Aromatic end-capped acceptor effects on molecular stacking and the photovoltaic performance of solution-processable small molecules

Dan Deng a, Yang Yang a, Wenjun Zou a, Yajie Zhang a, Zhen Wang a, Zaiyu Wang b, Jianqi Zhang a, Kun Lu *a, Wei Ma b and Zhixiang Wei *ac
aCAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China. E-mail: lvk@nanoctr.cn; weizx@nanoctr.cn
bState Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China
cUniversity of Chinese Academy of Sciences, Beijing 100049, China

Received 3rd May 2018 , Accepted 8th June 2018

First published on 15th June 2018


Aromatic end-capped acceptors are important in constructing donor materials and non-fullerene acceptors in organic solar cells. However, their features (such as electron-withdrawing ability and subtle change in planarity) and effects on molecular stacking and photovoltaic performance, lack a systematic study. This manuscript reports four molecules, namely, BT-RCN, BT-BA, BT-RA, and BT-ID, which are terminated by different acceptors in the same backbone. We quantify the molecular planarity through their dipole moment in the Z direction and investigate the effect of the degree of planarity on molecular properties and device performances. The four acceptors are classified into two groups based on acceptor strength: medium-strong and strong acceptors. Molecules based on medium-strong acceptors exhibit excellent efficiencies as follows: 10.1% for BT-ID, 9.6% for BT-RA, and 8.5% for BT-BA. Their decreased efficiency is quite consistent with their lowered hole mobility. Grazing incidence X-ray diffraction results demonstrate the positive relationship between the non-planarity of the acceptors and d-spacing distance in the π–π stacking direction, which are detrimental to mobility. BT-RCN, with a strong acceptor, obtains the lowest efficiency of 6.1%. These findings indicate the importance of matching the electron-donating ability of donor units and electron-withdrawing ability of acceptor units, and the subtle planarity change in molecular properties and aggregation.


1. Introduction

Acceptor–donor–acceptor (A–D–A) small molecules for organic solar cells (OSCs) have attracted considerable attention in recent years due to their extraordinary performance in active layers whether as donors1–8 or acceptors.9–13 As donors, they possess advantages of obtaining cheap and flexible devices in comparison with their inorganic counterparts and high-purity, as well as, definite molecular structures compared with polymers.11,14–19 They obtained impressive power conversion efficiency (PCE) and the highest efficiency based on fullerene and non-fullerene has surpassed 11%20,21 and 10%,22,23 respectively. As acceptors, they take advantage of easily tuneable molecular properties through chemical modification,9,24 and the highest PCEs based on polymers and small molecules have surpassed 13%10 and 10%,22 respectively. Either as donors or as acceptors, the end-capped acceptor units play a key role for optimizing materials.10,20

Selecting an appropriate (including further modifying) end-capped acceptor is an effective approach to regulate molecular photo-physical properties and microstructures, which are widely investigated in OSCs.10,25–32 As acceptors, Zhan et al. and Hou et al. have conducted detailed studies to tune the energy levels or molecular stacking by modifying the end-capped acceptor of 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IDCN);10,30,31 as the donor. Chen et al. experimented on various end-capped acceptors and finally improved the PCE from 1% to 10%.2,26–28 Recently, we reported the fluorination of the end-capped acceptor of 1H-indene-1,3(2H)-dione to modify their vertical and lateral morphology, which improved the inverted device performance from 8.3% to 11.3%.20 In addition, the boundaries between the donor materials and non-fullerene acceptors mainly depend on the end-capped acceptor strength. However, selection of an appropriate end-capped acceptor remains difficult due to the lack of systematic investigations on their structural features. Hence, investigations on the specific features (such as electron-withdrawing ability, degree of non-planarity) of end-capped acceptors, and their effects on their matching with donor units, molecular stacking, and photovoltaic performances, are significant.

