Approaching 16% Efficiency in All-Small-Molecule Organic Solar Cells Based on Ternary Strategy with a Highly Crystalline Acceptor

•A highly crystalline acceptor (4TIC) was added into the ternary devices•The crystallinity of the blend film was highly enhanced after the addition of 4TIC•Small-molecule organic solar cells achieved a power conversion efficiency of 15.88%•A certified power conversion efficiency of 15.5% was achieved Organic solar cells (OSCs) with great potential for producing light-weight, low-cost, flexible solar cells have attracted much attention in the past few years. Compared with polymer-based OSCs (P-OSCs), small-molecule organic solar cells (SM-OSCs) have unique advantages in commercialization, including well-defined molecular structures and low batch-to-batch variations, despite their power conversion efficiencies (PCEs) being lower. Here, we demonstrate ternary small-molecule organic solar cells (SM-OSCs) with a record-high PCE up to 15.88% (certified 15.5%) by adding 4TIC as the additional acceptor into porphyrin:6TIC blends. The highly crystalline 4TIC significantly enhances the crystallinity of the blend film and maintains the desired face-on orientations and proper multi-length scale morphology, which notably optimizes the charge extraction/recombination in the ternary devices. The ternary devices show a remarkably enhanced short-circuit current density and fill factor, resulting in over 20% enhancement of PCE. All-small-molecule organic solar cells (SM-OSCs) with a high power conversion efficiency (PCE) of 15.88% (certified 15.5%) are demonstrated by employing 4TIC as the additional non-fullerene acceptor (NFA) to construct ternary SM-OSCs. 4TIC is a highly crystalline acceptor with a similar molecular structure as 6TIC, the host NFA in the binary blend. The addition of 4TIC not only significantly enhances the crystallinity of the blend film but also maintains the desired face-on orientation in the proper multi-length scale morphology to improve both charge extraction and recombination in devices. As a result, the PCE of the ZnP-TBO: 6TIC-based SM-OSCs increases from 12.11% to 14.73% after the addition of 4TIC. For ZnP-TSEH: 6TIC-based SM-OSCs, the PCE increases from 13.54% to 15.88% (certified 15.5%) after the addition of 4TIC. The 15.88% efficiency is the best result for SM-OSCs reported to date. All-small-molecule organic solar cells (SM-OSCs) with a high power conversion efficiency (PCE) of 15.88% (certified 15.5%) are demonstrated by employing 4TIC as the additional non-fullerene acceptor (NFA) to construct ternary SM-OSCs. 4TIC is a highly crystalline acceptor with a similar molecular structure as 6TIC, the host NFA in the binary blend. The addition of 4TIC not only significantly enhances the crystallinity of the blend film but also maintains the desired face-on orientation in the proper multi-length scale morphology to improve both charge extraction and recombination in devices. 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In this communication, we demonstrate SM-OSCs with high PCEs up to 15.88% (certified 15.5%) and 14.73% by using a highly crystalline acceptor (4TIC) as the additional acceptor in two porphyrin: 6TIC blends, respectively. 4TIC is a highly crystalline low-band-gap fused-ring non-fullerene acceptor (NFA) with a similar molecular structure as that of 6TIC, the acceptor used in the host binary blend.45Shi X. Zuo L. Jo S.B. Gao K. Lin F. Liu F. Jen A.K.-Y. Design of a highly crystalline low-band gap fused-ring electron acceptor for high-efficiency solar cells with low energy loss.Chem. Mater. 2017; 29: 8369-8376Crossref Scopus (140) Google Scholar,46Shi X. Chen J. Gao K. Zuo L. Yao Z. Liu F. Tang J. Jen A.K.-Y. Fused-ring electron acceptor for highly efficient solar cells with a high short-circuit current density and low open-circuit voltage loss.Adv. Energy Mater. 