Ternary Organic Solar Cells Based on a Wide-Bandgap Polymer with Enhanced Power Conversion Efficiencies

A low-bandgap acceptor (ITIC) was added to a binary system composed of a wide-bandgap polymer (PBT-OTT) and an acceptor (PC71BM) to increase the light harvesting efficiency of the associated organic solar cells (OSCs). A ternary blend OSC with an acceptor ratio of PC71BM:ITIC = 8:2 was found to exhibit a power conversion efficiency of 8.18%, which is 18% higher than that of the binary OSC without ITIC. This improvement is mainly due to the enhanced light absorption and optimized film morphology that result from ITIC addition. Furthermore, an energy level cascade forms in the blend that ensures efficient charge transfer, and bimolecular and trap-assisted recombination is suppressed. Thus the use of ternary blend systems provides an effective strategy for the development of efficient single-junction OSCs.

In this study, we combined this mixed acceptor based on the narrow-bandgap ITIC and PC 71 BM with a wide-bandgap polymer, PBT-OTT 28 , and sought to optimize the light absorption and morphology of the resulting photoactive layer. The absorption of ITIC complements that of PBT-OTT (300 ≤ λ ≤ 800 nm) and forms an energy cascade that promotes charge transfer in the ternary blend. Here, we define [ITIC] as the ITIC content (wt/wt) relative to that of PC 71 BM. [ITIC] was systematically varied from 0 to 100%. For [ITIC] = 20%, ITIC is well-mixed with PBT-OTT and PC 71 BM, which results in an optimized film morphology and a ternary-blend-based OSC with a PCE of 8.18%, which is 18% higher than that of the binary-blend-based OSC. The charge generation, charge transport, and recombination dynamics of the OSC were characterized to determine the effects of the use of the ternary blend. These results demonstrate that the ternary blend approach is an effective strategy that enables the simple fabrication of highly efficient OSCs.

Results and Discussion
optoelectric properties and the charge transfer mechanism. The chemical structures of PBT-OTT, PC 71 BM, and ITIC are presented in Fig. 1a. The highest occupied molecular orbital (HOMO) energy levels of PBT-OTT and ITIC were determined from their onset oxidation potentials measured by cyclic voltammetry (CV), and their lowest unoccupied molecular orbital (LUMO) energy levels were determined from their optical bandgaps (Fig. 1b) 29 . The ternary blends provide broad and strong absorption covering the range of wavelengths from the visible to the near-infrared (Fig. 1c). The maximum absorptions of the PBT-OTT and ITIC films are at λ = 512 nm and 706 nm respectively. As the ITIC content of the PBT-OTT:PC 71 BM blend increases, the intensity of absorption in the range 680 ≤ λ ≤ 760 nm increases while that in the range 340 ≤ λ ≤ 510 nm decreases and the intensity of the PBT-OTT shoulder peak adjacent to the maximum absorption peak in the film state strengthens.
The presence of the ITIC acceptor results in cascaded energy levels, in contrast to those of the PBT-OTT:PC 71 BM blend (Fig. 1b). The LUMO energy level of ITIC is positioned between that of PBT-OTT and PC 71 BM. Cascaded LUMO energy level alignment promotes electron transfer between the components of the bulk heterojunction blend and ensures efficient exciton splitting and charge transport to the electrodes 30 . The HOMO energy level of ITIC lies between that of PBT-OTT and PC 71 BM, so holes are extracted efficiently from PC 71 BM. To demonstrate that the energy levels of the three components are cascaded, we obtained the film photoluminescence (PL) spectra of PBT-OTT, ITIC, PC 71 BM, PBT-OTT:ITIC (1:1), and PBT-OTT:PC 71 BM (1:1) with excitation at 570 nm, which corresponds to the maximum absorption of PBT-OTT, and of ITIC:PC 71 BM (1:1) with excitation at 705 nm, which corresponds to the maximum absorption of ITIC (Fig. 2). The emission of PBT-OTT is quenched without an increase in the ITIC PL signal and quenched completely without an increase in the PC 71 BM PL signal (Fig. 2a). These results confirm that photoinduced electrons can be transferred from PBT-OTT to ITIC and then to PC 71 BM. OSC devices based on the ITIC:PC 71 BM blend were found to exhibit photodiode characteristics in their J-V curves and EQE peaks in the ranges 300 ≤ λ ≤ 450 nm and 700 ≤ λ ≤ 800 nm, which correspond to the absorption ranges of PC 71 BM and ITIC respectively. These results reveal that holes and electrons transfer from PC 71 BM to ITIC and from ITIC to PC 71 BM respectively. Further, considering that there is evidence for hole transfer from ITIC to PBT-OTT in the demonstration of a PCE of 5.43% for a device based on PBT-OTT:ITIC ( Table 1) and also that the EQE increases in the range 700 ≤ λ ≤ 800 nm (Fig. 3b) because ITIC exhibits high absorption in the PBT-OTT:ITIC blend (1:1) (Fig. 1c), we conclude that holes can transfer from PC 71 BM to ITIC and finally to PBT-OTT. Thus, an energy level cascade forms in the ternary blend system. photovoltaic properties. The photovoltaic efficiencies of ternary blend OSCs fabricated with the inverted structure ITO/ZnO/active layer (PBT-OTT:ITIC:PC 71 BM)/MoOx/Al were evaluated. The overall donor-to-acceptor ratio in the active layer was fixed at 1:1.5 (wt/wt%). 