ITIC derivative acceptors for ternary organic solar cells: fine-tuning of absorption bands, LUMO energy levels, and cascade charge transfer

Abstract

For effective supplementary acceptor molecules (A₂) in ternary organic solar cell (TOSC) devices, a series of ITIC derivatives were designed and synthesized by incorporating symmetrically or asymmetrically functional termini of various electron-accepting 1,3-indanedione (IND) groups having distinct different electron affinities to the fused core unit, indacenodithieno[3,2-b]thiophene (IDTT). The absorption, thermal properties and LUMO energy level of the prepared ITIC derivatives were precisely controlled according to the terminal group as well as the open-circuit voltage (V_OC) in binary organic solar cells (BOSCs) employing PBDB-T as a host donor polymer and ITIC derivatives as a host acceptor material. In the PBDB-T:ITIC:A₂-based ternary system, simply adding ITIC derivatives, which can form a cascade energy alignment between the LUMO levels of the host PBDB-T donor polymer and ITIC acceptor molecule, as a small amount (approximately 5%) of A₂ acceptor molecules improved the photoelectric efficiency by up to 10% more than that of a PBDB-T:ITIC binary system of the same structure through enhancing V_OC, J_SC, and FF, synchronously. These results offer a route to find the selection criteria for A₂ acceptor molecules suitable for effective use in TOSC devices and develop high-performance TOSC devices.

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Introduction

Although organic solar cells (OSCs) have made significant progress, additional improvement in the efficiency and stability of OSC devices is required for commercialization. Hence, recently, studies have been actively conducted to develop a ternary OSC (TOSC) device with supplementary donor or acceptor molecules added to a binary OSC (BOSC) system.^1–3 TOSCs can combine the strengths of both tandem OSCs and BOSCs, i.e., TOSCs can improve further the photon harvesting efficiency by employing third components, also keeping the simple processing of single-junction devices. In a TOSC device system, so far, most efforts have been made to improve the efficiency of TOSC devices by finding appropriate ternary material systems and to understand the working mechanism of TOSCs.^1–3 To date, cascade charge transfer,^4–7 energy transfer,^8–11 parallel-linkage structures,^12–14 and alloy models^15–17 have been suggested as possible driving mechanisms for TOSC devices; however, owing to insufficient study on TOSC devices, research to identify their driving mechanism remains rudimentary. Nevertheless, clarifying the working mechanism of TOSC devices is crucial to developing a superior TOSC device in the long run. Therefore, a more diverse library of TOSC devices should be established in the future.

Recently, among TOSC devices, the D:A₁:A₂ TOSC device, where D is the donor and A₁ and A₂ are acceptors, using two types of acceptor molecules has received enormous attention.^2,18–21 The second acceptor (A₂) molecule used can play multiple roles of (i) increasing the effect of additional light absorption, (ii) enhancing the effect of open circuit voltage (V_OC) adjustment and charge transfer through an appropriate energy level (especially, lowest unoccupied molecular orbital (LUMO) energy level) alignment, and (iii) optimizing the morphology of the blend film, thereby improving the efficiency of the TOSC device. As mentioned earlier, however, owing to the lack of understanding of the driving mechanism of TOSC devices, selection criteria for a suitable A₂ acceptor molecule are still unclear.^2,3

In the early D:A₁:A₂ TOSC system, fullerene derivatives were widely used as A₂ molecules.^1,2 However, recently, non-fullerene acceptor (NFA) derivatives have received considerable attention for their high superiority as A₂ acceptor molecules.^2,3,20–22 Among the related NFA molecules, ITIC derivatives can be appreciated as a good candidate.^2,3,23–25 ITIC is referred to as one of the most excellent acceptor molecules for BOSC devices since it has both high light-absorbing properties and excellent charge transfer properties owing to strong intermolecular interaction. ITIC has an A–D–A structure and its backbone is composed of a fused core as a donor unit, indacenodithieno[3,2-b]thiophene (IDTT or IDDT), and two terminal 1,3-indanedione (IND) derivative units with an excellent electron affinity, 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC), as acceptor units.²³ However, so far, ITIC and its derivatives have been primarily developed as host acceptor molecules used in BOSC devices.^23–26 Therefore, for ITIC derivatives to also be used as A₂ acceptor molecules of TOSC devices, more kinds of superior ITIC derivatives that can perform the abovementioned roles of the A₂ acceptor molecules are needed to be developed and investigated in more detail.

