Suppressing nonradiative energy loss in ternary organic solar cells through elaborate disruption of guest acceptors planarity

Abstract

The relatively large non-radiative energy loss (ΔE_nr) in organic solar cells (OSCs) remains a major obstacle for improving the power conversion efficiency (PCE). Therefore, it is imperative to minimize ΔE_nr through rational molecular design and device engineering. In this work, three small-molecule acceptors with different terminal steric hindrance groups, namely, Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃, were designed as the third components to elaborately reduce the π–π interactions in the acceptor phase and improve the photoluminescence quantum yield (PLQY). All the third components effectively improved the fluorescence quantum yield of the acceptor phase and inhibited ΔE_nr. Among these systems, the Y-PH-CH₃ ternary system exhibited remarkable suppression of non-radiative energy loss, coupled with refined charge transport capabilities. Consequently, it achieved an impressive power conversion efficiency (PCE) of 18.63%, accompanied by a low non-radiative energy loss of merely 0.178 eV. Moreover, by adopting this third-component design strategy into a D18:L8-BO system, a significantly improved open circuit voltage (V_OC) of 0.924 V and a high PCE of 19.18% could be achieved. This study confirms that appropriately manipulating the planarity of acceptors by terminal steric hindrance groups is an effective approach for designing third components toward highly efficient ternary OSCs with low ΔE_nr.

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Yahui Liu

Yahui Liu received his BS degree from Beijing Normal University in 2013. He joined Prof. Zhishan Bo's group for his doctoral research and received his PhD degree from Beijing Normal University in 2018. He then worked as a research assistant in Prof Bo's group and joined Qingdao University in 2020. His research interest is focused on the design and application of functional materials such as conjugated polymers, fused and nonfused ring electron acceptors, and fullerene derivatives. He has published more than 70 papers as the first/corresponding author, which have been cited more than 4000 times.

Introduction

Organic solar cells (OSCs) have attracted tremendous attention from both industrial and academic societies due to their outstanding advantages of light weight, flexibility, transparency and solution processing, which make them promising candidates for flexible electronic devices and indoor photovoltaic technology.^1–4 Currently, with the rapid revolution of non-fullerene acceptors (NFAs) and device engineering strategies, the power conversion efficiency (PCE) of OSCs exceeds 20% in single-junction devices.^5–7 At present, a high external quantum efficiency (EQE) over 85% and a fill factor (FF) over 80% can be realized simultaneously in the best-performing OSC, which is comparable with the GaAs and perovskite solar cells.^8,9 Nonetheless, the PCE of OSCs is still inferior to that of inorganic solar cells (silicon, GaAs, and perovskites), which could be attributed to the low open-circuit voltage (V_OC) due to the significantly larger energy loss (E_loss) of 0.55 eV (which is 0.3 eV for GaAs, GalnP and perovskite solar cells).^10–13 The energy loss of photovoltaic devices can be divided into two parts: charge generation energy loss and charge recombination energy loss; here, the charge recombination energy loss consists of radiative energy loss (ΔE_r) and nonradiative energy loss (ΔE_nr). ΔE_r is inevitable, which sets the upper limit to V_OC (V_r). ΔE_nr is the key factor that hinders the V_OC of the OSC device. The ΔE_nr value of OSCs is around 0.24 eV, while other photovoltaic technologies (GaAs, GalnP and perovskite solar cells) exhibit ΔE_nr values less than 0.1 eV. Normally, ΔE_nr is expressed as ΔE_nr = −kT ln(EQE_EL), where k_B is the Boltzmann constant, T is the temperature and EQE_EL is the electroluminescence external quantum efficiency.^14,15 The OSC devices only exhibit a low EQE_EL value around 0.01%, which is far below that of the GaAs, GalnP and perovskite solar cells (over 10%).^16–18 Therefore, elevating EQE_EL of OSCs is of paramount importance for boosting V_OC and reaching a PCE over 20%.^19,20