In the present study, based on the molecular backbone we have reported recently20 (with thiophene-substituted benzodithiophene as the core and 2-(thiophen-2-yl)thieno[3,2-b]thiophene as π-bridges), we terminated the molecules with four commonly used end-capped acceptors: 2-(1,1-dicyanomethylene)rhodanine (RCN), ethylrhodanine (RA), barbituric acid (BA), and 1H-indene-1,3(2H)-dione (ID), Fig. 1(a). The corresponding small molecules are BT-RCN, BT-RA, BT-BA, and BT-ID, respectively, Fig. 1(a). The structural features (electron-withdrawing ability and degree of non-planarity) of the four end-capped acceptor units were investigated and compared. Their features were further confirmed to be retained in molecules through calculation of the dipole moment of half molecules (D–π–A structures) and the whole molecular dipole moment in the Z direction. Their effects on molecular properties, aggregations, and device performances were also explored in detail. For the strong end-capped acceptor of RCN, the strong electron-withdrawing ability would greatly down-shift the energy levels; however, its lowered highest occupied molecular orbital (HOMO) energy level increases the barrier of hole injection and its considerably strong acceptor strength decreases the electron-donating ability. Hence, the mobility and efficiency of BT-RCN are the lowest among the four molecules. For the medium-strong end-capped acceptors, the increased planarity would decrease the d-spacing distance in the π–π direction, which would cause the increase of the mobility and device performances. Consequently, all the devices based on molecules with medium-strong end-capped acceptors exhibited excellent PCEs as follows: 10.1% for BT-ID, 8.5% for BT-BA, and 9.6% for BT-RA.


image file: c8ta04102d-f1.tif
Fig. 1 (a) Molecular structures. (b) End-capped acceptor electron withdrawing ability calculated using their LUMO energy level. (c) Molecular planarity represented by their dipole moments in the z direction.

2. Results and discussion

2.1. Structural features of end-capped acceptors and their corresponding molecules

In D–A structured small molecules and polymers, the acceptor strength greatly affects the nature of coupling between the donor and the acceptor units, which determines the energetic alignment of frontier orbitals, intrinsic electronic and optical properties,33 and molecular assembly ability by tuning the strength of the D–A dipole moment.34 What is more, increasing the acceptor strength would convert the donor materials to acceptors. The planarity of the molecules causes the variation of molecular symmetry and aggregation hindrance.35 The four end-capped aromatic acceptors have similar molecular sizes without long alkyl chains; hence, their structural differences can be attributed to their diverse electron-withdrawing abilities, hindrance, and planarity.

According to the literature,33 the strength of the acceptor units can be characterized by the lowest unoccupied molecular orbital (LUMO) energy levels through density functional theory (DFT) calculations. The calculated energy levels and plots of the four acceptors are shown in Fig. 1(b) and Table S1 in the ESI, respectively. ID, BA, and RA exhibited similar LUMO levels (−2.47, −2.50, and −2.51 eV, respectively). After the strong electron-withdrawing group dicyanomethylene was substituted to ethylrhodanines, the LUMO energy level decreased from −2.51 eV (RA) to −2.83 eV (RCN). M. E. Köse et al. analysed the electron-withdrawing ability of a series of acceptor units combined with thiophene, and most of the calculated LUMO energy levels were between −2.0 and −3.0 eV.33 Hence, in this manuscript, we categorized that the acceptors with LUMO energy levels of ca. 2.5 eV as medium-strong acceptors, and those with LUMO energy levels of ca. 3.0 eV as strong acceptors. Consequently, in this work, RA, BA, and ID are classified as medium-strong acceptors, while RCN is considered as a strong acceptor. This acceptor strength trend is further supported by the calculated dipole moment of half of the small molecules (D–π–A; Fig. S1 in the ESI). In comparison with molecules BT-ID, BT-RA, and BT-BA (the dipole moments of half molecules are 3.5 D, 5.9 D, and 6.4 D, respectively), BT-RCN obtains much stronger push–pull ability [34], and consequently, the dipole moment of its half molecule is 10.0 D.

The planarity degrees of the four end-capped acceptors can be roughly represented by the number of sp3-hybridized N atoms. Because only the sp3-hybridized N atoms in the aromatic ring are tetrahedral and linked alkyl chains. Hence, ID (0 sp3-hybridized N) possesses the best planarity and the least hindrance among the four end-capped acceptors, which is opposite to that of BA (two sp3-hybridized N). RA and RCN (both one sp3-hybridized N) are in between ID and BA. The planarity degrees of the end-capped acceptors retained in the molecules as deduced from the molecular dipole along the Z direction, Fig. 1(c) and Table S2 in the ESI.36 With the increase of non-planarity degrees in the end-capped acceptors, the dipole moment of the corresponding molecules along the Z direction increased. For BT-ID, the high degree of planarity essentially cancels net dipole moments along the Z direction, while BT-BA exhibits the highest dipole moment (−1.54 D).36 Notably, the optimized molecular structures and the influence of structure on the electrostatic distribution among the different molecules were investigated via ab initio calculations by using DFT at the B3LYP/6-311G(d, p) level of theory (Fig. S2 in the ESI), and the dipole moments were calculated from the lowest energy conformations.36