3 vol % 1,8-diiodooctane (DIO) was used as a processing additive. In these ternary blends, the PC 71 BM:ITIC ratio was varied: [ITIC] = 10, 20, 30, 40, 50, and 100 wt %. Figure 3a shows the current density (J) vs. voltage (V) characteristics of the OSCs with the different ITIC contents, and their photovoltaic parameters are summarized in Table 1. The binary reference device based    Table S2). For [ITIC] = 20%, the hole mobility μ h is higher and μ h /μ e is close to 1 (Table S2). However, for [ITIC] = 50%, μ h is reduced and μ h /μ e deviates from 1. We conclude that under the optimal conditions when [ITIC] = 20%, photo-generated charge carriers are extracted more efficiently to the electrodes in the device than at other [ITIC]. These phenomena are also affected by bimolecular recombination, which is discussed in Section 2.4. For [ITIC] > 20%, increases in the [ITIC] of the blend up to 50% simultaneously degrade the J SC and FF values of the associated OSCs. V OC gradually increases as [ITIC] increases, possibly because the high-lying LUMO of ITIC leads to charge transfer (CT) state with higher energy than that of PC 71 BM at the donor polymer/ acceptor molecule interface. In ternary blends, CT states can form at both PBT-OTT/ITIC and PBT-OTT/PCBM interface, and average CT energy determines final V OC .
The overall EQEs for [ITIC] = 10, 20, and 30% in the ternary blend are significantly higher than those of the PBT-OTT:PC 71 BM binary blend (Fig. 3b). This result demonstrates that more photogenerated excitons in the active layer dissociate to free charges and are collected by the electrodes, as indicated by the significant increase in EQE for the range 630 ≤ λ ≤ 800 nm due to the increase in light absorption that results from the introduction of ITIC, and that the energy level cascade of PBT-OTT, ITIC, and PC 71 BM improves the charge carrier transport. charge generation and dissociation dynamics. To investigate the improved J SC of the ternary OSCs, the charge generation and dissociation of the OSCs with [ITIC] = 0, 20, 50, and 100% were assessed by determining the saturation current density J sat and the charge dissociation probabilities P(E, T). Figure 4a shows the photocurrent density (J ph ) versus effective voltage (V eff ) curves for these ternary devices. Here, J ph is defined as J ph = J L − J D , where V 0 is the voltage at which J ph is zero and V a is the applied bias voltage 31 . Generally, all photogenerated excitons are assumed to dissociate into free charge carriers at high V eff (approximately 2 V), and then J sat is only limited by the maximum exciton generation rate (G max ). As a result, J sat equals to qLG max , where q is the constant of the elementary charge and L is the active layer thickness 32  charge recombination dynamics. To further investigate the effects of charge recombination dynamics on the efficiencies of the ternary OSCs, we obtained the J SC -light illumination intensity plots for the four devices (Fig. 5a). It is known that J SC has a power-law dependence on the light intensity (P light ), i.e. J SC ∝ (P light ) S , in OSCs 34   www.nature.com/scientificreports www.nature.com/scientificreports/ The relationship between V OC and P light in OSCs with various [ITIC] is presented in Fig. 5b. The slope of each V OC versus ln(P light ) plot can be used to investigate the extent of trap-assisted recombination in the OSCs: a slope of k B T/q indicates whether trap-assisted recombination is dominant or not, where k B is Boltzmann's constant, T is the absolute temperature. For Shockley-Read-Hall recombination or trap-assisted, the dependence of V OC on P light is stronger and results in the slope of 2k B T/q 34,35 . In our case, the blend with [ITIC] = 20% produces the smallest slope, 1.44 k B T/q. These results show that the incorporation of a low concentration of ITIC in the host blend reduces the density of interfacial surface traps in the active layer; this reduction suppresses trap-assisted recombination and contributes to an increase in J SC .