The EQE_EL of the device is intimately correlated with the PLQY of NFA. Therefore, improving the PLQY of the acceptor phase is a feasible approach to improve EQE_EL and suppress ΔE_nr. A ternary strategy appeared to be an elegant approach to enhance the PLQY of the acceptor and suppress ΔE_nr.^21–23 In the OSC system, π–π interaction is considered as a significant nonradiative decay route, which should be restrained when constructing OSCs with low ΔE_nr. We have previously reported an NFA with a norbornene-modified end group, SM16, which shows a high PLQY and suppressed ΔE_nr in OSC devices due to the rigid three-dimensional structure of norbornene group, which successfully disturbed the π–π interaction of NFA.^24,25 The introduction of SM16 into the D18:eC9-4F system effectively improved the PLQY and exciton diffusion length of the acceptor phase, thus leading to a high efficiency over 18% with reduced ΔE_nr and higher FF in the ternary system. Similarly, we then introduced an NFA with a bulky side group (LA15) as the third component to construct ternary OSCs with low ΔE_nr of 0.18 eV and high efficiency over 19%. LA15 effectively improved the PLQY and exciton lifetime of the acceptor phase, thus leading to reduced nonradiative recombination loss and improved EQE_EL.²⁶ However, given the current scarcity of methods for enhancing the PLQY of the acceptor, it is significant to develop new approaches to improve the PLQY of the acceptor, thereby boosting the efficiency of OSCs.

In this work, three NFAs were designed as the guest acceptor to enhance the PLQY of the acceptor phase in a D18:eC9-4F system by coupling sterically hindered groups at the terminal position of acceptors (Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃Fig. 1a). By tuning the size of the steric groups, the planarity of guest components can be effectively tuned, which affects the molecular stacking and PLQY of the acceptor. As a result, the three guest acceptors exhibit a high PLQY when blended with eC9-4F, which results in suppressed ΔE_nr and higher V_OC in ternary systems. Particularly, the D18:eC9-4F:Y-PH-CH₃ ternary system demonstrated an improved PCE of 18.63% with a high V_OC of 0.881 V and a suppressed ΔE_nr of 0.179 eV. We further adopted this ternary strategy in the D18:L8-BO system and delivered a high PCE of 19.18% with a high V_OC of 0.924 V and an excellent FF of 77.17%. Our results demonstrate that elaborately manipulating the molecular stacking and PLQY of the acceptor by coupling the steric group to the end group could be an elegant approach to design the third component toward high-efficiency ternary OSCs with suppressed ΔE_nr.

Fig. 1 (a) Chemical structures of Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃. (b) Normalized absorption spectra of neat Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃, D18, and eC9-4F films. (c) Energy level diagrams of host and guest components. (d) Rotational barrier of Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃. (e) Torsion angle of Y-PH-H, Y-PH-CH₃ and Y-PH-2CH₃ thin films.

Results and discussions

Three acceptor molecules, namely, Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃, featuring distinct steric hindrance terminal groups, were synthesized by the introduction of varying quantities of methyl substituents onto the phenyl group of the terminal group, as illustrated in Fig. 1a. The comprehensive synthesis protocols are documented in the ESI (the NMR spectra of Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ are shown in Fig. S1–S3).† The terminal groups were obtained via Suzuki coupling reactions, followed by the synthesis of the target acceptors via Knoevenagel condensation with dialdehyde intermediates.

The normalized UV-visible absorption spectra of photoactive materials are shown in Fig. 1b. Y-PH-H, Y-PH-CH₃ and Y-PH-2CH₃ exhibited complementary absorption with D18 and eC9-4F, which was conducive to achieving high J_SC in ternary devices. Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ have gradually blue-shifted absorption peaks at 791, 780 and 765 nm along with increased methyl number, which could be attributed to the shortened effective conjugate length and suppressed molecular packing (vide infra) resulted from the large steric hindrance of the end group. The molecular geometries of Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ were probed by density functional theory (DFT) calculations. As shown in Fig. 1e, the dihedral angles between indandione and phenyl hindrance groups of Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ are 38.2°, 53.2° and 89.1°, respectively. We further calculated the rotational barrier of these guest acceptors. As shown in Fig. 1d, the rotation barriers of Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ are 0.005 hartree, 0.096 hartree and 0.239 hartree, respectively. As the number of methyl groups increases, the planarity of these three molecules gradually decreases, which tends to disrupt the strong π–π interaction in the acceptor phase and improve the PLQY of the acceptor. The absorption spectra of acceptor blend films were recorded to further probe the impact of these guest acceptors on the aggregation of eC9-4F. As shown in Fig. S4†, all the acceptor blend films showed blue-shifted absorption compared with the neat eC9-4F film, demonstrating the disturbed effective π conjugated length of acceptor. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ were measured by cyclic voltammetry (Fig. S5†), and the energy level alignment of photovoltaic materials is shown in Fig. 1c. Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ have higher LUMO energy levels of −3.98 eV, −3.96 eV, and −3.95 eV, which are higher than that of eC9-4F (−4.05 eV), thus being beneficial for obtaining high V_OC in ternary devices.