2.2. Design, synthesis, and characterization of small molecules

The detailed synthetic procedures and characterization data for the small molecules are presented in the ESI (Scheme S1 in the ESI). The normalized UV-visible optical absorption spectra of the four small molecules in chloroform solution and in solid films are shown in Fig. 2(a), and their coefficients and other important parameters are summarized in Table S3 in the ESI. In comparison with the absorption spectrum in solution, the four molecules showed obvious shoulder peaks (attributed to π–π stacking) in the film, especially for BT-ID and BT-BA, indicating that the molecules exhibit good aggregation. Among the four molecules, BT-ID presents the most red-shifted absorption spectrum and highest absorption coefficient both in solution and films, which can be due to its optimum molecular backbone conjugation (best planarity).
image file: c8ta04102d-f2.tif
Fig. 2 Molecular properties. (a) Absorption spectra and coefficient of small molecules in solution and films. (b) Energy levels of small molecules.

The spectra based on BT-RA and BT-RCN exhibit similar shape whether in solution or in films, which can be attributed to their structure analogues. However, in comparison with BT-RA, BT-RCN exhibits ca. 20 nm red-shifted absorption in solution, indicating its stronger internal charge transfer (ICT) which is due to its increased acceptor strength. However, this disparity is improved in the film through aggregation because the ICT peak for BT-RA red-shifts ca. 20 nm which is larger than that of BT-RCN. In addition, compared with BT-RA, the slope of the absorption edge is less sharp, and the shoulder peak is less obvious based on BT-RCN. Both the above results demonstrate higher disorder in BT-RCN compared with BT-RA. This phenomenon is further supported and analysed in the section below.

For the molecules BT-RA, BT-BA and BT-ID based on medium-strong acceptors, their absorption spectra slightly red-shifted with their acceptor strength being decreased. Therefore, in this work, the stronger electron-withdrawing ability and smaller band-gap should be limited in a similar acceptor structure, and this trend can be disrupted by molecular packing.

To investigate the influence of the end-capped acceptors on the energy levels of the molecules, CV (Fig. S3 in the ESI) was carried out to measure the HOMO and LUMO energy levels of the small molecules. The energy level diagram is shown in Fig. 2(b). The HOMO energy levels of BT-RCN, BT-RA, BT-BA, and BT-ID are −5.49, −5.30, −5.25, and −5.19 eV, respectively, and their corresponding LUMO levels are −3.71, −3.58, −3.50, and −3.48 eV, respectively. The energy levels were calculated from the onset oxidation (ϕox) and onset reduction (ϕred) vs. Ag+/Ag according to the equations: HOMO = −e (ϕox + 4.71) (eV) and LUMO = −e (ϕred + 4.71) (eV). With increasing acceptor strength (ID < BA ≈ RA < RCN), their corresponding LUMO and HOMO levels are gradually down-shifted though the reduced amplitude variation, Fig. 2(b). The results are nearly consistent with the calculation (Fig. S2 in the ESI). Notably, compared with BT-RA, the introduction of dicyanomethylene to rhodanine in BT-RCN lowered the LUMO level from −3.58 eV to −3.71 eV, and the HOMO level from −5.30 eV to −5.49 eV. However, the lowered HOMO would maximize the ideal Voc and reduce energy loss, but it would also increase the hole injection barrier and may lower the driving force (ΔLUMO) for efficient exciton separation.

