support from Hundred Talents Program (Chinese Academy of Sciences). Qicong Li and Yang Sun contributed equally to this work.

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Figures and Figure captions (a)

ITIC PC71BM PBDB-T

(b)

(c)

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E0 -EF 4.3eV 0

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4 16 18 20 Binding Energy (eV)

Figure 1. (a)The molecular structures of PBDB-T, ITIC and PC71BM. (b) The UPS spectrum of ternary active layer. (c) The energy level diagrams of PBDB-T, ITIC, PC71BM, PBDB-T:ITIC, PBDB-T:PC71BM and ITIC:PC71BM blend films.


(a)

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Figure 3. (a-d) 2D GIWAXS patterns of PBDB-T, blend films of PBDB-T:PC71BM, PBDB-T:ITIC and PBDB-T:ITIC:PC71BM. (e) 1D integrated scattering profiles for the corresponding films.


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Figure 4. (a) Transient PL spectra of PBDB-T, PBDB-T:ITIC, PBDB-T:PC71BM and PBDB-T:ITIC:PC71BM films (excitation light wavelength: 450 nm, probe light wavelength: 528 nm), respectively. (b) PC spectra of PBDB-T:ITIC, PBDB-T:PC71BM and PBDB-T:ITIC:PC71BM with the PC spectra of PBDB-T and ITIC in the inset zoom. (c) Pump-probe PC spectra of PBDB-T:ITIC system with different pump wavelengths. The upper-left inset shows the normalized PC spectra. (d) Pump-probe PC spectra of PBDB-T:ITIC:PC71BM system with different pump wavelengths. The inset shows the pump light wavelength dependence of the change in PC intensity. (e) Normalized spectra of the ternary system with different pump wavelengths. The inset shows the pump light wavelength dependence of the change in FWHM. (f) Pump-probe PC spectra of PBDB-T:ITIC:PC71BM system with different pump powers. The inset shows the pump power dependence of the change in PC intensity.



(b)

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(d) 500

PBDB-T:ITIC:PC71BM PBDB-T:PC71BM 6 4 2

0.1

D:A1:A2 D:A1

0

1

2 3 Voltage(V)

D:A2

4

4

Fitted PBDB-T:ITIC Fitted

300

100

0

5

0

5

PBDB-T:ITIC:PC71BM

200

e(10-4cm2V-1s-1)

PBDB-T:ITIC

1

0

Fitted PBDB-T:PC71BM

400

10

2 3 Voltage(V)

D:A2

h(10-4cm2V-1s-1)

PBDB-T:PC71BM

P(E,T)(%)

P(E,T)

PBDB-T:ITIC:PC71BM

J1/2(mA1/2cm-1)

1

-Z" (Ω)

(a)

J1/2(mA1/2cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 35

0

100 200 300 400 500 600 700 Z' (Ω)

Figure 5. (a) P(E,T)-Veff curves of the devices, the inset shows the dependence of the P(E,T) on different active layers. (b) J1/2-V curves of the hole only devices with PBDBT:ITIC, PBDB-T:PC71BM and PBDB-T:ITIC:PC71BM systems, the inset shows the hole mobility of these systems. (c) J1/2-V curves of the electron only devices with the same blend films, the inset shows the electron mobility of these systems. (d) Nyquist plots of the devices, the inset shows the equivalent circuit model.


Page 35 of 35

PBDB-T PBDB-T:ITIC PBDB-T:PC71BM

1000

PBDB-T:PC71BM

100

10 0

20

40 60 Time (ns)

ITIC

80

Normalized (a.u.)

TOC Image

PL intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100


1.5 1.0 0.5 0.0

600


PBDB-T

PC71 BM PBDB-T


Insights into Charge Separation and Transport in ... - ACS Publications

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