Very recently, along with ITIC, Y6 has been introduced as one of the most efficient NFA materials for OSCs.²⁶ Y6 consists of an A–DA′D–A structure where an electron deficient-core-based central fused ring with a benzothiadiazole unit is employed, and exhibited a very high photoelectric efficiency of 15.7% in BOSCs paired with the PM6 donor polymer.²⁷ However, owing to the very complicated chemical structure, up to 10 steps are required to prepare Y6 molecules from the starting compound,^27,28 which inevitably leads to a complex and high cost synthesis process. This is not only a major obstacle for the practical commercialization of OSC devices, but also makes it difficult to readily develop various types of Y6 derivatives compared to the ITIC series. Therefore, in research to systematically find efficient A₂ acceptor molecules for TOSCs via a trial-and-error method using various candidate molecules, it is considered that ITIC derivatives are more suitable than the Y6 series.

In consideration of these things, in this study, we tried to develop various ITIC derivatives that can be used as effective A₂ acceptor molecules in TOSC devices, especially where ITIC molecules are used as a host acceptor (A₁) molecule. To this end, it was assumed that the target ITIC derivatives should have a chemical structure similar to that of ITIC so that they may have mutually good compatibility, and eventually prevent “recombination centers” or “morphological traps” in the ternary active layer.^2,29,30 Thus, we fixed the largest and central part of the ITIC molecule, the electron-donating IDTT unit. In addition, in order to precisely regulate the light absorption region and LUMO energy level of the target compounds which affect profoundly the performance of the TOSC devices,^23,31–34 several types of IND derivatives with sufficiently different electron affinities were introduced in various combinations at the terminal end of the IDTT core unit. Therefore, the electron affinity of the IND derivatives introduced at the terminal end was increased by gradually introducing an element or a functional group with a high electron-accepting effect into the IND unit.

To achieve this purpose, we prepared benzo[b]thiophen-3(2H)-one 1,1-dioxide (BTD) (one ketone group of IND is substituted with a –SO₂ unit), 5-fluorobenzo[b]thiophen-3(2H)-one 1,1-dioxide (BTDF) (fluorine is introduced into the phenyl group of the BTD unit), 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC) (also so-called ‘INCN’, one ketone of IND is substituted with a –(CN)₂ unit), and 2-(1,1-dioxidobenzo[b]thiophen-3(2H)-ylidene)malononitrile (ICS) (the remaining ketone of IC is substituted with a –SO₂ unit) as terminal end groups (Fig. 1). Finally, we introduced these IND units symmetrically or asymmetrically at both terminal ends of the IDTT central unit and thus successfully synthesized various target symmetric and asymmetric ITIC derivatives (Fig. 1). Except ITIC and ITICS,³⁵ all the prepared ITIC derivatives are firstly reported for this study. Especially, the prepared asymmetric ITIC derivatives are unique in that there are only a few efforts to develop asymmetric ITIC analogues by terminal end group modulation owing to the complexity and difficulty of the synthesis.^36–38 Until now, most conventional asymmetric ITIC derivatives have introduced asymmetric parts into the central core unit instead of the terminal end group.^39–45 Moreover, given that the optical properties and the LUMO energy level of ITIC derivatives are strongly influenced by the terminal end groups, for this study, these asymmetric molecules comprising two different terminal end groups are expected to be more useful than the previous asymmetric ITIC derivatives in which the asymmetric core units are employed.

Fig. 1 Chemical structures of terminal IND units and symmetric/asymmetric ITIC derivatives.

In this study, we minutely and systematically analyzed the thermal, optical, and electrochemical properties, energy levels, and BOSC device characteristics of the prepared ITIC derivatives according to the terminal group. Based on these results, we prepared PBDB-T:ITIC:A₂-based TOSC devices using the selected ITIC derivatives, and evaluated their functions and roles as A₂ acceptor molecules. In particular, we realized that by simply adding a small amount, approximately 5% (wt), of the selected A₂ acceptor molecules that can form a cascade energy alignment between the LUMO energy levels of the PBDB-T donor polymer and ITIC acceptor molecule, regardless of the symmetric or asymmetric nature, could promote the effective charge transfer, and thus enhance the power conversion efficiency by up to 10% compared with PBDB-T:ITIC-based BOSC devices of the same structure.