2.3. Photovoltaic properties of OSC devices

To investigate the end-capped acceptor effects on device performance, bulk-heterojunction solar cells with a device structure of ITO/PEDOT:PSS/Donor (D):PC71BM (A)/Ca (ca. 20 nm)/Al (ca. 100 nm) were fabricated. The photovoltaic properties of the fabricated devices under optimal conditions are listed in Table 1. The highest PCEs for BT-RCN, BT-RA, BT-BA, and BT-ID are 6.1%, 9.6%, 8.5%, and 10.1%, respectively. The detailed optimization of the device performances based on BT-BA, BT-RA, and BT-RCN is shown in Tables S4–6 in the ESI, and the detailed optimization of the device performances based on BT-ID is shown in our previous work.20,37 The addition of 0.2% 1,8-diiodooctane (DIO) greatly enhanced the PCE for BT-RA/PC71BM and BT-RCN/PC71BM. Hot plate (40 °C) spin coating enhanced the PCE for BT-ID/PC71BM from 9.5% to 10.1%.37 Notably, the optimal device performance of 8.5% obtained from BT-BA was without any post-treatment, and this is the highest PCE reported for BA as an end-capped acceptor. The short current-density versus voltage (J–V) curves of these devices under illumination of AM 1.5 (100 mW cm−2) and their corresponding external quantum efficiency (EQE) are shown in Fig. 3(a and b). The errors between the calculated Jsc and measured Jsc are within 5%.
Table 1 Optimized photovoltaic performance (device structure: ITO/PEDOT:PSS/active layer/Ca/Al)
Donors Additives V oc [V] J sc [mA cm−2] FF [%] PCEmax (PCEave)a [%]
a The average PCEs are obtained from over 16 devices.
BT-RCN 0.2% DIO 1.01 11.7 51.5 6.1 (5.95 ± 0.06)
BT-RA 0.2% DIO 0.99 13.7 71.0 9.6 (9.57 ± 0.07)
BT-BA None 0.99 14.1 61.0 8.5 (8.33 ± 0.18)
BT-ID None 0.95 15.3 71.0 10.1 (9.94 ± 0.09)



image file: c8ta04102d-f3.tif
Fig. 3 Device performances: (a) JV curves of optimized devices based on the four molecules. (b) Corresponding EQE curves, (c) hole mobility of the pristine films based on small molecules. (d) Hole mobility of blend films under optimized device conditions.

All four materials obtained high Voc values (1.01 V for BT-RCN, 0.99 V for BT-RA, 0.99 V for BT-BA, and 0.95 V for BT-ID), which are among the highest Voc values based on fullerene as acceptors. In comparison with BT-ID, the higher Voc values of the devices based on the other three materials can be attributed to their lower HOMO energy levels. However, compared with BT-BA, with further decrease of HOMO energy levels of the small molecules, the Voc was nearly unchanged. For BT-RA, this phenomenon can be due to the addition of DIO, which may shift the energy levels by modification of aggregation, because its Voc can reach 1.05 V without DIO (ESI Table S6). The Voc loss for BT-RCN is the largest (ca. 0.2 eV higher than that of BT-ID), which can be partly due to the HOMO shifts caused by the addition of DIO and partly due to its more disordered state. The differences in Jsc (11.7 mA cm−2 for BT-RCN, 13.7 mA cm−2 for BT-RA, 14.1 mA cm−2 for BT-BA, and 15.3 mA cm−2 for BT-ID) of the devices based on the four molecules are partly due to the diverse absorption coefficients and spectra, and partly due to the varied efficiency of charge separation and transfer, which will be further supported by their different molecular stacking and blend morphologies in the following sections. In addition to Jsc, the huge variation in fill factor (FF) (52% for BT-RCN, 71% for BT-RA, 61% for BT-BA, and 71% for BT-ID) leads to device performance varying from 6.1% to 10.1%. The large difference in FF and Jsc loss of the four molecules will be analysed in detail.

2.4. Hole mobility of small molecules

FF is mainly determined by the competition between the charge extraction and recombination. And charge transport ability of the molecules is vital to suppress charge recombination, enhance the charge extraction efficiency and finally reduce FF and Jsc loss. Consequently, the significant difference in FF and Jsc loss can be mainly due to the variation in hole mobility. Hence, the hole mobility of the pristine small molecules and optimized conditions were measured using the hole-only space-charge-limited current method in a device structure of ITO/PEDOT:PSS/D or D[thin space (1/6-em)]:[thin space (1/6-em)]A (1.5[thin space (1/6-em)]:[thin space (1/6-em)]1) with or without DIO/Au, Fig. 3(c and d). As expected, the hole mobility for pristine films varied according to the following order: 2.4 × 10−5 cm2 V−1 s−1 for BT-RCN, 1.74 × 10−4 cm2 V−1 s−1 for BT-BA, 1.21 × 10−3 cm2 V−1 s −1 for BT-RA, and 1.29 × 10−3 cm2 V−1 s−1 for BT-ID. With the higher hole mobility in the pristine films, the charge can be efficiently extracted prior to recombination from mutual coulombic attraction. Consequently, devices based on BT-ID and BT-RA with high mobility obtained higher EQE (highest values of both surpass 75%) and FF (both surpass 70%). With reduced mobility, the FF (61% for BT-BA and 52% for BT-RCN) and the highest EQE (71% for BT-BA and 64% for BT-RCN) are decreased.