Thin film morphology and molecular ordering. To investigate how the presence of ITIC affects the film morphologies and photovoltaic properties of the blends, atomic force microscopy (AFM) was used. The PBT-OTT:PC 71 BM films are homogeneous with a root-mean-square roughness (RMS) of 3.02 nm (Fig. 6a), which resulted from the high miscibility of PBT-OTT and PC 71 BM 32 . For [ITIC] = 20%, the morphology is aggregated (RMS = 3.49 nm), which enables the development of an interpenetrating network and reduces the interfacial trap density in the active layer, and thereby improves the PCE of the associated OSCs. Increases in [ITIC] result in the formation of large aggregated regions and surfaces with high RMS values. These effects produce a significant reduction in FF (Section 2.2).
To further understand the results for the morphologies, optical microscopy (OM) was conducted for various weight ratios of ITIC and PC 71 BM (Figs 6a and S2). For [ITIC] ≤ 20%, the resulting morphologies are almost clear, which indicates that the three components are well mixed. In contrast, obvious ITIC crystals form for [ITIC] > 30%; they are largest for [ITIC] = 40%, but decrease in size and increase in number for [ITIC] = 50% and 100%. For [ITIC] > 30%, the ITIC molecules do not mix well with PBT-OTT, so exciton dissociation is presumably inefficient and efficient electron transport pathways do not form.
To investigate the compatibility of PBT-OTT with ITIC and PC 71 BM, the surface energies of PBT-OTT, ITIC, and PC 71 BM were measured ( Fig. S3; Table S5). Generally, the similar surface energies of components ensure good compatibility between the components. The obtained surface energies of PBT-OTT, ITIC, and PC 71 BM were The crystal orientations and crystallite sizes of ternary blend films with various ITIC contents were studied by using grazing-incidence wide-angle X-ray scattering (GIWAXS) (Fig. 6b). The PBT-OTT:PC 71 BM film exhibits a face-on orientation, which can increase the favorability of intra-and inter-molecular charge carrier transport by the polymers in OSC devices 32 . The addition of ITIC at a concentration of 20% increases the intensity of the scattering peaks attributed to the face-on orientation. This preferential face-on orientation enhances charge transport and thereby the photovoltaic properties, as demonstrated by the improved SCLC results (Fig. S1). Further increases in [ITIC] up to 100% result in gradual increases in the intensity of the peak due to the face-on orientation; this trend indicates that the face-on orientation of ITIC favors the face-on orientation of PBT-OTT, i.e. the presence of ITIC enhances the intensity of the peak due to the face-on orientation. Furthermore, for [ITIC] ≥ 50%, two separate peaks due to face-on PBT-OTT and ITIC in PBT-OTT:ITIC binary blend films are evident in the GIWAXS data (Figs S4 and 6b), which indicates that the miscibility of PBT-OTT and ITIC is reduced.
The sizes of the crystal domains of PBT-OTT and ITIC in the blend films were compared by using the Scherrer equation 36 to calculate the coherence lengths (CLs) (Table S6). Increases in [ITIC] from 40 to 100% result in gradually decreases in the CLs of ITIC. This trend is in agreement with the OM results in Fig. S4; for 0% ≤ [ITIC] ≤ 30%, the CLs of ITIC could not be calculated from the GI-WAX results, but the CLs of ITIC are expected to be small, as suggested by the OM results for 0% ≤ [ITIC] ≤ 30%. On the other hand, for 0% ≤ [ITIC] ≤ 20%, the CLs of PBT-OTT increase, but then decrease for 30% ≤ [ITIC]. As a result, the CLs of PBT-OTT mostly increase as the PC 71 BM content in the blends is increased, which indicates that an improvement in the degree of the molecular ordering of PBT-OTT is obtained. This effect may explain the increase in the intensity of the shoulder on the PBT-OTT peak adjacent to the maximum absorption peak in the UV absorption spectra of the film states (Fig. 2d). Although their enhanced face-on orientation is facilitated by the addition of ITIC, addition of ITIC at concentrations greater than 30% reduces the PCE due to the reduced miscibility of PBT-OTT and ITIC.

conclusion
We incorporated narrow-bandgap ITIC into the binary blend composed of wide-bandgap PBT-OTT and PC 71 BM. It was found that the addition of ITIC extends the light absorption of the active layer and increases photocurrent generation; it also establishes an energy level cascade with PBT-OTT and PC 71 BM, which promotes exciton dissociation and charge transfer. The optimum ITIC content (20%) results in a well-mixed and crystalline film morphology, which enhances the charge transport properties. Furthermore, the high-lying LUMO of ITIC, comparing with that of PC 71 BM, boosts the V OC of the ternary OSCs. The low electron mobility of ITIC is compensated by the high electron mobility of PC 71 BM and balanced by the hole mobility of PBT-OTT. Therefore, charge recombination is effectively reduced and photo-generated charge carriers are efficiently collected at each electrode. The optimized ternary OSC with [ITIC] = 20% yields the highest PCE, 8.18%, which is 18% higher than that of the PBT-OTT:PC 71 BM binary OSC. These results confirm the usefulness of the ternary blend approach to the development of OSCs.