Results and discussion

Electron affinity of 1,3-indanedione (IND)-type end groups

We prepared IND derivative units (BTD, BTDF, IC, and ICS) (Scheme S1 and see the ESI for details†) to be introduced at the terminal end of an IDTT core and compared the electron affinity strength of these molecules through cyclic voltammetry (CV) measurements and density functional theory (DFT) calculations. From the measurement of the reduction potentials of BTD, BTDF, IC, and ICS units via CV, we found that the reduction potential values gradually reduced in the order of BTD < BTDF < IC < ICS at constant gaps (Table S1 and Fig. S1†). Furthermore, in the LUMO energy level analysis obtained from the DFT calculations, we confirmed that the LUMO levels of the IND derivatives deepened in the same order as the reduction potential values (Table S1†) and the reduction potential and LUMO levels are highly correlated (Fig. 2), which demonstrates that the fine-tuning of the LUMO energy level of IND derivatives can be achieved by incorporating the electron accepting element or functional group. From these results, the electron affinity strength of the IND derivatives was, as expected, in the BTD < BTDF < IC < ICS order, and the difference in strength was almost constant. Therefore, we expected to effectively control the light-absorbing properties and LUMO levels of the target ITIC derivatives by varying the type and the introduction method (symmetric and asymmetric) of the IND derivatives introduced at the terminal end of the IDTT central unit.

Fig. 2 Correlation plot of the LUMO energy levels of IND-type electron withdrawing groups (BTD, BTDF, IC, and ICS) as determined using DFT calculations (B3LYP/6-31G*) and their reduction potentials as determined using cyclic voltammetry.

Fig. 3 UV/vis absorption spectra of symmetric and asymmetric ITIC derivatives in CHCl₃.

Synthesis of symmetric and asymmetric ITIC derivatives

The target ITIC derivatives were prepared by symmetrically and asymmetrically introducing the IND derivatives (BTD, BTDF, IC, and ICS) into IDTT central units (Scheme 1 and see the ESI for details†). The chemical structure of the ITIC derivatives was fully confirmed via¹H-NMR, ¹⁹F-NMR, FT-IR, mass, and elemental analysis. We prepared symmetric ITIC derivatives, namely, ITBTD, ITBTDF, ITIC, and ITICS compounds, respectively, by introducing BTD, BTDF, IC, and ICS units on both sides of the IDTT-CHO compound by Knoevenagel condensation under certain base conditions. The higher the electron affinity of the IND derivative units introduced, the lower the basicity of the base catalyst used (i.e., piperidine for ITBTD & ITBTDF, and pyridine for ITIC). Even in the case of the synthesis of ITICS, where the ICS units with the largest electron affinity were introduced, no base catalyst was used and the reaction was carried out only under Ac₂O solvent conditions.

Scheme 1 Synthetic routes of symmetric and asymmetric ITIC derivatives.

For asymmetric ITIC derivatives, namely, ITBTD-IC, ITBTD-IC, and ITIC-ICS, unlike symmetric ITIC derivatives, we used a method of introducing the terminal unit with lower electron affinity first into the IDTT-CHO central unit and then gradationally introducing the other terminal unit with higher electron affinities. This was because, when we first introduced the terminal unit with higher electron affinities, such as IC or ICS, obtaining intermediates with the terminal unit introduced on only one side of the central IDTT unit was difficult, and mostly the intermediates with the terminal unit introduced on both sides of the central IDTT unit were obtained. Therefore, for ITBTD-IC and ITBTD-ICS, after introducing the BTD unit on one side, the IC and ICS units were introduced sequentially into the intermediates, respectively. In the same manner, for ITIC-ICS, the introduction of the IC unit into the IDTT central unit was followed by introducing the ICS unit into the intermediate. However, the reaction yield for ITIC-ICS (14.0%) was lower compared to that of ITBTD-IC and ITBTD-ICS (26.2 and 44.8%, respectively) owing to the lower production yield of the intermediate which has the IC unit introduced on one side of the IDTT central unit. All synthesized ITIC derivatives had excellent solubility in general organic solvents, such as dichloromethane, chloroform, and chlorobenzene.