The trend of hole mobility in the blend films is nearly similar to their pristine films, as follows: 7.22 × 10−6 cm2 V−1 s−1 for BT-RCN/PC71BM with 0.2% DIO, 2.16 × 10−5 cm2 V−1 s −1 for BT-BA/PC71BM, 3.11 × 10−4 cm2 V−1 s−1 for BT-RA/PC71BM with 0.2% DIO, and 2.23 × 10−4 cm2 V−1 s −1 for BT-ID/PC71BM (hot plate). In comparison with pristine films, all of the mobility values in blend films are decreased. This phenomenon is common in solution-processable small molecule OSCs, because the addition of PC71BM interrupts molecular connectivity.38

The balance between the hole mobility and electron mobility in the blend films is also vital to suppress the charge recombination and enhance the FF. Consequently, the electron mobility was characterized based on a device structure of Al/active layer/Al, and the electron mobilities are as follows (Fig. S4 in the ESI): 1.78 × 10−4 cm2 V−1 s−1 for BT-RCN/PC71BM with 0.2% DIO, 8.64 × 10−5 cm2 V−1 s −1 for BT-BA/PC71BM, 7.99 × 10−5 cm2 V−1 s−1 for BT-RA/PC71BM with 0.2% DIO, and 5.29 × 10−4 cm2 V−1 s −1 for BT-ID/PC71BM (hot plate). Among the four, the μh/μe for BT-RCN/PC71BM with 0.2% DIO is only 0.04, which is one or two magnitudes lower than those of the other three blends. Its seriously unbalanced charge transport should be attributed to its much lower hole mobility, which we would further analyse in the following part.

2.5. Morphologies

One of the most important factors to determine the hole mobility is the microstructure of the molecules. The molecular packing mode decides the charge transport routes, the crystalline coherence length in the π–π stacking direction affects the activation energy of charge transport, and the π–π stacking distance significantly affects the electronic coupling. Thus, the molecular packing of the four molecules was investigated through grazing incidence X-ray diffraction (GIXRD) to elucidate the impacts of the end-capped acceptors on the hole mobility of the pristine and blend films, Fig. 4(a–c). The detailed parameters are summarized in Table S7 in the ESI. All four molecules preferred edge-on molecular packing, because multiple higher-order (n00, n = 1, 2, 3) reflections in the out-of-plane direction and an evident (010) reflection of π–π stacking in the in-plane direction were observed. The crystalline coherence lengths in the π–π stacking (010) direction are 40.3, 47.1, 70.2, and 51.0 Å for BT-RCN, BT-RA, BT-BA, and BT-ID, respectively. The π–π stacking distances are 3.71, 3.72, 3.83, and 3.65 Å for BT-RCN, BT-RA, BT-BA, and BT-ID, respectively. Among the four molecules, BT-BA possesses the longest order range but the largest π–π stacking distance. Obviously, its low hole mobility (one order smaller than those of BT-RA and BT-ID) should be due to its poorer coupling results from its considerably larger π–π stacking distance.38 Hence, for the pristine films based on BT-BA, the π–π stacking distance, and not the order of π–π stacking, is the limiting factor that governs its mobility. Comparison between BT-RA and BT-RCN shows that they exhibit a similar π–π stacking distance (ca. 3.71 Å), but BT-RCN exhibits a shortened order range (40.3 Å for BT-RCN and 47.1 Å for BT-RA). This greater disorder in BT-RCN is consistent with our previous analysis on the absorption spectrum and its higher Voc loss. The slightly poorer aggregations for BT-RCN possibly resulted from its lowered symmetry as deduced from the molecular dipole moment in x, y, and z directions (Table S2 in the ESI). In addition, in comparison with BT-RA, the considerably stronger acceptor strength (dipole moment of the half BT-RCN is 10.0 D, which is considerably larger than that of BT-RA, which is 5.9 D, Fig. S1 in the ESI) increased the electron density disparity along the molecular backbone and lowered the electron donating ability. Consequently, for the pristine films based on BT-RCN, the order of the π–π stacking direction and electron distribution along the conjugation, and not the π–π stacking distance, are the main factors that govern the difference in their mobility. BT-ID obtained the highest mobility due to its smallest π–π stacking distance and long ordered π–π stacking.
image file: c8ta04102d-f4.tif
Fig. 4 GIXRD figures. (a) GIXRD images of the pristine and blend films. (b) Out-of-plane GIXRD plots of the pristine and blend films. (c) In-plane GIXRD plots of the pristine and blend films.