2018; 8: 1702831Crossref Scopus (74) Google Scholar The addition of 4TIC significantly enhances the crystallinity of the blend film, while keeping the desired face-on orientation and a proper multi-length scale morphology, significantly optimizing charge extraction/recombination in the ternary devices. The spectroscopic studies further reveal that the enhanced crystallinity in the ternary blend is highly beneficial for both charge generation and recombination. High crystallinity and the proper multi-length scale morphology facilitate the excitons to propagate faster to the donor-acceptor interfaces, leading to a faster quenching of the initial photoexcitation via electron and hole transfer across the interfaces. Furthermore, the delayed onset for the decay of charged species and their prolonged lifetimes observed in the transient absorption (TA) spectra show that both the monomolecular (traps and geminate CT) and bimolecular recombinations can be significantly alleviated in the reduced Langevin recombination regime, leading to an enhanced FF and photocurrent extraction in the device. As a result, the PCE of the ZnP-TBO: 6TIC-based NFSM-OSCs increases from 12.11% to 14.73% after the addition of 4TIC. For ZnP-TSEH: 6TIC-based SM-OSCs, the PCE increases from 13.54% to 15.88% (certified 15.5%), with both high Jsc (25.95 mA cm−2) and FF (75.57%) after the addition of 4TIC. To the best of our knowledge, the PCE of 15.88% is the highest value reported for SM-OSCs. Figure 1A shows the chemical structures of ZnP-TBO (donor), 6TIC (primary NFA), and 4TIC (additional NFA). 4TIC has similar molecular structure as 6TIC, replacing the core terthieno [3,2-b]thiophene (6T) with thiophene-thieno[3,2-b]thiophene-thiophene (4T). Figures 1B and 1C show the grazing incidence wide-angle X-ray scattering (GIWAXS) of the pure 6TIC and 4TIC films, respectively. 6TIC pure film shows a face-on orientation, with an obvious π−π stacking peak located at 1.78 Å−1 in the out-of-plane (OOP) direction, the crystal coherence length (CCL) of which is 17.28 Å. The existence of an amorphous halo at ∼1.30 Å−1 indicates a poor structure order. For the in-plane (IP) part, a quite weak and board scattering peak at about 0.36 Å−1; and a sharp peak was recorded at 0.46 Å−1, with a CCL of 100.00 Å, which is ascribed to the lamellae packing of 6TIC molecules, forming a good packing of crystallines. 4TIC pure film shows a quite good crystalline order, as seen in 2D patterns that there exist large amounts of diffraction spots. To take the IP and OOP line cut, a well-developed (100) peak is observed at 0.35 Å−1 in IP direction, with a sharp π−π peak located at 1.86 Å−1 in the OOP direction, indicating a closer packing style. The CCLs of the two peaks are 260.05 Å and 36.82 Å, respectively, much larger than those of 6TIC, indicating a better crystalline quality. The different crystallization properties between 6TIC and 4TIC provide the opportunity to fine tune the morphology of the blend film, which will be discussed below. Figure 1D shows UV-vis absorption spectra of ZnP-TBO, 6TIC, and 4TIC. 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Sci. 2014; 7: 2005-2011Crossref Scopus (244) Google Scholar The photovoltaic performance of the ternary SM-OSCs were investigated based on the following device structure: indium tin oxide (ITO)/ PEDOT:PSS/ ZnP-TBO:6TIC:4TIC/ C60-bissalt/ Ag. The overall ratio between donors and acceptors was kept at 1:1.2 in this study. C60-bissalt was applied as an electron-transporting layer and its chemical structure is shown in Figure S1.48Li C.Z. Chueh C.C. Yip H.L. O’Malley K.M. Chen W.C. Jen A.K.-Y. Effective interfacial layer to enhance efficiency of polymer solar cells via solution-processed fullerene-surfactants.J. Mater. Chem. 2012; 22: 8574-8578Crossref Scopus (149) Google Scholar The photovoltaic parameters of the devices are shown in Table 1. Figure 2A illustrates the representative current density versus voltage (J-V) characteristics of devices with different 6TIC: 4TIC weight ratios (0%, 10%, and 20% 4TIC) under simulated AM 1.5 G illumination at 100 mW cm−2. The performance of devices with more 4TIC are shown in Figure S2; Table S2.Table 1Photovoltaic Parameters for ZnP-TBO: 6TIC: 4TIC Devices under Simulated AM 1.5 G Illumination at 100 mW cm−24TIC (%)Voc (V)Jsc (mA cm−2)FF (%)PCE (%) (Average)aAverage PCEs in brackets are based on 15 devices.00.8021.57 (20.51)bValues in brackets are calculated from EQE.70.1712.11 (11.54 ± 0.41)50.8023.17 (22.06)73.2913.58 (12.99 ± 0.42)100.8024.58 (23.37)74.9314.73 (14.09 ± 0.45)200.7922.41 (21.32)71.6212.68 (12.06 ± 0.44)10 (Certified)cIndependent certification results in NIM (Certificate no. GXtc2019-2205).0.80224.2971.413.9a Average PCEs in brackets are based on 15 devices.b Values in brackets are calculated from EQE.c Independent certification results in NIM (Certificate no. GXtc2019-2205). Open table in a new tab The binary ZnP-TBO: 6TIC control device exhibited a maximum PCE of 12.11% with an open-circuit voltage (Voc) of 0.80V, a Jsc of 21.57 mA cm−2, and an FF of 70.17%. Adding 10% of 4TIC (the ratio of 4TIC in acceptor, similarly