Thermal properties

All the synthesized ITIC derivatives had high thermal stability to the extent that they were suitable for use in OSC devices owing to the rigid structure of the IDTT-CHO central unit, but the thermal properties depended on the introduced terminal IND derivative type or the symmetry/asymmetry in the structure (Table 1, Fig. S2 and S3†). The symmetric ITIC derivative exhibited no specific thermal transition up to 250 °C on differential scanning calorimetry (DSC) (Table 1 and Fig. S2†) and had the thermal decomposition temperature at 5% weight loss (T_d) above 330 °C (Table 1 and Fig. S3(a)†). As the electron-accepting strength of the terminal IND derivatives increased, the T_d increased. However, ITBTD-F showed a slightly lower T_d than ITBTD, despite a higher electron-accepting strength than ITBTD, because the fluorine-substituted aromatic compounds exhibit lower thermal stability compared to hydrogen analogs.^46,47

Table 1 Summary of the thermal, optical and electrochemical properties of ITIC derivatives

Compound	T d [°C]	λ max [nm]	ε max [M^−1 cm^−1]	E ox [V]	E red [V]	E HOMO [eV]	E LUMO [eV]
a The decomposition temperature at 5% weight loss obtained from TGA measurements with a heating rate of 10 °C min^−1 under N2. b Measured in CHCl3 solution. c Onset potential vs. Fc/Fc^+. d Calculated using the equation of EHOMO/ELUMO = −(4.80 + E^oxonset/E^redonset) eV.
ITBTD	336	602	1.87 × 10^5	+0.68	−1.17	−5.48	−3.63
ITBTDF	325	612	1.56 × 10^5	+0.70	−1.11	−5.50	−3.69
ITIC	351	679	1.89 × 10^5	+0.67	−0.95	−5.47	−3.85
ITICS	372	719	1.85 × 10^5	+0.73	−0.77	−5.53	−4.03
ITBTD-IC	326	648	1.64 × 10^5	+0.67	−1.02	−5.47	−3.78
ITBTD-ICS	329	683	1.16 × 10^5	+0.69	−0.81	−5.49	−3.99
ITIC-ICS	334	699	1.44 × 10^5	+0.68	−0.78	−5.48	−4.02

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In all asymmetric derivatives, no melting points were detected up to 250 °C (Fig. S2†), and they had T_d above 320 °C (Table 1 and Fig. S3(b)†). However, their average T_d was lower than that of the symmetric derivatives, and like the symmetric derivatives, as the overall electron-accepting strength of the terminal IND derivatives increased, the T_d also increased.

Photophysical properties

We could ascertain that, depending on the type of terminal IND derivative and the symmetric/asymmetric introduction method, the maximum absorption wavelength of the ITIC derivatives was finely regulated over a wide range of 600–720 nm (Fig. 3). In the symmetric ITIC derivatives, the maximum absorption band of ITIC derivatives shifted toward the long-wavelength region as the electron-accepting strength of the terminal IND derivatives increased. This is because the intramolecular charge transfer (ICT) process in the ITIC molecule, in which charge transfers from the HOMO level of the electron-donating IDTT unit to the LUMO level of the electron-accepting IND derivative,^31,48–50 depends on the electron-accepting strength of the IND unit, that is, the stronger the electron-accepting effect in an IND unit the lower the LUMO in the IND, reducing the ICT absorption bandgap in the resulting ITIC derivatives.

The maximum absorption band of asymmetric ITIC derivatives having terminal IND units of different electron-accepting strength was located between the maximum absorption bands of two ITIC derivatives whose terminal units were introduced symmetrically. This means that, in the asymmetric ITIC derivatives, the ICT process from the IDTT-CHO central unit to the terminal IND unit does not occur only exclusively in the single terminal IND unit with a high electron-accepting strength but also in the IND unit with a low electron-accepting strength, i.e., all of which affect the ICT absorption band of ITIC derivatives.⁵¹ The light absorption properties of ITIC derivatives in the solution were the same as those in the film state (Fig. S4, S5 and Table S2†).