After blending with PC71BM (with/without DIO) under optimized device conditions, all four molecules adopted a mixed edge-on and face-on packing, which can be easily distinguished by the (010) peaks that emerged both in in-plane and out-of-plane GIXRD images. Among the four mixed-packing molecules, BT-RA exhibited the most obvious face-on packing, followed by BT-RCN, and BT-ID exhibited the least obvious (Table S7 in the ESI). The crystalline coherence lengths calculated from the π–π stacking (010) peaks are 54.6, 49.3, 38.3, and 44.3 Å for BT-RCN, BT-RA, BT-BA, and BT-ID. The trend of π–π stacking distance variation for the four molecules remained unchanged. The slightly higher hole mobility for BT-RA than BT-ID under the optimized device conditions might be due to the increased ratio of face-on packing in BT-RA (Table S7 in the ESI). After blending with PC71BM, BT-BA showed a coherence length that is only half of its pristine counterpart, indicating that BT-BA and PC71BM mixed quite well with each other, which was further confirmed by morphological analysis. BT-RCN exhibited an obvious face-on packing and increased crystalline coherence length in blends, which explained its slightly decreased mobility amplitude in blends than in pristine, in comparison with the other three molecules. The decreased hole mobility was due to the disrupted hole transport routes by PC71BM domains. In comparison with that in pristine, the different decreased degrees of hole mobility in blends can also be due to the varied scattered and blocked PC71BM domains in different blend films as well as varied change of coherence length and packing modes. In conclusion, after mixing with PC71BM (with/without DIO), the microstructure properties, such as packing modes and coherence length, changed; however, the π–π stacking distances exhibited a similar trend, and their hole mobility trends in blends were similar to those in pristine.

A transmission electron microscope (TEM) and an atomic force microscope (AFM) were used to further investigate the morphology of the active layer under the optimized device conditions based on the four molecules. The TEM images and AFM phase images are shown in Fig. S5 in the ESI. As seen from the AFM phase and TEM images, the film based on BT-BA exhibited the smallest domain size among the four; and by contrast, BT-RCN exhibited the largest domains, while BT-ID and BT-RA were in between. The AFM and TEM results further confirmed that BT-BA mixed quite well with PC71BM. The small domain size provided a large interface for exciton separation. Hence, though the hole mobility of BT-BA is low, its EQE still reached a high value at 70% and a good Jsc was obtained. The active layers based on BT-ID/PC71BM and BT-RA/PC71BM with 0.2% DIO exhibited an appropriate domain size, and thus obtained well-balanced exciton separation and charge transfer, and consequently, higher Jsc, improved FF, and better device performances.

3. Discussion and conclusions

Based on previous analysis, in this work, the acceptor strength does not obviously affect the absorption because different structures would lead to different electron conjugations. The stronger acceptor strength with red-shifted absorption is only limited to quite similar molecular structures, such as BT-RA and BT-RCN. However, strengthening the acceptor greatly influences the energy levels. The stronger the acceptor is, the higher the number of energy levels downshifted. Hence, the Voc values based on BT-BA, BT-RA, and BT-RCN are ca. 1 V. Notably, properly strengthening the acceptor would be an effective approach to lower the energy loss and improve the device efficiency. In addition, the acceptor strength was also reflected in the D–A dipole moment. The introduction of dicyanomethylene to rhodanine increased the D–π–A dipole moment from 5.9 D to 10.0 D. Hence, in comparison with BT-RA, BT-RCN increased the electron density disparity along the molecular backbone and lowered the electron-donating ability. Consequently, though BT-RA and BT-RCN exhibit minimal differences in molecular stacking whether in the pristine films or in blend films, the hole mobility of BT-RCN is 2-fold lower than that of BT-RA. In addition, though the HOMO level of BT-RCN decreased by ca. 0.2 and 0.25 eV in comparison with BT-RA and BT-BA, respectively, the Voc values based on the three molecules are nearly the same. Consequently, BT-RCN obtains the lowest EQE, FF, and device performances (6.1% with a Voc of 1.01 V, a Jsc of 11.7 mA cm−2, and a FF of 52%). In contrast, all the devices based on the other three molecules with medium-strong acceptors (RA, BA, and ID) obtain considerably better efficiencies (BT-RA with a PCE of 9.6%, a Voc of 0.99 V, a Jsc of 13.6 mA cm−2, and a FF of 71%; BT-BA with a PCE of 8.5%, a Voc of 0.99 V, a Jsc of 14.1 mA cm−2, and a FF of 61%; BT-ID with a PCE of 10.1%, a Voc of 0.95 V, a Jsc of 15.3 mA cm−2, and a FF of 71%) with a high Voc of ca. 1.0 V, and all the highest EQE values exceeded 70%. Hence, based on the same donor core (especially for the medium electron-donating ones), strengthening the acceptor should be within a certain scope, which would be an effective approach to lower energy loss. Beyond the range, a further increase in acceptor strength would decrease the electron density along the molecular backbone and lower the electron donating ability; much probably, changing the molecule from P to N type could be observed. Consequently, the molecule exhibits considerably lower hole mobility. Therefore, superior efficiency obtained for medium-strong acceptor showed that based on a medium-strong donor, a medium strong acceptor would be a better choice in terms of obtaining a high hole mobility