The photovoltaic performance of D18:eC9-4F binary and ternary devices was evaluated by a conventional device architecture. The current density–voltage (J–V) characteristics and corresponding device parameters are shown in Fig. 2a and Table 1, respectively. The D18:eC9-4F binary device delivered a moderate PCE of 17.21% with a V_OC of 0.869 V, a J_SC of 27.76 mA cm⁻² and an FF of 71.23%. After incorporating Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ as the guest acceptor, all the three ternary devices (namely, Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ ternary devices) exhibited improved V_OC of 0.88–0.89 V, which are much higher than that in binary devices. The Y-PH-H ternary devices delivered an improved PCE of 17.75% with a moderate J_SC of 27.89 mA cm⁻² and an FF of 72.77%. Encouragingly, the Y-PH-CH₃-based ternary device exhibited a high PCE over 18% with a simultaneously improved J_SC of 27.95 mA cm⁻² and an FF of 75.58%. Nonetheless, the Y-PH-2CH3 ternary devices showed an inferior PCE of 16.39% with a low J_SC of 26.52 mA cm⁻² and an FF of 69.59%. The EQE spectra and integrated J_SC with an AM 1.5 solar irradiation spectrum are plotted in Fig. 2b and S11† to investigate the photo-response of binary and ternary devices. The binary device, Y-PH-H and Y-PH-CH₃ ternary devices exhibited broad and intense photo-responses at 350–920 nm with a high EQE over 80%, demonstrating that the addition of guest acceptors did not disturb the efficient exciton generation and charge transport in the D18:eC9-4F host system. Nonetheless, the Y-PH-2CH₃ ternary device exhibited a lower EQE at 350–920 nm, indicating that the addition of Y-PH-2CH₃ might disrupt the exciton dissociation and charge transport process. The integrated J_SC (listed in Table 1) of binary and ternary devices are in well accordance with measured J_SC with an error less than 5%.

Fig. 2 (a) J–V curves of the binary and ternary devices. (b) EQE curves of the binary and ternary devices (c) PL curves of the binary and ternary devices. (d) J_ph–V_eff curves of the binary and ternary devices. Light intensity dependence of (e) J_SC and (f) V_OC of binary and ternary devices.

Table 1 Summary of the photovoltaic parameters of binary and ternary devices

	V OC (V)	FF (%)	J SC (mA cm^−2)	J cal (mA cm^−2)	PCE (%)
D18:eC9-4F	0.869	71.23	27.76	26.38	17.21
D18:eC9-4F:Y-PH-H	0.874	72.77	27.89	26.51	17.75
D18:eC9-4F:Y-PH-CH3	0.881	75.58	27.95	26.68	18.63
D18:eC9-4F:Y-PH-2CH3	0.887	69.59	26.52	25.22	16.39
D18:L8-Bo	0.915	75.07	26.47	25.14	18.14
D18:L8-Bo:Y-PH-CH3	0.924	77.17	26.52	25.23	19.18