Electrochemical properties & energy levels

Using the results obtained from electrochemical analysis (Table 1 and Fig. 4), we calculated the HOMO and LUMO energy levels of ITIC derivatives. From the calculations, it was confirmed that all HOMO levels of ITIC derivatives are similar, whereas the LUMO levels are finely modulated depending on the type of terminal IND derivative and symmetry of their introduction (Table 1 and Fig. 5). For the symmetric ITIC derivatives, the greater the electron-accepting effect of the terminal IND derivatives, the deeper the LUMO level (ITBTD > ITBTDF > ITIC > ITICS). For the asymmetric ITIC derivatives with different terminal IND units introduced, the LUMO level decreased with the increase in the sum of electron-accepting strength of both terminal IND units (ITBTD-IC > ITBTD-ICS > ITIC-ICS); in addition, they were located between the LUMO levels of the two ITIC derivatives with mutually different terminal units introduced symmetrically, for example, ITBTD > ITBTD-IC > ITIC. This showed that, as the ICT process, the LUMO levels of the asymmetric ITIC derivatives were undetermined by one IND terminal unit with large electron-accepting strength but by all mutually different terminal IND units. These results are because in HOMO states, most electrons are mainly delocalized in IDTT-CHO central units, whereas electrons extend to terminal IND derivative units to be delocalized in LUMO states.^52,53 This was also confirmed by changes in electron cloud density through DFT calculations. For the symmetric ITIC derivatives, electrons were equally concentrated in the same terminal IND units on both sides at the LUMO and LUMO+1 levels (Fig. S6†). However, in the LUMO of the asymmetric ITIC derivatives, electron clouds were concentrically delocalized in the IND terminal unit with a great electron-accepting effect, whereas, in the LUMO+1 level, electron clouds were concentrated primarily in the IND unit with relatively low electron-accepting strength (Fig. S7†). It is also noteworthy that the experimental LUMO levels of ITIC derivatives obtained through solution CV measurements are highly correlated with theoretical LUMO levels obtained through DFT calculations (Fig. 5), and these HOMO and LUMO levels of the ITIC derivatives tend to be the same in film states (Table S2 and Fig. S5†).

Fig. 4 (a) Cyclic voltammograms of ITIC derivatives in the solution of CH₂Cl₂. (b) HOMO and LUMO energy levels of ITIC derivatives calculated using the equation of E_HOMO/E_LUMO = −(4.80 + E^ox_onset/E^red_onset) eV.

Fig. 5 Black circles: correlation plot of the LUMO energy levels of ITIC derivatives as determined using their reduction potentials and DFT calculations (B3LYP/6-31G*). Grey circles: correlation plot of the LUMO energy levels of ITIC derivatives as determined using their reduction potentials and open-circuit voltages (V_oc) obtained from PBDB-T: acceptor-based binary solar cells.

Binary device photovoltaic properties

To examine the basic function of ITIC derivatives as solar cell acceptors, we fabricated conventional BOSC devices using PBDB-T donor polymer, which is one of the successful modeling donor polymers paired with ITIC,⁵⁴ and investigated their solar cell properties (Table 2 and Fig. S8†). The V_OC of the solar cells was between 0.75 and 1.11 V, and followed the same trend as the LUMO level of ITIC derivatives (Table 2 and Fig. 5). This would be a textbook example showing that the V_OC of an OSC is typically determined by the difference between the HOMO level of the donor polymer and the LUMO level of the acceptor molecule.^55–57 Additionally, it can be said that the energy loss generated by these devices also appears to be similar (Table S3†).

Table 2 Photovoltaic parameters of the PBDB-T:acceptor-based binary solar cells

Acceptor^a	V OC [V]	J SC [mA cm^−2]	FF	PCEave (max) [%]
a All the devices were thermally annealed at 100 °C.
ITBTD	1.11	5.99	0.37	2.36 (2.66)
ITBTDF	1.06	8.67	0.41	3.73 (3.81)
ITIC	0.90	14.93	0.67	8.96 (9.05)
ITICS	0.71	9.42	0.36	2.39 (2.80)
ITBTD-IC	0.97	10.53	0.42	4.34 (4.60)
ITBTD-ICS	0.77	5.23	0.34	1.37 (1.47)
ITIC-ICS	0.75	9.26	0.37	2.54 (2.61)