hereinafter) into the ZnP-TBO: 6TIC blend dramatically increased the Jsc to 24.58 mA cm−2 and FF to 74.93%, resulting in a promising PCE of 14.73%. The average PCE of ZnP-TBO: 6TIC :4TIC (10%)-based devices was 14.09%±0.45%, which was calculated from 15 individual devices prepared from different batches. To verify our result, a ternary device was sent for certification by the National Institute of Metrology (NIM), China, after being stored in air for 24 h, which obtained a PCE of 13.9% (Figure S3). Further addition of 4TIC (20%) led to decreased Jsc (22.41 mA cm−2) and FF (71.62%), but the PCE still remained at a higher value (12.68%) than the ZnP-TBO: 6TIC binary blend. The stability of encapsulated devices with the optimal 4TIC loading (10% in acceptor) was monitored and the results are shown in Figure S4. The ternary SM-OSCs exhibited good stability both in the nitrogen atmosphere and in air after being stored for 100 h. External quantum efficiency (EQE) measurements were conducted to confirm the Jsc of the SM-OSCs. As shown in Figure 2B, the ternary-blend-based devices showed remarkable enhancement in the whole test range compared with the corresponding control devices. The much-improved charge carrier collection and a reduced recombination contribute to the enhanced EQE values, which will be discussed below. The integrated Jsc values from EQE measurements are shown in Table 1, agreeing well with the value obtained from the J-V measurements. To understand the reasons for the achieved high-performance of ZnP-TBO: 6TIC: 4TIC ternary devices, the charge transport and extraction properties were investigated. The electron mobilities were measured by using the space-charge-limited-current (SCLC) method (Figure S5; Table S3). The electron-only devices were fabricated based on the device architecture of ITO/ZnO/active layer/C60-bissalt/Ag. The mobilities were extracted from the slopes of J1/2-V curves by modeling the dark current in the SCLC region. The electron mobilities of the ZnP-TBO: 6TIC :4TIC = (1:1.2:0), (1:1.14:0.06), (1:1.08:0.12), and (1:0.96:0.24) devices were 3.66 × 10−4, 5.22 × 10−4, 6.84 × 10−4, and 7.43 × 10−4 cm2 V−1 s−1, respectively. The enhanced crystallinity resulting from the addition of 4TIC contributed to the increased electron mobilities, leading to a higher FF for the device. To gain more insight into the exciton dissociation and charge-extraction processes, the photocurrent density (Jph) as a function of the effective voltage (Veff) was measured (Figure 2C). At a high Veff, it is assumed that all of the photo-induced excitons are dissociated, and the charge carriers are collected, resulting in a saturation current (Jsat). The charge-collection probability P(E,T) could be calculated by normalizing Jph with Jsat. The P(E,T) at the maximum power point (Mpp) dramatically increased from 77.3% to 84.3% (5% 4TIC), 88.3% (10% 4TIC), and 80.1% (20% 4TIC), respectively. The higher P(E,T) value for the ternary blend devices also suggested that the addition of 4TIC efficiently enhanced charge extraction, leading to an increased Jsc and higher FFs for the ternary SM-OSCs. The enhanced electron mobilities in the ternary blend devices contributed to the increased charge-extraction process. The bimolecular recombination in the binary and ternary devices was then analyzed by measuring Jsc as a function of the light intensity (Plight). In OSCs, Jsc shows a power-law dependence on light intensity (Jsc∝(Plight)S). For a device with a negligible recombination occurring before the charge extraction, the S value is closed to 1. Figure S6 shows the Jsc∼Plight relationship for the ZnP-TBO: 6TIC :4TIC = (1:1.2:0), (1:1.14:0.06), (1:1.08:0.12), and (1:0.96:0.24) devices. The extracted S values are 0.972, 0.988, 0.994, and 0.951, respectively, indicating that recombination was suppressed in ternary devices. To gain more physical insights, we have performed a charge-transfer investigation via ultrafast spectroscopy. In order to quantitatively investigate the photo-induced charge-transfer process, we employed time-resolved photoluminescence (TRPL) spectroscopy to probe the initial electron-transfer process. In order to avoid the influence of the acceptor’s emission, we irradiated with a laser at 400 nm and probed at 500–600 nm. As illustrated in Figure 2D, the ZnP-TBO neat film shows a fluorescence lifetime (τ) of 967.83 ± 6.89 ps. After being blended with 6TIC, the fluorescence lifetime significantly decreased, with a time constant of 178.22±2.8 ps, indi