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Then we plotted photocurrent density (J_ph) versus effective voltage (V_eff) to estimate the exciton dissociation and overall charge collection efficiencies of binary and ternary devices. As shown in Fig. 2d, J_ph of binary and ternary devices reached a saturation value (J_sat) at high V_eff > 2 V, which means that all the photo-generated charges are driven to the corresponding electrodes. The current density at maximum power point (J_max) and J_SCversus J_sat denotes the overall charge collection efficiency (P_coll) and exciton dissociation efficiency (P_diss), respectively.^27,28 As shown in Table 2, the Y-PH-CH₃ ternary device exhibited a P_coll of 87.6% and a P_diss of 98.5%, which are comparable to the binary device. The Y-PH-H ternary device exhibited a higher P_coll of 87.4% and a P_diss of 98.3% than those of the binary device, while the Y-PH-2CH₃ ternary device exhibited an inferior P_coll of 84.8% and a P_diss of 97.4%. These indicated that Y-PH-CH₃ could improve the exciton dissociation and charge transport process, while Y-PH-2CH₃ exerted a negative impact on the exciton dissociation and charge collection, which may be correlated with the BHJ morphology (vide infra). The bimolecular recombination process in binary and ternary devices was further characterized by plotting J_SCversus incident light intensity (P_Light). Typically, J_SC and P_light conform to the following equation: J_SC ∝ P_Light^α, where α approaching 1 demonstrates that the bimolecular recombination is minimal.^29,30 As shown in Fig. 2e, α of binary and Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ ternary devices are 0.992, 0.994, 0.997, and 0.990, respectively, demonstrating that Y-PH-CH₃ ternary devices could assist in suppressing the bimolecular recombination in the host binary system. As shown in Fig. 2f, the relationship between V_OC and P_light can be expressed as follows: V_OC ∝ (nkT/q)ln(P_Light), where n is correlated with the recombination type, and n approaching 1 denotes negligible trap-assisted recombination. The n values of the binary and Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ ternary devices are 1.25, 1.21, 1.17, and 1.29 respectively. Therefore, the addition of Y-PH-CH₃ can significantly reduce the trap-assisted recombination. According to the aforementioned analysis, Y-PH-CH₃ significantly reduced the recombination loss, while Y-PH-2CH₃ increased the recombination loss.

Table 2 Parameters of exciton dissociation efficiency and charge collection efficiency based on D18:eC9-4f, D18:eC9-4f:Y-PH-H, D18:eC9-4f:Y-PH-CH3, and D18:eC9-4f:Y-PH-2CH3 devices

	J sat (mA cm^−2)	J ph (mA cm^−2)	J max (mA cm^−2)	P diss (%)	P coll (%)
D18:eC9-4F	27.52	27.01	23.56	98.1	87.2
D18:eC9-4F:Y-PH-H	27.77	27.29	23.85	98.3	87.4
D18:eC9-4F:Y-PH-CH3	27.90	27.48	24.08	98.5	87.6
D18:eC9-4F:Y-PH-2CH3	26.93	26.23	22.83	97.4	84.8

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The electron and hole mobilities (μ_e and μ_h) in binary and ternary devices were calculated by a space charge limited current (SCLC) model.³¹ As shown in Fig. S6 and Table S1,† the Y-PH-H and Y-PH-CH₃ ternary devices exhibited higher μ_e and μ_h than the binary device, whereas μ_e/μ_h is more close to 1 in the Y-PH-CH₃ ternary device, indicating refined and more balanced charge carrier transport, thus leading to higher J_SC and FF. On the contrary, the Y-PH-2CH₃-based ternary device exhibited lower μ_e and unbalanced charge transport than the binary device, which led to the disrupted charge transport and low FF in ternary devices.

The compatibility between the guest acceptor and the host systems was analysed by water contact angle (WCA) measurement. The WCAs of D18, eC9-4F, eC9-4F:Y-PH-H, eC9-4F:Y-PH-CH3, and eC9-4F:Y-PH-2CH3 films were 107.6°, 96.4°, 101.9°, 105.7° and 103.9°, respectively. As shown in Fig. S7,† the acceptor blend exhibited a much larger WCA than that of the neat eC9-4F film. The eC9-4F:Y-PH-CH3 film exhibited a large WCA of 105.7°, which was much closer to that of D18 than to that of the eC9-4F film. According to the Flory Huggins theory, the closer WCAs of D18 (107.6°) and eC9-4F:Y-PH-H, eC9-4F:Y-PH-CH₃, eC9-4F:Y-PH-2CH₃ are beneficial for promoting the compatibility between the donor and acceptor phases, thus leading to efficient exciton dissociation. The overall exciton dissociation was characterized by fluorescence quenching.³² As shown in Fig. 2c, upon excitation at 550 nm, the fluorescence intensity of D18 was significantly reduced after the addition of Y-PH-H, Y-PH-CH₃ and Y-PH-2CH₃, which showed a good quenching behavior and confirmed the efficient exciton dissociation between the donor polymer and the small-molecule acceptor.