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In terms of device efficiency, derivatives other than ITIC molecules had low overall photoelectric efficiencies due to low J_sc and FF, even if they had high V_OC (for ITBTD, ITBTDF, and ITBTD-IC) or a similar light absorption range (for ITICS and ITIC-ICS) compared with ITIC. The main reason for the low efficiency is believed to be due to the bulky steric effects of BTD and ICS units introduced at the terminal end of the ITIC derivatives, resulting in lower intermolecular interactions of these molecules. In other words, the IC terminal group of ITICs can induce high intermolecular interactions due to their planar structure,^23–26 whereas the –S=O functional groups of BTD and ICS groups are placed perpendicular to the plane in terminal IND units (Fig. S9†), which disturbs the intermolecular interaction with neighboring molecules. This is presumed to result in preventing the formation of an effective charge transport channel for ITIC derivatives, which adversely affects charge mobility. Through space-charge-limited-current (SCLC) measurement, it was experimentally found that all ITIC derivative-based devices have lower hole and electron mobility than ITIC-based devices; in particular, their electron mobility is significantly lower than that of ITIC-based devices (Table S4†). Because of this, furthermore, ITIC-based devices had balanced hole and electron mobility while other ITIC derivative-based devices did not.

Ternary device photovoltaic properties

We evaluated the photovoltaic properties of the ternary OSC devices using the prepared ITIC derivatives as a third component with the same structure as the binary OSC device system, aiming to use the synthesized ITIC derivatives as the A₂ acceptor molecules and obtain higher performance than the ITIC-based BOSC devices (Table 3). Thus, we fabricated PBDB-T:ITIC:A₂ ternary devices after selecting ITBTD, ITBTDF, ITBTD-IC, and ITIC-ICS molecules as A₂ acceptor molecules, considering the essential role of A₂ acceptor molecules in the TOSC, light absorption enhancement and appropriate energy level alignment (Fig. 6 and S10†). In addition, it is here worth mentioning that the selected ITIC derivatives (as A₂ acceptors) and ITIC (as a host acceptor) have almost the same HOMO level but different LUMO levels, so we can clearly investigate the effects of the A₂ acceptor molecules with different LUMO levels.

Table 3 Photovoltaic parameters of the PBDB-T:ITIC:A₂ acceptor-based ternary solar cells^a

Donor (D)	Acceptors (A1 : A2)^b	D–A blend ratio (D : A1 : A2) (wt)	V OC (V)	J SC (mA cm^−2)	FF	PCEave (max) (%)	PCE change rate^b (%)
a All the devices were thermally annealed at 120–130 °C. b Calculated the PCE change rate using [(PCET − PCEb)/(PCEb)] × 100 (%), PCET: average conversion efficiency of PBDB-T:ITIC:A2-based ternary devices, and PCEB: average conversion efficiency of PBDB-T:ITIC binary devices.
PBDB-T	ITIC	1.00 : 1.00 : 0.00	0.896	15.01	0.66	8.90 (9.32)	0.00
	ITIC:ITBTD	1.00 : 0.80 : 0.20	0.921	14.07	0.57	7.41 (7.81)	−16.74
		1.00 : 0.90 : 0.10	0.917	14.58	0.63	8.36 (8.94)	−6.07
		1.00 : 0.95 : 0.05	0.926	15.20	0.69	9.71 (10.11)	+9.10
	ITIC:ITBTDF	1.00 : 0.80 : 0.20	0.934	14.79	0.62	8.59 (9.60)	−3.48
		1.00 : 0.90 : 0.10	0.944	15.32	0.66	9.56 (9.80)	+7.42
		1.00 : 0.95 : 0.05	0.933	15.15	0.69	9.79 (10.03)	+10.00
	ITIC:ITBTD-IC	1.00 : 0.80 : 0.20	0.916	13.89	0.56	7.15 (8.05)	−19.66
		1.00 : 0.90 : 0.10	0.908	14.55	0.64	8.40 (9.01)	−5.62
		1.00 : 0.95 : 0.05	0.923	15.28	0.69	9.69 (10.04)	+8.88
	ITIC:ITIC-ICS	1.00 : 0.80 : 0.20	0.854	13.81	0.46	5.40 (5.66)	−39.33
		1.00 : 0.90 : 0.10	0.887	15.10	0.58	7.79 (8.13)	−12.47
		1.00 : 0.95 : 0.05	0.893	15.29	0.60	8.13 (8.92)	−8.65

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Fig. 6 Molecular energy-level alignments in devices based on the PBDB-T:acceptor 1:acceptor 2-based ternary solar cells.