Based on the medium-strong end-capped acceptors (RA, BA, and ID), the disparity in device performances mainly resulted from their FF and not their HOMO energy levels and absorption spectrum. As analysed in the mobility and morphology parts for pristine films, the mobility differences of the three molecules mainly resulted from their π–π stacking distances. Interestingly, we found that with the increased planarity (dipole moment in the z direction decreased, Fig. 1(c)) the π–π stacking distance also decreased, Fig. 5(a). Consequently, BT-BA has the largest π–π stacking distance of 3.83 Å. BT-RA exhibits a π–π stacking distance of ca. 3.71 Å. The dipole moment in the Z direction for BT-ID is nearly 0. Consequently, BT-ID has the smallest π–π stacking distance of 3.65 Å. Hence, the subtle structural planarity change would cause a significant effect on electronic coupling even when only the π–π stacking distance changed and on device performances. The summarized comparison of PCE, FF, and hole mobility based on the four molecules are shown in Fig. 5(b).


image file: c8ta04102d-f5.tif
Fig. 5 (a) Relationship among molecular planarity, π–π stacking distance and hole mobility based on molecules with medium-strong acceptor units. (b) Reflection of hole mobility and device parameters with the four molecules.

In summary, we designed and synthesized four molecules, BT-RCN, BT-RA, BT-BA, and BT-ID, with the same backbone but different end-capped acceptors. We investigated their end-capped acceptor structural features (acceptor strength and non-planarity degree) and the structural features effects on molecular properties, microstructure properties, and device performances. BT-RCN, based on a medium-strong donor with a strong acceptor, possessed a similar absorption spectrum and morphology to those of BT-RA but obtained considerably lower hole mobility and low EQE, and its efficiency was only 6.1% with a Voc of 1.01 V, a Jsc of 11.4 mA cm−2, and a FF of 52%. All the other three molecules, BT-RA, BT-BA, and BT-ID, based on a medium-strong donor with a medium-strong acceptor, obtained considerably better efficiencies than BT-RCN, BT-BA, BT-RA, and BT-ID, with the obtained efficiencies of 8.5%, 9.6%, and 10.1%, respectively. The main disparity in the efficiency of the three molecules is their different hole mobilities, which resulted from their varied molecular planarity and absorption spectra. Our results indicate that a matching boundary should exist between the electron poor units and electron rich units. Instead of molecules constituted by the medium-strong donor units with strong acceptor units, molecules designed with a medium-strong acceptor is a more favourable strategy for donor materials design. The subtly varied planarity of the end-capped acceptors greatly affects the molecular stacking distances, which is an effective approach for tuning fine morphology.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

D. Deng and Yang. Y contributed equally to the manuscript. This work was supported by the Ministry of Science and Technology of China (No. 2016YFA0200700), the National Natural Science Foundation of China (Grant No. 51603051, 21534003, 21474022, 91427302, 51673049, 21603044 and 21504066), the Youth Innovation Promotion Association, CAS, and the Chinese Academy of Sciences.

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Footnote

Electronic supplementary information (ESI) available: Calculation data, synthesis schemes and procedures, characterization methods, device optimization details and AFM/TEM images. See DOI: 10.1039/c8ta04102d

This journal is © The Royal Society of Chemistry 2018