To further investigate the molecular orientation and crystallinity of the four acceptors, we performed grazing incidence wide-angle X-ray scattering (GIWAXS) measurements (Fig. S8†). The pristine Y-PH-H film exhibited dominant face-on orientation with an apparent π–π stacking peak at q_z = 1.755 Å⁻¹ along the out-of-plane (OOP) direction. On the contrary, only feeble π–π stacking signals appeared in Y-PH-CH₃ and Y-PH-2CH₃ films along the OOP direction, demonstrating weak crystallinity and π–π interactions of Y-PH-CH₃ and Y-PH-2CH₃. Clearly, increasing the dihedral angle of the end group could drastically affect the aggregation behavior of the guest acceptor, which is also in good accordance with the absorption spectra.³³ We then probed the GIWAXS diffraction patterns of acceptor blend films to investigate the impact of guest acceptors (10 wt% in the acceptor blend) on the molecular packing of the acceptor blends. As shown in Fig. 3a–e, the eC9-4F:Y-PH-H, eC9-4F:Y-PH-CH₃, eC9-4F:Y-PH-2CH₃ and eC9-4F films showed dominant face-on orientation with intense π–π diffraction peaks at q_z = 1.766, 1.747, 1.734 and 1.778 Å⁻¹ in the OOP direction, corresponding to the π–π stacking distances of 3.56, 3.59, 3.62 and 3.53 Å, respectively. Accordingly, the incorporation of guest acceptors (Y-PH-H, Y-PH-CH₃ and Y-PH-2CH₃) can slightly suppress the π–π interaction of the host acceptor (eC9-4F), which is beneficial for improving the PLQY of the acceptor blend.^34,35 Notably, the eC9-4F:Y-PH-CH₃ acceptor blend exhibited a comparable diffraction intensity and face-on orientation with neat eC9-4F, indicating that incorporating a small amount of Y-PH-CH₃ does not significantly affect the crystallinity of the acceptor phase, which is favorable for charge transport.

Fig. 3 GIWAXS scattering patterns of (a) eC9-4F, (b) eC9-4F:Y-PH-H, (c) eC9-4F:Y-PH-CH₃, and (d) eC9-4F:Y-PH-2CH₃. (e) Line profiles of GIWAXS in the in-plane (IP) and out-of-plane (OOP) directions.

Since the photoluminescence property of acceptors correlates intimately with EQE_EL and ΔE_nr in OSC devices, the PLQY of neat acceptors and acceptor blends were measured.³⁶ As shown in Table S2† and Fig. 4b, the neat eC9-4F film exhibited a relatively low PLQY of 4.19%, whereas eC9-4F:Y-PH-H, eC9-4F:Y-PH-CH₃ and eC9-4F:Y-PH-2CH₃ blend films exhibited higher PLQYs of 4.87%, 5.95%, and 7.45%, respectively. The gradually increased steric hindrance group effectively disrupted the planarity of the acceptor, which endowed the acceptors with reduced π–π interactions and enhanced PLQYs, thus being favorable for elevating EQE_EL and suppressing ΔE_nr. These results correlate well with the above-mentioned absorption and GIWAXS results.

Fig. 4 (a) Energy loss mechanism. (b) PLQY of neat films and blend films. (c) Band gaps calculated by the intersection of UV-vis and fluorescence spectra (FL). (d) EQE_EL curves of binary and ternary devices. (e) Comparison of different energy losses of devices. (f) Statistics of high-efficiency OSCs with low ΔE_nr (the corresponding references are cited in the ESI†).