In the PBDB-T:ITIC:ITBTD ternary system with 20% ITBTD content, the A₂ acceptor molecule showed increased V_OC (from 0.896 to 0.921) compared with the PBDB-T:ITIC binary system (Table 3 and Fig. 7), which is a key advantage of using the A₂ acceptor with a higher LUMO level than ITIC. However, unlike V_OC, J_sc and FF decreased, resulting in lower photoelectric efficiency PCE = 7.48% (approximately 17% decrease compared with the PBDB-T:ITIC binary system). This is because ITBTD molecules with lower acceptor performance than ITIC molecules adversely affected the device performance. In particular, a sharp decrease in FF (from 0.66 to 0.57 V) indicates that the formation of bulk-heterojunction layers of optimized PBDB-T:ITICs was disturbed by the participation of ITBTD molecules, and this often tended to appear when A₂ acceptor molecules with lower performance than the host acceptor molecule were used in the ternary system.^58–60 However, when the ITBTD content decreased to 10%, V_OC remained the same, whereas J_SC and FF improved compared with the 20% content, thereby increasing the photoelectric efficiency to 8.36% (approximately 6% lower than that of the PBDB-T:ITIC binary system). This is because the decrease in the ITBTD content naturally mitigated the adverse effect of the ITBTD A₂ acceptor molecules. Remarkably, when the ITBTD content decreased by 5%, both J_SC and FF, as well as the V_OC, were higher than those of the PBDB-T:ITIC binary system, resulting in an improvement in the photoelectric efficiency by more than 9% (PCE = 9.71%). This tendency also appeared in a ternary system with both ITBTDF and ITBTD-IC used as A₂ acceptor molecules (Table 3 and Fig. S11–S14†). Especially, it was confirmed that the TOSC system with 5% ITBTDF added has the highest value of 9.79%, which increased the photoelectric efficiency by approximately 10% compared with the PBDB-T:ITIC binary devices.

Fig. 7 The changes of (a) V_OC, (b) J_SC, (c) FF and (d) PCE as a function of ITBTD (A₂ acceptor) content in PBDB-T:ITIC:ITBTD-based ternary solar cells.

We can construe the fact that the photoelectric efficiency of the ternary devices was improved by up to 10% by only adding a very small amount, approximately 5%, of A₂ acceptor molecules which have inferior performance to the host ITIC acceptor molecule as follows. In fact, adding a small amount of A₂ molecules cannot significantly affect the bulk-heterojunction morphology in the PBDB-T:ITIC binary system, which is confirmed from the atomic force spectroscopy (AFM) measurements for PBDB-T:ITIC, PBDB-T:ITIC:ITBTD (1.00 : 0.95 : 0.05), and PBDB-T:ITBTD blend films (Fig. S15†). As shown in Fig. S15,† the root-mean-square (RMS) value of the PBDB-T:ITIC:ITBTD (1.00 : 0.95 : 0.05) blend film was found to be 3.0 nm, which is similar to that of the PBDB-T:ITIC blend film (3.3 nm) rather than that of the PBDB-T:ITBTD blend film (2.1 nm), indicating that the ternary blend largely reserves the original bulk-heterojunction framework of the PBDB-T:ITIC blend. Therefore, the holes and electrons produced after charge separation in the ternary system with a small amount of A₂ acceptor molecules added were expected to migrate in the channels created mainly by the host donor polymer (PBDB-T) and acceptor molecule (ITIC). This would be beneficial for maintaining efficient charge transfer channels related to the device J_SC and FF. As a result, it can be seen that the addition of a small amount of A₂ molecules did not deteriorate the good compatibility between active materials in the blended films, which play a very important role in the performance of the TOSC device.^61,62

However, the LUMO level of the A₂ acceptor molecule being located between the LUMO levels of the host donor polymer and (A₁) acceptor molecule, namely, the bridging effect, can create the cascade energy alignment,³ and then enable additional exciton dissociation at the D/A₂, D/A₁, and A₁/A₂ interfaces. In particular, we could confirm the efficient exciton dissociation at the A₁/A₂ interface by the photoluminescence (PL) quenching effect of ITIC (A₁):ITBTD (A₂) blended films as a function of ITBTD content (Fig. S16†).^63,64 More importantly, the cascade energy alignment can play an effective role in enhancing the charge transfer by decreasing the trap density between D and A₁.^1–7 Such generation of additional exciton dissociation and enhancement of charge transfer consequently can lead to increased J_SC and FF of the ternary system. Moreover, it was clearly verified that when we used an ITIC-ICS (LUMO = −4.02 eV) molecule, as an A₂ acceptor molecule, which cannot create the cascade energy alignment between the host donor polymers (PBDB-T) and A₁ acceptor molecules (ITIC) due to a deeper LUMO level than ITIC (LUMO = −3.85 eV), the photoelectric efficiency of the TOSC device, even after the added content decreased to 5%, was 8.13%, which was still lower than that of the PBDB-T:ITIC binary system (approximately 9% decrease) (Table 3 & Fig. S13†). This is because the ITIC-ICS A₂ acceptor molecules, having a deeper LUMO level than the host acceptor ITIC, could not enhance the V_OC of the ternary system; instead, they lowered the charge transfer properties by forming a deep charge trap between PBDB-T and ITIC. Therefore, despite the 5% addition of ITIC-ICS, the FF of the TOSC system only increased to 0.60, which is still lower than that of the PBDB-T:ITIC binary system (FF = 0.66).

From the energetic perspective, the energy transfer model can also work with the cascade charge transfer model in the TOSC system. In general, energy transfer can occur in various ways within the TOSC system, mainly through Förster resonance energy transfer (FRET).^1–3 For example, energy transfer can occur between the host donor polymer and A₂ acceptor molecule⁸ or between the host (A₁) and A₂ acceptor molecules.^58,65,66 The energy transfer process that has occurred at each interface can help generate more excitons, contributing to enhanced current in devices. For efficient FRET, the emission spectrum of the energy donor molecule must significantly overlap with the absorption spectrum of the energy acceptor molecule.^3,67 In this regard, it is unlikely that the energy transfer between PBDB-T donor polymer and A₂ acceptor molecules such as ITBTD, ITBTDF, and ITBTD-IC occurs effectively since there is a small overlap in the absorption and emission spectral regions of these materials (Fig. S17(a) and (b)†). However, it was revealed that the emission spectra of these A₂ acceptor molecules and the absorption spectrum of the host ITIC molecule overlap in large regions (Fig. S17(c)†), indicating that these molecules are more likely to transfer energy to the ITIC molecule than the host donor polymer (PBDB-T). Another promising positive role of the A₂ acceptor molecule in the ternary system is improving photon harvesting through additional light absorption. However, we found out that it is difficult to expect an effective photon harvesting improvement by adding a tiny amount, approximately 5%, of A₂ acceptor molecules into the ternary system from the insignificant difference between the absorption spectra of the PBDB-T:ITIC and PBDB-T:ITIC:A₂ blending films (Fig. S18†).

Conclusions

In this study, we synthesized various ITIC derivatives that can be used as third components (A₂ acceptors) in TOSC units by symmetrically or asymmetrically introducing functional termini of the IND group (BTD, BTDF, IC, and ICS) having different electron affinities to the IDTT central unit. The absorption bands and LUMO energy levels of the synthesized ITIC derivatives were precisely regulated according to the electron affinity strength of the terminal IND groups. In addition, the V_OC of the BOSC system that used these ITIC derivatives as host acceptors was controlled accurately by the LUMO energy level of the relevant ITIC derivatives. In the PBDB-T:ITIC:A₂-based ternary system, we ascertained that simply adding ITIC derivatives, which can form a cascade energy alignment between the LUMO levels of the host polymer (PBDB-T) and A₁ acceptor molecule (ITIC), as a small amount (approximately 5%) of A₂ acceptor molecules, regardless of the symmetric or asymmetric structure, can promote effective charge transfer while not disturbing the morphology of the PBDB-T:ITIC blend, and thus enhance V_OC, J_SC, and FF, synchronously, which resultantly improve the photoelectric efficiency by up to 10% more than that of a PBDB-T:ITIC binary system of the same structure. The results of this study are expected to help not only find the selection criteria for A₂ acceptor molecules suitable for effective use in TOSC devices but also develop high-performance TOSC devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Global Frontier R&D Program on Center for Multiscale Energy System through the NRF funded by the MSIP (2012M3A6A7055540) and Basic Science Research Program through the NRF funded by the Ministry of Science, ICT and Future Planning (2017R1E1A1A01075372).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1se01429c

‡ These authors contributed equally.

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