We then investigated the details of the energy loss in binary and ternary devices. The energy loss in OSC devices was determined using the following equation: E_loss = E_g − qV_OC, where E_g is the bandgap of the OSC device.³⁷E_g was determined by the intersection points of absorption and emission spectra (Fig. 4c). As shown in Table 3, the Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ ternary devices exhibited low E_loss of 0.513, 0.507 and 0.503 eV, respectively, which are significantly lower than that of the binary device (0.529 eV). The E_loss in OSCs is composed of three parts: (i) radiative recombination above the bandgap (ΔE₁); (ii) radiative recombination below the bandgap (ΔE₂) and (iii) ΔE_nr. The ΔE₁ value was calculated using the following equation: ΔE₁ = E_g − qV^SQ_OC, where V^SQ_OC is the maximum voltage according to the Shockley–Queisser (SQ) limit (Fig. 4a). As shown in Fig. 4e, the ΔE₁ values of binary film, Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ ternary devices were 0.256 eV, 0.269 eV, 0.262 eV, and 0.261 eV, respectively. ΔE_nr can be calculated using the following formula: ΔE_nr = −kT ln(EQE_EL). As depicted in Fig. 4d, the Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ ternary devices exhibited significantly higher EQE_EL corresponding to the ΔE_nr values of 0.181, 0.179 and 0.178 eV, respectively. In comparison, the D18:eC9-4F binary device only exhibited low EQE_EL with a large ΔE_nr of 0.206 eV. Therefore, it can be concluded that the reduced E_loss in ternary devices is mainly ascribed to the suppressed ΔE_nr.³⁸ The low ΔE_nr below 0.18 eV in ternary devices ranks one of the lowest in high-performance OSCs. These results demonstrated that incorporating a phenyl group at the tail of the end group to tune the packing pattern of the acceptor is a promising approach for enhancing the PLQY and suppressing ΔE_nr.

Table 3 Details of energy loss in D18:eC9-4f, D18:eC9-4f:Y-PH-H, D18:eC9-4f:Y-PH-CH3, and D18:eC9-4f:Y-PH-2CH3 devices

	ΔE1	ΔE2	ΔE3	E loss
D18:eC9-4F	0.256 eV	0.067 eV	0.206 eV	0.529 eV
D18:eC9-4F:Y-PH-H	0.269 eV	0.079 eV	0.181 eV	0.513 eV
D18:eC9-4F:Y-PH-CH3	0.262 eV	0.066 eV	0.179 eV	0.507 eV
D18:eC9-4F:Y-PH-2CH3	0.261 eV	0.064 eV	0.178 eV	0.503 eV

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The molecular stacking and phase scale of binary and ternary blend films were analyzed by GIWAXS measurements. As shown in Fig. 5b–f, all the binary and ternary films exhibited a comparable π–π stacking peak at a q_z value around 1.75 Å⁻¹ along the OOP direction and the (100) lamellar diffraction peak in the IP (q_xy = 0.31 Å⁻¹) direction, demonstrating preferably face-on orientation in binary and ternary films. As shown in Table S3,† the π–π stacking distances of the D18:eC9 binary film, and Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ ternary films are 3.64, 3.61, 3.61 and 3.64 Å, and the corresponding crystal coherence lengths are 22.56, 23.33, 23.87 and 19.92 Å, respectively. These indicated that the incorporation of the guest acceptor does not interfere with the ordered molecular stacking of D18 in the host binary systems.^24,39 The Y-PH-CH₃ ternary film exhibited stronger crystallinity and a slightly reduced π–π stacking distance of D18, which could be confirmed by the intense characteristic peak of D18 (q_xy = 0.31 Å), thus resulting in refined charge transport. The Y-PH-2CH₃ ternary films also exhibited strong diffraction peaks, which could be attributed to good compatibility between Y-PH-2CH₃ and the host acceptor. According to the contact angle measurement, Y-PH-2CH3 tended to be well blended with acceptor instead of D18. The strong diffraction peak observed at 0.31 Å⁻¹ in the GIWAXS results is the (100) diffraction characteristic peak of D18. Therefore, the strong diffraction peaks in the blend film could be attributed to the good crystallinity of D18. Furthermore, the phase separation scale in binary and ternary systems was quantified by Grazing Incidence Small-Angle X-ray Scattering (GISAXS) measurements. The average domain size in binary and ternary blend can be estimated by the Guinier radius of the fractal-like network R_g using the following equation: .^39,40 As shown in Fig. S9 and Table S4†, the R_g value of binary and Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ ternary films are 29.7, 30.1, 34.9, and 23.1 nm, respectively. Based on the discussions above, incorporating a small amount of Y-PH-H and Y-PH-CH₃ would refine the crystallinity and phase purity, leading to improved charge transport. However, the high R_g of 34.9 nm in the Y-PH-CH₃ ternary film is 51% higher than that of the binary film, which might lead to overlarge phase separation, thus hampering exciton diffusion and charge transport.

Fig. 5 (a) TEM and AFM images based on D18:eC9-4F, D18:eC9-4F:Y-PH-H, D18:eC9-4F:Y-PH-CH₃, and D18:eC9-4F:Y-PH-2CH₃ films. GIWAXS diffraction patterns based on (b) D18:eC9-4F, (c) D18:eC9-4F:Y-PH-H, (d) D18:eC9-4F:Y-PH-CH₃, and (e) D18:eC9-4F:Y-PH-2CH₃. (f) Line profiles of GIWAXS diffraction patterns.

Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were performed to study the surface morphology and phase separation in binary and ternary films. All the binary and ternary films exhibited similar root-mean-square roughness (RMS) values of 1.23, 1.14, 1.31 and 0.84 nm for binary film, Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ ternary films, respectively. The Y-PH-CH₃ ternary film exhibits a more elaborate fiber structure, which is conducive to charge transport. In contrast, the Y-PH-2CH3 ternary film exhibits a fuzzy nanofiber structure, which is unfavorable for charge transport and collection. As shown in Fig. 5a, the binary and ternary films exhibited homogeneous distribution with no large aggregation observed, demonstrating that the incorporation of third component did not induce overlarge aggregate formation in BHJ films.

We further adopted this ternary strategy in the D18:L8-BO system. As shown in Table 1 and Fig. S10,† the D18:L8-BO binary device exhibited a PCE of 18.14% with a high V_OC of 0.915 V, a J_SC of 26.47 mA cm⁻², and an FF of 75.07%. After introducing 10% of Y-PH-CH₃ as the third component, the ternary device delivered a high PCE of 19.18% with a significantly improved V_OC of 0.924 V and an FF of 77.17%, demonstrating that our ternary strategy has good universality and feasibility in fabricating high-efficiency OSCs with low ΔE_nr.

Conclusion

In this work, three NFAs with different phenyl steric groups, namely, Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃, were designed and synthesized as third components to disrupt the π–π interaction in the acceptor phase and improve the PLQY for suppressing ΔE_nr in OSCs. With the increase in the numbers of methyl on phenyl group, the planarity of Y-PH-H, Y-PH-CH₃, and Y-PH-2CH₃ gradually decreased, which effectively increased the π–π stacking distance and PLQY of acceptor blends. As a result, all the ternary devices exhibited significantly reduced ΔE_nr below 0.18 eV, which endowed ternary OSCs with higher V_OC values. Encouragingly, Y-PH-CH₃ could appropriately enhance the crystallinity and phase separation in the ternary system, which optimized the charge transport and delivered a higher FF than that of the binary devices. After introducing Y-PH-CH₃ into D18:L8-BO system, the ternary OSCs delivered high V_OC of 0.924 V with PCE over 19%. Our results demonstrated that elaborately disrupting the planarity of the guest acceptor could be a facile ternary strategy for improving the PLQY of acceptors while maintaining efficient charge transport in constructing high-performance OSCs with low ΔE_nr values.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Author contributions

Q. Liang carried out device fabrication and measurement. The synthesis was conducted by X. Wang. H. Lu, H. Li and A. Zhang contributed to a part of the device measurements. H. Jiang wrote the manuscript. Y. Liu and Z. Bo supervised and revised the manuscript. All authors discussed the results and provided feedback on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are deeply grateful to the National Natural Science Foundation of China (22309098, 52173174, 52433007, and 52403234), the Shandong Postdoctoral Foundation (SDBX2023026), the Natural Science Foundation of Shandong Province (ZR2022YQ45, ZR2023QB013 and ZR2022QF024), the Taishan Scholars Program (tstp20221121 and tsqnz20221134), and the State Key Laboratory of Bio-Fibers and Eco-Textiles (Qingdao University) (RZ2200002821).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta00232j

‡ Q. Liang, X. Wang and H. Li contributed equally to this work.

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